New Generation Silicon Solar Cells

New Generation Silicon Solar Cells
By Sarah Lindner Engineering Physics TUM New Generation Silicon Solar Cells

Table of contents 1. Introduction - Photovoltaics on the world market 2. Semiconductor 2.1 Electronic band structure 2.2 Metal – Isolator – Semiconductor 2.3 Definition 2.4 Doping 2.5 Intrinsic/Extrinsic 2.6 Conductivity 2.7 Direct/indirect band gap 2.8 Absorptioncoefficient 3. Solar cell – functionality 3.1 pn-junction 3.2 pn-junction under radiation 3.3 Solar cell characteristics 3.4 Equivalent circuit 3.5 Generation and recombination New Generation Silicon Solar Cells

Table of contents 3.6 Diffusion length 4. Solar cell – efficiency 4.1 Dilemma 4.2 Solar basics 4.3 Losses 4.4 Efficiency values 5. How to optimize silicon solar cells 5.1 Why still silicon? 5.2 Surface passivation 5.3 Reflection 5.4 Laser operations 5.5 Solar cell contacts 5.6 OECO-cell 5.7 Further prospects 6. Bibliography New Generation Silicon Solar Cells

1. Introduction - Photovoltaics on the world market
„In 2007 the photovoltaic market grew over 40% with ~ 2.3 GW of newly installed capacity“ (EPIA) Germany has the first position on the world market with 50% global market share power Installed by region: 80% Europe 16% North America 4% Asia Most dynamic market is Spain Seven Countries hosting the majority of large photovoltaic power plants: RoW, Italy, Japan, Korea, USA, Spain, Germany the cumulative power quadrupled Installed PV world wide 7300MWP Annual growth predicted ~ 25% Turnover by modules (2030) ~100billion €/a By 2030 a worldwide contribution of 1% is reached Global Solar Photovoltaic market estimated 2.3 GWp in 2007 The European Photovoltaic Industry Association (EPIA) reported that the photovoltaic world market (all types of PV systems, i.e. big power plants, private net connected systems and off grid PV) in 2007 grew by over 40 %, with approximately 2.3 gigawatt (GW) of newly installed capacity. Four countries mainly contributed to the global photovoltaic market growth: established countries such as Germany, Japan and the US; but also Spain, which made a large contribution by tripling its annual installations. Germany remains clearly in first position with a 50 % global market share. Japan’s market is estimated to have stagnated 2007, while Spain’s market approached 300 MW. The US may have registered a 260 MW market by the end of Other new European markets have confirmed the effectiveness of their feed-in tariff schemes:Italy, South Korea, France ~ 40-50MW The total installed capacity due to BSW-Solar reached roughly 3,8 gigawatt (in Germany) and the solar electricity produced by this systems is sufficient to supply the households of a metropolis like Hamburg. 80 % of all large photovoltaic plants (power related) are installed in Europe (700 MWp). The share of the USA accounts about 16 % (142 MWp) and in Asia 4 % (34 MWp) are installed. At present Germany hosts nearly 50 % of the world's installed photovoltaic power, but its market share was decreasing slowly within the last months. The most dynamic market is Spain - where an extreme increase of installed power has been observed in In the last decade only the USA and Germany created a steady growth of their photovoltaic market. The fast growth in Spain started about three years ago and led to an extreme increase in Further progress is visible in Europe and in South Korea. Italy, particulate France, and Greece turn out to be auspicious markets. The rest of the world (i.e. Africa, South America and Australia) represents less than 1 % of global installed PV power but shows significant potentials for future solar energy use in these regions. Countries with cumulative installed power more than 1 MW of large photovoltaic power plants (> 200 kWp each considered plant) are listed in Table 2 a the end of this report. Germany leads with more than 400 MW, followed by Spain (almost 250 MW) that displaced the USA (140 MW) at the second position. Italy and Japan (each about 17 MW) Korea (13 MW) and Portugal (12 MW) anyhow reached two digit figures. Countries with less than 1 MWp installed are Thailand, France (without overseas territories), United Kingdom, Malaysia, Saudi Arabia, Luxembourg, Rwanda, India and Mexico. Primary PV world markets are still Germany with about 45 % of the installed power, followed by Spain (28 %) and the USA with 16 % market share. Spain proved as the most dynamic PV market with an impressive growth that might be probably lower this year. The average installed capacity of a single large commercial power plant has increased from 400 kWp in 1997 to 1,64 MWp in The average capacity of sole commercial PV plants accounts for 1,14 MWp. Substantial market: Market will support employment on the order of several million people worldwide Global energy justice: Providing affordable power to everybody Annual installed power grew significantly from 2004

2.1 Electronic band structure
One single atom  discrete energy levels Bring atoms close together , e.g. crystall lattice  Interaction of the electrons Energy levels split up band structure Discrete energy levels Vokabel: Reciprocal lattice Die Bandstruktur beschreibt die Zustände von Elektronen und Löchern eines kristallinen Festkörpers im Impulsraum und damit die Beschaffenheit dessen elektronischer Struktur. Sie ist die Dispersionsrelation von Elektronen unter dem Einfluss des Festkörper-Gitterpotentials. Das Energiebändermodell eines Festkörpers ist im Wesentlichen die im Impulsraum dargestellte Bandstruktur. Number of orbitals become exceedingly large, Difference in energy between the orbitals very small  band structure. Inner bands are very small, because the electrons are bound very strong to the core. Different to the outer electrons, where the width of the band is a lot bigger. The band gaps behave reversed.  overlap of the outer bands The band structure and the occupation of the electrons determines the electrical and optical properties. Inner bands are all occupied, although the electrons have to follow some rules to occupation. In inner bands electrons can just exchange their place  no conduction Highest occupied band is the valence band. In order to have conductivity the electrons must get excited into the conduction band, where they can move and thereby create current. Band structure of silicon: A more complete view of the band structure takes into account the periodic nature of a crystal lattice. k is called the wavevector, and is related to the direction of motion of the electron in the crystal. The wavevector k takes on values within the Brillouin zone (BZ) corresponding to the crystal lattice. The available energies for the electron also depend upon k, as shown for silicon in the more complex energy band diagram at the right. In this diagram the topmost energy of the valence band is labeled Ev and the bottom energy in the conduction band is labeled Ec. .  indrect band gap  needs something to give it energy Ec − Ev and a change in direction/momentum. Merke: Although the number of states in all of the bands is effectively infinite, in an uncharged material the number of electrons is equal only to the number of protons in the atoms of the material. Therefore not all of the states are occupied by electrons ("filled") at any time. The likelihood of any particular state being filled at any temperature is given by the Fermi-Dirac statistics. The Fermi level naturally is the level at which the electrons and protons are balanced. Regardless of the temperature, f(EF) = 1 / 2. At T=0, the distribution is a simple step function: f(E) = \begin{cases} 1 & \mbox{if}\ 0 < E \le E_F \\ 0 & \mbox{if}\ E_F < E \end{cases} At nonzero temperatures, the step "smooths out", so that an appreciable number of states below the Fermi level are empty, and some states above the Fermi level are filled. Brillouin zone: Because electron momentum is the reciprocal of space, the dispersion relation between the energy and momentum of electrons can best be described in reciprocal space. It turns out that for crystalline structures, the dispersion relation of the electrons is periodic, and that the Brillouin zone is the smallest repeating space within this periodic structure. For an infinitely large crystal, if the dispersion relation for an electron is defined throughout the Brillouin zone, then it is defined throughout the entire reciprocal space. Band structure of Mg with potential well Band structure of silicon E(k) New Generation Silicon Solar Cells

2.2 Metal – Isolator – Semiconductor
either the conduction band is partly filled or no seperate conduction and valence band exist electrons can move freely T ↑  resistivity ↑ electrons give their energy to the phonons very fast ~ 10-12s Metal: The band is partly empty partly filled with electrons, regardless of Temperature Its resistivity is very low and increases with the Temperature, because of the rising thermal vibration caused by the atoms Fermi-level (the energy of the highest occupied quantum state in a system of fermions at T=0K) is within the conduction band Easy for metall electrons to give their energy to the phonons because they have a almost continuate band of energy, energies they can jump to  give energy fast to phonons in very small portions (10E-12 s) Phonons: quantized mode of vibration occuring in a rigid cristall lattice; can not be localized, have energy from 0 – 0,05 eV; major role in material’s thermal and electrical conductivity. Isolator: No free electrons which make conductivity possible. If the Temperature is high enough, than a very small conductivity is possible Fermi level between the to bands Isolator: at T = 0 the conduction band is empty  very high resistivity band gap EG > 3eV no conductivity despite doping possible Band structure New Generation Silicon Solar Cells

2.2 Metal – Isolator – Semiconductor
isolator for deep temperatures (T = 0) conduction band at low temperatures as good as empty, valence band almost full band gap 0,1eV < EG < 3eV T ↑  resistivity ↓ Electrons can stay in the conduction band for about 10-3s (intrinsic semiconductor) Semiconductor: conduction band as good as empty, valence band almost full  very low conductivity at the beginning band gap 0,1eV < EG < 3eV supply of thermal energy  easy for electrons to get into the conduction band  strongly increasing conductivity with rising temperature Fermilevel, for intrinsic semiconductor  intrinsic: no doped materials, tell you in a bit what doping means E-Photon >= E-Gap  electrons in conduction band, they give up their energy very fast to the phonons until they reach the Edge of the conductionband, than the energy they have to give to the phonons is to big  they stay a while in the Con-Band. 10 E-03 seconds. Band structure New Generation Silicon Solar Cells

2.3 Definition A semiconductor is a material that has electrical conductivity between that of a conductor and that of an insulator Its resistivity decreases with increasing temperature and therefore its conductivity increases. Sometimes ist conductivity can get as high as the one of metall, but this holds just if the band gap is Very small  Eg <0,1eV, so called semi – metal. New Generation Silicon Solar Cells

2.4 Doping Doping: Change in carrier concentration  change in electrical properties Donor - doping add an extra electron number of e- > number valence e- n – type dopant ED right under conduction band EC Acceptor - doping add an extra hole number of e- < number valence e- p – type dopant EA right above valence band EV Doping: intentionally introduce impurities into an extremely pure semiconductor  change conductivity because of higher concentration of carriers By this you can change the semiconductor‘s electrical properties. In order to do change its conductivity you just need a small amount of dopants. 1 out of 100,000,000  light/low doping 1 in 10,000  heavy or high doping N-type: add an extra electron by adding an donor atom which has one more valence electron than the semiconductor material the extra electron is bound very weak to the atom.  easy to become a free electron by this to distribute to the current electrons are the majority carrier and protons are the minority carrier these electrons are on a extra energy level called ED for donor and is right under the edge of the conduction band and above the Fermi level  electron goes from ED to Ec means donor-atom becomes inonized  higher conductivity even at room temperature , almost free electrons P-type: electrons with less valence electrons are added. Ectra level of energy EA (under Fermi level) therefore it is easier for electrons to get out of the valence band  more holes are created  holes majority carriers. concentration of holes is higher. acceptors tend to bond an extra valence electron Die Bewegungsrichtung der Löcher verhält sich dabei entgegengesetzt zu der Bewegungsrichtung der Elektronen und somit in Richtung der technischen Stromrichtung (von plus nach minus) Thermal equilibrium  concentration of n and p is equal n-type doping p-type doping New Generation Silicon Solar Cells

2.4 Doping New Generation Silicon Solar Cells 19.04.2017
One thing you should remember. Concentration of the electrons * c protons = const, which depends on the temperature, but just if you have no radiation. If c electron inreases, c proton decreases  but conductivity increases!! Merke: Bei einem intrinsischen Halbleiter ist die Bandlücke temperaturabhängig und ändert sich zwischen 0° und raumtemperatur um etwa 10° Grund: Wärmeausdehnung des kristalls  periodisches potential des kristalls ändert sich, New Generation Silicon Solar Cells

2.5 Intrinsic/Extrinsic Intrinsic Extrinsic pure semiconductor
doped semiconductor n = p n ≠ p self conduction Self conduction + conduction because of doping Conductivity depends on T Conductivity depends on T and on additional charge carriers (dopant) Change in EF At thermal equilibrium T>0 The carrier concentration depends on temperature  high T  many carriers can get into the conduction band (generation of holes and electrons) intrinsic: Atoms of a semiconductor usually are bound kovalent. (tetraedrical) Almost no free electrons  very low conductivity for T = 0 (kovalent bonds are intact) Because of the interaction of the atoms, the electrons belong to actually all atoms, not just to the kovalent bound of two atoms. T>0 With temperature increasing some kovalent bonds break.  electrons go into the conduction band and holes remain in the valenceband ≈ broken kovalent bonds Permanent exchange of electrons between kovalent bonds.  holes are in thermal movability e.g. Silicon has a little ability to self conduction, because not a lot free electrons are available at roomtemperature New Generation Silicon Solar Cells

2.5 Intrinsic/Extrinsic Intrinsic case Extrinsic case
Bilder aus Elementare Festkörperphysik und Halbleiterelektronik Intrinsic: T=0 Efi trennt die beiden Bereiche der Fermi-Dirac-Funktion. für T>0 there is a small possibility that some eletrons are in the conduction band. But always n=p because if an electron moves to the conduction band, at the same time a hole is created Fermi energy level is always in the middle!! Exact theory: fermi level depends on the temperature and you have to consider the effective mass of the electrons and holes Extrinsic (for n-type): T = 0 Fermi level between Ed and Ec. Because there are electrons in the donor energy level Efi musst be inbetween Ef ~Ed/2 T>0 two processes of excitation 1. electron – proton couple by excitation of an electron from the valence into the conduction band (Thermal excitation) 2. electrons from Ed to the conduction band For small T the 2. process dominants, for T = 300K the first dominants because almost all electrons are excited out of the donator band  seems like an intrinsic band Fermi dirac distribution  F(Ef) = 0,5 For p-type: Electrons get into the Ea band. The holes will be excited downwards into the acceptor level. Fermi-level for a) T = 0K and b) T > =K Fermi- level for n-doped semiconductor and T > 0K

2.5 Intrinsic/Extrinsic New Generation Silicon Solar Cells 19.04.2017
The fermi level depends on the temperature as you can see. It also depends on the concentration of the charge carriers and the width of the band gap. For high T the fermi level is equal to the intrinsic Fermi level Switch of the Fermi level with increasing temperature a) n-doped b) p-doped New Generation Silicon Solar Cells

2.6 Conductivity σi depends strongly on the temperature and the charge carrier densities extrinsic conductivity depends additionaly on excitation of dopants into the conduction band. The charge carrier density you get from the fermi dirac distribution which depends on the temperature. The density of n and p is the same, the last part of the equation describes the density. C is a constant and includes the effectve masses. μ is the mobility and one factor next to the temperature that influences the conductivity μ is limited because of the scattering by the phonons  T ↑ than μ ↓  μ ~ T-3/2 And n = C * T3/2 exp ( - Eg/2kT) concentration (number/Volume) Both of these equations give an intrinsic conductivity which strongly depends on the temperature. Extrinsic: For high temperatures all electrons are excited into the conduction band, donor level is empty.for an extrinsic semiconductor you can say, that the conductivity in low temperatures mostly depends on the conductivity of the dopants. For high T the intrinsic conductivity dominates, because all electrons have allready been excited.  try to be in the saturation range  around room temperature always the same electron density and therefore the same conductivity  you can adjust the conductivity. New Generation Silicon Solar Cells

2.7 Direct/indirect band gap
Material c-Si a-Si:H GaAs Band gap 1,12 eV (indirekt) 1,8 eV („direct“) 1, 43 eV (direct) Absorption coefficient (hν = 2,2) [cm-1] 6*103 2*104 5*104 Momentum and Energy are always conserved!!!!!!!!!!! Indirect: In semiconductor physics, an indirect bandgap is a bandgap in which the minimum energy in the conduction band is shifted by a k-vector relative to the valence band. The k-vector difference represents a difference in momentum.  a direct transition from one band to the other is forbidden, because of the conservation of momentum These recombinations will often release the bandgap energy as phonons, instead of photons, and thus do not emit light. As such, light emission from indirect semiconductors is very inefficient and weak. If you have a solar cell, you do not just need a photon with the right energy but you also need have a momentum of the right value.  absorb a phonon to compensate the k-vector Electrons stay longer at the bottom (energy minimum) of the conduction band, because in order to get back to the valence band they need a change In momentum which they can not get from the photons, because this momentum is negligible. This transition is less possible, because you need three bodys to happen (photon, phonon or crystall defects, charge carrier) e.g. crystalline silicon is an example. These cells have to be thicker in order to work, because the transition happens less often. You can also make it work if you get total reflection by a special surface characterisation. The absorption (read: color) of an indirect bandgap material usually depends more on temperature than that of a direct material, because at low temperatures (e.g. 4K) phonons are not available for a combined (vibronic) process. Silicon e.g. starts to transmit red light at these temperatures, because red photons do not have sufficient energy for a direct process. Indirect and direct band gap Indirect: need a photon, a phonon, and a charge carrier  happens more seldom  longer absorption length recombination at grain boundarys and point defects Direct: need just the right photon for band transition higher transition probability New Generation Silicon Solar Cells

2.8 Absorptioncoefficient
For semiconductors with a direct band gap the Absorption coefficient is high and the needed layer of the material can be relativly thin. This does not work for crystaline silicon  the usual width is about 300µm. If you want to have the same absorption effect for silicon as for GaAs you need at least about 100µm for silicon and 1µm for GaAs. New Generation Silicon Solar Cells

3.1 pn-junction Equilibrium condition, no bias voltage
diffusion current opposite to the E-field diffusion voltage V0 with ∆E = eV0 at diffusion force = E-field force V0 is the electrial voltage at the equlibrium state = diffusion voltage Equilibrium condition: Two differently doped materials n and p are brought together for a abrupt junction  than a diffusion current develops because of the potential difference of the junction. Electrons diffuse to the p-doped layer and protons diffuse to the ne doped layer. As electrons diffuse, they leave positively charged ions (donors) on the n region. Similarly holes near the PN interface begin to diffuse in the n-type region leaving fixed ions (acceptors) with negative charge. The regions nearby the PN interfaces lose their neutrality and become charged, forming the space charge region or depletion layer (see figure A). This space charge region has almost no free charge carriers. The width of this depletion layer depends on the doping.  and the space charge region expands more into the less doped side. The majority carriers become minority carriers.  potential gradient, which makes the now majority carriers to move away from the junction into the semiconductor. And there the can recombine with the still existing majority carriers. Because of the different types of now majority carriers in the depletion layer an electric field is formed.  equilibrium between the diffusion current and the current because of the electric field. The Fermi levels of both of the doped materials agline. The fermi levels of the inner semiconductor stay as they are  an electron in the p-area has now more energy as one in the n-area. The energy levels of the p-area are moved up. And the fermi levels are bend in the depletion area. The width of the space charge region and the hight of the voltage V0 depends on the doping. The space charge region has almost no free carriers anymore, therefore its resistance is huge, and all the voltage, V0 and the outer Voltage drop of at the junction. Some facts: Drift: electrons in the conductionband in the p-region, are there because of thermal energy, they move coincidently with kinetic energy. In the pn, some electrons will be moved to the n-region, and this depends on the number of electrons in the p-region, and this depends on the Temperature. Diffusion: depends on the number of electrons in the n-region and the probability that a electron can negotiate the potential barrier. New Generation Silicon Solar Cells

3.1 pn-junction Band structur for n-doped and p-doped semiconductor
before contact b) Band structure after contact c) Depletion area Bending of the conduction and valence band. im n-bereich verbleiben nach der diffusion die positiven donator-ionen  depletion layer poor in free charge carriers  complete charge in the depletion layer still zero. The width of the depletion layer depends on the doping density and on a bias voltage which usually is zero for solar cells. Xp * Na = Xn * Nd wenn Xn,p die breite der jeweiligen raumladungszone ist.  Überschuss an Minoritätsladungträgern.  Konzentrationsgradient  treibt die Minoritätsladungsträger noch weiter in die Kernregionen des Halbleiters  stop durch elektrisches Feld  Gleichgewicht zwischen Diffusionstrom und dem strom des e-feldes.  im Gleichgewicht fließt kein strom und die fermilevel gleichen sich an. New Generation Silicon Solar Cells

3.2 pn-junction under radiation
Absorption of light: If Eph < Eg  no electron-hole-creation If Eph > Eg  electron-hole-creation  drift and diffusion  current and voltage Electron-hole creation, In the space charge region the electric field seperates the carriers. Electron goes to the n-region.  n region becomes more and more negativ.  electric field  the diffusion potential decreases Outside of the space charge region: can get into the region by diffusion because of the thermal energy. But depends on the diffusion length of the electron in the special material It depends strongly on the recombination processes how many carriers get to the particular side. The potential barrier prevents that each carrier can go to the wrong side. At some time the created potential prevents more electrons to get to the negative region. That is when the created electrical potential equals the diffusion potential of the pn junction  open circuit voltage Band structure Solar cell under radiation New Generation Silicon Solar Cells

3.3 Solar cell characteristics
 Isc = -Iph for V = 0 The solarcell creates a photocurrent with the created free electrons and the electric field.  So Iph ~ number of photons Later we will see graphs where the current is positive, which is actually the conventional writing. But the current created is negative, therefore the current moves from negative to positive and the power generated is negative, thus generated. (4. quadrant) Diode: The Shockley ideal diode equation or the diode law is derived with the assumption that the only processes giving rise to current in the diode are drift (due to electrical field), diffusion, and thermal recombination-generation. It also assumes that the recombination-generation (R-G) current in the depletion region is insignificant. This means that the Shockley equation doesn’t account for the processes involved in reverse breakdown and photon-assisted R-G. Additionally, it doesn’t describe the “leveling off” of the I–V curve at high forward bias due to internal resistance. Under reverse bias voltages (see Figure 5) the exponential in the diode equation is negligible, and the current is a constant (negative) reverse current value of -IS. The reverse breakdown region is not modeled by the Shockley diode equation. For even rather small forward bias voltages (see Figure 5) the exponential is very large because the thermal voltage is very small, so the subtracted ‘1’ in the diode equation is negligible and the forward diode current is often approximated as I=I_\mathrm{S} e^{V_\mathrm{D}/(n V_\mathrm{T})} The use of the diode equation in circuit problems is illustrated in the article on diode modeling. Constants: Io is the saturation inverse current n = The emission coefficient n varies from about 1 to 2 depending on the fabrication process and semiconductor material and in many cases is assumed to be approximately equal to 1 (thus the notation n is omitted). By these two equations you can determine the short circuit current Isc and the open circuit voltage Vos. 1 can be neglected for low currentdensities in the voltage equation. Explanation: The more the light radiates on to the solar cell, the more the electrons move to the other side and create a voltage. The more the voltage raises, the less electrons can move to the other side. Than you can measure the two magnitudes if you add a load of the size of R = Vm/Im I-V characteristic of a solar cell  for I = 0 I0 is the saturation current n is the ideality factor k is the Boltzmann`s constant Isc is the short circuit current Voc is the open circuit voltage New Generation Silicon Solar Cells

3.3 Solar cell characteristics
Maximum power point (MMP) depends on: Temperature Irradiance Solar cell characteristics Wilson s. 209 MMP: If the radiation increases, the short circuit current current increase, the open circuit current increases too. and the power (mmp) of the solar cell increases slightly If the temperaturer increases, the voltage drops because the diffusion voltage drops strongly and diffusion current raises lightly, because of increasing mobility of the charge carriers, and the power drops to. e.g. T steigt um 50K  Voc sinkt um 85mV um 14% in ähnlicher weise sinkt der Wirkungsgrad The FF is the ratio from blue to yellow area. An ideal solarcell has constant current until ist maximum voltage. (yellow) thus FF = 1 the FF-factor tells you something about the quality of the solarcell. Usually ist about 0,75 – 0,85.  helps you to controll the process. bestimmt durch serienwiderstand : ohmsche Kontaktverluste an Emitter und Basis Shuntwiderstand: parasitäre Kurzschlüsse des pn übergangs Sättigungsstrom der zweiten Diode = rekombination in der Raumladungszone Efficency coefficient is determined under Standard Test Conditions (STC) where the radiation ist 1000W/m^2 and the air mass is 1,5 and the temperature of the solarcell is constantly 25°C.  Performance of solar cell Fill factor Efficency coefficent New Generation Silicon Solar Cells

3.4 Equivalent circuit New Generation Silicon Solar Cells 19.04.2017
RS and RP represent the loss a real solar cell is liable to For a good solar cell RP should be big, and RS should be small which means a gute Sperrwirkung und kleiner Io Sperrsättigungsstrom. First Diode: Without any radiation we have seen that the solar cell acts like a diode. (pn) this is still true for a solar cell under radiation. The first diode describes the losses by the recombination in the volume and the emitter of the solar cell. Rs, Rp, and sometimes a second diode describe the different processes where you can loose energy. Rp: represents cristall defects, non ideal doping or material defects.  so it represents the leaking currant at the edge of each solarcell. Rs:represents the resistance of the contact material, the service connection or also called the feed lines and the resistance of the semiconductor material. Second Diode: Describes the recombination of charge carriers in the space charge region. I01 and I02 are the saturation inverse current for each diode. Equivalent circuit New Generation Silicon Solar Cells

3.5 Generation and recombination
n0  n0 + ∆n = n Recombination and generation processes. Generation processes depend on absorption and on flow of photons   G = R  Life time of minority carriers: There are different recombination processes, the generated charge carriers will be built back into the cristall. Each recombination process i has it‘s own recombination rate, and ist own minority charge carrier living time τ Bulk: the first three processes go on parallel and do not depend on each other. The first three components describe the life time of a minority carrier in a special Volume. But you also have to think about the surface recom. Which goes on parallel too. you get a effective charge carrier life time of τeff Typical values for solarcells are 1-100μs Life time: The functionallity of a solar cell depends on the minority charge carriers, mostly the electrons, because for solar cell p-doped silicon is mostly used. Life time of an electron is important.  Because n0 is very small you can say n = ∆n The life time also depends on the density of th holes. p = p0 + ∆p  po depends on the doping, ∆p depends on the excitation  here ∆n = ∆p ∆n depends on the excitation and is also called injection.  ∆n<<p0 is low injection. ∆n is the surplus concentration Ri is the rate of recombination n0 is the concentration at equilibrium n is the charge concentration G is the rate of generation New Generation Silicon Solar Cells

Recombination by radiation Auger-recombination Radiation: The inverse process of absorption. A electron goes over into the valenceband by radiation of electromagnetic radiation. Proportional to occupied state in the conduction band and unoccupied states in the valence band. B = a material konstant N0 und p0 are the concentrations of electrons or holes in thermal equilibrium Ni is the intrinsic charge darrier concentration of the material  for silicium this plays a minor role because here you need a phonon as we know  very improbable Auger-recombination: Electron recombinates. Free energy but not in form of a electromagnetic radiation. A second Electron or hole in either band get the energy. This second charge carrier gives ist energy back in form of phonons to the cristall latice. Mainly in high doped material >1017 cm3  for high doped materials the life time of minority carreirs is highly reduced by this formular  Stärkster begrenzender faktor bei solarzellen mit dicke <1mm New Generation Silicon Solar Cells

Recombination by impurity τSRH depends on: Number of impurities Energy level of impurities Cross section of impurities Recombination on the surface Untreated silicon surfaces S > 106 cm/s Depends strongly on charge carrier injection and doping Recombination by (impurities shokley read hall) There are impurities because of cristall defects or impureness.  energy levels between the two bands in the forbidden zone  two steps recombination way Electrons fall into the energylevel of the impuritý and from there in the valence band. The life time SRH depends on the numper of the impurities and energetic level of these impurities and from the cross section. The more the level in the middel of the two bands, and the bigger the cross section the worse the life time. SRH theory believes in one defective state between the band gap which interacts with the bands by emission or einfang of electrons and holes. This process is the limiting recombination process in multicristall silicon and Czochralski-Silicon. If you use moncristall silicon or float zone silicon than there is almost no recombination because of impurities. FZ is monocristall and free of dislocation. One impurity on 1 million particles Surface: Correspond to high mistakes in the cristall!! Because the cristall structure breakes up very abruptly Dangling bonds: free bonds will be filled up with foreign atoms this means new impurities or the bonds will not been filled and stay unoccupied.  many different energy levels at the surface in the forbidden zone. Because of these energy levels many created electron hole couples will be destroyed. This works for the back and the front of the surface. Die defektniveaus in der Bandlücke bilden ein Kontinuum An der Oberfläche bildet sich eine Raumladungszone aus  ferminiveaus und ladungsträgerdichten unterscheiden sich von denen im Volumen Annahme, dass die einzelnen Defektzustände nicht miteinander wechselwirken,  Rekombinationsrate der Oberfläche ist die Summe aus den Beiträgen aller Defektzuständee zur SRH rekombination S is the surface recombination velocity and the velocity on the back and on the front surface is equally if you look at it simply S considers the behaviour of the SRH recombination and the influence of the depletion area. S= Oberflächenrekombinationsrate(Teilchenstromdichte welche in der Oberfläche fließt) / delta n S depends on the potential of the surface, but even stronger on the charge carrier injectionand the doping D is the diffusionn constant of the minority carriers which depends on the material, the Temperature and concentration of the doping W is the thikness of the material For a untreated silicon material is S > 106 cm/s New Generation Silicon Solar Cells

radiation Low p0  SRH is dominant and τ independet of p0 High p0  τ ~ p0-2 (Auger recombination) radiation recombination plays no role for silicon Normal sunlight radiation the basis of the solar cell is in the are of the SRH recombination 105 104 103 102 101 100 Auger τ [µs] SRH Experimental Low hole density  t is constant the SRH is dominant SRH is independent of the equilibrium hole density which is determined by the doping. High hole density  t ~ po^-2 (auger)# Because of the doping, and the low injection the basis of the solar cell is in the field of the SRH recombination. SRH theory believes in one defective state between the band gap which interacts with the bands by emission or einfang of electrons and holes. Bei niedriger injektion delta n<<p0 ist die lebensdauer unabhängig von der ladungsträgerdichte SRH dominiert. Wird delta n = p0 überschritten, dann fällt oder steigt die lebensdauer, je nach typ des dominanten rekombinationszentrum. Für hohe injektion dominiert Auger  lebensdauerabfall mi (delta n)^2 für delta n ~ 10^16-10^18 cm^-3 p0 [cm-3] Low innjection, depenence between hole equilibrium concentration and τ

3.6 Diffusion length Is the mean free length of path a charge carrier can travel in a volume of a crystall lattice before recombination takes place. depends on: The semiconductor material The doping The perfection of the crystall lattice D is the diffusion constant Merke: Ladungsträger bewegen sich aus Bereichen höherer Konzentration in Bereiche geringerer Konzentration D is the Diffusion constant After one Diffusion length, the number of light generated charge carriers decreases by 1/e exponentialy. Silizium Teff = 1μs - 100μs Bei 300K im p-type silicon Dn = 26,9 cm^2 /s und Dp=10,7cm^2/s Bei 300K in n-type Dp = 11,6 cm^2/s und Dn = 32,3cm^2/s New Generation Silicon Solar Cells

3.6 Diffusion length Silicon: (10 μm - 100 μm)
λ < 800nm light absorbed within 10μm λ > 800nm electron-hole generation all over the volume  for an effectiv solar cell the diffusion length has to be 2-3 times thicker than the actual solar cell Multichristall silicon τeff = 50μs Leff,n (cm) Leff,p (cm) p-type 0,037 0,023 n-type 0,040 0,024 Silicon: λ < 800nm a huge part of the sunlight will be absorbed in silicon within 10μm  the pn junction is right under the surface.  most of the light generated carriers will be generated under right next to the pn junction. They just have to diffuse a few microns towards the pn junction to distribute to the solar current λ > 800nm electron-hole generation all over the volume  longer way into the pn-junction and the probability to reach this junction before they recombinate ist very small. Silizium Teff = 1μs - 100μs Bei 300K im p-type silicon Dn = 26,9 cm^2 /s und Dp=10,7cm^2/s Bei 300K in n-type Dp = 11,6 cm^2/s und Dn = 32,3cm^2/s New Generation Silicon Solar Cells

4.1 Dilemma P = U * I ideal band gap size, depending on the solar spectrum  The usuall ideal band gap is supposed to be at EG = 1,5eV A small band gap causes a big short circuit current, because many photons will create electron-hole-couples. A big band gap causes a larger potential barrier and therefore a larger open circuit voltage. Try always to get maximum power. Power equals P = U*I Now the current increases the more the photons create electron-hole couples. This happens when the gap is pretty small. The Photocurrent is proportional to the absorbed photons  the gap becomes smaller, the current raises!! But the band gap limits the potential barrier you can get out of the pn-junction. A small band gap creates a small diffusion votage and therefore a small open curcuit Voltage. Voltage decreases with the width of the gap. New Generation Silicon Solar Cells

4.2 Solar basics AM0 solar spectrum 1353W/m2 Black body curve 5762K
Äquator: 320W/m^2 B.Deutschland: 114W/m^2 You can see the spectrum of the sun. Outside of the atmosphere it is similar to a black body radiation spectrum. And the sun has a T =5762K The radiátion is weakend by the scattering at the atoms in the atmosphere e.g. air molekules, water particles, monocristall, etc.  no energy transmission absorption in the atmosphere  energy will be converted into heat Reduced also by reflection  Out of this you get the global radiation on the earth = direct sky radiation and diffuse sky radiation The maximum of the Energy is in the visible array at 0,5-0,6 μm In the UV-area the radiant power decreases very fast. In the infrarot area the radiant power decreases more slowly.  The intensity decreases, because of the CO2 and the water in the atmosphere. In the visible area the absorption comes from the ozon. Can see some deep incisions  comes from the selective absorption within the atmosphere caused by some particles in the atmosphere hours of sun ín germany  more in the south of germany. Spectral distribution of solar radiation. New Generation Silicon Solar Cells

4.2 Solar basics AM = air mass = degree to which the atmosphere affects the sunlight received at the earth`s surface The factor behind tells you the length of the way when the light passes through the atmosphere. Standard Test Conditions (STC): Temperature of 25°; irradiance of 1000W/m2; AM1.5 (air mass spectrum) Outside of the atmosphere the sun assimilates an black body radiation with T = For AM0= the radiation power has a value of 135mW/cm^2 AM0 means the characteristic of the radiation outside the atmosphere. AM1 means that the light goes vertically trough the atmosphere. AM1,5 means the length of the way of the light is 1.5 times longer, thus the angle of incidence is 41.8° The efficiency of a solar cell is always measured under Standard Test Conditions: STC specifies a temperature of 25°C and an irradiance of 1000 W/m² with an air mass 1.5 (AM1.5) spectrum. These correspond to the irradiance and spectrum of sunlight incident on a clear day upon a sun-facing 37°-tilted surface with the sun at an angle of 41.81° above the horizon. The power reached under these conditions is called the peak power These idealized conditions can just be reached very sedlom, because if the solar cell is under radiation, than the solar cell warms up between 20 and 50 K. Thus in order to hold the temperature you need winter with ca. 0°C outside. But then the sun is very deep all day and the AM1.5 factor is no true anymore, the light has a longer way through the atmosphere  change in the solar spectrum. Im winter steht die sonne tiefer und wir haben AM4 – AM6 Different air mass numbers New Generation Silicon Solar Cells

4.3 Losses 1. Reflection: 2. Shadow 3. Recombination
the metall circuit path on the front of a solar cell reflects the light the solar cell itsself reflects the light 2. Shadow The metall circuit path obscures the front of the solar cell 3. Recombination On the surface  dangling bonds Inside the volume 4. Interaction with phonons Reflection: at the front of the solar cell there is a circuit path which works as a metall contact, because of the different refraction index between the air and the semic. there is reflection on the surface of the solar cell. The metall circuit path reflects some electromagnetic radiation. This metall path has to be very small, but if it is to small, than transition resistance between tht metall and the semiconductor is very high, because they are inversely proportional Shadow: this path puts shadow on the solar panel. Recombination: The stuff I told you just before. Interaction with phonons: If the energy of the photon is bigger than the energy of the band gap, than the electrons give back their kinetic energy by interaction with the phonons, the cristall becomes warmer.  an other reason to keep the band gap small. New Generation Silicon Solar Cells

4.3 losses 5. Resistance factors 6. Absorption and Transmission
short circuit between the front and the back of the solar cell transport of the charge carriers through the cables and contacts 6. Absorption and Transmission Other layers of the solar cell (e.g. ARC) can also absorb Light can totaly be transmitted trough the solar cell 7. Other factors Dirt on the solar cell No ideal conditions (STC) STC: There are usuall no ideal conditions: the solar cell will work best in the summer, because the sun´will be about AM1,5 but than the temperature of the cell is way to high. Besides the radiation in Germany is usually under 1000W/m^2 New Generation Silicon Solar Cells

4.4 Efficiency values Material η (laboratory) η (produktion)
Monocrystalline 24,7 14,0 – 18,0 Polycrystalline 19,8 13,0 – 15,5 Amorphous 13,0 8,0 Material Crystalline order Thickness Wafer Monocrystalline One ideal lattice 50μm - 300μm One single crystall Polycristalline Many small crystalls grain (0,1mm – Xcm) Amorphous No crystalline order; Groups of some regularly bound atoms < 1μm No wafer New Generation Silicon Solar Cells

5.1 Why still silicon? > 90% silicon and multisilicon
Silicon has the potential for high efficiency Silicon is available unlimited  second most element of the earth‘s crust The involved materials and processes are non-toxic and do not harm the environment The silicon technology already exists and is reliable Already exists a broad knowledge of the materials and the devices zweithäufigstes Element der Erdrinde in ausreichenden Mengen vorhanden Global PV-market New Generation Silicon Solar Cells

5.2 Surface passivation 1. Thermal oxidation:
Reduction of the density of states on the interface or surface Oxygen streams over the hot wafer surface and reacts with silicon to SiO2  This results in an amorphous layer Temperature of the process ~ 1000°C Thickness of the layer > 35nm  efficiency decreases Time goes on and the velocity of the growth of the oxidic layer decreases Disadvantage: Genereller Nachteil der Passivierung: diese Schicht absorbiert licht  verlust von photonen On the surface is the periodity of the cristall broken, therefore new states for electrons will be created (intinsic states) Besides foreign atoms bound themself to the free silicon atoms and also create new states for electrons. This has to be reduced.  life time of charge carriers will be increased  need highly pure semiconductor materials Reduce the density of states on the interface With this process you try to feed the dangling bonds, thus no free bond do exist any more. Time goes on and velocity decreases, because the growth is limited by the oxygen diffusion which comes from the already existing oxid layer. Thickness of the layer > 35nm  efficiency decreases more than 1% because short circuit current density decreases. New Generation Silicon Solar Cells

5.2 Surface passivation 2. Passivation with SiNx
Reduction of the density of states on the interface Gases silane SiH4 and methane NH3 form a layer of Si3N4 Temperature of the process ~ 350°C Passivation quality rises with silane amount S ~ 20 cm/s – 240 cm/s depending on the refraction index  advantages: lower production temperature Nitride seems also to work better as an anti reflection layer for solar cells  better passivation On the surface is the periodity of the cristall broken, therefore new states for electrons will be created (intinsic states) Besides foreign atoms bound themself to the free silicon atoms and also create new states for electrons. This has to be reduced. Reduce the density of states on the interface With this process you try to feed the dangling bonds, thus no free bond do exist any more. Time goes on and velocity decreases, because the growth is limited by the oxygen diffusion which comes from the already existing oxid layer. Am ISFH wird eine alternative Passivierung mit Siliciumnitrid (SiNx) eingesetzt. Hierbei handelt es sich um eine Schicht, die bei ~400° C aus einem Plasma der Gase Silan und Ammoniak abgeschieden wird. Aufgrund der im Vergleich zum SiO2 niedrigeren Abscheidetemperatur eignet sich die SiNx-Passivierung gut für eine kommerzielle Anwendung. Wir konnten zeigen, dass sich die SiNx-Passivierqualität mit wachsendem Silananteil verbessert. advantages: lower T because at the oxidation process foreign atoms have to diffuse in to the volume  can reduce the life time of the volume Merke: Eine der Voraussetzungen zur Herstellung hocheffizienter Siliciumsolarzellen ist eine qualitativ hochwertige Oberflächenpassivierung. Mit Si-reichem Remote-PECVD Siliciumnitrid (SiNx) mit Brechungsindizes im Bereich von n ≈ 2,4 konnten auf Silicium effektive Oberflächenrekombinationsgeschwindigkeiten (SRV) von Seff = 20 cm/s erzielt werden. Diese Schichten sind jedoch infolge ihrer hohen optischen Absorption nicht für die Vorderseite der BACK OECO-Solarzelle geeignet. Andererseits weisen die als Antireflexionsschicht geeigneten SiNx-Filme mit n = 2,05 und d = 105 nm nur mäßige Passiviereigenschaften mit Seff = 240 cm/s auf. Als möglicher Ausweg aus dieser Problematik wurde eine SiNx-Doppelschicht bestehend aus einer ersten dünnen Lage mit n = 2,5 und einer darauf folgenden 100 nm dicken Schicht mit n = 2,05 untersucht. Trotz der niedrigen Abscheidetemperatur von nur 400° C konnte die effektive SRV mit diesem Schichtsystem auf 50 cm/s auf einem texturierten 0,5 Ωcm Siliciumwafer reduziert werden. Die Absorptionsverluste der nur 5 nm dünnen ersten Schicht sind sehr gering und werden durch die hohe Passivierqualität mehr als aufgewogen. Damit können unter Verwendung dieses SiNx-Doppelschichtsystems gute optische Eigenschaften und eine hohe Oberflächenpassivierqualität miteinander verbunden werden. Es stellt somit ein für rückkontaktierte Siliciumsolarzellen verwendbares Passiviersystem dar, durch dessen Einsatz ein diffundierter sog. "Floating Junction" (Bezeichnung für einen elektrisch nicht kontaktierten passivierenden pn-Übergang an einer Halbleiteroberfläche) auf der Zellenvorderseite vermieden werden kann. PECVD Verfahren: Plasma Enhanced Chemical Vapour Deposition (PECVD) oder üblicherweise auch als Plasma Assisted Chemical Vapour Deposition (PACVD) bezeichnet, ist ein Begriff für eine Sonderform der chemical vapor deposition (CVD), bei der die Abscheidung von dünnen Schichten durch chemische Reaktion wie beim CVD-Verfahren erfolgt. Zusätzlich wird der Prozess durch ein Plasma unterstützt. Das Plasma kann direkt beim zu beschichtenden Substrat brennen (direkt Plasma Methode), oder in einer getrennten Vorkammer (remote Plasma Methode). Bei der direkt Plasma Methode wird zwischen dem zu beschichtenden Substrat und einer Gegenelektrode ein starkes elektrisches Feld angelegt, durch das ein Plasma gezündet wird. Das Plasma bewirkt ein Aufbrechen der Bindungen des Reaktionsgases und zersetzt es in Radikale, die sich auf dem Substrat niederschlagen und dort die chemische Abscheidereaktion bewirken. Dadurch kann eine höhere Abscheiderate bei geringerer Abscheidetemperatur als mit CVD erreicht werden. Bei der remote Plasma Methode ist das Plasma so angeordnet dass es keinen direkten Kontakt zum Substrat hat. Dadurch erzielt man Vorteile bzgl. selektiver Anregung von einzelnen Komponenten eines Prozessgasgemisches und verringert die Möglichkeit einer Plasmaschädigung der Substratoberfläche. Nachteile sind evtl. der Verlust von Radikalen auf der Strecke zwischen remote Plasma und Substrat und die Möglichkeit von Gasphasenreaktionen bevor die reaktiven Gasmoleküle die Substratoberfläche erreicht haben. PECVD Plasmen können auch induktiv / kapazitiv durch Einstrahlung eines elektromagnetischen Wechselfeldes erzeugt werden, wodurch Elektroden überflüssig werden. Mit PECVD lassen sich amorphes Silicium, Siliciumnitride, Siliciumoxide und Silicium-Oxid-Nitrid-Filme und vieles mehr abscheiden (z.B. Kohlenstoffnanoröhren). New Generation Silicon Solar Cells

5.2 Surface passivation 3. Passivation with only silane
The quality of the passivation is enormous Passivation layer on the emitter should be very thin (10nm)  high absorption  prefer SiNx-Process on the emitter The process temperature is ~225°C The passivation seems independet of contaminations of the silicon surface brought in during the manufacturing process An example is the HIT-Solar Cell from Sanyo  Layer of monocristalline silicon between amorphous silicon layers  Efficiency of ~ 18,5% Aus dieser Beobachtung resultierte der Versuch, allein mit Silan die Oberfläche zu passivieren. Die entstandene amorphe Silicium-Schicht (a-Si:H) zeigte eine hervorragende Passivierqualität. Die optimale Abscheidetemperatur ist mit 225° C nochmal niedriger als beim SiNx. Dazu konform zeigte sich die Passivierung unbeeinflusst von während der Solarzellenproduktion eingebrachten Verunreinigungen der Siliciumoberfläche mit Nickel oder Aluminium.Auf dieser Grundlage ließ sich ein stabiler Prozess entwickeln. Gut passivierende Eigenschaften wurden von uns auch bei einer Anwendung der a-Si:H-Schichten auf dem Emitter nachgewiesen. Weil der Emitter auf der Zellvorderseite ist, sollte die lichtabsorbierende a-Si:H-Schicht möglichst dünn ausgeführt werden. Abbildung 13 zeigt die gemessene Oberflächenrekombinationsgeschwindigkeit als Funktion der Dicke der a-Si:H-Schicht. Eine Dicke von ca. 10 nm ist für eine gute Passivierung erforderlich. Leider absorbiert eine 10 nm dicke Schicht einen erheblichen Anteil des Sonnenspektrums. Deswegen ist auf der Vorderseite eine SiNx-Passivierung vorzuziehen. Sanyo: combinates the good Absorption ability and the passivation ability from amorphous silicon with mono crystalline silicon Only 200µm and efficiency of 18,5 % Passivierqualität als Funktion der a-Si:H-Schichtdicke HIT solar cell New Generation Silicon Solar Cells

5.2 Surface passivation 4. Back Surface Field (BSF)
A thin layer of p-doped material to prevent the minorities from moving to the back contact where they recombinate e.g. use aluminium for a back contact, which melts (T ~ 500°C) into the silicon and creates a positive doped BSF. Besides it serves as a reflection layer. New Generation Silicon Solar Cells

5.2 Surface passivation Intrinsic gettering: Extrinsic gettering:
Contaminations will be collected at one area in the crystall and afterwards will be removed Extrinsic gettering: Contaminations will be transported to the crystall surface and afterwards be removed e.g. aluminium Foreign atom will be freed out of their bonds  diffuse into the Al-Si alloy 30 minutes at T = 800°C to eliminate most of the contaminations, depends on the diffusion length of the atom Metallic contamination or contamination in general in silicon This you want to reduce. Therefore you add a gettering step into the process. Aluminium: foreign atoms will be freed out of their bonds and diffuse because of their higher Löslichkeit (sollubility) into the Al-Si-phase. When temperature is low enough, they will be bound into this phase and afterwards the will be etched away at T = 60°C with HCL(37%) Huge volume  usually use extrinsic gettering Ni Fe do not need a lot of time to diffuse trough 300micro just a few minutes at T~900 but B, P need way longer New Generation Silicon Solar Cells

5.3 Reflection 1. Anti reflection layer 2. Texturing (light trapping)
One or more layers  reduction from 30-35% to 5%-10% Mainly 600nm transmission Silicon nitride or transparent layers, e.g. SiO2; TiO2; Ta2O5 ITO can be used as anti reflection layer and at the same time as a transparent contact Double anti reflection layers ZnS or MgF2 2. Texturing (light trapping) Use NaOH, KOH in etching baths The etching works anisotropic  2μm - 10μm big pyramids on (100) oriented crystall planes Anti reflection layer  The thickness and the refraction index will be choose so mainly 600nm will transmit, because as we have seen in the solar spectrum 600nm waves carry most of the energy With the help of silicon nitride  replaced titanium dioxide Variation of the thickness of the anti reflection layer the color of the laer can be changed  makes the solar cell more beautiful, but can redue the efficiency ITO usually use it as a layer for solar cells made out of a thin amorphous layer coated on a crystall or multicrystall thick layer. It also supports the conductivity crossways  later on 2. Texturizing Anisotropic: different etching velocities of the crystall planes KOH, NaOh mixed with isopropanol makes pyramid distributed at random For polychristall you use hydrofluric acid or nitric acid Reactive ion etching (RIE) is an etching technology used in microfabrication. It uses chemically reactive plasma to remove material deposited on wafers. The plasma is generated under low pressure (vacuum) by an electromagnetic field. High-energy ions from the plasma attack the wafer surface and react with it. Plasma is initiated in the system by applying a strong RF (radio frequency) electromagnetic field to the wafer platter. The field is typically set to a frequency of megahertz, applied at a few hundred watts. The oscillating electric field ionizes the gas molecules by stripping them of electrons, creating a plasma. In each cycle of the field, the electrons are electrically accelerated up and down in the chamber, sometimes striking both the upper wall of the chamber and the wafer platter. At the same time, the much more massive ions move relatively little in response to the RF electric field. When electrons are absorbed into the chamber walls they are simply fed out to ground and do not alter the electronic state of the system. However, electrons absorbed into the wafer platter cause the platter to build up charge due to its DC isolation. This charge build up develops a large negative voltage on the platter, typically around a few hundred volts. The plasma itself develops a slightly positive charge due to the higher concentration of positive ions compared to free electrons. Because of the large voltage difference, positive ions tend to drift toward the wafer platter, where they collide with the samples to be etched. The ions react chemically with the materials on the surface of the samples, but can also knock off (sputter) some material by transferring some of their kinetic energy. Due to the mostly vertical delivery of reactive ions, reactive ion etching can produce very anisotropic etch profiles, which contrast with the typically isotropic profiles of wet chemical etching. Etch conditions in an RIE system depend strongly on the many process parameters, such as pressure, gas flows, and RF power. A modified version of RIE is deep reactive-ion etching, used to excavate deep features. New Generation Silicon Solar Cells

5.3 Reflection  advantages: At least second reflection
The effective absorption length of the silicon layer will be reduced  the light way through the layer increases The area of the surface becomes bigger Total reflection on the inside of the front layer possible Reflection can be reduced about 9/10 of the former reflection Examples of light trapping  advantages: A light beam will after reflection be absorbed by the surface of an other pyramid  second reflection The effective absorption length of the silicon layer will be reduced because the light beam enters the layer sloped  the way through the layer increases The area of the surface becomes bigger Total reflection on the inside of the front layer in combination with the aluminium layer New Generation Silicon Solar Cells

5.3 Reflection New:  disadvantage:
More difficult to form it on multi-/polycrystalline silicon layers  no sufficient reflection reduction The surface area is increased  higher surface carrier recombination rates New: A focused laser scans the wafer surface to form a dotted matrix The damage on the surface of the crystall will be etched away afterwards Advantage: it is better for the environment and can be used on different materials Reflection can be reduced from ~35% to 20% The back contacts do not have to been all over the back p+ layer  you need an oxid layer to cover the „open“ parts of the p+ layer in order to reduce the velocity of recombination. The laser has to be a solid state body laser with 355nm refelction can be reduced from 35%-40% to 20% Laser texturized poly chrystall silicon New Generation Silicon Solar Cells

5.3 Reflection 3. Back side reflection  advantage:
Two different layers at the backside: Patterns of microscopic spheres of glass within a precisely designed photonic crystall Capture and recycle the photons Large-scale manufacturing techniques are being developed  advantage: Reflects more light than the aluminium layer Light reenters the silicon at low angle  light bounces around inside Efficiency can be increased up to 37% aims to capture and use photons that ordinarily pass through solar cells without generating electricity.  makes the silicon thinner The oxid layer on the back can be covered inside with a metall layer that reflects the light back into the bulk. Metall layer on the backside of the p+ Junction. New version:  Developed at the MIT  they say cost could be cut into a half (my opinion to optimistic) Photonic crystals are periodic optical (nano)structures that are designed to affect the motion of photons in a similar way that periodicity of a semiconductor crystal affects the motion of electrons.  can be engineered to reflect and diffract all the photons in specific wavelengths of light, Photonische Kristalle sind in prinzipiell transparenten Festkörpern vorkommende oder geschaffene periodische Strukuren, die u. a. durch Beugung und Interferenz die Bewegung von Photonen (in der Regel Licht oder Infrarot) beeinflussen. Photonische Kristalle sind nicht zwingend kristallin - ihr Name rührt von analogen Beugungs- und Reflexionseffekten von Röntgenstrahlung in Kristallen aufgrund deren Gitterkonstanten MIT researchers developed sophisticated computer simulations to understand how thin layers of photonic crystal could be engineered to capture and recycle the photons that slip through thin layers of silicon. Silicon easily absorbs blue light, but not red and infrared light. The researchers found that by creating a specific pattern of microscopic spheres of glass within a precisely designed photonic crystal, and then applying this pattern in a thin layer at the back of a solar cell, they could redirect unabsorbed photons back into the silicon. the light is diffracted by one layer of the material (larger dots). This causes the light to reenter the silicon at a low angle, at which point it bounces around until it is absorbed. The light that makes it through the first layer is reflected by the second layer of material (smaller dots) before being diffracted into the silicon. Advantage: The photonic crystal reflects more light than the aluminum does, especially once the aluminum oxidizes. And the photonic crystal diffracts the light so that it reenters the silicon at a low angle.( longer way trough silicon) The low angle prevents the light from escaping the silicon. Instead, it bounces around inside; this increases the chances of the light being absorbed and converted into electricity. As a result, the photonic crystal can increase the efficiency of solar cells by up to 37 percent, says Peter Bermel, CTO and a cofounder of StarSolar. What's needed is a way to cheaply arrange two materials in an orderly three-dimensional pattern. For example, microscopic spheres of glass would be arranged in rows and columns inside silicon. Currently, techniques such as e-beam lithography can be used, but that's too slow for large-scale manufacturing. The method, which employs optical lithography similar to that used in the semiconductor industry, works best for a type of solar cell that concentrates light onto a small chunk of expensive semiconductor material. a method for manufacturing eight-inch disks of photonic crystal Another potentially less-expensive method, called interference lithography, creates orderly patterns in the photonic-crystal materials. The method is fast and uses machines that are far less expensive than those used for conventional optical lithography. It also requires fewer steps represents the aluminium layer represents the new version New Generation Silicon Solar Cells

5.4 Laser operations Why using laser?
All for Si-PV-technology used materials absorb light A small optical/thermical penetration depth is given for λL < 1µm Laser can focuse very good (size of structure 10µm – 100µm) Minimal mechanical demands on the fragile Si-wafer Screen printing process can be prevented  Laser`s high quality output beams and unique pulse characteristics coupled with low cost –of-ownership - Laserverfahren sind für die Si-PV interessant, weil alle in der Si-PV-Technologie verwendeten Materialien (Si, Metalle, Dielektrika) bei geeigneter Wellenlänge absorbieren. - Bei einseitiger Bearbeitung ist eine geringe optische bzw. thermische Eindringtiefe bei kurzwelligem (< 1 µm) bzw. gepulstem Laser (< 1 µs) gegeben. - Bei der lokalen Strukturierung ist die Fokussierbarkeit der meisten Lasersysteme im geforderten Strukturgrößenbereich (10 bis 100 µm) möglich. Die minimale mechanische Beanspruchung der fragilen Si-Wafer durch die berührungslose Laserbearbeitung ermöglicht den Aufbau einer Fließfertigung bei Prozessierung und Charakterisierung. Laser based manufacturing enables higher efficiency of the cells. Being a non-contact technology, lasers are essential for the use of thinner wafers reducing raw material costs. The lasers’ high quality output beams and unique pulse characteristics, coupled with very low cost-of-ownerships, allow yields and throughput to be optimized by the cell manufacturer. New Generation Silicon Solar Cells

5.4 Laser operations p-doped layer is coated with an outer layer of n-doped silicon to form a large pn-junction n-doped layer coats the entire wafer  recombination pathways between front and rear surfaces Edge isolation: groove is continuously scribed completely through the n-type layer right next to the edge of the cell Requirements: Rp should be kept high; FF > 76% Little waste of solar cell area 1000 wafer/h Flexibility (thin wafers) Die Kantenisolation ist ein Standard-Industrieprozess für p-Si-Elemente, die etwa 200 bis 300 nm dick sind bei 125 bis 156 mm Kantenlänge. Die Laserkantenisolation (LKI) wird alternativ gegenüber traditionellen Bearbeitungsverfahren, z. B. dem Plasmakantenätzen, angewendet, das zwar eine sehr gute Kantenisolation liefert, aber hinsichtlich der Automatisierung als ungünstig eingeschätzt wird. Auch das Kantenschleifen bewirkt eine gute Kantenisolierung, hat jedoch eine Erhöhung der Bruchrate zur Folge. Ähnlich verhält es sich beim Kantenbrechen, das Aufgrund der Verringerung der Zellfläche nur für Laborzellen geeignet ist. Somit sind bei der Solarzellenbearbeitung grundsätzlich die Sprödität des Materials sowie die dünnen Materialstärken Als Grenzfaktoren zu bedenken. In manufacturing the cells, p-doped wafers are coated with an outer layer of n-doped silicon to form a large area p-n junction which ultimately generates the electrical power. However, this thin (10-20 microns) layer coats the entire wafer, including the edges, and often the rear surface, creating an unacceptable recombination pathway between the front and rear surfaces. This pathway can be eliminated by Edge Isolation, whereby a groove is continuously scribed completely through the n-type layer. In order to maximize the cell active area, and hence efficiency, this groove has to be as narrow and as close to the edge as possible. Zu den besonderen Laseranwendungen gehört die Kantenisolation der Si-Wafer. Die Problemstellung bei der Kantenisolation resultiert aus der notwendigen elektrischen Trennung von Vorder- und Rückseitenkontakt der Solarzelle. Einerseits soll damit vermieden werden, dass die Emitterdiffusion um die Waferkante greift. Andererseits soll durch einen Lasergraben entlang der Waferkante die Ausbildung eines Verluststrompfads vermieden werden, wodurch sonst ein niedriger Parallelwiderstand (RP) und eine Reduktion der Leistungsabgabe des p-Siliziums entstehen würden. Die Anforderungen an ein Kantenisolationsverfahren resultieren aus den physikalischen Randbedingungen wie hochohmige Trennung (RP > 4 kW cm', FF ≥ 76 %) und möglichst keine Reduktion der stromsammelnden Zellfläche. Zusätzlich sind technische Randbedingungen gegeben, z. B. der industrielle Durchsatz (> Wafer/h), Flexibilität (Wafergröße, dünne Wafer) oder ein optisch homogenes Erscheinungsbild. The usual laser of choice is a Q-switched solid state laser. Some manufacturers use 1064 nm lasers as these offer the highest power/cost. An increasing number of manufacturers have been using 532 nm and 355 nm lasers for this application for two reasons. First, these lasers can scribe narrower grooves. Nachteile: In addition, 1064 nm lasers create microcracks that emanate from the scribed groove. If these reach the edge of the wafer, structural integrity is potentially compromised. So this limits how close the 1064 nm-machined trenches can be placed to the edge of the wafer. Significance of the wavelength: The depth of penetration at 1200K is twice as small as the depth of penetration at 293,15K Smaller wavlenght can be foccused on a smaller dot. Hierzu wurden Versuche mit Nd:YAG-Lasern bei einer Wellenlänge von 532 nm sowie mit 1064 nm Wellenlänge ausgewertet. Weitere Untersuchungen betreffen die Bedeutung der LKI-Vorschubgeschwindigkeit, die Punktgröße (Durchmesser der ablatierten Fläche je Puls), die Pulsenergie in Abhängigkeit der Frequenz usw., jeweils für eine Bearbeitungskapazität von Zellen/h (etwa 2 s Bearbeitungszeit) und einer Lasergrabenlänge (156 mm x 4) von 624 mm. Groove to isolate the front and rear side of the cells New Generation Silicon Solar Cells

5.4 Laser operations Front surface contacts: Laser Fired Contacts
Burried contacts to minimize the area obscured by the front contacts  electrodes with a high volume and collection surface Depth and width 20μm – 30μm every 2mm-3mm Laser Fired Contacts Electrically and thermo-mechanically advantageous to include passivation layer, which is non-conducting laser creates localized Al/Si- alloys Efficiency of ~ 21% Buried electrical contacts represent a different approach to minimizing the area obscured by screen-printed front-side metallization. After the surface of the cell has been antireflection coated with silicon nitride, pulsed lasers are used to scribe narrow surface grooves which are then plated. This results in electrodes with a high volume and collection surface, relative to their width. The grooves may have a width and depth of microns and are cut at 2-3 mm intervals along the cell. In a recent adaptation of this approach, lasers cut only through the AR coating and the exposed semiconductor is electroplated.(galvanisiert) Passivation layer possible and small contacts possible too  recombination of the light generated carriers is limited The efficiency of the back contacts will be increased which is advantageous for low doping concentrations Passivatio Metallization of the whole back side with aluminium Laser fired contacts Use laser system NY – YAG  10cm x 10cm holes in 1s Can create thin layers  expensive semiconductor material can be saved nowadays use screen printing process for good back side passivation  bending  With LFC no bending and silicon wafer of 50µm-90µm silicon are possible  thin wafers can be bend and put on crocked surfaces With thinner wafers, it can be electrically and thermo-mechanically advantageous to include a passivation layer between the rear surface aluminum electrode layer and the silicon. However, this passivation layer is non-conducting. The Laser Fired Contacts (LFC) technique recently developed by Fraunhofer ISE now paves the way for full commercial production of this type of cell by using a laser to create localized contacts. At each site, a 1064 nm laser pulse drives the aluminum through the passivation layer and several microns deep into the silicon, creating a localized Al/Si alloy. Der LFC-Prozess (Laser-Fired Contacts) als ein weiterer Sonderfall des Lasereinsatzes dient der Anpassung der Kontakt-Anzahl an das Wafermaterial. Das bedeutet, dass per LFC die Erzeugung von bis zu Kontaktierungen in 1 Sekunde erfolgt, wobei eine Anpassung der Pulsenergie an die Schichtdicke des Wafers beachtet werden muss. Als LFC-Ergebniss entstehen z. B. hocheffiziente Zellstrukturen mit sehr guten Wirkungsgraden über einen weiten Dotierungsbereich. Laser generated groves on the cell surface Over 1000 rear side local metal point-contacts created per solar cell New Generation Silicon Solar Cells

5.5 Solar cell contacts Saturn-solar-cell  Laser Grooved Buried Contact (LGBC) Laser will burn a trench in the front side of the solar cell Trench is 35µm deep and 20µm wide and has form of a „U“ or a „V“ Trench will chemically be filled up with the front contact material, usually silver a large metal hight-to-width aspect ratio  allows closely spaced metal findgers low parasitic resistance losses advantages: Shading losses will only be 2% to 3% Reduction of metall grid and contact resistance Reduction of emitter resistance because of very close fingers Possible efficiency >17% Can find it in Australia Sydney The heavily doped layer n++ will lower the resistance between the kontact and the silicon The n+ allows the usage of the blue light. Advantages: Shading of the front side of the solar cell reduced  because the contacts are a lot smaller than screen printed contacts Constantly high power values  especially at a radiation of 200w/m^2 Aufgrund dieser besonderen Bauweise kann die BP-Saturnzellein den Morgen- und Abendstunden mehr Energieumwandeln (Bild 3) und sie reagiert im gesamten relevanten Spektralbereich empfindlicher auf die Einstrahlung als herkömmliche Solarzellen. A key high efficiency feature of the of the buried contact solar cell is that the metal is buried in a laser-formed groove inside the silicon solar cell. This allows for a large metal height-to-width aspect ratio. A large metal contact aspect ratio in turn allows a large volume of metal to be used in the contact finger, without having a wide strip of metal on the top surface. Therefore, a high metal aspect ratio allows a large number of closely spaced metal fingers, while still retaining a high transparency. For example, on a large area device, a screen printed solar cell may have shading losses as high as 10 to 15%, while in a buried contact structure, the shading losses will only be 2 to 3%. These lower shading losses allow low reflection and therefore higher short-circuit currents. In addition to good reflection properties, the buried contact technology also allows low parasitic resistance losses due to its high metal aspect ratio,  its fine finger spacing and its plated metal for the contacts. As shown in the Emitter Resistance page, the emitter resistance is reduced in a buried contact solar cell since a narrower finger spacing dramatically reduces the emitter resistance losses.  The metal grid resistance is also low since the finger resistance is reduced by the large volume of metal in the grooves and by the use of copper, which has a lower resistivity than the metal paste used in screen printing. As well, the contact resistance of a buried contact solar cell is lower than that in screen printed solar cells due to the formation of a nickel silicide at the semiconductor-metal interface and the large metal-silicon contact area. Overall, these reduced resistive losses allow large area solar cells with high FFs. When compared to a screen-printed cell, the metalization scheme of a buried contact solar cell also improves the cell's emitter. To minimise resistive losses, the emitter region of a screen-printed solar cell is very heavily doped and results in a "dead" layer at the surface of the solar cell. Since emitter losses are low in a buried contact structure, the emitter doping can be optimized for high open-circuit voltages and short-circuit currents. Furthermore, a buried contact structure includes a self-aligned, selective emitter, which thereby reduces the contact recombination and also contributes to high open-circuit voltages. The efficiency advantages of buried contact technology provide significant cost and performance benefits. In terms of $/W, the cost of a buried contact solar cell is the same as a screen-printed solar cell (Jordan, Nagle). However, due to the inclusion of certain area-related costs as well as fixed costs in a PV system, a higher efficiency solar cell technology results in lower cost electricity. An additional advantage of buried contact technology is that it can be used for concentrator systems (Wohlgemuth, Narayanan). Based on the sheet resistivity, the power loss due to the emitter resistance can be calculated as a function of finger spacing in the top contact. However, the distance that current flows in the emitter is not constant. Current can be collected from the base close to the finger and therefore has only a short distance to flow to the finger or, alternatively, if the current enters the emitter between the fingers, then the length of the resistive path seen by such a carrier is half the grid spacing. LGBC-cell

5.5 Solar cell contacts Prevent obscuration of the solar cell or high reflection and absorption of the silver grids. small and high grids, which will become smaller towards the edge of the cell COSIMA (Contacts to a-Si:H passivated wafers by means of annealing): Amorphous silicon (silane process) on monocrystalline silicon Aluminium on theses layers results in contacting the monocrystalline silicon Process temperature ~ 200°C No photolithography ITO (Indium Tin Oxid) Helps to improve the conductivity crossways along the layer ITO  imropves the conductivity between the grids. Thickness of at the most 70nm – 80nm The resistivity determines the distance between the grids  ITO usually use it as a layer for solar cells made out of a thin amorphous layer coated on a crystall or multicrystall thick layer. ITO is transparent and supports the grid on the front too, because it helps to develop ot ist conductivity crossways. Das a-Si:H besitzt noch einen weiteren Vorteil gegenüber SiNx und SiO2-Schichten: ein auf das an sich schlecht leitende a-Si:H aufgebrachter Aluminiumkontakt bildet bei ~200° C einen Kontakt zum darunter liegenden kristallinen Silicium. Dieser als COSIMA (Contacts to a-Si:H passivated wafers by means of annealing) bezeichnete Effekt vereinfacht den Herstellungsprozess weiter, da es damit möglich wird, die passivierende Schicht vor den Metallkontakten und damit auf das saubere Silicium aufzubringen. Während man bei konventionellen Passivierungen die Passivierschicht in einem aufwändigen Photolithographieverfahren öffnen muss, wird beim COSIMA-Verfahren die auf die a:Si-H-Schicht aufgebrachte Aluminium-Kontaktstruktur lediglich einige Minuten getempert. Damit erhält man auf einfache Weise eine Solarzelle mit sehr guten Kontakten. Abbildung 12 zeigt eine Solarzelle mit COSIMA-Rückseitenkontakten. Solche Solarzellen mit a-Si:H-Passivierung zwischen den Kontakten auf der Zellrückseite zeigten unabhängig bestätigte Wirkungsgrade von über 20%. Dabei wurde eine Standard-SiNx-Vorderseite verwendet. Das Formieren der Aluminium- Kontakte erfolgte simultan mit der Abscheidung der SiNx-Schicht auf der Vorderseite. Mit der COSIMA-Technik können also auf einfache Weise hocheffiziente kristalline Si-Solarzellen hergestellt werden. Wegen der niedrigen Präparationstemperaturen ist die COSIMA-Technik sehr gut für die Passivierung von Dünnschichtsolarzellen aus Schichttransferprozessen geeignet. Solar cell with a-Si:H-rear passivation and COSIMA contacts New Generation Silicon Solar Cells

5.5 Solar cell contacts Advantages: Combination with doted contacts:
Simplifies thin film manufacturing process Efficiency values about 20% Combination with doted contacts: Screen printed interface layer (little holes)  good passivation Aluminium on the interface layer  COSIMA Can be used on thinner wafers  no bending The passivation abbility of the amorphous layer will be kept after the annealing process The contact resistivity is 15mΩcm2 Increase of the quantum yield in the infrared wavelength range Reduces Seff to 100 cm/s (4% metallization) Interface layer : silicon oxide or silicon nitride or silicon carbide Dot contacted rear side of the Cosima Cell; connects the COSIMA cell, which simplifies the process with the doted contacts, which reduces the material again and advantages: Use on thinner wafers  no bending of the wafer, this would happen if you used a aluminium rea contact on the hole rear side Herstellung: Bei 190°C PECVD wird amorphous silicon abgeschieden. Passivation quality as good as SiO2 Sreen printing process for the interface layer The rear side will be printed or metallized with aluminium by means of vapour deposition. 1-3 h bei 210°c annealed  developing of the contact when the amorphous layer melts into the aluminium layer. (Cosima)  the aluminium interconncts with the silicon layer no seperate layers anymore. Oberflächengeschwindigkeit: Seff ist 10^6 cm/s; nur amorphe schicht ohne metallkontakte  35cm/s und bei 4% Metallkontakten (Punkten) liegt Seff bei 100cm/s The contact resistivity is 15mΩc^cm^2  this is the same like the resistivity without any passivation layer and a doping concentration of Bor of 1*10^16 cm^3 New Generation Silicon Solar Cells

EWT/MWT Emitter Wrap through (EWT)
Emitter on the front surface is wraped with the rear surface by little holes Edges of the holes are also emitter areas, which transport emitter current Power-conveying busbars and the grid are moved to the rear surface Use double sided carrier collection (n+pn+)  increases the efficiency 100µm holes are made by laser The Ewt wraps the emitter on the front surface through holes on the rear surface during the emitter difussion step. Current collected by the emitter on the front surface is transported trough the emitter in the holes to the contacts on the back surface N+pn+ The rest of the surface has an n+ emitter layer but not near the rear contacts. This structure provides for collection of carriers on both surfaes and therby improves the collection of photogenerated carriers in the base. high carrier collection despite low diffusion length –BSF not needed anymore no screen printed metallization on the surface  no low resistance contacts to a screen printed metallization needed  no heavily doping of the n+ area  better optimization for Jsc and Voc High efficiency comes from the n+pn+ structure and the prevented obscuration. Measurments showed, that a EWT cell has a broad peak at a thickness around half the diffusion length  EWT works well with low-lifetime material and a thin substrate. Another fast-growing laser application in solar cell manufacturing is via drilling. The simplest solar cells have contacts on the front and rear surfaces to collect the negative and positive charge carriers. But the screen-printed metal comprising the front-side contacts blocks a significant area from receiving sunlight. Two newer high-value device architectures address this. In metal wrap through (MWT) devices, the thin metal “fingers” are moved to the rear surface. In emitter wrap through (EWT) devices, power-conveying busbars are moved to the rear surface as well, leaving the front free of metal. This is made possible by drilling tiny vias to connect the front surface with rear-surface contacts. Laser drilling is the only process with the potential to meet the commercial-scale speeds required here. Interesting point: efficiency of an EWT cell degrades muss less as the diffusion length degrades with light induced degradation Front (left) and rear (right) of a EWT-solar cell. The front contacts are brought to the rear of the solar cell by many dots. EWT- cell with n+pn+ - structure New Generation Silicon Solar Cells

5.5 Solar cell contacts Advantages: Disadvantage:
Eliminate grid obscuration  no high doping  high Isc  high efficiency n+pn+- structure  use lower quality solar grade silicon Uniform optical appereance  improves asthetics Silicon solar cell < 200μm Efficiency around 18% gain in active cell area Diffusion length can be reduced to the half Disadvantage: Manufacturing process is very complex Metal wrap throug (MWT) Absence of the bus bars (on the rear side)  connection by holes Less serial resistance losses because of interconnection of the modules on the back FF ~77%; efficiency ~ 16% Hole drilling for interconection between front and back side and metallization of the holes. Contact isolation can be done in the same step as the edge isolation. Bus bars( Stromverschaltende schiene) are conduction paths needed to interconnect the different modules to a photovoltaic system. These bus bars are hughe and usually obscure a lot of the solar cell area. Bus bars are brazed lanes witch interconnect the modules and cover most of the metall area at the front side. Bus bars connect the contact grids Advantages: Same as EWT, just a little easier to produce, not as many holes needed, Having the n and p contact on the rear sied simplifies the interconnection of the cells in the module and allows the use of new interconnction technologies with the objective of serial resistance conduction. In an MWT technique there are three main stages, which partially shift the front side contacts over to the rear side, thus reducing the front side metallization almost by 50%. During the first stage a laser is used to drill holes into the cells. The through-connection of the cells is simultaneously achieved through the subsequent silkscreen printing process for the production of rear side contacts. After this a silk-screen printing paste is used to cover the holes, which completes the electrical connection to the front side. This isolation of the contacts reduces additional costs. MWT-cell New Generation Silicon Solar Cells

5.5 Solar cell contacts MWT EWT Voc [mV] 617 596 Jsc [mA/cm2] 36,1
37,7 FF [%] 75,1 72,8 η [%] 16,7 16,3 Area [cm2] 189,5 61,5 New Generation Silicon Solar Cells

5.5 Solar cell contacts New Generation Silicon Solar Cells 19.04.2017
Cross section of a partially plated laser groove. New Generation Silicon Solar Cells

5.5 Solar cell contacts A 300 solar cell:
Negative conducting silcon wafer Emitter and all contacts on the back side No obscuration on the front side Efficiency value > 21% A-300 is from sun power No obscuration on the front side because of metall lines New Generation Silicon Solar Cells

5.6 OECO-cell (Obliquely Evaporated COntacts)
Standard OECO cell: front contacts are evaporated on the flanks of the ditch by self obscurance flat homogeneous emitter because of one step phosphor diffusion very thin contacts of metall are possible development of a ultra thin tunnel oxid between metal and semiconductor, which forms high sufficient MIS contacts passivation layer on the front and rear side (SiNX or SiO2) efficiency ~ 20% Produktion: The ditches can be manufactured by etching or a laser  phosphor difffusion  vapor deposition  passivation The bridges obscure the inner area of the bitches when you evaporate the aluminium on the ditch  no masks or adjustments are required The single solar cells are in a round circular folwer bed and in the middle there is the source of aluminium which sprays the aluminium Because of one step difussion because of phosphor diffusion Homogeneous emitter with quantum yield in the short wavelength High sufficient MIS contacts Isfh hameln Reduziert die abschattung ; schräges aufdampfen entstehen an den flanken der gräben elektrische kontakte die den lichteinfall praktisch nicht mehr behindern 20% Das Institut für Solarenergieforschung Hameln (ISFH) verfolgt ein Konzept, welches gleichfalls die Abschattung durch Frontkontakte minimieren will. Durch schräges Aufdampfen entstehen auf den Flanken von Gräben elektrische Kontakte, die den Lichteinfall praktisch nicht mehr behindern. Auch mit dem Konzept des Fraunhofer ISE werden Wirkungsgrade jenseits der 20 %-Marke erreicht – unter Verwendung industrietauglicher Techniken. Es basiert auf einem Rückseitenkontakt, der nur punktuell hergestellt wird, um den größten Teil der Rückseite mit einer Schicht aus einer dielektrischen Siliziumverbindung (Siliziumoxid, -nitrid oder -carbid) zu vergüten. Dies führt zu besseren Ergebnissen als ein ganzflächiges Aluminium-Rückseitenfeld und kann gleichzeitig auf sehr viel dünneren Wafern verwendet werden, welche sich bei ganzflächiger Aluminiumbeschichtung stark verbiegen würden. Mechanisch erzeugte öberflächenstruktur New Generation Silicon Solar Cells

5.6 OECO-cell  Mass production Advantages: reduces the oscuration
easy manufacturing processes and environmentally friendly efficiency value > 20%  Mass production Standard OECO solar cell New Generation Silicon Solar Cells

5.6 OECO-cell Both contacts are on the rear side
The back of this cell accords to the standard OECO cell The front has a texturized surface Deep phosphorous emitter on almost the whole back side Advantages: Reduction of impurity shunt resistance and serial resistance Reduction of obscurance at the front Double sided light-sensitivity  bifaciale solar cell efficiency for both sides ~ 22% possible Herstellung BACK OECO-Solarzelle beidseitig lichtempfindlich. Beide Kontakte befinden sich auf der Rückseite der Solarzelle. In Abbildung 14 ist die BACK OECO-Solarzelle schematisch dargestellt. Sie wird ohne Photolithographie-, Maskierungsoder Justierschritte hergestellt. Die Zellenvorderseite weist eine chemisch hergestellte pyramidale Textur auf, während die Rückseite unter Einsatz eines schnell rotierenden Schleifwerkzeuges mechanisch strukturiert wird. Nach Herstellung einer lokalen Diffusionsbarriere wird ein tiefer Phosphor-Emitter erzeugt, der nahezu die gesamte rückseitige Fläche bedeckt. Die Aluminium-Kontaktfinger werden unter Einsatz der OECO-Technologie im Vakuum schräg aufgedampft. Der Emitterkontakt weist ein dünnes Tunneloxid zwischen Metall und Halbleiter auf. Im Bereich der Kontakte sind keine zusätzlichen lokalen Tiefdiffusionen bzw. selektive Emitter notwendig. Zur Passivierung der Halbleiteroberflächen wird bei 400° C abgeschiedenes Remote-PECVD Siliciumnitrid eingesetzt Most of the manufacturing steps are the same like the one for OECO cells * die Optimierung der Rückseitenstruktur - einerseits zur Erzielung geringster Serienwiderstände und andererseits, um qualitativ hochwertige Basis- und Emitterkontakte zu erhalten, * die Optimierung der vorderseitigen Oberflächenpassivierung unter Vermeidung einer zusätzlichen Diffusion sowie  mit hilfe con punkt 2 bei surface passivation * die Reduktion parasitärer Shunt-Widerstände. - front efficiency 18,3%; back efficiency 17,6% ; Mit diesen Simulationen entdeckten wir einen interessanten Effekt, der in Abbildung 15 illustriert wird: Schmale Stege zeigen im Gegensatz zu breiten Stegen unter Kurzschlussbedingung eine Verminderung der Minoritätsladungsträgerdichte Δn im Bereich der Basiskontakte. Die Konzentration reduziert sich um mehr als zwei Größenordnungen, was eine Reduktion des Rekombinationsstromes am Basiskontakt zur Folge hat. Dieser Effekt der "Minoritätsträger-Abschirmung" wird dadurch verursacht, dass die Elektronen von dem sich in der Nähe befindenden Emitter abgesaugt werden. Die rechte Hälfte von Abbildung 15 zeigt die Simulation für breite Stege. Weil der sammelnde Emitter weiter vom Basiskontakt entfernt ist, ergibt sich eine höhere Elektronendichte am Basiskontakt. Die "Minoritätsträger-Abschirmung" ist weniger effizient. Anwendungen: Can use this bifacial solar cell in combination with a white reflection area behind the solar cell, which´reflects the light towards the solar cell. Back – OECO - cell

5.7 Further prospects There is also high potential in improvents for the manufacturing process  development of a „solar silicon“ Sawing process has to be improved Automation processes have to be developed New contact processes Fast processes with low cycle time Solar silicon: 3 different grades of silicon: Solar graded silicon; float zone silicon, ultra pure silicon (czochralski prozess) Optimization between costs and efficiency; is cleared of metall; can not be used to build chips or mosfets, transistors There are currently many research groups active in the field of photovoltaics in universities and research institutions around the world. This research can be divided into three areas: making current technology solar cells cheaper and/or more efficient to effectively compete with other energy sources; developing new technologies based on new solar cell architectural designs; and developing new materials to serve as light absorbers and charge carriers. [edit] Silicon processing One way of reducing the cost is to develop cheaper methods of obtaining silicon that is sufficiently pure. Silicon is a very common element, but is normally bound in silica, or silica sand. Processing silica (SiO2) to produce silicon is a very high energy process - at current efficiencies, it takes over two years for a conventional solar cell to generate as much energy as was used to make the silicon it contains.[30] More energy efficient methods of synthesis are not only beneficial to the solar industry, but also to industries surrounding silicon technology as a whole. The current industrial production of silicon is via the reaction between carbon (charcoal) and silica at a temperature around 1700 degrees Celsius. In this process, known as carbothermic reduction, each tonne of silicon (metallurgical grade, about 98% pure) is produced with the emission of about 1.5 tonnes of carbon dioxide. Solid silica can be directly converted (reduced) to pure silicon by electrolysis in a molten salt bath at a fairly mild temperature (800 to 900 degrees Celsius).[31][32] While this new process is in principle the same as the FFC Cambridge Process which was first discovered in late 1996, the interesting laboratory finding is that such electrolytic silicon is in the form of porous silicon which turns readily into a fine powder, (with a particle size of a few micrometres), and may therefore offer new opportunities for development of solar cell technologies. Another approach is also to reduce the amount of silicon used and thus cost, as done by Professor Andrew Blakers at the Australian National University with their "Sliver" cells, by micromachining wafers into very thin, virtually transparent layers that could be used as transparent architectural coverings.[33] Using this technique, one silicon wafer is enough to build a 140 watt panel, compared to about 60 wafers needed for conventional modules of same power output. Yet another way to achieve cost improvements is to reduce wastes during the crystal formation by improved modelisation of the process, as done by FemagSoft, spin-off of the Université Catholique de Louvain. Another novel approach employed by Evergreen Solar is to grow silicon ribbons from specialized "string puller" furnaces. They claim to be able to produce thinner cells without machining waste plus the resulting cells are naturally rectangular in shape. Silicon wafer based solar cells Despite the numerous attempts at making better solar cells by using new and exotic materials, the reality is that the photovoltaics market is still dominated by silicon wafer-based solar cells (first-generation solar cells). This means that most solar cell manufacturers are equipped to produce these type of solar cells. Therefore, a large body of research is currently being done all over the world to create silicon wafer-based solar cells that can achieve higher conversion efficiency without an exorbitant increase in production cost. The aim of the research is to achieve the lowest $/watt solar cell design that is suitable for commercial production. [edit] Sliver cells Professor Andrew Blakers and Dr Klaus Weber, working at Australian National University and Origin Energy have developed a technique for slicing a single silicon wafer, which allows a significantly larger collector surface area from each wafer, compared to usual solar cells. The technique involves taking a silicon wafer, typically 1 to 2 mm thick, and making a multitude of parallel, transverse slices across the wafer, creating a large number of slivers that have a thickness of 50 micrometres and a width equal to the thickness of the original wafer. These slices are rotated 90 degrees, so that the surfaces corresponding to the faces of the original wafer become the edges of the slivers. The result is to convert, for example, a 150 mm diameter, 2 mm-thick wafer having an exposed silicon surface area of about 175 cm² per side into about 1000 slivers having dimensions of 100 mm x 2 mm x 0.1 mm, yielding a total exposed silicon surface area of about 2000 cm² per side. As a result of this rotation, the electrical doping and contacts that were on the face of the wafer are located the edges of the silicon Simlification of the cleaning process  lower quality and cheaper  but it has to undergo some processes to inactivate the higher number of impurities.  minimise recombination (gettering processes) Beim EFG-Verfahren (Edge-defined Film-fed Growth) werden aus einer elektrisch beheizten Graphitwanne aus flüssigem Reinstsilizium achteckige Röhren von etwa 6 bis 7 m Länge nach oben gezogen. Die Ziehgeschwindigkeit liegt im Bereich von ca. 1 mm/s. Die Kantenlänge der einzelnen Seiten beträgt 10 bzw. 12,5 cm, die Wandstärke ca. 280 µm. Nach Fertigstellung der Röhre wird diese entlang der Kanten mit NdYAG-Lasern geschnitten und in einem bestimmten Raster dann über die Breite der jeweiligen Seite. Daraus ergibt sich die Möglichkeit der Herstellung von Zellen mit unterschiedlichen Kantenlängen (zum Beispiel 12,5 x 15 cm oder 12,5 x 12,5 cm). Es wird eine Ausbeute von etwa 80 % des Ausgangsmaterials erzielt. Bei den so erzeugten Zellen handelt es sich ebenfalls um multikristallines Material, welche sich vom Aussehen her deutlich von den gesägten Zellen unterscheidet. Unter anderem ist die Oberfläche der Zellen welliger. Dieses Verfahren wird auch Bandzieh- oder Octagon-Verfahren genannt. Das EFG-Verfahren wird von der Firma Schott Solar (Deutschland) angewendet. Entwickelt wurde das Verfahren von der Firma ASE Solar(USA). String-Ribbon-Verfahren [Bearbeiten] Weiterhin gibt es noch ein Verfahren der US-amerikanischen Firma Evergreen Solar, bei dem die Wafer zwischen zwei Fäden direkt aus der Silizium-Schmelze gezogen werden. Hierbei entsteht weniger Abfall (wie Späne etc., die normalerweise direkt entsorgt werden) als bei den herkömmlichen Verfahren. Basiert auf dem hydrostatischen effekt: zwei drähte werden durch flüssiges silizium gezogen  silizium miniskus  silizium erstarrt in for eines bandes, welches dann in scheiben geschnitten wird. RGS: A band of silicon. A substrat will be moved under a pot of liquid hot silicon  silicon will be pulles with the substrate and crystallise on it. New Generation Silicon Solar Cells

5.7 Further prospects Annual consumption of electricity per person:
1000kWh/a Annual solar cell power 1000W/m2a 800 – 1200 hours of sun in Germany with 80% ca. 800kWh/m2a out of a photovoltaic system Efficiency of 15%  120kWh/m2a To cover the annual consumption of electricity per person you need ~ 8,3m2 Multicrystalline solar cell (15x15x0,03cm3) has a peak power of 3,5W and is made out of 24g silicon (+ loss during production)  6,8kg silicon Of all modules globaly sold about 90% have been made from silicon. By 2030 alt least 30% of the global demand coming from crystalline silicon  solar grade silicon consumption as high as t The annual hours of sun can in southern germany just used around 70% 2030 silicon needed per year = 160,000t ! New Generation Silicon Solar Cells

5.7 Further prospects Russia – Saint Petersburg Germany - Munich
Nominal power (crystalline silicon) 1kW Incline of the modules 42° 37° Losses because of temp. 6,4% 6,5% Losses because of reflection 2,9% Losses in general 15,0% 15% Complete losses 24,3% 24,4% Power production out of a PV constructed for 1kW per year 865kWh 1009kWh By New Generation Silicon Solar Cells

6. Bibliography New Generation Silicon Solar Cells 19.04.2017
Rudden M.N., Wilson J., „Elementare Festkörperphysik und Halbleiterelektronik“, Spektrum Akademischer Verlag, ©1995, 3. Auflage Würfel P., „Physik der Solarzellen, Spektrum Akademischer Verlag, ©2000, 2.Auflage Kaltschmitt M., Streicher W., Wiese W. (Hrsg.), „Erneuerbare Energien Systemtechnik, Wirtschaftlichkeit, Umweltaspekte“, Springer Verlag, ©1993, 3. Auflage New Generation Silicon Solar Cells

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