Applications of Photovoltaic Technologies Referenced website:

2 Why Solar Cells? Finite fossil fuel supply Less environmental damage No radiation risk (meltdown) Nearly infinite supply of FREE energy Sun gives us 32 x10 24 joules a year, Cover 0.1% of the Earth’s surface with 10% efficient solar cells with an efficiency of would satisfy our present needs.

3 Greenhouse Effect Human activities have now reached a scale where they are impacting on the planet's environment and its attractiveness to humans.

4 Spectrum of light h: Planck’s constant × (J-s) ν: frequency (s -1 ) λ: wavelength (m) c : light speed 3.0 × 10 8 (m/s)

5 Atmospheric Effects Hu, C. and White, R.M., "Solar Cells: From Basic to Advanced Systems", McGraw-Hill, New York, 1983.

6 Solar Radiation Power emitted from Sun =3.8×10 23 (kw) Power direct to Earth=1.8×10 14 (kW) Solar constant=1353 W/m 2 T=5762 K

7 Air Mass (AM) AM0 : The standard spectrum outside the Earth's atmosphere. AM 1: Light incident with the angle of 0 degree. AM 1.5: Light incident with the angle of 48 degree. I D : Direct beam intensity (W/m 2 ) I G : Global irradiance (W/m 2 ) Meinel A.B. and Meinel M.P., "Applied Solar Energy", Addison Wesley Publishing Co., 1976 Intensity

8 Standard Solar Spectra

9 The AM1.5G Global spectrum is designed for flat plate modules and has an integrated power of 1000 W/m 2 (100 mW/cm 2 ). The AM1.5 D The direct plus circumsolar spectrum has an integrated power density of 900 W/m 2. Standard Solar Spectra-cont.

10 Part of periodic table IIIIIIVVVI BC(6) AlSi(14)PS ZnGaGe(32)AsSe CdInSbTe

11 Compound semiconductors Elemental semiconductors: Si, Ge Compound semiconductors: GaAs, InP Ternary semiconductors: AlGaAs, HgCdTe Quaternary semiconductors: InGaAsP, InGaAlP ElementalIV Compounds Binary III-VBinary II-VI SiSiGeAlPCdTe GeSiCGaAsCdS AsInPZnS GaPCdSe

12 Crystal Structures Polycrystalline Amorphous Crystalline In a crystalline solid atoms making up the crystal are arranged in a periodic fashion Some solids are composed of small regions of single crystal material, known as polycrystalline. In some solids there is no periodic structure of atoms at all and called amorphous solids

13 Commercial Si solar cells SINGLECRYSTALPOLYCRYSTAL AMORPHOUS

14 Photoelectric effect Metal Photon Electron Photon is a particle with energy E = hv Semiconductor EgEg Photon E ph ( hv)>E g

15 Direct and indirect semiconductor High absorption probability Low absorption probability EvEv E P EcEc Direct Semiconductor photon EvEv E P EcEc Indirect Semiconductor phonon photon GaAs; InP etc. c-Si

16 Metal-insulator-conductor M etal →CB and VB overlap, Insulator and semiconductor CB and VB are separated by an Eg (energy band Eg). Eg for Si is eV (semiconductor) ;5eV for diamond (Insulator) Filled States (VB) Empty States (CB) EgEg metal semiconductorinsulator

17 Absorption of Light E ph < E G Photons with energy E ph less than the band gap energy E G interact only weakly with the semiconductor, passing through it as if it were transparent. E ph = E G have just enough energy to create an electron hole pair and are efficiently absorbed. E ph > E G Photons with energy much greater than the band gap are strongly absorbed

18 N- and P-type Addition of impurities with three valence electrons results in available empty energy state, a hole B, Al, In, Ga (Acceptor impurities) Addition of impurities with five valence electrons results an extra electron available current conduction P, As, Sb (donor impurities

19 Physics of Photovoltaic Generation ※ E hp > E G ※ Electron-hole pair (EHP). ※ Electrons go to negative electrode; hole to positive electrode.

n-type semiconductor p-type semiconductor Physics of Photovoltaic Generation Depletion Zone

21 Solar Cell-structure A solar cell is a P-N junction device Light shining on the solar cell produces both a current and a voltage to generate electric power. Busbar Fingers Emitter Base Rear contact Antireflection coating Antireflection texturing (grid pattern)

22 Solar cell structure How a solar cell should look like ?  It depends on the function it should perform, it should convert light into electricity, with high efficiency It should be a P-N junction P-type N-type There should be ohmic contact at both side It should absorb all light falling on it  It should reflect less light  Most of the light should go in It should convert all absorb light into electricity

23 Minimizing optical losses The optical path length in the solar cell may be increased by a combination of surface texturing and light trapping. Top contact coverage of the cell surface can be minimized Anti-reflection coatings can be used on the top surface of the cell. Reflection can be reduced by surface texturing The solar cell can be made thicker to increase absorption There are a number of ways to reduce the optical losses:.

24 Optical properties of surface What are optical losses:  Reflection  Shadowing due to metal contact  Partial absorption Photons in the spectrum can generate EHP, ideally all the sun light falling on the cell should be absorbed Short circuit current (I SC ) is usually reduced due to optical losses Design criteria for small optical losses : Mminimize optical loss

25 Air, n 0 Semiconductor, n 2 ARC, n 1 The thickness of a ARC is chosen such that the reflected wave have destructive interference  this results in zero reflected energy The thickness of the ARC is chosen so that the wavelength in the dielectric material is one quarter the wavelength of the incoming wave (destructive interference). n 2 > n 1 > n 0 Choice of ARC

26 Reflection from various combination Multilayer structure reduces the reflection losses Index of refraction is also a function of wavelength, minimum reflection is obtained for one wavelength More than one ARC can be used, but expensive Source: PV CDROM - UNSW

27 Surface texturing Any rough surface decreases the reflection by increasing the chances of the reflected rays bouncing back on the surface Surface texturing can be obtained by selective etching  a process by which material is removed by chemical reaction Selective etching is based on the concept of different material property in different direction in crystals, Etching rate are different in dir n than in dir n

28 Surface texturing Chemical etching in KOH results in pyramid formation on the Si surface  etching is faster in direction than in direction Using photolithography, inverted pyramids can be obtained, which are more effective surface

29 Light trapping Rear side reflector or rear side texturing is used to increase the optical path length in solar cell  Increased optical path is required for thin solar cell (thin solar cell have higher V oc. It saves expensive Si) Total internal reflection (TIR) condition are used to increase the optical path length Snell’s law (  1 for Si is 36 degree) For TIR

30 Lambertian Rear Reflectors Increases the path length by 4n 2, very good in light trapping, path ;length increases by about 50 Random reflector from the rear side TIR Lambertian reflector is one which reflects the lights in a random direction  this together with the front texturing increases the optical path length

31 P-N junction Current loss due to recombination Recombination areas  Surface recombination  Bulk recombination  Depletion region recombination Recombination of carriers reduces both short circuit current as well as open circuit voltage Bulk semiconductor rear surface Front surface Design criteria: The carrier must be generated within a diffusion length of the junction, so that it will be able to diffuse to the junction before recombining

32 w w h h d Emitter  finger and busbar spacing,  the metal height-to-width, aspect ratio,  the minimum metal line width and  the resistivity of the metal Top contact One example of top metal contact design Design criteria: minimize losses (resistive, shadow)

33 Resistive Losses Resistive effects (series and shunt resistance) in solar cells reduce the efficiency of the solar cell by dissipating power in the resistances. Both the magnitude and impact of series and shunt resistance depend on the geometry of the solar cell and solar cell area Resistance are given in Ω-cm 2 I L I f R s R sh V I Solar Cell model The key impact of parasitic resistance is to reduce fill factor.

34 Resistive Losses: Series resistance, R s 1. the movement of current through the emitter and base of the solar cell 3. resistance of the top and rear metal contacts 2. the contact resistance between the metal contact and the silicon Contributing factors to R s : Bus bar Finger s N- layer p- layer Base emitte r M-S contact

35 Contact resistance N Heavy doping under contact to minimize contact resistance Metal to semiconductor contact Contact resistance losses occur at the interface between the silicon solar cell and the metal contact. To keep top contact losses low, the top N+ layer must be as heavily doped as possible. A high doping creates a "dead layer“. Ohmic contact, High doping, tunneling contact

36 Sheet resistance In diffused semiconductor layers, resistivity is a strong function of depth. It is convenient to a parameter called the "sheet resistance" (Rs). Rs is called sheet resistance with unit of ohms/square or Ω/ □ (actual unit is Ohms) The L/W ratio can be thought of as the number of unit squares (of any size) Sheet resistance of a solar cell emitter is in the range of 30 to 100 Ω/ □ W L t

37 Emitter resistance: Power loss t P N d L x dx d/2 Zero current flow exactly at midpoint of fingers Maximum current density at the finger edge Resistance dR in infinitesimally thin layer of dx