1. Photovoltaic Cells: Solar Cells
EBB 424
Dr. Sabar D. Hutagalung
USM

4. Discovery and Development of Photovoltaic Power
“Photovoltaic” means “light energy”.
The photovoltaic effect has been known since 1839, but cell efficiencies remained around 1% until the 1950s Bell Lab quickly achieved 11% efficiency,
in 1958, the Vanguard satellite employed the first practical PV generator producing a modest one watt.
In the 1960s, the space program continued to demand improved PV power generation technology.

5. Converting Sunlight to Electricity
A typical PV cell consists of semiconductor material having a p-n junction.
Sunlight striking the cell raises the energy level of electrons and frees them from their atomic shells.
The electric field at the p-n junction drives the electrons into the n region while positive charges are driven to the p region.
A metal grid on the surface of the cell collects the electrons while a metal back-plate collects the positive charges.

6. Photovoltaics
Among a wide variety of renewable energy projects, photovoltaics is the most promising as future energy technology due to:
pollution-free,
the in-put energy is abundantly available everywhere,
it also can operate by diffused light.
A big problem in the photovoltaic system is the high price of the solar cell module.
Therefore, cost reduction of the solar cell is very importance.

7. Do you know?
Sunlight reaches the Earth’s outer atmosphere at a strength of 1367 watts per square meter, defined as AM0, or “air mass zero.”
Atmospheric losses reduce the sun’s power to about 1000 W/m2 when the sun is directly overhead on a cloudless day.

8. Solar Spectrum

9. Use of solar spectrum
Basically, the certain semiconductor materials are suited only for specific spectral ranges.
A specific portion of the radiant energy cannot be used, because the photons do not have enough energy to "activate" the charge carriers.
On the other hand, a certain amount of surplus photon energy is transformed into heat rather than into electrical energy.

10. Use of solar spectrum If bandgap is too high,
part of spectrum will wasted (transmitted)
But
if Eph >> Eg, the excess energy will wasted as heat.
Optimum intensity:
~1.3 to 1.4 eV.
Is GaAs better than Si?

11. Use of solar spectrum
Multijunction solar cells use the solar spectrum more efficiently.

12. Solar Cell : Basic design

13. Solar Cell : Basic design
Light
As temperature increases, intrinsic generation increases, light becomes less effective at generating excess carriers.
A solar cell with interdigitated contacts.
The load resistor is RL.

14. Calculation of maximum power
The current through the solar cell can be written as:
where Is is the saturation current and Iph is the photo current (which is assumed to be independent of the applied voltage Va).
This expression only includes the ideal diode current of the diode, thereby ignoring recombination in the depletion region.
The short circuit current, Isc, is the current at zero voltage equals Isc = -Iph and the open circuit voltage equals:

15. Calculation of maximum power
The total power is then:
The maximum power occurs at dP/dVa = 0. (See I-V characteristics)
The voltage and current corresponding to the maximal power point are Vm and Im.
This equation can be rewritten as:

16. Calculation of maximum power
Using equation for the open circuit voltage Voc.
The most accurate solution is obtained by solving this transcendental equation and substituting into equations above.
The maximum power can be approximated by:

17. Calculation of maximum poweror
where
The energy Em is the energy of one photon, which is converted to electrical energy at the maximum power point.
The total photo current is calculated as (for a given bandgap Eg)
The efficiency equals:

18. The Photocurrent
Consider a diode, illuminated but not biased (no external voltage applied)
Illuminated photons will be absorbed:
Within the depletion region, W
Within the neutral side p
The illuminated photons  e-h pairs generated:
e-h pairs in depletion region  separated by Eo
e-h pairs in p-region  diffuse to the depletion region to be separated by the Eo (Le and Lh are then important)
The separated e will drift to the n+ neutral-side, making the area highly negative (due to e-charge)
Holes that drift to the p-side will make the area positive.
Open circuit voltage develops
If external load is applied  electrons can travel around the external circuit to do work and reach p-side to recombine with the holes there.

19. The Photocurrent
Elecron-hole pairs generated as a function of cell length down the solar cell
The pairs generated decay exponentially with length & depends on the absorption coefficient
E-h pairs photogeneration rate, Gph = Goexp(-x) n p
If Lh (hole diffusion length) > thickness of n region, ln, then we use ln instead
Le & Lh = minority carrier diffusion lengths
If e-h generation occurs within the Lh+ W+Le
Photocurrent, Ip:
W= depletion layer width (e-h separation occurs here)
p-region, e-h must diffuse to the depletion region to be separated, diffusion length of e = Le and holes = Lh

20. Efficiency
Efficiency = Fraction of incident light energy converted to electrical energy.
Efficiency is the most important characteristic because it allows the device to be assessed economically in comparison to other energy devices.
For a given solar spectrum, the efficiency depends on:
Semiconductor material
Device structure
Ambient conditions (temperature)
High radiation damage
Sun spectrum
Efficiency alone is not enough, the cost of the cell is also important and the life time.

21. I-V characteristics & Efficiency
in dark (diode)
under illumination
Typical I-V characteristic of solar cells

22. I-V characteristics & Efficiency
As the intensity increases, the short-circuit current ISC increases linearly, but the open circuit voltage VOC increases sublinearly.
The I-V characteristics of a solar cell with varying illumination as a parameter.

23. I-V characteristics & Efficiency

24. I-V characteristics & Efficiency
(II)
(I)
(III)
(IV)
The I-Va characteristic of a solar cell.
The maximum power is obtained at Pm = ImVm

25. I-V characteristics & Efficiency

26. I-V characteristics & Efficiency

27. I-V characteristics & Efficiency
The I-V characteristics of a solar cell with varying illumination

28. I-V characteristics & Efficiency

29. Calculation, 
Consider a solar cell driving a 30 resistive load. The cell has area = 1 cm x 1 cm and is illuminated with light of intensity = 600Wm-2 and has I-V characteristic as shown below.
0.425V
14.2mA
Q: What are the current and voltage in the circuit?
A: I’ = mA , V’ = 0.425V
Q: Calculate the power delivered to the load (Pout)
A: Area under the I’, V’ rectangular = P
P= 14.2 x 10-3A x 0.425V = 6.035mW

30. Calculation,  Q: Calculate the input sun-light power
A: Pin = (light intensity x surface area)
Pin = 600Wm-2 x (0.01m)2 = 0.06W
Q: Calculate the efficiency of the device
A:  = 100x (Pout/Pin)
Pin = 0.06W
Pout = 6.035mW
 = 10.06%
Q: State why the efficiency is very small?

32. Natural Limits of Efficiency
In addition, there are optical losses, such as the shadowing of the cell surface through contact with the glass surface or reflection of incoming rays on the cell surface.
Other loss mechanisms are electrical resistance losses in the semiconductor and the connecting cable.
The material contamination, surface effects and crystal defects, however, are also significant.

33. Natural Limits of Efficiency
Theoretical maximum levels of efficiency of various solar cells at standard conditions

34. Efficiency: Effect of Series Resistance

35. Solar cells design structure

36. PV Technology Option PV Technologies can be divided into 2 main areas:
Flat plates
Concentrators
Flat plates:
Use crystalline silicon (ingots or sheet growth) or semiconductor compounds
Thin films on low cost substrates
The thinner the layer the better organic polymer and nanomaterials
Large scale
Lenses or reflectors needed to focus sunlight
MW power

37. PV Technology Option

38. Silicon - PERL
Single crystal PERL cells (Passivated Emitter Rear Locally diffused)
Inverted pyramids on the surface of the cell (etched (100) Si to produce 4 (111) planes).
Pyramids:
Reduce reflection of light
Increases refraction into the cell
Increase absorption

39. Silicon - PERL Structure
To increase the area being exposed to sun light – use slim finger electrodes
to further increase the absorption and reduce reflection
Inverted pyramids

42. Si:H Cell Glass Transparent Conducting Oxides
substrate can be anything really, window glass, stainless steel or polyamide
Si:H is not single crystal Si (amorphous) made by glow discharge of silane (SiH4)
Powering pocket calculator and digital watches
Problems with degradation of Si
Glass
Transparent Conducting Oxides p i n
metal contact (ohmic & reflector mirror)
high optical (high bandgap), low electrical resistivity, low reflection, high carrier mobility, low roughness, good crystallinity and adhesion (ITO, Cu2O, CuAlO2
p  Si:H doped B
i  the most important layer, must be able to absorbed as much light as possible
n  Si:H doped Sb
Amorphous silicon p-i-n solar cell structure
Ag or Al, Ag oxidise Al  low reflectance, use Al-Ag combine layer

43. Si:H Solar Cell: Incorporation of H2
Hydrogen may be incorporated in the silicon to reduce the number of dangling bonds.
This will create a material called hydrogenated amorphous silicon (Si:H)
The process to introduce H2  hydrogenation
Dangling bond is not desired as it is a place where EHP can recombine hence efficiency of the device will be reduced.
However once ‘treated’ with H2 the number of dangling bond can be reduced significantly and the EHP recombination reduces

44. Thin Film Solar Cells
Typical thin-film amorphous silicon construction.
Thin-film solar cells are manufactured by applying thin layers of semiconductor materials to a solid backing material.
Sunlight entering the intrinsic layer generates free electrons.
The p- and n-type layers create an electric field across the intrinsic layer.
The electric field drives the free electrons into the n-type layer while positive charges collect in the p-type layer.

45. Thin Film Solar Cells
Less efficient than single- and polycrystal Si, but offer greater promise for large-scale power generation because of ease of mass-production and lower materials cost.
Suitable for building-integrated systems because the semiconductor films may be applied to building materials such as glass, roofing, and siding.
The materials that have been used for thin film PV cells: a-Si:H, GaAs, CuInSe2, CdTe and TiO2.
Tin oxide is a conductive material that is transparent when in a thin layer.
Tin oxide is used in place of a metallic grid for the top layer of thin film PV sheets.

46. Thin Film Solar Cells
Schematic cross-section of thin film a-Si:H solar cell
Schematic cross-section of thin film CIGS (CuInGaSe2) solar cell
Schematic cross-section of a crystalline Si solar cell.

47. Thin Film Solar Cells

48. Thin Film Solar Cells
Manufacturing cost and production capacity projections for thin-film and non-thin-film modules based on data available in year 2001 (Chopra et al., 2004; Mitchell et al., 2002). Thin-film solar cell technologies have the potential for producing cheaper devices on a large scale.

51. CdS quantum dot sensitized solar cells
CdS quantum dots have been self-assembled on the surface of dispersed nanocrystalline TiO2 particles, and a light-harvesting electrode has been fabricated from the resulting sensitized P25 particles

52. ZnO/CdS/CuGaSe2 single-crystal solar cells
I-V characteristics of the analyzed solar cells with h= 3:5%, 6.0%, 6.7% & 9.7%. The illumination was performed using an AM1.5 solar simulator. The illuminated characteristics of the cells (A, B, C) were measured at an illumination intensity Iill= 83mW/cm2. The characteristics of cell (D) were measured at an illumination intensity of Iill= 100mW/cm2.
Solar Energy Materials & Solar Cells 80 (2003) 491–499

53. Nanostructured CdTe, CdS & TiO2 thin film solar cells
Device configuration of a Glass/ITO/n-Nano-CdTe/p-bulk CdTe/graphite solar cell.
I–V curve of a glass/ITO-nano CdS–Ag Schottky diode.

54. Design of thin film solar cells
Solar cells based on polycrystalline semiconductor thin films have great potential for decreasing the cost of photovoltaic energy.
However, this kind of solar cells has characteristics very different from those fabricated on crystalline Si for which the carrier-transport and behavior is clearly known.
Instead, for heterojunction solar cells made on less known polycrystalline materials the design is almost empirical.

55. Design of thin film solar cells
Solar cells made on polycrystalline materials,
which are built up by small crystallites with different orientations joint through
regions with a great amount of defects and impurities.
The regions between
crystallites are known as grain boundaries.
Schematics of polycrystalline thin films with (A) columnar grains, and (B) noncolumnar grains. The former is better for solar cell applications.

56. Design of thin film solar cells
Due to the high defect and impurity content at the grain boundaries, the photo generated charge carriers will recombine strongly there, since the defects cause deep energy levels in the band gap which act as recombination centers in any semiconductor material.
In order to quantify the recombination rate at grain boundaries, the concept of recombination velocity is used.

57. Design of thin film solar cells
Using the above concept was found, e.g, the design of a cell with structure glass/ FTO/CdS/CdTe/metal, where CdTe is the active material.
If CdTe film grain sizes are very small, i.e. less than 500 nm, it will be enough to have 0.5–1 mm thick layers, because in this case, the effective diffusion length will be < 0.5 mm.
In other words, there will be no contribution to the cell short circuit current for thickness of this material above 0.5 mm.
On the other hand, if the grain size is large enough, i. e., above 1 mm, a thicker cell will be necessary with a total thickness of around 2–5 mm for the CdTe layer.

58. CdS/CdTe thin film solar cells (15% Efficiency )
CdS/CdTe solar cell employing the CdS layer doped with various metal organic compounds, i.e., (CH3)2SnCl2, (C6H5)3GeCl, (CH3CO2)3In, [(C2H5)2NCS2]2Zn.
The CdS layer was deposited by MOCVD with a thickness of 80nm on a glass substrate (Corning glass -1737) coated by indium tin oxide (ITO).
Solar Energy Materials & Solar Cells 90 (2006) 3108–3114

59. CdS/CdTe thin film solar cells (15% Efficiency )
Photovoltaic performance of CdS/CdTe solar cells CdS layers doped with different MO compounds.

60. III–V compound multi-junction solar cells
Multijunction SC also called a cascade or tandem cell, can achieve a higher total conversion efficiency by capturing a larger portion of the solar spectrum.
A multijunction device is a stack of individual single-junction cells in descending order of bandgap (Eg). The top cell captures the high-energy photons and passes the rest of the photons on to be absorbed by lower-bandgap cells.
This multijunction device has a top cell of gallium indium phosphide, then a "tunnel junction" to allow the flow of electrons between the cells, and a bottom cell of gallium arsenide.

61. III–V compound multi-junction solar cells
European roadmap for the development of 3, 5 and 6-junction solar cells for space applications

62. III–V compound multi-junction solar cells
InGaP/InGaAs/Ge 3-junction cells with efficiency of 31.7% at AM1.5 were achieved on Ge substrates,
InGaP/GaAs//InGaAs mechanically stacked 3-junction cells with world-record efficiency of 33.3%.
Chronological improvements in AM0 conversion efficiencies and target of space solar cells.

63. Single- & Multi-junction solar cells

64. Thin Film Efficiency Improvement
a-Si:H/c-Si:H Cell Spectral Response
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2 20 40 60 80
100 1 2 3 4 5
Number of Sunlight Photons (m-2s-1micron-1) E+19
Relative External Quantum Efficiency, %
c-Si:H junction
a-Si:H junction
AM 1.5 global spectrum
Wavelength, microns
Glass Substrate
Transparent Conductor
Amorphous Silicon
Microcrystalline Silicon
Back Contact
Cost / m2
Watt / m2
Objectives
Maximize Light Collection
Increase Voltage
Increase Stability
Optimize Yield and Uniformity
Possible Directions:
a-Si/-Si tandem junction
New TCO & metal materials
Optimized interfaces
Advanced patterning/contacts
Advanced Si devices – BG, triple jn
Non Si absorbers

65. Nanostructured Cells

66. Dye Sensitised Solar Cell
Semiconducting oxides can also be used to generate electricity
For example the use of dye sensitised solar cell utilizing titanium dioxide
Dye-sensitized solar cells use photo-sensitization of wide-band-gap mesoporous oxide semiconductors
DSSC requires dyes, porous semiconductors and electrolyte
It has efficiency ~ 15% at the moment
But it is a low cost device
It is not a solid state device
Device has electrolyte in it.

67. DSSC Titania particles Photoactive electrode Counter electrode (Pt)
Titania particles are in contact with dye molecules
When shine with sunlight, the dye molecules will be senstised to produce electrons.
Dye molecules will loose electrons to the titanum dioxide
These electrons will be injected into the conduction band of titanium dioxide.
Excess electrons in the CB of TiO2 are used to generate photo current
Titania particles
Photoactive electrode
Counter electrode (Pt)
Redox Electrolyte
The redox electrolyte consists of iodine ions.
These ions will reoxidise the dye again (regenerate the dye)
Regeneration of the redox system is catalysed by platinum at the counter electrode.

68. Porous Layer of TiO2 with absorbed dye
DSSC
Conducting Glass Electrode
Porous Layer of TiO2 with absorbed dye
Electrolyte with redox mediator (I-/I-3)
Catalyst, Pt

69. DSSC

70. PV CONCENTRATOR
The FLATCON concentrator module developed at Fraunhofer ISE uses metamorphic GaInP/GaInAs dual-junction solar cells with an efficiency of 30 % at a concentration ratio of 500.
Several thousand cells, 2 mm in diameter, have been produced.
Picture of a FLATCON concentrator module with GaInP/GaInAs dual-junction solar cells developed at Fraunhofer ISE.

71. CPV Technology

72. CPV Technology

73. Terrestrial PV System Cost vs. Cell Cost
CPV Technology
Terrestrial PV System Cost vs. Cell Cost

74. CPV Technology

75. CPV Technology

76. CPV Technology Iso-efficiency contour
plots for 3-junction cells limited only by radiative recombination
Dependence of ideal efficiency on band gaps Eg2 and Eg3
Ideal efficiency contours → show limiting efficiencies possible without the effects of grid shadowing, series resistance, and non-radiative carrier recombination.
3-Junction Theoretical Efficiency - Vary Eg1 and Eg2

77. Photovoltaic Power

78. PHOTOVOLTAIC POWER GENERATION

79. Solar Spectrum

80. Conversion Efficiency
The most efficient PV modules usually employ single-crystal Si cells, with efficiencies up to 15%.
Poly-crystalline cells are less expensive to manufacture but yield module efficiencies of about 11%.
Thin-film cells are less expensive still, but give efficiencies to about 8% and suffer greater losses from deterioration.

81. Photovoltaic Applications
PV power generation has been most useful in remote applications with small power requirements.
As PV power becomes more affordable, the use of photovoltaics for grid-connected applications is increasing.
However, the high cost of PV modules and the large area they require continue to be obstacles to using PV power to supplement existing electrical utilities.
An interesting approach to both of these problems is the integration of photovoltaics into building materials.

82. Building-Integrated Systems
Building-integrated photovoltaic (BIPV) systems offer advantages in cost and appearance by incorporating photovoltaic properties into building materials such as roofing, siding, and glass.
When BIPV materials are substituted for conventional materials in new construction, the savings involved in the purchase and installation of the conventional materials are applied to the cost of the photovoltaic system.
BIPV installations are architecturally more attractive than roofmounted PV structures.

83. Solar Photovoltaic Plants
Solar Photovoltaic Plants convert sunlight into electricity. Photovoltaic (PV) panels are often referred to as “solar panels” because they are made up of several small sections called “solar cells”. Most “solar cells” are made of silicon and each cell is designed with a positive and a negative layer to create an electric field, just like in a battery.

84. Solar Photovoltaic Plants

85. How PV Cells Work Photovoltaic cells, modules, panels and arrays
Diagram of a photovoltaic cell
Major photovoltaic system components.

86. Benefits of a Solar PV Plant
Produces pollution free electricity
Reducing fuel consumption (i.e. gas) will displace fossil fuels, lower energy bills & cushion the user against increasing energy prices
Noise free “green energy”

87. Problem
Suppose that a particular family house in a sunny geographic location consumes a daily average electrical power of 1500 W supply by an 50 m2 are of solar panel. The average solar intensity incident per day is about 6 kWh m-2. What is the efficiency of solar cell?

88. Solution Total sunlight energy per day: = intensity x area
= 6 kWh m-2 x 50 m2
= 300 kWh
Solar cell output = 1.5 kW x 24 h = 36 kWh
Efficiency = Output/Total energy light
= 36 kWh/300 kWh
= 12 %

89. Solar Cells