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12 Cards in this Set
- Front
- Back
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Advantages of GaAs compared to Si |
1. Stronger absorption due to direct band gap - cells can be made thinner 2. Better temperature coefficient - good concentrator application 3. Better radiation hardness 4. Efficiency - 29.1% with concentrator. |
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Recombination in GaAs |
- Direct Eg so radiative dominant - Auger lower due to lower number of intrinsic carriers - SRH recombination dominant in depletion layer - Surface recombination also faster than Si |
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GaAs optimisation |
- Reduce front and rear surface recombination - Recombination in depletion layer also strong - Expensive |
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Features: Inorganic thin film PV |
- Short Ln and Lp, so there are several junctions needed to improve collection probability - can extend built in field by using p-i-n structure - defect states in band gap can make doping difficult |
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Amorphous Si: positives and negatives |
+ cheap, low T processing + Good absorption (direct, large band gap); so can be made thinner! - doping and charge transport difficult due to more dangling bonds than crystalline Si ~ hydrogenating ties up dangling bonds and extends band gap to 1.8 eV |
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Staebler-Wronski effect |
Light induced reversible degradation in amorphous solar cells. Efficiency drops from 10% to 7% (FF is also decreased). Can be reverted after heating to 150°C. Main causes: - metastable defects under illumination - extra trapping/recombination centres |
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CI(G)S: Features |
-Direct band gap - High absorption coefficient - Available in both p- and n-type - CIGS has tunable band gap, the more Ga, the higher Eg - efficiency: 15-18% module |
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Problems with CdS - CIGS heterojunctions |
- defect states = SRH recombination = lower Voc - diffusion of chemicals, unwanted alloys could be formed. |
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CZTS features (Not commercial) |
+ non-toxic and abundant elements + 12% efficiency + Same crystal structure as CIGS |
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CdTe: pros and cons |
+ Good T coefficient + Direct Eg + Good absorption = thin + Very stable, although Cd toxic - High surface recombination - CdS must be used as window layer ~ efficiency: 21% (one cell), module 9-14%. - defects also occur at heterojunction due to lattice mismatch + Shortest energy payback time! |
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Hot-spot heating |
If a string of series cells is short-circuited, there is heavy power dissipation in the bad cell (due to the reverse bias on it). This results in heating and damage to the solar cell material. In parallel all cells must be under Voc. |
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Bypass diodes |
Diode connected parallel to shaded cell, and with opposite bias. When shaded cell works fine, diode allows no current (reverse biased). When shaded cell doesn't work, diode allows current above 0.5 V (forward bias) |
