Losses in Solar Cells

1. 3-05-2006 Energy Systems Engg IIT Bombay 11 Losses in Solar Cells By Vikrant A.Chaudhari 05417001 Under the guidance of Prof C.S.Solanki Energy Systems Engg

2. 3-05-2006 Energy Systems Engg IIT Bombay 21 Direct Energy Conversion Direct conversion of sunlight to electricity is possible through Solar cells. No mechanical parts. Maintenance free. Longer life

3. 3-05-2006 Energy Systems Engg IIT Bombay 31 Solar cell Materials and technologies Silicon (Si). Gallium Arsenide (GaAs). a-Si. CIS. Wafer Based. Thin films. Compound Multi-junction

4. 3-05-2006 Energy Systems Engg IIT Bombay 41 Si solar cells Si is first choice for solar cells because for good knowledge of Si processing in micro electronics industry Efficiency- 26% theoretical. 24.7% obtained in laboratory. 12-14% commercial.

5. 3-05-2006 Energy Systems Engg IIT Bombay 51 Low efficiency of Si Losses in Si solar cells, causes efficiency to reduce. Reflection losses. Recombination losses. Resistive losses. Thermal losses.

6. 3-05-2006 Energy Systems Engg IIT Bombay 61 Reflection losses Occurs on the top surface of the cell. Light gets reflected due to reflectivity of Si(~0.35). Loss due to reflection is given as

7. 3-05-2006 Energy Systems Engg IIT Bombay 71 Reflection losses condt.. Internal Quantum efficiency.

8. 3-05-2006 Energy Systems Engg IIT Bombay 81 Reflection losses cond.. Anti reflective coating (ARC) and texturing on the top surface reduces the reflection. ARC in the form of SiO, SiN, MgF, ZnO. Texturing in the form of pyramids so that light is trapped at the surface. Typical thickness is in the range of <60nm. Texturing is in the done using Isotropic etching of the top surface with H2SO4 or HNO3 in H2O2.

9. 3-05-2006 Energy Systems Engg IIT Bombay 91 Reflection losses cond..

10. 3-05-2006 Energy Systems Engg IIT Bombay 101 Generated carriers gets recombine. Impurity present in Si causes dangling bonds. Recombination loss. Loss of carriers results in decrease Isc and Voc. Different mechanisms govern the recombination loss. Recombination loss occurs at various location in the cell. Loss depends on volume of the cell. Recombination losses

11. 3-05-2006 Energy Systems Engg IIT Bombay 111 Recombination losses

12. 3-05-2006 Energy Systems Engg IIT Bombay 121 Recombination losses cond.. Surface recombination loss depends on SRV.

13. 3-05-2006 Energy Systems Engg IIT Bombay 131 Recombination losses cond.. Bulk recombination due to impurity. Trap assisted recombination is dominant in this region. High purity of bulk material. Recombination at metal semiconductor contact. Due to large surface defects at the metal semiconductor contacts there is high SRV at the contacts. Heavy doping can help in reducing the SRV by creating an electric field.

14. 3-05-2006 Energy Systems Engg IIT Bombay 141 Recombination losses cond..

15. 3-05-2006 Energy Systems Engg IIT Bombay 151 Resistive losses Series resistance consists of Bulk resistance Emitter resistance Metal semiconductor contact resistance. Fingers and bus bar resistance. Fill factor is affected due to series resistance.

16. 3-05-2006 Energy Systems Engg IIT Bombay 161 Resistive Losses Emitter resistance

17. 3-05-2006 Energy Systems Engg IIT Bombay 171 Resistive Losses

18. 3-05-2006 Energy Systems Engg IIT Bombay 181 Resistive Losses Metal resistive contacts form Schotty contacts and ohmic contacts. Work function of metal semiconductor decides the type of contact. Ohmic contacts are required in Solar cells. Specific contact resistance (rc) dominates the metal semiconductor contacts.

19. 3-05-2006 Energy Systems Engg IIT Bombay 191 Resistive Losses Fingers thickness decides the contact resistance of the metal semiconductor. Thinner the finger thickness more number of fingers can be accommodated in the given cell area. This increases current collection by the fingers reducing resistance.

20. 3-05-2006 Energy Systems Engg IIT Bombay 201 Resistive Losses Shading loss reduces the light incident on the front surface of the cell. Shading can be reduced by different design of the cell. Both the contacts at the back (Interdigitated contacts). Buried contact cells.

21. 3-05-2006 Energy Systems Engg IIT Bombay 211 Thermal losses Thermal losses increases temperature of the cell. Intrinsic carrier concentration (ni) depends on the temperature. Voc depends on the temperature. Increased ni increases reverse saturation current Io and hence Voc.

22. 3-05-2006 Energy Systems Engg IIT Bombay 221 Efficiency and loss

23. 3-05-2006 Energy Systems Engg IIT Bombay 231

24. 3-05-2006 Energy Systems Engg IIT Bombay 241 Conclusion Losses mechanism in Si solar cells are studied. Losses tend to reduce the efficiency of the cells. Various techniques that are used to reduced the losses are discussed briefly.

25. 3-05-2006 Energy Systems Engg IIT Bombay 251 Thank you