Hello,
Kindly, check the details below related to the question:
Background:
If the HIT solar cell is front-illuminated, we need TCO at the anode (front) end of the solar cell for better transmission efficiency. However, in the HIT device structure ITO (TCO) is used at the rear end as well, even though we usually have Ag (100 nm) as back electrode.
Question: Why do we need ITO (transparent conductive oxide) at the rear end? Does it has something to do with back surface field (BSF) or any optical reasons?
Argument: The c-Si (crystalline Si) normally used in HIT solar cells is about 300 micrometer. Light can barely cross this to have any optical effects on the rear side?
I would appreciate your response
Regards
Image Source: https://pvlab.epfl.ch/heterojunction_solar_cells
Dear Hassen,
welcome,
I would add the ITO as a buffer layer between the silver and the amorphous silicon thin layers to prevent the silver to leak to silicon above the ITO layer. This may be required if the thin silicon layers have pinholes or any discontinuity. Silver is heavy metal and if it diffuses to silicon it kills the minority carrier lifetime.
Best wishes
Thank you so much for your kind response Prof. Abdelhalim Zekry i think that makes sense, and i am assuming a metal/oxide electrode combination is preferrably used, which we can also see in some other OLED structures to improve the charge injection and in case of solar cells improve the short circuit current density (Jsc). Otherwise, only ITO would have been sufficient on the top side as well.
Thank you once again
Dear Hassan Hafeez,
Along with Prof. Abdelhalim Zekry‘s statement, it can be said that only the photons absorbed in the active layer are considered as contributing to the photo generation of free electrons and holes. in other word, in order to maximize light absorption the active layer technology can be combined with a back contact architecture. So, for effective light trapping, a highly reflective back contact (BR) is necessary when the average photon is reflected many times at the back side of the solar cell. The reflectivity of the back reflector increases introducing a TCO layer between silicon layers and back metal contact. Usually more transparent and less conductive ITO (rather than front ITO contact) is used to optimize the solar cell’s optical properties in the longer wavelengths range.
I think this paper can be excellent fit for your question:
TCO and light trapping in silicon thin film solar cells. Solar Energy 77 (2004) 917–930
Also this can be suitable for your question:
ITO-free metallization for interdigitated back contact silicon heterojunction solar cells, Energy Procedia 124 (2017) 379–383
Dear @ Milad Fallah
Thank you so much for your lovely and detailed answer.
It seems to be a good reason and due to more of an infrared wavelength nature of the light at the rear end, reflection could help confine the photons within the active layer. And, i checked some articles were TCO was optimized to decrease reflection losses instead.
Also, according to that the front TCO would also reflect so much light back to the source which is not useful for solar cells.
Thank you again, i shall read these articles :)
At the front of the solar cells, these TCO layers act as the optically transparent electrode that allows photons into the solar cell and transports the photo-generated electrons to the external device terminals. Therefore, high lateral conductivity and low UV-IR absorption are crucial requirements of the front TCO of any solar cell device.
I understand that Mohammed Kadhim Jawad i was more concerned about the back side usage.
Probably to increase the back contact barrier high to improve the number of collected electrons.
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