I want to know the role of ZnO in these solar cells. I am confused as to how it acts as a Buffer/Contact layer.
Normally ZnO (i-ZnO) is used together with CdS or ZnS buffer layer. i-ZnO plays a role like shunting path covering layer between CdS and AZO and this makes improved Voc for CIGS solar cell. And also additionally, important thing - as a protection layer for CdS. Chemically deposited CdS is very easilly damaged during AZO sputtering deposition. Some researchers claim that ZnO reduce defect density of the surface and this leads to reduced surface recombination. But I'm not sure this is also applicable to CdS/CIGS case.
Do you think one solar cell would have nice efficiency without i-ZnO?
And... why some people use AZO 1% instead of i-ZnO?
Nice efficiency - yes, or might be better efficiency due to the high transparency, but with very low-possibility. During my work in LG we tried i-ZnO-less PV panel. But unfortunately only a few panels showed A-grade in efficiency. Some researchers claim that i-ZnO plays a role like a layer for built-in-field and this makes high Voc (unfortunately I cannot explain the detail why it is, sorry).
AZO (1~3% Al) is generally used TCO layer for CIGS PV, and normally it is used together with i-ZnO as a i-ZnO/AZO combination.
Hi Rekha,
This is a very interesting question! The ZnO/CdS/CIGS heterointerface is an interesting puzzle! There at less 3 points to consider :
1) Electrical : charges
2) chemical (also link to charge)
3) process as M. Dowon Bae indicate.
So I think it depend strongly to your device!
Best regards
Hi Dowon. Some author proposed AZO 1% instead of i-ZnO. I mean, the TCO bilayer will be formed by AZO 1% + AZO 2%. Do you see any advantage of this proposal instead of i-ZnO / AZO 2%?
I think alternating the i-ZnO/AZO to AZO/AZo also can work well. Sorry, I'm not sure, but ther are reports that low-doped AZO also can alternate ZnO with reduced resistance (see Kessels papers regarding surface passivation). Normally i-ZnO is deposited by using RF sputtering with low deposition rate, and this makes the whole process time longer in mass production. The advantage (just from the process point of view) of the AZO is its high deposition rate (by using DC sputtering), and this might increase the total annual capacitance of the factory.
The properties of ZnO depend crucially on the deposition technique. We deposited i-ZnO by ALD or MOMBE directly on epitaxial films of CuInSe2 (112) and (100) surfaces and determined nice band alignments. It seems possible to prepare Cd free junctions with good electronic properties at the interface.
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