Optimization of i-ZnO window layers for Cu(In,Ga)Se2 solar cells with ALD buffers

• Cu(In,Ga)Se2 solar cells are fabricated with different buffer layers while the intrinsic ZnO, i-ZnO,layer thickness is varied to optimize device performance. Measurements of the resulting cells show that there is astrong correlation between the increase in quantum efficiency for shorter wavelengths and the thinning of the i-ZnO layers for large band gap buffer layers. The thinning effect for low band gap buffer layers such as CdS done by chemical bath deposition, CBD, is however weak for short wavelengths. Omitting the i-ZnO layer for cells using CdS results in shunting 6 out of 24 cells. The thinning effect is significantly stronger for cells with larger band gap bufferssuch as Zn(O,S) and (Zn,Mg)O layers deposited by atomic layer deposition, ALD. Additionally these buffers show improved fill factors as a secondary thinning effect. As an example, the champion cell with (Zn,Mg)O and 0 nm of i-ZnO has an efficiency of 18.1% after anti reflective coating. Omitting the i-ZnO for cells with Zn(O,S) results in 3 out of 24 cells being shunted whereas 0 out of 24 cells are shunted for (Zn,Mg)O. The optimal thicknesses deducedfrom analyzing both performance and reproducibility is 80 nm for CdS, 20-40 nm for Zn(O,S) and 0 nm for(Zn,Mg)O.

Nyckelord

Buffer layer

Cu(In

Ga)Se2

ZnO

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