Evaluation of different intrinsic ZnO and transparent conducting oxide layer combinations in Cu(In,Ga)Se2 solar cells

We studied the interaction of four different window layer combinations in Cu(In,Ga)Se2 solar cells. Intrinsic ZnO (i-ZnO) layers were grown on CdS by either chemical vapor deposition (CVD) or magnetron sputtering. These were combined with sputtered ZnO:Al or In2O3:H grown by atomic layer deposition as transparent conducting oxides (TCO). It was found that the thickness of the CVD i-ZnO layer affects the open circuit voltage (Voc) significantly when using In2O3:H as TCO. The Voc dropped by roughly 30 mV when the i-ZnO thickness was increased from 20 to 160 nm. This detrimental effect on Voc was not as prominent when a ZnO:Al TCO was used, where the corresponding decrease was in the range of 5 to 10 mV. In addition, the Voc drop for the CVD i-ZnO/In2O3:H structure was not observed when using the sputtered i-ZnO layer. Furthermore, large fill factor variations were observed when using the In2O3:H TCO without an i-ZnO layer underneath, where already a thin (20 nm) CVD i-ZnO layer mitigated this effect. Device simulations were applied to explain the experimentally observed Voc trends.