TiO2 as intermediate buffer layer in Cu(In,Ga)Se2 solar cells

Abstract The application of TiO 2 as part of the buffer layer stack in thin film Cu(In,Ga)Se 2 (CIGS) solar cells is investigated for the improvement of the photovoltaic device performance. In a standard device configuration a CdS/ZnO/Al:ZnO layer stack is applied onto the CIGS absorber layer. By decreasing the CdS buffer layer thickness a higher photocurrent is expected from a reduced parasitic absorption. When the CdS layer is not fully covering the CIGS surface, losses in V OC and FF are observed in I-V measurements due to the arising unfavorable CIGS/ZnO band alignment and sputter damage on the CIGS surface. Here we present thin TiO 2 layers deposited by atomic layer deposition at low temperature as alternative to the unintentionally doped ZnO. With this approach, the photocurrent can be increased without adversely affecting V OC . Comparable device efficiency is achieved for the investigated structure and the reference process with the gain in current density being compensated by increased series resistance. Temperature dependent I-V measurements coupled with 1D-SCAPS simulations suggest a positive conduction band offset at the CdS/TiO 2 interface limiting the FF. ALD-TiO 2 is suggested as a more suitable intermediate buffer layer than sputtered ZnO when thin CdS buffer layers are applied.