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

- ALD-TiO2 is applied as intermediate buffer layer in CIGS solar cells.
- Replacing ZnO with TiO2 allows for a reduction in CdS buffer layer thickness.
- The current density could be improved without adversely affecting VOC.
- A positive conduction band offset at the CdS/TiO2 interface is proposed.

The application of TiO2 as part of the buffer layer stack in thin film Cu(In,Ga)Se2 (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 VOC 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 TiO2 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 VOC. 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/TiO2 interface limiting the FF. ALD-TiO2 is suggested as a more suitable intermediate buffer layer than sputtered ZnO when thin CdS buffer layers are applied.