Performance and Limits of 2.0 eV Bandgap CuInGaS2 Solar Absorber Integrated with CdS Buffer on F:SnO2 Substrate for Multijunction Photovoltaic and Photoelectrochemical Water Splitting Devices

We report on the electrical properties of 2.0 eV bandgap (EG) CuInGaS2 (CIGS) solar absorbers integrated on SnO2:F (FTO) substrates and interfaced with CdS buffer layers for multijunction solar cells and photoelectrochemical water splitting devices. The averaged short-circuit photocurrent density measured on nine ITO/ZnO/CdS/CIGS/FTO cells was 10.0 mA cm-2, a value corresponding to 70% of the optical limit for a 2.0 eV-bandgap absorber. However, the averaged power conversion efficiency was low (avg.: 2.4%) and for the most part limited by modest open circuit voltage values (avg.: 587 mV). Solid-state analyses performed at low temperatures revealed poor energetic alignment at the CdS/CIGS top interface. An activation energy (1.1 eV) for the dominant recombination mechanism significantly lower than the CIGS bandgap was measured, implying that recombination takes place near the CdS/CIGS interface. This finding is supported by the large difference found between the quasi Fermi level splitting of the bare absorber (1.17 eV) and the open circuit voltage of the device. Complementary theoretical calculations identified Fermi level pinning as a possible cause for the poor interface energetics through Cd incorporation on both the Cu and the group-III sites, resulting in a large conduction band offset (~0.9 eV) at the CdS/CIGS interface. This work underlines the need for new EG-tunable buffers coupled with optimized CIGS surface energetics (e.g. ordered vacancy compounds) for future chalcopyrite-based multijunction solar cells and photoelectrochemical water splitting devices.