Alternative buffer layers for Cu2ZnSn(S,Se)4 and Sb2Se3 solar cells: numerical and experimental study

Cu2ZnSn(S,Se)4 (CZTSSe) and Sb2Se3 stand as promising absorber materials for thin films solar photovoltaics (PV) thanks to their sustainability and their excellent optoelectronic properties. However, CZTSSe and Sb2Se3 PV devices are still far from the Shockley-Queisser limit, primarily due to a strong open circuit voltage deficit. One of the main bottlenecks in reaching higher Voc values is related to the choice of CdS as an n-type buffer layer: CdS parasitically absorbs in the blue range of light and displays a non-optimal energy band alignment; moreover, CdS is very toxic. Recently, thin film oxides, such as ZnSnO (ZTO) and TiO2, have been proposed as alternative buffer layers for producing more efficient and greener devices. In this work, CZTSSe/CdS and Sb2Se3/CdS junctions were first modelled through SCAPS-1D software, starting from experimental optoelectronic values, to derive the density and energetic levels of the absorbers’ defects. Then, CdS was substituted by ZTO and TiO2 as alternative buffer layers in the simulation to study their effective possibility to reach or outmatch CdS. The simulation has determined the optimal thickness of ZnSnO and TiO₂ and their ideal composition to achieve optimal band alignment with the absorber layers, ensuring the same performance as CdS as a buffer layer. Then these alternative buffer layers have been grown experimentally through atomic layer deposition (ALD). ALD technique was chosen as ensures the preparation of compact and flat layers with nanometric thickness control. ALD deposition processes of ZTO and TiO2 were optimized to grow uniform thickness and composition. These alternative buffer layers have been deposited on CZTSSe and Sb2Se3 to make prototype PV devices. The ALD-deposited materials and the devices have been fully characterized, showing promising preliminary results, indicating a pathway to improve the performance of CZTSSe and Sb2Se3 enabling the use of more environmentally friendly buffer layers.