Suppression of interfacial oxygen vacancies for efficient charge extraction at CZTS/TiO2 heterojunction

Abstract

Earth abundant CZTS (Cu2ZnSnS4) absorber layers are promising for the development of cost-effective and large area photovoltaics; however, interfacial nonradiative recombination is a major obstruction to the pathways toward high performing CZTS devices. Elimination of interfacial recombination losses via interface engineering is paramount to obtain efficient CZTS solar cells. Herein, we report a systematic investigation of the influence of oxygen vacancies (OV) settled at the CZTS/TiO2 interface on the charge transfer rate in heterostructures. Modulation of OV by varying oxygen flow rate during TiO2 deposition was confirmed by x-ray photoelectron spectroscopy. Lower OV concentration shifted the conduction band offset from negative to positive at the CZTS/TiO2 heterojunction, which is essential for efficient charge transportation through the interface. Photoluminescence quenching of the CZTS/TiO2 heterojunction also showed a strong correlation between charge dynamics and OV at the interface. Finally, we found the fast decay response of photogenerated charge carriers for the CZTS/TiO2 device with lower OV strongly favors the suppression of carrier trapping at the interface. This work provides a critical insight into interface engineering in CZTS solar cells through regulating interfacial OV, particularly when an oxide electron transport layer is applied.