Back contact optimization for Sb₂Se₃ solar cells

Abstract

Antimony selenide (Sb₂Se₃) has advantages of low-toxicity, abundant and excellent photoelectric properties. It is widely considered as one of the most promising light-harvesting materials for thin-film solar cells. However, the power conversion efficiency of the Sb₂Se₃ thin-film solar cell is still far inferior to that of cadmium telluride, copper indium gallium selenium and perovskite solar cells. As is well known, the Sb₂Se₃ solar cell performance is closely related to the light absorber layer (crystallinity, composition, bulk defect density, etc.), PN heterojunction quality (charge carrier concertation, energy band alignment, interface defect density, etc.) and back-contact barrier formation, which determines the process of carrier generation, excitation, relaxation, transfer and recombination. The low fill factor is one of the core problems that limit further efficiency improvement of Sb₂Se₃ solar cells, which can be attributed to the high potential barrier at the back contact between the Mo electrode and Sb₂Se₃ absorption layer. In this work, a heat treatment is applied to the Mo electrode to generate a MoO₂ buffer layer. It can be found that this buffer layer can inhibit MoSe₂ film growth, exhibiting better Ohmic contact with Sb₂Se₃, and reducing the back contact barrier of the solar cell. The Sb₂Se₃ thin film is prepared by an effective combination reaction involving sputtered and selenized Sb precursor. After introducing the MoO₂ buffer layer, it can also promote the formation of (<i>hk</i>1) (including (211), (221), (002), etc.) preferentially oriented Sb₂Se₃ thin films with average grain size over 1 μm. And the ratio of Sb to Se is optimized from 0.57 to 0.62, approaching to the stoichiometric ratio of Sb₂Se₃ thin film and inhibiting the formation of V_se and Sb_Se defects. Finally, it enhances the open-circuit voltage (<i>V</i>_OC) of solar cells from 0.473 to 0.502 V, the short-circuit current density (<i>J</i>_SC) from 22.71 to 24.98 mA/cm², and the fill factor (FF) from 46.90% to 56.18%, thereby increasing the power conversion efficiency (PCE) from 5.04% to 7.05%. This work proposes a facile strategy for interfacial treatment and elucidates the related carrier transport enhancement mechanism, thus paving a bright avenue to breaking through the efficiency bottleneck of Sb₂Se₃ thin film solar cells.