Interface-enhanced germanium selenide solar cells comprising an ultrathin and uniform antimony selenide buffer layer via hydrothermal approach

Germanium selenide (GeSe) is a promising thin film photovoltaic absorber material owing to its excellent optoelectronic properties, high stability, and low toxicity. Interface engineering by introducing an ultrathin antimony selenide (Sb₂Se₃) buffer layer between the CdS electron transport layer and GeSe absorber layer is an effective technique for enhancing solar cell performance. However, the key to this technique is the fabrication of a uniform and smooth Sb₂Se₃ buffer layer with minimal thickness. In this study, instead of the conventional closed-space sublimation method, a hydrothermal method was employed to slowly grow an Sb₂Se₃ buffer layer with a thickness of approximately 8 nm. The Se/Na₂SO₃ molar ratio in the selenium source during the hydrothermal synthesis was adjusted; a molar ratio of 1:2 led to an uneven Sb₂Se₃ buffer layer thickness, whereas a molar ratio of 1:10 resulted in the formation of Sb₂O₃ particles on the buffer layer surface. When the Se/Na₂SO₃ molar ratio was 1:6, a smooth, uniform, dense, and impurity-free Sb₂Se₃ buffer layer was obtained, achieving the highest efficiency of 3.33 % in a GeSe solar cell. Moreover, GeSe solar cells with hydrothermally grown Sb₂Se₃ buffer layers demonstrated superior device interface properties and efficiency comparable with those using Sb₂Se₃ buffer layers deposited via closed-space sublimation. This technique offers an effective method for steadily improving the performance of GeSe solar cells.