Enhanced charge transfer and stability in heterojunction perovskite solar cells with optimized Se/CuSe multi-layer hole transport layer using physical vapor deposition technique

A multi-layered hole transport layer (ML-HTL) composed of two chalcogenide thin films, Selenium (Se) and Copper Selenide (CuSe), is fabricated via the physical vapor deposition (PVD) technique as an economical substitute for the expensive Spiro-OMeTAD layer in heterojunction perovskite solar cells (HJPSC). The HTL of Se and CuSe thin films are alternately deposited to achieve a composite thickness totalling 150 nm by optimizing the individual layer thicknesses, yielding three distinct multi-layered Se/CuSe/Se thin film configurations: 30/60/30, 50/50/50, and 40/70/40 nm, respectively. Scanning electron microscopy (SEM) reveals its nanofibrous morphology with enhanced surface area, corroborating the reinforcement to enhance charge transfer and diminish recombination. Both Se and CuSe operate as p-type semiconductors exhibiting opto-electronic properties that encompass a work function alignment with the MAPbI₃ perovskite layer, in addition to their superior charge transfer characteristics. Consequently, the HJPSC device with an active area of 0.5 cm² utilizing an optimized multi-layered HTL structure achieves a maximum efficiency of 7.04 % and an outstanding fill factor of 70.4 % along with the open-circuit voltage (V_oc) of 1V and short circuit current (J_sc) measuring 9.9 mA/cm⁻². The optimized interface between the Se/CuSe layers and the perovskite absorber significantly minimizes charge recombination, leading to higher V_oc and overall device performance. All the fabricated devices set out an adequate stability up to 8 h after exposure to AM 1.5G of solar stimulation.