Planar Heterojunction Solar Cell Employing a Single-Source Precursor Solution-Processed Sb₂S₃ Thin Film as the Light...

We discuss here a solution-processed thin film of antimony trisulphide (Sb₂S₃; band gap ≈ 1.7 eV; electronic configuration: ns²np⁰) for applications in planar heterojunction (PHJ) solar cells. An alternative solution processing method involving a single-metal organic precursor, viz., metal-butyldithiocarbamic acid complex, is used to grow the thin films of Sb₂S₃. Because of excess sulphide in the metal complex, the formation of any oxide is nearly retarded. Sb₂S₃ additionally displays structural anisotropy with a ribbon-like structure along the [001] direction. These ribbon-like structures, if optimally oriented with respect to the electron transport layer (ETL)/glass substrate, can be beneficial for light-harvesting and charge-transport properties. A PHJ solar cell is fabricated comprising Sb₂S₃ as the light absorber and CdS as an ETL coated on to FTO. With varying film sintering temperature and thickness, the typical ribbon-like structures predominantly with planes hkl: l = 0 stacked horizontally along with respect to CdS/FTO are obtained. The morphology of the films is observed to be a function of the sintering temperature, with higher sintering temperatures yielding compact and smooth films with large-sized grains. Maximum photon to electricity efficiency of 2.38 is obtained for PHJ solar cells comprising 480 nm thick films of Sb₂S₃ sintered at 350 °C having a grain size of few micrometers (>5 μm). The study convincingly shows that improper grain orientation, which may lead to nonoptimal alignments of the intrinsic structure with regard to the ETL/glass substrate, is not the sole parameter for determining photovoltaics performance. Other solution-processing parameters can still be suitably chosen to generate films with optimum morphology, leading to high photon to electricity efficiency.