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

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.

Optimizing Photovoltaic Response by Tuning Light-Harvesting Nanocrystal Shape Synthesized Using a Quick Liquid-Gas Phase Reaction.

The electron recombination lifetime in a sensitized semiconductor assembly is greatly influenced by the crystal structure and geometric form of the light-harvesting semiconductor nanocrystal. When such light harvesters with varying structural characteristics are configured in a photoanode, its interface with the electrolyte becomes equally important and directly influences the photovoltaic efficiency. We have systematically probed here the influence of nanocrystal crystallographic structure and shape on the electron recombination lifetime and its eventual influence on the light to electricity conversion efficiency of a liquid junction semiconductor sensitized solar cell. The light-harvesting cadmium sulfide (CdS) nanocrystals of distinctly different and controlled shapes are obtained using a novel and simple liquid-gas phase synthesis method performed at different temperatures involving very short reaction times. High-resolution synchrotron X-ray diffraction and spectroscopic studies respectively exhibit different crystallographic phase content and optical properties. When assembled on a mesoscopic TiO2 film by a linker molecule, they exhibit remarkable variation in electron recombination lifetime by 1 order of magnitude, as determined by ac-impedance spectroscopy. This also drastically affects the photovoltaic efficiency of the differently shaped nanocrystal sensitized solar cells.