(Invited) Bi-Based Solar Absorbers for Thin-Film Photovoltaic and Solar Fuels

Abstract

Extensive deployment of solar energy conversion systems requires the development of materials and technologies which go beyond utility-scale Si photovoltaics (PV) solar cells. For instance, thin-film technologies (currently dominated by CIGS and CdTe) are particularly suited to flexible and building integrated PV, a sector predicted to grow to global market revenues of over $5bn by 2025.1 In the lower TRL scale, monolithic integrated III-V tandem devices have pushed the efficiency of photo-electrochemical (PEC) water-splitting to levels which can compete with PV powered electrolysers.2-4 However, PEC systems are yet to show the levels of stability, performance and balance of costs required for technological deployment. As elegantly exposed by Rajeshwar and colleagues,5-6 both fields require exploring the vast space provided by solid-state chemistry in order to identify new families of compound semiconductors with: (i) appropriate band gap for solar energy conversion, (ii) optimal electronic properties featuring high carrier mobility and defect tolerance, (iii) processable by solution-based methods and (iv) composed of Earth abundant and low-cost materials. Defect tolerance is a key descriptor in the search of new solar absorbers, in which Bi3+ and Pb2+ are key cations exhibiting large spin-orbit coupling, dielectric constants and band dispersion.7,8 In a recent study, we have shown that high quality polycrystalline PbI2 thin films exhibit ideal photoelectrode properties towards hydrogen evolution, following the Gartner-limit down to the flat-band potential.9 In this contribution, we will present our recent studies on Bi based absorbers including BiFeO3 and BiI3. BiI3 is prepared by spontaneous gas-phase iodination of Bi2S3 films at 200˚C.10 The films exhibit a high degree of phase purity, p-type conductivity (acceptor density of the order of 1015 cm-3) and a band gap of 1.7 eV. Thin-film devices with the structure Glass/FTO/TiO2/BiI3/F8/Au, where F8 is Poly(9,9-di-n-octylfluorenyl-2,7-diyl), display open-circuit voltage above 600 mV and record power conversion efficiency of 1.2% under AM 1.5G illumination. We have implemented a similar approach to the preparation of high quality BiFeO3 thin films, in which thermolysis of the sulphide precursor is carried out in air. Our early work on devices with the structure graphite/BiFeO3/ZnO/ITO/SLG yielded conversion efficiencies close to 4%, which is among the highest for all-oxide PV systems.11 We shall also contrast the PEC hydrogen generation of BiFeO3 with other poorly understood materials such as BiOCl.12