Perovskite-silicon tandem based photoelectrochemical systems for efficient solar hydrogen generation

Abstract

Direct solar hydrogen generation(DSTH) is a promising method for renewable hydrogen generation, where solar energy drives the generation of hydrogen and oxygen by molecular dissociation of water on a catalytic or semiconductor surface. Despite an enormous amount of work over the last few decades, DSTH has not yet been implemented on large scale due to low efficiency and high costs. This thesis investigates perovskite-silicon tandem PV and Earth abundant catalysts-based system for low-cost and high-efficiency direct-solar-hydrogen-generation. The thesis starts by identifying performance limitations and conceptualising practical device designs to improve the STH efficiency towards 20%. We develop a new theoretical framework to quantify and compare different loss mechanisms in photovoltaic(PV) based solar hydrogen generation systems and evaluate the potential of different loss mitigation techniques to improve the solar to hydrogen generation efficiency. Our analysis shows that the two largest losses in an ideal system are energy lost as heat in the photovoltaic component, and current and voltage mismatch between the PV and electrochemical (EC) components due to sub-optimal system configuration. Employing loss mitigation techniques targeting the two major efficiency losses results in predicted STH efficiencies above 20%, without the need for further improvement of the photovoltaic devices or catalyst. These results demonstrate where best to target interventions to mitigate the biggest losses and ensure maximum improvement in the performance. To achieve the STH target set by the DOE for the year 2020, we develop a system with perovskite-silicon tandem PV and Ni based Earth abundant catalysts with a record STH efficiency of 20%. The NiMo electrodes with a high density of NiMo active sites are fabricated in a flower-stem morphology and exhibit an exceptional HER performance. In addition, an improved perovskite top cell with a record open circuit voltage is achieved using n-dodecylammonium bromide. Further analysis is performed to assess the potential for improvement in the STH efficiency. With Si solar cell performance already close to theoretical limits, perovskite solar cell offer opportunities of improvement to improve the tandem performance. Improving the performance of perovskite solar cell alone could improve the STH efficiency to 25%. A technoeconomic analysis is performed to assess the cost competitiveness of the developed system. The levelized cost of hydrogen (LCOH) of DSTH system is calculated as $4.12/Kg. Combining efficiency improvements with projected cost reduction could further reduce the LCOH to $2.3/Kg, presenting a remarkable opportunity to realise cheap renewable hydrogen. Moving towards the aim of a fully integrated system for direct solar hydrogen generation, we develop a photoelectrochemical system based on perovskite PV and Si photocathode system in tandem configuration. We demonstrate a high-performance Si photocathode, incorporating state-of-the-art charge selective passivation and Earth abundant catalysts, with an ABPE of over 10%. Charge selective passivation layers improve the Si photocathode performance by roughly 70% compared to Si photocathode without any passivation, highlighting the importance of the efficiency loss due to recombination at the Si/catalyst interface. An overall water splitting efficiency of 17% is achieved for the photoelectrochemical system when combined with a previously reported Earth abundant OER catalyst and a wide bandgap perovskite solar cell in tandem. These results show that perovskite-silicon tandem based photoelectrochemical systems have the potential to make large scale direct solar hydrogen generation a reality. This work will encourage dedicated research with a clear awareness of components and system design that needs to be focused on, towards achieving the aim of low-cost, large scale solar hydrogen generation. Show more