Abstract
AbstractAs is obvious from previous work on semiconductor photoelectrochemistry, single junction semiconductors do not provide either the required maximum photovoltage or a high photocurrent for solar water splitting, which is required for efficient stand-alone devices. From these experiences we conclude, that multi-junction devices must be developed for bias-free water splitting. In this article we present our design considerations needed for the development of efficient photo-electro-synthetic cells, which have guided us during the DFG priority program 1613. At first, we discuss the fundamental requirements, which must be fulfilled to lead to effective solar water splitting devices. Buried junction and photoelectrochemical arrangements are compared. It will become clear, that the photovoltaic (PV) and electrochemical (EC) components can be optimized separately, but that maximized conversion efficiencies need photovoltages produced in the photovoltaic part of the device, which are adapted to the electrochemical performance of the electrolyzer components without energetic losses in their coupling across the involved interfaces. Therefore, in part 2 we will present the needs to develop appropriate interface engineering layers for proper chemical and electronic surface passivation. In addition, highly efficient electrocatalysts, either for the hydrogen or oxygen evolution reaction (HER, OER), must be adjusted in their energetic coupling to the semiconductor band edges and to the redox potentials in the electrolyte with minimized losses in the chemical potentials. The third part of our paper describes at first the demands and achievements on developing multijunction thin-film silicon solar cells. With different arrangements of silicon stacks a wide range of photovoltages and photocurrents can be provided. These solar cells are applied as photocathodes in integrated directly coupled PV-EC devices. For this purpose thin Pt and Ni catalyst layers are used on top of the solar cells for the HER and a wire connected RuO2counter electrode is used for the OER. Electrochemical stability has been successfully tested for up to 10,000 s in 0.1 M KOH. Furthermore, we will illustrate our experimental results on interface engineering strategies using TiO2as buffer layer and Pt nanostructures as HER catalyst. Based on the obtained results the observed improvements, but also the still given limitations, can be related to clearly identified non-idealities in surface engineering either related to recombination losses at the semiconductor surface reducing photocurrents or due to not properly-aligned energy states leading to potential losses across the interfaces.
Highlights
The energy demands of mankind cannot be delivered by C-based fossil fuels in the near future due to the limitations of given resources, even more if the expected growth in energy utilization of the developing countries is taken into account [1,2,3]
Despite the fact that fossil fuels are remains of solar photosynthesis running over millions of years the yearly yield of fuel generation based on biomass is considered to be not efficient enough to substitute the used resources on a short time scale
It can be deduced from fundamental considerations of the involved semiconductor physics and water oxidation and reduction electrochemistry that for working arrangements the PV component and the electrolyser must be connected in series
Summary
The energy demands of mankind cannot be delivered by C-based fossil fuels in the near future due to the limitations of given resources, even more if the expected growth in energy utilization of the developing countries is taken into account [1,2,3]. Identifying the right semiconductor materials with appropriate bandgaps and adjusted charge transfer properties allowing HER as well as OER without any additional electrical and chemical bias remains critical for the realization of efficient devices They have been called artificial inorganic leaves [16] to indicate their relation to the photosynthesis conversion process in leaves which produce fuels from water and sunlight. These studies mainly serve as a proof of principle: Heller and Vadimsky reported already in 1981 an efficiency of 12% for hydrogen production from p-InP photocathodes covered with small platinum metal islands They did not reach sufficient stability of their photoelectrode in contact to the electrolyte and even worse it did not deliver enough voltage on its own to drive the water splitting reaction: to reach the voltage for driving H2O electrolysis an external bias voltage had to be added to the photovoltage provided by the photocathode [19]. Further research efforts are needed which will be presented in an outlook at the end of this paper
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