Photoelectrochemical water splitting using adapted thin film silicon tandem junction solar cells

For the application as photocathodes in integrated photoelectrochemical water splitting devices the thin film silicon technology stands out as an attractive choice, because it combines low-cost production, earth-abundance and versatility. Since the electrochemical potential to electrolyze water generally lies above 1.23 V, great importance is given to the latter characteristic, as thin film silicon solar cells can be adjusted to provide an extended range of achievable voltages, without impairing device efficiency. Nevertheless, as integrated water splitting devices additionally require chemical-resistant electrodes, stability issues of the silicon solar cells in contact with aqueous solutions need to be addressed.We report on the optimization and usage of thin film silicon tandem junction solar cells. Tandem junction solar cells consist of two sub-cells connected in series. In this work, we investigate two types of tandem solar cells: (i) two amorphous (a-Si:H/a-Si:H) sub-cells with an open circuit voltage VOC of 1.87 V and a solar conversion efficiency of 10.0% (ii) and amorphous connected to microcrystalline (a-Si:H/µc-Si:H) sub-cells with a VOC of 1.42 V and an efficiency of 10.8%. a-Si:H and µc-Si:H layers were deposited by plasma enhanced chemical vapor deposition, using a mixture of SiH4, H2, CH4, B(CH3)3 and PH3 gases. The optical band gap E04 was evaluated using photothermal deflection spectroscopy measurements and the crystallinity ICRS of µc-Si:H was determined by means of Raman spectroscopy. Solar cells were investigated by current-voltage measurements under AM 1.5 illumination. The photoelectrochemical performance of the electrodes was evaluated in an aqueous 0.1 M H2SO4 solution under Xe halogen lamp irradiation (100 mW/cm2). By carrying out cyclic voltammetry measurements, we demonstrate the performance of the developed silicon based photocathodes, with respect to photocurrent densities and onset potentials for water reduction. a-Si:H/µc-Si:H photocathodes with a Pt back contact, for instance, exhibit a photocurrent onset potential of 1.3 V vs. the reversible hydrogen electrode (RHE) and a high photocurrent of 9.0 mA/cm2 at 0 V vs. RHE. However, the poor stability of the photocathodes, evaluated using chronoamperometric measurements, suggests that the application of protective layers on the silicon surface will be essential. During operation at 0 V vs. RHE, photocathodes without back contacts, i.e. direct contact of the silicon surface to the acidic electrolyte, generate stable photocurrents only for two hours. In this regard, various back contact interface designs are investigated, including silicon-silicon (µc-SiC:H, µc-SiOx:H), silicon-metal (Pt, Ag, Al, Ni, Mo) and silicon-TCO-metal interfaces. The corrosion behavior of both single layers and complete photovoltaic devices are studied in a broad pH range and in different electrolyte concentrations. Thereby, the electrochemical stability of the respective interfaces is evaluated