A monolithic all-silicon multi-junction solar device for direct water splitting

We present a silicon-based, monolithic multi-junction solar device that is suitable for the sustainable and reliable production of hydrogen. It is based on an interdigitated back-contact (IBC) solar cell which is modified, so that the p- and n-regions are connected in a combination of series and parallel connections, which triples the photovoltage compared to a single-junction cell. Thus, it provides a potential larger than the water redox potential of 1.23 V plus over-potentials at the electrodes. We fabricated a working demonstrator with an open-circuit voltage of 1.81 V and a short-circuit current density of 12.2 mA/cm2. The processing can be integrated in an existing IBC cell line with one additional fabrication step and a modified contact layout. Coupled to a 1 M H2SO4 electrolysis system, with Pt and RuO2 electrodes, our device shows a solar-to-hydrogen conversion efficiency of 8%. In contrast to existing solutions, which are based on external series connections, it does not suffer from ohmic losses and holds the potential to reach 16.6%.