A monolithic all-silicon multi-junction photovoltaic electrolysis device for solar hydrogen production by direct water splitting

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The aim of the present proposal is the demonstration, investigation and optimization of a monolithic all-silicon multi-junction solar hydrogen production device based on a modified interdigitated back-contact (IBC) crystalline silicon (Si) solar cell that is wireless, is based on a mature technology, an abundant material and thus allows combining the advantages of a photoelectrolytic system for direct water splitting with the benefits of a photovoltaic/electrolysis (PV/E) system. In contrast to existing IBC solar cells, in the present approach every second n-p junction is insulated from each other employing a deep trench isolation (DTI) with a depth significantly exceeding the depth of the doped contacts including the depletion regions. Three n and p sections that are insulated by the DTI are connected in series by metal contacts to generate a sufficiently high photovoltage which allows to provide the water redox potential of 1.23 V taking overpotentials and losses into account.Contrary to vertically-stacked multi-junction (MJ) cells our approach has the following advantages: First, shadowing is completely avoided since there are no front side contacts. Together with the high quality of the Si wafer material this leads to a highly efficient solar cell. Second, both contacts are on the back side and hence the front side is not in contact with the electrolyte which provides a larger degree of freedom in the preparation of the front side in terms of structure and material since corrosion is completely avoided. Third, the IBC design yields a separation of the light converting bulk of the solar cell and the contacts for electrolysis. Hence, the electrodes that are in contact with the electrolyte can be sealed in a chemically stable, thick insulator at the back side without affecting the light absorption properties of the solar cell. Fourth, the employment of a nickel silicidation process to connect the n and p sections in series allows for a true monolithic MJ implementation with good ohmic contacts, which avoids wiring completely. As a consequence, the Si wafer could in principle serve as the seal of the electrolyte container yielding a compact, robust hydrogen production device. Finally, our approach is based on a Si solar cell and thus on a non-toxic and abundant material.In a second device configuration, a porous Si membrane is implemented into the MJ-PV/E cell. Using a through-via technique, however, contacts are placed on both sides of the wafer such that the porous Si membrane can act as a proton exchange membrane that allows using the device as a PV/E cell and as a micro fuel cell.Under the assumption that our MJ-PV/E device operates under AM 1.5 G illumination at a photocurrent, which is a third of the reported short-circuit current density of 40.6 mA/cm2 of IBC cells, a solar-to-hydrogen conversion efficiency of STH ~ 16% is feasible. Under ideal conditions, a Si solar cell can provide a short-circuit current density of 44 mA/cm2 yielding a maximum STH of ~ 19%.