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

Subject Area Physical Chemistry of Solids and Surfaces, Material Characterisation
Term from 2012 to 2016
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 221482728
 
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%.
DFG Programme Priority Programmes
Participating Person Professor Dr. Joachim Knoch
 
 

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