My summer research project is Optimizing Buried Junction Cubic and Orthorhombic SnS Photocathodes. For many years my research group have been searching for semiconductors and solar absorber materials that are non-toxic, abundant, stable, which crystallize at low temperature and are fabricated at low cost, but also have favorable optical and electrical properties for solar energy conversion.
Figure 1: π-SnS photocathodes a) before and b) after coating with a passivation and protection layer.
When designing a system for solar water splitting it has been proposed that an idealized tandem device, consisting of two semiconductors with band gaps around 1.0 to 1.3 eV and 1.6 to 2.0 eV, will provide both the optimal photocurrent and photovoltage for high efficiency solar water splitting. Interestingly, it has been discovered that the earth abundant semiconductor SnS can possess two different polymorphs at room temperature; one being the metastable cubic π-SnS phase with a direct band gap of 1.7 eV, and the other being α-SnS ground state with an indirect band gap of 1.1 eV, in which both are native p-type semiconductors.
Figure 2: Measuring the PEC properties of π-SnS photocathodes under 1 sun illumination.
Here at the Institute for Solar Fuels my research group has been able to deposit highly crystalline thin films of both polymorphs with exceptional control and selectivity. Our initial investigations of these samples have shown that relatively high saturated photocurrents for hydrogen evolution can be achievable, reaching ~8 mAcm-2 at -0.3 VRHE under AM 1.5 illumination for the π-SnS phase.
My aim is to design surface treatment methods and screen protection layers/catalysts that can enhance the onset potential, passivate the surface and improve the stability of cathodic photocurrents for the π-SnS photo absorbers. These protection layers/catalysts have been deposited by AA-CVD methods. We have already begun to screen a series of protection layers, which are only 40-60 nm thick! I have been able to learn these new experimental techniques to not only fabricate thin film absorber and protection layers, but also a range of photo-electrochemical (PEC) characterization methods. We are making progress and it is extraordinary to see how such a thin layer of material on the surface of the photocathode can dramatically effect the water splitting performance of our devices.
Figure 3: Scanning Electron Microscopy (SEM) images of the π-SnS photocathodes with protection/passivation layers on top.