In this project I fabricated photoelectrodes for CO2 reduction and investigated these with a set of transient absorption spectroscopies and gas chromatography techniques. I found that buried junction type photoelectrodes provide higher photovoltages and hence driving force for the reactions and hence would be more promising for the photoelectrochemical approach to fuel generation from water, CO2 and sunlight. This finding is published in DOI: 10.1039/C8TA07036A J. Mater. Chem. A, 2018, 6, 21809-21826 (open access). I fabricated polymer photocathodes and decorated those with suitable hydrogen evolution and CO2 reduction catalysts to study reaction dynamics, track intermediates and understand reaction mechanisms. These works require some further experiments due to the complexity of transient IR spectroscopy, instability of some polymer blend materials and novelty of this class of photoelectrochemical cells for CO2 reduction. I will continue my research at Imperial College London in this field and will benefit from my experience, expertise and collaboration network I established during the Marie Curie Fellowship.
In the transient absorption studies performed on photoabsorber materials such as CIGS, polymer bulkheterojunctions, La,Sr:SrTiO3 and other oxide materials, it became clear that the efficiency of catalytic systems is strongly affected by charge carrier (and energy) loss mechanisms within the photoabsorber material. Photoabsorber material properties can cause charge carriers to slow down on their way (i.e. polaron formation, defect trapping) or loose their energy entirely via electron-hole recombination by generating heat (non-radiative recombination) or light (radiative recombination). Hence, the challenge is to use these carriers for catalysis before these are lost to recombination.
In the study on La,Rh:SrTiO3 I could understand important mechanisms of co-doping in perovskite materials that transform the material’s electronic structure and eliminate mid-gap trap levels when the oxidation state of the dopant is varied. This study revealed the unique properties of La,Rh:SrTiO3 to be able to store charge very well up to timescales relevant for catalysis (second timescale) and proved the suitability of such material and doping mechanisms for CO2 reduction photocatalysts. The work will be submitted shortly.
Working with CIGS photoabsorber materials, I have learned that band gap gradients are useful to extract charge carriers and improve the mobility of carriers in polycrystalline materials. Using transient absorption spectroscopy as a tool to track photo-generated charge carriers and calculate mobility within the photoabsorber was introduced into the field of CIGS research. Two works will be submitted soon.
Working with a newly developed technique called pump-push-photocurrent spectroscopy in collaboration with the Bakulin group at Imperial College London, we could observe polaron formation (self-trapping) in oxide materials. Using a short infrared light pulse we could prove that these self-trapped charges can be re-excited to the conduction band and eventually be extracted as photocurrent. This work is just accepted in Nature Communications.
