Charge Carrier Dynamics in Novel Solar Materials : Ultrafast Spectroelectrochemistry

Abstract: The constant increase in global energy demand makes it inevitable to find clean means of energy production. Among renewable energy sources, the Sun is the most available source which makes it a versatile solution for energy needs. Tremendous efforts are made to invent and develop new materials for light to energy conversion. The main question is how the newly developed materials react to light and what can be done to make them more efficient. Through this dissertation a summary of the results for charge carrier dynamics in novel solar materials is represented. We studied a range of new iron-based molecules which exhibit excited state lifetimes up to ns. They can be used for dye sensitized solar cells (DSSCs) to replace expensive rare elements such as Ruthenium. We studied conjugated polymers that can reach efficiency up to 14% in organic photovoltaics. We also studied how structure and composition modification may lead to better performance for light-harvesting in perovskites. Through our studies we realized that, apart from perfecting solar materials and their performances, there is still a greater need for finding the correct energy level’s configuration at interfaces. This can yield better and efficient charge separation and extraction which can directly influence the performance and efficiency of the solar cells. There are various methods for manipulation of energy levels and charge carrier dynamics in materials and interfaces. As an example, by changing their composition or structure. Manipulations of charges at interfaces can also be done on existing materials via external sources for example by application of a bias. It is important if changes are variable, controlled and can be reversible. Therefore, we examined electrochemistry as a versatile method for direct manipulation of charges at the interfaces and in the sample under study. CdSe quantum dots (QDs) sensitized on TiO2, were chosen as a model system, replicating the photoanode of QDs sensitized solar cells. Specific objective of our studies was to investigate how the photo-physics of the QDs might change under influence of extra charges provided by electrochemistry. We used ultrafast spectroelectrochemistry as the combination of electrochemistry for direct manipulation of charges and ultrafast pump-probe spectroscopy measurements to directly probe the charge carrier’s dynamics in QDs. By using this combination, we observed that slight increase in the charges at QDs-TiO2 interface, significantly affect charge extraction from QDs. If applied bias is sufficient enough, it leads to negative charging of the QDs. Where photo excitation of such negative charged QDs leads to rapid negative trion decay in QDs. We also demonstrated that positive charging of QDs can be achieved by using electrochemistry. Photo excitation of the positively charged QDs also induce rapid positive trion decay. We strived to present the potential of spectroelectrochemistry as a supplement to ultrafast spectroscopy methods. The unique advantage of spectroelectrochemical measurement is that induced changes can be reversed which enable comparison of different configurations with similar experimental conditions. Further studies are needed to develop the required knowledge of the photophysical processes in solar materials to achieve ultimate performance and efficiency for light conversion.