This Materials World Network project is a collaboration of groups from Argentina, Canada, Mexico and the US with the common goal of synthesizing and characterizing, at the single molecule/particle level, semiconductor-photosensitizer materials that can be used for photovoltaic and solar-to-fuel applications. Electron injection by photosensitizers into semiconductor materials has been studied extensively, mostly by electrochemical and spectroscopic techniques based on conventional measurements of bulk materials. These studies often reveal electron transfer processes that are both thermodynamically and kinetically indicative of heterogeneity in the sensitizer-semiconductor material. To characterize such heterogeneity we are systematically studying the electron transfer processes using single molecule/particle techniques. If sub-populations of sensitizer-semiconductor particles of high efficiency can be identified, it should be possible to distinguish structural features that make it so and thus improve the sensitizer-semiconductor material interface and overall performance of photovoltaic devices.

A contribution of the US team is the synthesis of sensitizer-semiconductor nanoparticle models. Initially, we will work with systems consisting of perylene-TiO2 and perylene-SnO2. With the experimental techniques in hand, the aim is to develop tetrapyrrole-based semiconductor models, which upon excitation of the sensitizer in the visible range, inject electrons into the semiconductor conduction band and thereby generate high oxidation potential (> 1.30 V vs. NHE) sensitizer radicals. High potential is necessary to drive water oxidation. In parallel with these systems, we plan to develop materials for photoanodes having a highly reducing output, needed for the reduction of H+ to H2. Conventional and novel single molecule/particle fluorescence spectroscopy (SMFS) methods will be used to study photoinduced electron transfer processes within the sensitizer-semiconductor nanoparticle models. In particular, we seek to identify distributions of electron transfer efficiencies as a function of parameters such as sensitizer-semiconductor nanoparticle separation distance, type of linkage, geometry and assembly methodology. Another objective of the project is to study the role played by the occupation of the semiconductor energy levels in the photoinduced electron transfer process. The main technique in this case will be the newly developed single molecule/particle spectroelectrochemistry (SMS-EC). The electrochemical aspect of the technique will be used to control the population of the energy levels of the semiconductor particles, while the SMFS aspect will be used to monitor the redox state(s) of the sensitizer(s) in individual molecules by detecting their fluorescence.

The common denominator is a solar-to-fuel project important to securing a sustainable energy future appropriate to each culture. Undergraduate and graduate students working on this integrated project are thinking and communicating across disciplines and national boundaries. This award is co-funded by DMR-EPM, DMR-OSP, and OISE Americas Region.