Organic photovoltaic cells rely upon molecular interfaces to separate charge. Efficient charge separation depends on molecular structure and electronic band alignment across donor-acceptor interfaces. Correlation of molecular structure determined with scanning tunneling microscopy (STM) and interfacial electronic structure measured with one- and two-photon photoemission techniques allows determination of the factors that control the efficacy of charge separation.
Solar energy has perhaps the greatest potential to provide a large capacity renewable source of clean, readily available energy. Alternative photovoltaic technologies, including dye-sensitized oxide nanocrystalline solar cells and organic/polymeric devices, offer the possibility of greatly reduced manufacturing cost compared to conventional inorganic semiconductor (Si) technology, with the additional potential for production on flexible substrates and use of roll-to-roll processing.
Fundamental differences in photocurrent generation processes in organic photovoltaics have been identified and compared to conventional inorganic semiconductor photovoltaic cells. Because of these differences, efficient organic photovoltaic device structures require optimization of interfacial electronic structure (band alignment, coupling across the interface, and dynamics of charge transfer) and nanoscale morphology (degree of phase separation, domain size). We are focusing on studying correlations between key electronic and structural characteristics of organic donor-acceptor interfaces. Photoelectron spectroscopies, scanning probe methods, and theoretical modeling of multiscale processes in molecular materials are used to investigate molecular structure and interface electronic features critical to photovoltaic device optimization.
Indeed, the energy offset between donor and acceptor exciton and charge transport levels provides the driving force for separation of charge and the generation of a photovoltage. Photoemission techniques can be used to determine the electronic structure and energy level alignment at the interface. We seek to directly measure charge separation and recombination processes at model heterojunctions by employing time-resolved pump-probe experiments. In this case, an ultrafast (~100 fs) visible pump pulse generates a bound electron-hole pair (exciton) in the donor, which can dissociate at the heterojunction and transfer an electron to the transport level (polaron) of the acceptor. A second time-delayed probe pulse measures the population of the charge-separated state. A set of measurements as a function of pump-probe delay yields information of the dynamics of charge separation (favorable) and recombination (unfavorable to device performance). We have employed photoemission techniques, primarily two-photon photoemission spectroscopy (2PPE) and related time-resolved pump-probe measurements, to examine these processes for thin organic films and heterojunctions of copper phthalocyanine (CuPc) with C60 fullerene.