Final Report on project Photoinduced Dipoles and Charge Pairs in Condensed Media

Abstract

Most of our work involves photogenerated donor (D+) and acceptor (A-) radical ion pairs and their escape and recombination. The geminate radical ions are produced by inter or intramolecular electron transfer quenching of photoexcited acceptors. We made great progress in understanding the effect of charge separation distance and other factors on free ion formation. For geminate radical ion pairs formed by intermolecular electron transfer quenching, we found that, at least in a medium polarity solvent lilce dichloromethane, free radical ions are mainly formed from solvent separated radical ion pairs (SSRIPs). Contact radical ion pairs (CRIPs) make a very small contribution to free radical ion formation. It is observed that CRIPs and SSRIPs are kinetically distinguishable species. There is a potential barrier between CRIPs and SSRIPs that prevents a fast equilibrium between the two during the recombination of radical ion pairs and free radical ion formation. We confirmed for the first time that, for the recombination of both CRIPs and SSRIPs that are in the Marcus inverted region, rate constants of SSRIPs are larger than those of CRIPs. Our study indicates that initial charge separation distance and the potential barrier between CRIPs and SSRIPs play a more important role than recombinationmore » rate on free ion formation. Temperature dependence experiments reveal that through-tunneling back electron transfer is the dominant approach for the SSRIP recombination process. We believe that our discovery of the roles played by the initial charge separation distance and the potential barrier between radical ion pairs of different separation have very important implications for the development of new concepts for the design of organic photovoltaic (OPV) cells. Photoinduced transient dipole experiments are used to probe the effective charge separation distance of flexible electron donor/acceptor systems, D-(CH2)n-A, where D is 4-N-N-dimethylaniline, A is 9-anthryl and n=3, 4. We find that the dipole moments increase strongly with solvent polarity. Under the simplifying assumption that the folded, contact configuration and the extended, solvent-separated configuration are the only two stable species after electron transfer quenching, the formation efficiencies of contact radical ion pairs (CRIPs) and solvent-separated ion pairs (SSRIPs) are estimated in different solvents. The results indicate that a significant fraction of the ion pairs exists as solvent-spearated ion pairs when the dielectric constant of the solvent is larger than 10 and that electron transfer quenching can indeed happen at large separations in polar solvents. Light-induced charge separation often occurs at the interfaces and surfaces in solar cells and other electro-optic devices. To produce a substantial photovoltaic effect, the electrically neutral excitons formed by photon absorption must diffuse to an interface and produce ion pairs by dissociation. Since low dielectric constants of 3-4 are typical for organic materials used to fabricate OPV cells, the geminate ion pairs formed after exciton dissociation are believed to have short separations and are strongly bound by Coulomb interaction. The binding energy can be estimated to be ~0.25 eV. However, the experimentally observed activation energies are typically less than 0.1 eV. The .weak temperature dependence of carrier photo generation indicates that, somehow, geminate ion pairs are formed at large separation or that some sort of energy assists separation of nearby geminate ion pairs into free ions. To understand the underlying mechanism behind the above observations; a model based on what we have learned from solution was developed for charge separation at interfaces or surfaces. It was proposed that, in order for loose radical ion pairs with long separations to escape in media with dielectric constants of 3-4, a potential barrier that can effectively stop their collapse is critically needed. Based on our model, it is concluded that closely separated radical ion pairs do not form free radical ions by themselves. However, they do play a very important role in · free ion formation. The closely separated radical ion pairs formed at the interface can effectively build a potential barrier that offers a boost for the escape of radical ion pairs formed at long distance. The counterproductive part of a high potential barrier is that a large portion of photoenergy is used to produce closely separated ion pairs. The existence of a high potential barrier is also expected to make the ·formation of distantly separated radical ion pairs less probable. In our opinion, an efficient OPV cell will be a device with a potential barrier that is enough to counterbalance the Coulomb interaction within the radical ion pairs. Meanwhile, the distantly separated (loose) ion pairs must be formed with high efficiency.« less