IMA Postdoc Seminar (May 26, 2009)

Speaker: Xiaoyang Zhu (University of Minnesota)

Title:The Coulomb Barrier in Excitonic Charge Separation

Abstract: When a molecular material is electronically excited by a photon, the Coulomb attraction between the excited electron and the hole gives rise to an atomic-H-like quasi-particle called an exciton. The bound electron-hole pair also forms across a material interface, such as the donor/acceptor (D/A) interface in an organic heterojunction solar cell; the results are charge-transfer (CT) excitons. In a conventional p-n junction cell, the exciton binding energy is very small and there is a built-in potential to ensure charge separation. In contrast, there is no a priori a built-in potential in a solar cell based on organic molecules and polymers. Charge separation requires an energetic driving force provided by the differences in the electronic levels at the D/A interface. From typical dielectric constants of organic semiconductors and sizes of conjugated molecules, one can estimate that the Coulomb energy of a CT exciton across a D/A interface is more than one order of magnitude greater than k_B T at room temperature. We can also estimate this by solving the Schrodinger equation in a dielectric continuum model. The results are a series of atomic-H like states with the binding energy of the 1s state on the order of a few hundred meV. How can the e-h pair escape this Coulomb trap? To answer this question, we carry out two experiments: femtosecond time-resolved two-photon photoemission (TR-2PPE) spectroscopy to probe the physical nature of the CT excitons and vibrational spectroscopy to probe charge transfer and electric field at buried interfaces. In the first experiment, we use a crystalline pentacene thin film as a model system and excite an electron above the surface. When there is only the excess electron, the electron is bound to the surface by the positive polarization cloud in the pentacene molecules; this attractive potential leads to a bound electronic state called an image potential state. When a positive hole is present on a pentacene molecule, the electron is attracted to the surface by both the positive hole and the polarization cloud; the result of this combined attractive potential is approximately the CT exciton referenced to the image band. We observe a series of CT excitons with binding energies <=0.5 eV below the image band minimum. Given the large binding energy of the lowest CT exciton state (1s), we conclude that hot CT exciton states must be involved in charge separation. The second experiment probes electric field at the D/A interface. This field may come from direct charge transfer, from unintentional doping, or from charge redistribution or polarization. Experimentally, the presence of charge transfer and interfacial field in a bulk heterojunction can be probed by vibrational spectroscopy and the Stark effect. We apply this approach to a model bulk heterojunctions systems of electron donating polymers and a soluble form of C60. We find that the electric field for at the D/A interface is as large as 4x10^6 V/cm in the wrong direction for charge separation. This removal of this Coulomb barrier occurs upon photoinduced charge separation in the presence of sufficient phase separation and local crystallinity. These findings suggest key design principles for successful organic bulk heterojunction solar cells.