Probing Charge Separation at the Nanoscale with Ultrafast Laser Spectroscopy

Image and article credit: University of Texas, Energy Frontier Research Center (EFRC)

In organic-heterojunction solar cell materials or organic photovoltaics (OPVs), the conversion of sunlight to electrical energy is largely controlled by charge-transfer excitons–electron-hole pairs that are bound together by Coulomb attraction. These excitons are distributed across organic donor/acceptor interfaces, with the electron residing in the acceptor layer and its corresponding hole in the donor layer. To generate photocurrent in OPVs, the binding energy of an exciton must be overcome, resulting in the separation of the two opposite charges.

To develop high-performance OPV materials, the efficiency of this charge separation must be maximized. To this end, Prof. Xiaoyang Zhu at The University of Texas at Austin uses an ultrafast laser probe technique to examine the binding energy and band structure of charge-transfer excitons in promising OPV materials. This method, known as time-resolved two-photon photoemission spectroscopy (TR-2PPE), uses ultrafast (femtosecond) laser pulses to generate charge-transfer excitons within a material and subsequently separate the Coulombically bound electrons from their corresponding holes. The binding energy as well as the band structure of these excited electrons is measured, allowing the direct analysis of charge separation dynamics within a sample.

Zhu, a co-leader of Research Thrust I in the EFRC:CST, has recently demonstrated the utility of TR-2PPE in studying exciton dissociation dynamics in thin films of crystalline pentacene (X.-Y. Zhu et al., Acc. Chem. Res. 2009, available online). For the EFRC:CST, Zhu and his co-workers are using TR-2PPE to study new model OPV interfaces, such as crystalline oligothiophene (donor) thin films grown on single C60 (acceptor) molecules. The results of these studies will provide new insight into the relationship between crystallinity, band structure and interfacial charge separation efficiency in the model interfaces, thus providing valuable information for the optimization of new OPV materials.