Structural Study of Polymer-Fullerene Solutions for Organic Photovoltaics
Margaret J. Sobkowicz, Ronald L. Jones, R. Joseph Kline, and Dean M. DeLongchamp
Bulk heterojunctions (BHJs) composed of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) are promising active layers for organic photovoltaics. The nanoscale morphology of the BHJ active layer is critical to the performance of organic photovoltaic devices because exciton diffusion, charge separation, and charge carrier transport processes require domains that have specific optimal sizes and connectivity. Processing parameters have been shown to have a large influence on the nanoscale morphology and thus device performance. Because P3HT crystallization behavior during film formation is the driving force for phase segregation, casting solution properties are vital to film electronic properties. Small angle neutron scattering (SANS) is an ideal measurement technique to study the blend morphology and phase formation in P3HT:PCBM solutions due to the large difference in neutron scattering length density between P3HT and PCBM, the length scale probed, and the excellent sensitivity of SANS to concentration fluctuations. In this work, SANS as well as solution rheology are developed as characterization tools for organic photovoltaic materials. Scattering data from P3HT:PCBM solutions compared with modulus measurements during crystallization to develop a picture of the nanoscale organization and the influence of processing parameters on morphology.