MECHANISMS OF ORGANIC SEMICONDUCTOR FILM IMPROVEMENT BY PROCESSING ADDITIVES
Nayool Shin,1 Xinran Zhang,1 R. Joseph Kline,1 Lee J. Richter,2 Dean M. Delongchamp1
1Polymers Division, Material Measurement Laboratory
2Surface and Microanalysis Science Division, Material Measurement Laboratory
Bulk heterojunction (BHJ) organic photovoltaic (OPV) cells based on polymer : fullerene blends have offered a promising solution for future energy needs. Recently, it has been shown that the power conversion efficiency of BHJ OPV devices can be increased by the addition of small amounts of processing additives, which are believed to affect the morphology of the BHJ active layer. In this work, we apply detailed morphology and microstructure measurements to illuminate the mechanism by which these additives improve the performance of BHJ OPV devices based on a poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blend. Two types of additives were examined: 1) good solvents for PCBM and poor solvents for P3HT (1,8‑diiodooctane and 1,8‑octanedithiol), and 2) a good solvent for both components (1‑chloronaphthanlene). Aspects of P3HT:PCBM film structure including order, orientation, and nanoscale morphology were measured by techniques including grazing incidence X-ray diffraction, near edge X-ray absorption fine structure spectroscopy, variable angle spectroscopic ellipsometry, and transmission electron microscopy. The effect of processing additive on the ordering and charge carrier mobility of neat P3HT films was also investigated. Our results on neat P3HT films indicate that these additives can form a non-ideal solution when mixed with typical polar organic solvents, with the result that they can significantly prolong the drying process and increase levels of polymer chain order and orientation, even if they are not themselves good solvents for P3HT.