Direct characterization of vertical molecular distributions of organic bulk heterojunction structure by photoemission spectroscopy combined with argon gas cluster ion beam sputtering

The characterization of the vertical molecular distribution of organic bulk heterojunction (BHJ) structures is crucial to the development of high-performance organic photovoltaic (OPV) devices. Herein, we report a novel and direct method for the characterization of the vertical composition gradient of a BHJ structure. Ar gas cluster ion beam (GCIB) sputtering provided a uniform sputtering yield that preserved the chemical structure of the organic semiconducting materials. The combination of X-ray photoelectron spectroscopy (XPS) and Ar GCIB sputtering facilitated the accurate analysis of the vertical molecular distribution of a regioregular poly(3-​hexylthiophene) (P3HT) and [6,6]-phenyl C₇₁ butyric acid methyl ester (PC₇₁BM) BHJ structure. The J-V characteristics and depth profiles of the as-deposited, ethanol-annealed, and chloroform-annealed devices were examined to demonstrate the usefulness of Ar GCIB sputtering in optimizing the BHJ morphology. Chloroform-annealed device exhibited both the best performance and the most homogeneous morphology among the three prepared samples, and the thickness of the P3HT-rich region was substantially reduced from 20 to 10 nm. Consequently, these results provide important information about the correlation between the vertical molecular distribution of a BHJ blend structure and power conversion efficiency of an OPV device.