Visualization of Tens of Nanometers Spaced Donor: Acceptor Bulk Heterojunctions across Submicrometer-Square Cross Sections of Organic Photovoltaic Cells

Abstract

Controlling the bulk heterojunction (BHJ) structure of organic photovoltaic cells (OPVs) is essential to improving their power conversion efficiency. Visualizing the interfaces of the organic semiconductor domains in BHJs is important for the development of OPVs. However, the evaluation of the BHJ interface structure is often challenging owing to the close proximity of donor and acceptor organic domains, which are typically spaced less than 100 nm for keeping efficient exciton separation at the junction. In this study, a submicrometer-thick OPV composed of metal-free phthalocyanine (H2Pc) as the donor and fullerene (C60) as the acceptor was adopted to visualize the interface structure of BHJs. This typical combination of donor and acceptor molecules consists of only three elements: H, C, and N. Phase plate scanning transmission electron microscopy (P-STEM) enabled the visualization of the interface structure of the BHJ with a spacing of tens of nanometers across the submicrometer-square cross section of the OPV. Furthermore, a hybrid layer of H2Pc and C60 was observed to form at the initial stage of the film growth until a “critical thickness”, followed by the growth of a phase-separated ordered BHJ. This observation clearly demonstrates the significant potential of P-STEM for analyzing organic hybrid materials.