Understanding the Phase Behavior of Conjugated Polymer Blends Using Infrared Nanospectroscopy

The performance of polymer-based solar cells hinges on the bulk heterojunction morphology of the electron donor and acceptor blend. Understanding the average domain size, crystallinity, and phase purity of the multiphase blend is the first step towards controlling donor-acceptor morphology and optimizing solar cell performance. Despite extensive research, the temperature-dependent dynamics of donor-acceptor blends remain poorly understood. This work explores the dynamic, temperature-dependent morphology of a high-performance conjugated polymer (PM6) blended with a small-molecule acceptor (Y6) using X-ray scattering, differential scanning calorimetry, and atomic-force microscopy paired with infrared microscopy (AFM-IR). AFM-IR is a relatively new technique for interrogating the nanoscale surface and chemical morphology of complex polymer blends at sub-10 nm resolution. As the blend is heated, isolated crystallization of the Y6 domains is observed, subsequently purifying the PM6 domain. Further, the temperature and time-dependent performance of the donor-acceptor blend is measured to understand the morphology-performance relationship. Because surface and bulk morphology can be different in polymer blends, the depth sensitivity of AFM-IR is explored. Through a series of experiments, the range of depth characterization was determined to be between 20-100 nm. Limiting the range of depth characterization opens up several opportunities for probing bulk heterojunction morphology used in organic solar cells without influence from the underlying bulk layer.