Real-time observation of drying kinetics and morphology evolution in organic bulk heterojunctions (Presentation Recording)

Abstract

In organic photovoltaics field, an optimized bulk heterojunction film consists of an electron-donating conjugated polymer and an electron-accepting fullerene derivative, which is organized in a well phase-separated, yet interconnected network. This sensitive morphology, affecting the light absorption, exciton dissociation and subsequent charge generation-extraction, is determined by the film formation during solution casting under certain processing conditions. Therefore, a number of previous studies focused on characterizing the thin film formation during solution casting, mainly with in-situ grazing-incidence X-ray scattering methods, accompanied by various optical methods, such as ellipsometry/reflectometry and UV-VIS absorption. Although these studies provided invaluable information on the matter, the development of nanoscale morphology is yet to be fully understood. The purpose of this study is to demonstrate a portable in-situ characterization chamber, which can characterize any organic/hybrid thin film during solution casting. The chamber is a miniature doctor blade under controlled atmosphere, equipped with white light reflectometry (WLR), photoluminescence (PL) and laser light scattering (LLS). WLR was used to monitor the thickness reduction of the thin film during the drying, enabling to establish a drying curve. LLS informed the time scale of aggregate/crystallite formation. PL monitored molecular arrangement and enabled the estimation of microstructure. The combined data is used to understand the competition between thermodynamics (e.g. solubility, miscibility) and kinetics of morphology formation. In this study, we measured different BHJ systems with binary and ternary solvent mixtures under different processing conditions, from which we built a roadmap for microstructure formation in organic thin films, used in organic photovoltaics.