Unambiguous determination of molecular packing in crystalline donor domains of small molecule solution processed solar cell devices using routine X-ray diffraction techniques

Abstract

The crystalline phase in crystalline donor domains of squaraine-based small molecule (SM) solution processed organic solar cell (OSC) devices has been accurately characterized. The active layers consisting of solution processed bulk-heterojunction (BHJ) films of squaraine (SQ)-PC₇₀BM blends and of pristine SQ thin films were characterized using a set of X-ray diffraction techniques, simple sample preparation methods, and routine least-square data refinement. The approach involves comparing experimental diffractograms recorded from active layers and from powdered active layers with those calculated from single crystal structures of the SQ molecule. The calculated diffractograms were fitted to the experimental powder diffractograms using least-square refinement routines, which allow assignment of crystalline phases with great accuracy, providing clear evidence that the packing in the active layer is similar in nature to that in the corresponding single crystal polymorphs. It was found that molecules of SQ exclusively pack into a P2₁/n phase in crystallites of BHJ active layer blends comprising PC₇₀BM as the acceptor and into two distinct phases in the case of pristine SQ films, the P2₁/n phase and a polymorphic Pbcn phase. In the latter case, the predominance of one or the other phase depends on the solvent used in the deposition step, the annealing process employed and the substrate the active layer is deposited onto. This information, together with that obtained from grazing incidence X-ray diffraction GIXRD, provided precise structural information in the nano- and sub-nanometer range.