Morphology of a thermally stable small molecule OPV blend comprising a liquid crystalline donor and fullerene acceptor

Recently organic photovoltaic (OPV) devices comprising the small-molecule liquid-crystalline donor, benzodithiophene-quaterthiophene-rhodanine (BQR), and fullerene acceptor, [6,6]-phenyl-C-71-butyric acid methyl ester (PC71BM), were demonstrated to achieve high performance when thermally processed, avoiding the need for slow-drying solvent additives or complex solvent-vapor annealing post-processing. In this investigation we explore the impact of thermal processing on thin film blends of BQR and PC71BM using differential scanning calorimetry (DSC), in situ grazing incidence X-ray scattering (GISAXS, GIWAXS), and photoluminescence spectroscopy (PL) to correlate thermal behavior with morphological changes and photoactivity. We develop a phase diagram of the crystalline and liquid crystalline transitions in BQR and the related high performing electron donor material, benzodithiophene-terthiophene-rhodanine (BTR), and are able to predict phase transitions using Flory-Huggins theory, including suppression of liquid crystalline phase formation in the presence of PC71BM. Further DSC measurements demonstrate the superior thermal stability of PC71BM blends with BQR over blends with BTR. OPV devices with the BQR:PC71BM active layer were prepared using the blade-coating deposition technique and exhibit optimal device performance when annealed at 120 degrees C for 5 min. The characteristic acceptor/donor domain size in an as-cast BQR:PC71BM film, estimated from GISAXS, was about 60 nm which is sufficient for exciton separation. Domain purity was enhanced by annealing at temperatures above approximate to 80 degrees C. Annealing at temperatures above approximate to 120 degrees C resulted in over-coarsening of the acceptor/donor-rich phases to domain sizes beyond 80 nm and reduced performance.