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

Recently organic photovoltaic (OPV) devices comprising the small-molecule liquid-crystalline donor, benzodithiophene-quaterthiophene-rhodanine (BQR), and fullerene acceptor, [6,6]-phenyl C₇₁ 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-coated deposition technique and exhibit optimal device performance when annealed at 120 °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 ≈ 80 °C. Annealing at temperatures above ≈ 120 °C resulted in over-coarsening of the acceptor/donor-rich phases to domain sizes beyond 80 nm and reduced performance.