In this presentation we evaluate the thermal and light-soaking stability of polymer:fullerene bulk heterojunctions (BHJs) based on a donor polymer recently developed by Merck Chemicals. Blade-coated thin films of pure polymer and BHJs incorporating either PC61BM or PC71BM were investigated using atomic force microscopy (AFM), grazing-incidence x-ray diffraction (GIXD), neutron reflectivity (NR), and UV-Visible spectroscopy. The addition of fullerene to the polymer at device-relevant concentrations (25-50% by weight) results in a 7-fold reduction in surface roughness relative to pure polymer films, attributed to a reduction in the pure polymer domain size with increasingly homogeneous polymer:fullerene mixing. Films subjected to accelerated stressing relevant to scalable OPV fabrication—thermal stressing at 120 °C for 1 hour or light-soaking under AM1.5 light for 8 hours—indicate robust stability to stressing, with thin films showing marginal but monotonic increases in roughness with increasing fullerene concentration. Wide-angle GIXD measurements indicate a contraction of 1-4% in features attributed to polymer crystalline domains upon annealing. Comparison of pre-and post-annealed films reveals increased correlation between polymer scattering features in the latter, consistent with improved ordering of the polymer backbone and sidechains. These findings, together with GIXD measurements of full device stacks and accelerated aging measurements of operational cells, support recent reports on the high photoconversion efficiency stability of this polymer BHJ system for highly scalable, long-lifetime OPVs.
Molecular engineering of discotic liquid crystals is discussed with relation to charge carrier mobility and generation efficiency. Steady State DC and Pulsed TOF measurements are reported as viable complementary methods in the determination of charge generation efficiencies. Data for the quantum efficiencies of charge carrier generation in 2,3,6,7,10,11-hexa(hexyloxy)triphenylene and its 1:1 mixture with 2,3,6,7,10,11-hexakis(nonylphenyl)triphenylene (a complementary polytopic interaction (CPI) stabilized compound) are presented. The wavelength, field and temperature dependence of the quantum efficiency of charge carrier generation are discussed in respect of the Onsager theory of geminate recombination.