Quantitative Bimolecular Recombination in Organic Photovoltaics through Triplet Exciton Formation

The nanoscale morphology and high charge densities in organic photovoltaics (OPVs) lead to a high rate of bimolecular encounters between spin-uncorrelated electrons and holes. This process can lead to the formation of low-energy triplet excitons on the donor polymer that decay nonradiatively and limit the device performance. We use time-resolved optical spectroscopy to characterize the effect of morphology through the use of solvent additives such as 1,8-octane­dithiol (ODT) on triplet dynamics and charge recombination in blends of poly­[2,6-(4,4-bis­(2-ethyl­hexyl)-4<i>H</i>-cyclo­penta­[2,1-<i>b</i>;­3,4-<i>b</i>′]-dithio­phene)-<i>alt</i>-4,7-(2,­1,3-benzo­thia­diazole)] and [6,6]-phenyl-C₇₁-butyric acid methyl ester. This is an attractive OPV system since the extended absorption of the polymer into the near-infrared gives good coverage of the solar spectrum, but nevertheless, the internal quantum efficiency (IQE) has not been reported to be higher than ∼65% under short circuit conditions. We find that, without ODT, the IQE is 48% and 16% of excitations decay via bimolecular triplet formation. With ODT treatment, which improves crystallinity and carrier mobility, the IQE increases to 65%, but bimolecular triplet formation significantly increases and now accounts for all of the recombination (35% of charges)