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Abstract

We present combined results of femtosecond transient photoluminescence (PL), femtosecond transient absorption and quasi-steady-state photoinduced absorption spectroscopy on the organic semiconductor poly-6, 6', 12, 12'-tetraalkyl-2, 8- indenofluorene (PIF). By control of interchain order via the choice of the side-chain substituents, we have investigated its effect on exciton and polaron dynamics in this model, electronic material. We show that interfaces between ordered and disordered domains play a significant role in the photophysics. At high photoexcitation fluence, a high yield (similar to10%) of polarons is only observed in the ordered semiconductor. This process arises from two-step photoexcitation, first to the lowest exciton, and then to a high-energy state of opposite symmetry. In contrast, triplet exciton population is generated via sequential excitation with smaller yield (<1%) in both ordered and disordered materials. In the low fluence regime, triplet excitons are found to arise from evolution of polarons generated with low efficiency (also <1%) by diffusion-limited processes. The triplet generation yield is strongly dependent on order, with the disordered material displaying a higher yield. Polaron decay is found to be thermally activated, with a higher activation energy and lower room-temperature recombination rate in the ordered material. Furthermore, we do not find that emissive keto defects play a defining role in the PL properties of our PIF samples. Instead, absorption features of aggregate-like species, which we believe to lead to sub-gap emission, are evident in the photocurrent action spectrum of the more ordered PIF derivative.