Effect of local and global structural order on the performance of perylene diimide excimeric solar cells

Herein, we present a detailed study of the structure-function relationship in the organic photovoltaic (OPV) blend film composed of N,N'-bis(1-ethylpropyl)-perylene-3,4,9,10-tetracarboxylic diimide (EP-PDI) and the low energy gap copolymer of poly[4,8-bis-substituted-benzo[1,2-b:4,5-b']dithiophene-2,6-diyl-alt-4-substituted-thieno[3,4-b]thiophene-2,6-diyl] (PBDTTT-E-O). The hierarchical organization in the photoactive layers and in extruded fibers of PBDTTT-E-O:EP-PDI was studied by fluorescence optical microscopy, atomic force microscopy, and wide-angle X-ray scattering (WAXS). WAXS revealed a nanophase-separated structure where PBDTTT-E-O domains of 4.3 nm in size coexist with EP-PDI domains of 20 nm size. Thermal annealing results in an increase of the PBDTTT-E-O domains, but it does not affect the size of the EP-PDI domains. Only the length of the EP-PDI columns in each domain is increased by thermal treatment. The photophysical characterization of the PBDTTT-E-O:EP-PDI layers and the electrical characterization of the corresponding OPV and unipolar carrier devices were performed. The quenching of the EP-PDI excimer luminescence is correlated with the photocurrent generation efficiency of the OPV devices. At high annealing temperatures the EP-PDI columnar length becomes larger than the previously reported diffusion length of the PDI excimer, and fewer excimers dissociate at the EP-PDI/polymer interfaces, leading to reduced photocurrent generation. The charge transport properties of the PBDTTT-E-O:EP-PDI blend film were studied as a function of the active layer microstructure that was tuned by thermal treatment. Thermal processing increases electron mobility, but the poor connectivity of the EP-PDI domains keeps hole mobility six times higher. In respect to the as-spun OPV device, a 3-fold increase is found in the power conversion efficiency of the device annealed at 100 °C. The high surface roughness of the PBDTTT-E-O:EP-PDI photoactive layer impedes the efficient extraction of charges, and a thin and smooth perylene-3,4,9,10-tetracarboxylic bisbenzimidazole overlayer is required for increasing the device performance to a power conversion efficiency (PCE) ∼ 1.7%. The inversion in the polarity of the device contacts resulted in an inverted device with PCE ∼ 1.9%. We provide rational guidelines for the accurate tuning of the layer microstructure in PDI-based photoactive layers of efficient OPV devices. Local disorder in the EP-PDI aggregates is essential (i) for the optimum electron transport that is ensured by the efficient connectivity of the EP-PDI columns in adjacent EP-PDI domains and (ii) for preventing the stabilization of the neutral photoexcitations in the EP-PDI domains in the form of slowly diffusive excimers. The high photocurrent generation efficiency achieved suggests the EP-PDI excimers are formed faster than the activation of triplet states, and photocurrent losses are minimized.