Unusual charge transport and reduced bimolecular recombination in PDTSiTzTz:PC₇₁BM bulk heterojunction blend

Abstract

Solar cells with bulk heterojunction active layers containing donor-acceptor copolymer PDTSiTzTz exhibit persistent high fill factors with thicknesses up to 400 nm. Transport and recombination in a blend of PDTSiTzTz and fullerene derivative PC 71 BM is studied using lateral organic photovoltaic structures. This material system is characterized by carrier-concentrationdependent charge carrier mobilities, a strongly reduced bimolecular recombination factor, and a negative Poole-Frenkel coefficient. The analysis provides an explanation for the relatively thickness-independent fill factor behaviour seen in solar cells using the copolymer PDTSiTzTz. Cumulative insights from this copolymer can be employed for future organic photovoltaic material development, study of existing high performance bulk heterojunciton blends, and improved solar cell design.

Impact of Active Layer Morphology on Bimolecular Recombination Dynamics in Organic Solar Cells

“…Several experimental observations ,− indicate that even when second-order kinetics is operative, the experimental rate constant (β exp ) is smaller than that estimated according to the Langevin model, β L , by two to four orders of magnitude. Previous reports have attributed this overestimation to 2D recombination in the lamellar structure of conjugated polymer, charge carrier concentration gradient within the device, local dielectric constant difference, slow dynamics of charge carrier with smaller mobility, , delocalization of opposite charges in an encounter complex, phase separation, domain purity, and back electron transfer to triplet excitons concurrent with redissociation of charge-transfer (CT) states back to free carriers. , …”

“…Deviations of experimental observations from Langevin theory were reported to occur in two ways: (i) deviation from second-order dependence on charge carrier density and (ii) discrepancy in the rate constant, each of which will be addressed below: Indeed, in many polymer–fullerene based devices, the charge-decay dynamics probed by charge extraction technique , and transient absorption spectroscopy , at open-circuit voltage , was found to exhibit approximately a third-order dependence on ρ­( t ). Therefore, in order to be consistent with the experimental data, the rate constant (β) in the Langevin model has to depend on charge density (or, equivalently, on t ). , This third-order dependence of the BMR rate on charge density has been suggested to arise as a result of either a carrier lifetime dependence on charge density, , recombination via an exponential tail of density of states (DoS), − or carrier trapping in an inhomogeneous distribution of localized states. ,, Also, it was suggested that traps can sometimes enhance the dissociation of geminate pairs into free carriers but can act as recombination centers as well, leading to Shockley–Read–Hall (SRH) recombination dynamics. − Several experimental observations ,− indicate that even when second-order kinetics is operative, the experimental rate constant (β exp ) is smaller than that estimated according to the Langevin model, β L , by two to four orders of magnitude. Previous reports have attributed this overestimation to 2D recombination in the lamellar structure of conjugated polymer, charge carrier concentration gradient within the device, local dielectric constant difference, slow dynamics of charge carrier with smaller mobility, , delocalization of opposite charges in an encounter complex, phase separation, domain purity, and back electron transfer to triplet excitons concurrent with redissociation of charge-transfer (CT) states back to free carriers. , …”