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Abstract
Kinetic Monte Carlo simulations are used to examine the effect of high-energy, 'hot' delocalised charge transfer (HCT) states for donor:acceptor and mixed:aggregate blends, the latter relating to polymer:fullerene photovoltaic devices. Increased fullerene aggregation is shown to enhance charge generation and short-circuit device current - largely due to the increased production of HCT states at the aggregate interface. However, the instances where HCT states are predicted to give internal quantum efficiencies in the region of 50% do not correspond to HCT delocalisation or electron mobility measured in experiments. These data therefore suggest that HCT states are not the primary cause of high quantum efficiencies in some polymer:fullerene OPVs. Instead it is argued that HCT states are responsible for the fast charge generation seen in spectroscopy, but that regional variation in energy levels are the cause of long-term, efficient free-charge generation.
Highlights
The performance of bulk heterojunction organic photovoltaic devices (OPVs) has improved rapidly since their invention in 1995.1,2 Today, lab-made, single junction OPVs have almost reached 10% power conversion efficiency,[3,4] seen by some as the threshold at which OPVs become commercially viable.[5]
We find that hot’ delocalised charge transfer (HCT) states do improve the efficiency of charge generation as anticipated, but that the degree of delocalisation required to obtain the performance seen in OPVs is in excess of that measured in experiment
Two recombination rates are examined, kr = 1 Â 107 sÀ1 which is similar to that found in all-polymer OPVs,[50] and kr = 1 Â 109 sÀ1, similar to that observed in polymer:fullerene OPVs.[15,16,17,18,19]
Summary
The performance of bulk heterojunction organic photovoltaic devices (OPVs) has improved rapidly since their invention in 1995.1,2 Today, lab-made, single junction OPVs have almost reached 10% power conversion efficiency,[3,4] seen by some as the threshold at which OPVs become commercially viable.[5] These improvements in efficiency have come about largely by better matching the absorption spectrum of the blend components to sunlight,[6] as well as optimising the processing conditions of the active layer.[7] there is still active discussion[8,9,10,11] as to the mechanism by which efficient, even 480% efficient,[12,13] freecharge generation can be achieved in organic materials. Efficient operation is perhaps unexpected because the dielectric constant is small (e B 3), meaning that photoabsorption creates excitons with a high binding energy[14] rather than free-charges, and the recombination rate of high-performance polymer:fullerene OPVs, is of the order 109 sÀ1.15–19.
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