between the donor and acceptor in bulk heterojunctions formed by blending donor and acceptor molecules, depends on blend morphology and occurs on timescales less than 100 fs to tens of picoseconds with longer times representing diffusion-assisted ET. [ 3b-m ] In organic semiconductors, it is of great interest to separate ET from exciton diffusion to get better understanding of the infl uence of driving force on the critical process of charge generation. This will allow the optimization of the blend materials' electrical properties based on a complete understanding of the photophysics, potentially leading to a break-through in the power conversion effi ciencies attainable in organic photovoltaic (OPV) solar cells.In this communication we address this issue by measuring ET rates from thermally relaxed excitons in a high photovoltaic effi ciency conjugated polymer, to an assortment of acceptors with a range of electron affi nities (EA). Very low loadings of acceptor are used to ensure that the acceptor sites are spread throughout the fi lm so that the blend morphology has no infl uence on the rate of quenching. Comparably low concentrations of similar quenchers have been shown to remain intimately mixed in the blend [ 3,6,7 ] and a plot demonstrating that acceptor 4 remains intimately mixed well above the concentration used of 0.005 nm −3 is shown in the Supporting Information. The donor polymer used was poly [[4,8-bis[(2-ethylhexyl) [3,4-b]thiophenediyl]] (PTB7), a highly effi cient photovoltaic material, [ 8 ] chosen to be as relevant as possible to the study of OPV optimization. The infl uence of quenching by Förster resonance energy transfer (FRET) was minimized and the effect of exciton diffusion was determined independently and taken into account. Fast ET in <2 ps is determined for excitons at a distance of <1 nm to an acceptor for driving force between 0.2 and 0.6 eV. Higher and lower driving forces give slower rates of ET. This dependence is well described by Marcus theory and a reorganization energy of ≈0.4 eV. Our results show that ET from a thermally relaxed exciton can be very effi cient for transitions occurring at the optimal driving force, and that the energy loss at the donor-acceptor interface can be reduced by reducing the reorganization energy while maintaining the optimum electron affi nity or ionization potential offset.Figure 1 a shows the chemical structures and the EA of the acceptors used in this study. EA values were measured by square wave voltammetry in solution and the details are given in Figure S1, Supporting Information. There is some discussion in the literature regarding the measurement of electrochemical potentials of organic semiconductors, [ 9 ] but in this communication, these concerns are mitigated because only the Organic semiconductors have great potential for the development of large-scale, fl exible, and semitransparent solar panels. The primary excitations in organic materials are strongly bound excitons therefore for effi cient charge carrier generation it is necessary ...
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