We calculate the radiative efficiency limits of organic bulk heterojunction solar cells according to the theory of Shockley and Queisser and compare the results with experimental device performance. The difference between limiting theory (23% power conversion efficiency) and experimental data (4%) is explained and quantified by five reasons, namely the energy level misalignment at the donor/acceptor heterointerface of the bulk heterojunction, insufficient light trapping, low exciton diffusion lengths, nonradiative recombination, and low charge carrier mobilities. Comparison of the impact of the different loss mechanisms by numerical simulation reveals that efficiencies above 10% using PF10TBT/PCBM blends will require mostly a strong reduction of nonradiative recombination. The energy misalignment and the low carrier mobilities appear as a second-order restriction in this type of blend.
Recent simulations of the efficiency of polymer/fullerene solar cells as a function of mobility predicted finite optimum mobilities due to a decrease in open circuit voltage for higher mobilities. We explain this decrease in open circuit voltage with two features of the commonly used model, namely, infinite surface recombination and an integration over a distribution of separation distances of electron and hole in a charge transfer state at the interface between donor and acceptor molecules. Especially, the assumption of a variable electron/hole pair separation at the interface has a considerable influence on the open circuit voltage.
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