Efficiency Limits of Organic Bulk Heterojunction Solar Cells

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.