FULL PAPER www.MaterialsViews.com www.advenergymat.de Adv. Energy Mater. 2012, 2, 477–486 transport distances remains short enough to avoid free carrier recombination. [4,5] However, this comes at the expense of light absorption efficiency, which gener- ally increases with d active and dictates the maximum achievable photocurrent. Traditionally, the loss of free carriers has been described as non-geminate (recombined electron and hole originate from different excited states) and bimo- lecular (second order recombination rate that depends on the square of the free carrier density). [6] While the loss of free carriers is intuitively non-geminate, bimo- lecular recombination can only be linked to free carrier loss if it depends on the distance electrons and holes are required to travel to reach their respective exit con- tacts. While this has been inferred from comparisons of separate devices where d active [7] or carrier mobilities [8,9] are varied, such comparisons may convolve unintentional morphological changes from altered processing conditions. In our recent work, [10] we also showed an indirect link between bimolecular recombination and the distance required for carriers to transit the active layer, but again comparisons were made between dif- ferent samples with different d active and electron/hole transport interlayers. Finally, device models [6,11] have also predicted that increasing d active will lead to greater bimolecular recombina- tion, [12] but this effect is to be expected since free carriers are assumed to be lost only through this process. Establishing a solid experimental connection between bimolecular recom- bination and carrier transport distances would provide new insight to this loss mechanism that critically influences the open-circuit voltage [13,14] and also describes the dark injected current. [15] In this work, we utilize the characteristic local light absorp- tion profile in the photoactive layer to control the carrier transport distance for BHJ solar cells. Using semi-transparent devices, we show a direct link between bimolecular recombi- nation and the distance carriers must travel in order to be extracted. Using light intensity dependent measurements, we determine if carrier loss through bimolecular recombination is balanced or unbalanced with respect to electron or hole transport distances. Furthermore, using a simple model for carrier extraction, we estimate the intrinsic mobility lifetime product of the restricted carrier species, where holes and DOI: 10.1002/aenm.201100677 John R. Tumbleston, Yingchi Liu, Edward T. Samulski, and Rene Lopez* Interplay between Bimolecular Recombination and Carrier Transport Distances in Bulk Heterojunction Organic Solar Cells In this work, it is demonstrated that bimolecular recombination depends on the distance that free carriers are required to travel in transit to the electrodes in bulk heterojunction organic solar cells. By employing semi-transparent devices, the carrier transport distance can be controlled via the local light absorption profile with an appropriate choice of the illumination side and inci- dent wavelength. Using a series of light intensity-dependent measurements, bimolecular recombination is shown to depend on the distance electrons or holes are required to transit the active layer. This effect is demonstrated for three different bulk heterojunction blends, where the restrictive carrier that causes the onset of recombination is identified. The mobility-lifetime products of the limiting carriers are also estimated using a simple model for carrier extraction, where similar values are obtained regardless of the absorp- tion profile. Implications for 1-sun performance are also discussed, which provide guidelines for fabricating devices with thicker active layers capable of maximizing light absorption without succumbing to recombination losses. 1. Introduction The characterization of recombination processes in bulk het- erojunction (BHJ) organic solar cells has been a central com- ponent to improving the power conversion efficiency (PCE) to levels now exceeding 8%. [1] Using a suite of steady-state and transient techniques, a complex picture of the underlying physical mechanisms leading to photocurrent generation has emerged that has helped guide the design of higher performing materials and devices. [2,3] This development has been coinci- dent with empirical evidence refining the optimal processing conditions for the ever-growing number of organic photoactive materials. One such empirical guideline is that the active layer thickness ( d active ) must be kept on the order of 100 nm so that Dr. J. R. Tumbleston, Y. Liu, Prof. R. Lopez Department of Physics and Astronomy University of North Carolina at Chapel Hill Phillips Hall, CB 3255, Chapel Hill, NC, USA E-mail: rln@physics.unc.edu Prof. E. T. Samulski Department of Chemistry University of North Carolina at Chapel Hill Caudill and Kenan Laboratories CB 3290, Chapel Hill, NC, USA © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 477 wileyonlinelibrary.com