Blends of electron-donating and -accepting organic semiconductors are widely used as photoactive materials in next-generation solar cells and photodetectors. The yield of free charges in these systems is often limited by the separation of interfacial electron–hole pairs. In order to establish strategies to optimize the dissociation of these so-called interfacial charge transfer (CT) states, it is of particular importance to understand how different parameters affect their separation yield and dynamics. Here we show [1], by measuring charge-generation and extraction losses [2] along with key transport parameters [3], that the charge-generation yield increases, in a series of bulk-heterojunction solar cells, with increasing slower carrier mobility, regardless of the carrier type. This is in direct contrast with the well-established Onsager-Braun model [4] where the CT state dissociation rate is proportional to the mobility sum, and recent models that underscore the importance of fullerene aggregation for coherent electron propagation [5]. Our data suggest that it is largely the ability of the slower charge carriers to leave the donor:acceptor interface that dictates the efficiency of CT state dissociation. Possible mechanisms that enable the slower carriers to leave the interface are as follows: a high enough mobility, a sufficiently large domain size and enough conduction pathways that lower the Coulomb barrier for dissociation because of entropic effects. The result underscore the need for high mobilities to maximize not only charge collection but also charge generation in these photovoltaic systems. 

References:
[1]          Stolterfoht, M. et al. Slower carriers limit charge generation in organic semiconductor light-harvesting systems. In Press Nature Communications (2016), DOI: 10.1038/ncomms11944.
[2]          Stolterfoht, M. et al. Charge transport without recombination in organic solar cells and photodiodes. J. Phys. Chem. C 119, 26866–26874 (2015).
[3]          Philippa, B. et al. The impact of hot charge carrier mobility on photocurrent losses in polymer-based solar cells. Sci. Rep. 4, 5695 (2014).
[4]          Braun, C. L. Electric field assisted dissociation of charge transfer states as a mechanism of photocarrier production. J. Chem. Phys. 80, 4157–4161 (1984).
[5]          Gélinas, S. et al. Ultrafast long-range charge separation in organic semiconductor photovoltaic diodes. Science 343, 512–516 (2014).