Effects of Solvent Additives on Morphology, Charge Generation, Transport, and Recombination in Solution-Processed Small-Molecule Solar Cells

The effects of solvent additive (1,8-diiodooctane (DIO)) on the morphology, charge generation, transport, and recombination in solution-processed small-molecule solar cells are studied and these parameters are correlated with device performance. In the optimum nanoscale morphology, which is processed with 0.4% DIO, the phase separation is large enough to create a percolating pathway for carrier transport, yet still small enough to form large interfacial area for efficient charge separation. Complete phase separation in this film reduces the interfacial defects, which occurs without DIO, and hence suppresses the monomolecular recombination. Moreover, balanced charge transport and weak bimolecular recombination lead to a high fill factor (72%). On the other hand, an excess amount of DIO (0.8%) in the solvent results in the over-aggregation of the donor phase, which disturbs the percolating pathway of the acceptor phase and reduces the electron mobility. The over-aggregation of the donor phase also shrinks the interfacial area for charge separation and consequently reduces the photocurrent generation.