Post‐Deposition Activation of Latent Hydrogen‐Bonding: A New Paradigm for Enhancing the Performances of Bulk Heterojunction Solar Cells

Small conjugated molecules (SM) are gaining momentum as an alternative to semiconducting polymers for the production of solution-processed bulk heterojunction (BHJ) solar cells. The major issue with SM-BHJs is the low carrier mobility due to the scarce control on the phase-segregation process and consequent lack of preferential percolative pathways for electrons and holes to the extraction electrodes. Here, a new paradigm for fi ne tuning the phase-segregation in SM-BHJs, based on the post-deposition exploitation of latent hydrogen bonding in binary mixtures of PCBM with suitably functionalized electron donor molecules, is demonstrated. The strategy consist in the chemical protection of the H-bond forming sites of the donor species with a thermo-labile functionality whose controlled thermal cleavage leads to the formation of stable, crystalline, phase-separated molecular aggregates. This approach allows the fi ne tuning of the nanoscale fi lm connectivity and thereby to simultaneously optimize the generation of geminate carriers at the donor‐acceptor interfaces and the extraction of free charges via ordered phase-separated domains. As a result, the PV effi ciency undergoes an over twenty-fold increase with respect to control devices. This strategy, demonstrated here with mixtures of diketopyrrolopyrrole derivatives with PCBM can be extended to other molecular systems for achieving highly effi cient SM-BHJ solar cells.