Nanoscale Energetic Mapping of Interfaces in Organic Bulk Heterojunction Solar Cells

The bulk heterojunction (BHJ) layer in organic solar cells is a delicate nanoscale blend of electron donating (D) and accepting (A) molecules which assembles to form a three-dimensional distributed D-A heterointerface. Yet, the real-space energetic landscape responsible for charge generation from excitons at the heart of these blends has remained elusive, fueling significant debate about the roles of sharp D-A interfaces versus the mixed (M) phase. In this talk, we discuss recent developments in the direct imaging of the real-space energetic landscape of BHJ layers via scanning tunneling microscopy and spectroscopy (STM/STS) measurements. This approach is shown to work on BHJ films directly usable in OPV devices and reveal several heterointerfaces with different energetics that surround aggregated D, A and and mixed (M) phase domains. We reveal the coexistence of three types of heterointerfaces within the BHJ and, in total, four types of D-A intermolecular interactions exhibiting different energetics. These scenarios are universally present in classical and modern BHJs exhibiting a wide range of power conversion efficiencies (PCE, ca. 3.5 – 10.8%; P3HT:PCBM, P3HT:O-IDTBR and PCE11:PCBM). We provide accurate HOMO/LUMO energy diagrams for D/M, A/M and D/A heterointerfaces in these systems and discuss the most likely pathways for charge generation and recombination. Our study also points to new pathways to engineer BHJ interfaces so as to maximize the Voc and the power conversion efficiency of next generation organic solar cells.