Limiting factors for charge generation in low-offset fullerene-based organic solar cells

Abstract

Abstract Free charge generation in organic solar cells generally proceeds via (1) the formation of charge transfer (CT) excitons after photoexcitation of donor or acceptor molecules, and (2) CT dissociation into the charge separated (CS) state. While the efficiency of CT formation depends on the energetic difference between local excitation (LE) singlet and CT states, the CT dissociation efficiency is determined by the energetic barrier between the CT and CS states. For a long time, research either studied the combined effect of CT formation and dissociation on photocurrents without distinguishing the individual processes, or primarily focused on understanding and improving CT formation to increase photocurrents, neglecting the efficiency of CT dissociation all-together. In this work, we provide evidence that CT dissociation rather than CT formation presents a major bottleneck for free charge generation in fullerene-based blends with low energetic offsets between LE and CT states. We fabricate devices based on dilute donor content blends of ZnPc or its fluorinated derivatives and C60. Fluorination of ZnPc simultaneously shifts the molecular orbitals away from the vacuum level, increasing the CT state energy at the donor-acceptor interface with C60. Through experimental device characterization, density functional theory (DFT) calculations, and time-resolved electron paramagnetic resonance (trEPR) measurements, we draw a comprehensive picture of how LE, CT, and CS state energies and the transitions among states change upon fluorination of ZnPc. We find that, upon fluorination of ZnPc, primarily CT dissociation decreases, resulting in a significant decrease in the photocurrents of the blends.