Role of Exciton Lifetime, Energetic Offsets, and Disorder in Voltage Loss of Bulk Heterojunction Organic Solar Cells

Abstract

Recently, the power conversion efficiency (PCE) of organic solar cells (OSCs) has significantly progressed with a rapid increase from 10 to 19% due to state-of-the-art research on nonfullerene acceptor molecules and various device processing strategies. However, OSCs still exhibit significant open circuit voltage loss (ΔVOC ∼ 0.6 V) due to high energetic offsets and molecular disorder. In this work, we present a systematic investigation to determine the effects of energetic offset and disorder on different recombination losses in open circuit voltage (VOC) using 13 different photoactive layers, wherein the PCE and ΔVOC vary in the ranges of 2.21-14.74% and 0.561-1.443 V, respectively. The detailed voltage loss analysis of all these devices was carried out, and voltage losses were correlated with energetic offset and disorder. This has enabled us to identify the key features for minimizing the voltage loss like: (1) a low energy offset between the donor and acceptor molecular states is essential to attain a nonradiative voltage loss (ΔVOC,nrad) as low as ∼200 meV and (2) Urbach energy, which is a measure of the materials' disorder and packing, should be low for the minimization of the radiative voltage loss (ΔVOC,rad). In addition, time-resolved photoluminescence spectroscopy was employed to further understand the exciton dynamics of pristine materials and donor-acceptor blends. It was observed that the absorbers with ultralong exciton lifetime (∼1000 ps) produce higher efficiencies. The current study emphasizes the importance of simultaneously testing photovoltaic performance and active layer exciton dynamics for rational device optimization and opens new prospects for designing novel molecules with fine-tuning of energetic offset and disorder with longer exciton lifetime which is the effective strategy to boost the efficiency of OSCs to their modified Shockley-Queisser (SQ) limit by minimizing radiative and nonradiative voltage losses.