Optimizing the ionization-energy offset for enhanced photovoltaic properties in bilayer organic solar cells

The ionization-energy (IE) offset significantly impacts exciton dissociation and charge transfer in organic solar cells (OSCs). However, the optimal IE offset for efficient hole transfer remains uncertain. Herein, we explored the effect of IE offset on exciton dissociation and charge transfer in bilayer OSCs using simulations and experiments, focusing on PM6 as the donor and BTP-based acceptors (eC9-0Cl, eC9-2Cl, and eC9-4Cl). Varying the number of chlorine substitutions in the acceptors' terminal groups results in IE offsets of 0.23, 0.30, and 0.47 eV for PM6/eC9-0Cl, PM6/eC9-2Cl, and PM6/eC9-4Cl, respectively. With the IE offset of 0.47 eV, a power conversion efficiency (PCE) of 17.7% was achieved, while a 0.3 eV offset still attained a modest 16.4%. Simulations reveal three key findings: the IE offset directly influences the maximum PCE a device potentially achieves; the PCE is affected by the mobility of charge carriers but not by their type; and lower bimolecular recombination rates correlate with higher PCE. We found a minimum IE offset of 0.3 eV, carrier mobility of $1\phantom{\rule{0.1em}{0ex}}\ifmmode\times\else\texttimes\fi{}\phantom{\rule{0.1em}{0ex}}{10}^{\ensuremath{-}8}\phantom{\rule{0.1em}{0ex}}{\mathrm{m}}^{2}/\mathrm{V}\phantom{\rule{0.1em}{0ex}}\mathrm{s}$, and bimolecular recombination rate of $1\phantom{\rule{0.1em}{0ex}}\ifmmode\times\else\texttimes\fi{}\phantom{\rule{0.1em}{0ex}}{10}^{\ensuremath{-}13}\phantom{\rule{0.1em}{0ex}}{\mathrm{cm}}^{3}/\mathrm{s}$ to achieve a PCE of over 20%. Our results provide guidelines for designing novel materials for high-efficiency bilayer OSCs.