How ‘Hot’ are The Charges in OPV?

The question of whether charge transport in operational organic solar cells (OSC) occurs far-from-equilibrium or not is of significant practical and fundamental importance. While the equilibrium picture of the OSC assumes that the photogenerated charge carriers quickly lose their energy and attain lattice temperature, kinetic Monte Carlo (kMC) simulations of OSC have consistently shown that photogenerated charge carriers are extracted before reaching thermal equilibrium energy: the population thermalizes, albeit to an effective temperature that exceeds that of the lattice [1].<br/>Probing the distribution of photogenerated charges possessing this excess energy has proven to be notoriously hard. In this work, we use Johnson thermometry to measure the temperature of the photogenerated carriers carried out by cross-correlated current noise spectroscopy. Two representative material systems, P3HT:PCBM and PM6:Y6, are tested against their inorganic counterpart, silicon. The experiments univocally prove, in stark contrast to silicon photovoltaics, that charges in operational OSC are not thermalised and are almost twice as hot as the lattice. The experimental findings are confirmed by kMC simulations. The simulations show that the energetic disorder in organic semiconductors is not only the reason for slow thermalisation but also for the high effective temperature observed in the Johnson thermometry experiments.<br/>Our results imply that OPVs are truly far from equilibrium systems, which opens realistic prospects to mitigate the thermalisation losses and eventually beat the near-equilibrium thermodynamic limit [2]. In fact, the results presented show that even regular OPVs are Hot-Carrier Solar Cells in the sense that excess energy contributes to output power.<br/><br/>References<br/>[1] A. Melianas, M. Kemerink, A. Melianas, and M. Kemerink, <i>PROGRESS REPORT 1806004 (1 of 23) Photogenerated Charge Transport in Organic Electronic Materials: Experiments Confirmed by Simulations</i>, (2019).<br/>[2] T. Upreti, C. Tormann, and M. Kemerink, <i>Can Organic Solar Cells Beat the Near-Equilibrium Thermodynamic Limit?</i>, J. Phys. Chem. Lett. <b>13</b>, 6514 (2022).