Koen Vandewal1
Hasselt University1
The currently best organic solar cells suffer from relatively large voltage losses due to non-radiative recombination as compared to inorganic or perovskite solar cells. Further enhancement of the power conversion efficiency to values over 20% will require a reduction of these losses, inevitably corresponding to an increase in the electroluminescence quantum efficiency of the devices.<sup>[1]</sup> For a large number of donor-acceptor combinations, we have observed that non-radiative voltage losses decrease with increasing charge-transfer-state energies, consistent with non-radiative decay being facilitated by a common high frequency molecular vibrational mode.<sup>[2] </sup>We further identify small molecule donor-acceptor blends with an optical gap in the visible spectral range, with strongly reduced non-radiative losses as compared to systems with a gap in the near infrared (NIR).<sup>[3]</sup> This highlights the possibility of a simultaneous occurrence of a high photovoltaic quantum efficiency as well as a high electroluminescence quantum efficiency, occurring in a single organic donor-acceptor blend. For photovoltaic blends with strong absorption in the NIR, we show that the lowest non-radiative decay rates correspond to systems with the narrowest emission linewidths and steepest absorption tails.<sup>[4]</sup><br/> <br/>[1] Vandewal et al, Nature Materials 8, 904 (2009)<br/>[2] Benduhn et al, Nature Energy 2, 17053 (2017)<br/>[3] Ullbrich et al, Nature Mater. 18, 459 (2019)<br/>[4] Liu et al, joule 5, 2365 (2021)