Ultra-narrow bandgap non-fullerene organic solar cells with low voltage losses and a large photocurrent

Organic solar cells (OSCs) with ultra-narrow band gaps (UN-BGs) often suffer from higher energy losses and resulting inferior power conversion efficiencies (PCEs) compared to mid-bandgap systems. Benefiting from an optimal balance between the photocurrent and energy losses, OSCs based on the bulk heterojunction PTB7-Th:IEICO-4F with an effective bandgap of 1.03 eV can produce an impressive PCE of greater than 11%, which is on a par with that found in mid-gap non-fullerene devices with an 3,9-bis(4-(1,1-dicyanomethylene)-3-methylene-2-oxo-cyclopenta[b]thiophen)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d′:2,3-d′]-s-indaceno[1,2-b:5,6-b′]-dithiophene (ITCC) acceptor. The supreme solar harvesting observed in the IEICO-4F device can be correlated to a reduced bi-molecular recombination rate constant alongside low voltage losses (<0.4 V). On the other hand, the poor fill factor observed when using an IEICO-4F acceptor seems to be connected to a stronger field-dependence on the charge extraction and an enlarged energetic disorder. To this end, further improvements in the PCE of OSCs using UN-BG acceptors could be prompted by improving the charge sweepout, which could be enabled by increasing the charge transport properties with optimized nanomorphology.