Energy levels in dilute-donor organic solar cell photocurrent generation: A thienothiophene donor molecule study

Abstract To investigate photocurrent generation mechanisms in these organic solar cells (OSCs), we design and synthesize four thienothiophene (TT)-based small-molecule donors with the highest occupied molecular orbital (HOMO) levels varying from −6.4 eV to −5.1 eV, which span across the HOMO value of the [6,6]-phenyl-C70-butyric acid methyl ester (PC71BM) acceptor. We measure TT-based donor:PC71BM films’ electronic and optical properties, OSC current density-voltage characteristic, and external quantum efficiency, and perform density functional theory (DFT) calculations. Our results show that photocurrent generation depends strongly on the substitutions of the center TT groups, cyano (-CN) versus hexyloxy (-OHex). With 1 wt% donor, TTOHex:PC71BM devices produce seven times, increasing to twelve times for 5 wt % donor, higher photocurrent than neat PC71BM devices. In contrast, TTCN:PC71BM devices do not generate additional photocurrent even with 10 wt% donor. The photocurrent generation in TT-based donor:PC71BM devices depends critically on the HOMO value of the donor molecule with respect to that of PC71BM, indicating the importance of type II energy level alignment to facilitate exciton dissociation at the donor-acceptor interface. The photovoltage of all TT:PC71BM devices are comparable to neat PC71BM devices, 0.85–0.90 V, with a low voltage loss due to non-radiative recombination. The fill factor of TTOHex:PC71BM devices are low due to the low hole mobility, ~10−8 cm2/V. Following exciton dissociation, hole transport is analyzed according to three possible mechanisms: tunneling, percolation pathways, and hole back transfer. We find that the hole back transfer mechanism can explain all experimental results and therefore is the primary hole transport mechanism for photocurrent generation in TT-based donor:PC71BM dilute-donor OSCs.