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 (PC 71 BM) acceptor. We measure TT-based donor:PC 71 BM 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:PC 71 BM devices produce seven times, increasing to twelve times for 5 wt % donor, higher photocurrent than neat PC 71 BM devices. In contrast, TTCN:PC 71 BM devices do not generate additional photocurrent even with 10 wt% donor. The photocurrent generation in TT-based donor:PC 71 BM devices depends critically on the HOMO value of the donor molecule with respect to that of PC 71 BM, indicating the importance of type II energy level alignment to facilitate exciton dissociation at the donor-acceptor interface. The photovoltage of all TT:PC 71 BM devices are comparable to neat PC 71 BM devices, 0.85–0.90 V, with a low voltage loss due to non-radiative recombination. The fill factor of TTOHex:PC 71 BM devices are low due to the low hole mobility, ~10 −8 cm 2 /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:PC 71 BM dilute-donor OSCs. • Design and synthesis of thienothiophene (TT)-based small molecule donors with varied HOMO levels. • Functional group substitutions on center TT groups have stronger effect on energy levels and photocurrent generation. • Favorable HOMO offset between donor and acceptor is required for exciton dissociation. • Hole back transfer is the primary hole transport mechanism for photocurrent generation in the TT:PCBM dilute-donor devices.