DPP-based small molecule, non-fullerene acceptors for “channel II” charge generation in OPVs and their improved performance in ternary cells†
We synthesized three diketopyrrolopyrrole-thiophene-based small molecules (p-, m-, and o-DPP-PhCN) substituted with electron-withdrawing cyanide groups on both end phenyl rings in different positions. The physical properties of the oligomers varied based on the position of the CN groups. Compared to m- and o-DPP-PhCN, the p-DPP-PhCN film had a more red-shifted, strong UV absorption (λmax = 670 nm). p-DPP-PhCN also exhibited a relatively well-aligned arrangement in the X-ray diffraction pattern, owing to a high degree of molecular packing in p-DPP-PhCN. Such an exceptionally strong aggregation of p-DPP-PhCN is expected to give rise to strong molecular orbital interactions and a subsequent decrease in the energy band gap (Eg). p-DPP-PhCN has a lower optical Eg (1.75 eV) than m- and o-DPP-PhCN (∼1.80 eV). Organic photovoltaic cells with the structure ITO/PEDOT:PSS/poly(3-hexylthiophene) (P3HT):DPP-PhCN/LiF/Al were fabricated. Two D/A-type binary cells using p- or o-DPP-PhCN showed similar power conversion efficiencies (PCEs) of 0.5% although the device parameters were different. A high open circuit voltage of 1.09 V in P3HT:o-DPP-PhCN comes from a high-lying lowest unoccupied molecular orbital energy level of o-DPP-PhCN. In contrast, the relatively high short circuit current density of P3HT:p-DPP-PhCN can be explained by the red-shifted UV absorption and superior molecular packing in p-DPP-PhCN. Furthermore, the maximum photocurrent response (13%) of P3HT:p-DPP-PhCN was observed at the λmax of p-DPP-PhCN. In other words, the light absorption of p-DPP-PhCN contributes to the photocurrent along with the absorption of P3HT (e.g., “channel II” charge generation). Finally, a PCE of 1.00%, more than twice that of binary cells, was achieved in the D/A/A-type ternary cells composed of P3HT, p-, and o-DPP-PhCN. The contribution of the electron acceptor to the photocurrent of the devices was enhanced by adding a second acceptor. Improved film morphology and better charge separation were observed in the ternary cells.