A bacteriochlorin-diketopyrrolopyrrole triad as a donor for solution-processed bulk heterojunction organic solar cells

Abstract

We have designed an A–π–D–π–A small-molecule triad consisting of a bacteriochlorin (BC) donor central core linked with two diketopyrrolopyrrole (DPP) acceptors via ethynyl bridges (BC-DPP-1). BC-DPP-1 has a narrow optical bandgap of 1.38 eV with highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels of −4.93 eV and −3.40 eV, respectively, and it was used as an electron donor along with [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) as an acceptor for solution-processed small-molecule organic solar cells. After optimizing the weight ratio between BC-DPP-1 and PC₇₁BM and pyridine as a solvent additive and subsequent solvent vapor annealing using THF, an organic solar cell based on the optimized BC-DPP-1:PC₇₁BM showed an overall power conversion efficiency of 7.48%. Since BC-DPP-1 shows a weak absorption band in the 650–750 nm wavelength region, we used a second small molecule having strong absorption in this spectral region and prepared the ternary active layer BC-DPP-1 : SM : PC₇₁BM, varying the weight ratio between the two donors and keeping the amount of PC₇₁BM constant. The ternary active layer BC-DPP-1 (70% w/w):SM (30% w/w):PC₇₁BM showed the best photovoltaic performance. After the optimization of the ternary active layer (i.e., the solvent additive and subsequent solvent vapor additive), the organic solar cell showed overall power conversion efficiency of 9.88%. The improved power conversion efficiency resulted from the enhancement of J_sc, V_oc and FF as compared to the binary counterpart. Since BC is an analog of porphyrins and chlorophylls, these results demonstrate that benefiting from the narrow band gap of BC-DPP-1 (i.e., organic solar cells with light harvesting in the NIR region of the solar spectrum) can be a real improvement. Moreover, the low energy loss (0.48 eV) as compared to the binary counterpart (0.58 eV) also confirms the suppressed recombination in the ternary organic solar cells.