New π-extended diketopyrrolopyrrole-based conjugated molecules for solution-processed solar cells: Influence of effective conjugation length on power conversion efficiency

• Diketopyrrolopyrrole dimer based small molecules were synthesized.
• The effective conjugation length on the performance of thin film transistors and organic photovoltaic devices was studied.
• The terthiophene bridged dimer exhibited high carrier mobility of 3.6 × 10−2 cm2V−1 s−1.
• The photovoltaic device provides the high PCE of 3.76%.

New organic semiconducting molecules containing diketopyrrolopyrrole and thiophene units were designed to investigate the influence of effective conjugation length on the performance of organic photovoltaics made with [6,6]-phenyl-C61-butyric acid methyl ester. New D–A–D–A–D type molecules (where; D = donor, A = acceptor) such as ter- and quarter-thiophene bridged diketopyrrolopyrrole dimers were synthesized with different numbers of conjugative units. The field-effect mobilities of these molecules were determined by fabricating thin film transistors. The new molecules displayed moderately high mobility between 3.6 × 10−2 and 1.1 × 10−2 cm2 V−1 s−1 without a thermal annealing process for TT(BTDPP)2 and QT(BTDPP)2. The TT(BTDPP)2 molecule possesses a longer effective conjugation length (c.f. QT(BTDPP)2) and shows a high carrier mobility. When employing the ter- and quarter-thiophene bridged diketopyrrolopyrrole dimers as donor molecule in combination with [6,6]-phenyl-C61-butyric acid methyl ester [TT(BTDPP)2:PC61BM = 1:0.8 w/w], the photovoltaic cell provides the highest power conversion efficiency of 3.76%.

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