Efficient polymer solar cells based on dialkoxynaphthalene and benzo[c][1,2,5]thiadiazole: A new approach for simple donoracceptor pair

We reported the synthesis of novel polymeric semiconductor materials based on [poly(4-(5-(1,5-bis(alkoxy)naphthalen-2-yl)thiophen-2-yl)-7-(thiophen-2-yl) benzo[c][1,2,5]-thiadiazole)] (PANTBT) and the fabrication of solar cells with a power conversion efficiency of 4.2% using the synthesized polymers blended with [6,6]-phenyl C₇₁ butyric acid methyl ester (PC₇₀BM) in bulk heterojunction geometry. By varying the side chains, three polymers were synthesized [poly(4-(5-(1,5-bis(2-ethylhexyloxy)naphthalen-2-yl)thiophen-2-yl)- 7-(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (PENTBT), [poly(4-(5-(1,5- bis(decyloxy)naphthalen-2-yl)thiophen-2-yl)-7-(thiophen-2-yl)benzo[c][1,2,5] thiadiazole)] (PDNTBT), and [poly(4-(5-(1,5-bis(tetradecyloxy)naphthalen-2-yl) thiophen-2-yl)-7-(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (PTDNTBT), maintaining a low highest occupied molecular orbital (HOMO) energy level and relatively low band gap, which lead to a high open circuit voltage and short circuit current of the resulting devices. Due to the superior miscibility of PANTBT derivatives with PC₇₀BM, favorable phase separation with a domain size of 1020 nm was achieved regardless of the crystalline nature of the pristine polymers. PDNTBT with alkyl side chain C10 and PTDNTBT with alkyl side chain C14 showed higher photovoltaic performances. In addition, the effects of the crystalline nature of polymers on the thermal stability of the resulting solar cell devices were discussed in terms of the influence of side chains.