Effect of spacer insertion in a commonly used dithienosilole/benzothiadiazole-based low band gap copolymer for polymer solar cells

•Alkylated thiophene spacer tunes photovoltaic properties.•Linear octyl chains on Si atom affords easy synthesis of dithienosilole monomer.•Increasing molar masses of low bandgap polymers increases efficiencies.•Thermal annealing or additive increases efficiencies.

Dithienosilole-benzothiadiazole based low bandgap copolymers remain promising material for organic photovoltaics. A new copolymer, poly[(4,4′-dioctyldithieno[3,2-b:2′,3′-d]silole-2,6-diyl)-alt-{4,7-bis[2-(3-hexyl)thienyl]-2,1,3-benzothiadiazole-5,5′-diyl}] (PDTSDTBT) was designed by introducing a thiophene spacer bearing a hexyl chain at β-position in the main backbone and compared to its analog poly[(4,4′-dioctyldithieno[3,2-b:2′,3′-d]silole-2,6-diyl)-alt-(2,1,3-benzothiadiazole-4,7-diyl)] (PDTSBT). In PDTSDTBT, linear alkyl chains on silicon were chosen due to facile and cheap access and the inserted 3-hexylthiophene units were chosen to increase solubility and molar mass, a weak point with PDTSBT. The two parameters are important to optimize photovoltaic performances. To compare characteristics, PDTSDTBT of molar masses greater than, and equal to a sample of PDTSBT, were prepared. Pd-catalyzed Stille cross-coupling reactions in a micro-wave reactor to promote an efficient copolymerisations. A strong absorption ranging from 370 nm to 800 nm and a good thermal stability were observed. PDTSDTBT showed better solubility and higher degree of crystallinity. Facile synthesis of high molar masses meant that higher efficiencies, around 40% greater, could be obtained with PDTSDTBT. The polymer was demonstrated to be susceptible to improvement through the use of device-additives. For example, under initial optimisations using PDTSDTBT:PC60BM blend at a ratio of 1:1 delivered a power conversion efficiency of 2.13% with JSC = 7.73 (mA/cm2), under AM 1.5 (100 mW/cm2) illumination.

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