A new small molecule donor for efficient and stable all small molecule organic solar cells

In recent years, solution processed bulk heterojunction (BHJ) organic solar cells (OSCs) have been intensively studied. The widely used fullerene derivatives as acceptors, (e.g. [6,6]-phenyl-C ₇₁ -butyric acid methyl ester (PC ₇₀ BM)), widely used, display a limited visible light absorption and unsatisfactory thermal stability in combination with amounts of donor materials. It was found that rational design rules for novel donors and introduced analysis of non-fullerene acceptors (NFAs) are employed to develop novel systems overcoming these limitations. Here we reported a new small molecule donor, W-CN, containing dithieno[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene (DTBDT) donor unit and 2-ethylhexyl-cyanoacetate (CN) acceptor unit with thiophene-thieno[3,2-b]thiophene (T-TT) bridge. We chose 2,2'-((2Z,2′Z)-((4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene-2,7-diyl)bis(methanylylidene))bis(3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (IDIC), 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis (4-hexylphenyl)-dithieno[2,3-d:2′, 3′-d']-s-indaceno[1,2-b:5,6-b']dithiophene) (ITIC) and PC ₇₀ BM as the acceptors. Furthermore, we demonstrated all-small molecule solar cells (all-SMSCs) based on W-CN:IDIC and W-CN:ITIC composites with decent performance with the power conversion efficiencies (PCEs) of 5.26% and 3.89%, respectively. In contrast, W-CN:PC ₇₀ BM solar cell shows a slightly higher PCE of 5.94% as compared to the investigated NFA systems. Nevertheless, W-CN:IDIC and W-CN:ITIC systems exhibit better thermal stability as compared to the W-CN:PC ₇₀ BM system. Notably, solar cells based on W-CN:IDIC system maintain almost 96% initial performance after baking at 100 °C for 24 h under inert atmosphere. The highly promising findings demonstrated the importance of novel structure-property relationships for the rational design strategies of photovoltaic materials and suitable selection of acceptors for high performance OSCs.