Dithienosilole-bridged small molecules with different alkyl group substituents for organic solar cells exhibiting high open-circuit voltage

Three new small organic molecules, I, II and III, consisting of dithienosilole as the central core, bithiophene bridge with different alkyl group substituents, and octyl cyanoacetate or dicyano unit as different end units, have been designed and synthesized. The thermal, optical, electrochemical and photovoltaic properties of these three compounds have been investigated. The solubility, absorption, energy levels and band gaps of these materials were effectively tuned by different alkyl groups substituted on the thiophene unit and/or different electron-withdrawing end groups. Bulk heterojunction solar cells with molecules I-III as electron donors and PC60BM ([6,6]-phenyl-C-60-butyric acid methyl ester) as an election acceptor exhibited power conversion efficiencies of 3.27, 2.88 and 3.81% for I, II and III, respectively. All of these solar cells showed very high V-oc values of 0.89-0.92 V, and the high V-oc is consistent with the low-lying HOMO level of the donor. These compounds also have low LUMO levels which ensure effective charge transfer from the donor to the fullerene acceptor. The structure-photovoltaic property relationships of these donor materials were investigated and discussed.