Phenothiazine-based small molecules for bulk heterojunction organic solar cells; variation of side-chain polarity and length of conjugated system

Abstract

Three small molecules denoted SM1, SM2 and SM3, with the phenothiazine donor moiety connected to benzothiadiazole and 3-ethylrhodanine acceptor units through thiophene π-linkers have been synthesized for use in organic solar cells with PC71BM as electron acceptor. SM1 has a 2-ethylhexyl group at the phenothiazine nitrogen, while SM2 and SM3 have a 2-(2-methoxyethoxy)ethyl group at that N. Opto-electronic and dielectric properties, charge carrier mobilities, morphology of active layers, and photovoltaic properties were investigated in detail. The three molecules have wide absorption bands with high molar absorption coefficients and relatively low HOMO levels (−5.21 to −5.27 eV). Compared with the N-alkylated SM1, both SM2 and SM3 exhibit red-shifts of the long-wavelength absorption band in thin films, and show enhanced crystallinity in thin films with smaller stacking distances, higher hole mobility, and higher dielectric constant. After solvent vapour annealing, the power conversion efficiencies (PCEs) were significantly improved for the solar cell devices, from 1.69 to 3.95% for SM1, 2.78–6.62% for SM2 and 3.22–7.16% for SM3. This increase in PCEs was due to the enhancement in Jsc and FF attributed to the formation of nanoscale domains of donor and acceptor resulting in efficient charge separation, balanced charge transport and suppressed charge recombination. These results demonstrate that use of an oxygen-containing side chain, in conjunction with alkyl side chains for solubility during solar cell processing, is an alternative and effective strategy for achieving high-performance small molecule donor materials.