Efficient bulk heterojunction solar cells based on solution processed small molecules based on the same benzo[1,2-b:4, 5-b′]thiophene unit as core donor and different terminal units

Abstract

We report the synthesis, characterization, and optical and electrochemical of properties of two novel molecules DRT3-BDT (1) and DTT3-BDT (2), comprising the same BDT central core (donor) and different end capped acceptor units, i.e. rhodanine with ethyl hexyl and thiazolidione with ethylhexyl, respectively, linked via an alkyl-substituted terthiophene (3 T) π-conjugation bridge. The electrochemical properties of these small molecules indicate that their energy levels are compatible with energy levels of PC₇₁BM for efficient exciton dissociation. These molecules have been used as electron donors along with PC₇₁BM as an electron acceptor, for the fabrication of solution processed “small molecule” bulk heterojunction (BHJ) solar cells (smOPV). The device prepared from optimized 1 : PC₇₁BM(1 : 1) processed cast from DIO (3%v)/CF solvent exhibited a power conversion efficiency of 6.76% with J_sc = 11.92 mA cm⁻², V_oc = 0.90 and FF = 0.63. The device with 2 : PC₇₁BM under the same conditions showed a lower PCE of 5.25% with J_sc = 10.52 mA cm⁻², V_oc = 0.86 and FF = 0.56. The AFM, TEM and PL quenching measurements revealed that the high J_sc is a result of the appropriate morphology and exciton dissociation. The performances were compared for the devices based on two small molecules. The higher J_sc for device 1 was attributed to its better nanoscale phase separation, smooth surface and higher carrier mobility in the 1 : PC₇₁BM blend film. Moreover, the higher value of FF for the 1 : PC₇₁BM based device was ascribed to a good balance between the electron and hole mobilities.