Phenothiazine functionalized fulleropyrrolidines: synthesis, charge transport and applications to organic solar cells

A series of phenothiazine-C_60/70 dyads containing fulleropyrrolidine tethered to C-3 position (C₆₀-PTZ and C₇₀-PTZ) or to the heteroatom N-position via either phenyl (C₆₀-Ph-PTZ and C₇₀-Ph-PTZ) or phenoxyethyl linkers (C₆₀-PhOEt-PTZ and C₇₀-PhOEt-PTZ) of the phenothiazine were synthesized and light-induced electron transfer events were explored. Optimized studies suggested that the highest molecular orbital (HOMO) resides on donor phenothiazine moiety while lowest molecular orbital (LUMO) on the acceptor fulleropyrrolidine moiety of the dyads. Optical and electrochemical properties suggested no electronic communication between the donor and acceptor moieties in the ground state. However, steady-state emission studies in solvents of varied polarity, involving selective excitation of C₆₀/C₇₀, disclosed that the emission intensity of C₆₀/C₇₀ was quenched in the dyads in the increasing order, C_60/70-PTZ > C_60/70-Ph-PTZ > C_60/70-PhOEt-PTZ as a consequence of the donor-acceptor distance resulted due to spacer lengths. Also, the emission quenching is more pronounced in polar solvents such as DMF compared to a non-polar solvent, toluene. With the support of parallel electrochemical studies, the emission quenching is attributed to intramolecular photo-induced electron transfer occurring from PTZ to (C₆₀/C₇₀)* generating a radical ion pair, PTZ^+⋅-C₆₀^-⋅/PTZ^+⋅-C₇₀^-⋅. Finally, bulk heterojunction (BHJ) solar cells devices inverted fashion prepared by employing the dyads as acceptors, and PTB7 as donor, suggested that the devices prepared from C₇₀ derivatives i.e., PTB7:C₇₀-PTZ and PTB7:C₇₀-PhOEt-PTZ exhibited better power conversion efficiency of 2.66% and 2.15%, respectively over C₆₀ derivatives i.e., PTB7:C₆₀-PTZ and PTB7:C₆₀-PhOEt-PTZ with the efficiencies of 1.80 and 1.72%, respectively. AFM studies revealed that the poor performance of PTB7:C₆₀-PTZ- and PTB7:C₆₀-PhOEt-PTZ-based devices can be ascribed to the lower solubility of the dyads in 1,2-DCB solvent leading to rough morphology.