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

Abstract

A series of phenothiazine-C 60/70 dyads containing fulleropyrrolidine tethered to C-3 position (C 60 -PTZ and C 70 -PTZ) or to the heteroatom N-position via either phenyl (C 60 -Ph-PTZ and C 70 -Ph-PTZ) or phenoxyethyl linkers (C 60 -PhOEt-PTZ and C 70 -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 C 60 /C 70 , disclosed that the emission intensity of C 60 /C 70 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 nonpolar solvent, toluene. With the support of parallel electrochemical studies, the emission quenching is attributed to intramolecular photo-induced electron transfer occurring from PTZ to (C 60 /C 70 )* generating a radical ion pair, PTZ +⋅ -C 60 −⋅ /PTZ +⋅ -C 70 −⋅. 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 C 70 derivatives i.e., PTB7:C 70 -PTZ and PTB7:C 70 -PhOEt-PTZ exhibited better power conversion efficiency of 2.66% and 2.15%, respectively over C 60 derivatives i.e., PTB7:C 60 -PTZ and PTB7:C 60 -PhOEt-PTZ with the efficiencies of 1.80 and 1.72%, respectively. AFM studies revealed that the poor performance of PTB7:C 60 -PTZand PTB7:C 60 -PhOEt-PTZ-based devices can be ascribed to the lower solubility of the dyads in 1,2-DCB solvent leading to rough morphology.

Excited Charge Separation in a π-Interacting Phenothiazine–Zinc Porphyrin–Fullerene Donor–Acceptor Conjugate

“…40 In contrast, fullerene is a rigid threedimensional symmetrical-shaped molecule, requiring very small reorganization energy, which is a prerequisite for efficient electron transfer reactions. 41 Further, the relatively strong absorption in the UV−visible region and multistep reduction along with its high electron affinity make this unit an attractive candidate in material chemistry, 42 mainly in studying photoinduced electron transfer in donor−acceptor systems 43 and in photovoltaic devices. 44 Key findings on the newly made SPS-PPY-C 60 system are summarized below.…”