Taking the place of perylene diimide: perylene tetracarboxylic tetraester as a building block for polymeric acceptors to achieve higher open circuit voltage in all-polymer bulk heterojunction solar cells

Abstract

As strong electron acceptors, perylene diimides (PDI) have been widely utilized to prepare polymeric acceptors with an alternating donor–acceptor (D–A) structure for all-polymer solar cell applications, and which are used to provide low open circuit voltage (V_oc). Cyclic voltammetry measurements reveal that both the HOMO and LUMO levels of a structurally related analogue to PDI, perylene tetracarboxylic tetraester (PTTE), are 0.3 eV higher than those of the corresponding PDI. It was envisioned that replacing PDI by the weaker electron acceptor of PTTE in making polymeric acceptors with D–A structure could help to raise the LUMO level of the resultant polymers, which would lead to a higher V_oc when these PTTE based polymers are used as acceptors in all-polymer solar cell devices. In this work, a new conjugated alternating copolymer, poly([perylene tetracarboxylic tetra(2-hexyldecyl)ester-1,7-diyl]-alt-5,5′′-(2,2′:5′,2′′-terthiophene)) (PPTTE-TerT), was synthesized via Stille coupling reaction and characterized as an electron acceptor. The copolymer shows good solubility in common aliphatic organic solvents and good thermal stability. Differential scanning calorimetry (DSC) and grazing incident X-ray diffraction (GIXRD) measurements indicate that the copolymer at solid state is amorphous. PPTTE-TerT has a relatively broad absorption in the visible region from 400–650 nm with an optical band gap of 1.91 eV. The energy levels of HOMO and LUMO derived from the onset of the first oxidation and reduction potential of the cyclic voltammograms are at −5.6 and −3.54 eV, respectively. All-polymer solar cells of regioregular poly(3-hexylthiophene) (RR-P3HT) as the donor and PPTTE-TerT as the acceptor at an optimized donor–acceptor weight ratio of 1 : 0.7 achieved the best power conversion efficiency of 0.76%. The V_oc is 0.83 V, the short-circuit current (J_sc) is 2.38 mA cm⁻² and the fill factor (FF) is 0.34 under 1 sun (100 mW cm⁻²) AM1.5G solar illumination. The V_oc value is about 0.3 V higher than that of the all-polymer solar cell based on blending RR-P3HT with the acceptor copolymer comprised of alternating PDI-terthiophene units, which is consistent with the fact that the LUMO level of PTTE is 0.3 eV higher than that of PDI. These results indicate that PTTE is a promising electron-accepting unit to construct polymeric acceptors to be used in all-polymer BHJ solar cells.