A Selenophene Analogue of PCDTBT: Selective Fine-Tuning of LUMO to Lower of the Bandgap for Efficient Polymer Solar Cells

Abstract

In an attempt to further improve the performance of the PCDTBT-based polymer solar cells (PSCs), we have synthesized a selenophene analogue of PCDTBT, namely, PCDSeBT, in which diselenienylbenzothiadiazole (DSeBT) monomer alternately flanks with a 2,7-carbazole unit. The intrinsic properties of PCDSeBT are not only characterized by UV–vis absorption, cyclic voltammetry (CV), and organic field-effect transistors (OFETs) but also the surface morphology, mobilities of space charge-limited current (SCLC) model, and polymer solar cells (PSCs) in its bulk-heterojunction (BHJ) active layer with [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) are evaluated in detail. It is found that PCDSeBT simultaneously has a low-lying highest occupied molecular orbital (HOMO) energy level at −5.4 eV and a low bandgap of 1.70 eV as required by the ideal polymers for BHJ PSCs. The high current of 11.7 mA/cm2 is obtained for PCDSeBT-based PSCs, to our knowledge, which is among the highest short-circuit current density (JSC) values obtained from a BHJ device consisting of PCDTBT derivatives and [6,6]-phenyl C61-butyric acid methyl ester (PCBM). The high JSC value, along with a moderate fill factor (FF) of 45% and a high open-circuit voltage (VOC) of 0.79 V, yields a power conversion efficiency (PCE) of 4.12%, which is about 37% increase in PCE from a PCDTBT-based reference device. On the basis of our results, one can be concluded that the DSeBT placement for construction of donor (D)-acceptor (A) polymers is an easy and effective way to realize both the higher JSC and VOC values in PSCs, as a consequence of the selective lower-lying lowest unoccupied molecular orbital (LUMO) with the HOMO being almost unchanged, together with the effective broadening on the absorption band.