Energy Level Tuning of Diketopyrrolopyrrole-based Conjugated Polymers for High Performance Polymer Solar Cells

Abstract

Diketopyrrolo[3,4-c]pyrrole (DPP) has emerged as a promising building block of low band-gap conjugated polymers for the optoelectronic applications including both organic field effect transistors (OFETs) and polymer solar cells (PSCs). Its highly conjugated structure leads to strong π-π interaction and the electron deficiency provides the DPP with the potential use of electron-accepting unit for synthesis of low band-gap conjugated polymers. We report novel low band-gap conjugated polymers, PDTTDPP composed of dithieno[3,2-b:2,3-d]thiophene (DTT) and DPP, and PBDTDPP composed of unsubstituted benzo[1,2-b:4,5-b]dithiophene (BDT) and DPP as well as small molecules composed of phenylene and DPP. We demonstrates that a novel low bandgap polymer (PDTTDPP), composed of electron-rich extended heteroarene unit (DTT) and electron-deficient unit (DPP), can be used as a p-type conjugated polymer for both OFETs and OPVs. The strong intermolecular π-π stacking of PDTTDPP contributes to achieve the high hole mobility of 0.68 cm2/Vs in OFETs without post treatment. The PDTTDPP also shows a low bandgap of 1.22 eV due to strong intra-molecular charge transfer between DTT and DPP. Consequently, the high hole mobility and broad light absorption of PDTTDPP achieves a power conversion efficiency (PCE) of 6.05% with Jsc = 13.9 mA/cm2, Voc = 0.66 V, and FF = 65.7%. This work clearly demonstrates that extended heteroarene unit (DTT) is a promising building block in the molecular design of new polymers for high performance organic electronics. PBDTDPP is another interesting polymer and expected to exhibit a low-lying HOMO level to afford high Voc, because alkoxy group is removed from alkoxy-substituted BDT. Alkoxy group has strong electron-donating power, and therefore it raises the HOMO level of D-A conjugated polymer. Since the Voc of PSCs is proportional to the difference between the HOMO level of donor and the LUMO level of acceptor, PBDTDPP with low-lying HOMO level due to removal of alkoxy group exhibits higher Voc while the conjugated polymer (PBDTDPP-OR) with alkoxy group exhibits lower Voc due to its high-lying HOMO level. Consequently, the removal of the alkoxy side group from alkoxy-substituted BDT leads to achieve a promising PCE of 5.16% with a high Voc of 0.82V. These values are much higher than those of PBDTDPP-OR (2.24% and 0.61 eV). A series of simple structured small molecules based on DPP were also synthesized and their photovoltaic properties were investigated in terms of the type of electron-donating unit. By introducing a donor unit with different electron-donating power, such as thiophene (T) and phenylene (Ph), the frontier orbital energy level of small molecules can effectively be tuned. The small molecule with a weak donor unit, Ph(TDPP)2, exhibited a PCE of 4.01% with a remarkabley high Voc of 0.93 eV when it was used as a donor of active layer material in bulk heterojunction solar cells.