Random copolymers containing tetrafluorophenylene unit with deep HOMO energy levels for solar cell applications

Abstract

Two Novel terpolymers poly{4,8-di(2-ethylhexyloxyl)benzo[1,2-b:4.5-b']dithiophene-3,6-bis-(thiophen-2-yl)-N,N'-bis(2-octyl-1-dodecyl)-1,4-dioxo-pyrrolo[3,4-c]pyrrole-co-2,6-ditetrafluorophenyl-4,8-di(2-ethylhexyloxyl)benzo[1,2-b:4.5-b']dithiophene} (PBDT-DPP-TFB) are designed and synthesized as donor materials in PSCs by incorporation of tetrafluorophenylene (TFB) unit with different mole ratio of 1% and 5%, namely PBDT-DPP-TFB1% and PBDT-DPP-TFB5%. It is found that through the introduction of small amount tetrafluorophenylene (TFB) into the BDT-DPP backbone, absorbance range, energy levels, the charge carrier mobility and the device performance are significantly manipulated and enhanced. Lower HOMO energy levels are emphasized for PBDT-DPP-TFB1% (−5.39eV) and PBDT-DPP-TFB5% (−5.43eV) compared to pristine PBDT-DPP without TFB unit, due to the strong electron-withdrawing feature of the TFB. Thanks to the more suitable energy levels, and a more favorable morphology with improved excitons dissociation and charge transport in the active layer, the inverted device based on PBDT-DPP-TFB with 1% TFB achieves a dramatic improved PCE of 2.9% with about 200% enhancement to the device with the pristine PBDT-DPP. This work demonstrates the potential of TFB in developing new organic materials for optoelectronic devices, as well as the variation of the third component can be another way to construct D-A polymers with favorable properties.