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

•Random polymers containing different ratio of tetrafluorophenylene (TFB) Unit on the backbone were prepared.•Two random polymers showed lower HOMO of −5.39 eV for PBDT-DPP-TFB1% and −5.43 eV for PBDT-DPP-TFB5%.•The introduction of small amount tetrafluorophenylene (TFB) into the BDT-DPP backbone, absorbance range, energy levels, the charge carrier mobility and the device performances are significantly manipulated and enhanced.

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% (−5.39 eV) and PBDT-DPP-TFB5% (−5.43 eV) 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.

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