Manipulate Chemical Structures And Energy Levels Of Conjugated Polymer For Polymer Solar Cells

Polymer solar cells(PSCs)with light weight,easy fabrication,and capable of large-scale flexibility,have greatly attracted considerable attention in recent decades.The highest record power conversion efficiencies(PCE)of more than 13% for single-junction structure in the bulk heterojunction(BHJ)-polymer solar cells(PSCs)have been successfully achieved,which is possible to realize its application.This is mainly attributed to the huge optimization of the active layer by both the successful development of functional materials and the manufacturing techniques of the blend films.There is a wide range of improving the PCE,if more excellent polymers are desirable to be designed,or to improve the defect of the existing high performance polymers,such as the energy levels.Thus,in this work,based on the correlations between the molecular structure of polymers and the photovoltaic properties,we mainly centre on the synthesis of D-A conjucted polymers to optimizate the enengy levels and further improve the performance via constructing ternary polymer.Firstly,two novel terpolymers are designed and synthesized as donor materials in PSCs by incorporation of tetrafluorophenylene(TFB)unit with different mole ratio into the BDT-DPP backbone,namely PBDT-DPP-TFB1% and PBDT-DPP-TFB5%.It is found that absorbance range,energy levels,and the device performance are significantly manipulated.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.Thanks to the more suitable energy levels,and more favorable films,the inverted device based on PBDT-DPP-TFB achieves higher open-circuit voltage and a dramatic improved PCE of 2.9%.In addition,the dithienopyrrole(DTP),carbazole(CZ)and fluorine(FL)units with different electron donating ability chosen as the second donor unit(D2)are embedded into the traditional D-A backbone to prepare novel regular acceptor-donor1-acceptor-donor2(A-D1-A-D2)terpolymers,in which the energy levels and molecular geometry are well to be manipulated.Versus the D-A bipolymer PDPP-BDT,the HOMO levels can be effectively modulated by only varying D2 unit.More importantly,as the ability of the donor units increased from FL to DTP,gradually decreasing HOMO levels are obtained.Density functional theory(DFT)simulation and X-ray diffraction(XRD)measurements also reveal the effect of the D2 units on the molecular geometry of the terpolymers and their molecular packing.Therefore,choosing suitable donor groups to construct terpolymers is an effective and facile strategy to manipulate the energy levels and molecular geometry and finally to develop promising organic semiconducting materials with favorable properties.