Design,Sythesis And Photovoltaic Properties Of Novel Organic Photovoltaic Materials Based On Isatin And Its Derivatives

Active layer material of Polymer solar cells（PSCs）is the most important part of the whole PSCs device,which directly determines the level of power conversion efficiency（PCE）.However,the structure-activity relationship between the chemical structure of active layer materials and their absorption,energy level and photovoltaic performance is still not clear.In this paper,a series of polymer donor materials and small molecule acceptor materials were designed and synthesized with isatin as the building blocks,aiming at studying the internal relationship between molecular structure and optical absorption,molecular energy level and photovoltaic performance.The main research works of this paper are as follows:1.Two A-D1-D2 donor materials,PTEI-S and PSEI-T,were synthesized from isatin.They have the same unit A,except that D1 and D2are reversed in relative positions.The results showed that the introduction of heavy atomic selenium reduces both the optical band gap and the molar extinction coefficient,and the extinction coefficient of PSEI-T is the lowest.PCE of devices based on PTEI-S:PC₇₁BM is higher up to 4.28%,while PCE of devices based on PSEI-T:PC₇₁BM is 3.36%.Two A-D1-D2 structure polymers PSEI-BDTT and PbiSEI-BDTT were synthesized.The increase of the number of selenophenes is conducive to wider absorption and higher absorption efficiency,but at the same time,it will rise HOMO level and thus reduce V_oc.PCE of devices based on PSEI-BDTT:ITIC is higher of 5.40%,while PCE of devices based on PbiSEI-BDTT:ITIC is 4.54%.The experimental results show that the introduction of heavy sulfur atoms in A-D1-D2 polymers based on semi-isoindigo,the arrangement of the two donors and the length of selenophene donor have important effects on the optical absorption,molecular level and photovoltaic performance of the polymer.The narrow band gap polymer based on semi-isoindigo can be used in the preparation of nonfullerene polymer solar cells2.On the basis of the previous work,in order to obtain donor materials with wider absorption and narrower band gap,two polymers PTEI（CN）-T and PTEI（CN）-S were synthesized.The LUMO levels were significantly lower than that of the corresponding non-cyano-substituted polymer,resulting in a narrower optical band gap of 1.49 eV and 1.46 eV,respectively.In general,this research investigated the structure-performance relationship of polymers based on a new cyanosubstituted monomer,and demonstrated the potential of this unit as a building unit of conjugated polymers.3.Two small molecule acceptors of A-D-A structure,IM-IDT and TIM-IDT,were synthesized by introducing dicyano group into isatin and its derivatives.This is the first time that electron deficient units based on isatin derivatives were used as building blocks for non-fullerene small molecule acceptors.The two receptors showed wide absorption range and high molar extinction coefficient.PCE of devices based on PTB7-Th:ITIC is 1.96%.Due to the low LUMO level of IM-IDT andTIM-IDT,the V_oc of the device is low,only about 0.6 V.So it is hopeful to improve the PCE.This work shows that the isatin structural unit can be used as an electron acceptor in the design and synthesis of small molecule acceptor materials,providing more selectivity for the acceptor unit of A-D-A small molecule acceptor.4.Three non-fullerene small molecule acceptors IDNO-IDT,ITAN-IDT and IM1-IDT were synthesized from isatin.With the increase of the number of cyano-groups,the molecular electron-deficient increases,the intramolecular ICT is intensified,and the absorption gradually redshifted.The optical bandgaps of IDNO-IDT,ITAN-IDT and IM1-IDT are 1.95 eV,1.68 eV and 1.48 eV,respectively.PCE of devices based on PBDB-T:ITAN-IDT is 2.00%.It is worth noting that the V_oc of devices based on PBDB-T:ITANIDT after thermal annealing is as high as 1.18 V,higher than the V_occ of devices based on PBDB-T previously reported.The results showed that the number of cyano groups has an important effect on the optical,electrochemical and photovoltaic properties of the non-fullerene acceptors.It provides an important reference for the synthesis of acceptor materials with appropriate energy levels and spectral absorption.5.Fluorination was introduced into the design of small molecule acceptors,and three new small molecule acceptor materials ITANF0-IDTT,ITANF1-IDTT and ITANF2-IDTT were synthesized.The effects of fluorine substitution on molecular absorption spectra,energy levels and photovoltaic performance of devices were studied.The results showed that the absorption spectra of fluorine-substituted ITANF1-IDTT and ITANF2-IDTT were redshifted,the optical band gap was narrowed,and the HOMO/LUMO energy level decreased simultaneously,and the LUMO energy level decreased faster than that of fluorine-substituted ITANF0-IDTT.PCE based on PTB7-Th:ITANF2-IDTT device is up to 3.10%.The results indicated that fluorine substitution is an effective measure to regulate molecular energy level,improve light absorption and improve photovoltaic performance.