Design And Synthesis Of Conformation Controlled Fused-ring Electron Acceptors And Their Photovoltaic Solar Cells

Bulk heterojunction organic solar cells（OSCs）based on non-fullerene acceptors have developed rapidly over the past few years.Nowadays,fused-ring electron acceptors（FREAs）featuring A-D-A（acceptor-donor-acceptor）structures have received more and more attention due to their significant advantages in terms of easy synthesis and purification.Meanwhile,FREAs based OSCs are facing some challenging issues to be resolved.For example:（1）The approaches for modulating the energy levels,absorption,band gap,solubility and crystallinity of FREAs need to be consolidated and perfected;（2）The poor device stability and high energy loss are still inevitable in OSCs;（3）The fabrication technology for commercial devices is not yet mature.Therefore,solving these problems through rational strategies from the aspects of material development,device optimization and interlayer modification should be the key to improve device performance in realizing commercial application.In this dissertation,the development of FREAs envolves along the main theme of conformation control in molecular design.The purpose of this thesis is to solve or alleviate the problems on OSCs by rational design of conformation controlled FREAs for high performance devices.Furthermore,the influence of conformation control on FREAs properties and their OSCs performance is systematically researched.First of all,the problems concerning conformation chaos of indacenodithieno[3,2-b]thiophene（IT）based FREAs（i.e.,ITIC）and slightly low open-circuit voltage(V_OC)of its devices are solved.We tuned the conformation of ITIC by introducing long alkyl sidechains on the end of IT core.The conformation is unified by exploiting the steric hindrance effect to limit the rotation of the single bonds between D-A.And the lowest unoccupied molecular orbital（LUMO）level can be upshifted owing to the weak electron-donating nature of alkyl chains,which is beneficial to the enhancement of V_OC.Moreover,the introduction of long alkyl chains can improve the solubility,reduce the crystallinity and enhance the light absorption,while blueshifts the absorption of ITIC.The device PCEs can be further improved to 11.61%with smaller energy loss.These results indicate that conformation control via alkyl chain engineering is an effective strategy for developing high performance FREAs.Next,in order to overcome the weak crystallinity resulting from side-chain and further improve the short circuit current(J_SC)of OSCs,we further conduct fluorination on end-capped groups of ITIC series FREAs to modulate the conformation.The crystallinity is indeed enhanced due to the formation of noncovalent intermolecular interactions.And the large electronegativity of fluorine can narrow the band gap,enhance the light absorption and strengthen the intramolecular charge transfer（ICT）effect.As a result,we can achieve enlarged phase separation in active layer,which contributes to the improvement of J_SC.In addition,the fluorine substitutions pull down the energy levels of FREAs,leading to the inevitable decrease of V_OC（0.90 V）.When coupling with suitable polymer donor to fabricate OSCs,the related devices achieve a high PCE of 14.47%with an enhenced J_SCof 21.64 m A·cm^-2.Our results demonstrate that the performance of FREAs based devices can be elevated by rational conformation control of FREAs.Furthermore,the conformation of ITIC-like FREAs is modified by introducing methyl or methoxy groups on the benzene of IT core to deal with the problem of V_OCreduction caused by fluorination.The noncovalent intramolecular interactions originated from methyl and methoxy groups can lock the conformation of IT core,and thus enchence the crystallinity of FREAs.The weak electron-donating capacity of methyl and methoxy groups is used to improve the LUMO level to achieve higher V_OC.The hyperconjugation and p-πconjugation of methyl and methoxy groups are used to reduce the band gap and enhance the absorption,so as to achieve the promotion of J_SC.It is also found that the introduction of methyl or methoxy groups could effectively enhance the miscibility of the aceptor and donor materials,resulting in a slight decrease in FF.In addition,the introduction of methyl or methoxy groups reduces the energy loss resulted from the recombination losses of radiation and non-radiation in the device.The research shows that conformation regulation by methyl and methoxy groups is one of the important strategies to design efficient FREAs and reduce the energy loss of devices.Finally,on the optimal results of conformation control on ITIC,the FREAs is further modified by replacing the benzene of IT core with other conjugated rings（such as1,4-dimethyl benzene,thieno[3,2-b]thiophene,etc.）.A series of novel conformation locked FREAs are designed,and the FREAs with the best performance is screened out.This strategy exerts a significant impact on the energy levels,light absorption intensity and band gap of the materials.The enhancement of the electron-donating capacity and the degree of conjugation of the central conjugate ring is beneficial to elevate the energy levels and reduce the band gaps.Moreover,the side chains at the end of the fused rings can generally reduce the crystallinity of FREAs and enhance the solubility,thus affecting the surface roughness and morphology of the active layer.The device performance of the material was characterized,and the PCEs of the devices are increased to 14.65%.Currently,a large number of researches reported in the literature mainly focus on the development of FREAs with new structures.This work mainly focuses on the conformation adjustment of traditional and classical FREAs structures,with the correlationship between structure and optoelectronic properties systematically studied.The above research shows that reasonable conformation regulation on existing classical FREAs is an effective strategy for the design of efficient FREAs.Meanwhile,this work is an important supplement to the development of efficient FREAs,which can provide significant guidance in further designing efficient FREAs for high-performance OSCs.