Design And Photovoltaic Performance Of Organic Donor Materials Containing Siloxane-Terminated Side Chain

Organic solar cells(OSCs)have attracted considerable attention due to their unique advantages of lightweight,flexibility,semi-transparency,and the promise of low-cost scalable printing fabrication.The most efficient type of OSC active layer is typically a blend made from two components,conjugated polymer donor(that transports the holes)and small molecule acceptor(that transports the electrons),which intimately mix in a so-called bulk heterojunction structure.Optimization of the bulk heterojunction structure through material and process engineering has resulted in OSCs with power conversion efficiency(PCE)over 18%,which shows the great vitality and prospect for this field.In the first chapter,the working principle of OSCs and the structure-property relationship of some important polymer and small molecule donors were described.In the second to the fifth chapter,the application of siloxane-terminated side chain in the preparation of new polymer and small molecule donor materials was studied.Some special aspects of the morphology regulation of the siloxane-terminated side chain modified donor materials were found,which would provide some guidance of the construction of the different types of high efficiency OSCs.The main contents are as follows:In Chapter 2,we introduced siloxane-terminated side chain into the low bandgap thieno[3,4-b]thiophene-benzodithiophene based polymer PTB7-Th,designed and synthesized both alternating(PTBSi100)and random copolymers(PTBSi20,PTBSi25,and PTBSi33)with different ratios of siloxane-terminated pentyl side chain to 2-ethylhexyl group on the BDT unit.It was found that with the increase in the content of the siloxane-terminated side chain,the corresponding polymers showed decreasing surface energy.The Flory-Huggins interaction parameter(?_D,A)of each polymer with non-fullerene acceptor IEICO-4F was assessed,and the miscibility between the polymer and IEICO-4F would vary in an order of PTB7-Th>PTBSi20>PTBSi25>PTBSi33>PTBSi100.In solar cells,three side chain random copolymers PTBSi20,PTBSi25,and PTBSi33 with more suitable phase separations that contribute PCE of 11.94%,12.61%,and 11.80%,respectively,all higher than that of PTB7-Th.The results indicate that the control of siloxane-terminated side chain content is very important to the morphology of active layer.This work provides a theoretical basis for how to make good use of siloxane-terminated side chain in bulk heterojunction active layer materials,and can also be used to explain the structure-property relationship in morphology control of many photovoltaic materials containing siloxane-terminated side chain.In Chapter 3,we systematically synthesized a series of wide bandgap polymer donors with3,5-,2,3-,and 2,6-difluorophenyl as conjugated side chains.Through DFT calculation and optical properties test,it is found that the dihedral angle between the side chain and the main chain is directly affected by the substituted position of the fluorine atoms,resulting in the difference of molecular aggregation and packing.Further characterization of the performance and active layer morphology of photovoltaic devices based on polymer:IT-4F indicates that the steric effect and the different miscibility between the polymer and acceptor which caused by the different fluorinated position on the difluorophenyl side chain have a great influence on the morphology of the blend film.As a result,the photovoltaic device based on the polymer 35EH with the smallest steric effect and median miscibility afford a high PCE of over 11%.Moreover,the morphology of the blend film was optimized by varying the alkyl chain to siloxane-terminated alkyl chain in the two-dimensional side chain.The highest PCE of 12.97%was achieved for the 35Si:IT-4F based devices.In Chapter 4,the symmetric and asymmetric two-dimensional side chain combinations of alkyl chain substituted fluorothiophene group,siloxane-terminated alkyl chain substituted fluorothiophene group,alkyl chain substituted difluorophenyl group,siloxane-terminated alkyl chain substituted difluorophenyl group on benzodithiophene unit were studied.A series of wide bandgap polymer donors(2FT-2EH,2FT-1EH-1Si,FT-1EH-FP-1Si,2FP-1EH-1Si,and 2FP-2Si)were designed and synthesized with difluoro-benzotriazole(ff BTA)as acceptor unit.Through gradual enhancing the electron withdrawing ability of the two-dimensional side chain,the open circuit voltage(V_OC)of the polymer:IT-4F based PSCs could be increased continuously.In addition,the symmetric siloxane terminated side chain modified polymer 2FP-2Si based devices exhibited the maximum power density,showing the effective suppressing of the trade-off between the V_OC and the short current density(J_SC)for the corresponding device.As a result,the PSCs based on 2FP-2Si:IT-4F achieved the highest PCE of 12.97%.Using Y6 as the acceptor,the 2FP-2Si:Y6 based device could afford a higher PCE of 14.40%.In Chapter 5,we designed and synthesized three new small molecule donors(SMDs)S35,S35-1Si and S35-2Si with the 3,5-difluorophenyl unit as the 2-dimensional side chain.By introducing different number of siloxane-terminated side chain into SMDs afforded the asymmetric S35-1Si and the symmetric S35-2Si,so as to compare the impact of the siloxane-terminated side chain on the optoelectronic properties of the SMDs.Compared with S35,the introduction of siloxane-terminated side chain could enhance the crystallinity of small molecules,leading to more orderly crystalline orientation and higher carrier mobilities in the S35-1Si and S35-2Si based blend films.As a result,the S35-2Si:Y6 based all-small-molecule organic solar cells(ASM-OSCs)exhibited the best PCE of 13.50%with a V_OC of 0.83 V,a J_SCof 23.59 m A/cm² and a FF of 68.8%in conventional device,and a PCE of 13.20%was also obtained in inverted device.This work revealed the influence of symmetric and asymmetric siloxane-terminated side chain on the stacking orientation of small molecule donors in the blend films,and proved the potential of siloxane-terminated side chains in construction of high efficiency SMDs for ASM-OSCs.