The Design And Synthesis Of Dicyanodistyrylbenzene-based Conjugated Polymers For Polymer Solar Cells

Organic/polymer solar cells based on bulk-heterojunction?BHJ?have many advantages,such as flexible,semi-transparent,all-solid,solution-processable,easy to prepare in large areas,which making them exhibited potential application in areas such as portable electronic products,color-changing windows,and building surfaces.Recently,polymer solar cells based on non-fullerene small molecular acceptor exhibited a power conversion efficiency?PCE?over 14%and remarkbale progress has been made.However,the current PCE is insufficient to realize the commercial application of polymer solar cells.The key to solve this situation is still develop high-performance active layer materials.The dicyanodistyrylbenzene?DCB?unit had been widely used in optoelectronic devices due to its strong electron-withdrawing properties,which could enhance intramolecular charge transport?ICT?.A series of novel polymer donors and acceptors based on DCB unit were designed and synthesized in this thesis.The relationship between molecular structure and device performance was systematically studied,which provided guidance for designing high-performance active layer materials.The two separated phenyl rings of qunioxaline?Q?unit connected by a single bond would significantly increase the polymer planarity and enhance molecular stacking,but it would diminish ICT properties of the polymer.In chapter 2,the strategy of enhancing ICT by utilizing strong electron-withdrawing properties of the cynao?-CN?functional group,a series of conjugated polymers composed of DCB-based and Q-based units were designed and synthesized.The effect of substituents on the material properties were systematically studied.It exhibited that the ICT peak of absorption spectrum of the copolymers were significantly enhanced,confirming that the purpose of the introduction of strong electron-withdrawing groups to enhance the ICT was achieved.Moreover,under the synergistic effect of cyano and fluorine atoms,the non-fullerene polymer solar cells?NF-PSCs?based on ITIC as acceptor exhibited open-circuit voltage(V_OC)of up to 1.04 V,which is the highest value to date for devices based on quinoxaline moiety copolymers.Meanwhile,NF-PSCs based on PDCB-DFQ812:ITIC exhibited a PCE of 8.37%.The-CN group on DCB unit contains“outer”and“inner”two substitution positions.The molecular planarity and stacking of polymer could be affected by changing the substitution position of the functional groups.In chapter 3,high-performance electron-deficient units 5,6-difluorobenzo[c][1,2,5]thiadiazole?DFBT?and naphtho[1,2-c:5,6-c?]bis[1,2,5]thiadiazole?NT?were selected to construct two series of copolymers?P-o-copolymers and P-i-copolymers?to discuss the effects of altering-CN substitution position on material properties.It showed that P-o-copolymers possesses more planar conjugated backbone,the blend films exhibited strong face-on stacking,while blend films based on P-i-copolymers were inclined to edge-on stacking.Combining these copolymers with small molecular acceptor?ITIC-4F?,an optimum PCE of 10%was achieved based on P-o-copolymer blend film.All polymer solar cells?all-PSCs?,possessed superior mechanical strength and flexibility,offer the commercialization opportunity of the PSCs for flexible and portable devices.Of these n-type polymers,naphthalene diimide?NDI?based copolymers are particularly promising candidates for use in all-PSCs.However,due to the high electron affinity of NDI,the lowest unoccupied molecular orbital?LUMO?of NDI-based polymers is often largely localized on the NDI units,which hinders efficient intermolecular electron transport.In chapter 4,the strategy of enhancing ICT by utilizing strong electron-withdrawing properties of the cynao?-CN?functional group.We designed and synthesised two copolymer acceptors based on DCB and the corresponding copolymer acceptors are denoted as PDCB-NDI812 and PDCB-NDI1014.By integrating these copolymers with a medium-band gap conjugated electron-donating polymer PBDB-T for investigation of the photovoltaic performance.The highest PCE of 4.26%was obtained for PBDB-T:PDCB-NDI812-based device.The polymer acceptor N2200 with high crystallinity resulting in large phase separation of the blend film is one of the reasons that limits the performance of the all-polymer solar cells.In chapter 5,we added the DCB unit as the third component,and prepared the linear N2200-derivaties to reduce its crystallinity.But the device performance is out of the desired result,which may be not formed better morphology.Then,we developed branched N2200-derivaties?NDIPh1 and NDIPh2?by incorporating 1,3,5-trisphenyl-benzene as the branched unit to reduce the crystallinity of N2200 and enhance the charge transport in the anisotropy.It showed that as the proportion of the third component increased,the intermolecular packing and crystallinity properties of the copolymers decreased and the miscibility between donor and acceptor improved.All-PSCs based on PTB7-Th:NDIPh1 presented an impressive PCE of6.87%,which obviously exceeded the 5.75%efficiency obtained from devices based on N2200.These observations indicate that the developed branched NDIPh copolymers can be promising n-type acceptors for the construction of high-performance all-PSCs.