The Study For Aggregation And Photovoltaic Performance Of Conjugated Polymers Modified By Siloxane Terminated Side Chains

Renewable energy plays an important role in balancing the rapid economic development and the environmental concerns.Among many candidates,organic solar cell（OSC）has drawn much attentions due to their huge potential of large-scale application,including abundant material species,light weight,flexibility,solution processing and etc.In recent years,the flourishing advances of low-bandgap non-fullerene acceptors have boosted the power conversion efficiency（PCE）up to 18%.Therefore,it is of great importance to develop high-performance and low-cost wide-bandgap polymer donors.In this thesis,siloxane-terminated side chain regulators were chosen to design new wide-bandgap polymer donors with good cost-effectiveness,from which the structure-property relationship of siloxane-terminated side chains have been also investigated.In chapter 2,siloxane-terminated alkylthiol as a new combinatory side chain was developed to construct a new wide-bandgap polymer CY1.Interestingly,the polymer CY1exhibited preferential compatibility with non-fullerene acceptor rather than with fullerene acceptor,as revealed by Flory-Huggins interaction parameter.The PCE of the CY1:IDIC-based device was much higher than that of CY1:PC₇₁BM-based one,even with the assistant of solvent additive.The contrasting performances were mainly ascribed to the different morphology induced by siloxane-terminated combinatory side chain.Our preliminary results demonstrated that a suitable combinatory side chain should be valuable to tune the compatibility between polymer donor and non-fullerene acceptor and finally promote the photovoltaic performance.In chapter 3,a new combinatory side chain integrating siloxane terminal and alkoxy group was developed to decorate easy-synthesis quinoxaline-thiophene（Qx-Th）-based polymers.Thus,four polymers,PQSi500,PQSi505,PQSi510,and PQS5i25,with 0,5%,10%,and 25%contents of the siloxane-terminated alkoxy side chain,respectively,were successfully synthesized.As the content of the combinatory side chain increased,the surface energy of the corresponding polymer film decreased and aggregation tendency increased,showing tunable miscibility in blend films with non-fullerene acceptor IT-4F.A maximum PCE of 13.56%was achieved in the PQSi505:IT-4F-based device.The minor（5%）combinatory side chain approach retained a low synthetic complexity（SC）of 16.58%for PQSi505.Due to the improved device performance,a low figure-of-merit（FOM）of 1.22 was obtained for the PQSi505:IT-4F blend.Furthermore,the contribution of the IT-4F acceptor was also considered for a comprehensive analysis,yielding an average SC（ASC）of 39.31%and an average FOM（AFOM）of 2.90.After calculations and statistical analyses,the PQSi505:IT-4F was the most cost-effective active layer in the state-of-art.It was revealed that the introduction of the minor combinatory side chain is a promising strategy to develop high-performing and cost-effective polymer donors.In chapter 4,we have found that the intrinsic polymer orientation transition could be delicately controlled by the branching point position of the irregular alkoxy side chain in Qx-Th-based conjugated polymers.Three polymers with branching points at the third,fourth,and fifth positions away from the backbone were synthesized and abbreviated as PHT3,PHT4,and PHT5,respectively.Temperature-dependent absorption behaviors manifested that the polymer aggregation abilities followed the order of PHT3<PHT4<PHT5.Surprisingly,the polymer orientation transition from typical face-on to edge-on emerged between PHT4 and PHT5,as evidenced by X-ray-scattering analysis.The enhanced face-on crystallinity of PHT4 endowed the non-halogenated solvent,o-xylene processed PHT4:IT-4Cl-based devices with the highest PCE of 13.40%.For PHT5 with stronger aggregation,the resulting o-xylene-processed OSC still showed a good PCE of 12.66%.Our results demonstrated that a delicate control of polymer orientation could be realized through a precisely controlled strategy of the side chain,yielding green-solvent-processed high-performance OSC.In chapter 5,inspired by the works of chapter 3&4,siloxane-terminated alkoxy side chains with different branching positions were introduced into the easy-synthesis Qx-Th conjugated systems.Hence,three wide-bandgap polymers,PQSi505,PQSi705 and PQSi905 were obtained.It was demonstrated that shifting the branching position of siloxane-terminated alkoxy side chain has an impact on molecular aggregation and ordered packing.Among three polymers,PQSi705 owned the strongest aggregation and highest packing order.Notably,toluene processed PQSi705:Y6-BO-based device gained the best PCE of 15.77%.This work suggested that siloxane-terminated side chains could serve as a powerful modulator and optimize photovoltaic performance by tuning the branching positions.The results indicated that,the siloxane-terminated side chain is a powerful side chain regulator to fine-tune polymer aggregation and packing,even with minor decoration content as low as 5%.It was found that the unique far-branched branching position may be the key of the structure-property relationship of siloxane-terminated side chains.In addition,the surface energy change induced by siloxane group could be an effective method to control the morphology of active layers.