Modifications Of Aggregation And Photovoltaic Performance Of Conjugated Polymer Donors Through Backbone And Side-chain Engineering

Organic solar cells（OSCs）,with unique advantages of high efficiency,low-cost,flexibility,and capability of roll-to-roll high-speed printing technique,have attracted considerable attention recently.From fullerene based acceptors（eg.PCBM）in the early stage to small molecule acceptors by now,great progress has been made for the efficiency of OSCs.Up to now,the highest efficiency reported has exceeded 18%.Polymer donors,as an important component in active layer,play an indispensable role in the developing process of OSCs.Efficient polymer donors mainly adopt D-A structure,which contains an electron-accepting unit and an electron-donating unit.D-A polymer donors can modify energy gap,energy level,and absorbance easily.Except backbone,side-chain engineering is also very important.It can impact crystallinity of polymers greatly by changing the types,sites,separation points of side-chains,leading to the morphology changing of active layer.This thesis has designed and synthesized a series of polymer donors by modifying backbone and side-chains.Focused on basic property and photovoltaic property,the results of the thesis can provide some guidance for the design and synthesis of new polymer donors.In the second chapter,side-chain-random PFBT4T copolymers PFBT4T-DT90-EH10and PFBT4T-DT80-EH20 were synthesized with DT as the long side chain in combination with partial 2-ethylhexyl（EH）as the short side chain.The introduction of short side chains increases the aggregation of the polymers.With PFBT4T-DT90-EH10 and PFBT4T-DT80-EH20 as the polymer donors,their photovoltaic performances in solvent additive-free processed 300 nm thick BHJ active layers were investigated.With PC₇₁BM as the acceptor,PFBT4T-DT90-EH10 and PFBT4T-DT80-EH20 showed good PCEs of 9.37%and 8.37%,respectively.The efficiency values are all higher than the reported PCE of 6.4%for PFBT4T-C₉C₁₃:PC₇₁BM active layer processed without a solvent additive.It should be noted that the stability of the two devices is great.Annealing at 85℃for 6 h,the efficiency of the PFBT4T-DT90-EH10 based device was elevated from initial 9.06%to 9.20%.Similar increasing was also found for the PFBT4T-DT80-EH20 based device.The results suggest that this side chain random strategy is promising for the design of new copolymers..In the third chapter,partial alkyl chains were replaced with benzene-terminal alkyl chains in PFBT4T based polymers.With benzene-terminal alkyl,the resulting polymers PFBT4T-C₄B-10 and PFBT4T-C₄B-20 can retain good photovoltaic performanceswith gradually changed solubility and aggregation ability at the same time.With PC₇₁BM as the acceptor,devices based on PFBT4T-C₄B-10 and PFBT4T-C₄B-20 showed efficiency of 8.55%and 6.77%,respectively.The lower performance of the PFBT4T-C₄B-20 based device can be attributed to the excessive crystallization in the active layer,which can lead to big domains and a low short-circuit current density(J_sc).The SCLC hole mobility of PFBT4T-C₄B-10:PC₇₁BM film is 4.40×10^-3 cm² V^-1 s^-1,indicating the big potential of PFBT4T-C₄B-10 as a polymer donor in thick active layer.In the fourth chapter,we synthesized PFBT4T-BDT and PFBT4T-TVT to introduce conjugated side-chains into PFBT4T copolymers.With PFBT4T-BDT as the polymer donor in combination with IDIC and PC₇₁BM as the acceptors,the ternary device shows a much better efficiency of 10.17%if compared with the 2.40%of the PFBT4T-BDT:PC₇₁BM binary system.It was found that IDIC could act as a morphology regulator in the ternary blends,not only restricting the aggregation of PC₇₁BM but also maintaining the crystallization of PFBT4T-BDT10.Polymer PFBT4T-TVT adopts side chain random copolymerization strategy to partially introduce thiophene vinyl as the substituents on thiophenes in the backbone.For the 420 nm thick active layer,the PFBT4T-TVT:IEICO-4F based device can achieve an efficiency of 10.33%without any solvent additive and thermal annealing.This is a rare efficiency for thick-film OSCs when using IEICO-4F as acceptor.In the fifth chapter,the influence of fluorination and chlorination on photovoltaic performances of NTD and BDT derived polymers were investigated.With IT-4F as the acceptor,devices based on NTD-BDT-F and NTD-BDT-Cl showed efficiency of 11.94%and7.48%,respectively.It was found that the NTD-BDT-Cl,with weak crystallization,displayed a much bigger lamellar stacking distance,which would decrease the charge mobility.Our study demonstrates that for a NTD and BDT based polymer,the fluorination and chlorination would show big difference,and the fluorination is more efficient in modifying aggregation and morphology for the resulting polymer in the IT-4F based active layers.In the sixth chapter,the influence of thiophene asπ-bridge in IDTV and BDD based polymers were investigated.With IDIC as the acceptor,OSC devices based on IDTV-BDD and IDTV-DTBDD showed efficiency of 2.73%and 1.62%,respectively.The hole and electron mobilities of the two blends are of the order of 10^-6 cm² V^-1 s^-1,which greatly limited the charge transporting ability.It was also observed that very rough surfaces and large domains existed in the IDTV-BDD and IDTV-DTBDD based active layers,which would influence J_sc and fill factor greatly,leading to the low photovoltaic performances.The results indicate that it is still a big challenge to introduce the IDTV unit with a large molecular skeleton in a polymer donor,and the introduction of the thiophene as theπ-bridge would further enhance the polymer aggregation in active layer,which results in poor device performance.