Study On The Correlations Among Main/Side Chain Structure,Film Preparation Process And Photovoltaic Property Of Benzothiadiazole-based Polymers

Developing and exploring the solar cell technology,which can directly convert the green solar energy into the electric energy,has become one of the effective strategies to deal with the increasingly serious energy crisis and environmental pollution in modern society.Although silicon based solar cells currently share 70%of the total photovoltaic market,the inherent defects including complex processing technology,high material loss assisted by an expensive vacuum evaporation deposition method and large energy consumption in the production process limit the large-scale application.As an alternative,owing to the attractive advantages of light-weight,flexibility,solution-processibility to fabricate large-area devices by roll-to-roll printing technology,bulk-heterojunction polymer solar cells?PSCs?have become one of the hotspots in the field of solar cells.For promoting PSCs towards industrialization and benefiting mankind at an early date,pursuing the power conversion efficiency?PCE?of PSCs still is the core of research.Because the active layer is the important component of PSCs where the light-to-electricity conversion process occurs,the design and optimization of the electron donor materials,i.e.,conjugated polymers?CPs?,is particularly of great importance.In this dissertation,utilizing the low-cost and versatile benzo[c][1,2,5]thiadiazole?BT?,and naphtho[1,2-c:5,6-c']bis[1,2,5]thiadiazole?NT?which possesses the more outstanding optoelectric property,as the acceptor moieties,five donor-?-acceptor?D-?-A?type CPs systems are designed and constructed by changing the donors,conjugated?-bridges,flexible side chains and acceptor units.Assisted by the preparation optimization methods,including the choice of donor/acceptor?D/A?weight ratio,adding 3%?volume?1,8-diiodoctane?DIO?and using the PC₇₁BM replacing of PC₆₁BM etc,the film-forming ability,absorption,energy level,molecular configuration and hole mobility,aggregation,morphology of the active layer and photovoltaic?PV?performance of the device are investigated systematically.The relationship between the structure and property is also further explored.The key research contents are listed as following:1)A series of low band gap?LBG?D-?-A type CPs PT-DTNT-DT,PTT-DTNT-DT,Pff2T-DTNT-DT and PNTz4T,with the optical band gaps?E_g?of 1.48¹.59 eV,using the varied electron-rich oligothiophene units,such as thiohene?T?,thieno[3,2-b]thiophene?TT?,2,2?-bithiophene?2T?and 3,3?-difluoro-2,2?-bithiophene?ff2T?moieties,were prepared and applied in PSCs field to investigate the influence of the varied donor units on the PV performance.By optimizing the device preparation,the highest PCEs of the inverted devices reached up to 5.10%⁶.88%under the same experimental conditions.It was observed that phase separation of the active layers in these CPs was optimized after adding the DIO additive,thus the circuit current density(J_SC)and PCE of the corresponding devices were increased by19%¹02%and 23%⁷2%,respectively.2)Conjugated?-bridge,which not only linked D and A units on the polymer backbone,but also affected the degree of charge separation,was used as the channel of charge transfer.Changing the?-bridges,two random copolymers PIDTT-TBT,PIDTT-TFBT,and an alternated copolymer PIDTT-DTBT as the contrast,were prepared.And the influence of reducing conjugated?-bridge on the PV property was further investigated.PIDTT-TFBT has the medium aggregation in the solid film state.The PCEs of the optimal devices based on PIDTT-DTBT,PIDTT-TBT and PIDTT-TFBT were 2.05%,3.50%and 2.57%,respectively.The enhanced capability for capturing the sunlight,the deepened highest occupied molecular orbital energy level(E_HOMO)(thus an increased V_OC),the improved hole mobility and the better morphology of active layer were obtained,and thus the 70%enhanced PCE was achieved when reducing the conjugated thiophene?-bridge of the polymer backbone.3)The flexible side chains were not restricted to affect the solubility and film-forming ability,in fact it had a significant effect on the opto-electric property,aggregation and morphology of active layer.To investigate the influence of shape and size for side chains on the PV property,a class of LBG CPs PIDTT-DTNT-C16,PIDTT-DTNT-HD and PIDTT-DTNT-OD,with E_g of approximately 1.65 eV,were prepared.It was found that the flexible side chains had little influence on the absorption and energy levels of the CPs,however,there was great influence on extinction coefficient(?_film)and packing interaction of the solid film.The PCEs of the best PSCs based on them were low?1.02%¹.90%?.The poor device performance could be mainly ascribed to the rough surface and bad phase separation morphology in the active layer,which could severely limit the corresponding J_SC and fill factor?FF?.4)Assisted by the strong intramolecular charge transfer?ICT?effect,PBDT-TT-DTNT-HD and PBDT-TT-DTNT-OD containing the different alkyl chains?HD:2-hexyldecyl,OD:2-octyldodecyl?showed the strong absorption in the range of 300⁸20 nm,simultaneously possessed the lower E_g of 1.51 eV and deepened E_HOMO of–5.51 eV.It was observed that there were the larger red-shifted and the higher?_film of PBDT-TT-DTNT-OD than those of PBDT-TT-DTNT-HD ongoing from solution to the solid film state.Although the flexible side chains had little influence on the energy levels and aggregation in chlorobenzene solution,PBDT-TT-DTNT-OD was found to possess the better molecular ordering and?-?stacking interaction in solid state.And thus the corresponding PCEs of the best PV devices based on PBDT-TT-DTNT-HD/PC₇₁BM and PBDT-TT-DTNT-OD/PC₇₁BM reached up to 3.99%and 5.21%,respectively.The PCE was increased by 30.6%when the side chain increased from HD to OD,and this enhancement mainly benefited from the more ordered packing of the polymer chains in the solid state,the further enhanced mobility,the improved phase separation and thus improved fill factor of the corresponding device.5)A series of D-?-A type CPs,PDTBDT-TIPS-DTBT-OD,PDTBDT-TIPS-DTFBT-OD and PDTBDT-TIPS-DTNT-OD bearing the high tunability with regard to the absorbance?in the range of 300⁶50 nm and 300⁷50 nm?and energy level?from–5.35 eV to–5.46 eV?,were developed and applied in the PSCs to investigate the influence of varied acceptor moieties on the PV property.It was revealed that the aggregation of these varied acceptor CPs showed an order of PDTBDT-TIPS-DTNT-OD>PDTBDT-TIPS-DTFBT-OD>PDTBDT-TIPS-DTBT-OD.The PCEs of the best device were ranging from 1.09%to 3.37%.The PCE was decreased by 25.9%after adding F into BT,however,the PCE of the corresponding PSCs was increased by 1.29 times after using NT replacing of BT.The remarkable enhancement of PCE for NT-based device could mainly benefit from capturing more sunlight,the deepened E_HOMO,and the favorable morphology of the active layer which was good for exciton dissociation and charge transfer.This dissertation tends to enrich and perfect how the material structure including the donor unit,conjugated?-bridge,flexible side chain and acceptor moiety influence onto the preparation technology of active layer films and the photovoltaic performance of PSCs,and further provide the guidelines for the future design of donor materials and optimization of film preparation in PSCs field.