Design And Synthesis Of Novel N-Type Organic Semiconductor Materials And Their Applications In Polymer Solar Cells

Polymer solar cells(PSCs)have attracted extensive attention due to their outstanding advantages such as flexibility,light-weight,and solution processing to fabricate large-area devices.Recently,with the development of efficient materials and the optimization of devices,the power conversion efficiency of PSCs has achieved breakthrough.And the best reported PCE of PSCs has exceeded 18%.However,on the road of commercial application of PSCs,there are still many problems to be solved.It is an important challenge to further develop active layer materials and cathode interface materials with excellent performance.This dissertation focuses on the design and synthesis of novel n-type organic semiconductor materials and the research of their photovoltaic performance,which can be divided into the following two parts: one is the synthesis of water/alcohol-soluble polymers/small molecules and their cathode interface modification properties;the other is the synthesis of novel nonfullerene fused-ring small molecule acceptors and their photovoltaic performance.In Chapter 2,a series of conjugated polyelectrolytes with different electron-withdrawing unit contents were synthesized by ternary copolymerization.By adjusting the proportion of electron-withdrawing unit in the conjugated main chain,the problem caused by imbalance between the electron mobilities of the conjugated polyelectrolytes and their absorption in the visible light region,can be resolved effectively.Meanwhile,these conjugated polyelectrolytes with weak absorption and good interface modification properties can be obtained.This provides a new method to develop wide-bandgap cathode interface materials with high electron mobilities.In Chapter 3,three water/alcohol-soluble polymers were synthesized on the basis of the novel unit,which was constructed by connecting the aminofluorene unit at the imide position of the naphthalene diimide unit.With the increasing of the electron-withdrawing ability of the copolymerization unit,the absorption spectra and the lowest unoccupied molecular orbital(LUMO)energy level of the three polymers exhibit gradient change.By adjusting the copolymerization units,the absorption edge of the polymers can be tuned from 400 nm to 800 nm.Density functional theory calculations show that conjugation of polymers was partially interrupted,resulting in deteriorated planarity of the backbones and low electron mobilities.Therefore,three polymers cannot be used as thick-film cathode interface layers.In Chapter 4,a wide-bandgap small molecule HATNA-F3 N based on the hexaazatrinaphthylene(HATNA)unit with n-type property was synthesized.The highest occupied molecular orbital(HOMO)energy level of the small molecule was very deep,which is beneficial to block the holes.PSCs of the 9.23% PCE are achieved through applying HATNA-F3 N as cathode interface layer.Due to the large twist angle between the HATNA and the aminofluorene unit,the small molecule has poor planarity and conjugation performance so that its mobility is low.In Chapter 5,the nine-fused ring unit based on benzotriazole and benzothiadiazole was synthesized by Cadogen reaction,which was then copolymerized with water/alcohol-soluble aminofluorene to obtain four wide-bandgap conjugated polymers.These four polymers have almost the same absorption spectra and their optical bandgaps are about 2.0 e V.Compared with the device without cathode interface layer,the devices based on these polymers as cathode interface layers exhibit the enhanced PCEs by more than 53%.In Chapter 6,four non-fullerene small molecule acceptors were synthesized on the basis of the nine-fused ring discussed in Chapter 5.And their solubility,absorption and energy level were successfully fine-tuned through the side chain engineering and the regulation of terminal halogen atoms.The four small molecule acceptors are all possessed of narrow optical band gaps and deep energy levels,which are well matched with the absorption spectra and energy level of the donor material PBDB-T.PSCs based on PBDB-T and the four small molecules were fabricated and the best PCE of 11.10% can be achieved.