Synthesis And Application Of Organic Photovoltaic Materials With N Type Large π-conjugated Structure

In recent years,organic solar cells have attracted people’s extensive attention due to their advantages of the diversity of materials,the controllability of device processing,the light and flexibility of devices,and the processibility on large area substrate at low cost.The photoelectric conversion efficiency of the organic solar cell device based on the single heterojunction has exceeded 13%via collaboratively optimizing the materials,regulating the active layer topography and improving the device processing.However,in order to achieve the commercial application of organic solar cell,there are more in-depth systematic research and exploration need to be conduct including materials（such as active layer materials,interfacial materials and electrodes,etc）,processing and stability of devices and other aspects.The earlier research of organic solar cells was focused on the design of P type donor materials.Through the design and synthesis of donor materials,as well as the systematically study of the morphology and mechanism of the devices,the organic solar cells have achieved a series of success.Normally,compared with P type materials,the N type materials are more appropriate for acceptor materials and cathode interfacial materials due to their good electron mobility and affinity.However,N type materials are limited to fullerenes and their derivatives(such as PC₆₁BM and PC₇₁BM)for a long time,which is mainly attribute to their excellent electron mobility and isotropic charge transport properties.However,the fullerene materials have the obvious shortcomings,such as the poor absorption,the limited tenability of energy level,the morphological instability and the expensive cost.These factors have restricted the further development and application of organic solar cells.Therefore,it is meaningful to design organic solar cell materials with large N-typeπ-conjugated skeleton for the future development of organic solar cells.1.In the chapter 2,we designed and synthesized two alcohol/water-soluble amphiphilic organic small molecules that employed the anthrathiadiazole-4,11-dione as the N type conjugated backbone,and the pyridinium salt and quaternary ammonium salt ions as polar groups,namely PBATD and TBATD,respectively.Both molecules were applied as cathode interlayers（CILs）to conventional organic solar cell devices.Because of the strong electron affinity and low LUMO（lowest unoccupied molecular orbital）energy level of the conjugated backbone,both compounds could obviously enhance the performance of photovoltaic devices.In addition,by systematically comparing the performance of the devices modified by two CILs with different terminal ions,it is revealed that the TBATD ended with quaternary ammonium ions exhibited better interfacial modification ability.And the TBATD modified PCDTBT:PC₇₁BM based organic solar cell device obtained an open circuit voltage(V_oc)of 0.93 V and a power conversion efficiency（PCE）of 7.26%.These were relative high values for organic solar cells with the same active layer.It is obvious that both materials have performed outstandingly in improving the V_oc of the device.By combining the study of interface dipoles and work function（WF）of the modified cathode,we found that larger interface dipoles and lower WF are the main reasons for the higher V_oc of TBATD modified devices.Our work not only obtained two highly effective cathode interfacial materials,but also indicated that the performance of the cathode interfacial materials could be adjusted by logically regulating the terminal ions in the polar groups of CILs,which could provide meaningful support for the future design of organic solar cells with higher V_oc.2.In the chapter 3,the anthracene-heterocyclic skeleton was applied as the conjugated skeleton for the compounds with N type largeπ-conjugated structure either.By changing the type of heterocyclic ring in the conjugated center and adding the flexible side chain to the skeleton,three N type nonfullerene acceptors were obtained.We carried out the UV-Vis spectroscopy,TGA and electrochemical tests on the three acceptors.It was found that the acceptors performed good absorption in the visible light region,have good thermal stability and matched well with P3HT and PC₆₁BM in the energy level.Subsequently,they were applied as the ternaries to the P3HT:PC₆₁BM device.Due to the match of the ternaries with P3HT and PC₆₁BM in the energy level,the potential loss at the interface of donors and acceptors was reduced,leading to the increase of V_oc of the devices.By comparing the performance of the ternary devices,it was found that the ternary solar cell device with the APOSi-PDI molecule which had the indolopyrazine conjugated center with the flexible side chain exhibited the best performance.The AFM tests exhibited that the increased performance of the devices was mainly attribute to the improved crystallinity of P3HT due to the addition of APOSi-PDI.And the increased phase separation size between between the donor and accepter lead the improvement of the charge transfer and collection in the devices,resulting in the higher J_sc and FF,and an optimum power conversion efficiency of 4.11%.3.In the chapter 4,the N type largeπ-conjugated skeleton was designed by the well-known indacenodithiophene（IDT）.Subsequently,it was used as the conjugated center of cathode interlayers（CIL）for polymer solar cells（PSC）.The strong electron-withdrawing dicyanomethylene groups was introduced to the IDT backbone to endow the backbone with electron-deficient characteristic.Connecting the skeleton with quaternary ammonium ions terminated 1,2,3-trihexyloxy groups by the Suzuki synthesis method afforded a unique small molecular CIL with low LUMOs,namely,TBIDTCN.And a reference electrolyte without dicyanomethylene groups,namely TBIDTD was also synthesized.Compared to the TBIDTD,the LUMO levels of TBIDTCN were lowered down 0.2 eV due to the introduce of electron-withdrawing dicyanomethylene groups,and reached-3.96 eV,which was almost the same as the LUMO level of PC₇₁BM,indicating that the electron transfer at the cathode interface can be maximized.EPR（electron paramagnetic resonance）measurements revealed that dicyanomethylenated TBIDTCN shown a single intense resonance signal,but TBIDTD displayed no EPR signal,which indicated that the intrinsic charge transfer was existed in the TBIDTCN due to the further lower LUMO levels.Such intrinsic electron transfer could benefit the electron extraction and mobility of CILs.The application of two molecules as cathode interfacial materials in conventional PTB7:PC₇₁BM devices found that both CILs could significantly improve the device performance.Particularly,the dicyanomethylenated TBIDTCN modified devices performed a power conversion efficiency（PCE）of 9.19%,which was improved by nearly66%compared to the Al-only devices.Compared with TBIDTD,the dicyanomethylenated TBIDTCN has a lower LUMO energy level,which could further reduce the energy barrier at the cathode interface,and had better conductivity facilitating the extraction and transport of electron at the cathode interface.It was the first time that the IDT core was used as the conjugated backbone to construct cathode interfacial materials with N type large largeπconjugated structure.Our work indicated that the properties of the CILs could be effectively adjusted by modulating the LUMO level of the N type conjugated backbone.4.From the previous studies,it was found that even if the conjugated skeletons of the above described cathode interfacial material have high electron mobility,the excessive introduction of polar side chains to the skeleton would still lead to the decrease of conductivity of the materials.In the Chapter 5,the perylenediimides（PDI）unit with high electron affinity and good electron mobility was chosen as the conjugated backbone of the cathode modification materials with N-type largeπ-conjugated structure.Three N type amphiphilic organic small molecules were designed and synthesized by changing the number of amino-functionalized flexible side chains on the phenyl rings of the polar groups,namely SBTPDI,BBTPDI and TBTPDI,respectively.Afterwards,three molecules were applied to PTB7:PC₇₁BM devices to study their interfacial modification capabilities.Among them,the BBTPDI-modified PSC which has two amino-functionalized side chains in the polar groups achieved the highest PCE value（8.87%）.By systematically studying the device performance modified by different cathode interlayers（CIL）with various amounts of amino-functionalized side chains,it was found that increasing the number of amino-functionalized side chains in the polar group appropriately could effectively increase the interface dipole in the devices and further reduce the work function of the cathode,resulting in the enhancement of the modification ability of the cathode interfacial material.However,the excessive introduction of the amino-functionalized side chain would reduce the intrinsic conductivity of the materials,leading to the increase of the sensitivity of the device to the thickness of CILs.Our work revealed the effect of the concentration of amino-functionalized side chains on the interfacial modification capabilities of CILs,and figures out the suitable number of polar side chains for an excellent CIL system with an N type largeπconjugated backbone,which has important guiding significance for the future development of novel interfacial materials.In summary,we have designed and synthesized a series of N type compounds with largeπ-conjugated skeleton based on the polyphenylene heterocyclic skeletons.Firstly,the light absorption,electrochemical and thermodynamic properties were studied to explore the feasibility as photovoltaic materials.Subsequently,we applied these materials to organic photovoltaic devices and carefully studied their effects on device performance.These results provide valuable information for the design of N type organic photovoltaic materials in the future.