Synthesis And Photovoltaic Properties Of Conjugated Polymers Based On Thieno[3,2-b] Thiophene

Polymer solar cells(PSCs)with bulk heterojunction(BHJ)architectures have gained attention largely due to their low cost,lightweight,rich source of raw materials,simple device structure,solution processability and flexibility.Efficient PSCs materials should have the following characteristics:broad and strong absorption,suitable highest occupied molecular orbital(HOMO)and lowest unoccupied molecular orbital(LUMO)energy levels,good planarity and good solubility.As the basis of the device,it is important to design and synthesize the conjugated polymers with excellent properties to improve the performance of the devices.In this paper,a series of conjugated polymers with different structures based on thieno[3,2-b]thiophene units were synthesized by coupling reaction and then used in the research of PSCs devices.The main work is as follows:1.Two types of D-A alternating conjugated polymers based on alkylated thieno[3,2-b]thiophene and isoindigo were synthesized and named PBDT-TT-IID and PBDTT-TT-IID.These two polymers were blended with PC71BM to prepare the PSCs devices.The performance of the devices were optimized by using different solvents,different ratio of polymers and PC71BM,the kinds of additives and the thickness of the active layers,and finally the best efficiency of PBDT-TT-IID and PBDTT-TT-IID was 6.31%and 8.05%,respectively.This is first time that efficiency over 8%based on isoindigo conjugated polymers in conventional devices,and it also has the possibility of further improvement.The introduction of the thieno[3,2-b]thiophene unit makes the polymer backbone maintain a good linear planar structure,which enhances the interaction between molecules,and has a good effect on improving the carrier mobility and current density.Then,a series of polymers with different lengths or structures alkyl chain substituted were designed and synthesized,and the effects of alkyl chains on device performance were studied.The effects of alkyl chains on the energy levels,intermolecular interactions,and device properties were investigated and analyzed by UV-vis absorption spectra,cyclic voltammetry,atomic force microscope,and the performance of devices,which provides guidance for the structural design of the polymers based on isoindigo.2.Based on the concept of two-dimensional conjugation,we introduce thieno[3,2-b]thiophene containing different alkyl or alkylthio groups into the benzodithiophene units as conjugated side chains.A series of conjugated polymers were obtained by coupling with different acceptor units.Then we investigated the effect of the number of alkyl sulfur chains and the structure on the energy level and the performance of the devices.When the conjugated side chain changed from thiophene to thieno[3,2-b]thiophene,the planarity of the main chain decreased slightly,but the coplanarity between main chain and the side chain increased,which enhanced intermolecular interaction and hole mobility.Another two kinds of D-A conjugated polymers were synthesized by using BDTTS1 as donor,isoindigo or ffBT as acceptor.Due to the influence of the alkyl side chain,the HOMO energy levels of the molecules are reduced and the open circuit voltage of the devices is improved.3.Three kinds of conjugated polymers were synthesized by using alkoxy-substituted thieno[3,2-b]thiophene as ?-bridge,BDT as donor,and isoindigo,BT or ffBT as acceptor.The effects of alkoxy groups on the molecular energy levels and band gaps were studied preliminarily.When introduced the excess number of alkoxy groups,the electron-donating ability of the thieno[3,2-b]thiophene is remarkably improved,leading to the increase of the HOMO energy level and decrease the open-circuit voltage and chemical stability of polymers,thus reducing the photovoltaic performance.The morphology and phase state of active layers were characterized by AFM.The surface roughness was found to be large and not formed an ideal interpenetrating network structure.The improvement in the absorption of alkoxy group is not as good as the adverse effect on the HOMO energy level.4.A new acceptor structure was designed and synthesized by using thieno[3,2-b]thiophene as a core and benzothiadiazole as a flank.Coupling with carbazole-based donor,the polymer with a structure of D-AA was obtained.The LUMO energy level and the band gap of the polymers are reduced by increasing the density of the acceptor units on the conjugated backbone.Then we investigated the effect of the D-AA structure on energy level,band gap and device performance by characterization of polymers and PSCs.Compared with D-A structure,the D-AA structure reduces the LUMO energy level without changing the HOMO energy level,and improve the open circuit voltage and short circuit current simultaneously.With the application of D-AA structure,it is expected to obtain high performance polymer solar cell materials.