Efficient Organic Solar Cells Processed From Environmentally Friendly Solvents

In recent years,organic solar cells have attracted a wide range of attention,mainly due to their intrinsic advantages,such as solution-processability,flexibility,semitransparency,etc.The power conversion efficiency of single junction organic solar cells has exceeded 16%.Nevertheless,most of the high-performance organic solar cells are prepared from chlorinated solvents,which are harmful to human health and environment,making them unsuitable for large-scale fabrication of organic solar cells.In another side,organic solar cells processed from non-chlorinated solvents generally suffer from inferior performance.Therefore,developing efficient organic solar cells processed from environmentally friendly solvents are of great importance for their widespread applications.Fine-tuning the chemical structures of active layer materials has been proved to be effective in modulating their properties such as absorption,energy levels and charge mobility,and therefore improving the power conversion efficiency.Considering the effects of molecular structure on photovoltaic performance and solubility,this thesis presents a fine-tuning approach with 5-alkyl-4-(methylthio)thiophene(MTT)as a conjugated side chain to develop polymeric donors for efficient polymer solar cells processed from non-chlorinated solvents.The derived polymer PMTT56 with MTT unit exhibits optimized performance in solubility,interchain interaction and energy levels.Thus,the PMTT56 devices deliver power conversion efficiency up to 12.6%.The contents of this thesis include:In the first section,fundamentals of organic solar cells are briefly introduced.Then device optimizing methods and recent progress of organic solar cells,as well as applications are reviewed.Based on the aforementioned discussions,the purpose and contents of this thesis are outlined.In the second section,MTT functionalized polymer PMTT56,along with the control polymers PMOT39 and PEHTT,are systematically characterized.Compared with PEHTT,PMTT56 and PMOT39 based on the 4-substitution of thiophene exhibit higher solubility in toluene.PMTT56,PMOT39 and PEHTT exhibit similar absorption spectra and wide band-gaps of around 1.87 eV.Additionally,PMTT56 presents stronger interchain interaction and higher mobility than PMOT39.Due to the hyperconjugation effect and large electron affinity of S-atom,PMTT56 exhibits deeper energy levels than those of PMOT39 and PEHTT.Theoretical simulation and calculation further confirm the deeper HOMO level of PMTT56,and also reveal the localized excitation characteristics of PMTT56,which explains the higher molar extinction coefficient of PMTT56 than those of PMOT39 and PEHTT.In the third section,organic solar cells based on the polymeric donors and small molecular acceptor IT-2F are fabricated and characterized.The device performance is optimized through adjusting the parameters including solvents,device configurations,materials concentrations and ratio,solvent additives and so on by spin-coating method.PMTT56 presents much better performance in the toluene-processed devices than the chlorobenzene-processed devices.The PMTT56 devices deliver power conversion efficiencies up to 12.6%with 8 mg ml~-11 donor concentration,1:1 donor/acceptor ratio and 1%DPE,superior to those of PMOT39(10.5%)and PEHTT(10.8%).The PMTT56 devices are further fabricated by blade-coating method.The devices exhibit similar performance(11.6%)with the spin-coated devices,and also present good reproducibility.The high-performance of the PMTT56 devices from both spin-coating and blade-coating methods indicates its potential in environmentally friendly solvent processed organic solar cells.MTT unit is an effective fine-tuning modification for constructing efficient organic solar cells.