Investigations On The Molecular Design And Photovoltaic Performances For Donor/Acceptor Materials In Organic Solar Cells

As the most promising clean and renewable energy sources,organic solar cells takes an important role in human production and life because of its advantages of simple device,easy processing,light weight and low cost.In order to investigate the effects of materials on photoelectric conversion efficiency in active layers,we designed some new molecules(including small molecular acceptor materials and polymer donor materials).Based on the quantum chemistry,density functional theory(DFT)and time-dependent density functional theory(TD-DFT),the electrical and optical properties for three molecules were simulated and calculated.At the same time,the charge transfer properties were also simulated by the Marcus transfer theory.The main purpose of this paper is to qualitatively explain how to affect the molecular properties of the materials by modifying the molecular structures.Our works may provide an effective design strategy and reliable theoretical basis for the experimental synthesis.In Chapter 1,the development histories and current situations of organic solar cells were mainly introduced.What's more,we also provided the device structures and its working principle for organic solar cells,the main donor/acceptor materials in active layers as well as the meaning of our works.In Chapter 2,we expound the main theories for computation in this work,which include DFT,TD-DFT and Marcus theory.In addition,the main parameters which directly affected the photoelectric conversion efficiency(PCE)of organic solar cell were also introduced in this chapter.In Chapter 3,the goal of our work is focusing the following questions:(1)How the substituent groups affect the ground state geometry of polymers?(2)What are the substituent effects on the photovoltaic performances including electronic,optical and charge transfer properties? For elaborating these two problems,we divided this chapter in two parts.In Part 1,we introduced three heterocyclic units(furyl,thienyl and phenyl,respectively)into the TPD unit based on the reported polymer X0(X1,X2 and X3,respectively).According to the calculated results,X2 possesses the excellent electronic property,optical absorption and hole transport ability.Furthermore,the strong electron-withdrawing substituents of-OCH3,-F and-CN were also introduced on the thienyl unit based on X2(X2-1,X2-2 and X2-3,respectively)to further tune the electronic,optical and charge transport properties for D-A polymers in Part 2.Our calculation results demonstrated that it was an efficient strategy to improve the absorption,mobility and photovoltaic performance by incorporating the thienyl substituent and banding with the-CN(X2-3)as acceptor moiety into the D-A type polymers.In a word,we may present an effective way to modulate the A-units based on TPD in D-A polymers for developing the performances for donor materials in this chapter.In Chapter 4,the electronic structures and optical properties for all polymers were theoretically studied by DFT-BMK/6-31G* and TDDFT-TPSSH/6-311G* in dimer model in this chapter.According to the experiments,we can find that the PCE of PDPP2 TzDTP was higher than the value of PDPP2 TDTP.The main reason may be that it has a higher Voc.The calculated results indicated that PDPP2 TzDTP has a lower HOMO level but weaker absorption in near-infrared region,which is perfectly consistent with the experiment data.To make clear that the influence of different electronic-withdrawing groups were introduced into the bridges of Tz,we designed two molecules to further validate the experimental results and improve the performance of device.On the basis of theoretical calculation,both PDPP2TzDTP-Me and PDPP2TzDTP-F have excellent optical properties and charge mobilities.But the HOMO level of PDPP2TzDTP-Me increased 0.1 eV,which is not conducive to the improvement of Voc.In summary,the PDPP2TzDTP-F not only has an outstanding absorption and ameliorative charge mobility,but also has seldom impacts on HOMO level.Consequently,PDPP2TzDTP-F may be a promising donor material for organic solar cells,which may contribute to the device application in commercial.In Chapter 5,we have discussed the properties of electronic and optical absorption of NDI-based oligomers(NDI-2T1DCRD—NDI-2T4DCRD)and their performances in applications for organic solar cells by using quantum-chemical calculations and Marcus theory.As can be seen from the calculated results,increasing the electron-withdrawing ability of electron-deficient units not only extends the intra-molecular ? conjugation and yields a stronger and broader light absorption in the visible and near-infrared region,but also enhances the exciton separation efficiency at the donor/acceptor interface and improves the electron mobility.Particularly,the NDI-2T3 DCRD has a promising charge transfer rate(5.71×10-3 cm2V-1S-1)and can be effectively complementary with the light absorption of donor(P3HT)materials.We conclude that the NDI-2T3 DCRD may be a good candidate for acceptors materials in applications for organic solar cells.The results indicate that adjusting the electron-withdraw ability of terminal groups is a feasible approach to improve the electronic and optical performance in NDI-based type oligomers,and it presents us with a designed guideline for designing efficient oligomers acceptors for organic solar cells.