Theoretical Investigation On Excited State Properties Of Donor Materials In Organic Polymer Solar Cells

Using resource-rich solar energy to achieve photoelectric conversion is an effective way to solve the energy crisis and climate changes in current world. Organic solar cells have received considerable attention, as these materials show great promise for low-cost and easy to synthesize. On the one hand, the organic semiconductor materials(donor and acceptor) are important carriers of photoelectric conversion process. The structure of material determines the photoelectric conversion efficiency. On the other hand, the excited state is a key intermediate state of photoelectric conversion process. And the nature of the excited state is an important means to improve photovoltaic conversion efficiency of organic polymer solar cells. Over the past decade, organic solar cells with organic conjugated polymers as the donor materials have developed rapidly and achieved improved energy conversion efficiency. Charge transfer excitons have a lot of advantages, such as wide absorption spectrum, longer lifetime and smaller binding energy of excitons. Making full use of intramolecular charge transfer excitons to further improve the efficiency of organic solar cells has reliable theoretical and experimental basis. In this paper, we select D-A conjugated polymer systems in organic polymer solar cells as study object. We employ long range correction(LRC) functionals to investigate the variation of photoelectric properties in these systems with increasing oligomer length, such as geometries and electronic structures of ground and excited states, frontier molecular orbitals, energy gap and absorption spectra.This paper includes four parts: the first chapter is the preface, in which gives a brief introduce of the development history, basic principle, device structure, active layer materials of organic solar cells and effective ways to improve the energy conversion efficiency.The second chapter introduces the basic theory of quantum chemical methods and theoretical basis of the main parameters involved in this study, such as the molecular orbital theory, density functional theory and the long-range-corrected functionals with tuned range-separation parameter.The third chapter introduces the results of simulation calculation. In this work, we select polymers(PBDT-BT, PDTG-BT, PDTG-PT and PDTG-BTz) as target molecules. These polymers are both DA copolymers with alternating electron rich(BDT and DTG) and electron poor(BT, PT and BTz) moieties. We employ quantum chemical methods to investigate the photoelectric properties in these systems.(1) The DA oligomeric structures(with n=1-4 repeat units) are evaluated at the density functional theory(DFT) level with the global hybrid B3 LYP functional in conjunction with a 6-31G(d,p) basis set. We focus on the influence of the acceptor strength on the geometric, electronic and optical properties. The coupling between donor and acceptor and the electronic transition are both affected by the strength of donor and acceptor and degree of conjugation.(2) According to the results of calculation, the energy of the frontier orbitals evolves linearly with inverse chain length.(3) We discuss the energy levels of the systems from the perspective of intramolecular charge transfer state. We employ long-range-corrected functional to describe the excited-state properties of oligomers. We consider not only energies of the first ten singlet states but also the first fifteen triplet states as a comparison.The chapter four is conclusion.(1) DTG-based oligomers are very planar systems with longer characteristic lengths. In the four systems, the energy of the frontier orbitals evolves linearly with inverse chain length as with the trend in usual π-conjugated systems. The same donor with the strongest acceptor makes the energy gap of oligomer smaller. The ground-state geometric structures and degree of conjugation of the DA oligomers are influenced by the wave function characteristics and energy levels of the fragments.(2) The triplet states of PDTG-BT and PDTG-PT have almost no CT character and a large number of CT states in singlet excited states can be seen as effective precursors for efficient charge separation. This sort of excited state contribution can effectively decrease the lost of energies due to the intersystem crossing.(3) In the PDTG-PT system, the number of charge transfer states in singlet excited states increases with increasing oligomer length. The levels of charge transfer states range gradually from high energy levels to low levels. While the number of charge transfer states in triplet excited states decreases with increasing oligomer length. The levels of charge transfer states remain in the high energy levels. This can reduces the energy loss between the singlets and triplets due to the intersystem crossing. Therefore, when the molecular chain length approaches infinity polymer(higher molecular weight), the weakly bound CT states will move to the lowest singlet excited states to promote excitons dissociation in the heterojunction interface. Above theoretical results are consistent with the higher conversion efficiency in polymer PDTG-PT by experimental observations and provide theoretical guidance for the design of new high-efficiency organic polymer solar cells.