Theoretical Study On Photoelectric Properties Of Organic Functional Materials

With the environmental and energy issues becoming increasingly severe,the solar energy has received increasing attention as a clean energy source.Organic solar cells will play an extremely important role in third-generation solar cell devices.Organic materials have obvious advantages over inorganic materials in solar cells,but compared with inorganic solar cells,the power conversion efficiency of organic solar cells are still low.Solving the problem of low photoelectric conversion efficiency of organic semiconductors in essence is the key in solar cell research.There are two main ideas for improving the efficiency of organic solar cells:(1)Improve the absorption of sunlight and charge carrier mobility of donor and acceptor materials through chemical modification or physical doping of functional molecules(2)Optimize the property of the active dielectric layer and the structure of the interface to promotes exciton dissociation and inhibits electron-hole recombination.Singlet exci-ton fission is a concrete embodiment of the idea 1,the material absorbs one photon to produce two low-energy excitons,and then produces two electron-hole pairs,which theoretically can double the conversion efficiency.In fact,the main parameters that affect the performance of organic solar cells are the short-circuit current J.8C,the open-circuit voltage Voc,the fill factor FF,the photoelectric conversion efficiency PCE and the external quantum efficiency EQE,et al.The charge separation at the interface of the acceptor is one of the dominating part in the photoelectric conversion,and it is al-so the key to determine the efficiency of the photovoltaic device.This is a concrete embodiment of the idea 2.This article starts with these two ideas,combining(time-dependent)density func-tional((TD)-DFT)and time-dependent wavepacket diffusion(TDWPD)methods to in-vestigate the newly synthesized singlet exciton fission materials from a microscopic point of view.Through theoretical and computational studies,we give reasonable ex-planations to the new experimental phenomena,inspire new material designs,provide an understanding of the relationship between material functions and electronic struc-tures.The interface between fullerene and non-fullerene receptors was analyzed and the electronic processes at the interface were analyzed.We point out the possible fac-tors that affect PCE,and put forward feasible suggestions for improving efficiency.The structure of this article is as follows:The first chapter introduces the background of the study,the basic principles of organic solar cells and the principle of singlet exciton fission,we briefly introduce their history of development and research status,and proposes existing problems and possible solutions.The second chapter introduces basic knowledge of density functional theory and the Frenkel exciton model and the photon and electron-photon interaction included mixed Frenkel and charge transfer exciton model,and give the methods to calculate the key parameters in the Hamiltonian.The third chapter introduces the basic knowledge of time-dependent wavepacket diffusion method and the classical and quantum models used to calculate the rates of charge separation and charge recombination.In the fourth chapter,the carrier properties and exciton splitting dynamics of thio-phene derivatives were studied by combining the electronic structure calculations and system modified time-dependent wavepacket diffusion method,and the relationship be-tween the function and structure of the materials was explained.Chapter 5 introduces the dissociation of singlet and triplet exciton and the recombi-nation of electron-hole pairs at the interface between the donor and the acceptor.From the theoretical level,the reason why the singlet exciton fission phenomenon in the de-vice does not reach the expected efficiency is given.Chapter 6 briefly summarizes the content of this paper,and looks forward to the existing problems and future research directions.