Theoretical Study On The Photoelectric Properties Of Organic Photovoltaic Materials

With the development of social economy,problems such as energy shortage and environmental pollution have become increasingly prominent.Organic photovoltaic materials have developed rapidly as new energy sources.How to further improve the photoelectric conversion efficiency of organic photovoltaic materials is the key to research in this field.The main factors affecting the photovoltaic performance of organic photovoltaic devices mainly include the energy level matching between the electrode and the active layer,the light absorption of the active layer,the carrier mobility,and the morphology of the device.Among them,the degree of energy level matching between the active layer materials has a major influence on the photoelectric performance of the battery.Improving the photoelectric properties of the active layer by the selection and regulation of the donor/acceptor materials is an effective way to improve the photoelectric conversion efficiency of the battery device.D-A copolymer molecules have strong design properties and are often used as active layer donor materials for organic fullerene polymer batteries.Theoretical and experimental results show that the photoelectric properties of polymer materials such as energy level structure and optical absorption can be controlled by designing the push-pull electronic structure of D and A units.Firstly,this paper analyzes the relationship between the energy level difference of experimentally high-efficiency organic copolymer photovoltaic materials and its photovoltaic performance,indicating that there is a correlation between the energy level difference between the active layer materials of organic polymer cells and various photovoltaic performance parameters of battery devices.When the energy level difference?E_LUMOUMO between the acceptor materials is in the range of 0.5 eV to0.7eV,the battery device is more likely to obtain high short-circuit current and photoelectric conversion efficiency.In addition,it has been found that the energy level difference between the donor/acceptor materials is not a limiting factor for charge separation.While the energy level difference?E_LUMO<0.3eV,the charge can still be effectively separated to form a photocurrent.The energy loss E_loss during charge transport has a large impact on its photovoltaic performance.The smaller the Eloss,the more it helps to improve the photoelectric conversion efficiency of the device.Secondly,in this paper,the D and A units of D-A polymer（PBDT-BT）are replaced by oxygen group elements respectively,that is,the oxygen group elements are respectively given to the X and Y positions on the donor and acceptor units of the D-Acopolymer.The replacement simulates the change in the electronic structure and optical properties of the polymer in two alternative ways.The relationship between the molecular structure and electronic structure of the copolymer material was analyzed by comparing its frontier molecular orbital,Bader charge transfer,optical absorption and other properties.The specific results are as follows:（1）The band gap values of the polymer before and after the replacement of the oxygen atom at the X position on the D unit decreases with the ordinal number of the oxygen atom,and the molecular frontier orbit changes significantly.When X=O,the energy band curvature is large,X=S,When Se,Te,the energy band curvature of the polymer is small.Moreover,the center C-C bond length and bond energy value of the polymer decrease in turn with the electronegativity of the replacement atom.（2）The charge transfer in the D-A copolymer was calculated by using the Bader charge analysis.The results show that the substitution of Se and Te atoms at the X site enhances the charge transfer between the D and A units.The substitution of the oxygen group element at the Y position of the A unit has little effect on the change of the molecular frontier orbital and the band gap value of the polymer.When Y=O,S,Se,Te,the charge transfer amount of the polymer is substantially unchanged.（3）The oxygen atom substitution at the X position on the D unit causes the blue light shift of the light absorption peak of the polymer as a whole,and the replacement of the Y position on the A unit causes the absorption peak at 600 nm to appear red shift,and the absorption peak at 300 nm does not change.Next,the paper studied the difference in photoelectric properties of the three copolymers（PNTz4T,PBTz4T and PDPP4T）copolymerized with the same donor unit tetrameric thiophene（P4T）and three different acceptor units（NTz,BTz,DPP）.The electronic structure,optical properties and intramolecular charge transfer of the three copolymers were obtained,and the photoelectric properties and ICT differences of the three copolymers were obtained.The calculation results are analyzed and discussed.The specific results are as follows:（1）The band gap values of the three polymers are similar,and the electronic states at the HOMO level of the three polymers are mainly derived from the contribution of C-2p orbital electrons on the D unit P4T,and the LUMO level is mainly on the A unit.The contribution of each atom to P orbital electrons.For the copolymer PDPP4T,the electrons on the A unit DPP contribute to both the HOMO of the copolymer and the electronic state at the LUMO level.（2）The theory of transition between semiconductor strips combined with the absorption spectrum of the polymer shows that the absorption peak 1 at 800 nm of the three copolymers mainly comes from the transition of D from the 2p orbital electrons from DOS1 to DOS3,at1500nm.The absorption peak 2 is mainly due to the transition of 2p orbital electrons from DOS1 to DOS2 on the A unit.（3）The analysis of the bader charge indicates that the C atom and the O atom connected to the C=O double bond on the DPP molecule play a strong electron loss and electron interaction respectively,that is,the bipolarity of the DPP molecule mainly comes from the lactam group.The contribution of the group.In this work,we explored the relationship between molecular structure and photoelectric properties of organic copolymer donor materials,and hope to provide a useful reference for the design and optimization of organic photovoltaic material active layer donor materials.