Synthesis And Photovoltaic Properties Of Non-fullerene Small Molecule Acceptor Materials Based On 4H-Cyclopenta[1,2-b:5,4-b']dithiophene

In recent years,organic solar cells(OSCs)have been developed rapidly and the corresponding photoelectric conversion efficiency(PCE)has been increasing.Bulk heterojunction mixtures of electron donor and electron acceptor materials are the key components of organic photovoltaic devices synthesized by solution processes.Over the past few decades,Conjugated polymers and fullerene derivatives have been the most commonly used electron donors and electron acceptors respectively.However,owing to the most of device performance improved by the design of new polymer donors.fullerenes and derivatives of them have always dominated the OSCs acceptor materials.Recently,non-fullerene acceptor materials,particularly small molecules and oligomers,have become a promising alternative to fullerene derivatives.Compared to fullerenes,these new acceptors typically prepared via a variety of synthetic routeswith the character of good chemical,thermal and optical stability,which are easily functionalized to provide excellent tunability for their photoelectron and electrochemical properties.In the past five years,as many non-fullerene receptor molecules were synthesized.the organic solar photoelectric conversion efficiency with non-fullerene as a receptor has been also increased dramatically from about 4% in 2013 to > 15% in 2018.its device efficiency also exceeds the fullerene system,making non-fullerene small molecule acceptor materials being attached more and more attention and research.This paper reviews the development history and research status of organic solar cells,the research progress of small molecule acceptor materials for bulk heterojunction(BHJ)solar cells,as well as the structure,principle and several main characterization parameters of BHJ solar cells.Based on this,a series of non-fullerene small molecule acceptor materials were designed and synthesized.The thermal stability,photophysical and electrochemical properties of these receptor materials were studied,and the relationship between the structure of non-fullerene small molecule receptor materials and photoelectric conversion efficiency was explored.Specific research work is as follows:(1)A new type of conjugated non-fullerene small molecular acceptor materials(CPDT,CPDT-Me,CPDT-4F and CPDT-4Cl)were designed and synthesized,with dimeric thiophene cyclopentadiene(CPDT)as the central electron donor unit(D),cyanoindene,methylated-cyanoindene,fluoro-cyanoindene and chloro-cyanoindene as the terminal electron-withdrawing group(A),respectively.The effects of terminal groups with different electron absorption abilities on the photophysical,electrochemical and photovoltaic properties of small molecules were studied in detail.Four kinds of small molecule receptors have high yield and simple synthesis.CPDT-4F and CPDT-4Cl have broad absorption spectra and red shift of absorption edge to 910 nm.This is due to the increase of electronegativity of terminal group substitution sites,which can effectively cause red shift of material absorption.After system optimization,PCE of organic solar cells with PBDB-T as donor,CPDT-4F and CPDT-4Cl as acceptor reached 9.47% and 9.28% respectively,and had higher short circuit current(Jsc).However,the open circuit voltage of the devices and the solubility of the materials are lower.In order to further improve the comprehensive performance of organic solar cells,we have improved the above two aspects from the molecular level,and synthesized the following two materials.However,the open circuit voltage and the solubility of the material are low.In order to further improve the overall performance of the organic solar cell,we have improved the above two aspects from the molecular level point of view,and synthesized the following two materials.(2)Combining the research contents of the previous chapter of this subject,two new non-fullerene small molecule receptor materials,CPDT-Me-2Eh and CPDT-4Cl-2Eh,were synthesized by improving CPDT-ME and CPDT-4Cl.The optimal efficiencies of devices based on CPDT-Me-2Eh and CPDT-4Cl-2Eh as acceptor and wide band gap PBDB-T as donor materials,respectively,were 4.39% and 7.39%.Imagine improving the solubility of the material and the open circuit voltage of the device by improving the side chain alkyl group of the D unit.The test results show that the open circuit voltage of the device based on the solubility of the two materials is expected to be improved,but the photoelectric efficiency of the device decreases.This is because the change of alkyl chain affects the accumulation of molecules and the morphology of membranes.