Study On Charge-Transfer States And Open-Circuit Voltage Loss In Non-Fullerene Organic Solar Cells

For organic solar cells,the charge generation mechanism and the recombination loss is heavily linked with charge transfer states(CTS).However,in the photovoltaic external quantum efficiency(EQEPV)spectra of low-driving-force non-fullerene(NFA)organic solar cells,it is difficult to obtain the energy of CTS(ECT)by fitting EQEPV through Marcus theory due to the absence of absorption signal of CTS.Faced with the key scientific issue that ECT cannot be reliably characterized,the development of a novel spectroscopic technique to characterizing ECT of non-fullerene heterojunctions with low driving force will be beneficial for further understanding of exciton dynamics,which is also crucial for suppressing energy loss and improving device performance.The authors have done the following research:(1)Reliable characterization of the energy of charge transfer states.In order to solve the difficulty of ECT measurement of NFA organic solar cells with low driving force,the authors proposed a measurement scheme based on electroabsorption spectroscopy.By fitting the external quantum efficiency spectra of the devices by Marcus theory,the authors investigated the CTS absorption characteristics of NFA organic solar cells and summarize the reasons for the failure of this characterization method;By fitting the electro-absorption spectra of the donor and acceptor materials,it is demonstrated that the excellent performance of PM6:Y6(2,20-((2Z,20Z)-((12,13-bis(2ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2,"30’:4’,50]thieno[20,30:4,5]pyrrolo[3,2-g]thieno[20,30:4,5]thieno[3,2-b] indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1diylid-ene))dimalononitrile)devices benefits from the good dipole characteristics and lower exciton binding energy(EB)of Y6;The authors studied the relationship between CTS and subbandgap signal of electroabsorption spectra.By fitting the subbandgap signal,the ECT of PM6:Y6(1:1.2)device was determined to be 1.27 eV;The electroabsorption spectra of bulk heterojunctions(BHJ)with different donor/acceptor weight ratios indicated that electroabsorption measurements can sensitively detect the changes of CTS caused by different heterojunction morphology.These results provide a new method to study the issues of ECT and photovoltage loss in NFA organic solar cells.(2)Research on open-circuit voltage loss.Driving force and open-circuit voltage loss of PM6:Y6 heterojunction obtained from electroabsorption spectroscopy were verified by using ultraviolet photoelectron spectroscopy,transient photovoltage and other measurement methods;By analyzing the relationship between the dipole moment changes of the acceptor materials and the driving force in three low-driving-force NFA heterojunctions,the authors proposed that the driving force is inevitable in NFA organic solar cells.How to reduce nonradiative recombination is the most effective way to reduce open-circuit voltage loss.The results have guiding significance for the design of organic photovoltaic materials.