| The photoelectric conversion process of organic photovoltaic cells mainly occurs in the active layer region,and the active layer material plays a major role in the photovoltaic performance of the device.In recent years,non-fullerene organic solar cells have attracted more and more attention due to their advantages of easily modified chemical structure,adjustable energy level structure and absorption spectrum,as well as relatively stable morphology.Optimizing the molecular structure of the materials in the active layer can effectively improve the photoelectric conversion efficiency of the battery.In this thesis,the effects of fluorination/chlorination and molecular configuration change on the photoelectrical properties of donor(D),acceptor(A)and donor/acceptor(D/A)complexes in the active layer were investigated based on first-principle density functional theory(DFT).In addition,according to the different combinations of donor and acceptor materials in the active layer,non-fullerene organic solar cells can be divided into the following four categories:all polymer(P_D/P_A)type,polymer D/small molecule A(P_D/M_A)type,small molecule D/polymer A(M_D/P_A)type and all small molecule(M_D/M_A)type.In this thesis,the polymer donor D18,polymer acceptor P(NDI2HD-T),small molecule donor DRTB-T and small molecule acceptor FDICTF-2Cl were used as examples to construct the above four types of D/A complexes.The main research contents are as follows:Firstly,this thesis has disscussed the effect of fluorination/chlorination on the photoelectric properties of polymer molecules J50 and N2200.The results reveal that the donor J50 and the acceptor N2200 all show direct band gap before and after fluorination/chlorination.After fluorination/chlorination of the donor,the slope of the density edge at the highest occupied molecular orbital(HOMO)level increases,and the electron localization improves;after fluorination/chlorination of the acceptor,the slope of the density edge at the HOMO level decreases and the electron delocalization enhances.According to Bader charge analysis,fluorination/chlorination in the donor J50 promotes the intra-molecular charge transfer,while fluorination/chlorination in the acceptor N2200 reduces the intra-molecular charge transfer;fluorination in donor J50 enhances the light absorption capacity of the molecule in the visible region,while fluorination/chlorination of the acceptor N2200 has little effect on the light absorption spectrum.Besides,the effect of N2200Cl molecule configuration change on electronic properties was studied(The corresponding molecular structures are denoted as N2200Cl and N2200Cl*according to the different positions of the NDI group and thiophene connection,which are shown in Figure 3.8),and found that it has little effect on the electronic structure,but has a great shift on the charge distribution within the molecule chain,and N2200Cl*has a more uniform charge distribution.At the same time,the variation of electronic properties of N2200 with the number of the thiophene ring had been studied,and found that the band gap increases and the intramolecular charge transfer decreases.Secondly,the influence of fluorination/chlorination on the electron structure and charge transfer of the D/A molecular pair J50/N2200 was analyzed.The results reveal that J50/N2200 pair has an indirect band gap whether or not fluorination/chlorination.In the six groups of D/A complex,the difference(ΔE1)of LUMO energy level between D and A,the difference(ΔE2)of HOMO energy level between D and A,and the difference(ΔE3)between the LUMO energy level of the A and the HOMO energy level of the D had been calculated,and the results prove that after fluorination/chlorination of donor J50,ΔE1 andΔE2 both decrease andΔE3 increases;after the substitution of fluorine/chlorine in the acceptor molecule N2200,ΔE1 andΔE2 both increase andΔE3 decreases.This suggests that donor J50fluorine/chlorination may be beneficial to the increase of open circuit voltage and the fluorination/chlorination of acceptor N2200 is beneficial to the separation of excitons.Furthermore,the fluorination/chlorination of donor promotes the intra-molecular charge transfer in D/A blend and increases molecular polarity;the fluorine/chlorine substitution in the acceptor promotes the inter-molecular charge transfer of D/A pairs,the molecular dipole electric field has been enhanced.The fluorination of donor J50enhances the light absorption of molecular pairs in visible region,while the chlorination of aeceptor N2200 weakens the light absorption of molecular pairs,and there is no significant difference among the other groups.Thirdly,in this thesis,the electronic structures of A-D-A type small molecule acceptors ITIC series(IDIC,ITIC,ITIC-4F,ITIC-6F),Y6 series(BTPT-4F,Y6,Y6-Cl)and SBDT series(SBDT,SBDT-F)were studied.The results show that the state densities at the bottom of the conduction band and the top of the valence band all show local peaks,and the electron localization is strong.When the number of thiophene rings increases,the band gap decreases,and the intra-molecular charge transfer increases,the light absorption of the central-symmetric small molecule is enhanced,while that of the axisymmetric small molecule is weakened,and the light absorption peaks are red shifted;as the number of terminal-group fluorine atoms increases,the band gap decreases,and the amount of intra-molecular charge transfer increases,as well as the light absorption peaks are red shifted;fluorination in Y6can promote intra-molecular charge transfer,and the amount is better than chlorination,by compared with chlorination,the light absorption peak of fluorination is blue shifted.Fluorination of the BT group of SBDT increases the band gap and promotes intra-molecular charge transfer,as well as the light absorption peak is blue shifted.Finally,four groups of D/A complexes were constructed through different combinations of polymer D18,P(NDI2HD-T)and small molecules DRTB-T and FDICTF-2Cl,whichwereD18/P(NDI2HD-T),D18/FDICTF-2Cl,DRTB-T/P(NDI2HD-T)and DRTB-T/FDICTF-2Cl,and the photoelectric properties were explored.P_D/P_Apair D18/P(NDI2HD-T)has the largest absolute value of average binding energy,stronger interlayer interaction and more stable structure.Both the polymer monomers D18 and P(NDI2HD-T)show direct band gap with curved energy bands at the LUMO level and HOMO level,the slope of the edge of the density band is smaller and the electron delocalization is higher.The state of small molecular monomers DRTB-T and FDICTF-2Cl densities show local peaks and reveal high electron localization.The band of P_D/P_Apair D18/P(NDI2HD-T)is flexural,while the band of M_D/M_Apair DRTB-T/FDICTF-2Cl is horizontal.P_D/M_Apair D18/FDICTF-2Cl has a curved top valence band and a flat bottom conductance band,while M_D/P_Apair DRTB-T/P(NDI2HD-T)is just the opposite,these show that in the band structure of D/A molecular pair,donor molecules mainly affect the molecular orbital at the top valence band,simultaneously the acceptor molecules mainly affect the molecular orbital at the bottom conductance band.The intra-molecular charge transfer of small molecules is larger than that of polymer molecules.The existence of small molecules to D/A complexes is beneficial to promote intra-molecular charge transfer.Meanwhile,the inter-molecular charge transfer effect in all-polymer molecular pairs is stronger.Compared to the other three complexes,P_D/M_Atype pair D18/FDICTF-2Cl has better absorption complementarity and light absorption property in visible region.(Note:In this thesis,“D/A”is used to represent the donor/acceptor molecular pair,also namely the donor/acceptor complex)... |
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