Electronic,optical,and charge transport properties of A-p-A electron acceptors for organic solar cells:Impact of anti-aromatic p structures

Yan Zeng,Ruihong Duan,Yuan Guo,Guangchao Han,Qingxu Li*,Yuanping Yi,*

a School of Science,Chongqing University of Posts and Telecommunications,Chongqing 400065,China

b CASKey Laboratory of Organic Solids,CASResearch/Education Center for Excellence in Molecular Sciences,Institute of Chemistry,Chinese Academy of Sciences,Beijing 100190,China

Key words:A-p-A electron acceptor Anti-aromatic structure Strong absorption Reorganization energy Organic solar cells

ABSTRACT Organic solar cells based on acceptor-p-acceptor(A-p-A)electron acceptors have attracted intensive attention due to their increasing and record power conversion ef fi ciencies.To date,almost all of the reported A-p-A electron acceptors are based on aromatic p structures.Here,we have investigated the impact of anti-aromatization of the p-bridges on the optoelectronic properties of A-p-A electron acceptors by(time-dependent)density functional theory.Our calculationsshow that besidesthe frontier molecular orbitals corresponding to the aromatic p-bridge based acceptors(“aromatic”acceptors),additional and unique occupied and unoccupied frontier orbitals are found for the acceptors based on the anti-aromatic p-bridges(“anti-aromatic”acceptors).Moreover,by tuning isomeric structures of the p-bridges(e.g.,fusion orientations or linking positions of thiophene moieties),the optical excitation energies for the transition between the additional occupied and unoccupied levels turn to be close to or substantially lower with respect to those for the transition between the “aromatic”frontier orbitals.The optical absorption of the “anti-aromatic”acceptorsisthus either stronger or broader than the “aromatic”acceptors.Finally,the reorganization energies for electron transport are tunable and dependent on the p-bridge structures.These results indicate a great potential of “anti-aromatic”electron acceptors in organic photovoltaics.

Organic solar cells(OSCs)are regarded as a potential photovoltaic technology to convert sunlight into electricity due to their advantages of fl exibility,light weight,large-area capability,and easy fabrication[1–6].Typically,the active layer of an organic solar cell is a bulk-or bilayer-heterojunction consisting of tw o components,an electron-donating and an electron-accepting material[7–12].Ow ing to the design of new active materials and optimization of processing conditions,great advances have achieved in the power conversion ef fi ciencies(PCEs)of OSCs[13–29].Over the past twenty years,fullerene derivatives(e.g.,PCBM and ICBA)were dominantly used as electron acceptors due to their superior electron af fi nity and high electron mobility[30–34].However,these acceptor materials have some intrinsic limitations,including weak optical absorption,untunable energy levels,and high costs[35,36].

In recent years,non-fullerene small-molecule acceptors have attracted increasing attention due to their strong and broad absorption and highly tunable electronic energy levels[35,37–39].A large number of electron acceptors were developed on the base of fused or non-fused aromatics.Most strikingly,the indacenodithiophene(IDT)-based A-p-A electron acceptors,such as ITIC and IEIC[40,41],have achieved remarkable breakthrough in organic photovoltaics(OPVs).To date,the most efficient binary non-fullerene OPV devices using IEICand ITICas electron acceptor have gained PCEs up to 10%and 13.1%[29,42],respectively.The PCEs can be further improved by fabricating ternary or tandem OPV devices[42–45].

Relative to aromatic systems,anti-aromatics are characteristic of smaller energy gap and deeper electron af fi nity[46–53],which is bene fi cial for broadening optical absorption and increasing electron-accepting ability.For instance,quinoidal indeno fl uorenes(IFs)based on the s-indacene anti-aromatic framework exhibit electron af fi nity higher than PCBM[54–56].What is more,IFs possess broad absorption with the absorption edge extending to the near infrared[57–60].In addition,good and ambipolar charge transport properties have been found in the IFs based fi eld-effect transistors[61–63].Despite of these desirable properties,very few work was reported on the OPVs based on “anti-aromatic”electron acceptors[64–66].

In this contribution,we have designed a series of A-p-A smallmolecule electron acceptors,which consist of anti-aromatic sindacene based p-bridges and the electron-withdraw ing groups of 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile(INCN).In order to assess the potential of the “anti-aromatic”acceptors in OPVs,we have calculated their electronic,optical,and charge transport properties in comparison with the corresponding“aromatic” electron acceptors by (time-dependent)density functional theory(the computational details see Supporting information).

The chemical structures of the anti-aromatic and aromatic fused p-units are show n in Scheme 1.All the aromatic polycyclic p-units(1-4 and 40)contain a core of 1,5-dihydro-s-indacene(in blue).When the core is fused with tw o thiophene or benzene moieties,we get units 1(indacenodithiophene,IDT)or 2(indacenodibenzene,IDB).Further fusion of tw o thiophenes onto units 1 or 2 w ill return units 3(indacenodithienothiophene,IDTT)or 4/40(indacenodibenzothiophene).Isomers 4 and 40are different in sulfur orientations of the fused thiophenes.Units 1(IDT)and 3(IDTT)have been reported as the backbone components of IEICand ITIC,respectively.Correspondingly,the anti-aromatic polycyclic p-units(1a-4a and 40a)are designed by replacing the aromatic 1,5-dihydro-s-indacene core with the anti-aromatic s-indacene(in red),as show n in Scheme 1.The bond lengths of the optimized geometries for the p-units are show n in Fig.S1(Supporting information).The bond length alternations are signi fi cant in the sindacene core.Overall,the C??Cbonds of benzene are longer in the anti-aromatic indacene than in the aromatic indacene;in contrast,the C??C bonds of cyclopentene are much shorter in the antiaromatic indacene than the aromatic indacene(Fig.S2 in Supporting information).

The calculated frontier orbital energy diagram and pictorial representation of these p-units are displayed in Fig.1.The HOMOs(centrosymmetric,g)and LUMOs(anti-centrosymmetric,u)of all the aromatic p-units are delocalized over the w hole backbones while the LUMO of 40show s relatively weak distribution on the lateral atoms of the fused thiophenes.When replacing thiophene with benzene fused onto the aromatic indencene core,the HOMO level w ill descend appreciably by 0.31eV but the LUMO level w ill ascend slightly by 0.11eV,leading to an increment of 0.42eVin the LUMO-HOMOenergy gap(Egap)from 1 to 2.Similar energy changes are found from 3 to 4,but more apparent from 3 to 40;particularly,the increase of the LUMOenergy from 4 to 40even reaches 0.24eV due to the relatively weakened delocalization.As expected,extension of the p conjugation from 1(2)to 3(4/40)results in lower LUMO and higher HOMO levels and smaller gaps.

Interestingly,the anti-aromatic p-units 1a-4a and 40a have tw o pairs of important frontier molecular orbitals.One pair of the frontier orbitals correspond to the HOMO and LUMO of the aromatic units 1-4 and 40,which we name as “aromatic”orbitals.Compared with 1-4 and 40,the “aromatic”unoccupied orbital energiesare significantly up-shifted while the“aromatic”occupied orbital energies are hardly changed for 1a-4a and 40a.The other pair of frontier orbitals of 1a-4a and 40a originate from the antiaromatic s-indacene,which are named as“anti-aromatic”orbitals.These “anti-aromatic”orbitals display a quinoidal p-conjugation character dictated by the signi fi cant bond length alternation in the s-indacene core.The “anti-aromatic”unoccupied orbital levels(centrosymmetric,g)are lower than the “aromatic”unoccupied orbitals by ca.1.3–2.0eV.As a result,the LUMO energies are substantially(at least 1.2eV)decreased for the anti-aromatic units relative to the aromatic units,indicating that introducing antiaromatic structurescan enhance electron-accepting abilitiesof the p-units.In contrast,the energy differences are much smaller between the “anti-aromatic”and “aromatic”occupied orbitals,ca.0.6–0.7eV for 2a,4a and 40a and even only 0.07eV and 0.16 eV for 1a and 3a,respectively.Both the occupied and unoccupied“antiaromatic”orbitals are mainly localized on the s-indacene core for 1a and 3a,but more extended for 2a,4a and 40a.Also,the electron distributions of the “anti-aromatic”orbitals can be modulated by the fused thiophene orientations;higher electron densities are found on the peripheral atoms of the fused thiophenes for 40a relative to 4a.

The UV–vis absorption spectra of all the p-units are show n in Fig.S3(Supporting information),and the electronic transition properties of the lowest excited singlet states(S1)are summarized in Table S1(Supporting information).For the aromatic p-units,only a single absorption peak appears in the UV region,corresponding to the S1excitation dominated by the HOMO!LUMO transition.Consistent with the ordering of Egap,the w avelength at the maximum absorption is red-shifted as follow s 2<1 and 40<4<3.For the anti-aromatic p-units,the absorption from the transition between the “aromatic” occupied and unoccupied frontier orbitals is significantly blue-shifted and much stronger due to larger energy gap with respect to the aromatic p-units.Besides,an additional weaker absorption appears in the visible region ow ing to the lower-energy transition between the“anti-aromatic”occupied and unoccupied frontier orbitals.It should be noticed that the S1excitation for the anti-aromatic units is dominated by the transition between the “aromatic”occupied orbital and “anti-aromatic”unoccupied orbital and is symmetry forbidden.To summarize,both isomerization and antiaromatization have important impact on the electronic and optical properties of the p-units.

The chemical structures of all the A-p-Aacceptor molecules are show n in Scheme 2.For the acceptors containing the short fusedring units(A1/A1a,A2/A2a,and A20/A20a),the p-units are extended by linking tw o additional thiophene moieties.The structure difference between A2 and A20or A2a and A20a is the linking positions on the fused-ring units.Most of those molecules show a completely fl at backbone(Figs.S4 and S5 in Supporting information).In the case of A2/A20and A2a/A20a,the fused-ring core and the thiophene moieties exhibit a tw isted angle of22.4?/26.6?for the aromatic molecules and[79_TD DIFF]22.7?/21.3?for the antiaromatic molecules.

Scheme 1.Chemical structures of the aromatic fused p-units(top)and corresponding anti-aromatic fused p-units(bottom).

Fig.1.Energy diagram and pictorial representation for the frontier orbitals of the aromatic 1-4/40 and anti-aromatic 1a-4a/40a p-units.

Fig.2 and Fig.S6(Supporting information)show the frontier molecular orbitals and corresponding energies for the acceptors based on the short and long fused units,respectively.As seen from Fig.S7(Supporting information),the HOMO/LUMO energies of INCN (?9.00eV/?1.65 eV)are substantially lower than the“aromatic”frontier levels of the p-units,especially for the HOMO.As a result,the HOMO(g-symmetry)of the “aromatic”acceptors is determined by the p-bridges.Consistent with the trends in the p-units,the HOMO levels of the acceptors are successively decreased as follow s:A1[83_TD DIFF]>A2>A20and A3>A4>A40(Fig.3).In contrast,the LUMO(u-symmetry)and LUMO +1(g-symmetry)of the “aromatic”acceptors are dominated by the tw o terminal INCN units.Compared with A1(IEIC)or A3(ITIC),the LUMO electron densities are more localized on the INCN groups for A2/A20or A4/A40,in particular for A20and A40.Because of the reduced electronic delocalization,the LUMOlevel isgradually up-shifted by nearly 0.3 eV(A1

Scheme 2.Chemical structures of the “aromatic”and “anti-aromatic”acceptors.

Fig.2.Frontier orbital energies and pictorial representation for the A-p-A acceptors based on(a)the aromatic(A1,A2 and A20)and(b)the anti-aromatic s-indacene cores(A1a,A2a,and A20a).

Compared to the “aromatic”acceptors,the “anti-aromatic”acceptors have one more anti-centrosymmetric occupied frontier orbital and one more centrosymmetric unoccupied frontier orbital(Fig.2 and Fig.S6),which are brought about by the antiaromaticity of the s-indacene core.The “aromatic”occupied frontier orbitals of the “anti-aromatic”acceptors are similar to the HOMOs of the “aromatic”acceptors;except A4a,the orbital energies are slightly higher(Fig.3).The “anti-aromatic”occupied frontier orbitals are localized on the fused-ring cores for A1a/A2a and A3a/A4a while extended to the INCN groups for A20a and A40a due to effective couplings of the fused-ring units with the rest moieties caused by the large electron densities on the linking atoms;the energy change trend appears to be opposite to the“aromatic”occupied frontier orbitals.Consequently,the HOMO is“aromatic”for A1a and A3a but“anti-aromatic”for A2a/A20a and A4a/A40a.The anti-centrosymmetric unoccupied frontier orbitals are similar to those of the “aromatic”acceptors while their energies are a bit lower.In the case of the centrosymmetric unoccupied frontier orbitals,the “anti-aromatic”orbitals are concentrated on the s-indacene core and isolated from the INCN-dominant ones for A1a and A3a;on the contrary,the“anti-aromatic”and INCN-dominant components are hybridized for A2a/A20a and A4a/A40a.Especially for A20a and A40a,large energy splitting(>0.6eV)is found between these tw o centrosymmetric unoccupied orbitals due to strong electronic interaction;hence the LUMO becomes to be of g-symmetry.The remarkable changes in the frontier molecular levels are expected to have profound influence on the optical absorption properties of the electron acceptors.

Fig.3.Frontier orbital energies of the A-p-A acceptors based on(a)the aromatic and(b)the anti-aromatic s-indacene cores(red:u-symmetry,black:g-symmetry).

The UV–vis absorption spectra of all the A-p-A acceptors are show n in Fig.4 and corresponding excitation properties are summarized in Table 1.All the acceptorshave similar and moderate absorption in the UV range of 250–350nm,which is composed of many high-energy excitations.Here we focus on the absorption spectra at the long wavelengths above 350nm.As seen from Fig.4a,the “aromatic”acceptors A1,A2,and A20exhibit a single absorption peak,which is attributed to the S1excitation.From A1 to A2 and A20,the contribution to the S1excitation is decreased for the HOMO!LUMO transition while increased for the other higher-energy transitions(Table 1);along with the enlarged energy gap(Fig.3),the absorption peak is then obviously blueshifted with decreased intensity.Similar trend in the absorption is found from A3 to A4 and A40.We noticed that the absorption of A40arises from both S1and S3excitations,which have close energies and consist of the same main transitions with different percentages.

Fig.4.UV–vis absorption spectra and oscillator strengths of the excited states for the A-p-A acceptors.

Because of additional“anti-aromatic”frontier molecular levels,more excitations can contribute to the absorption spectra of the“anti-aromatic”acceptors.For A1a,A2a,A3a,and A4a,the optically allowed transitionshave similar energy gaps between the occupied and unoccupied orbitals(Fig.3),so the corresponding excitation energies show small variation and are located near the S1excitation of the “aromatic”acceptors.Hence the pro files of the absorption spectra for these “anti-aromatic”acceptors are similar to the related “aromatic”acceptors.However,ow ing to the additional contributions of the excitations from the“antiaromatic”levels,the absorption intensities are enhanced.Interestingly,for A20a and A40a,the energy gap is much smaller for the“anti-aromatic”HOMO!LUMOtransition with respect to the other optically allowed transitions,arising from strong electronic coupling between the fused-ring units and the rest moieties for the “anti-aromatic”frontier orbitals.Consequently,besides the absorption at the w avelengths similar to A20and A40,an extra absorption appears at much longer w avelengths;thus A20a and A40a are hopeful to be a panchromatic sunlight absorber.

For organic solar cells,high charge carrier mobility w ill facilitate charge separation to improve short-circuit current,fi ll factor,and hence power conversion ef fi ciency.Reorganization energy is one of the key parameters to determine charge transport performance in organic semiconductors;small reorganization energy isbene fi cial for achieving high mobility.Here,we are interested in the reorganization energiesfor electron transport in the A-p-Aacceptors.The calculated results are show n in Fig.5.The reorganization energies of A1(IEIC)and A3(ITIC)are ca.0.14eV and 0.16 eV;these values are similar to those of fullerenes and tw ice smaller than those of perylenediimides[67].When the fused thiophene on s-indacene isreplaced by benzene,the reorganization energies are decreased for A2/A20and A4/A40.Moreover,the decrease dependson the linking positions or fusion orientations of thiophene onto indacenodibenzene,and the A20and A40acceptors exhibit the smallest reorganization energies of<0.1eV.As seen in Fig.S8(Supporting information),the reorganization energy decrease can be attributed to reduced variation of the bond lengths in the w hole backbone upon charge.

Compared withA1 and A3,the reorganization energies for the“anti-aromatic”counterparts(A1a and A3a)are about twice increased,reaching ca.0.3eV.This can be due to big changes of the bond lengths in the anti-aromatic s-indacene core(Fig.S9 in Supporting information)since the electron accepting LUMO/degenerate LUMO +1 is completely localized on the s-indacenecore for A1a/A3a(Fig.2 and Fig.S6).Relatively,the(undegenerate)LUMO is more extended for A20a and A40a,so the reorganization energies are slightly decreased.In the case of A2a and A4a,the LUMO or degenerate LUMO +1 is delocalized over the electronwithdraw ing INCN groups,leading to similar reorganization energies as the “aromatic”acceptors A2 and A4.

Table 1 Excitation wavelengths(l,nm),oscillator strengths(f),and main electronic transitions and corresponding weights for the excited states of the A-p-A acceptors.

Fig.5.Reorganization energies of electron transport for the A-p-A acceptors.

To summarize,we have theoretically studied the impact of p-bridge structures on optoelectronic properties of the A-p-A electron acceptors.In the case of the “aromatic”acceptors,incorporation of benzene instead of thiophene in the fused p-units w ill result in much deeper HOMO(dominated by the p-bridges)and slightly higher unoccupied frontier levels(dominated by the tw o terminal electron-withdraw ing groups);thus the long-w avelength absorption,corresponding to the HOMO!LUMO transition,is significantly blue-shifted.At the same time,the reorganization energy for electron transport is decreased,which w ould be bene fi cial for improving the electron mobility.The calculations show that these optoelectronic properties are also very dependent on isomerization of the p-bridges,such asmodification of linking or fusion modes of thiophene moieties.We underline that,antiaromatization of the p-bridges w ill lead to additional and unique frontier levels for the“anti-aromatic”acceptorscompared with the“aromatic”acceptors(Fig.3).The multiplication of unoccupied frontier levels w ould be bene fi cial for achieving ultrafast “hot”charge separation[68,69].Moreover,the optical excitation energy for the transition between these additional“anti-aromatic”levelsis in the proximity of or much smaller than that for the transition between the“aromatic”frontier levelsfor the different isomers.The optical absorption of the A-p-A acceptors is hence enhanced or broadened by anti-aromatization of the p-bridges.Our theoretical resultspoint out theimportance of p-bridgeengineering,especially isomerization and anti-aromatization on tuning the electronic,optical,and electron transport properties of A-p-A electron acceptors.This w ould be very useful for the development of newelectron acceptors for organic solar cells.

Acknow ledgments

This work was fi nancially supported by the National Natural Science Foundation of China(No.51773208),the Ministry of Science and Technology of China(No.2014CB643506),and the Strategic Priority Research Program of the Chinese Academy of Sciences(No.XDB12020200).

Appendix A.Supp lementary data

Supplementary materialrelated to thisarticlecan befound,in the online version,at doi:https://doi.org/10.1016/j.cclet.2018.05.029.