Simple non-fullerene electron acceptors with unfused core for organic solar cells

Yao Li,Yunhua Xu*,Fan Yang,Xudong Jiang,Cheng Li,Shengyong You*,Weiwei Li,**[42_TD DIFF]

a Department of Chemistry,School of Science,Beijing Jiaotong University,Beijing 100044,China

b Beijing National Laboratory for Molecular Sciences,CASKey Laboratory of Organic Solids,Institute of Chemistry,Chinese Academy of Sciences,Beijing 100190,China

c Institute of Applied Chemistry,Jiangxi Academy of Sciences,Nanchang 330096,China

Keywords:Organic solar cells Non-fullerene electron acceptors Unfused core High crystallinity Low cost materials

ABSTRACT Tw o simple electron acceptors based on unfused bithiophene core and 1,1-dicyanomethylene-3-indanone end group were easily prepared via three synthetic steps.These acceptors exhibited broad absorption in the range of 300 nm to 800 nm,aligned energy levels and high crystallinity.When combined with a w ide band gap donor polymer in non-fullerene solar cells,an initial power conversion ef fi ciency of 2.4%was achieved.The relatively lowef fi ciencies were due to the large phase separation in blended thin fi lms,which is originated from their high aggregation tendency in thin fi lms.Our results suggest that these electron acceptors with unfused core are promising candidates for commercial application of solar cells due to the low cost starting materials and facile synthesis.

Non-fullerene organic solar cells(NFOSCs)have attracted tremendous attention in recent years[1–7]due to the huge amounts of non-fullerene acceptors developed in recent years[8–17].Among them,small molecules based on acceptor-donoracceptor(A-D-A)structures are w idely studied with high power conversion ef fi ciencies(PCEs)over 14%in single junction organic solar cells[18]and over 17%in tandem junction cells[19].These electron acceptors contain a fused donating backbone that is bene fi cial for charge delocalization,bulky aromatic side units to prevent aggregation and facilitate phase separation,and tw o electron-withdraw ing end groups in order to tune frontier energy levels.In addition,the pull-push–pull structure in these acceptors can induce intramolecular charge transfer so as to extend absorption spectrum to near-infrared region[20–22].All these merits make them to lead the research of NFOSCs.

However,these A-D-A type non-fullerene acceptors always have complex chemical structures so that multistep reactions are required to obtain the fi nal molecules.This w ill severely limit their large scale production and hence large-area devices for industry application.Obviously,it w ill be an important task to realize high performance non-fullerene acceptors with simple synthetic procedures.For these A-D-A type acceptors,an efficient route to reduce the synthetic steps is to use unfused conjugated backbone to replace fused backbone.For example,Chen and coworkers have developed a novel electron acceptor with an unfused core containing tw o cyclopentadithiophene(CPDT)moieties and one 2,5-di fl uorobenzene(DFB)group[23].Ahigh PCEof 10.14%in solar cells based on this acceptor could be obtained[24].Although this molecule still has complex synthetic stepsdue to the cyclo-core,its outstanding photovoltaic performance demonstrate that nonfused structures can also realize high performance in NFOSCs.

Herein,we intend to develop simple A-D-A electron acceptors with unfused cores for application in NFOSCs.We select 2,20-bithiophene(BT)as central core,in which tw o alkyl side chains,nhexyl(H)or ethylhexyl(EH),are used to enhance the solubility of the molecules.Side chains were attached to the 3 and 30position of BT units to produce appropriate dihedral angel,which is bene fi cial for creating a good phase separation in organic solar cells.Electronwithdraw ing end group 1,1-dicyanomethylene-3-indanone was used to tune the absorption and energy levels[25].The resulting molecules BTIC??H and BTIC-EH were found to show near-infrared absorption and aligned energy levels,and their initial application in NFOSCs was studied.Our results demonstrate that these simple unfused electron acceptors have the potential application in NFOSCs.

The synthetic routes of BTIC-H and BTIC-EH were show n in Scheme 1 and the detailed procedures were present in the Supporting information.In general,Starting from 2-bromo-3-alkylthiophene(1-H or 1-EH),the BTderivatives 2-H or 2-EH could be obtained,which were then converted into 3-H or 3-EH with CHO groups.BTIC-H and BTIC-EH were then synthesized via Knoevenagel condensation in the reasonable yield of 80%–90%.Therefore,tw o new non-fullerene acceptors were obtained via three simple synthetic steps,which is much improved compared to other fused and unfused acceptors[1,24].BTIC-H with hexyl side chains perform very poor solubility in chloroform and chlorobenzene(<2 mg/m L),while BTIC-EH with branched EH side chains show good solubility in these solvents(>20 mg/m L).

Absorption spectra of the new acceptors were present in Figs.1a and b.In chloroform solution,BTIC-H and BTIC-EH have identical absorption spectra with the peaks at 520 nm and absorption onset at 625 nm.In thin fi lms,BTIC-H has the peak at 638 nm with a shoulder at 706 nm and the onset at 825 nm,while BTIC-EH presents the peak at 573 nm and the onset at 757 nm.The significantly red-shifted absorption in thin fi lms indicate that both molecules have tw isted backbones in solution,while in thin fi lms they tend to form planar backbones.BTIC-H has the optical band gap(Eg)of 1.50 eV,which is much lower than BTIC-EH with Egof 1.64 eV.This can be attributed to the tw isted BT units caused by adjacent alkyl side chains.BT units with hexyl side chains show a dihedral angle of 47.2?,while dihedral angle can increase to 55.5?w hen using branched EH units(Figs.1c and d).The large dihedral angle indicates less crystallinity in thin fi lms,explaining relatively large Egof BTIC-EH.This can also be con fi rmed by their X-ray diffraction(XRD)patterns,as show n in Fig.1b.BTIC-H show s multiple diffraction peaks in thin fi lms,while BTIC-EHonly hasone diffraction peak around 2u=6?.From absorption and XRD measurement,we can conclude that although their short conjugated length,these simple unfused electron acceptors can still perform near-infrared absorption spectra that is comparable with other fused electron acceptors.

The frontier energy levels of the tw o acceptors in thin fi lmswere determined by cyclic voltammograms(CV)measurement(Fig.2),in which highest occupied molecular orbital(HOMO)levels of BTIC-H and BTIC-EHare?6.00 eVand?5.98 eV,and their lowest unoccupied molecular orbital(LUMO)levels are?3.85 eV and?3.82 eV.Therefore,the different side chains have little impact on their frontier energy levels.The w ide band gap polymer PBDB-T(Fig.S1 in Supporting inforamtion)that w ill be used as electron donor in this work show s HOMOand LUMOlevelsof?5.37 eVand?3.57 eVunder the same measurement condition[26].The energy different between donor and the BTIC-based acceptor is close to 0.3 eV that should be enough for exciton dissociation into free charge.

Schem e 1.The chemical structures of unfused electron acceptors BTIC-H and BTICEH and their synthetic procedures.(i)Ni(PPh3)2Cl2,Zn,KIand PPh3 in THF,re fl uxed overnight.(ii)TMEDA,n-BuLi,DMF,THF.(iii)Pyridine in chloroform,40?C,12 h.

Fig.1.(a)Optical absorption spectra in chloroform solution and in thin fi lms,(b)XRD patterns and(c,d)density functional theory calculations based on BTIC-H and BTIC-EH.The dihedral angle between tw o thiophenes is also included,47.2?for BTIC-H and 55.5?for BTIC-EH.

The tw o molecules aselectron acceptorswere then applied into NFOSCswith the donor polymer PBDB-T.An inverted con fi guration with ITO/Zn Oand Mo O3/Ag as electrode wasused.The photoactive layers based on PBDB-T:BTIC-H thin fi lms were solution-processed from high boiling point 1,1,2,2-tetrachloroethane(TCE)due to the poor solubility of BTIC-H,while PBDB-T:BTIC-EH thin fi lms can be easily fabricated from CB solution.We perform careful optimization of photoactive layers,including the amount of additive,the ratio of donor to acceptor and the thickness of active layers(Table S1 in Supporting information).It was found that 1,8-diiodooctane(DIO)as additive can provide the best performance.

PBDB-T:BTIC-H solar cells fabricated from TCE with 1%DIO showed the best PCEof 0.96%with a short-circuit current density(Jsc)of 2.68 m A/cm2,open-circuit voltage(Voc)of 0.69 V and fi ll factor(FF)of 0.52.BTIC-EH with branched side chains as electron acceptor provided the enhanced PCE of 2.4%with a Jscof 6.33 m A/cm2,Vocof 0.71 V and FFof 0.54.The improved PCEin BTIC-EH based cells is mainly due to the increased photocurrent,which can be further re fl ected by their external quantum ef fi ciencies(EQEs).As show n in Fig.3b,both cells have broad photoresponse from 300 nm to 800 nm,in which BTIC-H showed EQEs below 0.1 and BTIC-EH had EQEs close to 0.3.We also selected other conjugated polymers as donor and BTIC-EH as acceptor to fabricated solar cells,in which the PCEs up to 1.79%could be obtained(Fig.S2 and Table S2 in Supporting information).

Fig.2.Cyclic voltammogram of the electron acceptors(a)BTIC-H and(b)BTIC-EH.Potential vs.Fc/Fc+.The calculated HOMO and LUMO levels were also included.

Fig.3.(a)J-V characteristics in dark(dashed line)and under w hite light illumination(solid line).(b)EQE of the optimized solar cells.(c,d)AFM height images(3?3 m m 2)of PBDB-T:BTIC-H and PBDB-T:BTIC-EH based thin fi lms.

It is very interesting to observe that photoresponse in PBDB-T:BTIC-H cells is up to 800 nm(Fig.3b),which is inconsistent with the absorption spectra of BTIC-H fi lm with absorption over 800 nm(Fig.1a).This should be due to different crystalline behavior of BTIC-H in pure and thin fi lms.We speculate that the crystallinity of BTIC-H in blended thin fi lms w ill be hampered by the donor polymer PBDB-T,resulting in blue-shifted absorption.This can be con fi rmed by the absorption spectra of PBDB-T:BTIC-H thin fi lm(Fig.S3 in Supporting information)that is consistent with their EQE spectra in solar cells.

We further use atomic force microscopy(AFM)to study the microphase separation of BHJsurface,as show n in Figs.3c and d.PBDB-T:BTIC-H thin fi lms exhibit large domain size on the surface with a high roughness of RMS=11.50 nm,while the roughness in PBDB-T:BTIC-EH thin fi lms could reduce to 8.87 nm with decreased domain size.Since small domain size in BHJthin fi lm s can facilitate the exciton diffusion into the interface of donor and acceptor,thus responsible for the high photocurrent.It is w orthy mentioned that PCEs of BTIC-EH based cells are still much low compared to other high performance fused-ring based NFOSCs.From AFM images in Fig.3d,we infer that BTIC-EH still show s strong aggregation tendency in blended thin fi lms,resulting in relatively large domain size compared to other high performance solar cells.Therefore,it w ill be important to reduce the aggregation of these simple acceptors from chemical design and device optimization in order to improve the photovoltaic performance.For example,the crystallinity of these conjugated molecules can be reduced by using asymm etric structures or introducing furan moiety.

In conclusion,in this work,we successfully developed tw o simple electron acceptors based on bithiophene as unfused core,in which tw o side chains,hexyl and ethylhexyl units,were used to tune the crystalline properties.The tw o molecules show broad absorption in the range of 300 nm to 800 nm,aligned frontier energy levels and strong aggregation in thin fi lms.The tw o molecules as electron acceptor were applied into organic solar cells,in which BTIC-H with linear side chains only achieved a PCE of 0.96%.When using branched side chains in BTIC-EH,the corresponding solar cells showed enhanced PCEs up to 2.4%,which can be due to the better microphase separation from less crystalline BTIC-EH.Although the PCE is lag behind fused-ring based electron acceptor,these simple unfused electron acceptors with broad absorption and aligned energy levels demonstrate their great potential application in high performance NFOSCs.

Acknow ledgm ents

This study is jointly supported by MOST(No.2017YFA0204702)and the National Natural Science Foundation of China(Nos.51773207,21574138,51603209,91633301).This work was further supported by the Strategic Priority Research Program (No.XDB12030200)of the Chinese Academy of Sciences and the Recruitment Program of Global Youth Experts of China.

Appendix A.Supplem entary data

Supplementary material related to this article can be found,in the online version,at doi:https://doi.org/10.1016/j.cclet.2018.09.014.