Alleviating non-radiative losses in organic solar cells through side-chain regulation of low-bandgap non-fullerene acceptors

Domain: physics
Published: 2026-01-10
Editorial outcome: minor_revision

Abstract
Non-radiative losses present a significant hurdle limiting the performance of organic solar cells (OSCs). Selenium substitution in non-fullerene acceptors (NFAs) is an effective approach for tuning intermolecular stacking and energy levels in state-of-the-art near-infrared absorbing OSCs. However, the inter-system crossing (ISC) induced by selenium may enhance the formation of triplet excitons in NFAs, aggravating non-radiative losses in OSCs. Herein, we have structurally engineered selenium-containing NFAs through introducing achiral N-alkyl substituents with varied branching sites at C2 or C3 position onto the pyrrole moiety of NFA core. We find that the C2-branched ones in our material system exhibit more intimate molecular packing, improved luminescence efficiency and reduced triplet exciton generation in both neat and blend films, reflected as improved external quantum efficiencies and open-circuit voltages in higher-performance devices. We utilize the best-performing NFA as a ternary component into the benchmark D18:L8-BO absorber to realize a high efficiency of 20.4% (certified as 19.88%). This work shows how precisely regulating the microstructure of NFAs by side-chain modification can reduce the non-radiative losses of OSCs.

Introduction
Organicsolarcells(OSCs)exhibitdistinctadvantagesoverotherphotovoltaic technologies, attributed to the facilely tunable bandgap of photovoltaic materials, solution processability, and potential applicationinflexibledevice1–5.Inthepastfewyears,therapidprogressioninOSCshasbeenprimarilydrivenbytheadvancementofnarrow-bandgapnon-fullereneacceptors(NFAs),whichenableefficientconversionofnear-infrared (NIR) photons to electricity6–11. Recently, OSCs have achievedapowerconversionefficiency(PCE)exceeding20%,renderingthem viable candidates for commercial applications12–16. However, a significant hurdle impeding OSC performance is the substantial non-radiativeenergylosses17–20.TofurtherpromotetheOSCperformance,rigidmoleculesthatcanformaclosely-packedandorderedstructure21,22 have been pursued to mitigate the vibration of environmental molecules and suppress the electron–phonon coupling23–25, rendering the pathways of non-radiative relaxations less accessible. Moreover, the generatedlowest-energytripletexcitons(T₁)inOSCshavebeenwidelyreported to mediate non-radiative relaxations26–28. Efforts have been madeformitigatingT formation,includingmodulationofinterfacialsinglet-tripletenergeticsandengineeringthedegreeofdonor-acceptormixingsinOSCs29–31.

Inthedesignoforganicphotovoltaicmaterials,theincorporationofSeisanefficientapproachtotunetheenergygapandmolecularinteractions32–35.SinceSeismorepolarizable,itsloneelectronpairsenableastrongintermolecularSe-Seinteraction,whichpromotestheformationofcharge-transportingnetworksbetweenmolecules36,37.SeveralstudiesonintroducingSeintoNFAshaverevealedtheadvantages of red-shifted photoresponse, closer molecular packing, and reduced energetic disorder38–42. These not only help realize high photocurrent, but also minimize the photovoltage-photocurrent trade-off in narrow-bandgap OSCs. Moreover, the near-infrared absorptionofSe-containingNFAsrendersthemhighlypromisingforapplicationintandemsolarcells43.However,theincorporationofSeatommaypromotespin-orbitalcouplings(SOCs)intheNFAsandthusenhance the formation of T excitons from singlet (S) excitons throughinter-systemcrossing(ISC),openingupapathwaythatcouldleadtoadditionalnon-radiativelossesinOSCs44,45.

Sidechainengineeringhasbeenappliedasaneffectivestrategytotune the intermolecular interaction and crystallinity of NFAs46–48. Regulationofthelengthandbranchingpositionofthealkylsidechaincan alter its steric hindrance, thereby influencing the molecular stackingbehavior.RecentworkofSunetal.49onthemodificationofstate-of-the-artL8-BOhasreflectedthepotentialofdelicatesidechainmodificationfortuningmolecularcrystallinityandimprovingphotoluminescence quantum yield (PLQY). To suppress the non-radiative relaxations in Se-containing NFAs, we here attempted to optimize molecularstacking andcrystallinitythrough side chainengineering. We incorporated 3 Se atoms into the central core and introduced achiral N-alkyl substituents with varied lengths and branching positionsonthepyrrolemoietytotunethesterichindrance.Ultimately,aseriesofNFAs,namelyS9SBH-F,S9S3B-F,S9SHO-F,andS9SHN-F,weresuccessfullysynthesized(Fig.1a)withthesyntheticroutedepictedinSupplementaryFig.1.

Results
Materialscharacterization
Figure1ashowsthemolecularstructuresoftheNFAssynthesizedinthisstudy. The steric hindrance of side chains is modulated by both the branchingpositionandthechainlength.TheUV-VisabsorptionspectraoftheseNFAsinthinfilmsandsolutionsareshowninFig.1bandSupplementaryFig. 2. From the absorption profiles in solution,all NFAs exhibitamaximumabsorptionpeakat~760nm.Theabsorptionspectraoffilmsaremuchred-shiftedtothenear-infraredregion,amongwhichS9SBH-FandS9SHO-F,featuringbranchedN-alkylchainsatC2position(noted as 2nd-branched NFAs), exhibit more red-shifted absorption peaksalongsideblue-shiftedabsorptiononsetswhencomparedtotheothertwoNFAs that possess N-alkyl chains branched at C3 position (notedas3rd-branchedNFAs),asdetailedinSupplementaryTable1.Theshapesofabsorptionspectraoffilmsarealsodifferent,wheretheC2-branched NFAs have a more pronounced peak with lower-intensity shoulderscorrespondingtoamoreFrenkel-likenature,whiletheC3-branched NFAs have a broader peak with higher-intensity shoulderscorrespondingtoamorecharge-transfer(CT)-likenature,accordingtothepreviousliterature50.TheelectrochemicalpropertiesoftheNFAs were investigated by cyclic voltammetry (Supplementary Fig. 3). As showninFig.1c,thehighestoccupiedmolecularorbital(HOMO)/lowestunoccupiedmolecularorbital(LUMO)energylevelsaredeterminedas−5.66/−3.83and−5.68/−3.82eVforS9SBH-FandS9SHO-F,−5.63/−3.86and −5.64/−3.86eV for S9S3B-F and S9SHN-F. The up-shifted LUMO energylevelsofS9SBH-FandS9SHO-FarepotentiallyadvantageousforachievingahighV intheircorrespondingdevices.

To investigate the molecular self-assembly, single-crystal X-ray diffraction experiment was conducted. Single crystals of S9SBH-F, S9S3B-F,S9SHO-F,andS9SHN-Fweregrownthroughaternarysolventsystem, and the diffraction data were collected (Supplementary Tables2–5).Besides,theORTEPviewofallsingle-crystalstructuresispresentedinSupplementaryFigs.4–7.AsshowninFig.1dandSupplementaryFigs.8–10,S9SBH-FwiththeshortestN-alkylchainformsanordered3Dhoneycombnetwork,similartothepreviouslyreportedY633.ComparedwithS9SBH-F,S9SHO-F,withlongerbranchedN-alkylchains,exhibitsalong-rangeorderedpackingintheobliquedirectionacross the crystal lattice (Supplementary Fig. 11), forming columnscomposed of π-stacked NFA molecules with a small average π-πstackingdistance(3.38ÅforS9SBH-F,and3.20ÅforS9SHO-F).Thetransitionofcrystalpackingstructurefroma3Dhoneycombnetwork(S9SBH-F)intoanetworkofratherindependentcolumns(S9SHO-F)iscausedbythelongN-alkylchainsoccupyingtheintervalbetweentheadjacentcolumnsinS9SHO-Fsinglecrystal,andresultsinamismatchbetweenthetwoparallelmolecules.WhenchangingthebranchingsitefromC2-positiontoC3-position,althoughthebranchingsiteofN-alkylsmovesfartherfromtheπ-system,theystilloccupytheintervalbetweentheadjacentmolecularcolumnsthusthepackingpatternsofS9S3B-FandS9SHN-FaresimilartothatofS9SHO-F(SupplementaryFig.12).Wealsoinferthatthedistancebetweentheneighboringcolumns,whichisassociatedwiththesizeofN-alkylsubstituents,mayoffer the flexibility for NFA molecules to pack in a tighter manner. Therefore, we observe a reverse trend between the distance in the neighboring columns (10.8Å<15.3Å<19.5Å) and π-π stacking distance (3.52Å>3.50Å>3.20Å) in S9S3B-F, S9SHN-F, and S9SHO-F (Fig.1eandSupplementaryTable6).

Subsequently,thegrazing-incidencewide-angleX-rayscattering(GIWAXS) measurement was performed to further verify the π-πstackingintheC2-branchingS9SHO-FandC3-branchingS9SHN-Finthinfilms.AsshowninSupplementaryFig.13,S9SHO-Fshowsa(010)diffractionpeakat1.76Å−1correspondingwithad-spacingof3.57Åintheout-of-plane(OOP)direction.WhenchangingthebranchingsitetoC3-position,S9SHN-Fexhibitsa(010)diffractionpeakat1.73Å−1withad-spacing of 3.63Å in the OOP direction, indicating a slightly moreintimatepackinginC2-positionS9SHO-F.Theseresultsareconsistentwithourconclusionsobtainedfromthesingle crystal analysis.

ThetightpackingofS9SBH-FandS9SHO-Fmayreducethevibrationof environmental molecules and suppress electron–phonon coupling23,51,52.ItisfoundthattheC2-branchedS9SBH-FandS9SHO-Fexhibit significantly higher PLQYs (2.1% and 2.8%) than the C3-branchedS9S3B-FandS9SHN-F(1.2%and1.4%)infilms(SupplementaryFig.14).Accordingtotheenergy-gaplaw,thereexistsaninversecorrelation between the PLQY and static disorder23,53,54. Whenincreasingthedegreeofenergeticdisorder,itwillbroadentheenergylevels and make more low-energy electronic states accessible. As aresult, these additional low-energy electronic states facilitate non-radiativerecombination,ultimatelyresultinginthesuppressedPLQY.ComparedwiththeC2-branchingNFAs,theC3-branchingNFAsexhibitaratherblue-shiftedabsorptionpeak(Fig.1b)andthecorrespondingdevicesdisplayamoreobvioustailingintheEQEspectra,indicatingalargerenergeticdisorder49.Asaresult,themoreintimatepackingandlowerenergeticdisorderinS9SHO-FshouldenableahigherPLQY.

In addition, computed intermolecular electronic couplings by densityfunctionaltheory(SupplementaryFig.15)showthatS9SHO-Fpossessesahighernumberofelectroncouplingsinducedbythelargeπ-coretorsionangle,potentiallyleadingtomoreefficient3Dchargetransport. The space-charge-limited-current (SCLC) mobility measurement (Supplementary Fig. 16) also supports our findings here,showingthehighestelectronmobilityof3.3×10−4cm2/V·sforS9SHO-F,and3.2×10−4,2.8×10−4,and3.0×10−4cm2/V·sforS9SBH-F,S9S3B-F,andS9SHN-F(SupplementaryTable7),respectively,agreeingwellwiththeirpackingproperties.

Deviceperformance
ThephotovoltaicperformancebasedontheseNFAswasinvestigatedbyfabricatingOSCdeviceswithanarchitectureofITO/2PACz/D18:NFAs/PDINN/Ag.AsshowninFig.2aandTable1,theD18:S9SBH-F-baseddevicesexhibitaPCEof18.9%withaV of0.833VandaJ of29.65mAcm−2, while the devices based on D18:S9SHO-F deliver a highestPCEof19.2%withaJ of29.68mAcm−2andaV of0.835V.TheD18:S9SHN-F-basedcellexhibitsalowerPCEof18.5%withalowV of0.824V,whilethecellcontainingS9S3B-FpossessanevenlowerV of0.800Vandafillfactor(FF)of70.30%,leadingtoalowestPCEof16.6%.Utilizingthenear-infraredabsorptionofSe-containingNFAs,weincorporatedthebest-performingS9SHO-FintoL8-BOwithaweightratioof15%foraternarydevice.TheD18:L8-BO:S9SHO-FbasedcelldisplaysahighV of0.884VandahighJ of28.85mAcm−2,achievingaPCEof20.4%(SupplementaryFig.17),whichisamongthebestvaluesfor the OSCs. The ternary device was also sent to an independent laboratoryandcertifiedwithaPCEof19.88%(SupplementaryFig.18).Furthermore,thelong-termstoragestabilityinanitrogen-filledglovebox and thermal stability annealed at 70°C of the S9SHO-F basedbinaryandternarydeviceswereperformed(SupplementaryFig.19).Compared with the binary devices based on S9SHO-F, the ternaryD18:L8-BO:S9SHO-F-basedOSCsexhibitimprovedstability,indicatingtheenhancedmolecularpackinghelpsstabilizethemorphologyintheternaryactivelayer.

ItwasfoundthatthedevicesbasedonC2-branchedNFAsdeliverahighPCEandV amongthecellsbasedonthefourSe-containingNFAs(Table1).AccordingtotheEQEspectraofthedevices(Fig.2b),despitealltheNFAsdemonstratingsimilarJ sof~29mAcm−2,theC3-branchedNFAsexhibitweakerphotoresponseintherangeof610–900nmandbroaderbandtailscomparedtotheircounterpartswithC2-branchedN-alkylchains(Fig.2b).Thesebroaderbandtails,analogoustothered-shiftedabsorptiononsetsinneatfilms(Fig.1b),suggestthelargerratiosofbelow-gapstates,whichareprobablyassociatedwithvoltagelosses55.

Furthermore, we explored the impact of S9SHO-F on the photoresponse characteristics in the ternary device. As shown in SupplementaryFig.17b,theD18:L8-BOexhibitsanEQEtailataround900nm;however, the devices based on D19:S9SHO-F have a strong photoresponseintherangeof900–1000nm.WhenS9SHO-FisintegratedintothebinarymaterialsystemofD18:L8-BO,iteffectivelyenhancesthelight-harvesting capability in the range of 830–930nm, significantlycontributingtotheimprovedJ observedintheternarydevice.

Furtherin-depthelectricalcharacterizationswereconductedtoevaluate the exciton separation and charge recombination in the devices.Thephotocurrentdensity(J )plottedagainsttheeffectivevoltage(V )wasconductedtoaccesstheexcitondissociationprobability(P )andchargecollectionprobability(P )inthedevices.AsillustratedinFig.2c,theD18:S9SBH-FandD18:S9SHO-F(C2-branched)baseddevicespossessalargerP valuesof93.6%and93.7%,aswellaslargerP valuesof89.4%and90.2%thantheothertwodevices(C3-branched),which willbe beneficialforefficientexciton dissociationandchargegeneration.Toinvestigatethechargerecombinationprocessinthecells,thedependenceofJ andV onlightintensitywasperformed(Fig.2d,e).TheslopesoftheJ versuslightintensitycurve(α)are0.972fortheS9SHO-F-andS9SBH-F-baseddevices,whicharehigherthanthoseofD18:S9S3B-F(0.970)andD18:S9SHN-F(0.968),indicatingsuppressedbimolecularrecombinationinthedevicesbasedonC2-branchedNFAs.FromtheplotofV versuslightintensity,theslopeβkT/qwascalculatedtoelaboratetherecombinationmechanism. The β values of D18:S9SHO-F, D18:S9SBH-F, D18:S9S3B-F, andD18:S9SHN-Fare1.03,1.03,1.10,and1.04,respectively,suggestinglesstrap-assistedrecombinationintheC2-branchedS9SHO-F-andS9SBH-F-baseddevices.

Energyloss(E )wasanalyzedtostudythedifferencesofV inthedevices.TheE valuewasderivedfromthenormalizedabsorptionand photoluminescence (PL) emission spectra (SupplementaryFig. 20). As depicted in Supplementary Table 8, the E values fordevices based on D18:S9SBH-F, D18:S9S3B-F, D18:S9SHO-F, andD18:S9SHN-F are 0.536eV, 0.559eV, 0.531ev, and 0.534eV, respectively.Furthermore,electroluminescenceexternalquantumefficiency(EQE )wasmeasuredtoverifythenon-radiativeenergylossesinthedevices.AsshowninFig.2f,thedeviceswithC2-branchedNFAsexhibitlargerEQE sthanthosewithC3-branchedNFAs,benefitingfromthehigherPLQYsintheirneatNFAfilms(SupplementaryFig.14)56.Thenon-radiativeV losseswerecalculatedusingtheequation:ΔV = (kT/q)ln(1/EQE ),giving0.180,0.191,0.175,and0.185eVforD18:S9SBH-F,D18:S9S3B-F,D18:S9SHO-F,andD18:S9SHN-Fcells,respectively.Thesephenomenaillustratethattheside-chainmodulationisausefulstrategyforSe-containingNFAstoimprovethephotoluminescence efficiencies not only in neat NFA films but also inblends,suppressingnon-radiativelossesindevices.

Morphologyinvestigation
Toinvestigatethemorphologiesofblendfilms,atomicforcemicroscopy(AFM)andGIWAXSmeasurementswereperformed.AsdepictedinFig.3a,theblendfilmscomposedofC2-branchedNFAsexhibitwell-mixedmorphologywithclearfibrousnetworkcomparedtotheothertwoblendfilms.Thecalculatedsurfaceroot-mean-squareroughness(R ) in D18:S9SBH-F and D18:S9SHO-F are 1.16nm and 1.01nm,respectively, which are much smaller than those of D18:S9S3B-F(1.43nm)andD18:S9SHN-F(1.36nm).

The2DGIWAXSpatternsandthecorrespondingline-cutprofilesareshowninFig.3b,c.Itisfoundthatalltheblendsexhibitaπ−πdiffractionpeakat~1.71Å−1intheOOPdirectionandalamellar(100)peakat~0.31Å−1 in the in-plane (IP) direction, indicating a face-onorientation,whichisbeneficialforthechargetransport.Thecrystalcoherence length (CCL) of the (010) diffraction peak in theOOPdirectionwasalsocalculated(SupplementaryTable9),andthevalues are 32.0 and 34.0Å for blend films based on C2-branchedS9SBH-FandS9SHO-F,respectively,largerthanthoseofD18:S9S3B-F(30.8Å)andD18:S9SHN-F(31.2Å).Thisindicatesabettermorphologycompatibility between S9SHO-F and D18, which can form a highdegreeofmolecularorderandfurtherimprovethechargetransport.

Furthermore, we conducted the contact angle measurements toverify the miscibility between the polymer donor and smallmolecularacceptor.AsshowninSupplementaryFig.21andSupplementaryTable 10, the Flory–Huggins interaction parameter (χ) wascalculatedbythesurfaceenergy,andtheχparametersofD18:S9SBH-F, D18:S9S3B-F, D18:S9SHO-F, and D18:S9SHN-F are 0.20K, 0.51K,0.15K, and 0.40K, respectively. This indicates a high miscibility ofD18:S9SHO-F blend with a more homogeneous morphology, whichwould benefit the charge transport and exciton dissociation in theactivelayer.

Subsequently, the GIWAXS measurements of D18:L8-BO andD18:L8-BO:S9SHO-F blend films were employed to investigate the molecularpackingandcrystallizationbehavior.AsshowninSupplementaryFig.22,theD18:L8-BO:S9SHO-Fblendexhibitsaπ−πstackingdistanceof3.62ÅintheOOPdirection,whichisslightlysmallerthanthatofD18:L8-BOblend(3.64Å).Additionally,theCCLsofthe(010)diffractionpeakintheOOPdirectionfortheD18:L8-BOandD18:L8-BO:S9SHO-F blends are 30.2 and 34.9Å, respectively. These resultsindicatetheincorporationofS9SHO-Fbeneficiallyregulatesthecrystallinityoftheblendfilms,whichmayleadtomoreefficientcharge-transportingproperties.

Excited-statekineticsanalysis
Toelucidatehowthesidechainengineeringinfluencestheexcited-statekineticsintherelatedmaterials,femtosecond-resolvedtransientabsorptionspectroscopy(TAS)wasemployedtoinvestigatetheneatNFAfilmsaswellasthecorrespondingblendfilms.ThebroadbandTAScolor maps of neat NFAs films pumped at850nm are presented inFig.4a,withthecorrespondingdatashowninSupplementaryFig.23.After photoexcitation, a negative ground-state bleaching signal ofacceptor (GSB ) emerges around 850nm, accompanied by theappearanceoftheexcited-stateabsorption(ESA)signaloflocalexcitons(LEs)near950nm.Then,anewESAbandaround1600nmcorresponding to the delocalized singlet excitons (DSEs) generates togetherwiththedecayoftheLEinhundredsoffemtoseconds30,57.Then, as the DSE signal vanishes in ~100ps, a new ESA signal near1450nm gradually emerges, reaching its peak intensity in~1000ps.Thissignal,characterizedbyanexceptionallyprolongedlifetime,canbe unequivocally attributed to T₁ excitons arising from the ISC ofsingletexcitons,ascorroboratedbyourpriorinvestigationsonanalogousSe-containingmolecules30.

ForthecomparisonofdifferentNFAfilms,allthedataarenormalized to the minima of GSB s, respectively (SupplementaryFig.24a).AsdepictedinthenormalizedTASspectraprobedat0.5ps(Fig.4b),theC2-branchedS9SBH-FandS9SHO-FfilmsexhibitweakerDSE signalsbut stronger LE signalscompared to their C3-branchedcounterparts, which is also supported by the dynamics of DSEs inSupplementary Fig. 24b. Such differences in exciton delocalizationindicate that the C2-branched side chains in S9SBH-F and S9SHO-FfacilitatestheformationofmoreFrenkel-likeexcitonsratherthanCT-like excitons. This finding aligns with our prior analysis of theabsorption spectra in Fig. 1b, as supported by relevant literature50.Moreimportantly,thesefilmsexhibitgreatdifferencesingenerationofT excitons,atthetimedelayof1000ps(Fig.4c).TheC2-branchedS9SBH-F and S9SHO-F films exhibit markedly reduced T signalscomparedtotheS9S3B-FandS9SHN-Ffilms,ascorroboratedbytheT dynamicspresentedinSupplementaryFig.24c.

Inaddition,itisobservedthattheGSB peaksoftheNFAfilmscontinuouslyredshiftatdifferenttimescalesalongwiththeevolutionofphotoexcitedexcitons(Fig.4a).Throughconductinganin-depthanalysisofthered-shiftprocessesinNFAfilms,wefoundthattheycanbe categorized into three stages, as depicted by the temporal progressionoftheGSB peakpositioninFig.4d.Thefirststageoccurswithin ~0.3ps,predominantly involving the cooling process of photogeneratedexcitonsandtheconversionfromLEtoDSE57.Inthelaststage(50–1000ps),theprimaryprocessisthegenerationofT excitons.Thesecondstageisthemostnoteworthyone,wheretheS9S3B-FandS9SHN-FfilmsexhibitadistinctredshiftofGSB ,whiletheS9SBH-FandS9SHO-Ffilmsshownegligibleredshift.InviewofthattheT excitonsaregeneratedoverarelativelylongertimescale(asfittedinSupplementaryFig.24d),thesecondstagecannotbeascribedtotheISCprocess.Instead,itismorelikelytobeattributedtothefurtherinteraction of DSEs with surrounding environments, leading to theformationoflower-energystates,whichisconsistentwiththelargerStrokesshiftsinS9S3B-FandS9SHN-Ffilms(SupplementaryFig.23).Thelossofenergyduringthisprocessmaycausethebroaderbandtails of EQE spectra (Fig. 2b) and lower V s (Table 1) observed inS9SHN-F- and S9S3B-F-based devices. Moreover, the low-energyproperty and separated electron-holedistribution of DSEs probablyfacilitatetheISCtoT excitons,accordingtotheliteratures58,59.CombiningtheresultsofTASandabsorptionspectra,weconcludethattheC2-branched NFAs generate more Frenkel-like excitons with higherenergy,suppressingtheISCprocesstoT excitons.Incontrast,theC3-branched NFAs generate more CT-like (delocalized) excitons withlowerenergy,facilitatingtheformationofT excitons.

WefurtherinvestigatedtheT formationintheblendfilmsbasedontheSe-containingNFAs.AsshownbytheTASdatainFig.4eandSupplementary Fig. 25, the blend films were pumped at 850nm toselectivelyexcitetheNFAs.AlltheblendfilmsexhibitaDSE-mediatedhole transfer process closetothepreviousreports57,wheretheGSBsignal of D18 (GSB at 590nm) and the ESA signal of CS (630 and800nm)emergeastheGSB (850nm)andtheESAofDSE(1600nm)decayin~100ps.Inthelongertimescales(after100ps),theESAsignalofT excitons(1450nm)emergesandgraduallyrisestoitsmaximumwiththeadditionalcontributionofnon-geminaterecombination31.To comparetheT excitonsgeneratedindifferentblendfilms,wenormalized the TAS spectra and dynamics to the minima of GSB s,respectively(Fig.4f,g).SimilartothecasesofneatNFAfilms(Fig.4c),we still observed suppressed formation of T excitons in the C2-branchedNFA-basedblendscomparedtothoseintheC3-branched-basedblends.TheD18:S9S3B-FfilmexhibitsthemostintenseT formation,whiletheD18:S9SHO-FfilmexhibitstheleastT formation.

We also note that T excitons could form through differentpathways (i.e., ISC and non-geminate recombination) in these Se-containing NFAs. To provide a profound understanding into themechanismofT formationindifferentmaterialsystems,wefurtherinvestigatedtwoSe-freeNFAs,T9TBO-F(C2-branchedN-alkylchainonthe pyrrole unit) and T9TBN-F (C3-branched N-alkyl chain on thepyrrole unit) (Supplementary Fig. 26). As shown in SupplementaryFig.27,intheabsenceofheavy-atom-mediatedISC,thereisnegligiblegenerationofT excitonsinneatNFAfilms.IntheSe-freeblendfilms(Supplementary Fig. 28), relatively fewer T excitons are formedcomparedtotheSe-containingblendfilms,withthemajorityarisingfromnon-geminaterecombinationoffreecarriesatD-Ainterfaces29.IncontrasttothecaseofSe-containingNFAs,theSe-freeNFAwithC2-branchedN-alkylchainsisassociatedwithanincreasedgenerationofT excitonsinblendfilmsandahighernon-radiativeV lossindevice(0.228and0.220eVfordevicesbasedonC2-branchedD18:T9TBO-FandC3-branchedD18:T9TBN-F,respectively)(SupplementaryFig.29).

Inaddition,thechargecarrierdynamicsinthebinaryD18:L8-BOand ternary D18:L8-BO:S9SHO-F blends were investigated by TASmeasurements.As showninSupplementaryFig. 30, compared withD18:L8-BOfilm,strongGSB andCSsignalsareobservedinD18:L8-BO:S9SHO-F blend after hole transfer, suggesting more efficientcharge separation in the ternary film, which should benefit theimprovedJ inthecorrespondingdevices.

Methods
Materials
Allchemicalsandreagentswerepurchasedfromcommercialresources and used without further purification. Polymer donor D18 wassynthesizedaccordingtothepreviousreport60.ThesyntheticproceduresofS9SBH-F,S9S3B-F,S9SHO-F,S9SHN-F,andthecorrespondingstructural characterization are presented in SupplementaryFigs.1and31–82.

Generalmeasurements
The 1H NMR spectrometry was conducted on a Bruker 600MHzAVANCEIIIspectrometerusingCDCl .High-resolutionmassspectrometry was performed by a Q Exactive Focus Hybrid Quadrupole-OrbitrapMassSpectrometer(QEOrbitrapMS,ThermoFisherScientific).UV-VisabsorptionspectrawerecollectedbyaUltra-VioletVisibleScanning Spectrophotometer (Shimadzu 1700). Cyclic voltammetry(CV) measurements were performed utilizing an ElectrochemicalAnalyzer System (CH 1660C) in an anhydrous acetonitrile solutioncontainingtetra-n-butylammoniumhexafluoro-phosphate(Bu NPF )a concentrationof0.1Munderascanrateof50mVs−1.Aconventionalthree-electrodecellwasappliedwithglassycarbon,platinumwire,andAg/AgCl serving as the working electrode, counter-electrode, andreference electrode, respectively. Furthermore, FeCp₂/FeCp₂⁺ wasutilizedasaninternalreferencesystem.AFMimageswereinvestigatedbyaBrukerMultimode&Microscope.ForGIWAXSmeasurement,thefilmswerepreparedonthesiliconsubstrate.

Growthofsinglecrystals
The single crystals were grown using a ternary solvent diffusionmethod.Asolutioncontaining~0.5mgS9SBH-F,S9S3B-F,S9SHO-ForS9SHN-Fdissolvedin0.3mLCHCl ,wasmeticulouslytransferredintoacleanNMRtube.Around0.2mLCH Cl wasslowlylayeredontopofthe CHCl solution, followed by layering of acetone to fill the NMRtube.TheCH Cl layerservesasabuffertoestablishasolubilitygradient,therebymodulatingcrystalgrowthkineticsandpreventingrapidsolidprecipitation.TheNMRtubewasthensealedandlefttostandforafewdays(7–14days)untilthesolution’scolorfaded.

Devicefabricationandcharacterization
The OSCs werefabricated with a structure ofITO/2PACz/D18:NFAs/PDINN/Ag. The ITO/glass substrates underwent ultrasonic cleaningusing detergent, deionized water, acetone, and isopropanol. Afterbeingdriedinanovenat110°Covernight,theITOglassesweresubjectedtoplasmatreatmentfor25min.Athinlayerof2PACzwasspin-castontheITOsubstratesat5000rpmfor25s,thenannealedat70°Cfor5mininair.TheweightratioofD18:NFAsblendswas1:1.2andthe optimal weight ratio of D18:L8-BO:S9SHO-F was 1:1.05:0.15. Themixturewasdissolvedinchloroform(thetotalconcentrationofblendsolutionswere7mgmL−1forallblends),withtheadditionof0.5%CNasadditive,andstirred2honahotplateat55°Cinanitrogen-filledglovebox.Thesolutionwerespin-castonthe2PACzlayerat2000rpmfor20sandaccompaniedbyathermalannealingstepat90°Cfor5min.,followedbyathinlayerofPDINNcoatedontopoftheactivelayer.Finally,alayerofAg(~90nm)wasthermallyevaporatedat5×10−5Pathroughashadowmaskataratebetween2.5and5Ås−1.TheJ–VcurvesofthedeviceswereobtainedusingaKeithley2400sourcemeterunderroomtemperatureinanN -filledglovebox,withascanstepof0.02Vandadwelltimeof10ms.ThesimulatedsunlightwascalibratedusinganAM1.5Gsolarsimulator(Enlitech,SS-F5,Taiwan),whichwasmeasured with an Si diode applied with KG-2 filter calibrated by theNational Renewable Energy Laboratory. The EQE curves were measured by an EQE measurement system manufactured by EnLiTech(Taiwan).

Transientabsorptionspectroscopy(TAS)
TheTASmeasurementswereconductedusingaYb:KGWlaser(Pharos,Light Conversion). The wavelength of fundamental output is at~1030nm.Weusedasecondharmonicnoncollinearopticalparametricamplifier(2HNOPA,LightConversion)togeneratethepumppulsesat850nm. The probe beam was supercontinuum by focusing a smallfractionofthefundamental1030beamtoasapphireplateforvisibledetectionoranyttriumaluminumgarnet(YAG)platefornear-infrared(NIR)detection.TheexperimentswereconductedwithacommercialTASsystem(TA100,TIMETECHSPECTRA).Thetimedelaybetweenthepumpandprobepulseswascontrolledbyadelayline.Thepulse-to-pulsespectralanalysiswasconductedat10kHz.Thesignal-to-noiseratio(ΔO.D.)wasbetterthan1×10−5afteraveragingover5000pump-onandpump-offshotsforeachdatapoint.ThepumpdensitiesinTASexperiments was set at ~2μJcm−2 unless otherwise specified. ThesamplesforTASmeasurementwerespin-coatedonquartzsubstrates.The samples were kept in a nitrogen atmosphere during the measurementtopreventphoto-degradation.

EQE measurements
TheEQE datawerecollectedusinganin-house-builtsystem,comprising a Hamamatsu silicon photodiode 1010B, a Keithley 2400SourceMeterforvoltagessupplyandcurrentinjectionrecording,andaKeithley485Picoammeterformeasuringtheemittedlightintensity.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript entitled “Alleviating non-radiative losses in organic solar cells through side-chain regulation of low-bandgap non-fullerene acceptors” presents a systematic investigation of how N-alkyl side-chain steric in Se-substituted non-fullerene acceptors (NFAs) influence molecular packing, excited-state dynamics, and ultimately photovoltaic performance. In this work, Qi et al. designed and synthesized a series of NFAs (S9SHO-F, S9SBH-F, S9SHN-F and S9S3B-F) bearing different alkyl side-chains. Through single-crystal X-ray diffraction, device-physics analysis and morphology characterization, they found that larger steric hindrance contributes to a reduction in intersystem crossing (ISC) to triplet excitons (T₁) and suppresses non-radiative loss, which in turn improves photovoltage and efficiency. However, several issues need to be addressed before publication.\n> In this manuscript, Qi et al. claim that NFAs with bulkier side-chains exhibit more intimate molecular packing, which contradicts previous reports. Li et al. observed that the π–π stacking d-spacing increases gradually as the branched alkyl chain length increases (Nat. Energy 2021, 6, 605-613). Moreover, reducing steric interactions between the core and the branches has been shown to strengthen π–π stacking (Joule 2019, 3, 3020-3033; Chem. Rev. 2022, 122, 14180−14274; Nat. Mater. 2025, 24, 433-443). I suggest that the authors provide more convincing evidence to rationalize their observations, which would enhance the scientific rigor of the manuscript.\n> The NFA, S9SHO-F, with more sterically hindered substituents exhibits the more compact stacking and the highest luminescence efficiency among the four NFAs. Considering that organic photovoltaic materials typically follow the aggregation-caused quenching (ACQ) principle, stronger aggregation is generally expected to reduce luminescence efficiency. However, the trend reported in this work is the opposite. Could the authors provide a reasonable explanation for this apparent contradiction?\n> In this work, the authors attribute the suppressed formation of T₁ to a steric-hindrance-induced reduction in ISC. However, the involvement of ISC in T₁ generation remains questionable because heavy-atom-free analogues were not investigated. Therefore, I recommend that the authors prepare and characterize Se-free control NFAs to clarify the origin of T₁ states and the associated energy losses. Such a comparison would greatly aid readers in distinguishing heavy-atom-mediated ISC from other possible pathways (e.g., charge-transfer-mediated triplet formation or geminate recombination).", "reviewer_2": "The authors synthesized high-performance acceptor materials with near-infrared absorption properties via substitution with three selenophenes. Further studies revealed that alkyl substituents with greater steric hindrance can induce the formation of tight and ordered molecular packing and suppress non-radiative energy loss. Eventually, the binary and ternary devices based on D18:S9SHO-F achieved high efficiencies of 19.2% and 20.4% respectively, which provides significant reference for the design and synthesis of narrow-bandgap acceptor materials with low energy loss. Therefore, I suggest that the manuscript can be published in Nature Communications after minor revisions.\nSome comments and suggestions are following:\n(1) The performance of the D18:S9SHO-F -based binary and ternary devices in this paper is excellent, the author needs to provide the stability test data.\n(2) The authors need to analyze the reasons for the performance improvement of D18:L8-BO:S9SHO-F based ternary devices, including aspects of morphology and exciton dynamics.\n(3) In describing the increase in CCL observed in the GIWAXS of the blend films, the authors infer that “indicating a better morphology compatibility between S9SHO-F and D18”. It is necessary to conduct thin film surface contact angle experiments to verify the compatibility between the donor and acceptor.\n(4) Generally, side chains with larger steric hindrance tend to hinder molecular packing, which is not conducive to the formation of a tight and ordered packing network. In this paper, how do the starting position of the branched chains and the length of the branched chains affect molecular packing? The author needs to carefully analyze the molecular single crystal structure.\n(5) In most cases, selenium atom substitution results in the red shift of absorption, however, the inferior PCEs (FFs) comparing to their sulfur counterparts, especially for 5Se acceptor with 5-selenium substitution. What is the inner reason for that?\n(6) Some format suggestions: for abbreviations that appear for the first time in the article, their corresponding English full names should be listed, such as the abbreviation of “GIWAXS”. It is suggested that the authors carefully check the entire text."}, "author_response": "Response to the Reviewer 1\n1. In this manuscript, Qi et al. claim that NFAs with bulkier side-chains exhibit more intimate molecular\npacking, which contradicts previous reports. Li et al. observed that the π–π stacking d-spacing\nincreases gradually as the branched alkyl chain length increases (Nat. Energy 2021, 6, 605-613).\nMoreover, reducing steric interactions between the core and the branches has been shown to strengthen\nπ–π stacking (Joule 2019, 3, 3020-3033; Chem. Rev. 2022, 122, 14180−14274; Nat. Mater. 2025, 24,\n433-443). I suggest that the authors provide more convincing evidence to rationalize their observations,\nwhich would enhance the scientific rigor of the manuscript.\nReply: We appreciate the reviewer for the insightful comment. For the reported NFAs (Nat. Energy\n2021, 6, 605-613), the authors engineered the branching alkyl chains flanking outward the NFA core,\ngiving a closer π–π distance in L8-BO (3.19 Å) when comparing with L8-HD (3.30 Å) and L8-OD (3.40\nÅ). In our work, we focus on the regulation of the branching position of N-alkyl chains on the pyrrole\nunit, which locate at the inner side of the molecule. It is notable that in our previous works, we observed\nfrom the single crystals of NFAs that the size of N-alkyl chains may not necessarily affect the value of\nπ–π distance. For example, the π–π distance of Y6 (N-2-ethylhexyl, 3.3-3.6 Å, ref: J. Am. Chem. Soc.\n2020, 142, 15246-15251) is close to that of T9TBO-F (N-2-butyloctyl, 3.3-3.5 Å, ref: Nat. Energy 2022,\n7, 1076-1086). This should be attributed to the complicated intermolecular interactions of NFA\nmolecules in condensed phase, and there are factors beyond the size of N-alkyl chains that may affect\nmolecular packing. To better explain how the steric effect of N-alkyl substituents affects the molecular\npacking in our materials and gain clarity to the readers, we have analyzed the single-crystal data of\nNFAs in a more in-depth manner and provided detailed explanations. We have also made modifications\nto relevant statements to clarify this.\nFrom the single-crystal analysis, S9SBH-F with the shortest N-alkyl chain forms an ordered 3D\nhoneycomb network, similar to the previously reported Y6 (J. Am. Chem. Soc. 2020, 142, 15246-15251).\nCompared with S9SBH-F, S9SHO-F with longer branched N-alkyl chains exhibits a long-range ordered\npacking in the oblique direction across the crystal lattice (Supplementary Fig. 11), forming columns\ncomposed of π-stacked NFA molecules with a small average π-π stacking distance (3.38 Å for S9SBH-\nF, and 3.20 Å for S9SHO-F). The transition of crystal packing structure from a 3D honeycomb network\n(S9SBH-F) into a network of rather independent columns (S9SHO-F) is caused by the long N-alkyl\nchains occupying the interval between the adjacent columns in S9SHO-F single crystal, and results in\na mismatch between the two parallel molecules. When changing the branching site from C2-position to\nC3-position, although the branching site of N-alkyls moves farther from the π-system, they still occupy\nthe interval between the adjacent molecular columns thus the packing patterns of S9S3B-F and S9SHN-\nF are similar to that of S9SHO-F (Supplementary Fig. 12). We also infer that the distance between the\nneighboring columns, which is associated with the size of N-alkyl substituents, may offer the flexibility\nfor NFA molecules to pack in a tighter manner. Therefore, we observe a reverse trend between the\ndistance in the neighboring columns (10.8 Å < 15.3 Å < 19.5 Å) and π-π stacking distance (3.52 Å >\n3.50 Å > 3.20 Å) in S9S3B-F, S9SHN-F, and S9SHO-F (Fig. 1e and Supplementary Table 6).\nTo further investigate the π-π stacking in the neat NFAs, we conducted grazing-incidence wide-angle\nX-ray scattering (GIWAXS) measurement of the C2-position and C3-position acceptors. As shown in\nSupplementary Fig. 13, S9SHO-F shows a (010) diffraction peak at 1.76 Å-1 corresponding with a d-\nspacing of 3.57 Å in the out-of-plane (OOP) direction. When changing the branching site to C3-position,\nS9SHN-F exhibits a (010) diffraction peak at 1.73 Å-1 with a d-spacing of 3.63 Å in the OOP direction,\nindicating a slightly more intimate packing in C2-position acceptor. These results are consistent with\nour conclusions obtained from the single crystal analysis.\nWe have modified the manuscript and supporting information as below.\nS9SHO-F\nS9SBH-F\na b\n19.5 Å\n5.9 Å\nc\nb a\nc\na b\nd\nc\nSupplementary Fig. 11 Side view of the molecular packing of (a,c) S9SBH-F and (b,d) S9SHO-F,\nwhere the N-alkyl chains on the pyrrole unit are (a,b) presented or (c,d) omitted.\na S9S3B-F b S9SHN-F\n15.3 Å\n10.8 Å\nc\na\nb\nc\na\nb\nc d\nSupplementary Fig. 12 Side view of the molecular packing of (a,c) S9S3B-F and (b,d) S9SHN-F,\nwhere the N-alkyl chains on the pyrrole unit are (a,b) presented or (c,d) omitted.\na S9SBH-F S9SHO-F b 104\nOOP\n)\nu.103 S9SHO-F\na.\n(\ny\nt\nsi\nn\ne\nS9SHN-F n t102 S9SHN-F\nI\n101\n1 2\nQ Vector [A-1]\nSupplementary Fig. 13 GIWAXS characterizations for C2-branching S9SHO-F and C3-branching\nS9SHN-F. a 2D GIWAXS patterns of the NFA neat films. b The corresponding OOP line-cut profiles\nof the 2D GIWAXS patterns from the neat films.\nManuscript, Page 2, Line 6:\n“…we have structurally engineered selenium-containing NFAs through introducing achiral N-alkyl\nsubstituents with varied branching sites at C2 or C3 position onto the pyrrole moiety of NFA core. We\nfound that the C2-branched ones in our material system exhibit more intimate molecular packing,”\nManuscript, Page 6, Line 21:\n“S9SBH-F with the shortest N-alkyl chain forms an ordered 3D honeycomb network, similar to the\npreviously reported Y633. Compared with S9SBH-F, S9SHO-F with longer branched N-alkyl chains\nexhibits a long-range ordered packing in the oblique direction across the crystal lattice (Supplementary\nFig. 11), forming columns composed of π-stacked NFA molecules with a small average π-π stacking\ndistance (3.38 Å for S9SBH-F, and 3.20 Å for S9SHO-F). The transition of crystal packing structure\nfrom a 3D honeycomb network (S9SBH-F) into a network of rather independent columns (S9SHO-F)\nis caused by the long N-alkyl chains occupying the interval between the adjacent columns in S9SHO-F\nsingle crystal, and results in a mismatch between the two parallel molecules. When changing the\nbranching site from C2-position to C3-position, although the branching site of N-alkyls moves farther\nfrom the π-system, they still occupy the interval between the adjacent molecular columns thus the\npacking patterns of S9S3B-F and S9SHN-F are similar to that of S9SHO-F (Supplementary Fig. 12).\nWe also infer that the distance between the neighboring columns, which is associated with the size of\nN-alkyl substituents, may offer the flexibility for NFA molecules to pack in a tighter manner. Therefore,\nwe observe a reverse trend between the distance in the neighboring columns (10.8 Å < 15.3 Å < 19.5\nÅ) and π-π stacking distance (3.52 Å > 3.50 Å > 3.20 Å) in S9S3B-F, S9SHN-F, and S9SHO-F (Fig.\n1e and Supplementary Table 6). ”\nManuscript, Page 7, Line 18:\n“Subsequently, the grazing-incidence wide-angle X-ray scattering (GIWAXS) measurement was\nperformed to further investigate the π-π stacking in the C2-branching S9SHO-F and C3-branching\nS9SHN-F in thin films. As shown in Supplementary Fig. 13, S9SHO-F shows a (010) diffraction peak\nat 1.76 Å-1 corresponding with a d-spacing of 3.57 Å in the out-of-plane (OOP) direction. When\nchanging the branching site to C3-position, S9SHN-F exhibits a (010) diffraction peak at 1.73 Å-1 with\na d-spacing of 3.63 Å in the OOP direction, indicating a slightly more intimate packing in C2-branching\nS9SHO-F. These results are consistent with our conclusions obtained from the single crystal analysis.”\n2. The NFA, S9SHO-F, with more sterically hindered substituents exhibits the more compact stacking\nand the highest luminescence efficiency among the four NFAs. Considering that organic photovoltaic\nmaterials typically follow the aggregation-caused quenching (ACQ) principle, stronger aggregation is\ngenerally expected to reduce luminescence efficiency. However, the trend reported in this work is the\nopposite. Could the authors provide a reasonable explanation for this apparent contradiction?\nReply: We appreciate the reviewer for the insightful comment. We understand that a clear correlation\nbetween packing behavior and luminescence efficiency would benefit the readers. According to the\nenergy-gap law, there exists an inverse correlation between the PLQY and static disorder (Phys. Rev. B\n2005, 71, 165202; Nat. Energy 2017, 2, 17053; Nat. Commun. 2021, 12, 3642). When increasing the\ndegree of energetic disorder, it will broaden the energy levels and make more low-energy electronic\nstates accessible. As a result, these additional low-energy electronic states facilitate non-radiative\nrecombination, ultimately resulting in lower PLQY. For example, Sun et al. exploited a series of\nacceptor materials by regulating the branching site of the flanking alkyl chains on the central core of\nL8-BO. The acceptor based on the C4-position branched alkyl chains exhibited the smallest π-π stacking\naverage distance and lower energetic disorder, leading to a high PLQY (Nat. Mater. 2025, 24, 433). In\nour work, the C3-branching NFAs exhibit a blue-shifted absorption peak than the C2-branching ones\n(Fig. 1b); meanwhile, the corresponding devices of C3-branching NFAs display a more obvious EQE\ntailing compared to the C2-branching ones (Fig. 2b). This is indicative of a larger energetic disorder in\nthe C3-branching NFAs in condensed phase. As a result, the more intimate packing and lower energetic\ndisorder in S9SHO-F should enable a higher PLQY.\nWe have modified the manuscript as below.\nManuscript, Page 8, Line 7:\n“According to the energy-gap law, there exists an inverse correlation between the PLQY and static\ndisorder23, 53-54. When increasing the degree of energetic disorder, it will broaden the energy levels and\nmake more low-energy electronic states accessible. As a result, these additional low-energy electronic\nstates facilitate non-radiative recombination, ultimately resulting in the suppressed PLQY. Compared\nwith the C2-branching NFAs, the C3- branching NFAs exhibit a rather blue-shifted absorption peak\n(Fig. 1b) and the corresponding devices display a more obvious tailing in the external quantum\nefficiency (EQE) spectra (Fig. 2b), indicating a larger energetic disorder49. As a result, the more intimate\npacking and lower energetic disorder in S9SHO-F should enable a higher PLQY.”\n3. In this work, the authors attribute the suppressed formation of T₁ to a steric-hindrance-induced\nreduction in ISC. However, the involvement of ISC in T₁ generation remains questionable because\nheavy-atom-free analogues were not investigated. Therefore, I recommend that the authors prepare\nand characterize Se-free control NFAs to clarify the origin of T₁ states and the associated energy losses.\nSuch a comparison would greatly aid readers in distinguishing heavy-atom-mediated ISC from other\npossible pathways (e.g., charge-transfer-mediated triplet formation or geminate recombination).\nReply: We thank the reviewer for the insightful comment. To address the concern raised, we have\nadopted Se-free NFAs, T9TBO-F (C2-branched N-alkyl chain) and T9TBN-F (C3-branched N-alkyl\nchain) for comparison to clarify the origin of T state in different materials systems with different degree\n1\nof ISC. The chemical structures are presented in Supplementary Figure 26, and TAS measurements\nhave been performed on both neat and blend films. Supplementary Fig. 27 shows the TAS data of neat\nNFA films pumped at 800 nm. Different from the case of Se-containing NFAs (Fig. 4), there are only\nnegligible ESA signals associated with T excitons observed at the time delay of 1000 ps in Se-free\nno T generation in neat NFA films. Still, the very weak T signals in Supplementary Fig. 27d can be\n1 1\nattributed to the bimolecular recombination of trace amount of self-dissociated free charges in neat\nNFAs, as reported in previous studies (Nat. Commun. 2022, 13, 2827). Supplementary Fig. 28 shows\nthe TAS data of blend films pumped at 800 nm. The ESA signals of T excitons (~1500 nm) gradually\n1\nemerge after 100 ps and rise to their maxima in about 1000 ps. Due to the absence of ISC in Se-free\nNFAs, these formed T excitons should mostly originate from spin-triplet charge-transfer states (3CT)\n1\nformed by non-geminate recombination of free carries at D-A interfaces (Nature 2021, 597, 666). In\naddition, these Se-free blend films generally exhibit less T formation (Supplementary Fig. 28d) in\n1\ncomparison with the Se-containing blend films (Fig. 4) due to the lack of the ISC pathway.\nThe comparison between T9TBO-F and T9TBN-F also helps clarify the effects of steric hindrance,\nwhere we observed more T generation in the blend film of C2-branched NFA (Supplementary Figs.\n1\n28d and e), echoing with the severe recombination of GSB in the sample (Supplementary Fig. 28f).\nIt D\nappears that in Se-free blend films, the larger steric hindrance of the N-alkyl substituents leads to\nincreased generation of T excitons, which differs from what we observed in the Se-containing blend\n1\nfilms (Fig. 4). The electroluminescence external quantum efficiency (EQE ) was also measured to\nEL\nverify the non-radiative energy losses in the devices based on the Se-free NFAs. As shown in\nSupplementary Fig. 29, the non-radiative V losses of C2-branched D18:T9TBO-F and C3-branched\noc\nD18:T9TBN-F based devices are 0.228 and 0.220 eV, respectively.\nWe have also modified the manuscript as below.\nSupplementary Fig. 26 Chemical structures of T9TBO-F and T9TBN-F.\na\nT9TBO-F ΔO.D. (10-3) T9TBO-F\n1.4 0.0-1.4 1\n103\nel a y ( p\ns)\n102\n-3)\nD. ( 1 00 0.1 ps\nTi m e\nd101\nΔ\nO.\n-1\n1\n1 1 0 0\np\n0\ns\np s ps\n5000 ps\nLE\n0 -2\n600 700 800 900 1200 1400 1600 600 700 800 900 1200 1400 1600\nWavelength (nm) Wavelength (nm)\nb\nT9TBN-F ΔO.D. (10-3) T9TBN-F\n1.4 0.0-1.4 1\n103\nel a y ( p\ns)\n102\n-3)\nD. ( 1 00 0.1 ps\nTi m e\nd101\nΔ\nO.\n-1\n1\n1 1 0 0\np\n0\ns\np s ps\n5000 ps\nLE\n0 -2\n600 700 800 900 1200 1400 1600 600 700 800 900 1200 1400 1600\nWavelength (nm) Wavelength (nm)\nc d\nProbe @ 0.1 ps Probe @ 1000 ps\n0.5 0.4\nLE\nm\nal.) DSE\nm\nal.)\n0.2\nor0.0 or\nn n\nD.\n(\nD.\n(0.0\nΔ\nO.-0.5\nT T 9 9 T T B B O N- - F F Δ\nO.\n-0.2 GSBA T T 9 9 T T B B O N- - F F\n-1.0 GSBA -0.4\n600 700 800 900 1200 1400 1600 600 700 800 900 1200 1400 1600\nWavelength (nm) Wavelength (nm)\nSupplementary Fig. 27 Excited-state kinetics in neat films of Se-free NFAs measured by TAS pumped\nat 800 nm. TAS color maps and spectra at different time delays of a T9TBO and b T9TBN-F neat films.\nNormalized TAS spectra probed at c 0.1 ps and d 1000 ps. The spectra in c and d are normalized to the\nminima of GSB s at 0.1 ps, respectively. There are only negligible ESA signals associated with T\nA 1\nexcitons observed at the time delay of 1000 ps in Se-free NFAs (Supplementary Fig. 27d). Therefore,\nthe absence of heavy-atom-mediated ISC leads to almost no T generation in neat NFA films. Still, the\n1\nvery weak T signals in Supplementary Fig. 27d can be attributed to the bimolecular recombination of\n1\ntrace amount of self-dissociated free charges in neat NFAs, as reported in previous studies.\na\nD18:T9TBO-F ΔO.D. (10-3) D18:T9TBO-F\n1.4 0.0-1.4\nel a y ( p s) 102GSBD CS -3) D. ( 1 00 0.1 ps\nm e\nd101\nΔ\nO.\n-1\n1\n1 0\np s\nps\nTi 100 ps\n5000 ps\nLE\n0\n600 700 800 900 1200 1400 1600 600 700 800 900 1200 1400 1600\nWavelength (nm) Wavelength (nm)\nb\nD18:T9TBN-F ΔO.D. (10-3) D18:T9TBN-F\n1.4 0.0-1.4 1\nel a y ( p s) 102GSBD CS -3) D. ( 1 00 0.1 ps\nm e\nd101\nΔ\nO.\n-1\n1\n1 0\np s\nps\nTi 100 ps\n5000 ps\n600 700 800 900 1200 1400 1600 600 700 800 900 1200 1400 1600\nWavelength (nm) Wavelength (nm)\nc d\nProbe @ 0.1 ps Probe @ 1000 ps\n0.5\n0.5\nm\nal.) DSE\nm\nal.)\nT1\nor0.0 or0.0\nn n\nD.\n(\nD.\n(\nO.-0.5 D18:T9TBO-F O. D18:T9TBO-F\nΔ D18:T9TBN-F Δ-0.5 D18:T9SBN-F\n-1.0\nGSBA\n-1.0\n600 700 800 900 1200 1400 1600 600 700 800 900 1200 1400 1600\nWavelength (nm) Wavelength (nm)\ne f\nT GSB\n0.2 0.0\nD18:T9TBO-F\nm\nal.) D18:T9TBN-F\nm\nal.) D\nD\n1\n1\n8\n8\n:\n:\nT\nT\n9\nT\nB\nB\nO\nN-\n-\nF\nF\nD. ( n\nor\n0.1 T1 formation\nD. ( n\nor\n-0.5\nCharge\nO. O. recombination\nΔ Δ\n-1.0\n0.0\n0 50 102 103 0 50 102 103\nTime delay (ps) Time delay (ps)\nSupplementary Fig. 28 Excited-state kinetics in blends based on Se-free NFAs measured by TAS\npumped at 800 nm. TAS color maps and spectra at different time delays of a D18:T9TBO and b\nD18:T9TBN-F blend films. Normalized TAS spectra of blend films probed at c 0.1 ps and d 1000 ps. e\nNormalized TAS dynamics probed around 1500 nm (T ) in different blend films. The data in c, d and e\n1\nare normalized to the minima of GSB s at 0.1 ps, respectively. f Normalized TAS dynamics probed\nA\naround 590 nm (GSB ) in different blend films. The data in f are normalized to their minima\nD\nrespectively for the comparison of recombination kinetics. The ESA signals of T excitons (~1500 nm)\n1\ngradually emerge after 100 ps and rise to their maxima in about 1000 ps. Due to the absence of ISC in\nSe-free NFAs, these formed T excitons should mostly originate from spin-triplet charge-transfer states\n1\n(3CT) formed by non-geminate recombination of free carries at D-A interfaces6. In addition, these Se-\nfree blend films generally exhibit less T formation (Supplementary Fig. 28d) in comparison with the\nT9TBN-F also helps clarify the effects of steric hindrance, where we observed more T generation in\n1\nthe blend film of C2-branched NFA (Supplementary Figs. 28d and e), echoing with the severe\nrecombination of GSB in the sample (Supplementary Fig. 28f). It appears that in Se-free blend films,\nD\nthe larger steric hindrance of the N-alkyl substituents leads to increased generation of T excitons, which\n1\ndiffers from what we observed in the Se-containing blend films.\n3.0E-4\nD18:T9TBO-F 0.228 eV\nD18:T9TBN-F 0.220 eV\n2.0E-4\nL\nE\nE\nQ\nE 1.0E-4\n0.0\n0.000 0.005 0.010 0.015\nSample Current (mA/cm2)\nSupplementary Fig. 29 EQE curves.\nEL\nManuscript, Page 17, Line 6:\n“We also note that T excitons could form through different pathways (i.e., ISC and non-geminate\n1\nrecombination) in these Se-containing NFAs. To provide a profound understanding into the mechanism\nof T formation in different material systems, we further investigated two Se-free NFAs, T9TBO-F (C2-\n1\nbranched N-alkyl chain on the pyrrole unit) and T9TBN-F (C3-branched N-alkyl chain on the pyrrole\nunit) (Supplementary Fig. 26). As shown in Supplementary Fig. 27, in the absence of heavy-atom-\nmediated ISC, there is negligible generation of T excitons in neat NFA films. In the Se-free blend films\n1\n(Supplementary Fig. 28), relatively fewer T excitons are formed compared to the Se-containing blend\n1\nfilms, with the majority arising from non-geminate recombination of free carries at D-A interfaces29. In\ncontrast to the case of Se-containing NFAs, the Se-free NFA with C2-branched N-alkyl chains is\nassociated with an increased generation of T excitons in blend films and a higher non-radiative V loss\n1 oc\nin device (0.228 and 0.220 eV for devices based on C2-branched D18:T9TBO-F and C3-branched\nD18:T9TBN-F, respectively) (Supplementary Fig. 29).”\nResponse to the Reviewer 2\n1. The performance of the D18:S9SHO-F -based binary and ternary devices in this paper is excellent,\nthe author needs to provide the stability test data.\nReply: We appreciate the reviewer for the comment. To verify the stability of the D18:S9SHO-F-based\nbinary and ternary devices, the long-term stability of the cells stored in a nitrogen-filled glovebox and\nthermal stability under annealing at 70 ℃ were evaluated. As shown in Supplementary Fig. 19a, the\nD18:S9SHO-F-based device maintains 80% of the initial efficiency after 300 h storage in the nitrogen-\nfilled glovebox. On the other hand, the D18:L8-BO:S9SHO-F-based cell maintains 82% of the original\nPCE after 300 h. Furthermore, when annealing at 70 ℃ for 200 h, the efficiency exhibits a decay of 30%\nand 26% for devices based on D18:S9SHO-F and D18:L8-BO:S9SHO-F (Supplementary Fig. 19b),\nrespectively. These results should be attributable to the improved molecular packing in ternary cells,\nproviding a more robust morphology and thereby improving the device stability.\nWe have also modified the manuscript and Supporting Information as below.\na b\n1.0\n1.0 D18:S9SHO-F\nD18:L8-BO:S9SHO-F\nC E 0.8 C E 0.8\nP P\nd d\ne e\nm ali\nz\n0.6 D18:S9SHO-F m ali\nz\n0.6\nor D18:L8-BO:S9SHO-F or\nN N\n0.4 0.4\n0 100 200 300 400 500 0 100 200 300\nTime (h) Time (h)\nSupplementary Fig. 19 Stability characterization. a Long-term storage stability of the device in a\nnitrogen-filled glove box. b Thermostability of the devices annealed at 70 °C. The D18:S9SHO-F based\ndevice maintains 80% of the initial efficiency after 300 h in the nitrogen-filled glovebox. On the other\nhand, the D18:L8-BO:S9SHO-F-based cell maintains 82% of the original PCE after 300 h. Furthermore,\nwhen annealing at 70 ℃ for 200 h, the efficiency exhibits a decay of 30% and 26% for devices based\non D18:S9SHO-F and D18:L8-BO:S9SHO-F, respectively.\nManuscript, Page 9, Line 13:\n“Furthermore, the long-term storage stability in a nitrogen-filled glovebox and thermal stability\nannealed at 70 ℃ of the S9SHO-F based binary and ternary devices were performed (Supplementary\nFig. 19). Compared with the binary devices based on S9SHO-F, the ternary D18:L8-BO:S9SHO-F-\nbased OSCs exhibit improved stability, indicating the enhanced molecular packing helps stabilize the\nmorphology in the ternary active layer.”\n2. The authors need to analyze the reasons for the performance improvement of D18:L8-BO:S9SHO-F\nbased ternary devices, including aspects of morphology and exciton dynamics.\nReply: We appreciate the reviewer for the comment. To address the concern from the reviewer, we\nhave conducted GIWAXS measurements on the binary and ternary blend films to investigate the\nmolecular packing and crystallization behavior. As shown in Supplementary Fig. 22 and Supplementary\nTable 9, the D18:L8-BO:S9SHO-F blend exhibits a π−π stacking distance of 3.62 Å in the OOP\ndirection, which is slightly smaller than that of D18:L8-BO blend (3.64 Å). Additionally, the CCLs of\nthe (010) diffraction peak in the OOP direction for the D18:L8-BO and D18:L8-BO:S9SHO-F blends\nare 30.2 and 34.9 Å, respectively. These results indicate the incorporation of S9SHO-F beneficially\nregulates the crystallinity of the blend films, which may lead to more efficient charge-transporting\nproperties. To further corroborate this, TAS measurements were performed to investigate the charge\ncarrier dynamics in the binary and ternary blend films, as shown in Supplementary Fig. 30. The spectra\nand dynamics in Supplementary Fig. 30c-d were normalized to the minima of GSB at 0.1 ps for the\nA\ncomparison of hole-transfer efficiency. Compared with D18:L8-BO film, strong GSB and CS signals\nD\nare observed in D18:L8-BO:S9SHO-F blend after hole transfer, suggesting more efficient charge\nseparation in the ternary film, which will be beneficial for the improved J in the devices.\nsc\nWe have also modified the manuscript as below.\na b\nOOP IP\nD18:L8-BOS9SHO-F 106\nu.)\nD18:L8-BO:S9SHO-F\na.\ny\n(\nsit\nnt e 105\nI D18:L8-BO\nD18:L8-BO\n1 2\nQ Vector [A-1]\nSupplementary Fig. 22 GIWAXS characterizations. a 2D GIWAXS patterns of D18:L8-BO and\nD18:L8-BO:S9SHO-F blends. b The corresponding IP and OOP line-cut profiles of the 2D GIWAXS\nbased on the blend films.\na\nD18:L8-BO ΔO.D. (10-3) D18:L8-BO\n1.4 0.0 -1.4\n103 1\nel a y ( p\ns)\n102GSBD CS\n-3)\nD. ( 1 00 0.1 ps\nm e\nd101\nΔ\nO.\n-1\n1\n1 0\np s\nps\nTi 100 ps\n5000 ps\nLE\n0\n600 700 800 900 1200 1400 1600 600 700 800 900 1200 1400 1600\nWavelength (nm) Wavelength (nm)\nb\nD18:L8:S9SHO-F ΔO.D. (10-3) D18:L8:S9SHO-F\n1.4 0.0 -1.4\n103\n1\nel a y ( p s) 102GSBD CS -3) D. ( 1 00 0.1 ps\nm e\nd101\nΔ\nO.\n-1\n1\n1 0\np s\nps\nTi 100 ps\n5000 ps\nLE\n0\n600 700 800 900 1200 1400 1600 600 700 800 900 1200 1400 1600\nWavelength (nm) Wavelength (nm)\nc d\nProbe @ 100 ps GSB\nD\nD18:L8-BO\n0.2 D18:L8:S9SHO-F 0.0\nΔ\nO.\nD. ( n\nor m\nal.)\n-\n-\n0\n0\n0\n.\n.\n.\n4\n2\nO. D.\n(nor\nmal.)\nProbe @ 0.1 ps\nΔ\nO.\nD. ( n\nor m\nal.)\n-\n-\n0\n0\n.\n.\n4\n2\ns e\nC\np\nh\na\na\nr\nr\na\ng\nt\ne\nio n\nD\nD\n1\n1\n8\n8\n:\n:\nL\nL\n8\n8\n- :S\nB\nSHO-F\n-0.6 Δ -0.6\nGSBD\n600 700 800 900 1200 1400 1600 0 50 102 103\nWavelength (nm) Time delay (ps)\nSupplementary Fig. 30 Excited-state kinetics in binary and ternary blends measured by TAS. TAS\ncolor maps and spectra at different time delays of a D18:L8-BO and b D18:L8-BO:S9SHO-F blend\nfilms. c Normalized TAS spectra of blend films probed at 100 ps. d Normalized TAS dynamics probed\naround 590 nm (GSB ) in different blend films. The spectra and dynamics in Supplementary Fig. 30c-\nD\nd were normalized to the minima of GSB at 0.1 ps for the comparison of hole-transfer efficiency.\nA\nCompared with D18:L8-BO film, strong GSB and CS signals are observed in D18:L8-BO:S9SHO-F\nD\nblend after hole transfer, suggesting more efficient charge separation in the ternary film, which will be\nbeneficial for the improved J in the devices.\nsc\nSupplementary Table 9. Summary of GIWAXS parameters of the binary and ternary blend films.\nOut-of-Plane In-Plane\nUnit cell long axis\nπ-π stacking cell axis (010)\n(100)\nd- d-\nFWHM Coherence\nq (Å-1) spacing q (Å-1) spacing\n(Å-1) length (Å)\n(Å) (Å)\nD18:S9SBH-F 1.713 3.668 0.17845 32.0 0.305 20.61\nD18:S9S3B-F 1.708 3.679 0.18562 30.8 0.318 19.76\nD18:S9SHO-F 1.713 3.668 0.16846 34.0 0.303 20.73\nD18:S9SHN-F 1.713 3.668 0.18309 31.2 0.310 20.67\nD18:L8-BO 1.728 3.636 0.18975 30.2 0.309 20.33\nD18:L8-BO:S9SHO-\n1.735 3.621 0.16416 34.9 0.298 21.09\nF\nManuscript, Page 14, Line 8:\n“Subsequently, the GIWAXS measurements of D18:L8-BO and D18:L8-BO:S9SHO-F blend films\nwere employed to investigate the molecular packing and crystallization behavior. As shown in\nSupplementary Fig. 22, the D18:L8-BO:S9SHO-F blend exhibits a π−π stacking distance of 3.62 Å in\nthe OOP direction, which is slightly smaller than that of D18:L8-BO blend (3.64 Å). Additionally, the\nCCLs of the (010) diffraction peak in the OOP direction for the D18:L8-BO and D18:L8-BO:S9SHO-\nF blends are 30.2 and 34.9 Å, respectively. These results indicate the incorporation of S9SHO-F\nbeneficially regulates the crystallinity of the blend films, which may lead to more efficient charge-\ntransporting properties.”\nManuscript, Page 18, Line 17:\n“In addition, the charge carrier dynamics in the binary D18:L8-BO and ternary D18:L8-BO:S9SHO-F\nblends were investigated by TAS measurements. As shown in Supplementary Fig. 30, compared with\nD18:L8-BO film, strong GSB and CS signals are observed in D18:L8-BO:S9SHO-F blend after hole\nD\ntransfer, suggesting more efficient charge separation in the ternary film, which should benefit the\nimproved J in the corresponding devices.”\nsc\n3. In describing the increase in CCL observed in the GIWAXS of the blend films, the authors infer that\n“indicating a better morphology compatibility between S9SHO-F and D18”. It is necessary to conduct\nthin film surface contact angle experiments to verify the compatibility between the donor and acceptor.\nReply: We appreciate the reviewer for the comment. In response to the reviewer's concern, the contact\nangle measurements were performed to investigate the miscibility among the polymer donor and small\nmolecular acceptor. As shown in Supplementary Fig. 21 and Supplementary Table 10, the Flory–\nHuggins interaction parameter (χ) was calculated by the surface energy though the equation: χ =\n𝐾(√𝛾\nA\n−√𝛾\nB\n)2, in which K is a constant, γ\nA\nand γ\nB\nrepresents the surface energy of pure materials.\nThe χ parameters of D18:S9SBH-F, D18:S9S3B-F, D18:S9SHO-F and D18:S9SHN-F are 0.20K, 0.51K,\n0.15K and 0.40K, respectively. This indicates a high miscibility of D18:S9SHO-F blend with a more\nhomogeneous morphology, which would benefit the charge transport and exciton dissociation in the\nactive layer.\nWe have modified the manuscript and supporting information as below.\nD18 S9SBH-F S9S3B-F S9SHO-F S9SHN-F\nWater\nGlycol\nSupplementary Fig. 21 Contact angles for D18, S9SBH-F, S9SEB-F, S9SHO-F and S9SHN-F blend\nfilms measured by water and glycerol.\nSupplementary Table 10. Summary of contact angle, surface energy and Flory–Huggins interaction\nparameter of D18, S9SBH-F, S9S3B-F, S9SHO-F and S9SHN-F.\nθ (°) θ (°) γ (mJ/m2) χa) (× K mJ/m2) with D18\nwater glycerol\nD18 108.42 95.21 15.11 -\nS9SBH-F 92.51 84.97 18.76 0.20\nS9S3B-F 93.72 82.01 21.20 0.51\nS9SHO-F 98.92 87.29 18.23 0.15\nS9SHN-F 92.40 82.49 20.44 0.40\na) Flory–Huggins interaction parameters (χ) is calculated by the surface energy with the equation: χ =\n𝐾(√𝛾\nA\n−√𝛾\nB\n)2, in which K is a constant, γ\nA\nand γ\nB\nrepresents the surface energy of pure materials.\nManuscript, Page 18, Line 17:\n“Furthermore, we conducted the contact angle measurements to verify the miscibility between the\npolymer donor and small molecular acceptor. As shown in Supplementary Fig. 21 and Supplementary\nTable 10, the Flory–Huggins interaction parameter (χ) was calculated by the surface energy and the χ\nparameters of D18:S9SBH-F, D18:S9S3B-F, D18:S9SHO-F and D18:S9SHN-F are 0.20K, 0.51K,\n0.15K and 0.40K, respectively. This indicates a high miscibility of D18:S9SHO-F blend with a more\nhomogeneous morphology, which would benefit the charge transport and exciton dissociation in the\nactive layer.”\n4. Generally, side chains with larger steric hindrance tend to hinder molecular packing, which is not\nconducive to the formation of a tight and ordered packing network. In this paper, how do the starting\nposition of the branched chains and the length of the branched chains affect molecular packing? The\nauthor needs to carefully analyze the molecular single crystal structure.\nReply: We appreciate the reviewer for the insightful comment. To better explain how side-chain steric\nhindrance affects the molecular packing in our work, we have analyzed the single-crystal data of NFAs\nin a more in-depth manner and provided detailed explanations.\nFrom the single-crystal analysis, S9SBH-F with the shortest N-alkyl chain forms an ordered 3D\nhoneycomb network, similar to the previously reported Y6 (J. Am. Chem. Soc. 2020, 142, 15246-15251).\nCompared with S9SBH-F, S9SHO-F with longer branched N-alkyl chains exhibits a long-range ordered\npacking in the oblique direction across the crystal lattice (Supplementary Fig. 11), forming columns\ncomposed of π-stacked NFA molecules with a small average π-π stacking distance (3.38 Å for S9SBH-\nF, and 3.20 Å for S9SHO-F). The transition of crystal packing structure from a 3D honeycomb network\n(S9SBH-F) into a network of rather independent columns (S9SHO-F) is caused by the long N-alkyl\nchains occupying the interval between the adjacent columns in S9SHO-F single crystal, and results in\na mismatch between the two parallel molecules. When changing the branching site from C2-position to\nC3-position, although the branching site of N-alkyls moves farther from the π-system, they still occupy\nthe interval between the adjacent molecular columns thus the packing patterns of S9S3B-F and S9SHN-\nF are similar to that of S9SHO-F (Supplementary Fig. 12). We also infer that the distance between the\nneighboring columns, which is associated with the size of N-alkyl substituents, may offer the flexibility\nfor NFA molecules to pack in a tighter manner. Therefore, we observe a reverse trend between the\ndistance in the neighboring columns (10.8 Å < 15.3 Å < 19.5 Å) and π-π stacking distance (3.52 Å >\n3.50 Å > 3.20 Å) in S9S3B-F, S9SHN-F, and S9SHO-F (Fig. 1e and Supplementary Table 6).\nTo further investigate the π-π stacking in the neat NFAs, we conducted grazing-incidence wide-angle\nX-ray scattering (GIWAXS) measurement of the C2-position and C3-position acceptors. As shown in\nSupplementary Fig. 13, S9SHO-F shows a (010) diffraction peak at 1.76 Å-1 corresponding with a d-\nspacing of 3.57 Å in the out-of-plane (OOP) direction. When changing the branching site to C3-position,\nS9SHN-F exhibits a (010) diffraction peak at 1.73 Å-1 with a d-spacing of 3.63 Å in the OOP direction,\nindicating a slightly more intimate packing in C2-position acceptor. These results are consistent with\nour conclusions obtained from the single crystal analysis.\nWe have modified the manuscript and supporting information as below.\nS9SHO-F\nS9SBH-F\na b\n19.5 Å\n5.9 Å\nc\nb a\nc\na b\nd\nc\nSupplementary Fig. 11 Side view of the molecular packing of (a,c) S9SBH-F and (b,d) S9SHO-F,\nwhere the N-alkyl chains on the pyrrole unit are (a,b) presented or (c,d) omitted.\na S9S3B-F b S9SHN-F\n15.3 Å\n10.8 Å\nc\na\nb\nc\na\nb\nc d\nSupplementary Fig. 12 Side view of the molecular packing of (a,c) S9S3B-F and (b,d) S9SHN-F,\nwhere the N-alkyl chains on the pyrrole unit are (a,b) presented or (c,d) omitted.\na S9SBH-F S9SHO-F b 104\nOOP\n)\nu.103 S9SHO-F\na.\n(\ny\nt\nsi\nn\ne\nS9SHN-F n t102 S9SHN-F\nI\n101\n1 2\nQ Vector [A-1]\nSupplementary Fig. 13 GIWAXS characterizations for C2-branching S9SHO-F and C3-branching\nS9SHN-F. a 2D GIWAXS patterns of the NFA neat films. b The corresponding OOP line-cut profiles\nof the 2D GIWAXS patterns from the neat films.\nManuscript, Page 6, Line 21:\n“S9SBH-F with the shortest N-alkyl chain forms an ordered 3D honeycomb network, similar to the\npreviously reported Y633. Compared with S9SBH-F, S9SHO-F with longer branched N-alkyl chains\nexhibits a long-range ordered packing in the oblique direction across the crystal lattice (Supplementary\nFig. 11), forming columns composed of π-stacked NFA molecules with a small average π-π stacking\ndistance (3.38 Å for S9SBH-F, and 3.20 Å for S9SHO-F). The transition of crystal packing structure\nfrom a 3D honeycomb network (S9SBH-F) into a network of rather independent columns (S9SHO-F)\nis caused by the long N-alkyl chains occupying the interval between the adjacent columns in S9SHO-F\nsingle crystal, and results in a mismatch between the two parallel molecules. When changing the\nbranching site from C2-position to C3-position, although the branching site of N-alkyls moves farther\nfrom the π-system, they still occupy the interval between the adjacent molecular columns thus the\npacking patterns of S9S3B-F and S9SHN-F are similar to that of S9SHO-F (Supplementary Fig. 12).\nWe also infer that the distance between the neighboring columns, which is associated with the size of\nN-alkyl substituents, may offer the flexibility for NFA molecules to pack in a tighter manner. Therefore,\nwe observe a reverse trend between the distance in the neighboring columns (10.8 Å < 15.3 Å < 19.5\nÅ) and π-π stacking distance (3.52 Å > 3.50 Å > 3.20 Å) in S9S3B-F, S9SHN-F, and S9SHO-F (Fig.\n1e and Supplementary Table 6).”\nManuscript, Page 7, Line 18:\n“Subsequently, the grazing-incidence wide-angle X-ray scattering (GIWAXS) measurement was\nperformed to further investigate the π-π stacking in the C2-branching S9SHO-F and C3-branching\nS9SHN-F in thin films. As shown in Supplementary Fig. 13, S9SHO-F shows a (010) diffraction peak\nat 1.76 Å-1 corresponding with a d-spacing of 3.57 Å in the out-of-plane (OOP) direction. When\nchanging the branching site to C3-position, S9SHN-F exhibits a (010) diffraction peak at 1.73 Å-1 with\na d-spacing of 3.63 Å in the OOP direction, indicating a slightly more intimate packing in C2-branching\nS9SHO-F. These results are consistent with our conclusions obtained from the single crystal analysis.”\n5. In most cases, selenium atom substitution results in the red shift of absorption, however, the inferior\nPCEs (FFs) comparing to their sulfur counterparts, especially for 5Se acceptor with 5-selenium\nsubstitution. What is the inner reason for that?\nReply: We appreciate the reviewer for the insightful comment. Selenium is more polarizable than sulfur,\nand the lone pairs on selenium facilitates stronger interatomic interactions between adjacent selenium\natoms, resulting in enhanced intermolecular Se⋯Se interactions (J. Am. Chem. Soc. 2015, 137, 1314;\nAcc. Chem. Res. 2021, 54, 3906; Sol. RRL 2022, 6, 2200156). When introducing 5 selenium to the\ndesign of small-molecule acceptors, it may induce excessively strong intermolecular Se⋯Se\ninteractions, leading to over-aggregation of the acceptor phase and inadequate miscibility between the\ndonor and acceptor, resulting in a non-ideal morphology. Therefore, the generation and transport of\ncharge carrier could be prohibited and leads to an inferior FF, as well as PCE.\n6. Some format suggestions: for abbreviations that appear for the first time in the article, their\ncorresponding English full names should be listed, such as the abbreviation of “GIWAXS”. It is\nsuggested that the authors carefully check the entire text.\nReply: We appreciate the reviewer for the careful reviewing. We have checked the manuscript carefully\nand modified the manuscript accordingly as below.\nManuscript, Page 7, Line 18:\n“the grazing-incidence wide-angle X-ray scattering (GIWAXS)”\nManuscript, Page 11, Line 5:\n“charge collection probability (P )”\ncoll\nResponse to the Reviewer 1\n1. The D18:L8-BO:S9SHO-F ternary blend with a weight ratio of 1:1.05:0.15 achieved a surprisingly\nhigh JSC of 28.85 mA cm-2 and maintained a modest VOC of 0.884 V. would like to know the differences\nin the EQE spectra among the D18:L8-BO and D18:S9SHO-F binary devices and the D18:L8-\nBO:S9SHO-F ternary device. Could the authors provide a plot involving these devices for\nstraightforward comparison?\nReply: We appreciate the reviewer for the contributive comment. S9SHO-F exhibits a strong\nphotoresponse in the near-infrared region, which highly benefits the J . To make a clear comparison of\nsc\nthe differences on the photocurrent, we combine the EQE spectra of the binary and ternary devices into\na single figure. As shown in Supplementary Fig. 17b, the D18:L8-BO exhibits an EQE tail at around\n900 nm, however, the devices based on D19:S9SHO-F have a strong photoresponse in the range of 900-\n1000nm. When S9SHO-F is integrated into the binary material system of D18:L8-BO, it effectively\nenhances the light-harvesting capability from 830-930 nm, significantly contributing to the improved\nJ observed in the ternary device.\nsc\nWe have modified the manuscript and supporting information as below.\na b\n100\n-2)\nm 0 80\nc\nA\nm\n( y %) 60\nd e n\nsit\nD18:L8-BO:S9SHO-F E Q E\n(\nD18:L8-BO:S9SHO-F\nnt 40 D18:S9SHO-F\nurr e -20 D18:L8-BO\nC\n20\n0\n0.0 0.2 0.4 0.6 0.8 1.0 400 600 800 1000\nVoltage (V) Wavelength (nm)\nSupplementary Fig. 17 a Optimized J-V curve of D18:L8-BO:S9SHO-F based OSC. b EQE curves of\nD18:L8-BO:S9SHO-F, D18:S9SHO-F and D18:L8-BO based OSCs.\nManuscript, Page 10, Line 3:\n“Furthermore, we explored the impact of S9SHO-F on the photoresponse characteristics in the ternary\ndevice. As shown in Supplementary Fig. 17b, the D18:L8-BO exhibits an EQE tail at around 900 nm;\nhowever, the devices based on D19:S9SHO-F have a strong photoresponse in the range of 900-1000nm.\nWhen S9SHO-F is integrated into the binary material system of D18:L8-BO, it effectively enhances the\nlight-harvesting capability in the range of 830-930 nm, significantly contributing to the improved J\nsc\nobserved in the ternary device.”\n2. In Supplementary Table 8, the VOC of D18:S9SHO-F should be “0.835” instead of “0.935”. I\nsuggest that the authors double-check the manuscript and supplementary information to avoid similar\nerrors.\nReply: We appreciate the reviewer for the careful reviewing. We have checked the manuscript carefully\nand modified the manuscript accordingly as below.\nSupporting Information, Page 82:\nSupplementary Table 8. Summary of the detailed energy losses of the OSCs based on the\nD18:NFAs blend.\nV E V ΔE ΔE ∆V E\nOC g OC, SQ 1 2 oc,non-rd loss\nDevices\n(V) (eV) (V) (eV) (eV) eV) (V)\nD18:S9SBH-F 0.833 1.369 1.110 0.259 0.097 0.180 0.536\nD18:S9S3B-F 0.800 1.359 1.100 0.259 0.109 0.191 0.559\nD18:S9SHO-F 0.835 1.366 1.106 0.260 0.096 0.175 0.531\nD18:S9SHN-F 0.824 1.358 1.099 0.259 0.090 0.185 0.534"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The author has addressed most of my concerns, but there are still a few minor issues that require revision.\n> The D18:L8-BO:S9SHO-F ternary blend with a weight ratio of 1:1.05:0.15 achieved a surprisingly high JSC of 28.85 mA cm-2 and maintained a modest VOC of 0.884 V. would like to know the differences in the EQE spectra among the D18:L8-BO and D18:S9SHO-F binary devices and the D18:L8-BO:S9SHO-F ternary device. Could the authors provide a plot involving these devices for straightforward comparison?\n> In Supplementary Table 8, the VOC of D18:S9SHO-F should be “0.935” instead of “0.935”. I suggest that the authors double-check the manuscript and supplementary information to avoid similar errors.", "reviewer_2": "The authors have addressed all the concerns from the reviewers. So I recommend its publication without further scientific revisions."}, "author_response": ""}]

Decision letter