LUMO’s modulation by electron withdrawing unit modification in TAT dumbbell-shaped molecules

Journal ofMaterials Chemistry A

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LUMO's modulat

aInstitut de Chimie et Procedes pour l'Energ

Departement d'Ingenierie Polymere, UMR 7

de Chimie, Polymeres et Materiaux, 25 ru

E-mail: [email protected] ICube, Universite de Strasb

Strasbourg, FrancecInstitut de Physique et de Chimie des Mate

84047, 67034 Strasbourg Cedex 02, FrancedICPEES-LCOSA, Ecole Europeenne de Ch

LCOSA, UMR 7515 associee au CNRS, 25

02, France

† Electronic supplementary informatiomolecules and characterization datavoltammograms, and DFT gures). See D

Cite this: J. Mater. Chem. A, 2015, 3,6620

Received 25th January 2015Accepted 15th February 2015

DOI: 10.1039/c5ta00624d

www.rsc.org/MaterialsA

6620 | J. Mater. Chem. A, 2015, 3, 662

ion by electron withdrawing unitmodification in amorphous TAT dumbbell-shapedmolecules†

Ibrahim Bulut,a Patrick Leveque,b Benoıt Heinrich,c Thomas Heiser,b Rony Bechara,b

Nicolas Zimmermann,b Stephane Mery,c Raymond Ziesseld and Nicolas Leclerc*a

The synthesis and characterization of a series of dumbbell-shaped molecules constructed from paraffin-

free central dyes linked to two triazatruxene (TAT) units are described. The photovoltaic properties of

these new glassy electron-donating molecules in a bulk heterojunction with fullerene derivatives have

been studied. For this series of molecules, the local-range molecular packing in the amorphous solid

state is mainly influenced by the TAT units bearing bulky side-chains, while the optical properties and

energy levels are tuned by the choice of the central core. Therefore, TAT acts as a structuration platform

in thin films for pure or blended active layers and independently, the optoelectronic properties are driven

by the choice of the central dye. Photovoltaic cells based on these materials exhibited power conversion

efficiencies of up to 3.5%, among the top values reported so far for soluble amorphous small molecule

based organic photovoltaic devices. These TAT-based derivatives are a promising platform to reach high

solar cell efficiencies as well as to understand in detail the impact of the chemical structure on the

optoelectronic properties.

Introduction

The eld of soluble semiconducting small molecules fororganic photovoltaic (OPV) applications has blossomed over thelast decade. An intensive effort in the design and synthesis ofadvanced molecules together with the device optimization hasallowed the power conversion efficiency of bulk heterojunction(BHJ) solar cells processed from soluble molecules to rapidlyexpand from 4.4% (ref. 1) in 2009 to more than 9% nowadays.2

Optimization of their performances requires a convergenteffort from chemists and physicists, as it usually requires acompromise between optoelectronic and physico-chemicalproperties. Small molecules used as an electron donor in a BHJsolar cell should meet several prerequisites, in particular (i) a

ie, l'Environnement et la Sante (ICPEES),

515 associee au CNRS, Ecole Europeenne

e Becquerel, 67087 Strasbourg, France.

ourg, CNRS, 23 rue du Loess, 67037

riaux de Strasbourg, 23 rue du Loess, BP

imie, Polymeres et Materiaux, ICPEES-

rue Becquerel, 67087 Strasbourg Cedex

n (ESI) available: Synthesis of the(1H and 13C NMR, DSC, cyclic

OI: 10.1039/c5ta00624d

0–6628

broad absorption spectrum in the visible wavelength rangealong with a high extinction coefficient, (ii) suitable frontierenergy levels (HOMO and LUMO levels) to promote excitondissociation and a high open circuit voltage, (iii) a sufficientsolubility for wet processing and (iv) an appropriate solid statestructuration to sustain charge transport and collection.3

The ne-tuning of the optoelectronic properties of electron-donor polymers or small molecules has been performedfrequently by using the alternating donor–acceptor (D–A)approach.4 The strength of the intramolecular charge transfer(ICT) resulting from this D–A alternation can be easily modu-lated by the nature of electron-rich and electron-decient unitsconstituting the D and A building blocks. However, it has beenshown recently that an accurate prediction of the structure–optoelectronic property relationship is difficult, as a smallchange in chemical structure might inuence these propertiesstrongly.5 For instance, alkyl side chains, which are necessary topromote solubility in organic solvents, have been found toimpact the electronic properties as well. Indeed, their nature orgraing position affected markedly both, the intra-chain delo-calization and the inter-molecular packing by steric hindrance,inuencing in turn the charge carrier transport properties.6

Alkyl side chains could also act on the electron-donor/electron-acceptor miscibility and therefore impact on the optimumelectron-donor/-acceptor ratio.7

Recently, we introduced the triazatruxene (TAT) unit as achemical moiety for OPV applications that combine both aplanar shape, inuencing favorably the active layer morphology,

This journal is © The Royal Society of Chemistry 2015

Scheme 1 Suzuki cross-coupling reaction step between the boronated-TAT derivative and the dibrominated-central moiety including Btz, Pytzand Ttz unit.

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and a good solubility due to the three nitrogen-carried alkyl sidechains.8 The good performances showed that the TAT core canbe used as an effective electron-donating building block formolecular materials in BHJ solar cells. We aimed at developinga soluble TAT core as an electron-rich platform that allows thecombination with a wide choice of side-chain-free electron-decient units. The TAT core is expected to provide by its ownenough solubility for the solution processing and to act as astructuring agent in the solid state. Note that the TAT fragmenthas also been used by other groups as building blocks for effi-cient star-shaped electroluminescent9 and OPV10 materials. Inthis paper, we report on the synthesis, material characterizationand photovoltaic properties of a new series of dumbbell-shapedsolution-processable conjugated molecules using TAT as theend-capper and various “naked” central electro-decient units A(Scheme 1). This approach is expected to bring high chemicaldesign exibility as it allows us to handle almost separately themorphological (driven by the TAT) and optoelectronic issues.We anticipate, by changing the naked central group, to tunequasi-independently the optoelectronic properties of the elec-tron donor molecule without changing signicantly itsmorphological behavior in the BHJ solar cell active layer.

Fig. 1 UV-visible absorption spectra of the TAT-based molecules recor

This journal is © The Royal Society of Chemistry 2015

Herein, we used three classical electron-decient units A withincreasing electron affinity: benzo[2,1,3]thiadiazole (Btz) < pyr-idal[2,1,3]thiadiazole (Pytz) < thieno[1,2,5]thiadiazole (Ttz).4

These units were sandwiched between two TATs including onethiophene unit to form the central core of the dumbbell-shapedmolecule, as shown in Scheme 1. All new molecules wereblended with [6,6]-phenyl-C71-butyric acid methyl ester(PC71BM) to form BHJ solar cells, and gave rise to efficientpower conversion efficiencies, with a maximum value of 3.5%.

Results and discussionSynthesis

The general synthetic protocol for the preparation of TAT-basedcompounds is sketched in Scheme 2.

TAT being a carbazole analogue, two isomers of the boronatederivative, the 2-boronate-5,10,15-triazatruxene (1,meta-isomer)and the 3-bromo-5,10,15-triazatruxene (2, para-isomer), areaccessible. In a previous study, we observed almost no differ-ence in the optoelectronic properties between the meta- andpara-isomers when substituted with a diketopyrrolopyrrole(DPP) core.8a

ded in chloroform (a) and in thin film (b).

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Scheme 2 Synthetic pathway for the preparation of the TAT-based chromophores. (i) K2CO3, Pd2(dba)3, P(o-tolyl)3, toluene/1,4-dioxane, reflux.

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However, a slight improvement in the nal power conversionefficiency (PCE) measured with the meta-isomer was estab-lished. Because of the invariance of properties measured by UV-visible and cyclic-voltammetry and conrmed by density func-tional theory (DFT) calculations, we suggest that this increase inthe PCE was most probably due to a slight morphologicaldifference at the nanometer scale. In order to help clarifyingthis result, we decided to synthesize the two TAT-isomers in thecase of the benzothiadiazole-based dye.

Hence, the two boronated-TAT isomers were synthesizedfollowing a previously reported protocol.8a The synthesis of thethree central chromophores was undertaken according to theprocedure described in the ESI section.†

The dumbbell-shaped molecules were obtained using a nalSuzuki–Miyaura cross-coupling reaction between the boro-nated-TAT derivative and the dibrominated central dye, in thepresence of Pd2dba3 catalyst, P(o-tolyl)3 ligand and K2CO3 in areuxing toluene/1,4-dioxane solvent mixture for 48 hours(Scheme 2). All compounds were isolated in good purity andwith moderate yields (ranging from 19 to 45%), aer purica-tion steps including column chromatography and precipitation.

6622 | J. Mater. Chem. A, 2015, 3, 6620–6628

Novel compounds were characterized by NMR and MALDI-TOF(see ESI†), and unambiguously correspond to the drawnstructures.

Material properties

The TAT-derivatives 6m and 6p–8p have been intensivelyinvestigated by a number of techniques in order to characterizetheir thermal and optical properties, as well as to determinetheir energy levels and hole mobility values. The results aregathered in Table 1.

UV-vis absorption spectra of the molecules in chloroformsolutions and in thin lms are given in Fig. 1, and the peakmaximum of the low energy transitions is reported in Table 1.

All absorption spectra of the TAT compounds in the solutionand in thin-lm exhibit three maxima. Two of which are locatedbelow 500 nm and the third one appears between 500 and 900nm. The latter is dependent on the electron affinity of thecentral acceptor unit and is therefore likely to originate from aninternal charge transfer (ICT).11 This ICT band is more red-shied when the strength of the electron acceptor increases

This journal is © The Royal Society of Chemistry 2015

Table 1 Thermal, optical and electrochemical properties of the TAT-based compounds and the measured hole mobility values

TAT derivative

lmax (nm)

Eg optb (eV) HOMOc (eV) LUMOc (eV) Eg CV

d (eV) Tge (�C)

Mobility(cm2 V�1 s�1)

Solutiona Film Saturationf Linearg

6m 547 571 1.8 �5.2 �3.6 1.6 69 6 � 10�5 4 � 10�5

6p 547 571 1.8 �5.2 �3.6 1.6 83 6 � 10�5 4 � 10�5

7p 602 612 1.7 �5.3 �3.8 1.5 85 8 � 10�5 6 � 10�5

8p 747 761 1.4 �5.1 �3.9 1.2 85 1 � 10�5 1 � 10�5

a Chloroform solution. b Calculated from the absorption edge in the thin lm. c Determined from cyclic voltammetry measurements indichloromethane. d Eg CV ¼ Eox � Ered.

e Glass transition as determined by the DSC. f Determined on as-cast OFET from output characteristicsin the saturation regime. g Determined on as-cast OFET from output characteristics in the linear regime.

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from 6p to 7p and then to 8p. It is interesting to note that almostno difference could be observed between the spectrum of TAT-meta (6m) and TAT-para (6p) regioisomers having the benzo-thiadiazole (BTz) unit as an electron acceptor unit. This result isin agreement with previous observations on similar architec-tures using DPP as the central chromophore.8a All thin-lmspectra are broadened and red-shied compared to their solu-tion counterparts (from 20 to 40 nm) indicating higher inter-molecular interactions within the solid state. One can note thatthe bathochromic shi between the solution and solid-state issmaller for compound 8p (Dl ¼ 20 nm) compared to the othercompounds (Dl ¼ 40 nm).

The solid-state energy bandgaps (Eg opt) estimated from thelow energy band edges of the thin-lm absorption spectra rangefrom 1.8 eV to 1.4 eV (see Table 1). As expected, the energybandgap of compound 8p is the lowest and makes it a goodcandidate for near-infrared optoelectronic devices. Moreover,the energy bandgap of compound 8p is close to the optimalvalue predicted by Queisser et al. for homojunction solar cells.12

As anticipated, changing the central core electron withdrawingstrength (from Btz to Pytz and to Ttz) broadens the absorptionspectra distribution and therefore allows covering a wider rangeof the UV-visible spectrum.

Cyclic voltammetry characterizations of all compounds havebeen performed in solution (dichloromethane) with a platinum

Fig. 2 Cyclic voltammograms of the TAT-based molecules indichloromethane solution using TBAPF6 as the supporting electrolyte.

This journal is © The Royal Society of Chemistry 2015

electrode to determine the HOMO and LUMO levels of themolecules. PC71BM and ferrocene were used as internal refer-ences, enabling the determination of the frontier molecularorbital energies. Each central dye (3, 4 and 5) with its respectiveelectron acceptor unit (Btz, Pytz and Ttz) has been also char-acterized individually (see Fig. S4 in the ESI†). The cyclic vol-tammograms corresponding to the novel compounds (6m and6p–8p) are reported in Fig. 2. The HOMO and LUMO levels andthe electrochemical gap calculated from the electrochemicaldata are given in Table 1. The TAT fragment is not electroactivein reduction in the potential ranges explored (down to �2.1 V).For all compounds, only the central sub-units exhibit reductionpotentials with two reduction waves. In contrast, the positivepotential region of the voltammograms presents more featuressince both, the central unit and the TAT fragments, showoxidation waves. Indeed, the TAT fragment presents threereversible mono-electronic oxidations in the range of 0.4 to 1.4V, while all central units present two mono-electronic reversibleand/or quasi-reversible oxidation waves in a similar range of 0.3to 1.4 V.

In practice for compounds 6m and 6p–7p, the HOMO energylevels are mainly governed by the rst oxidation potential of theTAT fragment. This leads to a similar HOMO level for all theseTAT-compounds in the order of �5.25 eV.

Fig. 3 Energy level diagram of TAT-based compounds and PC71BM, asdetermined by cyclic voltammetry.

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Fig. 4 Calculated LUMO (above) and HOMO (below) levels forcompound 7p.

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Compound 8p presents a particular behavior as the higherHOMO level value of �5.11 eV is due to the oxidation of the Ttz-based central unit. Unlike HOMO levels, the LUMO levels arestrongly dependent on the nature of the central withdrawingunit. Thus, as the electron withdrawing strength increases, theLUMO values decreases from�3.60 to�3.84 and to�3.92 eV forcompounds 6p, 7p and 8p, respectively. Altogether, the elec-trochemical bandgap of the TAT compounds is found toincrease from 8p (1.2 eV), 7p (1.42 eV) to 6p–6m (1.64 eV), inagreement with UV-vis absorption measurements (see Fig. 3).

Finally, it is interesting to note similar electrochemical dataobtained for 6p and 6m, showing the invariance of redoxproperties on the functionalization position (meta or para) tothe TAT fragment. This observation is in agreement again withprevious studies on similar compounds.8a

Density functional theory (DFT) calculations have been per-formed on different compounds at the B3LYP/6-311+G* level oftheory in a vacuum using the Spartan 10 soware.13 Tomaintainthe calculation time reasonable, the alkyl chains were replacedby methyl groups. The resulting HOMO and LUMO level posi-tions along the conjugated backbone for compound 7p areexemplied in Fig. 4. The HOMO frontier orbitals for all mole-cules extend slightly onto the TAT units, while the LUMO

Table 2 Calculated HOMO and LUMO levels for the different TAT-based molecules and intermediate compounds

TAT-based moleculesand intermediate compounds HOMO (eV) LUMO (eV)

1a and 2a �4.80 �0.503b �5.30 �2.614b �5.49 �2.895b �4.91 �3.006m �4.61 �2.546p �4.67 �2.477p �4.71 �2.738p �4.40 �2.83

a Unboronated triazatruxene fragment. b Unbrominated central dyes.

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frontier orbitals remain essentially on the central part of themolecule, similarly to what has been observed also previously.8a

All molecules presented similar energy level features (seeFig. S5–S7 in the ESI†).

Despite the usual offset between the calculated andmeasured HOMO and LUMO levels (0.6 � 0.1 eV for the HOMOlevel and 1.10 � 0.04 eV for the LUMO level), the trend observedfor the measured and calculated levels of compounds 6m and6p–8p is the same (compare values in Tables 1 and 2). In orderto understand how the central unit affects the HOMO andLUMO levels, calculations using the same formalism were per-formed on intermediate compounds 1–5.

The energy offset between the TAT LUMO and all the centralcores' LUMO is too high (more than 2.1 eV) to promotehybridization between LUMOs of different constituents.Therefore, the LUMO of the dumbbell-shaped molecules (6mand 6–8p) is driven by the chemical fragment with the deepestLUMO, i.e. by the central unit. The situation is different for theHOMO levels as the offset between the TAT HOMO and thecentral dye HOMO ranges from 0.11 eV (1 and 5) to 0.69 eV (1and 4). Hybridization between HOMOs of different constituentscan therefore be very efficient, especially between dyes 1 (or 2)and 5. The consequence is a high lying HOMO level in thedumbbell-shaped molecules with thieno[1,2,5]thiadiazole (Ttz)as a central unit (8p). This is consistent with a previous exper-imental observation highlighting the role of the loweringoxidation potential effect when using the Ttz moiety.14

These data draw an interesting scenario, in which it appearsthat the optical properties and energy levels are mainly gov-erned by the nature of the central core. Moreover, the HOMOlevel of compounds 6m, 6p and 7p is theoretically well posi-tioned to reach a high open circuit voltage (Voc) in organic solarcells since the offset between the LUMO level of PC71BM and theHOMO level of aforementioned compounds is above 1 V.15

Thermal properties have been characterized by differentialscanning calorimetry (DSC, see Fig. S1 in the ESI section†). Theonly thermal event observed in the thermograms is a glass

Fig. 5 Powder SAXS patterns recorded for the TAT-based molecules6m and 6p–8p at room temperature after annealing at 100 �C for 2hours.

This journal is © The Royal Society of Chemistry 2015

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transition, thus revealing that all molecules are amorphous.The three TAT para-isomers 6p–8p exhibit the same glasstransition temperature around Tg ¼ 85 �C, while the meta-isomer 6m exhibit a slightly lower value at Tg ¼ 69 �C (seeTable 1).

The amorphous character of the TAT-based molecules hasbeen conrmed by (powder) small-angle X-ray scattering (SAXS)experiments at room temperature. As seen in Fig. 5, all mole-cules exhibit quite similar patterns.

Obviously, the four SAXS patterns show the same local-rangecorrelated organization, as they contain no sharp reections,but several broad scattering signals with nearly identical loca-tions and widths. In particular a unique maximum at ca. 4.4 A(2q z 20�) is observed in the wide-region that characterizes thelateral packing of neighboring segments. This signal mainlycomes from lateral distances between alkyl chains and thenotable absence of a second maximum at 3.8 A conrms theabsence of p-stacked assemblies. Such molecular assembliesare indeed hindered by the sterically demanding aliphaticchains (especially large for ramied chains), whose overallcross-section would far exceed the available area for face-to-facestacked TAT units.16

The small-angle scattering maxima (at ca. 11–12 and 20–22A) nevertheless prove the persistence of the rigid core/so chainsegregation, the high area requirement of the aliphaticperiphery being then fullled by shis and tilts apart of therigid moieties. This disordered arrangement of conjugatedsegments then results in irregular interfaces with aliphaticdomains, which explains the broad shape of scattering signalsand the macroscopic amorphous state. Thus, it is reasonable toassume that the two bands systematically observed in the SAXSpatterns at ca. 11–12 and 20–22 A (correlation length of 2 to 3molecules) should correspond to irregular piling up of chain-substituted elemental TAT fragments and TAT dumbbell-sha-ped molecules, respectively. A deeper understanding of thesegregated structure would require additional information,such as systematic variations in the aliphatic periphery, goinghowever beyond the scope of this contribution that deals withthe variation of the electro-decient unit in a TAT dumbbell-shaped molecule for OPV applications. The important result tokeep in mind here is that the aliphatic chains nano-segregate inthe periphery of domains formed by semiconducting segments.Even though these segments are not p-stacked, the local orderis sufficient to preserve semiconducting pathways and preservedecent charge transport properties in pure materials (seebelow).

Charge transport properties have been determined inorganic eld effect transistors (OFET) devices with a bottomcontact–bottom gate conguration. Hole-mobility values (Table1) were estimated from the transfer characteristics in the linearas well as the saturation regime. Details on the OFET devicesfabrication can be found in the ESI.† For all TAT-based mole-cules, mobility values ranging from 10�5 to 8 � 10�5 cm2 V�1

s�1 have been obtained on as-cast devices, conrming the weakimpact of the central chromophore on the morphologicalissues. In addition, both meta- and para-isomers (6m, 6p) showthe same mobility values conrming the negligible effect of the

This journal is © The Royal Society of Chemistry 2015

TAT connecting position on the material optoelectronic prop-erties, as evidenced above from the optical and electrochemicalcharacterization results.

Photovoltaic properties

Organic photovoltaic (OPV) devices were prepared in an inver-ted device conguration using the recently developed 80%ethoxylated polyethyleneimine (PEIE) interface layer.17 The thinlayer of this polymer having pendant alkylamine groups isknown to reduce the work function of ITO and can be processedin air from non-toxic alcoholic solutions. It is a highly inter-esting alternative to standard low-work function metal-oxidelms. A reective MoO3/Ag electrode has been used to collectholes on the top surface of the device. The active layers made upof TAT-based molecular compounds:PC71BM blends have beendeposited by spin-coating from chlorobenzene solutions, withand without 1,8-diiodoctane (DIO) as the additive. A thermalannealing step of the active layer has been performed beforeMoO3/Ag evaporation. The optimal thermal annealing temper-atures and times result from experimental adjustments.Although our molecules are amorphous, annealing above theglass transition temperature soens the thin lm and somehowimproves the device performances. Each OPV device had anactive area of 12 mm2. Details on the OPV device fabrication canbe found in the ESI.† Electrical parameters of OPV devices aresummarized in Table 3. The (J–V) curves for the best deviceswith 0.6% DIO are reported in Fig. 6.

For each TAT-based compounds (6m and 6–8p), theoptimum molecule : PC71BM ratio was found around 1 : 5,indicating that a high PC71BM loading is required to extractelectrons efficiently. Such a low donor molecule : PCBM ratiomight probably point out an intercalation of the fullerenederivative into the molecular structure, as already reported in anumber of polymer:fullerene systems.18 In these referencematerials, intercalation occurred only with conjugatedsegments devoid of alkyl chains, which is actually the case withthe naked central core between the two TAT units in ourmaterials. In these chain-free zones, a fraction of PCBM couldcome in contact with the aromatic core of the dumbbell-shapedmolecules to modify the material's morphology and nally thedevice performances. Moreover, the incorporation of PCBMwould favorably compensate the overcrowded chains in the TATfragments in order to obtain better-dened aromatic andaliphatic domains with a sharper interface. This assumption issupported by previous results obtained on TAT-based similarmolecules (see ref. 8a and Fig. 7), in which the use of the centralDPP core sterically hindered by two ramied alkyl chains led toa more balanced optimum molecule : PCBM ratio of about1 : 0.75.

The inuence of the nature and graing position of alkylside-chains on the morphology of TAT-based dumbbell-shapedmolecules as pure materials or in PCBM blends deserves anextensive study that will be published in a forthcoming paper.

Although presenting similar XRD patterns and optoelec-tronic properties, the two benzothiadiazole-based isomers 6mand 6p show signicant differences in OPV performances. The

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Table 3 Photovoltaic properties of the TAT-based molecules under AM 1.5G, 100 mW cm�2 illumination, and after annealing

Molecule Molecule : PC71BM ratio DIO (% vol) Voc (V) Jsc (mA cm�2) FF (%) PCE (%) Thickness (nm)

6ma 1 : 5 0 0.80 6.4 41 2.1 �751 : 5 0.6 0.79 6.8 40 2.1

6pb 1 : 5 0 0.82 8.0 41 2.71 : 5 0.6 0.80 8.1 45 2.9

7pb 1 : 5 0 0.81 7.9 45 2.9 �701 : 5 0.6 0.83 9.4 45 3.5

8pc 1 : 5 0 0.51 3.3 29 0.5 �851 : 5 0.6 0.55 2.2 29 0.4

6p + 7pb 0.5 + 0.5 : 5 0.6 0.79 10.2 39 3.1

a 15 minutes at 110 �C. b 10 minutes at 110 �C. c 10 minutes at 120 �C.

Fig. 6 Current density versus voltage for the best devices with 0.6%DIO. The dashed lines correspond to devices in the dark while thecontinuous lines correspond to standard AM 1.5G (100 mW cm�2)illumination conditions.

Fig. 7 TAT–DPP molecule studied in ref. 8a.

Fig. 8 External quantum efficiency measurements for devices using0.6% DIO (see Table 3).

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origin of the difference is still unclear, but as already noticed ina former study (ref. 8a), we found again a higher PCE for the TATpara-isomer (2.9% for 6p against 2.1% for the meta-isomer 6m)under similar optimized conditions. In agreement with energylevels measured by cyclic voltammetry, the open-circuit voltage(Voc) shows high and constant values around 0.8 V forcompounds 6m, 6p and 7p. The ll-factors are also similar tovalues ranging from 40 to 45%. The highest PCE value of 3.5%has been achieved with dye 7p in blend with PC71BM using 0.6%v/v of 1,8-diiodooctane (DIO) as the high boiling point pro-cessing additive.19 As shown by AFM, adding DIO improves the

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lm morphology by reducing the number of aggregates (seeFig. S8 and S9 in the ESI†). This leads to a better mixing betweenthe donor and acceptor phases and is likely to be at the origin ofthe increase in photocurrent. The increase of PCE between 6p(2.9%) and 7p (3.5%) (relative increase of 20%) is mainlyattributed to the increase of short-circuit current density (Jsc)from 8.1 to 9.4 mA cm�2 (relative increase of 16%). This Jscincrease could be attributed to the lower energy bandgap ofcompound 7p implying a red shi in the absorption spectrum,all other parameters (molecule : fullerene ratio and holemobility) being almost identical.

Unlike other donor molecules, compound 8p behavesdifferently and nally exhibits rather poor OPV performances.

As expected from its lower HOMO level, its Voc is signicantlylower than the one obtained for other molecules. The measuredJsc value for 8p was also found to be rather low despite its lowbandgap. Moreover, a low FF of 29% suggests a poor chargeextraction that could be indicative of a signicant chargerecombination. It has to be noted that for this compound 8p incombination with PC71BM as the electron acceptor, the LUMOoffset calculated from cyclic voltammetry is close to 0.3 eV, thatis considered to be the lowest acceptable offset value for effi-cient exciton dissociation.20 This low LUMO offset could be anexplanation for a signicant charge recombination incompound 8p. On the other hand, a lower hole mobility for dye

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8p can also explain the low Jsc and FF values measured on OPVdevices using this compound.

Finally, a combination of the electron donors 6p and 7p hasbeen used in blend with PC71BM in order to broaden theabsorption spectra of the donor compounds. Indeed, ternaryblend-based devices are quite promising systems under theconditions of a good compatibility of molecules. From thesimilarity of chemical structures and of the amorphous nature,these two molecules 6p and 7p appear to be good candidates forelaborating the ternary blend with enough compatibility. A ratioof 1 : 1 between both molecules has been used while the globaldonor : acceptor ratio has been kept constant to the optimized1 : 5 ratio. Thereof devices lead to a rather good PCE of 3.1%with a Voc of almost 0.8 V. Interestingly, the Jsc increasessignicantly as expected from the addition of two absorptionspectra complementing each other. This is also highlighted bythe External Quantum Efficiency (EQE) measurements dis-played in Fig. 8. Indeed, as expected from the superposition ofUV-visible spectra in thin lms (Fig. 1b), the EQE of the OPVdevice with the mixture 6p + 7p is slightly higher than the onewith 7p. Unfortunately, the FF is slightly decreasing in regard tothe best 7p cell, leading to a PCE that does not exceed the onefor 7p.

Altogether, the results of the mixture show that both mole-cules are well miscible and open up the possibility to optimizedevice performances by mixing complementary amorphousTAT-based dumbbell-shaped molecules.

Overall observations and results tend to conrm the envi-sioned scenario, i.e. OPV properties of these TAT-based dumb-bell-shaped molecules are essentially governed by the nature ofthe central side-chain-free dye segment. Direct relationshipscould be established between the OPV and optoelectronicproperties of compounds 6p–8p without any signicant input ofthe morphology aspects, as all systems exhibit the same struc-tural behavior and show the optimal performances for similarhigh molecule : PC71BM ratios in the range of 1 : 5.

Conclusions

We have designed and synthesized a series of amorphousdumbbell-shaped molecules made of central dyes of differentnature in between two triazatruxene (TAT) platforms, to be usedas electron donor small molecules for photovoltaic applica-tions. Alkyl chains (2-ethylhexyl) have been inserted, only on theTAT fragments giving amorphous states, to ensure good solu-bility and processability of the molecules. These stericallydemanding branched alkyl chains were found to hinder theface-to-face stacking of the TAT fragments. Nevertheless,segregation of conjugated segments and aliphatic chains takesplace, ultimately leading to local segregation of TAT-rich anddye-rich domains. Whatever be the chemical modications ofthe central units, all compounds exhibit the same structuralfeatures with the formation of an amorphous character, asevidenced by SAXS and DSC analyses. In particular, the absenceof molecular p-stacking was not detrimental for charge trans-port as the hole mobility could be clearly extracted from OFETmeasurements on the pure compounds.

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The very similar thermal, structural and charge-transportproperties observed for all TAT-based dumbbell-shaped mole-cules therefore conrm that these properties are essentiallydriven by the general molecular architecture, without a signi-cant inuence of the nature of the central dye and the inherentoptoelectronic properties.

Regarding photovoltaic property studies, the devices fabri-cated from blends between the dumbbell-shaped donor mole-cules and the electron acceptor PC71BM showed optimalperformances when the PC71BM ratios were of about 5 to 1.These high PC71BM contents presumably result from thegeneral donor molecule architecture, for which a tentativeexplanation was proposed.

In the series, the best performances were obtained for themolecule with the pyridal[2,1,3]thiadiazole central electro-de-cient unit (maximum PCE of 3.5%). The improvement withrespect to the molecules containing the benzo[2,1,3]thiadiazolecore is mainly due to a higher Jsc value, in relation to a lowerenergy bandgap.

Amorphous nature engineered by molecular design wasfound to promote a good compatibility between molecules,allowing elaborating a ternary blend with broader UV-visibleabsorption.

On the whole, it is demonstrated that by using a well-designed electron donor molecule, the morphological issuescan be identied and distinguished separately from optoelec-tronic aspects. This is of particular importance to understand indepth the impact of the chemical structure on the optoelec-tronic properties as, for most of the systems, a slight change ofthe molecular structure strongly affects both the morphologyand optoelectronic properties.

Acknowledgements

This work has been supported by the French National ResearchAgency (ANR ORION project, ANR-13-PRGE-0001) and theInterreg IV-A program under project C25 Rhin-Solar.

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