Journal of Materials Chemistry A - POSTECHohgroup.postech.ac.kr/data/file/br_21/1917192400_Gzu05xq1_54.pdf · View Journal | View Issue. hamper charge transport and extraction, resulting

Journal ofMaterials Chemistry A

PAPER

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aDepartment of Chemical and Biomolecular

Science and Technology (KAIST), Daejeon

kaist.ac.krbSchool of Nano-Bioscience & Chemical Engi

Energy, Low Dimensional Carbon Materials C

and Technology (UNIST), Ulsan 689-798, Ko

† Electronic supplementary informationprocedures of materials and additionalresults). See DOI: 10.1039/c3ta13266h

Cite this: J. Mater. Chem. A, 2013, 1,14538

Received 18th August 2013Accepted 26th September 2013

DOI: 10.1039/c3ta13266h

www.rsc.org/MaterialsA

14538 | J. Mater. Chem. A, 2013, 1, 1

Influence of intermolecular interactions of electrondonating small molecules on their molecular packingand performance in organic electronic devices†

Ki-Hyun Kim,a Hojeong Yu,b Hyunbum Kang,a Dong Jin Kang,a Chul-Hee Cho,a

Han-Hee Cho,a Joon Hak Oh*b and Bumjoon J. Kim*a

Intermolecular interactions have a critical role in determining the molecular packing and orientation of

conjugated polymers and organic molecules, leading to significant changes in their electrical and optical

properties. Herein, we investigated the effects of intermolecular interactions of electron-donating small

molecules on their structural, optical, and electrical properties, as well as on their performance in

organic field-effect transistors (OFETs) and organic photovoltaics (OPVs). A series of dithienosilole-based

small molecule donors were synthesized by introducing different terminal groups of ester and amide

groups combined with three different versions of alkyl side chains. In comparison to dithienosilole-based

small molecules with ester terminal groups, those with amide terminal groups exhibit strong

intermolecular interaction by hydrogen bonding in a non-destructive manner. In addition, in order to

control the intermolecular distance during assembly and thus fine-tune the interaction between the

small molecule donors, three different alkyl side chains (i.e., n-octyl, n-decyl, and 2-ethylhexyl chains)

were introduced into both small molecules with amide and ester terminal groups. The molecular

packing and orientation of the small molecule donors were dramatically changed upon modifying the

terminal groups and the alkyl side chains, as evidenced by grazing incidence X-ray scattering (GIXS)

measurements. This feature significantly affected the electrical properties of the small molecules in

OFETs. The trends in the activation energies for charge transport and the hole mobilities in OFETs were

consistent with the molecular ordering and orientation propensity. In addition, the nano-scale

morphology of small molecules blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) was also

influenced by the intermolecular interaction of small molecule donors. Power conversion efficiencies of

more than 4.3% in OPVs were obtained from dithienosilole-based small molecules with ester terminal

groups and linear side chains due to the optimized intermolecular interaction and morphology of the

active layer.

1 Introduction

Organic photovoltaics (OPVs) have attracted a great deal ofresearch interest as they are solution-processable, low-cost,lightweight and exible sources for clean and sustainableenergy.1–3 Conjugated polymers have been extensively utilizedas electron donors in bulk heterojunction-type (BHJ-type)OPVs due to the high power conversion efficiency (PCE)

Engineering, Korea Advanced Institute of

305-701, Korea. E-mail: bumjoonkim@

neering, KIER-UNIST Advanced Center for

enter, Ulsan National Institute of Science

rea. E-mail: [email protected]

(ESI) available: Detailed syntheticdata (UV-vis absorption, UPS, GIXS

4538–14547

exceeding 7–8%.4–14 Recently, solution-processable smallmolecule donors have been recognized as competitive alter-natives to conjugated polymers, due to the diverse merits ofgood reproducibility with monodisperse molecular weight,well-dened molecular structure, higher purity, and easierfunctionalization.15–18 In particular, small molecule donorswith an electron acceptor–donor–acceptor (A–D–A)-typeconjugated framework oen exhibit relatively high PCEs.Although great strides have been made for the improvement ofthe performance of small molecule donor-based OPVs viastructural modications,19–27 their PCEs are still lower thanthose of conjugated polymer-based OPVs.

The intermolecular interaction between small moleculedonors is an important factor in determining their molecularpacking structures, thereby signicantly affecting the electricalproperties and device performances. For instance, weak inter-molecular interactions between small molecule donors can

This journal is ª The Royal Society of Chemistry 2013

http://dx.doi.org/10.1039/c3ta13266h

http://pubs.rsc.org/en/journals/journal/TA

http://pubs.rsc.org/en/journals/journal/TA?issueid=TA001046

Paper Journal of Materials Chemistry A

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hamper charge transport and extraction, resulting in low llfactor (FF) values.28 To enhance the intermolecular interac-tions between small molecule donors, extended p-stackingmoieties, such as pyrene, were introduced at the terminalpositions of the small molecule donors.28,29 These extendedp-stacking moieties typically enhance the intermolecularinteractions that are strong enough to induce their self-assembly into thin lms, leading to the enhancement of FFand PCE values. Although the effects of p–p stacking inter-actions between small molecule donors on the performance ofOPVs have been explored frequently, the effects of the otherintermolecular interactions between small molecule donorshave scarcely been investigated.

Among the intermolecular interactions, hydrogen bonding isone of the strongest (typically H ¼ 1–10 kcal mol�1) with non-destructive characteristics. Therefore, hydrogen-bonding inter-actions can provide an efficient way to control intermolecularinteractions and the molecular ordering of materials withoutdisturbing the intrinsic properties of the parent materials. Forexample, self-assembly of oligothiophenes via hydrogen-bonding interaction could control their polymorphism andlong-range molecular orientation.30,31 These changes of molec-ular ordering and orientation via hydrogen-bonding interac-tions can signicantly affect the performance of organicelectronic devices. The dominant polarity and the charge carriermobility in organic eld-effect transistors (OFETs) based onambipolar semiconducting polymers could be tuned selectivelyby controlling the intermolecular interactions via hydrogenbonds.32,33 The charge transfer, transport, and thermal stabilityof OPV devices could be changed by introducing hydrogen-bonding interactions between the polymer donors and thefullerene acceptors.34,35

Herein, we developed a series of new small molecules inorder to perform a systematic study of the effects of theintermolecular interactions of small molecule donors ontheir molecular morphology and performance in OFETs andOPVs. We utilized a conjugated framework of small moleculedonors with a symmetrical A–p–D–p–A-type structurecomprising dithienosilole and terthiophene moieties asthe electron-rich (D) and p-conjugated bridge (p)units, respectively. Two different terminal groups of octyl2-cyanoacetate and 2-cyano-N-octylacetamide as control andhydrogen bonding terminal groups, respectively, were intro-duced into the small molecule donors. The strength ofintermolecular interactions was ne-tuned by introducingvarious alkyl side chains (i.e., n-octyl, n-decyl, and 2-ethyl-hexyl chains) on the terthiophene p-conjugated bridge unitsthat control the distance between the small molecule donors.The changes in the intermolecular interactions of the smallmolecule donors had a signicant effect on the molecularordering, orientation, and morphology in the thin lm,resulting in a dramatic difference in the performances of theOFETs and OPVs. The thin lm morphologies of the activelayers were studied using grazing incidence X-ray scattering(GIXS), atomic force microscopy (AFM), and transmission elec-tron microscopy (TEM) in order to investigate the structure–property relationships.


2 Experimental2.1 Characterization methods

UV-visible absorption spectra. UV-visible (UV-vis) absorptionspectra were obtained using a UV-1800 spectrophotometer(Shimadzu Scientic Instruments) at room temperature. Thesolution-phase UV-vis spectra of small molecule donors wererecorded in CHCl3. The thin lms of small molecule donorswere prepared by spin coating CHCl3 solution (10 mg mL�1) ofsmall molecule donors at 1500 rpm for 20 s onto a glasssubstrate.

Ultraviolet photoelectron spectroscopy (UPS). UPS sampleswere prepared by spin coating CHCl3 solution (10 mg mL�1) ofsmall molecule donors onto a Si substrate. The thin lms wereprepared in a N2-lled glove box and immediately transferred tothe UPS chamber, which was maintained at an ultra-highvacuum of 1 � 10�10 Torr. UPS analysis was performed using aHe I (21.22 eV) source. The HOMO energy levels of smallmolecule donors were calculated using the following equation:

HOMO ¼ hv � |Ecutoff � HOMOonset |

(hv: 21.22 eV as He I source, Ecutoff: high binding energy cutofffrom the vacuum level, HOMOonset: low binding energy cutofffrom the sample).

Grazing incidence X-ray scattering (GIXS). GIXS measure-ments were performed at beamline 9A in the Pohang Acceler-ator Laboratory (South Korea). GIXS samples were prepared byspin coating CHCl3 solution (10 mg mL�1) of materials at 1500rpm for 20 s onto a PEDOT:PSS/Si substrate. X-rays with awavelength of 1.1179 A were used. The incidence angle (�0.12�)was chosen to allow for complete penetration of X-rays into thelm.

Atomic force microscopy (AFM). AFM measurements wereperformed using a Veeco Dimension 3100 instrument intapping mode. The samples were prepared by spin coatingCHCl3 solution of small molecule donors blended with PCBMonto a PEDOT:PSS/Si substrate.

Transmission electron microscopy (TEM). TEM measure-ments were performed using a JEM-3011 instrument from JEOLLtd. The samples were spun cast on a PEDOT:PSS/Si substratefrom CHCl3 solution, and then the thin lms were oated ontothe air/water interface and transferred to a TEM grid.

2.2 Device fabrication and measurements

Organic photovoltaics. OPV devices were fabricated using anITO/PEDOT:PSS/active layer/LiF/Al structure. ITO-coated glasssubstrates were subjected to ultrasonication in acetone and 2%Helmanex soap in water, followed by extensive rinsing withultrasonication in deionized water and then in isopropylalcohol. Finally, the substrates were dried for several hours in anoven at 80 �C. The ITO substrates were treated with UV-ozoneprior to PEDOT:PSS deposition. A ltered dispersion ofPEDOT:PSS in water (PH 500) was applied by spin-coating at4000 rpm for 40 s and baking for 20 min at 150 �C in air. Aerdeposition of the PEDOT:PSS layer, all subsequent procedureswere performed in a glove box under a N2 atmosphere. Solutions

J. Mater. Chem. A, 2013, 1, 14538–14547 | 14539


Fig. 1 Chemical structure of six small molecule donors with various alkyl sidechains (n-octyl, n-decyl, and 2-ethylhexyl chains) and two different terminalgroups (ester and amide groups); the amide terminal groups can form hydrogenbonds.

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of small molecule donors as the donor and PCBM as theacceptor were prepared in CHCl3 and stirred at 40 �C for 2 h toensure complete dissolution. Prior to solution deposition, thesolutions were passed through a Polytetrauoroethylene (PTFE)syringe lter with a pore diameter of 0.2 mm. Each donor andacceptor solution was blended under optimized conditions(C8-Ester:PCBM¼ 1 : 0.7 w/w and 17 mgmL�1, C10-Ester:PCBM¼ 1 : 0.7 w/w and 18.7 mg mL�1, EH-Ester:PCBM ¼ 1 : 0.4 w/wand 14 mg mL�1, C8-Amide:PCBM ¼ 1 : 0.5 w/w and 13.5 mgmL�1, C10-Amide:PCBM ¼ 1 : 0.7 w/w and 15.3 mg mL�1,EH-Amide:PCBM ¼ 1 : 0.6 w/w and 11.2 mg mL�1). And theblend solutions were then spun-cast onto the ITO/PEDOT:PSSsubstrates at 1500 rpm for 20 s. Average thicknesses of activelms were about �100 nm. The substrates were then placed inan evaporation chamber and held under a high vacuum (<10�6

Torr) for more than 1 h before evaporating �0.8 nm of LiF/100nm of Al. The conguration of the shadow mask afforded fourindependent devices on each substrate. The photovoltaicperformances were characterized using a solar simulator (New-port Oriel Solar Simulators) with an air-mass (AM) 1.5 G lter.The intensity of the solar simulator was carefully calibratedusing an AIST-certied silicon photodiode. Current–voltagebehavior was measured using a Keithley 2400 SMU. The activearea of the fabricated devices was 0.09 cm2.

Organic eld-effect transistors. OFETs based on small mole-cule donors were fabricated with heavily n-doped Si wafers(<0.004U cm) covered with a 300 nm thick SiO2 dielectric (Ci¼ 10nF cm�2). The SiO2/Si wafers were treated with n-octadecyl-trimethoxysilane (OTS) in solution phase. 3 mM OTS solution intrichloroethylene (TCE) was spin-coated onto a UV/ozone treatedSiO2/Si wafer aer cleaning with piranha solution (7 : 3 v/v ofH2SO4 andH2O2). Then, the wafer was exposed to ammonia vaporin a vacuum desiccator for 12 h at room temperature. Thesubstrates were then rinsed with toluene, acetone and isopropylalcohol, followed by drying with N2 blowing. The thin lms ofsmall molecule donors were drop-cast onto the OTS-treated SiO2/Si substrates using CHCl3 solutions (3 mgmL�1) and then placedin a glove box for complete drying. Aer drying, 40 nm goldelectrodes were deposited onto the semiconductor layer bythermal evaporation through a shadow mask. The current�volt-age (I–V) characteristics of the devices were measured in aN2-lled glove box using a Keithley 4200 semiconductor para-metric analyzer. The eld-effect mobility was calculated in thesaturation regime using the following equation:

ID ¼ W

2LmCiðVG � VTÞ2

where ID is the drain current, W and L are the semiconductorchannel width and length, respectively, m is the mobility, Ci isthe capacitance per unit area of the gate dielectric, and VG andVT are the gate voltage and threshold voltage, respectively.

3 Results and discussions3.1 Synthesis

Fig. 1 shows the molecular structures of six small moleculedonors. The details of the synthesis routes to the six small

14540 | J. Mater. Chem. A, 2013, 1, 14538–14547

molecule donors are included in the ESI.† The intermediate(CHO–p–D–p–CHO) was synthesized through the Stillecoupling reaction between the dithienosilole and terthiophenemoieties. Then, the accepting units (octyl 2-cyanoacetate forthe ester terminal group or 2-cyano-N-octylacetamide for theamide terminal group) were combined with an intermediate bythe Knoevenagel condensation to produce carbon–carbondouble bonds. For convenience, these six small moleculedonors were denoted as C8-Ester, C10-Ester, EH-Ester,C8-Amide, C10-Amide, and EH-Amide, based on their alkyl sidechains (C8: n-octyl, C10: n-decyl, and EH: 2-ethylhexyl chains) andterminal groups. Aer purication, the synthesized small mole-cule donors were characterized using 1H-NMR spectroscopy andMALDI-TOF mass spectrometry. The 1H-NMR spectroscopyresults showed that most proton resonances of the ester-termi-nated small molecule donors in the aromatic region (7–8 ppm)were slightly downeld-shied compared to those of the amide-terminated small molecule donors (see ESI†). This indicated thatthe electron density in the conjugated framework of ester-terminated small molecule donors was reduced to a greaterextent than it was in the amide-terminated derivatives. Thisfeature originated from the fact that the oxygen atom in the esterterminal group has a stronger electron-withdrawing propertythan the nitrogen atom in the amide terminal group.

3.2 Optical properties

Fig. 2(a) shows the UV-vis absorption spectra of six smallmolecule donors in CHCl3 solution, and their absorptionproperties are summarized in Table 1. All the compoundsabsorbed intensely in the same visible region, i.e., between 400and 600 nm, mainly due to the intramolecular charge transfer(ICT) from the dithienosilole donor moiety to the cyano ester oramide acceptor moiety. A closer examination of the absorptionspectra showed that the vibrational transition appeared as ashorter-wavelength shoulder around 400 nm, while themaximum absorption wavelength ranged from 503 to 516 nm.The absorption properties in CHCl3 solution were affectedslightly by the terminal group of the materials, but the effects of



Table 1 Optical properties and energy levels of small molecule donors

Compound

Optical properties

Energy levelSolutiona Thin lmb

lmax (nm) lonset (nm) Eg (eV) lmax (nm) lonset (nm) Eg (eV) HOMOc (eV) LUMOd (eV)

C8-Ester 516 627 1.98 586 706 1.76 �5.28 �3.52C10-Ester 516 627 1.98 598 707 1.75 �5.27 �3.52EH-Ester 510 623 1.99 566 683 1.82 �5.47 �3.65C8-Amide 500 607 2.04 558 669 1.85 �5.35 �3.50C10-Amide 511 613 2.02 572 684 1.81 �5.34 �3.53EH-Amide 503 607 2.04 496 645 1.92 �5.02 �3.10

a In CHCl3 at room temperature. b As-spun cast lm from CHCl3 solution onto a glass substrate. c Calculated results from UPS measurements.d LUMO ¼ HOMOUPS + Eg

lm.

Fig. 2 UV-vis absorption spectra of six small molecule donors in (a) CHCl3 solution and (b) thin films as cast from CHCl3; ester-terminated (blue line), amide-terminated(red line), n-octyl side chain (square), n-decyl side chain (circle), and 2-ethylhexyl side chain (triangle) derivatives.


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the alkyl side chains on absorption were trivial. Changing theterminal group from the ester group to the amide group resul-ted in a slight hypsochromic shi (�10 nm) of the maximumabsorption peaks (lmax) because the amide group had a rela-tively weak acceptor characteristic than the ester group; theelectron accepting strengths of the ester and amide groups wereconsistent with the proton resonances of 1H-NMR. The opticalband gaps of ester-terminated materials in solution were alsofound to be about 0.05 eV smaller than those of the amide-terminated materials.

The terminal groups and alkyl side chains had signicanteffects on the light absorption ability of the small moleculedonors in the thin lm state due to the changes in themolecularordering and stacking structure caused by the different degreesof intermolecular interactions. Upon changing from the solu-tion to the thin lm solid states (Fig. 2(b) and Table 1), the lmax

values of most small molecule donors were shied bath-ochromically by signicant amounts, i.e., 50–80 nm, due to theplanarization of the conjugated framework and enhancedintermolecular p–p interactions.36–38 For most small molecule


donors, the vibronic peaks in the low-energy region were clearlyshown because of the formation of intermolecular p–p stacks.In contrast, the lmax value of EH-Amide was shied hyp-sochromically slightly, i.e., by 7 nm, with a broadened absorp-tion band and without vibronic peaks, which might beattributed to the formation of unexpected H-type aggre-gates.39–41 This result is also related to the structural character-istics of 2-ethylhexyl side chains which disturb the backboneplanarity.42 Thus, weak p–p interaction in EH-Amide may allowit to be more strongly inuenced by hydrogen bonding thanother small molecules, with a rather reduced p-planar overlaps.The optical band gaps in the thin-lm state were 1.76, 1.75,1.85, and 1.81 eV for C8-Ester, C10-Ester, C8-Amide, and C10-Amide, respectively. In particular, the decyl side chain deriva-tives (C10-Ester and C10-Amide) had more intense vibronicpeaks and slightly lower optical band gaps than the octyl sidechain derivatives (C8-Ester and C8-Amide). These results mightbe related to the fact that the packing structure of the decyl sidechain derivatives has a higher degree of order than does thepacking structure of the octyl side chain derivatives. However,

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the EH-Ester and EH-Amide that contain branched side chainshad optical band gaps (1.82 and 1.92 eV, respectively) that wererelatively large compared to those of the linear alkyl side chainderivatives. This observation may indicate that the branchedside chains interrupted the ordered structures of the smallmolecules in the thin lm state.42,43

Table 2 Measured domain spacings of small molecule donors via GIXS

Compound (100) d-spacinga (A) (010) d-spacinga (A)

C8-Ester 20.3 3.8C10-Ester 21.6 3.8EH-Ester 16.8 3.8C8-Amide 18.4 3.9C10-Amide 21.2 3.9

a Measured from in-plane line proles.

3.3 Molecular energy levels

The highest occupied molecular orbital (HOMO) and the lowestunoccupied molecular orbital (LUMO) levels of organic semi-conductors are among themost important factors in determiningthe performance of organic electronics. The HOMO levels ofsmall molecule donors in the thin lm state weremeasured usingultraviolet photoelectron spectroscopy (UPS). The LUMO levelswere calculated from the difference between the HOMO level andthe optical band gap of the thin lm state. The molecular energylevels are listed in Table 1, and the detailed UPS results are shownin Fig. S1.† In the case of linear alkyl side chain derivatives, theLUMO levels were similar (�3.5 eV), whereas the HOMO levels ofthe amide-terminated derivatives (C8-Amide and C10-Amide)were 0.07 eV lower than those of the ester-terminated derivatives(C8-Ester and C10-Ester) due to the larger optical band gap of theamide-terminated derivatives. In the case of the branchedside chain derivatives, the energy levels were remarkably differentfrom those of the linear alkyl side chain derivatives. Among thesix small molecule donors, EH-Ester and EH-Amide had thelowest and highest HOMO levels of �5.47 and �5.02 eV,respectively. The EH-Ester had the lowest HOMO level among thesix small molecule donors, presumably due to the relativelydisordered structure of the thin lm caused by the presence ofbranched side chains.42,43 In contrast, the EH-Amide had thehighest HOMO level, probably due to the strong hydrogenbonding interaction between the amide terminal groups. Thesedifferent HOMO levels of small molecule donors can change the

Fig. 3 GIXS images of thin films of small molecule donors prepared by spin-castin

14542 | J. Mater. Chem. A, 2013, 1, 14538–14547

open-circuit voltage (VOC) in OPVs because the VOC is proportionalto the difference between the LUMO level of the electron acceptor(i.e., PCBM) and the HOMO level of the electron donor.44–46

3.4 Structural properties

The effects of terminal groups and alkyl side chains of smallmolecule donors onmolecular packing and thin lmmorphologywere investigated using grazing incidence X-ray scattering (GIXS)analysis. Fig. 3 shows the GIXS images of the small moleculedonors in the thin lm state, and the structural characteristics ofthese materials are summarized in Table 2. The 2D GIXS lineproles can be divided into the in-plane (qxy) and out-of-plane (qz)components. Most samples exhibited strong out-of-plane reec-tions with the presence of (100), (200), and (300) reections. Thelamellar spacing calculated from the qxy value was closely relatedto the length of the alkyl side chains.47,48 For example, strong (100)reections of C8-Ester, C10-Ester, and EH-Ester at qxy values of0.31, 0.29, and 0.37 A�1 corresponded to 20.3, 21.6, and 16.8 A oflamellar spacing, respectively. The incorporation of amide groupsat the terminal positions of the small molecule donors inducedadditional intermolecular interactions, resulting in evidentlydecreased lamellar spacing values. The lamellar spacings of C8-Amide and C10-Amide were 18.4 and 21.2 A, respectively.

g from CHCl3 solution.




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Interestingly, compared with the case of octyl side chain deriva-tives from C8-Ester to C8-Amide with a decrement of 1.9 A, thereplacement of C10-Ester with C10-Amide showed a smallerdecrement (by 0.4 A) of the lamellar spacing. This differencecould be attributed to the fact that the longer decyl side chainscould allow a longer distance between the intermolecularamide groups and better screen the hydrogen bonding interac-tions. The distinct (010) reection of C8-Ester and C10-Ester inthe in-plane direction indicated a greater preference for “edge-on” molecular orientation with a domain spacing of 3.8 A.49,50 Incomparison, additional intermolecular interactions of the amide-terminal groups signicantly inuenced the molecular orienta-tion in the thin lm. C8-Amide and C10-Amide exhibited (010)reection along all polar angles, indicating more random orien-tation of themolecular assembly than the C8-Ester and C10-Ester.In addition, the branched 2-ethylhexyl side chain group couldfurther affect the molecular packing and orientation in the thinlm. EH-Ester showed (010) reection in both the in-plane andout-of-plane directions, which means the co-existence of “edge-on” and “face-on”molecular orientations. EH-Amide having bothbranched side chains and an amide terminal group showeddistinct scattering peaks without (010) reection; this resultmight originate from the formation of H-type aggregates.

Table 3 OFET performance of small molecule donor films prepared by a drop-cas

Samplea mh,maxb (cm2 V�1 s�1) mh,avg

c (cm2 V

C8-Ester 1.37 � 10�2 9.69 (�3.52)d

C10-Ester 2.82 � 10�2 2.05 (�0.32) �EH-Ester 2.70 � 10�4 2.62 (�0.07) �C8-Amide 2.18 � 10�3 2.04 (�0.28) �C10-Amide 2.99 � 10�3 2.81 (�0.26) �EH-Amide 6.54 � 10�6 4.67 (�1.37) �a The p-channel characteristics of small molecule donor FETs were meamaximum mobility of the FET devices (L ¼ 50 mm and W ¼ 1000 mm). c Td The standard deviation. e The activation energy for charge transport.

Fig. 4 Transfer characteristics of OFETs based on the drop-cast films of smallmolecule donors: (a) C8-Ester, (b) C10-Ester, (c) EH-Ester, (d) C8-Amide, (e) C10-Amide and (f) EH-Amide.


3.5 OFET performance

To investigate the effects of molecular ordering and orientationon the charge-transporting ability of small molecule donors,OFETs based on small molecule donors were fabricated inbottom-gate, top-contact conguration by drop casting the CHCl3solution onto a n-octadecyltrimethoxysilane (OTS)-treated SiO2/Sisubstrate. The current–voltage (I–V) characteristics of the OFETsbased on the drop-cast thin lms (�45 nm thickness) showedtypical p-channel operation (Fig. 4), and the electrical perfor-mances of the OFETs are summarized in Table 3. Compared withother small molecule donors, C8-Ester and C10-Ester exhibitedrelatively high hole mobilities (mh), up to 1.37 � 10�2 and 2.82 �10�2 cm2 V�1 s�1, respectively. This feature is closely related tothe well-ordered, “edge-on” molecular orientations for thesedonor molecules. It is also noteworthy that the GIXS patterns ofthe C8-Ester and C10-Ester thin lms exhibited distinct (010) in-plane reections, indicating that 3D conduction channels wereformed effectively.51,52 The charge carrier mobilities of the OFETsbased on the amide-terminated derivatives with linear alkyl sidechains were lower by an order of magnitude (C8-Amide: 2.18 �10�3 cm2 V�1 s�1; C10-Amide: 2.99 � 10�3 cm2 V�1 s�1)compared to those of C8-Ester and C10-Ester. These results canbe attributed to the increased p–p stacking distances and therandomly oriented molecular structures engendered by thestrong hydrogen bonding interactions. In addition, the branchedside chains of EH-Ester and EH-Amide led to the deterioration ofhole mobilities (10�4 to 10�6 cm2 V�1 s�1) due to the signicantlyreducedmolecular orderingwith disturbedp–p stacking. Overall,the trend in the hole mobility was in good agreement with themolecular ordering and orientation propensity of each smallmolecule donor.

Temperature-dependent charge transport behaviors inOFETs based on small molecule donors were investigated attemperatures between 80 and 280 K. The measurements wereconducted with a time delay of 1 h for stabilization undervacuum (1 � 10�5 Torr). The hole mobilities of all the smallmolecule donors showed a noticeable increase as the temper-ature increased, indicating that the charge transport of theseOFETs followed a thermally activated, charge-hopping transportmodel (Fig. 5(a)).52,53 The activation energies (EAs) for charge-hopping transport were extracted from temperature-dependentaverage mobilities tted to the Arrhenius relationship, m f

exp(EA/kBT). The EA values can also be considered as a trap

ting method

�1 s�1) Ion/Ioff VT (V) EAe (meV)

� 10�3 >104 16.2 12.9810�2 >105 6.0 6.8410�4 >103 �6.0 13.0710�3 >104 �5.5 20.2910�3 >105 1.0 16.1410�6 >102 �16.4 23.56

sured with VDS ¼ �100 V. Thermal annealing was not applied. b Thehe average mobility of the FET devices (L ¼ 50 mm and W ¼ 1000 mm).

J. Mater. Chem. A, 2013, 1, 14538–14547 | 14543


Fig. 5 Temperature effects on the electrical characteristics of the small moleculedonor-based OFETs. (a) Temperature dependence of hole mobilities measured atvarious temperatures (80 K to 280 K) with VDS ¼ �100 V in a vacuum (1 � 10�5

Torr). (b) Arrhenius plot of the temperature dependence of the FET mobilities. Thelinear fits to the Arrhenius plot resulted in EA values of 12.98, 6.84, 13.07, 20.29,16.14, and 23.56 meV for C8-Ester, C10-Ester, EH-Ester, C8-Amide, C10-Amide,and EH-Amide, respectively.


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depth for hopping transport. Fig. 5(b) shows the Arrheniusplots of the charge carrier mobilities as a function of recip-rocal temperature. Linear ts of the OFET performances ofC8-Ester and C10-Ester provided EA values of 12.98 and 6.84meV, respectively. EH-Ester had a slightly deeper trap depth,with an EA of 13.07 meV. The amide-terminated derivativesOFETs yielded EA values of 20.29, 16.14, and 23.56 meV forC8-Amide, C10-Amide, and EH-Amide, respectively. The highdensity of charge carriers lls a fraction of the trapping sites,consequently lowering the Fermi level towards the HOMOband edge, which, in turn, decreases the hopping EA value.54

Hence, OFETs with higher carrier density generally havelower activation energy. For small molecules with the sameterminal group, the OFETs followed a trend of decreasingactivation energy with increasing mobility (see Table 3, Holemobility: C10-Ester > C8-Ester > C10-Amide > C8-Amide >EH-Ester > EH-Amide; EA value: EH-Amide > C8-Amide >C10-Amide > EH-Ester > C8-Ester > C10-Ester). Intriguingly,

Fig. 6 (a) Current density–voltage (J–V) curves and (b) external quantum efficienillumination at 100 mW cm�2; ester-terminated (blue line), amide-terminated (redchain (triangle) derivatives.

14544 | J. Mater. Chem. A, 2013, 1, 14538–14547

the EA values of all the amide derivatives were larger thanthose of the ester derivatives. It is noteworthy that the EA ofEH-Ester was smaller than that of C10-Amide, even thoughEH-Ester had a lower mobility. The blue-shi of the UV-visabsorption spectra (Fig. 2(b)) and GIXS data (Fig. (3)) revealthat hydrogen bonding induces a decrease in the planarity ofthe conjugated backbone of the amide-terminated derivativesin the thin lm state, resulting in a certain degree ofdisruption in p–p stacking. This leads to an increment in theenergetic barrier of charge hopping. In addition, therestricted freedom in the molecular organization of amide-terminated molecules might increase the defect states forcharge transport, resulting in higher EAs.55 The lower mobilityof EH-ester with a lower EA than the amide-terminatedderivatives can be attributed to the lower degree of molecularordering in the thin lm. This underlines the importance ofconsidering the interplay between energetic and kineticfactors for charge transport.56

3.6 OPV performance

To elucidate the relationship between the molecularmorphology of the small molecule donors and their photovol-taic performances, BHJ-type OPVs (ITO/PEDOT:PSS/active layer/LiF/Al) were fabricated by using the blends of C8-Ester,C10-Ester, EH-Ester, C8-Amide, C10-Amide, and EH-Amide aselectron donors with PCBM as the electron acceptor. The activelayers were spin-coated from the CHCl3 blend solution. Fig. 6(a)shows the current density versus voltage (J�V) curves for theoptimized devices under AM 1.5 illumination at 100 mW cm�2.The characteristics of the OPV devices are summarized inTable 4. The PCEs of the ester-terminated small molecule-baseddevices were higher than those of the amide-terminated smallmolecule-based devices. The devices consisting of the C8-Esterand C10-Ester blended with PCBM showed the highest PCE of4.31% (C8-Ester: VOC ¼ 0.82 V, JSC ¼ 9.79 mA cm�2, FF ¼ 0.54;

cies (EQEs) of OPVs based on small molecule donors with PCBM under AM 1.5line), n-octyl side chain (square), n-decyl side chain (circle), and 2-ethylhexyl side



Table 4 Optimized characteristics of OPVs composed of small molecules aselectron donors and PCBM as electron acceptors under AM 1.5 G-simulated solarillumination (100 mW cm�2)

Activelayer VOC (V)

JSC(mA cm�2) FF

PCE(%)

Integrated JSCa

(mA cm�2)

C8-Ester 0.82 9.79 0.54 4.31 9.67C10-Ester 0.82 9.30 0.57 4.31 9.19EH-Ester 0.94 7.75 0.41 3.00 7.63C8-Amide 0.87 7.94 0.47 3.22 7.83C10-Amide 0.86 8.38 0.52 3.75 8.23EH-Amide 0.64 1.25 0.26 0.21 1.22

a Calculated by integration of EQE curves.


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C10-Ester: VOC ¼ 0.82 V, JSC ¼ 9.30 mA cm�2, FF ¼ 0.57).Conversely, the EH-Amide:PCBM device exhibited the lowestPCE of 0.21%. The VOC values of the devices increased as theHOMO levels of the small molecule donors decreased. Forexample, the VOC values of C8-Amide-based and C10-Amide-based devices were higher by �0.05 V than those of C8-Ester-based and C10-Ester-based devices. In addition, the highest andlowest HOMO levels of EH-Amide (�5.02 eV) and EH-Ester(�5.47 eV) resulted in the lowest and highest VOC values of 0.64and 0.94 V, respectively.

External quantum efficiencies (EQEs) were measured forsmall molecule donor-based devices under optimized condi-tions (Fig. 6(b)). The wavelength ranges of the EQE spectra ofthe OPV devices were consistent with the UV-vis absorptionrange of the donor–acceptor blend lms (Fig. S3†). Themaximum EQEs of most OPV devices were estimated to be50–60% at around 550 nm. However, the EQE values of EH-Amide:PCBM devices were very low (<�10%) between 300 and650 nm. The measured JSC values for the OPV devices were wellmatched (<2% error) with the integrated JSC (Table 4) obtainedfrom the EQE spectrum.

Fig. 7 TEM images of the optimized active layer consisting of small molecule don


To gain a deeper insight into the photovoltaic perfor-mances of small molecule donor-based OPV devices, theblend morphologies of the optimized active layers wereexamined using TEM and tapping-mode AFM (Fig. 7 and 8,respectively). In addition, the molecular packing and orien-tation structure of the small molecule donors and PCBM inoptimized active layers were characterized by GIXS (Fig. S4and S5†). The TEM images indicated that the structuralchanges of the small molecule donors signicantly affectedthe degree of nano-scale phase separation in the active layers(Fig. 7). C8-Ester and C10-Ester blended with PCBM exhibitedan interpenetrating, bi-continuous BHJ morphology with asmall domain length-scale of 20–30 nm. This feature couldlead to efficient separation of the photogenerated excitonsand charge transport, leading to the high PCEs of the OPVs.57

The AFM measurements also showed that the C8-Ester andC10-Ester blends had distinct BHJ morphologies with a root-mean-square (RMS) roughness value of 2.7 nm (Fig. 8).However, the C8-Amide and C10-Amide blended with PCBMexhibited less distinct features in the phase-separatedmorphology with much lower RMS roughness valuescompared to the ester-terminated molecules. This wasprobably due to the enhanced miscibility with fullerenes,which is partly attributable to the more randomly orientedmolecular stacking in the thin lm, as shown in the GIXSdata. Furthermore, the EH-Amide blend lm showed asmooth surface without a distinct interface between donorand acceptor materials, which could explain the low JSC andFF values of the OPVs.57 This observation was well supportedby the results obtained from the GIXS measurements (Fig. S4and S5†). The ester-terminated derivatives in the active layerexhibited well-ordered molecular orientations, whereas theamide-terminated derivatives in the active layer showedrelatively ill-dened orientations. These results suggest apositive inuence on the charge transport capability of the

ors and PCBM. The scale bar is 200 nm.

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Fig. 8 AFM height images of the optimized active layer consisting of small molecule donors and PCBM: The scale bar is 1 mm. RMS roughness: C8-Ester ¼ 2.77 � 0.05nm, C10-Ester ¼ 2.68 � 0.05 nm, EH-Ester ¼ 0.46 � 0.02 nm, C8-Amide ¼ 0.55 � 0.02 nm, C10-Amide ¼ 0.48 � 0.02 nm, EH-Amide ¼ 0.32 � 0.01 nm.


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ester-terminated derivatives, consistent with their higher JSCand FF values. In contrast, the distinct scattering peaks of theEH-Amide in the pristine lm disappeared when the EH-Amide was blended with PCBM molecules.

4 Conclusions

We developed a series of dithienosilole-based small moleculeswith modied terminal and side-chain groups and investi-gated the effects of intermolecular interactions of smallmolecule donors on their structural properties and perfor-mances in solution-processed OFETs and OPVs. The amideterminal groups induced strong intermolecular interactionsvia hydrogen bonds in small molecules, whereas the esterterminal groups induced relatively weak intermolecularinteractions. In addition, the intermolecular interactionsbetween the small molecules were ne-tuned by incorporatingdifferent alkyl side chains to control the distance betweenadjacent molecules. These small structural modicationsyielded signicant changes in the molecular packing andorientation in the thin lms as well as in the optical propertiesand energy levels of the lms. Our studies on the chargetransport kinetics of the OFET devices also revealed that thesestructural modications greatly affected the hole mobility andthe activation barriers for charge transport. The trend in holemobility is also consistent with the molecular ordering andorientation propensity of each small molecule donor.Furthermore, the control of the intermolecular interactions insmall molecule donors led to the different interfacial inter-actions with PCBM, resulting in dramatic changes in the BHJmorphology and PCE values in the OPVs. The blend lm ofC8-Ester and C10-Ester had a bi-continuous BHJ morphologywith a well-dened interface and domain size, and it producedan optimized PCE exceeding 4.3%. Our ndings demonstratethat the intermolecular interactions of small molecule donors

14546 | J. Mater. Chem. A, 2013, 1, 14538–14547

can be selectively tuned by rational molecular design and canalso induce the well-ordered/oriented molecular packing ofthe active layer that are highly suited for optimizing theperformance in OPVs and OFETs.

Acknowledgements

This research was supported by the National Research Founda-tion of Korea (NRF) funded by the Korean Government(2012M1A2A2671746, 2011-0017174), and by the Grants from theCenter for Advanced So Electronics (2013M3A6A5073175) andthe Center for Multiscale Energy System (2012M3A6A7055540)under the Global Frontier Research Program of the Ministry ofScience, ICT & Future Planning, Korea.

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Journal of Materials Chemistry A - POSTECHohgroup.postech.ac.kr/data/file/br_21/1917192400_Gzu05xq1_54.pdf · View Journal | View Issue. hamper charge transport and extraction, resulting