Solar Energy Materials & Solar Cells 102 (2012) 159–166

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Solar Energy Materials & Solar Cells

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Push-pull organic semiconductors comprising of bis-dimethylfluorenylamino benzo[b]thiophene donor and various acceptors for solution processedsmall molecule organic solar cells

Jooyoung Kim a, Nara Cho a, Haye Min Ko a, Chulwoo Kim a, Jae Kwan Lee b,n, Jaejung Ko a,nn

a Department of New Material Chemistry, Korea University, Chungnam, 330-700, South Koreab Department of Chemistry Education, Chosun University, Gwangju, 501-759, South Korea

a r t i c l e i n f o

Article history:

Received 19 January 2012

Received in revised form

8 March 2012

Accepted 9 March 2012Available online 4 April 2012

Keywords:

Bulk heterojunction

Intramolecular charge transfer

Electron-withdrawing acceptor

Organic solar cell

Organic semiconductor

48/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.solmat.2012.03.007

esponding author. Tel.: þ82 62 230 7319; fax

o to be corresponding author. Tel.: þ82 41 860

ail addresses: chemedujk@chosun.ac.kr (J. Kw

ea.ac.kr (J. Ko).

a b s t r a c t

New efficient push-pull organic semiconductors comprising of on the N,N-(6-bis(9,9-dimethyl-

9H-fluoren-2-yl)amino-benzo[b]thiophen (bisDMFABT) donor and the various acceptors such as NO2,

DCBP, and TCF, which were linked with thiophene or vinyl thiophene p-conjugation bridges, were

synthesized, and their photovoltaic characteristics were investigated in solution processed small

molecule organic solar cells (SMOSCs). The intramolecular charge transfers of these materials were

effectively appeared in between bisDMFABT donor and acceptors, depending on the electron-with-

drawing strength of acceptors. The organic semiconductor having DCBF acceptor exhibited the best

power conversion efficiency of 3.22% with short circuit current of 9.64 mA/cm2, fill fator of 0.42, and

open circuit voltage of 0.80 V in SMOSC devices with TiOx thin layer.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

Solution processed organic solar cells (OSCs) fabricated bymeans of versatile printing techniques such as doctor blade,inkjet, and roll-to-roll can provide the most attractive advantagesof device such as low cost, light weight, solution processability.Over the last few years, enormous efforts have intensively focusedon improving the device performances toward power conversionefficiency (PCE) of 10% though developments of photoactivematerials such as p-conjugated (semiconducting) polymer andfullerene derivatives, or functional layers such as buffering,charge transporting, and optical spacing [1–9]. And recently greatachievement of promising PCEs of above 8% in OSC has offeredspecial opportunities as a strong candidate on next generationsolar cell with threatening inorganic thin film solar cell as well asdye-sensitized solar cells (DSSCs) [10]. Most of high efficiencieshave been reported in OSC fabricated with bulk-heterojunction(BHJ) materials comprising of low-bandgap semiconducting poly-mers and [6,6]-phenyl-C61 (or 71)�butyric acid methyl ester (C61

(or 71)�PCBM) [11–15]. Nevertheless, small molecule organicsemiconductor seems to fascinate more than these polymers from

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: þ82 62 232 8122.

1337; fax: þ82 41 860 5396.

an Lee),

the viewpoint of mass production for commercial application dueto their low reproducibility for characteristics such as averagemolecular weight (Mw) and polydispersity index (PDI) as well asdifficulty in purification. Motivated by this, considerable researcheffort has been focused on developing efficient small moleculematerials for improved device performance, with the near-termgoal being a power conversion efficiency (PCE) comparable topolymer-based solar cells (PSCs) [16–20]. Indeed, recent break-throughs in realizing PCEs of above 6% have placed solutionprocessed small molecule organic solar cells (SMOSCs) squarelyin competition with PSCs [21].

The efficient organic semiconductors reported in SMOSC oftenhave the structural symmetric motifs containing the electron-withdrawing cores such as benzothiadiazole, [22] squaraine, [23]or diketopyrrolopyrrole, [18] which were motivated by low-bandgap semiconducting polymers or push-pull molecular struc-tures in nonlinear optics (NLO) and DSSC due to their superioroptoelectronic properties. Recently, we have also reported varioussymmetric push-pull organic semiconductors comprising triphe-nylamine (TPA) donor and squaraine (or diketopyrrolopyrrole)acceptor for solution processed SMOSC [24,25]. These push-pullstructures in SMOSC enable an efficient intramolecular charge-transfer (ICT) to give the better molar absorptivity. And an TPAelectron donating unit can play an important role of stabilizingthe separated hole from exciton and improving the transportingproperty of hole carrier [17,26–30].

Scheme 1. Molecular structures of the bisDMFABT-Th-Acceptors and device architecture of solution processed small molecule organic solar cell .

J. Kim et al. / Solar Energy Materials & Solar Cells 102 (2012) 159–166160

On the other hand, we have interestingly found that the unsym-metric push-pull organic semiconductor having the different endgroups of TPA donor and squaraine acceptor presented the betterPCE than the symmetric structures of these donor and acceptor[31], and that the new organic semiconductor, bisDMFA-diTh-MMNcomprising bis(9,9-dimethyl-9H-fluoren-2-yl)aniline (bisDMFA)donor, methylene-malononitrile (MMN) acceptor, and bithiophenebridge exhibited the superior photovoltaic performance in solutionprocessed BHJ SMOSC even its small molecular size, which shouldbe a obstacle for forming the self-network domains in BHJ compo-site with PCBM, showing the PCE of 3.6% [32] Also, very recently,we reported the improved efficiency from the efficient structuralmodification of bisDMFA-diTh-MMN by the new donor material,thiophene-fused bisDMFA to enhance p-conjugation length [33].In this study, we also tried to develop new semiconductingpush-pull type organic small molecules having various acceptormotifs, especially 4-dicyanomethylene-6-tert-butyl-4H-pyran (DCBP),2-dicyanomethylene-3-cyano-5-dimethyl-2,5-dihydrofuran (TCF),and nitrogen dioxide (NO2). To the best our knowledge, theelectron accepting features can significantly affect the energybandgap and molar absorptivity as well as the ICT strength ofpush-pull organic materials. Beside, the structural variation ofacceptor motifs often afford its unique features that when inte-grated into bulk heterojuction composite with PCBM can criticallyaffect the performance characteristics of BHJ OSC.

Herein, we report the synthesis and photovoltaic characteristicsof new efficient push-pull organic semiconductors comprisingof on the N,N-(6-bis(9,9-dimethyl-9H-fluoren-2-yl)amino-benzo[b]thiophen (bisDMFABT) donor and the various acceptors, N,N-(6-bis(9,9-dimethyl-9H-fluoren-2-yl)amino-benzo[b]thiophen-2-yl)-5-nitrothiophen (bisDMFABT-Th-NO2, 1), (E)-2-(2-(2-(5-(6-(bis(9,9-dimethyl-9H-fluoren-2-yl)amino)benzo[b]thiophen-2-yl)thiophen-2-yl)vinyl)-6-tert-butyl-4H-pyran-4-ylidene)malononitrile (bisDM-FABT-Th-DCBP, 2), and (E)-2-(4-(2-(5-(6-(bis(9,9-dimethyl-9H-fluoren-2-yl)amino)benzo[b]thiophe-2-yl)thiophen-2-yl)vinyl)-3-cyano-5,5-dimethylfuran-2(5H)-ylidene)malononitrile (bisDMFABT-Th-TCF, 3), which were linked with thiophene or vinyl thiophenep-conjugation bridges, in solution processed SMOSC. Scheme 1shows the structures of the synthesized organic semiconductorsand device architecture of solution processed SMOSC.

2. Material and methods

2.1. Materials

Solvents were distilled from appropriate reagents. All reagentswere purchased from Sigma-Aldrich, TCI and Alfa Aesar. All reactions

were carried out under a nitrogen atmosphere. And [6,6]-phenyl-C71-butyric acid methyl ester (C71-PCBM) was obtained from Nano-C.

2.2. Instruments and measurements

1H NMR and 13C NMR spectra were recorded on a VarianMercury 300 spectrometer. Chemical shift values were recordedas parts per million relative to tetramethylsilane as an internalstandard unless otherwise indicated, and coupling constants inHertz. Elemental analyses were performed with a Carlo Elba Instru-ments CHNS-O EA 1108 analyzer. Mass spectra were recorded on aJEOL JMS-SX102A instrument. UV–vis. data was measured with aPerkin-Elmer Lambda 2 S UV–vis spectrometer. Optimized struc-tures calculated by TD-DFT using the B3LYP functional and the6–31 Gn basis set. The highest occupied molecular orbital (HOMO)and the lowest unoccupied molecular orbital (LUMO) energies weredetermined using minimized singlet geometries to approximate theground state. Cyclic voltammetry was carried out with a BAS 100B(Bioanalytical System, Inc.). A three electrode system was used andconsisted of non-aqueous Reference Electrode (0.1 M Ag/Agþ acet-onitrile solution; MF-2062, Bioanalytical System, Inc.), platinumworking electrode (MF-2013, Bioanalytical System, Inc.), and aplatinum wire (diam. 1.0 mm, 99.9% trace metals basis, Sigma-Aldrich) as counter electrode. Redox potentials of materials weremeasured in CH2Cl2 with 0.1 M (n-C4H9)4N-PF6 with a scan ratebetween 100 mV s�1 (vs. Fc/Fcþ). Atomic Force Microscope (AFM)measurements were performed with a Digital Instruments Nano-Scope IV in the tapping mode.

2.3. Synthesis of materials

2.3.1. N,N-(6-bis(9,9-dimethyl-9H-fluoren-2-yl)amino-benzo[b]-

thiophen-2-yl)-5-nitrothiophen (bisDMFABT-Th-NO2, 1)

Under nitrogen atmosphere and at �10 1C, n-BuLi (0.36 ml,1.6 M in hexane) was added dropwise to a dry THF solutioncontaining (N,N-(6-bis(9,9-dimethyl-9H-fluoren-2-yl)amino)-ben-zo[b]thiophen (i, 0.253 g, 0.47 mmol). After 30 min stirring at�10 1C, 0.15 ml (0.74 mmol) of 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane was added slowly to the reaction solutionat �10 1C. The temperature of the solution was warmed to roomtemperature and the reaction was stirred for 1 h. Then, the reactionwas quenched with water. The solution was extracted withdichloromethane, dried with MgSO4 and evaporated solvent. Alight yellow solid (silica gel TLC plate, ethyl acetate/hexane¼1:10,Rf¼0.2) was obtained. Without further purification, this compoundwas mixed with 2-bromo-5-nitrothiophene (0.06 g, 0.29 mmol),Pd(PPh3)4 (0.02 g, 0.17 mmol), K2CO3 (0.16 g, 1.16 mmol), a drop ofAliquat 336 and degassed water (2 M) in dry toluene (10 ml), then

J. Kim et al. / Solar Energy Materials & Solar Cells 102 (2012) 159–166 161

refluxed under nitrogen atmosphere for overnight. After cooling toroom temperature, the solution was extracted with dichloro-methane, dried with MgSO4 and subjected to column chromato-graphy (silica gel, dichloromethane/hexane¼1:4, Rf¼0.1). A purplesolid was obtained. Yield: 85% (0.16 g). Mass: m/z 660.16 [Mþ]. IR:1322 cm�1, 1446 cm�1. 1H NMR (300 MHz, CDCl3): d 7.86 (d, 1H,J¼4.2 Hz), 7.67 (d, 2H, J¼6.6 Hz), 7.64 (d, 2H, J¼8.4 Hz), 7.55 (d,2H, J¼6.9 Hz), 7.41 (d, 2H, J¼7.5 Hz), 7.35-7.24 (m, 8H), 7.15 (dd,2H, J¼6, 2.7 Hz), 7.11 (s, 1H), 1.42 (s, 12H). 13C NMR (75 MHz,CDCl3): d 155.01, 153.59, 149.63, 147.01, 146.96, 145.48, 141.67,138.89, 134.98, 134.89, 132.76, 129.85, 127.15, 126.81, 124.84,123.59, 123.13, 123.01, 122.63, 120.85, 119.63, 119.09, 115.94,47.04, 27.21. Anal. Calcd for C44H34N2O2S2: C, 76.94; H, 4.99; N,4.24. Found: C, 76.71; H, 4.88; N, 4.08.

2.3.2. (E)-2-(2-(2-(5-(6-(bis(9,9-dimethyl-9H-fluoren-2-yl)amino)-

benzo[b]thiophen-2-yl)thiophen-2-yl)vinyl)-6-tert-butyl-4H-pyran-

4-ylidene)malononitrile. (bisDMFABT-Th-DCBP, 2)

2-(2-tert-butyl-6-methyl-4H-pyran-4-ylidene)malononitrile(iii, 45 mg, 0.21 mmol) and piperidine (3–5 drops) were addedto a solution of 5-(6-(bis(9,9-dimethyl-9H-fluoren-2-yl)amino)ben-zo[b]thiophen-2-yl)thiophene-2-carbaldehyde (ii, 90 mg, 0.14 mmol)in CHCl3:CH3CN (1:1, 10 ml:10 ml) at room temperature. The reac-tion mixture was heated under reflux for 24 h. Concentration andpurification of the residue by flash column chromatography (hex-ane:ethyl acetate¼5:1, Rf¼0.3) gave the product (87 mg, 74%). Mass:m/z 839.25 [Mþ]. 1H NMR (300 MHz, DMSO-d6): d 7.81-7.66 (m,7H), 7.58-7.45 (m, 5H), 7.35-7.24 (m, 5H), 7.19-7.02 (m, 5H), 6.98 (s,1H), 6.44 (s, 1H), 1.36 (s, 9H), 1.35 (s, 12H). 13C NMR (75 MHz,DMSO-d6): d 172.4, 159.1, 156.3, 154.8, 153.1, 146.8, 145.2, 140.2,139.9, 139.5, 138.2, 135.7, 134.4, 133.9, 133.1, 129.9, 127.1, 126.7,126.1, 124.7, 122.8, 122.6, 122.4, 121.1, 120.9, 119.5, 118.2, 117.9,116.6, 115.3, 115.2, 106.9, 101.8, 56.7, 46.4, 36.3, 27.4, 26.6. Anal.Calcd for C56H45N3OS2: C, 80.06; H, 5.40; N, 5.00. Found: C, 79.78; H,5.29; N, 4.89.

2.3.3. (E)-2-(4-(2-(5-(6-(bis(9,9-dimethyl-9H-fluoren-2-yl)amino)

benzo[b]thiophe-2-yl)thiophen-2-yl)vinyl)-3-cyano-5,5-dimethylfuran-

2(5H)-ylidene)malononitrile (bisDMFABT-Th-TCF, 3)

2-(3-cyano-4,5,5-trimethylfuran-2(5H)-ylidene)malononitrile(iv, 41.8 mg, 0.21 mmol) and piperidine (3–5 drops) were addedto a solution of 5-(6-(bis(9,9-dimethyl-9H-fluoren-2-yl)amino)-benzo[b]thiophen-2-yl)thiophene-2-carbaldehyde (ii, 90 mg,0.14 mmol) in CHCl3:CH3CN (1:1, 10 ml:10 ml) at room tempera-ture. The reaction mixture was heated under reflux for 24 h.Concentration and purification of the residue by flash columnchromatography (hexane:ethyl acetate¼2:1, Rf¼0.2) gave theproduct (83.2 mg, 72%). Mass: m/z 824.20 [Mþ]. 1H NMR(300 MHz, DMSO-d6): d 8.14 (d, 1H J¼16.8 Hz), 7.91 (s, 1H),7.85-7.74 (m, 5H), 7.66 (s, 1H), 7.59 (d, 1H, J¼3.9 Hz), 7.51 (d, 2H,J¼7.5 Hz), 7.35-7.16 (m, 8H), 7.06 (d, 2H J¼7.8 Hz), 6.82 (d, 1HJ¼15.9 Hz), 1.80 (s, 6H), 1.37 (s, 12H). 13C NMR (75 MHz, DMSO-d6): d 176.8, 174.3, 154.8, 153.2, 146.6, 145.8, 144.2, 140.8, 139.7,139.4, 138.2, 137.7, 135.5, 134.1, 133.7, 127.1, 126.7, 125.1, 123.1,122.7, 121.2, 119.6, 118.5, 116.2, 113.4, 112.8, 112.0, 110.9, 98.9,98.0, 62.1, 53.7, 46.4, 34.3, 26.6, 25.3. Anal. Calcd for C54H40-N4OS2: C, 78.61; H, 4.89; N, 6.79. Found: C, 78.39; H, 4.78; N, 6.58.

2.4. Fabrication and characterization of solar cell devices

2.4.1. Fabrication

The BHJ films were prepared under optimized conditionsaccording to the following procedure reported previously [34]:The indium tin oxide (ITO)-coated glass substrate was first cleanedwith detergent, ultrasonicated in acetone and isopropyl alcohol,

and subsequently dried overnight in an oven. PEDOT:PSS (Heraeus,Clevios P VP.AI 4083) in aqueous solution was spin-cast to forma film with thickness of approximately 35 nm. The substrate wasdried for 10 min at 140 1C in air, then transferred into a glove boxto spin-cast the photoactive layer. The optimized bisDMFABT-Th-Acceptor series:C71-PCBM BHJ materials were blended with 1:3weight ratio in chlorobenzene at a concentration of 30 mg/ml.These solutions were then spin-cast on top of the PEDOT layer.The substrate was dried for 10 min at 80 1C in air. Then, the devicewas pumped down to lower than 10–7 Torr and a �100 nm thickAl electrode was deposited on top.

2.4.2. Measurement

Solar cells efficiencies were characterized under simulated100 mW/cm2 AM 1.5 G irradiation from a Xe arc lamp with an AM1.5 global filter. The light intensity was adjusted with a Si solarcell that was double-checked with an NREL-calibrated Si solar cell(PV Measurement Inc.). The applied potential and measured cellcurrent were measured using a Keithley model 2400 digital sourcemeter. The current–voltage characteristics of the cell under theseconditions were determined by biasing the cell externally and mea-suring the generated photocurrent. This process was fully automatedusing Wavemetrics software. The IPCE spectra for the cells weremeasured on an IPCE measuring system (PV measurements).

3. Results and discussion

3.1. Synthesis and characterization

The synthetic methods are outlined in Scheme 2. All reactionswere carried out under a nitrogen atmosphere. The N,N-(6-bis(9,9-dimethyl-9H-fluoren-2-yl)amino-benzo[b]thiophen (i), 5-(6-(bis(9,9-dimethyl-9H-fluoren-2-yl)amino)benzo[b]thiophen-2-yl)thiophene-2-carbaldehyde (ii), 2-(2-tert-butyl-6-methyl-4H-pyran-4-ylidene)-malononitrile (iii), 2-(3-cyano-4,5,5-trimethylfuran-2(5H)-ylidene)-malononitrile (iv) could be effectively synthesized through modify-ing the procedures reported previously [35–37].

The bisDMFABT-Th-DCBP (2) (and bisDMFABT-Th-TCF (3))were readily prepared through Knoevenagel condensation reactionwith i and iii (and iv). And the borate compound of ii, which wasproduced from reaction of ii and pinacol borate using the n-BuLi,successfully performed the palladium-catalyzed Suzuki couplingreaction with 2-bromo-5-nitrothiophene using phase-transfer cata-lyst, Aliquat 336 mixture in biphase solution of anhydrous tolueneand deionized water, producing the bisDMFABT-Th-NO2 (1) withgood yield above 80%. The chemical structure of the synthesizedproducts, bisDMFABT-Th-Acceptor series, (1, 2, and 3) were verifiedwith 1H NMR, 13C NMR and MALDI-TOF mass analysis. And thesematerials have good solubility in common organic solvents, such asmethylene chloride, chloroform, chlorobenzene, and toluene.

3.2. Optical and electrochemical properties

Fig. 1 shows the UV–vis absorption spectra of the bisDMFABT-Th-Acceptor series, (1, 2, and 3) in chlorobenzene solution andthin films, and the corresponding optical properties are summar-ized in Table 1. As shown in Fig. 1, the absorption spectra of1 (black) and 2 (red) in cholorobenzene (solid line) showedtypical two transition bands with high molar absorptivities inwavelength region of 300�700 nm [38]: (1) 60,000 M�1 cm�1 at361 nm, 32,000 M�1 cm�1 at 514 nm and (2) 58,400 M�1 cm�1

at 371 nm, 51,100 M�1 cm�1 at 514 nm. The first peak could beassigned to the p–p* transition and the second peak at longerwavelength should result from an ICT between TPA donor andBT acceptor. These p–p* and ICT transitions are originate in

Scheme 2. Schematic diagram for the synthesis of the bisDMFABT-Th-Acceptors (NO2 (1), DCBP (2), TCF (3)) .

Fig. 1. UV–vis. absorption spectra of the bisDMFABT-Th-Acceptors [1 (black),

2 (red), and 3 (blue)] in chlorobenzene solution (solid line) and thin films (dashed

line). (For interpretation of the references to colour in this figure legend, the

reader is referred to the web version of this article.)

J. Kim et al. / Solar Energy Materials & Solar Cells 102 (2012) 159–166162

HOMO-1-LUMO and the HOMO-LUMO monoexcitations,respectively. The ICT band between bisDMFABT donor and DCPBacceptor of 2 showed the 1.6 times higher intensity than that of1 due to the more elongated p-conjugation length, but its positionseems to be almost similar with that of 1, indicating similar or alittle higher electron-withdrawing strengths of DCPB acceptorcompared to that of NO2 acceptor. Meanwhile, the 3 (blue)exhibited three transition bands, showing the absorption in wholevisible region. The three peaks at 367 nm, 481 nm, and 653 nmcould be also assigned to the transition conducted by the HOMO-2-LUMO (first), HOMO-1-LUMO (second) and the HOMO-

LUMO (third) monoexcitations, respectively [39]. Also, the ICT

transition observed as third peak of 3 was much more intensivethan the other p–p* transitions, which provide the ICT transitionstate between TPA donor and TCF acceptor as dominant excita-tion, even though the ICT bands of 1 and 2 showed the lessintensive than their p–p* transition bands. Moreover, the ICTband of 3 was significantly red-shifted �100 nm compared tothose of 1 and 2 because of the better electron withdrawingcharacteristics by three CN groups of TCF acceptor.

In solid-state thin film, the p-conjugated organic materials oftenshow the red-shifted absorption bands and broaden spectra by theintermolecular p–p packing interaction than those in solution [40].But, the absorption spectra of these materials in solid state exhibitedonly broaden spectra without band-shift to longer wavelengthregion. These observations seem to be caused by the amorphousnon-planar bisDMFABT that interrupts the intermolecular packinginteractions in solid-state film.

Fig. 2 shows the optimized structure of bisDMFABT-Th-Acceptorseries, (1, 2, and 3), which were calculated by the time dependent-density functional theory (TD-DFT) using the B3LYP functional/6-31 G* basis set. The orbital density of highest occupied mole-cular orbital (HOMO) of the 1, 2, and 3 were evenly distributed onbisDMFABT-thiophene-acceptor, but the orbital density of lowestunoccupied molecular orbital (LUMO) of these materials was pre-dominantly located on the electron accepting core and thiophenebridge, showing a general orbital distribution like push-pull organicsemiconductors. These calculations reveal that the ICT from bis-DMFABT to acceptor core can be effectively occurred in 1, 2, and 3absorbed the photon energy, and the bisDMFABT can especially playa role for stabilizing of hole separated from exciton and improve thetransport property of hole carrier.

Fig. 3 shows the the cyclic voltammograms of the bisDMFABT-Th-Acceptor series, (1, 2, and 3) in methylene chloride solution.And the corresponding electrochemical properties are also sum-marized in Table 1. The energy levels of HOMO and LUMO of thesematerials were determined from these cyclic voltammetry (CV)spectra. A platinum rod electrode, a platinum wire and an

Table 1Optical, redox parameters of the bisDMFABT-Th-Acceptors [1,2, and 3].

Compounds labsa/nm (e/M�1 cm�1) Eonset, ox (V)/HOMO (eV)b Eonset, red (V)/LUMO (eV) Eopt (eV)c E0�0 (eV)c

1 361 (60,000), 512 (32,000) 0.280/-5.080 �1.193/�3.608 2.00 1.92

2 371 (58,400), 514 (51,100) 0.192/-4.992 �1.194/�3.606 2.05 1.92

3 367 (18,800), 481 (13,700), 653 (34,400) 0.217/-5.017 �0.985/�3.815 1.66 1.54

a Absorption spectra were measured in chlorobenzene solution.b Redox potential of the compounds were measured in CH2Cl2 with 0.1 M (n-C4H9)4NPF6 with a scan rate of 100 mVs�1 (vs. Fc/Fcþ).c E0�0 was calculated from the absorption thresholds from absorption spectra in chlorobenzene solution.

Fig. 2. Isodensity surface plots of bisDMFABT-Th-Acceptors [1,2, and 3], calcu-

lated by the time dependent-density functional theory (TD-DFT) using the B3LYP

functional/6-31 G* basis set.

Fig. 3. Electrochemical characterization of the bisDMFABT-Th-Acceptors [1 (black),

2 (red), and 3 (blue)] in dichloromethane/TBAHFP (0.1 M), scan speed 100 mV/s,

potentials vs. Fc/Fcþ . (For interpretation of the references to colour in this figure

legend, the reader is referred to the web version of this article.)

J. Kim et al. / Solar Energy Materials & Solar Cells 102 (2012) 159–166 163

Ag/AgNO3 (0.10 M) electrode were used as the working electrode, asthe counter electrode and as the reference electrode, respectively.The analysis were performed in an electrolyte consisting ofa solution of 0.1 M tetrabutylammonium hexafluorophosphate(TBAPF6) in methylene chloride at room temperature undernitrogen with a scan rate of 100 mV/s and ferrocenium/ferroceneredox couple was used as an internal reference. The HOMO andLUMO levels can be deduced from the oxidation and reductiononsets with the assumption that the energy level of ferrocene (Fc)is 4.8reV below vacuum level. The CV of these materials in solid-state thin film could not be measured due to the stripping of filmon electrode. Therefore, we determined the optical bandgap fromcalculation of the absorption thresholds from absorption spectraof bisDMFABT-Th-Acceptor series in chlorobenzene. The HOMOlevel of the 1, 2, and 3 determined in solution by CV are calculatedas 4.99 eV, 5.02 eV, and 5.08 eV, respectively. These results notethat the HOMO level of bisDMFABT-Th-Acceptor could beslightly reduced by the electron withdrawing strength of accep-tors, leading to the increase of the open circuit voltage of devicesfabricated using these materials/PCBM BHJ active layers [41].These results will be discussed in more details below.

3.3. Photophysical characteristics of bisDMFABT-Th-acceptor/C71-PCBM

BHJ film

The photophysical characteristics of bisDMFABT-Th-Acceptorseries, (1, 2, and 3) were investigated through the application ofphotovoltaic device fabricated with these materials/C61(or 71)-PCBM BHJ films. In the course of studying the characteristics ofover 300 solar cells, the most efficient photovoltaic cells

fabricated using of [bisDMFABT-Th-Acceptor/C71-PCBM wereoptimized at ratio of 1:3 approximately. These BHJ films werecast on top of PEDOT:PSS (Heraeus, AI 4083) layer. The optimumthickness of these materials, 1, 2, and 3 based BHJ films obtainedunder these conditions was approximately 90 nm, 95 nm, and80 nm, respectively. Fig. 4(a) shows the UV–vis. absorptionspectra of the bisDMFABT-Th-Acceptor [1 (black), 2 (red), and3 (blue)]/C71-PCBM (1:3) films. As shown in Fig. 4(a), theabsorption bands of these materials in BHJ composites withC71-PCBM were red-shifted �10 nm and broaden to the longerwavelength compared to that of its pristine films shown inFig. 1.

To investigate the space-charge effects, we extracted the holemobilities of these organic semiconductor from the space chargelimitation of current (SCLC) J–V characteristics obtained in thedark for hole-only devices. Fig. 4(b) shows the dark-currentcharacteristics of hole-only ITO/PEDOT:PSS/bisDMFABT-Th-Acceptor:C71-PCBM/Au devices as a function of the bias cor-rected by the built-in voltage determined from the difference inwork function between Au and the HOMO level of thesematerials. The Ohm’s law can be observed at low voltages asan effect of thermal free carriers. For the presence of carriertraps in the active layer, there is a trap-filled-limit (TFL) regionbetween the ohmic and the trap-free SCLC regions. The SCLC

behavior in the trap-free region can be characterized using theMott–Gurney square law (1) [42,43].

J¼ ð9=8Þe mðV2=L3Þ ð1Þ

where e is the static dielectric constant of the medium and m is carriermobility. The hole mobility of bisDMFABT-Th-Acceptor series,[1 (black), 2 (red), and 3 (blue)] evaluated using the above Mott-Gurney Law (e¼3e0) were 4.37�10�5 cm2/V s, 2.11�10�5 cm2/V s,

Fig. 4. (a) UV–vis absorption spectra and (b) space charge limitation of current J–V

characteristics of the bisDMFABT-Th-Acceptors/C71-PCBM [1 (black), 2 (red), and

3 (blue)] BHJ (weight ratio of 1:3) films, which hole-only devices (ITO/PEDOT:PSS/

Donor:C71-PCBM/Au). (For interpretation of the references to colour in this figure

legend, the reader is referred to the web version of this article.)

Fig. 5. (a) Current (J)–voltage (V) curves under AM 1.5 conditions (100 mW/cm2)

and (b) IPCE spectra of the bisDMFABT-Th-Acceptors/C71-PCBM [1 (black), 2 (red),

and 3 (blue)] BHJ solar cells fabricated under optimized processing condition with

(solid line)/without (dashed line) insertion of TiOx layer. (For interpretation of the

references to colour in this figure legend, the reader is referred to the web version

of this article.)

Table 2Photovoltaic performances of the devices fabricated with the bisDMFABT-Th-Acceptors [1,2, and 3]/C71-PCBM BHJ filmsa.

Organic

semiconductor

Functional

layer

Jsc

(mA cm�2)

Voc

(V)

FF

(%)

Z (%)

1 None 8.54 0.75 33 2.14(70.30)

2b None 8.22 0.79 34 2.22(70.24)

3b TiOx 9.64 0.80 42 3.22(70.22)

4 None 7.32 0.82 32 1.92(70.15)

a The photovoltaic characteristics are performed under simulated 100 mW/

cm2 AM 1.5 G illumination. The light intensity using calibrated standard silicon

solar cells with a proactive window made from KG5 filter glass traced to the

National Renewable Energy Laboratory (NREL). The masked active area of device is

4 mm2.b The devices fabricated with this BHJ films were optimized after post-

annealing at 100 1C for 10 min.

J. Kim et al. / Solar Energy Materials & Solar Cells 102 (2012) 159–166164

and 1.24�10�5 cm2/V s, respectively. The 1 exhibited about 2 timesand 3.5 times higher hole mobility than those of the 2 and 3,respectively. It was interestingly found that these hole mobilitieswere contrary to the electron withdrawing strength of acceptors.The hole mobilities of p-type organic semiconductors observed inthese BHJ films might be closely related to the charge stabilityseparated from excition as well as the intermolecular packinginteraction, leading to the increase of transporting property of holecarrier. The bisDFMABT donor can stabilize the separated holefrom exciton. Therefore, we note that the bulk substituents such astert-butyl group of DCBP acceptor or dimethyl group of TCF acceptormight spatially interrupt the intermolecular packing interaction inBHJ system with PCBM.

3.4. Photovoltaic performances of bisDMFABT-Th-acceptor/C71-

PCBM BHJ film

Fig. 5 shows the current (J)–voltage (V) curves under AM1.5 conditions (100rmW/cm2) and incident-photon-to-currentefficiency (IPCE) spectra of bisDMFABT-Th-Acceptor [1 (black),2 (red), and 3 (blue)]/C71-PCBM BHJ solar cells fabricated underoptimized processing condition. The corresponding values aresummarized in Table 2. The IPCE spectra of these devices as

shown in Fig. 5(b) exhibit well-matched curves with their opticalabsorptions, resulting in the close correlation with their photo-currents in J–V curves.

As shown in Fig. 5 and Table 2, the devices conventionallyfabricated with 1/C71-PCBM, 2/C71-PCBM, and 3/C71-PCBM exhibiteda PCE of 2.14% (70.3) with short circuit current (Jsc)¼8.54 mA/cm2,

J. Kim et al. / Solar Energy Materials & Solar Cells 102 (2012) 159–166 165

fill fator (F.F)¼0.33, and open circuit voltage (Voc)¼0.75 V, a PCE of2.22% (70.24) with Jsc¼8.22 mA/cm2, FF¼0.34, and Voc¼0.79 V,and a PCE of 1.92% (70.15) with Jsc¼7.32 mA/cm2, FF¼0.32, andVoc¼0.82 V, respectively. Although the 3 showed the absorptionband covering the whole visible region, the solution processedSMOSCs fabricated with 3/C71-PCBM exhibited the less Jsc valuethan those of 1/C71-PCBM or 2/C71-PCBM BHJ film. This might bemainly originated from the low IPCE value in longer wavelengthregion, indicating the facile recombination of separated chargesfrom the excitons conducted by monoexcitation in ICT transitionband especially. These results were consistent with their holemobilities obtained by SCLC J–V characteristics shown in Fig. 4(b).Meanwhile, the Voc of 3/C71-PCBM BHJ solar cells showed the highervalues than those of 1/C71-PCBM or 2/C71-PCBM BHJ film, whichwere often affected by their HOMO levels observed in Fig. 2. Forthese results, the best performances were observed in the devicesfabricated with 2/C71-PCBM BHJ material. We tried to increase theefficiency of the device by means of insertion of TiOx thin layer,which can play the effective roles of optical spacer and buffer layer[44], between 2/C71-PCBM BHJ layer and Al electrode. The best a PCEof 3.22% (70.22) in devices fabricated using 2/C71-PCBM film withTiOx layer was observed with Jsc¼9.64 mA/cm2, F.F¼0.42, andVoc¼0.80 V, which showed the further improvements of photocur-rent and fill factor, resulting in �45% higher efficiency than thatwithout TiOx layer.

4. Conclusions

We have demonstrated the synthesis and photovoltaic charac-teristics of new efficient push-pull organic semiconductors com-prising of on the N,N-(6-bis(9,9-dimethyl-9H-fluoren-2-yl)amino-benzo[b]thiophen (bisDMFABT) donor and the various acceptors,bisDMFABT-Th-NO2 (1), bisDMFABT-Th-DCBP (2), and bisDM-FABT-Th-TCF (3), which were linked with thiophene or vinylthiophene p-conjugation bridges, in solution processed SMOSC.These materials showed the strong intramolecular charge transferbetween bisDMFABT donor and acceptors (NO2, DCBP, or TCF).Especially, the ICT band of 3 was significantly red-shifted �100 nmcompared to those of 1 and 2 because of the better electronwithdrawing characteristics by three CN groups of TCF acceptor.However, newly synthesized push-pull organic semiconductorshaving these acceptors (NO2, DCBP, TCF) exhibited rather lowefficiencies compared to that of MMN acceptor even they havesame donor unit, bisDMFABT. These results should be influencedwith many factors such as difference of charge-carrier mobilities,surface morphology, long-term stability of separated hole andelectron, or energy level in BHJ system. Among them, the electronwithdrawing strength of acceptor could effectively reduce theHOMO energy level, increasing the Voc of devices fabricated withthese materials/PCBM BHJ films. We also found that the bulksubstituents such as tert-butyl group of DCPB acceptor or dimethylgroup of TCF acceptor might spatially interrupt the intermolecularpacking interaction in BHJ system with PCBM. The best perfor-mances were observed in the devices fabricated with 2/C71-PCBMBHJ material that could be significantly improved by means ofinsertion of TiOx thin layer between photoactive layer and Alelectrode. These results obtained from the facile structural mod-ifications using various accepting units of organic semiconductorshown in this study can give an important guide for developingnew materials in solution-processed small molecule BHJ solar cell.

Acknowledgments

This research was supported by World Class University programfunded by the Ministry of Education, Science and Technology

through the National Research Foundation of Korea (R31-2011-000-10035-0) and the New & Renewable Energy of the KoreaInstitute of Energy Technology Evaluation ad Planning (KETEP)grant funded by the Korea government Ministry of KnowledgeEconomy (no. 20103060010020).

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