High-Performance Organic Solar Cells with Efficient Semiconducting

Mar 7, 2014 - High-performance organic semiconductors based on an electron-rich alkylsilylethynyl benzodithiophene (TIPSBDT) core were newly synthesiz...
0 downloads 6 Views 4MB Size
Download PDF
Article pubs.acs.org/cm

High-Performance Organic Solar Cells with Efficient Semiconducting Small Molecules Containing an Electron-Rich Benzodithiophene Derivative Namwoo Lim,† Nara Cho,† Sanghyun Paek,† Chulwoo Kim,‡ Jae Kwan Lee,*,§ and Jaejung Ko*,† †

Department of Materials Chemistry, Korea University, Sejong 339-709, South Chungcheong, Republic of Korea Nanotechnology Research Center, Konkuk University, Chungju 380-701, North Chungcheong, Republic of Korea § Department of Carbon Materials and Department of Chemistry Education, Chosun University, Kwangju 501-759, Gyeonggi, Republic of Korea ‡

S Supporting Information *

ABSTRACT: High-performance organic semiconductors based on an electron-rich alkylsilylethynyl benzodithiophene (TIPSBDT) core were newly synthesized and characterized for use in solution-processed small-molecule organic solar cells. The rigid and extended π-conjugation of the TIPSBDT motifs facilitates intramolecular charge transfer and intermolecular π−π packing interactions of semiconducting small molecules in the BHJ film enhanced by processing additives and induces deep HOMO levels producing high open-circuit voltage of ∼1.0 V, exhibiting notable power conversion efficiency of 5.84% from a bulk-heterojunction film with PC61BM.

1. INTRODUCTION Over the past few years, considerable effort has been focused on achieving power conversion efficiencies (PCE) above 10% in organic solar cells (OSCs), which are readily fabricated through facile and inexpensive manufacturing processes using versatile solution-processed printing methods such as the doctor blade, inkjet, and roll-to-roll.1−3 The following strategies have been adopted for this purpose:4−9 1) development of photoactive materials such as π-conjugated semiconducting polymers and fullerenes; 2) use of functional layers for buffering, charge transporting, optical spacing, and surface plasmon resonance; and 3) morphological tuning of photoactive films by postannealing, solvent drying, or by use of processing additives. One of the more promising strategies is use of low-bandgap semiconducting polymers composed of thieno[3,4-b]thiophene and benzodithiophene (poly(thieno[3,4-b]thiophene-alt-benzodithiophene) (PTB) series in bulk-heterojunction (BHJ) OSCs. BHJ OSC fabricated using these materials and [6,6]-phenylC(61 or 71)-butyric acid methyl ester (PC(61 or 71)BM) have achieved PCEs of up to 9.2%.10 These polymers have potential applications in next-generation solar cells and can compete with inorganic thin-film and dye-sensitized solar cells. However, polymer-based materials suffer from poor reproducibility of the weight-average molecular weight, high dispersity, and difficulties in purification. Since, fabrication methods with semiconducting small-molecules are more suited to mass production compared to those for polymer-based materials, significant research efforts in recent years have been directed at developing efficient smallmolecule organic semiconductors in an effort to improve the © 2014 American Chemical Society

performance of solution-processed small-molecule OSCs (SMOSCs) and with a near-term goal of achieving a PCE comparable to that of polymer solar cells (PSCs).11−14 We have also reported various organic semiconductors having push−pull molecular skeletons composed of electron-donating groups bridged with electron-accepting groups via π-conjugated motifs for SMOSCs.15,16 However, considering the low fill factors (F.F) observed for most small molecules used for SMOSCs, efficient molecular self-networking and phase separation with PCBM in BHJ morphology may be essentially required polymerlike materials. Very recently, Chen et al. have reported highly efficient semiconducting small molecules containing a benzo[1,2-b:4,5-b′]dithiophene (BDT) unit, affording OSCs with PCEs of 8.02% and high F.F,17 thus making solution-processed SMOSCs strong competitors to PSCs. Interestingly, these molecular frameworks with BDT units have exhibited remarkable photovoltaic performance in PSCs as well as SMOSCs.18 Thus, we aimed to develop efficient small molecule organic semiconductors based on a BDT derivative, 3,6-bis(triisopropylsilylethynyl)benzo[1,2-b:4,5-b′]dithiophene (TIPSBDT),19 which is quite similar to the soluble and crystallizable 6,13-bis(triisopropylsilylethynyl)anthracene (TIPSAnt) reported previously for use in SMOSCs.20 We expected that the TIPS motif might extend π-conjugation and induce a rigid structure, producing a more Received: January 14, 2014 Revised: March 7, 2014 Published: March 7, 2014 2283

dx.doi.org/10.1021/cm5004092 | Chem. Mater. 2014, 26, 2283−2288

Chemistry of Materials

Article

4,8-Bis[2-[tris(1-methylethyl)silyl]ethynylbenzo[1,2-b:4,5b′]dithiophenebis[4,4″-dihexyl[2,2′:5′,2″-terthiophene]-5-carboxaldehyde (T3CHO-TIPSBDT, iii). A solution of BrT3-CHO (ii) (1.02 g, 1.95 mmol) and stannyl derivative (i) (0.57 g, 0.65 mmol) in dry toluene (20 mL) was degassed twice with argon followed by the addition of Pd(PPh3)4 (37.5 mg, 0.032 mmol). After being stirred at 120 °C for 48 h, the reaction mixture was poured into water (50 mL) and extracted with CH2Cl2. The organic layer was washed with water and then dried over MgSO4. After removal of solvent, the crude product was purified by column chromatography on silica gel using a mixture of CH2Cl2 and hexane (1:1) as eluent to afford compound T3CHO-TIPSBDT (iii) (1.12g, 40%) 1H NMR (300 MHz, CHCl3): δ 10.0(s, 2H), 7.60(s, 2H), 7.26(d, 2H), 7.17(d, 2H), 7.11(s, 2H), 7.07(s, 2H), 2.91(m, 8H), 1.70−1.23 (m, 74H), 0.90(m, 12H). 13C NMR (75 MHz, CHCl3): δ 181.44, 153.97, 145.59, 142.34, 140.88, 138.73, 137.68, 135.89, 135.69, 135.13, 130.71, 127.62, 126.88, 126.45, 124.91, 120.93, 111.49, 102.49, 102.36, 55.16, 76.73, 31.94, 31.75, 31.52, 30.72, 30.01, 29.61, 29.20, 28,76, 22.85, 22.76, 19.04, 14.30, 13.10, 11.57. Anal. Calculated for C82H106O2S8Si2: C, 68.57; H, 7.44; O, 2.23; Found: C, 68.42; H, 7.54; O, 2.31. 4,8-Bis[2-[tris(1-methylethyl)silyl]ethynylbenzo[1,2-b:4,5b′]dithiophenebis[4,4″-dihexyl[2,2′:5′,2″-terthiophene]-2-cyanohexyl ester (DCAT3-TIPSBDT, 1). T3CHO-TIPSBDT (iii) (0.38 g, 0.264 mmol) was dissolved in a solution of dry CHCl3 (30 mL), a few drops of piperidine and then hexyl cyanoacetate (0.45 mL, 2.64 mmol) were added, and the resulting solution was refluxed and stirred for 24 h under argon. The reaction mixture was then extracted with CH2Cl2, washed with water, and dried over MgSO4. After removal of solvent it was chromatographed on a silica gel column using CH2Cl2 and hexane (1:1) as eluent, the crude black solid to afford DCAT3-TIPSBDT (1) (0.27g, 58%). mp: 276 °C. 1H NMR (300 MHz, CHCl3): δ 8.35 (s, 2H), 7.57 (s, 2H), 7.32 (d, 2H, J = 3.9 Hz), 7.17 (d, 2H, J = 3.6 Hz), 7.10 (s, 2H), 7.10 (s, 2H,), 4.28 (q, 4H), 2.86 (t, 4H), 2.81 (t, 4H), 1.75−1.66 (m, 8H), 1.36−1.26 (m, 82H), 0.932 (m, 18H). 13C NMR (75 MHz, CHCl3): 192.55, 163.71, 156.07, 145.67, 143.82, 143.06, 141.06, 138.95, 138.68, 137.76, 135.92, 134.60, 130.92, 129.47, 127.62, 125.78, 125.12, 121.17, 116.56, 115.37, 111.29, 102.47, 96.01, 66.64, 31.61, 31.60, 30.69, 29.51, 29.24, 28.60, 25.82, 25.55, 22.79, 19.10, 14.49, 14.23, 11.73, 11.46. Anal. Calculated for C100H132N2O4S8Si2 ; C, 69.07; H, 7.65; N, 1.61; O, 3.68; Found: C, 69.22; H, 7.58; N, 1.64; O, 3.72. 4,8-Bis[2-[tris(1-methylethyl)silyl]ethynylbenzo[1,2-b:4,5b′]dithiophenebis[4,4″-dihexyl[2,2′:5′,2″-terthiophene]-3ethyl-4-thiazolidinone (DRT3-TIPSBDT, 2). T3CHO-TIPSBDT (iii) (0.16 g, 0.11 mmol) was dissolved in a solution of dry CHCl3 (30 mL), a few drops of piperidine and then 3-ethylrhodanine (0.27 g, 1.65 mmol) were added, and the resulting solution was refluxed and stirred for 24 h under argon. The reaction mixture was then extracted with CH2Cl2, washed with water, and dried over MgSO4. After removal of solvent it was chromatographied on a silica gel column using CHCl3 and hexane (1:1) as eluent, and the crude solid was recrystallized from the hexane and ethyl acetate mixture four times to afford orange DRT3-TIPSBDT (2) (0.12 g, 62%). mp: 298 °C. 1H NMR (300 MHz, CHCl3): δ 7.89 (s, 2H), 7.57 (s, 2H), 7.23 (d, 2H, J = 3.9 Hz), 7.16 (d, 2H, J = 3.6 Hz), 7.10 (s, 2H), 7.09 (s, 2H,), 3.83 (q, 4H), 2.89 (t, 4H), 2.79 (t, 4H), 1.75−1.66 (m, 8H), 1.36−1.26 (m, 72H), 0.932 (m, 12H). 13C NMR (75 MHz, CHCl3): 167.33, 151.254, 143.58, 140.77, 138.68, 137.88, 137.69, 135.67, 135.04, 131.32, 130.69, 127.41, 126.43, 126.17, 124.95, 123.264, 119.39, 111.01, 102.54, 102.262, 40.01, 32.00, 31.83, 31.24, 30.65, 30.11, 29.93, 29.30, 22.91, 22.83, 19.07, 14.32, 12.50, 11.58. Anal. Calculated for C92H116N2O2S12Si2 ; C, 64.14; H, 6.79; N, 1.63; O, 1.86; Found: C, 64.22; H, 6.84; N, 1.72; O, 1.92. 4,8-Bis[2-[tris(1-methylethyl)silyl]ethynylbenzo[1,2-b:4,5b′]dithiophenebis[4,4″-dihexyl[2,2′:5′,2″-terthiophene]-3ethyl-2,4-thiazolidinedione (DTT3-TIPSBDT, 3). T3CHO-TIPSBDT (iii) (0.11 g, 0.07 mmol) was dissolved in a solution of dry CHCl3 (30 mL), a few drops of piperidine and then 3-ethyl-2,4-thiazolidinedione (0.11 g, 0.76 mmol) were added, and the resulting solution was refluxed and stirred for 24 h under argon. The reaction mixture was then extracted with CH2Cl2, washed with water, and dried over MgSO4. After removal of solvent it was chromatographied on a silica gel column using CHCl3

electron-rich BDT core that would facilitate intramolecular charge transfer (ICT) as well as molecular self-networking. Herein we report the synthesis and photovoltaic characteristics of three novel organic semiconductors: DCAT3-TIPSBDT (1), DRT3-TIPSBDT (2), and DTT3-TIPSBDT (3) consisting of a TIPSBDT donor and hexyl cyanoacetate, 3-ethylrhodanine, and 3ethyl-2,4-thiazolidinedione, respectively, linked via an alkylsubstituted terthiophene π-conjugation bridge. The acceptor− donor−acceptor (A-D-A) skeletons with these acceptors and bridge were also adapted for comparison with previously reported efficient materials, by constructing molecular structures with the TIPSBDT donor motif in organic semiconductors for SMOSCs (Scheme 1). These new materials were employed for fabrication of Scheme 1. Molecular Structure of TIPSBDT Series and Device Architecture of Solution-Processed Small Molecule Organic Solar Cell

photovoltaic devices using PC61BM BHJ films, exhibiting the high open-circuit voltage (Voc) of ∼1.0 V. Moreover, processing additives such as 1-chloronaphthalene (CN) and 1,8-diiodooctane (DIO) significantly affected the performance of the TIPSBDT series (1−3):PC61BM BHJ film, achieving markedly enhanced short-circuit current (Jsc) and high F.F in the SMOSCs.9 The most efficient small molecule organic semiconductor based on the novel TIPSBDT incorporated DCAT3-TIPSBDT (1) and exhibited a high PCE of 5.84% using BHJ film with PC61BM and 0.4 v/v% CN additive.

2. EXPERIMENTAL SECTION Synthesis and Characterization of Materials. The relevant synthetic methods are outlined in Scheme 2.

Scheme 2. Synthetic Procedure of TIPSBDT Series

All reactions were carried out under an argon atmosphere. Solvents were distilled from appropriate reagents. The molecules 2,6-bis[trimethyltin]4,8-bis[2-[tris(1-ethylethyl)silyl]ethynylbenzo[1,2-b:4,5-b′]dithiophenedihexyl[2,2′:5′,2″-terthiophene] (i) and 5″-bromo-4,4″-dihexyl-[2,2′:5′,2″-terthiophene]-5-carboxaldehyde (ii) were synthesized using a modified procedure of previous references.19,21 2284

dx.doi.org/10.1021/cm5004092 | Chem. Mater. 2014, 26, 2283−2288

Chemistry of Materials

Article

and hexane (1:1) as eluent, the crude solid was recrystallized from the ethyl acetate mixture four times to afford orange DTT3-TIPSBDT (3) (0.08 g, 62%). mp: 283 °C. 1H NMR (300 MHz, CHCl3): δ 8.06 (s, 2H), 7.57 (s, 2H), 7.23 (d, 2H, J = 3.9 Hz), 7.16 (d, 2H, J = 3.6 Hz), 7.10 (s, 2H), 7.09 (s, 2H,), 3.83 (q, 4H), 2.89 (t, 4H), 2.79 (t, 4H), 1.75−1.66 (m, 8H), 1.36−1.26 (m, 72H), 0.93 (m, 12H). 13C NMR (75 MHz,): 166.17, 165.42, 150.55, 143.01, 142.40, 141.44, 139.31, 138.54, 138.02, 136.84, 136.18, 131.83, 130.88, 128.29, 127.28, 126. 81, 125.94, 123.76, 121.54, 119.33, 111.75, 103.18, 103.03, 37.64, 32.71, 32.63, 31.90, 31.33, 30.68, 30.60, 30.34, 30.10, 29.84, 29.57, 29.33, 29.07, 26.76, 23.73, 19.50, 14.95, 13.64, 12.26. Anal. Calculated for C92H116N2O4S10Si2 ; C, 65.36; H, 6.92; N, 1.66; O, 3.79; Found: C, 65.37; H, 7.12; N, 1.69; O, 3.81. Measurements and Instruments. 1H and 13C NMR spectra were recorded on a Varian Mercury 300 spectrometer. Elemental analyses were performed with a Carlo Elba Instruments CHNS-O EA 1108 analyzer. Mass spectra were recorded on a JEOL JMS-SX102A instrument. The absorption and photoluminescence spectra were recorded on a Perkin-Elmer Lambda 2S UV−visible spectrometer and a Perkin LS fluorescence spectrometer, respectively. Cyclic voltammetry was carried out with a BAS 100B (Bioanalytical System, Inc.). A three electrode system was used and consisted of a nonaqueous reference electrode (0.1 M Ag/Ag+ acetonitrile solution; MF-2062, Bioanalytical System, Inc.), a platinum working electrode (MF-2013, Bioanalytical System, Inc.), and a platinum wire (diameter 1.0 mm, 99.9% trace metals basis, Sigma-Aldrich) as counter electrode. Redox potential of dyes was measured in CHCl3 with 0.1 M (n-C4H9)4N-PF6 with a scan rate between 100 mV s−1 (vs external Fc/Fc+). Solar cells efficiencies were characterized under simulated 100 mW/cm2 AM 1.5G irradiation from a Xe arc lamp with an AM 1.5 global filter. Simulator irradiance was characterized using a calibrated spectrometer, and illumination intensity was set using an NREL certified silicon diode with an integrated KG1 optical filter: spectral mismatch factors were calculated for each device in this report to be less than 5%. Short circuit currents were also found to be with 5% of values calculated using the integrated external quantum efficiency (EQE) spectra and the solar spectrum. The EQE was measured by underfilling the device area using a reflective microscope objective to focus the light output from a 75 W Xe lamp, monochromator, and optical chopper; photocurrent was measured using a lock-in amplifier, and the absolute photon flux was determined by a calibrated silicon photodiode. Device Fabrication. The BHJ films were prepared under optimized conditions according to the following procedure reported previously:9 The indium tin oxide (ITO)-coated glass substrate was first cleaned with 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 form a film with thickness of approximately 35 nm. The substrate was dried for 10 min at 140 °C in air and then transferred into a glovebox to spin-cast the photoactive layer. Semiconductors were combined with PC 61 BM blend in several ratios (1:0.5−1:3 w/w) in chlorobenzene at a concentration of 30 mg/mL. The blend solutions were spin-cast on the PEDOT layer, and the substrate was dried for 2 h in the glovebox. Then, the device was pumped down to lower than 10−6 Torr, and a ∼100 nm thick Al electrode was deposited on top.

Figure 1. (a) UV−vis absorption spectra of the 1 (black), 2 (blue), and 3 (red) in chlorobenzene (solid line) and thin-film (dashed line) and (b) cyclic voltammograms of the 1 (black), 2 (blue), and 3 (red) in CHCl3 with 0.1 M (n-C4H9)4NPF6 with a scan rate of 100 mVs−1 (vs external Fc/Fc+).

Table 1. Optical and Electrochemical Properties of TIPSBDT Series (1−3) mat.

λabsa/nm (ε/M‑1·cm‑1)

λmaxb/nm

HOMOc (eV)

LUMOd (eV)

E0−0e (eV)

1 2 3

526(111,800) 509(134,000) 500(138,000)

564 578 547

−5.15 −5.15 −5.12

−3.00 −3.06 −2.91

2.14 2.09 2.21

UV-vis absorption spectra in 1 × 10−5 M chlorobenzene. bFilm spincast on a quartz plate from a solution in chlorobenzene at 2500 rpm for 1 min. cRedox potential for HOMO of the compounds were measured in CHCl3 with 0.1 M (n-C4H9)4NPF6 with a scan rate of 100 mVs−1 (vs external Fc/Fc+). dLUMO was calculated by HOMOE0−0. eE0−0 was calculated from the thresholds from absorption spectra in chlorobenzene. a

3. RESULTS AND DISCUSSION Figure 1 shows the (a) UV−visible absorption spectra for the TIPSBDT series (1−3) in both chlorobenzene and thin-film form and (b) cyclic voltammograms in chloroform; the corresponding optical and electrochemical properties are summarized in Table 1. The absorption spectra for 1 (black), 2 (blue), and 3 (red) exhibited highly intensive ICT transition bands between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) in the 400−700 nm range. These bands were further analyzed by time-dependent density-functional theory (TD-DFT) calculations using the B3LYP/6-31G* model. Figure S1 and

Table S1 show the calculated energy levels of these materials in vacuo (see the Supporting Information). These ICT bands for 1, 2, and 3 in solution had high molar absorption coefficients of 111,800 M−1·cm−1 at 526 nm, 134,000 M−1·cm−1 at 509 nm, and 138,000 M−1·cm−1 at 500 nm, showing the twice molar absorptivity of BDT-based materials reported by Chen et al. previously.6 These results indicate that the rigid and extended π-conjugation of these TIPSBDT motifs facilitated ICT. In the thin-film state (dashed line), the absorption bands for 1, 2, and 3 were broader and red-shifted by about 50−70 nm compared to those in solution. In particular, the ICT spectra for 2285

dx.doi.org/10.1021/cm5004092 | Chem. Mater. 2014, 26, 2283−2288

Chemistry of Materials

Article

limited current (SCLC) J-V characteristics obtained in the dark for hole-only devices. Figure 2b shows the dark-current characteristics of ITO/PEDOT:PSS/1−3:PC61BM(1:1) with additive/Au devices as a function of the bias corrected for the built-in voltage determined from the difference in work function between Au and the PEDOT:PSS-coated ITO. Ohm’s law is obeyed at low voltages owing to the presence of thermal free carriers. In the presence of carrier traps in the active layer, a trap-filled-limit region exists between the Ohmic and trap-free SCLC regions. The SCLC behavior is characterized by the Mott−Gurney law (1)23

1, 2, and 3 in the thin-film state showed distinct vibronic shoulders at 601, 603, and 575 nm, respectively, indicating effective intermolecular π−π packing interactions. From the cyclic voltammograms of 1, 2, and 3 in Figure 1b, the HOMO and the orbital density of LUMO were deduced from the oxidation and reduction onsets under the assumption that the energy level of the standard ferrocene/ferrocenium (Fc/Fc+) system was 4.8 eV below the level in vacuo.22 Since the voltammograms of these materials in solid-state thin films could not be measured owing to the stripping of the film from the electrode, the bandgaps were determined from the solutionstate voltammograms. The calculated HOMO/LUMO levels of 1, 2, and 3 were 5.15/3.00, 5.15/3.06, and 5.12/2.91 eV, respectively. These HOMO levels were deeper than those of our previously reported materials15,16,20 and could generate high Voc values in BHJ OSCs. Figure 2a shows the UV−visible absorption spectra of 1/PC61BM, 2/PC61BM, and 3/PC61BM films, which were

J = (9/8)ε ·μ(V2/L3)

(1)

where ε is the static dielectric constant of the medium, and μ is the carrier mobility. The hole mobilities of 1, 2, and 3 evaluated using the Mott-Gurney law (ε = 3ε0) were 7.62(±1.843) × 10 −6 , 2.99(±1.938) × 10 −6 , and 4.82(±1.945) × 10 −6 cm2•V−1•s−1, respectively. Although these values were much lower than those (10−3−10−4cm2/V·s) of the current stateof-the art for small molecules in OSCs reported previously, the materials exhibited some comparable photocurrent.13 We thought that these results might be governed from well balanced electron and hole mobility in an optimized BHJ system with PCBM rather than those in small molecules.24 Next, molecules 1−3 were employed for the fabrication of BHJ photovoltaic devices using PC61BM films. Comparison of more than 200 solar cells revealed that the most efficient photovoltaic cells were based on the BHJ system with structures of 1/PC61BM, 2/PC61BM, and 3/PC61BM which exhibited a better performance than their PC71BM counterparts in terms of a better spectral response in the visible region. Figure 2 shows the J-V curves under AM 1.5 irradiation (100 mW•cm−2) and the incident photon-to-current efficiency (IPCE) spectra for the TIPSBDT series (1−3)/PC61BM BHJ solar cells fabricated under optimized conditions with (solid line) and without (dashed line) processing additives; the corresponding values are summarized in Table 2. These values Table 2. Photovoltaic Performances of SMOSCs Fabricated with TIPSBDT Series (1−3)/PC61BM BHJ Filmsa mat.

add.

Jsc (mAcm‑2)

Voc (V)

F.F

ηmax/ave (%)

1

no CN no DIO no DIO

6.63 10.32 6.75 8.67 6.3 8.91

0.99 0.96 1.02 0.97 1.00 0.97

0.51 0.59 0.34 0.60 0.38 0.62

3.35/3.15(±0.20) 5.84/5.50(±0.34) 2.36/2.21(±0.16) 5.03/4.70(±0.32) 2.40/2.18(±0.21) 5.31/5.07(±0.24)

2

Figure 2. (a) UV−vis absorption spectra and (b) space charge limitation of current J-V characteristics of the 1 (black), (2 (blue), or 3 (red))/PC61BM BHJ (weight ratio of 1:1) films, which hole-only devices (ITO/PEDOT:PSS/1−3:PC61BM with additive/Au).

3

The performances are determined under simulated 100 mW•cm−2 AM 1.5G 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. These values were determined from the average of 25 individual solar cells fabricated under each optimized conditions. a

optimized at ∼131, 128, and 110 nm of BHJ film thickness with/without additive, respectively, at a ratio of 1:1 for the best performance in BHJ OSCs. The normalized absorption bands observed in 1−3/PC61BM BHJ films without an additive exhibited spectral characteristics similar to that of their pristine solid-state film, but the peak intensities of their vibronic shoulders were significantly deceased. These peaks presented in 1−3/PC61BM BHJ films with an additive, indicating that the additive affected effectively intermolecular π−π packing interactions of these small molecules in the BHJ morphology. The hole mobilities of 1, 2, and 3 in these BHJ films optimized with processing additive extracted from the space-charge-

were determined from the average of individual 25 solar cells fabricated under each optimized conditions. The 1/PC61BM and 2(or 3)/PC61BM BHJ morphology was significantly affected by the CN and DIO additives, respectively, improving the performance of the BHJ OSCs. The IPCE spectra for 2286

dx.doi.org/10.1021/cm5004092 | Chem. Mater. 2014, 26, 2283−2288

Chemistry of Materials

Article

Figure 3. (a) Current (J)-voltage (V) curves under AM 1.5 conditions (100 mW/cm2) and (b) IPCE spectra of the 1 (black), (2 (blue), or 3 (red))/PC61BM BHJ solar cells fabricated under optimized processing conditions without (solid line)/with (dashed line) processing additives.

Figure 4. Tapping mode AMF images of the 1 (2 or 3)/PC61BM BHJ morphologies without/with additives.

4. CONCLUSIONS We demonstrated the synthesis and photovoltaic characteristics of a series of novel organic semiconductors based on electronrich TIPSBDT cores for solution processed SMOSCs. The rigid and extended π-conjugation of the TIPSBDT motifs facilitates ICT and intermolecular π−π packing interactions of semiconducting small molecules in the BHJ film enhanced by processing additives and induces deep HOMO levels producing high Voc values. The findings of this study introduce a new direction for development of novel materials for use in efficient solution-processed SMOSCs. Further studies of photophysics and molecular distribution and orientation of these materials in BHJ systems are in progress.

these devices were consistent with their optical absorptions in the BHJ film with PC61BM (Figure 3), closely correlating with the photocurrents in the J-V curves. The conventionally fabricated devices incorporating 1/PC61BM, 2/PC61BM, and 3/PC61BM had notably high Voc of ∼1.0 V: PCE (maximum/average) of 3.35/3.15% with a Jsc of 6.63 mA·cm−2, a F.F of 0.51, and Voc of 0.99 V; PCE of 2.36/2.21% with Jsc = 6.75 mA·cm−2, F.F = 0.34, and Voc = 1.02 V; and PCE of 2.40/2.18% with Jsc = 6.33 mA·cm−2, F.F = 0.38, and Voc = 1.00 V, respectively. All devices fabricated using these materials:PC61BM BHJ films in conjunction with CN or DIO additives showed significantly improved photovoltaic performances compared to those of the conventional devices without additives. The best PCEs were obtained for devices fabricated using 1/PC61BM, 2/PC61BM, and 3/PC61BM BHJ film with 0.4 v/v% of CN and 0.35 and 0.45 v/v% of DIO, respectively, resulting in 70−120% improvements in PCE and strongly enhanced Jsc and F.F compared to the films without additives: PCE(maximum/ average) of 5.84/5.50% with a Jsc of 10.32 mA·cm−2, a F.F of 0.59, and Voc of 0.96 V; PCE of 5.03/4.70% with Jsc = 8.67 mA·cm−2, F.F = 0.60, and Voc = 0.97 V; and PCE of 5.31/ 5.07% with Jsc = 8.91 mA·cm−2, F.F = 0.62, and Voc = 0.97 V. Thus, the CN and DIO additives substantially increase intermolecular π−π packing interactions of the semiconducting small molecules in the BHJ film compared to the devices without additives, inducing highly phase-segregated morphologies as shown in atomic force microscope (AFM) images (Figure 4).

■

ASSOCIATED CONTENT

S Supporting Information *

TD-DFT calculations using the B3LYP/6-31G* model and optimization of device fabrication with additives. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the Converging Research Center Program through the Ministry of Science, ICT and Future Planning, 2287

dx.doi.org/10.1021/cm5004092 | Chem. Mater. 2014, 26, 2283−2288

Chemistry of Materials

Article

Korea (2013K000203), the International Science and Business Belt Program through the Ministry of Education, Science and Technology (no. 2012K001573), the ERC (the Korean government (MEST)) program (no. 2013004800), and Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by NRF-2013R1A1A4A01005961.

■

REFERENCES

(1) Krebs, F. C. Sol. Energy Mater. Sol. Cells 2009, 93, 394−41. (2) Arias, A. C.; Mackenzie, J. D.; McCulloch, I.; Rivnay, J.; Salleo, A. Chem. Rev. 2010, 110, 3−24. (3) Helgesen, M.; Søndergaard, R.; Krebs, F. C. J. Mater. Chem. 2010, 20, 36−60. (4) Price, S. C.; Stuart, A. C.; Yang, L.; Zhou, H.; You, W. J. Am. Chem. Soc. 2011, 133, 4625−4631. (5) Zhou, H.; Yang, L.; Stuart, A. C.; Price, S. C.; Liu, S.; You, W. Angew. Chem., Int. Ed. 2011, 50, 2995−2998. (6) Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. Adv. Funct. Mater. 2005, 15, 1617−1622. (7) Yang, C.; Lee, J. K.; Heeger, A. J.; Wudl, F. J. Mater. Chem. 2009, 19, 5416−5423. (8) Lee, K.; Kim, J. Y.; Park, S. H.; Kim, S. H.; Cho, S.; Heeger, A. J. Adv. Mater. 2007, 19, 2445−2449. (9) Lee, J. K.; Ma, W. L.; Brabec, C. J.; Yuen, J.; Moo, J. S.; Kim, J. Y.; Lee, K.; Bazan, G. C.; Heeger, A. J. J. Am. Chem. Soc. 2008, 130, 3619− 3623. (10) He, Z.; Zhong, C.; Su, S.; Xu, M.; Wu, H.; Cao, Y. Nat. Photonics 2012, 6, 593−597. (11) Roncali, J. Acc. Chem. Res. 2009, 42, 1719−1730. (12) Demeter, D.; Rousseau, T.; Leriche, P.; Cauchy, T.; Po, R.; Roncali, J. Adv. Funct. Mater. 2011, 21, 4379−4387. (13) Sun, Y.; Welch, G. C.; Leong, W. L.; Takacs, C. J.; Bazan, G. C.; Heeger, A. J. Nat. Mater. 2012, 11, 44−48. (14) Li, W.; Du, C.; Li, F.; Zhou, Y.; Fahlman, M.; Bo, Z.; Zhang, F. Chem. Mater. 2009, 21, 5327−5334. (15) Paek, S.; Cho, N.; Song, K.; Jun, M. J.; Lee, J. K.; Ko, J. J. Phys. Chem. C 2012, 116, 23202−23213. (16) Paek, S.; Cho, N.; Cho, S.; Lee, J. K.; Ko, J. Org. Lett. 2012, 14, 6326−6329. (17) Zhou, J.; Zuo, Y.; Wan, X.; Long, G.; Zhang, Q.; Ni, W.; Liu, Y.; Li, Z.; He, G.; Li, C.; Kan, B.; Li, M.; Chen, Y. J. Am. Chem. Soc. 2013, 135, 8484−8487. (18) Chen, H. Y.; Hou, J. H.; Zhang, S. Q.; Liang, Y. Y.; Yang, G. W.; Yang, Y.; Yu, L. P.; Wu, Y.; Li, G. Nat. Photonics 2009, 3, 649−653. (19) Kim, J. H.; Song, C. E.; Kim, H. U.; Grimsdale, A. C.; Moon, S. J.; Shin, W. S.; Choi, S. K.; Whang, D. H. Chem. Mater. 2013, 25, 2722−2732. (20) Choi, H.; Ko, H. M.; Cho, N.; Song, K.; Lee, J. K.; Ko, J. ChemSusChem 2012, 5, 2045−2052. (21) Zhou, J.; Wan, X.; Liu, Y.; Long, G.; Wang, F.; Li, Z.; Li, C.; Chen, Y. Chem. Mater. 2011, 23, 4666−4668. (22) Chen, Z.; Debije, M. G.; Debaerdemaeker, T.; Osswald, P.; Würthner, F. ChemPhysChem 2004, 5, 137−140. (23) Mihalietchi, V. D.; Xie, H.; de Boer, B.; Koster, L. J. A.; Blom, P. W. M. Adv. Funct. Mater. 2006, 16, 699−708. (24) Abbas, M.; Tekin, N. Appl. Phys. Lett. 2012, 101, 073302.

2288

dx.doi.org/10.1021/cm5004092 | Chem. Mater. 2014, 26, 2283−2288