Dyes and Pigments 115 (2015) 17e22
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Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
Solution-processable thienoisoindigo-based molecular donors for organic solar cells with high open-circuit voltage Oleh Vybornyi a, Yue Jiang a, b, François Baert a, Dora Demeter a, Jean Roncali a, ment Cabanetos a, * Philippe Blanchard a, *, Cle a b
CNRS UMR 6200, MOLTECH-Anjou, University of Angers, 2 Bd Lavoisier, 49045 Angers, France South China University of Technology, 381 Wushan Rd, Tianhe, Guangzhou, Guangdong, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 September 2014 Received in revised form 27 November 2014 Accepted 5 December 2014 Available online 18 December 2014
Two acetylene-bridged DonoreAcceptoreDonor (D-A-D) type small pi-conjugated molecules involving triphenylamine or N-phenylcarbazole as donor blocks (D) and thienoisoindigo as the acceptor unit (A) were synthesized and characterized by UVeVis absorption and cyclic voltammetry. These donor materials were mixed with [6,6]-phenyl-C61-butyric acid methyl ester to prepare bulk heterojunction solar cells by simple solution processing. Due to their low-lying highest occupied molecular orbital energy levels, high open-circuit voltages up to 0.99 V were measured. The triphenylamine end-capped derivative led to the best power conversion efficiency of ca 2.20%, which ranks among the highest reported value for thienoisoindigo-based materials. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Thienoisoindigo Acetylene bridge Small p-conjugated molecule Organic photovoltaics Bulk heterojunction Organic electronics
1. Introduction Over the past decade, the design of active materials based on discrete p-conjugated molecules has generated considerable research efforts due to the possibility of developing cost-effective, light-weight and low environmental impact organic solar cells (OSCs) [1e3]. Compared to conjugated polymers, molecular systems present some specific advantages in terms of reproducibility of synthesis and purification, and allow a more precise analysis of the structure-property relationships owing to their monodisperse nature [4e7]. Based on a common design strategy shared with low band gap polymers involving the combination and alternation of electron-donating (D) and electron-accepting (A) building blocks [8e12], discrete p-conjugated molecules recently led to OSCs with power conversion efficiencies (PCE) above 8% [12e14]. In the past few years, isoindigo dyes have drawn attention due to their strong electron accepting properties, large optical transition dipoles and potential interest for organic photovoltaics (OPVs), organic field effect transistors (OFETs) and organic memory
* Corresponding authors. E-mail addresses:
[email protected] (P. Blanchard), clement.
[email protected] (C. Cabanetos). http://dx.doi.org/10.1016/j.dyepig.2014.12.004 0143-7208/© 2014 Elsevier Ltd. All rights reserved.
applications [15e19]. In order to improve the electronic p-delocalization of indigo-based conjugated systems, the outer phenyl rings of isoindigo have recently been swapped with thiophene moieties affording the so-called thienoisoindigo block (TII) [20,21]. Although this building block was found to be particularly promising for OFETs [22], very few examples of thienoisoindigo-based donor for OPVs have been reported [23e26] and to the best of our knowledge, only one concerns a discrete molecular donor [27]. The latter, published by Chen et al., shows moderate PCE of ca 1.14% when blended with PC61BM mainly due to a low open-circuit voltage (Voc ¼ 0.44 V) associated with the high-lying HOMO level of the donor material (4.87 eV). In this context, we report here on the synthesis and the characterization of two DeAeD type thienoisoindigo-based small pconjugated molecules as well as their preliminary evaluation as molecular donors in bulk heterojunction (BHJ) solar cells in the presence of PC61BM as electron acceptor material. Herein, the triphenylamine (TPA) and N-phenylcarbazole (PCz) units were selected as end-capping moieties (D) for their electron-donating character and hole-transport properties (Fig. 1) whereas thienoisoindigo was used as electron-accepting building-block (A). Moreover, to ensure a low-lying HOMO level to the target donors and thus maximize the Voc, acetylenic linkages were inserted
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Fig. 1. Chemical structures of TII-TPA and TII-PCz.
between the D and A blocks. Indeed, this strategy was recently applied to diketopyrrolopyrrole-based molecular donors [28] and was found to: i) improve the backbone planarity, ii) extend the conjugation, iii) enhance light absorption and iv) lower the HOMO energy level, thus increasing the Voc [29]. Finally, the 2-ethylhexyl branched side chains, born by the central TII unit, were selected to bring sufficient solubility for solution-processing. 2. Experimental section 2.1. Materials All reagents and chemicals from commercial sources were used without further purification. Reactions were carried out under nitrogen atmosphere unless otherwise stated. Solvents were dried and purified using standard techniques. 2.2. Measurements and characterization Flash chromatography was performed with analytical-grade solvents using Aldrich silica gel (technical grade, pore size 60 Å, 230e400 mesh particle size). Flexible plates ALUGRAM® Xtra SIL G UV254 from MACHEREY-NAGEL were used for TLC. Compounds were detected by UV irradiation (Bioblock Scientific) or staining with I2, unless stated otherwise. NMR spectra were recorded with a Bruker AVANCE III 300 (1H, 300 MHz and 13C, 75 MHz). Chemical shifts are given in ppm relative to TMS and coupling constants J in Hz. IR spectra were recorded on a Bruker spectrometer Vertex 70 and UVeVis spectra with a Perkin Elmer 950 spectrometer. Matrix Assisted Laser Desorption/Ionization was performed on MALDI-TOF MS BIFLEX III Bruker Daltonics spectrometer using dithranol as matrix. Cyclic voltammetry was performed in 0.10 M Bu4NPF6/ CH2Cl2 (HPLC grade). Solutions were degassed by nitrogen bubbling prior to each experiment. Experiments were carried out in a onecompartment cell equipped with platinum electrodes and a saturated calomel reference electrode (SCE) using a Biologic SP-150 potentiostat with positive feedback compensation. Transmission Electron Microscopy (TEM) images were recorded in bright field mode with a microscope operating at 120 keV (JEOL JEM 1400). A 20 mm objective aperture was used in to improve the contrast. Donor/PC61BM Films of ~60 nm thickness were spun cast on PEDOT: PSS coated glass substrates. The films were floated off in deionized water and collected on copper TEM grids (Electron Microscopy Sciences). 2.3. Synthetic procedures 2,20 -Dibromo-4,40 -bis(2-ethylhexyl)[6,60 bithieno[3,2,b]pyrrolylidene]-5,50 (4H,4H0 )-dione 1, 4-ethynyl-N,N-diphenylaniline 2 and 9-(4-ethynylphenyl)-9H-carbazole 3 were synthesized according to previously reported methods [20,21,30,31].
2.3.1. General procedure for the synthesis of TII-TPA and TII-PCz Dibromothienoisoindigo 1 (100 mg, 152.2 mmol, 1 eq), alkyne compound 2 or 3 (319.6 mmol, 2.1 eq), copper (I) iodide (0.29 mg, 1.52 mmol, 0.01 eq) and tetrakis-(triphenylphosphine) palladium (0) (8.79 mg, 7.61 mmol, 0.05 eq) were combined in a Schlenk flask. Then 20 mL of a freshly distillated and degassed solution of triethylamine: toluene (1:1) was added to the flask and the reaction mixture was stirred overnight at 45 C. The solution was then poured into 150 mL of brine and extracted with diethyl ether (2 100 mL). The organic layers were combined, washed with brine, dried over anhydrous MgSO4 and concentrated under vacuum. The crude product was finally purified by column chromatography (dichloromethane/pentane 3:7) to afford the desired dark green compound. TII-TPA: (75 mg, 47.7% yield) 1H NMR (300 MHz, CDCl3, ppm): d: 7.30 (m, 14H), 7.11 (m, 12H), 7.00 (d, J ¼ 8.7 Hz, 4H), 6.85 (s, 2H), 3.66 (d, J ¼ 7.2 Hz, 4H), 1.83 (s, 2H), 1.35e1.21 (m, 16H), 0.92e0.81 (m, 12H). 13C NMR (75 MHz, CDCl3, ppm) d: 170.80, 150.71, 148.44, 146.91, 132.37, 131.72, 129.46, 125.28, 123.89, 121.71, 120.13, 115.58, 114.87, 114.21, 99.31, 83.91, 45.77, 38.44, 30.53, 28.64, 23.88, 23.03, 14.05, 10.49. ESI HRMS: calculated for C68H64N4O2S2 1032.4471, found 1032.4467. IR (neat): y ¼ 1676 cm1 (C]O). TIIPCz: (84,6 mg, 54% yield) 1H NMR (300 MHz, CDCl3, ppm): d: 8.15 (d, J ¼ 7.7 Hz, 4H), 7.75 (d, J ¼ 8.4 Hz, 4H), 7.62 (d, J ¼ 8.4 Hz, 4H), 7.45 (q, J ¼ 8 Hz, 8H), 7.32 (m, 4H), 6.98 (s, 2H), 3.73 (d, J ¼ 7.4 Hz, 4H), 1.89 (m, 2H), 1.35e1.21 (m, 16H), 0.92e0.81 (m, 12H). 13C NMR (75 MHz, CDCl3, ppm): d: 170.78, 150.93, 140.43, 138.15, 132.91, 132.31, 131.11, 128.74, 126.90, 126.10, 123.65, 121.37, 120.60, 116.27, 114.95, 109.75, 97.81, 85.94, 45.90, 38.52, 30.59, 28.68, 23.96, 23.07, 14.07, 10.63. ESI HRMS: calculated for C68H60N4O2S2 1028.4158, found 1028.4158. IR (neat): y ¼ 1675 cm1 (C]O). 2.4. Device fabrication 2.4.1. Solar cells Indium-tin oxide coated glass slides of 24 25 1.1 mm with a sheet resistance of RS ¼ 7 U/sq were purchased from Praezisions Glas & Optik GmbH. The ITO layer was patterned via a 37% hydrochloric acid solution and zinc powder etching. The substrates were then washed with a dilluted Deconex® 12 PA-x solution (2% in water) and scrubbed using dishwashing soap before being cleaned by a series of ultrasonic treatments for 15 min in distilled water (15.3 MU cm1), acetone and isopropanol. Once dried under a steam of nitrogen, a UV-ozone plasma treatment (UV/Ozone ProCleaner Plus, Bioforce Nanosciences) was performed for 15 min. A filtered aqueous solution of poly(3,4-ethylenedioxy-thiophene)poly(styrenesulfonate) (PEDOT:PSS; Clevios P VP. AI 4083) through a 0.45 mm PTFE membrane (Millex®) was spun-cast onto the patterned ITO surface at 5000 rpm for 40 s before beeing baked at 140 C for 30 min. Then, blends of molecular donor TII-TPA or TIIPCz and PC61BM were dissolved in chloroform (10 mg/mL), stirred at 45 C for 2 h and spun-cast at different spin-speeds onto the
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Scheme 1. Synthetic route to TII-TPA and TII-PCz.
PEDOT:PSS layer. Finally, devices were completed by the successive thermal deposition of lithium fluoride (1 nm) and aluminum (120 nm) at a pressure of 1.5 105 Torr through a shadow mask defining six cells of 27 mm2 each (13.5 mm 2 mm). J vs V curves were recorded in the dark and under illumination using a Keithley 236 source-measure unit and a home-made acquisition program. The light source is an AM1.5 Solar Constant 575 PV simulator (Steuernagel Lichttecknik, equipped with a metal halogen lamp). The light intensity was measured by a broad-band power meter (13PEM001, Melles Griot). EQE was measured under ambient atmosphere using a halogen lamp (Osram) with an Action Spectra Pro 150 monochromator, a lock-in amplifier (PerkineElmer 7225) and a S2281 photodiode (Hamamatsu). 2.4.2. Space charge limited current (SCLC) measurements A solution of neat molecular donor TII-PCz or TII-TPA (20 mg/ mL) in chloroform was spun cast at different spin-rates on the above described PEDOT: PSS substrates to provide organic layers of various thickness. Gold cathodes (150 nm) were thermally evaporated under a vacuum of 1.5 105 Torr, through a shadow mask defining actives area of 12.60 mm2, 3.10 mm2, and 0.78 mm2 per substrates. Hole mobilities mh were evaluated using the MottGurney law, ie, JSCLC ¼ (9/8)ε0εrmh(V2/d3) where εr is the static dielectric constant of the medium (εr ¼ 3) and d, the thickness of the active layer [32,33].
cross-coupling reaction with the corresponding alkyne derivative (2 or 3) affording dark green powders. Both materials show good solubility in common organic solvents such as chloroform, tetrahydrofuran, chlorobenzene and o-dichlorobenzene.
3.2. Optical properties UVeVis absorption spectra of TII-PCz and TII-TPA were recorded on chloroform solutions and spin-cast films (Fig. 2). These molecules show a green color both in solution and in the solidstate. The optical data are summarized in Table 1. Two characteristic absorption bands are observed, ie, one at the 350e450 nm region and a broader one at longer wavelengths (500e800 nm). The former can be assigned to a pep* transition and the latter to an internal charge transfer (ICT) transition between the D and A blocks [27]. TII-TPA shows a higher molar extinction coeffcient (ε) of 44,900 M1 cm1 at 657 nm and 45,500 M1 cm1 at 382 nm vs 37,600 M1 cm1 at 632 nm and 40,400 M1 cm1 at 391 nm for TII-PCz. Moreover, the ca 25 nm red shift of the low energy maximum on going from TII-PCz to TII-TPA is consistent with the stronger donor effect of the triphenylamine block (vs the N-phenylcarbazole), ensuring a better light harvesting ability. Finally, Table 1 UVevis absorption data. Compound TII-PCz
3.1. Synthesis TII-TPA
The common synthetic route to TII-TPA or TII-PCz is depicted in Scheme 1. Dibromothienoisoindigo 1 was engaged in a Sonogashira
lmax (nm)in
ε(M1$cm1)
chloroform
3. Results and discussions
391 631 382 654
40,400 37,600 45,400 43,900
lmax (nm) thin film
lonset (nm) thin film
Eopt g (eV)
637
763
1.62
653
799
1.55
Fig. 2. Normalized UVeVis absorption spectra of the TII-PCz (black squares) and TII-TPA (red circles) in solution (a) and as thin film on glass (b). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Table 2 Electrochemical properties. 0.5 mM in 0.10 M Bu4NPF6 CH2Cl2, 100 mV s1, Pt working electrode, ref. SCE. Compound E1pa (V) Oxonset (V) E1pc (V) Redonset (V) HOMOa (eV) LUMOb (eV) TII-PCz TII-TPA a b
0.92 0.69
0.82 0.59
0.93 0.98
0.80 0.83
5.81 5.58
4.19 4.16
Table 3 Photovoltaic properties of TII-PCz and TII-TPA blended with PC61BM (1:3 w/w). Donor
Voc (V)
Jsc (mA$cm2)
FF (%)
PCEmax/avea) (%)
mh (cm2 V1 s1)
TII-PCz TII-TPA
0.99 0.82
4.11 5.48
29.9 39.8
1.52/1.48 2.23/2.11
1.9 105 4.1 105
a
Average value recorded over 12 optimized devices.
HOMO ¼ (Eox(onset) þ 4.99) (eV). ELUMO ¼ (Ered(onset) þ 4.99) [36].
gathered in Table 2. The cyclic voltammogram (CV) of both molecules presents a reversible reduction wave with cathodic peak potentials at E1pc ¼ 0:95 V for TII-PCz and 0.98 V for TII-TPA (Fig. 3). The similarity of the reduction potential for the two compounds suggests that the LUMO is essentially located on the TII acceptor moiety. In the positive potentials region, the CV of TII-TPA exhibits two reversible one-electron oxidation waves peaking at E1pc ¼ 0:69 V and E2pa ¼ 0.82 V assigned to the formation of stable radical cation and dication. The CV of TII-PCz shows a quasiirreversible oxidation wave at E1pa ¼ 0:92 V while a cathodic waves of weak intensity is observed in the reverse scan. This suggests that the high reactivity of the carbazole cation radical leads to some follow-up chemical coupling reactions [34,35]. The 0.23 V negative shift of E1pa observed for TII-TPA reveals its stronger donor character. From the onsets of oxidation and reduction, HOMO levels of 5.58 eV and 5.81 eV and LUMO levels of 4.16 eV and 4.19 eV were estimated for TII-TPA and TII-PCz respectively [36]. With relatively similar LUMO levels it turns out that the Nphenylcarbazole derivative displays, as expected, the lowest-lying HOMO. Furthermore, both molecules exhibit deeper HOMO levels than the thienoisoindigo-based molecules published by Chen et al. (4.87 eV), which should be beneficial for the Voc [29]. 3.4. Photovoltaic properties Fig. 3. Cyclic voltammograms of a) TII-PCz (black) and b) TII-TPA (red) 0.5 mM in 0.10 M Bu4NPF6 CH2Cl2, 100 mV s1, Pt working electrode. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
compared to the solution spectra, the UVeVis spectra of the films show broader absorption bands and red-shifted absorption edges with absorption onsets at ca 763 and 799 nm corresponding to optical energy gaps ðEopt g Þ of 1.62 eV and 1.55 eV for TII-PCz and TIITPA respectively.
3.3. Cyclic voltammetry Cyclic voltammetry was performed in methylene chloride solution in the presence of Bu4NPF6 as the electrolyte. Data are
The potential of the two compounds as donor material in OPV cells has been investigated through the fabrication of BHJ solar cells of configuration: ITO/PEDOT:PSS (ca 40 nm)/TII-PCz or TIITPA:PC61BM blend/LiF (1 nm)/Al (120 nm). The best devices were obtained with films spun-cast at 1000 rpm for TII-PCz and 1200 rpm for TII-TPA from chloroform solutions containing 10 mg/ mL of donor and acceptor in a 1:3 w/w ratio. Fig. 4a shows the current densityevoltage (J-V) characteristics of these devices under AM. 1.5 simulated solar illumination (80 mW cm2) and the corresponding photovoltaic properties are gathered in Table 3. In optimized conditions, TII-TPA/PC61BM-based devices exhibit a maximum PCE of 2.23% with a Jsc of 5.48 mA cm2 and FF of ca 40%. Nevertheless, in agreement with its deeper HOMO energy level, TIIPCz leads to a higher open-circuit voltage than TII-TPA (0.99 V vs
Fig. 4. J eV characteristics and EQE curves of TII-PCz (black squares, opened: under illumination, filled: in the dark) and TII-TPA (red circles, opened: under illumination, filled: in the dark) based OSCs. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 5. Bright-field TEM images of the nanoscale morphologies of optimized thin films of TII-PCz/PC61BM (a) and TII-TPA/PC61BM (b).
Fig. 6. JeV characteristics of hole only devices ITO/PEDOT:PSS/TII-TPA (a) or TII-PCz (b)/Au. Solid lines: ohmic regime (slope ¼ 1); SCLC regime (slope ¼ 2).
0.82 V respectively). To the best of our knowledge these Voc and PCE values are the highest reported so far for thienoisoindigo based materials. The external quantum efficiencies (EQE) of the cells based on the two donors are compared in Fig. 4b. Both spectra exhibit a peak around 365 nm associated mainly with the contribution of the fullerene derivative followed by a broad band extending from 520 to 800 nm corresponding to the absorption of the donors. The EQE spectrum of the TII-TPA-based cell shows a first maximum of ca 75% at ~365 nm and a second maximum of ca 20% at ~650 nm. For the TII-PCz cell these values decrease to 60% and 15%. These results agree well with the higher Jsc values obtained for TII-TPA compared to TII-PCz (5.48 mA cm2 vs 4.11 mA cm2). To gain further insight into these differences, nano-scale morphological analyses were conducted on the optimized active layers using bright-field transmission electron microscopy (TEM). As shown in Fig. 5, with darker areas attributed to the PC61BM-rich regions [37e39], it turns out that the TII-PCz:PC61BM blend exhibits a very homogeneous morphology with domains size 1 nm. On the other hand the TII-TPA:PC61BM blend reveals a rising percolated two-phase nanostructure network with nano-domains size above 10 nm conducive for better charge transport and therefore to higher current densities [8,40,41]. The hole-mobility (mh) of the two donors was determined on hole-only devices (ITO/PEDOT: PSS/donor/gold) using the spacecharge limited current (SCLC) method (Fig. 6). As shown in Table 3, TII-TPA shows a hole-mobility ca twice larger than that of TII-PCz. This results combined with the morphological characteristics of the TII-TPA based OSCs probably contributes to the better FF and Jsc values obtained the triphenylamine based donor.
4. Conclusion Two DeAeD type small p-conjugated molecules based on a central thienoisoindigo end-capped with triphenylamine or Nphenylcarbazole moieties attached by acetylenic linkers have been synthesized. The results obtained with BHJ solar cells using PC61BM as acceptor show that both donors can lead to high open-circuit voltages owing to low-lying HOMO levels. However, the low-lying LUMO level of these compounds could limit the efficiency of the exciton dissociation process and thus explain the relatively modest short-circuit current densities. Nevertheless, thanks to the higher hole-mobility and improved nanoscale morphology when blended with PC61BM, TII-TPA based-devices have reached a PCE of 2.23%, which is currently the highest reported value for thienoisoindigobased materials. Consequently further work focused on both device optimization (morphology and interfaces) and molecular engineering of the relevant electronic properties of the donor is now underway in order to further improve these first results. Acknowledgments The University of Angers is acknowledged for financial support (AAP CS project SolarIs). Y. Jiang thanks the Chinese Government Scholarship (CGC) programs for the Ph-D grant. The Minist ere de la Recherche is acknowledged for the Ph-D grant of F. Baert. The PIAM (Plateforme d’Ing enierie et Analyses Mol eculaires) and the SCIAM (Service Commun d’Imagerie et d’Analyse Microscopique) of the University of Angers are thanked for the characterization of organic compounds and TEM images respectively. C. Cabanetos thanks Dr. Sylvie Dabos for helpful discussion. Jonhson Mattey is
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