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Diketopyrrolopyrrole-based small molecules with simple structure for high VOC organic photovoltaics
Organic Electronics 13 (2012) 3060–3066
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Diketopyrrolopyrrole-based small molecules with simple structure for high VOC organic photovoltaics Jong Won Lee 1, Yoon Suk Choi 1, Won Ho Jo ⇑ Department of Materials Science and Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-744, Republic of Korea
a r t i c l e
i n f o
Article history: Received 12 July 2012 Received in revised form 7 September 2012 Accepted 8 September 2012 Available online 29 September 2012 Keywords: Small molecule HOMO Open circuit voltage Diketopyrrolopyrrole
a b s t r a c t A series of simple structured small molecules based on diketopyrrolopyrrole (DPP) are synthesized and their photovoltaic properties are investigated in terms of the type of electron donating unit. By introducing a donor unit with different electron-donating power such as thiophene (T) and phenylene (Ph), into ADA type small molecule, the frontier orbital energy levels of small molecules can effectively be tuned. The small molecule with a weak donor unit of Ph, Ph(TDPP)2 exhibits a power conversion efficiency of 4.01% with a remarkably high open circuit voltage of 0.93 V when it is blended with [6,6]-phenyl-C71-butyric acid methyl ester as an active layer material in bulk heterojunction solar cells. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction The efficiency of organic photovoltaic (OPV) cells has been rapidly increased in the past few years [1,2]. Bulk heterojunction (BHJ) OPVs using semiconducting polymers as electron donor and fullerene derivatives as electron acceptor have achieved the power conversion efficiency (PEC) over 7–8% [3–11]. This remarkable recent progress arises from not only development of device fabrication techniques such as thermal annealing [12,13], solvent annealing [14], and solvent additives [15,16] for optimization of nanoscale morphology but also development of new donor–acceptor type semiconducting organic materials with an optical bandgap of 1.3–1.8 eV and a deep highest-occupied molecular orbital (HOMO) energy level (<5.1 eV) for high short-circuit current density (JSC) and open-circuit voltage (VOC), respectively [17]. In developing new conjugated organic materials for BHJ solar cells, a well-known architecture, alternatively composed of weak electron-donating and strong electronaccepting units, has been utilized to manipulate their elec⇑ Corresponding author. Tel.: +82 2 880 7192; fax: +82 2 876 6086. 1
E-mail address: [email protected] (W.H. Jo). These authors contributed equally.
1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2012.09.004
tronic properties for high VOC and efficient charge separation [18]. Electron-donating units such as benzo[1,2b:4,5-b0 ]dithiophene [19], 2,7-carbazole [20,21], and 2,7fluorene [22,23] are considered as weak electron-donating molecules [24], in which phenylene rings are fused with hetero-aromatic rings. Since phenylene has high aromatic resonance stabilization energy, p-electrons are relatively localized within the phenylene ring in the molecule. Thus, molecular units containing phenylenes can be classified as weak electron-donating units. When the weak electrondonating unit is used as a building block for alternating conjugated copolymer, the copolymer is expected to have a deep HOMO energy level and as a result afford high VOC in OPVs [25]. On the other hand, a hetero-aromatic ring such as thiophene has less aromatic resonance stabilization energy; thus, p-electrons are delocalized along the conjugated backbone. As a result, the molecular units containing thiophene raise the HOMO energy level as compared to those containing phenylene [26]. Such synthetic strategy can effectively be used to tune the frontier molecular orbital energy levels for high efficiency OPV solar cells. It has recently been reported that a family of solutionprocessable semiconducting polymers containing a diketopyrrolopyrrole (DPP) unit achieved a high PCE up to 6.05% in OPVs and high hole mobility of 0.60 cm2/V s in organic
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field effect transistor [27] due to their high extinction coefficient, strong electron-accepting ability and coplanarity of polymer backbone for solid-state molecular ordering [28,29]. However, conjugated polymers are subjected to inevitable drawbacks such as batch-to-batch variation of molecular weight, broad polydispersity and complicated purification. As a consequence, OPVs based on small molecules (SMs) have emerged as an alternative to polymer solar cells. Recently, BHJ solar cells based on small molecules have achieved PCEs over 6.0% [30,31]. We have postulated that the introduction of electrondonating units with different aromatic resonance stabilization energy into the ADA type SM, where A is thiophene-capped diketoppyrolopyrrole (TDPP) and D is an electron-donating unit such as thiophene (T) and phenylene (Ph), would precisely control the HOMO energy levels of the SMs for efficient OPVs. For this approach to be realized, a series of TDPP-based SMs, (TDPP)2, T(TDPP)2, and Ph(TDPP)2, were synthesized (Scheme 1) and denoted as SM1, SM2, and SM3, respectively. As expected, the HOMO energy level of SM2 (5.17 eV) is slightly higher than that of SM1 (5.19 eV) due to a strong electron-donating thiophene bridge, while the HOMO energy level of SM3 (5.28 eV) is deeper than that of SM1 due to the introduction of weak electron-donating phenylene bridge between two TDPPs. Consequently, the solar cell based on SM3 achieved a PCE of 4.01% with high VOC of 0.93 V. This performance is one of the highest ones among OPVs fabricated from DPP-based SMs. 2. Experimental section 2.1. Materials 3-(5-Bromo-thiophene-2-yl)-2,5-bis-(2-ethyl-hexyl)-6thiophene-2-yl-2,5-dihydro-pyrrole-1,4-dion (1) and 2,5bis-trimethylstannanyl-thiophene (2) were synthesized
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following the same procedure as reported in the literature [32]. 1,4-Benzenediboronic acid bis(pinacol) ester (3), Pd(PPh3)4, sodium, N-bromosuccinimide (NBS) and 2-ethylhexyl bromide were purchased from Sigma–Aldrich and used without further purification. Common organic solvents were purchased from Daejung. Tetrahydrofuran (THF) was dried over sodium/benzophenone prior to use. Poly(3,4-ethylenedioxy-thiophene):poly(styrene sulfonate) (PEDOT:PSS) (Clevios P VP AI 4083) was purchased from H.C. Stark and passed through a 0.45 m PVDF syringe filter before spin-coating. [6,6]-Phenyl-C71-butyric acid methyl ester (PC71BM) was obtained from Nano-C. All other reagents were purchased from Tokyo Chemical Industry and used as received. 2.2. Synthesis 2.2.1. Synthesis of (TDPP)2 (TDPP)2 was synthesized by the Yamamoto coupling. The compound 1 (0.30 g, 0.33 mmol), 2,20 -bipyridiyl (0.12 g, 0.79 mmol) and Ni(COD)2 (0.21 g, 0.79 mmol) were dissolved in THF (10 mL) and the solution was flushed with N2 for 20 min. The reaction mixture was stirred at 150 °C for 3 h in a microwave reactor. After being cooled to room temperature, the mixture was poured into acidic methanol (200 mL methanol and 10 mL HCl) and stirred 1 h to remove nickel catalyst. The crude product was obtained by vacuum filtration and purified by column chromatography on silica gel (100% dichloromethane as eluent) to afford (TDPP)2 as a deep blue solid (0.14 mmol, 87%). 1H NMR (300 MHz, CDCl3, d): 8.96 (d, J = 4.2 Hz, 2H), 8.93 (d, J = 3.8 Hz, 2H), 7.65 (d, J = 0.8 Hz, 2H), 7.44 (d, J = 4.2 Hz, 2H), 7.29 (d, J = 4.0 Hz, 2H), 4.02–3.88 (m, 8H), 1.84 (m, 4H), 1.37–1.23 (m, 32H), 0.90–0.85 (m, 24H); 13C NMR (125 MHz, CDCl3, d): 161.68, 140.89, 140.61, 139.23, 136.63, 135.54, 130.79, 129.83, 129.73, 128.51, 126.07, 108.91, 45.99, 39.34, 30.35, 28.50, 23.71, 23.06, 14.05,
Scheme 1. Synthesis of TDPP-based SMs.
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10.57; HRMS (FAB, m/z): [M + H]+ calcd for C21H38N4O6S, 1047.55; found, 1047.49. 2.2.2. Synthesis of T(TDPP)2 T(TDPP)2 was synthesized by the Stille coupling. The compounds 1 (0.30 g, 0.33 mmol) and 2 (0.06 g, 0.15 mmol) were dissolved in toluene (8 mL). After the solution was flushed with N2 for 20 min, 10 mg of Pd(PPh3)4 was added. The reaction mixture was stirred at 150 °C for 3 h in a microwave reactor. After being cooled to room temperature, the solvent was evaporated under vacuum. The residue was purified by column chromatography on silica gel (100% dichloromethane as eluent). The product T(TDPP)2 was obtained as a deep blue solid (0.12 mmol, 80%). 1H NMR (300 MHz, CDCl3, d):8.96 (d, J = 8.7 Hz, 2H), 8.93 (d, J = 4.8 Hz, 2H), 7.63 (d, J = 4.8 Hz, 2H), 4.04–3.97 (m, 8H), 1.91–1.87 (m, 4H), 1.39–1.25 (m, 32H), 0.94–0.83 (m, 24H); 13C NMR (125 MHz, CDCl3, d): 161.73, 141.68, 140.22, 139.57, 136.70, 135.31, 130.57, 129.89, 128.74, 128.46, 126.11, 125.17, 108.52, 108.22, 45.99, 39.33, 30.41, 28.60, 23.74, 23.11, 14.08, 10.58; HRMS (FAB, m/z): [M + H]+ calcd for C64H80N4O6S5, 1129.67; found, 1129.48. 2.2.3. Synthesis of Ph(TDPP)2 Ph(TDPP)2 was synthesized by the Suzuki coupling. The compounds 1 (0.30 g, 0.33 mmol) and 3 (153 mg, 0.15 mmol) were dissolved in a mixture of aqueous K2CO3 solution (2 M, 2 mL) and THF (8 mL). After the solution was flushed with N2 for 20 min, 10 mg of Pd(PPh3)4 was added. The reaction mixture was stirred at 150 °C for 3 h in a microwave reactor. The residue was purified by column chromatography on silica gel (100% dichloromethane as eluent). The product Ph(TDPP)2 was obtained as a reddish blue solid (0.11 mmol, 73%). 1H NMR (300 MHz, CDCl3, d): 8.97 (d, J = 4.1 Hz, 2H), 8.91 (d, J = 3.7 Hz, 2H), 7.72 (s, 4H), 7.63 (d, J = 4.9 Hz, 2H), 7.53 (d, J = 4.1 Hz, 2H), 7.29 (d, J = 4.0 Hz, 2H), 4.14–3.98 (m, 8H), 1.94–1.88 (m, 4H), 1.42–1.27 (m, 32H), 0.95–0.88 (m, 24H); 13C NMR (125 MHz, CDCl3, d): 161.79, 148.54, 140.25, 139.89, 136.82, 135.28, 133.43, 130.53, 129.90, 129.29, 128.45, 126.68, 124.85, 108.34, 108.19, 45.98, 39.30, 30.40, 28.57, 23.74, 23.09, 14.06, 10.60; HRMS (FAB, m/z): [M + H]+ calcd for C66H82N4O4S4, 1123.64; found, 1123.52. 2.3. Fabrication of photovoltaic cells The organic solar cells in this study were fabricated with the standard device configuration of glass/ITO/PEDOT:PSS/ SM:PC71BM/Al. Prior to device fabrication, the ITO-coated glass was cleaned with acetone and isopropyl alcohol. After complete drying at 150 °C for 30 min, the ITO-coated glass was treated with UVozone for 15 min. PEDOT:PSS was spin-coated on the ITO glass at 4000 rpm for 1 min and annealed at 120 °C for 30 min to obtain a 40 nm thick film. A 2 wt% blend solution of SM:PC71BM was dissolved in chloroform. This solution was stirred for 6 h at room temperature and then passed through a 0.2 lm PTFE syringe filter before spin coating. This solution was spin-coated on the top of PEDOT:PSS at 4000 rpm for 60 s. Finally aluminium (100 nm) was thermally evaporated on the top of the active layer under vacuum (<106 Torr).
2.4. Characterization and measurement The chemical structure of compound was identified by H NMR (Avance DPX-300) and 13C NMR (Avance DPX500). Elemental analysis was recorded with a Jeol JMS600w in fast atom bombardment mode. The absorption spectra were obtained by a UV–vis spectrophotometer (Lambda 25, Perkin Elmer). CV was measured using a potentiostat/galvanostat (VMP 3, Biologic). Dichloromethane and 0.1 M tetra-butyl ammonium hexafluorophosphate (Bu4NPF6) were used as solvent and the electrolyte, respectively. Platinum wires (Bioanalytical System Inc.) were used as both counter and working electrodes, and silver/silver ion (Ag in 0.1 M AgNO3 solution, Bioanalytical System Inc.) was used as a reference electrode. DSC analysis was performed on a TA Instruments 2910 Modulated DSC (Dupont) under N2 at a heating rate of 10 °C/min. Second-heating DSC scans are reported for all SMs. The crystallinity of active layer was investigated by X-ray diffractometer (M18XHF-SRA, Mac Science Co.) using Cu Ka (k = 0.154 nm) radiation. The morphology of active layer films was observed by TEM (JEM-1010, JEOL) with an accelerating voltage of 80 kV. The current density voltage (JV) characteristics were measured with a Keithley 4200 source-meter under AM 1.5G (100 mW/cm2) simulated by a Newport–Oriel solar simulator. The light intensity was calibrated using a NREL certified photodiode and light source meter prior to each measurement. The active area was 0.04 cm2. The hole- and electron-only devices were fabricated with a device configuration of glass/ITO/PEDOT:PSS/ SM:PC71BM/Au and glass/Al/SM:PC71BM/Al, respectively. Both devices were annealed at 120 °C for 10 min. The hole and electron mobilities of blend films were measured from the space-charge limited JV curve using the Mott–Gurney law. The EQE was measured using a lock-in amplifier with a current preamplifier (K3100, Mac Science Co.) under short circuit current state with illumination of monochromatic light. 1
3. Results and discussion Three TDPP-based SMs, SM1, SM2 and SM3 were synthesized by the Yamamoto coupling, the Stille coupling and the Suzuki coupling, respectively (scheme 1). All three SMs are soluble in common organic solvents, including chloroform, dichloromethane, tetrahydrofuran and toluene. When the UV–vis absorptions of three SMs are compared, as shown in Fig. 1a, the maximum absorption wavelength (kmax) and the absorption edge of SM1 are redshifted as compared to SM2 and SM3 in solution. Particularly, SM1 exhibited a discernible vibronic shoulder in solution, indicating that SM1 is partially aggregated in solution state. The red-shift in film state of all SMs (Fig. 1b) as compared to the solution state (Fig. 1a) suggests that p–p stacking due to the intermolecular interaction in solid state is more favorable than in the solution. When the optical bandgaps (Eopt g ) of the SMs are estimated from the absorption edges of thin films, the bandgaps of
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Fig. 2. Cyclic voltammograms of SMs.
Fig. 1. UV–vis absorption spectra of SMs in chloroform solution (a) and in film (b).
SM1, SM2 and SM3 are 1.51 eV, 1.51 eV and 1.66 eV, respectively. Particularly, the Eopt of SM3 (Ph(TDPP)2) is g significantly larger than those of SM1 and SM2. In donor– acceptor type SMs based on push–pull structure, the LUMO energy level is controlled by electron accepting (deficient) unit, while the HOMO energy level is governed by electron rich (donating) unit. Since SM1, SM2 and SM3 have the same electron accepting unit (DPP), it is expected that the LUMO energy levels of three SMs are almost the same,
Fig. 3. XRD diffractograms of SMs in thin film.
whereas the HOMO energy levels are different because of different electron-donating unit in conjugated SMs. SM3 has deeper HOMO energy level and thus larger bandgap than SM1 and SM2, because the electron-donating power of phenylene is weaker than thiophene. Electrochemical properties of three SMs are measured by cyclic voltammetry (CV) in the presence of Bu4NPF6
Table 1 Optical and electrochemical properties of SMs. SMs
UV–vis absorption kmax,
SM1 SM2 SM3 a b
625 615 601
CHCl3
(nm)
kmax,
film
628 638 612
Determined from the onset of UV–vis absorption spectra. Calculated from LUMO = HOMO + Eopt g .
Eopt (eV)a g
HOMO (eV)
LUMO (eV)b
1.51 1.51 1.66
5.19 5.17 5.31
3.66 3.68 3.65
(nm)
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Fig. 4. JV curves of SMs/PC71BM BHJ solar cells under AM 1.5G, 100 mW/cm2 (a) before and (b) after annealing at 120 °C for 10 min.
with Ag/Ag+ as a reference electrode (Fig. 2). The HOMO and LUMO energy levels are determined from CV using the equation EHOMO/LUMO = [(Eonset 0.35) 4.8] eV, and the results are summarized in Table 1. Since the electrondonating power of phenylene is weaker than thiophene in ADA type conjugated SMs, the HOMO energy levels of these three SMs are deeper in the order of SM3 (5.31 eV), SM1 (5.19 eV) and SM2 (5.17 eV). While the HOMO energy levels are different, the LUMO energy levels of SM1, SM2 and SM3 are almost the same (around 3.70 eV) because they have the same electron-accepting unit. When the crystallinities of the conjugated SMs are examined by differential scanning calorimetry (DSC), SM1 exhibits the highest melting temperature (265 °C) with the largest enthalpy of melting while SM2 does the lowest
melting temperature (178 °C) with the smallest enthalpy of melting (Fig. S1), indicating that SM1 has the highest degree of crystallinity. All the three SMs exhibit a strong Xray diffraction peak at 2h = 6.10°, corresponding to the (100) diffraction with an interlayer spacing of 14.4 Å, as shown in Fig. 3. Photovoltaic properties of the SMs were measured at least 8 times with the general device structure of ITO/PEDOT:PSS/SM:PC71BM/Al under AM 1.5G simulated light. The current density–voltage (J–V) curves of the devices with blends of the SMs and PC71BM are shown in Fig. 4, and the photovoltaic parameters are summarized in Table 2. Since SM3 has deeper HOMO energy level than others, SM3 exhibits the highest VOC of 0.93 V, and as a consequence a high PCE of 4.01% after thermal annealing at 120 °C for 10 min. Thermal annealing improves the JSC due to nanoscale phase separation between SMs and PC71BM. The length scale of phase separation is critically important for exciton dissociation and charge transport. If the length scale is too large, excitons may recombine before reaching the interface between donor and acceptor phases, while too small phase separation length scale may also block the efficient charge transport [33]. Thus, the nanoscale phase separation with 20 nm domain size in BHJ materials is optimum for charge separation and transport because the exciton diffusion length is about 10 nm [34]. When the morphologies of SM1:PC71BM, SM2:PC71BM and SM3:PC71BM blends after thermal annealing at 120 °C for 10 min are compared, as shown in Fig. 5 where bright and dark regions correspond to SM-rich phase and PC71BM-rich domain, respectively, SM3/PC71BM exhibits bicontinuous two-phase nanostructure (<20 nm), which is essential for effective exciton dissociation and charge transport [35], whereas SM1:PC71BM and SM2:PC71BM show homogeneous one-phase morphology and macrophase separated morphology, respectively, both of which prevent exciton dissociation and charge transport. Considering the charge carrier mobility and phase morphology of blend films, it is reasonable that SM3 exhibits higher JSC than SM1 and SM2, although it has rather higher bandgap than other two SMs. When the charge carrier mobilities of SM:PC71BM are determined from the space-charge limited current J–V curves as obtained in the dark using the Mott–Gurney law (Fig. 6 and Table 2), the hole mobilities of SM3:PC71BM (8.8 105 cm2/V s) is higher than those of SM1:PC71BM (6.0 105 cm2/V s) and SM2:PC71BM (2.5 105 cm2/ V s), whereas the electron mobilities of SM1:PC71BM
Table 2 Photovoltaic properties of devices under standard AM 1.5G illumination and charge carrier mobilities after thermal annealing at 120 °C for 10 min under dark condition. SMs SM1 SM2 SM3 a b c
SM:PC71BM (w/w) 1:1 1:1 1.25:1
VOC (V) 0.84 0.80 0.93
JSC (mA/cm2) 7.40 4.30 9.09
Hole mobility measured by SCLC method. Electron mobility measured by SCLC method. Average value over eight devices for OPVs under the optimized condition.
FF 0.37 0.43 0.47
lh (cm2/V s)a 5
6.0 10 2.5 105 8.8 105
le (cm2/V s)b 4
2.2 10 2.0 104 2.4 104
PCEmax/avec 2.31/2.05 1.49/1.30 4.01/3.88
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Fig. 5. TEM images of SMs/PC71BM blend films after thermal annealing at 120 °C for 10 min: (a) SM1:PC71BM (1:1 w/w), (b) SM2:PC71BM (1:1 w/w) and (c) SM3:PC71BM (1.25:1 w/w).
Fig. 7. External quantum efficiency spectra of SMs/PC71BM solar cells: SM1:PC71BM (1:1 w/w), SM2:PC71BM (1:1 w/w) and SM3:PC71BM (1.25:1 w/w).
Fig. 6. Dark JV characteristics of SM1: PC71BM (1:1 w/w), SM2:PC71BM (1:1 w/w) and SM3:PC71BM (1.25:1 w/w) blends with (a) hole- and (b) electron-only device after thermal annealing at 120 °C for 10 min. The solid lines represent the best linear fit of the data points.
(2.2 104 cm2/V s), SM2:PC71BM (2.0 104 cm2/V s) and SM3:PC71BM (2.4 104 cm2/V s) are almost the
same. These results are consistent with morphological characteristics of SMs:PC71BM film which are observed by TEM images. The SM1:PC71BM blend exhibits homogeneous one-phase morphology and thus cannot form interpenetrating network, while SM2:PC71BM blending film shows a phase separation with large and discontinuous domains, which prevents effective charge transport. However, the SM3:PC71BM blend exhibits a percolated twophase nanostructure with the domain size of less than 20 nm, which is an optimum for effective charge transport [34]. Since SM3:PC71BM exhibits higher hole mobility and better balance between electron and hole mobility, the device from SM3:PC71BM shows higher JSC and better fill factor (FF) than others (Table 2) [36]. To further confirm higher JSC of SM3, the external quantum efficiencies (EQEs) of SM1, SM2, and SM3 blended with PC71BM are measured and compared, as shown in Fig. 7. SM3 shows higher EQE in the range of 400– 700 nm and a maximum EQE of 50.3% at 590 nm. Integration of EQE spectrum of SM3:PC71BM yields JSC = 8.82 mA/ cm2, which is well consistent with the JSC value (9.09 mA/
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cm2) obtained from JV measurement. Further device optimization such as the interface engineering and the morphology control using an additive is needed to improve further the solar cell performance. 4. Conclusions Conjugated SMs with a simple ADA structure based on TDPP were successfully synthesized and characterized. The LUMO energy levels of three SMs (SM1, SM2 and SM3) were almost the same (3.70 eV) because all three SMs have the same electron accepting unit in the molecules. However, since the electron-donating power of phenylene between DPP units is weaker than that of thiophene, SM3 has deeper HOMO energy level, which consequently leads to a high VOC of 0.93 V. Since SM3 shows higher hole mobility than other two molecules with well-developed nanoscale phase-separated morphology, it exhibits higher JSC than others, yielding a PCE of 4.01%, although the bandgap of SM3 is higher than those of SM1 and SM2. This is one of the highest photovoltaic performances obtained from DPP-based SMs for organic solar cells [37]. Acknowledgements The authors thank the Ministry of Education, Science and Technology (MEST), Korea for financial support through the Global Research Laboratory (GRL) and the World Class University (WCU) programs. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.orgel.2012.09.004. References [1] L. Huo, J. Hou, S. Zhang, H.Y. Chen, Y. Yang, Angew. Chem. Int. Ed. 49 (2010) 1500. [2] Q. Peng, X. Liu, D. Su, G. Fu, J. Xu, L. Dai, Adv. Mater. 23 (2011) 4554. [3] Y. Lee, Y.M. Nam, W.H. Jo, J. Mater. Chem. 21 (2011) 8583. [4] H.Y. Chen, J.H. Hou, S.Q. Zhang, Y.Y. Liang, G.W. Yang, Y. Yang, L.P. Yu, Y. Wu, G. Li, Nat. Photon. 3 (2009) 649. [5] Y. Liang, Z. Xu, J. Xia, S.T. Tsai, Y. Wu, G. Li, C. Ray, L. Yu, Adv. Mater. 22 (2010) E135. [6] Z. He, C. Zhong, X. Huang, W.Y. Wong, H. Wu, L. Chen, S. Su, Y. Cao, Adv. Mater. 23 (2011) 4636.
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Diketopyrrolopyrrole-based small molecules with simple structure for high VOC organic photovoltaics Jong Won Lee 1, Yoon Suk Choi 1, Won Ho Jo ⇑ Department of Materials Science and Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-744, Republic of Korea
a r t i c l e
i n f o
Article history: Received 12 July 2012 Received in revised form 7 September 2012 Accepted 8 September 2012 Available online 29 September 2012 Keywords: Small molecule HOMO Open circuit voltage Diketopyrrolopyrrole
a b s t r a c t A series of simple structured small molecules based on diketopyrrolopyrrole (DPP) are synthesized and their photovoltaic properties are investigated in terms of the type of electron donating unit. By introducing a donor unit with different electron-donating power such as thiophene (T) and phenylene (Ph), into ADA type small molecule, the frontier orbital energy levels of small molecules can effectively be tuned. The small molecule with a weak donor unit of Ph, Ph(TDPP)2 exhibits a power conversion efficiency of 4.01% with a remarkably high open circuit voltage of 0.93 V when it is blended with [6,6]-phenyl-C71-butyric acid methyl ester as an active layer material in bulk heterojunction solar cells. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction The efficiency of organic photovoltaic (OPV) cells has been rapidly increased in the past few years [1,2]. Bulk heterojunction (BHJ) OPVs using semiconducting polymers as electron donor and fullerene derivatives as electron acceptor have achieved the power conversion efficiency (PEC) over 7–8% [3–11]. This remarkable recent progress arises from not only development of device fabrication techniques such as thermal annealing [12,13], solvent annealing [14], and solvent additives [15,16] for optimization of nanoscale morphology but also development of new donor–acceptor type semiconducting organic materials with an optical bandgap of 1.3–1.8 eV and a deep highest-occupied molecular orbital (HOMO) energy level (<5.1 eV) for high short-circuit current density (JSC) and open-circuit voltage (VOC), respectively [17]. In developing new conjugated organic materials for BHJ solar cells, a well-known architecture, alternatively composed of weak electron-donating and strong electronaccepting units, has been utilized to manipulate their elec⇑ Corresponding author. Tel.: +82 2 880 7192; fax: +82 2 876 6086. 1
E-mail address: [email protected] (W.H. Jo). These authors contributed equally.
1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2012.09.004
tronic properties for high VOC and efficient charge separation [18]. Electron-donating units such as benzo[1,2b:4,5-b0 ]dithiophene [19], 2,7-carbazole [20,21], and 2,7fluorene [22,23] are considered as weak electron-donating molecules [24], in which phenylene rings are fused with hetero-aromatic rings. Since phenylene has high aromatic resonance stabilization energy, p-electrons are relatively localized within the phenylene ring in the molecule. Thus, molecular units containing phenylenes can be classified as weak electron-donating units. When the weak electrondonating unit is used as a building block for alternating conjugated copolymer, the copolymer is expected to have a deep HOMO energy level and as a result afford high VOC in OPVs [25]. On the other hand, a hetero-aromatic ring such as thiophene has less aromatic resonance stabilization energy; thus, p-electrons are delocalized along the conjugated backbone. As a result, the molecular units containing thiophene raise the HOMO energy level as compared to those containing phenylene [26]. Such synthetic strategy can effectively be used to tune the frontier molecular orbital energy levels for high efficiency OPV solar cells. It has recently been reported that a family of solutionprocessable semiconducting polymers containing a diketopyrrolopyrrole (DPP) unit achieved a high PCE up to 6.05% in OPVs and high hole mobility of 0.60 cm2/V s in organic
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field effect transistor [27] due to their high extinction coefficient, strong electron-accepting ability and coplanarity of polymer backbone for solid-state molecular ordering [28,29]. However, conjugated polymers are subjected to inevitable drawbacks such as batch-to-batch variation of molecular weight, broad polydispersity and complicated purification. As a consequence, OPVs based on small molecules (SMs) have emerged as an alternative to polymer solar cells. Recently, BHJ solar cells based on small molecules have achieved PCEs over 6.0% [30,31]. We have postulated that the introduction of electrondonating units with different aromatic resonance stabilization energy into the ADA type SM, where A is thiophene-capped diketoppyrolopyrrole (TDPP) and D is an electron-donating unit such as thiophene (T) and phenylene (Ph), would precisely control the HOMO energy levels of the SMs for efficient OPVs. For this approach to be realized, a series of TDPP-based SMs, (TDPP)2, T(TDPP)2, and Ph(TDPP)2, were synthesized (Scheme 1) and denoted as SM1, SM2, and SM3, respectively. As expected, the HOMO energy level of SM2 (5.17 eV) is slightly higher than that of SM1 (5.19 eV) due to a strong electron-donating thiophene bridge, while the HOMO energy level of SM3 (5.28 eV) is deeper than that of SM1 due to the introduction of weak electron-donating phenylene bridge between two TDPPs. Consequently, the solar cell based on SM3 achieved a PCE of 4.01% with high VOC of 0.93 V. This performance is one of the highest ones among OPVs fabricated from DPP-based SMs. 2. Experimental section 2.1. Materials 3-(5-Bromo-thiophene-2-yl)-2,5-bis-(2-ethyl-hexyl)-6thiophene-2-yl-2,5-dihydro-pyrrole-1,4-dion (1) and 2,5bis-trimethylstannanyl-thiophene (2) were synthesized
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following the same procedure as reported in the literature [32]. 1,4-Benzenediboronic acid bis(pinacol) ester (3), Pd(PPh3)4, sodium, N-bromosuccinimide (NBS) and 2-ethylhexyl bromide were purchased from Sigma–Aldrich and used without further purification. Common organic solvents were purchased from Daejung. Tetrahydrofuran (THF) was dried over sodium/benzophenone prior to use. Poly(3,4-ethylenedioxy-thiophene):poly(styrene sulfonate) (PEDOT:PSS) (Clevios P VP AI 4083) was purchased from H.C. Stark and passed through a 0.45 m PVDF syringe filter before spin-coating. [6,6]-Phenyl-C71-butyric acid methyl ester (PC71BM) was obtained from Nano-C. All other reagents were purchased from Tokyo Chemical Industry and used as received. 2.2. Synthesis 2.2.1. Synthesis of (TDPP)2 (TDPP)2 was synthesized by the Yamamoto coupling. The compound 1 (0.30 g, 0.33 mmol), 2,20 -bipyridiyl (0.12 g, 0.79 mmol) and Ni(COD)2 (0.21 g, 0.79 mmol) were dissolved in THF (10 mL) and the solution was flushed with N2 for 20 min. The reaction mixture was stirred at 150 °C for 3 h in a microwave reactor. After being cooled to room temperature, the mixture was poured into acidic methanol (200 mL methanol and 10 mL HCl) and stirred 1 h to remove nickel catalyst. The crude product was obtained by vacuum filtration and purified by column chromatography on silica gel (100% dichloromethane as eluent) to afford (TDPP)2 as a deep blue solid (0.14 mmol, 87%). 1H NMR (300 MHz, CDCl3, d): 8.96 (d, J = 4.2 Hz, 2H), 8.93 (d, J = 3.8 Hz, 2H), 7.65 (d, J = 0.8 Hz, 2H), 7.44 (d, J = 4.2 Hz, 2H), 7.29 (d, J = 4.0 Hz, 2H), 4.02–3.88 (m, 8H), 1.84 (m, 4H), 1.37–1.23 (m, 32H), 0.90–0.85 (m, 24H); 13C NMR (125 MHz, CDCl3, d): 161.68, 140.89, 140.61, 139.23, 136.63, 135.54, 130.79, 129.83, 129.73, 128.51, 126.07, 108.91, 45.99, 39.34, 30.35, 28.50, 23.71, 23.06, 14.05,
Scheme 1. Synthesis of TDPP-based SMs.
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10.57; HRMS (FAB, m/z): [M + H]+ calcd for C21H38N4O6S, 1047.55; found, 1047.49. 2.2.2. Synthesis of T(TDPP)2 T(TDPP)2 was synthesized by the Stille coupling. The compounds 1 (0.30 g, 0.33 mmol) and 2 (0.06 g, 0.15 mmol) were dissolved in toluene (8 mL). After the solution was flushed with N2 for 20 min, 10 mg of Pd(PPh3)4 was added. The reaction mixture was stirred at 150 °C for 3 h in a microwave reactor. After being cooled to room temperature, the solvent was evaporated under vacuum. The residue was purified by column chromatography on silica gel (100% dichloromethane as eluent). The product T(TDPP)2 was obtained as a deep blue solid (0.12 mmol, 80%). 1H NMR (300 MHz, CDCl3, d):8.96 (d, J = 8.7 Hz, 2H), 8.93 (d, J = 4.8 Hz, 2H), 7.63 (d, J = 4.8 Hz, 2H), 4.04–3.97 (m, 8H), 1.91–1.87 (m, 4H), 1.39–1.25 (m, 32H), 0.94–0.83 (m, 24H); 13C NMR (125 MHz, CDCl3, d): 161.73, 141.68, 140.22, 139.57, 136.70, 135.31, 130.57, 129.89, 128.74, 128.46, 126.11, 125.17, 108.52, 108.22, 45.99, 39.33, 30.41, 28.60, 23.74, 23.11, 14.08, 10.58; HRMS (FAB, m/z): [M + H]+ calcd for C64H80N4O6S5, 1129.67; found, 1129.48. 2.2.3. Synthesis of Ph(TDPP)2 Ph(TDPP)2 was synthesized by the Suzuki coupling. The compounds 1 (0.30 g, 0.33 mmol) and 3 (153 mg, 0.15 mmol) were dissolved in a mixture of aqueous K2CO3 solution (2 M, 2 mL) and THF (8 mL). After the solution was flushed with N2 for 20 min, 10 mg of Pd(PPh3)4 was added. The reaction mixture was stirred at 150 °C for 3 h in a microwave reactor. The residue was purified by column chromatography on silica gel (100% dichloromethane as eluent). The product Ph(TDPP)2 was obtained as a reddish blue solid (0.11 mmol, 73%). 1H NMR (300 MHz, CDCl3, d): 8.97 (d, J = 4.1 Hz, 2H), 8.91 (d, J = 3.7 Hz, 2H), 7.72 (s, 4H), 7.63 (d, J = 4.9 Hz, 2H), 7.53 (d, J = 4.1 Hz, 2H), 7.29 (d, J = 4.0 Hz, 2H), 4.14–3.98 (m, 8H), 1.94–1.88 (m, 4H), 1.42–1.27 (m, 32H), 0.95–0.88 (m, 24H); 13C NMR (125 MHz, CDCl3, d): 161.79, 148.54, 140.25, 139.89, 136.82, 135.28, 133.43, 130.53, 129.90, 129.29, 128.45, 126.68, 124.85, 108.34, 108.19, 45.98, 39.30, 30.40, 28.57, 23.74, 23.09, 14.06, 10.60; HRMS (FAB, m/z): [M + H]+ calcd for C66H82N4O4S4, 1123.64; found, 1123.52. 2.3. Fabrication of photovoltaic cells The organic solar cells in this study were fabricated with the standard device configuration of glass/ITO/PEDOT:PSS/ SM:PC71BM/Al. Prior to device fabrication, the ITO-coated glass was cleaned with acetone and isopropyl alcohol. After complete drying at 150 °C for 30 min, the ITO-coated glass was treated with UVozone for 15 min. PEDOT:PSS was spin-coated on the ITO glass at 4000 rpm for 1 min and annealed at 120 °C for 30 min to obtain a 40 nm thick film. A 2 wt% blend solution of SM:PC71BM was dissolved in chloroform. This solution was stirred for 6 h at room temperature and then passed through a 0.2 lm PTFE syringe filter before spin coating. This solution was spin-coated on the top of PEDOT:PSS at 4000 rpm for 60 s. Finally aluminium (100 nm) was thermally evaporated on the top of the active layer under vacuum (<106 Torr).
2.4. Characterization and measurement The chemical structure of compound was identified by H NMR (Avance DPX-300) and 13C NMR (Avance DPX500). Elemental analysis was recorded with a Jeol JMS600w in fast atom bombardment mode. The absorption spectra were obtained by a UV–vis spectrophotometer (Lambda 25, Perkin Elmer). CV was measured using a potentiostat/galvanostat (VMP 3, Biologic). Dichloromethane and 0.1 M tetra-butyl ammonium hexafluorophosphate (Bu4NPF6) were used as solvent and the electrolyte, respectively. Platinum wires (Bioanalytical System Inc.) were used as both counter and working electrodes, and silver/silver ion (Ag in 0.1 M AgNO3 solution, Bioanalytical System Inc.) was used as a reference electrode. DSC analysis was performed on a TA Instruments 2910 Modulated DSC (Dupont) under N2 at a heating rate of 10 °C/min. Second-heating DSC scans are reported for all SMs. The crystallinity of active layer was investigated by X-ray diffractometer (M18XHF-SRA, Mac Science Co.) using Cu Ka (k = 0.154 nm) radiation. The morphology of active layer films was observed by TEM (JEM-1010, JEOL) with an accelerating voltage of 80 kV. The current density voltage (JV) characteristics were measured with a Keithley 4200 source-meter under AM 1.5G (100 mW/cm2) simulated by a Newport–Oriel solar simulator. The light intensity was calibrated using a NREL certified photodiode and light source meter prior to each measurement. The active area was 0.04 cm2. The hole- and electron-only devices were fabricated with a device configuration of glass/ITO/PEDOT:PSS/ SM:PC71BM/Au and glass/Al/SM:PC71BM/Al, respectively. Both devices were annealed at 120 °C for 10 min. The hole and electron mobilities of blend films were measured from the space-charge limited JV curve using the Mott–Gurney law. The EQE was measured using a lock-in amplifier with a current preamplifier (K3100, Mac Science Co.) under short circuit current state with illumination of monochromatic light. 1
3. Results and discussion Three TDPP-based SMs, SM1, SM2 and SM3 were synthesized by the Yamamoto coupling, the Stille coupling and the Suzuki coupling, respectively (scheme 1). All three SMs are soluble in common organic solvents, including chloroform, dichloromethane, tetrahydrofuran and toluene. When the UV–vis absorptions of three SMs are compared, as shown in Fig. 1a, the maximum absorption wavelength (kmax) and the absorption edge of SM1 are redshifted as compared to SM2 and SM3 in solution. Particularly, SM1 exhibited a discernible vibronic shoulder in solution, indicating that SM1 is partially aggregated in solution state. The red-shift in film state of all SMs (Fig. 1b) as compared to the solution state (Fig. 1a) suggests that p–p stacking due to the intermolecular interaction in solid state is more favorable than in the solution. When the optical bandgaps (Eopt g ) of the SMs are estimated from the absorption edges of thin films, the bandgaps of
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Fig. 2. Cyclic voltammograms of SMs.
Fig. 1. UV–vis absorption spectra of SMs in chloroform solution (a) and in film (b).
SM1, SM2 and SM3 are 1.51 eV, 1.51 eV and 1.66 eV, respectively. Particularly, the Eopt of SM3 (Ph(TDPP)2) is g significantly larger than those of SM1 and SM2. In donor– acceptor type SMs based on push–pull structure, the LUMO energy level is controlled by electron accepting (deficient) unit, while the HOMO energy level is governed by electron rich (donating) unit. Since SM1, SM2 and SM3 have the same electron accepting unit (DPP), it is expected that the LUMO energy levels of three SMs are almost the same,
Fig. 3. XRD diffractograms of SMs in thin film.
whereas the HOMO energy levels are different because of different electron-donating unit in conjugated SMs. SM3 has deeper HOMO energy level and thus larger bandgap than SM1 and SM2, because the electron-donating power of phenylene is weaker than thiophene. Electrochemical properties of three SMs are measured by cyclic voltammetry (CV) in the presence of Bu4NPF6
Table 1 Optical and electrochemical properties of SMs. SMs
UV–vis absorption kmax,
SM1 SM2 SM3 a b
625 615 601
CHCl3
(nm)
kmax,
film
628 638 612
Determined from the onset of UV–vis absorption spectra. Calculated from LUMO = HOMO + Eopt g .
Eopt (eV)a g
HOMO (eV)
LUMO (eV)b
1.51 1.51 1.66
5.19 5.17 5.31
3.66 3.68 3.65
(nm)
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Fig. 4. JV curves of SMs/PC71BM BHJ solar cells under AM 1.5G, 100 mW/cm2 (a) before and (b) after annealing at 120 °C for 10 min.
with Ag/Ag+ as a reference electrode (Fig. 2). The HOMO and LUMO energy levels are determined from CV using the equation EHOMO/LUMO = [(Eonset 0.35) 4.8] eV, and the results are summarized in Table 1. Since the electrondonating power of phenylene is weaker than thiophene in ADA type conjugated SMs, the HOMO energy levels of these three SMs are deeper in the order of SM3 (5.31 eV), SM1 (5.19 eV) and SM2 (5.17 eV). While the HOMO energy levels are different, the LUMO energy levels of SM1, SM2 and SM3 are almost the same (around 3.70 eV) because they have the same electron-accepting unit. When the crystallinities of the conjugated SMs are examined by differential scanning calorimetry (DSC), SM1 exhibits the highest melting temperature (265 °C) with the largest enthalpy of melting while SM2 does the lowest
melting temperature (178 °C) with the smallest enthalpy of melting (Fig. S1), indicating that SM1 has the highest degree of crystallinity. All the three SMs exhibit a strong Xray diffraction peak at 2h = 6.10°, corresponding to the (100) diffraction with an interlayer spacing of 14.4 Å, as shown in Fig. 3. Photovoltaic properties of the SMs were measured at least 8 times with the general device structure of ITO/PEDOT:PSS/SM:PC71BM/Al under AM 1.5G simulated light. The current density–voltage (J–V) curves of the devices with blends of the SMs and PC71BM are shown in Fig. 4, and the photovoltaic parameters are summarized in Table 2. Since SM3 has deeper HOMO energy level than others, SM3 exhibits the highest VOC of 0.93 V, and as a consequence a high PCE of 4.01% after thermal annealing at 120 °C for 10 min. Thermal annealing improves the JSC due to nanoscale phase separation between SMs and PC71BM. The length scale of phase separation is critically important for exciton dissociation and charge transport. If the length scale is too large, excitons may recombine before reaching the interface between donor and acceptor phases, while too small phase separation length scale may also block the efficient charge transport [33]. Thus, the nanoscale phase separation with 20 nm domain size in BHJ materials is optimum for charge separation and transport because the exciton diffusion length is about 10 nm [34]. When the morphologies of SM1:PC71BM, SM2:PC71BM and SM3:PC71BM blends after thermal annealing at 120 °C for 10 min are compared, as shown in Fig. 5 where bright and dark regions correspond to SM-rich phase and PC71BM-rich domain, respectively, SM3/PC71BM exhibits bicontinuous two-phase nanostructure (<20 nm), which is essential for effective exciton dissociation and charge transport [35], whereas SM1:PC71BM and SM2:PC71BM show homogeneous one-phase morphology and macrophase separated morphology, respectively, both of which prevent exciton dissociation and charge transport. Considering the charge carrier mobility and phase morphology of blend films, it is reasonable that SM3 exhibits higher JSC than SM1 and SM2, although it has rather higher bandgap than other two SMs. When the charge carrier mobilities of SM:PC71BM are determined from the space-charge limited current J–V curves as obtained in the dark using the Mott–Gurney law (Fig. 6 and Table 2), the hole mobilities of SM3:PC71BM (8.8 105 cm2/V s) is higher than those of SM1:PC71BM (6.0 105 cm2/V s) and SM2:PC71BM (2.5 105 cm2/ V s), whereas the electron mobilities of SM1:PC71BM
Table 2 Photovoltaic properties of devices under standard AM 1.5G illumination and charge carrier mobilities after thermal annealing at 120 °C for 10 min under dark condition. SMs SM1 SM2 SM3 a b c
SM:PC71BM (w/w) 1:1 1:1 1.25:1
VOC (V) 0.84 0.80 0.93
JSC (mA/cm2) 7.40 4.30 9.09
Hole mobility measured by SCLC method. Electron mobility measured by SCLC method. Average value over eight devices for OPVs under the optimized condition.
FF 0.37 0.43 0.47
lh (cm2/V s)a 5
6.0 10 2.5 105 8.8 105
le (cm2/V s)b 4
2.2 10 2.0 104 2.4 104
PCEmax/avec 2.31/2.05 1.49/1.30 4.01/3.88
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Fig. 5. TEM images of SMs/PC71BM blend films after thermal annealing at 120 °C for 10 min: (a) SM1:PC71BM (1:1 w/w), (b) SM2:PC71BM (1:1 w/w) and (c) SM3:PC71BM (1.25:1 w/w).
Fig. 7. External quantum efficiency spectra of SMs/PC71BM solar cells: SM1:PC71BM (1:1 w/w), SM2:PC71BM (1:1 w/w) and SM3:PC71BM (1.25:1 w/w).
Fig. 6. Dark JV characteristics of SM1: PC71BM (1:1 w/w), SM2:PC71BM (1:1 w/w) and SM3:PC71BM (1.25:1 w/w) blends with (a) hole- and (b) electron-only device after thermal annealing at 120 °C for 10 min. The solid lines represent the best linear fit of the data points.
(2.2 104 cm2/V s), SM2:PC71BM (2.0 104 cm2/V s) and SM3:PC71BM (2.4 104 cm2/V s) are almost the
same. These results are consistent with morphological characteristics of SMs:PC71BM film which are observed by TEM images. The SM1:PC71BM blend exhibits homogeneous one-phase morphology and thus cannot form interpenetrating network, while SM2:PC71BM blending film shows a phase separation with large and discontinuous domains, which prevents effective charge transport. However, the SM3:PC71BM blend exhibits a percolated twophase nanostructure with the domain size of less than 20 nm, which is an optimum for effective charge transport [34]. Since SM3:PC71BM exhibits higher hole mobility and better balance between electron and hole mobility, the device from SM3:PC71BM shows higher JSC and better fill factor (FF) than others (Table 2) [36]. To further confirm higher JSC of SM3, the external quantum efficiencies (EQEs) of SM1, SM2, and SM3 blended with PC71BM are measured and compared, as shown in Fig. 7. SM3 shows higher EQE in the range of 400– 700 nm and a maximum EQE of 50.3% at 590 nm. Integration of EQE spectrum of SM3:PC71BM yields JSC = 8.82 mA/ cm2, which is well consistent with the JSC value (9.09 mA/
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cm2) obtained from JV measurement. Further device optimization such as the interface engineering and the morphology control using an additive is needed to improve further the solar cell performance. 4. Conclusions Conjugated SMs with a simple ADA structure based on TDPP were successfully synthesized and characterized. The LUMO energy levels of three SMs (SM1, SM2 and SM3) were almost the same (3.70 eV) because all three SMs have the same electron accepting unit in the molecules. However, since the electron-donating power of phenylene between DPP units is weaker than that of thiophene, SM3 has deeper HOMO energy level, which consequently leads to a high VOC of 0.93 V. Since SM3 shows higher hole mobility than other two molecules with well-developed nanoscale phase-separated morphology, it exhibits higher JSC than others, yielding a PCE of 4.01%, although the bandgap of SM3 is higher than those of SM1 and SM2. This is one of the highest photovoltaic performances obtained from DPP-based SMs for organic solar cells [37]. Acknowledgements The authors thank the Ministry of Education, Science and Technology (MEST), Korea for financial support through the Global Research Laboratory (GRL) and the World Class University (WCU) programs. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.orgel.2012.09.004. References [1] L. Huo, J. Hou, S. Zhang, H.Y. Chen, Y. Yang, Angew. Chem. Int. Ed. 49 (2010) 1500. [2] Q. Peng, X. Liu, D. Su, G. Fu, J. Xu, L. Dai, Adv. Mater. 23 (2011) 4554. [3] Y. Lee, Y.M. Nam, W.H. Jo, J. Mater. Chem. 21 (2011) 8583. [4] H.Y. Chen, J.H. Hou, S.Q. Zhang, Y.Y. Liang, G.W. Yang, Y. Yang, L.P. Yu, Y. Wu, G. Li, Nat. Photon. 3 (2009) 649. [5] Y. Liang, Z. Xu, J. Xia, S.T. Tsai, Y. Wu, G. Li, C. Ray, L. Yu, Adv. Mater. 22 (2010) E135. [6] Z. He, C. Zhong, X. Huang, W.Y. Wong, H. Wu, L. Chen, S. Su, Y. Cao, Adv. Mater. 23 (2011) 4636.
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