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DOI: 10.1039/C6RA01103A (Paper) RSC Adv., 2016, 6, 28658-28665

Low band gap diketopyrrolopyrrole-based small molecule bulk heterojunction solar cells: influence of terminal side chain on morphology and photovoltaic performance†

Quoc Viet Hoangab, Chang Eun Songab, In-Nam Kangc, Sang-Jin Moonab, Sang Kyu Leeab, Jong-Cheol Leeab and Won Suk Shin*ab
aEnergy Materials Research Center, Korea Research Institute of Chemical Technology (KRICT), 141 Gajeong-ro, Yuseong-gu, Daejeon, 305-343, Korea. E-mail: shinws@krict.re.kr
bDepartment of Nanomaterials Science and Engineering, University of Science & Technology (UST), 217 Gajeong-ro, Yuseong-gu, Daejeon, 305-350, Korea
cDepartment of Chemistry, The Catholic University of Korea, Gyeonggido 90-2, Korea

Received 13th January 2016 , Accepted 9th March 2016

First published on 11th March 2016

Abstract

Two new low band gap small molecules BDT(DPP-TTHex)2 and BDT(DPP-TT)2 with different terminal side chain are designed and synthesized for bulk heterojunction solar cells. BDT(DPP-TTHex)2 is flanked by hexyl at head and tail of backbone while BDT(DPP-TT)2 is free attachment. Both small molecules exhibit similar optical, electrochemical properties and charge carrier mobilities. Nevertheless, BDT(DPP-TT)2 performs optimal film morphology leading to higher short circuit current (JSC) comparing with BDT(DPP-TTHex)2, thus 5.12% power conversion efficiencies (PCE) of BDT(DPP-TT)2 is achieved, higher than 2.36% PCE of BDT(DPP-TTHex)2 under AM1.5G illumination (100 mW cm−2).

1. Introduction

Bulk heterojunction (BHJ) organic solar cells (OSCs) have been a rapidly developing field of research due to low-cost manufacturing and capability to fabricate flexible large-area devices.1–4 Current research in organic photovoltaic (OPV) materials focuses mainly on the design and synthesis of conjugated polymers capable of low band-gap and high charge transport. BHJ polymer solar cells (PSCs) have demonstrated promising device efficiency. However, they can suffer from drawbacks such as batch-to-batch variation,5,6 chain-end contamination,7 and entanglement problem,8,9 which can reduce overall performance and device consistency.10

Solution processable conjugated small molecules (SMs) have recently been recognized as an efficient class of donor material for bulk heterojunction solar cells with several reports achieved 10% PCE.11,12 SMs which have advantages including intrinsic monodispersity, easily purification and characterization using standard organic chemistry protocols.6 However, almost small molecules often have a tendency to self-assemble into large domains, so that it is challenging to obtain the desired nanoscale morphology for efficient charge carrier generation and transport. The low fill factor (FF) of SMs due to hindered charge transfer through the external contacts is another issue.13 To try solving these problems, several device process techniques also employed to get ordered nanoscale morphology such as thermal annealing,6 solvent vapor annealing,14 choice of solvents,15 use of solvent additives,16–18 fine tuning active layer thickness and interfacial layer.13

To achieve high PCE of single OSCs, C. J. Brabec et al. reported that the band gap of organic semiconductors should be close to 1.5 eV.19 SMs based diketopyrrolopyrrole (DPP) unit is one of the most promising compounds to obtain band gap of 1.5 eV among all electron deficient moieties. SMs based on DPP normally exhibit high extinction coefficient in visible range, well-ordered molecular structure in solid state, and high hole mobility.20,21 It is essential to reach high short circuit current density (JSC) as well as good fill factor (FF) in OSCs. Generally, DPP based SMs are designed in two ways: D-DPP-D or DPP-D-DPP (D is electron-rich units). In the first case, some D units were employed such as dithiophene,22 benzofuran,6 benzodithiophene (BDT),23 triphenylamine (TPA),23 and pyrene.23 The resulting PCE devices with D-DPP-D SMs were often lower than 5%. The second way is link of two DPP units with central D unit. According to this design, Lin et al. and Huang et al. presented the same molecule with BDT as a central core and PCE was achieved 5.8%.24,25 However, this SM still has higher band gap (1.64 eV) comparing to the ideal band gap (∼1.50 eV).19 Very recently, Jung et al. introduced stronger electron-donating unit dithienopyran (DTP) as a central core to approach near optimized band gap (1.52 eV), and resulting PCE of 6.9% was reached due to high JSC.26 To extend light absorption, D2-DPP-D1-DPP-D2 framework is an effective way with D1 and D2 as electron-donating units.16 This strategy may also reduce self-assembly of SMs in active layer due to long π-conjugated system and therefore, nanoscale morphology and high JSC can be obtained. Following this design, Tang et al. recently reported DPA-diDPPBDT SM with a high JSC (15.64 mA cm−2), low band gap and optimized domain size (∼20 nm).27

Similar to polymers, side chain engineering is a clue to achieve high efficiency SMs. Side chains are necessary to make SMs solution processable and formation of optimized film morphology in active layer. Some groups reported that optical, crystalline, electrochemical properties and film morphology of SMs were affected by length,28 and position22,29 of side chains. Recently, VS Gevaerts et al. observed that the anchor side chain strongly influenced on crystallinity, film morphology as well as solar cell performance of SMs.22

In this paper, we report the synthesis of two new SMs based on D2-DPP-D1-DPP-D2 framework consisting of a 4,8-di(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene as central core (D1) and dithiophene/2-hexyldithiophene as external donor moieties (D2). BDT was chosen as core donor moiety due to its excellent performance in both BHJ PSCs30–34 and SMs.11,27,35

2. Experimental section

2.1. Device fabrication and testing

Solar cells were fabricated using an ITO/PEDOT:PSS (40 nm)/SM[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM (w/w 1[thin space (1/6-em)]:[thin space (1/6-em)]1) (90–100 nm)/Ca (2 nm)/Al (100 nm). The ITO-coated glass substrates were first cleaned with detergent, ultrasonicated in water, acetone and isopropyl alcohol, and subsequently dried overnight at 130 °C in the oven. A filtered dispersion of PEDOT:PSS in water was spin-cast at the rate of 3000 rpm and baking for 30 min at 140 °C in air. A solution containing a mixture of SM[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM in CF with SM and PC71BM concentrations of 16 mg ml−1 was spin-cast on top of PEDOT:PSS film at the rate of 2000 rpm. The thickness of the active layer is approximately 90 nm (using Alpha Step IQ Contact Surface Profile Meter). Annealing process was conducted at 120 °C for 10 min before the deposition of the top electrode. Finally, calcium (∼2 nm) and then aluminium (∼100 nm) were deposited through a shadow mask by thermal evaporation under a vacuum of about 3 × 10−6[thin space (1/6-em)] Torr. The active area of the device was ∼9 mm2. During the measurement, the mask was used. Current density–voltage (J–V) characteristics were measured using a Keithley 236 Source Measure Unit. The solar cells were characterized using a Newport Air Mass 1.5 Global (AM 1.5G) full spectrum solar simulator with an irradiation intensity of 100 mW cm−2. IPCE spectra were measured using a QEW7 Solar Cell QE measurement system (PV Measurements). The integrated IPCE values are always in good agreement with the measured short-circuit current.

3. Results and discussion

3.1. Synthesis and thermal stability of the compounds

The simple synthetic procedure and the chemical structures of BDT(DPP-TTHex)2 and BDT(DPP-TT)2 are shown in Scheme 1. DPP precursor 1 is prepared according to a reported literature.36 Intermediates 2a and 2b are achieved by reacting precursor 1 with 5′-hexyl-2,2′-bithiophene-5-boronic acid pinacol ester and 2,2′-bithiophene-5-boronic acid pinacol ester via the palladium-catalyzed Suzuki coupling, respectively. The yield of these reactions is only obtained around 30–35% due to bis-coupling products are formed as byproducts. Target SMs BDT(DPP-TTHex)2 and BDT(DPP-TT)2 are synthesized via the Stille coupling of 2a and 2b with a benzo[1,2-b;4,5-b′]dithiophene 3, respectively.
image file: c6ra01103a-s1.tif
Scheme 1 Synthetic procedure and chemical structures of small molecules.

Thermogravimetric analysis (TGA) suggests that BDT(DPP-TTHex)2 and BDT(DPP-TT)2 exhibit good stability under N2 atmosphere with decomposition temperature (Td) of 412 and 417 °C, respectively (ESI-Fig. S1a†). Differential scanning calorimetry (DSC) of two small molecules is showed in Fig. S1b (ESI†). Melting and crystalline temperature of BDT(DPP-TTHex)2 are 276.7 and 249.5 °C, while those of BDT(DPP-TT)2 are 307.0 and 269.6 °C, respectively. BDT(DPP-TTHex)2 are lower in melting and crystalline temperature than BDT(DPP-TT)2 about 30 and 20 °C, respectively, due to its flexible hexyl side chain.

3.2. Optical and electrochemical properties

Optical properties are investigated using UV-Vis absorption spectroscopy as showed in Fig. 1a. One absorption maximum is apparent in the range of 370–420 nm for the π–π* transition of the oligothiophene and BDT moiety, and the other absorption maximum in the visible range is for the intramolecular charge transfer (ICT) transition. UV-Vis spectra of both BDT(DPP-TTHex)2 and BDT(DPP-TT)2 are similar. It reveals the hexyl substitution on conjugated backbones has limited impact on optical properties of small molecules. In solid film, both SMs have new peaks at near 750 nm, which indicated J-aggregation of SMs. Both BDT(DPP-TTHex)2 and BDT(DPP-TT)2 exhibit broad low energy transitions with favorable overlap with the solar spectrum in solid state.
image file: c6ra01103a-f1.tif
Fig. 1 (a) Absorption spectral of BDT(DPP-TTHex)2 and BDT(DPP-TT)2 solution in CF (circle) and thin film cast from CF solution (triangle). (b) Energy band diagram relative to the vacuum level.

Energy level alignment of SMs, PC71BM with cathode and anode buffer layers play an important role for a correct device operation in OPVs and thus necessary to characterize frontier energy level of SMs.37 To determine the HOMO and LUMO energy levels of both SMs, cyclic voltammetry (CV) is characterized by thin film in anhydrous acetonitrile with 0.1 M tetrabutyl ammonium hexafluorophosphate (ESI-Fig. S2†). HOMO, LUMO energy level and electrochemical band gap (Eecg) are calculated from the values of oxidation potential onset (Eoxo) and reduction potential onset (Eredo) after calibrated by the standard ferrocene/ferrocenium redox couple (Fc/Fc+).38 The results are summarized in Table 1 and energy band diagram of full device with 2 SMs is showed in Fig. 1b. The frontier energy level of 2 SMs indicate that they are suitable as donors of BHJ OSCs. By introducing hexyl group in BDT(DPP-TTHex)2, the HOMO and LUMO slightly reduce from −5.15 to −5.16 eV and −3.61 to −3.64 eV, respectively, comparing to BDT(DPP-TT)2 without terminal side chain. It indicates hexyl group has limited impact on electrochemical properties of these SMs. The result also shows that the electrical band gaps (Eecg) of BDT(DPP-TTHex)2 and BDT(DPP-TT)2 are consistent with Eoptg.

Table 1 Energy levels and band gaps of BDT(DPP-TTHex)2 and BDT(DPP-TT)2
SM Eoxo (V) Eredo (V) HOMO (eV) LUMO (eV) Eecg (eV) Eoptg (eV)
BDT(DPP-TTHex)2 0.36 −1.16 −5.16 −3.64 1.52 1.49
BDT(DPP-TT)2 0.35 −1.19 −5.15 −3.61 1.54 1.53

3.3. Crystallinity study

To evaluate crystallinity and ordered structure, X-ray diffraction (XRD) analysis of both compounds is investigated as showed in Fig. 2. Both pristine BDT(DPP-TTHex)2 and BDT(DPP-TT)2 films exhibited a strong first-order diffraction peak (100) at 2θ = 5.31 and 5.07° corresponding to lamellar distance (d100) of 16.63 and 17.42 Å, respectively. These XRD patterns of pristine compounds indicate a highly organized assembly and crystallinity at solid state. BDT(DPP-TTHex)2 with hexyl at 5 position of thiophene has favourite stacking direction along the backbone while BDT(DPP-TT)2 without hexyl at 5 position possibly shows stacking direction both along and across backbone.22 The crystal orientation of BDT(DPP-TT)2 is more random than that of BDT(DPP-TTHex)2. It might be the reason why BDT(DPP-TTHex)2 shows higher crystallinity comparing with BDT(DPP-TT)2. The crystallinity of two SMs blending with PC71BM in 1[thin space (1/6-em)]:[thin space (1/6-em)]1 weight ratio was decayed due to the interruption of pristine network by PC71BM. Consequently, lamellar distances are slightly increase to 17.05 and 17.98 Å for BDT(DPP-TTHex)2 and BDT(DPP-TT)2, respectively. By using 1-chloronaphthalene (CN) as additive, the change of BDT(DPP-TTHex)2 and BDT(DPP-TT)2 films are different. The (100) peak of BDT(DPP-TTHex)2 is slightly recovered and lamellar distance reduced to 16.72 Å because some pure SM parts are self-assembled. In contrast, the (100) peak sharpness of BDT(DPP-TT)2 is decreased and shift to 18.87 Å, and one more peak is appeared at 24.65 Å. It indicates that molecular packing of BDT(DPP-TT)2 and PC71BM changed.
image file: c6ra01103a-f2.tif
Fig. 2 X-Ray diffraction of (a) BDT(DPP-TTHex)2 and (b) BDT(DPP-TT)2 film spin-coated from CHCl3 onto glass substrate.

3.4. Photovoltaic performance

Photovoltaic characteristics are investigated with the conventional architecture of ITO/PEDOT:PSS/BDT(DPP-TT)2 or BDT(DPP-TTHex)2[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM/Ca/Al. Current density–voltage (J–V) characteristics under one sun (simulated AM 1.5G irradiation at 100 mW cm−2) are shown in Fig. 3a, and device performance parameters are summarized in Table 2.
image file: c6ra01103a-f3.tif
Fig. 3 (a) Current–voltage (J–V) characteristics and (b) EQE spectra of solar cells of BDT(DPP-TTHex)2[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM (1[thin space (1/6-em)]:[thin space (1/6-em)]1) without (A) and with (B) 0.5% CN in CF; BDT(DPP-TT)2[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM (1[thin space (1/6-em)]:[thin space (1/6-em)]1) without (C) and with (D) 0.3% CN in CF.
Table 2 Summary of device characterization
SM Additive VOC [V] JSC [mA cm−2] JSCa [mA cm−2] FF [%] PCEmax (ava)b [%]
a Calculated from the EQE spectra.b Average values of eight devices. Active layer of[thin space (1/6-em)]SM :[thin space (1/6-em)]PC71BM (1[thin space (1/6-em)]:[thin space (1/6-em)]1) was fabricated with CF as host solvent.
BDT(DPP-TTHex)2 — 0.65 2.52 2.23 60 0.97 (0.89)
0.3% CN 0.65 6.08 5.13 60 2.36 (2.12)
BDT(DPP-TT)2 — 0.68 8.37 7.53 51 2.91 (2.73)
0.5% CN 0.69 13.39 12.32 56 5.12 (5.03)

The device based on BDT(DPP-TTHex)2[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM exhibits a JSC of 2.52 mA cm−2, an open circuit voltage (VOC) of 0.65 V and FF of 60%, which yields a PCE of 0.97%. When using additive, the PCE of device is increased to PCE of 2.36% due to the enhancement of JSC to 6.08 mA cm−2. In the case BDT(DPP-TT)2[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM, the device exhibits JSC of 8.37 mA cm−2, VOC of 0.68 V and FF of 51%, leading a PCE of 2.91%. When using CN as an additive in BDT(DPP-TT)2[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM system, the enhancement of JSC to 13.39 mA cm−2 and FF to 56% is observed, leading the improvement of PCE to 5.12%. The relative photon energy loss (Eloss) of both BDT(DPP-TTHex)2 and BDT(DPP-TT)2 which is calculated by Eoptg − eVOC, is 0.84 eV (typical Eloss of 0.7–1.0 eV).39 This result indicates terminal hexyl group not influence on Eloss.

Fig. 3b shows the external quantum efficiency (EQE) spectra of devices based on BDT(DPP-TTHex)2 and BDT(DPP-TT)2. The integral current density values calculated from the EQE spectra and the standard solar spectrum (AM 1.5G) agreed well with the JSC values obtained from the J–V curves. Both SMs exhibit efficient photoconversion efficiency over 800 nm and the significant enhanced EQEs are observed by using CN as additive which are consistent with the improvement of JSC.

The higher PCEs of BDT(DPP-TT)2 based devices compared to BDT(DPP-TTHex)2 cases are mainly due to improved JSC. The value of JSC in the PSCs was influenced by several factors, including the optical absorption, the charge carrier mobility, and the nanoscale morphology of active layer. Both SMs have similar optical absorption spectra, and therefore, to understand the reason for high JSC, morphology and charge carrier mobility of active layers are studied.

3.5. Film morphology

Transmission electron microscopy (TEM) (Fig. 4) and atomic force microscopy (AFM) (Fig. 5) are characterized to understand the large difference of two SMs in solar cell performance. In TEM images, film cast from pure solvent base on BDT(DPP-TTHex)2[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM shows domains with diameter of ∼140 nm. By adding CN, the domain size becomes smaller with diameter ∼80 nm, which is still far from desired domain size (∼20 nm).40,41 This one leads to less efficient exciton separation, charge transport, and thus low JSC. It is consistent with OPVs' performance. In comparison, BDT(DPP-TT)2[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM blend film shows better morphology with reduction of domain size from ∼70 nm without CN to ∼20 nm with CN. Furthermore, some nanoscale fibril feature is observed from all SMs[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM blend films, which supports charge transportation in BHJ (ESI – Fig. S3†).42,43
image file: c6ra01103a-f4.tif
Fig. 4 TEM images of BDT(DPP-TTHex)2[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM 1[thin space (1/6-em)]:[thin space (1/6-em)]1 (a) without and (b) with 0.5% CN. BDT(DPP-TT)2[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM 1[thin space (1/6-em)]:[thin space (1/6-em)]1 (c) without and (d) with 0.3% CN (scale bar: 200 nm).

image file: c6ra01103a-f5.tif
Fig. 5 Phase images of BDT(DPP-TTHex)2[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM 1[thin space (1/6-em)]:[thin space (1/6-em)]1 (a) without CN and (b) with 0.5% CN. BDT(DPP-TT)2[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM 1[thin space (1/6-em)]:[thin space (1/6-em)]1 (c) without CN and (d) with 0.3% CN. (e–h) are the topographic imagines of (a–d), respectively.

Fig. 5 represents phase and topographic AFM images of BDT(DPP-TTHex)2 and BDT(DPP-TT)2 blend with PC71BM (1[thin space (1/6-em)]:[thin space (1/6-em)]1 weight ratio without and with CN as an additive). These films are prepared by spin-coating their CF solutions on top of the PEDOT:PSS layer. The surface phase images of the blend films obtained from AFM measurements are consistent with the morphologies observed in the TEM images. In topographic images, after adding CN, the surface roughnesses (rms) are reduced from 8.20 nm to 1.24 nm for BDT(DPP-TTHex)2, from 1.32 nm to 0.44 nm for BDT(DPP-TT)2. Smoother surface might help active layer have better contact with interlayer.

3.6. Charge carrier mobilities

The hole mobility (μh) of two small molecules blended with PC71BM in 1[thin space (1/6-em)]:[thin space (1/6-em)]1 wt ratio in CF without and with CN as additive are measured by the space charge limited current (SCLC) method, giving mobility values of 1.54 × 10−4, 1.01 × 10−5 cm2 V−1 s−1 for BDT(DPP-TTHex)2 and 2.88 × 10−4, 1.28 × 10−5 cm2 V−1 s−1 for BDT(DPP-TT)2 as shown in Fig. S4 and Table S2 (ESI†). By using additive, the hole mobilities of both materials are reduced dramatically. It is possible due to changed molecular ordering. These results also indicate that hole mobilities of BDT(DPP-TTHex)2 in both condition without or with additive has similar values of BDT(DPP-TT)2 blended film, respectively.

The electron mobility (μe) of two SMs blended with PC71BM is shown in Fig. S5 (ESI†). The low μe of the BDT(DPP-TTHex)2[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM film (7.22 × 10−8 cm2 V−1 s−1) at without additive condition might influence on the low JSC of device. The μe of BDT(DPP-TTHex)2[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM is increased to 2.39 × 10−5 cm2 V−1 s−1 by adding 0.5% CN in CF solvent and the ratio μh/μe achieves 0.42. Meanwhile, BDT(DPP-TT)2 shows similar μe of 5.71 × 10−5 and 5.46 × 10−5 cm2 V−1 s−1 in both without and with additive case, respectively.

On the other hand, the hole mobilities of BDT(DPP-TTHex)2 and BDT(DPP-TT)2 pristine are measured by organic field effect transistor (OFETs) devices as shown in Fig. S6 and S7 (ESI†). BDT(DPP-TTHex)2 and BDT(DPP-TT)2 pristine show similar hole mobilities of 2.2 × 10−2 and 2.1 × 10−2 cm2 V−1 s−1, respectively. Consequently, the similar charge carrier mobilities of two SMs suggest that the high JSC of BDT(DPP-TT)2 is mainly caused by optimal morphology.

3.7. Photocurrent

To further evaluate the effect of terminal side chain on BHJ OSCs, saturation photocurrent density (Jsat), maximum exciton generation rate (Gmax), and exciton dissociation probabilities [P(E, T)] are characterized according to literatures.44 The dependence of photocurrent density (Jph) and P(E, T) on effective voltage (Veff) under illumination of 100 mW cm−2 are showed in Fig. 6a and b, respectively. Gmax is obtained in case of no barrier for charge separation at the donor–acceptor interface, all excitons can be dissociated in to free carriers.45 Interface area of donor–acceptor is one of the factors that influences on Gmax of BHJ OSCs. The results are summarized in Table 3 indicate that smaller Gmax of BDT(DPP-TTHex)2 comparing to BDT(DPP-TT)2 is consistent with the large phase separation of BDT(DPP-TTHex)2[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM morphology. P(E, T) of two SMs at short circuit current condition is over 85% that indicate small barrier at donor–acceptor interface.
image file: c6ra01103a-f6.tif
Fig. 6 (a) Photocurrent density (Jph) and (b) exciton dissociation probability [P(E, T)] versus effective bias (Veff).
Table 3 Summary of photocurrent related data
SMa Jsat (A m−2) Gmax (m−3 s−1) P(E, T)b (%)
a Characterization of optimized devices.b Under short circuit condition.
BDT(DPP-TTHex)2 63.3 3.96 × 1027 91.8
BDT(DPP-TT)2 152.3 10.02 × 1027 87.7

4. Conclusions

Two new small molecules, BDT(DPP-TTHex)2 and BDT(DPP-TT)2, are designed and synthesized with favorable properties of light harvesting. We investigate the effect of end conjugated groups and solvent additives on photovoltaic performance. The results show that both SMs have similar optical property, electrochemical property, and charge carrier mobilities. We demonstrated that BDT(DPP-TT)2 flanked by 2,2′-bithiophen-5-yl end-group performed better efficiency than BDT(DPP-TTHex)2 blanked by 5-hexyl-2,2′-bithiophen-5′-yl due to optimized morphology. This strategy can be applied to other molecular systems with DPP as electron-withdrawing group to improve PCEs.

Acknowledgements

This research was supported by the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy (No. 20133030011330) and the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2015M1A2A2056214), Republic of Korea.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra01103a
This journal is © The Royal Society of Chemistry 2016
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