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DOI: 10.1039/C5RA07099F (Communication) RSC Adv., 2015, 5, 45755-45759

Natural-photosynthesis-inspired photovoltaic cells using carotenoid aggregates as electron donors and chlorophyll derivatives as electron acceptors†

Taojun Zhuangb, Shin-ichi Sasakic, Toshitaka Ikeuchic, Junji Kidob and Xiao-Feng Wang*a
aKey Laboratory of Physics and Technology for Advanced Batteries (Ministry of Education), College of Physics, Jilin University, Changchun 130012, P. R. China. E-mail: xf_wang@jlu.edu.cn
bDepartment of Organic Device Engineering, Graduate School of Engineering, Research Center for Organic Electronics (ROEL), Yamagata University, Yonezawa, 992-8510, Japan
cNagahama Institute of Bio-Science and Technology, Nagahama, Shiga 526-0829, Japan

Received 20th April 2015 , Accepted 14th May 2015

First published on 15th May 2015

Abstract

Carotenoids (Cars) and chlorophylls (Chls) are major pigments playing key roles in light-harvesting and energy transfer processes in natural photosynthetic apparatus. We demonstrated, in this study, photovoltaic cells with entire active layers consisting of a linear Car, lycopene, as the electron donor and Chl derivatives, either methyl 32,32-dicyano-pyropheophorbide-a (Chl-1) or methyl 131-deoxo-131-(dicyanomethylene)pyropheophorbide-a (Chl-2), as the electron acceptor.

Natural photosynthetic systems have evolved over billions of years, and they can therefore provide hints regarding some possible strategies for developing new type of photovoltaic cells.1,2 Photosynthetic pigments, in particular, chlorophylls (Chls) and carotenoids (Cars), play key roles in the initial stage of photosynthetic reactions including light-harvesting in the pigments–protein complexes, excitation energy delocalization among pigment–protein complexes, and electron transfer and charge separation between different Chls within the reaction center.3–5 Importantly, the fact that Chls and Cars are the most abundant pigments found in nature guarantees future large-scale production and application of these pigments in photovoltaic devices.

Chls are cyclic tetrapyrrole-based molecules with spectral properties that can be explained in terms of Gouterman's four-orbital model. The four orbitals are the highest occupied molecular orbital (HOMO)−1, HOMO, the lowest unoccupied molecular orbital (LUMO) and LUMO+1, which can be termed a2u, a1u, egx and egy, respectively.6,7 A major photophysical feature of Chls is that they can more efficiently use the solar energy at the near infrared region with their Q absorption bands.8,9 By contrast, Cars are the ideal accessory pigments to chlorophylls and together they can have complementary absorption covering almost the entire 300–800 nm wavelength region.10 Moreover, Cars have a C2h symmetry allowing efficient exciton delocalization among Car molecules and this character can regulate the application of Cars in opto-electrical devices.11 Natural photosynthetic systems employ both Chls and Cars to not only fully utilize the solar energy from visible to the near infrared regions but also efficiently convert solar energy into chemical energy and protect the organism from the photo-degradation.

Inspired from the usage of Chls and Cars in natural photosynthetic systems, we herein explore the possibility to use one Car, namely lycopene, as an electron donor and Chl derivatives, namely methyl 32,32-dicyano-pyropheophorbide-a (Chl-1) and methyl 131-deoxo-131-(dicyanomethylene)pyropheophorbide-a (Chl-2), as the electron acceptors in the blend films, to fabricate photovoltaic cells with entire photosynthetic active layers for the first time. In fact, previous studies have demonstrated that lycopene, Chl-1 and Chl-2 can be employed as the p-type donor in conjunction with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as the acceptor in organic photovoltaic cells, where the relatively lower LUMO level of PCBM allows electron transfer to take place from the above photosynthetic donors to PCBM.10,12,13 Moreover, it is also found that Chl-1 and Chl-2 can function as the n-type acceptor with copper phthalocyanine (CuPc) as the donor, suggesting ambipolar transport property of Chl-1 and Chl-2.12 Therefore we designed such a device architecture using lycopene as the donor and Chl as the acceptor for photosynthetic photovoltaic cells. As a result, distinctly different photovoltaic performance has been realized in these photosynthetic cells when using different Chl acceptors. It is found that the carrier mobilities of lycopene and Chl derivatives play a crucial role in determining the photovoltaic performance by balancing the carrier transport in such lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl photovoltaic cells.

Chl derivatives were synthesized as reported before.14 Other materials including MoO3 and lycopene were commercially available. All the photovoltaic cells were fabricated with the following structure: ITO/MoO3 (5 nm)/lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl-1 (or Chl-2) (80 nm)/Ca (20 nm)/Al (100 nm). Patterned ITO glass with a sheet resistance of 15 Ω per square were pre-cleaned by deionized water, acetone, and isopropanol in sonication, in this order, and then treated by ultraviolet-ozone for 30 min prior to use. MoO3, Ca and Al cathode were deposited via thermal evaporation in high vacuum at a base pressure of ∼5 × 10−5 Pa. The lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl-1 (or Chl-2) blend films were prepared via spin coating the mixed chloroform solution. The Al cathode was deposited through a shadow mask, which defined an active area of 3 × 3 mm2. After the deposition of the Al cathode, the devices were transferred directly to a nitrogen-filled glovebox for encapsulation without exposure to air. The external quantum efficiency (EQE) and current density–voltage (J–V) characteristics were measured by an integrated characterization system for thin film photovoltaic cells, CEP-2000 by Bunkoukeiki Co. The power conversion efficiency (PCE) was measured under 100 mW cm−2 illumination of AM 1.5 G solar spectrum. A reference silicon diode with a KG-5 filter certified by the National Institute of Advanced Industrial Science and Technology in Japan was used to calibrate the incident light intensity. Absorption spectra were measured by a SHIMADZU MPC-2200 UV-Visible spectrophotometer. Surface roughness profiles were measured with AFM (Veeco). The carrier mobility was calculated from single carrier devices by using space charge limited current (SCLC) method. The device structures for hole and electron mobility are ITO/MoO3 (5 nm)/lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl-1 (or Chl-2) (80 nm)/MoO3 (5 nm)/Al (100 nm) and ITO/Ca (1 nm)/lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl-1 (or Chl-2) (80 nm)/BCP (8 nm)/Al (100 nm), respectively. The HOMO levels of the materials were determined by using Riken Keiki AC-3. The LUMO levels were then derived by subtracting the optical band gaps from the HOMO levels.

Fig. 1 shows the absorption spectra of lycopene, Chl-1, and Chl-2 as solid thin films obtained by spin coating and their molecular structures. Owing to the existence of the cyclic tetrapyrrole structures,8,15 both Chl-1 and Chl-2 exhibit intense absorption in the near-ultraviolet and near-infrared regions, which can compensate the visible-range absorption of lycopene for intense light harvesting in the solar spectrum. Compared with Chl-1, Chl-2 gives relatively broader Soret peaks and stronger Q peaks, which may partly contribute to higher photocurrent in lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl-2 photovoltaic cells.12 It is well documented that lycopene monomers give clear absorption peaks corresponding to transition to different vibrational levels of S2 state.10 Given the large red- and blue-shift of the lycopene absorption peaks in Fig. 1(a), lycopene molecules must form aggregates in the film.10 Generally speaking, the difference between the energy level offsets of the donor and acceptor should be at least comparable to the exciton binding energy, so as to provide sufficient driving force for the exciton dissociation by charge transfer at the interface.16 Fig. 2(a) shows the energy alignment of each molecule of lycopene, Chl-1 and Chl-2. The HOMO and LUMO levels of lycopene are −4.8 eV and −2.9 eV, respectively. Although the energy levels of Chl-1 are slightly deeper than those of Chl-2, the LUMO offsets between lycopene and both Chl acceptors are much more than 0.3 eV, which is usually considered as the minimum value for efficient exciton dissociation at the donor/acceptor interface.17 Therefore, exciton dissociation process in all these photovoltaic cells based on lycopene and Chl pair should be efficient.


image file: c5ra07099f-f1.tif
Fig. 1 (a) The absorption spectra of 20 nm-thick lycopene, Chl-1 and Chl-2 film and (b) their molecular structures.

image file: c5ra07099f-f2.tif
Fig. 2 (a) The energy level diagrams of lycopene and Chl-1 (or Chl-2) heterojunction; (b) the device structures of photovoltaic cells using lycopene as the donor and Chl-1 (or Chl-2) as the acceptor.

We then fabricated photovoltaic cells using these photosynthetic pigments with the device configuration shown in Fig. 2(b). Fig. 3(a) and (b) present the J–V characteristics of bulk heterojunction (BHJ) photovoltaic cells based on lycopene and these Chl derivatives, i.e., Chl-1 and Chl-2. The relevant key photovoltaic parameters are given in Table 1. It is noted that the all lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl-1 photovoltaic cells have given much worse performance compared to lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl-2 photovoltaic cells. The values of open-circuit voltage (Voc) in lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl-1 and lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl-2 photovoltaic cells with varied blend ratio are around 0.54 V and 0.83 V, respectively. It is well established that Voc is mainly determined by the energy gap between the HOMO level of donor and LUMO level of acceptor in donor–acceptor heterojunction based photovoltaic cells.17,18 Therefore, the different Voc values of photovoltaic cells using different Chl acceptors are consistent with the LUMO levels of Chl-1 and Chl-2 at −4.0 eV and 3.7 eV, respectively. The most striking divergence between photovoltaic cells with two kinds of Chls is the short-circuit current density (Jsc). Compared to the small Jsc values of lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl-1 BHJ cells, lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl-2 cells present one order of magnitude larger Jsc values, which is hard to be explained just from the perspective of different absorption or energy levels between Chl-1 and Chl-2, as we discussed above. As shown in Fig. 3(c) and (d), the remarkably difference of photoresponses can be also clearly demonstrated from the EQE spectra of two kinds of photovoltaic cells. Here, if we compare the absorption spectra of lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl blend films with varied ratios (see Fig. S1 in the ESI† for the details) to those EQE spectra in Fig. 3(c) and (d), it is noted that the EQE response peaks in the lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl photovoltaic cells are corresponding to the absorption peaks of lycopene and Chls, especially the absorption peak of lycopene in short-wavelength range and the infrared absorption peak of Chls, suggesting that both lycopene and Chls are photoexcited in the blend active layer. Besides, when the blend ratio changes from 2[thin space (1/6-em)]:[thin space (1/6-em)]1 to 1[thin space (1/6-em)]:[thin space (1/6-em)]4, the long-wavelength photoresponses (∼720 nm) in both lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl-1 and lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl-2 BHJ cells increase due to the enhanced near-infrared light-harvesting as the Chl content increases, which should have a certain contribution to increased Jsc from 0.039 to 0.052 mA cm−2 in lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl-1 cells and from 0.15 to 0.23 mA cm−2 in lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl-2 cells.


image file: c5ra07099f-f3.tif
Fig. 3 J–V curves and EQE spectra of photovoltaic cells using (a), (c) Chl-1 or (b), (d) Chl-2 as the acceptor with varied blend ratios of lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl-1 (or Chl-2).
Table 1 The key parameters of photovoltaic cells using (a) Chl-1 or (b) Chl-2 as the acceptor with varied blend ratios of lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl-1 (or Chl-2)
Donor[thin space (1/6-em)]:[thin space (1/6-em)]acceptor Blend ratio Jsc (mA cm−2) Voc (V) FF PCE (%)
Lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl-1 2[thin space (1/6-em)]:[thin space (1/6-em)]1 3.9 × 10−2 0.54 0.26 5.5 × 10−3
1[thin space (1/6-em)]:[thin space (1/6-em)]1 4.0 × 10−2 0.54 0.24 5.0 × 10−3
1[thin space (1/6-em)]:[thin space (1/6-em)]2 4.9 × 10−2 0.54 0.26 7.0 × 10−3
1[thin space (1/6-em)]:[thin space (1/6-em)]4 5.2 × 10−2 0.55 0.26 7.4 × 10−3
Lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl-2 2[thin space (1/6-em)]:[thin space (1/6-em)]1 0.15 0.81 0.27 3.2 × 10−2
1[thin space (1/6-em)]:[thin space (1/6-em)]1 0.18 0.81 0.28 4.1 × 10−2
1[thin space (1/6-em)]:[thin space (1/6-em)]2 0.21 0.83 0.24 4.3 × 10−2
1[thin space (1/6-em)]:[thin space (1/6-em)]4 0.23 0.85 0.23 4.5 × 10−2

In organic photovoltaic cells with various device architectures, carrier mobility is always one key parameter that dominates charge carrier extraction and photovoltaic performance.13,19 Table 2 shows the carrier mobility values of thin films containing lycopene and Chl-1 (or Chl-2) in different blend ratios by using SCLC method.20 The hole mobility for pure lycopene film is quite high, with the value of 2.1 × 10−2 cm2 (V−1 s−1).10 The hole mobility and electron mobility for Chl-1 are 9.5 × 10−7 cm2 (V−1 s−1) and 4.6 × 10−6 cm2 (V−1 s−1), respectively, and those for Chl-2 are 2.0 × 10−6 cm2 (V−1 s−1) and 3.0 × 10−5 cm2 (V−1 s−1).12 The huge difference of mobility values between hole and electron makes carrier transport and extraction be the dominated processes that govern the performance in these lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl photovoltaic cells. As shown in Table 2, the electron mobility of Chl-1 is too low compared with the hole mobility of lycopene. Therefore the hole mobility and electron mobility are extremely imbalanced in lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl-1 blend films, leading to inefficient charge extraction and quite low photocurrent output.21–23 Even in the lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl-1 blend film with the ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]4, the hole mobility is still two order of magnitude larger than electron mobility. By contrast, the electron mobility of Chl-2 is much higher than that of Chl-1, which can provide a more balanced charge transport in the lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl-2 blend films and result in an improved photocurrent. Due to the same reason, the increased Jsc value with the blend lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl ratio varying from 2[thin space (1/6-em)]:[thin space (1/6-em)]1 to 1[thin space (1/6-em)]:[thin space (1/6-em)]4 should be partly originated from the improved charge extraction caused by more and more balanced carrier mobilities in the blend films with increased Chl content. Furthermore, it is found that the fill factor (FF) values of both lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl-1 and lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl-2 cells are quite low (<0.3) and their photocurrent is strongly field-dependent. This should be owing to the fact that high mobility and imbalanced carrier transport in the blend active region can induce serious bimolecular recombination,20,21 and in all these photovoltaic cells, the hole mobility is far beyond the electron mobility. Thus, in both lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl-1 and lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl-2 photovoltaic cells, the best photovoltaic performance were always given by cells with a lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl blend ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]4. More specifically, the optimal values of Jsc, Voc, FF, and PCE are 0.052 mA cm−2, 0.55 V, 0.26 and 0.0074% for lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl-1 photovoltaic cells and they are 0.23 A cm−2, 0.85 V, 0.23 and 0.045% for lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl-2 photovoltaic cells. It is noteworthy that further increase of the Chl content after 1[thin space (1/6-em)]:[thin space (1/6-em)]4-blend ratio exerts little influence on the electron mobility of the blend film, and as a result, the photovoltaic performance is more or less similar.

Table 2 Carrier mobility data derived from single carrier devices using SCLC model
Carrier mobility Donor Blend ratio Acceptor
Lycopene 2[thin space (1/6-em)]:[thin space (1/6-em)]1 1[thin space (1/6-em)]:[thin space (1/6-em)]1 1[thin space (1/6-em)]:[thin space (1/6-em)]2 1[thin space (1/6-em)]:[thin space (1/6-em)]4 Chl-1
μh (cm2 (V−1 s−1)) 2.1 × 10−2 1.6 × 10−3 4.7 × 10−4 2.3 × 10−4 2.8 × 10−5  
μe (cm2 (V−1 s−1))   3.9 × 10−8 9.2 × 10−8 4.1 × 10−7 9.5 × 10−7 4.6 × 10−6
  Lycopene 2[thin space (1/6-em)]:[thin space (1/6-em)]1 1[thin space (1/6-em)]:[thin space (1/6-em)]1 1[thin space (1/6-em)]:[thin space (1/6-em)]2 1[thin space (1/6-em)]:[thin space (1/6-em)]4 Chl-2
μh (cm2 (V−1 s−1)) 2.1 × 10−2 1.8 × 10−3 5.1 × 10−4 3.0 × 10−4 3.4 × 10−5  
μe (cm2 (V−1 s−1))   2.3 × 10−7 8.6 × 10−7 3.8 × 10−6 8.2 × 10−6 3.0 × 10−5

So far, it can be seen that in these photovoltaic cells based on lycopene and Chls, the photovoltaic performance was mainly dominated by carrier mobility and the performance is relatively low. However, compared to previously reported single-active-layer devices based on Chl-1 or Chl-2,12 obviously improved photocurrent and PCE values in these lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl photovoltaic cells have indicated charge transfer process between lycopene and Chl by introducing lycopene and Chl blend active layer (see Table S1 in the ESI† for the details), suggesting the feasibility of photosynthetic active layers-based photovoltaic cells based on lycopene and Chls.

Conclusions

In conclusion, a typical carotenoid, lycopene, and two chlorophylls derivatives, Chl-1 and Chl-2, were employed as the electron donor and the acceptors respectively, for fabricating photosynthetic active layers-based photovoltaic cells. In this case, lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl-2 photovoltaic cells show much better performance than lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl-1 photovoltaic cells. Those results should be ascribed to much higher electron mobility of Chl-2 than that of Chl-1, which leads to a much balanced charge transport for holes and electrons as well as better charge extraction in lycopene[thin space (1/6-em)]:[thin space (1/6-em)]Chl-2 photovoltaic cells. In this paper, we mainly focused on the photovoltaic performance of these natural photosynthetic pigments based photovoltaic cells. In addition to the device performance, the photo stability of Cars and Chls is also crucial for the devices, which is now still under studying. We would like to discuss the stability of Cars and Chls in detail in our future papers. This innovative application of natural-photosynthetic pigments based photovoltaic cells provides further possibilities for developing high-performance, environmental-friendly photovoltaics.

Acknowledgements

This work was supported by Grants-in-Aid for Young Scientists (A) (23686138) (to X.-F.W) and for Scientific Research (C) (25410152) (to S.S) from the Japan Society for the Promotion of Science (JSPS). This work was also supported by the Open Project of State Key Laboratory of Supramolecular Structure and Materials (no. sklssm2015019).

References

  1. H. Tamiaki, R. Shibata and T. Mizoguchi, Photochem. Photobiol., 2007, 83, 152–162 CAS.
  2. X.-F. Wang, H. Tamiaki, L. Wang, N. Tamai, O. Kitao, H. Zhou and S. Sasaki, Langmuir, 2010, 26, 6320–6327 CrossRef CAS PubMed.
  3. G. Cerullo, D. Polli, G. Lanzani, S. De Silvestri, H. Hashimoto and R. J. Cogdell, Science, 2002, 298, 2395–2398 CrossRef CAS PubMed.
  4. A. Gall, R. J. Cogdell and B. Robert, Biochemistry, 2003, 42, 7252–7258 CrossRef CAS PubMed.
  5. X.-F. Wang, Y. Koyama, O. Kitao, Y. Wada, S. Sasaki, H. Tamiaki and H. Zhou, Biosens. Bioelectron., 2010, 25, 1970–1976 CrossRef CAS PubMed.
  6. M. Gouterman, J. Chem. Phys., 1959, 30, 1139–1161 CrossRef CAS PubMed.
  7. M. Gouterman, J. Mol. Spectrosc., 1961, 6, 138–163 CrossRef CAS.
  8. H. Imahori, T. Umeyama and S. Ito, Acc. Chem. Res., 2009, 42, 1809–1818 CrossRef CAS PubMed.
  9. X.-F. Wang and H. Tamiaki, Energy Environ. Sci., 2010, 3, 94–106 CAS.
  10. X.-F. Wang, L. Wang, Z. Wang, Y. Wang, N. Tamai, Z. Hong and J. Kido, J. Phys. Chem. C, 2013, 117, 804–811 CAS.
  11. Y. Koyama, Y. Kakitani, T. Miki, R. Christiana and H. Nagae, Int. J. Mol. Sci., 2010, 11, 1888–1929 CrossRef CAS PubMed.
  12. Y.-W. Wang, S. Sasaki, T. Zhuang, H. Tamiaki, J.-P. Zhang, T. Ikeuchi, Z. Hong, J. Kido and X.-F. Wang, Org. Electron., 2013, 14, 1972–1979 CrossRef CAS PubMed.
  13. Y.-W. Wang, X.-F. Wang, J.-P. Zhang, Z. Hong, J. Kido, O. Kitao, T. Ikeuchi, H. Tamiaki and S. Sasaki, J. Phys. Chem. C, 2012, 116, 21244–21254 CAS.
  14. S. Sasaki, M. Yoshizato, M. Kunieda and H. Tamiaki, Eur. J. Org. Chem., 2010, 2010, 5287–5291 CrossRef PubMed.
  15. M. G. Walter, A. B. Rudine and C. C. Wamser, J. Porphyrins Phthalocyanines, 2010, 14, 759–792 CrossRef CAS.
  16. M. C. Scharber, D. Mühlbacher, M. Koppe, P. Denk, C. Waldauf, A. J. Heeger and C. J. Brabec, Adv. Mater., 2006, 18, 789–794 CrossRef CAS PubMed.
  17. G. Dennler, M. C. Scharber and C. J. Brabec, Adv. Mater., 2009, 21, 1323–1338 CrossRef CAS PubMed.
  18. S. R. Cowan, A. Roy and A. J. Heeger, Phys. Rev. B: Condens. Matter Mater. Phys., 2010, 82, 245207 CrossRef.
  19. P. Peumans, A. Yakimov and S. R. Forrest, J. Appl. Phys., 2003, 93, 3693–3723 CrossRef CAS PubMed.
  20. P. W. M. Blom, V. D. Mihailetchi, L. J. A. Koster and D. E. Markov, Adv. Mater., 2007, 19, 1551–1566 CrossRef CAS PubMed.
  21. G. Chen, H. Sasabe, Z. Wang, X.-F. Wang, Z. Hong, Y. Yang and J. Kido, Adv. Mater., 2012, 24, 2768–2773 CrossRef CAS PubMed.
  22. W. Tress, A. Merten, M. Furno, M. Hein, K. Leo and M. Riede, Adv. Energy Mater., 2013, 3, 631–638 CrossRef CAS PubMed.
  23. H. Kageyama, H. Ohishi, M. Tanaka, Y. Ohmori and Y. Shirota, Appl. Phys. Lett., 2009, 94, 063304 CrossRef PubMed.

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra07099f
This journal is © The Royal Society of Chemistry 2015
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