Mesoporous SnO₂ nanoparticle films as electron-transporting material in perovskite solar cells

Abstract

Perovskite solar cells with mesoporous metal oxide films as scaffold layers have demonstrated very impressive advances in performance recently. Here, we present an investigation into mesoporous perovskite solar cells incorporating mesoporous SnO₂ nanoparticle films as electron-transporting materials and scaffold layers, to replace traditional mesoporous TiO₂ films. We have optimized the SnO₂ film thickness and treated the surface of the SnO₂ film with an aqueous solution of TiCl₄. Due to the TiCl₄ treatment the recombination process was significantly retarded. The short-circuit current density (J_sc) and open-circuit voltage (V_oc) reached nearly 18 mA cm⁻² and 1 V, respectively. Consequently, the power conversion efficiency of the device with the SnO₂ film exceeded 10%.

---

1. Introduction

Organic–inorganic halide lead perovskite (CH₃NH₃PbX₃, X = Cl, Br, I) materials have shown great potential in thin-film photovoltaic devices owing to their excellent light absorption coefficient, appropriate band gap, long diffusion length, and the ease by which these materials can be fabricated with a solution process.^1–11 Recently, the power conversion efficiencies (PCEs) of mesoporous perovskite solar cells have skyrocketed from 3.8% to more than 20% within 5 years.^1,5,10,11 In the perovskite solar cells based on the mesoporous configuration, mesoporous metal oxide thin films are used as either electron-transporting materials (ETMs), to accept photogenerated electrons from perovskites and then transmit them to the conductive substrates, or mesoporous scaffold layers for perovskite material formation. Mesoporous TiO₂ nanoparticle films are usually used as ETMs and scaffold layers for high-efficiency perovskite solar cells. In addition to mesoporous TiO₂ films, a variety of materials are also used in perovskite solar cells as ETMs and/or the scaffold. For example, one-dimensional TiO₂ nanowires, rutile TiO₂ nanorods, ZnO nanorods, [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM), Al₂O₃ and ZrO₂, etc. have been successfully introduced into mesoporous solar cells with different PCEs.^12–18 Despite the remarkable achievements in the field of perovskite solar cells, the development of alternative ETMs/scaffolds to TiO₂ is a promising avenue to further improve the performance of perovskite solar cells.

Previously, mesoporous tin oxide (SnO₂) thin film electrodes were widely applied in dye-sensitized solar cells (DSCs) as ETMs.^19,20 SnO₂ has a deeper conduction band than TiO₂ and in principle should facilitate a more efficient transfer of photo-generated electrons from the perovskite light absorber to the SnO₂ conduction band.²¹ Furthermore, bulk SnO₂ has an electron mobility of up to 240 cm² V⁻¹ s⁻¹, which is 100 times higher than that of TiO₂,²² making it conceptually a more likely candidate for highly efficient solar cells. The DSCs based on SnO₂ electrodes usually give a lower PCE compared to those based on TiO₂ electrodes, due to the degree of recombination between the “bare” SnO₂ and the hole-transporting materials (HTMs).¹⁹ However, by treating the surface of the SnO₂ nanoparticles with an aqueous solution of TiCl₄ or wide band gap “insulating” oxides, such as MgO or Al₂O₃, suppression of the backward reaction enables a significant enhancement in the PCE of the DSCs by over 7%.²³ There are still no reports about mesoporous SnO₂ being used as an ETM in perovskite solar cells despite the distinguished advantages. In this study, we present perovskite solar cells utilizing mesoporous SnO₂ electrodes as the ETM. By optimizing the thickness of the SnO₂ film and treating the surface of the SnO₂ film with an aqueous solution of TiCl₄, the short-circuit current density and open-circuit voltage have been increased to nearly 18 mA cm⁻² and 1 V, respectively, resulting in devices with a PCE of over 10%. Furthermore, we employed impedance spectroscopy (IS) to investigate the recombination kinetics of devices based on mesoporous SnO₂ electrodes before and after treatment of the SnO₂ with an aqueous solution of TiCl₄.

2. Results and discussion

We present a solid state mesoporous perovskite solar cell incorporating a mesoporous SnO₂ film as the ETM, CH₃NH₃PbI₃ as the light absorber and spiro-MeOTAD as the hole-transporting layer. A schematic structure of the device is depicted in Fig. 1. It is imperative that the mesoporous perovskite solar cells contain a compact blocking layer to retard the combination of the FTO substrate and the hole conductor.²⁴ In this perovskite solar cell, mesoporous SnO₂ thin films are used as both the ETMs, to accept photogenerated electrons from the CH₃NH₃PbI₃ perovskites and then transmit them to the conductive substrate, and the mesoporous scaffold layer for perovskite material formation. We fabricated the CH₃NH₃PbI₃ perovskite on the mesoporous SnO₂ films via a two-step spin-coating procedure,²⁵ as shown in Fig. 2. Firstly, the mesoporous SnO₂ film was infiltrated with a PbI₂-dissolved DMF solution by spin coating; the mesoporous SnO₂ layer was completely covered by the PbI₂ overlayer with a thickness of about 200 nm, as can be seen in Fig. 2b. Secondly, a 100 μL CH₃NH₃I-dissolved isopropyl alcohol solution was spin-coated onto the SnO₂/PbI₂ film. The reaction of PbI₂ with CH₃NH₃I forms void-free CH₃NH₃PbI₃ cuboids on the mesoporous SnO₂ film, as shown in Fig. 2c.

Fig. 1 Schematic device structure of the CH₃NH₃PbI₃ perovskite solar cells using mesoporous SnO₂ as the ETM.

Fig. 2 SEM images of (a) the mesoporous SnO₂ film; (b) the SnO₂/PbI₂ film; and (c) the SnO₂/CH₃NH₃PbI₃ film. The mesoporous SnO₂ film thickness is about 200 nm.

In order to investigate the effect of the SnO₂ film thickness on the photovoltaic performance of perovskite solar cells, we diluted the spin-coating SnO₂ paste with ethanol in different weight ratios: 1 : 1, 1 : 3 and 1 : 5, to obtain the SnO₂ electrodes with different thickness. The cross section SEM images of these SnO₂ electrodes are shown in Fig. 3. As can be seen, the thicknesses of these SnO₂ electrodes were 100 nm, 200 nm and 300 nm, respectively. Fig. 4 shows the absorption spectra of the SnO₂/CH₃NH₃PbI₃ films with SnO₂ thickness of 100 nm, 200 nm and 300 nm, respectively. From the results shown in Fig. 4, it can be seen that the absorption spectra of the CH₃NH₃PbI₃ goes up to 800 nm wavelength owing to its band gap of 1.57 eV. In addition, light absorption increases with increasing the thickness of the mesoporous SnO₂ films from 100 nm to 300 nm, which is mainly due to the fact that increasing the SnO₂ thickness increases the amount of CH₃NH₃PbI₃ deposited into the SnO₂ film.

Fig. 3 Cross-sectional SEM images of the mesoporous perovskite solar cells using porous SnO₂ films with different thicknesses as the ETMs. SnO₂ films were obtained by spin-coating SnO₂ paste diluted with ethanol to different weight ratios: (a) 1 : 1; (b) 1 : 3; (c) 1 : 5. The structures of the devices are FTO/SnO₂/CH₃NH₃PbI₃/spiro-MeOTAD/Au.

Fig. 4 Effect of different mesoporous SnO₂ films thicknesses on the absorption spectra of SnO₂/CH₃NH₃PbI₃ films.

The photovoltaic performance of the perovskite solar cells using a mesoporous SnO₂ electrode as the ETM was evaluated. Firstly, the dependence of the photovoltaic performance on the thickness of the SnO₂ electrode was investigated. The PCE of the FTO/SnO₂/CH₃NH₃PbI₃/spiro-MeOTAD/Au solar cells is slightly improved from 6.42% (100 nm) to 6.50% (200 nm), which is mainly due to the remarkable increase in J_sc from 16.78 mA cm⁻² to 18.69 mA cm⁻², though V_oc and FF are slightly decreased from 0.753 V and 55.4% (100 nm) to 0.701 V and 53.4% (200 nm), respectively, as shown in Fig. 5a and Table 1. The increased J_sc and PCE might arise from the higher light absorption caused by the increased loading of CH₃NH₃PbI₃ (Fig. 4). However, with a further increase in the thickness of the SnO₂ electrode from 200 nm to 300 nm, the PCE of the perovskite solar cells is substantially decreased from 6.50% to 0.90%, which mainly results from the significant decrease in V_oc, J_sc and FF from 0.701 V, 17.39 mA cm⁻² and 53.4% to 0.272 V, 8.85 mA cm⁻² and 37.3%, respectively (as shown in Fig. 5a and Table 1). The decreased efficiency may be explained by the fact that a thicker SnO₂ film means a longer distance for charge carriers to travel which also increases the probability of electron–hole recombination. As has been known in the field, the morphology of the perovskite capping layer significantly affects the photovoltaic performance.^25–28 The main reason for effecting the PCE of the perovskite solar cell is that the thinner CH₃NH₃PbI₃ capping layer is prepared on the 300 nm-thick SnO₂ electrode, as can be seen in Fig. 3. In Fig. 5b, the incident photon-to-electron conversion efficiency (IPCE) spectra follow the same trend observed for the short-circuit photocurrent with the SnO₂ based perovskite solar cells. Specially, the perovskite solar cells based on a SnO₂ thin film with 200 nm thickness exhibit IPCE values of over 80% between 400 and 600 nm. Integrating this IPCE spectrum over the AM 1.5 solar spectra at 100 mW cm⁻² estimates a J_sc of 17.59 mA cm⁻², which is in close agreement with our measured maximum value of 17.39 mA cm⁻² under simulated solar conditions.

Fig. 5 (a) Current density–voltage (J–V) characteristics of the perovskite solar cells depending on the thickness of the SnO₂ electrodes. (b) The corresponding IPCE spectra.

Table 1 Photovoltaic performance parameters of the perovskite solar cells depending on the thickness of the SnO₂ electrodes

Thickness of SnO2 (nm)	Voc^a (V)	Jsc^b (mA cm^−2)	FF^c	PCE^d (%)
a Open-circuit photovoltage.b Short-circuit photocurrent.c Fill factor.d Power conversion efficiency.
100	0.753	15.39	0.554	6.42
200	0.701	17.39	0.534	6.50
300	0.272	8.85	0.373	0.90

---

For the spiro-MeOTAD-based perovskite solar cell composed of a “bare” SnO₂ electrode as the ETM, we found that the performance of this device is not satisfactory, which may be caused by the degree of electron recombination occurring between the SnO₂ and HTMs or perovskite. In order to inhibit the charge recombination, we optimized the interface between the SnO₂ and perovskite layer via treatment with an aqueous solution of TiCl₄.²⁴ The photovoltaic performance of the perovskite solar cells with the mesoporous SnO₂ electrode, with and without treating the SnO₂ film surface with an aqueous solution of TiCl₄, was evaluated, as can be seen in Fig. 6 and Table 2. Upon treating the SnO₂ film surface with an aqueous solution of TiCl₄, the PCE of the FTO/SnO₂/CH₃NH₃PbI₃/spiro-MeOTAD/Au solar cell is substantially improved from 6.50% to 10.18% (33% increment), which is mainly due to the significant increase in V_oc from 0.701 V to 0.933 V, with a slight increase in FF from 53.4% to 62.8% and a nearly equal J_sc value.

Fig. 6 Current–voltage curves for the mesoporous perovskite solar cells with and without treating the SnO₂ film surface with an aqueous solution of TiCl₄. The thickness of the SnO₂ electrodes is 200 nm.

Table 2 Photovoltaic performance parameter values, extracted from the current–voltage curves presented in Fig. 6

Surface treatment of SnO2	Voc (V)	Jsc (mA cm^−2)	FF	PCE (%)
None	0.701	17.39	0.534	6.50
TiCl4	0.933	17.38	0.628	10.18

---

We employed impedance spectroscopy (IS) to investigate the effect of the TiCl₄ treatment on the dramatic improvement of the performance of the SnO₂-based perovskite solar cells. Fig. 7a shows representative IS spectra (Nyquist plots) for the SnO₂-based perovskite solar cells before and after TiCl₄ treatment at low applied forward bias (0.7 V) under dark conditions. The obtained IS spectra includes two arcs, where the first arc in the high frequency region is related to the charge transfer behavior at the counter electrode and the second arc is due to a combination of the recombination resistance (R_rec) and the chemical capacitance of the film (C_μ).^24,29,30 Excellent fitting results (Fig. 7a) were obtained using a simplified equivalent circuit (Fig. 7b).³¹ The R_rec for a perovskite solar cell incorporating a “bare” SnO₂ electrode shows a lower value than for that incorporating a SnO₂ electrode coated with an ultrathin TiO₂ layer at the same applied bias voltage, which indicates that the recombination process in the device based on a TiO₂-coated SnO₂ electrode is remarkably retarded via TiCl₄ treatment. Thus, the perovskite solar cells with a “bare” SnO₂ electrode represent a lower open-circuit voltage due to the higher recombination. Likewise, by treating the SnO₂ electrode surface with an aqueous solution of TiCl₄, the device shows lower recombination kinetics due to the ultrathin TiO₂ between the SnO₂ and CH₃NH₃PbI₃ layers enabling ease of electron transfer from the TiO₂ to the SnO₂ and also avoiding any extra internal trap sites,²⁴ and thus decreasing the charge recombination, leading to a 230 mV higher V_oc and a 10% higher FF than that exhibited by the perovskite solar cell incorporating a “bare” SnO₂ electrode.

Fig. 7 (a) Nyquist plots for the perovskite solar cells with mesoporous SnO₂ as the ETM before and after treating the SnO₂ electrode surface with an aqueous solution of TiCl₄, measured under dark conditions at 0.7 V bias. The thickness of the SnO₂ electrodes is 200 nm. (b) A simplified equivalent circuit model employed for impedance analysis of the perovskite solar cells.

3. Conclusions

In summary, a mesoporous SnO₂ film has been successfully utilized in the CH₃NH₃PbI₃ perovskite solar cell as an electron-transporting material and scaffold layer. By optimizing the SnO₂ thickness and treating the surface of the SnO₂ film with an aqueous solution of TiCl₄, the perovskite solar cell exhibits an increased V_oc of around 1 V and a PCE of 10.18%. Impedance spectroscopy results indicated that the ultrathin TiO₂ coating the SnO₂ film as a result of TiCl₄ treatment significantly retards the charge recombination process via avoiding any extra internal trap sites and facilitating electron transfer from the CH₃NH₃PbI₃ perovskite to the SnO₂ conduction band. These results make the mesoporous SnO₂ based perovskite solar cells competitive with the mesoporous TiO₂ based devices, and a much more promising concept for future optimization of perovskite solar cells.

4. Experimental section

4.1. Materials

Lead(II) iodide (PbI₂, 99%), lithium bis(trifluoromethylsulphonyl) imide (Li-TFSI) (99.95%), 4-tert-butylpyridine (t-BP, 96%), butyl-(tin chloride), N,N-dimethylformamide (DMF, 99.9%) and hydriodic acid (57 wt% in water) were purchased from Sigma-Aldrich. Methylamine solution (40% in methanol) was purchased from TCI. Tin(IV) oxide (SnO₂) nanoparticle powders (NanoArc) were purchased from Alfa Aesar and ranged in size from 22 to 43 nm. Spiro-MeOTAD was purchased from Merck KGaA. All chemicals were used as received.

Synthesis of CH₃NH₃I. CH₃NH₃I was synthesized according to the reported method.⁶ Hydroiodic acid (30 mL, 57 wt% in water) was reacted with methylamine (27.86 mL, 40% in methanol) in a 250 mL round-bottom flask at 0 °C for 2 h. The precipitate was recovered by putting the solution on a rotary evaporator and carefully removing the solvent at 50 °C for 1 h. The generated yellowish powder was dissolved in ethanol, recrystallized from diethyl ether, and finally dried at 60 °C in a vacuum oven for 24 h.

4.2. Solar cell fabrication

Substrate preparation. Fluorine-doped tin oxide (FTO, 15 Ω square⁻¹) glass substrates with dimensions of 2 cm × 1.5 cm were partially etched with Zn powder and 2 M HCl to reveal the electrode pattern. The etched substrates were cleaned with detergent, followed by ultrasonication in pure water and ethanol for 30 min, and were then rinsed with deionized water and ethanol and dried by air. Finally, the substrates were annealed at 500 °C for 30 min. A thin (60 nm) blocking layer of SnO₂ (bl-SnO₂) was deposited via spray pyrolysis deposition at 450 °C from a solution of butyl-(tin chloride) in anhydrous ethanol at a 1 : 10 volume ratio and then annealed at 500 °C for 30 min.

SnO₂ nanoparticle powders were used to synthesize the SnO₂ paste by following the method for making TiO₂ paste.³² Briefly, 5 g SnO₂ nanoparticles was redispersed in 150 mL of anhydrous ethanol and mixed with 20 g of terpineol and 15 g of ethyl cellulose to prepare the SnO₂ paste. The mesoporous SnO₂ electrodes were obtained by spin-coating at 5000 rpm for 30 s onto the bl-SnO₂ substrates using the SnO₂ paste diluted in ethanol (1 : 1, 1 : 3 and 1 : 5, weight ratio). After drying at 100 °C, the resulting mesoporous SnO₂ films were annealed at 500 °C for 30 min to remove the organic part. Some of SnO₂ films resulting from the SnO₂ paste diluted in ethanol (1 : 2, weight ratio) were treated with a 0.04 M aqueous solution of TiCl₄ at 60 °C for 1 h, rinsed with deionized water and annealed at 500 °C for 30 min. Prior to their use, the SnO₂ films were again dried at 500 °C for 30 min.

462 mg of PbI₂ was dissolved in 1 mL DMF under stirring at 70 °C overnight, followed by filtering with a 0.22 μm pore PVDF syringe filter. The solution was kept at 70 °C during the whole procedure. A 25 μL PbI₂ solution was spin-coated on the mesoporous SnO₂ films at 3000 rpm for 20 s, and dried at 50 °C for 3 min and 100 °C for 5 min, consecutively. After cooling to room temperature, a 100 μL CH₃NH₃I solution in 2-propanol (10 mg mL⁻¹) was loaded onto the PbI₂-coated SnO₂ films for 20 s, which was spun at 4000 rpm for 30 s and then dried at 100 °C for 5 min.

The spiro-MeOTAD HTM was deposited on the SnO₂/CH₃NH₃PbI₃ film by spin coating at 4000 rpm for 30 s. The composition of the spiro-MeOTAD HTM was 72.3 mg spiro-MeOTAD, 28.8 μL TBP, and 17.5 μL of a solution of 520 mg mL⁻¹ LiTFSI in acetonitrile in 1 mL chlorobenzene.

Finally, 60 nm-thick Au was thermally evaporated on top of the device to form the back contact. The active area of the devices was 9 mm² determined by a black mask with dimensions of 3 mm × 3 mm.

4.3. Characterization

The surface morphology of the film was observed with a field emission scanning electron microscope (FE-SEM, sirion200, FEI Corp., Holland). The UV-vis spectrum of the films was obtained using a UV-vis spectrophotometer (U-3900H, HITACHI, Japan). The incident-photon-to-electron conversion efficiency (IPCE) measurement was conducted using a QE/IPCE measurement kit (Newport Corporation, CA). The current–voltage characteristics (J–V curves) were measured with a Keithley model 2420 digital source meter (Keithley Instruments, Inc., USA) under the illumination of 100 mW cm⁻² (AM 1.5) provided by a solar simulator (solar AAA simulator, Oriel USA). The impedance spectra were measured with an electrochemical analyzer (Autolab 320, Metrohm, Switzerland) with a bias potential at 0.7 V. AC 20 mV perturbation was applied with a frequency from 1 MHz to 1 Hz. The obtained impedance spectra were fitted with ZView software (v2.8b, Scribner Associates, USA).

Acknowledgements

This work is financially supported by the National Basic Research Program of China under Grant no. 2011CBA00700 and the National Natural Science Foundation of China under Grant no. 21403247, 21173228, 61204075 and 61404142.

Notes and references

1. A. Kojima, K. Teshima, Y. Shirai and T. Miyasaka, J. Am. Chem. Soc., 2009, 131, 6050–6051.
2. H.-S. Kim, C.-R. Lee, J.-H. Im, K.-B. Lee, T. Moehl, A. Marchioro, S.-J. Moon, R. Humphry-Baker, J.-H. Yum, J. E. Moser, M. Gratzel and N.-G. Park, Sci. Rep., 2012, 2, 591.
3. J. Burschka, N. Pellet, S.-J. Moon, R. Humphry-Baker, P. Gao, M. K. Nazeeruddin and M. Gratzel, Nature, 2013, 499, 316–319.
4. S. D. Stranks, G. E. Eperon, G. Grancini, C. Menelaou, M. J. P. Alcocer, T. Leijtens, L. M. Herz, A. Petrozza and H. J. Snaith, Science, 2013, 342, 341–344.
5. H. Zhou, Q. Chen, G. Li, S. Luo, T.-b. Song, H.-S. Duan, Z. Hong, J. You, Y. Liu and Y. Yang, Science, 2014, 345, 542–546.
6. J. H. Im, C. R. Lee, J. W. Lee, S. W. Park and N. G. Park, Nanoscale, 2011, 3, 4088–4093.
7. J. M. Ball, M. M. Lee, A. Hey and H. J. Snaith, Energy Environ. Sci., 2013, 6, 1739–1743.
8. H.-S. Kim, I. Mora-Sero, V. Gonzalez-Pedro, F. Fabregat-Santiago, E. J. Juarez-Perez, N.-G. Park and J. Bisquert, Nat. Commun., 2013, 4, 2242.
9. G. Xing, N. Mathews, S. Sun, S. S. Lim, Y. M. Lam, M. Grätzel, S. Mhaisalkar and T. C. Sum, Science, 2013, 342, 344–347.
10. http://www.nrel.gov/ncpv/images/efficiency_chart.jpg, The National Renewable Energy Laboratory (NREL), 2015.
11. W. Nie, H. Tsai, R. Asadpour, J.-C. Blancon, A. J. Neukirch, G. Gupta, J. J. Crochet, M. Chhowalla, S. Tretiak, M. A. Alam, H.-L. Wang and A. D. Mohite, Science, 2015, 347, 522–525.
12. H.-S. Kim, J.-W. Lee, N. Yantara, P. P. Boix, S. A. Kulkarni, S. Mhaisalkar, M. Grätzel and N.-G. Park, Nano Lett., 2013, 13, 2412–2417.
13. M. H. Kumar, N. Yantara, S. Dharani, M. Graetzel, S. Mhaisalkar, P. P. Boix and N. Mathews, Chem. Commun., 2013, 49, 11089–11091.
14. J. You, Z. Hong, Y. Yang, Q. Chen, M. Cai, T.-B. Song, C.-C. Chen, S. Lu, Y. Liu and H. Zhou, ACS Nano, 2014, 8, 1674–1680.
15. K. Wojciechowski, M. Saliba, T. Leijtens, A. Abate and H. J. Snaith, Energy Environ. Sci., 2014, 7, 1142–1147.
16. D. Bi, S.-J. Moon, L. Haggman, G. Boschloo, L. Yang, E. M. J. Johansson, M. K. Nazeeruddin, M. Gratzel and A. Hagfeldt, RSC Adv., 2013, 3, 18762–18766.
17. X. Zhang, Z. Bao, X. Tao, H. Sun, W. Chen and X. Zhou, RSC Adv., 2014, 4, 64001–64005.
18. K. Manseki, T. Ikeya, A. Tamura, T. Ban, T. Sugiura and T. Yoshida, RSC Adv., 2014, 4, 9652–9655.
19. A. N. Green, E. Palomares, S. A. Haque, J. M. Kroon and J. R. Durrant, J. Phys. Chem. B, 2005, 109, 12525–12533.
20. A. Kay and M. Grätzel, Chem. Mater., 2002, 14, 2930–2935.
21. M. Gratzel, Nature, 2001, 414, 338–344.
22. S. Gubbala, V. Chakrapani, V. Kumar and M. K. Sunkara, Adv. Funct. Mater., 2008, 18, 2411–2418.
23. M. K. I. Senevirathna, P. K. D. D. P. Pitigala, E. V. A. Premalal, K. Tennakone, G. R. A. Kumara and A. Konno, Sol. Energy Mater. Sol. Cells, 2007, 91, 544–547.
24. H. J. Snaith and C. Ducati, Nano Lett., 2010, 10, 1259–1265.
25. J.-H. Im, I.-H. Jang, N. Pellet, M. Grätzel and N.-G. Park, Nat. Nanotechnol., 2014, 9, 927–932.
26. F. Huang, Y. Dkhissi, W. Huang, M. Xiao, I. Benesperi, S. Rubanov, Y. Zhu, X. Lin, L. Jiang, Y. Zhou, A. Gray-Weale, J. Etheridge, C. R. McNeill, R. A. Caruso, U. Bach, L. Spiccia and Y.-B. Cheng, Nano Energy, 2014, 10, 10–18.
27. G. E. Eperon, V. M. Burlakov, P. Docampo, A. Goriely and H. J. Snaith, Adv. Funct. Mater., 2014, 24, 151–157.
28. J.-H. Im, H.-S. Kim and N.-G. Park, APL Mater., 2014, 2, 081510.
29. E. M. Barea, M. Shalom, S. Giménez, I. Hod, I. Mora-Seró, A. Zaban and J. Bisquert, J. Am. Chem. Soc., 2010, 132, 6834–6839.
30. V. González-Pedro, X. Xu, I. Mora-Seró and J. Bisquert, ACS Nano, 2010, 4, 5783–5790.
31. M. A. Hossain, J. R. Jennings, Z. Y. Koh and Q. Wang, ACS Nano, 2011, 5, 3172–3181.
32. L. H. Hu, S. Y. Dai, J. Weng, S. F. Xiao, Y. F. Sui, Y. Huang, S. H. Chen, F. T. Kong, X. Pan and L. Y. Liang, J. Phys. Chem. B, 2007, 111, 358–362.

---