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Nano Energy (2015) 11, 728–735

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RAPID COMMUNICATION

TiO2 nanotube arrays based flexible perovskite solar cells with transparent carbon nanotube electrode Xiaoyan Wanga,1, Zhen Lia,1, Wenjing Xuc, Sneha A. Kulkarnia, Sudip K. Batabyala, Sam Zhangd, Anyuan Caoc, Lydia Helena Wonga,b,n a

Energy Research Institute @ NTU (ERI@N), Nanyang Technological University, Techno Plaza, 50 Nanyang Drive, 637553 Singapore, Singapore b School of Materials Science and Engineering, Nanyang Technological University (NTU), Block N4.1, Nanyang Avenue, 639798 Singapore, Singapore c Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, People’s Republic of China d School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore Received 4 September 2014; received in revised form 5 November 2014; accepted 25 November 2014 Available online 9 December 2014

KEYWORDS

Abstract

Perovskite solar cell; Flexible; Titanium foil; TiO2 nanotubes; Carbon nanotubes; Anodization

A solid-state, flexible solar cell based on titanium (Ti) foil/TiO2 nanotubes (TNTs) with organic– inorganic halide perovskite absorber and transparent carbon nanotube electrode is demonstrated. TNT arrays together with an inherent blocking layer were simultaneously formed on Ti foil during one-step anodization. TNT arrays serve as deposition scaffold and electron conductor for perovskite absorber. Transparent conductive carbon nanotube network is laminated on top of perovskite and serves as hole collector as well as transparent electrode for light illumination. Under AM 1.5, 100 mW cm  2 illumination, power conversion efficiency of 8.31% has been achieved, which is among the highest for TiO2 nanotube based flexible solar cells. Interestingly, up to 100 mechanical bending cycles show little deterioration to the device performance, demonstrating good flexibility of the Ti foil based perovskite solar cells. The Ti foil based

n Corresponding author at: Energy Research Institute @ NTU (ERI@N), Nanyang Technological University, Techno Plaza, 50 Nanyang Drive, 637553 Singapore, Singapore. E-mail address: [email protected] (L.H. Wong). 1 Equal contribution to the work.

http://dx.doi.org/10.1016/j.nanoen.2014.11.042 2211-2855/& 2014 Elsevier Ltd. All rights reserved.

TiO2 nanotube arrays based flexible perovskite solar cells with transparent carbon nanotube electrode

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solid-state, flexible perovskite solar cells have great potential for applications in building photovoltaics and wearable electronic devices. & 2014 Elsevier Ltd. All rights reserved.

Introduction As a new member of the next generation photovoltaic materials, organometal halide perovskite (e.g., CH3NH3PbI3, CH3NH3PbI3 xClx) was first demonstrated for efficient solar cells in 2009 [1], and soon become the most important candidate to replace silicon, with low material cost and high efficiency. Owing to the high light absorption coefficient (1.5  104 cm  1 at 550 nm) [2] and long electron–hole diffusion length ( 100 nm for CH3NH3PbI3 [3,4] and 1 μm for CH3NH3PbI3 xClx [4]), perovskite solar cells have achieved a stunning success in a very short period in terms of efficiency, i.e., from 4% in 2009 [1] to 10% in 2012 [5,6], then 15% in 2013 [7] and 19.3% at present [8]. The rapid and significant progress in perovskite solar cell triggers vigorous research interest, as well as commercialization efforts. Flexibility is one of the development directions, which can be beneficial for both production (with roll-to-roll approaches) and applications (easy installation on buildings and integration on wearable devices). Flexible perovskite solar cells have been reported by a few groups since end of 2013 [9–14]. Polyethylene terephthalate (PET) coated by conductive indium tin oxide (ITO) was adopted as the flexible substrates [9–13]. PCBM and PEDOT:PSS were used as electron and hole transport layers, yielding efficiencies of 4.5%  9.2% [9–12]. In addition, low-temperature processed ZnO layer has also been used as electron transport layer in flexible perovskite solar cells on PET substrate [13]. The efficiency limitation of the flexible perovskite solar cells on PET substrates may lie in the high series resistance of ITO on PET substrates. In contrast, conductive metal foils possess better conductivity and mechanical robustness compared to PET substrates. Furthermore, metal substrate can tolerate high temperature treatment for sintering TiO2. Among the electron transport materials used in perovskite solar cell, TiO2 is still holding the efficiency record [8]. Meso-porous TiO2 scaffold is also essential for solving the hysteresis problem of perovskite solar cells [15]. Titanium (Ti) foils with TiO2 nanoparticles, nanotubes, or nanowires have been applied in flexible dye-sensitized solar cells (DSSCs) as photoanodes [16–23]. In particular, TiO2 nanotube (TNT) arrays can be grown on Ti foil by a facile electrochemical anodization, which is a scaleable production technique. However, flexible DSSCs with TNT arrays have shown relatively poor performance with efficiencies lower than 4% [17,19] and all these devices required liquid electrolyte, which will cause sealing difficulty in large-scale production. To replace dye absorber in DSSCs, efficient perovskite absorbers will greatly increase the light absorption. More importantly, the monolithic all-solid-state device structure for perovskite absorber will render the flexible solar cells better performance stability during deformations. Recently Gao and co-authors reported perovskite sensitized liquid solar cells with TNTs on rigid FTO glass [24]. Fiber shaped, flexible perovskite solar cells have also been demonstrated with TiO2 nanoparticle coated on

stainless steel fibers, yielding efficiency of 3.3% [14]. However, solid-state flexible perovskite solar cells based on TiO2 nanotubes have not been reported so far. In Ti foil based perovskite devices, the opaque Ti foil hinders the light absorption from the photoanode, so the device will not work using a conventional metallic counter electrode and therefore a transparent counter electrode is required. Transparent graphene [25] and carbon nanotubes (CNT) [20–23,26,27] have been successfully employed as the counter electrode in DSSC devices. Recently, transparent CNT networks had been proved to be good hole conductor for perovskite solar cells [14,28]. These works inspired our proposed flexible perovskite solar cell architecture with Ti foil as a working electrode, TNT as mesoporous layer for perovskite loading and carbon nanotubes as hole conductor and transparent electrode for light illumination. To date, it is the first attempt of Ti metal foil substrate based flexible perovskite solar cell and a decent power conversion efficiency of 8.31% has been achieved.

Experimental Fabrication of TiO2 nanotube arrays Two kinds of Ti foils with different thicknesses (125 μm, 99.7% purity, Sigma-Aldrich; 25 μm, 99.98% purity, Sigma-Aldrich) were employed. Prior to anodization, Ti foils were degreased ultrasonically in acetone, ethanol and deionized (DI) water for 20 min each and dried by air stream. Highly-ordered TiO2 nanotube arrays were prepared by electrochemically anodization at 20 V for 10 min at room temperature ( 20 1C). The anodizations were carried out with a two-electrode configuration with Ti foil as the working electrode and platinum gauze as the counter electrode. The electrolyte solution was ethylene glycol (extra pure, Merck) containing 0.3 wt% ammonium fluoride (98+%, Reagent, Sigma-Aldrich) and 2 vol% DI water. After anodization, the as-anodized TNT samples were rinsed in DI water to remove the electrolyte and then dried in air. For application in solar cells, the as-grown TNTs were subjected to thermal annealing at 450 1C for 3 h to convert amorphous titania into anatase phase. For better cell performance, the TNTs were also treated in 40 mM TiCl4 aqueous solution at 70 1C for 10 h and then rinsed with ethanol and DI water.

Synthesis of carbon nanotubes CNT network films were synthesized using the floating catalyst chemical vapor deposition (CVD) method using a tube furnace [29]. Ferrocene (0.36 M) as catalyst and sulfur (0.036 M) as growth promotion agent were dissolved in xylene to form a uniform precursor solution. The temperature for CVD was set to 1150 1C. Then 2500 sccm Ar and 600 sccm H2 were introduced into the quartz tube as carrier

730 gas. When the temperature and gas flow stabilized, the precursor solution was injected into the quartz tube at a preheating zone of the furnace with a temperature of 180 1C. The precursor vaporized and was transported into the center zone of the furnace by the gas flow for CNT growth. CNTs grew from the floating Fe catalyst in the carrier gas flow. As shown in the Supporting materials (Fig. S1), the attained carbon nanotubes are mainly single and doublewalled carbon nanotubes with diameters between 1 and 2 nm. After growth, the CNTs formed an aerosol carried by

X. Wang et al. the gas flow to the end of the quartz tube with a temperature of 100–150 1C, where it was collected on a nickel foil. Individual nanotubes assembled into bundles with diameter of tens of nanometers and interweave to form a free standing CNT film.

Deposition of perovskite absorber Perovskite absorber was deposited on TNT arrays by a sequential method. 1 M lead iodide (PbI2) were dissolved in N,N-dimethylformamide overnight under stirring condition at 70 1C. The PbI2 solution was spin coated on TNTs at 6000 rpm for 5 s, followed by drying on a hot plate at 70 1C for 30 min. In order to convert PbI2 into CH3NH3PbI3, the PbI2 loaded TNT samples were immersed in 8 mg mL  1 CH3NH3I solution in 2-propanol for 30 min. Subsequently, the samples were rinsed with 2-propanol and then dried at 70 1C for 30 min again.

Assembly of perovskite solar cell device

Fig. 1 Schematic of solid-state perovskite solar cells based on Ti foil/TiO2 nanotubes and carbon nanotubes.

CNT films on nickel foil was lifted off by a taped substrate and transferred on the top of perovskite sensitized TNT/Ti foil [24]. Several drops of toluene were used to wet CNT film for improving the contact between CNTs and perovskite surface. After toluene vaporization, a hole transport materials, namely

Fig. 2 (a) XRD patterns of the phase structures of TiO2 nanotubes/Ti, PbI2/TiO2 nanotubes/Ti and perovskite/TiO2 nanotubes/Ti. TiO2, PbI2 and perovskite peaks are marked by black box, circle and star respectively and the rest peaks from Ti substrate; (b) cross-sectional morphology of perovskite/TiO2 nanotubes/Ti electrode; (c) tilted scanning electron image of CNT film covering partially on perovskite surface; (d) magnified top morphology of CNT covered perovskite.

TiO2 nanotube arrays based flexible perovskite solar cells with transparent carbon nanotube electrode spiro-OMeTAD (2,20 ,7,70 -tetrakis-(N,N-di-p-methoxyphenylamined) 9,90 -spirobifluorene) in chlorobenzene (120 mg mL  1) were spin coated on CNTs covered substrate with a speed of 4000 rpm for better hole collection. Prior to cell testing, both Ti foil and CNT electrodes were soldered for better electrical contact.

Film characterization and cell testing The phase structure of the TNT arrays, the deposited PbI2 and perovskite films were investigated by X-ray diffraction (XRD, Bruker-AXS D8 Advance). The cross-sectional morphology of perovskite sensitized TNTs and top views of CNT covered perovskite layers were examined by field-emission scanning electron microscope (FESEM, JSM-7600). The photocurrent density–voltage (J–V) performance of the cell devices was characterized using solar simulator (San-EI Electric, XEC-301S) under AM 1.5 with illumination from the CNT side. The illumination area was determined by the black mask with an area of 0.16 cm2 (small area testing) and 0.36 cm2 (larger area for bending testing). Incident photon to current conversion efficiency (IPCE) was determined using PVE300 (Bentham), with a dual xenon/quartz halogen light source, measured in DC mode and no bias light used.

Results and discussion The cell configuration is shown in Fig. 1. From bottom to top in sequence are Ti foil, TNT arrays loaded with perovskite absorber

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and CNT networks composite with spiro-OMeTAD. Dense TNT arrays grown on Ti foil by electrochemical anodization serve both as a scaffold for perovskite deposition and as an electron collector. CNT network acts as hole collector and transparent electrode. For better hole collection, the hole transport material spiro-OMeTAD is infiltrated in carbon nanotubes network [28]. Light comes from CNT side, as indicated by arrow in Fig. 1. Since Ti foil and CNT network are flexible materials, the integrated solar cell device is expected to show good flexibility. Highly ordered TiO2 nanotubes arrays are formed on Ti foil by electrochemical anodization [30,31]. The as-anodized TNTs are amorphous in nature. To facilitate electron transport, amorphous nanotubes are converted into anatase phase by thermal annealing at 450 1C for 3 h, as shown in the XRD patterns in Fig. 2(a). CH3NH3PbI3 perovskite are formed on TNT arrays by a sequential deposition method [7]. In the first step, PbI2 was deposited on nanotubes by spin coating, as revealed by the two peaks at 12.72 1 and 39.52 1 in the XRD patterns Fig. 2(a)). Thereafter, soaking of PbI2 loaded nanotube substrates in CH3NH3I solution and subsequent drying process at 70 1C lead to formation of CH3NH3PbI3. The characteristics XRD peaks of CH3NH3PbI3 are indicated by black stars in Fig. 2(a). It is in well agreement with previous report [32]. Cross-sectional morphology of perovskite loaded TiO2 nanotubes is presented in Fig. 2(b). The tube arrays formed on Ti foil are 300 nm in length and 60 nm in diameter. A dense layer of perovskite nanocrystals with a size of 100–400 nm completely covers the nanotubes. The flexible CNT network is transferred on top of the perovskite layer as the counter electrode, as shown in the tilted SEM image of Fig. 2(c). The CNT transfer

Fig. 3 (a) Characteristic photocurrent–voltage curves of TNT and CNT based perovskite solar cells with different Ti foil thickness and with/without TiCl4 treatment; (b) IPCE of 25-μm-thick Ti based perovskite solar cells with/without TiCl4 treatment. The table summarizes performance parameters with standard deviations calculated from different batches of devices fabricated under identical fabrication conditions, together with the attained best value shown in brackets.

732 procedure is described in previous report [28]. The CNT film is highly transparent with transmittance between 60% and 80% all over the CH3NH3PbI3 absorption wavelength range from 300 to 800 nm (see Fig. S2 in Supporting information). The CNT network is closely adhered to the perovskite by van der Waal force. From the magnified top morphology of CNT/perovskite in Fig. 2(d), it shows that the bundled CNT networks are sparse with pores for light transmittance. In order to enhance hole collection in perovskite solar cells, spiro-OMeTAD are infiltrated into CNT networks by spin coating [28]. Noticeably, there is a very thin TiO2 compact layer formed between the TNT arrays and Ti foil during anodization (Fig. 2(b)) [33–35]. The simultaneous anodic formation of the TiO2 blocking layer and nanotube scaffold has great advantages. The one-step anodization exempts the complex fabrication process of sequential depositing blocking layer and meso-porous TiO2 layers. It is also highly controllable with the ability of forming uniform coating over large area, which is desirable for large scale production. The thickness of Ti foil affects the device flexibility. Herein, two kinds of Ti foils with different thicknesses

X. Wang et al. (125 μm and 25 μm) were used in perovskite solar cell fabrication. For both thick and thin Ti foil based perovskite devices, TiCl4 treatment was employed to improve the photovoltaic performance. TiCl4 treatment has been widely used in dye-sensitized solar cells [36–38] and perovskite solar cells [5,7,28]. It fills the voids and cracks in TiO2 blocking layer and therefore decreases recombination in solar cells. Fig. 3 shows the device performances of the TNT/CNT perovskite solar cells with different Ti foil thicknesses and with/without TiCl4 treatment. The combined J–V curves are presented in Fig. 3(a) and the corresponding photovoltaic parameters are summarized in the table. Solar cells fabricated on 25 μm Ti foil exhibited higher photovoltage (0.83 vs. 0.70 V for non-TiCl4-treated tubes; 0.99 vs. 0.78 V for TiCl4-treated tubes) and improved fill factor (0.63 vs. 0.62 for non-TiCl4-treated tubes; 0.68 vs. 0.62 for TiCl4treated tubes), in comparison to 125 μm Ti foil based solar cells. The Voc and fill factor improvement may be ascribed to the smaller surface roughness of the thinner Ti foil, as shown in Fig. S3 of the Supporting information. It is presumable that the smoother Ti surface improves the

Fig. 4 (a) Photograph of Ti foil/TNT and CNT based flexible perovskite solar cells; (b) combined J–V curves of flexible device with different bending cycles; (c) plot of photovoltaic parameters as a function of bending cycles.

TiO2 nanotube arrays based flexible perovskite solar cells with transparent carbon nanotube electrode flatness of perovskite layer and reduces the unfavorable current shunting, thus in return increases the Voc and fill factor. As evident in Fig. 3(a), TiCl4 treatment after anodization is beneficial to enhance all photovoltaic parameters, including photocurrent, Voc, fill factor and thus power conversion efficiency. Notably, Voc is greatly improved after TiCl4 treatment (0.78 vs. 0.70 V for 125-μm-thick Ti; 0.99 vs. 0.83 V for 25-μm-thick Ti). It can be ascribed to the reduction of recombination sites by the newly formed nanoparticles from TiCl4 treatment. The slight improvement of photocurrent can be also shown from the IPCE of perovskite solar cells with/without TiCl4 treatment, as displayed in Fig. 3(b). From 400 nm to 700 nm, TiCl4 treated TNTs exhibit higher IPCE, which indicates a higher charge separation efficiency thus results in photocurrent increase. Noticeably, the IPCE is relatively low at wavelength between 300 and 400 nm due to the strong light absorption of spiro-OMeTAD in this wavelength region (Fig. S2 in the Supporting information). Further improvement of photocurrent can be expected by adapting HTM materials with better transparency. For the 25-μm-thick Ti foils, the best perovskite solar cells with TiCl4 treatment yield efficiency of 8.31%. The obtained efficiency is among the highest reported for flexible perovskite solar cells [10–13]. Besides photovoltaic performance, tolerance to mechanical bending is another important factor of consideration for flexible perovskite solar cells. A photograph of flexible TNT/ CNT perovskite solar cell is shown in Fig. 4(a). The 25-μmthick Ti foil based devices were used to investigate solar cell flexibility. The solar cell with length of 2.5 cm was bended to a bending radius of 0.75 cm by mechanical force up to 100 cycles. The dependence of the device performances on bending cycles is presented in Fig. 4(b and c). J–V curves in Fig. 4(b) show that the photocurrent remains identical and the photovoltage is slightly decreased through the bending tests. As the J–V curve shape moves inwards with bending, as indicated by the arrow, the most affected device parameter after bending is the fill factor. During repeating bending, micro-sized cracks and delamination could be generated at the interfaces between different layers of the solar cells, which would deteriorate the interface and increase the series resistance of solar cells. The decrease of fill factor could be a result of the increased series resistance in solar cells after bending. The photovoltaic parameters with bending cycles are plotted in Fig. 4(c). After 100 bending cycles, the photocurrents are just slightly decreased from 9.56 to 9.37 mA cm  2 and photovoltage is reduced from 0.98 V to 0.95 V. The fill factor is mostly affected, from 0.64 to 0.57, with 11% decrease. The efficiency is thus decreased from 6.01% to 5.06%. The results show that the mechanical bending does not significantly affect the cell performance. Ti foil/TNT/CNT based perovskite solar cells maintain good performance after 100 bending cycles, demonstrating their high flexibility.

Conclusions In summary, flexible, solid-state perovskite solar cells based on Ti foil/TNTs and CNTs have been demonstrated. To our best knowledge, it is the first demonstration of flexible

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perovskite solar cells on Ti metal foil substrate. The Ti foil/ TNTs act as scaffold for perovskite loading and electron transport layer, while the transparent CNT top electrode acts as hole collecting layer and light transmission. With 25 μm Ti foil and TiCl4 treatment to TiO2 nanotube arrays, power conversion efficiency up to 8.31% has been achieved. The solar cells on Ti foil maintain good performance after 100 mechanical bending cycles, indicating their excellent flexibility. Considering the high efficiency, good flexibility and simple fabrication technique, Ti foil/TNTs based flexible perovskite solar cells holds a promising future for roof-top photovoltaics and power sources for wearable devices.

Acknowledgement Funding from National Research Foundation (NRF), Singapore, is acknowledged through CRP Award no. NRF-CRP42008-03 and the Singapore-Berkeley Research Initiative for Sustainable Energy (SinBeRISE) CREATE program. X. Wang wishes to thank the support from the World Future Foundation (WFF) as a recipient for the 2014 WFF PhD Prize.

Appendix A.

Supporting information

Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.nanoen.2014.11.042.

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Dr. Xiaoyan Wang received her Bachelor and Master degree in Materials Science and Engineering from BeiHang University, Beijing, in 2006 and 2009, respectively. In February 2014, she received her PhD degree from Nanyang Technological University, Singapore. She won the 2014 “World Future Foundation” outstanding Ph.D thesis prize. She is currently a research fellow in Energy Research Institute @ NTU (ERI@N). She is now working on CIGS thin film solar cell and organic–inorganic perovskite solar cells.

Dr. Zhen Li received his B.S. and Ph.D. degree in Material Science and Engineering from Tsinghua University, China in 2008 and 2013. Currently, He is research fellow in Energy Research Institute (ERI@N), Nanyang Technological University. His research interest includes perovksite-based solar cells and synthesis and photovoltaic applications of carbon nanomaterials (carbon nanotubes, graphene etc.) Miss. Wenjing Xu received her B.S. degree in School of Physics And Engineering from Zhengzhou University, China in 2013. She is currently a Ph. D. student, majoring in Material Science and Engineering in College of Engineering in Peking University. Her research interest focuses on fabrication and characterization of carbon-based thin-film solar-cell for efficient energy conversion.

Dr. Sneha Avinash Kulkarni received her Ph.D. in Physical and Materials Chemistry from National Chemical Laboratory (NCL), University of Pune, India in 2008. She worked as a Postdoctoral Fellow in National University of Singapore (NUS) from 2008– 2011. Presently, she is working as a Senior Research Fellow in Energy Research Institute (ERI@N), Nanyang Technological University. Her research interests focus in synthesis and application of nano materials for energy harvesting and storage. Her current research is involved in fabrication of the perovskite based solar cell. Dr. Sudip Kumar Batabyal obtained his PhD degree in physics from Indian Association for the Cultivation of Science (Jadavpur University), India in 2007. After completing the Postdoctoral Research work in National University of Singapore and in Nanyang Technological University he joined as a senior scientist in Energy Research Institute in NTU (ERI@N). His research work is in synthesis and application of nanostructured materials for energy harvesting and storage. Solution processing of inorganic and hybrid materials for device fabrication is of his special interest. His research focus is on the development of absorber materials and electrode materials for photovoltaic’s devices. Prof. Sam Zhang received Ph.D. degree in Ceramics in 1991 from The University of Wisconsin-Madison, USA and is a tenured full professor (since 2006) at School of Mechanical and Aerospace Engineering, NTU. He serves as Editor-in-Chief for Nanoscience and Nanotechnology Letters and Principal Editor for Journal of Materials Research (USA). He has been in processing and characterization of nanocomposite thin films and coatings for more than 20 years and has published more than 280 peer reviewed international papers, 12 books, 20 book chapters and guest-edited more than 10 Journal volumes. His papers have been cited 4532 times and his h-index is 37.

TiO2 nanotube arrays based flexible perovskite solar cells with transparent carbon nanotube electrode Prof. Anyuan Cao received his PhD degree in Mechanical Engineering from Tsinghua University. He has spent 3 years in Rensselaer Polytechnic Institute as a postdoc researcher, and 3 years in the University of Hawaii at Manoa as an assistant professor. He is currently a professor in the Department of Materials Science and Engineering, College of Engineering, Peking University. His research areas include controlled synthesis of macroscopic structures based on carbon nanotubes and graphene, selfassembly, nanocomposites, nanoelectronics, energy and environmental applications. He has published over 100 peer-reviewed journal papers.

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Prof. Lydia Helena Wong received B. Appl. Sci. with Honors and Ph.D. in Materials Science and Engineering from NTU. Afterwards, she worked as a Senior Engineer at the Technology Development Department of Chartered Semiconductor Manufacturing (Global Foundries) and was a Visiting Scientist at Stanford University developing organic photovoltaic materials at the Department of Chemical Engineering. She is currently an Assistant Professor at the School of Materials Science and Engineering, NTU. Her research group currently focuses on the investigation of non-toxic and abundant metal oxides and chalcopyrite materials for solar harvesting applications.