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Abstract
In this work, TiO2 nanotubes were assembled in the hole-conductor-free, carbon counter electrode-based (CH3NH3)PbI3 perovskite solar cells based on the TiO2 nanotube/TiO2 nanoparticle hybrid photoanode under ambient conditions, on which TiO2 nanotubes were uniformly embedded in the hybrid photoanode, and their structure was not affected by the assembly process. Compared with cells based on TiO2 nanoparticles, the cells exhibited a power conversion efficiency of 9.16% that was improved by 40% under similar conditions. The optimized device structure provided effective transport pathways for carrier and carrier recombination was effectively suppressed. In addition, the TiO2 hybrid photoanode ensured efficient light trapping by the sensitizers, leading to an increase in the number of photogenerated carriers.
© 2017 Optical Society of America
1. Introduction
It was reported recently that organic-inorganic hybrid perovskite solar cells (PSCs) would finally move out of the laboratory and undergo commercialization in 2017 [1]. Since 2009 when perovskite materials were first employed as light sensitizers in solar cells, the power conversion efficiency (PCE) of these devices has been increased from 3.8% to 22.1% [2–12]. Organic-inorganic PSCs have attracted a lot of attention because of their numerous desirable characteristics, including their low cost, simple structure, tunable properties, and superior photoelectrical performance, such as a high quantum efficiency and short-circuit current density [13–15]. Perovskite-based materials exhibit the appropriate band gap, have a long carrier diffusion length, and show high carrier mobility and a high light-harvesting coefficient [16–18]. Furthermore, they can also transfer electrons and holes, which provide the possibility for the development of PSCs [19–21].
To increase the efficiency of charge separation and collection, PSCs are formed using a sandwiched structure, with the an optical absorption layer (I) being placed between the electron-transport layer (N) and the hole-transport layer (P). As mentioned above, the perovskite material can transfer electrons and holes. Therefore, the development of a hole-conductor-free structure is important to the PSCs [22–25]; accordingly, the electron-transport layer is of significant importance. Generally, TiO2 nanoparticles are used in the electron-transport layer and the electron-transfer process plays an essential role in determining the device performance [26–28]. In conventional PSCs, TiO2 nanoparticles not only increase the length of the transmission path for the carriers but also increase their recombination rate, thus hindering improvements in cell performance. In contrast to TiO2 nanoparticles, TiO2 nanotubes can provide a one-dimensional transmission channel for the charge carriers; this not only greatly reduces the recombination rate of the carriers, but also provides a channel for fast carrier transport. In addition, it has been shown that TiO2 nanotubes also promote visible-light absorption, which also greatly improves device performance [29–37].
TiO2 nanotubes are widely used in dye-sensitized solar cells but have rarely been employed in PSCs. One reason for this is the length mismatch between the TiO2 nanotubes and the PSC structure. Another reason is the fact that it is difficult to assemble TiO2 nanotubes successfully within PSCs. In 2014, Wang et al. formed PSCs based on Ti sheets with TiO2 nanotube arrays; the devices exhibited an efficiency of 8.31% [38]. Furthermore, Qin et al. prepared TiO2 nanotubes by sputtering on a fluorine-doped tin oxide (FTO) substrate and used it to fabricate PSCs, which showed an efficiency of 14.8% [39]. Similarly, in 2017, Gao et al. transformed an anodic TiO2 nanotube film into a TiO2 nanotube network film and used it in PSCs, which showed an efficiency of 13.8% [40]. At present, there have been little research on this topic in PSCs.
In this work, we report a (CH3NH3)PbI3 PSC based on a transparent TiO2 nanotubes/TiO2 nanoparticles hybrid photoanode. By using the TiO2 hybrid photoanode, TiO2 nanotubes could be successfully included in the PSC and the structure of the nanotubes was not damaged. To the best of our knowledge, this is the first report on the use of a hybrid photoanode containing TiO2 nanotubes/TiO2 nanoparticles to fabricate PSCs. The carrier recombination rate influences the device performance, as it determines the optoelectronic properties. In the case of our proposed structure, carrier recombination is effectively suppressed and the charge-collection efficiency is increased. Moreover, the TiO2 nanotubes can enhance the ability of the light scattering to promote the light trapping and absorption by the sensitizers, while the TiO2 nanoparticles in the hybrid photoanode acted as a role of adhesive which cannot only effectively improve the filling factor of cells but also increase the light scattering ability of devices. Finally, with the hole-conductor-free and carbon counter electrode, the PCE of the device was 9.16% response to AM1.5 radiation (100 mW/cm2), exceeding that of a device based on TiO2 nanoparticles (6.56%) and a device based on TiO2 nanotubes (7.63%). Further, the highest open-circuit voltage and short-circuit current of the PSC based on the hybrid photoanode were 0.9 V and 19.85 mA/cm2, respectively. These results indicate that the hybrid photoanode was highly effective in improving cell performance.
2. Experimental
2.1 Fabrication of films of transparent TiO2 nanotube arrays
Films of transparent TiO2 nanotube arrays were fabricated by the two-step anodization of Ti foil (0.2 mm in thickness, 99.8% purity) in an NH4F electrolyte solution [41]. Prior to the anodization process, the Ti foil was polished with sandpaper and degreased by successive sonication in acetone, ethanol, and deionized water for 15 min. It was then rinsed with deionized water and dried in the air. For the anodization process, a piece of Ag foil was used as the counter electrode. The ethylene glycol electrolyte solution used consisted of 0.5 wt% NH4F and 3 vol% deionized water. The Ti foil was anodized at 50 V for 12 h at room temperature. After anodization, the Ti substrate was rinsed with deionized water and ethanol and then annealed at 450°C for 30 min in the air. It was then re-anodized in the same solution for approximately 30 min until the film of the TiO2 nanotube arrays separated from the Ti substrate. This yielded a film of free-standing, transparent TiO2 nanotube arrays.
2.2 Synthesis of slurry for producing TiO2 hybrid photoanode consisting of TiO2 nanotubes and TiO2 nanoparticles
First, to fabricate the TiO2 hybrid photoanode, the film of the transparent TiO2 nanotube arrays was placed in a mortar and ground into a uniform powder. Then, 0.02g of this TiO2 nanotube powder and TiO2 nanoparticle slurry (0.4 g, 20 wt%) were weighed and dissolved in 0.2 g of terpineol and 2 g of ethanol. Next, ethyl cellulose (0.4 g, 10 wt%) was added to this mixture. The mixture was then magnetically stirred for 30 min and sonicated for 30 min. This step was repeated thrice to ensure that the slurry was mixed evenly.
2.3 Fabrication of perovskite solar cells
Etched FTO glass plates (8Ω/sq, Filkington) were used as the substrates in this study. The plates were pretreated by successive sonication in acetone, alcohol, and deionized water for 30 min. Next, the plates were subjected to an ultraviolet (UV) O3 treatment for 30 min. Then, the dense TiO2 precursor solution (1mL titanium diisopropoxide bis added in 19mL ethanol) was deposited on the glass substrates by spin coating. The substrates were subsequently heated at 150°C for 5 min to allow the solvent to evaporate. This step was repeated once. Next, the substrates were treated with TiCl4. For this treatment, the substrates were immersed in a 0.2 M TiCl4 solution for 30 min at 70°C, rinsed with methanol and deionized water, and dried in the air. They were then annealed in the air at 500°C for 30 min. Then, a uniform layer of the hybrid photoanode slurry was deposited on the top of the compact TiO2 film formed on the substrates by spin coating at 4000 rpm for 30 s. The substrates were then annealed in the air at 500°C for 30 min. Next, a ZrO2 spacer layer was deposited on the substrates by the same method, and the substrates were sintered at 500°C for 30 min. The perovskite (CH3NH3)PbI3 film was formed next by a one-step spin-coating method. First, the perovskite precursor solution (178 mg of (CH3NH3)I and 462 mg of PbI2 dissolved in 78 mg of dimethyl sulfoxide and 600 mg of dimethylformamide) was deposited on the substrates at 4000 rpm for 25 s. During 15 s, 1 mL of diethyl ether was dropped on this coating to remove the solvent and to ensure that the perovskite crystallized readily. The substrates were then heated at 100°C for 10 min. Finally, a carbon counter electrode was printed on the device, which was heated at 100°C for 30 min.
2.4 Device characterization and performance measurement
X-ray diffraction (XRD) analysis (Advance D8, AXS) used to characterize the purity of the crystalline phases of the samples with Cu Kα radiation. The morphologies of the as-prepared devices were measured using scanning electron microscopy (SEM, JSM-IT300, JEOL), while UV-visible (UV-vis) spectrophotometry (UV3600, Shimadzu) was performed to characterize the light-absorption spectra of the devices. The photocurrent-voltage (J-V) characteristics of the devices were measured using a solar simulator (91192−1000, Oriel) and a source meter (2400, Keithley) under simulated AM 1.5G radiation (irradiance of 100 mW/cm2). Electrochemical impedance spectroscopy (EIS) was performed using an electrochemical workstation (IM6, Zennium) under simulated AM 1.5G radiation (irradiance of 100 mW/cm2) over frequencies of 10 mHz to 2 MHz.
3. Results and discussion
A schematic showing the configuration of the fabricated devices and the energy band diagram of the devices are shown in Fig. 1. We would like to emphasize that the length of the TiO2 nanotubes in the hybrid photoanode matched the device structure well, and the photoanode layer acted not only as a scaffold for perovskite loading but also as an electron collector. The mesoporous ZrO2 layer served as a barrier layer to prevent the photoexcited electrons from reaching the back contact [42]. Moreover, the device contained a hole-conductor-free carbon counter electrode, with the (CH3NH3)PbI3 perovskite absorber layer acting as both the hole collector and the transparent electrode.
Figure 2(a) shows the XRD pattern of a TiO2 hybrid photoanode on an FTO substrate and those of (CH3NH3)PbI3 layers with different mesoporous structures formed by the one-step spin-coating method; these were fabricated in order to evaluate the crystalline quality of (CH3NH3)PbI3. It can be seen that the diffraction peaks of the hybrid photoanode correspond to the anatase phase (JCPDS Card No. 21-1272), which was formed by heating at 450°C for 30 min. A series of diffraction peaks characteristic of tetragonal (CH3NH3)PbI3 were observed at 14.1°, 19.7°, 28.5°, 31.9°, 40.6°, and 43.5°; these corresponded to the reflections of the (110), (112), (220), (310), (224), and (314) planes, respectively, and were in keeping with previous results [43]. It was interesting to note that the device based on the TiO2 hybrid photoanode exhibited higher-intensity diffraction peaks than did the similar devices based on TiO2 nanoparticles and TiO2 nanotubes alone. This indicated that the crystallinity of the perovskite was higher in the former cell.
Fig. 2 (a) XRD patterns of TiO2 hybrid photoanode grown on FTO substrate and devices with different mesoporous structures; (b-c) SEM image and cross-sectional SEM image of as-prepared highly ordered TiO2 nanotube arrays; and (d-f) SEM images of different mesoporous structures after perovskite deposition.
SEM images of the as-prepared film of vertically aligned TiO2 nanotubes arrays formed by anodization are shown in Fig. 2(b)-2(c). The pore diameter was approximately 77 nm. We believe that the porous structure of the TiO2 nanotube not only provided a pathway for rapid charge transport and facilitated efficient electron collection but also effectively suppressed electron recombination [27,28]. Figure 2(d)-2(f) show the SEM images of the cells based on TiO2 nanoparticles, TiO2 nanotubes and the TiO2 hybrid photoanode, respectively, after the deposition of the perovskite layer. It can be clearly seen the sizes of the perovskite particles formed on TiO2 nanoparticles、TiO2 nanotubes and TiO2 hybrid photoanode are approximately 250 nm, 350nm and 450 nm, respectively.
To reduce the device cost, the cell based on the TiO2 hybrid photoanode was fabricated using a hole-conductor-free carbon counter electrode. A cross-sectional SEM image of the cell is shown in Fig. 3(a). It can be seen that the thickness of the TiO2 hybrid photoanode film is ~300 nm. Further, the mesoporous scaffold layer is completely filled with the perovskite particles and the thickness of perovskite deposition is ~700nm. As a result, the boundary between the TiO2 and ZrO2 layers is difficult to identify, suggesting that there is intimate contact between the two layers, which should facilitate the transmission of electrons [44]. Figure 3(b)-3(d) show the different surface morphology structures of TiO2 nanoparticles, TiO2 nanotubes, and TiO2 hybrid photoanode film, respectively, before the deposition of the perovskite layer. It can be seen clearly that the structure of the nanotubes is intact and that TiO2 nanograins are uniformly coated on the dispersed TiO2 nanotubes in TiO2 hybrid photoanode film.
Fig. 3 (a) Cross-sectional FE-SEM image of PSC based on TiO2 hybrid photoanode; (b-d) Surface mesoporous structures of TiO2 nanoparticles, TiO2 nanotubes, and TiO2 hybrid photoanode, respectively.
Cells based on the TiO2 hybrid photoanode, TiO2 nanotubes and TiO2 nanoparticles were tested under AM 1.5G radiation (100 mW/cm2) at room temperature to evaluate the photovoltaic performance of the device. Figure 4(a) shows the current density-voltage (J-V) characteristics of the devices; the corresponding photovoltaic parameters are listed in Table 1. It can be seen that the device based on the TiO2 hybrid photoanode shows better photovoltaic performance, with a PCE being 9.16%, short-circuit photocurrent density (JSC) of 19.85 mA/cm2, open-circuit voltage (VOC) of 0.9 V, and fill factor (FF) of 0.51. In contrast, a device based on TiO2 nanoparticles exhibits relatively poorer characteristics. Its PCE was 6.56%, while its JSC was 17.20 mA/cm2, VOC was 0.83 V, and FF was 0.46. Moreover, a device based on TiO2 nanotubes showed the PCE of 7.63%. These results show that using the TiO2 hybrid photoanode increased the photocurrent density and fill factor, resulting in a higher PCE. We believe that this enhancement can be attributed to the TiO2 nanotubes, which effectively improved the light-scattering capability and provided the pathway for charge transfer, thus facilitating charge collection efficiency and fast charge transport. Meanwhile, charge recombination during the carrier transfer process was also effectively suppressed by the porous structure of TiO2 nanotubes [45]. However, the irregular surfaces of the nanotubes may affect the crystallization and deposition of perovskite. With respect to this scenario, the TiO2 nanoparticles in the TiO2 hybrid photoanode acted as an adhesive, effectively modifying the interface between the TiO2 nanotubes and the perovskite particles, resulting in the perovskite completely infiltrating the hybrid photoanode scaffold and the FF is effectively improved. This is consistent with the SEM and XRE observations. The photovoltaic parameters of the PSCs based on the TiO2 hybrid photoanode are shown in Fig. 4(b) (a total of 12 devices were tested) .
Table 1. Photovoltaic parameters of PSCs based on TiO2 nanoparticles, TiO2 nanotubes, and TiO2 hybrid photoanodea
The increased photocurrent density of the device based on the hybrid photoanode can probably be attributed to the enhancement of light absorption of perovskite in the visible region by hybrid photoanode. To confirm this, UV-vis absorption spectroscopy was performed; the results are shown in Fig. 5(a). It can be seen that the cell based on the TiO2 hybrid photoanode exhibits relatively higher absorption over the entire visible-light region than do the cells based on TiO2 nanotubes and TiO2 nanoparticles alone. This result can be ascribed to the enhancement of scattering capacity by TiO2 hybrid photoanode.
Fig. 5 (a) UV-vis absorption spectra of PSCs based on TiO2 nanotubes, TiO2 nanoparticles, and TiO2 hybrid photoanode and (b) EIS Nyquist plots of PSCs based on TiO2 nanoparticles, TiO2 hybrid photoanode, and TiO2 nanotubes.
EIS measurements were performed for frequencies of 10 mHz to 2 MHz at a bias of 0.8 V under simulated AM 1.5G radiation (irradiance of 100 mW/cm2) to understand charge transport and charge recombination in the PSCs in greater depth. The Nyquist plots of the cells based on the TiO2 hybrid photoanode, TiO2 nanotubes, and TiO2 nanoparticles are shown in Fig. 5(b). As reported in this study, the devices included a hole-conductor-free carbon counter electrode. Hence, the perovskite layer acted not only as the light absorber but also as the hole conductor. Therefore, a circuit model of the layer was constructed for analysis. The two distinct semicircles in the high-frequency region and low-frequency region represent charge transfer and charge recombination at the perovskite/counter electrode interface and the TiO2/perovskite interface, respectively [46,47]. The values of the related parameters are given in Table 2. Based on the semicircles in the high-frequency region, it can be concluded that the three devices with different structures showed similar characteristics, suggesting that the charge-transfer resistance, Rtr, values of the devices were similar. This indicates that the perovskite/counter electrode interface was an intimate one in all the devices. On the other hand, in the low-frequency region, major differences were observed in the devices. The low-frequency-region semicircle corresponding to the cell based on the TiO2 hybrid photoanode has a larger radius than that of the semicircle for the cell based on TiO2 nanoparticles. Calculation results showed that, in the case of the former, the recombination resistance, Rrec, at the TiO2/perovskite interface was higher, confirming that the TiO2 hybrid photoanode effectively suppressed carrier recombination. It is known that the Rs value affects the FF value and the RS value is inversely related to FF value.44 It was observed that the series resistance, Rs, values of the three devices were different. To elucidate the role of the TiO2 nanoparticles in the TiO2 hybrid photoanode, EIS measurements were performed on the cell based on the hybrid photoanode and the cell based on TiO2 nanotubes. The former exhibited a lower Rs value, which can be attributed to the effect of the TiO2 nanoparticles in the hybrid photoanode. This observation is in keeping with the previously discussed results.
Table 2. Resistivity values of PSCs based on TiO2 nanoparticles, TiO2 nanotubes, and TiO2 hybrid photoanodea
4. Conclusions
In conclusion, a hybrid TiO2 photoanode was used in hole-conductor-free PSCs based on a carbon counter electrode, such that the TiO2 nanotubes in the hybrid photoanode matched the device structure well in terms of size. In the fabricated cell, the TiO2 nanotubes not only greatly reduced carrier recombination but also provided a one-dimensional transmission channel for rapid carrier transport. Further, the TiO2 nanoparticles in the hybrid photoanode acted as an adhesive in improvement of the interfacial contact and facilitated the deposition of perovskite, which can also enhance the absorption of the cell to visible light. Consequently, a PCE as high as 9.16% could be achieved in the PSCs based on the hybrid TiO2 photoanode. In contrast, a device based on TiO2 nanoparticles having the same device structure only exhibited a PCE of 6.56%. This confirmed that the hybrid TiO2 photoanode significantly improved the photoelectric properties and should aid the development of PSCs.
Funding
National Natural Science Foundation of China (NSFC) (51572072 and 21402045); Fundamental Research Funds for the Central Universities under Grant WUT (2017IB017,2017IB018).
Acknowledgments
The authors thank Dr. H. F. Lv for his helpful discussion. The authors are also grateful to S. L. Zhao in Material Research and Testing Center of WHUT for her help with SEM.
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