Controllable Preparation of Rutile TiO2

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Controllable Preparation of Rutile TiO2 Nanorods Array for Enhanced Photovoltaic Performance of Perovskite Solar Cells Shufang Wu, Chi Chen, Jinming Wang, Jiangrong Xiao, and Tianyou Peng ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00106 • Publication Date (Web): 26 Mar 2018 Downloaded from http://pubs.acs.org on March 26, 2018

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ACS Applied Energy Materials

Controllable Preparation of Rutile TiO2 Nanorods Array for Enhanced Photovoltaic Performance of Perovskite Solar Cells Shufang Wu,† Chi Chen,† Jinming Wang,† Jiangrong Xiao,*‡ and Tianyou Peng*† † ‡

College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, P. R. China College of Urban Construction, Wuchang Shouyi University, Wuhan 430070, P. R. China

ABSTRACT: Vertically oriented rutile TiO2 nanorods (NRs) array as an efficient electron transport layer (ETL) has been used in the perovskite solar cells (PSCs), and its microstructure has a great impact on the corresponding photovoltaic conversion efficiency (PCE). Here we employ a facile control strategy to modulate the microstructures of rutile TiO2 NRs arrays hydrothermally grown on the fluorine tin oxide (FTO) glass from a water–HCl solution of titanium n-butoxide (TBOT). It was found that introducing commercial TiO2 nanoparticles (P25, Degussa) into the hydrothermal reaction system can efficiently slow down the growth rate of rutile TiO2 NRs, and thus causing the controllable preparation of NRs array on the FTO substrate. The device fabricated with an optimized NRs array derived from the hydrothermal reaction solution containing P25 exhibits an improvement of 26.5% in PCE compared with the device fabricated with the NRs array from the hydrothermal solution without P25, which is mainly attributed to the reduced charge recombination and the enhanced fill factor stemming from the better contact at the NRs array/perovskite interface. This successful finding demonstrates that the introduction of TiO2 nanoparticles into the hydrothermal reaction solution of TBOT slows down the growth rate and the electron recombination process of the rutile TiO2 NRs array, and thus acts as a facile control strategy for improving the photovoltaic performance of the rutile TiO2 NRs array film-based PSCs. KEYWORDS: rutile TiO2 nanorod array, controllable preparation, electron transporting layer, perovskite solar cell, photovoltaic conversion efficiency

1. INTRODUCTION Perovskite solar cells (PSCs), employing organometal trihalide perovskite materials with the composition of ABX3 (A = Cs+, CH3NH3+ or NH=CHNH3+; B = Pb2+ or Sn2+; C = Cl-, Br- or I-) as light-harvesting layer, have attracted worldwide research interest due to their excellent photoelectric properties and unprecedented increase in the photovoltaic conversion efficiency (PCE).1-11 In the past few years, the PCE of PSCs has boosted from the first report of 3.8% by Miyasaka’s group in 2009 to a certified record of 22.1%, comparable with the commercial Si-based devices and thin-film ones.12-15 Although various device architectures have been applied in PSCs, the dominant ones are still the mesoporous-typed and planar-typed structures.7 For the both device structures, the electrontransport layer (ETL) is an indispensable component to ensure high performance since it can effectively interfacial contact with the perovskite layer to selectively extract and transport the photogenerated electrons.12-15 Up till now, many efforts have been made to explore efficient ETL materials such as TiO2, SnO2, ZnO, BaSnO3 and In2S3.16-21 Among which, mesoporous film made of TiO2 nanoparticles is the most widely used ETL and the holder of higher PCE.7,12-16 Nevertheless, the random distribution of

TiO2 nanoparticles leads to a random walk for electrons transporting through the ETL and the difficulty for perovskite to fill the unsteady pores, both of them will increase the probability of charge recombination.16 To overcome this issue, one-dimensional nanorods or nanotubes could be the desirable choice since one-dimensional ETL materials can provide a direct pathway for the electron transport and improve the infiltration of perovskite in the ETL due to the aligned openpore structures, which would be a benefit to the enhancement of interfacial contact of the ETL/perovskite layers for improving the electron extraction.10,18,22 Compared to the reported nanorods (NRs) materials such as ZnO, WO3 and CdS, more attention has been paid to the rutile TiO2 NRs arrays in the field of PSCs due to its tunable morphology, ease of synthesis on the conductive substrate, and better chemical stability.10,23-26 It has been proved that the microstructures (such as length, diameter and packing density) of rutile TiO2 NRs array play important role in the photovoltaic performance of PSCs,23-26 and thus various modification strategies have been developed to the solvothermal, hydrothermal and chemical bath deposition methods so as to modulate the microstructure and morphology of rutile TiO2 NRs array.10,27-30 For instance, rutile TiO2 NRs arrays with controllable length-to-diameter ratio and wideopen spaces have been solvothermally synthesized by using 2-

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butanone replacing water, and the relative PSC fabricated with 900 nm NRs array film achieved an optimum PCE of 11.7%.27 Mao’s group has compared the effects of ethanol and water on the rutile TiO2 NRs’ morphology, and an optimum PCE of 11.8% was achieved using TiO2 NRs array derived from the water–HCl solution containing titanium n-butoxide.28 By adding different organic acids into a modified ketone-HCl hydrothermal system, Chen’s group has modulated the rutile TiO2 NRs array, and demonstrated that a high PCE (18.22%) under reverse scan direction achieved from the TiO2 NRs with optimum interfacial contact.29 Also, hollow and split nanorods/nanoflowers of rutile TiO2 have been synthesized using hydrothermal and chemical etching processes, and the optimized PCE of 15.87% was obtained from the etched TiO2 with a split structure.30 The above investigations demonstrated that the optimization of rutile TiO2 NRs’ microstructure provides an efficient approach for improving the photovoltaic performance of PSCs, and those rutile TiO2 NR arrays achieved greater potential for its fewer grain boundaries, more open structure and faster electron extraction compared with the mesoporous TiO2 film.10,27-30 Nevertheless, the interfacial contact of the rutile TiO2 NRs array/perovskite layers is still the most important limiting factor for highly efficient NRs film-based PSCs,29 and thus it is necessary to produce an ideal NRs array with more excellent microstructures so as to provide optimum interfacial contact, wide-open spaces and large surface area. Herein, we report a simple method to retard the rapid growth of rutile TiO2 NRs hydrothermally grown on the FTO substrate via adding commercial TiO2 nanoparticles (P25, Degussa) into water–HCl solution of titanium n-butoxide (TBOT). A relative slower NRs growth rate due to the addition of TiO2 nanoparticles can promote the formation of NRs array with well length-to-diameter ratio and wide-open space, and there is no need an additional TiO2 compact layer pre-deposited on the FTO layer. The MAPbI3-xClx-based device fabricated with an optimized NRs array derived from the hydrothermal solution containing P25 exhibits an improvement of 26.5% in PCE compared with the device fabricated with the NRs array from the hydrothermal solution without P25, and the PCE of the optimized NRs array filmbased PSC can be further increased to 14.3% by the traditional TiCl4 treatment. To the best of our knowledge, there are few reports on the controllable preparation of NRs array by adding TiO2 nanoparticles into the hydrothermal solution of TBOT. The present results demonstrate a facile control strategy for the growth of optimized rutile TiO2 NRs array as promising ETL for highly efficient rutile TiO2 NRs film-based PSCs.

2. EXPERIMENTAL SECTION Materials. Commercial TiO2 nanoparticles (P25) were obtained from Degussa. Lead chloride (PbCl2) was purchased from Heptachroma Co, Ltd. Spiro-OMeTAD (2,2’,7,7’tetrakis-(N,N-di-p-methoxyphenylamine)-9,9’spirobifluorene) was purchased from Shenzhen Feiming Sci & Tech Co, Ltd. Most other chemicals are analytical grade (Sinopharm Chemical Reagent Corporation) and used as received unless noted otherwise. Before being used, FTO glass was cleaned with detergent, rinsed with water several times, and then treated with ultrasonication in acetone, 2-propanol

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and ethanol successively for 15 min. After that, FTO glass was dried with clean dry air, and then subjected to O3/ultraviolet treatment for 30 min. Rutile TiO2 Nanorods Array Preparation. Rutile TiO2 nanorods (NRs) array film was directly grown on the FTO substrate via a hydrothermal process. Typically, titanium nbutoxide (TBOT) was added into a water-HCl mixed solution containing 37% HCl solution (15 mL) and water (15 mL) with or without addition of P25. After magnetically stirring for 30 min, the mixed solution was transferred into a Teflon-lined autoclave (50 mL) with the cleaned FTO substrate immersed in the reaction solution and the conductive coating facing down, and then the hydrothermal process was performed at 150 oC for 1 to 4 h in a laboratory oven. After that, the resultant film was taken out and rinsed with water and ethanol several times, and then dried in air and annealed at 500 oC for 30 min. To optimize the microstructures of rutile TiO2 NRs arrays, three series of products were grown on the FTO substrate through 150 °C hydrothermally treating 0.5 mL of TBOT, 1.0 mL of TBOT and the mixture (1.0 mL of TBOT + 0.5 g of P25) in the water-HCl solution, which are denoted as NRs-1, NRs-2 and NRs-3, respectively. Perovskite Precursor Solution Preparation. Methylammonium iodide (MAI) was prepared according to the previous report.2 Typically, 24 mL of methylamine (CH3NH2) ethanol solution (27~32 wt%) was reacted with 10 mL of HI solution (57 wt%) in anhydrous ethanol (100 mL) at 0 oC for 2 h under stirring continuously. The solvent and the excess CH3NH2 were removed via an evaporation procedure. The resulted precipitate was then dissolved in ethanol followed by re-crystallized in diethylether under magnetically stirring for 30 min. This process was repeated three times and then dried at 60 oC in a vacuum oven overnight. The resultant MAI and PbCl2 at a 3:1 molar ratio were dissolved in anhydrous N,Ndimethylformamide (DMF) to obtain the perovskite (MAPbI3xClx) precursor solution with a final concentration of 40 wt%, which was filtered through 0.22 µm PVDF filter before the film deposition process. Device Fabrication. Before the preparation of perovskite layer, the rutile TiO2 NRs film was treated with O3/ultraviolet for 15 min and subsequently pre-heated at 60 oC on a hotplate. The perovskite layer was deposited on the NRs film by spinning the above MAPbI3-xClx precursor solution at 3000 rpm in a glovebox, and then annealed at 70 oC for 15 min, 90 o C for 10 min, and 100 oC for 1 h in sequence. After that, spiro-OMeTAD chlorobenzene solution (72.3 mg mL-1) containing 28.8 mL of 4-tert-butylpyridine (Sigma-Aldrich) and 17.5 mL of lithium bis(trifluoromethyl sulphonyl)imide (J & K Chemical) acetonitrile solution (520 mg mL-1) as additives was spin-coated to prepare the hole transport material (HTM) layer. Finally, a thin Au layer was deposited on the HTM layer through a thermal evaporation process. Material Characterization. Crystal phase analyses were based on the X-ray diffraction (XRD) measurements using a Miniflex 600 X-ray diffractometer with Cu Kα irradiation (λ = 0.154056 nm) at 40 kV and 15 mA and a scan rate of 5o min-1 in the range of 10o ≤ 2θ ≤ 70o. The microstructure and morphology of the film were investigated using a field emission scanning electron microscope (FESEM, Zeiss Sigma), and the high-resolution transmission electron microscopy (HRTEM) was performed at 200 kV on a JEM2100(HR) ﬁeld-emission electron microscope equipped with


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an ultrahigh-resolution pole piece (JEOL). UV-vis absorption spectra were measured on a Shimadzu UV-3600 spectrophotometer under scanning scope from 400 to 850 nm. Photoluminescence (PL) spectra were determined at room temperature by using a fluorescence spectrophotometer (FP6500, Jasco, Japan). Photoelectrochemical Measurement. The devices were illuminated using a 300-W solar simulator (Newport, 91160) with a power density of 100 mW cm-2 (AM1.5). The photocurrent-voltage (J-V) curves of perovskite solar cells were collected using a Computer-controlled Keithley 2400 sourcemeter. The active area (0.09 cm-2) is defined with a shadow mask. The light intensity was determined using a reference monocrystalline Si cell (Oriel, U.S.). The incident photon-to-electron conversion efficiency (IPCE) was recorded in a QE-R 3011 system (Enli Technology Co. Ltd. China). The electrochemical impedance spectroscopy (EIS) measurements were conducted in the dark at a bias of 0.6 V with a frequency range of 0.1-105 Hz and an AC amplitude of 10 mV. For the photoinduced open-circuit voltage decay (OCVD) measurements, the illumination was turned off using a shutter after the device was first illuminated to a steady voltage, and then the OCVD curve was recorded using a CHI-604C electrochemical analyzer.

3. RESULTS AND DISCUSSION To explore an optimized microstructure and morphology of the rutile TiO2 nanorods (NRs) arrays for an efficient electron transport layer (ETL), three series of products were grown on the FTO substrate through 150 °C hydrothermally treating 0.5 mL of TBOT, 1.0 mL of TBOT and the mixture (1.0 mL of TBOT + 0.5 g of P25) in a water-HCl solution for different times, which are denoted as NRs-1, NRs-2 and NRs-3, respectively. Figure 1 depicts the X-ray diffraction (XRD) patterns of the representative NRs-1, NRs-2 and NRs-3 products derived from hydrothermal treatment at 150 °C for 3 h. Except for the diffraction peaks of FTO layer on the conductive substrate, all products exhibit obvious diffraction peaks at 2θ = 36.1°, 41.2°, 54.3° and 62.7°, corresponding well to the (101), (111), (211) and (002) planes of the tetragonal rutile TiO2 (JCPDS, 21-1276).29 The (002)/(101) peak intensity ratio of NRs-3 is obviously higher than that of NRs-1 and NRs-2, implying the rutile TiO2 NRs in NRs-3 are more preferentially grown along the [001] direction.10,29 Figure 2 show the top-view and the corresponding crosssection FESEM images of those products. As for the NRs-1 series of products, only sparse nanorods (NRs) with diameter of ∼10 nm are grown on the FTO substrate after 1 h hydrothermal reaction (Figure 2a), and vertically aligned rutile TiO2 NRs array with obvious enhanced diameter (~78 nm) and length (∼750 nm) are covered on the FTO layer with prolonging the hydrothermal time to 2 h (Figure 2b). After hydrothermal treatment for 3 and 4 h, the NRs arrays become more uniform and dense with a tight arrangement and significant increases in length and diameter, which cause an obvious reduction in the inter-rod spaces (Figure 2c and d). The NRs-2 series of products derived from a higher TBOT content show a faster formation process of NRs as shown in

Figure 1. XRD patterns of the representative NRs-1, NRs-2 and NRs-3 products derived from hydrothermal treatment at 150 °C for 3 h.

Figure 2e-h. The NRs-2 product derived from 1 h hydrothermal reaction is already vertically aligned rutile TiO2 NRs array with an average diameter of ~105 nm and length of ∼550 nm (Figure 2e). After hydrothermal reaction for 2 h, the NRs begin to fuse with adjacent ones while the lengths of NRs increase to ∼2.5 µm (Figure 2f), and then grow in a fast rate to form a denser surface morphology with obvious cracks after hydrothermal reaction for 3 and 4 h (Figure 2g and h). These fast growths of nanorods during the present hydrothermal process can be ascribed to the quick hydrolysis of TBOT as titanium precursor, which would produce numerous large seeds as nucleation sites on the FTO substrate in a short time, and then facilitate the crystallization of NRs. Nevertheless, these long and densely packed NRs provide very limited interrod spaces for the infiltration of perovskite, which then would cause poor contact between perovskite and NRs and increase the transport path of electrons, and thus increase the charge recombination probability. The rapid crystallization and growth processes during the hydrothermal process of the single TBOT in the water-HCl solution is unfavorable to obtain an ideal microstructure of rutile TiO2 NRs array, and thus it would take more attention and effort to optimize the reaction condition. For this consideration, commercial TiO2 nanoparticles (P25, Degussa) are introduced into the hydrothermal system to try to slow down the growth of rutile TiO2 NRs so as to improve the microstructure and morphology. The primary experiments by hydrothermally treating a mixture (0.5 mL of TBOT + 0.1 (or 0.5) g P25) in the water-HCl solution at 150 oC for 4 h show that no vertically aligned rutile TiO2 NRs array form on the FTO substrate as shown in Figure S1 in the Supporting Information. The mixture (0.5 mL of TBOT + 0.1 g P25) produces some irregular aggregation consisting of nanorods and nanoparticles loaded on the top of NRs array (Figure S1a), while the product derived from the mixture (0.5 mL of TBOT + 0.5 g P25) is consist of a large number of powders dispersed on the FTO surface (Figure S1b), which cannot be rinsed away by water. After wiping off those deposited powder, there is almost nothing left on the FTO surface. Obviously, the above films derived from a low TBOT content are not a suitable ETL for the PSCs, and optimization of the reaction solution components are necessary. Therefore, NRs-3 series of products are prepared by hydrothermally treating a mixture



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Figure 2. Top-view and cross-section (inset) FESEM images of NRs-1 (a-d), NRs-2 (e-h) and NRs-3 (i-l) films derived from hydrothermal treatment at 150 °C for 1 h (a, e, i), 2 h (b, f, j), 3 h(c, g, k), and 4 h (d, h, l).

Figure 3. TEM (a) and HRTEM (b) images of the NRs-3 derived from hydrothermal treatment at 150 °C for 3 h. The inset in (b) is the corresponding FFT pattern.


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Table 1. Characteristic Parameters of the NRs Films Derived from Different Reaction Conditions time (h)

1 diameter/len gth (nm) NRs-1 10/-NRs-2 105/550 60/500 NRs-3

2 diameter/len gth (nm) 78/750 230/2500 76/800

3 diameter/len gth (nm) 115/1300 --/4000 105/1100

4 diameter/len gth (nm) 200/2400 --/7500 125/1250

(1.0 mL of TBOT + 0.5 g P25), and the corresponding topview and cross-section FESEM images are shown in Figure 2i-l. For comparison, the length and diameter of nanorods in various NRs products are measured and shown in Table 1. As can be seen, the addition of 0.5 g P25 into the hydrothermal solution containing 1.0 mL of TBOT significantly decreases the growth rate of nanorods in the NRs-3 series of products, and more uniformly and vertically NRs array can be grown on the FTO substrate. For example, the length of NRs in the NRs3 product derived from 1 h hydrothermal reaction is similar to that of NRs-2 derived from 1 h reaction time but having much smaller diameter, and the length and diameter increase at a much slower speed with prolonging the reaction time (Table 1). The TEM image (Figure 3a) of NRs-3 derived from 3 h hydrothermal reaction shows those NRs have uniform microstructures, those obvious lattice fringes (with spacing of ∼0.324 nm) and the Fast Fourier transform (FFT) pattern (inset) in the HRTEM image (Figure 3b) correspond well to the (110) planes of rutile TiO2,30 demonstrating those NRs in the NRs-3 are grown along the (110) crystal planes. The above results demonstrate that a high P25 addition amount (0.5 g) in a low TBOT content (0.5 mL) can almost completely impede the growth of rutile TiO2 NRs (Figure S1b). The most likely reason for this situation is that the most of TBOT prefer to interact with P25 nanoparticles via hydrogen bonding, so very limited free TBOT left in the reaction solution can’t form enough seeds on FTO at the early stage of reaction. Furthermore, P25 nanoparticles is absorbed on the substrate through the hydrolysis of TBOT interacted with P25 nanoparticles, which further hinder the production of nucleation sites. Whereas a higher TBOT content (1.0 mL) means more free TBOT can hydrolyze to form seeds for the nucleation of NRs in the NRs-3 series of products, and thus causing the formation of uniform NRs array. In addition, the existence of P25 leads to slower growth rate of NRs as the consumption of TBOT, and then resulting in the formation of NRs-3 arrays with smaller diameter, shorter length and larger inter-rod spaces compared to NRs-1 and NRs-2 (Figure 2 and Table 1). Accordingly, a schematic diagram of the growth processes of rutile TiO2 NRs on FTO substrates without and with P25 in the reaction solution was depicted in Scheme S1, and it can be conjectured that P25 may take effect in the present system as follows: 1) interacting with TBOT through hydrogen bonding to slow down the seed formation by decreasing the hydrolysis of TBOT;31 2) absorbing on the FTO surface to hinder the seeds growing too large; 3) acting as heterogeneous nucleation sites to facilitate the reaction in the solution, which competes with the nucleation crystallization of NRs on the FTO substrate. These effects of P25 on the nucleation and growth processes of NRs are conducive to producing an ideal microstructure of NRs array on the FTO substrate, which would offer enough contact surface area and inter-spacing to ensure efficient charge separation, and thus it

can be concluded that addition of P25 is an effective strategy to control the growth rate and microstructure of NRs array, and the NRs-3 series of products would be a more suitable ETL for PSCs, which will be further discussed below. The perovskite (MAPbI3-xClx) layer as sensitizer was prepared on the rutile TiO2 NRs array through a one-step spincoating process.2,8 The XRD pattern of the MAPbI3-xClx layer prepared on the NRs film (take NRs-3 film derived from 3 h hydrothermal reaction as an example) is depicted in Figure 4a. As can be seen, strong diffraction peaks at 2θ = 14.2° and 28.5° can be assigned to the (110) and (220) crystal plane reflections of the tetragonal perovskite MAPbI3-xClx.2,3 Figure 4b show a typical cross-section EFSEM image of the device based on the NRs-3 film derived from 3 h duration time. The perovskite penetrates well into the inter-rod spaces and form an upper layer, which can avoid the contact between NRs and the hole transport layer (HTL) and guarantee sufficient light absorption. The photovoltaic performances of PSCs fabricated with NRs-1, -2 and -3 array films derived from different reaction times were recorded by current density-voltage (J-V) measurements under simulated AM 1.5G solar irradiation at 100 mW cm-2 (Figure 5). The corresponding photovoltaic performance parameters are summarized in Table S1-3 in the Supporting Information. As for the NRs-1 series of products, the PCE of the corresponding PSC first boosts from 4.2% for the NRs-1 film derived from 1 h hydrothermal treatment (denoted as NRs-1 (1 h), same as below) to 10.3% for the NRs-1 (2 h), and then declines along with the extension of the reaction time to 3 or 4 h (Table S1). The worst performance of PSC fabricated with NRs-1 (1 h) may be due to those sparse NRs on the FTO substrate (Figure 2a), which neither provide sufficient surface area to effectively separate and transport electrons, nor avoid the charge recombination stemming from the inevitable interfacial contact between FTO and sensitizer layers, and thus causing the lower photocurrent density (Jsc) and fill factor (FF).26 As for the NRs-2 series of products, the PCE of the devices decreases quickly from 10.2% to 3.1% with substantially reduced Jsc and FF, which can be due to the NRs grown rapidly upon prolonging the reaction time (Figure 2e-h). This decline trend is very similar to that of the PSCs fabricated with NRs-1 film after 2 h hydrothermal treatment due to the increased length and decreased inter-spaces. The denser NRs in NRs-2 film arising from the coalescence of larger NRs due to the longer reaction time result in the poor infiltration of perovskite, and then the reduction of loaded perovskite and worse contact between NRs and perovskite, which directly decrease the light absorption and the charge separation.23,26 Also, the longer NRs in NRs-2 film cause more probability of charge recombination during the long electron transport pathway,10,26 and then the decrease of the photovoltaic performance (Table S2). Compared with the devices fabricated with NRs-1 and NRs-2 films, the fluctuation in the photovoltaic performance parameters of the PSCs fabricated with NRs-3 films derived from different reaction times is much minor, which is consistent with slower change in the NRs microstructure of those NRs-3 films (Figure 2i-l). The highest PCE (12.9%) is achieved from the PSCs fabricated with NRs-3 (3 h) as its appropriate length and surface morphology can facilitate the loading of perovskite and the interfacial contact with NRs-3 array, which then causing the reduction of the charge recombination.29



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Figure 4. (a) XRD pattern of the perovskite layer loaded on the NRs-3 film derived from hydrothermal treatment at 150 °C for 3 h. (b) Cross-section FESEM image of the corresponding PSC.

Figure 5. Typical J-V curves of the PSCs fabricated with NRs-1 (a), NRs-2 (b), and NRs-3 (c) arrays derived from different hydrothermal times, as well as the comparison of J-V curves (d) of the best-performing PSCs fabricated with NRs-1, NRs-2, and NRs-3 film from their respective optimum growth time.

Table 2. Performance Parameters of the Best-Performing PSCs Fabricated with NRs-1, NRs-2 and NRs-3 film device Jsc (mA cm-2) NRs-1(2 h) 21.2 22.1 NRs-2(1 h) 22.4 NRs-3(3 h) 23.8 NRs-3(3 h)a a

Voc (V) 0.92 0.92 0.91 0.93

NRs-3 film with TiCl4 post-treatment.

FF (%) 52 50 63 65

PCE (%) 10.3 10.2 12.9 14.3

For comparison, the typical J-V curves of the bestperforming devices fabricated with NRs-1 (2 h), NRs-2 (1 h) and NRs-3 (3 h) film derived from their respective optimum growth time are shown in Figure 5d, and the corresponding photovoltaic performance parameters are listed in Table 2. All of the devices show a similar high Jsc, in agreement with the strong and similar light absorption of the perovskite layer on those NRs films (Figure 6a) and the similar results of incident photon-to-electron conversion efficiency (IPCE) measurement (Figure 6b). The NRs-3 (3 h) film has a longer NRs than the


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Figure 6. (a) UV-vis absorption spectra of the MAPbI3-xClx layer loaded on NRs-1, NRs-2 and NRs-3 film derived from their respective optimum growth time and (b) IPCE curves of the corresponding PSCs, respectively.

Figure 7. (a) Photoluminescence spectra of the MAPbI3-xClx layer loaded on NRs-1, NRs-2 and NRs-3 film derived from their respective optimum growth time and (b) Nyquist plots of the corresponding PSCs measured in the dark at a bias voltage of 0.6 V.

NRs-1 (2 h) or NRs-2 (1 h) as shown in Table 1, which might be the reason for its relatively lower Voc (Table 2). Previous works have demonstrated the Voc depend on the length of NRs and usually decrease with the increase of NRs’ length.23,27 Nevertheless, the device based NRs-3 (3 h) also exhibits a significantly improved FF due to the better interfacial contact between NRs array and perovskite, which is beneficial for the improvement of electron extraction and the reduction of the charge recombination.26,29 The steady-state photoluminescence (PL) spectra of MAPbI3-xClx layer is shown in Figure 7a, in which NRs-3 (3 h) shows the most obvious PL quenching effect among those films tested, indicating that this rutile TiO2 NRs film can more efficiently separate and extract the photogenerated electrons of the MAPbI3-xClx layer, which then lead to the improved photovoltaic performance of the corresponding PSC. Electrochemical impedance spectroscopy (EIS) was also conducted to investigate the charge transfer and recombination behaviors in PSCs based on various NRs ETLs. The corresponding Nyquist plots of devices are shown in Figure 7b. The dominated large semicircle in the low frequency range refers to the recombination resistance (Rrec) and capacitance.32 PSC based on NRs-3 (3 h) exhibits the largest Rrec, indicating it has the lowest charge recombination.10,32 Considering the longer length of NRs-3 (3 h) than NRs-1 (2 h) and NRs-2 (1 h), the reduced charge recombination stems from the improved interfacial contact and

electron extraction. In summary, the device fabricated with the optimized NRs-3 (3 h) derived from the hydrothermal solution containing P25 exhibits the best PCE (12.9%), improved by 26.5% as compared to that (10.2%) of the device fabricated with the NRs-2 (1 h) filmy from the hydrothermal solution without P25. It is mainly ascribed to the reduced charge recombination and the enhanced fill factor stemming from the better contact at the NRs array/perovskite interface. The above results and discussion demonstrate that the introduction of TiO2 nanoparticles into the hydrothermal solution of TBOT can slow down the growth rate and the electron recombination process of the rutile TiO2 NRs film, and thus acts as a facile control strategy for improving the photovoltaic performance of the rutile TiO2 NRs array film-based PSCs. In addition, the photovoltaic performance of the device fabricated with the optimized NRs-3 (3 h) film can be further improved through a traditional TiCl4 solution post-treatment according to the previous reports.33 As shown in Figure 8, the PCE can be enhanced to 14.3% with all photovoltaic performance parameters improved and little hysteresis phenomenon, demonstrating that the TiCl4 post-treatment can ameliorate the surface of ETL and prevent the back electron transport.33 This conjecture about the effects of TiCl4 posttreatment on the charge recombination can be confirmed by the open-circuit voltage decay (OCVD) curves of the devices fabricated with NRs-3 (3 h) (Figure S2a in the Supporting



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Supporting Information. Additional schematic diagram of the growth processes, FESEM images of some films, the OCVD and τn-Voc curves of PSCs, as well as the performance parameters (Table S1-3) of PSCs (PDF). This material is available free of charge on the ACS Publications website at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (T. P). [email protected] (J. X)

ORCID Tianyou Peng: 0000-0002-2527-7634 Figure 8. Typical J-V curves of the PSCs fabricated with NRs-3 film derived from hydrothermal treatment at 150 oC for 3 h followed by TiCl4 treatment measured under different scanning directions.

Information). As can be seen, the TiCl4 post-treatment can obviously decrease the decay trend of Voc value, indicating the effective suppression of the charge recombination.8,9 The corresponding electron lifetime (τn)-Voc curves (Figure S2b) also suggest that the device fabricated with the NRs-3 (3 h) with TiCl4 post-treatment has longer electron lifetime in the whole region of the Voc test. Therefore, TiCl4 post-treatment can further hinder the charge recombination at the NRs array/perovskite interface and increase the electron lifetime, and then improve the performance of the whole device.

The manuscript was written through contributions of all authors.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (21573166, 21271146, 20973128, 20871096), the Funds for Creative Research Groups of Hubei Province (2014CFA007), and the Natural Science Foundation of Jiangsu Province (BK20151247), China.

REFERENCES (1) Kim, H. S.; Lee, C. R.; Im, J. H.; Lee, K. B.; Moehl, T.; Marchioro, A.; Moon, S. J.; Humphry-Baker, R.; Yum, J. H.; Moser, J. E.; Gratzel, M.; Park, N. G. Lead Iodide Perovskite Sensitized All-Solid-

4. CONCLUSIONS

State Submicron Thin Film Mesoscopic Solar Cell with Efficiency

In summary, a facile control strategy is developed to modulate the microstructures of rutile TiO2 NRs arrays hydrothermally grown on the FTO substrate from a water–HCl solution of titanium n-butoxide (TBOT). By introducing commercial TiO2 nanoparticles (P25, Degussa) into the hydrothermal system, the growth rate of rutile TiO2 NRs can be efficiently slowed down since the added nanoparticles can not only slow down the formation of nucleation sites but also act as heterogeneous nucleation sites to facilitate the reaction in the solution, which competes with the nucleation crystallization of NRs on the FTO substrate. As a result, the microstructure and morphology (such as length-to-diameter ratio, surface area and inter-rod spaces) of the rutile TiO2 NRs array can be adjusted easily. The device fabricated with an optimized NRs array derived from the hydrothermal reaction solution containing P25 exhibits an improvement of 26.5% in PCE compared with the device fabricated with the NRs array from the hydrothermal solution without P25, which is mainly ascribed to the reduced charge recombination and the enhanced fill factor stemming from the better contact at the NRs array/perovskite interface. In addition, the PCE of the optimized NRs array film-based PSC can be further increased to 14.3% by the traditional TiCl4 post-treatment. The results presented here demonstrate a new and facile control strategy for the growth of optimized rutile TiO2 NRs array as promising ETL for highly efficient rutile TiO2 NRs film-based PSCs.

ASSOCIATED CONTENT

Author Contributions

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Graphic entry for the Table of Contents (TOC).


Controllable Preparation of Rutile TiO2

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