Efficient Perovskite Solar Cells Depending on TiO2 Nanorod Arrays

Aug 2, 2016 - Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University, Xiamen 361005, China. ‡ State Key Lab for Physical Che...
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Efficient Perovskite Solar Cells Depending on TiO2 Nanorod Arrays Xin Li,*,†,# Si-Min Dai,‡,# Pei Zhu,† Lin-Long Deng,† Su-Yuan Xie,*,‡ Qian Cui,† Hong Chen,† Ning Wang,*,§ and Hong Lin∥ †

Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University, Xiamen 361005, China State Key Lab for Physical Chemistry of Solid Surfaces, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China § State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China ∥ School of Material Science and Engineering, Tsinghua University, Beijing 100084, China ‡

S Supporting Information *

ABSTRACT: Perovskite solar cells (PSCs) with TiO2 materials have attracted much attention due to their high photovoltaic performance. Aligned TiO2 nanorods have long been used for potential application in highly efficient perovskite solar cells, but the previously reported efficiencies of perovskite solar cells based on TiO2 nanorod arrays were underrated. Here we show a solvothermal method based on a modified ketone−HCl system with the addition of organic acids suitable for modulation of the TiO2 nanorod array films to fabricate highly efficient perovskite solar cells. Photovoltaic measurements indicated that efficient nanorod-structured perovskite solar cells can be achieved with the length of the nanorods as long as approximately 200 nm. A record efficiency of 18.22% under the reverse scan direction has been optimized by avoiding direct contact between the TiO2 nanorods and the hole transport materials, eliminating the organic residues on the nanorod surfaces using UV−ozone treatment and tuning the nanorod array morphologies through addition of different organic acids in the solvothermal process. KEYWORDS: TiO2, nanorod array, organic lead halide perovskite, solar cell, one-dimensional material

1. INTRODUCTION Since Miyasaka’s pioneering1 work on organic lead halide perovskite absorbers for sensitized solar cells in 2009, much attention has been paid to the development of efficient nextgeneration solar cells with various organometallic halide perovskite materials.2,3 Rapid improvements have been achieved in the field especially over the last four years,4,5 with a recently announced certified efficiency of over 20%.6 Competing with traditional silicone-based solar cells and thinfilm solar cells, perovskite solar cells promise a tantalizing future based on a wide range of recently presented methods/ technologies for fabrication of various perovskite-type devices with higher photovoltaic performance and lower costs.7 Early reported perovskite solar cells were dominantly based on mesoporous metal oxide structures that have been prevalent in solid-state dye-sensitized solar cells (ssDSSCs). Although diversity in the architectures of perovskite has been increasingly © 2016 American Chemical Society

reported in the past two years, both mesoscopic perovskite solar cells and planar perovskite solar cells remain as the dominant structures.8,9 Planar perovskite solar cells have given extremely high efficiencies of over 18% recently,10 whereas the mesoscopic perovskite solar cells with the presence of a mesoporous scaffold such as TiO2-containing film have delivered even higher efficiencies.6 In the TiO2-involving devices, the mesoporous layer not only played as a scaffold providing mechanical support to the perovskite layer but also acted as an electron transporter. Initially, nanoparticles with a diameter of approximately 20 nm were declared suitable for mesoscopic perovskite solar cells, similar to those ssDSSCs.11 Recently Seok et al. reported that mesoscopic perovskite solar Received: May 19, 2016 Accepted: August 2, 2016 Published: August 2, 2016 21358

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obtained by evaporating the solutions at 50 °C for 1 h, which were then dissolved in ethanol and recrystallized using diethyl ether. The process was repeated three times, and the product was finally dried at 60 °C in a vacuum oven overnight. The MAPbI3 solution was prepared by adding CH3NH3I and PbI2 into the solvent of γ-butyrolactone and DMSO (7:3 volume ratio). The MAPbI3 solution was heated at 60 °C for 3 h with stirring. The molar concentration of MAPbI3 solution used for the comparison between planar and TiO2 nanorod devices was 1.2 M, while the molar concentration of MAPbI3 solution used for other devices in this paper was optimized at 1.3 M. Synthesis of the TiO2 NR array: The TiO2 NR array was prepared by solvothermal treatment according to a procedure described elsewhere.22 First, FTO glass (Grene, Qingdao, China) was cleaned successively with deionized water, propanol, and acetonitrile. Titanium butoxide (740 μL, TBT), 10 μL of deionized water, 10 μL of HCl (37% in water, Sinopharm, China), and 10 mL of ethanol were mixed to prepare a clear precursor sol. The precursor sol was spin-coated at 2000 rpm onto a Zn/HCl-etched FTO substrate, followed by annealing at 450 °C to form a compact TiO2 layer. The as-annealed compact layer acts not only as the seed layer for the TiO2 NR arrays but also as a hole-blocking layer for the perovskite solar cells. TiO2 NR arrays were grown on the TiO2-compact-layer-seeded FTO substrates by hydrolysis of TBT in a mixture of HCl/acetic acid/ H2O with a volume ratio of 4:2:3 in a Teflon-lined stainless steel autoclave at 200 °C for 17 min. To tune the structure of the TiO2 NR arrays, acetic acid was replaced with several organic acids having different alkane groups, i.e., butyric acid, iso-butyric acid, n-hexanoic acid, and 2-ethyl-butyric acid. The resulting NR arrays were washed repeatedly with deionized water until no excess ions were detected and then dried at 60 °C under vacuum. Fabrication of perovskite solar cells: Organo-metal halide perovskite solution was coated onto the FTO/compact TiO2 layer/TiO2 NR array substrate by two consecutive spin-coating steps of 1000 and 5000 rpm for 15 and 25 s, respectively. Toluene (1 mL) was dropped onto the substrate quickly at the 16 s mark of the second spin coating step, in accordance to a reported reference.23 The substrate was then dried on a hot plate at 100 °C for 10 min. A solution of spiro-oMeTAD (90 mg), Li-bis(trifluoromethanesulfonyl) imide (23.5 mg), acetonitrile (45 μL), and 4-tert-butylpyridine (10 μL) in chlorobenzene (1 mL) was spin-coated onto the FTO substrate/compact TiO2 layer/TiO2 NR array/perovskite layer at 3000 rpm for 30 s. Finally, a Ag metal layer (∼80 nm) was deposited by thermal evaporation. The area of the Ag electrode was fixed at 0.10 cm2. Characterization. XRD spectra of the TiO2 NR films were recorded using a Rigaku Ultima-IV X-ray diffractometer. The morphology of the films was observed using a field-emission scanning electron microscope (SEM, Hitachi S4800, Japan) and a transmission electron microscope (TEM, JEM 2100F, Japan). The J−V curves were measured using a 300 W xenon solar simulator (Newport Oriel Solar Simulators) with a source meter (Keithley 2420, USA) under the illumination of AM 1.5 G, 100 mW/ cm2, and a calibrated Si-reference cell. Unless stated otherwise, the J− V curves were measured by reverse scan (forward bias (1.2 V) to short circuit (−0.5 V)). The step voltage was fixed at 100 mV. The active area for all devices was fixed by using a mask (area of 0.06 cm2). The external quantum efficiency (EQE) was recorded by a Merlin lock-in amplifier coupled with a CS260 monochromator and a 300 W xenon lamp. The light intensity at each wavelength was calibrated with a standard single-crystal Si photovoltaic cell. For the stability test, an unencapsulated cell was kept under controlled ambient conditions at approximately 30% relative humidity, with which J−V measurements were recorded every 4 days. Electrochemical impedance spectra were measured using a CHI 660D electrochemical station (Chenhua, China). The bias potential, which was imposed upon the FTO substrate, was set from −0.75 to −1.0 V. The frequency range was 0.1 to 10 kHz. The magnitude of the alternative signal was set at 10 mV. The measuring temperature was kept at 25 °C. The Nyquist spectra were fitted with Z-View software (v310, Scribner Associate, Inc., USA) in terms of an appropriate equivalent circuit. Steady-state PL spectra were recorded on a F7000

cells based on nanoparticles with a diameter of approximately 50 nm exhibited superior photovoltaic performance.7 Due to random distribution of nanoparticles within mesoporous metal oxide films, filling the “unsteady” pore with halide metal perovskite in the films remains a persistent difficulty. By contrast, highly oriented nanorod (NR) or nanotube, with controllable aligned porous structures and opened spaces, could be a better choice for optimizing the pore filling condition of perovskites or hole transport material.12,13 Moreover, oriented one-dimensional (1D) TiO2 films have exhibited better performance with regards to electron transport and recombination behavior in DSSCs.14,15 Hence, several works on the application of highly crystalline, oriented rutile TiO2 NRs in perovskite solar cells have been reported.16−18 Park and co-workers reported an efficient solar cell based on oriented submicrometer rutile TiO2 NRs sensitized with perovskite nanodots, with an efficiency of 9.4%.19 In the structure, a spiro-oMeTAD material was easily infiltrated into perovskite-sensitized NR films, which positively influenced the short-circuit current. Later, Xu et al.20 used different lengths of rutile TiO2 nanowires (NWs) with wide-open spaces as photoanodes for perovskite solar cells to improve material filling, obtaining an optimum efficiency of 11.7% when the length of the NRs reached 900 nm. Very recently, Hong et al.21 presented highly efficient perovskite-sensitized solar cells with the highest efficiency of 13.45%, based on hydrothermally grown oriented TiO2-NR films with the surface passivated by an ultrathin atomic TiO2 layer. However, although the 1D materials exhibited advantages of controllable pore filling and uniform distribution for perovskite materials, the photovoltaic performances of perovskite solar cells incorporating 1D materials still lag far behind those using traditional mesoporous structures. Even though the TiO2 NR array exhibited greater potential for its more open structure and faster electron extraction over TiO2 mesoporus film,12−15 the most important limiting factor for achieving highly efficient TiO2-NR-based devices plausibly lies in the contact at the perovskite/TiO2-NR interface. Hence it is necessary to produce an optimized perovskite/TiO2-NR interface to enhance the photovoltaic performance of TiO2-NRbased devices. Herein, we carefully tuned the TiO2-NR arrays, including NR diameters and NR densities and surface, via a facile solvothermal method. Surface treatment with UV-ozone on TiO2 NRs was also applied to improve the interface properties of perovskite/TiO2 NRs. Better photovoltaic performance of TiO2-NR-based perovskite solar cells than that of mesoscopic perovskite solar cells has been realized. A champion efficiency of 18.22% under standard one sun illumination has been achieved, which, to the best of our knowledge, is higher than any of the previously reported perovskite solar cells based on 1D structured photoanodes.

2. EXPERIMENTAL DETAILS Materials. Spiro-oMeTAD was purchased from Lumtec (Taiwan). Methylamine solution (40 wt % in methanol), hydroiodic acid (57 wt % in H2O), and lead(II) iodide (99.999%) were purchased from AlfaAesar. Chlorobenzene (99.8%) and acetonitrile (99.8%) were received from Sigma-Aldrich. Other reagents and chemicals were purchased from Alfa-Aesar or Sigma-Aldrich Co. and used as received without further purification unless otherwise noted. Synthesis of the organo-metal halide perovskite: CH3NH3I was synthesized by reacting 30 mL of hydroiodic acid (57% in water) and 27.86 mL of CH3NH2 (40% in methanol) in a 250 mL roundbottomed flask at 0 °C for 2 h with stirring.18 The precipitates were 21359

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Figure 1. Surface view of oriented TiO2 NRs, grown from different precursors containing (a) acetic acid, (b) butyric acid, (c) iso-butyric acid, (d) nhexanoic acid, and (e) 2-ethyl-butyric acid. The scale bar is 2 μm. fluorescence spectrophotometer (HITACHI), excited with a Xe lamp at 550 nm. UV−vis absorption spectra were obtained on a Varian Cary 5000 UV−vis−NIR spectrophotometer. Infrared spectroscopy measurements were conducted on Nicolet 380.

glass is presented in Figure S2, where the TiO2 NR array film is almost transparent. TEM images of selected TiO2 NRs are shown in Figure S3. The TiO2 NR has a (110) interplanar distance of 0.327 nm, which indicates a rutile characteristic.22 X-ray diffraction patterns of the NR arrays grown from different precursor solutions were obtained to evaluate the structural features of the TiO2 NRs upon varying the organic acid used (Figure 2). All of the arrays exhibited typical

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RESULTS AND DISCUSSION TiO2-NR arrays can be synthesized in the solvothermal process with n-butanone, hydrochloric acid, and tetrabutyl titanate as starting materials. As shown in Feng’s previous work,22 singlecrystal rutile TiO2 NR arrays with an average length of approximately 1.6 μm and a diameter of 40 nm were obtained within 30 min in the solvothermal reaction, in which hydrochloric acid acted as strongly acidic media to slow the growth of TiO2 NRs and the Cl ions suppressed the plane growth of nanowires resulting in oriented growth by selectively adsorbing onto the (110) plane of TiO2 NRs. Accordingly, the growth of TiO2 NRs could be controllable depending on the amount of precursor and reaction time. In our present work, the starting materials in the ketone−HCl solvothermal reaction22 were modified to finely tune the TiO2-NR array film morphologies. Hydrochloric acid was partially replaced by organic acids having different alkane (CnH2n) group lengths (e.g., acetic acid (n = 2), butyric acid (n = 4), iso-butyric acid (n = 4), n-hexanoic acid (n = 6), and 2-ethyl-butyric acid (n = 6)). Figure 1 shows the plain-view SEM images of the TiO2-NR arrays with preferred [001] orientation and uniform distribution on FTO substrates vertically grown in different organic acids. In the solvothermal reaction time of 17 min, diameter and length of the TiO2 NRs were determined to be ∼20−40 nm and ∼180 nm, respectively (shown in Figure S1). The diameter and density of the NRs will change on varying the length or size of the alkane chain of the organic acid (Table S1). Supposedly the addition of organic acid into the ketone− acid precursors could replace the ligand of the TBT precursor and thereof affect the nucleation density of TiO2 NRs by changing the Ti−O−Ti linkage of the TBT/organic acid. A typical optical photograph of a TiO2 NR array growing on FTO

Figure 2. X-ray diffraction patterns of the as-synthesized TiO2 NR arrays on FTO substrates grown from different starting precursors containing (a) acetic acid, (b) butyric acid, (c) iso-butyric acid, (d) nhexanoic acid, and (e) 2-ethyl-butyric acid. All of the XRD patterns are indicative of a rutile phase.

diffraction peaks related to the (101), (211), and (002) plains that are identified as those of tetragonal rutile (JCPDS 211276). Differences concerning the relative intensities of the (101) plains are obvious in the patterns of TiO2 NRs obtained from various starting precursors, which is ascribed to the more or less oriented growth of the NRs in the preferred direction, as supported by the SEM images in Figure 1. The XRD patterns shown in Figure 1 and the TEM image of the TiO2 NR shown in Figure S2 implied that the NRs exhibit rutile crystalline 21360

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to the passivation of traps at perovskite surfaces by metal oxides. Though the NR-structured solar cells exhibited a higher photovoltaic performance, the relatively poorer FF (Figure 3) necessitates further improvement to compete with state-of-theart mesoscopic perovskite solar cells. In previous reports11−21 related to TiO2-NR array-based PSCs, the FFs were normally lower than 0.70. The tips of the TiO2 NRs were easily contacted with the hole transport material, leading to a shunt pathway that could explain the poorer FF. In our work, we also found that perovskite film could not fully cover the NR film, which may lead to a direct contact between TiO2 NRs and hole transport material. Bulovic et al.25 noted that, using 1D NR arrays to fabricate high-performance colloidal quantum dot solar cells, the uniformity of the NR array should be of particular concern to avoid increasing shunt pathways due to longer NRs or increasing recombination losses because of shorter NRs. Hence, carefulness should be taken to inhibit the direct contact between TiO2 NRs and the hole transport material (e.g., spiro-oMeTAD). To address these concerns, we kept well-crystallized perovskite with a thickness of approximately 100 nm atop the TiO2-NR array as a capping layer to eliminate TiO2/hole transport material contact, by increasing the concentration of perovskite precursor solution for making the perovskite capping layer (Figure 4a). The device with perovskite capping exhibited an impressive enhancement in photovoltaic performance, as shown in Figure 4b. An improved Jsc of 20.10 mA/cm2, Voc of 1.02 V, and FF of 0.671 (giving a PCE of 13.80%) were recorded, superior to the NR-structured device without the capping layer atop the NR array (with a Jsc of 19.3 mA/cm2, a Voc of 0.99 V, and a FF of 0.599, resulting in a PCE of 11.53%). For the capped device, obviously, improvement in PCE mainly contributes to higher FF. To scrutinize the mechanism responsible for the higher FF, both Rsh and Rs values were calculated for the devices capped or not. As extracted from the J−V curves, the Rsh and Rs of the capped device (3046 and 10.01 ohm cm2) are higher than those of the uncapped device (2210 and 6.92 ohm cm2), equating to a more efficient reduction of shunt pathways by a well-capped NR array. It is worthy of note that a thicker capping layer atop the TiO2-NR array (thickness approximately 200 nm) barely improved the photovoltaic performance. Although the capping layer effectively improved solar cell performance,26 the FF and PCE of the NR-structured device were still lower than those of conventional TiO2-nanoparticle mesoscopic solar cells. The difference may be ascribed to the

characteristics, a favorable property for application as an effective electron transport material in PSCs.22 A previous report has indicated that TiO2 used in PSCs can provide an n-type doping of the perovskite material close to the TiO2/perovskite interface, leading to an n-type/intrinsic homojunction within the perovskite layer.2,24 Hence, the use of TiO2-NR arrays may also provide an effective n-type/ intrinsic homojunction by increasing surface area of the internal TiO2-NR/perovskite interface. A comparison was made between a structure comprising FTO/TiO2 compact layer/ perovskite/spiro-oMeTAD/Ag (Planar-structured) and a structure comprising FTO/TiO2 compact layer/TiO2 NR array/ perovskite/spiro-oMeTAD/Ag (NR-structured) shown in Figure 3. A typical planar-structured device exhibited a Jsc of

Figure 3. Photocurrent density−voltage (J−V) curves of a representative NR-structured cell (circles) and a representative planar-structured cell (triangles) under AM1.5 simulated light at 100 mW/cm2 and under dark. The thickness of the perovskite layer was approximately 200 nm, as determined by SEM. Acetic acid was used to synthesize the NR array, and the mean length of the NR was approximately 180 nm.

17.5 mA/cm2, a Voc of 1.01 V, and a FF of 0.453, resulting in a PCE of 8.10%. If the TiO2 NR array was added between the TiO2 compact layer and the perovskite, the photovoltaic performance was greatly improved. A typical NR-structured device exhibited a significant increase in the Jsc compared with that of the planar-structured device, to a value of 19.9 mA/cm2, a Voc of 1.03 V, and a FF of 0.576, resulting in a PCE of 11.83%. Though the origin of the n-doping of perovskite by TiO2 remains an open question, it can be hypothesized that the aforementioned phenomenon is due to the undercoordinated iodine ions acting as shallow donors at gain boundaries or due

Figure 4. Morphological and photovoltaic properties. (a) Surface morphology image of the TiO2-NR array with good perovskite film capping. The scale bar is 200 nm. (b) J−V curves of a control NR-structured cell without a good capping layer (circles) and a representative NR-structured cell having an optimized capping layer atop the NR array (squares), under AM1.5 simulated light at 100 mW/cm2. Acetic acid was used to synthesize the NR array, and the mean length of the NR was approximately 180 nm. 21361

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of the devices were recorded in the dark at bias potentials ranging from −0.75 to −1.0 V. Figure 6a presents Nyquist plots of typical NR-based devices with or without UV−ozone treatment at a bias of −0.95 V in the dark. Interestingly, both Nyquist plots present two distinct features occurring at the high- and low-frequency zones. The patterns are quite different from traditional mesoporous TiO2-based perovskite solar cells, which always exhibit three semicircular features representing 3RC elements in typical Nyquist plots.28 Previous works have indicated that the patterns in Figure 6a normally belong to PIN-structured perovskite solar cells.29 The Nyquist plots support our hypothesis that in the present configuration the TiO2-NR array may act as an n-doping structure to effectively form an n-type/intrinsic homojunction within the perovskite layer. An equivalent circuit (shown in Figure 6b, inset) was used to fit the EIS data, and detailed information related to the interface properties was obtained. Rs denotes the series resistance from the FTO substrate, silver layer, and wires. Ccon is the contact capacitance owing to the charge buildup at the interfaces of perovskite/TiO2 and perovskite/hole-transport material. Rrec accounts for the recombination resistances both at the electronselective contact and at the hole-selective contact. Finally, the capacitance Cps and resistance Rps are responsible for the Debye dielectric relaxation of the perovskite film, as identified by Bisquert et al., normally reflecting the charge densities within the perovskite film. To evaluate the impact of UV−ozone treatment on the device performance, Rrec and Ccon were extracted from the EIS spectra, as displayed in Figure 6b. Due to the exponential dependence of carriers on the bias voltage, the carrier recombination across the interfaces should increase exponentially with increasing bias voltage and lead to an exponentially decreasing Rrec, as shown in Figure 6b. Upon UV−ozone treatment of the NR array, the recombination was effectively constrained. On the other hand, Ccon of both devices was kept almost constant, representing the Helmholtz response of the electrical double layer at the contact interfaces. Because the perovskite/spiro-oMeTAD/Ag configurations are the same for both cells, Ccon in Figure 6b can directly reflect the TiO2NR/perovskite contact. As shown in Figure 5, UV−ozone treatment can reduce the Ccon values, effectively reducing charge accumulation at the TiO2/perovskite interface as a result of surface cleaning. As shown in Figure 6, the TiO2/perovskite interface can affect the performance of the devices. It is reasonable to suppose that optimization of the total interfacial area of TiO2 NRs and perovskite by varying the NR diameters and the

NR−perovskite interface. For improving the metal oxide− perovskite interface in previously reported mesoporous TiO2 films, an annealing post-treatment or TiCl4 coating followed by an annealing post-treatment were routinely conducted to burn out the organic residues. To simplify the preparation of the devices, the NR arrays used in this research were not subjected to thermal annealing at high temperature and were washed by deionized water only. Apparently some organic residues from the hydrothermal precursor solution distributed on the NR surface. To remove these organic residues and to increase the NR wettability, UV-ozone treatment, a well-established cleaning method,27 was adopted to modify the NR surface. As shown in the infrared spectra inside Figure 5, the organic

Figure 5. J−V curve of a NR-structured cell with UV−ozone treated TiO2-NR array compared with that of a control NR-structured cell under AM1.5 simulated light at 100 mW/cm2. Acetic acid was used to synthesize the NR array, and the mean length of the NR was approximately 180 nm. The perovskite capping layer was approximately 100 nm. The inset presents the infrared spectra of NR arrays before and after UV−O3 treatment.

residues on NR arrays were efficiently reduced after UV-ozone treatment, which further optimized the NR/perovskite interfaces, indicating that the effect of the UV−ozone treatment on the NR array was beneficial for improving photovoltaic performance (shown as the J−V curves in Figure 5). The surface treatment of the NRs led to an increase in PCE from 13.58% to 14.72%. The adhesion between the NR array and the perovskite layer was strengthened by the surface modification, which could facilitate faster electron injection between the perovskite layer and TiO2. Thus, enhancements in the Jsc, Voc, and especially the FF were obtained on the devices with surface-treated TiO2 NR arrays, leading to a higher PCE. To further evaluate the evolution of the interfacial property between TiO2/perovskite, electrochemical impedance spectra

Figure 6. (a) Nyquist impedance plots of a NR-structured cell with UV-ozone treatment of the TiO2-NR array and the control NR-structured cell from Figure 5, in a frequency range from 1 MHz to 0.1 Hz at −0.95 V in the dark. (b) Recombination resistance values (curves in black) and interface capacitance related to the charge separation within PI&NI contact interfaces (curves in red) at different biases under dark for both devices. 21362

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Figure 7. Calculated VSF in TiO2-NR film dependence of solar cell photovoltaic parameters (a) Jsc, (b) Voc, (c) FF, and (d) PCE under AM1.5 simulated light at 100 mW/cm2. All of the NR arrays were subjected to UV-ozone before the penetration of perovskite solution. The NR arrays were well-capped with a perovskite layer having thickness of approximately 100 nm. (e) J−V curve and (f) IPCE for a best-performing device wherein the TiO2-NR array was synthesized from a precursor containing 2-ethyl-butyric acid. For each condition, 12 cells were characterized.

HOMO of spiro-oMeTAD) varied slightly, a similar changing tendency with that of Jsc and FF was also observed. The effect of VSF on Jsc was first considered. Different VSF may affect the loading of perovskite within the NR arrays when the length of NRs was fixed in this research as around 180 nm, which would further affect the light absorption of the perovskite active layer. However, a UV−vis characterization showed that VSF affects the light absorption of perovskite layers with limited range, shown in Figure S5. Hence, the differences of short-circuit current densities with different VSFs may come from the change of interface between the TiO2 NR and neighbor perovskite layer. The evolution of FF shows a similar tendency with the Jsc, which is also affected by the TiO2/perovskite interface. From the experimental results given in Figure 7, an optimum VSF of 0.682 was determined. The average PCE of the cells with a mean calculated film VSF of 0.682 was approximately 18.0%, with the best device exhibiting 18.22%, as shown in Figure 7e. The IPCE spectrum of the best device, which was integrated with the AM1.5G solar photon flux yielding a current density of 22.09 mA/cm2, is shown in Figure 7f. The calculated Jsc is in good agreement with the measured photocurrent density given in Figure 7e. To the best of our knowledge, this is the highest PCE reported, by far, for metal oxide arrays for organic or hybrid inorganic−organic solar cells. Like many other reports, hysteresis was observed in our cells, as shown in Figure S6, which may originate from the TiO2 itself, spiro-oMeTAD, or from Li salt additives. Future work is

density of the NR arrays can further optimize the photovoltaic performance of the solar cells. Morphological characterization of the NR arrays indicated that the diameter of TiO2 NRs and the areal density of the NR arrays were tunable based on the addition of different organic acids. No doubt various diameters and areal densities will affect the penetration of perovskite crystals in the TiO2-NR array film and lead to different photovoltaic performance. Hereinafter, a calculated value of void-space-fraction (VSF) in NR-array film was introduced to evaluate the effect of NR diameter and the areal density on the photovoltaic performance. Because of the unique geometrical feature of the 1D NR array, the calculated VSF can be obtained through the following eq 1 VSF = 1 − ρ × Dav 2

(1)

where ρ is the average NR density and Dav is the average NR diameter. The photovoltaic responses of devices fabricated from the different NR arrays grown in different solutions as functions of VSF were plotted, shown in Figure 7 and Table S1. The Ag layer, spiro-oMeTAD layer, and perovskite layer of the devices in Figure S4 were removed after J−V measurements, and the remaining NR arrays were characterized by SEM to obtain the average NR diameters and the average NR densities for the calculation of film porosities. As seen, the calculated VSF of NR-array film is closely related to the Jsc and FF. Though Voc (determined by the energetic difference between the quasi-Fermi level of TiO2 and the 21363

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TiO2-NR arrays reported herein competitive with conventional nanoparticle-based films for photovoltaic application. We anticipate further improvement in the performance of NRstructured perovskite solar cells by metal doping in combination with the optimization of the perovskite composition as well as the hole-transport materials.

required to minimize or eliminate the hysteresis, such as via the replacement of spiro-oMeTAD with NiO4, or via the replacement of MAPbI3 with a mixture of FAPbI3, Cs+, and FAPbBr3,30 although this goes beyond the scope of this work. The unsealed device exhibited good stability for up to 16 days under controlled ambient conditions at approximately 30% relative humidity, as shown in Figure S7. Jsc and FF decreased slightly over time. The similar tendencies of the two parameters hint that they may be linked to the same degradation mechanism. It is possible that partial decomposition of perovskite due to humidity occurred at the TiO2− NR interface. Interestingly, the Voc increased slightly with increasing time, owing to the reduced trap density under ambient illumination.31 Hence, it can be concluded that the decrease in PCE is due to the decrease in both Jsc and FF. Even so, the NR-structured photovoltaic device maintained more than 92% of its initial PCE after a period of 16 days. To have a better understanding of the perovskite solar cells using tunable TiO2-NR arrays, the mesoscopic solar cell with mesoporous TiO2 film was fabricated for comparison. The mesoscopic perovskite solar cell obtained a PCE of 16.37% (shown in Figure S8) by the same solar cell manufacturing method, which is relatively lower than that of the TiO2-NR array device. The morphology of perovskite of the capping layer and pore filling condition of perovskite in the TiO 2 nanostructured with mesoporous structure or NR array were considered. It is found that the morphology of perovskite on mesoporous TiO2 film, shown in Figure S9, is similar to that on NR array film. However, the pore filling condition in mesoporous TiO2 was different from that in NR array film, presented in Figure S9. Several small voids were observed in the perovskite/mesoporous TiO2 film, which would impair the perovskite/TiO2 interface and then the photovoltaic performance.32 To get a deeper insight into the electron transfer between perovskite and the TiO2 underlayer, including a compact TiO2 layer, mesoporous TiO2, and TiO2 NR, steady-state photoluminescence (PL) measurement was performed. As shown in Figure S10, the steady-state PL intensity of perovskite is obviously quenched when perovskite deposited on TiO2 electron conductors. The quenching efficiency of perovskite CH3NH3PbI3 film with TiO2-NR arrays is higher than that with compact TiO2 or mesoporous TiO2, which indicated better electron tranfer process between perovskite and TiO2-NR arrays, corresponding to the better photovoltaic performance. The fastest electron transfer process may be a main factor which would be responsible for the better photovoltaic performance of devices with the TiO2-NR underlayer.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b05971. Optical photographs of FTO and TiO2 nanowire array film and TEM photograph of TiO2 nanorods. J−V curves from forward and reverse scan directions, evolution of photovoltaic performance over time, average diameters, and densities of different TiO2-NW arrays (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Author Contributions #

X.L and S.-M.D contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support from the National Natural Science Foundation of China (Grant 21203159), the funding from the guiding project of Fujian Province of China (No. 2016H0036), and the funding from Xiamen Southern Oceanographic Center (Grant 14GHS016NF16).

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REFERENCES

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CONCLUSION In summary, highly transparent and TiO2-NR array films were prepared and used to fabricate efficient perovskite solar cells. An optimized perovskite capping layer atop the NR array improved the photovoltaic performance. Electrochemical impedance analysis indicated that UV−ozone treatment of the as-prepared TiO2 can improve the NR/perovskite interface to enhance the cell performance. With the optimized NR structure, a short-circuit current density of 22.9 mA/cm2 and an encouraging PCE of 18.22% under standard full sun illumination were obtained, which is the highest efficiency reported so far on 1D metal oxide nanostructured perovskite solar cells. Importantly, shorter growing times, nonsintering treatment, high performance, and long-term stability make the 21364

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Research Article

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