Research Article www.acsami.org
Ternary Oxides in the TiO2−ZnO System as Efficient ElectronTransport Layers for Perovskite Solar Cells with Efficiency over 15% Xiong Yin,*,†,‡,§ Zhongzhong Xu,† Yanjun Guo,† Peng Xu,† and Meng He*,†,∥ †
CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China ‡ State Key Laboratory of Chemical Resource Engineering, School of Science, Beijing University of Chemical Technology, Beijing 100029, China § State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University, Changsha 410082, China ∥ School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *
ABSTRACT: Perovskite solar cells, which utilize organometal−halide perovskites as light-harvesting materials, have attracted great attention due to their high power conversion efficiency (PCE) and potentially low cost in fabrication. A compact layer of TiO2 or ZnO is generally applied as electron-transport layer (ETL) in a typical perovskite solar cell. In this study, we explored ternary oxides in the TiO2−ZnO system to find new materials for the ETL. Compact layers of titanium zinc oxides were readily prepared on the conducting substrate via spray pyrolysis method. The optical band gap, valence band maximum and conduction band minimum of the ternary oxides varied significantly with the ratio of Ti to Zn, surprisingly, in a nonmonotonic way. When a zinc-rich ternary oxide was applied as ETL for the device, a PCE of 15.10% was achieved, comparable to that of the device using conventional TiO2 ETL. Interestingly, the perovskite layer deposited on the zinc-rich ternary oxide is stable, in sharp contrast with that fabricated on a ZnO layer, which will turn into PbI2 readily when heated. These results indicate that potentially new materials with better performance can be found for ETL of perovskite solar cells in ternary oxides, which deserve more exploration. KEYWORDS: perovskite solar cell, electron-transport layer, hole-blocking layer, ternary oxide, zinc titanate
■
INTRODUCTION The hybrid perovskite-based photovoltaic solar cells utilizing organometal−halide perovskites as light harvesters have been considered as promising alternatives to traditional silicon-based solar cells due to their high efficiencies, solution processability, and potential low-cost fabrication processes.1−10 Previous studies revealed that the high efficiency mainly results from the high charge carrier mobility and long charge carrier diffusion length of the perovskites.11,12 A certified PCE of 22.1% has been recently reported for perovskite solar cell.13 In a typical perovskite solar cell, a compact layer of binary oxide, usually TiO2 or ZnO, is generally applied as ETL which selectively extracts photoexcited electrons from the lightharvesting perovskite layer, and then transports them to the conducting substrate. Meanwhile, this compact layer also blocks the electrons transporting reversely from the conducting © 2016 American Chemical Society
substrate to recombine with the holes in the light-harvesting layer. By introducing such a layer into the perovskite solar cell, the efficiency of device could be considerably enhanced.14 TiO2 has been the most widely employed materials as the ETL for perovskite solar cells. ZnO was also used as an alternative material in a few studies.15,16 Although TiO2 and ZnO have suitable conduction band minimum (CBM), valence band maximum (VBM), and band gaps, they also have limitations to be applied as ETLs for perovskite solar cells. TiO2 has rather low conductivity and electron mobility, which are unfavorable for the electron collection.17 The electron mobility of ZnO is orders of magnitude higher than TiO2.17 Received: July 28, 2016 Accepted: October 14, 2016 Published: October 14, 2016 29580
DOI: 10.1021/acsami.6b09326 ACS Appl. Mater. Interfaces 2016, 8, 29580−29587
Research Article
ACS Applied Materials & Interfaces
substrate by spraying 0.2 mol L−1 Zn(OAc)2 at 350 °C. In the case of ternary oxides, Ti(i-OPr)2(acac)2 (Aldrich) was added into the Zn2+ isopropyl alcohol precursor solution with different molar ratios of Ti4+ to Zn2+. Prior to spraying, the solution was ultrasonically mixed for 15 min. The thickness of the compact oxide layers was controlled by changing the number of spray cycles, and the thickness for these films was controlled to be ∼60 nm. The nonstoichiometric perovskite precursor was spin-coated on the compact oxide layer in an Ar-filled glovebox, at 2000 rpm for 45 s. After spin-coating, the films were left to dry at room temperature in the glovebox for 30 min, to allow slow solvent evaporation. After that, they were then annealed on a hot plate in the glovebox at 100 °C for 90 min. The hole transferring layer (HTL) was prepared by spin-coating a solution containing 72.3 mg of spiro-OMeTAD, 28.8 μL of 4-tert-butylpyridine, and 0.06 M lithium bis(trifluoromethylsulfonyl)imide per milliliter of chlorobezene on top of the perovskite layer at 3000 rpm for 30 s. Finally, Au contact electrode (around 80 nm) was deposited on the HTL under vacuum of 1 × 10−6 mbar at a rate of 0.1 nm s−1 using thermal evaporation technique. Characterization. The morphology of the compact layers was observed with a field emission scanning electron microscope (FESEM, Hitachi S-4800). X-ray photoelectron spectroscopy (XPS) measurements were conducted with an ESCA Lab250xi spectrometer using Al Kα (1486.6 eV) irradiation as X-ray source. All of the spectra were calibrated to the binding energy of the adventitious C 1s peak at 284.8 eV. The structure of the compact layers and the perovskite films was characterized with an X-ray diffractometer (Rigaku D/MAX-TTRIII (CBO)). The ultraviolet photoelectron spectroscopy measurements were carried out with a Kratos AXIS Ultra DLD spectrometer. The energy is calibrated with respect to He I photon energy (21.22 eV). The optical transmittance of ternary oxide compact layers on FTO substrate was measured by using a UV−vis spectrophotometer (Hitachi U3010). The rectifying behaviors of compact layers were characterized using a CHI660D electrochemical workstation over a scanning range from −0.8 to 0.8 V with a scanning rate of 75 mV s−1. The photocurrent−voltage measurements of perovskite solar cells were recorded by a Keithley 2420 source meter with a solar simulator (Oriel Newport, 150 W, AM 1.5) as the light source. The light intensity was calibrated using a Si reference cell (Oriel Newport PN91150V). The J−V curves were measured by reverse scan (forward bias 1.1 V to short circuit 0 V). IPCE measurements of the devices were performed using a 300 W xenon lamp (Newport 66902), a lockin amplifier (Newport Merlin digital 70140), and a monochromator (Cornerstone CS260). The active area of each perovskite solar cell was 2 mm × 5 mm. Electrochemical impedance spectra were recorded using an Autolab PGSTAT302N potentiostat. The impedance measurements of perovskite solar cells were recorded at an applied DC potential bias with a sinusoidal AC potential perturbation of 20 mV in a frequency range from 2 MHz to 0.05 Hz. The applied DC potential increased from 0 to 0.9 V with a step of 0.1 V. The resultant impedance spectra were analyzed using the ZView software (Scribner Associates Inc.) and the corresponding equivalent circuit. All of the measurements were performed under ambient conditions.
Nevertheless, ZnO is harmful to the stability of the lightharvesting perovskie layer.18 Therefore, ETLs with better performance are still strongly desired although great progress has been made in the development of perovskite solar cells. In comparison with binary oxides, ternary oxides will have more freedom to tune their properties by adjusting the chemical stoichiometry. Actually, ternary oxide Zn2SnO4, both in crystalline and amorphous forms, has been applied as photoanodes for dye-sensitized solar cells in previous studies,19−21 and was further applied as electron-transporting electrodes/layers for perovskite solar cells recently.22−25 Nevertheless, no attempts have been made in all these studies to tune the properties of the ternary oxide by adjusting its composition. Here we report our exploration for alternative ETLs of perovskite solar cells in ternary oxides in the TiO2− ZnO system. Although TiO2/ZnO bilayer and TiO2/ZnO/TiO2 sandwich multilayer have been reported as ETLs for perovskite solar cells,26,27 to the best of our knowledge, films of ternary oxides in the TiO2−ZnO system have never been applied as ETLs for perovskite solar cells. We focus our first exploration on the TiO2−ZnO system because both TiO2 and ZnO have been successfully applied as ETLs for perovskite solar cells, and they are low-cost, nontoxic, and stable. Up to now, three ternary compounds, TiZn2O4, TiZnO3, and Ti3Zn2O8, have been reported in the TiO2−ZnO system. Among them, TiZnO3 crystallizes in a trigonal system whereas TiZn2O4 and Ti3Zn2O8 adopt a spinel-type structure.28 We prepared ternary compact layers with nominal compositions corresponding to the three previously reported compounds. Characterizations reveal that CBM, VBM, and band gap of ternary oxides strongly depend on the compositions, and interestingly, they vary nonmonotonically with the ratios of Ti/Zn. This implies that materials with desired properties can potentially be found in ternary oxides by just altering the compositions. In this case, the compact layer with Ti/Zn = 0.5 was identified to be suitable ETL for perovskite solar cells, and a device with such an ETL delivered a power conversion efficiency of 15.10%, which is comparable with that of devices using conventional TiO2 compact layers as ETLs. It is worth noting that although such a compact layer is Zn-rich, perovskite layers deposited on it do not suffer from the problem of decomposition when heated, as observed in the case of a ZnO compact layer. Our results indicate that ternary oxides are promising materials to be applied as ETLs for perovskite solar cells, and even other devices, and deserve further exploration.
■
EXPERIMENTAL SECTION
Device Fabrication. Methylamine iodide (MAI) was synthesized by reacting methylamine, 33 wt % in ethanol (Aladdin), with hydroiodic acid (HI) 57 wt % in water (Aladdin) for 2 h, at room temperature. HI was added dropwise while stirring. Upon heating at 100 °C, a white powder was formed, which was dried overnight in a vacuum oven prior to usage.29,30 To form the precursor solution of nonstoichiometric CH3NH3PbI3−xClx, the as-synthesized methylammonium iodide and lead(II) chloride (Sigma-Aldrich) were dissolved in anhydrous N,N-dimethylformamide (DMF), at a 3:1 molar ratio of MAI to PbCl2, with final concentrations of 0.88 M lead chloride and 2.64 M MAI. This solution was heated at 90 °C with stirring for 3 h and, then, stored under a dry nitrogen atmosphere before usage. Fluorine-doped SnO2 (FTO) substrate was first etched using Zn powder and HCl (2 M) to form the desired electrode patterns (15 mm × 10 mm). The resultant FTO substrates were ultrasonically cleaned with detergent, deionized water, acetone, ethanol, and 2-propanol, successively. ZnO ETL was then deposited on the cleaned FTO
■
RESULTS AND DISCUSSION The device configuration is shown in Scheme 1a. Commercially available FTO glass was used as the conducting substrate in the study. A compact layer of ZnO or ternary oxides was first deposited on the FTO surface via spray pyrolysis technique at 450 °C with corresponding precursors. A light-harvesting layer, CH3NH3PbI3−xClx, was then deposited on the compact layer by a spin-coating process, followed by heating as a post-treatment according to previous reports.7,29 After the perovskite layer was annealed under Ar atmosphere in a glovebox, the commercial p-type hole-transport material, spiro-OMeTAD, was spincoated on it. Finally, the metal gold was evaporated on the hole-transport layer, working as a top contact electrode. 29581
DOI: 10.1021/acsami.6b09326 ACS Appl. Mater. Interfaces 2016, 8, 29580−29587
Research Article
ACS Applied Materials & Interfaces Scheme 1. (a) Schematic of the Device Structure and (b) Energy Level Diagram of Device in the Study
Figure 1a shows the scanning electron microscopy (SEM) image of the compact ZnO layer. Images presented in Figure
Figure 2. XRD patterns of the perovskite layer on ZnO (a−c) and ternary oxide TiZnO12 ETL (d, e) without and with different heat treatment. Insets show the corresponding photographs for the perovskite layer.
began to appear after the perovskite layer was heated at 70 °C for 2 min. As for the sample heated at 100 °C for 90 min, the reflections of PbI2 dominate the XRD pattern, and only weak reflections for perovskite were observed (Figure 2c). The morphology also changes with the phase evolution of the lightharvesting layer, as revealed by the SEM images shown in Figure 1c−f. It is important to note that the FTO substrate has been severely exposed in Figure 1f. The decomposition of the perovskite deposited on ZnO has been observed and discussed previously,18 and here we emphasize that the decomposition takes place even when the preovskite is heated in an Ar-filled drybox. Ternary oxide layers with nominal compositions of TiZn2O4, TiZnO3, and Ti3Zn2O8 are named as TiZnO12, TiZnO11, and TiZnO32, respectively. When the precursor of perovskite was spin-coated on the ternary oxide layer, perovskite formed spontaneously, as observed in the case of the ZnO compact layer. Nevertheless, an important difference between both cases was also noted. The transformation from the precursor deposited on ternary oxides to perovskite was incomplete. The X-ray powder pattern and photograph of the sample TiZnO12 are presented in Figure 2d. In addition to peaks from perovskite and FTO, strong reflections are observed in the powder pattern, which should result from the unidentified intermediate phase (or phases). Consistent with the incomplete transformation, the sample was light black (inset of Figure 2d). When this sample was heated at 100 °C for 90 min, it turned dark black. The X-ray powder pattern (Figure 2e) indicates that the precursor has been converted into perovskite completely. Precursors of perovskite deposited on TiZnO11 and TiZnO32
Figure 1. SEM images of ZnO electron-transport layer (a) and the perovskite layer deposited on ZnO without (b) and with heat treatment (c−f).
1b−f show the evolution of the perovskite layer deposited on the compact ZnO. The precursor spin-coated on a ZnO layer does not need further heat treatment and turns into perovskite spontaneously, as revealed by the X-ray powder pattern shown in Figure 2a. Only strong reflections of perovskite are identified in addition to signals from the FTO substrate. In agreement with the XRD result, the substrate covered with perovskite is black (inset of Figure 2a). SEM image (Figure 1b) reveals that the perovskite layer consists of cube-like crystallites in micrometer scale. After heating at 70 or 100 °C in Ar for different time duration, the samples faded and finally turned into bright yellow (insets of Figure 2b,c). As revealed by the XRD pattern (Figure 2b), reflections characteristic for PbI2 29582
DOI: 10.1021/acsami.6b09326 ACS Appl. Mater. Interfaces 2016, 8, 29580−29587
Research Article
ACS Applied Materials & Interfaces exhibit a similar evolution process, and the corresponding XRD patterns are presented in Figure S1 as Supporting Information. It is interesting to note that the precursor of perovskite deposited on the Zn-rich ternary oxide behaves so differently from that deposited on ZnO. The morphology of the ternary oxide layers was characterized with SEM, and typical images are shown in Figure 3. All three kinds of ternary oxide layers are
Figure 3. SEM images of ternary oxide ETLs: (a) TiZnO11, (b) TiZnO32, (c) TiZnO12, and (d) perovskite film on TiZnO12 layer. Figure 4. Core level XPS spectra of Ti 2p (a) and Zn 2p (b) from ternary oxide films TiZnO11 (black), TiZnO32 (blue), and TiZnO12 (pink). (c) UPS spectra of ternary oxide films. (d) Plots of (αhv)0.5 versus photon energy (hv) for ternary oxide films prepared on quartz substrates.
smooth and homogeneous, which are significantly different from the ZnO layer (Figure 1a) and FTO (Figure S2a). These ETLs were formed by the stacking of nanoparticles with the size of a few tens of nanometers (Figure S2b−d). No obvious difference in morphology was observed among these three kinds of ternary oxide layers. SEM image of the perovskite formed on the top of ternary oxide layers is shown in Figure 3d. It is worth noting that the pervoskite layer shown in Figure 3d is smooth and compact, which is favorable for the performance of solar cells. XPS analysis was performed to characterize the chemical composition of the ternary oxide films and chemical states of the constitute elements. The core level spectra of Ti 2p are presented in Figure 4a. Two peaks are observed at binding energies of 458.29 and 464.13 eV, which are characteristic for Ti4+, and no signals of Ti3+ and Ti2+ are identified in each sample.31 The core level spectra of Zn 2p are shown in Figure 4b. Two peaks with binding energies of 1021.37 and 1044.46 eV are detected, which are assigned to Zn2+.32 The atomic ratios of Ti to Zn are also estimated for each sample using XPS, and the results are summarized in Table 1. The compositions of all three samples agree with the Ti/Zn ratios in the precursors in overall tendency. Nevertheless, the content of Ti determined by XPS is consistently higher than the nominal values in the precursors. All ternary oxide films exhibit high transmittance in the visible wavelength range, which will be beneficial to the light harvesting (Figure S3). Ultraviolet photoemission spectra (UPS) are recorded to estimate the VBM of the ternary oxides and presented in Figure 4c. The optical band gaps of ternary oxide films are determined from the plots of (αhv)1/2 as a function of photon energy, which are shown in Figure 4d. The CBM of the ternary oxides are then calculated based on VBM and band gaps.33 The VBM, band gap, and CBM of each ternary oxide are listed in Table 1. The Zn-rich compact layer TiZnO12 has a band gap of 3.29 eV and a CBM of −4.13 eV with respect to the vacuum level. Both values are very close to the corresponding ones of TiO2 and ZnO. The other two samples, TiZnO11 and TiZnO32, have smaller band gaps and
Table 1. Atomic Ratios of Ti/Zn and Energy Levels of Ternary Oxide ETLs ETL
atom ratio (Ti:Zn)a
VBM/eVb
Eg/eV
CBM/eV
TiZnO32 TiZnO11 TiZnO12
1.64:1 1.06:1 0.73:1
−7.79 −8.26 −7.42
2.95 2.86 3.29
−4.84 −5.40 −4.13
a Data obtained from XPS measurements. bData obtained from UPS measurements.
significantly lower VBM in comparison with TiO2 and ZnO. Consequently, the CBM of these two samples are much lower than those of ZnO and TiO2. The CBMs of TiZnO32 and TiZnO11 are −4.84 and −5.40 eV, respectively. As illustrated in the energy level diagram (Scheme 1b), TiZnO11 and TiZnO32 are not suitable to be applied as ETLs for perovskite solar cells in terms of their CBMs, whereas TiZnO12 seems to be a good choice due to its favorable band gap and CBM. It is particularly interesting to note that the VBM, CBM, and band gaps of ternary oxides in the TiO2−ZnO system vary significantly with the Ti/Zn ratios. Both band gaps and VBM change nonmonotonically with the compositions of ternary oxides. This implies that we have opportunities to find new ternary materials with properties which are quite different from those of the binary end members. Although TiZnO11 and TiZnO32 films are not suitable ETLs for perovskite solar cells, their visible wavelength transparency and extraordinarily low CBM may make them promising materials for other devices. The nonmonotonic variation of band gaps and VBM with the composition of the ternary oxide ETLs has not been fully understood yet. Here we give a tentative explanation: The band gap and VBM of a substance depend on both its constituent and structure. Because both TiZn2O4 and Ti3Zn2O8 adopt a spinel structure while TiZnO3 crystallizes in an ilmenite-type structure, we speculate that TiZnO12 is similar to TiZnO32 in 29583
DOI: 10.1021/acsami.6b09326 ACS Appl. Mater. Interfaces 2016, 8, 29580−29587
Research Article
ACS Applied Materials & Interfaces
Interestingly, rectifying behaviors are also observed in the cases of TiZnO11 and TiZnO32, although the rectification ratios are much worse than that observed for TiZnO12. We have expected that ohmic contacts will be formed between FTO and TiZnO11 or TiZnO32 films in terms of their energy levels. The rectifying behaviors observed in the cases of TiZnO11 and TiZnO32 have not been fully understood yet. Nevertheless, it has been found previously that energy levels of the individual materials may shift significantly with respect to each other upon contact within devices.36 It is more likely to take place for the very thin layers. Possibly, it is the shift of energy levels that results in the rectifying behaviors observed in the cases of TiZnO11 and TiZnO32. In terms of the rectifying characteristic, the compact layer of TiZnO12 is the most efficient among three ternary oxides in blocking the backward transport of electrons. The solar cells without and with ETLs, including ZnO, TiO2, and ternary oxide films, were fabricated under the controlled conditions. The current−voltage characteristics of devices were measured under simulated solar light with an intensity of 100 mW cm−2 (AM 1.5G). Shown in Figure 5b are the representative J−V curves of devices using ternary oxides as ETLs, and the corresponding photovoltaic parameters are listed in Table 2. The device using TiZnO12 as the ETL exhibits the best photovoltaic performance. As listed in Table 2, the opencircuit voltage (Voc), the short-circuit current density (Jsc), and fill factor (FF) are 0.969 V, 21.13 mA cm−2, and 0.742, respectively, resulting in a PCE of 15.10%. An inferior photovoltaic performance is observed on the device based on the TiZnO32 ETL. The Voc, Jsc, and FF of this device are 0.903 V, 18.29 mA cm−2, and 0.713, respectively, leading to a PCE of 11.78%. The TiZnO11-based device has the lowest PCE of 5.05%, resulting from a Jsc of 14.45 mA cm−2, a Voc of 0.760 V, and a FF of 0.460. The incident photon-to-electron conversion efficiency (IPCE) curves of these three devices are presented in Figure 5c. The Jsc values obtained by integrating IPCE curves are in good agreement with those determined from the J−V measurement, as summarized in Table 2. Histograms of PCEs for each group of 25 devices using TiZnO12 and TiZnO32 ETLs, respectively, are shown in Figure 5d. The detailed photovoltaic parameters for each group of 25 devices are shown in Figure S4. For comparison, the J−V curves of devices without ETLs and with binary ZnO or TiO2 layer are presented in Figure S5. The device without an ETL showed a PCE of 2.23% with a Voc of 0.515 V, a Jsc of 13.0 mA cm−2, and a FF of 0.33. The low Voc and FF could be attributed to the severe recombination at the FTO/perovskite interface. When the ZnO compact layer was applied as ETL, the Voc increased to 0.767 V, whereas, the Jsc decreased to 4.36 mA cm−2, resulting in a PCE of 0.67% with a FF of 0.20. The decline of Jsc was due to the decomposition of the perovskite layer during the fabrication process, as revealed by the X-ray powder pattern and photograph (Figure 2c). The device with a conventional
structure, but much different from TiZnO11. Possibly, this is the reason for the nonmonotonic variation of band gaps and VBM of ternary oxide ETLs. An ideal ETL for perovskite solar cells should efficiently extract electrons from the light harvester and then transport electrons to FTO substrate, meanwhile, strongly blocking the backward transport of electrons from FTO to the lightharvester or hole-transport materials. The rectifying property of a contact provides important information on its performance. To investigate the rectifying behavior of the contact between FTO and ternary oxide films, simple devices with a structure of “FTO substrate/ternary oxides/hole-transport layer/Au electrode” were fabricated.29,34,35 All three ternary oxide layers have the same thickness of 60 nm. The potential between the FTO electrode and Au electrode continuously increased from −0.8 to 0.8 V. The typical current−voltage curves measured under the dark for different ternary oxide layers are shown in Figure 5a. In the case of a blank FTO (absence of an oxide layer), a
Figure 5. (a) Current−voltage curves for devices without (blank FTO) and with three types of ternary oxide ETLs. (b) Photocurrent− voltage characteristic curves and (c) incident photon-to-electron conversion efficiency (IPCE) spectra of perovskite solar cells with three types of ternary oxide ETLs. (d) Histogram of power conversion efficiencies for each group of 25 devices using TiZnO12 and TiZnO32 ETLs, respectively.
linear J−V relationship is observed, indicating that an ohmic contact is formed under this condition. When the compact layer of TiZnO12 is deposited on the FTO substrate, strong rectifying characteristic is observed due to the Schottky contact between the metallic FTO and semiconducting TiZnO12.
Table 2. Photovoltaic Parameters of Perovskite Solar Cells with Different Ternary Oxide ETLs ETL
Voc/Va
Jsc/(mA cm−2)b
FF/%c
PCE/%d
Jsc/(mA cm−2)e
TiZnO11 TiZnO32 TiZnO12
0.760 0.903 0.969
14.45 18.29 21.13
0.460 0.713 0.742
5.05 11.78 15.10
13.87 17.56 20.69
a
Open-circuit potential, Voc. bShort-circuit current, Jsc. cFill factor, FF. dPower conversion efficiency, PCE. eShort-circuit current obtained by integrating IPCE curves shown in Figure 5c. 29584
DOI: 10.1021/acsami.6b09326 ACS Appl. Mater. Interfaces 2016, 8, 29580−29587
Research Article
ACS Applied Materials & Interfaces TiO2 ETL exhibited a PCE of 14.63% with a Voc of 0.963 V, a Jsc of 21.98 mA cm−2, and a FF of 0.691. These photovoltaic parameters are comparable with those of the TiZnO12-based device. Combining the results of rectification and device performance measurements, we conclude that an ETL, which can efficiently block the backward transport of the electrons, is distinctly important to achieve the high efficiency of a perovskite solar cell. The effect of different ETLs on the photovoltaic performance of perovskite solar cells was investigated using electrochemical impedance spectroscopy (EIS).14,37,38 EIS has been extensively applied in the characterization of solid-state dye/quantum dotsensitized solar cells and perovskite solar cells to gain associated parameters and analyze the internal electron-transport processes. The EIS spectra of devices with ternary oxide ETLs were measured under light illumination at different applied biases. The typical Nyquist plots are presented in Figure S6a,b with the corresponding equivalent circuit shown in Figure S6c.14,37,38 The fitting results of selective contact resistance (Rsc) and recombination resistance (Rrec) for three devices are shown in Figure 6a,b, respectively. Rsc of each device
complete devices are consistent with their photovoltaic parameters. The device based on TiZnO12, which has the largest Rrec and the least Rsc, exhibited the best photovoltaic performance. It is interesting to note that although TiZnO11 is between TiZnO32 and TiZnO12 in composition, it is quite different from them in properties. On the contrary, TiZnO12 and TiZnO32 are close to each other in properties, though they are much different in composition. To find structural reasons for this, we have tried to characterize the structures of these thin ternary oxide films with XRD. Unfortunately, no significant reflections from the thin layers are identified even in grazing incidence XRD measurements. Therefore, we speculate that the ternary oxide layers are amorphous. Although no detailed information on the atomic structure of the thin ternary oxide layers is available, the fact that both compounds of TiZn2O4 and Ti3Zn2O8 adopt a spinel-type structure implies that TiZnO12 and TiZnO32 ETLs may be similar in structure. Such a speculation is supported by the similar properties exhibited by them.
■
CONCLUSIONS In conclusion, compact films of ternary oxides in the TiO2− ZnO system were prepared on FTO substrates by spray pyrolysis and applied as ETLs for perovskite solar cells for the first time. The compact film with the nominal composition of TiZn2O4 has suitable energy levels to extract electrons from perovskite. And meanwhile, the contact between this film and the FTO substrate can efficiently block the backward transport of electrons. A perovskite solar cell with such a compact film as ETL has achieved a high PCE of 15.10%. More importantly, results obtained in this study indicate that ternary materials can be quite different in properties from the binary end members, and the properties of ternary materials can be tuned over a wide range by simply altering the composition. This implies that ternary materials, especially ternary oxides, deserve further exploration to find more appropriate new materials of ETLs for perovskite solar cells.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b09326. XRD patterns of perovskite layers on TiZnO11 and TiZnO32 covered FTO substrates, SEM images, UV−vis transmittance of the blank and ternary oxide covered FTO substrates, photovoltaic parameters of solar cells with TiZnO12 or TiZnO32 ETLs, respectively, J−V curves of solar cells without ETL or with ZnO and TiO2 ETLs, and typical Nyquist plots (PDF)
Figure 6. (a) Selective contact resistance (Rsc) and (b) recombination resistance (Rrec) of perovskite solar cells with three types of ternary oxide ETLs, extracted from electrochemical impedance spectra measured under light illumination with different applied forward biases.
■
changed slightly with the applied biases in the whole range of applied biases. The trend of Rsc is in good agreement with the previously reported results.14,29 The Rsc value of the devices increases in the order of TiZnO12 < TiZnO32 < TiZnO11. In contrast, the Rrec value of the devices decreases in the order of TiZnO12 > TiZnO32 > TiZnO11. In addition, Rrec also decreases with the applied bias. The improved Rrec indicates less electron−hole recombination in the device, whereas the decreased Rsc contributes less to the total internal series resistance of the devices.14 The less electron−hole recombination will lead to higher Voc and Jsc, and the lower internal series resistance will result in higher FF. The EIS results of the
AUTHOR INFORMATION
Corresponding Authors
*(X.Y.) E-mail:
[email protected]. *(M.H.) E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51272049, 21103032, and 11574060) and CAS Key Lab of Nanosystem 29585
DOI: 10.1021/acsami.6b09326 ACS Appl. Mater. Interfaces 2016, 8, 29580−29587
Research Article
ACS Applied Materials & Interfaces
Photoanodes for Dye-Sensitized Solar Cells. ACS Nano 2014, 8, 2261−2268. (18) Si, H. N.; Liao, Q. L.; Zhang, Z.; Li, Y.; Yang, X. H.; Zhang, G. J.; Kang, Z.; Zhang, Y. An Innovative Design of Perovskite Solar Cells with Al2O3 Inserting at ZnO/Perovskite Interface for Improving the Performance and Stability. Nano Energy 2016, 22, 223−231. (19) Tan, B.; Toman, E.; Li, Y.; Wu, Y. Zinc Stannate (Zn2SnO4) Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2007, 129, 4162−4163. (20) Choi, S. H.; Hwang, D.; Kim, D. Y.; Kervella, Y.; Maldivi, P.; Jang, S. Y.; Demadrille, R.; Kim, I. D. Amorphous Zinc Stannate (Zn2SnO4) Nanofibers Networks as Photoelectrodes for Organic DyeSensitized Solar Cells. Adv. Funct. Mater. 2013, 23, 3146−3155. (21) Li, Z.; Zhou, Y.; Bao, C.; Xue, G.; Zhang, J.; Liu, J.; Yu, T.; Zou, Z. Vertically Building Zn2SnO4 Nanowire Arrays on Stainless Steel Mesh toward Fabrication of Large-Area, Flexible Dye-Sensitized Solar Cells. Nanoscale 2012, 4, 3490−3494. (22) Oh, L. S.; Kim, D. H.; Lee, J. A.; Shin, S. S.; Lee, J. W.; Park, I. J.; Ko, M. J.; Park, N. G.; Pyo, S. G.; Hong, K. S.; Kim, J. Y. Zn2SnO4Based Photoelectrodes for Organolead Halide Perovskite Solar Cells. J. Phys. Chem. C 2014, 118, 22991−22994. (23) Bera, A.; Sheikh, A. D.; Haque, M. A.; Bose, R.; Alarousu, E.; Mohammed, O. F.; Wu, T. Fast Crystallization and Improved Stability of Perovskite Solar Cells with Zn2SnO4 Electron Transporting Layer: Interface Matters. ACS Appl. Mater. Interfaces 2015, 7, 28404−28411. (24) Mali, S. S.; Shim, C. S.; Hong, C. K. Highly Porous Zinc Stannate (Zn2SnO4) Nanofibers Scaffold Photoelectrodes for Efficient Methyl Ammonium Halide Perovskite Solar Cells. Sci. Rep. 2015, 5, 11424. (25) Shin, S. S.; Yang, W. S.; Noh, J. H.; Suk, J. H.; Jeon, N. J.; Park, J. H.; Kim, J. S.; Seong, W. M.; Seok, S., II. High-Performance Flexible Perovskite Solar Cells Exploiting Zn2SnO4 Prepared in Solution below 100 °C. Nat. Commun. 2015, 6, 7410. (26) Xu, X.; Zhang, H.; Shi, J.; Dong, J.; Luo, Y.; Li, D.; Meng, Q. Highly Efficient Planar Perovskite Solar Cells with a TiO2/ZnO Electron Transport Bilayer. J. Mater. Chem. A 2015, 3, 19288−19293. (27) Duan, J.; Wu, J.; Zhang, J.; Xu, Y.; Wang, H.; Gao, D.; Lund, P. D. TiO2/ZnO/TiO2 Sandwich Multi-Layer Films as a Holeblocking Layer for Efficient Perovskite Solar Cells. Int. J. Energy Res. 2016, 40, 806−813. (28) Bartram, S. F.; Slepetys, R. A. Compound Formation and Crystal Structure in the System ZnO-TiO2. J. Am. Ceram. Soc. 1961, 44, 493−499. (29) Yin, X.; Guo, Y. J.; Xue, Z. S.; Xu, P.; He, M.; Liu, B. Performance Enhancement of Perovskite-Sensitized Mesoscopic Solar Cells Using Nb-doped TiO2 Compact Layer. Nano Res. 2015, 8, 1997−2003. (30) Malinkiewicz, O.; Yella, A.; Lee, Y. H.; Espallargas, G. M.; Grätzel, M.; Nazeeruddin, M. K.; Bolink, H. J. Perovskite Solar Cells Employing Organic Charge-Transport Layers. Nat. Photonics 2014, 8, 128−132. (31) Hashimoto, S.; Tanaka, A. Alteration of Ti 2p XPS Spectrum for Titanium Oxide by Low-energy Ar Ion Bombardment. Surf. Interface Anal. 2002, 34, 262−265. (32) Biesinger, M. C.; Lau, L. W. M.; Gerson, A. R.; Smart, R. St. C. Resolving Surface Chemical States in XPS Analysis of First Row Transition Metals, Oxides and Hydroxides: Sc, Ti, V, Cu and Zn. Appl. Surf. Sci. 2010, 257, 887−898. (33) Miller, E. M.; Kroupa, D. M.; Zhang, J.; Schulz, P.; Marshall, A. R.; Kahn, A.; Lany, S.; Luther, J. M.; Beard, M. C.; Perkins, C. L.; van de Lagemaat, J. Revisiting the Valence and Conduction Band Size Dependence of PbS Quantum Dot Thin Films. ACS Nano 2016, 10, 3302−3311. (34) Peng, B.; Jungmann, G.; Jager, C.; Haarer, D.; Schmidt, H. W.; Thelakkat, M. Systematic Investigation of the Role of Compact TiO2 Layer in Solid State Dye-Sensitized TiO2 Solar Cells. Coord. Chem. Rev. 2004, 248, 1479−1489. (35) Zhang, W.; Cheng, Y. M.; Yin, X.; Liu, B. Solid-State DyeSensitized Solar Cells with Conjugated Polymers as Hole-Transporting Materials. Macromol. Chem. Phys. 2011, 212, 15−23.
and Hierarchical Fabrication. X.Y. thanks Dr. Wei Zhang for valuable discussions.
■
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.; Grätzel, M.; Park, N.-G. Lead Iodide Perovskite Sensitized AllSolid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591. (2) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338, 643−647. (3) Liu, M. Z.; Johnston, M. B.; Snaith, H. J. Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition. Nature 2013, 501, 395−398. (4) Burschka, J.; Pellet, N.; Moon, S. J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M. Sequential Deposition as a Route to High-Performance Perovskite-sensitized Solar Cells. Nature 2013, 499, 316−319. (5) Sepalage, G. A.; Meyer, S.; Pascoe, A.; Scully, A. D.; Huang, F.; Bach, U.; Cheng, Y.; Spiccia, L. Copper(I) Iodide as Hole-Conductor in Planar Perovskite Solar Cells: Probing the Origin of J−V Hysteresis. Adv. Funct. Mater. 2015, 25, 5650−5661. (6) Zhao, L.; Luo, D.; Wu, J.; Hu, Q.; Zhang, W.; Chen, K.; Liu, T.; Liu, Y.; Zhang, Y.; Liu, F.; Russell, T. P.; Snaith, H. J.; Zhu, R.; Gong, Q. High-Performance Inverted Planar Heterojunction Perovskite Solar Cells Based on Lead Acetate Precursor with Efficiency Exceeding 18%. Adv. Funct. Mater. 2016, 26, 3508−3514. (7) Yang, W. S.; Noh, J. H.; Jeon, N. J.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S., II. High-Performance Photovoltaic Perovskite Layers Fabricated through Intramolecular Exchange. Science 2015, 348, 1234−1237. (8) Park, N. G. Organometal Perovskite Light Absorbers toward a 20% Efficiency Low-Cost Solid-State Mesoscopic Solar Cell. J. Phys. Chem. Lett. 2013, 4, 2423−2429. (9) Wu, W.; Huang, F.; Chen, D.; Cheng, Y.; Caruso, R. A. Thin Films of Dendritic Anatase Titania Nanowires Enable Effective HoleBlocking and Efficient Light-Harvesting for High-Performance Mesoscopic Perovskite Solar Cells. Adv. Funct. Mater. 2015, 25, 3264−3272. (10) Xia, X.; Li, H. C.; Wu, W. Y.; Li, Y. H.; Fei, D. H.; Gao, C. X.; Liu, X. Z. Efficient Light Harvester Layer Prepared by Solid/Mist Interface Reaction for Perovskite Solar Cells. ACS Appl. Mater. Interfaces 2015, 7, 16907−16912. (11) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342, 341− 344. (12) Xing, G.; Mathews, N.; Sun, S.; Lim, S. S.; Lam, Y. M.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C. Long-Range Balanced Electronand Hole-Transport Lengths in Organic-Inorganic CH3NH3PbI3. Science 2013, 342, 344−347. (13) National Center for Photovoltaics, Research Cell Efficiency Records, http://www.nrel.gov/ncpv/, accessed June 2016. (14) Juarez-Perez, E. J.; Wussler, M.; Fabregat-Santiago, F.; LakusWollny, K.; Mankel, E.; Mayer, T.; Jaegermann, W.; Mora-Sero, I. Role of the Selective Contacts in the Performance of Lead Halide Perovskite Solar Cells. J. Phys. Chem. Lett. 2014, 5, 680−685. (15) Liu, D.; Kelly, T. L. Perovskite Solar Cells with a Planar Heterojunction Structure Prepared Using Room-Temperature Solution Processing Techniques. Nat. Photonics 2014, 8, 133−138. (16) Kumar, M. H.; Yantara, N.; Dharani, S.; Grätzel, M.; Mhaisalkar, S.; Boix, P. P.; Mathews, N. Flexible, Low-Temperature, Solution Processed ZnO-Based Perovskite Solid State Solar Cells. Chem. Commun. 2013, 49, 11089−11091. (17) Chandiran, A. K.; Abdi-Jalebi, M.; Nazeeruddin, M. K.; Grätzel, M. Analysis of Electron Transfer Properties of ZnO and TiO2 29586
DOI: 10.1021/acsami.6b09326 ACS Appl. Mater. Interfaces 2016, 8, 29580−29587
Research Article
ACS Applied Materials & Interfaces (36) Wang, J. T. W.; Ball, J. M.; Barea, E. M.; Abate, A.; AlexanderWebber, J. A.; Huang, J.; Saliba, M.; Mora-Sero, I.; Bisquert, J.; Snaith, H. J.; Nicholas, R. J. Low-Temperature Processed Electron Collection Layers of Graphene/TiO2 Nanocomposites in Thin Film Perovskite Solar Cells. Nano Lett. 2014, 14, 724−730. (37) Gonzalez-Pedro, V.; Juarez-Perez, E. J.; Arsyad, W. S.; Barea, E. M.; Fabregat-Santiago, F.; Mora-Sero, I.; Bisquert, J. General Working Principles of CH3NH3PbX3 Perovskite Solar Cells. Nano Lett. 2014, 14, 888−893. (38) Kim, H. S.; Mora-Sero, I.; Gonzalez-Pedro, V.; FabregatSantiago, F.; Juarez-Perez, E. J.; Park, N.-G.; Bisquert, J. Mechanism of Carrier Accumulation in Perovskite Thin-Absorber Solar Cells. Nat. Commun. 2013, 4, 2242.
29587
DOI: 10.1021/acsami.6b09326 ACS Appl. Mater. Interfaces 2016, 8, 29580−29587
