Efficient Planar Perovskite Solar Cells with ... - ACS Publications

Research Article www.acsami.org

Efficient Planar Perovskite Solar Cells with Reduced Hysteresis and Enhanced Open Circuit Voltage by Using PW12−TiO2 as Electron Transport Layer Chun Huang,† Canjun Liu,‡ Yunxiang Di,† Wenzhang Li,‡ Fangyang Liu,*,†,§ Liangxing Jiang,† Jie Li,†,‡ Xiaojing Hao,§ and Haitao Huang*,∥ †

School of Metallurgy and Environment, Central South University, Changsha, Hunan 410083, China College of Chemistry and Chemical Engineering, Central South University, Changsha, Hunan 410083, China § School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney, New South Wales 2052, Australia ∥ Department of Applied Physics, The Hong Kong Polytechnic University, Kowloon, Hong Kong 999077, China ‡

S Supporting Information *

ABSTRACT: An electron transport layer is essential for effective operation of planar perovskite solar cells. In this Article, PW12−TiO2 composite was used as the electron transport layer for the planar perovskite solar cell in the device structure of fluorine-doped tin oxide (FTO)-glass/PW12− TiO2/perovskite/spiro-OMeTAD/Au. A proper downward shift of the conduction band minimum (CBM) enhanced electron extraction from the perovskite layer to the PW12− TiO2 composite layer. Consequently, the common hysteresis effect in TiO2-based planar perovskite solar cells was significantly reduced and the open circuit voltage was greatly increased to about 1.1 V. Perovskite solar cells using the PW12−TiO2 compact layer showed an efficiency of 15.45%. This work can contribute to the studies on the electron transport layer and interface engineering for the further development of perovskite solar cells. KEYWORDS: planar perovskite solar cells, composite, electron transport layer, interface, electron extraction

■

INTRODUCTION Due to the high efficiency and the low cost, organic−inorganic perovskite solar cells have rapidly become the focus of emerging photovoltaic techniques in recent years.1 Despite being processed via solution techniques at low temperature,2,3 perovskite materials have comparable properties with those of an inorganic semiconductor of high crystallinity,4,5 such as high absorption coefficient,5 unusual defect chemistry,6 high carrier mobility,7 long carrier diffusion length, and lifetime.8,9 For example, the carrier diffusion length of mixed halide perovskite was reported to be exceeding 1 μm, which is 1 order of magnitude greater than the absorption depth.9 These supreme properties underlie the rapidly reached energy conversion efficiencies of more than 19% for planar structure perovskite solar cells,1,10 in which the perovskite layer is sandwiched between an electron transport layer (ETL) and a hole transport material (HTM). After absorption of the incident photons in the perovskite layer, the ETL and HTM extract the photogenerated electrons and holes from the perovskite layer at the corresponding interface and then transport these charges through each layer to the outside circuit as current. When the roles they play in the devices are considered, the electrical © 2016 American Chemical Society

properties of ETL, as well as the interface of ETL/perovskite, are essential for the operation of the planar perovskite solar cells. Thanks to the optimized device architecture and improved deposition of perovskite film, great success has been achieved in using ZnO,11 [6,6]-phenyl-C61-butyric acid methyl ester (PCBM),12,13 and TiO210 as ETL. As perovskite solar cells have evolved from dye sensitized solar cells, TiO2 has been intensively investigated and used.10,14−16 However, the conductivity of pristine TiO2 is much lower than that of perovskite materials and the hole collecting materials (such as well doped spiro-OMeTAD or PSS−PDOTS),17 resulting in unbalanced charge transport in the device18 as in the case of dye sensitized solar cells. Moreover, it was also found that the electron extraction from the perovskite layer to TiO2 was not as efficient as to PCBM or doped TiO2,13,19,20 which may partly be due to the low conductivity of TiO221 and partly due to the conduction band misalignment proposed by Correa Baena et Received: January 21, 2016 Accepted: March 8, 2016 Published: March 8, 2016 8520

DOI: 10.1021/acsami.6b00846 ACS Appl. Mater. Interfaces 2016, 8, 8520−8526

Research Article

ACS Applied Materials & Interfaces

perovskite films, 3% excess PbCl2 was added in the perovskite precursor. Perovskite precursor was then spin coated on the TiO2 coated FTO substrate at a speed of 3000 rpm for 30 s, followed by annealing on a hot plate at 105 °C for 1.5 h. Then, hole transport material (HTM) spiro-OMeTAD was deposited onto the perovskite layer by spin coating 50 μL of spiro-OMeTAD solution at a speed of 4500 rpm for 40 s. The spiro-OMeTAD solution was prepared according to the previous report.16 Finally, Au electrode was evaporated at a speed of 1 nm/s using a thermal evaporator under a vacuum of 10−4 Pa. The active area of the devices is 0.1 cm2. Material and Device Characterization. The optical absorption spectra of the PW12−TiO2 composite layer was carried out using a UV−vis spectrophotometer (Shimadz 2450 series). The morphology of the PW12−TiO2 composite layer and the TiO2 compact layer and perovskite thin films is measured using a FEI scanning electron microscope (X-Max20/H1002). The crystal structures of TiO2 and PW12−TiO2 thin films were characterized by using X-ray diffraction (Bruker). The thickness of the PW12−TiO2 and TiO2 films were determined by a surface profiler. The XPS spectra were recorded by Xray photoelectron spectroscopy (ESCALAB 250Xi). The Fourier transform infrared (FTIR) spectra were recorded using a Fourier infrared spectrometer (Nicolet6700). The Mott−Schottky measurements were performed on an FTO/composite device in a traditional three-electrode cell including FTO/PW12−TiO2 composite as the working electrode, a saturated calomel electrode (SCE) as the reference electrode, and a graphite plate as counter electrode (Princeton Applied Research PARSTAT 4000 Potentiostat). J−V characterization of the perovskite solar cells was carried out at a scan rate of 100 mV/s using a Keithley 2400 source meter under the simulated AM 1.5G illumination (100 mW·cm−2; Oriel Sol3A Class AAA Solar Simulator). Photoluminescence spectra of the perovskite films were recorded by using an Edinburgh FLSP920 spectrophotometer equipped with the excitation source of a 485 nm picosecond pulsed diode laser.

al.22 The inefficient electron extraction at the ETL/perovskite interface, together with the low conductivity of TiO2, is detrimental to device performance and is possibly responsible for some problematic issues, such as the significant hysteresis phenomenon in current−voltage characteristics in TiO2-based planar perovskite solar cells.23−25 Aiming to solve such issues, efforts have been made to increase the conductivity of TiO2 by Y,10 Mg,26 or Zr27 doping. However, these devices still suffer from hysteresis to a certain extent, implying that low mobility of TiO2 may not necessarily dominate the hysteresis in planar perovskite solar cells. On the other hand, increasing investigations have provided evidence that the interface of ETL/perovskite is the origin of hysteresis. It has been reported that modification of the TiO2 surface by PCBM19 or fullerene20 can significantly reduce the hysteresis and improve device performance, because of the improved electron extraction from the perovskite layer to the PCBM or fullerene. Hysteresis can also be reduced by band offset engineering, such as lowering the TiO2 conduction band edge by Li-doping28 or replacing the conduction band misalignment of TiO2/perovskite with a barrier-free energetic configuration of SnO2/perovskite.22 Phosphotungstic acid (PW12) is a well-known electron acceptor, which has usually been used to accelerate electron transport from TiO2 to fluorine-doped tin oxide (FTO) in dye sensitized solar cells.29,30 In addition, tungsten (W) in TiO2 can lead to the downward shift of conduction band minimum (CBM) of TiO2.31 In this Article, we manipulated the electron extraction along the interface of perovskite layer/ETL by using the PW12−TiO2 composite layer in planar perovskite solar cells. The formation of TiO2−PW12 composite leads to the downward shift of CBM and Fermi energy level of TiO2, facilitating the electron extraction from perovskite to the PW12−TiO2 composite layer. By using this PW12−TiO2 composite, the hysteresis effect of planar perovskite solar cell was greatly reduced and the open circuit voltage was significantly enhanced. This effort toward the elimination of the hysteresis effect is essential for further progress of perovskite solar cells.

■

■

RESULTS AND DISSCUSION To determine the compositions of the samples, FTIR spectra were recorded for pure PW12 and the prepared TiO2 samples, as shown in Figure 1. For pure PW12, there are four characteristic peaks of Keggin-structure anion located at 1080, 978, 888, and 794 cm−1. For the TiO2 sample, two characteristic peaks of the pure PW12 at 1080 and 978 cm−1 can be observed, thus confirming the existence of a separate PW12 phase in the PW12−TiO2 composite.

EXPERIMENTAL METHOD

Preparation of PW12−TiO2 Composite Precursor. PW12−TiO2 composite precursor was prepared as follows. First, 25.3 mL of ethanol was mixed with 3.67 mL of isopropoxide titanium (97%) to form solution A. Meanwhile, solution B was prepared by adding tungsten chrolide (WCl6) and PW12 (AR) to a mixture of 25.3 mL of ethanol and 5 mL of concentrated HCl solution (12 M). The molar ratio of WCl6 and PW12 was 1:1. Finally, solution B was dropped into solution A to form a composite precursor under strong stirring for 0.5 h. The PW12 concentration in the precursor was about 1% (molar ratio of PW12/ isopropoxide titanium). For comparison, pristine TiO2 sol was also prepared without the addition of WCl6 and PW12. Preparation of Perovskite Solar Cells. FTO glass sheets (14 Ω/ square, Nippon) were etched by fine Zn powder and HCl solution (2 M). The etched FTO glass sheets were then cleaned sequentially by 2% MICRO-90 solution, acetone, isopropanol, and DI water for 30 min. A 60 nm thick compact TiO2 layer was then deposited by spin coating the composite precursor solution onto the patterned FTO and then annealed at 450 °C for 1.5 h. The fabrication process of the perovskite solar cells can be referred to our previous works.32 First, methylammonium iodide (MAI) was synthesized by reacting hydroiodic acid (HI, 57 wt % in water) with methylamine (33 wt % in ethanol) under stirring in an ice−water bath for 2 h. After drying in vacuum at 60 °C for 24 h, MAI was mixed with PbCl2 (98%) in anhydrous dimethylformamide (DMF) (98%) to form 0.88 M perovskite precursor. For the formation of dense and pinhole free

Figure 1. FTIR spectra of composite film and the pristine TiO2 film. Inset: enlarged spectrum between 1284 and 850 cm−1. 8521


Research Article


and the pristine TiO2 film, respectively, revealing that the substrates were both well covered by the perovskite layer with few pin holes. In fact, our previous study demonstrated that excess PbCl2 in precursors can greatly enhance the morphology of perovskite thin films by altering the thin film formation pathway according to our recent report.32 This compact and pinhole free perovskite layer is the prerequisite for high performance of planar perovskite solar cells. A close inspection of the SEM images suggested that perovskite films on PW12− TiO2 have slightly, if not significantly, larger grain size, in comparison with the pristine TiO2/perovskite which has smaller grains and increased boundaries. We proposed that the increased size of the grain may be due to the H2O absorbed by PW12 during sample transfer,12 as dried PW12 can easily absorb H2O. Large grains will help to reduce defects existing on the grain boundaries, which is a commonly employed strategy to improve the performance of perovskite solar cells. Using the PW12−TiO2 film as ETL, perovskite solar cells were fabricated in the structure of FTO glass/PW12−TiO2/ perovskite/spiro-OMeTAD/Au, where the spiro-OMeTAD was used as the hole transport materials. For comparison, control devices were also fabricated using pristine TiO2 as ETL in the identical structure. The structure of the perovskite solar cell device is sketched in Figure 3a. Figure 3c shows the performance of perovskite solar cells. The PW12−TiO2-based device showed an efficiency of 15.4%, with the open circuit voltage, short current density, and fill factor being 1.097 V, 19.97 mA cm−2, and 0.705, respectively. For comparison, the pristine TiO2-based device showed an efficiency of 12.32%, with the open circuit voltage, short current density, and fill factor being 0.95 V, 19.81 mA cm−2, and 0.655, respectively (Figure 3b). In addition, it is notable that the J−V hysteresis was greatly reduced for the device based on PW12−TiO2 ETL. Comparing their J−V curves in the scan direction of SC (short circuit)−FB (forward bias) with that of FB−SC, the PW12− TiO2-based device showed almost identical J−V curves, with a comparable efficiency of 15.0% but very slightly reduced Voc (open-circuit voltage) and FF. In contrast, the pristine TiO2based device exhibited significant hysteresis, with greatly reduced Voc and FF of only 0.85 V and 0.520, respectively, at SC−FB direction scan. Besides, it is also noted that the Voc was significantly increased from 0.95 to 1.095 V for the PW12− TiO2-based device. The reproducibility of these results was confirmed by the statics on the efficiency, Voc, Jsc (short-circuit current density), and FF of 20 individual devices based on PW12−TiO2 films (Figure S5). We believe that it is the improved properties of the composite and the corresponding favorable band alignment of the PW12−TiO2/perovskite interface that gives rise to the enhanced device performance. To investigate the effects of PW12 on the properties of PW12−TiO2 films and thus on the device properties, the valence band maximum of PW12−TiO2 was determined by XPS techniques. It could be observed that the valence band edge of pristine TiO2 and PW12/TiO2 was centerd at 2.65 and 2.87 eV, respectively, confirming the downshift of the valence band maximum. The absorption spectra of pristine and PW12−TiO2 films were shown in the inset of Figure 4a. Both samples had a steep edge at around 330 nm. It is very interesting that the transmission of both samples were nearly the same, indicating that the optical band gap of the PW12−TiO2 is almost the same as that of pristine TiO2. Therefore, it is reasonable to deduce that the conduction band minimum of the PW12−TiO2 was shifted downward by about 0.22 eV. In fact, the Mott−Schottky

XPS characterization was carried out to further investigate the chemical composition of the PW12−TiO2 composite and pristine TiO2 layer (Figure S1). Again, effective incorporation of W and P was confirmed by the XPS scan. The XPS peaks located at around 37.7 and 35.6 eV could be ascribed to the binding energies of W4f5/2 and W4F7/2,33 and the peaks located at 133.9 eV could be ascribed to the binding energy of P2p.34 Meanwhile, the XPS peaks located at around 464.7 and 458.7 eV could be ascribed to the binding energies of Ti2p1/2 and Ti2p3/2, and the peak separation between the 2p1/2 and 2p3/2 lines is 6 eV, which is also consistent with the +4 oxidation state.35 It should be noted that the XPS peaks of O and Ti were changed (Figure S2c,d), indicating the change of the chemical environment of TiO2 in the composite. These differences may due to W6+ insertion in the TiO2 lattice.33 Because the radius (0.0600 nm) of W(VI) is close to that (0.0605 nm) of Ti(IV), doping of W(VI) into the TiO2 lattice is ideally viable through lattice replacement. From the fitting of the O 1s XPS spectrum, the peak corresponding to the binding energy (531.09 eV) of O1s can not be defined to that of the WO336 (Figure S3). Besides, the growth of the (101) direction of TiO2 was not preferred in the PW12−TiO2 sample, indicating the W6+ doping in TiO237 (Figure S4). Therefore, some of W6+, which may be derived partly from WCl6 and partly from PW12, may exist in the form of a substitution of Ti4+ in the TiO2 structure, rather than in the form of a separate phase of WO3. Figure 2 shows SEM images of the pristine TiO2 films, PW12−TiO2 composite films, and the corresponding perovskite

Figure 2. SEM images of PW12−TiO2 film (a), PW12−TiO2/ perovskite (b), pristine TiO2 (c), and TiO2/perovskite (d) (the scale bar is 1 μm for (a) and (c) and 5 μm for (b) and (d).

films. It could be seen that the FTO was well covered by the PW12−TiO2 layer (Figure 2a). This compact and pinhole free composite layer is critical for the performance of planar perovskite solar cells, as it can avoid the direct contact between perovskite and FTO, which may lead to recombination between holes in perovskite and electrons in FTO. No obvious difference can be observed between the morphologies of the pristine TiO2 (Figure 2c) and PW12−TiO2 film, which may be due to the low content of PW12 in the TiO2. Figure 2b,d shows the SEM images of perovskite layer on the PW12−TiO2 film 8522


Research Article


Figure 3. (a). Schematic of the planar perovskite device; (b) and (c) represent the J−V characteristics of devices based on pristine TiO2 ETL and PW12−TiO2 ETL, respectively.

Figure 4. (a) The conduction band cut off by XPS for PW12−TiO2 and pristine TiO2 films on Si substrate; the inset represents the absorption spectra of PW12−TiO2 and pristine TiO2 film on FTO; (b) illustration of the band level matching between perovskite and pristineTiO2 or PW12− TiO2.

measurement performed on the FTO/PW12−TiO2 device also confirmed a 0.127 eV downward shift of the Fermi energy level of PW12−TiO2, in comparison with that of the pristine TiO2 (Figure S6). On the basis of these results, the band diagrams of the perovskite and PW12−TiO2 are sketched in Figure 4b, in which the conduction band level of pristine TiO2 is based on the reported value.1 As shown in Figure 4b, the CBM of the PW12−TiO2 composite is about 0.29 eV lower than the reported LOMO

of the perovskite, while the CBM of the pristine TiO2 is only 0.07 eV lower than the reported LOMO of the perovskite. As the driving force is much larger, it is reasonable that the electron is more readily extracted from the perovskite to the PW12−TiO2 layer than to the pristine TiO2 layer. To confirm this, the steady-state photoluminescence measurements (PL) were carried out, as shown in Figure 5a. First, steady-state PL strength was significantly reduced as the perovskite films were deposited on both TiO2 and PW12−TiO2, indicating that the 8523


Research Article


Figure 5. (a) Steady-state PL quenching of perovskite films on PW12−TiO2 (black line), pristine TiO2 (red line), and glass (blue line). (b) J−V curves for composite and pristine TiO2 films in the structure of FTO/PW12−TiO2 (or TiO2)/Au. The thickness of the pristine TiO2 or the PW12− TiO2 is about 60 nm.

band of PW12−TiO2 will quickly be transported to FTO due to the enhanced conductivity. Furthermore, for PW12−TiO2-based devices, the large energy barrier would inhibit the injected electrons from moving back to perovskite from PW12−TiO2 (Figure 4b), reducing the possibility of recombination.33 This is in accordance with decreased dark current density (Figure S7). In sum, the improved morphology of perovskite and the reduced recombination at the PW12−TiO2/perovskite interface contribute to the improved Voc and the enhanced device performance eventually.

photogenerated electrons were effectively extracted from the perovskite layer into the PW12−TiO2 or TiO2. Notably, a stronger degree of PL quenching for perovskite on the PW12− TiO2 layer was observed, confirming that electron extraction from perovskite to the PW12−TiO2 was more significant than to pristine TiO2. With enchanced electron extraction, charge accumulation in the perovskite layer or at the surface of the perovskite38,39 is less likely to occur.32,33 We believe that it is this enhanced electron extraction efficiency that contributes to the reduced hysteresis in the PW12−TiO2-based device, as proposed by McGehee and co-workers24 and Im and coworkers.13 Reduced hysteresis has been reported by interface engineering of using PCBM-modified19 or fullerene-modified20 TiO2 to facilitate the electron extraction at the TiO2/perovskite interface. Similar studies on the HTM/perovskite interface have also demonstrated reduced hysteresis in the case of better hole extraction.21,40 To deepen the understanding of hysteresis, further experiments will be needed to clarify the details of charge injection, charge transport, and charge distribution at the interface of the composite/perovskite. It has been reported that the mobility and the conductivity of the PW12 was higher than that of TiO2.30 In our experiments, J−V curves for the FTO/PW12−TiO2/Au device showed that the conductivity of PW12−TiO2 was slightly larger than that of pristine TiO2 (Figure 5b), although Mott−Schottky measurement indicated that the effective density of states in conduction band of PW12−TiO2 was slightly smaller than in pristine TiO2 (Figure S5). Using the diode equation and J−V curves, series resistance was estimated to be 5.87 Ω·cm−2 for the PW12− TiO2-based device and smaller than 6.54 Ω·cm−2 for the pristine TiO2-based device. The decreased series resistance would partly contribute to the increased FF of PW12−TiO2based devices. Previous studies revealed that the Voc under 1 sun was determined by the properties of perovskite films, irrespective of the employed ETL.41,42 Therefore, the proper downward shift of CBM would not decrease Voc in our experiments. Under working conditions, a large amount of photogenerated electron will be injected into the PW12−TiO2 layer immediately after they are produced. These injected electrons in the conduction

■

CONCLUSIONS In summary, PW12−TiO2 was used as a compact ETL in a planar perovskite solar cell. XPS and FTIR characterizations showed that the PW12 can be effectively composited in TiO2. XPS characterization and Mott−Schottky measurement confirmed the downshift of the conduction band edge of the composite, leading to a better band alignment between the PW12−TiO2 and perovksite. The better conduction band alignment can facilitate electron extraction from the perovskite layer to PW12−TiO2. Using the composite as electron transport material, the planar perovskite solar cells showed significantly reduced hysteresis and increased Voc up to 1.1 eV, achieving an efficiency of 15.45%.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b00846. Detailed description of the experimental procedure, XPS spectra, Mott−Schottky plot, and dark J−V curves (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Fax/Tel: +86 731 88876454 (F.L.). *E-mail: [email protected] (H.H.). 8524


Research Article

ACS Applied Materials & Interfaces Notes

Implications to Dye-Sensitized and Organic Solar Cells. Adv. Mater. 2013, 25, 3227−3233. (18) Ponseca, C. S., Jr.; Savenije, T. J.; Abdellah, M.; Zheng, K.; Yartsev, A.; Pascher, T.; Harlang, T.; Chabera, P.; Pullerits, T.; Stepanov, A.; et al. Organometal Halide Perovskite Solar Cell Materials Rationalized: Ultrafast Charge Generation, High and Microsecond-Long Balanced Mobilities, and Slow Recombination. J. Am. Chem. Soc. 2014, 136, 5189−5192. (19) Ip, A. H.; Quan, L. N.; Adachi, M. M.; McDowell, J. J.; Xu, J.; Kim, D. H.; Sargent, E. H. A Two-Step Route to Planar Perovskite Cells Exhibiting Reduced Hysteresis. Appl. Phys. Lett. 2015, 106, 143902. (20) Wojciechowski, K.; Stranks, S. D.; Abate, A.; Sadoughi, G.; Sadhanala, A.; Kopidakis, N.; Rumbles, G.; Li, C. Z.; Friend, R. H.; Jen, A. K.; et al. Heterojunction Modification for Highly Efficient OrganicInorganic Perovskite Solar Cells. ACS Nano 2014, 8, 12701−12709. (21) Kim, J. H.; Liang, P.-W.; Williams, S. T.; Cho, N.; Chueh, C.-C.; Glaz, M. S.; Ginger, D. S.; Jen, A. K. Y. High-Performance and Environmentally Stable Planar Heterojunction Perovskite Solar Cells Based on a Solution-Processed Copper-Doped Nickel Oxide HoleTransporting Layer. Adv. Mater. 2015, 27, 695−701. (22) Correa Baena, J. P.; Steier, L.; Tress, W.; Saliba, M.; Neutzner, S.; Matsui, T.; Giordano, F.; Jacobsson, T. J.; Srimath Kandada, A. R.; Zakeeruddin, S. M.; et al. Highly Efficient Planar Perovskite Solar Cells through Band Alignment Engineering. Energy Environ. Sci. 2015, 8, 2928−2934. (23) Snaith, H. J.; Abate, A.; Ball, J. M.; Eperon, G. E.; Leijtens, T.; Noel, N. K.; Stranks, S. D.; Wang, J. T.-W.; Wojciechowski, K.; Zhang, W. Anomalous Hysteresis in Perovskite Solar Cells. J. Phys. Chem. Lett. 2014, 5, 1511−1515. (24) Unger, E. L.; Hoke, E. T.; Bailie, C. D.; Nguyen, W. H.; Bowring, A. R.; Heumüller, T.; Christoforo, M. G.; McGehee, M. D. Hysteresis and Transient Behavior in Current-Voltage Measurements of Hybrid-Perovskite Absorber Solar Cells. Energy Environ. Sci. 2014, 7, 3690−3698. (25) Azpiroz, J. M.; Mosconi, E.; Bisquert, J.; De Angelis, F. Defect Migration in Methylammonium Lead Iodide and Its Role in Perovskite Solar Cell Operation. Energy Environ. Sci. 2015, 8, 2118−2127. (26) Wang, J.; Qin, M.; Tao, H.; Ke, W.; Chen, Z.; Wan, J.; Qin, P.; Xiong, L.; Lei, H.; Yu, H.; et al. Performance Enhancement of Perovskite Solar Cells with Mg-Doped TiO2 Compact Film as the Hole-Blocking Layer. Appl. Phys. Lett. 2015, 106, 121104. (27) Nagaoka, H.; Ma, F.; deQuilettes, D. W.; Vorpahl, S. M.; Glaz, M. S.; Colbert, A. E.; Ziffer, M. E.; Ginger, D. S. Zr Incorporation into TiO2 Electrodes Reduces Hysteresis and Improves Performance in Hybrid Perovskite Solar Cells While Increasing Carrier Lifetimes. J. Phys. Chem. Lett. 2015, 6, 669−675. (28) Heo, J. H.; You, M. S.; Chang, M. H.; Yin, W.; Ahn, T. K.; Lee, S.-J.; Sung, S.-J.; Kim, D. H.; Im, S. H. Hysteresis-Less Mesoscopic CH3NH3PbI3 Perovskite Hybrid Solar Cells by Introduction of LiTreated TiO2 Electrode. Nano Energy 2015, 15, 530−539. (29) Wang, S.-M.; Liu, L.; Chen, W.-L.; Su, Z.-M.; Wang, E.-B.; Li, C. Polyoxometalate/ TiO2 interfacial Layer with the Function of Accelerating Electron Transfer and Retarding Recombination for Dye-Sensitized Solar Cells. Ind. Eng. Chem. Res. 2014, 53, 150−156. (30) Wang, S. M.; Liu, L.; Chen, W. L.; Wang, E. B.; Su, Z. M. Polyoxometalate-Anatase TiO2 Composites Are Introduced into the Photoanode of Dye-Sensitized Solar Cells to Retard the Recombination and Increase the Electron Lifetime. Dalton Trans. 2013, 42, 2691−2695. (31) Roose, B.; Pathak, S.; Steiner, U. Doping of TiO2 for Sensitized Solar Cells. Chem. Soc. Rev. 2015, 44, 8326−8349. (32) Huang, C.; Fu, N.; Liu, F.; Jiang, L.; Hao, X.; Huang, H. Highly Efficient Perovskite Solar Cells with Precursor Composition-Dependent Morphology. Sol. Energy Mater. Sol. Cells 2016, 145 (Part 3), 231− 237. (33) Zhang, X.; Liu, F.; Huang, Q.-L.; Zhou, G.; Wang, Z.-S. DyeSensitized W-Doped TiO2 Solar Cells with a Tunable Conduction

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the Hong Kong Polytechnic University (Project Nos. G-UC69 and G-UC71) and National Natural Science Foundation of China (Grant No. 51204214).

■

REFERENCES

(1) Park, N.-G. Perovskite Solar Cells: An Emerging Photovoltaic Technology. Mater. Today 2015, 18, 65−72. (2) Stranks, S. D.; Nayak, P. K.; Zhang, W.; Stergiopoulos, T.; Snaith, H. J. Formation of Thin Films of Organic-Inorganic Perovskites for High-Efficiency Solar Cells. Angew. Chem., Int. Ed. 2015, 54, 3240− 3248. (3) Hodes, G. Applied Physics. Perovskite-Based Solar Cells. Science 2013, 342, 317−318. (4) Filippetti, A.; Delugas, P.; Mattoni, A. Radiative Recombination and Photoconversion of Methylammonium Lead Iodide Perovskite by First Principles: Properties of an Inorganic Semiconductor within a Hybrid Body. J. Phys. Chem. C 2014, 118, 24843−24853. (5) De Wolf, S.; Holovsky, J.; Moon, S.-J.; Löper, P.; Niesen, B.; Ledinsky, M.; Haug, F.-J.; Yum, J.-H.; Ballif, C. Organometallic Halide Perovskites: Sharp Optical Absorption Edge and Its Relation to Photovoltaic Performance. J. Phys. Chem. Lett. 2014, 5, 1035−1039. (6) Yin, W.-J.; Shi, T.; Yan, Y. Unusual Defect Physics in CH3NH3PbI3 Perovskite Solar Cell Absorber. Appl. Phys. Lett. 2014, 104, 063903. (7) Stoumpos, C. C.; Malliakas, C. D.; Kanatzidis, M. G. Semiconducting Tin and Lead Iodide Perovskites with Organic Cations: Phase Transitions, High Mobilities, and near-Infrared Photoluminescent Properties. Inorg. Chem. 2013, 52, 9019−9038. (8) Xing, G.; Mathews, N.; Sun, S.; Lim, S. S.; Lam, Y. M.; Gratzel, M.; Mhaisalkar, S.; Sum, T. C. Long-Range Balanced Electron- and Hole-Transport Lengths in Organic-Inorganic CH3NH3PbI3. Science 2013, 342, 344−347. (9) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J.; 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. (10) Zhou, H.; Chen, Q.; Li, G.; Luo, S.; Song, T. B.; Duan, H. S.; Hong, Z.; You, J.; Liu, Y.; Yang, Y. Photovoltaics. Interface Engineering of Highly Efficient Perovskite Solar Cells. Science 2014, 345, 542−546. (11) Liu, D.; Kelly, T. L. Perovskite Solar Cells with a Planar Heterojunction Structure Prepared Using Room-Temperature Solution Processing Techniques. Nat. Photonics 2013, 8, 133−138. (12) You, J.; Yang, Y.; Hong, Z.; Song, T.-B.; Meng, L.; Liu, Y.; Jiang, C.; Zhou, H.; Chang, W.-H.; Li, G.; et al. Moisture Assisted Perovskite Film Growth for High Performance Solar Cells. Appl. Phys. Lett. 2014, 105, 183902. (13) Heo, J. H.; Han, H. J.; Kim, D.; Ahn, T. K.; Im, S. H. HysteresisLess Inverted CH3NH3PbI3planar Perovskite Hybrid Solar Cells with 18.1% Power Conversion Efficiency. Energy Environ. Sci. 2015, 8, 1602−1608. (14) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050−6051. (15) Im, J. H.; Lee, C. R.; Lee, J. W.; Park, S. W.; Park, N. G. 6.5% Efficient Perovskite Quantum-Dot-Sensitized Solar Cell. Nanoscale 2011, 3, 4088−4093. (16) 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. (17) Leijtens, T.; Lim, J.; Teuscher, J.; Park, T.; Snaith, H. J. Charge Density Dependent Mobility of Organic Hole-Transporters and Mesoporous TiO2 Determined by Transient Mobility Spectroscopy: 8525


Research Article

ACS Applied Materials & Interfaces Band and Suppressed Charge Recombination. J. Phys. Chem. C 2011, 115, 12665−12671. (34) Lo, P. H.; Tsai, W. T.; Lee, J. T.; Hung, M. P. The Electrochemical Behavior of Electroless Plated Ni-P Alloys in Concentrated Naoh Solution. J. Electrochem. Soc. 1995, 142, 91−96. (35) Sanjinés, R.; Tang, H.; Berger, H.; Gozzo, F.; Margaritondo, G.; Lévy, F. Electronic Structure of Anatase TiO2 Oxide. J. Appl. Phys. 1994, 75, 2945−2951. (36) Vasilopoulou, M.; Soultati, A.; Georgiadou, D. G.; Stergiopoulos, T.; Palilis, L. C.; Kennou, S.; Stathopoulos, N. A.; Davazoglou, D.; Argitis, P. Hydrogenated under-Stoichiometric Tungsten Oxide Anode Interlayers for Efficient and Stable Organic Photovoltaics. J. Mater. Chem. A 2014, 2, 1738−1749. (37) Sathasivam, S.; Bhachu, D. S.; Lu, Y.; Chadwick, N.; Althabaiti, S. A.; Alyoubi, A. O.; Basahel, S. N.; Carmalt, C. J.; Parkin, I. P. Tungsten Doped TiO2 with Enhanced Photocatalytic and Optoelectrical Properties Via Aerosol Assisted Chemical Vapor Deposition. Sci. Rep. 2015, 5, 10952. (38) Edri, E.; Kirmayer, S.; Mukhopadhyay, S.; Gartsman, K.; Hodes, G.; Cahen, D. Elucidating the Charge Carrier Separation and Working Mechanism of CH3NH3PbI3‑XClX Perovskite Solar Cells. Nat. Commun. 2014, 5, 3461. (39) 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. (40) Christians, J. A.; Fung, R. C.; Kamat, P. V. An Inorganic Hole Conductor for Organo-Lead Halide Perovskite Solar Cells. Improved Hole Conductivity with Copper Iodide. J. Am. Chem. Soc. 2014, 136, 758−764. (41) Gouda, L.; Gottesman, R.; Ginsburg, A.; Keller, D. A.; Haltzi, E.; Hu, J.; Tirosh, S.; Anderson, A. Y.; Zaban, A.; Boix, P. P. Open Circuit Potential Build-up in Perovskite Solar Cells from Dark Conditions to 1 Sun. J. Phys. Chem. Lett. 2015, 6, 4640−4645. (42) Chang, S. H.; Lin, K.-F.; Cheng, H.-M.; Chen, C.-C.; Wu, W.-T.; Chen, W.-N.; Wu, P.-J.; Chen, S.-H.; Wu, C.-G. Influence of Organic Cations on High-Performance CH3NH3PbI3 Based Photovoltaics. Sol. Energy Mater. Sol. Cells 2016, 145 (Part 3), 375−381.

8526
