www.advmat.de
COMMUNICATION
www.MaterialsViews.com
Perovskite Photovoltaics with Outstanding Performance Produced by Chemical Conversion of Bilayer Mesostructured Lead Halide/TiO2 Films Chenyi Yi,* Xiong Li, Jingshan Luo, Shaik M. Zakeeruddin, and Michael Grätzel* Metal halide perovskites have recently emerged as attractive solar cell materials.[1] Although methylammonium lead iodide (MAPbI3) has been most frequently employed as light harvester,[2] the use of formamidinium lead iodide (FAPbI3) is rising because of its greater thermal stability and red-shifted absorption spectrum which can potentially produce higher photocurrents than MAPbI3.[3] To realize high quality MAPbI3 perovskite films, a variety of methodologies have been developed, such as sequential deposition,[1c] vacuum evaporation,[4] vapor-assisted deposition[5], and solvent engineering.[2j] The sequential deposition was also applied to produce FAPbI3 yielding good power conversion efficiencies.[3a] It involves depositing a layer of PbX2 and transforming it into perovskite by reacting with the organic ammonium halide solution.[1c] In general, a mesoscopic oxide scaffold such as mesoporous TiO2 is applied to accelerate the transformation from lead halide to perovskite.[1c] Recent studies show that a thick (≥300 nm) uniform perovskite capping layer on top of the mesoporous TiO2 film boosts the power conversion efficiency of the solar cells,[3e] as it harvests the photons in the red and near IR range that are transmitted by the TiO2/ perovskite nanocomposite layer. Accordingly, a sufficiently thick PbX2 film is needed atop the nanocrystalline TiO2 scaffold to form a smooth perovskite capping layer. However, the penetration of the organic ammonium iodide solution into the thick compact PbX2 over-layer hardly proceeds beyond the film surface, resulting in incomplete conversion of the PbX2 to perovskite.[1c] In order to improve the conversion of PbI2, Huang and co-workers developed an interdiffusion method for producing MAPbI3 films.[2f ] Very recently, Seok and co-workers reported an intramolecular exchange method to produce FAPbI3 films.[3f ] It mainly involves preparation of a dense PbI2(DMSO) film, followed by molecular exchange reaction and annealing. During the exchange reaction, the dimethyl sulfoxide (DMSO) ligand in PbI2(DMSO) complex directly exchanges with formamidinium iodide (FAI) to form the FAPbI3 perovskite film.
Dr. C. Yi, Dr. X. Li, Dr. J. Luo, Dr. S. M. Zakeeruddin, Prof. M. Grätzel Laboratory for Photonics and Interfaces Institute of Chemical Sciences and Engineering School of Basic Sciences Ecole Polytechnique Federale de Lausanne CH-1015, Lausanne, Switzerland E-mail: [email protected]; [email protected]
DOI: 10.1002/adma.201506049
2964
wileyonlinelibrary.com
Here, we present a new method of producing high quality perovskite films via sequential deposition, introducing a PbX2 capping layer that is endowed with a network of interconnected nanopores. We prepare the mesoporous film by spin-coating a PbX2 solution containing PbI2 and PbBr2 from a mixture of N,N’-dimethylformamide (DMF) and DMSO onto a nanocrystalline TiO2 scaffold and subsequent annealing. During the conversion reaction, the formamidinium/methylammonium halide solution infiltrates the entire pore space accessing rapidly the interior of the mesoscopic PbI2 layer rather than being retained on its surface. Thus, the mesoporous lead halide architecture provides a powerful tool to accomplish rapid and complete transformation of lead halide into the perovskite. These pores also provide the space needed to accommodate the large volume expansion accompanying the insertion of the cations into the PbX2 lattice. The pores are filled during the conversion step, resulting in the formation of a compact perovskite film. We note that during the refereeing procedure of our manuscript, two papers appeared reporting on planar heterojunction solar cells with best efficiencies in the 15%–16% range, based on smooth MAPbI3 films that were produced by conversion of nanostructured PbI2 layers.[6] With our new approach, we produce the desired contiguous and dense perovskite capping layer consisting of FA1–xMAxPb(I1–xBrx)3 on top of a mesoporous TiO2 scaffold, which in turn is well-infiltrated with the perovskite. Using this procedure, we fabricate perovskite photovoltaics with solar to electric power conversion efficiencies (PCEs) exceeding 20% with negligible hysteresis in their J–V curve. We deposit the PbX2 films by spin-coating a solution containing PbI2 and PbBr2 of 85/15 molar ratio in a DMSO and DMF mixture (2/8 v/v) on a nanocrystalline TiO2 film prepared as described below in the Experimental Section and heating it at 70 °C for 10 min. Figure 1a,b shows scanning electron microscope (SEM) top-view and cross-sectional images of the porous PbX2 film. The top-view image shows that the PbX2 crystals are in the size range of 40–50 nm and that a variety of pores are distributed over the surface. The cross-sectional image demonstrates that the voids penetrate down to the nanocrystalline TiO2 scaffold. In addition, the back-scattered SEM image in Figure 3b reveals that the lead halide is well infiltrated into the TiO2 scaffold, serving as a precursor for perovskite to fill the mesoporous TiO2. For comparison, we also deposited a PbX2 film by using pure DMF or DMSO as solvents maintaining otherwise the same conditions. Figure 1c illustrates a SEM top-view image of the PbX2 film produced from DMF solution showing the absence of any voids. The SEM cross-sectional image
© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2016, 28, 2964–2970
www.advmat.de www.MaterialsViews.com
COMMUNICATION
DMSO appears to be responsible for the pore formation within the lead halide films. From the dense structure of the freshly prepared film in Figure S2 (Supporting Information), it can be inferred that the mesopores are produced during the annealing when the DMSO is evaporated. Figure S3 (Supporting Information) demonstrates that the pores become larger and the quantities of pores are increasing by increasing the volume ratio of DMSO/DMF from 5/95 to 30/70. It is known that PbX2 forms PbX2(DMSO) complexes such as PbI2(DMSO) and PbBr2(DMSO) with DMSO, where the DMSO ligand is sufficiently labile to be ejected upon heating.[3f,7] We propose that such PbX2(DMSO) complexes are present after spin-coating and that the DMSO is released during the annealing step, producing a porous PbX2 film. The formation of pores is facilitated by the large volume contraction accompanying the ligand loss. In order to investigate more closely the 3D Figure 1. a) SEM top-view of a porous PbX2 film deposited from the DMF/DMSO solvent mixnanostructure of the pores within the PbX2, ture. b) Cross-sectional image of the porous PbX2. c) Top-view of the dense PbX2 film deposited we performed 3D tomography of the film from DMF. d) Cross-sectional image of the dense PbX2. using a dual focused ion beam (FIB) to slice the layer and characterizing it by SEM (FIBSEM). We deposited a thin layer of carbon on top of the lead Figure 1d also confirms the compact structure of the film. SEM halide film to protect it from damage by the ion beam before top-view images of the film produced from pure DMSO solustarting the FIB. Both standard SEM and back-scattered SEM tion (Figure S1, Supporting Information) show the presence are collected during the FIB. Figure 2c shows a 3D view of the of large striated structures covering only part of the nanocrysfilm cut through the PbX2 layer. It reveals the presence of welltalline TiO2 film most likely due to solvent dewetting. To sum up, under otherwise identical conditions, DMF solutions of the developed pores that are evenly spread throughout the film lead halide produces dense lead halide films while DMSO soluforming an interconnected network, consistent with the SEM tion produces striated deposits. By contrast, the DMF/DMSO cross-sectional image of the lead halide. To gain deeper insight mixture produces mesoporous PbX2 films. The presence of into the morphology of the pores, we cut slices at different
Figure 2. 3D FIB tomography of porous PbX2 deposited on the TiO2 scaffold. a) SEM cross-sectional of the PbX2 layer (the red lines indicate the slice positions of the corresponding panels (c), (d), and (e). b) The same measured by back-scattered method (the red line indicates the position of the slice of (f). c) 3D image of the porous PbX2 cut through the PbX2 layer. d) Horizontal slice of the PbX2 overlayer close to the surface. e) Horizontal slice of PbX2 taken from within the inner part of the porous PbX2 layer. f) Horizontal back-scattered image in the mesoporous PbX2/TiO2 composite layer showing complete infiltration of the TiO2 by the lead halide.
Adv. Mater. 2016, 28, 2964–2970
© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
2965
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
positions of the lead halide layer. Figure 2d is from a slice close to the surface, revealing a structure similar to the surface itself, even though the size and number of pores are larger. Figure 2e shows a slice from the interior of the PbX2 layer demonstrating the connectivity of the pores. Figure 2f presents a slice of the PbX2/TiO2 composite layer imaged by the back-scattering method. The grey background representing TiO2 is filled with white spots representing the lead halide. The overall picture emerging from this analysis is that the nanocrystalline TiO2 scaffold is well infiltrated by PbX2, and covered by a highly porous PbX2 layer. These pores become smaller when they are closer to the surface. Thus, only a small percentage of the holes are visible on the surface. The interconnected pores not only serve as channels for transporting the
FAI/MABr (methylammonium bromide) solution, allowing all the PbX2 to be impregnated rapidly, but also offer a large surface area to promote the conversion of the halide to the perovskite by insertion of the FA and MA cations. It should be noted that the size of PbI2(DMSO) is comparable to that of the FAPbI3. Thus, the pores created by the release of the DMSO provide needful space for the volume expansion occurring during the conversion reaction, resulting in a comparable thickness for the compact perovskite and the porous PbX2 film. The green and orange curves in Figure 3b show X-ray powder diffraction (XRD) spectra of the PbX2 film produced from DMSO/DMF, before and after heating respectively. The XRD patterns of the as-coated PbX2 film resemble that of the PbX2(DMSO),[3f ] while that of the annealed film clearly shows
Figure 3. a) Absorption spectra of PbX2 films and perovskite films. b) XRD of PbX2 and perovskite films. c) J–V curves of forward, backward scan, and the average of the best performing perovskite solar cell. d) IPCE spectrum (red) with integrated current curve (black) of the perovskite solar cell exhibiting the highest performance.
2966
wileyonlinelibrary.com
© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2016, 28, 2964–2970
www.advmat.de www.MaterialsViews.com
Adv. Mater. 2016, 28, 2964–2970
spiro-MeOTAD functions as a hole-transport layer. The top-view reveals the compact morphology of the perovskite capping layer which is formed by 150–300 nm-sized densely packed grains without voids. The dense structure of the capping layer is also confirmed by the SEM cross-sectional image of the whole device shown in Figure 4b. The perovskite layer thickness is comparable to the thickness of its precursor PbX2. To investigate the infiltration of the perovskite into the porous TiO2, we examined its structure by elemental mapping using transmission electron microscope (TEM) and STEMEDS. Figure 4c shows a HAADF-STEM image of the nanocomposite formed by TiO2 scaffold and the perovskite. Energy-dispersive X-ray spectroscopy demonstrates that all the elements contained in the perovskite, i.e., lead, iodine, bromine, carbon, and nitrogen show a similar spatial pattern, signifying that the lead, formamidinium, and methylammonium cations as well as the iodide and bromide anions are evenly distributed in the film. By contrast, the distribution of the lead and titanium is complementary. Merging the distribution of these two elements generates the pattern of the HAADF image. In Figure S4 (Supporting Information), we selected a Ti and a Pb rich area for elemental mapping. In both areas, all the perovskite and TiO2 related elements coexist even though the relative intensities are different. Together with the SEM cross-sectional image, they prove the excellent infiltration of the perovskite in the pore of the TiO2 nanoparticles. The high-resolution TEM image in Figure S5b (Supporting Information) reveals the highly crystalline characteristics of the perovskite particles. We examined the photovoltaic properties of the devices by measuring their current–voltage (J–V) curves and their incidentphoton-to-current conversion efficiency (IPCE). All the devices fabricated from the porous lead halide show a much higher PCE than those prepared from dense lead halide. Perovskite solar cells prepared from dense PbX2 films show large PCE variation between 9% and 16%, indicating the low reproducibility of the method, probably due to the incapability of controlling PbX2 conversion rate and the morphology of the perovskite films. Figure S6 (Supporting Information) shows representative J–V curve of perovskite solar cells made from pure DMF and pure DMSO solution, with a PCE of 13.1% (DMF, Voc, Jsc, and FF being 1.04V, 20.0 mA cm−2, and 61.4%) and 9.5% (DMSO, Voc, Jsc, and FF being 0.84V, 16.1 mA cm−2, and 67.6%). Both devices show strong hysteresis. The relatively low Jsc is attributed to the incomplete conversion of the PbX2 (DMF) or inhomogeneous film morphology (DMSO), which agrees well with the absorption measurement and the IPCE spectrum. By contrast, the perovskite solar cells made from the porous PbX2 show an average efficiency of 19.1% with 40 devices. A histogram of the devices is shown in Figure S7 (Supporting Information); while the PV metrics of individual cells are listed in Table S1 (Supporting Information). The best cell shows PCE (average of forward and reverse sweeps) of 20.4% with a short circuit current density (Jsc) of 23 mA cm−2, an open circuit voltage (Voc) of 1.11V, and a fill factor of 78.8% under simulated standard air mass 1.5 G illuminations. Importantly, the hysteresis between the forward and backward scan is negligible as shown in Figure 3c. The stabilized maximum power output measurements presented in Figure S8 (Supporting Information) show good agreement between the measured PCE and power output.
© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
COMMUNICATION
the absence of the PbX2(DMSO) reflections. The series of new diffraction peaks that are in good agreement with literature data on the hexagonal 2H polytype phase of lead iodide[1c] appear during the heating, implying that the bromide ions are incorporated as a dopant in the PbI2 upon annealing and do not form a separate phase. The transformation of PbX2(DMSO) into PbX2 on heating is apparent also from the UV–Vis absorption spectra. The green and orange curves in Figure 3a show the absorption of the film before and after annealing, respectively. The freshly coated film shows an absorption peak at 380 nm with an onset at 440 nm matching the spectrum of PbX2(DMSO).[2k] Upon heating, the peak at 380 nm disappears and a new absorption appears with a plateau at 490 nm and an absorption onset at 510 nm, which fits the band edge absorption of PbX2. The PbX2 composition of the annealed film is further confirmed by the agreement of its absorption spectrum with that of a PbX2 reference sample produced from DMF. We prepared the perovskite film by exposing the porous lead halide film to a mixture of methylammonium bromide and formamidinium iodide in isopropanol. The color of the film changed from yellow to dark brown immediately upon contacting it with the organic ammonium halide solution, indicating rapid conversion of lead halide to perovskite. In contrast, when the same solution was applied to the compact lead halide over-layer produced from a DMF solution of the lead halides, the brown color appeared very slowly reaching a much weaker intensity than that of the perovskite produced from the porous lead halide layer. The orange, blue, and red curves in Figure 3a show the UV–Vis absorption spectra of the porous PbX2 and the perovskite films produced from the dense and porous PbX2 film, respectively. After the conversion, the films absorb light from the UV region to 800 nm with a plateau in the 600–750 nm range. The perovskite film produced from the porous PbX2 shows about twice the absorbance than the one produced from the condensed PbX2, suggesting a better conversion of the porous with respect to the compact precursor. We also observe a shoulder at around 490 nm for the blue curve, which probably arises from residual PbX2, confirming the incomplete conversion of the dense film. Figure 3b shows that a series of new X-ray diffraction (XRD) peaks corresponding to the trigonal phase of the FA1-xMAxPb(I1-xBrx)3 perovskite formed during the conversion. However, the XRD pattern of the blue curve corresponding to the perovskite film produced from the dense lead halide maintains an intense peak at 12.7°, arising from the diffraction by (001) planes of PbI2. This demonstrates that the conversion of the dense PbX2 film is incomplete. On the other hand, the peaks of PbI2 disappear in the red curve in Figure 3b, corresponding to the XRD spectra of the perovskite film formed from the porous PbX2. It indicates the complete conversion of porous PbX2 to the perovskite. Figure 4a shows a SEM top-view of the perovskite film produced by conversion of the porous PbX2 precursor while Figure 4b shows a cross-sectional image of a complete device composed of superimposed layers of fluorine doped tin oxide (FTO)/compact TiO2/mesoporous TiO2-perovskite nanocomposite/perovskite capping layer/spiro-MeOTAD/Au. The
2967
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
Figure 4. a) SEM top-view image of the perovskite layer. b) SEM cross-sectional image of the complete device. c) High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and scanning transmission electron microscopy coupled with energy dispersive X-ray spectroscopy (STEM-EDS) of the perovskite infiltrated TiO2.
The IPCE spectrum demonstrates a high photon-to-current conversion efficiency from 400 to 800 nm. Integration of the IPCE over the AM 1.5 G spectrum yields a photocurrent density of 22.7 mA cm−2 as shown in Figure 3d, which is in excellent agreement with the short circuit photocurrent density of 23 mA cm−2 derived from the J–V curve. Initial stability test of the device (in the dark under dry condition) shows promising result with only 5% PCE decrease over 1000 h. The superior performances of the perovskite solar cells exemplify the great potential of the new approach to produce high quality perovskite films by sequential deposition using porous PbX2 film as intermediate. In conclusion, we introduce a new and effective strategy to produce high quality perovskite films by chemical conversion of mesoporous PbX2 precursor films that can be prepared by simple solution processing. The interconnected mesopores serve as channels to allow for rapid infiltration by the solution of the reagents and provide a large internal surface area, enhancing the insertion of the organic ammonium halides into the lead halide lattice, resulting in rapid and quantitative conversion to perovskite. In this fashion, we realized perovskite
2968
wileyonlinelibrary.com
films of excellent optoelectronic properties, allowing fabrication of perovskite solar cells with a power conversion efficiency of 20.4%. Even higher PCEs appear feasible by further refinements of the method, which is presently being implemented. This strategy is expected to be of general importance for the realization of semiconductor films with outstanding optoelectronic properties.
Experimental Section Materials: MABr was synthesized by stirring a mixture of methylamine (5 mL, 40% in methanol) and HBr (6 mL, 48% in water) at room temperature for 2 h. The solvent was evaporated in a rotavap and the remaining solid was poured into 50 mL ether and filtrated. The precipitate was washed three times with ether and dried under vacuum to get a white crystalline MABr product. FAI was synthesized according to a reported procedure.[3d] All the other materials were purchased from commercial sources and used as received. Device Fabrication: FTO-coated glass substrates were patterned by laser etching and cleaned by ultrasonication in detergent solution and subsequent rinsing with deionized water, acetone, and ethanol. A TiO2
© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2016, 28, 2964–2970
www.advmat.de www.MaterialsViews.com
Acknowledgements The authors would like to thank Dr. Marco Cantoni from the Interdisciplinary Centre for Electron Microscopy (CIME) in EPFL for the assistance in the FIB-SEM measurements. Financial support from CCEM-CH in the 9th call proposal 906: CONNECT PV, the Swiss National Science Foundation (SNF)-NRP70 “Energy Turnaround”, a grant from King Abudulaziz City of Science and Technology (KACST) Saudi Arabia, and the GRAPHENE project supported by the European Commission Seventh Framework Program under contract 604391 is gratefully acknowledged. J.L. would like to acknowledge financial
Adv. Mater. 2016, 28, 2964–2970
support by an EPFL Fellowship co-funded by Marie Curie from the European Union’s Seventh Framework Program for research, technological development, and demonstration under grant agreement no. 291771. Received: December 5, 2015 Revised: January 8, 2016 Published online: February 19, 2016
[1] a) H. S. Kim, C. R. Lee, J. H. Im, K. B. Lee, T. Moehl, A. Marchioro, S. J. Moon, R. Humphry-Baker, J. H. Yum, J. E. Moser, M. Gratzel, N. G. Park, Sci. Rep.-U.K 2012, 2, 591; b) M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. J. Snaith, Science 2012, 338, 643; c) J. Burschka, N. Pellet, S. J. Moon, R. Humphry-Baker, P. Gao, M. K. Nazeeruddin, M. Gratzel, Nature 2013, 499, 316; d) D. B. Mitzi, J. Mater. Chem. 2004, 14, 2355; e) F. Hao, C. C. Stoumpos, R. P. H. Chang, M. G. Kanatzidis, J. Am. Chem. Soc. 2014, 136, 8094; f) A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 2009, 131, 6050; g) A. Y. Mei, X. Li, L. F. Liu, Z. L. Ku, T. F. Liu, Y. G. Rong, M. Xu, M. Hu, J. Z. Chen, Y. Yang, M. Gratzel, H. W. Han, Science 2014, 345, 295; h) I. C. Smith, E. T. Hoke, D. Solis-Ibarra, M. D. McGehee, H. I. Karunadasa, Angew. Chem. Int. Ed. 2014, 53, 11232; i) E. L. Unger, E. T. Hoke, C. D. Bailie, W. H. Nguyen, A. R. Bowring, T. Heumuller, M. G. Christoforo, M. D. McGehee, Energy Environ. Sci. 2014, 7, 3690; j) C. C. Stoumpos, C. D. Malliakas, M. G. Kanatzidis, Inorg. Chem. 2013, 52, 9019; k) Y. Yang, Y. Yan, M. Yang, S. Choi, K. Zhu, J. M. Luther, M. C. Beard, Nat. Commun. 2015, 6, 7961. [2] a) G. C. Xing, N. Mathews, S. Y. Sun, S. S. Lim, Y. M. Lam, M. Gratzel, S. Mhaisalkar, T. C. Sum, Science 2013, 342, 344; b) H. P. Zhou, Q. Chen, G. Li, S. Luo, T. B. Song, H. S. Duan, Z. R. Hong, J. B. You, Y. S. Liu, Y. Yang, Science 2014, 345, 542; c) J. H. Im, I. H. Jang, N. Pellet, M. Gratzel, N. G. Park, Nat. Nanotechnol. 2014, 9, 927; d) Y. Z. Wu, A. Islam, X. D. Yang, C. J. Qin, J. Liu, K. Zhang, W. Q. Peng, L. Y. Han, Energy Environ. Sci. 2014, 7, 2934; e) Z. G. Xiao, Q. F. Dong, C. Bi, Y. C. Shao, Y. B. Yuan, J. S. Huang, Adv. Mater. 2014, 26, 6503; f) Z. G. Xiao, C. Bi, Y. C. Shao, Q. F. Dong, Q. Wang, Y. B. Yuan, C. G. Wang, Y. L. Gao, J. S. Huang, Energy Environ. Sci. 2014, 7, 2619; g) T. Y. Zhang, M. J. Yang, Y. X. Zhao, K. Zhu, Nano Lett. 2015, 15, 3959; h) M. D. Xiao, F. Z. Huang, W. C. Huang, Y. Dkhissi, Y. Zhu, J. Etheridge, A. Gray-Weale, U. Bach, Y. B. Cheng, L. Spiccia, Angew. Chem. Int. Ed. 2014, 53, 9898; i) K. Y. Yan, M. Z. Long, T. K. Zhang, Z. H. Wei, H. N. Chen, S. H. Yang, J. B. Xu, J. Am. Chem. Soc. 2015, 137, 4460; j) N. J. Jeon, J. H. Noh, Y. C. Kim, W. S. Yang, S. Ryu, S. Il Seol, Nat. Mater. 2014, 13, 897; k) W. Li, J. Fan, J. Li, Y. Mai, L. Wang, J. Am. Chem. Soc. 2015, 137, 10399; l) N. Ahn, D. Y. Son, I. H. Jang, S. M. Kang, M. Choi, N. G. Park, J. Am. Chem. Soc. 2015, 137, 8696; m) J. H. Kim, P. W. Liang, S. T. Williams, N. Cho, C. C. Chueh, M. S. Glaz, D. S. Ginger, A. K. Y. Jen, Adv. Mater. 2015, 27, 695; n) H. S. Kim, I. Mora-Sero, V. Gonzalez-Pedro, F. FabregatSantiago, E. J. Juarez-Perez, N. G. Park, J. Bisquert, Nat. Commun. 2013, 4, 2242; o) J. M. Azpiroz, E. Mosconi, J. Bisquert, F. De Angelis, Energy Environ. Sci. 2015, 8, 2118; p) Q. Q. Lin, A. Armin, R. C. R. Nagiri, P. L. Burn, P. Meredith, Nat. Photonics 2015, 9, 106; q) P. W. Liang, C. Y. Liao, C. C. Chueh, F. Zuo, S. T. Williams, X. K. Xin, J. J. Lin, A. K. Y. Jen, Adv. Mater. 2014, 26, 3748; r) D. Shi, V. Adinolfi, R. Comin, M. J. Yuan, E. Alarousu, A. Buin, Y. Chen, S. Hoogland, A. Rothenberger, K. Katsiev, Y. Losovyj, X. Zhang, P. A. Dowben, O. F. Mohammed, E. H. Sargent, O. M. Bakr, Science 2015, 347, 519; s) J. S. Manser, P. V. Kamat, Nat. Photonics
© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
COMMUNICATION
compact layer was then deposited on the FTO by aerosol spray pyrolysis at 450 °C using a titanium diisopropoxide bis(acetylacetonate) solution (75% in 2-propanol, Sigma-Aldrich) in ethanol (1:19, weight ratio) as precursor and oxygen as carrier gas. The mesoporous TiO2 layer was deposited by spin-coating at 5000 rpm for 20 s using a TiO2 paste diluted in ethanol (1:6, weight ratio). After drying at 125 °C, the TiO2 films were gradually heated to 500 °C, annealed at this temperature for 15 min, and cooled to room temperature. The films were sintered at 500 °C for 20 min before use. The PbX2 films were prepared by consecutive two-step spin-coating process at 1000 and 4000 rpm with a solution of 1.2 M PbX2 (85:15 molar ratio of PbI2:PbBr2) in DMF/DMSO (80/20 v/v) mixture (porous film) or DMF (dense film), DMSO (inhomogeneous film). The films were left to dry at room temperature in the dry box and then annealed at 70 °C for 10 min. After cooling to room temperature, the film was exposed to 0.1 M of FAI and MABr (85:15 molar ratio) mixture in isopropanol for 90 s, then the film was spun at 4000 rpm and annealed at 100 °C for 10 min. After cooling to room temperature, the HTM was deposited by spin-coating at 4000 rpm for 20 s. The HTM solution consists of 72.3 mg (2,29,7,79-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene)(spiro-OMeTAD), 28.8 µL 4-tert-butylpyridine, and 17.5 µL of 520 mg mL−1 lithium bis(trifluoromethylsulphonyl)imide acetonitrile solution dissolved in 1 mL chlorobenzene. Finally, 80 nm of gold was thermally evaporated on top of the device to form the back contact. Materials Characterization: The PbX2 and perovskite films used for UV–Vis absorption spectra, XRD, SEM, and TEM were prepared by spin-coating on mesoporous TiO2 films following the above-mentioned procedure. The UV–Vis spectra were measured with a PerkinElmer Lambda 950 spectrophotometer. The XRD patterns were recorded with a Bruker D8 Discover diffractometer in Bragg–Brentano mode, using Cu K α radiation (1.540598 Å) and a Ni β-filter. Spectra were acquired with a linear silicon strip “Lynx Eye” detector from 2θ = 10°–60° at a scan rate of 2° min−1, step width of 0.02°, and a source slit width of 1 mm. The morphology of the films was characterized using a highresolution scanning electron microscope (ZEISS Merlin). The FIB-SEM was characterized using Zeiss NVision 40 CrossBeam with FIB and high-resolution field emission SEM. The perovskite structures and compositions were examined by a high-resolution transmission electron microscope (Technai Osiris, FEI), equipped with an Oxford EDS detector. Device Characterization: Current–voltage characteristics of the devices were measured with a solar simulator composed of light source and a digital source meter (Keithley Model 2400). The light source was a 450 W xenon lamp (Oriel) equipped with a Schott K113 Tempax sunlight filter (Praezisions Glas & Optik GmbH) to match the emission spectrum of the lamp to the AM1.5G standard. Before each measurement, the light intensity was calibrated by a Si reference diode equipped with an infrared cut-off filter (KG-3, Schott). IPCE spectra were recorded as functions of wavelength under a constant white light bias of ≈5 mW cm−2 supplied by an array of white light emitting diodes. The excitation beam coming from a 300-W xenon lamp (ILC Technology) was focused through a Gemini-180 double monochromator (Jobin Yvon Ltd) and chopped at ≈2 Hz. The signal was recorded using a Model SR830 DSP Lock-In Amplifier (Stanford Research Systems). All measurements were conducted using a nonreflective metal aperture of 0.16 cm2 to define the active area of the device and avoid light scattering through the sides.
2969
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
2970
2014, 8, 737; t) C. Law, L. Miseikis, S. Dimitrov, P. Shakya-Tuladhar, X. E. Li, P. R. F. Barnes, J. Durrant, B. C. O’Regan, Adv. Mater. 2014, 26, 6268; u) F. Hao, C. C. Stoumpos, Z. Liu, R. P. H. Chang, M. G. Kanatzidis, J. Am. Chem. Soc. 2014, 136, 16411; v) X. Li, M. I. Dar, C. Yi, J. Luo, M. Tschumi, S. M. Zakeeruddin, M. K. Nazeeruddin, H. Han, M. Gratzel, Nat. Chem. 2015, 7, 703. [3] a) N. Pellet, P. Gao, G. Gregori, T. Y. Yang, M. K. Nazeeruddin, J. Maier, M. Gratzel, Angew. Chem. Int. Ed. 2014, 53, 3151; b) F. Wang, H. Yu, H. H. Xu, N. Zhao, Adv. Funct. Mater. 2015, 25, 1120; c) G. E. Eperon, S. D. Stranks, C. Menelaou, M. B. Johnston, L. M. Herz, H. J. Snaith, Energy Environ. Sci. 2014, 7, 982; d) J. W. Lee, D. J. Seol, A. N. Cho, N. G. Park, Adv. Mater. 2014, 26, 4991; e) N. J. Jeon, J. H. Noh, W. S. Yang, Y. C. Kim, S. Ryu,
wileyonlinelibrary.com
[4] [5] [6]
[7]
J. Seo, S. I. Seok, Nature 2015, 517, 476; f) W. S. Yang, J. H. Noh, N. J. Jeon, Y. C. Kim, S. Ryu, J. Seo, S. I. Seok, Science 2015, 348, 1234. M. Z. Liu, M. B. Johnston, H. J. Snaith, Nature 2013, 501, 395. Q. Chen, H. P. Zhou, Z. R. Hong, S. Luo, H. S. Duan, H. H. Wang, Y. S. Liu, G. Li, Y. Yang, J. Am. Chem. Soc. 2014, 136, 622. a) H. Zhang, J. Mao, H. He, D. Zhang, H. L. Zhu, F. Xie, K. S. Wong, M. Grätzel, W. C. H. Choy, Adv. Energy Mater. 2015, 5, 1501354; b) T. Liu, Q. Hu, J. Wu, K. Chen, L. Zhao, F. Liu, C. Wang, H. Lu, S. Jia, T. Russell, R. Zhu, Q. Gong, Adv. Energy Mater. 2015, 5, 1501890. J. Selbin, W. E. Bull, L. H. Holmes, J. Inorg. Nucl. Chem. 1961, 16, 219.
© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2016, 28, 2964–2970
COMMUNICATION
www.MaterialsViews.com
Perovskite Photovoltaics with Outstanding Performance Produced by Chemical Conversion of Bilayer Mesostructured Lead Halide/TiO2 Films Chenyi Yi,* Xiong Li, Jingshan Luo, Shaik M. Zakeeruddin, and Michael Grätzel* Metal halide perovskites have recently emerged as attractive solar cell materials.[1] Although methylammonium lead iodide (MAPbI3) has been most frequently employed as light harvester,[2] the use of formamidinium lead iodide (FAPbI3) is rising because of its greater thermal stability and red-shifted absorption spectrum which can potentially produce higher photocurrents than MAPbI3.[3] To realize high quality MAPbI3 perovskite films, a variety of methodologies have been developed, such as sequential deposition,[1c] vacuum evaporation,[4] vapor-assisted deposition[5], and solvent engineering.[2j] The sequential deposition was also applied to produce FAPbI3 yielding good power conversion efficiencies.[3a] It involves depositing a layer of PbX2 and transforming it into perovskite by reacting with the organic ammonium halide solution.[1c] In general, a mesoscopic oxide scaffold such as mesoporous TiO2 is applied to accelerate the transformation from lead halide to perovskite.[1c] Recent studies show that a thick (≥300 nm) uniform perovskite capping layer on top of the mesoporous TiO2 film boosts the power conversion efficiency of the solar cells,[3e] as it harvests the photons in the red and near IR range that are transmitted by the TiO2/ perovskite nanocomposite layer. Accordingly, a sufficiently thick PbX2 film is needed atop the nanocrystalline TiO2 scaffold to form a smooth perovskite capping layer. However, the penetration of the organic ammonium iodide solution into the thick compact PbX2 over-layer hardly proceeds beyond the film surface, resulting in incomplete conversion of the PbX2 to perovskite.[1c] In order to improve the conversion of PbI2, Huang and co-workers developed an interdiffusion method for producing MAPbI3 films.[2f ] Very recently, Seok and co-workers reported an intramolecular exchange method to produce FAPbI3 films.[3f ] It mainly involves preparation of a dense PbI2(DMSO) film, followed by molecular exchange reaction and annealing. During the exchange reaction, the dimethyl sulfoxide (DMSO) ligand in PbI2(DMSO) complex directly exchanges with formamidinium iodide (FAI) to form the FAPbI3 perovskite film.
Dr. C. Yi, Dr. X. Li, Dr. J. Luo, Dr. S. M. Zakeeruddin, Prof. M. Grätzel Laboratory for Photonics and Interfaces Institute of Chemical Sciences and Engineering School of Basic Sciences Ecole Polytechnique Federale de Lausanne CH-1015, Lausanne, Switzerland E-mail: [email protected]; [email protected]
DOI: 10.1002/adma.201506049
2964
wileyonlinelibrary.com
Here, we present a new method of producing high quality perovskite films via sequential deposition, introducing a PbX2 capping layer that is endowed with a network of interconnected nanopores. We prepare the mesoporous film by spin-coating a PbX2 solution containing PbI2 and PbBr2 from a mixture of N,N’-dimethylformamide (DMF) and DMSO onto a nanocrystalline TiO2 scaffold and subsequent annealing. During the conversion reaction, the formamidinium/methylammonium halide solution infiltrates the entire pore space accessing rapidly the interior of the mesoscopic PbI2 layer rather than being retained on its surface. Thus, the mesoporous lead halide architecture provides a powerful tool to accomplish rapid and complete transformation of lead halide into the perovskite. These pores also provide the space needed to accommodate the large volume expansion accompanying the insertion of the cations into the PbX2 lattice. The pores are filled during the conversion step, resulting in the formation of a compact perovskite film. We note that during the refereeing procedure of our manuscript, two papers appeared reporting on planar heterojunction solar cells with best efficiencies in the 15%–16% range, based on smooth MAPbI3 films that were produced by conversion of nanostructured PbI2 layers.[6] With our new approach, we produce the desired contiguous and dense perovskite capping layer consisting of FA1–xMAxPb(I1–xBrx)3 on top of a mesoporous TiO2 scaffold, which in turn is well-infiltrated with the perovskite. Using this procedure, we fabricate perovskite photovoltaics with solar to electric power conversion efficiencies (PCEs) exceeding 20% with negligible hysteresis in their J–V curve. We deposit the PbX2 films by spin-coating a solution containing PbI2 and PbBr2 of 85/15 molar ratio in a DMSO and DMF mixture (2/8 v/v) on a nanocrystalline TiO2 film prepared as described below in the Experimental Section and heating it at 70 °C for 10 min. Figure 1a,b shows scanning electron microscope (SEM) top-view and cross-sectional images of the porous PbX2 film. The top-view image shows that the PbX2 crystals are in the size range of 40–50 nm and that a variety of pores are distributed over the surface. The cross-sectional image demonstrates that the voids penetrate down to the nanocrystalline TiO2 scaffold. In addition, the back-scattered SEM image in Figure 3b reveals that the lead halide is well infiltrated into the TiO2 scaffold, serving as a precursor for perovskite to fill the mesoporous TiO2. For comparison, we also deposited a PbX2 film by using pure DMF or DMSO as solvents maintaining otherwise the same conditions. Figure 1c illustrates a SEM top-view image of the PbX2 film produced from DMF solution showing the absence of any voids. The SEM cross-sectional image
© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2016, 28, 2964–2970
www.advmat.de www.MaterialsViews.com
COMMUNICATION
DMSO appears to be responsible for the pore formation within the lead halide films. From the dense structure of the freshly prepared film in Figure S2 (Supporting Information), it can be inferred that the mesopores are produced during the annealing when the DMSO is evaporated. Figure S3 (Supporting Information) demonstrates that the pores become larger and the quantities of pores are increasing by increasing the volume ratio of DMSO/DMF from 5/95 to 30/70. It is known that PbX2 forms PbX2(DMSO) complexes such as PbI2(DMSO) and PbBr2(DMSO) with DMSO, where the DMSO ligand is sufficiently labile to be ejected upon heating.[3f,7] We propose that such PbX2(DMSO) complexes are present after spin-coating and that the DMSO is released during the annealing step, producing a porous PbX2 film. The formation of pores is facilitated by the large volume contraction accompanying the ligand loss. In order to investigate more closely the 3D Figure 1. a) SEM top-view of a porous PbX2 film deposited from the DMF/DMSO solvent mixnanostructure of the pores within the PbX2, ture. b) Cross-sectional image of the porous PbX2. c) Top-view of the dense PbX2 film deposited we performed 3D tomography of the film from DMF. d) Cross-sectional image of the dense PbX2. using a dual focused ion beam (FIB) to slice the layer and characterizing it by SEM (FIBSEM). We deposited a thin layer of carbon on top of the lead Figure 1d also confirms the compact structure of the film. SEM halide film to protect it from damage by the ion beam before top-view images of the film produced from pure DMSO solustarting the FIB. Both standard SEM and back-scattered SEM tion (Figure S1, Supporting Information) show the presence are collected during the FIB. Figure 2c shows a 3D view of the of large striated structures covering only part of the nanocrysfilm cut through the PbX2 layer. It reveals the presence of welltalline TiO2 film most likely due to solvent dewetting. To sum up, under otherwise identical conditions, DMF solutions of the developed pores that are evenly spread throughout the film lead halide produces dense lead halide films while DMSO soluforming an interconnected network, consistent with the SEM tion produces striated deposits. By contrast, the DMF/DMSO cross-sectional image of the lead halide. To gain deeper insight mixture produces mesoporous PbX2 films. The presence of into the morphology of the pores, we cut slices at different
Figure 2. 3D FIB tomography of porous PbX2 deposited on the TiO2 scaffold. a) SEM cross-sectional of the PbX2 layer (the red lines indicate the slice positions of the corresponding panels (c), (d), and (e). b) The same measured by back-scattered method (the red line indicates the position of the slice of (f). c) 3D image of the porous PbX2 cut through the PbX2 layer. d) Horizontal slice of the PbX2 overlayer close to the surface. e) Horizontal slice of PbX2 taken from within the inner part of the porous PbX2 layer. f) Horizontal back-scattered image in the mesoporous PbX2/TiO2 composite layer showing complete infiltration of the TiO2 by the lead halide.
Adv. Mater. 2016, 28, 2964–2970
© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
2965
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
positions of the lead halide layer. Figure 2d is from a slice close to the surface, revealing a structure similar to the surface itself, even though the size and number of pores are larger. Figure 2e shows a slice from the interior of the PbX2 layer demonstrating the connectivity of the pores. Figure 2f presents a slice of the PbX2/TiO2 composite layer imaged by the back-scattering method. The grey background representing TiO2 is filled with white spots representing the lead halide. The overall picture emerging from this analysis is that the nanocrystalline TiO2 scaffold is well infiltrated by PbX2, and covered by a highly porous PbX2 layer. These pores become smaller when they are closer to the surface. Thus, only a small percentage of the holes are visible on the surface. The interconnected pores not only serve as channels for transporting the
FAI/MABr (methylammonium bromide) solution, allowing all the PbX2 to be impregnated rapidly, but also offer a large surface area to promote the conversion of the halide to the perovskite by insertion of the FA and MA cations. It should be noted that the size of PbI2(DMSO) is comparable to that of the FAPbI3. Thus, the pores created by the release of the DMSO provide needful space for the volume expansion occurring during the conversion reaction, resulting in a comparable thickness for the compact perovskite and the porous PbX2 film. The green and orange curves in Figure 3b show X-ray powder diffraction (XRD) spectra of the PbX2 film produced from DMSO/DMF, before and after heating respectively. The XRD patterns of the as-coated PbX2 film resemble that of the PbX2(DMSO),[3f ] while that of the annealed film clearly shows
Figure 3. a) Absorption spectra of PbX2 films and perovskite films. b) XRD of PbX2 and perovskite films. c) J–V curves of forward, backward scan, and the average of the best performing perovskite solar cell. d) IPCE spectrum (red) with integrated current curve (black) of the perovskite solar cell exhibiting the highest performance.
2966
wileyonlinelibrary.com
© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2016, 28, 2964–2970
www.advmat.de www.MaterialsViews.com
Adv. Mater. 2016, 28, 2964–2970
spiro-MeOTAD functions as a hole-transport layer. The top-view reveals the compact morphology of the perovskite capping layer which is formed by 150–300 nm-sized densely packed grains without voids. The dense structure of the capping layer is also confirmed by the SEM cross-sectional image of the whole device shown in Figure 4b. The perovskite layer thickness is comparable to the thickness of its precursor PbX2. To investigate the infiltration of the perovskite into the porous TiO2, we examined its structure by elemental mapping using transmission electron microscope (TEM) and STEMEDS. Figure 4c shows a HAADF-STEM image of the nanocomposite formed by TiO2 scaffold and the perovskite. Energy-dispersive X-ray spectroscopy demonstrates that all the elements contained in the perovskite, i.e., lead, iodine, bromine, carbon, and nitrogen show a similar spatial pattern, signifying that the lead, formamidinium, and methylammonium cations as well as the iodide and bromide anions are evenly distributed in the film. By contrast, the distribution of the lead and titanium is complementary. Merging the distribution of these two elements generates the pattern of the HAADF image. In Figure S4 (Supporting Information), we selected a Ti and a Pb rich area for elemental mapping. In both areas, all the perovskite and TiO2 related elements coexist even though the relative intensities are different. Together with the SEM cross-sectional image, they prove the excellent infiltration of the perovskite in the pore of the TiO2 nanoparticles. The high-resolution TEM image in Figure S5b (Supporting Information) reveals the highly crystalline characteristics of the perovskite particles. We examined the photovoltaic properties of the devices by measuring their current–voltage (J–V) curves and their incidentphoton-to-current conversion efficiency (IPCE). All the devices fabricated from the porous lead halide show a much higher PCE than those prepared from dense lead halide. Perovskite solar cells prepared from dense PbX2 films show large PCE variation between 9% and 16%, indicating the low reproducibility of the method, probably due to the incapability of controlling PbX2 conversion rate and the morphology of the perovskite films. Figure S6 (Supporting Information) shows representative J–V curve of perovskite solar cells made from pure DMF and pure DMSO solution, with a PCE of 13.1% (DMF, Voc, Jsc, and FF being 1.04V, 20.0 mA cm−2, and 61.4%) and 9.5% (DMSO, Voc, Jsc, and FF being 0.84V, 16.1 mA cm−2, and 67.6%). Both devices show strong hysteresis. The relatively low Jsc is attributed to the incomplete conversion of the PbX2 (DMF) or inhomogeneous film morphology (DMSO), which agrees well with the absorption measurement and the IPCE spectrum. By contrast, the perovskite solar cells made from the porous PbX2 show an average efficiency of 19.1% with 40 devices. A histogram of the devices is shown in Figure S7 (Supporting Information); while the PV metrics of individual cells are listed in Table S1 (Supporting Information). The best cell shows PCE (average of forward and reverse sweeps) of 20.4% with a short circuit current density (Jsc) of 23 mA cm−2, an open circuit voltage (Voc) of 1.11V, and a fill factor of 78.8% under simulated standard air mass 1.5 G illuminations. Importantly, the hysteresis between the forward and backward scan is negligible as shown in Figure 3c. The stabilized maximum power output measurements presented in Figure S8 (Supporting Information) show good agreement between the measured PCE and power output.
© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
COMMUNICATION
the absence of the PbX2(DMSO) reflections. The series of new diffraction peaks that are in good agreement with literature data on the hexagonal 2H polytype phase of lead iodide[1c] appear during the heating, implying that the bromide ions are incorporated as a dopant in the PbI2 upon annealing and do not form a separate phase. The transformation of PbX2(DMSO) into PbX2 on heating is apparent also from the UV–Vis absorption spectra. The green and orange curves in Figure 3a show the absorption of the film before and after annealing, respectively. The freshly coated film shows an absorption peak at 380 nm with an onset at 440 nm matching the spectrum of PbX2(DMSO).[2k] Upon heating, the peak at 380 nm disappears and a new absorption appears with a plateau at 490 nm and an absorption onset at 510 nm, which fits the band edge absorption of PbX2. The PbX2 composition of the annealed film is further confirmed by the agreement of its absorption spectrum with that of a PbX2 reference sample produced from DMF. We prepared the perovskite film by exposing the porous lead halide film to a mixture of methylammonium bromide and formamidinium iodide in isopropanol. The color of the film changed from yellow to dark brown immediately upon contacting it with the organic ammonium halide solution, indicating rapid conversion of lead halide to perovskite. In contrast, when the same solution was applied to the compact lead halide over-layer produced from a DMF solution of the lead halides, the brown color appeared very slowly reaching a much weaker intensity than that of the perovskite produced from the porous lead halide layer. The orange, blue, and red curves in Figure 3a show the UV–Vis absorption spectra of the porous PbX2 and the perovskite films produced from the dense and porous PbX2 film, respectively. After the conversion, the films absorb light from the UV region to 800 nm with a plateau in the 600–750 nm range. The perovskite film produced from the porous PbX2 shows about twice the absorbance than the one produced from the condensed PbX2, suggesting a better conversion of the porous with respect to the compact precursor. We also observe a shoulder at around 490 nm for the blue curve, which probably arises from residual PbX2, confirming the incomplete conversion of the dense film. Figure 3b shows that a series of new X-ray diffraction (XRD) peaks corresponding to the trigonal phase of the FA1-xMAxPb(I1-xBrx)3 perovskite formed during the conversion. However, the XRD pattern of the blue curve corresponding to the perovskite film produced from the dense lead halide maintains an intense peak at 12.7°, arising from the diffraction by (001) planes of PbI2. This demonstrates that the conversion of the dense PbX2 film is incomplete. On the other hand, the peaks of PbI2 disappear in the red curve in Figure 3b, corresponding to the XRD spectra of the perovskite film formed from the porous PbX2. It indicates the complete conversion of porous PbX2 to the perovskite. Figure 4a shows a SEM top-view of the perovskite film produced by conversion of the porous PbX2 precursor while Figure 4b shows a cross-sectional image of a complete device composed of superimposed layers of fluorine doped tin oxide (FTO)/compact TiO2/mesoporous TiO2-perovskite nanocomposite/perovskite capping layer/spiro-MeOTAD/Au. The
2967
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
Figure 4. a) SEM top-view image of the perovskite layer. b) SEM cross-sectional image of the complete device. c) High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and scanning transmission electron microscopy coupled with energy dispersive X-ray spectroscopy (STEM-EDS) of the perovskite infiltrated TiO2.
The IPCE spectrum demonstrates a high photon-to-current conversion efficiency from 400 to 800 nm. Integration of the IPCE over the AM 1.5 G spectrum yields a photocurrent density of 22.7 mA cm−2 as shown in Figure 3d, which is in excellent agreement with the short circuit photocurrent density of 23 mA cm−2 derived from the J–V curve. Initial stability test of the device (in the dark under dry condition) shows promising result with only 5% PCE decrease over 1000 h. The superior performances of the perovskite solar cells exemplify the great potential of the new approach to produce high quality perovskite films by sequential deposition using porous PbX2 film as intermediate. In conclusion, we introduce a new and effective strategy to produce high quality perovskite films by chemical conversion of mesoporous PbX2 precursor films that can be prepared by simple solution processing. The interconnected mesopores serve as channels to allow for rapid infiltration by the solution of the reagents and provide a large internal surface area, enhancing the insertion of the organic ammonium halides into the lead halide lattice, resulting in rapid and quantitative conversion to perovskite. In this fashion, we realized perovskite
2968
wileyonlinelibrary.com
films of excellent optoelectronic properties, allowing fabrication of perovskite solar cells with a power conversion efficiency of 20.4%. Even higher PCEs appear feasible by further refinements of the method, which is presently being implemented. This strategy is expected to be of general importance for the realization of semiconductor films with outstanding optoelectronic properties.
Experimental Section Materials: MABr was synthesized by stirring a mixture of methylamine (5 mL, 40% in methanol) and HBr (6 mL, 48% in water) at room temperature for 2 h. The solvent was evaporated in a rotavap and the remaining solid was poured into 50 mL ether and filtrated. The precipitate was washed three times with ether and dried under vacuum to get a white crystalline MABr product. FAI was synthesized according to a reported procedure.[3d] All the other materials were purchased from commercial sources and used as received. Device Fabrication: FTO-coated glass substrates were patterned by laser etching and cleaned by ultrasonication in detergent solution and subsequent rinsing with deionized water, acetone, and ethanol. A TiO2
© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2016, 28, 2964–2970
www.advmat.de www.MaterialsViews.com
Acknowledgements The authors would like to thank Dr. Marco Cantoni from the Interdisciplinary Centre for Electron Microscopy (CIME) in EPFL for the assistance in the FIB-SEM measurements. Financial support from CCEM-CH in the 9th call proposal 906: CONNECT PV, the Swiss National Science Foundation (SNF)-NRP70 “Energy Turnaround”, a grant from King Abudulaziz City of Science and Technology (KACST) Saudi Arabia, and the GRAPHENE project supported by the European Commission Seventh Framework Program under contract 604391 is gratefully acknowledged. J.L. would like to acknowledge financial
Adv. Mater. 2016, 28, 2964–2970
support by an EPFL Fellowship co-funded by Marie Curie from the European Union’s Seventh Framework Program for research, technological development, and demonstration under grant agreement no. 291771. Received: December 5, 2015 Revised: January 8, 2016 Published online: February 19, 2016
[1] a) H. S. Kim, C. R. Lee, J. H. Im, K. B. Lee, T. Moehl, A. Marchioro, S. J. Moon, R. Humphry-Baker, J. H. Yum, J. E. Moser, M. Gratzel, N. G. Park, Sci. Rep.-U.K 2012, 2, 591; b) M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. J. Snaith, Science 2012, 338, 643; c) J. Burschka, N. Pellet, S. J. Moon, R. Humphry-Baker, P. Gao, M. K. Nazeeruddin, M. Gratzel, Nature 2013, 499, 316; d) D. B. Mitzi, J. Mater. Chem. 2004, 14, 2355; e) F. Hao, C. C. Stoumpos, R. P. H. Chang, M. G. Kanatzidis, J. Am. Chem. Soc. 2014, 136, 8094; f) A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 2009, 131, 6050; g) A. Y. Mei, X. Li, L. F. Liu, Z. L. Ku, T. F. Liu, Y. G. Rong, M. Xu, M. Hu, J. Z. Chen, Y. Yang, M. Gratzel, H. W. Han, Science 2014, 345, 295; h) I. C. Smith, E. T. Hoke, D. Solis-Ibarra, M. D. McGehee, H. I. Karunadasa, Angew. Chem. Int. Ed. 2014, 53, 11232; i) E. L. Unger, E. T. Hoke, C. D. Bailie, W. H. Nguyen, A. R. Bowring, T. Heumuller, M. G. Christoforo, M. D. McGehee, Energy Environ. Sci. 2014, 7, 3690; j) C. C. Stoumpos, C. D. Malliakas, M. G. Kanatzidis, Inorg. Chem. 2013, 52, 9019; k) Y. Yang, Y. Yan, M. Yang, S. Choi, K. Zhu, J. M. Luther, M. C. Beard, Nat. Commun. 2015, 6, 7961. [2] a) G. C. Xing, N. Mathews, S. Y. Sun, S. S. Lim, Y. M. Lam, M. Gratzel, S. Mhaisalkar, T. C. Sum, Science 2013, 342, 344; b) H. P. Zhou, Q. Chen, G. Li, S. Luo, T. B. Song, H. S. Duan, Z. R. Hong, J. B. You, Y. S. Liu, Y. Yang, Science 2014, 345, 542; c) J. H. Im, I. H. Jang, N. Pellet, M. Gratzel, N. G. Park, Nat. Nanotechnol. 2014, 9, 927; d) Y. Z. Wu, A. Islam, X. D. Yang, C. J. Qin, J. Liu, K. Zhang, W. Q. Peng, L. Y. Han, Energy Environ. Sci. 2014, 7, 2934; e) Z. G. Xiao, Q. F. Dong, C. Bi, Y. C. Shao, Y. B. Yuan, J. S. Huang, Adv. Mater. 2014, 26, 6503; f) Z. G. Xiao, C. Bi, Y. C. Shao, Q. F. Dong, Q. Wang, Y. B. Yuan, C. G. Wang, Y. L. Gao, J. S. Huang, Energy Environ. Sci. 2014, 7, 2619; g) T. Y. Zhang, M. J. Yang, Y. X. Zhao, K. Zhu, Nano Lett. 2015, 15, 3959; h) M. D. Xiao, F. Z. Huang, W. C. Huang, Y. Dkhissi, Y. Zhu, J. Etheridge, A. Gray-Weale, U. Bach, Y. B. Cheng, L. Spiccia, Angew. Chem. Int. Ed. 2014, 53, 9898; i) K. Y. Yan, M. Z. Long, T. K. Zhang, Z. H. Wei, H. N. Chen, S. H. Yang, J. B. Xu, J. Am. Chem. Soc. 2015, 137, 4460; j) N. J. Jeon, J. H. Noh, Y. C. Kim, W. S. Yang, S. Ryu, S. Il Seol, Nat. Mater. 2014, 13, 897; k) W. Li, J. Fan, J. Li, Y. Mai, L. Wang, J. Am. Chem. Soc. 2015, 137, 10399; l) N. Ahn, D. Y. Son, I. H. Jang, S. M. Kang, M. Choi, N. G. Park, J. Am. Chem. Soc. 2015, 137, 8696; m) J. H. Kim, P. W. Liang, S. T. Williams, N. Cho, C. C. Chueh, M. S. Glaz, D. S. Ginger, A. K. Y. Jen, Adv. Mater. 2015, 27, 695; n) H. S. Kim, I. Mora-Sero, V. Gonzalez-Pedro, F. FabregatSantiago, E. J. Juarez-Perez, N. G. Park, J. Bisquert, Nat. Commun. 2013, 4, 2242; o) J. M. Azpiroz, E. Mosconi, J. Bisquert, F. De Angelis, Energy Environ. Sci. 2015, 8, 2118; p) Q. Q. Lin, A. Armin, R. C. R. Nagiri, P. L. Burn, P. Meredith, Nat. Photonics 2015, 9, 106; q) P. W. Liang, C. Y. Liao, C. C. Chueh, F. Zuo, S. T. Williams, X. K. Xin, J. J. Lin, A. K. Y. Jen, Adv. Mater. 2014, 26, 3748; r) D. Shi, V. Adinolfi, R. Comin, M. J. Yuan, E. Alarousu, A. Buin, Y. Chen, S. Hoogland, A. Rothenberger, K. Katsiev, Y. Losovyj, X. Zhang, P. A. Dowben, O. F. Mohammed, E. H. Sargent, O. M. Bakr, Science 2015, 347, 519; s) J. S. Manser, P. V. Kamat, Nat. Photonics
© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
COMMUNICATION
compact layer was then deposited on the FTO by aerosol spray pyrolysis at 450 °C using a titanium diisopropoxide bis(acetylacetonate) solution (75% in 2-propanol, Sigma-Aldrich) in ethanol (1:19, weight ratio) as precursor and oxygen as carrier gas. The mesoporous TiO2 layer was deposited by spin-coating at 5000 rpm for 20 s using a TiO2 paste diluted in ethanol (1:6, weight ratio). After drying at 125 °C, the TiO2 films were gradually heated to 500 °C, annealed at this temperature for 15 min, and cooled to room temperature. The films were sintered at 500 °C for 20 min before use. The PbX2 films were prepared by consecutive two-step spin-coating process at 1000 and 4000 rpm with a solution of 1.2 M PbX2 (85:15 molar ratio of PbI2:PbBr2) in DMF/DMSO (80/20 v/v) mixture (porous film) or DMF (dense film), DMSO (inhomogeneous film). The films were left to dry at room temperature in the dry box and then annealed at 70 °C for 10 min. After cooling to room temperature, the film was exposed to 0.1 M of FAI and MABr (85:15 molar ratio) mixture in isopropanol for 90 s, then the film was spun at 4000 rpm and annealed at 100 °C for 10 min. After cooling to room temperature, the HTM was deposited by spin-coating at 4000 rpm for 20 s. The HTM solution consists of 72.3 mg (2,29,7,79-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene)(spiro-OMeTAD), 28.8 µL 4-tert-butylpyridine, and 17.5 µL of 520 mg mL−1 lithium bis(trifluoromethylsulphonyl)imide acetonitrile solution dissolved in 1 mL chlorobenzene. Finally, 80 nm of gold was thermally evaporated on top of the device to form the back contact. Materials Characterization: The PbX2 and perovskite films used for UV–Vis absorption spectra, XRD, SEM, and TEM were prepared by spin-coating on mesoporous TiO2 films following the above-mentioned procedure. The UV–Vis spectra were measured with a PerkinElmer Lambda 950 spectrophotometer. The XRD patterns were recorded with a Bruker D8 Discover diffractometer in Bragg–Brentano mode, using Cu K α radiation (1.540598 Å) and a Ni β-filter. Spectra were acquired with a linear silicon strip “Lynx Eye” detector from 2θ = 10°–60° at a scan rate of 2° min−1, step width of 0.02°, and a source slit width of 1 mm. The morphology of the films was characterized using a highresolution scanning electron microscope (ZEISS Merlin). The FIB-SEM was characterized using Zeiss NVision 40 CrossBeam with FIB and high-resolution field emission SEM. The perovskite structures and compositions were examined by a high-resolution transmission electron microscope (Technai Osiris, FEI), equipped with an Oxford EDS detector. Device Characterization: Current–voltage characteristics of the devices were measured with a solar simulator composed of light source and a digital source meter (Keithley Model 2400). The light source was a 450 W xenon lamp (Oriel) equipped with a Schott K113 Tempax sunlight filter (Praezisions Glas & Optik GmbH) to match the emission spectrum of the lamp to the AM1.5G standard. Before each measurement, the light intensity was calibrated by a Si reference diode equipped with an infrared cut-off filter (KG-3, Schott). IPCE spectra were recorded as functions of wavelength under a constant white light bias of ≈5 mW cm−2 supplied by an array of white light emitting diodes. The excitation beam coming from a 300-W xenon lamp (ILC Technology) was focused through a Gemini-180 double monochromator (Jobin Yvon Ltd) and chopped at ≈2 Hz. The signal was recorded using a Model SR830 DSP Lock-In Amplifier (Stanford Research Systems). All measurements were conducted using a nonreflective metal aperture of 0.16 cm2 to define the active area of the device and avoid light scattering through the sides.
2969
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
2970
2014, 8, 737; t) C. Law, L. Miseikis, S. Dimitrov, P. Shakya-Tuladhar, X. E. Li, P. R. F. Barnes, J. Durrant, B. C. O’Regan, Adv. Mater. 2014, 26, 6268; u) F. Hao, C. C. Stoumpos, Z. Liu, R. P. H. Chang, M. G. Kanatzidis, J. Am. Chem. Soc. 2014, 136, 16411; v) X. Li, M. I. Dar, C. Yi, J. Luo, M. Tschumi, S. M. Zakeeruddin, M. K. Nazeeruddin, H. Han, M. Gratzel, Nat. Chem. 2015, 7, 703. [3] a) N. Pellet, P. Gao, G. Gregori, T. Y. Yang, M. K. Nazeeruddin, J. Maier, M. Gratzel, Angew. Chem. Int. Ed. 2014, 53, 3151; b) F. Wang, H. Yu, H. H. Xu, N. Zhao, Adv. Funct. Mater. 2015, 25, 1120; c) G. E. Eperon, S. D. Stranks, C. Menelaou, M. B. Johnston, L. M. Herz, H. J. Snaith, Energy Environ. Sci. 2014, 7, 982; d) J. W. Lee, D. J. Seol, A. N. Cho, N. G. Park, Adv. Mater. 2014, 26, 4991; e) N. J. Jeon, J. H. Noh, W. S. Yang, Y. C. Kim, S. Ryu,
wileyonlinelibrary.com
[4] [5] [6]
[7]
J. Seo, S. I. Seok, Nature 2015, 517, 476; f) W. S. Yang, J. H. Noh, N. J. Jeon, Y. C. Kim, S. Ryu, J. Seo, S. I. Seok, Science 2015, 348, 1234. M. Z. Liu, M. B. Johnston, H. J. Snaith, Nature 2013, 501, 395. Q. Chen, H. P. Zhou, Z. R. Hong, S. Luo, H. S. Duan, H. H. Wang, Y. S. Liu, G. Li, Y. Yang, J. Am. Chem. Soc. 2014, 136, 622. a) H. Zhang, J. Mao, H. He, D. Zhang, H. L. Zhu, F. Xie, K. S. Wong, M. Grätzel, W. C. H. Choy, Adv. Energy Mater. 2015, 5, 1501354; b) T. Liu, Q. Hu, J. Wu, K. Chen, L. Zhao, F. Liu, C. Wang, H. Lu, S. Jia, T. Russell, R. Zhu, Q. Gong, Adv. Energy Mater. 2015, 5, 1501890. J. Selbin, W. E. Bull, L. H. Holmes, J. Inorg. Nucl. Chem. 1961, 16, 219.
© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2016, 28, 2964–2970
