Research Update: Hybrid organic-inorganic perovskite (HOIP) thin films and solar cells by vapor phase reaction , Po-Shen Shen, Yu-Hsien Chiang, Ming-Hsien Li, Tzung-Fang Guo, and Peter Chen Citation: APL Mater 4, 091509 (2016); doi: 10.1063/1.4962142 View online: http://dx.doi.org/10.1063/1.4962142 View Table of Contents: http://aip.scitation.org/toc/apm/4/9 Published by the American Institute of Physics Articles you may be interested in Research Update: Large-area deposition, coating, printing, and processing techniques for the upscaling of perovskite solar cell technology APL Mater 4, 091508091508 (2016); 10.1063/1.4962478 Research Update: Strategies for improving the stability of perovskite solar cells APL Mater 4, 091503091503 (2016); 10.1063/1.4961210 Research Update: Behind the high efficiency of hybrid perovskite solar cells APL Mater 4, 091505091505 (2016); 10.1063/1.4962143 Preface for Special Topic: Perovskite solar cells—A research update APL Mater 4, 091201091201 (2016); 10.1063/1.4960670 APL MATERIALS 4, 091509 (2016) Research Update: Hybrid organic-inorganic perovskite (HOIP) thin films and solar cells by vapor phase reaction Po-Shen Shen,1 Yu-Hsien Chiang,1 Ming-Hsien Li,1 Tzung-Fang Guo,1,2,3 and Peter Chen1,2,3,a Department of Photonics, National Cheng Kung University, Tainan 701, Taiwan Research Center for Energy Technology and Strategy (RCETS), National Cheng Kung University, Tainan 701, Taiwan Advanced Optoelectronics Technology Center (AOCT), National Cheng Kung University, Tainan 701, Taiwan (Received 31 May 2016; accepted 11 August 2016; published online 20 September 2016) With the rapid progress in deposition techniques for hybrid organic-inorganic perovskite (HOIP) thin films, this new class of photovoltaic (PV) technology has achieved material quality and power conversion efficiency comparable to those established technologies Among the various techniques for HOIP thin films preparation, vapor based deposition technique is considered as a promising alternative process to substitute solution spin-coating method for large-area or scale-up preparation This technique provides some unique benefits for high-quality perovskite crystallization, which are discussed in this research update C 2016 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4962142] Hybrid organic-inorganic perovskites (HOIPs) have been the focus of research with tremendous amount of scientific publications in the past few years The HOIPs have shown numerous remarkable characteristics such as wide absorption range compatible with solar spectrum,1 low exciton binding energy with long carrier diffusion length,2 which make them as promising materials for emerging photovoltaic technology The extraordinary power conversion efficiency (PCE) over 20% of HOIP solar cells has been achieved with credited certification.3,4 The deposition techniques of HOIP thin films have progressed from simple spin-coating process,5–8 sequential dipping method9 to ultrasonic spray coating,10 and vapor phase deposition.11–17 In the 1990s, the layered organic-inorganic halide perovskite was first studied by Mitzi and co-workers for the electronic applications.18 Until 2009, the first application of hybrid organicinorganic perovskites (HOIPs) (CH3NH3PbI3 and CH3NH3PbBr3) as light absorber for photovoltaic activities was carried out by Miyasaka and co-workers in liquid-electrolyte type sensitized solar cells with efficiencies of 3.8% and 3.1%, respectively.19 In 2012, a breakthrough was made by Kim et al using CH3NH3PbI3 perovskite in combination with 2,29,7,79-tetrakis-(N,N-di-pmethoxyphenylamine)9,99-spirobifluorene (spiro-MeOTAD) as hole transporting material (HTM) to fabricate mesoscopic solid-state solar cells and to prevent the decomposition of perovskite in the polar electrolyte solvent with device efficiency exceeding 9%.5 Meanwhile, Lee et al reported a meso-superstructured perovskite solar cells by utilizing mesoporous Al2O3 as non-injecting scaffold and demonstrated a power conversion efficiency (PCE) over 10%.6 These results demonstrated that HOIPs not only act as an efficient light absorber for charge separation but also a good charge transporting material Since then, extensive efforts have been endeavored to investigate the HOIPs’ material and solar cells with a variety of architectures During the vast developments in the past few years, it has been realized that the film morphology, thickness, crystallinity, and crystal size of the deposited perovskite layer have crucial influences on the photovoltaic performance.20,21 a Author to whom correspondence should be addressed Electronic mail: petercyc@mail.ncku.edu.tw 2166-532X/2016/4(9)/091509/14 4, 091509-1 © Author(s) 2016 091509-2 Shen et al APL Mater 4, 091509 (2016) Therefore, many attempts have emphasized in producing uniform, densely packed, defect-less, and large grain size perovskite thin films by modifying the deposition parameters and techniques Nowadays, solution-processed method remained the major approach to prepare perovskite films due to its advantages of simplicity and low-cost For instance, solvent engineering using toluene, diethyl ether, or other solvents was conducted to produce shiny mirror-like perovskite film for efficient perovskite solar cells with excellent reproducibility.7,8,22 Despite the extremely high device efficiency achieved by solution-processed approach, fabrication of uniform large-area perovskite thin film remained challenging by spin-coating process Vapor-based deposition technique is considered as a promising alternative process to substitute solution spin-coating for large-area or scale-up preparation This technique provides some unique benefits for high-quality perovskite crystallization First, the vapor deposition route offers higher purity of precursor reactants during evaporation process under high vacuum environment Second, the reaction process for chemical vapor deposition is much slower than solution-processed methods, and it is advantageous to the formation of ordered perovskite crystallites Meanwhile, controllable macroscopic parameters such as pressure, evaporation rate, and deposition temperature make vapor deposition as an elaborate and reproducible approach In the late 1990s, vacuum deposition and thermal ablation methods have been developed to fabricate HOIPs thin films for stoichiometry control and 2-D quantum well.23–25 In this manuscript, we will review the HOIP solar cells that are made with vapor deposition or reaction process The various methods are classified as shown in Figure These methods are mainly categorized by highly vacuum evaporation process, vapor-assisted solution process (VASP) which can be divided into atmospheric VASP (AP-VASP) and low-pressure VASP (LP-VASP), and unique deposition techniques (flash evaporation and ultrasonic spray coating (USC)) The highly vacuum evaporation processing involves dual-source co-evaporation, sequential two-step evaporation, and unique flash evaporation LP-VASP can be divided into single zone and double zone based on the heating sources In 2013, a dual-source co-evaporation deposition system was reported by Liu et al in order to fabricate flat CH3NH3PbI3−xClx layer by co-evaporating organic (CH3NH3I) and inorganic source (PbCl2) in high vacuum chamber (Fig 2(a)) The X-ray diffraction (XRD) results indicated similar crystal structure for perovskite films deposited by co-evaporation technique and solution process (Fig 2(b)) The HOIP films deposited by co-evaporation process showed a denser and more uniform morphology (as referred to Figs 2(c) and 2(e)) than the solution-processed one (as referred to Figs 2(d) and 2(f)) Moreover, the planar HOIP solar cells incorporating with a hole conductor, spiro-MeOTAD, achieved a remarkable PCE of 15% (Fig 2(g)).11 On the other hand, the inverted planar HOIP solar cells incorporating with an organic hole blocking layer have also been reported by Malinkiewicz et al., applying dual-source evaporation route with PbI2 (250 ◦C) and CH3NH3I (70 ◦C) as evaporation sources The evaporation-deposited film is semi-transparent with small roughness measured by AFM The device with 285-nm-thickness perovskite layer reached an efficiency of 12%.17 The device efficiency with the same structure was then further optimized to 14.8% for small-area device (0.065 cm2) and 10% for large-area device (∼1 cm2) by carefully adjusting the evaporation conditions.16 The evaporated FIG Classifications of the various vapor phase deposition methods for HOIP solar cells 091509-3 Shen et al APL Mater 4, 091509 (2016) FIG (a) Figure of dual-source thermal evaporation system for depositing the perovskite absorbers (b) XRD spectra of a solution-processed (blue) and vapor-deposited (red) perovskite film SEM top view images of vapor-deposited (c) and solution-processed perovskite film (d) Cross-sectional SEM images under lower magnification of completed solar cells constructed from a vapor-deposited perovskite film (e) and a solution-processed perovskite film (f) (g) J –V curves of the best-performing solution-processed (blue lines, triangles) and vapor-deposited (red lines, circles) planar heterojunction perovskite solar cells measured under simulated AM1.5G sunlight of 101 mW/cm2 irradiance (solid lines) and in the dark (dashed lines) Reproduced with permission from Liu et al., Nature 501, 395 (2013) Copyright 2013 Nature Publishing Group PbI2:CH3NH3I ratio was modified by varying the evaporation temperature of inorganic source (PbI2) while the temperature of the organic source kept constant at 70 ◦C The stoichiometric perovskite crystallites were fabricated in an optimum condition when the evaporation temperature for PbI2 is 250 ◦C On the other hand, Ono et al reported a home-built instrument for evaporation deposition of perovskite films.26 The stoichiometry and film thickness could be more precisely monitored by the quartz crystal microbalance (QCM), which is mounted inside the vacuum chamber More recently, Lin et al studied the effect of organic p-type interlayers on the structure and composition of co-evaporation deposited perovskite films.27 It was found that the crystal structure of deposited perovskite films would be slightly different according to the choice of p-type interlayer coated on the substrate By incorporating ultrathin PC60BM and PCPDTBT as n- and p-type work-function modifying layer, respectively, the co-evaporation deposited perovskite films achieved 16.5% using planar HOIP solar cells Similar to the two-step coating method in a solution-based process, the sequential evaporation method is a vapor phase analogy of layer-by-layer deposition process for HOIPs In general, co-evaporation technique requires elaborate control on the evaporation conditions for both material sources to maintain the quality of the deposited HOIPs To overcome this issue, a modified layer-by-layer sequential vacuum evaporation method was proposed The lead halide and methylammonium halide were thermally sublimated one after another onto the substrate sequentially to 091509-4 Shen et al APL Mater 4, 091509 (2016) FIG (a) Schematic illustration of perovskite solar cells fabricated by sequential layer-by-layer vacuum deposition (b) Tilt-angle SEM image of the perovskite thin film fabricated at 75 ◦C substrate temperature (magnification: 4500×) (c) J –V characteristics of perovskite solar cells fabricated at 65–85 ◦C substrate temperatures measured under sun AM 1.5G illumination (solid lines) and in the dark (dashed lines) Reproduced with permission from Chen et al., Adv Mater 26, 6647 (2014) Copyright 2014 Wiley-VCH fabricate planar device It was first applied by Chen et al to form CH3NH3PbI3−xClx absorber layer by sequentially sublimating PbCl2 and CH3NH3I onto the PEDOT:PSS/ITO substrate (Fig 3(a)).12 This vacuum layer-by-layer deposition method enables the formation of large-scale homogeneous crystalline structure (Fig 3(b)) by sublimating 150-nm-thick PbCl2 layer first and converting into perovskite film (430 nm) after CH3NH3I vapor reaction Meanwhile, the substrate temperature was found to have significant influences on the quality of the deposited films By optimizing the substrate temperature (65–85 ◦C), the simple planar HOIP solar cells delivered a remarkable PCE of 15.4% (Fig 3(c)) Similar work was reported by Abbas et al using the sequential evaporation deposition of PbI2 and CH3NH3I The PbI2 film (200 nm) was first evaporated on the compact TiO2 layer coated FTO substrate in the vacuum chamber and then transferred into a graphite vessel for perovskite formation in a nitrogen-filled glove box.28 Yang et al reported a modified sequential evaporation process by alternating precursor layer of PbCl2 and CH3NH3I (Fig 4).29 This method allows more flexible FIG Manufacture processes of perovskite solar cells by alternating layer-by-layer vacuum deposition Reproduced with permission from Yang et al., J Mater Chem A 3, 9401 (2015) Copyright 2015 Royal Society of Chemistry 091509-5 Shen et al APL Mater 4, 091509 (2016) FIG The high resolution SEM images of perovskite films for vapor CH3NH3I reaction with different thicknesses of PbCl2: (a) 50 nm, (b) 100 nm, (c) 125 nm, and (d) 150 nm; the scan bar is 500 nm (e) The PCE stability of a high performance perovskite solar cell without encapsulation stored under ambient conditions Reproduced with permission from Yang et al., J Mater Chem A 3, 9401 (2015) Copyright 2015 Royal Society of Chemistry deposition monitoring and superior uniformity in film morphology, surface coverage, and crystalline phase purity It was also found that the PbCl2 layer thickness affects the surface morphology of the synthesized perovskite films When the PbCl2 thickness is less than 100 nm, the perovskite film is uniform with full coverage However, when the PbCl2 thickness is larger than 100 nm, pinholes appear in the perovskite films and the size of voids increases with PbCl2 thickness (Figs 5(a)–5(d)) The best device performance achieved as high as 16.03% and the devices made by this approach exhibited high reproducible efficiency of 15.37% with small deviation of 0.37% While the active area increased from 0.1 cm2 to cm2, the photovoltaic parameters of devices showed negligible differences Furthermore, the bare device without encapsulation demonstrated superior stability performance with only 9% degradation over 62 days under ambient condition (Fig 5(e)) More recently, Hsiao et al reported HOIP solar cells of 17.6% with photovoltaic parameters of 1.06 V, 22.7 mA/cm2, and fill factor of 0.73, which currently is the best record efficiency for vapor-based techniques.30 By manipulating the partial pressure of organic halide vapor during the sequential evaporation process, it was found that the metal halide film can fully convert into perovskite only at a pressure range 10−3–10−4 Torr in h Perovskite films with smooth surface and crystallite size of micrometer were obtained under vapor pressure of 10−4 Torr At low pressure of 10−5 Torr, the transformation of metal halide into perovskite is incomplete The results demonstrated the narrow window on partial pressure for perovskite film formation by vacuum vapor deposition process In contrast to the high vacuum evaporation system, the perovskite film grew by hybrid chemical vapor deposition (HCVD) was usually performed under atmosphere or low vacuum (∼10−2 Torr) in a close environment, e.g., close container or quartz tube rather than a complicated vacuum chamber This HCVD perovskite formation proceeds at the gas-solid interfaces and prevents the presence of solvation intermediates (CH3NH3PbI3-DMF), which was responsible for the incomplete coverage of resulting films in solution-based process Vapor-assisted solution process (VASP) conceptually contains the advantages of solutionprocessed method and vapor evaporation The perovskite crystallization is carried out by placing the metal halide coated substrate which is prepared by spin-coating beforehand, into a CH3NH3I vapor-filled environment (Fig 6) The first work using VASP for HOIP thin films synthesis was reported by Chen et al and the planar HOIP solar cells showed a PCE of 12.1%.13 Annealing temperature of 150 ◦C was required for the as-deposited PbI2 film to effectively react with CH3NH3I vapor under atmospheric pressure The reaction time for full conversion from PbI2 (200 nm) into CH3NH3PbI3 (∼350 nm) is h This two-step CVD method provides favorable nucleation kinetics and avoids fast crystallization observed in solution processing approaches Meanwhile, the report for HTM-free HOIP solar cells with PCE over 10% also confirms the superior perovskite films prepared by VASP method.31 In this work, it demonstrated high reproducibility with negligible deviation efficiency of 0.1% for 30 cells in total Except for the iodide-based HOIPs, the 091509-6 Shen et al APL Mater 4, 091509 (2016) FIG Schematic illustration of perovskite film formation through vapor-assisted solution process Reproduced with permission from Chen et al., J Am Chem Soc 136, 622 (2014) Copyright 2014 American Chemistry Society bromide-based perovskite films were also successfully synthesized by alternating the precursor materials with PbBr2 and CH3NH3Br The PbBr2 framework was spin-cast from a DMF solution and then transferred into a close container facing downwards to the CH3NH3Br vapor The bromide-based HOIP solar cells delivered a high open-circuit voltage of 1.45 V because of its wide bandgap.32 The photoluminescence results showed longer diffusion length for CH3NH3PbBr3 films, indicating better crystallinity Subsequently, the processing environment for the gas-solid perovskite crystallization was further modified from a close container to a tubular furnace A VASP-based in situ tubular deposition was demonstrated and the time for full conversion from PbI2 to CH3NH3PbI3 in the heated furnace (145 ◦C) was h, leading to a 12.2% PCE.33 Recently, the VASP method was applied to investigate the formation of intermediate phases during the hybrid vapor reaction.34,35 Jain et al performed VASP method for perovskite films growth on planar (Fig 7(a)) and mesoscopic architectures, respectively (Fig 7(b)).35 Surprisingly, the process for vapor inter-diffusion in PbI2 films proceeds faster with the presence of mesoporous scaffold and completes the conversion of PbI2 into CH3NH3PbI3 in 30 On the other hand, the remaining PbI2 was observed for planar samples after 60 vapor reaction and resulted in deteriorated photovoltaic performances The PbI2 residues near the compact TiO2 layer would act as a filter of incident light and a defect-rich layer Meanwhile, according to the evolution of absorption spectra as a function of reaction time, the results suggest the presence of the (CH3NH3I)0.5(PbI2) intermediate phase, which leads to increased absorption within the range from 500 to 650 nm Very recently, the evidence for the present intermediate phase between PbI2 polytype and perovskite was found.36 Similarly, Raga et al demonstrated the role and interplay for PbI2 films, methylamine (MA) gas, and hydroiodic (HI) gas for superior HOIPs formation.34 The 2D layered structure of PbI2 crystallites reacts with MA gas to complete conversion of CH3NH3PbI3 within few seconds with additional PbO and Pb(OH)2 phases in the resulting films Also, treatments of introducing sequential and simultaneous HI gas exposure favorably convert the additional PbO and Pb(OH)2 phases to PbI2 and CH3NH3PbI3 The best-performing devices using resultant films by simultaneous exposure of MA and HI gases achieved a 15.3% PCE.34 A chlorine (Cl)-incorporated HOIPs have been realized to form better morphology control and defect repair with PCE of 13.76%.37 In comparison with atmospheric VASP (AP-VASP), low pressure VASP (LP-VASP) provides unique advantages including faster vapor diffusion rate, lower sublimation temperature, and shorter reaction time for perovskite films growth Most works applying LP-VASP method for HOIP films 091509-7 Shen et al APL Mater 4, 091509 (2016) FIG Schematic illustration of the MAPbI3 formation on a glass substrate (a) without and (b) with mesoporousTiO2 scaffold layer, respectively The yellow-green lines on the surface of MAPbI3 represent grain boundaries Reproduced with permission from Jain et al., J Mater Chem A 4, 2630 (2016) Copyright 2016 Royal Society of Chemistry formation were conducted in a tubular furnace, where either single or double heating zones were utilized for heating organic precursor powders and substrates LP-VASP was first reported by Leyden et al using dual heating zones system (Fig 8(a)).38 The CH3NH3I vapor was created by vaporizing the precursor in the high temperature heating zone (185 ◦C) and carried by purging with inert gas to the second low temperature zone (reaction zone 160–170 ◦C) In the system, the CH3NH3I vapor steadily diffuses and infiltrates through the PbCl2 film from surface to the bottom for hybrid perovskite formation Under low pressure of ∼1 Torr, the conversion of perovskite is complete in h and a device with 11.8% PCE was demonstrated Furthermore, the efficiency was maintained almost the same for stability test after 1100 h The devices were kept in the dark and N2-filled glove box and the measurement was performed in ambient air with a relative humidity ∼50% In a later work, the same group fabricated formamidinium iodide-based perovskite films using dual zone system with moderate modification of heating temperature (160 ◦C).39 A shorter reaction time less than 30 was needed and devices showed a best PCE up to 14.2% The efficiency of device with larger area (1 cm2) reached 7.7% Except for the dual zone system, simple single one system was reported for HOIPs preparation by LP-VASP (Fig 8(b)) Under reaction pressure of mTorr, it took h to complete the transformation of PbI2 to CH3NH3PbI3 at 82 ◦C and the best-performing device is up to 14.7%.40 A similar work reported the fabrication of CH3NH3PbI3 perovskite films under working pressure of Torr with reaction time of h and temperature of 120 ◦C The optimized planar device demonstrated a 15.37% PCE.14 Another merit for single zone LP-VASP is the high utilization of the vaporized precursor that around 50% yield can be obtained.14 The aerosol-assisted chemical vapor deposition (AACVD) is an alternative technique, for which the solution precursor is vaporized into ultrafine droplets as building blocks by ultrasonic aerosol generation The humidifier transfers the precursor into aerosol mist and transports to the CVD reactor where the precursors decompose.41,42 The ultrasonic spraying coating (USC) fabrication process involves the precursor solution transport through the syringe pump and using ultrasonic nozzle to spray the sample on the substrate (Fig 9(a)).43 AACVD is a simple, low-cost, ambient-pressure processable, and 091509-8 Shen et al APL Mater 4, 091509 (2016) FIG (a) Diagram of the HCVD furnace and MAI deposition onto metal halide seeded substrates for double heating zone system Reprinted with permission from Leyden et al., J Mater Chem A 2, 18742 (2014) Copyright 2014 Royal Society of Chemistry (b) Low-pressure heating tube for single zone hybrid chemical vapor deposition process Reproduced with permission from Shen et al., Adv Mater Interfaces 3, 1500849 (2016) Copyright 2016 Wiley-VCH large-scale feasible method for thin film deposition The choice of precursor materials can be flexible based on its volatility condition.44 The first ambient pressure AACVD research for perovskite film was published by O’Brien group who deposited the CH3NH3PbBr3 on the glass substrate.45 The precursor of CH3NH3PbBr3 was prepared by mixing CH3NH3Br and PbBr2 in N,N-dimethylformamide (DMF) and heated at 60 ◦C for h After nebulization of CH3NH3PbBr3 precursor, the carrier gas of argon transferred aerosol onto the glass substrate placed in the hot tube furnace at 250 ◦C The powder X-ray diffraction showed that the CH3NH3PbBr3 film was confirmed with the distinct peak at 14.77◦ and 29.98◦ which can be assigned to the (100) and (200) planes, respectively The scanning electron microscopic images observed uniform surface of CH3NH3PbBr3 film by using AACVD method At the same year, the group of Palgrave reported AACVD for large scale deposition of CH3NH3PbI3 film on glass and TiO2 coated glass substrate with 40 cm2 area.46 The scalable technique is a possible route for industrial application The deposition of CH3NH3PbI3 film occurred at 200 ◦C The XRD pattern showed the cubic structure of CH3NH3PbI3 with lattice parameter of a = 6.2993 Å which agreed with previous reports.47,48 However, after they store the sample under the dry box for few days, the cubic structure of CH3NH3PbI3 film transforms to tetragonal phase with the appearance of distinct peak at FIG (a) Schematic diagram of ultrasonic spray coating process (b) J –V curve of a typical perovskite solar cell on the glass substrates Reproduced with permission from Das et al., ACS Photonics 2, 680 (2015) Copyright 2015 American Chemistry Society 091509-9 Shen et al APL Mater 4, 091509 (2016) 23.5◦ The stoichiometry of final film was determined by X-ray photoemission spectra, which revealed the surface composition ratio of Pb and I to be 0.325 These results proved that the AACVD method prepared the CH3NH3PbI3 film successfully A two-step aerosol deposition has also been proposed to obtain better control on the deposition condition of each precursor with enhanced perovskite crystal film density.42 Nonetheless, the optimized deposition should be further studied as the thickness of two-step AACVD perovskite film was 1.5 µm which was greatly larger than optimum thickness (∼400 nm).49 The AACVD has been proved as a useful method to fabricate a dense, pinhole-free, and large grain perovskite film onto large-scale substrate Barrows et al reported the mix-halide perovskite film deposition by USC method The MAI and PbCl2 precursor sprayed on the planar-heterojunction surface with the ITO/PEDOT:PSS/perovskite/PCBM/Ca/Al device architecture.10 They observed that the boiling point of organic solvent in spray solution affected the film morphology The low boiling point of spray solution could dry out during the spray process which causes non-uniform of droplet distribution, the formation of pin-hole surface, and variation of film thickness In their report, the author deposited the perovskite film at 75 ◦C substrate temperature using DMF as spray solvent and annealed at 90 ◦C for 90 min, sequentially The high performance of USC-prepared perovskite solar cells obtained 11.1% PCE with Voc of 0.92 V, Jsc of 16.8 mA/cm2, and FF of 72% Das et al reported using the USC method to deposit high quality CH3NH3PbI3−xClx film on the ITO glass or flexible PET substrate with 13.0% and 8.1% of PCE, respectively (Fig 9(b)).43 Flash evaporation is another novel technique to fabricate smooth and flat perovskite film Longo et al reported the flash evaporation with the following deposition process: the perovskite precursor is deposited on the tantalum foil at 80 ◦C by meniscus coating.50 The perovskite-coated tantalum foil was then transferred to vacuum chamber and applied with high current through tantalum to evaporate perovskite This method is favorable for multilayer structure and film thickness control The flash-evaporation-based perovskite solar cells can reach PCE of 12.2% with Voc of 1.067 V, Jsc of 18.0 mA/cm2, and FF of 68.04% Understanding the formation mechanism of vapor-processed perovskite is crucial for controlling film quality as well as its device optimization In situ XRD was conducted to real-time monitor the crystal nucleation and growth via the structural evolution or phase transition By tracking the variation of diffraction peaks, the resultant perovskite crystalline can be precisely identified, as seen in Fig 10 For the dual-source co-evaporation process, the real-time XRD is schematically presented in Fig 10(a) and mapped the structural evolution under various reaction temperature of PbCl2 shown in Fig 10(b).51 With further characterization by energy-dispersive X-ray spectroscopy (EDX), a miscibility gap for the MAPbI3 and MAPbCl3 phases in the mixed-halide MAPbI3(1−y)Cl3y perovskite was estimated to be 0.05 < y < 0.5 during co-evaporation of MAI and PbCl2 The formation chemistry of co-evaporated CH3NH3PbI3(1−y)Cl3y was examined utilizing in situ X-ray photoelectron spectroscopy (XPS).52 A negligible amount of Cl was detected due to the ionic radii mismatch between Cl and I ions Borchert et al employed in situ XRD to record the growth and thermal annealing of co-evaporated MAPbI3 and MAPbI3(1−y)Cl3y From the temperature-controlled real-time XRD contour, the tetragonal β-phase MAPbI3 at room temperature underwent the β-α phase transition as the temperature increases to 50 ◦C The authors also indicated that the flux ratio between MAI and PbI2 had significant influences on the orientation of the deposited MAPbI3 perovskite films, while the flux ratio governed the phase formation of mixed halide MAPbI3(1−y)Cl3y perovskite By further applying thermal annealing process, increasing XRD intensities and decreasing FWHMs were observed for both films, delivering well-ordered crystallizations of perovskite.53 Teuscher et al also introduced inductively coupled plasma mass spectrometry (ICP-MS) to provide quantitative I/Pb ratio after co-evaporation of PbI2 and MAI.54 To precisely control the PbI2/MAI stoichiometric ratio of the resulting perovskite, the proportional-integral-derivative (PID) driven thermal evaporator equipped with feedback loop was introduced to well control the deposition rate of PbI2 and MAI, respectively Through changing the chamber pressure, perovskite films with well-controlled composition of I/Pb ratio was achieved from 2.5 to 3.5 The perovskite solar cells were thus fabricated with different stoichiometries, and the results revealed that formation of MAPbI3 (stoichiometries close to 3) yielded the best device performance and reproducibility.54 Time-resolved in situ XRD, schematically illustrated in Fig 10(c), was also employed to reveal the MAPbI3 perovskite formation during VASP process.35 From the tracing of diffraction peaks 091509-10 Shen et al APL Mater 4, 091509 (2016) FIG 10 (a) Schematic illustration of in situ XRD setup for a dual-source evaporation of perovskite; (b) time sequences of the X-ray diffraction contour of dual-source evaporated perovskite Reprinted with permission from Pistor et al., J Phys Chem Lett 5, 3308 (2014) Copyright 2014 American Chemistry Society (c) Schematic illustration of in situ XRD setup for vapor-assisted solution processed (VASP) perovskite; (d) temperature dependent X-ray diffraction contour of VASP-based perovskite Reproduced with permission from Yang et al., J Am Chem Soc 138, 5028 (2016) Copyright 2016 American Chemistry Society shown in Fig 10(d), the transition of PbI2 to perovskite crystal was concluded as follows MAI vapor molecules are decomposed into gas phase methylamine (CH3NH2) and gas phase hydrogen iodide (HI) by controlling the reactive temperature close to the MAI boiling point The decoupled MAI vapor diffused into the spun-cast PbI2 film to initiate the reaction The MAI vapor initially reacts with the PbI2 layer on the upper surface to produce a perovskite thin film as crystal nuclei, followed by diffusion of MAI vapor penetrating the upper surface to react with PbI2 below, resulting in a top-down reaction The perovskite grains mainly grow along the direction perpendicular to the substrate with a one-dimensional growth path and laterally merge to form large grains Additionally, the atmosphere of CH3NH2 gas had benefits to transform the defective MAPbI3 crystal into a dense morphology.55 Eventually, a compact perovskite film was fabricated by filling up large perovskite grains The real-time formation dynamics of perovskite film prepared using sequential thermal evaporation of PbI2 and MAI was exploited by Patel et al in virtue of UV-Vis spectra, IR, XRD, and time-resolved PL measurements.56 To monitor the time-dependent formation of perovskite film, the as-deposited PbI2 substrate was controlled at ◦C during the deposition of MAI, and the formation of MAPbI3 perovskite in vacuum was subsequently examined at room temperature Initial interdiffusion of PbI2 and MAI leads to the formation of nascent MAPbI3 with remnant of unreacted MAI as revealed from the UV-Vis spectra, IR, and XRD When the film was further exposed to ambient atmosphere (T = 21 ± ◦C, relative humidity = 40 ± 10%), moisture benefited the ion diffusion of MAI to fully crystallize MAPbI3 and reacted with excess MAI to form the 091509-11 Shen et al APL Mater 4, 091509 (2016) TABLE I List of the photovoltaic characteristics of various vapor phase deposited HOIP devices Performance JSC (mA/cm2) FF Perovskite type/thickness (nm) VOC (V) PCE (%) References Planar CH3NH3PbI3−xClx/330 1.07 21.5 0.68 15.4 11 Planar (inverted) Planar (inverted) Planar (inverted) Planar (inverted) Planar Planar (inverted) Planar (inverted) Planar (inverted) Planar CH3NH3PbI3/285 CH3NH3PbI3/285 CH3NH3PbI3/285 CH3NH3PbI3/285 CH3NH3PbI3−xClx/135 CH3NH3PbI3−xClx/390 CH3NH3PbI3/320 CH3NH3PbI3/370 CH3NH3PbI3−xClx/500 1.05 1.09 1.08 1.07 1.09 1.03 1.12 1.05 0.97 16.12 18.2 16.53 18.8 17.0 16.0 18.1 21.9 18.0 0.67 0.75 0.65 0.63 0.54 0.66 0.68 0.72 0.64 12.0 14.8 11.6 12.7 9.9 10.9 13.7 16.5 11.1 17 16 62 63 26 64 65 27 66 Sequential evaporation Planar (inverted) Planar Planar (inverted) Planar (inverted) Planar Planar Planar CH3NH3PbI3−xClx/430 CH3NH3PbI3/480 CH3NH3PbI3/350 CH3NH3PbI3/400 CH3NH3PbI3−xClx/410 CH3NH3PbI3/320 CH3NH3SnBr3/200–300 1.02 1.06 0.8 0.96 1.0 1.10 0.498 20.9 22.7 13.6 21.76 22.27 18.9 4.27 0.72 0.73 0.5 0.65 0.72 0.75 0.491 15.4 17.6 5.4 13.7 16.03 15.7 1.12 12 30 67 28 29 68 69 Vapor-assisted solution process (VASP) Planar CH3NH3PbI3/350 0.92 19.8 0.66 12.1 13 Mesoscopic Mesoscopic Planar Planar Mesoscopic Mesoscopic Planar CH3NH3PbI3/250 CH3NH3PbBr3/325 CH3NH3PbI3/320 CH3NH3PbI3/n.a CH3NH3PbI3/450 CH3NH3PbI3/450 CH3NH3PbI3−xClx/400 0.82 1.45 0.952 0.89 1.05 0.91 1.005 18.3 9.75 21.0 15.7 20.6 22.0 21.94 0.71 0.62 0.61 0.58 0.71 0.65 0.624 10.6 8.7 12.2 8.1 15.3 13.3 13.76 31 32 33 70 34 35 37 Planar CH3NH3PbI3−xClx/350 1.04 21.7 0.75 16.8 71 Mesoscopic Planar CH3NH3PbI3/250 CH3NH3PbI3/400 1.02 0.97 21.4 21.15 0.68 0.75 14.7 15.37 72 14 Planar CH3NH3PbI3/300 0.91 21.7 0.65 12.7 73 Planar Planar CH3NH3PbI3−xClx/296 CH(NH2)2PbI3/324 0.92 1.03 19.1 20.9 0.62 0.66 10.8 14.2 38 39 USC spray deposition Planar Planar Planar CH3NH3PbI3−xClx/340 CH3NH3PbI3−xClx/300 CH3NH3PbI3/500 0.92 1.03 1.00 16.8 20.6 15.08 0.72 0.615 0.53 11.1 13 7.99 10 43 74 Flash evaporation Planar (inverted) CH3NH3PbI3/200 0.95 21.0 0.61 12.2 50 Multilayered deposition Planar (inverted) CH3NH3PbI3−xClx/330 0.99 21.58 0.78 15.12 75 Planar (inverted) CH3NH3PbI3/473 0.96 21.8 0.60 12.5 76 Deposition technique Dual-source co-evaporation Low pressure VASP (single zone) Low pressure VASP (double zone) Architecture hydrate (CH3NH3)4PbI6·2H2O The resultant hydrate phase would decompose into water and PbI2, resulting in the MAPbI3 film degradation Yang et al further studied the competition of halogens during the synthesis of mixed-halide perovskite via time-dependent isothermal XRD contour Neither Br− nor Cl− was incorporated into the resultant perovskite crystal structure Instead, they were dispersed in the grain boundaries of MAPbI3 perovskite to promote the perovskite crystal growth.57 091509-12 Shen et al APL Mater 4, 091509 (2016) FIG 11 The development roadmap of reported efficiency versus publication date for vapor-based deposition techniques The star marker indicates the best-performing device efficiency of 17.6% for vapor-based deposition technique The chemical reaction during the formation of MAPbI3 perovskite was investigated by using in situ XPS on successive depositions of thermally evaporated MAI on PbI2 film, in which the N1s and C1s core level peaks were monitored to comprehend the transition from PbI2 to MAPbI3.35,58 Initially, a low binding energy feature of C1s at 285 eV from the surface contamination species was observed, while no nitrogen peak was presented before reaction with MAI After MAI deposition on the PbI2 film, the XPS peaks from C and N elements, respectively, appeared at 286 eV and 402 eV and increased with the reaction time, which were assigned to the methyl group and nitrogen in CH3NH3PbI3 Furthermore, Jalin et al introduced resonance Raman spectroscopy to show gradual shift of Pb-I vibrations with reaction time, demonstrating the intercalation of MA+ inside PbI64− during formation of perovskite crystalline.35 In our recently published works, we summarized the relation between annealing temperature, working pressure, and corresponding PCEs achieved by VASP-based methods.14 It is worth noticing that the working pressure for perovskite formation is 760 Torr for those using atmosphere VASP method or below Torr for low-pressure VASP Surprisingly, no perovskite formation was successfully conducted under working pressure of several hundreds torr so far In conclusion, for the future of industry-scale mass production, well-defined process for large area uniformity and quality control is of crucial importance Hence, high quality of perovskite films prepared by the vapor-deposition technology is promising to be considered for commercial production route A list of the recent vapor phase HOIP solar cells is presented in Table I with their photovoltaic parameters and device configurations Figure 11 illustrates the roadmap on the evolution of the various techniques discussed in this research update The record efficiency of 17.6% for vapor-based HOIP solar cells has been achieved, though it is still lower than that of solution-processed counterpart Improvements on the photovoltaic performance of vapor-based HOIP solar cells are expected with further effort It is worth noting that the state-of-the-art perovskite solar cells use multiple components such as mixed-cation and mixed-halide-based perovskites to form perovskite crystal.59–61 These complex systems are not yet investigated by vapor deposition method We believe that higher efficiency can be achieved with mixed-cation and multiple halide materials processed by vapor deposition methods The large-area module of commercial scale HOIP solar cells can be realized by designing the subcell architecture, reducing the transport layer resistivity, and optimizing the deposition parameters The vapor-based deposition method has 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