research update hybrid organic inorganic perovskite hoip thin films and solar cells by vapor phase reaction

The extraordinary power conversion efficiency PCE over 20% of HOIP solar cells has been achieved with credited certification.3,4 The deposition tech-niques of HOIP thin films have progre

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

With the rapid progress in deposition techniques for hybrid organic-inorganic perov-skite (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, va-por 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 crys-tallization, 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 tremen-dous 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,2which 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 tech-niques of HOIP thin films have progressed from simple spin-coating process,5 8sequential dipping method9to ultrasonic spray coating,10and 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 organic-inorganic perovskites (HOIPs) (CH3NH3PbI3and CH3NH3PbBr3) as light absorber for photovoltaic activities was carried out by Miyasaka and co-workers in liquid-electrolyte type sensitized so-lar 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-p-methoxyphenylamine)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 Al2O3as non-injecting scaffold and demonstrated a power conversion efficiency (PCE) over 10%.6These 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.

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 Nowa-days, 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 e ffi-ciency 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 va-por 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 stoichiom-etry 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 Figure1 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 fabri-cate flat CH3NH3PbI3−xClxlayer 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)and2(e)) than the solution-processed one (as referred to Figs.2(d)and2(f)) Moreover, the planar HOIP solar cells incorporating with a hole conductor, spiro-MeOTAD, achieved

a remarkable PCE of 15% (Fig.2(g)).11On the other hand, the inverted planar HOIP solar cells incor-porating 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%.17The 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.16The evaporated

FIG 1 Classifications of the various vapor phase deposition methods for HOIP solar cells.

FIG 2 (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 per-ovskite solar cells measured under simulated AM1.5G sunlight of 101 mW/cm 2 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 crys-tallites were fabricated in an optimum condition when the evaporation temperature for PbI2is 250◦C

On the other hand, Ono et al reported a home-built instrument for evaporation deposition of perovskite films.26The 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 depos-ited perovskite films.27It 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 evapora-tion method is a vapor phase analogy of layer-by-layer deposievapora-tion process for HOIPs In general, co-evaporation technique requires elaborate control on the evaporation conditions for both mate-rial 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 methy-lammonium halide were thermally sublimated one after another onto the substrate sequentially to

FIG 3 (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 1 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−xClxabsorber layer

by sequentially sublimating PbCl2and 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 PbCl2layer 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 PbI2and CH3NH3I The PbI2film (200 nm) was first evaporated on the compact TiO2layer 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 4 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.

FIG 5 The high resolution SEM images of perovskite films for vapor CH 3 NH 3 I reaction with different thicknesses of PbCl 2 : (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 PbCl2layer thickness affects the surface morphology of the synthesized perovskite films When the PbCl2thickness is less than 100 nm, the perovskite film is uniform with full coverage However, when the PbCl2thickness is larger than 100 nm, pinholes appear

in the perovskite films and the size of voids increases with PbCl2thickness (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 cm2to 1 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.30By 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 10−3–10−4Torr in 2 h Perovskite films with smooth surface and crystallite size of micrometer were obtained under vapor pressure of 10−4Torr At low pressure of 10−5Torr, 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 (∼10−2Torr) 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 solution-processed 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 PbI2film to effectively react with CH3NH3I vapor under atmospheric pressure The reaction time for full conversion from PbI2(200 nm) into

CH3NH3PbI3 (∼350 nm) is 2 h This two-step CVD method provides favorable nucleation ki-netics 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 negli-gible deviation efficiency of 0.1% for 30 cells in total Except for the iodide-based HOIPs, the

FIG 6 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 solu-tion 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.32The photoluminescence results showed longer diffusion length for CH3NH3PbBr3films, 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 situtubular deposition was demonstrated and the time for full conversion from PbI2to CH3NH3PbI3

in the heated furnace (145◦C) was 2 h, leading to a 12.2% PCE.33Recently, 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 meso-scopic architectures, respectively (Fig.7(b)).35Surprisingly, the process for vapor inter-diffusion in PbI2films proceeds faster with the presence of mesoporous scaffold and completes the conversion

of PbI2into CH3NH3PbI3in 30 min On the other hand, the remaining PbI2was observed for planar samples after 60 min vapor reaction and resulted in deteriorated photovoltaic performances The PbI2residues near the compact TiO2layer 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 interme-diate phase between PbI2polytype and perovskite was found.36Similarly, Raga et al demonstrated the role and interplay for PbI2films, methylamine (MA) gas, and hydroiodic (HI) gas for superior HOIPs formation.34The 2D layered structure of PbI2crystallites reacts with MA gas to complete conversion of CH3NH3PbI3within few seconds with additional PbO and Pb(OH)2phases in the resulting films Also, treatments of introducing sequential and simultaneous HI gas exposure favor-ably convert the additional PbO and Pb(OH)2phases to PbI2and CH3NH3PbI3 The best-performing devices using resultant films by simultaneous exposure of MA and HI gases achieved a 15.3% PCE.34A 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

FIG 7 Schematic illustration of the MAPbI 3 formation on a glass substrate (a) without and (b) with mesoporousTiO 2

scaffold layer, respectively The yellow-green lines on the surface of MAPbI 3 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 Ley-den et al using dual heating zones system (Fig.8(a)).38The CH3NH3I vapor was created by vapor-izing 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 PbCl2film from surface to the bottom for hybrid perovskite formation Under low pressure of ∼1 Torr, the conversion of perovskite is complete in

1 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).39A shorter reaction time less than 30 min 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 2 mTorr, it took

3 h to complete the transformation of PbI2to CH3NH3PbI3at 82◦C and the best-performing device

is up to 14.7%.40A similar work reported the fabrication of CH3NH3PbI3perovskite films under working pressure of 1 Torr with reaction time of 1 h and temperature of 120◦C The optimized planar device demonstrated a 15.37% PCE.14Another 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 gener-ation The humidifier transfers the precursor into aerosol mist and transports to the CVD reactor where the precursors decompose.41,42The 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)).43AACVD is a simple, low-cost, ambient-pressure processable, and

FIG 8 (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.44The first ambient pressure AACVD research for perovskite film was published by O’Brien group who deposited the CH3NH3PbBr3on the glass substrate.45The precursor

of CH3NH3PbBr3was prepared by mixing CH3NH3Br and PbBr2in N,N-dimethylformamide (DMF) and heated at 60◦C for 2 h After nebulization of CH3NH3PbBr3precursor, 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 CH3NH3PbBr3film 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 micro-scopic images observed uniform surface of CH3NH3PbBr3film by using AACVD method At the same year, the group of Palgrave reported AACVD for large scale deposition of CH3NH3PbI3film on glass and TiO2coated glass substrate with 40 cm2area.46The scalable technique is a possible route for industrial application The deposition of CH3NH3PbI3film occurred at 200◦C The XRD pattern showed the cubic structure of CH3NH3PbI3with lattice parameter of a= 6.2993 Å which agreed with previous reports.47 , 48However, after they store the sample under the dry box for few days, the cubic structure of CH3NH3PbI3film transforms to tetragonal phase with the appearance of distinct peak at

FIG 9 (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.

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 Vocof 0.92 V, Jscof 16.8 mA/cm2, and FF of 72% Das et al reported using the USC method to deposit high quality CH3NH3PbI3−xClxfilm 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 thick-ness control The flash-evaporation-based perovskite solar cells can reach PCE of 12.2% with Vocof 1.067 V, Jscof 18.0 mA/cm2, and FF of 68.04%

Understanding the formation mechanism of vapor-processed perovskite is crucial for controll-ing 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 PbCl2shown in Fig.10(b).51With further characterization by energy-dispersive X-ray spectroscopy (EDX), a miscibility gap for the MAPbI3and MAPbCl3phases 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).52A 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 MAPbI3and MAPbI3(1−y)Cl3y From the temperature-controlled real-time XRD contour, the tetragonal β-phase MAPbI3at 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 PbI2had 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 decreas-ing FWHMs were observed for both films, deliverdecreas-ing 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 PbI2and MAI.54To 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 MAPbI perovskite formation during VASP process.35 From the tracing of diffraction peaks