Stable Triple-Cation (Cs+âMA+âFA+) Perovskite ... - ACS Publications

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A stable triple-cation (Cs+-MA+-FA+) perovskite powder formation under ambient conditions for a hysteresis-free high efficiency solar cells Ranbir Singh, Sanjay Sandhu, Hemraj Yadav, and Jae-Joon Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09121 • Publication Date (Web): 26 Jul 2019 Downloaded from pubs.acs.org on July 26, 2019

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ACS Applied Materials & Interfaces

A Stable Triple-Cation (Cs+-MA+-FA+) Perovskite Powder Formation Under Ambient Conditions for a Hysteresis-Free High Efficiency Solar Cells Ranbir Singh, Sanjay Sandhu, Hemraj Yadav, Jae-Joon Lee* Department of Energy Materials and Engineering, Research Center for Photoenergy Harvesting & Conversion Technology (phct), Dongguk University, Seoul, Korea Corresponding author e-mail: [email protected]

Abstract. Organometallic halide perovskite materials have promising photovoltaic properties and have emerged as a cost-effective solar cell technologies. However, a synthesis protocol to fabricate high quality perovskite thin film under ambient conditions remains a critical issue and hinders commercialization of the technology. Therefore, this paper proposes efficient and stable fabrication of triple-cation perovskite photoactive solid-state thin film for solar cells using preformed perovskite powder under ambient conditions. Highly crystalline triple-cation perovskite powder was synthesized by a solution processed anti-solvent recrystallization technique, and films were prepared following a previously reported recipe for an efficient triple-cation perovskite. The synthesized perovskite powder was characterized using UV-visible absorption spectroscopy, Xray diffraction, time resolved photoluminescence, and field emission scanning electron microscope. 1

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Fabricated solar cells were investigated for photovoltaic characteristics, including current densityvoltage hysteresis, recombination losses, and thermal stability. The improved photovoltaic characteristics and thermal stability were attributed to the superior proposed perovskite film quality and crystalline properties. Keywords: Perovskite powder, triple-cation perovskite, solar cells, thermal stability, hysteresisfree.

Introduction. The emergence of perovskite based light absorbing material has generated a revolution in organicinorganic hybrid solar cell technology. Many research groups have developed remarkable perovskite materials for energy harvesting, with broad light absorption range (300–780 nm), high extinction coefficient, long diffusion length (˃ 1 μm), ambipolar charge transport, long carrier lifetime, and high mobility.1-4 The highest power conversion efficiency (PCE) achieved for perovskite solar cells (PSCs) is currently 24.2%.5 However, despite the impressive efficiency, thin film PSCs still face several critical challenges, like morphology control, scalable high quality production, reproducible device fabrication, and thermal and moisture instability, which prevents them competing with established technologies. Many studies have considered contributing factors, including chemical composition variation, crystallization processes, small amounts of additives, device architectures, and perovskite deposition techniques to enhance the film quality.6-10 Deposition of high quality perovskite film by a simple and scalable fabrication process is one of the most important requirements for commercial PSCs.

2


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Various factors strongly affect PSCs performance, including perovskite film homogeneity, crystallinity, grain size, and surface coverage, which largely depend on the film deposition method employed. In early stages, MAPbX3 (X = Cl, Br, or I) PSCs were prepared by spin coating, with subsequent 100°C thermal annealing for perovskite conversion. Other deposition approaches were subsequently developed to facilitate better quality and easier fabrication of the perovskite thin films, including physical vapor deposition, spray coating, vapor assisted solution processing, solvent-solvent extraction, perovskite powder and solid-state chemistry.11-17 The most promising approach has been solution processed spin coating, providing ease of fabrication and low cost. Solution processed spin coating combines solvent engineering and Lewis acid-base adduct approaches to form uniform, flat, and pinhole-free perovskite films. Anti-solvents, such as chlorobenzene (CB), toluene, diphenyl ether, etc. have been employed to provide uniform crystallization, and control over nucleation and grain growth.18-19 Pinhole-free perovskite layers considerably improve cell parameters, particularly open circuit voltage (VOC) and fill factor (FF). High efficiency PSC development is largely focused on organicinorganic Cs based triple-cation perovskites, due to their superior stability, with some lesser attention on methylammonium (MA) based perovskites, and very few on formamidine (FA) based perovskites.20-22 Most fabrication is conducted in controlled nitrogen filled glovebox conditions to ensure outcome quality, as knowledge regarding the fabrication of an efficient and stable perovskite photoactive layer under ambient conditions is somewhat limited. Although a few attempts have been made to fabricate uniform and pinhole-free perovskite films under ambient conditions, they have not generally been very successful, and the process remains a challenge. Previous attempts have invariably suffered from defects and pinholes in perovskite films, 3



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increasing leakage paths, charge carrier recombination, and increasing radiation less charge carrier decay.23 Low defect and pinhole concentration is a primary requirement for efficient and stable device operation. This paper introduces a synthetic process to prepare high quality solid-state thin perovskite films from triple-cation perovskite powder under ambient conditions. The resultant fabricated planar PSCs achieved significantly improved performance and thermal stability at 90°C in 38% relative humidity (RH) conditions. Triple-cation perovskite powder was synthesized by a solution processed anti-solvent recrystallization process, adding hot triple-cation based perovskite precursors dropwise to hot chlorobenzene (CB) anti-solvent (120°C) to produce highly crystalline perovskite powder. Perovskite solid-state thin film was deposited by single step spin coating using an equimolar concentration solution of perovskite powder. We subsequently investigated perovskite film quality experimentally, measuring PSC performance, and perovskite layer thermal stability.

Materials and methods Chemicals and materials. Lead (II) iodide 99% (PbI2), lead(II) bromide (PbBr2), methylammonium

bromide

(MABr),

bis(trifluoromethane)sulfonimidelithium

salt

cesium

(Li-TFSI),

iodide

4-tert-butylpyridine

(CsI), (TBP),

Dimethylformamide (DMF), dimethyl sulfoxide (DMSO), CB, and titanium diisopropoxide bis(acetylacetonate) solution were purchased from Sigma-Aldrich, and used as received. Po-SpiroMeOTAD was purchased from 1-materials company (Nano Clean Tech, Korea). Formamidinium iodide 4

(FAI),

and

FK209

(tris[2-(1H-pyrazol-1-yl)-4-tert-butylpyridine]cobalt(III)


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tris[bis(trifluoromethylsulfonyl)imide]) were purchased from Dyesol (GreatCell Solar Ltd, Australia). Perovskite solar cell preparation. Normal solar cell structures were fabricated with stack glass / Fluorine-doped Tin Oxide (FTO) / TiO2 (60 nm)/ perovskite/ po-Spiro-MeOTAD (80 nm) / Au (80 nm) architecture under ambient condition at controlled humidity (RH~ 22%). ͠First, cleaned FTO coated glass substrates were treated with UV/ozone for 30 min, and then a hole blocking layer of compact TiO2 was deposited by spin coating at 3000 rpm from 0.15 M titanium diisopropoxide dis(acetylacetonate) in 1-butanol, and subsequently annealed at 500°C for 30 min. TiO2 film was treated with 40 mM aqueous TiCl4 solution at 70°C for 45 min, cleaned with distilled water, and further annealed at 500°C for 10 min. Triple mixed cation perovskite precursor solution was prepared according to the recipe in SI. To prepare precursor solution from triple-cation perovskite powder (following the synthesis process in Figure 1), an equimolar ratio of perovskite powder was mixed in DMF:DMSO (4:1) solvent and stirred overnight at 65°C. Equimolar ratio of perovskite powder was calculated by adding the molar concentration of the components as used to prepare primary solution described in supporting information. Both perovskite precursor solutions were spin coated in two steps at 1000 and 5000 rpm for 10 and 20 sec, respectively. During the second step, 200 μl CB was poured on the spinning substrate 10 sec prior to the end of the program. The films were then annealed at 110°C for 1 h under controlled humidity, RH = 22%. The solution of hole extracting material was prepared by dissolving 30 mg po-spiro-MeOTAD, 21.5 µl dilute 4-tert-butylpyridine (1ml 4-tert-butylpyridine mixed with 1ml of acetonitrile), 21.5 µl stock solution (170 mg ml-1 lithium bis(trifluoromethylsulphonyl)imide in acetonitrile) and 6 µl stock solution

(300

mg

ml-1

FK209

(tris[2-(1H-pyrazol-1-yl)-4-tert-butylpyridine]cobalt(III)

5



tris[bis(trifluoromethylsulfonyl)imide]) in acetonitrile) in 1 ml anhydrous CB. The hole transporting layer was deposited from po-spiro-MeOTAD solution at 3000 rpm. Finally, an Au (80 nm) anode was thermally deposited under high vacuum (< 1×10-6 Torr) through a shadow mask to create devices with 0.22 cm2 total area. Optical characterization. UV-Vis absorption and PL spectra of the spin coated films were recorded using a Scinco UV-Visible spectrophotometer (S-3100, Korea) and Varian Cary Eclipse fluorescence spectrophotometer with 15 W Xenon lamp, respectively. Time resolved photoluminescence (TRPL) measurements were performed using a time correlated single photon counting (TCSPC) system (FL920, Edinburgh Instruments) with 40 nm pulsed laser source. Characterization of perovskite solar cells. Electrical characteristics were measured using a Solar simulator (LAB50, MacScience) for one-sun conditions with attached Keithley source meter unit under ambient conditions. Light was calibrated using a standard mono-Si solar cell (PVM-396, PV Measurements Inc.) certified by the National Renewable Energy Laboratory. Incident photon converted electron (IPCE) efficiency was measured using photomodulation spectroscopy (McScience, K3100 Spectral IPCE Measurement System) with monochromatic light from a Xenon lamp, where the source power density was calibrated using a Si photodiode certified by the National Institute for Standards and Technology. Photoexcitation intensity dependent J-V curves were measured using the same solar simulator setup by varying light intensity 0.1–1 sun equivalent. Morphological characterizations. FESEM images were obtained using JSMJSM-6700F (JEOL) Field Emission Scanning Electron Microscope. One-dimensional XRD measurements were performed using an X-ray diffractometer (Rigaku D/Max-2500) with Cu-Kα X-rays. 6


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SCLC measurement. Hole mobility data were extracted from dark J–V characteristics of holeonly devices: ITO/ poly(3,4-ethylenedioxythio-phene):polystyrene sulfonate (PEDOT:PSS)/ perovskite/Au. Electrical dark J-V characteristics were measured using a Source/Meter Keithley 2400.

Results and discussion Figure 1(a) shows the proposed perovskite powder synthesized route. Precursor solution was prepared by mixing triple-cation perovskite components (PbI2, PbBr2, FAI, MABr, and CsI) in DMF:DMSO (4:1) as reported elsewhere.24 The triple-cation perovskite precursor was optimized with different CsI concentrations to obtain good quality perovskite film

and high power

conversion efficiency (PCE) under ambient conditions, as shown in Supporting Information (SI) Figure. S1 and Table S1. To prepare perovskite powder, precursor solutions with different CsI concentrations were added dropwise (1 drop/5 s) to 50 ml hot CB previously heated to 120°C, and then the solution was stirred for 2 min at 120°C. The solution color changed from yellow to brown, and finally black perovskite nanocrystal precipitates. The precipitates were collected by washing three times with fresh CB followed by drying at 90°C in a vacuum oven for 12 h. Resultant perovskite powders were stored in ambient conditions for further use. Figures 1(b) and (c) show perovskite powder sample and FESEM image of optimal CsI concentration powder, respectively.

7



Figure. 1 (a) Schematic diagram for the preparation of triple-cation (TP) perovskite powder. (b) Picture of the synthesized perovskite powder in a vial (c) field emission scanning electron microscope (FESEM) images of the perovskite powder and (d) XRD pattern of the perovskite powder.

Figure 1(d) shows one-dimensional (1D) perovskite powder XRD. The intense (220) plane peak suggest preferential perovskite growth along the (220) plane. Other XRD peaks for (110), (220), and (310) planes are characteristic perovskite peaks, and the major and minor perovskite powder XRD peaks are consistent with conventional perovskite.1 Figures S2–S4 show different CsI concentration effects on morphology and crystalline properties. We have noticed that perovskite film morphology improved for an optimum concentration of CsI (8%). Similarly, Gratzel and Choi et al. have explained that a small amount of CsI concentration 8


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can effectively suppress the yellow phase impurities and defects in the perovskite films, and produce a good quality perovskite films.24-25 Figure S5 compares photovoltaic performances for PSCs prepared from the reference recipe (P-I) and perovskite powder (P-II). PSC device performance significantly improved for all CsI ratios when photoactive layers were fabricated from perovskite powders. Particularly, perovskite powders with 8% CsI exhibited highest VOC, short circuit current density (JSC), FF, and PCE. Therefore, we have investigated optimal perovskite films and PSCs for both processes.

Figure 2. Schematic for preparing perovskite film with two different processes (Process-1 (P-I) and Process-II (P-II)) under ambient condition. P-I: perovskite film fabricated from the triple-cation perovskite precursor solution. P-II: perovskite film fabricated from the perovskite precursor solution prepared from perovskite powder.

9



We evaluated fabricated perovskite powder based photoactive layer and PSC devices following two candidate processes, P-I and P-II, shown in Figure 2. P-I prepared the perovskite precursor solution with optimum 8% CsI as described in SI, and then deposited the solution over a TiO2 blocking layer using spin coating followed by anti-solvent coating, as described in the experimental section. Subsequent annealing at 110°C for 1 h changed the deposited layer color to black, indicating perovskite formation. P-II dissolved equimolar concentration of perovskite powder in DMF:DMSO (4:1) solvent at 65°C to prepare an alternative precursor solution. This solution was then spin coated following the same process as for P-I, including annealing.

10


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Figure 3. (a, b) Top-view FESEM images, (c) 1D-XRD (d) UV-vis absorption spectra (e) photoluminescence (PL) spectra (f) TRPL decay curve for the perovskite films prepared by the P-I and PII. Both the perovskite films were spin coated on the glass/FTO/ TiO2 as presented in P-I and P-II (Figure 2).

P-I and P-II produced perovskite thin film surface morphologies were examined using field emission scanning electron microscope (FESEM), as shown in Figures. 3(a) and (b), respectively. Perovskite film prepared following P-II exhibited considerably smoother and more compact surface, with larger grains. Even in large area scans, P-I perovskite based films had more pinholes and smaller grains compared with P-II (see SI, Figure S6). XRD spectra for films from the two processes exhibited very similar diffraction angles (2θ), indicating the same perovskite material was present. However, significant differences in the XRD spectra, including increased peak (110), (220), and (310) for P-II confirm better morphology. The predominant (110) peak suggests preferential perovskite growth along the (110) plane, and full width at half maximum decreased from 0.204° for P-I to 0.195° for P-II, indicating superior perovskite film crystallinity. This improved crystallinity was a consequence of decreased pinhole (or defect) concentration, and increased structural order.26-27 Both process XRD patterns exhibit no extra peaks for PbI2 (001) at 12.6° or FAPbI3 (δ-phase at 11.8°), which implies good purity for both process routes.28 We evaluated space charge limited current (SCLC) mobility for both films using the Mott-Gurney model,29-30 estimating mobility from dark current density-voltage (Jd-V) characteristics obtained from controlled devices fabricated with architecture glass/ITO/ poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)/perovskite layer/Au (Figure. S7). The mobility for the perovskite film prepared by P-II exhibited mobility µ = 8.7×10-3 cm2 V-1s-1, P-I film had µ = 11



3.9×10-3 cm2 V-1s-1. Higher mobility for the P-II route was attributed to better film morphology and improved crystallinity. Figure 3(d) shows absorption coefficient spectra for both films. P-II perovskite layer absorption is significantly higher than P-I. The more dense and larger crystal grain sizes in P-II film enhances absorption across a wider spectral range.31 Thus, the P-II route is more advantageous to produce compact and dense photoactive layers for photovoltaic (PV) devices. Figure 3(e) shows photoluminescence (PL) spectra to investigate light induced halide phase segregation, where the films were excited with a 500 nm laser. P-II prepared perovskite layers exhibited weaker luminescence at 773 nm than P-I, indicating better exciton quenching at the perovskite (P-II)/TiO2 interface, which could be due to better excitons diffusion bringing them closer to the charge extracting TiO2 layer,12 i.e., improved charge transfer at the interfaces. Figure 3(f) shows time resolved PL (TRPL) to investigate charge transfer rate from perovskite to TiO2. Decay traces for both process routes were fitted with bi-exponential decay, as detailed elsewhere,29 and the charge carrier lifetime, τ, associated with charge transfer from perovskite to TiO2 was extracted,32 with τ = 4.23 and 2.19 ns for P-I and P-II route films, respectively. Thus, the P-II films achieved significantly superior charge transfer rate, suggesting that photogenerated charge carriers could more effectively transfer for P-II than P-I prepared films. This improved transfer was attributed to improved morphology and charge transport properties for P-II prepared perovskite films.29

12


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Figure 4. (a, b) Cross-sectional FESEM images (c) current density-voltage (J-V) (d) incident photon to current efficiency (IPCE) spectra (e) light intensity-dependent JSC and (f) light intensity-dependent VOC characteristics of PSC devices fabricated with P-I and P-II perovskite films.

Table.1 Photovoltaic parameters of the perovskite solar cell fabricated with P-I and P-II perovskite films. Averages were taken over eight devices. Active layer Process

VOC (V)

JSC (mA cm-2)

FF (%)

PCEavg (max) (%)

RS RSh (Ohm cm2) (Ohm cm2)

P-I

1.09 ± 0.004 21.05 ± 0.41 67.3 ± 1.03

15.58 (15.92)

7.33 ± 0.58 474.1 ± 7.72

P-II

1.11 ± 0.007 22.54 ± 0.86 72.2 ± 0.69

18.07 (18.68)

6.67 ± 0.16 746.5 ± 9.68

Triple-cation Perovskite

13



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To further investigate the photovoltaic performance of fabricated P-I and P-II perovskite films we fabricated PSC devices with glass/FTO/TiO2/Perovskite/po-Spiro-MeOTAD/Au architecture, as shown in Figures. 4(a) and (b). Figure 4(c) shows the fabricated device PV features, Table 1 summarizes the relevant PV parameters. The J-V curve for the highest performing devices under one-sun conditions confirm P-II efficiency and excellence, with VOC = 1.11 ± 0.007V, JSC = 22.54 ± 0.86 mA.cm-2, FF = 72.2 ± 0.69%, and PCE = 18.07 ± 0.61%; whereas P-I based devices achieved 1.09 ± 0.004V, 21.05 ± 0.41 mA.cm-2, 67.3 ± 1.03%, and 15.58 ± 0.34%, respectively. Thus, P-II based devices produced approximately 5% more FF than P-I, due to better charge transport and extraction properties; and JSC and VOC improvements (1.49 mA.cm-2 and 0.02 V, respectively) were due to better absorption, improved morphology and charge transfer characteristics, as discussed above. Perovskite solar cells can have significant hysteresis between forward and reverse J-V scans for many reasons, including ion migration, ferroelectric polarization, slow transient capacitive current, trapping and detrapping, etc.

33-34

The J-V characteristics for

different scanning directions (Figure. S8) shows that P-II based PSCs exhibited smaller hysteresis than to P-I based devices, consistent with the improved perovskite layer morphology. Figure 4(d) shows incident photon to current efficiency (IPCE) spectra, with almost 5% higher quantum efficiency for P-II based PSCs compared with P-I (400–650 nm). The calculated JSC(EQE) spectra (by integrating the IPCE) showed similar trend to JSC values under one-sun conditions. Generally, PSCs are represented with a single diode equivalent-circuit model consists of photocurrent source, diode, series resistance (RS) and shunt resistance (RSh), as shown in Figure S9a.35 Under illumination conditions, typical behavior of J-V characteristic of a PSC can be represent using the Shockley equation, where JSC and VOC are defined by eq. 1 and eq. 2.36 14


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[ (

𝐽𝑆𝐶 = 𝐽𝑃ℎ ― 𝐽0 exp

𝑉OC =

𝑛kT 𝑞

( )

𝑞(𝑉 ― 𝐽𝑅𝑆)

𝑉 ― 𝐽𝑅𝑆

𝑛𝑘𝑇

𝑅𝑆ℎ

𝐽𝑃ℎ

{ (

𝑙𝑛

𝐽0

) ― 1] ― 𝑉𝑂𝐶

)}

1 ― 𝐽𝑃ℎ 𝑅𝑆ℎ

…………………….. (1)

…………………….. (2)

where JPh is photogenerated current density, J0 is reverse bias saturation current density, e is elementary charge, n is diode ideality factor, k is Boltzmann’s constant (1.38 × 10-23 JK-1), T is room temperature (298K). RS originates from the internal resistances in PSCs such as the lightabsorbing material, interface barriers resistance, charge-collecting interlayers, and transparent or metal-based electrodes, whereas RSh is accounted to leakage elements in PSCs, for example, the pinholes of each functional layer and recombination losses.36 For an ideal case, RSh should approach to infinity whereas RS must be zero. In our devices, we found that RS (P-I) = 7.33 ± 0.58 Ohm-cm2 decreased to RS (P-II) = 6.67 ± 0.16 Ohm cm2, whereas RSh (P-I) = 474.1 ± 7.72 Ohmcm2 increased to RSh (P-II) = 746.5 ± 9.68 Ohm cm2 that is one of the main reasons for improved JSC values in devices. The higher VOC for the P-II can be directly correlated with lower J0, when RSh >> RS and JPh = JSC implies 𝑉OC =

𝑛kT

𝐽SC

( 𝑞 ) 𝑙𝑛( 𝐽 ) . 0

It can be clearly noticed from Figure. S9b

that devices for P-II have showed slightly lower J0 values due to the better compact morphology of the perovskite film formation from perovskite powder. Figure 4(e) shows measured intensity-dependent J-V characteristics under light intensities 10– 100 mW.cm-2. The JSC relationship to light intensity (I), follows a power law, i.e., JSC ∝ Iα, and we extracted α from the log-linear graphs,27 with α = 0.93 and 0.98 P-I and P-II based PSCs, respectively. The larger deviation of α from 1 for P-I devices indicates that JSC was limited by 15



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bimolecular recombination losses.27, 29 Figure 4(f) shows VOC light intensity dependence, fitted to VOC ∝ m(kBT/q)ln(I).27 The extent of trap assisted recombination can be estimated by extracting m, where m = 1 implies no trap assisted recombination and m > 1 indicates that trap assisted recombination is a dominating mechanism.37 We found m = 1.76 and 1.35 for P-I and P-II, respectively. Smaller m for P-II based PSCs suggests less trap-assisted recombination losses, which can be attributed to improved morphology, better charge carrier mobility, and improved charge extraction.38 To investigate trap density, Dtrap, we fabricated single charge carrier devices by sandwiching P-I and

P-II

films

between

electron

transporting

materials

like

FTO/compact

TiO2/perovskite/PC60BM/Au, reported earlier.39-40 Figure 5 shows the corresponding dark currentvoltage (Id-V) characteristics. Three distinct regions occurred under SCLC conditions, corresponding to Id ∝ Vn, where n varied with the slope.41-42 When n = 1, the region was considered ohmic, n > 2 for trap limited, and n = 2 for trap free. Voltage corresponding to the intersection of two regions (e.g. ohmic and trap limited regions) is known as trap filled limit voltage (VTFL), and Dtrap can be expressed by eq. 3.

𝐷𝑡𝑟𝑎𝑝 =

2𝜀0𝜀 𝑉TFL 𝑒 𝐿2

……………………………… (3)

where L is the thickness of perovskite layer, ε = 46.9 is perovskite relative dielectric constant43-44, ε0 = 8.854×10-14 F.cm-1 is vacuum permittivity, and e is the elementary charge. The extracted VTFL from Figure 5 and calculated Dtrap values are summarized in Table 2. We found electron Dtrap ≈ 4.26×1016 cm-3 and 5.88×1016 cm-3 for P-II and P-I based PSCs, respectively. Decreased P-II Dtrap 16


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could be partly attributed to decreased grain boundaries, defect passivation, and suppressed ion migration.45-46 The variation in Dtrap was consistent with decreased trap assisted recombination losses, and hysteresis for P-II.33 Overall, PSC devices prepared using P-II exhibited relatively lower recombination losses and hence improved photovoltaic performance.

Figure 5. Dark current-voltage (Id-V) characteristics for the controlled electron-only devices. Inset figure shows the device structure used for making the electron-only devices.

Table 2. VTFL, Dtrap, τ, α, and m values were summarized for the P-I and P-II processed perovskite films, respectively. Process

VTFL (V)

Dtrap (cm-3)

τ (ns)

α

m

P-I

1.02

5.88 × 1016

4.23

0.93

1.76

P-II

0. 74

4.26 ×1016

2.19

0.98

1.35

17



Figure 6. Photovoltaic parameters (a) JSC, (b) VOC, (c) FF and (d) PCE versus thermal annealing time of the PSCs, where perovskite films were fabricated via P-I and P-II. Both the PSCs were kept on a hot plate at 90˚C in atmospheric conditions (RH 38%). Averages were taken over six devices.

We tested device thermal stability by annealing the unencapsulated devices at 90°C under ambient conditions (RH ∼ 38%). Figure 6 shows the resulting PV parameters (JSC, VOC, FF, and PCE) as a function of thermal annealing time, by their initial values. P-II based PSCs exhibited significantly improved stability than P-I, with JSC, VOC, FF, and PCE reduced by 61.4, 51.3, 29.4, and 85.1% 18


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of their initial values for P-I, and 18.4, 6.3, 3.9, and 19.8% for P-II, respectively. Thus, P-II based films were more resistive against degradation and provided superior device stability, attributed to improved film compactness and morphology due to pinhole-free structures with high crystallinity.47-48

Figure 7. FESEM images of the perovskite films prepared with P-I and P-II (a, c) before aging and (b, d) after aging and magnified image in inset. (e) XRD scan for the fresh perovskite (before aging - solid lines) and degraded-perovskite films (after aging - dotted lines). (f) Powder XRD pattern for the fresh and after 96 days of the perovskite powder, where it was kept in atmospheric condition.

Figures 7(a)–(d) show perovskites film morphologies by FESEM, indicating that thermodynamic deterioration leads to large pinhole formation and aging defects, which was more severe for P-I 19



than P-II. Degradation increased grain size as well as forming many voids that appeared to propagate deeper into the bulk from the surface (Figure. 7(b), inset). Void formation was partly attributed to release of gaseous perovskite decomposition products (HI, CH3NH2) during aging,4950

which induced formation of a large number of bright spots due to decomposed high atomic

number elements, such as PbI2, identified at 2θ = 12.6° diffraction by XRD (Figure 7(e)). Variations in the XRD pattern suggest significant reduction of the (110) diffraction peak, representing typical perovskite aging, with concomitant appearance of a secondary peak at 2θ = 12.6°. Degradation and PbI2 formation was less serious for P-II than P-I based films, suggesting that perovskite films formed with the higher quality powder (P-II) are more resistive against thermal aging under ambient atmospheric conditions. Further, we have also investigated perovskite powder stability under ambient atmospheric conditions (RH ~ 38%) by monitoring XRD peaks over 96 days. Less variant peak positions and intensities between fresh and aged samples indicated higher stability, consistent with previous studies on enhanced atmospheric stability of singlecrystal perovskite powder.51 The improved intrinsic stability could be attributed to higher quality and superior crystallinity P-II perovskite powder, resulting in strong ionic interactions within the crystal lattices.

Conclusion. We successfully synthesized highly crystalline, thermally stable, and photovoltaic efficient triplecation perovskite films using perovskite powder for solar cell applications. Structural investigations confirmed the formation of highly crystalline perovskite material with all XRD 20


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peaks well matched to reference perovskite prepared by conventional solution process. Perovskite thin films fabricated using the perovskite powders exhibited highly crystalline compact morphology, good absorption in visible region (500–750 nm), improved exciton dissociation and charge transfer at the perovskite/TiO2 interface. Planar PSCs fabricated in ambient conditions from perovskite powder precursor solutions achieved a high PCE of 18.68% under one-sun conditions with negligible J-V hysteresis, low recombination losses, and enhanced thermal stability. Perovskite powder prepared by the proposed method exhibited significant stack stability enhancement under ambient conditions even after 96 days. Thus, the proposed preparation method provides an efficient and scalable mass production route for high quality thin film perovskite solar cells to facilitate commercialization. Acknowledgment This research was supported by the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education (grants NRF2015M1A2A2054996, NRF-2016R1A2B2012061), and the Technology Development Program to Solve Climate Changes of the National Research Foundation, funded by the Ministry of Science, ICT & Future Planning (grant NRF-2016M1A2A2940912).

Supporting Information. Perovskite precursor solution preparation; CsI concentration dependent SEM images and 1-D XRD scans; SCLC mobility calculation; J-V hysteresis curves.

21



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Table of content figure. A highly crystalline, stable, and photovoltaic efficient triple-cation perovskite film fabricated using perovskite powder for solar cell applications. Planar perovskite solar cells fabricated in ambient conditions from the synthesized perovskite powder precursor solution achieved high PCE of 18.68% under one-sun conditions, including negligible J-V hysteresis, low recombination losses, and better thermal stability. The proposed perovskite powder also exhibited impressive ambient stability, retaining initial crystalline behavior even after 96 days exposure.


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