Versatile perovskite solar cell encapsulation by low-temperature ALD-Al₂O₃ with long-term stability improvement

Abstract

The outstanding advancement of perovskite solar cells (PSCs) in recent years makes them one of the currently most interesting topics in photovoltaic (PV) technology. However, some instability issues must be solved for future application. Encapsulation techniques have been an important solution to avoid or, at least, to limit those concerns for other PV technologies and are a promising way to circumvent extrinsic instability in the case of PSCs as well. Here, we introduce a low temperature (60 °C) encapsulation process based on a single thin coating of Al₂O₃ prepared by atomic layer deposition (ALD-Al₂O₃). Our procedure is compatible with the individual PSC components, and hence a mean of 93.6% of the original peak power conversion efficiency (PCE) was retained after encapsulation, with the champion device exhibiting a PCE of 17.4%. Moreover, the long term stability of the encapsulated devices was remarkably lengthened by preserving the fill factor and reducing the hysteresis usually observed in non-encapsulated samples. The improvements due to the ALD-Al₂O₃ encapsulation process were achieved for a standard PSC configuration: methylammonium lead triiodide (MAPbI₃), additivated-Spiro-OMeTAD and porous TiO₂ and could rapidly be extrapolated to other, more stable PSC architectures, paving a route towards commercialization.

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Introduction

Perovskite Solar Cells (PSCs) have emerged in recent years as one of the most promising solar cell technologies,^1–5 exceeding 22% power conversion efficiency (PCE). The increasing interest in this technology relates to some of the outstanding optical and electrical properties of hybrid organic–inorganic perovskite materials, e.g. direct band gap absorption,^6–8 a tunable band gap,⁹ ambipolar conductivity,³ high radiative recombination,¹⁰ and long charge carrier lifetimes,¹¹ a great variety of deposition methods,^2,12–15 cell architectures and transport layer and electrode materials that can be employed for PSC fabrication.^16–18 In spite of the notable progress of PSCs, their limited stability remained a principal bottleneck for the future development of this technology, requiring focused efforts from the research community.^19–21

Generally speaking, instabilities are commonly differentiated into two types: intrinsic instability, i.e. caused by the structure of the perovskite material, and extrinsic instability, that originates from undesirable environmental impacts. Various sources for intrinsic instability can be cited, such as trap states,²² ion migration,²³ defect migration,²⁴ thermal instability^25,26 or light instability.^27,28 Many of them are convoluted with each other, since trap states can be modified by light exposure²⁸ and ion migration is also strongly dependent on temperature and illumination.^29,30 In contrast, extrinsic instability typically arises from moisture^31–33 and oxygen ingress.³⁴ Many efforts have been made to solve or, at least, to limit instability issues. One of the main approaches is based on the compositional tuning of the perovskite formula (ABX₃, with A being a monovalent cation, B a divalent cation and X a monovalent anion), usually referred to as compositional engineering. In this case, different halides⁵ or A-site cations (either organic³⁵ or inorganic^36,37) were reported to improve the stability. Also, modifications of the perovskite thin-film (such as the formation of two-dimensional (2D) Ruddlesden–Popper perovskites³⁸ and inclusion of protecting agents in the perovskite³⁹) and modifications of the PSC device layout (such as the use of nanostructured materials^40,41 and interface engineering^21,42,43) can improve the stability. Finally, the exploration of new electron and hole selective materials has also attracted much attention.^44–46

With respect to the charge transport layers, 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (Spiro-OMeTAD) has been employed as a hole selective material from the early beginning of this technology.^2,3 However, it has notable shortcomings such as temperature instability,⁴⁷ migration of metal atoms from the cathode to the perovskite⁴⁸ or the hygroscopy of the dopant salts.³² For the active layer, methylammonium lead iodide (MAPbI₃) serves as the prototypical perovskite absorber material, even though it suffers from crystal transitions at low temperature (∼55 °C),⁴⁹ thermal instability^25,26 and severe moisture-induced degradation.^31,33 Hence, in the present work, we focus on a cell configuration based on MAPbI₃ and Spiro-OMeTAD, since they have both been extensively tested, remain highly relevant, and yet exhibit critical instability issues. Thus, our encapsulation process has the potential to be applied to any intrinsically more stable and less-challenging configurations, such as alternative perovskite film compositions and hole-selective layers.

Various approaches of encapsulation have been proposed to shield PSCs from external stressors, from basic early approaches using Surlyn®,⁵⁰ to more complex ones with flexible barrier encapsulant thermoplastic polymeric films with integrated adhesives.^51,52 Improved mechanical stability after thermal cycling of PSCs can be achieved by encapsulation with ethylene vinyl acetate (EVA) compared to the Surlyn® encapsulation,⁵³ while methods using glass–glass encapsulation showed limited success.⁵⁴ The utilization of UV-curable epoxy resin as an encapsulant has recently reached acceptable long-term stability,⁵⁵ passing standard protocols (ISOS-L2 and ISOS-O1).⁵⁶ However, UV-based methods are less favorable, especially if TiO₂ is employed, due to accelerated degradation²⁷ and alternative epoxies curable under visible light, polymeric coatings and hydrophobic encapsulants have been proposed.^57–59

The application of permeation barrier layers deposited by atomic layer deposition (ALD) against water and moisture helps in improving PSC stability. Indeed, ALD is a thin-film deposition technique based on self-limiting surface reactions that enables the fabrication of continuous, dense and pinhole-free inorganic thin films in a large range of deposition temperatures. Its self-limiting nature results in excellent surface coverage and conformality, and allows a subnanometer control of the film thickness and composition.^60,61 ALD of materials like Al₂O₃, TiO₂ or SiN_x thin layers or nanolaminates (inorganic or hybrid)⁶² provides efficient moisture barriers for devices such as organic light emitting diodes (OLEDs).^63–65

Detailed information about ALD processes applied to PSCs can be found in the ESI (Table S3†), where a summary of the PSC architecture, ALD process, encapsulation details and efficiency variation after the ALD process is given. Amorphous TiO₂ or SnO_x layers from ALD have already been used in PSC devices as barriers against moisture infiltration^66,67 or in organic-PVs.⁶⁸ ALD-Al₂O₃ coatings have demonstrated great versatility, being grown over both hydrophilic and hydrophobic substrates,⁶⁹ and tuned to present either hydrophilic or hydrophobic surfaces.⁷⁰ ALD-Al₂O₃ films provide good optical transmittivity which is a requirement for many PV device configurations and typical WVTR (Water Vapor Transmission Rate) values of 1 × 10⁻⁶ g per m² per day are reported. However, those films are typically obtained from trimethylaluminum (TMA) and water as precursors, in the process temperature range from 80 to 300 °C, which could be reduced even down to room temperature.⁷¹ Consequently, some of those protocols need to be modified and adapted to PSC encapsulation. So far, despite their simplicity and applicability to PV technologies, only few reports have demonstrated an improvement of PSC lifetime using ALD-Al₂O₃ coatings. One of the first examples was reported by Kim and Martinson, where different precursors for ALD-Al₂O₃ coatings were tested at deposition temperatures of around 100 °C directly on perovskite thin-films.⁷² In working PSCs, ALD-Al₂O₃ thin layers have been deposited on a perovskite absorber by Dong et al. using O₃ instead of water, which eventually acted as a strong oxidizer and degraded the perovskite.⁷³ Cells with improved stability over 24 hours were obtained when ALD-Al₂O₃ layers were deposited on top of Spiro-OMeTAD, but while 1–3 ALD cycles had no evident effect on the cell performance, a higher number of cycles induced parasitic resistances that led to a decrease in photocurrent. Later, Koushik et al. used an ultrathin (0.8 nm) Al₂O₃ layer deposited from TMA/H₂O at 100 °C directly on a perovskite absorber to make a hydrophobic tunneling insulating layer and enhanced the air stability of a PSC with planar architecture.⁷⁴ X-ray photoemission spectroscopy (XPS) studies have shown that the ALD precursors passivate the perovskite surface.⁷⁵ In each of the examples cited above, a sub-nm thickness of the ALD layer is required to prevent parasitic resistances, but such values are not sufficient to create a continuous film and hence a suitable encapsulation layer. Indeed, although critical thicknesses to obtain a continuous, defect-less film acting as an effective barrier against moisture and oxidation are very dependent on the underlying material, most studies agree that a > 10 nm films are needed.^76,77 One example of an efficient “thick” encapsulation layer deposited by ALD for PSCs is a 50 nm ALD-Al₂O₃ coated PET substrate applied on the top of a Ag-NW/ZnO/perovskite/PEDOT:PSS/ITO device architecture that showed improved stability over 40 days.⁷⁸ Another recent example of encapsulation including ALD-Al₂O₃ processes has been provided by Lee et al.,⁷⁹ where a system composed of 4 dyads alternating with ∼250 nm of polymeric pV3D3 deposited by initiated chemical vapor deposition (iCVD) with ∼20 nm ALD-Al₂O₃ was employed. In such organic/inorganic encapsulation architectures, the polymer layer separates the defects arising from the inorganic film deposited by physical vapor deposition (PVD), and the encapsulation coating hinders the diffusion of water and oxygen and reaches very low WVTR values, similar to the standard requirements for OLEDs.^80,81 A single thin ALD-Al₂O₃ encapsulation layer for OLEDs with remarkably low WVTR has been reported in the literature,⁸² and should ensure long-term stability for the encapsulation of PV devices. In the case of PSCs, the application of an ALD-Al₂O₃ coating at 90 °C led to catastrophic failure for Spiro-OMeTAD-containing devices and a 40% drop in PCE for PTAA-based ones. A reduction of the processing temperature to 60 °C helped to suppress this reduction of performance.⁷⁹

In our study, we report an efficient ALD-Al₂O₃ based encapsulation procedure for perovskite solar cells as a damage-less alternative for encapsulation and improving the long-term device stability without any significant decrease in the initial photovoltaic performance. The ALD-Al₂O₃ coatings were deposited on top of the PSCs after cathode evaporation, ensuring a full encapsulation of the device. The thin, pinhole-free, conformal barrier (approximately 16 nm) was obtained using industry-relevant precursors (TMA/H₂O) and applying a low reaction temperature of 60 °C. We further produced thin layers of the ALD-Al₂O₃ film on top of half-cell layer stacks to probe the impact of the ALD process on the material composition and electronic properties by XPS. The XPS measurements also probe the immediate interfaces between the forming Al₂O₃ film and the active layer components of the PSC for chemical reactions that could potentially occur through pin-holes in the subsequent layers. We report our approach to work on the particularly heat and moisture sensitive MAPbI₃/Spiro-OMeTAD based cell architecture and hence consider that this low-temperature ALD-Al₂O₃ encapsulation could be extrapolated to other PSC architectures in order to enhance the extrinsic instability of PSCs.

Experimental

Device fabrication

Fluorine-doped tin oxide (FTO) glass (Solems) was etched with Zn powder and HCl (4 M). The substrates were sonicated for one hour in RBS® detergent solution (2 vol%), rinsed with deionized water and ethanol, and then ultrasonicated in absolute ethanol, dried and heated at 500 °C for cleaning. A TiO₂ hole blocking layer was prepared by spray pyrolysis deposition (SPD) at 450 °C employing a precursor solution made from 0.6 mL of titanium diisopropoxide bis(acetyl acetonate) (75% in 2-propanol, Sigma Aldrich) and 0.4 mL of acetyl acetone (Sigma Aldrich), employing 9 mL of absolute ethanol as a solvent and O₂ as a carrier gas. The mesoporous TiO₂ layer was prepared by spin-coating a solution of TiO₂ paste (30 NR-D from Dyesol) in absolute ethanol (1 : 7 by weight) at 4000 rpm for 30 s. Subsequently, the films were sintered in a sequential heating process (5 min at 125 °C, 5 min at 325 °C, 5 min at 375 °C, 15 min at 450 °C and 30 min at 500 °C). Then, the samples were transferred into a glovebox to continue the device preparation. MAPbI₃ solution was prepared by mixing an equimolar concentration (1.42 M) of PbI₂ (TCI Chemicals), methyl ammonium iodide (MAI, Dyesol) and DMSO (anhydrous) in DMF. The precursor solution was spin-coated onto the mesoporous TiO₂ substrates and a few seconds later, washed by dripping 0.5 mL of anhydrous diethylether to induce the intermediate adduct formation of the films, as reported previously.⁸³ Spiro-OMeTAD (2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene) from Merck was employed as a hole transport material. 110 mg of Spiro-OMeTAD were dissolved in 1 mL of chlorobenzene containing tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)cobalt(III) bis(trifluoromethylsulphonyl)imide (FK209, Dyesol), lithium bis(trifluoromethylsulphonyl)imide (LiTFSI, Sigma Aldrich) and 4-tert-butylpyridine (t-BP, Sigma Aldrich 96% of purity) as additives in relative molar concentrations of 5%, 50% and 330% respectively, with respect to Spiro-OMeTAD. Then, 35 μL of that solution were deposited by spin coating at 3000 rpm for 20 s. Finally, 100 nm of gold was thermally evaporated under vacuum as the top contact of the solar cells.

Encapsulation by atomic layer deposition of Al₂O₃ (ALD-Al₂O₃ encapsulation)

Devices were hermetically transferred to a Savannah 200 ALD system from Ultratech/Cambridge Nanotech, embedded into an MBraun glovebox, to deposit a ca. 16 nm-thick Al₂O₃ encapsulation layer. Trimethylaluminium (TMA, AlMe₃, from Sigma Aldrich) and H₂O were respectively used as Al and O precursors and kept at room temperature. Nitrogen (N₂, 99.9999%, Air Liquide) was used as both carrier and purging gas. For encapsulation purposes, ALD cycles (H₂O pulse/N₂ purge/TMA pulse/N₂ purge) with a typical time of 30 s were repeated 250 times (number of cycles) for a total run duration of approximately 2 hours with an average Growth Per Cycle (GPC) of ∼0.64 Å per cycle for a final thickness of ∼16 nm. The deposition temperature was set at either 60 °C or 90 °C. Samples were taken out of the reactor right after the end of the run. For the devices containing the ALD-Al₂O₃ layer used for X-ray photoelectron spectroscopy (XPS) characterization, the number of cycles of that process was carefully reduced to prepare films only ∼2 nm thick. The glovebox with the Savannah system is located inside a cleanroom environment, so as to ensure a low level of particle contamination. Besides, the reactor contamination is particle-controlled so as not to add any particles to the thin film encapsulation process.

Stability test

The reliability of the ALD-Al₂O₃ encapsulation process was evaluated by comparing the photovoltaic performance of PSCs before and after the ALD-Al₂O₃ step. PSC fabrication and characterization were done at IPVF whereas ALD-Al₂O₃ encapsulation was accomplished at CEA-LETI. Samples were shipped under an inert atmosphere from IPVF to CEA-LETI after an initial J–V characterization (before ALD-Al₂O₃ encapsulation) and shipped back to IPVF for further characterization (after ALD-Al₂O₃ encapsulation). To elucidate the origin of efficiency losses during the encapsulation process, reference PSC samples were fabricated and shipped together with the samples destined to be encapsulated. Accordingly, different kinds of reference samples were distinguished: reference samples with original efficiency or simply reference samples (coded as “Ref”), samples shipped to CEA-LETI and stored inside a glovebox under a protecting N₂ atmosphere (coded as “Shipment”) and samples shipped to CEA-LETI and introduced inside the ALD reactor and heated under the same heating protocols used for the encapsulation process but without being exposed to ALD precursors/chemicals (coded as “H@T”). This way, we were able to discriminate the intrinsic degradation of PSCs (in Shipment) from the degradation related to the heating protocols of the ALD process (H@T) and the one associated with the combined effect of heating and ALD precursors (coded as “ALD-Al₂O₃@T”). Long-term stability tests were performed by comparing the photovoltaic performances of the encapsulated devices and reference ones after long term storage (2256 hours, ∼94 days). For this, both ALD-Al₂O₃ encapsulated and reference samples were stored in a normal air atmosphere (no moisture or oxygen control, relative humidity around 80%) in a laboratory room to check environmental stability of the encapsulated devices vs. reference ones. The stability results are considered as the mean of four encapsulated solar cells.

Photovoltaic characterization

For J–V characterization, an AAA sun simulator (Oriel Sol3A) with an AM1.5G spectrum (1 sun) was used as a light source. A digital source meter (Keithley Model 2400) was employed to realize a voltage sweep rate of ∼85 mV s⁻¹ (unless different conditions are explicitly specified) while the generated photocurrent of the solar cell was recorded. External quantum efficiency (EQE) measurements were acquired on an Oriel IQE200 system equipped with a source meter (Keithley 2400) using a digital lock-in amplifier (Oriel Merlin) without any additional light bias. For chronopotentiometry measurements, consecutive cycles of 30 minutes were carried out during 33.5 h. In this case, PSCs were kept at open circuit conditions with the help of a potentiostat (Autolab) under continuous illumination (0.65 Suns) also with an AM1.5G spectrum. Between cycles, the illumination was always turned on, but the voltage was set to 0 V before starting the next chronopotentiometry step to monitor the transient behavior before reaching V_OC. The active area of the cells was 0.16 cm² (4 × 4 mm) while for J–V, EQE and chronopotentiometry measurements the illuminated area was fixed at 0.09 cm² (3 × 3 mm) with a metal black-painted mask.

Time resolved photoluminescence (TRPL) measurements

For the time-resolved photoluminescence (TRPL) experiments, a monochromatic (λ = 532 nm) pulsed laser beam (TALISKER, Coherent) was used as a light source. The laser was coupled into a single-mode optical fibre connected to a homemade microscope to focus the light on the sample. The photoluminescence decay dynamics were recorded by time-correlated single-photon counting (TCSPC). An Aurea infrared single photon detector was coupled to a PicoHarp 300 for opto-electrical conversion of the decay signal to achieve a time resolution of 130 ps. The output signal was normalized and fitted between the peak of the pulse and the end of the signal using a biexponential model (see the ESI†).

X-ray photoelectron spectroscopy (XPS) analysis

XPS measurements were performed on a K-Alpha⁺ (Thermo Fisher Scientific) spectrometer using a monochromatic Al-Kα X-ray source (1486.6 eV). The spectrometer was calibrated using the Thermo Fisher Scientific K-Alpha⁺ procedure and verified on sputter-clean Cu and Au samples following the ASTM-E-902-94 standard procedure.⁸⁴ High energy resolution peaks were acquired with a 400 μm spot size with a Constant Analyser Energy mode of 20 eV and a 0.1 eV energy step size. No low-energy electron flood gun was necessary for the analysis. Quantification was performed based on the peak areas after a Shirley type background subtraction using a Thermo Fisher Scientific Avantage© data system.

Scanning Electron Microscopy (SEM)

The completed devices were observed by SEM with a Zeiss Merlin VP compact microscope.

X-Ray Diffraction (XRD) analysis

The crystalline properties of the absorber were determined by XRD studies with a PanAnalytical Empyrean diffractometer using Cu-Kα radiation.

Results and discussion

ALD deposition

The PSC device stack used in this study is comprised of a glass/FTO substrate (400–450 nm of FTO) with a few nanometers thick blocking layer made of TiO₂, a mesoporous TiO₂ structure (130–150 nm) as an electron selective contact, MAPbI₃ (ca. 400 nm) as a perovskite absorber over the mesoporous TiO₂, Spiro-OMeTAD as a hole selective layer (200–250 nm) and Au (100 nm) as the cathode of the solar cell. The thin ALD-Al₂O₃ encapsulation layer was deposited onto the completed solar cells as depicted in the scheme of the perovskite solar cell architecture (Fig. 1a) together with its respective cross-sectional SEM image (Fig. 1b and S1†). A continuous, pinhole-free, covering Al₂O₃ coating was achieved over the whole surface. The film thickness was uniform and in the range of ∼16 nm by ALD for the deposition temperature set to 60 °C. This is surprisingly low considering the number of cycles applied (250) and corresponds to an average GPC of ∼0.64 Å per cycle, while typical GPC values are in the range of ∼0.85 Å per cycle for this process when silicon wafer or SiO₂ is used as a substrate. This can be attributed to a very slow nucleation process. Indeed, while the ALD-Al₂O₃ growth mechanism is well understood on oxide surfaces and is based on acid–base reactions,^71,85,86 it is quite different on metal surfaces. For instance, when grown on Pd surfaces, H₂ release during the H₂O pulse of the first cycle⁸⁷ and Pd–Al alloy formation by dissociative adsorption on TMA have been observed.^87,88 The adsorbed CH₃ species can inhibit the ALD-Al₂O₃ growth by blocking the potential TMA adsorption sites within the first cycles, which can impact the nature of the film as confirmed by STM measurements.⁸⁷ Similar observations have been reported on Ir and Pt surfaces.⁸⁷ In the case of the Au surface, a detailed growth mechanism has not been reported yet, but though the layer thickness seems to increase linearly with ALD cycles, similar mechanisms can lead to nucleation delays and account for the relatively low GPC.^89,90

Fig. 1 (a) Schematic of the FTO/bl-TiO₂/mp-TiO₂/MAPbI₃/Spiro-OMeTAD/Au architecture with the ALD-Al₂O₃ encapsulation layer proposed in this work (not to scale). (b) Cross-sectional Scanning Electron Microscopy (SEM) image of a perovskite solar cell containing an ALD-Al₂O₃ encapsulation layer deposited at 60 °C.

Indeed, after a few minutes of exposure to air, gold surfaces become hydrophobic due to surface adsorption of species in the environment, and surface cleaning pre-treatments such as UV and/or O₂ plasma can be applied to improve the quality of the film.⁹¹

PSC performance before/after encapsulation and optimization of the deposition temperature

In this work, PSCs were fabricated with minor modifications to the procedure reported by Ahn et al.,⁸³ achieving very similar performances in comparison with the state-of-the-art devices, i.e. efficiencies in the range of 16–18%, with open-circuit voltages (V_OC) exceeding 1 V, short-circuit current densities (J_SC) over 21 mA cm⁻², and fill factors (FF) exceeding 70% with very good reproducibility (see the Ref devices in Fig. S3† and 2c).

Fig. 2 Study of the ALD-Al₂O₃ encapsulation process at different temperatures. (a) Current density–Voltage (J–V) characteristics for the champion PSCs ALD-Al₂O₃@90 °C (red) and ALD-Al₂O₃@60 °C (blue); PV parameters are specified in the legend. (b) External quantum efficiency for the champion PSCs ALD-Al₂O₃@90 °C (red) and ALD-Al₂O₃@60 °C (blue). (c) J–V characterization of the ALD-Al₂O₃@60 °C device with average characteristics before (dashed blue line) and after the encapsulation (solid blue line) process compared with the Ref PSC (black). (d) Time-Resolved Photoluminescence (TRPL) measurements on ALD-Al₂O₃@60 °C and Ref.

Firstly, to proceed with the encapsulation, an ALD-Al₂O₃ layer was grown on the top of these PSCs using a deposition temperature of 90 °C inside the ALD reactor. This temperature was prudently chosen by reducing the standard temperature reported for ALD-Al₂O₃ routines using TMA + H₂O (normally set at 130–150 °C ⁷¹), and slightly decreased for the ALD procedures applied to PSCs (normally fixed at 100 °C), where significant degradation was still observed.^72,74,78 In Fig. S2,† the normalized photovoltaic properties (J_SC, V_OC, FF and PCE) for the encapsulation process at 90 °C and the corresponding reference samples are shown. Reference PSCs before any encapsulation (named as Ref) were taken as a base for the normalized graphs and data.

The Shipment sample, stored in a protective atmosphere and measured again after its return, shows negligible losses (Relative change in PCE amounts to only ∼2.5%), demonstrating sufficient stability of all PV parameters under the shipping conditions used here. For H@90 °C samples, significant performance losses were detected, mainly in photocurrent density (ΔJ_SC = −36.2%) but also in voltage and fill factor (ΔV_OC = −6.5%, ΔFF = −5.1%), resulting in an overall relative decrease in efficiency of ΔPCE = 43.3%. Thus, the heating process at 90 °C inside the ALD reactor during the typical time needed for obtaining an ALD-Al₂O₃ encapsulation layer is not suitable for a successful and harmless PSC ALD encapsulation. Moreover, for the samples encapsulated at 90 °C (ALD-Al₂O₃@90 °C), two different trends can be clearly distinguished. One batch of the ALD-Al₂O₃@90 °C encapsulated devices, around half the samples, exhibited losses very similar to those of H@90 °C, with a total ΔPCE of −53.9%; i.e. primarily a loss in photocurrent (ΔJ_SC = −37.3%), while the decrease in fill factor detected is slightly higher (ΔFF = −19.6%) than for the H@90 °C samples, without a significant decay in voltage (Table 1 and Fig. S2†). The other batch of ALD-Al₂O₃@90 °C devices showed critical device failure linked to a pronounced loss in J_SC (non-working devices). A priori, we consider this general device failure in non-working devices to be related to a detrimental effect of the precursors (see Fig. S2†), previously observed in the literature^72,79 and linked to degradation of the material, since Dong et al. found that when the number of ALD cycles increases, the PbI₂ content increases as well.⁷³ Here, we further elaborate on this hypothesis in our XPS section.

Table 1 Normalized average variation of photovoltaic properties (ΔJ_SC, ΔV_OC, ΔFF and ΔPCE) before and after ALD-Al₂O₃ encapsulation at different temperatures

Temperature of ALD-Al2O3 encapsulation	Normalized average variation of PV properties before/after encapsulation
	ΔJSC (%)	ΔVOC (%)	ΔFF (%)	ΔPCE (%)
ALD-Al2O3@90 °C (excluding non-working cells)	−37.3	−8.1	−19.6	−53.9
ALD-Al2O3@60 °C	−3.2	0.3	−3.8	−6.4

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To avoid these detrimental effects, we lowered the deposition temperature of the ALD-Al₂O₃ encapsulation process to 60 °C (ALD-Al₂O₃@60 °C). The current density–voltage characteristics of the best-performing PSCs after encapsulation in both ALD-Al₂O₃@90 °C and ALD-Al₂O₃@60 °C are shown in Fig. 2a. We find an overall performance of 17.40% from the J–V curve for the champion ALD-Al₂O₃@60 °C sample with J_SC = 21.56 mA cm⁻², V_OC = 1.120 V and FF = 72.1%, practically identical to the performance observed in non-encapsulated samples (Ref). In Fig. 2c, the evolution before/after encapsulation in ALD-Al₂O₃@60 °C is compared with a Ref cell. The ALD-Al₂O₃@60 °C process leads to an only very minor reduction in photocurrent in contrast to the encapsulation performed at 90 °C (Fig. 2a and b). Further statistical information is given by the box and whisker diagrams of the PSC characteristics before and after ALD-Al₂O₃ encapsulation at 60 °C (Fig. S3†), confirming the low dispersion and high reproducibility of the process. In contrast, for the champion ALD-Al₂O₃@90 °C device, a PCE of only 6.94% was recorded after encapsulation, due to a significant reduction in current as laid out earlier (only J_SC = 11.12 mA cm⁻²) with a reduction in fill factor of up to 62% mainly due to increased series resistance. In summary, we found that the normalized average change of PCE before and after ALD-Al₂O₃ encapsulation (ΔPCE) was critical for the ALD-Al₂O₃@90 °C samples resulting in ΔPCE = −53.9%, even after excluding the non-working cells, while the change amounted only to ΔPCE = -6.48% in the case of ALD-Al₂O₃@60 °C samples (complete information in Table 1). This remarkable performance-maintenance achieved by applying a single ALD coating used as an encapsulation technique for PSCs indicates the key role of temperature control in these procedures.

To further investigate the impact of the ALD-Al₂O₃ process on the optoelectronic properties of the devices and especially how it affects the charge carrier dynamics, TRPL measurements were carried out. In Fig. 2d, TRPL decays are presented for both Ref and ALD-Al₂O₃@60 °C samples. The curves were fitted using a biexponential model obtaining very good correlation for both devices at any time interval. Moreover, comparing both models, i.e. for Ref and ALD-Al₂O₃@60 °C cells, practically identical correlation parameters were obtained. As a result, time constants for rapid (τ₁) and slow (τ₂) decay processes were similar to reported values¹¹ and almost equivalent between the Ref sample, with τ₁ = 5.16 ns and τ₂ = 32.89 ns, and the ALD-Al₂O₃@60 °C sample, with τ₁ = 5.68 ns and τ₂ = 33.45 ns. This shows that internal mechanisms and carrier dynamics in the PSC are not significantly modified by the 60 °C ALD-Al₂O₃ encapsulation (more details in the ESI†). Consequently, the ALD-Al₂O₃@60 °C encapsulation process altered neither the optical nor the transport processes inside the PSC.

Ageing process and stability data

As the innocuousness of the encapsulation procedure (ALD-Al₂O₃@60 °C) to the PSC performance was demonstrated, we then evaluated the improvement of the long-term stability. Both reference (Ref) and ALD-Al₂O₃@60 °C devices were kept in an ambient atmosphere without further precautions and repeatedly characterized by J–V measurements. The evolution of the PV properties both for the Ref. and ALD-Al₂O₃@60 °C devices is compared in Fig. 3a–d. The global evolution of PCE is mostly determined by the FF rather than V_OC and J_SC. The measured V_OC remained high for both types of samples and stayed remarkably constant for the encapsulated devices with an initial V_OC of 1.039 V and V_OC = 1.038 V after 2256 h of storage. In contrast, for the reference device V_OC dropped from 1.090 V to <1 V. In the case of J_SC, we observed a linear decrease for the first 1000 hours and subsequent relative stabilization independent of any encapsulation. Curiously, while lowering the ALD deposition temperature was critical to limit J_SC losses during the ALD-Al₂O₃ process, once encapsulated, J_SC was a little bit less stable for the ALD-Al₂O₃ encapsulated samples than for the reference ones.

Fig. 3 Stability test; evolution of J_SC (a), V_OC (b), FF (c) and PCE (d) with storage time for Ref (black) and ALD-Al₂O₃@60 °C (blue). Evolution of hysteresis in Ref (e) and ALD-Al₂O₃@60 °C (f) devices, before (dotted line) and after 14 days (∼335 h) of ageing under ambient conditions (solid line); for hysteresis measurements, the scan rate was fixed at 20 mV s⁻¹. Hysteresis Index, HI = (PCE_reverse − PCE_forward)/PCE_reverse. (g) Chronopotentiometry of ALD-Al₂O₃@60 °C under continuous illumination.

However, in both cases the losses were on the same order of magnitude with J_SC dropping from 21.57 to 18.19 mA cm⁻² for the ALD-Al₂O₃@60 °C device and from 21.83 to 19.28 mA cm⁻² for the reference. Thus, the extension of lifetime in the encapsulated devices is primarily attributed to a stabilization of the fill factor, as in the ALD-Al₂O₃ protected devices the FF only decreased from 73.4% to 66.4% after 2256 h, while for the reference, it substantially decayed from 75.2% to 34.2%. This FF behavior leads to an overall PCE loss in the case of the reference sample from almost 18% to 6.5% (i.e. 64% of the initial PCE on a normalized scale), whereas ALD-Al₂O₃@60 °C exhibited a final PCE of 12.4% after more than 2250 hours of storage under ambient conditions (from original PCE = 16.4%), which corresponds to a relative loss of <25% of the initial PCE.

To further elucidate the origin of the differences in the FF decay between the Ref and ALD-Al₂O₃@60 °C devices, reverse and forward J–V scans were performed for both types of samples to investigate the hysteresis phenomenon immediately after cell fabrication and after ageing (plotted in Fig. 3e–f). In non-encapsulated PSC systems, hysteresis is well known to increase with time,⁹² which holds true for our reference devices, since the hysteresis index (HI = (PCE_reverse − PCE_forward)/PCE_reverse) was increased from 0.122 to 0.381 after 14 days under ambient conditions. However, in the case of the ALD-Al₂O₃@60 °C sample, the situation was improved as the typical increase in hysteresis was limited from an initial HI = 0.116 to only 0.192 after 14 days of storage under ambient conditions. An additional chronopotentiometry experiment shown in Fig. 3g reveals that 96.5% of the initial V_OC is retained after 33.5 h (in cycles of 0.5 h) under continuous illumination for the ALD-Al₂O₃@60 °C sample.

Finally, we investigated using XRD whether structural changes occurred in the MAPbI₃ films after ∼2250 hours of storage in an ambient atmosphere. Representative XRD patterns of the Ref and ALD-Al₂O₃@60 °C devices are presented in Fig. 4a. In both cases, diffraction peaks of a tetragonal perovskite phase can be observed at 2θ = 14.1° and 28.4°. However, only in the case of the non-encapsulated device is a peak corresponding to the formation of a PbI₂ phase observed at 2θ = 12.6°, which is in agreement with the occurrence of yellow areas visually detected in the aged reference sample, and which has not been witnessed for the samples encapsulated at 60 °C (Fig. 4b). Additional ageing tests at elevated temperatures should be conducted in order to more accurately reflect the operating conditions of the solar cell.

Fig. 4 (a) XRD patterns of the Ref (black) and ALD-Al₂O₃@60 °C (blue) devices stored for ∼2256 h under ambient conditions. Diffraction peaks of FTO and TiO₂ are indicated by + and # symbols, respectively. (b) Photograph of the Ref devices on top and ALD-Al₂O₃@60 °C in the bottom part stored for ∼2256 h (94 days) under ambient conditions.

XPS analysis

To evaluate the formation and impact of the initial Al₂O₃ ALD layer deposition on the critical material components (MAPbI₃ and Spiro-OMeTAD), XPS measurements were performed on glass/FTO/bl-TiO₂/mp-TiO₂/MAPbI₃ and glass/FTO/bl-TiO₂/mp-TiO₂/MAPbI₃/Spiro-OMeTAD (Fig. S5†) samples coated with ALD Al₂O₃ films of 2 nm nominal thickness, deposited both at 60 °C and 90 °C. This very low thickness allows us to capture the XPS signal of the underlying MAPbI₃ and Spiro-OMeTAD layers. We also note that the potential of forming these various interfaces can lead to critical changes in the device functionality.⁹³ First, the presence of the AlO_x layer was confirmed by the Al 2p and O 1s contributions in the spectra (74.7 eV and 532.2 eV respectively, Fig. S6†). The AlO_x films deposited at 90 °C exhibit a lower concentration of oxygen and carbon, indicative of a higher conversion factor of the ALD precursor. We also show the XPS high-resolution spectra of the Pb 4f, N 1s and I 3d core level regions for the MAPbI₃ samples with and without coating by a 2 nm thick Al₂O₃ layer (Fig. 5). All peaks present one major chemical contribution in good agreement with the literature and notably we do not find any formation of metallic lead evidenced by the absence of any Pb⁰ contribution in the Pb 4f core level spectra.⁹⁴

Fig. 5 XPS high resolution spectra of the Pb 4f (a), N 1s (b) and I 3d (c) regions for FTO/bl-TiO₂/mp-TiO₂/MAPbI₃ stacks not encapsulated (black), encapsulated at 60 °C with a 2 nm Al₂O₃ layer (blue) and encapsulated at 90 °C with a 2 nm Al₂O₃ layer (red).

Furthermore, we do not observe any significant core level shifts in the spectra, which suggests that the respective chemical coordination for all components in the MAPbI₃ film is conserved. However, this statement cannot be applied to the carbon component because the C 1s region is convoluted with contributions from the TMA precursor and adventitious carbon (see Fig. S6†). Notably, we do not find any shift of the Fermi level (ΔE_F < 0.1 eV) in the MAPbI₃ layer, which is in contrast to MAPbI₃ interfacing more reactive oxides such as MoO₃.⁹⁵ However, some chemical changes of the MAPbI₃ films can be inferred from the elemental ratios (Table 2). Both devices without encapsulation and after a 60 °C encapsulation present similar Pb/I ratios. Indeed, for those two sets of devices, the Pb/I ratio obtained is equal to the theoretical ratio of a MaPbI₃ device (0.31 ± 0.03 and 0.34 ± 0.03 respectively). However, an increase of the Pb/I ratio is noted when the deposition is performed at 90 °C. Indeed, the value reaches 0.42 ± 0.03, which suggests that we form a substoichiometric PbI₂-rich phase at the interface with the adjacent layer for the high temperature process. Regarding the N/Pb ratio, we observe a decrease for both encapsulation temperatures, leading to values in better agreement with the nominal stoichiometry of the layer. Indeed, the N/Pb ratio for the non-encapsulated device reaches 1.30 ± 0.03 while the nominal stoichiometry is expected to give a ratio of 1.00. The excess in the nitrogen content probably originates from excess MAI in the fabrication process. After contact with the ALD precursors some of the excess MA might be removed, as for the encapsulated devices, the N/Pb ratio is lower (1.19 ± 0.03 and 1.16 ± 0.03 for the encapsulation at 60 °C and 90 °C, respectively). The decomposition of methylammonium has previously been described in the literature.⁹⁶ Here, in summary the loss in nitrogen for the ALD processing at 60 °C points to a loss of surface excess MA, potentially linked to the interface formation between the AlO_x film and MAPbI₃, while the concomitant loss of I and N in the case of the 90 °C ALD deposition describes a thorough degradation process.

Table 2 XPS atomic ratio evolution for FTO/bl-TiO₂/mp-TiO₂/MAPbI₃ stacks not encapsulated, encapsulated at 60 °C and encapsulated at 90 °C with a 2 nm Al₂O₃ layer

Compounds	Before encapsulation	ALD-Al2O3@60 °C (2 nm)	ALD-Al2O3@90 °C (2 nm)
Pb/I	0.31 ± 0.03	0.34 ± 0.03	0.42 ± 0.03
N/Pb	1.30 ± 0.03	1.19 ± 0.03	1.16 ± 0.03

---

Addressing the differences in the degradation of the Spiro-OMeTAD layer underneath the ALD Al₂O₃ film is inherently more challenging as carbon and oxygen contributions of the Spiro-OMeTAD molecule to the XPS spectra are convoluted by the contribution of the ALD precursors and adventitious species. Nonetheless, we do see some minor qualitative changes in the carbon signal that are absent in the scans of the AlO_x/MAPbI₃ interfaces: upon deposition of the AlO_x film on top of the Spiro-OMeTAD layer we find a new contribution of C–O bonds at 289 eV binding energy (ESI, Fig. S5†). This C–O component is more pronounced at higher processing temperatures (90 °C) than at lower processing temperatures, indicating a stronger reaction/degradation of the Spiro-OMeTAD surface. Moreover, we find that the N 1s signal of the nitrogen containing moieties in the Spiro-OMeTAD is attenuated to the same degree and remained unchanged for both AlO_x overlayers (Fig. S5†). Overall, we conclude from the XPS chemical analysis (full elemental composition in the ESI, Tables S1 and S2†) that the encapsulation performed at 60 °C is more compatible for the full device encapsulation as the chemical integrity of the MAPbI₃ layer seems to be better retained.

Conclusions

In summary, we report an efficient method for encapsulation of perovskite solar cells by atomic layer deposition of Al₂O₃ thin-films. By the careful control of deposition temperatures of the ALD-Al₂O₃ process, a well-covering continuous encapsulation coating of approximately 16 nm thickness was achieved. In particular, by reducing the ALD-Al₂O₃ deposition temperature from 90 °C, a typical temperature for this kind of processes, to only 60 °C, we retained over 93% of the initial device efficiency. We attribute this effect to the fact that in the low-temperature process the measured chemical composition and optoelectronic properties relevant for charge collection at the interfaces remain mostly unperturbed, whereas decomposition of the MAPbI₃ film into PbI₂ sets in for the high temperature ALD process. This demonstrates that our process is compatible with inherently heat sensitive materials used for PSC fabrication, i.e. MAPbI₃ and Spiro-OMeTAD. Thus, we project that the results obtained here for our ALD-Al₂O₃ encapsulation are universally applicable to other and more robust PSC configurations. Finally, we demonstrate that the method is effective in extending the long-term stability since losses in efficiency for the devices encapsulated at 60 °C were limited to <25% of the initial PCE vs. 64% in the case of non-encapsulated devices after >2250 hours of storage under ambient conditions. The reduction in PCE losses is correlated with a better preservation of the fill factor and is accompanied by a mitigation of hysteresis. We ultimately link this improvement to the suppression of PbI₂ formation in the encapsulated devices. Hence, our method provides a new encapsulation route which not only extends the lifetime of PSCs for lab applications but also paves the way for future commercialization efforts.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was partly carried out in the framework of a project of IPVF, which has been supported by the French Government in the frame of the program of investment for the future (Programme d’Investissement d’Avenir ANR-IEED-002-01). P.S. thanks the French Agence Nationale de la Recherche for funding under the contract number ANR-17-MPGA-0012.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8se00282g

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