Design of High-Efficiency and Environmentally Stable Mixed

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Design of high-efficient and environmentally stable mixed-dimensional perovskite solar cells based on cesium-formamidinium lead halide component Guozhen Liu, Haiying Zheng, Xiaoxiao Xu, Liang-Zheng Zhu, Xianxi Zhang, and Xu Pan Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02970 • Publication Date (Web): 29 Oct 2018 Downloaded from http://pubs.acs.org on October 29, 2018

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Chemistry of Materials

Design of high-efficient and environmentally stable mixed-dimensional perovskite solar cells based on cesium-formamidinium lead halide component Guozhen Liu,ab Haiying Zheng,ab Xiaoxiao Xu,ab Liang-Zheng Zhu,ab Xianxi Zhangc and Xu Pan*a aKey

Laboratory of Photovoltaic and Energy Conservation Materials, Institute of Applied Technology,

Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, China. bUniversity

of Science and Technology of China, Hefei 230026, China.

cShandong

Provincial Key Laboratory/Collaborative Innovation Center of Chemical Energy Storage

&Novel Cell Technology, Liaocheng University, Liaocheng 252000, China.

Abstract Long-term instability of the high-efficiency perovskite solar cells (PSCs) has been the intractable hindrance for their further commercialized applications. Here, we successfully introduced 2-hydroxyethylamine cation (HEA+) into Cs/FA mix-cations three-dimensional (3D) perovskite to form new-type stable mixed-dimensional (MD) perovskite structure based on the general formula of (HEA)2(Cs0.1FA0.9)n-1PbnI3n+1. FAPbBr3 component was also employed to improve the quality of thin films. The designed MD perovskite films exhibit better crystallization, outstanding optical properties and uniform morphology with less grain boundaries. Especially, due to inheriting the advantages of high-performance FA-based 3D perovskite, the PSCs made from MD-Br10 (n=30) perovskite achieve the

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optimal power conversion efficiency (PCE) as high as 19.84% and an average PCE of 19.02% among 30 devices. Surprisingly, it is found that the MD PSCs show superior long-term stability when exposed to heat and moisture, attributing to the high-quality films and intrinsic stability structure. After aging at the condition of 85 °C (about 10% relative humidity in the dark) for 400 hours and 55±5% relative humidity (about 25 °C in the dark) for 2160 hours, the unencapsulated devices can maintain 82% and 87% of the initial PCE, respectively. This work provides a feasible design direction of new-type MD perovskites to obtain efficient and stable PSCs for next-generation photovoltaic devices.

Introduction Organic-inorganic halide perovskites, a kind of spotlighted materials in the field of photovoltaics, have attracted remarkable attention in all over the academic community in the past several years.1-8 Due to the unique optoelectronic properties such as outstanding carrier diffusion length, low defect density, suitable band gaps and excellent absorption coeffcient,9-14 perovskite solar cells (PSCs) display a praiseworthy certified power conversion efficiency (PCE) reaching up to 23.3%.15 Particularly, FA-based 3D perovskites exhibit brilliant photoelectric property.16-19 Unsatisfactory, the issue that they are prone to degradation associated with heat and humidity has become the stumbling block to the commercial development.20-24 The imminent challenge is to exploit different available strategies to improve both the PCE and stability of PSCs for further actual applications.


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Up to now, Ruddlesden–Popper layered two-dimensional (2D and quasi-2D) and mixed-dimensional (MD) perovskites have been broadly investigated and employed as light absorber materials in photovoltaics owing to the advantage of enhanced moisture stability.8,25-31 In the 2D perovskite general formula of A2A′n−1BnX3n+1, A is always alkylammonium cation, A′ is generally CH3NH3+ (MA) or HC(NH2)2+ (FA), B is divalent metal ion and X is halide anion.29 Due to the stronger interaction between the [PbI6]4octahedra and organic molecules and the hydrophobicity of alkylammonium cation, 2D perovskites present prominent resistance to moisture degradation.32,33 However, the low PCE caused by the mediocre charge transport and poor absorption spectra has hindered the extensive use of 2D perovskites.27 More recently, some studys reported that MD perovskites showed great potential both in the efficiency and humidity stability.34-38 Boix et al.35 demonstrated a strategy to fabricate nanostructured MD perovskites (IC2H4NH3)2(CH3NH3)n−1PbnI3n+1 and used in photovoltaics. The charge transport and extraction could be drastically improved. Snaith et al.37 prepared 2D-3D heterostructured films by introducing 2D perovskite platelets into the highly orientated 3D perovskite grains. The devices achieved a champion efficiency of 20.6% and operational stability under simulated sunlight. In addition, Huang et al.39 reported the construction of 2D/3D stacking structures to enhance the thermal stability in PSCs with the structure of p-i-n. Thermal stability of the n-i-p devices with 3D perovskites also have been widely studied.17,40,41 Nevertheless, the thermal stability is reraly discussed in the passing researchs about MD PSCs with the structure of n-i-p. It remains a great challenge to acquire high PCE and



superior environmental stability for MD PSCs. In this article, we report the [HO(CH2)2NH3]2(Cs0.1FA0.9)n-1Pbn(I0.9Br0.1)3n+1 MD perovskite exhibiting excellent performance and superior long-term stability. Based on the general formula of (HEA)2(Cs0.1FA0.9)n-1PbnI3n+1, HO(CH2)2NH3+ (HEA+) was introducing as suitable ammonium salt cation into 3D perovskites to achieve MD perovskites. The results show that the MD perovskite films with Br component exhibit better crystallization and improved morphology, resulting in the excellent optical properties and reduced defecttrap sites. An outstanding solar cell with PCE as high as 19.84% can be gained by employing the MD-Br10 (n=30) perovskite as the light-absorbing materials under AM 1.5G solar illumination. In addition, the unencapsulated devices present remarkable thermal stability at the aging condition of 85 °C and obviously enhanced tolerance to humidity when exposed to 55±5% RH.

Results and Discussion Herein, we prepared MD perovskite films by incorporating HO(CH2)2NH3I (HEAI) into the 3D perovskite Cs0.1FA0.9PbI3 based on the general formula of (HEA)2(Cs0.1FA0.9)n1PbnI3n+1

(n=10, 20, 30 and 40). Then the Br component was introduced by employing

FAPbBr3 with the content ranging from 0 to 5%, 10%, 15% and 20% (labeled as MD-Br0, MD-Br5, MD-Br10, MD-Br15 and MD-Br20 for easy recognition, respectively). For a legible understanding of the crystalline structure of MD perovskites, Figure 1a shows the schematic illustration of self-assembled MD perovskite structure. 2D perovskite phase


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owns a well interface with 3D phases due to both the 2D and 3D domains exhibit a priority alignment with (010) plane perpendicular to the substrate plane.37 In addition, the XRD image of 2D (HEA)2Pb(I0.9Br0.1)4 (n=1) is shown in Figure S1. The crystal exhibit characteristic diffraction peaks at 8.83°, 17.64°, 26.63° and 35.78°, representing the crystallographic (002), (004), (006) and (008) planes of 2D perovskite, respectively.

Figure 1. (a) Schematic illustration of the self-assembled MD perovskite structure. (b) XRD patterns of 3D-Br10 and MD-Br10 (n=10, 20, 30 and 40) perovskite films. (c) The magnified XRD patterns of the (020) crystallographic plane of MD-Br10 perovskite films. (d) XRD patterns of MD-Brx (x=0, 5, 10, 15 and 20) perovskite films. To determine the crystal structure of the different perovskite films, we performed Xray diffraction (XRD) measurements. To characterize the signatures of MD perovskites



layered structure which presents unambiguous features at low diffraction-angle, the test range was selected from 2θ= 5° to 50°. As shown in Figure 1b, for the 3D-Br10 perovskite film, the peaks located around 14.10°, 28.40° and 31.82° are identified as the crystallographic planes of (110), (220), and (310), respectively, representing the tetragonal perovskite phase.16 When refer to the MD-Br10 (n=10, 20, 30, 40) perovskite, a new diffraction peak at ca. 9.3° appears in all films. It can be assigned to the (020) reflections for the MD perovskite phase.45 Because of the competition of ammonium salt compounds, the HEA+ cation can play a part of flux and then accelerate the crystal growth, indicating a self-assembled MD perovskite structure. Moreover, all the MD perovskite films display the smaller full width at half maximum (FWHM) values than that of 3D film (Figure S5), indicating the better crystallization of MD films with excellent orientated at the (110) direction.2,16 As the n value decreases from 40 to 10, the diffraction peak of (020) plane at ca. 9.3° (Figure 1b) display the slight shift to lower angle, attributting to the expansion of the MD perovskite crystal lattice. The same phenomenon can be got from the diffraction peaks of (111) and (222) planes (Figure S2a, b). In Figure 1d, we exhibited XRD images of perovskite films with different Br concentrations. For MD-Br0 film, two additional diffraction peaks can be observed at around 11.76° and 12.70°, attributting to the δ-phase of FAPbI3 and residual PbI2, respectively. After introducing Br component, all the films display great crystallinity without any additional diffraction peaks. Besides, with the increase of Br fraction (from 0 to 20%), the diffraction peak at ca. 14.10° present little shift to higher angle (Figure S2c),


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in agreement with the smaller size of bromide relative to iodide, which shrinks the crystal lattice. We may draw a conclusion that the introducing of Br component can effectively stabilize the perovskite phase of MD perovskite films.

Figure 2. (a-c) Top surface SEM images of MD-Brx (x=0, 10 and 20) perovskite films. (d-g) Top surface SEM images of MD-Br10 (n=10, 20, 30 and 40) perovskite films. (h-i) Cross-sectional SEM images of 3D-Br10 and MD-Br10 (n=30) perovskite films. The film morphology was performed by using scanning electron microscopy (SEM) measurements. To examine the effect of Br fraction on the morphology of MD perovskite films, we tested the surface morphology of MD-Br0, MD-Br10 and MD-Br20 (n=30) films. As shown in Figure 2a, MD-Br0 display a rough surface with nonuniform grains. After introducing 10% Br (Figure 2b), the film present smooth, compact and uniform surface morphology with less grain boundaries and defect sites, which can result in enhanced



photoelectricity properties.40 However, when the Br fraction increases to 20% (Figure 2c), the film begins to be uneven and rough, which may be caused by the appearing of another phase or material, corresponding to the new diffraction peak around 12.2° the XRD pattern (Figure S4).47 In addition, the AFM images and the particle size statistic distributions of MD-Brx (x=0, 10 and 20) perovskite films are shown in Figure S6 and S7. It can be found that the surface roughness of MD-Brx (x=0, 10 and 20) are 20.3, 15.7 and 19.0 nm, respectively. The MD-Br10 displayed more uniform distribution of particle size, which are consistent with the SEM images. The top surface SEM images of MD-Br10 (n=10, 20, 30 and 40) perovskite films are shown in Figure 2d-e. Compared with 3D perovskite (Figure S3), smooth and compact films without pinhole can be formed after introducing HEA+ cations because of the selfassembly property of MD perovskites. Meanwhile, we can observe that the grain size is gradually becoming larger and more uniform with the gradual increase of n value. The MD-Br10 (n=30) exhibits the best morphology with less grain boundaries and defect-trap sites, resulting in reduced recombination between electron and hole and better photoelectric property. Moreover, the differences of morphology between MD-Br10 (n=30) and MD-Br10 (n=40) are caused by more beneficial effect of HEA+ cation introduction in the MD-Br10 (n=30). The different slabs and texture structure between 3D and MD perovskites can also be surveyed from the cross-sectional SEM images in Figure 2h and i. We further studied the optical properties and band gaps of the MD perovskite films and that with different Br concentrations. The Ultraviolet−visible (UV-vis) absorption


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spectra of the 3D-Br10 and MD-Br10 perovskite films are shown in Figure 3a. Comparing with the 3D-Br10 perovskite film, slight shift of the absorption band edge to shorter wavelengths can be observed in the MD-Br10 films when the n values reduce from 40 to 10. When n values reach 20 or 10, a significant reduction appears in the absorption. All the change of absorption in the wavelength range will be reflected in the short-circuit current density (Jsc). The blue-shift occurring in MD films can be put down to the introduction of larger organic ammonium salt cation into perovskite crystal lattice, which will also make the band gap wider.33,35 As can be seen in Figure 3b, we determined the optical band gap (Eg) of those perovskite films by using Tauc’s equation (αhν)2 = B(hν － Eg) based on the UV-Vis spectra.37 The band gaps of 3D-Br10 and MD-Br10 films (n=40, 30, 20, 10) can be calculated as 1.62, 1.63, 1.64, 1.66, and 1.68 eV, respectively. It can lead to a higher opencircuit voltage (Voc) due to a better match of energy band with electron and hole transfer materials.36 A similar trend can be found in the photoluminescence (PL) spectra of those films. In Figure 3c, there are slight blue-shift in the PL position when the n values reduce from 40 to 10, according to the band gap of each perovskite film.



Figure 3. (a) UV-vis absorption spectra, (b) Tauc plot from the UV-vis spectra and (c) PL spectra of 3D-Br10 and MD-Br10 (n=10, 20, 30 and 40) perovskite films. (d) UV-vis absorption spectra, (e) Tauc plot from the UV-vis spectra and (f) PL spectra of MD-Brx (x=0, 5, 10 and 20, n=30) perovskite films. Figure 3d and f further illustrate the influence of Br concentrations on the absorption and PL of the MD (n=30) perovskite films. In Figure 3d, all the films display strong light absorption and obvious blue-shift as the increase of Br contents. The optical band gaps of MD (Br concentration from 0 to 20%) perovskite films can be calculated as 1.56, 1.59, 1.64, 1.66, and 1.69 eV (Figure 3e), respectively. The PL spectra of corresponding films are shown in Figure 3f. Increasing Br from 0 to 20% results in a 51-nm shift of the PL peaks from 893 nm to 742 nm for the MD (n=30) films. Energy dispersive X-ray spectroscopy (EDS) was used to reveal the presence and distribution of elements on the top surface of MD-Br10 perovskite films.45 Figure 4a shows


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the EDS mapping of all the elements existing in the MD-Br10 film, which summarizes that the elements of C, N, Cs, Pb, I, Br, O are homogeneously distributed inside the crystals, suggesting that the Cs and Br are uniformly incorporated into the MD perovskite films. The distribution of O content indicates that the HEA+ cations exist in the perovskite film and owns a uniform distribution. In order to make a clear insight into the recombination kinetics between electron and hole in the MD perovskite films, we further carried out transient absorption (TA) measurements with the construction of glass/perovskite.46 Figure 4b shows the TA response of MD-Br10 (n=10, 20, 30 and 40, respectively) films, along with a singleexponential fitting. The MD (n=30) perovskite displays the longest lifetime of 60.3 ns (Table S4) which is about three times longer than the lifetime of 3D perovskite (Figure S10), indicating a slower recombination of electron and hole and a better device performance.47 A good explanation can be gained based on the difference of band gaps. Since the band gaps of layered 2D perovskite phase are wider than that of 3D perovskite phases, the charges from 3D phases to the grain boundaries will be reﬂected back instead of trapping and recombination.34,37 Furthermore, the TA response of MD-Brx (x=0, 5, 10 and 20) films deposited on glass are displayed in Figure 4c to make certain the influence of Br concentrations. After introducing Br component, all the films show the longer fitting lifetimes than MD-Br0. The MD-Br10 perovskite owns the longest lifetime of 60.3 ns, which are 38.0 ns, 44.2 ns and 52.9 ns for MD-Br0, MD-Br5 and MD-Br20 perovskites, respectively. The importing of Br component can effectively improve the film quality and



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reduce the nonradiative recombination rates.

Figure 4. (a) EDS mapping of MD-Br10 (n=10) perovskite film. (b-c) Normalized transient absorption (TA) responses of MD-Br10 (n=10, 20, 30 and 40) and MD-Brx (x=0, 5, 10 and 20, n=30) with the structure of glass/perovskite films. In

our

experiment,

PSCs

with

the

architecture

of

FTO/c-TiO2/m-

TiO2/perovskite/spiro-MeOTAD/Au (Figure 5a) were implemented to assess the photovoltaic performance of devices based on different MD perovskites light-absorbing materials. Under the measurement condition of standard AM1.5G illumination at an irradiance of 100 mW cm-2 (0.09 cm2 active area), the Figure 5b and Table S1 displayed the current density-voltage (J-V) curves and corresponding photovoltaic parameters of


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MD-Br10 devices, respectively. In Figure 5b, the 3D-Br10 device presents a PCE of 19.02%. However, the MD (n=10) perovskite device displays an unsatisfactory PCE of 15.62%. Comparing with the 3D-Br10, the obviously reduced of Jsc (20.18 mA cm-2) and fill factor (FF, 68.14%) are caused by the poor light absorption and charge transport. The enhanced Voc (1.14 V) is attributed to the larger band gap. Fortunately, when the n value increases to 30, MD device presents a superior PCE of 19.84%, with a Jsc of 22.81 mA cm-2, a Voc of 1.10 V and a FF of 79.63%. This improved performance with Jsc increased and FF is in close agreement with the better band alignment with TiO2 (Figure S8), the excellent light absorption and the retardation of charge recombination at the grain boundaries and interfaces. The increase in Jsc can be verified from the incident photon to current conversion efficiency (IPCE) spectra demonstrated in Figure 5d. Besides, as shown in Figure 5f, the MD-Br10 device display a low PCE hysteresis of 0.97% which can be explained by the decrease in trap density.46 The property and PCE of MD (n=40) PSCs are inferior when comparing to the MD (n=30) ones, causing by the slightly deficient crystallization and morphology. To have an insight into the effect of MD (n=30) perovskites with different Br concentrations on the performance of devices, Figure 5c gives out the J-V curves of MDBrx (x=0, 5, 10, 15 and 20) devices. After introducing Br component, all the devices showed a high PCE over 18.50%. Interestingly, compared to MD-Br0, the devices display gradual increase of Voc from 1.06 to 1.13 V as the increase of Br concentrations from 0 to 20% (Table S2), which is attributed to the corresponding increase in band gaps. Moreover,



the Jsc of MD-Br15 and MD-Br20 decreased to 21.70 mA cm-2 and 21.32 mA cm-2, respectively. It is caused by the blue-shift absorption in Figure 3d, which can also be observed in IPCE spectra (Figure 5e).

Figure 5. (a) Schematic architecture of PSCs. (b, c) J-V curves and (d, e) IPCE spectra of PSCs based on MD-Br10 and MD-Brx (n=30) perovskite films. (f) J–V curves of MD-Br10 (n=30) PSC under reverse and forward scan directions. (g) PCE histogram fitted with a Gaussian distribution and (h) the statistical parameters of corresponding photovoltaic parameters of the MD-Br0, 3D-Br10 and MD-Br10 PSCs among 30 measured devices. We further measured the uniformity and reproducibility of MD-Br10 PSCs. The PCE histograms of MD-Br0, 3D-Br10 and MD-Br10 devices with a Gaussian distribution are presented in Figure 5g. The statistical distributions of corresponding photovoltaic


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parameters are also shown in Figure 5h. For MD-Br10, an average PCE of 19.02% can be obtained among 30 devices, which are 15.59% and 18.01% for the MD-Br0 and 3D-Br10 at the same condition, respectively. Notably, the fairly narrow distributions of the MD perovskite with Br component reveals a better reproducibility in the measured PSCs.

Figure 6. Normalized PCE variation curves of different unsealed PSCs when exposure to (a) 85 °C and (c) 55±5%RH. (b) Photographs of different unsealed devices before and after 400-h aging under 85 °C. (d) Photographs of different unsealed devices before and after 2160-h aging under 55±5% RH. The long-term stability of PSCs at the environment of moisture and heat is the most important inspection standard of industrial application. In our experiment, the thermal and humidity stability of MD-Br10 (n=30) was compared to that of MD-Br0 (n=30), 3D-Br10 and 3D-Br0 at the same condition. Firstly, the thermal stability was evaluated without any



encapsulation at 85 °C. As shown in Figure 6a, the four devices present marked differences in normalized PCE variation curves. At the first 160 hours (h), all the PCEs display slow downward trend and maintain above 80 % of the initial values. But then the PCEs of MDBr0, 3D-Br10 and 3D-Br0 begin to decrease quickly. Fortunately, the MD-Br10 device display the most superior stability. After aging at 85 °C for 400 h, the MD-Br10 still keep 82% of the initial PCE, which only 59%, 40% and 20% for the 3D-Br10, MD-Br0 and 3DBr0, respectively. In the meantime, the badly degraded performance is caused by the decompose of perovskite to PbI2 (Figure 6b). Although HTM material is also one of the main factors affecting thermal stability of PSCs, the four kind of devices owned the same architecture of FTO/c-TiO2/mTiO2/perovskite/spiro-MeOTAD/Au, the stability of devices can reflect the thermal resistance of the absorption layer materials. To further prove the thermal stability of MDBr10 (n=30) perovskite materials, the XRD patterns before and after exposing to 85 °C for 400 h are shown in Figure S16. After 216-h aging, it can be found that the film still presents a strong diffraction peak at about 14.04°. The weak peaks at about 11.28° and 12.71° are assigned to the δ-phase perovskite and PbI2, respectively. When refer to 400 h, the peak at 11.28° is significantly enhanced and the peak assigned to PbI2 is still weak. It can be concluded that the MD-Br10 (n=30) displayed higher intrinsic stability resisting temperature. During the aging process, part of perovskite would undergo phase transition to δ-phase, but the degradation of perovskite to PbI2 can be effectively suppressed. Moreover, the film presents good absorption spectra (Figure S14) and morphology


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(Figure S17) after aging teat, corresponding to the XRD patterns. The humidity stability of four kinds of devices were further assessed under 55±5% RH at room temperature without encapsulation. As shown in Figure 6c, the MD-Br10 device exhibits the most outstanding moisture-proof stability and the PCE presents a slow downward trend. Whereas, the others show marked decline at different levels after 720 h. The corresponding photographs of degraded devices are shown in Figure 6d. After 2160h aging test, the PCE of MD-Br10 decreases by 14%, which are 30%, 47% and 63 % for the 3D-Br10, MD-Br0 and 3D-Br0, respectively. In comparison with the other 3D PSCs, the designed MD-Br10 PSCs display excellent photovoltaic properties and brilliant long-term stability against heat and moisture. The improved performances are mainly relying on the compositional structure and the advantages of MD perovskites. Firstly, the compose we employed in the experiment based on Cs/FA mix cations owns a stable internal structure than that including MA cation. Secondly, the introducing of HEA+ cation can efficiently promote the crystal of selfassembly MD perovskites, resulting in the superior optical property and enhanced morphology with less grain boundaries and defect-trap sites. Thirdly, the introduction of Br component further develops the crystallization and phase stability. Last, the HEA-based MD perovskites display better internal stability and the layered 2D perovskite slabs can retard the attack of moisture.

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In summary, we have demonstrated a new-type MD perovskite composition exhibiting excellent photoelectric properties and prominent long-term stability against heat and moisture. The contents of 2D phase and Br component are adjusted based on the formula of (HEA)2(Cs0.1FA0.9)n-1Pbn(I3-xBrx)3n+1. The MD-Br10 perovskite films show better crystallization and outstanding optical properties with suitable band gaps. Owing to the self-assembly stability of MD perovskites, the morphologies display smoother and more uniform than that of 3D perovskites. Equally importantly, by recombination kinetics measurement, it is found that MD perovskites own longer carrier lifetimes as a result of the heterostructures of 3D and 2D domains. Finally, when the n value increases to 30, MDBr10 device achieves the best PCE of 19.84% with a Jsc of 22.81 mA cm-2, Voc of 1.10 V and FF of 79.63%. Without encapsulation, the MD PSCs can keep 82% and 87% of the initial efficiency after exposing to 85 °C for 400 h and 55±5% RH for 2160 h, respectively, which are obviously improved than other reference devices.

Associated Content *Supporting Information Experimental section; XRD patterns of HEAI and 2D perovskite; Magnified XRD patterns of the (111) and (222) peaks; FWHM for the (110) peaks; XRD pattern of MD-Br20 film; Top-view SEM images of 3D-Br0 and 3D-Br10 perovskite films; AFM images and Statistic distributions of the particle size for MD-Brx (x=0, 10 and 20) perovskite films; Steadystate PL spectra for different perovskite on glass or TiO2/glass; EDS of MD-Br10 perovskite;


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Normalized TA response of 3D perovskite; Integrated Jsc spectra; J–V curves under reverse and forward scan directions; Steady-state measurement of PCE at the maximum power point; UV-vis absorption spectra, XRD patterns and SEM image for the aging tests; J-V curves before and after the aging; Photovoltaic parameters of perovskite solar cells; Specific aging data.

Author Information Corresponding Author *E-mail: [email protected] ORCID Xu Pan: 0000-0003-3770-7918 Notes The authors declare no competing financial interest.

Acknowledgements This work was financially supported by the National High Technology Research and Development Program of China under Grant No.2015AA050602, STS project of Chinese Academy of Sciences (KFJ-SW-STS-152).

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