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ARTICLE DOI: 10.1038/s41467-018-03169-0 OPEN Surface passivation engineering strategy to fully-inorganic cubic CsPbI perovskites for high-performance solar cells 1 1 1 1 1 1 1 Bo Li , Yanan Zhang , Lin Fu , Tong Yu , Shujie Zhou , Luyuan Zhang & Longwei Yin Owing to inevitable thermal/moisture instability for organic–inorganic hybrid perovskites, pure inorganic perovskite cesium lead halides with both inherent stability and prominent photovoltaic performance have become research hotspots as a promising candidate for commercial perovskite solar cells. However, it is still a serious challenge to synthesize desired cubic cesium lead iodides (CsPbI ) with superior photovoltaic performance for its thermo- dynamically metastable characteristics. Herein, polymer poly-vinylpyrrolidone (PVP)-induced surface passivation engineering is reported to synthesize extra-long-term stable cubic CsPbI . It is revealed that acylamino groups of PVP induce electron cloud density enhancement on the surface of CsPbI , thus lowering surface energy, conducive to stabilize cubic CsPbI even 3 3 in micrometer scale. The cubic-CsPbI PSCs exhibit extra-long carrier diffusion length (over 1.5 μm), highest power conversion efficiency of 10.74% and excellent thermal/moisture stability. This result provides important progress towards understanding of phase stability in realization of large-scale preparations of efficient and stable inorganic PSCs. Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials, Ministry of Education, School of Materials Science and Engineering, Shandong University, Jinan 250061, P. R. China. Correspondence and requests for materials should be addressed to L.Y. (email: yinlw@sdu.edu.cn) NATURE COMMUNICATIONS (2018) 9:1076 DOI: 10.1038/s41467-018-03169-0 www.nature.com/naturecommunications 1 | | | 1234567890():,; ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-03169-0 ue to suitable direct bandgap, high absorption coefficient, Results and extra-long carrier diffusion length, excellent optoe- PVP-induced cubic phase stability studies. The specific cubic- Dlectronic property, simple and reproducible solution/ phase CsPbI films were prepared via a simple and reproducible 1–3 vapor-chemistry processing , organic–inorganic hybrid halide one-pot solution spin-coating process using a mixture of CsI, perovskite materials (ABX ,A=CH NH ,B=Pb, X=Br, I) have PbI , and PVP as a precursor. X-ray diffraction (XRD) patterns of 3 3 3 2 been deemed as a promising candidate for light harvester for the CsPbI films coated on the F-doped SnO (FTO) substrates 3 2 4–8 next-generation high-performance solar cells . Despite great present the difference in the presence and absence of PVP. In the progress in photovoltaic performance in the last few years, one-pot solution process without PVP, the CsPbI film exhibits a commercial application of perovskite solar cell (PSC) still suffers prompt transition from cubic phase to orthorhombic phase when from moisture and thermal instability owing to the degradation prolonging anneal time or cooling to room temperature, as shown and volatilization of organic component, which presents the in the XRD pattern (black line) in Fig. 1a and the photograph in uppermost obstacle in further development and mass produc- Fig. 1b. After adding PVP and gradually increasing the con- tion . For this reason, all inorganic halide perovskite formed by centration to 10 wt%, the CsPbI can keep stable cubic phase both substituting the organic cation with cesium (Cs) is an optimal at high and room temperature, even stable at exceeding 80 days alternative for its native inorganic structure stability, and has (Fig. 1a, b; Supplementary Fig. 2). Similarly, the prominent phase demonstrated equally efficient and more stable compared to stability is demonstrated achievable in full series of inorganic 10–13 organic–inorganic halide perovskites . perovskite compositions from CsPbI to CsPbBr shown in 3 3 Of the various inorganic lead halide perovskites, especially, Supplementary Figures 3 and 4. cesium lead iodide (CsPbI ) in cubic phase (α phase) with a We further fabricate CsPbI film on mesoporous TiO via one- 3 3 2 bandgap of around 1.73 eV and a visible-light-absorption spec- step solution spin-coating process with different PVP concentra- trum up to 700 nm is the mostly desired light harvester in solar tions. As shown in SEM images of Fig. 1c, d, both orthorhombic 14–16 cells . However, cubic CsPbI can only keep stable at high and cubic CsPbI exhibit high-surface coverage. Compared with 3 3 temperature of above 300 °C . As temperature decreasing to irregular grain size distribution of orthorhombic ones, the ambient temperature, CsPbI suffers from thermodynamically obtained PVP-induced cubic CsPbI film presents a dense 3 3 phase transition to undesired orthorhombic phase (δ phase) with grained uniform morphology with comparatively small grain size a wide bandgap of 2.82 eV (Supplementary Figure 1), exhibiting of around 100 nm. The cross-section morphology of the an extremely poor photovoltaic conversion efficiency (PCE) of fabricated solar device architecture is shown in Fig. 1e, consisting only 0.09% in PSC . To overcome this obstacle, composition mainly of two uniform layers containing a 400 nm mesoporous engineering which pursues a certain amount of bromide (Br) to TiO /CsPbI nanocomposite film and a 100 nm pure CsPbI 2 3 3 substitute iodide (I) can be one of efficient methods by balancing perovskite overlayer. It is shown that the inorganic perovskite the tolerance coefficient between PbX octahedron and Cs materials are fully permeated into TiO mesoporous layer, 6 2 18–20 18 ions . For example, Sutton et al. developed a full set of forming a very uniform overlayer with 100% coverage. Intrigu- cesium lead halide films from CsPbBr to CsPbI , demonstrating ingly, tuning anneal time range, the spin-coating obtained CsPbI 3 3 3 a stabilized power output of 5.6% and J–V efficiency reaching exhibits crystalline size of over 1 μm and high-crystalline with 9.8% for PSC based on cubic CsPbI Br, although CsPbI Br still cubic phase structure (Supplementary Figs. 5, 6 and 7), which is 2 2 reverts to δ phase over prolonged exposure in air. Increasing different from the previous reports involving of phase transition continuously bromide proportion induces more prominent phase of perovskite materials in large grain size . stability/moisture-stability, dispiritingly, which brings Br- In order to gain insight into the PVP stabilization mechanism widened bandgap near or above 2.0 eV compared with the ideal on cubic CsPbI , we examine the infrared transmittance spectra solar spectrum (from 1.1 eV to 1.7 eV) . Another effectual of CsPbI films (Fig. 2a) for pure PVP, CsPbI film synthesized in 3 3 method to stabilize cubic phase CsPbI is synthesizing colloidal the presence of PVP, and the CsPbI film after removing PVP 3 3 quantum dots (CQDs) with well-controlled size via hot injection washed with isopropanol (IPA). The IR spectrum of pure PVP process, and best-performance CsPbI solar cells are achieved by shows absorption bands in the region of 1668, 1421, and 1297 cm 22–25 −1 assembling cubic phase CsPbI CQDs as photoactive layer . , which are attributed to typically functional groups of C=O, Unfortunately, the undesired α-to-δ phase transition of Cs-based C–H, and C–N stretching vibration in acylamino of PVP, 26,27 inorganic perovskite has not been inhibited in the solution- respectively . For the IR spectrum of the CsPbI film chemistry processed film. It is of great and fundamental challenge synthesized in the presence of PVP, these characteristic vibrations −1 to develop effective and facile route to synthesize cubic Cs-based are still persisted, but only blue-shifting to 1652 cm for the −1 inorganic perovskite film for high-performance PSC for potential C=O stretching, and 1282 cm for the C–N stretching, large-scale industrial application. respectively, indicating an interaction effect between functional Herein, poly-vinylpyrrolidone (PVP)-induced surface passiva- groups of PVP and precursor ions of CsPbI . For the CsPbI film 3 3 tion strategy is reported to stabilize inorganic perovskite CsPbI washed with IPA, characteristic bands for C=O, C–N, and C–H with cubic crystal structure via a reproducible solution-chemistry stretching decreases considerably in terms of intensity, while it reaction process. The surface chemical state of cubic CsPbI remains at the same location. A similar binding energy variation crystals synthesized in the presence of PVP is investigated via of CsPbI surface elements can be found in X-ray photoelectron Fourier transformed infrared (FTIR) and nuclear magnetic reso- spectroscopy (XPS) measurements in Supplementary Figure 9. nance (NMR) techniques, demonstrating that decreased surface The variation tendency demonstrates that PVP is not only tension can be conducive to stabilize cubic CsPbI even in large absorbed on the surface of CsPbI physically, but also functions 3 3 scale of film with micrometer scale, due to enhanced electron chemically in formation and stabilization of cubic CsPbI through cloud density on the surface of CsPbI originated from chemical N–C=O bond of acylamino group . bonding between acylamino group in PVP and CsPbI . The On the basis of the above IR information, it is indicated that obtained cubic CsPbI exhibits extra-long carrier lifetime of 338.7 acylamino group of PVP plays a dominant role on the nucleation ns and diffusion length of greater than 1.5 μm, up to an order of and growth of cubic CsPbI perovskite film. For further magnitude compared to the active layer depth. The fabricated understanding this specific effect of acylamino group of PVP, 1 13 PSCs based cubic CsPbI achieves the highest power conversion the liquid-state H/ C NMR measurement is conducted on pure efficiency of 10.74% and excellent thermal/moisture stability. PVP samples and CsPbI perovskite synthesized in the presence 2 NATURE COMMUNICATIONS (2018) 9:1076 DOI: 10.1038/s41467-018-03169-0 www.nature.com/naturecommunications | | | NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-03169-0 ARTICLE ab FTO Fresh 80 day α with PVP (80 day) α with PVP α CsPbI δ w/o PVP Fresh 1 h Cubic CsPbl Orthorhombic CsPbl 10 20 30 40 50 δ CsPbI 2 (degrees) cd δ CsPbI α CsPbI 3 3 1 μm 1 μm Au spiro-OMeTAD m-TiO & CsPbI 2 3 c-TiO FTO 500 nm Fig. 1 Structure and morphology of CsPbI films and CsPbI perovskite solar cell. a X-ray diffraction (XRD) spectra of CsPbI with orthorhombic phase (δ, 3 3 3 black line), cubic phase (α, red line) and stable cubic phase aging 80 days (blue line). The reference powder pattern for CsPbI (cubic and orthorhombic phase) is from Swarnkar et al. b Images of prepared orthorhombic and cubic CsPbI films aging for different times. Scale bar, 1 cm. c, d Scanning electron microscope (SEM) images of the overlayers for orthorhombic and cubic CsPbI films deposited on the meso-TiO annealing for 5 min at 300 °C. e The 3 2 typical cross-section SEM image of fabricated inorganic perovskite CsPbI solar cell a bc PVP PVP CsPbl -PVP CsPbl -PVP IPA treatment 2 3 2 CsPbl -PVP 3 1 1 O O N N β H β H α n n Pure PVP C–N stretch 2 CH scissor 1 1 2 α α β 32 4 3 2 C=O stretch 1000 1200 1400 1600 1800 2000 4.0 3.5 3.0 2.5 2.0 1.5 1.0 200 180 160 40 0 –1 Wavenumber (cm ) Chemical shift (ppm) Chemical shift (ppm) Fig. 2 Fourier transform infrared and nuclear magnetic resonance spectra of CsPbI . a Fourier transform infrared (FTIR) spectroscopy of pure PVP, cubic 1 13 phase CsPbI films synthesized in the presence of PVP, and cubic CsPbI films after IPA treatment. b, c H and C liquid-state nuclear magnetic resonance 3 3 (NMR) spectra of PVP solution and CsPbI perovskite solution in the presence of PVP dissolved with DMSO-d 3 6 NATURE COMMUNICATIONS (2018) 9:1076 DOI: 10.1038/s41467-018-03169-0 www.nature.com/naturecommunications 3 | | | Transmission (a.u.) Intensity (a.u.) Intensity (a.u.) Intensity (a.u.) ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-03169-0 on the above experiment facts. It is known that the acylamino ab group in N-vinylpyrrolidone molecule of PVP has donated lone PVP pairs related to oxygen and nitrogen atoms, which offer a large O O number of coordination centers. As shown in Fig. 3a, the Cs coordination modus indicates the polymer molecules coordinate CsPbl CsPbl CsPbl 3 3 3 onto the surface of CsPbI through the oxygen atoms, to a lesser DMSO/DMF extent, via the nitrogen of N–C=O groups, resulting in a weakening of the C=O bonding and an increasing of N–C bond. Pbl At the initial stage (Fig. 3b), PVP molecules initiate to attract cations of CsPbI precursors due to long backbone chain and electronegative acylamino group structure. The positive and dc negative ions of CsPbI tend to assemble and bond to form cubic CsPbI a metastable state around the N–C=O coordination CsPbl 3 centers of PVP. With time increasing, more nuclei of CsPbI are crystal promptly launched with PVP attached. The PVP molecules, in PVP surface the meantime, stabilize the CsPbI nanocrystals from aggregation owing to the intermolecular rejection effect, as shown in Fig. 3c. For the grown CsPbI nanocrystals, long-chain PVP molecules CsPbl 3 with a large number of acylamino groups anchored at the surface nano-crystal of CsPbI provide more coordination polymer units for interactions between oxygen, nitrogen in acylamino and cesium Fig. 3 Mechanism of PVP-induced cubic phase stability. a Schematic ions of inorganic perovskite. With the growth of CsPbI stabilized diagram of the chemical bonding between CsPbI and PVP molecules. PVP 3 with PVP, the interactions between N–C=O of acylamino and Cs molecule contains of long-chain alkyls and acylaminos. The unbounded lone of inorganic perovskite exposed at the surface are enhanced, pairs for N/O atoms in acylaminos offer excess electrons and interact with contributing increasing negative field in Cs -PVP complex on the Cs ions in CsPbI . Mechanism and scheme for the formation of cubic phase surface of CsPbI (Fig. 3d), which results in the enhancement of CsPbI with the assistant of PVP in three stages. b PbI and Cs ions in 3 3 2 + 26,32 the electron cloud for Cs of CsPbI . According to study by DMF/DMSO solvent assemble and interact with PVP molecules 3 Grätzel’s group that the surface free energy is a function of the spontaneously, and maintain a metastable state. c CsPbI nanocrystals surface tension . While the surface tension is related to charge formed attached on PVP molecules, and remain relatively independent and density . An increase in charge density decreases the surface stable under the effect of PVP molecules. d PVP anchored at the surface of tension. Therefore, in the CsPbI -PVP complex, the increase in CsPbI crystals via the combination between N/O and Cs. The negative 3 the electron cloud density may result in low surface tension, thus state in CsPbI crystals surface reduces surface tension significantly and greatly reduces the surface energy of CsPbI . As a result, the cubic stabilizes cubic phase 3 CsPbI can be maintained at ambient temperature. Furthermore, of PVP in deuterated DMSO-d solution, as shown in Fig. 2b, c. cubic structure of CsPbI can even be still maintained after 6 3 In H NMR spectra (Fig. 2b) of neat PVP sample, resonance 80 days for the PVP chemically functionalized CsPbI . Owing to signals attributed to the acylamino group appear at δ = 2.5 and the increase of surface charge originated from the interaction 3.35 ppm, which are characteristic of CH attached to C=O group between PVP and CsPbI , the surface tension of CsPbI grains 2 3 3 and N atom, respectively . The interaction of unique groups in reduced significantly, which plays an essential role in the PVP with precursor ions of CsPbI induces a downfield chemical stabilization of cubic phase CsPbI . 3 3 shift of Δδ ≈ 0.5 ppm for CH adjoining with acylamino group. Reversely, almost no variation for the resonances of hydrogen in backbone chain appears. This can be rationally explained in terms Optical and photovoltaic performance. Weighing the phase of strengthening effects of resonance for organic constituents stability and power conversion efficiency (Supplementary Figs. 8, through the interaction between cesium cations of perovskite and 14, 15 and 16), the optimal synthetic condition (10 wt% of PVP, atoms in organic molecules of PVP, reflecting on the shift of 5 min annealing, and 30 min IPA treatment) was selected and chemical resonances . Moreover, the result indicates that the N applied for the following optical, electrical and photovoltaic and O atoms in acylamino group are jointly responsible for the investigation. Figure 4a presents the ultraviolet–visible absorption chemical shift and can be as two possible centers for coordination spectra of the obtained cubic and orthorhombic phase CsPbI with cesium ions. Furthermore, C NMR spectroscopy in Fig. 2c films. The orthorhombic CsPbI exhibits limited visible-light- show that resonance signal of δ = 175 ppm arising from C=O absorption range less than 450 nm, demonstrating that it is group undergoes a significant downfield shift of Δδ ≈ 2 ppm on unsatisfactory as an optical active material for solar devices. interaction of PVP with CsPbI , which is indicative of the Oppositely, the cubic CsPbI shows a desired absorption width to 3 3 coordination-bonding interaction between oxygen atoms of 700 nm, nearly covering full visible-light region. Furthermore, we acylamino group and cesium ions in perovskite . In contrast, investigate the effect of anneal time (crystalline grains) on the the resonances at δ = 41 and 43 ppm for C(1)H and C(α)H optical properties and carry out the PL measurement of cubic attached to nitrogen atoms exhibit slight chemical shift. Such CsPbI perovskite films. The result shows that, tuning size of variation of PVP molecule structure further confirms that there cubic CsPbI grains, the emission peaks red-shift gradually until exist two potential centers, i.e. the nitrogen and the carbonyl to a constant value of around 710 nm (Supplementary Fig. 10). + 31 oxygen, interacting with Cs ions of perovskite exposed . The time-resolved photoluminescence (TRPL) measurement Moreover, the oxygen in acylamino group occupies a dominate (Fig. 4b; Supplementary Figure 18) is conducted to investigate the position in the formation of C=O···Cs bonds, since nitrogen in carrier lifetime of cubic and orthorhombic CsPbI films. To planar conformation of internal amide can only have relatively eliminate the effect of quenching treatment, the CsPbI films are weak influence. deposited on glass slides under the same solution-method and the A conceivable PVP-induced surface tension-driven mechanism same thickness. The corresponding steady-state PL spectra of for the formation of stable cubic phase CsPbI is proposed based cubic and orthorhombic CsPbI films are shown in 3 3 4 NATURE COMMUNICATIONS (2018) 9:1076 DOI: 10.1038/s41467-018-03169-0 www.nature.com/naturecommunications | | | N –2 Integrated J (mA cm ) sc NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-03169-0 ARTICLE ab c δ CsPbl δ CsPbl α CsPbl α CsPbl 0.1 = 338.7 ns 0.01 = 19.3 ns 400 500 600 700 0 200 400 600 300 400 500 600 700 800 Wavelength (nm) Times (ns) Wavelength (nm) de Forward Reverse J V FF PCE sc oc –2 (mA cm ) (V) (%) Forward 14.79 1.08 0.65 10.38 Reverse 14.88 1.11 0.65 10.74 Average 14.83 1.10 0.65 10.61 0.0 0.2 0.4 0.6 0.8 1.0 1.2 4 5 6 7 8 9 10 11 12 Voltage (V) PCE (%) Fig. 4 Optical and photovoltaic performance of cubic CsPbI . a The ultraviolet–visible (UV) absorption spectra of orthorhombic and cubic CsPbI films. b 3 3 Time-resolved photoluminescence (TRPL) spectra of orthorhombic and cubic CsPbI films deposited on glass substrates. The excitation wavelength was fixed at 300 nm, the emission wavelengths were set at 412 and 704 nm for orthorhombic and cubic, respectively. c The incident photon-to-current efficiency (IPCE) spectrum and corresponding integrated J for the best-performance cubic CsPbI solar cell. d The J–V curves for the best cubic CsPbI sc 3 3 cell measured by forward and reverse scans. The average photovoltaic performance values form the two J–V curves are summarized (inset). e Histogram of average efficiencies for 30 devices of cubic CsPbI Supplementary Figure 11. The PL decay for neat orthorhombic Table 1 The carrier diffusion constant (D) and diffusion CsPbI film exhibits a time-constant of τ = 19.3 ns. In contrast, 3 e length (L ) simulated form PL decays using the diffusion cubic CsPbI film shows an extra-long carrier lifetime of τ = 3 e model 338.7 ns. To simulate the carrier diffusion length in perovskite films, only electron/hole extraction layers and inorganic per- 2 −1 Phase Species D (cm s ) L (nm) ovskite layer (i.e., TiO /CsPbI and CsPbI /spiro-OMeTAD) are 2 3 3 Cubic Electrons 0.061 ± 0.016 1566 ± 254 fabricated via same solution-chemistry processing and the same Holes 0.057 ± 0.013 1427 ± 238 thickness with the fabricated cell, the PL decay curves with Orthorhombic Electrons 0.014 ± 0.009 121 ± 51 electron/hole extraction layers are shown in Supplementary Holes 0.011 ± 0.007 117 ± 35 Figures 12 and 13, the PL decay dynamics are modeled via The errors arise predominantly from perovskite film thickness variations, which is ±50 nm for accounting the excitations number and distributions based on the both orthorhombic and cubic CsPbI films 1 3 one-dimensional diffusion equation . ∂nðx; tÞ ∂ nðx; tÞ ð1Þ ¼ D ktðÞnðx; tÞ 1,8 and 813 nm, respectively . In addition, in pure/mixed Br based ∂t ∂x inorganic perovskite, the carrier diffusion length is less than 200 nm . The ultra-long carrier diffusion length not only originates in which n(x,t) is the number of excitations within a certain from the excellent carrier transport capability of cubic CsPbI , but thickness of perovskite film, k(t) is the PL decay rate without also from the inhibition of defect recombination via the surface quenching layer, and D is the diffusion coefficient. Table 1 shows passivation effect, which provides the feasibility in planar- the carrier diffusion length for both orthorhombic and cubic structure PSCs or even thicker light-absorption layers. CsPbI films, which depends on electron or hole quenching layer On the basis of optical and carrier transport properties, we used, and it is assumed that all photogenerated carriers reach the conduct the photovoltaic measurements of the cubic CsPbI PSCs quenching interface. It is clear the diffusion length of both fabricated with mesoporous TiO scaffold. Of the solar devices electron and hole in orthorhombic CsPbI film is around 120 nm. acquired, Fig. 4d depicts the current–voltage (J–V) curves However, for cubic CsPbI film, the carriers exhibit a diffusion measured via forward and reverse bias sweep for the best- length for electrons over 1 μm, and even over 1.5 μm. As reported performance PSCs. The corresponding photovoltaic parameters (Supplementary Table 1), the average carrier diffusion length in under the optimized conditions with an active area of 0.09 cm , organic–inorganic hybrid perovskites MAPbI and FAPbI is 129 including of short-circuit current density (J ), open-circuit 3 3 sc NATURE COMMUNICATIONS (2018) 9:1076 DOI: 10.1038/s41467-018-03169-0 www.nature.com/naturecommunications 5 | | | –2 Abs intensity (a.u.) Current density (mA cm ) PL intensity (norm.) Counts IPCE (%) ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-03169-0 a b 1.0 0.8 1.0 0.6 0.4 MAPbl Humidity: 45–55% (RT) 0.8 CsPbl 0.2 0 50 100 150 200 250 300 350 400 450 500 Aging time (h) 0.6 1.0 0.4 0.8 0.6 MAPbl CsPbl 0.4 MAPbl 3 0.2 CsPbl Temperature : 60 °C (N ) 0.2 0 50 100 150 200 250 300 350 400 450 500 20 40 60 80 100 Aging time (h) Temperature (°C) Fig. 5 Moisture and thermal stability investigation of perovskite solar cells based cubic CsPbI . a Efficiency evolution of the devices exposed in ambient air under relative humidity of 45–55% without any sealing. The measurements were carried every 50 h during 500 h. b Efficiency variation as a function of temperature from 20 to 100 °C. The PCEs were measured under nitrogen atmosphere after an equilibration time of 30 min at each temperature setting. c Efficiency evolution of the cells in a nitrogen atmosphere at 60 °C during 500 h voltage (V ), fill factor (FF), and PCE values are summarized in 26 °C fixed). Fig. 5a shows the device moisture-stability as a oc the insert of Fig. 4d. The J , V , and FF for forward sweep of the function of aging time in terms of normalized power conversion sc oc −2 device are 14.79 mA cm , 1.08 V, and 65%, respectively, efficiency (PCE). During 500 h, the cell of MAPbI shows a corresponding to a PCE of 10.38% under standard AM 1.5 G dramatic drop with 70% efficiency loss with respect to the fresh condition. With faint hysteresis, the solar device for reverse sweep solar cell. Comparatively, cubic CsPbI based device exhibits a −2 exhibits a J of 14.88 mA cm ,a V of 1.11 V and a PCE of better moisture-stability with 75% retention after 500 h. Figure 5b sc oc 10.74%, which are higher than those of previous reports on shows the thermal-stability measurement of PSCs with cubic 18,19,35 CsPbBr and CsPbBr I (Supplementary Table 2) . CsPbI and MAPbI , which was conducted at different tem- 3 3−x x 3 3 Moreover, the stability of J and PCE for both devices is shown perature ranging from 20 to 100 °C. It is clear that, with the sc in Supplementary Figure 19. The cubic CsPbI device shows a increasing of temperature, the inorganic cubic perovskite exhibits −2 stable output with a J of 13.1 mA cm and a PCE of 10.0%. The more prominent thermal stability, showing over 90% efficiency sc corresponding incident photo-to-current efficiency (IPCE) spec- retention even at 80 °C. It is worth noting that, as increasing the trum in Fig. 4c for the best cell exhibits a broad plateau of over temperature to 100 °C, the devices of both CsPbI and MAPbI 3 3 −2 60% between 350 and 700 nm. The integrated J of 14.7 mA cm show obvious decay in PCE, which might result from the failure sc is in good agreement with the current density acquired from the of the organic hole transport material. For further investigating current–voltage measurement. Intriguingly, compared with other the long-term thermal stability of the inorganic perovskite CsPbI 18,19,35 inorganic PSCs , the CsPbI based solar cell exhibits a much solar cell, we measured the device performance as stored at high higher J , which can be attributed to the extended visible-light- temperature (60 °C) under normal sunlight exposure, which is sc absorption range and extra-long carrier diffusion length for shown in Fig. 5c. During 500 h measurement, the cubic CsPbI CsPbI , beneficial to more photoelectrons/hole generations and based PSC shows a slight efficiency decay of only around 10%, captures by corresponding transport layers. Moreover, the PVP demonstrating an outstanding superiority in thermal stability covered on CsPbI grains decreases surface defects and suppresses compared to MAPbI based solar cell (70% efficiency loss). 3 3 nonradiative recombination, significantly (Supplementary Notably, the thermal-stability efficiency test for inorganic per- Figure 17). Figure 4e shows a histogram of average PCEs from ovskite exhibits a slower decay rate than the humidity stability. all of the cubic CsPbI PSCs fabricated under the same condition The result demonstrates that the CsPbI inorganic perovskite 3 3 for the repeatability purpose. Over 70% of the devices exhibit over possesses more outstanding stability, especially in thermal 8% PCE, and the average PCE summarized shows 8.50%, which is stability. better than most current stable and efficient CsPbI Br perovskite cells (Supplementary Figure 20). Discussion In summary, we developed a surface passivation engineering for Moisture and thermal stability. The excellent stability of PSCs is preparing long-term stable cubic phase CsPbI films via a an essential factor for the reproducibility and commercial appli- reproducible solution-chemistry process with the assistant of cation. To investigate the moisture and thermal stability under PVP. We proposed a plausible mechanism for the formation of different conditions, the performance of inorganic cubic per- stable cubic CsPbI by investigating the surface chemical states of ovskite (CsPbI ) PSCs with average PCE is comparatively mea- the perovskite crystals. The decreased surface tension can be sured with that of solar cells based on typical organic–inorganic obtained to stabilize CsPbI grains in cubic phase even in hybrid perovskite (MAPbI ). The ambient-humidity-stability test micrometer scale, due to electron cloud density enhancement on was conducted under ambient condition for 500 h without the surface of CsPbI originated from chemical bonding between encapsulation (average humidity of 45–55% with temperature of acylamino in PVP and CsPbI . Furthermore, we found the 6 NATURE COMMUNICATIONS (2018) 9:1076 DOI: 10.1038/s41467-018-03169-0 www.nature.com/naturecommunications | | | Normalized PCE (%) Normalized PCE (%) Normalized PCE (%) NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-03169-0 ARTICLE obtained cubic CsPbI exhibits prominent photoelectronic 4. Kim, H.-S. et al. Lead iodide perovskite sensitized all-solid-state submicron thin film mesoscopic solar cell with efficiency exceeding 9%. Sci. Rep. 2, 591 properties with extra-long carrier lifetime of 338.7 ns and diffu- (2012). sion length of greater than 1.5 μm, up to an order of magnitude 5. Lee, M. M. et al. 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NATURE COMMUNICATIONS (2018) 9:1076 DOI: 10.1038/s41467-018-03169-0 www.nature.com/naturecommunications 7 | | | ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-03169-0 Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in Acknowledgements published maps and institutional affiliations. We acknowledge support from the project supported by the State Key Program of National Natural Science of China (No.: 51532005), the National Nature Science Foundation of China (No.: 51472148, 51272137), and the Tai Shan Scholar Foundation of Shandong Province. Open Access This article is licensed under a Creative Commons Author contributions Attribution 4.0 International License, which permits use, sharing, L.Y. initiated and directed the study. B.L. conceived the original research idea. B.L. and adaptation, distribution and reproduction in any medium or format, as long as you give Y.Z. conducted most of the device fabrication and measurements. S.Z. contributed to the appropriate credit to the original author(s) and the source, provide a link to the Creative deposition of electron extraction layer. T.Y. contributed to the deposition of hole Commons license, and indicate if changes were made. The images or other third party extraction layer. L.F. contributed to the structural characteristics. L.Z. provided the material in this article are included in the article’s Creative Commons license, unless mechanism idea. The manuscript was co-written by L.Y. and B.L. All authors contributed indicated otherwise in a credit line to the material. If material is not included in the to the discussion and revising of this manuscript. article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/ Additional information Supplementary Information accompanies this paper at https://doi.org/10.1038/s41467- licenses/by/4.0/. 018-03169-0. © The Author(s) 2018 Competing interests: The authors declare no competing interests. Reprints and permission information is available online at http://npg.nature.com/ reprintsandpermissions/ 8 NATURE COMMUNICATIONS (2018) 9:1076 DOI: 10.1038/s41467-018-03169-0 www.nature.com/naturecommunications | | |
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Published: Mar 14, 2018
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