Self-passivation of low-dimensional hybrid halide perovskites guided by structural characteristics and degradation kinetics

Wang, Xiao; Li, Zhimin; Liu, Zhou; Qiao, Zhiqiang; Wang, Chunwu; Hu, Youdi; Li, Shunde; Cheng, Lei; Zhai, Yufeng; Chen, Gang

doi:10.1039/d0ee03836a

Cited by 19 publications

(20 citation statements)

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“…Notably, we found that the PbI 2 layers observed in the kink segment exhibit 6H lattice stacking (hexagonal, space group: R 3̅ m : H , a = b = 0.4557 nm, c = 2.0937 nm, α = β = 90°, γ = 120°), distinct from the 2H phase observed in the stem segment (Figure S1). Figure d displays a 6H PbI 2 atomic arrangement superimposed on the magnified HRTEM image, in which each trilayer is repeated at the third layer in the stacking sequence, together with the emergence of (101̅4) planes ( d spacing = 0.32 nm) , parallel to the TB and the broad sidewall facets (Figure a).…”

Section: Resultsmentioning

confidence: 99%

Br-Induced Orientation Control of PbI₂ van der Waals Nanowires and Their Optoelectronics

2021

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van der Waals (vdW)-layered direct bandgap semiconductor materials such as lead iodide (PbI 2 ) possess highly anisotropic optoelectronic properties that are dependent on their layer stacking orientations. Here, we report an in situ control of PbI 2 layer growth orientation, or stacking axis, via the vapor− liquid−solid (VLS) growth. The PbI 2 vdW nanowires initially grow with transverse stacking relative to the [0001]-oriented growing axis, and the controlled introduction of PbBr 2 induces threedimensional twin formation, which is responsible for the change in the layer stacking axis (i.e., kinking). By varying the duration of the PbBr 2 introduction, two different types of kinking modes are confirmed: Type I kink in parallel with the twin boundary (TB) propagation accompanying two mirrored stacking c-axes and Type II kink, in which the layers are stacked parallel to the growing segment. Optoelectronic analyses using photoluminescence and cathodoluminescence reveal that the kinked PbI 2 nanostructures exhibit asymmetric band-edge excitonic emission profiles, which are distinct from those of the single-oriented nanowires. These results demonstrate a new methodology to modulate the structuredependent optoelectronic/photonic properties, as well as phonon transport, which cannot be achieved using conventional 2D design platforms for the vdW-layered materials.

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Section: Resultsmentioning

confidence: 99%

Br-Induced Orientation Control of PbI₂ van der Waals Nanowires and Their Optoelectronics

2021

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“…Perovskite solar cells (PSCs) have attracted increased attention in recent years due to their high efficiency. [ 1–50 ] However, various instabilities under normal operating conditions (exposure to ambient air/moisture, illumination, elevated temperature, etc.) have been hindering the efforts in PSC commercialization.…”

Section: Introductionmentioning

confidence: 99%

Improvement in the Performance of Inverted 3D/2D Perovskite Solar Cells by Ambient Exposure

Wang

Lin

et al. 2022

Solar RRL

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Perovskite solar cells (PSCs) are known to be sensitive to the exposure to ambient humidity, which typically results in the degradation and deterioration of performance, although positive effects of exposure to moisture have also been reported, due to recrystallization of the perovskite. Common approach to improve stability is to use 3D/2D perovskite active layer, where 2D capping layer is prepared by spin coating the bulky organic cation halide. Herein, it is shown that optimizing the exposure of the capping layer prepared by spin coating phenylethylammonium iodide (PEAI) to ambient atmosphere results in substantial improvement of the PSC performance. Furthermore, the initial effects of PEAI treatment are dependent on the NiO x /perovskite interface, but in all cases except at very high humidity (80–85% RH) optimized exposure to ambient results in improved performance. The variations in device performance with PEAI treatment and ambient exposure can be attributed to defect passivation and changes in the charge extraction due to energy‐level alignment at the interfaces. The best performing devices have passivation of NiO x /perovskite interface and PEAI treatment of top surface followed by exposure to ambient atmosphere at RH of 40–45%, which results in the power conversion efficiency increase from 20.3% to 22.4%.

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“…14 Despite the importance of HP degradation reactions, a formal mechanism with rate orders and rate constants has not been reported. Some reports of effective rate constants and activation energies of perovskite degradation exist from fits to either inverse degradation times 30 or an Avrami model 31 based on the time-evolution of optical absorbance, 32 XRD patterns, [33][34][35][36] XPS peaks, 37 and AFM features of degrading HP films. 38 However, quantitative relationships between the rate of perovskite degradation and ambient partial pressure of oxygen, the illumination intensity, and the partial pressure of water vapor are still unknown.…”

Section: Introductionmentioning

confidence: 99%

Water-Accelerated Photo-oxidation of CH3NH3PbI3 Perovskite: Mechanism, rate orders, and rate constants

Siegler

Dunlap-Shohl

Meng

et al. 2021

Preprint

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Understanding the chemical reactions that hybrid organic-inorganic halide perovskite (HP) semiconductors undergo in the presence of moisture, oxygen, and light are essential to the commercial development of HP solar cells and optoelectronics. Here we use optical absorbance to study the kinetics of methylammonium lead iodide (MAPbI3) degradation in response to combinations of moisture, oxygen, and illumination over a range of temperatures. We identify two primary reaction pathways that dominate MAPbI3 material degradation in these mixed environmental conditions: (1) dry photooxidation (DPO) due to the combined role of oxygen and photoexcited electrons (with a rate of 2 x 10-9 mol/m2∙s in dry air at 25 C and an effective activation energy of 0.62 eV), and (2) a water-accelerated photooxidation (WPO) process due to the combined role of water, oxygen, and photoexcited electrons (with a rate of 1 x 10-7 mol/m2s in 50% RH air at 25 C and observed effective activation energy of 0.07 eV). Commonly reported humidity-only, blue light, and thermal degradation pathways are demonstrated to have rates that are respectively 100, 1000, and >1000 times slower than predominant photooxidation processes in ambient conditions. Extracting kinetic rate constants from the dynamics of the initial degradation, we calculate that in dry air, photooxidation rate of MAPbI3 follows a f(x)∝x/(1+Kx) relationship with respect to oxygen in the vapor phase (PO2) and excess concentration of photoexcited electrons (n). In humid air, photooxidation of MAPbI3 exhibits first order kinetics with respect to the partial pressure of water in the vapor phase (PH2O). However, with respect to PO2 and n, kinetics follow a f(x)∝x/(1+kx)^2 relationship with respect to rate. We then identify a plausible reaction mechanism for degradation of MAPbI3 material that is consistent with these rate functionalities. The rate determining step for both DPO and WPO is proton abstraction by photogenerated superoxide radicals. However, proton donation by adsorbed water proceeds much more rapidly than donation by methylammonium, resulting in faster degradation rates for WPO at typical ambient conditions (~50% RH). Rate laws derived from this mechanism were fit to the entire dataset to extract rate constants for DPO and WPO processes. Accurate predictions of material degradation rates, with narrow confidence intervals of fit parameters as identified by the Bootstrap algorithm, provide the first experimental estimates of the equilibrium constants of oxygen adsorption on MAPbI3 (Keq ≈ 3 x 10-3 kPa-1) and superoxide generation from adsorbed oxygen and photoexcited electrons in MAPbI3 (Keq ≈ 5 x 10-15 (photons/m2∙s)-0.7). Given that water has been reported as a degradation product of DPO, the results reported here highlight the need for the development of encapsulation schemes that rigorously block oxygen, as over longer time periods, product water (if generated) may accumulate inside the packaging and initiate the much faster WPO process.

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Self-passivation of low-dimensional hybrid halide perovskites guided by structural characteristics and degradation kinetics

Abstract: The low-dimensional perovskite (LDP) holds great potential to deliver the highly stable and efficient solar cells. However, the fundamental understanding of the structure, composition and degradation mechanism of the LDP...

Cited by 19 publications

References 50 publications

Br-Induced Orientation Control of PbI₂ van der Waals Nanowires and Their Optoelectronics

Br-Induced Orientation Control of PbI₂ van der Waals Nanowires and Their Optoelectronics

Improvement in the Performance of Inverted 3D/2D Perovskite Solar Cells by Ambient Exposure

Water-Accelerated Photo-oxidation of CH3NH3PbI3 Perovskite: Mechanism, rate orders, and rate constants

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Self-passivation of low-dimensional hybrid halide perovskites guided by structural characteristics and degradation kinetics

Abstract: The low-dimensional perovskite (LDP) holds great potential to deliver the highly stable and efficient solar cells. However, the fundamental understanding of the structure, composition and degradation mechanism of the LDP...

Cited by 19 publications

References 50 publications

Br-Induced Orientation Control of PbI2 van der Waals Nanowires and Their Optoelectronics

Br-Induced Orientation Control of PbI2 van der Waals Nanowires and Their Optoelectronics

Improvement in the Performance of Inverted 3D/2D Perovskite Solar Cells by Ambient Exposure

Water-Accelerated Photo-oxidation of CH3NH3PbI3 Perovskite: Mechanism, rate orders, and rate constants

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Br-Induced Orientation Control of PbI₂ van der Waals Nanowires and Their Optoelectronics

Br-Induced Orientation Control of PbI₂ van der Waals Nanowires and Their Optoelectronics