Organometal halide perovskite solar cells have evolved in an exponential manner in the two key areas of efficiency and stability. The power conversion efficiency (PCE) reached 20.1% late last year. The key disquiet was stability, which has been limiting practical application, but now the state of the art is promising, being measured in thousands of hours. These improvements have been achieved through the application of different materials, interfaces and device architecture optimizations, especially after the investigation of hole conductor free mesoporous devices incorporating carbon electrodes, which promise stable, low cost and easy device fabrication methods. However, this work is still far from complete. There are various issues associated with the degradation of Omh-perovskite, and the interface and device instability which must be addressed to achieve good reproducibility and long lifetimes for Omh-PSCs with high conversion efficiencies. A comprehensive understanding of these issues is required to achieve breakthroughs in stability and practical outdoor applications of Omh-PSCs. For successful small and large scale applications, besides the improvement of the PCE, the stability of Omh-PSCs has to be improved. The causes of failure and associated mechanisms of device degradation, followed by the origins of degradation, approaches to improve stability, and methods and protocols are discussed in detail and form the main focus of this review article.

Organometal halide perovskite solar cells: degradation and stability

Two-inch-sized perovskite crystals, CH3 NH3 PbX3 (X=I, Br, Cl), with high crystalline quality are prepared by a solution-grown strategy. The availability of large perovskite crystals is expected to transform its broad applications in photovoltaics, optoelectronics, lasers, photodetectors, LEDs, etc., just as crystalline silicon has done in revolutionizing the modern electronics and photovoltaic industries.

Two-Inch-Sized Perovskite CH3NH3PbX3 (X = Cl, Br, I) Crystals: Growth and Characterization

The photoluminescence, transmittance, charge-carrier recombination dynamics, mobility, and diffusion length of CH3NH3PbI3 are investigated in the temperature range from 8 to 370 K. Profound changes in the optoelectronic properties of this prototypical photovoltaic material are observed across the two structural phase transitions occurring at 160 and 310 K. Drude-like terahertz photoconductivity spectra at all temperatures above 80 K suggest that charge localization effects are absent in this range. The monomolecular charge-carrier recombination rate generally increases with rising temperature, indicating a mechanism dominated by ionized impurity mediated recombination. Deduced activation energies Ea associated with ionization are found to increase markedly from the room-temperature tetragonal (Ea ≈ 20 meV) to the higher-temperature cubic (Ea ≈ 200 meV) phase adopted above 310 K. Conversely, the bimolecular rate constant decreases with rising temperature as charge-carrier mobility declines, while the Auger rate constant is highly phase specific, suggesting a strong dependence on electronic band structure. The charge-carrier diffusion length gradually decreases with rising temperature from about 3 μm at -93 °C to 1.2 μm at 67 °C but remains well above the optical absorption depth in the visible spectrum. These results demonstrate that there are no fundamental obstacles to the operation of cells based on CH3NH3PbI3 under typical field conditions. The photoconductivity in CH3NH3PbI3 thin films is investigated from 8 to 370 K across three structural phases. Analysis of the charge-carrier recombination dynamics reveals a variety of starkly differing recombination mechanisms. Evidence of charge-carrier localization is observed only at low temperature. High charge mobility and diffusion length are maintained at high temperature beyond the tetragonal-to-cubic phase transition at ≈310 K.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/adfm.201502340

Temperature-Dependent Charge-Carrier Dynamics in CH3NH3PbI3 Perovskite Thin Films

We report the low-frequency resonant Raman spectrum of methylammonium lead-iodide, a prototypical perovskite for solar cells applications, on mesoporous Al2O3. The measured spectrum assignment is assisted by DFT simulations of the Raman spectra of suitable periodic and model systems. The bands at 62 and 94 cm–1 are assigned respectively to the bending and to the stretching of the Pb–I bonds, and are thus diagnostic modes of the inorganic cage. We also assign the librations of the organic cations at 119 and 154 cm–1. The broad, unstructured 200–400 cm–1 features are assigned to the torsional mode of the methylammonium cations, which we propose as a marker of the orientational disorder of the material. Our study provides the basis to interpret the Raman spectra of organohalide perovskites, which may allow one to further understand the properties of this important class of materials in relation to their full exploitation in solar cells.

The Raman Spectrum of the CH3NH3PbI3 Hybrid Perovskite: Interplay of Theory and Experiment

We determine high charge-carrier mobilities ≥ 33 cm2 V−1 s−1 and bi-molecular recombination rates about five orders of magnitude below the prediction of Langevin's model for vapour-deposited CH3NH3PbI3−xClx using ultrafast THz spectroscopy. At charge-carrier densities below ∼1017 cm−3 intrinsic diffusion lengths are shown to approach 3 microns, limited by slow mono-molecular decay processes.

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Charge-carrier dynamics in vapour-deposited films of the organolead halide perovskite CH3NH3PbI3-xClx

Phase transitions and crystal dynamics in the cubic perovskite CH3NH3PbCl3

Vibrational study of hydrogen bonding and structural disorder in Na2H(SO4)2, K3H(SO4)2 and (NH4)3H(SO4)2 crystals

DSC and Raman spectroscopy studies of high temperature structural phase transitions in (C3H7NH3)2PbCl4

Low-frequency Raman spectra (5–250 cm−1) of a polycrystalline sample of bis(n-propylammonium) lead tetrachloride were recorded at different temperatures between 4 and 300 K. A new first-order reentrant phase transition was observed near 35 K. The phase diagram is: Both phases II and IV present the same degree of disorder and are believed to be isomorphic with a monoclinic cell symmetry. Spectral evolution analysis and analogy with a manganese homologous compound suggest an incommensurate character for phase III between 35 K and 91 K. The pseudo-phase mode characteristic of the incommensurability was identified at about 10 cm−1

Low temperature reentrant phase transition in the perovskite type layer compound (C3H7NH3)2PbCl4

Butylenediammonium lead tetrachloride exhibits three structural phase transitions according to the sequence I-(324 K) to gamma to (341 K) to beta to (420 K) to alpha . These transitions are characterized by DSC measurements and Raman scattering on polycrystalline samples. The space groups and the cell parameters of I, gamma and beta phases were determined by X-ray diffraction on single crystals and powder samples. The three phases I, gamma and beta have the same space group P21/c with Z = 4. The first-order phase transition at 324 K is driven by orientational and conformational motions of the butylenediammonium chains. The transition at 341 K is also of first-order type and involves both organic cation motions and PbCl6 octahedra deformations. The transition at 420 K was evidenced by Raman scattering; it leads to a plastic phase similar to that observed in the homologous monoammonium compound (C3H7NH3)2PbCl4. This transitional behaviour and the crystal dynamics will be discussed and compared with those of homologous compounds.

https://iopscience.iop.org/article/10.1088/0953-8984/5/33/009/pdf

DSC, X-ray diffraction and Raman studies of high-temperature phase transitions in the perovskite-type layer compound butylenediammonium lead tetrachloride

A. Lautie

Papers

Phase transitions and crystal dynamics in the cubic perovskite CH3NH3PbCl3

Vibrational study of hydrogen bonding and structural disorder in Na2H(SO4)2, K3H(SO4)2 and (NH4)3H(SO4)2 crystals

DSC and Raman spectroscopy studies of high temperature structural phase transitions in (C3H7NH3)2PbCl4

Low temperature reentrant phase transition in the perovskite type layer compound (C3H7NH3)2PbCl4

DSC, X-ray diffraction and Raman studies of high-temperature phase transitions in the perovskite-type layer compound butylenediammonium lead tetrachloride