Letter pubs.acs.org/JPCL
Probing the Intrinsic Thermal and Photochemical Stability of Hybrid and Inorganic Lead Halide Perovskites Azat F. Akbulatov,† Sergey Yu. Luchkin,‡ Lyubov A. Frolova,† Nadezhda N. Dremova,† Kirill L. Gerasimov,§ Ivan S. Zhidkov,∥ Denis V. Anokhin,§,† Ernst Z. Kurmaev,∥,⊥ Keith J. Stevenson,‡ and Pavel A. Troshin*,‡,† †
IPCP RAS, Semenov Prospect 1, Chernogolovka 142432, Russia Skolkovo Institute of Science and Technology, Nobel Street 3, Moscow 143026, Russian Federation § Faculty of Fundamental Physical and Chemical Engineering, Moscow State University, Leninskie Gory, Moscow 119991, Russia ∥ Institute of Physics and Technology, Ural Federal University, Mira 19 Street, Yekaterinburg 620002, Russia ⊥ M. N. Mikheev Institute of Metal Physics of Ural Branch of Russian Academy of Sciences, S. Kovalevskoi 18 Street, Yekaterinburg 620990, Russia ‡
S Supporting Information *
ABSTRACT: We report a careful and systematic study of thermal and photochemical degradation of a series of complex haloplumbates APbX3 (X = I, Br) with hybrid organic (A+ = CH3NH3) and inorganic (A+ = Cs+) cations under anoxic conditions (i.e., without exposure to oxygen and moisture by testing in an inert glovebox environment). We show that the most common hybrid materials (e.g., MAPbI3) are intrinsically unstable with respect to the heat- and light-induced stress and, therefore, can hardly sustain the real solar cell operation conditions. On the contrary, the cesium-based all-inorganic complex lead halides revealed far superior stability and, therefore, provide an impetus for creation of highly efficient and stable perovskite solar cells that can potentially achieve pragmatic operational benchmarks.
L
problem of material hydrolysis or photo-oxidation can be solved using appropriate encapsulation isolating the active layer of the perovskite solar cells from the aggressive environment.10,11 Under realistic operational conditions, solar cells face a number of other stress factors such as field-induced degradation of the active layer materials,12,13 elevated temperatures leading to thermal aging effects,14,15 and, finally, photon flux inducing photochemical damage of the material.16,17 We want to emphasize that the aforementioned degradation processes have intrinsic nature and can hardly be prevented by using encapsulation or any other technological tricks. Therefore, one has to make sure that the perovskite films are intrinsically stable under the real solar cell operation conditions before considering their commercialization and massive practical application in the PV industry. In this Letter, we present a systematic study of the intrinsic thermal and photochemical stability of a series of lead halide based perovskite materials in order to compare their behavior and estimate suitability for practical application in emerging perovskite PVs. To investigate the thermal and photochemical
ead halide based perovskites APbX3 (where A is a univalent organic or inorganic cation and X is Br or I) have recently accomplished a revolution in the field of optoelectronics: solution processed solar cells, light-emitting diodes, photodetectors, and even X-ray detectors with exciting characteristics were demonstrated.1−4 The best laboratory prototypes of the perovskite solar cells have demonstrated certified power conversion efficiencies exceeding 22%.5 Therefore, they represent now the only photovoltaic (PV) technology that surpassed the 20% efficiency barrier and still has not entered the PV market. Unfortunately, practical application of the perovskite solar cells is limited severely by their short operation lifetimes. The device stability is influenced to some extent by a rather high sensitivity of the hybrid lead halide based perovskites (e.g., MAPbI3 or FAPbI3, where MA and FA are the methylammonium and formamidinium cations, respectively) toward moisture and oxygen. Several research groups reported the facile hydration of MAPbI3, affording some crystal hydrates first with a different stoichiometric composition and, finally, with the formation of the parent PbI2 along with a number of other degradation byproducts.6−8 More recently, it has been found that the MAPbI3 films undergo facile photobleaching even in dry air, which suggests photochemical oxidation of the iodide species.9 These reports on the poor extrinsic stability of the hybrid perovskite films deserve particular attention. Nevertheless, the © 2017 American Chemical Society
Received: December 23, 2016 Accepted: February 21, 2017 Published: February 21, 2017 1211
DOI: 10.1021/acs.jpclett.6b03026 J. Phys. Chem. Lett. 2017, 8, 1211−1218
Letter
The Journal of Physical Chemistry Letters aging of perovskite films, we have designed special experimental setups. While the description of the setups and experimental procedures are given in Supporting Information (SI), we want to outline that all of the experiments and measurements were performed under an anoxic inert atmosphere inside of a dedicated glovebox that is not used for other purposes (e.g., film preparation). The sample temperature was adjusted during the photochemical aging experiments by using specially designed sample stages with heating and cooling capabilities. Performing these aging tests under well-controlled conditions was crucially important for achieving reliable and reproducible results. It is known that the temperature of operating solar panels can easily reach 90 °C and even go beyond this value in sunny areas with a high average solar exposure (e.g., Negev desert in Israel). The solar cell active layer materials must sustain such elevated temperatures without any noticeable degradation. Therefore, while testing the thermal stability of the perovskites, we kept their thin films deposited on glass or ITO substrates at 90 °C for 20 h in the dark. Figure 1a−d shows that all investigated hybrid perovskite materials (i.e., ones containing organic cations) undergo dramatic thermal degradation regardless of their chemical composition. This degradation is evidenced, first, by the strong changes in the absorption spectra of the films. One can notice the disappearance or a strong bleaching of the “perovskite” bands at longer wavelengths (600−800 nm) and emergence of new bands (Figure 1a−d) (e.g., spectra characteristic of PbI2 for all systems except for MAPbBr3, where PbBr2 is expected to be formed). In addition, strong evolution of the film structure was revealed by scanning electron microscopy (SEM) and Kelvin probe force microscopy (KPFM). In particular, the films lose their homogeneous structure and reveal more complex morphology (similar to thin-film-based dewetting processes), suggesting the presence of at least two distinct phases (Figures S1−S5, SI). It is also notable that the average surface potentials measured by KPFM of the thermally annealed samples of MAPbI3, MAPbBr3 and MAPbI2.7Br0.3 are shifted to negative values as compared to that of the fresh non-aged films (Figure S6, SI). These dramatic shifts in surface potential relate to changes in both the phase and chemical composition of the materials. Collectively, the obtained results suggested that all hybrid perovskites undergo a substantial thermal decomposition. EDX analysis of the MAPbX3 films demonstrates that the X/ Pb atomic ratio (X = Br, I, I + Br, I + Cl) decreases from ∼3 to
