The effects of doping and coating on degradation kinetics in perovskites

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Highlights

  • A systematic study of the degradation kinetics in hybrid perovskites.

  • Two stability enhancement strategies applied: (i) addition of Cl during synthesis and (ii) use of a protective coating layer.

  • The process of degradation is successfully described by simple exponential kinetics.

  • The addition of chlorine slows down the oxygen diffusion-related degradation by approximately 3 times.

  • The use of a thin (50 nm) Al2O3 protective layer slows down the degradation by at least one order of magnitude.

Abstract

In the last few years, perovskite-based solar cells have seen rapid improvements of their power conversion efficiencies (PCE) that have already reached silicon cell values. The commercial use of perovskite cell technology remains, however, a challenge due to durability issues. In fact, perovskite films when exposed to moisture and oxygen undergo degradation within a couple of hours or days. In order to address these problems, several strategies have been developed in recent years, including surface passivation and coating. Herein, we analyze and compare two perovskite stability improvement approaches, each aimed at the reduction of the oxygen diffusion, one by adding chlorine to perovskite precursors in order to block iodine vacancy sites, and the other by coating the perovskite layers with protective Al2O3 films using Atomic Layer Deposition (ALD). The study provides an experimental verification of the degradation mechanism linking it to oxygen diffusion into the perovskite films. As shown by X-ray photoelectron spectroscopy (XPS) analysis, the addition of chlorine into the iodine-based perovskite film slows down the oxygen diffusion, while X-ray diffraction (XRD) measurements show improved stability of such iodide perovskite with chlorine addition. Characteristic times for oxygen diffusion into subsurface region as well as for degradation process of film volume are provided and discussed. XRD measurements are also used to show that an additional significant stability improvement can also be achieved when the films are coated with a protective ALD Al2O3 layer.

Introduction

In recent years, organic – inorganic (hybrid) perovskites, such as CH3NH3PbX3, known as methylammonium lead halides (MAPbX3, where X = I, Br, Cl), have gained tremendous attention as highly efficient absorber materials for next-generation solar cells. MAPbI3 is a member of this family, with well-adjusted bandgap to the solar spectrum radiation. Perovskite based solar cells have undergone exceptionally fast advancements of power conversion efficiencies (PCE) in the last few years. Currently, such cells have reached 25.2% of PCE listed in the efficiency chart published by National Renewable Energy Laboratory [1]. Furthermore, it has been shown that the binding energy of excitons in MAPbI3 is lower than thermal energy at room temperature (a few meV compared to 25 meV) [2]. Therefore, light excitation leads to the formation of free carriers in this material. The typically long diffusion length and lifetime of carriers in perovskites allows for the migration of both electrons and holes to electrodes without significant losses due to non-radiative recombination [3,4]. Moreover, the possibility of using low temperature ink-jet printing for the preparation of perovskite solar cells [5] could potentially lower the price of this technology at commercial scale in the future. However, despite all these advantages, serious questions concerning stability of perovskites remain to be addressed for their commercial application and cost-competitive deployment at a large scale. In particular, it is well known that exposure of perovskite films to moisture and oxygen causes their degradation within a couple of hours or days [6]. This widely recognized problem of poor stability of perovskites requires that an in-depth understanding of the chemistry and physics behind the degradation mechanism is developed and appropriate steps to prevent the degradation process are undertaken. Although, a number of significant advances have already been made to achieve this goal.

Recently, it has been demonstrated that exposure of MAPbI3 layers to light and oxygen leads to the formation of superoxide (O2) species [7]. These reactive O2 species are able to deprotonate the methylammonium cation (MA+) of photo-excited perovskite (MAPbI3*), leading to the material decomposition into PbI2, water, methylamine and iodine [8], according to the following reaction:4 MAPbI3* + O2 → 4PbI2 + 2I2 + 2H2O + 4CH3NH2

To investigate the perovskites degradation process, formation energy of superoxide from O2 adsorbed at vacancy sites in methylammonium lead iodide structure has been calculated and the highest energy gain has been found for the O2 located at an iodine vacancy VI [8] (following O2 reduction sites has been taken into account: face site neighboring four iodide ions, and iodide, lead and methylammonium vacancies). According to Aristidou et al. [8], two main findings emerge. Firstly, superoxide formation is energetically favorable due to a direct electron transfer from perovskite to oxygen. Secondly, formation energies of O2 species indicate that vacant iodine sites are the preferred location for the reduction process. Therefore, it seems that presence of iodine vacancies is crucial for the perovskite degradation.

Furthermore, chlorine (Cl)-containing perovskite (MAPbI3(Cl), called mixed perovskite) has attracted a lot of interest of the photovoltaics community due to its improved crystal structure, charge transport and stability, in comparison with pure iodine material [9]. Recent Density Functional Theory (DFT) calculations by Mosconi et al. have suggested that Cl atoms in MAPbI3(Cl) preferentially occupy the apical positions of PbI6 octahedra [10,11], substituting iodine atoms. Combined results of those two studies, showing preferred locations of superoxide species and positions of Cl atoms, lead to a conclusion that the presence of chlorine improves not only the film quality and optoelectronic properties, but it also boosts the stability of mixed perovskites by reducing the concentration of iodine vacancy sites, and therefore suppressing the diffusion of oxygen into the perovskite layers. An alternative and a promising method of perovskite stabilization is covering the layer with Al2O3 deposited by Atomic Layer Deposition (ALD) technique [12,13]. Such coating provides a robust isolation from the influence of environment and blocks the diffusion of water and oxygen, two main factors responsible for the degradation of perovskite layers.

Here, in this study, we analyze the impact and compare the effects of two factors on perovskite stability: (1) addition of chlorine to perovskite precursors which blocks iodine vacancy sites and (2) encapsulation by ALD Al2O3 coating layers to prevent water and oxygen diffusion.

Section snippets

Material preparation

Lead halide hybrid perovskites were prepared using different precursor solutions based on commercially available compounds, in particular, lead (II) iodide (PbI2) (Sigma Aldrich), lead (II) chloride (PbCl2) (Sigma Aldrich), and methylammonium iodide (MAI) (Ossila).

Methylammonium lead iodide (MAPbI3) perovskite was synthesized using solutions of 600 mg mL-1 with equimolar mixtures of methylammonium iodide (MAI) and lead (II) iodide (PbI2) in DMF (N,N-dimethylformamide).

Methylammonium lead iodide

Results and Discussion

XRD patterns of measured samples are presented in Fig. 1a and b; Fig. 1c shows the reference X-ray powder diffraction pattern of MAPbI3 in the tetragonal structure, as calculated in Ref. 15. By comparison of XRD patterns in Fig. 1a and c, one can identify the MAPbI3 film as MAPbI3 crystallized in the tetragonal perovskite structure (in agreement with the MAPbI3 JCPDS data) [15]. The observation of (211) reflection indicates tetragonal phase [16]. The presence of numerous peaks originating from

Conclusions

In this work, films of methylammonium lead iodide perovskite and mixed perovskite with chlorine were prepared and analyzed in terms of their structure and stability. XRD analysis performed on these samples confirmed that all perovskite films were synthesized successfully and that they crystallized in the tetragonal structure. The obtained films of mixed perovskite were highly textured and displayed only strong diffraction peaks from the (1 1 0) and (2 2 0) lattice planes, whereas the films

Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding sources

The authors would like to acknowledge the support of the Conn Center for Renewable Energy Research and of the TECHMATSTRATEG Program (TECHMATSTRATEG1/347431/14/NCBR/2018) funded by The National Centre for Research and Development, Poland. The authors would like also to thank Dr. Dominika A. Buchberger for helping with the acquisition of SEM and EDS data.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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