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Metal halide perovskites (MHPs) has splendid optoelectronic properties and ease of processing, enabling efficiently fabricating high-performance photovoltaic devices with low-cost. MHPs are easy preparation and processing, but also face inherent instability issues, such as ion migration, halide segregation, phase transition and degradation.
- perovskite
- halide segregation
1. Introduction
Metal halide perovskites (MHPs), as a class of unique semiconductor materials, have sparked unprecedentedly intense research activities globally over the past decade. This is mainly due to their splendid optoelectronic properties and ease of processing, enabling efficiently fabricating high-performance photovoltaic devices with low-cost. Tremendous advances of perovskite solar cells (PSCs) have been achieved in terms of power conversion efficiencies (PCEs), large-area fabrication and device stability since their emergence in 2009, especially after 2012, making PSCs become significant candidates for next-generation photovoltaic technologies [1,2,3,4,5,6,7,8,9,10,11,12]. In addition, MHPs are attractive and promising for other applications beyond photovoltaics, including light-emitting diodes (LEDs) [13,14,15], photocatalysis [16,17], photodetectors [18,19], lasers [20,21], transistors [22,23], thermoelectrics [24], and even nonlinear optics [25], spintronics [26] and so forth, demonstrating MHPs are indeed a type of versatile and fascinating materials.
In general, MHPs feature a chemical formula of ABX3, where A refers to methylammonium, formamidinium or cesium cation (MA+ = CH3NH3+, FA+ = HC(NH2)2+, or Cs+), B is lead or tin cation (Pb2+ or Sn2+), X denotes halide anion (I−, Br− or Cl−). The BX6 octahedra with corner-sharing form a three-dimensional framework, and A-site cations are centrally located in the cuboctahedral cavities [27,28]. The unique crystal structure, moderate Pb-I bonding energy, and weak electrostatic interaction together with hydrogen bonding between the organic A cations and halogen anions coherently dictate MHPs featuring soft nature [27]. This character makes MHPs be of easy preparation and processing, but also face inherent instability issues, such as ion migration, halide segregation, phase transition and degradation [29,30,31,32,33].
As is known, bandgap tunability is one of the most attractive characteristics for MHPs (from about 1.2 to 3.0 eV) [34,35], which can be readily achieved through compositional engineering, offering immense opportunities to fabricate perovskite based multijunction photovoltaics and multicoloured LEDs by meticulously regulating the energy bandgap to the target values of the perovskites. However, an apparitional phenomenon existing in MHPs, called halide segregation, makes such anticipations far from favorableness [31]. The halide segregation occurs when MHPs exposed to photoirradiation or subjected to charge carrier injection. As a result, coessential aggregation of iodide (I) and bromide (Br) occurs, leading to the formation of I-rich and Br-rich regions. The I-rich domains feature narrower energy bandgaps, acting as radiative recombination centers due to carriers funneling from adjacent Br-rich regions, thus giving rise to redshifted photoluminescence (PL) emission. What is even more peculiar is that when the perovskite films are kept in the dark on a timescale of from minutes to hours, remixing processes take place and the corresponding films could recover to their original states. Since photoinduced halide segregation in MHPs was first reported by Hoke et al. in 2015 [31], numerous studies have been devoting to this subject, striving to uncover the microscopic mechanism and reveal what is the driving force behind the phenomenon or seek effective strategies to suppress it.
2. Visualization of Halide Segregation
Several years ago, Hoke and coworkers first noticed that the PL spectra of MAPb(BrxI1−x)3 MHPs could undergo a red-shift when the perovskite films were exposed to illumination in less than one minute, which, interestingly, would recover to their original states when the films were kept in the dark for a few minutes [31]. Inspired by this behavior, they discovered the light-induced halide segregation phenomenon in MHPs. Since then, significant research activities have been focusing on this subject. During this process, recording the variation of steady-state PL spectra, ultraviolet–visible (UV–vis) absorption spectra, and transient absorption spectra are simple and effective ways to monitor and evaluate the phase segregation. While, in-depth understanding of this phenomenon necessitates locally multimodal microscale imaging of halide segregation. In this section, the advances in visualized investigation of halide demixing in many cases based on different characterization methods are systematically reviewed and discussed.
2.1. Photoluminescence Mapping
Photoluminescence (PL) measurements represent the most extensively adopted method to characterize the charge carrier recombination behavior of perovskites. A photoexcited electron from the excited state returns to ground state through radiative pathway, leading to a photon emitting. As electron–hole pairs generally thermalize to the band edges before recombination, the wavelength of emission peak directly correlates with the bandgap of the perovskite material. Thus, detecting and analyzing the emitted photons provides insight into the photoelectric quality and band gap of the perovskite in terms of emission intensity and wavelength [46]. Furthermore, spatially resolved PL mapping enabled by PL microscopy technique can give significant information of spatial variations in the photogenerated carrier recombination dynamics, which can be used to determine the film quality, heterogeneity, trap state distribution, ion migration, local phase transformation and so forth [29,47,48,49,50,51,52,53,54,55]. PL microscopy generally includes confocal and widefield types. The former can deliver PL images with much higher spatial resolution than the latter, owing to adopting a point light source and single point detector. For local visualization of halide segregation in MHPs under external stimuli, PL mapping, featuring noninvasive, nondestructive characteristics, desirable operational flexibility, and relative ease of access, is naturally becoming the most widely used characterization method.
2.2. Cathodoluminescence Imaging
Similar to PL mapping, cathodoluminescence (CL) imaging technique is also an important way to probe local halide segregation at the nanoscale. CL microscopy is generally equipped into scanning electron microscopy (SEM) system, imaging the luminescence of a material induced by electron beam. CL is quite sensitive to the optoelectronic properties of the materials and can achieve high spatial resolution with about 20 nm, compared with a low spatial resolution of around 300 nm given by PL mapping at typical excitation/emission wavelengths due to diffraction limitation [88,89,90]. Simultaneously detecting secondary electrons and fluorescence emission enables to directly correlate microstructure with local carrier radiative recombination characteristics of the perovskites, thus shedding light on the spatial distributions of segregated I-rich phase with nanoscale resolution. While, of particular note is that care must be taken to avoid beam-induced damage during measurement, especially for hybrid perovskites with soft nature. Therefore, significantly reducing acceleration voltage and the probing current, even coupled with low-temperature cooling are required [88,89].
2.3. Transmission Electron Microscopy
Transmission electron microscopy (TEM) is also a powerful and widely used technique to probe the microstructures of materials by detecting the transmitted electrons through the specimen, enabling to achieve a resolution down to sub-angstrom [92]. Thus, the thickness of sample is generally controlled to be in the range of 5 to 100 nm [93], which depends on the elemental composition of the observed material and the acceleration voltage, to guarantee enough electron transparency. Funk et al. [94] have monitored halide segregation behavior in CsPb(Br0.8I0.2)3 perovskite under electron beam irradiation using in situ TEM. The authors hold the viewpoint that light and electron irradiation are equivalent in the aspect of inducing phase separation in various MHPs. They directly prepared the CsPb(BrxI1−x)3 samples on a carbon-coated TEM grid by spin-coating. After five minutes of electron beam irradiation, the abundance maps derived from the HRTEM images showed that the phase in the center of the nanoparticle was converted into CsPbBr3, while the phase close to the edges was in agreement with CsPb(Br0.6I0.4)3, indicating a Br/I substitution process upon electron beam exposure, consistent with previous works using PL and CL imaging techniques as discussed above [76,77,86].
2.4. Energy-Dispersive X-ray Spectroscopy
Energy-dispersive X-ray spectroscopy (EDS or EDX) is a basic and widely used technique to probe elemental composition and distribution, which is a typical accessory equipment integrated into the SEM and TEM systems. When the electron beam is irradiated onto the specimen, detecting and analyzing the characteristic X-rays generated from the atoms using energy dispersive detector allows performing qualitative and quantitative elemental analysis [92]. The constant advance of SEM technique enables a spatial resolution down to one nanometer. While for general EDS, the resolutions are still restricted to about several hundred nanometers or beyond one micrometer for bulk specimens, which depend on material characteristics, sample thickness, acceleration voltage, and so forth. This restriction is due to the scattering of electrons inside the sample, leading to a large tear-drop shaped excitation volume below the surface where the characteristic X-rays are generated [95]. By contrast, the spatial resolution of several nanometers for EDS in TEM is realizable due to the sample size is generally in the order of a few tens to hundred nanometers in thickness. As such, it is impracticable to get insight into the local halide segregation at the nanoscale using SEM-EDS, but in principle, feasible by means of TEM-EDS system.
2.5. Atomic Force Microscopy
Atomic force microscopy (AFM) is an imperative and the most extensively used scanning probe microscopy (SPM) technique that has been well developed for imaging various material surfaces with nanoscale spatial resolution. This technique works based on the interaction forces between the atoms at sample surface and the probe tip, usually made of Si, SiO2, or Si3N4, which is attached to a cantilever [46]. When closing to a material surface, the tip and the cantilever can be deflected by forces such as mechanical contact forces, van der Waals forces, and electrostatic forces. Meanwhile, a displacement sensor measures the deflection of the cantilever with a sensitivity better than 0.1 nanometer by detecting the angle variation of a laser beam reflected from the backside of the cantilever, thus enabling to determine the specimen surface topography with resolutions in the order of a few nanometers [93]. Moreover, some derivative techniques, including kelvin probe force microscopy (KPFM), conductive-AFM (c-AFM), have been exploited. KPFM measures the contact potential difference (CPD) between the material surface and the tip, allowing to map the work function or surface potential [97,98]. While c-AFM method is an electrical model in the SPM family, which is used to study the conductivity and imaging electrical properties of the samples, such as charge transport and charge distribution at the nanoscale. AFM based techniques have been used to unveil variations of the topography, phase contrast, surface potential or photocurrent of the MHPs under illumination or electric field.
This entry is adapted from the peer-reviewed paper 10.3390/electronics11050700
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