Photoinduced Phase Segregation Research Articles

Mechanism of Photoinduced Phase Segregation in Mixed-Halide Perovskite Microplatelets and Its Application in Micropatterning.

Photoinduced phase segregation (PPS) is considered as a dominant factor that greatly deteriorates the performances of mixed-halide perovskite devices. However, the mechanism of PPS is still under fierce debate. Herein, CsPb(Brx/Cl1-x)3 microplatelets (MPs) with homogeneous and heterogeneous surfaces are obtained by controlling the growth conditions. Under continuous irradiation, a new photoluminescence (PL) band at 516 nm gradually appears in the heterogeneous MPs, accompanied with the decreased emission of the mixed phase at 480 nm, revealing the occurrence of PPS, while the photoirradiation only leads to slight PL dimming without PPS in the homogeneous MPs. The direct correlation between PPS and the structural heterogeneity indicates that the localized electric field-induced drift (LEFD) of halide ions/carriers is responsible for the PPS. In situ microfluorescence images evidence that the migration of halide ions is directed by the structural heterogeneity-induced localized electric field. Our refined model not only consolidates that PPS can be suppressed by eliminating the defects but also reveals that PPS can be directed by the distribution of defects. Therefore, a fluorescence micropatterning technique is developed based on PPS.

Improved photostability of metal halide perovskites by microstructure modulation for photovoltaic application

Despite the excellent optoelectronic properties of metal halide perovskites in a photovoltaic device, they suffer from serious and poor photostability in real-scenario operating conditions during test and storage. As common, perovskite undergoes complex variation under successive light radiation, the most conspicuous effect is the photo-induced ion migration and phase segregation, which happens the quickest, dramatically influences build-in field, and hence remarkable poor photostability of photovoltaic devices. Herein, we adopted thiourea as an intermediate regulator to slow down the nucleation process of perovskite and form larger grain sizes, thus, fewer grain boundaries, namely fewer migration channels, decrease the ion migration possibility under illuminance and strengthen the photostability of perovskite films. Experiment results showed that such a strategy improves device reproducibility and photostability under continuous scanning and prolongs device lifetime stored at air condition. • Thiourea was used as an intermediate regulator for perovskites. • Thiourea could slow down the nucleation process of perovskites. • The ion migration possibility was suppressed. • Thiourea could improve the device reproducibility and photostability.

Suppressing photoinduced phase segregation in mixed halide perovskites

Suppressing photoinduced phase segregation in mixed halide perovskites

Suppression of Photoinduced Phase Segregation in Mixed-Halide Perovskite Nanocrystals for Stable Light-Emitting Diodes.

Halide segregation is a critical bottleneck that hampers the application of mixed-halide perovskite nanocrystals (NCs) in both electroluminescent and down-conversion red-light-emitting diodes. Herein, we report a strategy that combines precursor and surface engineering to obtain pure-red-emitting (peaked at 624 nm) NCs with a photoluminescence quantum yield of up to 92% and strongly suppresses the halide segregation of mixed-halide NCs under light irradiation. Red-light-emitting diodes (LED) using these mixed-halide NCs as phosphors exhibit color-stable emission with a negligible peak shift and spectral broadening during operation over 240 min. By contrast, a dramatic peak shift and spectral broadening were observed after 10 min of operation in LEDs based on mixed-halide NCs synthesized by a traditional method. Our strategy is critical to achieving photo- and band-gap-stable mixed-halide perovskite NCs for a variety of optoelectronic applications such as micro-LEDs.

Role of electrochemical reactions in the degradation of formamidinium lead halide hybrid perovskite quantum dots.

Organic-inorganic hybrid perovskites are widely utilized in solar driven chemistry such as photocatalysis, hydrogen evolution, and oxygen reduction. Hybrid perovskites contain various components with high polarity and/or charge values, which undergo transformations due to ion exchange, photoinduced phase segregation, or ion migration. These variable characteristics make perovskites "soft materials". Meanwhile, optoelectronic devices often operate under electrochemical reactions in the presence of an electrical field. To examine the effect of this field on the material/photophysical properties of hybrid perovskites, hybrid FAPbBr3 (FA+: CH(NH2)2+) perovskite quantum dots (PQDs) were synthesized. In this study, we report the spectroelectrochemical investigation of the hybrid FAPbBr3 PQDs to understand the electrochemical stability and degradation process. We also found that the electrochemical condition played an important role in inducing defect-mediated oxidation/reduction reactions, changing the photophysical properties of hybrid PQDs, and causing their irreversible transformations to various lead halide plumbate complexes. These findings can help develop a strategy for enhancing the operational performance of PQDs for the solar driven chemistry.

Regulating synthesis and photochromic behavior via interfacial Eu3+/Eu2+-Pb0/Pb2+ redox of the CsPbCl1.5Br1.5@ Ca0.9Eu0.1MoO4 porous composites

Halogen vacancies are regarded to play a vital role in photo-induced phase segregation and the resulting switchable emission colors in the soft mixed-halide perovskites; however, its control strategy via the balanced Pb0 defects remains a big challenge. The research reports the regulation of synthesis and photochromic behavior via interfacial Eu3+/Eu2+-Pb0/Pb2+ redox in composites of porous Ca0.9Eu0.1MoO4 and nominal mixed-halide perovskite CsPbCl1.5Br1.5. The composite takes full advantage of Eu3+ ions with the concerns of its luminescence and variable valences. It provides an additional emission color besides the halide perovskite, manipulates the Pb0 defects and the resulting Br-rich domain via interfacial redox reaction in the composites. The more contents of surfaced Eu3+ caused by substituting the unequivalent Ca2+ ions and the high volume-to-surface ratio of the porous Ca0.9Eu0.1MoO4 guarantees the interfacial access for the Eu3+ and the halides. The research provides some perspectives on the regulation of ionic valence and photoluminescence of halide perovskites with the use of lanthanide ions. The composites may find potential applications in the anti-counterfeiting field.

Beyond the Phase Segregation: Probing the Irreversible Phase Reconstruction of Mixed-Halide Perovskites.

Mixed‐halide perovskites can undergo a photoinduced phase segregation. Even though many reports have claimed that such a phase segregation process is reversible, what happens after phase segregation and its impact on the performance of perovskite‐based devices are still open questions. Here, the phase transformation of MAPb(I1− x Br x )3 after phase segregation and probe an irreversible phase reconstruction of MAPbBr3 is investigated. The photoluminescence imaging microscopy technique is introduced to in situ record the whole process. It is proposed that the type‐I band alignment of segregated I‐rich and Br‐rich domains can enhance the emission of the I‐rich domains by suppressing the nonradiative recombination channels. At the same time, the charge injection from Br‐rich to I‐rich domains drives the expulsion of iodide from the lattice, and thus triggers the reconstruction of MAPbBr3. The work highlights the significance of ion movements in mixed‐halide perovskites and provides new perspectives to understand the property evolution.

Recent Advances and Perspectives of Photostability for Halide Perovskite Solar Cells

AbstractHalide perovskite solar cells (PSCs) have attracted much attention in the next generation of photovoltaic devices due to their low material costs, simple preparation process, and excellent power conversion efficiency (PCE). However, the poor photostability severely limits their commercial application. In this review, the degradation mechanism of PSCs under different light conditions, and various recovery mechanisms are discussed in detail, including i) the degradation of hybrid perovskite under visible light and ultraviolet (UV) light; ii) the degradation of inorganic perovskite under the mixed conditions of oxygen, moisture, and light; iii) the superoxide promoted degradation of perovskite under the combined action of photo‐oxygen; iv) photoinduced ion migration and phase segregation in mixed halide perovskites. Moreover, recent approaches to improve the photostability of PSCs are further summarized and discussed. Among them, the interface modification, the structural optimization of perovskite layer, the material selection of carrier transport layers, and the associated encapsulation technology of PSCs are essential to the photostability of PSCs. A perspective and outlook toward more photo‐stable PSCs are further given.

Short-Range Migration of A-Site Cations Inhibit Photoinduced Phase Segregation in FAxMAyCs1–x–yPbI3–zBrz Single Crystals

Mixed halide perovskites, particularly for mixed iodine (I) and bromine (Br), are promising semiconductor materials for multi-junction solar cells due to their band gap modulations; however numerous research studies have shown that halides undergo ion migration under light excitation, resulting in undesired phase segregation, which severely affects the device long-term stability. In high-Br-content (40%) mixed halide perovskites, I-rich and Br-rich regions simultaneously come into being under illuminations, whereas in a low Br content (<10%), the photoinduced phase segregation process is still unclear until now. On the other hand, the severity of phase segregation relies on defect density, especially prone to occur at grain boundaries in thin films. Here, we investigated the regulation of the phase segregation process by A-site cations in FAxMAyCs1–x–yPbI3–zBrz single crystals (SCs) under illuminations that evade disturbance of grain boundaries. It reveals that the introduction of MA, Cs, and Br into FAxMAyCs1–x–yPbI3–zBrz SCs leads to crystal lattice distortion, which confins the long-range migration of ions, and only the short-range migration at a local range occurs under illuminations. This short-range migration further affects the directional polarization of MA and brings lattice relaxation, which is helpful for eliminating local stress imbalance and shallow level traps, ultimately enhancing the stability under illumination.

Suppressed light-induced phase transition of CsPbBr2I: Strategies, progress and applications in the photovoltaic field

The rapid rise in the power conversion efficiency (PCE) of CsPbBr2I-based perovskite solar cells (PSCs), from 4.7% in 2016 to 11.08% in 2020, render it a promising material for use in photovoltaic devices. However, the phase stability and current hysteresis caused by photo-induced phase segregation in CsPbBr2I represent major obstacles to further improvements in the PCE for such devices. In this review, we describe the basic structure and optical properties of CsPbBr2I, and systematically elaborate on the mechanism of the phase transition. We then discuss the strategies in progress to suppress phase transition in CsPbBr2I, and their potential application in the photovoltaic field. Finally, challenges and application prospects for CsPbBr2I PSCs are summarized in the final section of this article.

Spacer Cations Dictate Photoinduced Phase Segregation in 2D Mixed Halide Perovskites

Spacer Cations Dictate Photoinduced Phase Segregation in 2D Mixed Halide Perovskites

Halide Ion Mobility of Mixed Halide Perovskites in Light and Dark

The intrinsic ionic defects, specifically halide ion vacancies, often dictate the mobility of halide species within the perovskite film during the operation of solar cells. Of particular interest is the halide ion mobility in metal halide perovskites, which plays an important role in determining the performance of perovskite solar cells. Photoinduced phase segregation seen in mixed halide perovskite films under steady state irradiation offers a convenient way to visualize halide ion segregation. Interestingly, upon storage in dark, the process is reversed and the original mixed halide composition gets restored. Whereas entropy of mixing explain the thermally activated mixing of halide ions to yield mixed halide perovskite, the opposite trend observed during photoirradiation remains an intriguing phenomenon. The threshold energy of incident light to observe halide segregation increases with increasing temperature. The diffusion of these halide species, which is tracked through changes in the absorption spectra at different temperatures, offers a direct measurement of thermally activated halide diffusion in perovskite films. The thermally activated halide exchange shows the challenges of stabilizing metal halide perovskites with varying bandgap in tandem solar cells.

Linking Phase Segregation and Photovoltaic Performance of Mixed-Halide Perovskite Films through Grain Size Engineering

Mixed-halide perovskites are attractive candidates as wide-bandgap absorber layers in tandem solar cells. However, photoinduced phase segregation leads to the formation of Br-rich and I-rich domain...

Stabilizing Mixed Halide Lead Perovskites against Photoinduced Phase Segregation by A-Site Cation Alloying

Mixed halide perovskites as top absorbers with relatively high bandgaps (Eg > 1.70 eV) are essential for tandem and multijunction solar cells. Light-induced phase segregation of mixed halide perovs...

Suppressing the Phase Segregation with Potassium for Highly Efficient and Photostable Inverted Wide-Band Gap Halide Perovskite Solar Cells.

Wide-band gap (WBG) mixed-halide perovskites have drawn much attention because of their excellent optoelectronic properties and the potential to be deployed in tandem solar cells. Nevertheless, the bromine incorporation inevitably leads to photoinduced phase segregation in WBG mixed-halide perovskites. Herein, potassium is used to effectively suppress photoinduced phase segregation, which is visualized with confocal photoluminescence microscopy imaging. Strikingly, the potassium passivation not only inhibits the formation of the narrow-band gap subphase but also enhances the crystallinity of the WBG mixed-halide perovskite. In addition, the potassium-passivated WBG perovskite exhibits lower defect density, longer charge carrier lifetime, and better photostability. As a result, the optimized KI (2 mol %)-passivated WBG perovskite solar cells (PSCs) deliver a champion power conversion efficiency of 18.3% with negligible hysteresis. They maintain 98% of their initial efficiency after 400 h under 100 mW·cm-2 white light illumination in nitrogen.

Photoinduced Phase Segregation in Mixed Halide Perovskites: Thermodynamic and Kinetic Aspects of Cl–Br Segregation

AbstractSuppression of halide ion mobility remains a key issue in defining the device performances and stability of perovskite solar cells. The halide ion migration is facilitated by the lattice‐distortion‐mediated halide vacancy and hence dictated by the polarizability (or rigidity) of the halide sublattice and framework. Photoinduced halide ion segregation and dark recovery of MAPb(Cl0.5Br0.5)3 films are now probed. The temperature dependence of the rate constants of segregation and dark remixing allow to determine the activation energy barriers (Ea) for photosegregation (38 kJ mol−1) and dark recovery (42 kJ mol−1). A comparison of the segregation activation energy (Ea) and excitation intensity threshold between MAPb(Cl0.5Br0.5)3 and MAPb(Br0.5I0.5)3 films reflects that the presence of Cl stabilizes mixed halide composition. The thermodynamic stabilization is attributed to the formation of more rigid [PbX6]4− frameworks with an increased barrier for halide ion migration. The thermodynamic rationale behind the intriguing halide ion mobility offers a fundamental insight into the role of Cl and design principle of perovskite solar cells with improved efficiency and long‐term stability.

Minimized surface deficiency on wide-bandgap perovskite for efficient indoor photovoltaics

Abstract Wide-bandgap perovskite solar cells (PVSCs) possess significant potential in providing reliable power sources for applications in the Internet of Things (IoT) ecosystem under indoor light illumination. However, the wide-bandgap PVSCs usually suffer from photo-induced phase segregation and non-radiative energy loss caused by the Shockley-Read-Hall (SRH) type trap-assisted recombination at the interfaces in the devices. To address these issues, a simple strategy by applying phenethylammonium halides to reduce the energy loss and suppress the phase segregation of wide-bandgap PVSCs is developed. The devices incorporated with phenethylammonium chloride (PEACl) is revealed to achieve a high open-circuit voltage (VOC) of 1.26 V, leading to a merit power conversion efficiency (PCE) of 18.3%, which is the best performance among the inverted wide-bandgap PVSCs (~1.75 eV) under one sun illumination. Meanwhile, the photovoltaic performance of the device is also significantly enhanced, especially under a white light-emitting diode (LED) with an illumination of 1000 lux, showing a PCE of 35.6% with a high VOC of 1.08 V. Impressively, the device delivers a minimum energy loss of 670 meV, which is among the smallest value reported for perovskite-based indoor photovoltaics.

Photoinduced Phase Segregation Leading to Evident Open-Circuit Voltage Loss in Efficient Inorganic CsPbIBr2 Solar Cells.

The photoinduced phase segregation (PIPS) of mixed-halide perovskites (MHPs), due to halogen migration, has reaped considerable attention for its retroaction on film photostability and photovoltaic output. Nevertheless, the original mechanism is still unclear. Herein, taking the representative CsPbIBr2 material as an example, a confocal laser scanning microscope (CLSM) technique was adopted to track the PIPS and dark recovery procedures. Besides the aggregation of iodide-rich (I-rich) domains at grain boundaries (GBs), some sporadic iodide "islands" with a swifter light response also appear throughout the polycrystalline films. It illustrates again that GBs are not essential for iodide aggregation. Furthermore, the iodide "islands" have substantial influence on a device's open-circuit voltage (Voc), resulting in an obvious plunge in the first tens of seconds. Results reveal the internal reason for the failure to reach the larger Voc outputs expected from wide-bandgap perovskites. Importantly, this finding can help promote the exploration of an efficient means to stabilize MHPs.

Inhibition of Phase Segregation in Cesium Lead Mixed-Halide Perovskites by B-Site Doping.

SummaryThe emergence of all-inorganic halide perovskites has shown great potential in photovoltaic and optoelectronic devices. However, the photo-induced phase segregation in lead mixed-halide perovskites has severely limited their application. Herein, by real-time monitoring the photoluminescence (PL) spectra of metal mixed-halide perovskites under light irradiation, we found that the photo-induced phase transition can be significantly inhibited by B-site doping. For pristine mixed-halide perovskites, an intermediate phase of CsPbBrxI3-x can only be stabilized under low excitation power. After introducing Sn2+ ions, such intermediate phase can be stabilized in nitrogen atmosphere under high excitation power and phase segregation can be started after the exposure in oxygen due to oxidization of Sn2+. Replacing Sn2+ by Mn2+ can further improve the intermediate phase's tolerance to oxygen proving that B-site doping in perovskites structure by Sn2+ or Mn2+ could effectively minimize the light-induced phase segregation and promote them to serve as promising candidates in photovoltaic and light-emitting devices.

How Chloride Suppresses Photoinduced Phase Segregation in Mixed Halide Perovskites

Halide ion mobility in metal halide perovskites plays an important role in dictating the overall device performance and long-term stability of perovskite solar cells. Alloying with chloride (Cl), w...