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Photoinduced iodide repulsion and halides-demixing in layered perovskites

Mixing halides in metal halide perovskites (MHPs) is an effective approach to adjust MHPs bandgap for applications in tandem solar cells. However, mixed-halide (MH-) MHPs undergo light-induced-phase-segregation (LIPS) under continuous illumination. Therefore, understanding the mechanism of LIPS is necessary for developing stable MH-MHPs. In this work, we investigated LIPS in layered (L) MHPs and discovered a critical role of spacer cations in LIPS. Through probing chemical changes of LIPS, we unveil light-induced-iodide-repulsion and the formation of Br-rich-phase in illuminated regions during LIPS. This discovery also gives insight into LIPS process in three dimensional (3D) MHPs. By further investigating LIPS in 3D MHPs, we reveal that LIPS induces not only the formation of Br-rich and I-rich domains but also an overall change of halide distribution along the film thickness direction, which can affect the electronic energy alignment and consequently MHPs devices performance. Moreover, LIPS is more significant in the bulk due to larger population of photogenerated charge carriers. Overall, this study reveals the chemical mechanism of LIPS in MHPs and its potential effect on device performance, offering insight into understanding LIPS mechanism and improving the stability of MHPs.

Metal halide perovskites (MHPs) community has witnessed a rapid growth in the development of photovoltaics in the past decade, with a power conversion efficiency (PCE) increase from 3.8 % to 25.5 % in single-junction MHPs solar cells. 1 MHPs-based multijunction solar cells are now attracting enormous research interests to further boost the PCE of MHPs solar cells. [2][3][4][5] The possibility of tuning bandgap via adjusting and mixing halides 6 makes MHPs a promising candidate for applications in tandem solar cells with various structures, such as MHPs/MHPs, 7 organic photovoltaics/MHPs, 8 silicon/MHPs, 2,4 and Copper Indium Gallium Selenide (CIGS)/MHPs multijunctions. 9 However, mixed-halide MHPs undergo phase segregation under photoillumination, [10][11][12] which is detrimental to achieving stable photovoltaic devices. Therefore, understanding the process of light-induced phase separation (LIPS) will be a vital step for application of mixed-halide MHPs in stable solar cells.
LIPS in MHPs was discovered by Hoke et al., 12 showing as a bandgap transition and photoluminescence (PL) redshift of mixed-halide MHPs under continuous illumination. Later, extensive researches investigated how LIPS affects optical properties of MHPs, such as emission, absorption, and so on. 10,11,[13][14][15][16] LIPS originates from the formation of separated iodine (I -) rich and bromine (Br -) rich domains. 10,11,[13][14][15][16][17] Therefore, LIPS is a chemical change due to halide migration driven by illumination. However, to date, direct observation of chemical changes due to LIPS in MHPs is missing. This is because LIPS is a reversible process, 10,11,[13][14][15][16] which recovers (remixing of separated I-domain and Br-domain) quickly in several minutes or hours when the samples store in dark condition, leading to challenges in probing the chemical changes directly. For this reason, irreversible LIPS processes will be good model systems to investigate chemical changes associated with LIPS and hence understand the underlying mechanism.
Currently, to improve the stability of MHPs, large spacer cations have been widely used, which lead to the formation of layered (L) MHPs structure and play roles in suppressing ion migration. 18,19 In addition to improve the stability of traditional 3D MHPs, L-MHPs have also established themselves as an excellent material for the application of light-emitting field. In this regard, the emission color of L-MHP can also be controlled via mixing halides, resulting in mixed-halides L-MHP and consequently raising a question regarding the LIPS in mixed-halides L-MHP. However, to date, most research into LIPS mainly focused on 3D-MHPs, [20][21][22][23][24][25][26][27] understanding of LIPS in layered MHPs is currently missing.
In this work, we investigated LIPS in layered MHPs (L-MHPs). We reveal that the spacer cation in L-MHPs plays a crucial role in LIPS, suggesting that adjusting cation is an effective approach to manage LIPS. In contrast to reversible LIPS in 3D-MHPs that recovers quickly under dark, LIPS in L-MHPs is irreversible and lasts for several days. This allows us to directly probe the chemical redistribution due to LIPS, which has not been thoroughly studied yet. Here, we use time-of-flight secondary ion mass spectrometry (ToF-SIMS) to probe the chemical distribution in MHPs. We find that illumination repels iodide ions and induces the formation of Br-rich in the illuminated region. Iodide repulsion is dependent on the light intensity, which results in a variation of halide distribution in both vertical and lateral directions of MHPs films.
We further reveal that the chemical environment (e.g., oxygen penetration into MHPs) also affects LIPS. To comprehensively explore chemical changes due to LIPS in an operando condition, we illuminated a MAPb(Br0.5I0.5)3 (MA: methylammonium) film by a uniform white light LED and probed its chemical changes in both vertical and lateral directions. In addition to the formation of I-rich and Br-rich domains that has been discussed extensively, we also found a significant vertical halide redistribution due to LIPS, which can play critical roles in charge carrier transport, recombination, and extraction in these materials. Moreover, we discovered that LIPS occurs more significantly in the bulk of MHPs film than the surface, which is probably due to a higher density of photogenerated charge carriers in the bulk.

Results and Discussions
We investigated LIPS in L-MHPs with different spacer cations, as shown in Figure 1a investigate LIPS in these L-MHPs. PL is a widely utilized spectroscopy to investigate LIPS phenomena in MHP. 12,16 LIPS shows as a continuous red-shift of PL peak due to the formation of separated Br-rich and I-rich phases; photogenerated carriers in Br-rich phase transfer to I-rich phase before recombining and emitting (as shown in Figure S1). As a result, PL spectra predominantly (or only) show the emission peak of the I-rich phase (that is red-shifted compared to the original emission peak of mixed Br-I phase) indicating chemical changes after LIPS.
In our PL measurement, we used a 375 nm laser with an intensity of 54.7 mW cm -2 to illuminate and excite the L-MHP samples. We observed a significant red-shift in the PL peak A distinct difference between LIPS in L-MHPs (EA-and BA-MHPs) and 3D-MHPs is its reversibility: LIPS in 3D-MHPs is reversible while LIPS in L-MHPs is irreversible. As shown in Figure S6, LIPS induced PL changes are still very stable after keeping the L-MHPs samples under dark condition for 1 hour. It is well known that LIPS is a chemical phenomenon, which is associated with photoinduced ion redistribution. However, owing to the reversibility of LIPS in 3D-MHPs, it is challenging to directly detect the chemical changes due to LIPS. Thus, most previous studies only focused on investigating the changes in optical properties due to LIPS. In contrast, irreversible LIPS in L-MHPs offers us an opportunity to directly detecting chemical changes, allowing us to understand the exact chemical changes and the detailed mechanism of LIPS. Irreversible LIPS in L-MHPs is likely due to the higher activation energy for ion migration in L-MHPs, therefore, automatic ion redistribution under dark condition is inhabited in L-MHPs.
Next, we study the nature of chemical changes in L-MHPs due to LIPS using ToF-SIMS.
We used a rectangular-shape laser spot (375 nm) to illuminate the L-MHPs samples, where the rectangular width is less than the maximum field of view of ToF-SIMS (500 x 500 μm).
Therefore, we can cover both the illuminated and non-illuminated regions in a ToF-SIMS dataset, this allowed us to directly compare the chemical changes associated with illumination. The rectangular-shape illumination is shown in Figure 2a To explore the effect of illumination intensity on chemical changes, we varied the laser intensity and performed chemical characterization after various intensity illumination. According to Beer-Lambert law, we expect the chemical change in the vertical direction to be depthdependent due to the decay of laser intensity inside the materials. Note that the laser used here is not perfectly uniform, the intensity of this laser decays from the middle to the edge of the rectangular due to gaussian profile of the laser intensity within the laser spot. Therefore, depending on whether or not the laser can penetrate the MHP films, the expected illumination conditions and corresponding resultant chemical distributions under strong illumination (can penetrate) and weak illumination (cannot penetrate) are schematically shown in Figure 2b and 2c, respectively. Previous works revealed that laser illumination can expel iodide in MHPs into solvent when the MHP sample was in contact with a solvent. 29 Our PL results show that strong laser illumination can lead to the formation of pure Br-phase (the appearance of PL from pure Br-phase, Figure 1 and Figure S5), also implies an iodide-repulsion effect. Therefore, we speculate that the laser treatment results in an I-deficient region in the illumination region.  Figure 2e) indicates that oxygen in MHP also affects iodide-repulsion. It has been previously revealed that oxygen in MHPs can lead to an up-shift of conduction band but has a negligible effect on valence band. 30 Accordingly, an oxygen-rich condition (which is near the substrate and in illuminated region, as shown in Figure 2i) in EA-MHP will lead to an electronic band bending and results in hole-rich area in the corresponding region, as schematically shown in Figure 2j. It is revealed that the hole carriers have a role in repulsion iodides in MHPs. 29,31 Therefore, we propose that the correlation between oxygen distribution and IDBR region is due to oxygen-induced hole accumulation, which repels iodide. In summary, under illumination, oxygen from substrate penetrates to the EA-MHP and leads to a local hole-rich environment which results in iodide-repulsion (we refer to this phenomenon as oxygen-induced iodiderepulsion). This oxygen-induced iodide-repulsion, together with light-induced iodide-repulsion, results in the abnormal IDBR regions in EA-MHP (Figure 2f-g).
The difference in oxygen distribution in BA-MHP (Figure 2h) and EA-MHP (Figure 2i) also indicates that spacer cations (e.g., BA vs. EA) effect on oxygen penetration in MHPs, which is important to MHP performance and stability. Therefore, the role of spacer cation in ion migration and stability of MHPs is worth further exploring in the future.
It has been suggested that the formation of Br-rich and I-rich domains (as schematically shown in Figure 3a) due to LIPS detrimentally affects the photovoltaic performance of MHPs by trapping charge carriers in I-domains. 32 Our chemical investigations show that LIPS can also induce a chemical change along vertical direction of MHP films (as schematically shown in Figure 3a), which will significantly affect charge carrier transport and extraction. In LIPS studies above (Figure 1-2), we illuminated MHP samples by a laser, which is not a uniform illumination.
Thus, an IDBR region corresponding to the illuminated region formed due to light-induced iodide-repulsion. However, in an operando condition, MHP films should be under a uniform illumination (that is, the whole MHP film is under illumination). Therefore, we next systematically investigate light-induced chemical distribution change in MHP under a uniform illumination in order to understand light-induced chemical changes in operando devices. We performance. This is consistent with recent literature; 33 which revealed that an improper energy alignment due to LIPS is are responsible for the photovoltaic performance loss. 33 Here, our results on vertical redistribution of halides elucidates the chemical origin of this improper energy alignment. The energy alignment related to halide redistribution will affect charge carrier transport and consequently photovoltaic performance of MHPs. 33 In the depth profiles in Figure 3b, we explicitly see that light-induced changes in halide distribution mainly located at the bulk of MHP film, suggesting that MHP's bulk undergoes most significant LIPS. Therefore, we further analyzed the lateral halide segregation in the bulk and surface of MHP separately. As shown in Figure S7a, we show the lateral distribution of Br and I near surface and bulk of the MHP film. Consistent with our speculation, a more significant lightinduced halide segregation (exhibits as the formation of Br-rich and I-rich domains) is seen in bulk ( Figure S7c) than near surface ( Figure S7b). Since it has been revealed that halide migration and segregation are associated with charge carriers, 29, 31 the more significant LIPS in bulk is probably because of a larger population of photogenerated charge carriers at bulk than the surface.

Conclusions
In conclusion, we investigated LIPS in L-MHPs, unveiling that spacer cation plays a crucial effect on LIPS. We observed irreversible LIPS in L-MHPs; this irreversible LIPS offers an opportunity to directly investigate chemical redistribution due to LIPS, which is challenging in 3D MHPs that show quickly reversible LIPS behavior. Our chemical characterization of LIPS in L-MHPs revealed an iodide-repulsion effect and consequently the formation of pure-Br-phase or extremely Br-rich phase in the illuminated region, where iodide repulsion is dependent on light intensity and chemical environment (e.g., oxygen penetration into MHPs affect iodide repulsion process). In order to understand chemical redistribution due to LIPS in an operando condition, we illuminated a MAPb(Br0.5I0.5)3 film using a uniform white LED light. In addition to the formation of I-rich and Br-rich domains (that is a generally believed mechanism of LIPS), we discovered that LIPS also leads to a significant vertical chemical redistribution, which is critical to charge carrier transport and extraction in MHPs devices. Moreover, we discovered that LIPS is more significant in the bulk of the MHP film, which is probably due to a large population of photogenerated charge carriers in the bulk. Overall, by directly characterizing chemical changes due to LIPS, we for the first time revealed the chemical changes associated with LIPS in MHPs, which is the key for understanding LIPS mechanism and developing stable mixed-halides MHPs.
Our studies about the role of spacer cation in LIPS and extrinsic ion penetration also offers insight into improving the stability of MHPs.   Br and I distribution due to illumination is observed. c, lateral distribution of I and Br under dark was shown as the ratio; the ratio was calculated as: e.g. iodide ratio, RI = (I count)/(I count + Br count). d, lateral distribution of I and Br under illumination. Microscale I and Br segregation is observed under illumination (d) compared to those under dark (c), the I-rich and Br-rich regions are marked by circles in I ratio map and Br ratio map in (d), respectively.