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

The 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.

Introduction

Photovoltaic and optoelectronic devices composed of organic-inorganic hybrid perovskites have achieved incredible development during the past 10 years (Green et al., 2014; Snaith, 2013; Tan et al., 2014). The power conversion efficiency of perovskite solar cells has skyrocketed from 3.8% to 25.2% (Bi et al., 2015; Jiang et al., 2017, 2018; NREL, 2020; Ono et al., 2018; Wu et al., 2016). Meanwhile, the external quantum efficiency of perovskite-based light-emitting diodes (LEDs) has also been reported to exceed 20% (Cao et al., 2018; Lin et al., 2018). However, the major challenge that remained for these materials is the poor stability, which mainly originated from the non-negligible vapor pressure of the organic moieties and their complex interactions with the moisture in air (Han et al., 2015; Leijtens et al., 2015; Mosconi et al., 2015). To improve stability, substituting the organic cations by inorganic Cs+ ions has been regarded as an effective method. Cs-based perovskites solar cells have been reported to show high stability against humidity and heat (Liang et al., 2016; Wang et al., 2018).

Nevertheless, there are still some drawbacks limiting the application of Cs-based all-inorganic perovskites. One of them is the large band gap (∼2.3 eV for CsPbBr3), which only allows CsPbBr3 to absorb light from the UV region. And most part of the visible light in the solar spectrum cannot be utilized, which seriously limits the improvement of power conversion efficiency (Chang et al., 2016; Duan et al., 2018; Liang et al., 2016). Comparatively, CsPbI3 owns a narrower band gap (∼1.73 eV), leading to a wider light absorption range. However, the most ideal black-CsPbI3 (cubic phase) is thermodynamically less favorable and tends to spontaneously turn into the orthorhombic phase δ-CsPbI3 under ambient condition (Eperon et al., 2015). Mixed-halide perovskite CsPbIBr2 with a narrower band gap than CsPbBr3 and a better thermodynamic stability than CsPbI3 is proposed to be a better choice of light absorbers. Unfortunately, light-induced halide segregation still cannot be avoided in these all-inorganic mixed-halide perovskite absorbers (Beal et al., 2016; Hoke et al., 2015; Noh et al., 2013; Zhang et al., 2019). Several previous studies have documented that the engineering of band gap in perovskites by interchanging bromide and iodide did not yield a corresponding increase in photo-conversion efficiency because of light-induced halide phase segregation (Hoke et al., 2015).

Recently, it is reported that the partial substitution of Pb by Sn in CsPbIBr2 perovskite can not only help to improve the efficiency but also show good long-term stability in photovoltaic devices (Li et al., 2018b; Liang et al., 2017). To find out how the exceptional improvement happens in Sn-doped all-inorganic halide mixed perovskites, we prepared CsPbIBr2 and CsPb0.9Sn0.1IBr2 perovskites and made a comparison between them by monitoring their real-time photoluminescence (PL) spectrum. Under continuous light irradiation, we observed that the phase-segregation of CsPbIBr2 happened rapidly, while CsPb0.9Sn0.1IBr2 can be stabilized at an intermediated state, which was consistent with the metastable CsPbBrxI3-x formed under low excitation power density. With the further oxidization of Sn2+ ions in CsPb0.9Sn0.1IBr2 material, the phase-segregation started again, verifying that the Sn2+ doping in mixed-halide perovskites can effectively minimize the light-induced phase segregation. Substitution of Sn2+ by Mn2+ in mixed-halide perovskites was further proved to be able to increase the intermediated state's tolerance to oxygen indicating that B-site doping is an effective way to promote the performance of the mixed-halide perovskite materials as photovoltaic and light-emitting devices.

Results and Discussion

Material Characteristics of Sn2+-Doped Perovskites

The morphology of as-prepared CsPb0.9Sn0.1IBr2 films was characterized by scanning electron microscopy (SEM), as shown in Figure 1A. The corresponding elemental mappings measured by energy-dispersive X-ray spectroscopy (EDX) were also taken and presented in Figure S1. The results of the quantitative analysis in Table S1 demonstrate that the compositions in the film are in agreement with CsPb0.9Sn0.1IBr2. As shown in Figure 1B, X-ray diffraction (XRD) pattern displays several peaks corresponding to (100), (110), and (200) planes, respectively, confirming the crystal structure is based on perovskite phase (Li et al., 2018b; Liang et al., 2017). The angles in CsPb0.9Sn0.1IBr2 films (14.96°, 21.07°, 30.20°) are slightly larger than those in CsPbIBr2 films (14.84°, 20.99°, 30.08°) indicating the shrinkage of the lattice after the doping of Sn2+ at the B site of the crystal structure. This result is in good agreement with the relative smaller ionic size of Sn2+ than Pb2+.

Figure 1

Characterization of the Perovskite Films

(A) Morphology of CsPb0.9Sn0.1IBr2 film taken by SEM. The scale bar is 100

(B) XRD pattern of CsPb0.9Sn0.1IBr2 film (red solid line) and CsPbIBr2 film (black solid line).

Real-Time PL Variations of CsPb0.9Sn0.1IBr2 and CsPbIBr2 Films

We first measured the PL spectra of CsPbIBr2 and CsPb0.9Sn0.1IBr2 films under N2 atmosphere with a 450-nm-wavelength laser and with the power density of 5 W/cm2. As shown in Figure 2A, the initial PL spectrum of CsPb0.9Sn0.1IBr2 showed a single peak with a maximum value at 576 nm (the inset of Figure 2A). With continuous light irradiation, the PL intensity significantly increased and the maximal position shifted to 676 nm within 10 min (Figures 2A and 2B), which might be caused by the photo-induced halide ions redistribution processes, similar to that in MAPbBrxI3-x (Hoke et al., 2015), due to the lattice mismatch with different halide anion radius in fresh CsPb0.9Sn0.1IBr2 samples. Also, the variation of the FWHM of the PL spectra indicates the transition process from a single PL peak to another one (Figure 2C). Then, the PL spectra kept stable and the intensity continuously increased for more than 100 times compared with the initial intensity (the inset of Figure 2B). The enhanced PL intensity has been widely reported, which could be due to two reasons: (1) energy transfer from large band gap phase to the newly formed stable low-energy phase, which has higher quantum yield; (2) passivation of the quenching defects by optimization of the crystal structure. Both of them can also correspond to the reported performance improvement in photovoltaic devices (Stranks, 2017). The PL emission of CsPbIBr2 film also showed a single peak with maximum value around 584 nm at the initial time (Figure 2D). However, under continuous light irradiation, the PL intensity gradually decreased and a new peak appeared (Figure 2E) which can also be proved from the evaluation of PL FWHM (Figure 2F). A clear intensity decrease of the initial PL peak and a clear intensity increase of the new peak without spectral shift can be observed, indicating a phase transition process. The transition process stopped after 100 s with the final PL spectrum containing two peaks locating at 584 and 694 nm, respectively. These two peaks are proposed to be corresponding to bromide-rich phase and iodine-rich phase (Hoke et al., 2015) formed by the photo-induced phase-segregation behaviors (Slotcavage et al., 2016). Such fast phase segregation process in CsPbIBr2 verifies that the lead mixed-halide perovskite is in a dynamically unfavorable state as reported (Bischak et al., 2017; Brivio et al., 2016). The PL enhancement and the single peak featured PL spectra of CsPb0.9Sn0.1IBr2 suggests the formation of a stable phase, which is different from either the bromide-rich phase or the iodine-rich phase, as discussed below. As well known, PL properties of the perovskites are correlated to many factors including crystal structure (Klug et al., 2017), atmospheres (Fang et al., 2016; Galisteo-López et al., 2015; Tian et al., 2015), and degradation (Matsumoto et al., 2015). Since both PL variations were first taken under N2 atmosphere, they should be free from the interactions between atmospheres and crystals. In addition, these measurements were performed under the same experimental conditions except the difference in the sample. That is to say, the difference of the PL properties between CsPbIBr2 and CsPb0.9Sn0.1IBr2 can be only due to the doping of Sn2+ ions. Therefore, we can conclude that the substitution of partial Pb2+ in CsPbIBr2 with Sn2+ ions can successfully prevent it from segregating into two different phases under light soaking.

Figure 2

Real-Time PL Monitoring of CsPb0.9Sn0.1IBr2 Films

(A) PL variations of CsPb0.9Sn0.1IBr2 film under continuous irradiation. The inset in (A) shows the PL variation during the first 100 s in detail.

(B) PL peak wavelength in (A) varied as irradiation time. The inset in (B) shows the variation of PL intensity amplitude under continuous irradiation. The amplitude is calculated by (I – I)/I(I-I)I, in which I indicates the initial PL intensity and I indicates the varied PL intensity.

(C) The full width at half maximum (FWHM) of the PL spectra in (A) varied as irradiation time.

(D) PL variations of CsPbIBr2 under continuous irradiation.

(E) PL peak wavelength in (D) varied as irradiation time.

(F) The full width at half maximum (FWHM) of the PL spectra in (D) varied as irradiation time. Both samples were excited by a 450-nm laser excitation with a power density of 5 W/cm2, and the PL measurements were taken under nitrogen atmosphere.

Because the stable PL emission of CsPb0.9Sn0.1IBr2 films located just between the PL emissions of bromide-rich regions and iodine-rich regions in CsPbIBr2 films, it must be a stable phase with the content of mixed-halide CsPb0.9Sn0.1IxBr3-x. Another evidence for this conclusion is that the spectrum of CsPb0.9Sn0.1I3 phase should locate at a longer wavelength than CsPbI3 due to the doping of Sn2+(Hao et al., 2014). Actually, this mixed-halide phase has also been observed in CsPbIxBr3-x (Slotcavage et al., 2016). We did not observe the intermediate state during the phase segregation process of CsPbIBr2 films, possibly owing to the faster phase transition under high excitation power density. We thus rechecked the phase transition process of CsPbIBr2 films with a low excitation power density of 0.4 W/cm2 in order to slow down the halide ions migration processes. As shown in Figure 3A, the PL emission started from 580 nm and redshifted to 622 nm. The stable FWHM in Figure 3B also indicates there is only one emission peak. That is to say, with low excitation power density, the halide redistribution process was successfully slowed down and could form an intermediate mixed-halides state (CsPbIxBr3-x). Such mixed-halides state was quite stable unless higher excitation power density was applied. As shown in Figure 3C and 3D, under a higher excitation power density of 4 W/cm2, fast phase separation process happened, similar to the results shown in Figure 2D. The single peak located at 622 nm, which is stabilized under low power excitation (0.4 W/cm2) proved the formation of the intermediate state (CsPbIxBr3-x) during the phase segregation process, which was very similar to the stable PL emission in CsPb0.9Sn0.1IBr2. The redshifted PL emission in CsPb0.9Sn0.1IBr2 intermediate state compared with that in CsPbIBr2 is consistent with the fact that the occupation of Sn2+ ions in B sites of perovskite structure can decrease the conduction band minimum and make the band gap narrower (Brandt et al., 2015). The difference is that the peak position of PL emission in CsPb0.9Sn0.1IBr2 film would keep stable, whereas that in CsPbIBr2 film would start to separate into two peaks under high excitation power. In other words, the doping of Sn2+ ions in lead mixed-halide perovskites can effectively stabilize the intermediate state induced by light irradiation. The increased PL intensity of the intermediate state in CsPb0.9Sn0.1IBr2 indicates its high tolerance properties to photo-excitation, which can also explain the significantly improved long-term stability relative to lead mixed-halide perovskites (Niezgoda et al., 2017; Yang et al., 2016). In principle, doping of Sn2+ ions in B sites will significantly shift down the CBM of lead mixed-halide perovskite decreasing the band gap of CsMX3. This is in good agreement with the intermediate states that CsPb0.9Sn0.1IBr2 film has longer wavelength than CsPbIBr2. However, the initial PL for CsPb0.9Sn0.1IBr2 (576 nm) is slightly blueshifted than CsPbIBr2 (584 nm), which is contradictory. This should be due to the exciton binding energy of CsPbIBr2 is much higher than that of Sn2+-doped perovskite (Li et al., 2018a; Long et al., 2019; Galkowski et al., 2019), which can also be verified by the comparisons from the absorption spectra of these two samples (Figure S2). In addition, the detailed structure and band gap of the intermediate state is still unclear. It is possible that the ratio between the halide elements could be different for CsPbIBr2 and CsPb0.9Sn0.1IBr2, which significantly affects the band gap of the intermediate state. Therefore, the redshifted PL spectrum from CsPb0.9Sn0.1IBr2 intermediate state compared with that from CsPbIBr2 intermediated state can be due to decrease of band gap after Sn2+ doping and/or different I/Br ratio of the intermediate state. Characterization of the intermediate state is still undergoing in our group by employing more in situ measurements.

Figure 3

Real-Time PL Monitoring of CsPbIBr2 Films

(A) PL variations of CsPbIBr2 film under the excitation power density of 0.4 W/cm2.

(B) The FWHM variation of the PL spectra in (A).

(C) PL variation of CsPbIBr2 film under the excitation power density of 4 W/cm2.

(D) The FWHM variation of the PL spectra in (C). The PL emissions in (A) and (C) were collected from the same location, the low-power excitation in (A) was first measured, and then the high-power excitation in (C) was measured after the low-power excitation was turned off.

Reversibility of the PL Properties of CsPb0.9Sn0.1IBr2 Films

As can be seen from the above discussion, the stable PL emission from CsPb0.9Sn0.1IBr2 films was achieved by light soaking. To confirm that the emission is from a state of CsPb0.9Sn0.1IBr2 material stabilized by Sn2+ doping, we have to exclude any light-induced irreversible variation that may permanently change the properties of the films. Thus, we checked the reversibility of this light soaking process, as shown in Figure 4. Under continuous light excitation, the PL intensity kept increasing and the spectrum redshifted to ∼650 nm, similar to the results shown in Figure 2A. When the excitation light was turned off, the PL intensity decreased and the spectrum blueshifted back to the initial state. To investigate the recovery process without significantly affecting it, we detected the PL spectra and PL intensity for 1 s and then kept in dark with a time interval of 30 s, as shown in Figure 4. Moreover, if we keep the film in dark for ∼5 min and then switch on the excitation light again, the material can recover to the initial state and the PL enhancement and spectral shift process can be well repeated, as shown in Figure S3. Therefore, we can conclude that the intermediate state phase in the Sn2+-doped lead mixed-halide perovskites can only be stabilized under light irradiation.

Figure 4

Reversibility of the PL Variations in CsPb0.9Sn0.1IBr2 Films

(A) PL intensity variations of CsPb0.9Sn0.1IBr2 films. The first 600 s was measured under continuous irradiation, whereas the following 1,800 s was the PL intensity measured for 1 s and then kept in dark for the duration of 30 s.

(B) The corresponding PL wavelength variations of the two stages. The excitation wavelength was 450 nm and the power density was 5 W/cm2. The measurements were taken under N2 atmosphere.

Spectral Variation after the Oxidization of Sn2+ Ions

Photo-induced phase segregation behavior has been widely observed in organic-inorganic hybrid lead mixed-halide perovskites (Hoke et al., 2015). All-inorganic Cs-based mixed-halide perovskites that exhibit brilliant stability against moisture also have the same problem (Li et al., 2017). The phase segregation process was proposed to be originated from the ion migration affected by lattice strain (Hoke et al., 2015). The expansion of lattice strain could result from the incompatible sizes of halide atoms, which will increase the Helmholtz free energy of CsPbIxBr3-x (Beal et al., 2016; Yin et al., 2014). Under illumination, the interatomic separations along distinct directions (e.g., Pb-I and Pb-Br) will take place to minimize the internal strain (Bischak et al., 2017; Brivio et al., 2016). However, the partial substitution of Pb2+ by Sn2+ in CsPbIxBr1-x will lead to the shrinkage of perovskite lattice, indicating that the reduction of lattice strain in CsPb0.9Sn0.1IxBr3-x should also correlate with the decrease of Helmholtz free energy (Li et al., 2018b). Therefore, with the substitution of Sn2+, the photo-excitation can drive CsPb0.9Sn0.1IBr2 to form a stable intermediate phase state (CsPb0.9Sn0.1IxBr3-x), which possesses the minimum lattice strain and will not continue to segregate into two different phases. Besides the light-induced segregation, the photo-stability of CsPb0.9Sn0.1IBr2 was also significantly improved, which can also be explained by the reduction of the lattice strain.

To further verify the effect of Sn2+ doping, we tried to oxidize the Sn2+ ions to Sn4+ by two different ways to check the effect on phase segregation. As well known, Sn2+ cations can be easily oxidized to Sn4+ under ambient environment (Jung et al., 2016; Kwoka et al., 2005). To confirm whether Sn2+ played a significant role in stabilizing the intermediate phase of CsPb0.9Sn0.1IBr2, we introduced an oxygen flow surrounding the CsPb0.9Sn0.1IBr2 film and simultaneously detected the PL variations. In Figure 5A, the PL emission started from a single peak and slowly transformed into two peaks under continuous irradiation in an oxygen atmosphere, which showed the same tendency as the CsPbIBr2 film exposed under oxygen atmosphere (see Figure S4). Although the oxygen atmospheres will complicate the PL emissions of the perovskite, they mainly involve with the PL intensity rather than with the PL spectra, which can also be verified from the PL variation of CsPbIBr2 film measured under oxygen atmosphere. The PL variation only differs from that under nitrogen atmosphere in the PL intensity, because the defects in CsPbIBr2 were always susceptible to different atmosphere (Fang et al., 2016; Galisteo-López et al., 2015; Tian et al., 2015). The small peak near 500 nm could be derived from the PL emissions of PbI2 and PbBr2 formed by the degradation of perovskite crystals. As discussed before, by comparing the PL emissions of CsPbIBr2 and CsPbSnIBr2 under N2 atmospheres we can conclude the effects of the doped Sn2+ ions. Then, according to that the photoinduced phase segregation of CsPb0.9Sn0.1IBr2 perovskite only happens in O2 atmosphere, which should be due to the destructive effects of Sn2+, the inhibition effects of Sn2+ doping can be further confirmed. In addition, we took another method to directly oxidize the Sn2+ ions. Before the perovskite film was fabricated, the precursor solution was pre-oxidized under ambient environment. Figure 5B showed the PL emissions of the CsPb0.9Sn0.1IBr2 film prepared by spin-casting the pre-oxidized precursor solution. The photo-induced phase segregation processes of this perovskite film under nitrogen atmosphere can be directly observed, which is similar to that in Figure 2D. Subsequently, we further changed the atmosphere to oxygen (see Figure S5) and verified that the PL variation of CsPb0.9Sn0.1IBr2 film prepared by pre-oxidized solution showed no difference with that of CsPbIBr2 film after the oxidization of Sn2+. To confirm the valence state change from Sn2+ to Sn4+ in CsPb0.9Sn0.1IBr2, we also performed XPS analysis for comparison. Three different samples were measured by XPS: (1) film fabricated from fresh precursor solution (named as “fresh sample”); (2) film fabricated from fresh precursor solution and then treated under UV light in oxygen atmosphere for 30 min (named as “UV-treated sample”); (3) film fabricated from pre-oxidized precursor solution (named as “oxidized sample”). The binding energies of C1s bands were all recalibrated to 284.6 eV (Briggs et al., 1981). As shown in Figure 5C, compared with the fresh sample, the binding energies of Sn species in the UV-treated sample moved to lower levels, which shall be ascribed to the formation of SnO2, in which the electronegativity of oxygen is lower than that of halide, thus leading to the decrease of binding energy of Sn4+. As showed in Figure S6, the increase of oxygen contents in the UV-treated sample was also detected by XPS, further confirming the formation of SnO2 on the surface of perovskite films. In contrast, the binding energies of Sn species in the oxidized sample moved to higher levels, corresponding to the valence state change from Sn2+ to Sn4+, which indicated the presence of Sn4+ in the oxidized sample. The oxidization from Sn2+ to Sn4+ would destroy the octahedron structures of mixed-halide perovskites (Leijtens et al., 2017; Qiu et al., 2017) and thus result in the increase of lattice strain; consequently, the light-induced phase segregation reoccurred. As a whole, the above results proved the effective inhibition functions of Sn2+ dopants in CsPb0.9Sn0.1IBr2.

Figure 5

PL Variations of CsPb0.9Sn0.1IBr2 Films Treated by Different Conditions

(A) PL emissions of CsPb0.9Sn0.1IBr2 film exposed under an oxygen atmosphere.

(B) PL emissions of CsPb0.9Sn0.1IBr2 film fabricated by using the precursor solution that was pre-oxidized before spin-casting. And the PL measurements in (B) were taken under nitrogen atmosphere. The samples in both (A) and (B) were excited by a 450-nm-wavelength laser with a power density of 5 W/cm2.

(C) XPS analysis of Sn element in CsPb0.9Sn0.1IBr2 films, and the three samples were fresh sample, UV-treated sample, and oxidized sample, respectively.

Destructive Effects of Moisture on Sn2+ Based Mixed-Halide Perovskite Films

The study presented above has successfully assessed the impact of doped Sn2+ ions in CsPb0.9Sn0.1IBr2. However, it also reflected the shortcoming of mixed-halide perovskites that is easily susceptible to the ambient atmosphere. Although CsPb0.9Sn0.1IBr2 exhibits brilliant stability against phase segregation and photo-degradation under nitrogen atmosphere, it is still labile when getting in touch with oxygen and moisture. Figure S7 shows the time-dependent PL traces and variations of PL spectra when exposed in ambient atmosphere with a relative humidity of 40%. Both photo-induced phase segregation and photo-degradation gradually appeared after 10 min, which should be attributed to the synergistic influence of oxygen and H2O in air. We also checked the PL variation in oxygen atmosphere mixed with H2O vapor, as shown in Figure S8. The photo-induced segregation even cannot catch up with photo-degradation processes, and finally, the whole perovskite phase turned into lead halide phase (Baibarac et al., 2015). For the fact that CsPbI3 perovskite was thermodynamically unfavorable under room temperature and very prone to exhibit phase transition in ambient atmosphere (Eperon et al., 2015; Kulbak et al., 2015; Luo et al., 2016; Yunakova et al., 2012), the photo-induced phase segregation was almost masked owing to the fast disappearance of iodine-rich phase in the mixed atmosphere of oxygen and H2O. These results also explain why the long-term stability of CsPbxSn1-xIBr2 perovskite solar cells can be well maintained when protected by encapsulation (Li et al., 2018b; Liang et al., 2017), which can help to isolate the perovskite materials from the oxygen and humidity in air.

Improved Tolerance to Ambient Atmosphere by Doping Mn2+ in B Site

As discussed above, the Sn2+ is easy to be oxidized into Sn4+ or SnO2 after interacting with oxygen and moisture (Ke et al., 2017; Liao et al., 2016) leading to the light-induced phase segregation behaviors. To solve this problem and improve the tolerance of mixed-halide perovskite to oxygen and moisture, we tried to use Mn2+ instead of Sn2+ because Mn2+ is very difficult to be directly oxidized into MnO2 by oxygen (Diem and Stumm, 1984; Feng et al., 2004) and can be effectively introduced into the lattice of CsPbX3 perovskites (Liang et al., 2018). We prepared the CsPb0.99Mn0.01IBr2 films by one-step method and further characterized the X-ray diffraction pattern (Figure S9). Also, the HR-TEM images were taken and shown in Figure S10. From the comparisons among undoped, Sn2+-doped, and Mn2+-doped samples, we can observe that, as the ionic radius of B-site ions decreases, the diffraction angles are increased and the lattice distances are reduced. And according to Bragg's law we can also conclude that the lattice distance obtained in Figure S10 corresponds to (200) planes of cubic Pm-3m phase (PDF#84-0464). Thus the results obtained from XRD and HR-TEM confirm that the Sn2+ and Mn2+ ions are successfully doped into the perovskite crystals leading to the lattice shrinkage that can effectively help to release the lattice strain. Then we measured the PL spectra under ambient atmosphere to compare with CsPb0.9Sn0.1IBr2 films. As shown in Figure 6A, the PL behavior of CsPb0.99Mn0.01IBr2 is very similar to that of CsPb0.9Sn0.1IBr2 shown in Figure 2, which was measured in N2 atmosphere. The initial PL spectrum of CsPb0.99Mn0.01IBr2 film also showed a single peak (the inset of Figure 6A), and the PL intensity significantly increased for more than 100 times under light irradiation with the maximal position shifted to 676 nm in 10 min as shown in Figure 6B. The most important difference between these two samples was the atmosphere effect. As shown in Figure S7 and discussed in previous sections, clear phase segregation of CsPb0.9Sn0.1IBr2 can be observed owing to the oxidization of Sn2+. However, for CsPb0.99Mn0.01IBr2, no phase segregation was observed after 20 min light irradiation in air verifying the improved tolerance to oxygen and moisture with doped Mn2+ in the B site of the perovskite. Therefore, B-site doping with Mn2+ ions should be a promising way to inhibit the light-induced phase segregation of mixed-halide perovskite materials and can simultaneously improve the stability in ambient atmosphere. In addition, other divalent cations including Zn2+, Cd2+, and alkaline-earth metals are reported to have been successfully incorporated into CsPbX3 nanocrystals (Cai et al., 2018; Chen et al., 2019; van der Stam et al., 2017). We believe that the doping of these ions will also affect the stability of the materials. However, it is difficult to predict whether they will have positive or negative effect before we perform systematic investigation.

Figure 6

Real-Time PL Monitoring of CsPb0.99Mn0.01IBr2 Films

(A) PL variations of CsPb0.99Mn0.01IBr2 film under continuous irradiation.The inset in (A) shows the initial PL spectrum.

(B) PL peak wavelength in (A) varied as irradiation time. The inset in (B) shows the variation of the PL intensity amplitude under continuous irradiation. The amplitude is calculated by , where I indicates the initial PL intensity and I indicates the varied PL intensity. The sample was excited by a 450-nm laser with a power density of 5 W/cm2, and the PL measurements were taken under ambient atmosphere.

Conclusion

In summary, we investigated the light-induced phase segregation processes in Cs-based mixed-halide perovskites by monitoring the real-time PL spectra. We found that the partial substitution of Pb2+ by Sn2+ in cesium-based lead mixed-halide perovskites can significantly inhibit the photo-induced phase segregation. As a typical Sn2+-doped mixed-halide perovskite, CsPb0.9Sn0.1IBr2 exhibits brilliant photo-stability in inert gas atmosphere from two aspects: (1) the stable mixed-halide perovskite phase without phase segregation; 2) the high tolerance to high excitation power with nearly no photo-degradation after long-time light irradiation. The photo-stability brought by Sn2+ doping could be originated from the release of lattice strain in the crystal structure of Sn2+-doped perovskite film. However, such stability can be easily destroyed by the oxidization of Sn2+ into Sn4+ when exposed to oxygen atmosphere. Control experiments and XPS analysis proved that the oxidization of Sn2+ could lead to phase separation confirming that B-site doping was an effective method to inhibit the light-induced phase separation. To improve the stability of ions in B sites, we replaced dopant ions Sn2+ by Mn2+ and successfully improved the tolerance against ambient atmosphere. We believe our findings provide an efficient way to prevent the phase separation process of mixed-halide perovskites and have significant values for the development of stable and efficient all-inorganic perovskite photovoltaic and light-emitting devices.

Limitations of the Study

In this work, we found that B-site doping by Sn2+ or Mn2+ can effectively improve the photo-stability and inhibit the light-induced phase segregation of the mixed-halide perovskite. However, according to our results, it takes time to reach the stable phase depending on the light intensity, which could affect the application of these materials. Further optimization of the stable phase is needed.

Resource Availability

Lead Contact

Further information and requests for resources and reagents should be directed to and will be fulfilled by the Lead Contact, Prof. Yuxi Tian (tyx@nju.edu.cn).

Materials Availability

The study did not generate new unique reagents.

Data and Code Availability

The published article includes all datasets/code generated or analyzed during this study. Supplemental Informationtyx@nju.edu.cn

Methods

All methods can be found in the accompanying Transparent Methods supplemental file.