Stabilization of Mixed-Halide Lead Perovskites Under Light by Photothermal Effects

Abstract

Mixed-halide lead perovskites (MHLPs) are semiconductor materials with bandgaps that are tunable across the visible spectrum and have seen promising applications in photovoltaics and optoelectronics. However, their segregation into phases with enriched halide components, under resonant light illumination and/or electric field, have hindered their practical applications. Herein, we demonstrate the stabilization of the MHLP photoluminescence (PL) peak as a function of their excitation intensities. This effect is associated with the phase segregation of MHLPs and their subsequent remixing by photothermal heating. We conclude that the balance between these opposing processes dictates the equilibrium PL peak of the MHLPs. The findings in this work could serve as a potential approach to obtain MHLP with stable emission peaks under operating conditions.

Graphical Abstract

The steady-state photoluminescence of mixed-halide lead perovskites under illumination is dictated by the balance between photo-induced phase segregation and photothermal effects.

Mixed-halide lead perovskites (MHLPs) are semiconductor materials with a chemical formula of AB(X_1-xY_x)₃, where A is typically an organic cation, like CH₃NH₃⁺ (MA⁺), or an inorganic cation, like Cs⁺ (or a mixture of both), B is a central cation, typically Pb²⁺, and X+Y is a mixture of halides (Cl⁻, Br⁻, and I⁻). MHLPs have tunable bandgaps across the visible spectrum by controlling their halide composition and ratios.[1] For this reason, MHLPs have found promising applications in tandem solar cells and color-tunable LEDs.[2–4] However, reversible phase segregation on MHLPs has limited its practical utility.[1,5–12] Under continuous excitation, MHLPs segregates into domains with enriched halide contents.[1,5–7,9,13]

The spatiotemporal behavior of individual I-rich domains during phase segregation of Br-I MHLPs has been measured for single microcrystals, nanoplatelets, as well as thin films.[5,7,14] Bischak and co-workers observed that the evolution of I-rich nano-clusters on single MHLP nanoplatelets occurs stochastically in time and space,[15] supporting the idea that the I-rich domains are promoted by local lattice strains induced by polarons[15–17] Mao and co-workers have shown that I-rich domains formed in the same locations over several light-dark cycles, suggesting that a fixed structural feature promotes the segregation of the I-rich domains.[7] These features are often associated with halide vacancies on the surface and grain boundaries of MHLPs,[18–21] and passivation of these defects have been utilized to mitigate phase segregation on MHLPs.[22–27]

The phase stabilization of MHLPs at elevated temperatures has been demonstrated previously by Elmelund and co-workers when they discovered faster phase remixing of CH₃NH₃Pb(Br_xI_1-x)₃ in the dark as the temperature is increased.[12] A similar effect was also reported by Wang and co-workers for CsPb(Br_xI_1-x)₃ when the samples were subjected to different temperatures under illumination.[28] A recent report by Mao and co-workers revealed the reversal of phase segregation of CH₃NH₃Pb(Br_xI_1-x)₃ single crystals under intense excitation intensities.[17] In these reports, the stabilization effects were believed due to either increased temperature or intense photo-excitation. In our previous study, we observed the initial segregation of I-rich domains disappear and a new stable phase emerged under mild excitation and room temperature.[14] We hypothesized that the new phase is a consequence of the photothermal remixing of the segregated phases. In this work, we provide compelling evidence for photothermal remixing using in situ temperature-dependent photoluminescence (PL) measurements. When we lower the temperature, the stabilized new phase disappears, and the segregated phases reappear. The moment the film temperature is brought back to room temperature, the segregated phases remix. On this basis, we develop a model that explains the dependence of a stable emission peak of MHLPs under illumination on their excitation intensity.

To begin with, 40 ± 2 nm ultrathin films of CH₃NH₃Pb(Br_xI_1-x)₃ MHLPs were prepared with x = 0.90 compositions by solvent engineering method (Supporting Information, SI).[29] This composition was chosen mainly due to its more drastic spectral response during phase segregation. All samples are capped with a layer of polystyrene film (~1 μm) to isolate them from their immediate atmosphere, which has been shown to have a significant impact on the phase segregation of MHLPs.[22] All preparations were done in an Argon-filled glovebox. The initial PL and UV-Vis extinction spectra of the as-prepared MHLP samples are shown in SI, Fig. S5. A PL peak at ~573 nm was observed due to the Br-rich composition of the film, consistent with our previous measurements and in the literature.[1,14,30] The thin film is relatively smooth (RMS roughness = 3.2 nm) and composed of tightly packed grains, with some pinholes (~10 nm in size) seen under AFM and SEM (SI, Fig. S5), which is normal for a film prepared under similar conditions. X-ray diffraction (XRD) patterns of the as-prepared film also show a single Gaussian peak for crystal plane (200)C indicating insignificant phase segregation before illumination (SI, Fig. S5).[31]

The as-prepared MHLP thin film is then illuminated under different excitation intensities (1 – 16 W/cm²) for 300 seconds (Fig. 1). The appearance of the peak associated with the initial phase (t = 0) is followed by a red-shifted peak attributed to I-rich domains (SI, Fig. S6). Eventually, the major peak blue-shifts and appears stable over time. This stable peak is attributed to a new mixed-halide phase as we and others reported previously.[14,32] Interestingly, when the sample is illuminated at higher excitation intensities (I_exc ⩾ 8 W/cm²), an I-rich peak at ~760 nm re-emergent (Fig. 1). Our two-channel PL imaging shows that these I-rich domains are localized to a few grains (SI, Fig. S7).

Fig. 1.

(a) The time-resolved PL of CH₃NH₃Pb(Br_xI_1-x)₃ (MHLP) films with x = 0.90 over an illumination period of 300 seconds at various intensities of excitation laser (473 nm, CW). The PL signal is normalized against the maximum for each series. The illumination area is ~0.005 mm² on a sample with a total area of ~6 cm². (b) The initial PL spectra (t=0) for all excitation intensities are similar (573 ± 1 nm).

The corresponding blue-shift in the PL peak as a function of increasing excitation gives a clue as to how the segregated phases (I- and Br-rich) interact with each other under illumination. Similar blue-shifting behavior has been observed recently by Suchan and others when they illuminated CH₃NH₃Pb(Br_xI_1-x)₃ (x = 0.50) between 1 – 100 mW/cm² of 458 nm CW laser.[32] It is established that the bandgap energy of I-Br MHLPs increases with the increase in Br-content.[1,30] Hence, we attribute this blue shift to the increasing amount of the Br-rich phase re-homogenizes with the I-rich phase.

When the film is sufficiently cooled down during illumination, temperature-dependent PL spectra measurements show that the remixing of the segregated phases is reversed (Fig. 2). It is important to note that the temperatures reported here are the measured nominal temperatures of the film without illumination. The local temperature of the illuminated spot must be higher than the measured values and increases with an increase in the excitation intensity.[33] The temperature of the spot under illumination is also dependent on other factors such as illumination spot size, the thermal conductivity of the substrate, heat accumulation, as such very difficult to estimate. It is also challenging to measure the local temperature at the illuminated spot due to the small illumination area and temperature gradient around it. However, we can approximate that the temperature difference under the same illumination roughly as the measured ΔT of the substrate with and without cooling within a relatively small temperature window.

Fig. 2.

Temperature-controlled PL spectra measurements of CH₃NH₃Pb(Br_xI_1-x)₃ thin film with x = 0.90 at 22 W/cm² illumination over time. (a, b) The PL of the film when the temperature of the film was kept at 298 K (R.T.) for 200 seconds and switched to 262 K. (c, d) When the temperature was kept at 262 K for 200 seconds and switched to 298 K. The white line corresponds to the time with the temperature was switched. The plots in b and d correspond to the dashed lines with the same color in a and c, respectively. A complete set of data for various excitation intensities are shown in SI, Fig. S8.

As shown in Fig. 2(a), a stable PL at around 630 nm is observed when the MHLP film is illuminated at room temperature (298 K), alongside weaker Br-rich and I-rich peaks at 555 nm and 760 nm, respectively. The moment the film temperature is switched to 262 K at ~200 s, drastic changes in the PL spectra are evident. The stable 630 nm peak intensity is significantly reduced, associated with an increase in both the Br and I-rich peaks. The inverse effect is observed when the film is first kept at 262 K under illumination before switching to 298 K at 200 s (Fig. 2(c, d)). Multiple 298 K to 262 K thermal cycles shown in SI (Fig. S8) reveal small variations in PL spectra between each cycle suggesting the reversibility of the phenomenon. When cycled from 298 K to 323 K, a blue shift in the major peak is observed. A significant irreversible effect is observed in the heating cycle than in the cooling cycle (SI, Fig. S8), suggesting that the actual temperature during heating under illumination may be above temperatures where MHLP could undergo thermal degradation.[34]

The temperature-dependence of PL evolution observed in Fig. 2 (and Fig. S7), could not be explained completely by polaronic effects.[17] For instance, from a pure polaronic effects standpoint, under constant illumination intensity, lowering the temperature to 262 K of a re-mixed MHLP at 298 K (Fig. 2a) is not expected to result in phase segregation. The decrease in the film temperature is only expected to slow down the remixing process, but not reverse it as we have observed. Hence, we propose a kinetic model that accounts for the temperature effects due to illumination (Fig. 3). For an area of interest with a unit number of Br + I that undergoes anion exchange with its surroundings, the rate constant of the exchange is dictated by the diffusion of the anions between the two phases which reaches thermodynamic equilibrium. The equilibrium composition is represented by the emission peak center of the new phase (Fig. 3(a)). These values, as we can see from the proposed model, are determined by the ratio between k₁ and k₂ and the initial Br/I ratio. Assuming the photothermal heating is proportional to the excitation intensity,[33,35] this model can fit the composition vs laser intensity curve with a lot of possible solutions (Fig. 3(c)). However, by confining a few parameters, we obtain a physically reasonable fit. Specifically, we confine E_a2 to be 0.52 eV (50 kJ/mol) based on the literature.[1s2] From our previous measurements,[14] we also confined the dependence of temperature change with excitation intensity to be ΔT ≈ 3.8I_exc. This small temperature increase over high illumination power density is reasonable because we shine the light with an area only the size of the cross-section of a human hair onto an inch size film and substrate, thus, most energy is dissipated into the rest of the film, and we do not see a phase change on the coated polymer film. If the whole film had been illuminated, the temperature would have raised much more. The segregation activation energy E_a1 is fitted at 53 kJ/mol or 0.55 eV, 0.03 eV, or ~1RT more than E_a2. Interestingly, the proposed model requires an additional weak dependence of k₁ on laser intensity to achieve a good fit that satisfies the expected temperature rise, such that, k₁ ′ = I^0.08k₁. This dependence could be attributed to the increased mobility of the halide ions by light energy, which results to lower activation barrier causing phase-segregation to occur faster.[12] Another possibility is that the new phase gets hotter than its surroundings under illumination due to the energy funneling from the surroundings to the new phase, as we previously suggested.[14]

Fig. 3.

(a) The dependence of equilibrium composition of CH₃NH₃Pb(Br_xI_1-x)₃ thin film with x = 0.90 as a function of excitation intensity after 200 seconds of illumination. (b) The proposed kinetic model between the new mixed-phase (M) and its surroundings (S) where ΔT is proportional to the laser intensity (I). (c) The equilibrium fraction of Br as a function of laser intensity (scattered red dots, 4 trials) and the fitted kinetic model (blue curve). The fraction of Br⁻ is estimated from the PL peaks in (a) using the calibration curve shown in SI, Fig. S9.

The fitted results are thermodynamically viable. For example, at x = 0.5 (Fig. 3(c)), the fitted laser intensity is ~6 W/cm² with a corresponding fitted temperature T = 321 K. At this temperature, the free energy difference before and after the phase separation is ΔG = −RTln(k₁^’/k₂) = −2.8 RT = −7.6 kJ/mol (−0.08 eV). The entropy contribution is TΔS = 1.4RT = 3.6 kJ/mol. So, there is a ΔH = ΔG + TΔS = −1.4 RT (−0. 04 eV) gap from the enthalpy and photo-induced bias within the grains. However, it is important to note that the current data are not enough for the model to confine every parameter in the model, i.e., a few assumptions based on the literature values are needed for the fitting to converge. It does support a possibility that the whole process can be explained by only the thermal effect, the thermal side of the debate.

In summary, we demonstrate in this letter direct evidence of photothermal remixing of segregated phases in MHLP by changing the environmental temperature under constant illumination. Our temperature-dependent PL spectroscopy measurements show that the photothermal remixing is highly suppressed at low environment temperatures and promoted at elevated temperatures. On this basis, we developed a mechanism that explains the dependence of equilibrium PL emission of MHLPs on its excitation intensity. We believe that the findings presented in this letter have provided further vital insights into the phase segregation behavior of MHLPs. An approach based on the dependence of the MHLP emission peak on the excitation intensity could be designed to fine-tune the material’s optical properties under operating conditions.