Unraveling the degradation process of 2D passivated and Cs stabilized FAPbI<sub>3</sub> by optical pump THz probe spectroscopy

Hao Sun; Hongchuan He; Ming Yin; Ming Yin; Hao Cheng; Xiaoxia Lin; Qiuping Huang; Qiuping Huang; Qiuping Huang; Yalin Lu; Yalin Lu; Yalin Lu

doi:10.1364/OME.426085

1. Introduction

Benefited from the outstanding optoelectronic properties and easy fabrication techniques, hybrid organic-inorganic lead halide perovskites (HOIPs) have been widely studied in photovoltaic and nanophotonic applications [1–4], with the power conversion efficiency (PCE) of perovskite solar cells reaching a phenomenal advancement [5–7]. The general formula of HOIPs is ABX₃, where A is a monovalent cation, usually including methylammonium (MA⁺, CH₃NH₃⁺), formamidinium (FA⁺, CH(NH₂)₂⁺) or cesium (Cs⁺). The B-site cation usually is Pb²⁺ or Sn²⁺, and the final component is an anion (Cl⁻, Br⁻, I⁻). Despite the achievement of impressive PCE, one primary problem which hinders the commercialization of HOIPs is their environmental instability. For example, it has been reported that pure FAPbI₃ (FAPI) perovskite exhibited poor stability after exposure to humidity, and the black trigonal perovskite $\mathrm{\alpha }$ phase readily turned into undesirable yellow hexagonal non-perovskite $\mathrm{\delta }$ phase [8–12]. It’s demonstrated that the δ phase shows a poor absorbance in visible spectra range compared to $\mathrm{\alpha }$ phase [13]. This unpleasant transformation severely affects its photoelectric performance and hinders its further applications. In order to improve the stability of HOIPs, additive and compositional engineering have been actively explored [14]. Additive engineering usually introduces a controlled amount of organics into the precursor. The added component can contribute to the reduction of trap-state density [15] and improvement of morphology [16]. And sometimes the hydrophobic additives are prone to distribute along grain boundaries and surfaces, which can protect the perovskite from moisture [17]. By contrast, compositional engineering incorporates different ions into the structure and thus provides more freedom to improve the intrinsic properties of perovskite materials [6]. Hence great efforts utilizing the compositional engineering have been widely made in recent years [18–20].

To reveal the effect of additive and compositional engineering on the performance of HOIPs, various methods have been employed [3,5,6,19,21–25]. The maximum power point tracking method is frequently adopted to demonstrate the degradation process of solar cells based on HOIPs [14]. However, when a perovskite film is incorporated in a layered solar cell, it has been shown that the adjacent layers [21] and their interfaces [26] also have an effect on the perovskite stability. It’s unfriendly for us to observe the degradation process clearly. Therefore, it’s important to identify directly the properties changes of the photoactive layer in the transformation period. The anelastic and dielectric measurements were performed to study the transformation of cubic FAPbI₃, showing that loose α-FAPbI₃ powder could remain stable in normally humid air at room temperature (RT) for at last 100 days [27]. Some other optical techniques are also developed to investigate the performance of HOIPs. For example, the effect of doping on the phase stability and on the photo-physical properties of CsPbI₂Br perovskites was explored using photoluminescence and absorbance spectra [28]. Moreover, the ultrafast transient absorption spectrum (TAS) and optical pump THz probe (OPTP) spectroscopy were performed to provide information on carrier dynamics of HOIPs [29–33]. For instance, Liu et al. demonstrated that the internal charge transfer can efficiently separate electrons and holes to the upper and bottom surfaces of 2D perovskite film by TAS [30]. Milot et al. found that monomolecular carrier recombination rate firstly decreased, and then increased significantly with increasing PEA fraction by OPTP spectroscopy [34]. However, among these researches, more attention is focused on the optimization of optoelectronic performance by additive and compositional engineering. Little understanding has been established on the charge carrier dynamics and photo-physical changes in the natural degradation period.

In this report, we employed the time-resolved spectroscopy, that is, optical pump THz probe spectroscopy, to observe the long-term degradation process of 2D passivated and cesium stabilized FAPbI₃ films. It’s found that although both the incorporation of PEA and Cs have a positive effect on the stability of $\alpha $-FAPI perovskite phase, photoconductivity of the FAPI and FAPEAPI films exhibits an accelerated decay rate, while that of FACsPI film decreases slowly at a relatively steady rate in the degradation process. To further enhance the understanding of the degradation process, Drude-Smith model was adopted to fit the complex photoconductivity of different samples, obtaining the evolution of carrier mobility. It’s found that the evolution of mobility shows a similar rule as the photoconductivity, and it’s also inferred that the localization of carrier increases with the degradation. These results indicate Cs incorporation can provide sustainable stabilized effect, while the passivated effect of PEA₂PbI₄ can be weakened by the newly emerged phase boundaries. This report helps understand the influence about additive and composition engineering on the freedom in the sample preparation or examination procedure. It’s thus expected that further advancement would be inspired through these findings.

2. Experimental methods

The samples were prepared as follows. A 0.5 mm-thick quartz substrate was cleaned with deionized water, ethanol and acetone, then treated with UV-ozone cleaning for 20 minutes. The FAPI perovskite solution was prepared by dissolving 172 mg formamidinium iodide (FAI) and 461 mg lead iodide (PbI₂) in a 1 ml mixture of N,N-dimethylformamide (DMF) /dimethyl sulfoxide (DMSO) (9:1 volume ratio). The FACsPI perovskite solution was prepared by dissolving 155 mg FAI, 26 mg cesium iodide (CsI) and 461 mg PbI₂ in a 1 ml mixture of DMF/ DMSO, while the FAPEAPI solution was prepared by dissolving 4.1 mg phenethylannonium iodide (PEAI), 169 mg FAI and 461 mg PbI₂ in the DMF/DMSO mixture. Thin films were fabricated on the quartz substrate by one-step spin-coating method. All films were coated at 1000 rpm for 10 s at the beginning, then 3000 rpm for 30 s with 30 $\mu$l toluene as the anti-solvent dripped in the last 15 s. The FACsPI film was annealed at 110${^\circ{\textrm C}}$ for 15 minutes while the FAPI and FAPEAPI films were annealed at 150${^\circ{\textrm C}}$ for 30 minutes. all the steps were performed in a N₂ filled glove box. The samples were kept in the dark under normal laboratory air conditions during the degradation period.

The crystal structure of all samples was obtained by X-ray diffraction (XRD, Rigaku) with Cu K$\mathrm{\alpha }$ radiation. To observe the surface morphology of the perovskite films on the quartz substrate, surface scanning electron microscope (SEM, JSM-6700F) was characterized. A homebuilt OPTP setup was utilized, in which the femtosecond pulses were provided by a Ti:sapphire regenerative amplifier (Spitfire Ace-35F, Spectra-physics) with a center wavelength of 800 nm and pulse duration of 35 fs. The generation and detection of THz radiation was performed by two pieces of 0.5 mm thick (110)-oriented zinc telluride crystals. The THz peak electric field $E(\tau )$ and the photo-induced change $\Delta E(\tau )$ of samples as a function of pump-probe time delay $\tau $ was measured by controlling the delay line, with measurement performed in a box filled with dry air (RH 3% - RH 5%).

3. Results and discussion

Figure 1(a)–1(c) shows the XRD results of our samples. It’s clear that no $\delta $ and PbI₂ diffraction peaks can be found in the XRD patterns, which demonstrates the high quality of our samples. Besides, the central thickness of the FAPI, FAPEAPI, FACsPI films is about 476 nm, 453 nm and 440 nm, respectively. With similar thickness of thin film, it can be seen that the${\; }\alpha $ perovskite phase diffraction peaks in the FAPEAPI and FACsPI XRD patterns are higher than the one of FAPI, which suggests that the addition of PEAI and CsI into the precursor solution effectively improves the crystallinity of $\alpha $ perovskite phase. As shown in Fig. 1(d)–1(f), obvious cracks are observed in the FAPI, FAPEAPI surface and cracks in the FAPI film are larger. Meanwhile, no obvious cracks are found in the FACsPI film and it has a more uniform grain size. Cracks and boundaries are related to the crystallization kinetics of different material compositions [20]. On the other hand, cracks are also possibly attributed to the thermal coefficient mismatch between the perovskite films and the quartz substrate [35], which perhaps cause an obvious difference in corresponding photoelectric performance. So it may be effective to suppress the formation of cracks by choosing different substrates [36].

Fig. 1. XRD patterns of (a) FAPI, (b) FAPEAPI, (c) FACsPI films. SEM images of (d) FAPI, (e) FAPEAPI, (f) FACsPI films.

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It’s widely acknowledged that the FAPI perovskite phase tends to transform from $\alpha $ phase to photo-inactive $\delta $ phase at RT, resulting in a serious degradation of photovoltaic performance [37]. In this report, OPTP measurement was performed to observe the degradation process. The schematic structure of the samples is presented in Fig. 2(a). Simple double-layer structure makes sure that the degradation is mostly caused by the storage environment. All the three kinds of films were illuminated with 800 nm femtosecond pump pulse (540µJ cm⁻²) to ensure an obvious signal changes in the degradation period. Figure 2(b)–2(d) shows the transient THz transmission changes of the three films in a degradation period. It’s found that the FACsPI film shows the best phase stability, and the maximal change of THz transmission electric amplitude (-$\Delta E/E$) retains about 8% from 19% after 43 days (Fig. 2(d)). Meanwhile, the FAPI film decreases to 9% from 15.5% (Fig. 2(b)) only after 5 days. The addition of PEAI also enhances the phase stability, the FAPEAPI film retains 9.5% from 17% after 10 days (Fig. 2(c)). The enhancement of maximal -$\Delta $E/E of the FAPEAPI and FACsPI films is caused by the improved crystallinity, which can be demonstrated in the XRD patterns.

Fig. 2. The time-dependent THz characterization of the perovskite films. (a) Graphical representation of the sample structure with the pump-probe laser beams. The transient THz transmission change of (b) FAPI, (c) FAPEAPI, (d) FACsPI films.

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The free carrier excitation and relaxation dynamics of FAPI, FAPEAPI and FACsPI perovskite films are included in the transient transmission amplitude evolution when the femtosecond pump pulse strikes the perovskite films. Therefore, to analyze the ultrafast free carrier dynamics of the three samples, a bi-exponential function given as

- \frac{{\Delta E}}{E} = {A_F}exp\left( { - \frac{t}{{{\tau_F}}}} \right) + {A_S}exp\left( { - \frac{t}{{{\tau_S}}}} \right)

was adopted to describe the observed temporal response of the THz electric field, where ${\tau _F}$ and ${\tau _S}$ represent the time constant for the fast and slow process, respectively. The fitted bi-exponential curves are shown with the traces in Fig. 2(b)–2(d). The changes of ${\tau _F}$ and ${\tau _S}$ in a degradation period is depicted in Fig. 3. It’s seen that the fast time constant ${\tau _F}$ is several ps and decreases gradually with the degradation reaction for all the three samples. The slow time constant ${\tau _S}$ is around 180 ps and fluctuates slightly during the whole degradation period. Comparable values have been reported on similar materials [38,39]. Since the fluence (540µJ cm⁻²) is much higher than the amplified spontaneous emission (ASE) threshold of most perovskites [40], The ${\tau _F}$ may be related with the ASE process and the observed decreasing trend can be explained by the degradation of the gain properties of the perovskite films [38]. As for the reason why ${\tau _S}$ does not change obviously with the degradation process, it may be caused by the phonon bottleneck effect, which induces the carrier cooling [41]. Besides, although we tried to concentrate the THz spot on the center area of the 10 mm × 10 mm samples by our eyes, it’s difficult to ensure the completely same spot because the diameter of the spot where THz wave passing through is 2 mm. Additionally, an uneven color change happened during the degradation process which can be found in Fig. 4(a). so we attribute the fluctuations in the curves of ${\tau _F}$ and ${\tau _S}$ to the difference in the measured area at different measured time.

Fig. 3. The time-dependent ${\tau _F}$, ${\tau _S}$ of the perovskite films. FAPI (a) and (d), FAPEAPI (b) and (e), FACsPI (c) and (f). The error bars are the standard error of the fitting analysis.

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Fig. 4. Improved stability of the FAPEAPI, FACsPI films. (a) Photographs of the Perovskite films in a degradation period. Schematics of the (b) FAPEAPI and (c) FACsPI films. Evolution of photoconductivity of (d) FAPI, (e) FAPEAPI, (f) FACsPI film.

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It’s clearly observed that the FACsPI film kept black after 43 days while the FAPI and FAPEAPI films turned bleached during 6 and 12 days, respectively (Fig. 4(a)). The photo-induced THz conductivity is used to display the decay process. The peak photoconductivity is estimated by [42,43]

\Delta \sigma (t )\approx{-} {\varepsilon _o}\frac{c}{d}({1 + {n_{sub}}} )\frac{{\Delta E(t )}}{{E(t )}}

where the $c,\,d,\,{\varepsilon _o},\,{\; }{n_{sub}}{\; }$are the speed of light in vacuum, sample thickness, the free space permittivity and the refractive index of the quartz substrate, respectively. Figure 4(d)–4(f) shows the photoconductivity variation of the three samples. We can see that the FAPI and FAPEAPI films show a critical time point of 2nd and 7th day, beyond which the photoconductivity decreases rapidly. While the FACsPI film shows a comparatively steady decay rate. It has been observed that the degradation in FAPI film initiates with the water condensation at grain boundaries [11,44], and the decay reaction seems autocatalytic through the degradation products [12,27]. Water condensation was reported to occur preferentially at cracks and grain boundaries [6,34]. Moreover, grain boundaries often have a higher density of defects, which help both the adsorption and reaction of water molecules [27]. In the beginning of degradation process, only few $\alpha \to \delta $ transformations happened and this would not result in a rapid decrease of photoconductivity. However, with the continuous phase transformation, more $\delta $ phase appears with more phase boundaries. These newly born boundaries can act as new centers for water condensation. Meanwhile, the $\delta $ phase shows a higher hygroscopicity [12], which speeds up the water absorption. This presumably is why the reaction sequence shows an accelerated transformation rate. As for the FAPEAPI film shown in Fig. 4(e), the critical time point beyond which the photoconductivity decreases rapidly is elongated. It has been pointed out that at small to moderate concentrations, complete 2D layered perovskite can’t be formatted and the 2D perovskite just spontaneously forms at grain boundaries and surfaces to passivate the ingression pathway of moisture and suppress ion migration [25,45]. A schematic in Fig. 4(b) shows the formatted 2D perovskite at the grain boundaries of the 3D FAPI. As the PEA fraction in the precursor solution of FAPEAPI film is just 1.67 mol%, we attribute the delay of the critical time point and enhanced stability to the passivated effect of 2D PEA₂PbI₄ perovskite at the grain boundaries. However, with the reaction of phase transition, more boundaries emerge, but the amount of the 2D perovskite is not changed. So the effect of protection is weakened with the emerging phase boundaries and the degradation rate jumps. As for the FACsPI film, the crystal structure schematic is shown in Fig. 4(c). It seems that more grain boundaries are formatted because of the smaller grain size (Fig. 2(f)), increasing possibility of water absorption. However, the incorporation of Cs also optimizes the Goldschmidt tolerance factor [20], which favors the formation of $\alpha $ perovskite phase thermodynamically [22] and suppresses the cracks effectively. What’s more, it’s energetically unfavorable for the unpleasant transformation reaction for the FACsPI film [22]. In other words, the introduction of Cs can reduce the sensitivity to moisture effectively, that is why the FACsPI film shows a steady decay rate even with more grain boundaries, as well as the best performance on stability in normal ambient conditions.

By recording the transmitted THz signals through the sample E_sam, and through the quartz substrate E_sub, we can abstract the complex transmission function and then the complex refractive index. Finally, the complex photoconductivity $\Delta \sigma (w )= \Delta {\sigma _1}(w )+ i\Delta {\sigma _2}(w )$ is obtained through the complex refractive index [43,46]. As the real photoconductivity ($\Delta {\sigma _1}(w )$) increases with the frequency and the imaginary part ($\Delta {\sigma _2}(w )$) is negative, Drude-Smith (DS) model is adopted to extract the carrier density and mobility. In the DS model, the complex photoconductivity is given by [46,47]

\Delta \tilde{\sigma } = \frac{{N{e^2}}}{{{m^{\ast }}\gamma }}{\; }\frac{1}{{1 - iw/\gamma }}\left[ {1 + \frac{{{c_1}}}{{1 - iw/\gamma }}} \right]

Where N is the carrier density, $\gamma $ is the scattering rate between free carrier and ions, ${m^{\ast }} = 0.2m$ is the effective carrier mass [23,42,48], w is the angular frequency, and e is the elementary charge. The parameter ${c_1}$, which is defined as the fraction of initial velocity of carrier after a collision, is presumably decided by the grain boundaries or defects of the perovskite films [49]. The fitted data are displayed in Fig. 5(a) and 5(b). It’s calculated that initial ${c_1}$ for the FAPI, FAPEAPI and FACsPI films is -0.98, -0.89, -0.91, respectively. The smaller${\; }|{{c_1}} |$ values of FAPEAPI and FACsPI imply that the addition of PEAI and CsI can both improve the crystallinity, as shown in the XRD patterns. It’s also found that the absolute value of ${c_1}$ increases with the phase transformation reaction, meaning the localization of carrier is increasing [42]. The ${c_1}$ of FAPI film reaches -1 in two days, suggesting most of the carriers were localized [50]. So we don’t display its time-dependent effective carrier mobility in the graph. The fitted carrier concentrations for the FAPI, FAPEAPI and FACsPI films are 1.42${\times} $10²¹ cm⁻³, 1.56${\times} $10²¹ cm⁻³ and 1.55${\times} $10²¹ cm⁻³, respectively, which are attributed to the higher optical pump intensity (540µJ cm⁻²) [31]. The carrier mobility can be extracted by the formula $\mu = \frac{e}{{\gamma {m^{\ast }}}}$. Considering the influence of disorder, Fig. 5(c) and 5(d) shows the time dependence of $({1 + {c_1}} )\mu $. It’s clear that the time-dependent carrier mobility also shows a critical time point in the FAPEAPI film while that of the FACsPI film shows a steady decreasing rate. Meanwhile, the initial mobility of FAPEAPI is almost twice as that of FACsPI film, it’s possibly caused by the smaller grain size of FACsPI film (Fig. 1(f)). In the end, we estimated the diffusion length ${L_D}$via ${L_D} = \sqrt {D/{R_{total}}} $, where the total recombination rate is ${R_{total}} ={-} \frac{1}{{N(\tau )}}\frac{{dN(\tau )}}{{d\tau }}$, and $N(\tau )$ is the carrier concentration at different delay time. The diffusion constant D is calculated by the carrier mobility and the temperature T via $D = \frac{{\mu {k_b}T}}{e}$, where${\; }{k_b}$ is the Boltzmann’s constant. It’s reported that the quantum efficiency of HOIPs is between 0.03 and 0.065 [48,51,52], and we chose 0.05 as a possible value. As a result, the diffusion length of the FAPI, FAPEAPI and FACsPI films is 0.49 µm, 0.57 µm, 1.20 µm, respectively. This relative result is possibly caused by cracks which are shown in the SEM images (Fig. 1(d)–1(f)), and demonstrates that the addition of PEAI and CsI can both improve the optoelectronic properties.

Fig. 5. The complex photoconductivity and time-dependent carrier mobility of different samples. (a) Real and (b) imaginary conductivity of the perovskite films. Experimentally extracted data and the fitted data are displayed by points and solid lines, respectively. The effective carrier mobility evolution of the (c) FAPEAPI and (d) FACsPI films.

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4. Conclusions

In conclusion, the charge-carrier dynamics of the 2D passivated and Cs stabilized FAPI perovskite films has been determined by the optical pump THz probe technique. We observed the critical time point beyond which the optoelectronic performance decreased rapidly in the FAPI and FAPEAPI films while the FACsPI film showed a steady decay rate. It’s inevitable to format grain boundaries in the crystallization process, and water condensation tends to happen at the boundaries and accelerates the degradation reaction. So the photoconductivity of the FAPI film showed an accelerated decay rate. When the 2D PEA₂PbI₄ forms at the grain boundaries of FAPI, it can effective suppress the water condensation and enhance the stability performance. However, the effect of protection will be weakened with the emerging phase boundaries. As for the FACsPI film, the incorporation of Cs effectively reduced its sensitivity to water and no obvious cracks formatted in the annealing process, so the FACsPI film showed the best stability and no critical point found in the degradation process. Moreover, Drude-Smith model was applied to fit the complex conductivity. Similar rules are also found in the evolution of carrier mobility. It’s also revealed that the localization of carrier increased with the degradation process, and the time of complete localization is consistent with the observed stability performances. This report offers a new perspective on the degradation process of the HOIPs, and it can promote our understanding on the degradation process. Furthermore, subsequent works can be investigated to obtain more information and improve the environmental instability of HOIPs, by varying amounts of PEA or Cs in the HOIPs, employing heterogeneous (multi-layered) preparation with PEA, and utilizing different substrates to avoid cracks, and so on.

Funding

National Natural Science Foundation of China (11704373, 51627901); National Key Research and Development Program of China (2020YFA0710100); Anhui Initiative in Quantum Initiative in Quantum Information Technologies (AHY100000); Fundamental Research Funds for the Central Universities (WK2340000071).

Disclosures

The authors declare that there are no conflicts of interest related to this article.

Data availability

Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.

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Unraveling the degradation process of 2D passivated and Cs stabilized FAPbI₃ by optical pump THz probe spectroscopy

Abstract

1. Introduction

2. Experimental methods

3. Results and discussion

4. Conclusions

Funding

Disclosures

Data availability

References

Data availability

Cited By

Figures (5)

Equations (3)

Optical Materials Express