A special issue on Optoelectronics in Energy

1 Introduction

Organometal trihalide organic-inorganic hybrid perovskites with general formula ABX₃, where the “A” site is an organic-molecule cation, were first discovered in 1978 [1,2]. The structural understanding, solution-processing, and properties of this remarkable family of materials were further developed in the 1990s [3], but their use in solar cells in 2009 [4] has sparked tremendous interest in these hybrid organic-inorganic materials for photovoltaics [5–13], and also other applications [14–17]. The most popular and the most well-studied perosvkite is methylammonium lead triiodide (CH₃NH₃PbI₃ or MAPbI₃). This is primarily because of the relative ease by which high-quality thin films of MAPbI₃ can be deposited using a variety of solution-based techniques [18–20], and the high power conversion efficiencies (PCEs) that can be obtained in perovskite solar cells (PSCs) based on thin films of MAPbI₃ [10–13]. While MAPbI₃ perovskite has been characterized extensively using a battery of analytical methods, there is relatively little effort on its Raman spectroscopy in the open literature [21–25]. Raman spectroscopy is a very powerful and sensitive vibrational spectroscopy characterization tool for gaining insight into the structure of materials, in particular organic-inorganic hybrid materials like MAPbI₃ perovskite, and complements other characterization tools such as infrared (IR) spectroscopy, X-ray diffraction (XRD), and nuclear magnetic resonance (NMR) [26]. Strong Raman signal can be obtained from small volume of material, making it highly sensitive [26]. Also, Raman signal is sensitive to strain, which can be used to measure the stress state in the material [26].

MAPbI₃ crystallizes in tetragonal perovskite structure under ambient conditions (Fig. 1) [27,28] because its Goldschmidt tolerance factor, t, is less than the ideal unity (t~0.83) [3,29]:

(1)$$t=\left[\right(r_{\mathrm{M}{\mathrm{A}}^{+}})+(r_{{\mathrm{I}}^{-}}\left)\right]/\left[\sqrt{\mathrm{2}}\right(r_{\mathrm{P}{\mathrm{b}}^{\mathrm{2}+}})+(r_{{\mathrm{I}}^{-}}\left)\right],$$

where r is the respective ionic radii. The space group I4/mcm indicates a⁰a⁰c^- (Glazer notation [30]) PbI₆-octahedra tilting (one-tilt) in MAPbI₃ to accommodate the non-ideal t. Furthermore, the presence of the polar MA⁺ cation in the PbI₆-octahedra “cage” could result in the loss of inversion symmetry in MAPbI₃ at room temperature [27]. While experimental Raman spectra of MAPbBr₃ [31] and MAPbCl₃ [32] perovskites under ambient conditions have been reported in the literature, ambient Raman scattering from MAPbI₃ perovskite has not been studied until very recently. However, there are significant discrepancies between the reported Raman spectra of MAPbI₃ perovskite [21,22,24,25]. This has motivated us to perform systematic Raman spectroscopy experiments on MAPbI₃ perovskite thin films in the ambient to investigate the possible sources of these discrepancies.

Fig.1 Crystal structure of MAPbI3 perovskite: (a) 〈100〉 view and (b) 〈001〉 view

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2 Experimental procedure

A smooth 100-nm thickness Al layer was deposited on quartz plates using electron beam deposition (Lesker Laboratory 18, Kurt Lesker, USA), which constitutes the substrates. Standard MAPbI₃ thin film with smooth, full-coverage morphology was then deposited on the substrates using a method reported elsewhere [33,34]. Briefly, 40 wt% precursor solution of MAI (Dyesol, Australia) and PbI₂ (Acros Organics, USA) (MAI:PbI₂ = 1:1, molar ratio) was dissolved in a mixed solvent of 1-N-methyl-2-pyrrolidinone (NMP)/g-butyrolactone (GBL) (7:3, weight ratio). The precursor solution was then spin-coated on the Al-coated quartz substrates by spin-coating at 4500 r/min for 10 s. The solution-coated substrate was then dipped into a stirring diethyl ether (Fisher Scientific, USA) bath for 2 min. The as-crystallized dark film was then taken out of the bath and rapidly dried using nitrogen, and annealed at 100°C for 5 min in a nitrogen-fill glovebox (<0.1 ppm<FootNote>

1 ppm= 1×10^-6

</FootNote> H₂O). The freshly-made film were characterized immediately.

The as-prepared MAPbI₃ thin films were characterized using XRD (Bruker D-8 Discover, Karlsruhe, Germany) with Cu Ka₁ radiation (l = 1.5406 Å) at a step size of 0.02°. The surface and cross-sections (fractured) morphology of the as-prepare MAPbI₃ thin films were characterized using a scanning electron microscope (SEM; LEO 1530VP, Carl Zeiss, Germany).

A confocal Raman spectrometer (Witec Alpha 300, Ulm, Germany) was used to collect Raman spectra in the ambient, using two different laser excitations: Nd:YAG 532 nm and He:Ne 633 nm. A 100× objective with a holographic grating of 600 grooves·mm^-1 (BLZ 500 nm), and a 100 mm diameter pinhole were used. In some cases, a holographic notch-filter was used with the 532 nm laser to collect Raman spectra at low wavenumber. The laser power at the sample for the 532 nm laser without and with the low-wavenumber notch-filter was 0.61 and 0.89 mW, respectively. For the 633 nm laser power at the sample was 4.31 mW (without the low-wavenumber notch-filter). All Raman measurements were performed under ambient conditions (~20°C, ~30% relative humidity (RH)).

3 Results and discussion

Figure 2(a) shows indexed XRD pattern of the as-prepared MAPbI₃ thin film confirming the presence of phase-pure tetragonal MAPbI₃ perovskite. The location of the main PbI₂ peak is marked, confirming the absence of PbI₂ within the resolution of the XRD [35]. The phase-purity of the MAPbI₃ thin film is of particular importance, primarily because the precursor (MAI and PbI₂) phases are Raman active, which will be discussed later. The top-view and cross-section SEM images in Figs. 2(b) and 2(c), respectively, show uniform coverage of the MAPbI₃ thin film on the substrate, indicative of a near-ideal thin-film sample characteristics. Note that the smooth Al layer of 100 nm thickness deposited on the quartz substrate prior to MAPbI₃ film deposition is Raman inactive (within the measured wavenumber range). This precludes any substrate-related artifacts in the Raman spectra that can occur when using other common substrates such as glass, TiO₂, ZrO₂, etc.

Fig.2 Characterization of MAPbI3 perovskite thin films deposited on Al substrate: (a) indexed XRD pattern, (b) top-view SEM image, and (c) cross-sectional SEM image

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Since Raman scattering is inelastic, which is independent of the wavelength of the excitation monochromatic light [26], detailed Raman studies are typically carried out using two lasers with different wavelengths (e.g., 532 and 633 nm, which are readily available commercially). This helps isolate any spurious effects such as fluorescence [26]. Figures 3(a) and 3(b) are Raman spectra of as-prepared MAPbI₃ thin films excited with two different lasers, and at two different integration times. The right panel shows corresponding optical images of the areas before and after Raman characterization. Raman spectra of the bare Al-coated quartz substrates (short integration time: 0.2 s, 10 accumulations) are also included as reference in Figs. 3(a) and 3(b), confirming no Raman signal from the substrate in the wavenumber range of interest. Figure 3(a) shows Raman spectra with short (0.2 s, 10 accumulations) and long (2.0 s, 10 accumulations) integration times using 532 nm laser excitation (0.61 mW power). The prominent band around ~110 cm^-1 in all the spectra is a spectral artifact due to the cut-off edge of the filter, which becomes more pronounced with long integration time, as expected. Note that the filter with different cut-off edges may result in such bands occurring at different wavenumbers. Besides the “filter-edge” effect, another prominent band around ~130 cm^-1 becomes better resolved at long integration time. However, the film has been significantly damaged by the laser, as seen in the corresponding optical images in the right panel. This suggests that this evolved band in Fig. 3(a) is primarily from degradation products of MAPbI₃ perovskite under the laser, and it can be the result of fluorescence and/or Raman scattering from those products. Again, such a band can also be the result of spectral artifacts, combined with signal integration, considering that it is near the filter cutoff. For Raman spectra using 633 nm laser excitation (4.31 mW power) in Fig. 3(b), similar results are obtained.

Fig.3 Raman spectra (left) of MAPbI3 thin films on Al-coated quartz substrates at short (0.2 s, 10 accumulations) and long (2.0 s, 10 accumulations) integration times (with regular notch filter), and corresponding before and after optical micrographs (right) of the sample: (a) 532 nm laser excitation, (b) 633 nm laser excitation

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To reduce the “filter-edge” effect of the regular notch filter, a low-wavenumber holographic notch filter is used to conduct more reliable Raman spectroscopy characterization (533 nm laser excitation), and the results are presented in Fig. 4. As seen in Fig. 4, the prominent bands at ~100 and ~130 cm^-1 that appear in Fig. 3(a) are absent, regardless of the integration time. While no observable peaks are shown in the short-integration time case, a shoulder at ~100 cm^-1 evolves with longer integration time. However, pronounced damage to the film is observed in the optical images at long integration time in the right panel of Fig. 4. This, again, indicates that these bands may originate from the degradation products rather than the MAPbI₃ perovskite itself.

Fig.4 Raman spectra (left) of MAPbI3 thin film on Al-coated quartz substrate at short integration time (0.2 s, 10 accumulations), with low-wavelength holographic notch filter (532 nm laser excitation)

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MAPbI₃ perovskite absorbs the laser radiation strongly, and it also has a low thermal conductivity (0.5 W·K^-1·m^-1) [36]. These factors can result in local heating of the film [25], which in turn is highly likely to result in the thermal decomposition of MAPbI₃ perovskite via MAPbI₃→PbI₂ + MAI reaction. Thus, PbI₂ and MAI are expected to be the first degradation products. Figures 5(a) and 5(b) show typical ambient Raman spectra of PbI₂ and MAI (short integration time: 0.2 s, 10 accumulations; 532 nm laser; with low-wavelength holographic notch filter), respectively. The three main bands in PbI₂ occur at ~73, ~97, and ~112 cm^-1 in Fig. 5(a). These are fully consistent with those observed by Khilji et al. [37] for PbI₂ at room temperature. For MAI, a major band occurs at ~114 cm^-1 (Fig. 5(b)). The coherent anti-Stokes lines are also observed in their Raman spectra (not shown here), which suggests that those bands are indeed from Raman scattering from PbI₂ and MAI. It has been suggested that PbI₂ can degrade into Raman-active lead oxides (PbO_x) in the ambient under laser irradiation [38]. These considerations support the argument that the observed shoulder band in Fig. 3(a) (long integration time) is most likely from primary (PbI₂, MAI) and secondary (lead oxides) products of MAPbI₃ perovskite degradation.

Fig.5 Fig. 5 Raman spectra of (a) PbI2 and (b) MAI thin films on Al substrates: short integration time (0.2 s, 10 accumulations); 532 nm laser excitation; with low-wavelength holographic notch filter. Dashed lines mark the centers of the PbI2 bands

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While identical characterization conditions have been used for the MAPbI₃, PbI₂ and MAI samples (short integration time: 0.2 s, 10 accumulations; 532 nm laser; with low wavelength holographic notch filter) in Figs. 4 and 5, well-resolved Raman scattering spectra have been observed only in the case of PbI₂ and MAI. This implies that significant Raman scattering under laser excitation does not occur in the MAPbI₃ perovskite case under ambient conditions. Note that we also conducted Raman spectroscopy on phase-pure MAPbI₃ perovskites (single crystals or thin films) that were deposited using other typical methods (“one-step,” “two-step,” solvent engineering, etc.) [10–13], with consistent results. Thus, the characterization of the relatively weak Raman scattering in MAPbI₃ perovskite has been highly challenging using typically available laser excitation sources. While the tradeoff between sample damage (chemical stability in the ambient under laser irradiation) and signal intensity by adjusting the integration time is well known, this tradeoff is particularly severe in the MAPbI₃ perovskite case — low integration time yield little to no Raman signal, while longer integration time needed for obtaining Raman signal damages the sample, and the spectra are essentially from the degradation products rather than the MAPbI₃ perovskite itself. This appears to be responsible for the striking variability in the Raman spectra of MAPbI₃ perovskite reported in the literature. To circumvent this issue, Ledinský et al. [25] used a 785 nm laser, but still found damage to MAPbI₃ perovskite at room temperature. Brivio et al. [23] also used a 785 nm laser, but they were able to perform their Raman experiments on MAPbI₃ perovskite only at 100 K (orthorhombic phase), where they found no damage. Ledinský et al. [25] have suggested that the optimal laser wavelength is 830 nm, which is above the absorption edge and peak photoluminescence (PL) in MAPbI₃ perovskite. Unfortunately, such long-wavelength lasers are not readily available commercially. While one can experiment with power and integration time parameters using conventional excitation lasers used in commercial Raman spectrometers, questions will remain regarding the validity of those measurements. Thus, to remove any ambiguity regarding the Raman results for MAPbI₃ perovskites under ambient conditions, it is imperative that infrared (>830 nm wavelength) excitation lasers must be used in the future.

4 Conclusion

A systematic ambient Raman spectroscopy characterization study of MAPbI₃ thin films is conducted by varying the excitation laser wavelength, integration time, and filter characteristics. The comprehensive results, which are obtained using available characterization resources, indicate that Raman scattering under laser excitation under ambient conditions from MAPbI₃ perovskite is insignificant compared to that from the precursor phases and/or degradation products (MAI, PbI₂, PbO_x, etc.). This does not exclude the possibility of the existence of the weak Raman scattering in MAPbI₃ perovskite. It is found that the use of typically available laser excitation sources (532 and 633 nm) easily damages the MAPbI₃ thin films, and resulting Raman spectra contain spectral artifacts and bands from Raman scattering by primary (PbI₂, MAI) and secondary (lead oxides) degradation products rather than the MAPbI₃ perovskite itself. This becomes a challenging issue in the ambient Raman characterization of MAPbI₃ perovskite, and it is likely to be the source of the discrepancies found in the Raman spectra of MAPbI₃ perovskite reported in the literature. This challenge may be overcome by using the less common infrared (>830 nm wavelength) excitation lasers. The lessons learnt from this Raman spectroscopy studies of MAPbI₃ perovskite can be extended to other laser-sensitive light-absorbing compounds such as CsSnI₃, HC(NH₂)₂PbI₃, etc.

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