1. Introduction

In recent years, solution-processed perovskite semiconductors in the form of thin films, nanocrystals and single crystals have attracted great attention for their potential applications in solar cells, light-emitting diodes (LEDs), photodetectors and lasers [1], because of their excellent optical and electrical properties, such as long carrier lifetimes, micrometer-scaled diffusion lengths, high charge mobilities and large optical absorption [2]. In particular, organic-inorganic hybrid perovskites (CH₃NH₃PbX₃, X = Cl, Br, and I) have enabled solution-processed solar cells with power conversion efficiency of 23.7% [3]. Meanwhile, perovskites have shown high photoluminescence quantum yield (PLQY) and large optical gain [4]. A red hybrid perovskite LED with an external quantum efficiency (EQE) of 20.7% has been demonstrated by Wang et al. [5]. Moreover, wavelength-tunable amplified spontaneous emission (ASE) in hybrid organic-inorganic CH₃NH₃PbX₃ films at low thresholds of photoexcitation was demonstrated [6]. Even more, low threshold single-crystal CH₃NH₃PbX₃ nanowires lasers at room temperature (RT) were obtained [7], and also two-photon pumped CH₃NH₃PbBr₃ microwire lasers [8]. However, hybrid organic-inorganic perovskites present poor thermal stability due to the volatile organic constituents [9]. Inorganic perovskites CsPbX₃ (X = Cl, Br, and I) have been studied intensively for their applications in solar cells, photodetectors, LEDs and lasers, due to their enhanced stability and excellent optoelectronic properties compared to their hybrid organic-inorganic counterparts. They have also drawn great attention as laser gain medium. Benefiting from various processing methods, such as solution processing [10], chemical vapor deposition (CVD) [11] and atomic layer deposition (ALD) [12], many inorganic micro/nano perovskite laser devices have been developed, such as perovskites quantum dot lasers [13], wavelength tunable CsPbX₃ (X = Cl, Br, and I) nanowire lasers [14], microdisk lasers fabricated using solution self-assembly method [15–18] and a single-mode laser in a cesium lead halide perovskite submicron sphere [19]. Thin film perovskite laser array with two dimension (2D) photonic crystal resonator [20] and random CsPbBr₃ laser have also been demonstrated [21]. Compared with their inorganic-organic hybrid counterpart CH₃NH₃PbX₃, all-inorganic perovskites present much stronger environment stability and photo-thermal stability (thermal decomposition temperature over 500 K in air) [22]. They have also very high physical and chemical stability. Under 1.2 times threshold pumping excitation, the integrated emission intensity of CsPbBr₃ nanowire lasers maintained the same after over 4.4×10⁸ excitation cycles under atmosphere condition [23]. Besides CsPbX₃ perovskite nanomaterials, polycrystalline perovskite CsPbX₃ thin films used for ASE or lasing have also been reported [24–26]. Compared with CsPbX₃ nanomaterials, the synthesis of CsPbX₃ thin films using one or two-step method is much simpler and the organic capping ligands are not needed to stabilize the nanocrystals during the preparation process. However, the performance of the ASE or lasing in CsPbX₃ thin films is not as efficient as CsPbX₃ nanocrystal materials since the internal quantum efficiency of CsPbX₃ thin films (∼7%) is much lower than that of CsPbX₃ nanocrystals (∼90%) [21] . The conventional one-step method only needs direct spin-coating of a CsPbX₃ precursor solution, it is usually difficult to control perovskite crystallization and film quality due to low solubility of the cesium bromide (CsBr) precursor [22], causing voids and pinholes in the films. Polymer additive poly(ethylene oxide) (PEO) has been introduced into CsPbX₃ thin films to improve the quality of the films, i.e., the film morphology and the PLQY, thus to enhance EQE of the CsPbX₃ LEDs and the ASE or lasing performance of the films [27]. Inorganic solids ZnO nanoparticles (NPs) were also introduced into the CsPbX₃ precursor solution to enhance the crystallization and to improve the photoluminescence (PL) intensity of the final CsPbBr₃:ZnO films [21], the introduction of ZnO NPs provided an effective route for CsPbBr₃ nucleation during film forming process and led to compact and smooth films with no obviously large voids and pinholes. ASE was also investigated in CsPbBr₃ thin films at various temperatures and trap densities [26]. Better photophysical properties, such as PLQY, photoluminescence relaxation dynamics, and electroluminescence properties have been observed in films deposited from CsBr-rich solution, which was ascribed to a reduction of halogen vacancies [26]. Recently, You et al. incorporated a small amount of organic cation (MA) into the CsPbBr₃ lattice and prepared a mixed-cation Cs_0.87MA_0.13PbBr₃ film as the emitting layer in a green LED, the optimized perovskite composition modulation together with electron-injection interface passivation with a polymer polyvinyl pyrrolidine (PVP) minimized pinholes in the perovskite film and reduced the device leakage current, thus resulted in obtaining a high brightness of 91,000 cdm⁻² and a high external quantum efficiency (EQE) of 10.4% [28]. The improved device performance was attributed to improved film morphology, reduced non-radiative recombination as well as improved charge injection balance. By far, all of the reported inorganic perovskite film waveguides were fabricated by one step method. Two step method has never been employed to fabricate inorganic perovskite ASE waveguides, especially at room temperature and atmospheric conditions. Compared with one-step method, films of better flatness and compactness can be more easily fabricated with the two-step method. The solvent volatilization and perovskite crystal crystallization occur in the same process for the one-step method, which is adverse to the formation of compact and flat films due to film shrinkage. But for the two-step method, the final films quality is mainly determined by the PbX₂ films fabricated during the first step [29].

In this work, inorganic CsPbBr₃ polycrystalline films were facilely prepared via a two-step solution synthesis method. During the synthesis process, we incorporated a series of organic cations of various chain lengths, i.e., methylammonium (MA), ethylammonium (EA), butylammonium (BA) and octylammonium (OA) into CsPbBr₃ films to improve film quality [28,30]. By introducing ammonium halides of different chain lengths into the CsBr precursor solution, we demonstrate an effective route for CsPbBr₃ nucleation during the two-step synthesis process to better control the film morphology and reduce the occurrence of pinholes or voids. The crystalline structural properties and photophysical properties, as well as the ASE properties of the resulted films were investigated in detail. X-ray diffraction (XRD) confirmed the crystalline phase of the perovskite nanocrystals in the films. Multiple spectroscopy measurements, including UV-visible optical absorption, steady-/transient-state fluorescence and power-dependent PL, indicate that notable green emission originates from excitonic radiative recombination of the CsPbBr₃ nanocrystals. Owing to the improved film compactness and smoothness, the CsPbBr₃ films with organic cations show enhanced PL and ASE efficiency. The ASE thresholds decreased from 404 µJ/cm² for the pristine CsPbBr₃ films, to 271 µJ/cm², 300 µJ/cm², 338 µJ/cm² and 399 µJ/cm² for MA, EA, BA and OA doped CsPbBr₃ films, respectively. We expect these results can provide an effective route to the synthesis of high-quality perovskite films and promote their applications in waveguide lasers.

2. Experimental section

Lead bromide (PbBr₂, 99.99%) was purchased from Sigma. Cesium bromide (CsBr, 99.99%) was purchased from Alfa Aesar. Methylammonium Bromide (MABr, CH₃NH₃Br) (>99.5%), Ethylammonium Bromide (EABr, CH₃CH₂NH₃Br) (>99.5%), Butylammonium Bromide (BABr, CH₃(CH₂)₃NH₃Br)(>99.5%), Octylammonium Bromide (OABr, CH₃(CH₂)₇NH₃Br) (>99.5%) were purchased from Xi’an Polymer Light Technology Corp. Dimethyl formamide (DMF, >99.9%) and methanol were purchased from Sigma Aldrich. All solvents were used without further purification.

The CsPbBr₃ thin films were fabricated by two step solution method since it is a facile and low-cost approach for large-scale production of perovskite materials (Fig. 1). Briefly speaking, 300 mg PbBr₂ dissolved in 1 mL DMF solvent and 10 mg/mL CsBr in methanol were prepared as precursor solutions [31]. For films incorporated with the organic cations, MABr, EABr, BABr and OABr were introduced into CsBr solution at 1:20 molar ratio of the organic cations and Cs⁺ (MA, EA, BA and OA: Cs⁺).

 

Fig. 1. Schematic diagram of the fabrication process of inorganic perovskite films.

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The PbBr₂ solution was firstly stirred at 60 °C for 5 h, then filtered by hydrophobic filter head (PTFE) (0.45 µm, Titan) for using. Quartz substrates were cleaned by sonication in detergent, deionized water, acetone, and isopropanol for 20 min successively. Subsequently, substrates were dried by nitrogen flow and cleaned with ultraviolet ozone plasma for 15 min and heated at 75 °C before use. The PbBr₂ solution was heated up and kept at 75 °C, then spin-coated on the quartz substrates at 3000 rpm for 30 s, followed by annealing on a hot plate at 75 °C for 30 min. Then the PbBr₂ film was dipped and immersed into the CsBr solution at 50 °C for 5 min to induce perovskite crystallization and enable the growth of the pristine CsPbBr₃ films. Finally, the films were air annealed at 150 °C for 10 min to remove the organic residues and to obtain the final perovskites granular films. The whole process of film fabrication was carried out under ambient environment at room temperature.

The crystalline structure of the CsPbBr₃ perovskite films were characterized by XRD with CuKa radiation (Smartlab, Rigaku). SEM images were taken by a scanning electron microscope (JSM-7800F, JEOL, Japan). Absorption spectra were recorded by a UV-vis-NIR spectrophotometer (SHIMADZU UV-1750, Shimadzu, Japan), PL spectra of the perovskite films were obtained by a fluorescence spectrophotometer (F-4600, Hitachi, Japan), and time-resolved fluorescence spectra were achieved using a fluorescence lifetime spectrofluorometer (Edinburgh Instruments Ltd., FLS980, United Kingdom). The surface roughness of the films was measured using atomic force microscope (XE-70, Park). The PLQY of the perovskite films were obtained by a home-made PLQY measurement system.

A nanosecond Nd:YAG laser system in combination with an optical parametric oscillator (OPO) system (Surelite-II-10 Continuum) was used as the pumping source for the ASE excitation. The output wavelength of 355 nm of the Nd:YAG laser (repetition rate: 10 Hz, pulse width: 5 ns) was used to pump the OPO to generate a lasing output of 400 nm (repetition rate: 10 Hz, pulse width: 5 ns), which was employed to excite ASE of the CsPbBr₃ films. The laser beam passed successively through diaphragm (4 mm in diameter), slit (4 mm in width), neutral density filter and was then focused by a cylindrical lens (focal length: 10 cm) into a narrow strip (4.7 mm ×0.27 mm), which irradiated onto the sample surface. The light signals emitted from the samples were collected by an optical fiber and detected by a spectrometer in conjunction with a CCD detector (SR-500i, Andor) with a spectral resolution of 0.1 nm. All the measurements were performed in atmosphere air and at room temperature (293 K).

3. Results and discussion

CsPbBr₃ perovskite thin films were fabricated by spin-coating a PbBr₂ precursor in DMF onto quartz substrates, followed by annealing at 75 °C for 30 min to remove the solvent. Then the coated PbBr₂ layers were dipped into CsBr precursor in methanol. The soaking time was firstly optimized in order to achieve pure phase CsPbBr₃ films. Figure 8 shows the images of the obtained films prepared from various soaking time. XRD was also utilized to monitor the crystalline structure of the films during their growth. The films prepared from 5 min soaking time presented the best film quality, as verified by the XRD patterns Fig. 9). The CsPbBr₃ perovskite films deposited onto quartz substrates exhibited a high density of pinholes (Fig. 2(a)). Then we added a small amount of organic additives, i.e., CH₃NH₃Br (MABr), CH₃CH₂NH₃Br (EABr), CH₃(CH₂)₃NH₃Br (BABr), CH₃(CH₂)₇NH₃Br (OABr), into the CsBr precursor solution to control the crystallization kinetics of the CsPbBr₃ films through molecular pinning [32,33], thus to reduce the pinholes and to improve the film morphology. The molar ratios of the organic cations and Cs⁺ were optimized at 1:20. Figures 2(b)–(e) show the SEM images of the pristine CsPbBr₃ films and CsPbBr₃ films doped with organic cations. The morphology of the CsPbBr₃ films was improved considerably once we added the additives. They presented a negligible density of pinholes and smooth and compact surface. The gullies between crystalline grain boundaries were also largely reduced. The CsPbBr₃ films prepared with ammonium halides presented relatively smaller crystalline grains. The size statistics of the corresponding films are given in the inserts of Fig. 2(b)-(e). We noticed that organic cations with longer chains were beneficial to achieve smaller crystal grain size, the CsPbBr₃ crystalline size reduced from 528 nm for the films without additives to 447 nm, 387 nm, 334 nm and 268 nm for MA, EA, BA and OA doped films, respectively. We ascribed the size reduction of grains to the introduction of organic cations.

 

Fig. 2. SEM images of different organic cations doped perovskite films ((a). CsPbBr₃, (b). CsPbBr₃:MA, (c). CsPbBr₃:EA, (d). CsPbBr₃:BA, (e). CsPbBr₃:OA).

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Surface roughness affects the optical characteristics of the films significantly. The surface roughness of the films was measured with AFM. From the AFM images shown in Fig. 10, we can see the surface of the CsPbBr₃ films doped with additives were improved compared to the pristine CsPbBr₃ films. The roughness of the CsPbBr₃ films (scanning area 5 µm × 5 µm) was 38.46 nm. while the films doped with organic cations were 28.02 nm, 28.83 nm, 29.94 nm, 30.51 nm for films doped with MA, EA, BA and OA, respectively. The pinholes were filled, and the films surface was smoothed out. The smoother surface will decrease the scattering, reflection and deflection loss of incident pumping light at air-film interface and enhance the proportion of the pump light propagating in the films, which would be beneficial for the generation of ASE in the CsPbBr₃ films with organic cations. The non-radiative transition loss was also reduced at grain boundaries due to the filling effect of the organic cations.

We characterized the crystal structure of the CsPbBr₃ films and films fabricated with organic additives. XRD measurement revealed the effect of doped organic cations on the crystallization properties of the films. Figure 9 shows the X-ray diffraction patterns of the pristine films prepared at different soaking time, which were almost identical, with all of the main crystallographic signatures matching that of the pure CsPbBr₃ phase. Figure 3 shows the X-ray diffraction patterns of the doped films. The diffraction peaks at 21.69° and 34.62° are from (110) and (210) planes of the CsPbBr₃ crystal in Fig. 3. Besides of the pure CsPbBr₃ phase, the diffraction peak located at 11.6° can be assigned to the resident tetragonal crystal of CsPb₂Br₅ in the films, the diffraction peaks at 12.66°, 12.94° and 22.48° belong to (012) (110) and (300) planes of Cs₄PbBr₆ crystal [30,31]. The generation of metastable CsPb₂Br₅ phase has also been observed by other groups [27,31,34]. It is attributed to the nonstoichiometric material transfer and structural rearrangement during the solution synthesis and thermal annealing process [35,36]. The transformation from CsPb₂Br₅ to CsPbBr₃ happened when the reaction continued and there always exists an impure phase CsPb₂Br₅ in the final films, this secondary phase can reduce the trap state density of the CsPbBr₃ and benefit to the light emission of the CsPbBr₃ nanocrystals [37]. 5 min soaking time was found to be the optimized reaction time. The ASE mainly originates from CsPbBr₃ rather than Cs₄PbBr₆ [38]. We found that the relative intensities of the peaks decreased with increasing chain length, which meant long chain organic amine ions were in favour of the formation of perovskite crystals. In addition, all of the XRD diffraction peaks were quite sharp and narrow, indicating the great crystallinity of the perovskite films. The intensity of the peaks became stronger, which implied the ratio of orthorhombic CsPbBr₃ crystals increased as prolonger reaction time. We found that the XRD patterns from the CsPbBr₃ films doped with different organic cations were almost identical, as the XRD diffraction peak positions did not change as the variation of the chain length of the additives. We considered that most of the organic cations did not incorporate into the perovskite lattice structure in the two-step solution process, they only cladded on top of the surface of perovskite crystalline grains similar as surface-active agent [39], thus the introduction of the organic cations did not change the intrinsic perovskite crystalline structure. On the other hand, from the SEM images, we did not find any quasi two-dimension perovskite crystalline structure as expected, when introducing longer chain cations. In addition, longer chain ammonium with larger steric hindrance prevented ions (Cs⁺, Pb²⁺) from continuously entering into the forming perovskite crystals and prevented crystal growth, so the films doped longer chain length had smaller grain size. The intensity of the XRD diffraction peaks from films doped with additives enhanced, indicating better crystallization in the additives doped films. The diffraction peak intensity of films doped with additives with shorter chain length was stronger than those of films doped with additives with longer chain length, which indicated that organic cations with shorter chain length were more beneficial to the crystallization of the CsPbBr₃ nanocrystals.

 

Fig. 3. XRD patterns of the additives doped perovskite films.

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Combined with the SEM and XRD results, all of the characteristics imply that high-quality perovskite films can be obtained by introducing a certain amount of ammonium halides into the CsBr precursor. The addition of ammonium halides affects the crystal nucleation on the substrate, hindering the crystal grains to grow up. The CsPbBr₃ films with organic cations have less pinholes, and the crystal grains of the CsPbBr₃ films with additives became smaller than that of the pristine films. The organic chains of the ammonium halides play a similar role to the organic ligands on nanocrystals surface (or quantum dots) during the CsPbBr₃ crystal growth [40].

The absorption, photoluminescence (PL) spectra and decay lifetime of the CsPbBr₃ films and CsPbBr₃ films doped with additives are shown in Fig. 4. The absorption and fluorescence spectra showed that the doped organic cations had no effect on the band gap structure of the CsPbBr₃ films. The absorption spectra of the CsPbBr₃ films with additives showed almost no change in comparison with the pristine films, with the absorption peak at around 518 nm, indicating that the addition of organic cations into CsPbBr₃ precursor solution does not affect the original absorption property of CsPbBr₃ films. We then carried out steady-state PL on CsPbBr₃ films prepared with different additives, negligible shifts of the emission peaks’ positions were observed from the doped CsPbBr₃ films relative to the pristine films. The emission peak was around 535 nm. We also observed the full width half maximum (FWHM) of the PL was around 16 nm for MA, EA, BA, and OA doped films. The absorption and PL characterization results are summarized in Table 1. In our experiments, we also studied the absorption and PL spectra of the CsPbBr₃ films with organic cations prepared from the CsBr precursors doped with different amounts of ammonium halides, the molar ratios of CsBr: MABr/EABr/BABr/OABr were set at 40:1, 20:1, 10:1, 5:1, 2.5:1, 1.7:1 and 1:1 (Fig. 11). We found that the absorption spectra of them were almost identical, which suggests that during the two-step synthesis process of the CsPbBr₃ films, the organic cations did not incorporate into the CsPbBr₃ crystal lattice and no Ruddlesden-Popper layered perovskite structures were generated in the final films [28,40].

 

Fig. 4. Absorption spectra (a) and normalized PL spectra (b) of the perovskite films doped with different organic cations (Excitation wavelength: 400 nm).

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Table 1. Absorption spectra and fluorescence spectra of the CsPbBr₃ films doped with organic cations

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We next acquired time-resolved PL decay spectra of the different perovskite films. The time-resolved PL curves were fit to bi-exponential decays, where the fast decay component is associated with trap-assisted recombination at grain boundaries or surfaces, and the slow decay is ascribed to radiative recombination inside the bulk perovskite phase [41,42]. Figure 5 showed the decay curves for the different perovskite films. The decay times for the pristine films are τ₁=1.15 ns, τ₂=3.16 ns. For the films doped with MA, EA, BA and OA, the decay times are (τ₁=1.51 ns, τ₂=3.99 ns), (τ₁=1.93 ns, τ₂=4.66 ns), (τ₁=1.56 ns, τ₂=4.36 ns), (τ₁=2.36 ns, τ₂=6.43 ns), respectively. Generally, it was found that the PL lifetime of the perovskite films increased after the addition of the organic cations. The longer the chain length of the organic cations, the longer the PL lifetime. We hypothesize that this might be a result of reduced bulk defects. In addition, stronger PL intensity was achieved from the films with organic cations. The PLQY of the CsPbBr₃ films were measured to be ∼1.9% at a light intensity of 3.1 mW/cm². Together with the improvement of film morphology, it can be proved that the ASE properties of the CsPbBr₃ films with additives could be improved compared to the pristine films.

 

Fig. 5. Time-resolved fluorescence spectra of the perovskite films with organic cations.

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To evaluate the ASE properties of the CsPbBr₃ films, laser excitation experiments were conducted. The laser output pulses with a pulse width of 5 ns, central wavelength of 400 nm and repetition rate of 10 Hz from an optical parametric oscillator (OPO) pumped by a Nd:YAG laser at 355 nm were focused onto the films by a cylindrical lens to form a narrow strip (4.7 mm×0.27 mm) pumping zone. Measured emission spectra of the films are given in Fig. 6. Figure 6(a) shows the power-dependent emission spectra of the films under excitation. For the CsPbBr₃ films with a thickness of 210 nm, a broad spontaneous emission (SE) spectrum centred at about 533 nm with a FWHM of about 22 nm can be observed under relatively low pump excitation (< 404 µJ/cm²). When the pump intensity increased, a relatively sharp and narrow peak at about 539 nm appeared and then became the dominant spectral component of the emission spectrum. Meanwhile, the FWHM of the emission spectra gradually reduced to about 4 nm, indicating that the SE transformed into ASE. Similarly, the emission of the CsPbBr₃ films doped with organic cations also changed from SE into ASE with the increasing intensity of the pump laser. The ASE emission intensity of the CsPbBr₃ films doped with organic cations was stronger than that of pristine films as shown in the Fig. 6, which agrees well with the results of PL emission. The ASE thresholds for the CsPbBr₃ films doped with organic cations were lower than those for the pristine films, which were 271 µJ/cm², 300 µJ/cm², 338 µJ/cm² and 399 µJ/cm², respectively. The MA doped perovskite films had the lowest ASE threshold. There are several aspects contributing to the ASE enhancement. Firstly, by introducing the organic cations, the crystal grains became smaller and the larger pinholes or voids have been effectively reduced, therefore the waveguide loss has been reduced. Secondly, the CsPbBr₃ films with organic cations showed more compact and smoother surface, which makes the light in the films have higher chance to achieve total reflection and reflect back to the films, thus the escaping light at air/film interface could be decreased. As a result, the light is expected to undergo oscillation with much more gain before it outputs as ASE from the edge of the films, which can decrease the ASE threshold and enhance the ASE intensity. We also measured the gain and loss coefficients for the CsPbBr₃ films using variable stripe lengths (VSL) method [43]. Figure 12 shows the stripe-length dependence of PL detected at peak wavelengths of 539 nm of the ASE spectra. The stripe width was ∼2 mm, excitation intensity was about 1200 µJ/cm², which was nearly 3 folds above the ASE thresholds. The gain and loss coefficients were deduced from the classical VSL equation I = I₀/g·{exp(g·L)-α}. The measurement results are summarized in Fig. 12 and Table 2. The gain coefficients were much higher for the CsPbBr₃ films with organic cations than that for the pristine films, while the losses were lower. We found that the CsPbBr₃ films with additives of longer chain lengths achieved lower gain coefficients and lower optical loss. The net gain for the CsPbBr₃ films doped with MA was the highest, which was complying with the ASE measurement results. The gain and loss of the doped perovskite films showed different trends as the variation of the chain length of the organic cations. From the SEM and AFM measurement results, we found that the surface roughness of the doped films increased with the chain length. The introduction of organic amine cations could reduce the perovskite grain size, which can then much better fill the holes and voids in the films, thus resulting in lower intrinsic optical loss of the film waveguides. However, smaller grains resulted in the unevenness of the film surface (as seen in Fig. 2). Increased surface roughness of the films led to larger scattering optical loss and interface reflection loss of the incident pump beam, which reduced the optical gain of the film waveguides in the end, thus the CsPbBr₃ films doped with OA achieved the lowest gain. From the lifetime measurement, we found that introducing organic cations with long chain length would significantly reduce the defects in the bulk, which is beneficial to reducing the nonradiative transition loss due to the energy transfer to defects. But the interface scattering loss increased with introducing the cations with longer chain length, the contribution from the scattering loss to the loss was less than that from the defects’ reduction. Thus, the reduction of the nonradiative transition to defects cancels out the scattering loss. Thus the OA doped films had the smallest loss. However, MA and EA doped films showed better film uniformity and flatness, thus lower scattering loss, which results in their larger net gain. In addition, from the XRD measurements, we found that with the increasing of the organic chain length, the diffraction peak intensity of Cs₄PbBr₆ (300) increased, especially for BA doped films, which means the increasing of Cs₄PbBr₆ phase. According to the previous work, Cs₄PbBr₆ phase has little contribution to ASE, which may be another cause of reducing the net gain.

 

Fig. 6. ASE spectra of the CsPbBr₃ films pumped by a 400 nm lasing output with a 5 ns pulse width at a repetition rate of 10 Hz. The top shows the fluorescence spectra at different pump energies and the down shows the fluorescence intensity variation as the pump energy increased. (CsPbBr₃ films (a)&(f), CsPbBr₃:MA films (b)&(g), CsPbBr₃:EA films (c)&(h), CsPbBr₃:BA films (d)&(i), CsPbBr₃:OA films (e) &(j)).

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Table 2. Gain and loss coefficients of the CsPbBr₃ films with different organic cations

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We measured photostability of the CsPbBr₃ films after introducing organic cations. The ASE intensities was monitored as the number of the pump pulse increased. Figure 7 shows the results. We can see the ASE intensity from the perovskite films with organic cations did not change significantly after being excited with 18k pulses at an excitation intensity of 1127 µJ/cm², 2.8 folds of thresholds. The increment of the ASE thresholds of the films were less than 10% after keeping the films for 5 days under ambient atmosphere (Fig. 13 and Table 3 in supporting information). The ASE intensity fluctuation in the photostability test may result from the unstable pump energies and film degradation under continuous pumping.

 

Fig. 7. Stability of different organic cations doped perovskite films (Pump energy: 1127 µJ/cm² above threshold).

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Table 3. Photostability of perovskite films at room temperature in air

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4. Conclusions

In summary, we have fabricated CsPbBr₃ perovskite thin film waveguides doped with a series of organic cations of various chain lengths, namely, MA, EA, BA and OA with a two-step solution process. The crystalline structure, surface morphology and photophysical properties of the films were characterized in detail. Organic cations could fill pinholes, thus reduce loss from nonradiative recombination at grain boundary and surface. We achieved ASE from all of different CsPbBr₃ films at RT, with the minimum thresholds of 271 µJ/cm², 300 µJ/cm², 338 µJ/cm², and 399 µJ/cm² for MA, EA, BA and OA doped CsPbBr₃ films, respectively. We also measured the gain coefficients for the doped CsPbBr₃ films, which were 48.2 ± 5.9 cm⁻¹, 45.3 ± 2.4 cm⁻¹, 27.9 ± 2.0 cm⁻¹, and 25.4 ± 2.0 cm⁻¹ for MA, EA, BA and OA doped CsPbBr₃ thin films, respectively. The intrinsic light loss for MA, EA, BA and OA doped films were 9.6 ± 1.0 cm⁻¹, 8.2 ± 0.6 cm⁻¹, 7.5 ± 0.4 cm⁻¹ and 5.2 ± 0.4 cm⁻¹, respectively. The organic cations incorporated films presented higher gain and lower optical loss, due to reduced bulk defects and non-radiative recombination in the films as well as improved surface quality of the films.

Appendix

A. CsPbBr₃ film image

 

Fig. 8. Images of the CsPbBr₃ films prepared at different soaking time.

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B. XRD of CsPbBr₃ film

 

Fig. 9. XRD patterns of CsPbBr₃ films prepared at different soaking time.

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C. AFM image of perovskite films

 

Fig. 10. AFM images of different organic cations doped perovskite films.

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D. Absorption and PL spectra of perovskite films

 

Fig. 11. Absorption spectra ((a). CsPbBr₃, (c). CsPbBr₃:MA, (e). CsPbBr₃:EA, (g). CsPbBr₃:BA, (i). CsPbBr₃:OA) and PL spectra ((b). CsPbBr₃, (d). CsPbBr₃:MA, (f). CsPbBr₃:EA, (h). CsPbBr₃:BA, (j). CsPbBr₃:OA) of different organic cations doped perovskite films (excitation wavelength: 400 nm).

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E. Gain and loss of the perovskite films

 

Fig. 12. Gain and loss of the CsPbBr₃ films (CsPbBr₃ films (a)&(b), CsPbBr₃:MA films (c)&(d), CsPbBr₃:EA films (e)&(f), CsPbBr₃:BA films (g)&(h), CsPbBr₃:OA films (i) &(j)).

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F. Stability of perovskite film after 5 days in air

 

Fig. 13. Stability study of perovskite films at room temperature in air (The left column shows the initial ASE thresholds; the right column shows the ASE thresholds after 5 days) ((a),(b) CsPbBr₃ film, (c),(d) CsPbBr₃:MA film, (e),(f) CsPbBr₃:EA film, (g),(h) CsPbBr₃:BA film, i,j CsPbBr₃:OA film).

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Funding

National Natural Science Foundation of China (2015CB932200, 61605075).

Disclosures

The authors declare no conflicts of interest.

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