Simple Approach to Improving the Amplified Spontaneous Emission

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A simple approach to improve the amplified spontaneous emission properties of perovskite films Jing Li, Junjie Si, Lu Gan, Yang Liu, Zhi-Zhen Ye, and Haiping He ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13289 • Publication Date (Web): 08 Nov 2016 Downloaded from http://pubs.acs.org on November 9, 2016

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ACS Applied Materials & Interfaces

A simple approach to improve the amplified spontaneous emission properties of perovskite films Jing Li, Junjie Si, Lu Gan, Yang Liu, Zhizhen Ye* and Haiping He*

State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China.

KEYWORDS: Perovskite; Thin films; Amplified spontaneous emission; Surface passivation; Waveguide

ABSTRACT: Organo-lead halide perovskite has emerged as a promising optical gain media. However, continuous efforts are needed to improve the amplified spontaneous emission (ASE) even lasing properties to evade the poor photo- and thermal instability of the perovskites. Herein, we report that simply through the coating of polymer layer, the CH3NH3PbBr3 polycrystalline films prepared by a modified sequential deposition process show remarkably enhanced photoluminescence and prolonged decay lifetime. As a result, under nanosecond pulse pumping the ASE threshold of the perovskite films is significantly reduced from 303 to 140 µJ/cm2.

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Furthermore, the light exposure stability is improved greatly after the polymer coating. We confirmed that the polymer layer plays the roles of both surface passivation and symmetric waveguides. Our results may shed light upon the stable and sustained output of laser from perovskite materials.

INTRODUCTION Organo-lead halide perovskites (CH3NH3PbX3, where X=Cl, Br, I) have recently attracted great attention as a new class of photovoltaic materials. Due to their excellent optoelectronic properties, such as high absorption coefficients, balanced long-range electron/hole transport lengths and low defect density, rapid developments in thin-film organo-lead perovskites were enabling high-efficiency and low-cost solar cells with high conversion efficiency exceeding 20%.1-7 Interestingly, this class of materials also show high photoluminescence (PL) quantum efficiencies8,9 and large optical gain10, making them promising candidates for the applications in efficient light emitting devices.10-26 For examples, Cho et al recently demonstrated the green light emitting diode utilizing CH3NH3PbBr3 perovskite with external quantum efficiency of 8.57%.20 Xing et al reported wavelength-tunable amplified spontaneous emission (ASE) in CH3NH3PbX3 films at room temperature.10 Even more, Zhu et al obtained room-temperature lasing from single-crystal lead halide perovskite nanowires with very low lasing threshold (220 nJ/cm2) and high quality factor (Q ~3600).26 One of the exciting goals of semiconductor lasing studies is to achieve continuous wave pumping, taking advantages of the cheapness and convenient access of the pumping sources. So far most of the reported ASE or lasing from perovskite materials is pumped by femtosecond laser, and that pumped by much longer nanosecond laser pulses is much less.24, 25 Due to the short Auger recombination lifetime27, the ASE and lasing threshold fluence would significantly


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increase when using nanosecond laser pumping. A higher ASE and lasing threshold under nanosecond laser pumping means higher requirements for photo- and thermal-stability of the optical gain materials, which are particularly serious for organo-lead halide perovskites.28-32 In this regard, it is desired to reduce the ASE and lasing threshold of perovskite materials to obtain stable and sustained ASE or lasing. Considering the well-known “green gap” in solid state lighting, green emitting CH3NH3PbBr3 is of particular interest owing to its high PL quantum efficiency. There are many reports on perovskite ASE at present, and also some studies on the green lasing from CH3NH3PbBr3 under femtosecond laser pumping26, 37-40. However, green ASE from CH3NH3PbBr3 polycrystalline films pumped by nanosecond pulses has not been reported so far. Herein, we report nanosecond laser pumped ASE from CH3NH3PbBr3 polycrystalline films prepared by a modified sequential deposition process (MSDP), and improve the ASE properties by simply coating the perovskite films with polymer layer. Due to the surface passivation and symmetric waveguide effects of the polymer coating, we reduce the ASE threshold by half and significantly improve the light exposure stability of the perovskite films.

EXPERIMENTAL DETAILS Materials and Methods Lead bromide (PbBr2, 99.9%), isopropanol (IPA, 99.9%) and polymethyl methacrylate (PMMA) were purchased from Sigma-Aldrich. CH3NH3Br (MABr, 99.9%) were purchased from Xi'an Polymer Light Technology Corp. and N, N-dimethylformamide (DMF, 99.9%) were purchased from Alfa Aesar. The perovskite films were prepared via a modified sequential deposition process (MSDP), as schematically described in Fig. 1a. PbBr2 precursor solution in DMF (200


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mg/ml) was deposited on glass substrate by spin-coating at 2500 rpm for 60 s, and then annealed at 70oC for 30 min. Subsequently, an appropriate amount of MABr solution in IPA were dropped on the PbBr2 films, stood for 120 s, and then spin coated at 5000 rpm for 30 s. The resulting films were annealed at 100oC for 1 h to form the yellow CH3NH3PbBr3 perovskite layers with good transparency. In this preparation, we combined drop casting and spin coating, and increased the annealing temperature to improve the quality of the CH3NH3PbBr3 films. Especially, for the concentration of the MABr solution, we tried four different concentrations (5, 10, 20, 30 mg/mL) to improve the morphology of CH3NH3PbBr3 films. After the perovskite layer was prepared, we conducted a polymer coating. An appropriate amount of PMMA solution in toluene (50 mg/ml) were dropped on the CH3NH3PbBr3 films and spin coated at 2000 rpm for 60 s, then annealed at 100oC for 30 min. The PMMA layer was formed with a thickness of ~350 nm. The whole process of thin films preparation was performed in a nitrogen filled glove box. Film Characterization Field emission scanning electron microscope (FE-SEM, Hitachi-S4800) was used to study the film morphology. X-ray diffraction (XRD) measurements were performed on a PANalytical X'Pert PRO diffractometer operated at 40 kV and 40 mA using Cu Kα radiation (λ=1.5406 Å). Ultraviolet-visible (UV-vis) absorption spectra were collected on a UV-3600 spectrometer in the wavelength range of 300–800 nm. Atomic force microscope (AFM, Bruker dimension edge) was used to measure the surface roughness of the perovskite films. Fluorescence spectrometer (Edinburgh Instruments FLS-920) was employed to perform steady-state and transient PL measurements. Semiconductor laser diode (EPL405) with a wavelength of 404.2 nm and pulse width of 58.6 ps was used as the excitation source. The highest excitation fluence is ~4 nJ cm-2, corresponding to photocarrier density of 2.3×1014 cm-3 by taking the film thickness of 350 nm.


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A pulsed laser (FTSS 355-50) with a wavelength at 355 nm, pulse duration of 1 ns and repeating frequency of 100 Hz was employed as the excitation source for ASE measurements. The PL signal was collected from edge of the sample, which perpendicular to the direction of excitation light. The excitation density was tuned by a series of neutral attenuators. The gain coefficient of the perovskite films were measured by variable stripe length (VSL) method. The pump laser was shaped into a stripe on the sample surface by a cylindrical lens, and the length of the stripe was varied by a tunable slit. The emission intensity was then measured as a function of stripe length.

RESULTS AND DISCUSSION The morphology of perovskite films prepared by MSDP is shown in Fig. 1(b-e). From the SEM results we find that 10 mg/mL is the optimum concentration to achieve good film morphology. As shown in Fig. 1c, the CH3NH3PbBr3 film is very uniform and compact without any pinhole, with average grain size of about 300 nm. In the subsequent experiments, we all use this concentration to prepare CH3NH3PbBr3 films. As will be demonstrated later, we indeed get a good optical performance through this MSDP method. Room temperature PL (green) and absorption (blue) spectra of the fabricated perovskite thin films (Fig.2a) show prominent exciton feature and a slight Stokes shift, which were consistent with the previous reports.33 The corresponding XRD pattern (Fig. 2b) shows a set of strong diffraction peaks that can be indexed to a cubic CH3NH3PbBr3.20 The narrow linewidth of the diffraction peaks indicates good crystallinity of the film. Combined with the good film morphology revealed by the SEM image, these results suggest that we obtained high-quality perovskite films by the MSDP method. The influence of polymer coating on the PL and lifetime of photocarriers is shown in Fig. 3. We restricted the excitation fluence to a low level (< 4 nJ/cm-2) to ensure obtaining the intrinsic


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properties of materials (Fig. S1). It is found that the PL of the film is enhanced greatly after the coating, and the lifetime also becomes longer accordingly. We conduct a bi-exponential decay fitting to the lifetime of the film before and after PMMA coating. The fit gives an effective PL lifetime of 7.3 and 14.7 ns for CH3NH3PbBr3 without and with PMMA coating, respectively. These results indicate a surface passivation effect of the PMMA coating, which suppresses the nonradiative recombination sites on the surface. The passivation of surface defects would be beneficial to the achievement of ASE from our perovskite thin films. Figure 4(a, b) shows the evolution of the PL spectra for CH3NH3PbBr3 perovskite films before and after the PMMA coating upon increasing the pump density. For the film after the coating, the spectra for low pump fluences (

Simple Approach to Improving the Amplified Spontaneous Emission

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