Stable Green Perovskite Vertical-Cavity Surface-Emitting Lasers on

Article pubs.acs.org/journal/apchd5

Stable Green Perovskite Vertical-Cavity Surface-Emitting Lasers on Rigid and Flexible Substrates Songtao Chen† and Arto Nurmikko*,†,‡ †

School of Engineering and ‡Department of Physics, Brown University, Providence, Rhode Island 02912, United States S Supporting Information *

ABSTRACT: Solution processed thin film organo-metal halide perovskites are gaining attention as attractive material prospects for low cost visible and near-infrared lasers while benefiting from recent large investments in photovoltaics of this micro/nanocrystalline material class. A number of reports have demonstrated optically pumped laser action in various optical resonator configurations at comparative low thresholds that might ultimately be compatible with corresponding injection currents required for junction lasers. One major question, however, is the inherent material robustness and stability of the perovskites under what are much higher optical and electronic excitation conditions than encountered in photovoltaics, yet typical in the technologically mature epitaxially grown III−V compound semiconductor lasers. Here we assess CH(NH2)2PbBr3 (FAPbBr3) solid thin films embedded within two sputtered planar dielectric HfO2/SiO2 distributed Bragg reflectors (DBRs) in green perovskite verticalcavity surface-emitting lasers (PeVCSELs). The high quality factor single mode resonator (Q ∼ 1420) enables us to reach lasing threshold at low optical pumping (∼18.3 μJ/cm2) while delivering a temporally and spatially well-defined output beam. The device fabrication approach automatically self-encapsulates the perovskite active layer from the ambient. We report here achieving a device lasing lifetime for up to 20 h (∼108 laser shots) at room temperature under sustained illumination of 355 nm pulsed laser excitations (0.34 ns, 1 kHz). We also show how this PeVCSEL microfabrication route can be generally adapted to many substrates, specifically demonstrating a green perovskite thin film surface-emitting laser on a flexible polymer substrate. KEYWORDS: perovskite, distributed Bragg reflectors, vertical-cavity surface-emitting laser, flexible substrate, photostability

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rgano-metal halide perovskites (ABX3, where A = Cs+, CH3NH3+, CH(NH2)2+; B = Pb2+, Sn2+; X = Cl−, Br−, I−) have been widely exploited recently for optoelectronics, catapulted to prominence mainly by their photovoltaic application.1 Within short research period, low-cost solutionprocessed perovskite based solar cells have achieved remarkable power conversion efficiency of 22.1%.2 Beyond photovoltaic efforts, encouraging light emitting properties of perovskites have led to early demonstration of perovskite LEDs covering near-infrared, red and green spectrum,3 with preliminary work on assessing prospects of lasing now under way in many laboratories.4,5 Optical gain from perovskites was first evidenced via amplified spontaneous emission (ASE) from polycrystalline thin films with encouragingly low threshold energy density albeit under ultrashort (fs) pulse excitations.6 Fabricated by solution growth or chemical vapor deposition method, a range of single-crystal micro/nanoforms of perovskites such as nanowire,7−10 microdisk,11,12 and microplatelet13 structures were demonstrated to lase under similar ultrashort pulse (extreme transient) optical pumping. Whether exploiting naturally facet-formed Fabry−Perot or whispering gallery mode cavity, and notwithstanding the remarkable achievement of ultralow lasing thresholds, the spatially random distribution of © XXXX American Chemical Society

these types of microstructures collected from solution onto solid substrates and the parallel challenge of efficient light extraction may hinder corollary practical laser applications. Initial steps have been taken toward thin film perovskite lasers equipped with proposed, microfabricated optical resonators using vertical cavity,14−16 distributed feedback (DFB) gratings,17−19 and two-dimensional photonic crystal20−22 geometries. With optically engineered cavity configurations embracing the active gain media, perovskite lasers are shown to emit a well-defined spatially coherent laser output such as Gaussian mode beams15 and to operate under much longer pulsed quasi-steady state (∼ns) excitations. However, obstructing the next major milestone of room temperature continuous-wave operation one core issue is the material′s chemical- and photostability. By placing the device into a N2filled inert and sealed environment, the perovskite (CH3NH3PbI3) DFB laser can sustain up to 15 h.19 As far as we know, in the ambient condition the device lifetime barely exceeds a few hours16,20 in the best of cases. We address this matter here by using specific vertical cavity structures while also Received: July 3, 2017

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Figure 1. Green perovskite polycrystalline solid thin film characterization. (a) Linear absorption coefficient and photoluminescence spectrum obtained from a green FAPbBr3 perovskite film with ∼235 nm thickness. (b) Scanning electron microscope image of surface morphology of a FAPbBr3 thin film on planar quartz substrate. (c) Photographic images of a film taken after thermal annealing (Brown University logo is used with permission). (d) Indexed X-ray diffraction pattern of the green perovskite thin film, reflecting its polycrystalline nature, and confirming a single-phase sample with cubic crystal structure at room temperature. (e) Time-correlated single photon counting measurement of the FAPbBr3 film for recording the spontaneous emission lifetime under different excitation levels. Sample is photoexcited by pulsed excitation with λ = 408 nm, τpulse ∼ 68 ps, 100 kHz repetition rate. Symbols are experimental data and black lines are the exponential decay fittings (with one or two time constants).

platelets,25 quantum dots,26,27 and organic polymers28 in the same wavelength range under similar optical pumping conditions. In broader terms, we note that VCSELs are inherently among the most challenging semiconductor device testbeds given their stringent requirements of high optical gain and very low losses.29−31 In aiming to synthesize robust green perovskite thin films we note that in the organo-metal halides (ABX3), the organic cation (A) can be changed from CH3NH3+ (MA+) to CH(NH2)2+ (FA+) complex for better temperature and moisture/oxygen stability. This has been shown to result from stronger hydrogen bonding between FA+ and the halide ions within BX6 octahedral configuration.32−34 In addition, we change the X anion from I− to Br− for increasing the bandgap to the green.35,36 For comparison of chemical stability, we recorded the absorption spectra of FAPbBr3 and MAPbI3 fresh thin film samples in ambient conditions on a daily basis. The FAPbBr3 sample remained stable up to 2 months while the MAPbI3 sample began to decompose after 1 week (visible film color change from dark brown to yellow; see also Supporting Information, Figure S1). Both perovskite films were fabricated by an in-house developed method previously reported20 combining solution based spin-cast and dripping procedure (see Supporting Information). Figure 1a shows the linear absorption and photoluminescence (PL) spectrum of the FAPbBr3 solid thin film. A well-defined absorption edge can be observed, with the putative lowest excitonic transition manifested by a small peak at the edge. The absorption

focusing on green wavelengths, given the still modest performance of conventional epitaxial III-nitride diode lasers in this spectral range. As an extension, we probe the potential for perovskite thin film lasers on flexible substrate for another technological opportunity.23 We also note recent endeavors that explore a vertical cavity composed of a flexible cholesteric liquid crystal reflector and metal back-reflector, achieving enhanced ASE from a perovskite thin film embedded within the cavity.24 Dielectric DBRs are materially compatible with solution extracted perovskite thin films. “Sandwiching” a perovskite film between pairs of discrete, separate DBRs has been demonstrated.15,16 However, the ambient air (oxygen/moisture) has lateral access to the perovskite layer through air gaps, leading to material degradation under high optical excitations. For improved sealing yet achieving a high-Q cavity, as described below, we developed a fabrication process using sputtered highreflectivity (∼99.6%) dielectric DBR consists of 10 pairs alternating HfO2/SiO2 layers. By growing the top DBR directly onto the perovskite thin film layer, the completed device selfencapsulates and is shown below to improve the device lasing lifetime up to ∼20 h. As detailed below, we were able to reach low lasing thresholds (∼18.3 μJ/cm2) at green wavelength (λ = 552.4 nm) under sub-nanosecond pulse excitations with welldefined spectrally and spatially coherent outputs. This threshold value is comparable or lower than not only perovskite thin film based vertical cavity lasers,16 but also other solution processed lasing materials such as II−VI colloidal nanoB

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Figure 2. Vertical cavity with HfO2/SiO2 dielectric DBR mirrors. (a) Schematic fabrication flow of green PeVCSEL devices. Ten pairs of alternating HfO2 and SiO2 films are sputtered onto quartz substrate. Immediately after coating of the DBR with FAPbBr3, the sample is transferred into sputtering chamber for another 10 pairs SiO2/HfO2 layers to complete the vertical cavity. The SiO2 and HfO2 layer thickness are 96.6 and 67.7 nm, respectively. (b) Cross-sectional SEM image of green PeVCSEL device at 52° titled view after focused ion beam milling process. The thickness of green perovskite is 235 nm ± 15.9 nm. (c) Standalone HfO2/SiO2 dielectric DBR reflectivity spectrum, with peak reflectivity of 99.6 ± 0.1%. (d) Zoom-in DBR reflectivity spectrum (black line), covering the anticipated optical gain region of the perovskite film (as deduced from, e.g., ASE emission), with a broad spectral window (525∼600 nm) having R > 99% reflectivity. (e) Spontaneous emission from complete device under excitation well below the lasing threshold; the narrow peak shown at λ = 552.4 nm is indicative of the single cavity mode. Inset shows the Lorentz fitting (orange line) of the peak, yielding mode linewidth Δλ = 0.39 nm.

coefficient α(ℏω) near the band edge can be fitted under Elliot′s theory of Wannier excitons37,38 (Supporting Information, eq S1 and Figure S2), yielding estimated exciton binding energy Eb ∼ 25 meV. Considering the thermal energy fluctuation (26 meV) at room temperature, these weakly bonded excitons are likely to dissociate to form an electron hole plasma at the level of optical excitation described below.38 Nonetheless, we speculate that the finite electron−hole (e−h) Coulomb correlation is likely to enhance the oscillator strength of the interband transition, contributing to the measured high absorption coefficient (>104 cm−1) near the band edge. This strong absorption feature in turn enables the highly efficient optical pumping for achieving pronounced material gain at relatively low pump energy densities. Room temperature PL of FAPbBr3 film locates at λ = 542.7 nm, with a full-width halfmaximum (fwhm) linewidth of Δλ = 26.6 nm (111.7 meV). Note that the relatively large Stokes shift of 13.5 nm (58.1 meV) is of benefit for light emission/amplification due to a reduced self-absorption process. The surface morphology of the film shows a closely packed ensemble of nanograins which suggests a void-free continuous polycrystalline film (Figure 1b). The optical quality is also reflected qualitatively by the transparent and shiny photographic images of the films after fabrication (Figure 1c). By comparing with powder X-ray diffraction (XRD) results of FAPbBr3 single crystals,39,40 we confirm the pure cubic phase of our polycrystalline films (XRD peaks shown in Figure 1d). The near-continuum, close-packing film quality also contributes to the relatively high refractive

index of our FAPbBr3 green perovskite thin films (n ∼ 2.32 near λ = 550 nm; Supporting Information, Figure S3). As for electron−hole pair recombination processes, Figure 1e characterizes the PL decay process under different pump fluences (i.e., initial carrier densities). The experiment results are well fitted by two-time constant exponential decay (Supporting Information, eq S2; Table S1 shows the time constants and their corresponding ratios). At very low pumping levels (excess bulk equivalent e−h pair density n0 = 1.12 × 1016 cm−3), the PL decays slowly with lifetime τ = 104.8 ns, which is most likely due to the slow trap-related process. At higher excitation (n0 = 4.07 × 1018 cm−3) corresponding to lasing regime in a fully fabricated device, we observe an initial faster PL decay component (τ1 = 4.8 ns) followed by a slower decay (τ2 = 24.0 ns). The initial suggests an influence from the Auger effect (three particles inelastic process) although relatively modest one when compared with, for example, similarly excited CdSe-based colloidal quantum dots (τAuger is on the order of 100 ps). To estimate the internal PL quantum yield, we analyzed the carrier dynamics together with the PL decay process41,42 (see Supporting Information, eq S3 and Figure S4a). The internal quantum efficiency (IQE) first increases with carrier concentration as the trap states are filled, then begins to decrease when n > 2 × 1018 cm−3 due to the presumed Auger recombination loss (Supporting Information, Figure S4b). The peak IQE is estimated to be ∼67% at a carrier density n = 1.88 × 1018 cm−3. We note that the fitted Auger recombination constant (k3 ∼ 10−29 cm6s−1) of our FAPbBr3 thin film is one order magnitude lower than reported values (k3 ∼ 10−28 C

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Figure 3. Green FAPbBr3 PeVCSEL characteristics on a rigid substrate. (a) Device light output with increasing pump fluence in a log-log plot, showing the threshold region as the “kink” between the two linear regimes of spontaneous emission and lasing, with threshold energy density determined to be 18.3 ± 1.5 μJ/cm2. Open circles monitor the fwhm of the emission spectrum which collapses to Δλ = 0.28 nm after crossing the threshold. The solid line is a rate equation fitting, yielding an estimated value of spontaneous emission factor β (∼0.02). (b) Emission spectrum (550−558 nm) under different pump fluences. Spectrally coherent single-mode lasing peak at 552.4 nm is observed. Inset shows the spectrum on logarithmic plot emphasizing the single mode VCSEL operation. (c) Near-field images of a device with pump energy densities below, near, and above the threshold. (d) Far-field pattern recorded of the green PeVCSEL, with the spatial coherence expressed as an approximate fundamental transverse Gaussian beam mode. The output beam is emitted perpendicular to the device plane, with a divergence angle ∼6°. (e) Intensity polar plot of lasing through a rotational analyzer (green symbol), showing linear polarization with DOP to be 96%. The solid black line is the fitting based on sin2θ. (f) Green PeVCSEL device lifetime (longevity) measured under continuous sub-nanosecond pulse pumping (355 nm, 0.34 ns, 1 kHz). The incident pump energy density is Epump ∼ 1.5Eth. Solid green squares record the output power evolution; The fluctuations originate from instability of the pumping source. Open circles record the fwhm linewidth during the experiment, underscoring the clean lasing operation.

cm6s−1) for MAPbI3 polycrystalline films,43,44 which we anticipate as a result from reduced Auger recombination cross-section in the wider bandgap bromide based perovskite. Due to its inherently short single pass optical gain path, a VCSEL structure poses stringent low-loss requirements for the active medium and its optical cavity. Discrete high reflectivity multilayer dielectric DBR mirrors were used in early trials of perovskite vertical cavity lasers.15,16 Here we directly integrate high reflectivity (>99%) multilayer dielectric DBRs by developing a dedicated fabrication scheme which includes sputtering of 10 pairs of alternating HfO2/SiO2 layers to microfabricate the PeVCSELs (Note the large index contrast between two dielectric materials nHf O2 = 2.08, nSiO2 = 1.45). Figure 2a shows the schematic process flow for the dielectrically self-encapsulated PeVCSEL devices. The first stack of HfO2/ SiO2 layers are sputtered onto quartz substrate to form the bottom DBR, with thickness of 67.7 and 96.6 nm for each layer, respectively. After proper hydrophilic surface treatment, the sample is coated with FAPbBr3 solid thin film and then speedily transferred back into the sputtering chamber. We sputtered the

top DBR (same recipe/parameters as bottom DBR) after chamber pressure reached ∼10−8 Torr to ensure minimal residue of oxygen or moisture in the final device. Given the lower index and better adhesion of SiO2 to the perovskite film, we design both the bottom and top DBRs so that each surface of the perovskite thin film contacts with SiO2 layer. Focused ion beam milling process is employed to enable SEM imaging the cross-section of the devices (direct sample cleaving tends to shatter the top DBR structure due to embedded strain). One image is shown as the tilted (52°) view in Figure 2b. The perovskite thickness is measured to be d = 235 ± 15.9 nm and the overall structure corresponds to a ∼1λ cavity. Separately, the reflectivity maximum of sputtered dielectric DBRs is measured as R = 99.6 ± 0.1% while maintaining the design goal of R > 99% within a broad spectral range (525∼600 nm) to ensure full coverage of the anticipated optical gain window of FAPbBr3 film as informed approximately by the ASE spectrum (Figure 2c,d). We interrogate the optical properties of the completed cavity by recording the spontaneous emission spectrum under low level optical excitation (λ = 355 nm, D

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τpulse = 0.34 ns, 1 kHz), as shown in Figure 2e, with presence of a single cavity mode peak clearly observed at λ = 552.4 nm superposed on background PL (long detector exposure time is used to get a clean spectrum). The cavity mode is fitted with Lorentzian shape45 (Figure 2e, inset), yielding linewidth Δλ = 0.39 nm and indicating a “cold cavity” quality factor Q ∼ 1420. The optical field distribution is obtained from a FDTD simulation (Supporting Information, Figure S5) to extract the optical mode confinement factor of Γ = 0.455 (Supporting Information, eq S4). Characteristics and analysis of our green PeVCSEL devices are assembled into Figure 3. A compact diode-pumped solidstate laser provides pulsed excitations with λ = 355 nm, τpulse = 0.34 ns; repetition rate = 1 kHz. The excitation beam is focused to a spot (r ∼ 15 μm) at surface normal direction to the device and the spatially coherent emission of the PeVCSEL beam is collected coaxially in transmission with a long-pass filter blocking leakage pump laser light. Figure 3a shows the double logarithmic plot of PeVCSEL lasing output versus pump pulse fluence (solid green symbols). The light-out versus light-in (L− L curve) results exhibit the clear S-shape nonlinear behavior characteristics of the transition from spontaneous emission via ASE to lasing. The open green circles record the fwhm linewidth of the emission spectrum, which collapse from Δλ ∼ 15 nm down to Δλ = 0.28 nm (spectrometer limited) after crossing the lasing threshold Eth = 18.3 ± 1.5 μJ/cm2. The corresponding equivalent carrier density at lasing threshold is estimated as n = (1.28 ± 0.11) × 1018 cm−3, which converts to a continuous current of 5.53 ± 0.48 kA/cm2 if considering electrical injection (assuming each injected e-h pair 100% recombines by radiative emission). Note that the laser linewidth at threshold (system loss balanced with optical gain) differs from the cold cavity case (Figure 2e), which results from the damping of the system due to finite photon lifetime. Rather, in the lasing regime the linewidth is fundamentally limited by and originates from the phase and amplitude noise as spontaneous emission is coupled into the lasing mode.46 For microscopic insight, we apply a classic rate-equation analysis47−51 to model the coupled carrier and photon density dynamics upon excitation (Supporting Information, eqs S4−S6, Figures S5−S7). By fitting the L−L curve data (both threshold energy density and phase transition “kink” from spontaneous emission to lasing, green line in Figure 3a), we estimate the spontaneous emission factor β ∼ 0.02, which is consistent with values from other similar optically pumped inorganic semiconductor VCSEL structures, such as III−V nitrides.52 Figure 3b displays the lasing spectra of the PeVCSEL at different pumping levels. After crossing the threshold, a single-mode peak at λ = 552.4 nm emerges and increases sharply in intensity. Under higher excitations, we observe slight blueshifting and broadening of the PeVCSEL lasing spectrum, which could be attributed to band-filling, carrier density modulated refractive index change of perovskite layer, and electron/hole many-body effect,7,51 or some combination thereof. Inset of Figure 3b plots the PeVCSEL lasing spectrum at pump energy density 42.4 μJ/cm2, underscoring the intensity contrast between a single lasing mode and the background PL. In addition to spectral coherence, our PeVCSEL devices also show well-defined spatially coherent outputs; a requirement for a credible laser demonstration. Figure 3c shows the near-field photographic images of emission from a PeVCSEL device. Weak and diffuse PL emission is seen below threshold, whereas above threshold, a crisp and high-brightness beam emerges

perpendicular to device plane. In the far-field, we measure a nearly fundamental Gaussian beam lasing mode (Figure 3d) from the PeVCSEL device, with a measured ∼6° beam divergence angle across the transverse plane. The divergence contains a contribution due to the finite optical aperture in the cavity. The emission from the PeVCSEL is typically linearly polarized as shown in Figure 3e, with the degree of polarization DOP = (Imax − Imin)/(Imax + Imin) to be ∼96%. We surmise this strong linear polarization effect may originate from the local anisotropy in perovskite film thickness and/or top DBR (conformal coating on perovskite). Different linear polarization configurations (i.e., polarization angle, DOP) are observed when various lasing spots are tested (Supporting Information, Figure S8). To check the self-encapsulation of the perovskite medium within the vertical cavity device structure, we test the green PeVCSEL device longevity under sustained pulsed excitations (355 nm, 0.34 ns, 1 kHz) at room temperature and in ambient condition, without any further protection/heat sinking schemes. The pump energy density is set at 1.5 times the threshold so as to compare our device lifetime with conditions similar to recent reports. We record both the laser intensity and fwhm of lasing spectra during the testing runs, as shown in Figure 3f. It can be seen that the laser intensity decreases rather slowly, with 90% of initial condition after ∼10 h and 60% of initial intensity after ∼20 h (∼108 laser shots) sustained excitations. That the device lasing is further confirmed by simultaneous monitoring of the fwhm linewidth (green open circles in Figure 3f). We summarize other recent perovskite laser device lifetime work from literatures in Supporting Information (Table S2, showing lifetime improvement under N2 protection, whereas in ambient condition, the devices only last no more than few hours). As the top DBR (total thickness ∼ 1.6 μm) in our PeVCSELs conformally coats the perovskite film surface to prevent direct contact between perovskite material to ambient oxygen and moisture, we speculate the device degradation has little contribution from the chemical reactions. Although a systematic statistical study would require a large number of PeVCSEL samples (perhaps not yet warranted), we also speculate that thermal effects may now be the prime culprit. During each pulsed event, the interaction with lattice excitations (phonons) in the release of excess photoexcitation energy (∼1.25 eV per excitation photon in green PeVCSEL case) may result in transient heating, leading to slow degradation/decomposition of the photoexcited perovskite. Due to the large photon flux inside cavity during lasing, the photodegradation may also affect the perovskite stability, possibly further accelerated by the heating process. Parenthetically, we stored green PeVCSEL devices in ambient condition for roughly three months, and observe that the device can still show healthy lasing action at similar thresholds compared with initial results at first day. We also conduct preliminary tests employing a 450 nm CW laser diode pump to excite the PeVCSEL device (at room temperature and in ambient environment). The FAPbBr3 film appears irreversibly damaged when the pump power reached 11.3 kW/cm2 without having reached a lasing threshold. Even when chopping the CW excitation (∼2 μs pulses, 125 Hz repetition rate) and increasing excitation to 30.0 kW/cm2 peak power (average power, 7.5 W/cm2), damage incur without achieving lasing action. This points to a main remaining challenge: under harsh conditions such as in CW (or quasiCW) regime, the generated heat is excessive and likely leads to E

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Figure 4. PeVCSEL characteristics of FAPbBr3 device built on the flexible polymer substrate. (a) Flexible PeVCSEL device emission spectrum well below the threshold. Cavity mode peak (λ = 552.7 nm) is fitted by Lorentzian line shape (inset) leading to linewidth of Δλ = 0.60 nm. (b) Photograph of measuring setup, illustrating the flexible PeVCSEL device as the bending membrane. (c) Near-field images of a flexible PeVCSEL device with pump energy densities below, near, and above the threshold. (d) Spectrum (550−558 nm) evolution under different pump fluences. Spectrally coherent single-mode lasing peak at 552.7 nm is observed. Inset shows the spectrum on logarithmic plot demonstrating single mode operation. (e) Laser output under increasing pump fluences, yielding threshold energy density of 40.6 ± 3.4 μJ/cm2. Open green circles record the fwhm of emission spectrum, indicating onset of lasing as abrupt collapse of the linewidth.

552.7 nm in spontaneous emission. The Lorentzian fit yields a linewidth of Δλ = 0.60 nm and a cavity quality factor Q ∼ 920. Above threshold, the device emits a bright laser beam to the surface normal direction (Figure 4c), highlighting the spatial coherence of the output. Spectral and threshold characteristics of flexible PeVCSEL devices are exhibited in Figure 4d,e. Temporally coherent single mode lasing is achieved, with the intensity of the lasing peak growing superlinearly after crossing the lasing threshold of 40.6 ± 3.4 μJ/cm2 (corresponds to equivalent carrier density n = (2.84 ± 0.24) × 1018 cm−3), accompanied by the spectral collapse of the fwhm (Figure 4e). The somewhat lower value of cold-cavity Q, compared with rigid substrate case above, may be influenced by device bending process, leading to increase escaping (loss) spontaneous emitted photons and higher threshold gain of the device (i.e., increasing laser thresholds). This leakage also results in stronger PL background, as shown in the semilog plot of lasing spectrum (inset of Figure 4d), and near-field images from the PeVCSEL devices (Figure 4c), compared with PeVCSEL device on rigid substrate (Figure 3). The realization of the flexible PeVCSEL devices help us to envision further practical applications of perovskite based laser devices, such as large-area laser emitter arrays on curved surfaces (i.e., wallpaper).

perovskite decomposition or accelerates other chemical- or photodegradation processes. We surmise that innovative heat management strategies, such as using high thermal-conductivity substrates,53,54 are likely next essential steps for realizing real CW operation of perovskite-based lasers. Of course, and in parallel, further lowering the lasing threshold by designing even higher Q-factor cavity should be pursued along with improvement of the thermal stability of gain medium by using mixed cation perovskite thin films.35,55,56 Finally, we point to the opportunity of using the fabrication approach described above to microfabricate PeVCSELs on other types of substrates. As a proof of concept, we construct green PeVCSEL devices onto commercially available flexible transparent polymer substrate (thickness d = 76.2 μm). Measured transmission spectrum of pure polymer substrate achieves 85∼90% across the visible range (Supporting Information, Figure S9, see also the 3D schematic structure of flexible PeVCSEL device). The DBR sputtered onto the flexible substrate reached comparable high reflectivity of R = 99.5 ± 0.1% as those deposited on quartz substrates. Enabled by the flexible substrate, the devices are bent and mounted on glass slides with tape (Figure 4b). We first extract the device emission spectrum under optical excitation well below the lasing threshold (Figure 4a) to isolate the cavity mode at λ = F

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In summary, we demonstrate optically pumped green lasers based on polycrystalline perovskite (FAPbBr3) solid thin films on both the rigid and flexible substrates for the first time. The all-planar sputtered high-reflectivity DBR stacks form a high-Q cavity to realize low threshold lasing operation, and largely improve the PeVCSEL device lasing lifetime under ambient condition by self-encapsulation of environment-sensitive perovskite thin films. Our degradation test is of broader relevance to the current questions to the material stability of perovskites, especially under high electronic excitation conditions. The modest energy density needed for lasing in PeVCSELs may be reachable in direct or indirect (i.e., diode-pumped) electrical injection in the future. The device fabrication process is readily applicable to different types of substrates, broadening the possible applications of perovskite-based lasers. Device heat management and perovskite film quality improvement are now underway, to further enhance the perovskite laser performance and realize CW operation of perovskite lasers in the near future.

lasing spectrum are obtained by Princeton Instrument ACTON Inspectrum spectrometer (resolution ≥ 0.125 nm).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.7b00713. Additional details about green perovskite thin film synthesis and characterization, carrier recombination dynamics and carrier dependent IQE, device fabrication, modal analysis of vertical cavity, carrier/photon rate equation analysis, PeVCSEL output polarizations, device lifetime comparisons, and the structure of flexible PeVCSEL device (PDF).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

METHODS Fabrication of Perovskite Polycrystalline Films and PeVCSEL Devices. Lead bromide (PbBr2, 99.999%) is purchased from Alfa Aesar. Anhydrous dimethyl sulfoxide (DMSO, 99.9%) is purchased from Sigma-Aldrich. Formamidinium bromide (FABr, >98%) is purchased from Dyesol. All the materials are used as received. In a N2 filled 1 atm glovebox (with H2O < 0.1 ppm and O2 < 20 ppm), we mix the FABr and PbBr2 with 1.1:1 molar ratio into anhydrous DMSO for a 25 wt % solution. A 50 μL aliquot of mixed solution is spread onto the quartz or flexible polymer substrate (transparent polyester film) and spin-cast at 2000 rpm for 63 s. During spin-casting, 200 μL of toluene is dripped vertically, targeting the center of substrate at 55 s after start of the spin-casting. Right after the spin-casting and dripping process, the film is transferred onto a 100 °C hot plate for 5 min annealing. For PeVCSEL devices fabrication, the bottom DBR (10 pairs of HfO 2 /SiO 2 alternating layers) is directly sputtered onto substrates (quartz or polymer film). And after surface hydrophilic treatment and perovskite film deposition, the top DBR (same recipe/ parameters as bottom one) is sputtered directly onto perovskite film surface. All DBRs are fabricated by Angstrom Sputter Deposition system. Characterization of Perovskite Polycrystalline Films and PeVCSEL Devices. All experiments are conducted using fresh perovskite films at room temperature and under ambient conditions. Perovskite film′s linear absorption and DBR reflection spectra are measured by Cary 500i UV−vis-NIR spectrophotometer. The effective refractive index of the perovskite films is measured with variable angle spectroscopic ellipsometer (J. A. Woollam M-2000DI). The PL lifetime is measured by time-correlated single photon counting (TCSPC) setup, using pump laser source (λ = 408 nm, τpulse ∼ 68 ps, 100 kHz repetition rate) from PicoQuant and avalanche photon counting module from PerkinElmer. The surface morphology images are taken by LEO 1530 field emitter scanning electron microscope with 10 kV acceleration voltage. The device crosssection SEM images are taken by FEI Helios−FIB system. XRD pattern is measured (perovskite thin film deposited on quartz substrate) by Bruker D8 Discover X-ray diffraction system. The green PeVCSEL devices are optically pumped by subnanosecond (Teem Photonics, Powerchip laser, frequency tripled, 355 nm, 0.34 ns, 1 kHz) excitation pulses. The PL and

ORCID

Songtao Chen: 0000-0003-4206-6131 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Department of Energy (Basic Energy Sciences) under Grant DE-FG02-07ER46387 and the Air Force Office of Scientific Research (AFOSR), and Quantum Metaphotonics and Metamaterials MURI (AFOSR Award No. FA9550-12-1-0488).

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