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Distributed Feedback Lasers and Light Emitting Diodes Using 1-naphthylmethylamonium Low-Dimensional Perovskite Matthew R. Leyden, Shinobu Terakawa, Toshinori Matsushima, Shibin Ruan, Kenichi Goushi, Morgan AUFFRAY, Atula S.D. Sandanayaka, Chuanjiang Qin, Fatima Bencheikh, and Chihaya Adachi ACS Photonics, Just Accepted Manuscript • Publication Date (Web): 07 Jan 2019 Downloaded from http://pubs.acs.org on January 7, 2019

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Distributed Feedback Lasers and Light Emitting Diodes Using

1-naphthylmethylamonium

Low-Dimensional

Perovskite Matthew R. Leyden, Shinobu Terakawa, Toshinori Matsushima, Shibin Ruan, Kenichi Goushi, Morgan Auffray, Atula S. D. Sandanayaka , Chuanjiang Qin, Fatima Bencheikh, Chihaya Adachi

TOC FIGURE

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Abstract This work investigates the feasibility of using low-dimensional perovskites for electrically driven lasers given the current status of perovskite light emitting diodes and optically pumped lasers. In our progress towards electrically driven lasers, we performed a variety of measurements on bulk and low-dimensional perovskite films to give a baseline for expectations. This included the measurement of amplified spontaneous emission, lasing, as well as near infrared light emitting diodes operated at low and high current density. We considered power density thresholds needed for amplified spontaneous emission and lasing, and compared this to light emitting diodes operated at high current density to speculate on the future of electrically driven perovskite lasers. We concluded that our current perovskite devices will need current densities ~4 to 10 kA/cm2 to achieve lasing. Future devices will most significantly benefit from architectures that accommodate higher current, but meaningful reductions in threshold may also come from improved film quality and confinement. Keywords: Quasi-2D, Ruddlesden-Popper, Roll-off, Auger recombination Lead halide perovskites are a family of materials with the structure of APbX3, where A is a cation incorporated into the perovskite lattice such as methylammonium (MA) or formamidinium (FA), and X is a halide such as iodide. These materials have recently become a popular research subject because of their application in solar cells.1 Low dimensional (low-D) perovskites are also known as Ruddlesden-Popper perovskites or quasi-2D perovskites but are essentially any perovskite material defined by the inclusion of a large cation that is too big to fit into the perovskite lattice. These cations define the crystal grain boundary and reduce the dimensionality of the crystals. This is in contrast to the continuous structure of MAPbI3, which we may refer to as 3D perovskite. Low-D perovskites have the general formula B2A(i-1)PbiX(3i+1), where B is the large cation and i is the order of the perovskite. Films made from solutions of low-D perovskite will typically have a distribution of different orders that will depend on fabrication conditions, but we will define the order i by the stoichiometry of the solution. It is commonly assumed that the lower order material will funnel its energy to higher orders where it will emit radiatively. Recently there have been a handful of reports on amplified spontaneous emission (ASE) and lasing in low-D perovskite materials. ASE has been demonstrated in butylamine (BA),2 1-naphthylmethylamine (NMA)3, octylamine OA,4,5 and phenylethylamine (PEA)6 based low-D perovskite materials. In this work, we used the


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material system of NMA2MA(i-1)PbI(3i-+1). Typically these materials show ASE and lasing thresholds in the range of ~10 J/cm2. In Table 1, we present a collection of publications demonstrating lasing behavior in 3D and low-D perovskite materials to provide context on the current status of perovskite lasers in a way where results can be meaningfully compared. 1-12 Power density is calculated by dividing the energy per pulse by the pulse duration. One work on FAPbI3 showed explicitly that the threshold will increase by a factor of ~13 moving from fs to ns pulse lengths.7 This shows that different measurement systems cannot be simply compared by energy per pulse. Additionally, lasing behavior potentially changes depending on if the pulse is longer or shorter than the photoluminescent (PL) lifetime. Pulses that are significantly longer than the PL lifetime may be considered to be in the quasi-continuous regime. For example, films of MAPbI3 were reported to undergo lasing death when pumped continuously at room temperature.8 Both power density and pulse duration should be considered when evaluating different materials and their relative performance.2-12

Table 1. Comparison of different perovskite-based lasing materials in terms of excitation power density. Power density should be considered in addition to the energy per pulse when comparing materials. This work investigates the feasibility of using low-D perovskites for electrically driven lasers. Specifically, we used NMA based low-D perovskite to make light emitting diodes (LEDs) and optically driven distributed feedback (DFB) lasers. We used high order perovskite films to fabricate lasers with an emission where the full width half maximum


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(FWHM) was under 1 nm. Lasing has been demonstrated using low-D perovskite, in single crystals,4 and a ring lasers structure.2 To confirm film quality, we demonstrated an LED with an external quantum efficiency (EQE) up to ~8% in the near infrared. To probe the feasibility of electrically driven low-D perovskite lasers, we measured high current density under pulsed operation up to ~400 A/cm2, but this fell significantly short of the ~10 kA/cm2 predicted for lasing. From these measurements, we concluded that the bottleneck is the device structure, which uses organic transport layers to improve the EQE at low current, but inhibits the use of the high current needed for lasing applications. Further reductions in the lasing threshold resulting from improved film crystallinity and confinement are likely possible, allowing for lasing current density thresholds that may be simpler to reach.

Results and Discussion At lower excitation energies low-D perovskite films show increased photoluminescent quantum yield (PLQY) compared to that of 3D perovskite, which is usually attributed to an energy transfer process from low-D perovskite to higher orders. Lower order perovskite within a film has a 2 component PL exponential decay curve, where the fast component is attributed to energy transfer, while the emitting material will only have a single decay constant associated with radiative emission.13 However, when using the high power densities needed for ASE and lasing, 3D perovskite materials can outperform lowD materials. We recently showed that the ASE threshold approximately linearly increased with the concentration of PEA cation.6 This trend is also confirmed when reviewing published thresholds using similar pulse durations (Table 1). Here we observed a similar threshold increase with NMA concentration, but it was less significant than for PEA. Specifically, the films of order 10 showed a threshold of 19 J/cm2 (24 kW/cm2, Figures 1a and b), or about half that on an analogous PEA based film. We speculate that the most likely cause of this threshold increase is Auger recombination, which will be discussed in more detail later. To better understand this threshold increase, we would like to know if energy transfer contributes to ASE. To investigate the energy transfer process within our films we performed transient absorption spectroscopy using a 30 ps laser with a pump intensity above the ASE threshold of the film (Figures S1 and 1c). From this measurement, we can see bleaching features corresponding to the low-D material (Feature A, ~650 nm, predominantly i= 4) that may transfer its energy to higher orders, as well as high order perovskite material that contributes to the spontaneous PL (Feature B, ~750 nm). Also, there is a sharp dip in the spectrum corresponding to the emission wavelength of ASE


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(Feature C, 760-780 nm). We can track each of these features by looking at a 20 nm section as a function of time (Figure 1c). We can see that features B and C overlap for the first ~30 ps during excitation, but feature C continues to drop to lower values indicating higher carrier concentrations at this energy. This is followed by a rapid increase in absorption, which we will tentatively associate with stimulated emission. We were able to see that the peak of feature A precedes feature C by about ~30 ps. Assuming energy is transferred from low-D to higher orders, this shows that this transfer process is potentially fast enough to contribute to the ASE process, similar to an existing report.3 This measurement by itself does not confirm energy transfer, or ASE, but does give us an idea about the timescales for carriers at relevant energies. Due to equipment constraints, we could not confirm times scales down to ps level accuracy. Despite a possible energy transfer contribution, the ASE thresholds for films containing significant amounts of NMA are still higher than our films of pure MAPbI3 (17 kW/cm2). Even if there is a contribution to ASE from the lower order films, the net effect is still an increased threshold. This threshold increase could be explained by the fraction of energy that is radiatively emitted by lower orders, the fraction that is transferred too slowly to contribute to ASE, or Auger recombination. Also, at high carrier concentrations, the traps within 3D perovskite are filled allowing efficient radiative emission, diminishing the advantage of using low-D perovskite. The EQE of our LEDs including NMA would need to be ~50% higher than pure MAPbI3 LEDs at high current densities to justify the inclusion of NMA for lasing applications. At low current densities, low-D perovskite films typically show significant improvement over 3D counterparts, so the inclusion of NMA may be warranted. However, performance at very high current density is currently not well investigated.


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Figure 1. Amplified spontaneous emission in low-D NMA (i=10) perovskite films. Energy transfer from lower order films may contribute to ASE, but the threshold remains higher than pure 3D films. a) Shows amplified spontaneous emission in a low-D film on glass. Insert shows the NMA cation. b) The ASE threshold is measured at the fluence where we see a rapid rise in PL intensity (19 J/cm2). c) Shows the evolution of the three relevant features in the transient absorption spectra including low-D material (A - 640-


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660 nm), higher order emitting material associated with spontaneous emission (B - 730750 nm), and the region associated with ASE (C - 760-780 nm). We can see that the low order feature precedes the ASE feature and appears to be fast enough to contribute to ASE. In our progress towards electrically driven perovskite lasers, we first fabricated optically pumped DFB lasers. The simplest case uses a film of 3D perovskite and provides a reference, which we compare to the more complex system of low-D perovskite. We used a 2nd order distributed feedback (DFB) grating and a film of MAPbI3. We were able to reproducibly make MAPbI3 DFB lasers using a grating pitch of 350 to 370 nm with a PL emission ranging from 780-810 nm using a film thickness of ~200-230nm (Figures 2a-c). This gave an effective refractive index of the film of ~2.2, which was slightly lower than the refractive index of MAPbI3 films measured with ellipsometry (~2.3). These values were all reasonably consistent with expectations. The effective refractive index is defined in the following equation, mBraggλBragg = 2neffΛ, where mBragg is the order, λBragg is the PL wavelength, neff is the effective refractive index, and Λ is the grating pitch. This pitch was slightly shorter than existing works on MAPbI3 lasers,8 likely due to our use of a thicker film. We found that the thicker films more consistently produced a narrow emission with a FWHM under 1 nm, and was still able to achieve a reasonably low threshold of ~9 kW/cm2. This threshold was similar to that observed for ASE using optimized films (17 kW/cm2),6 and comparable to the report on continuous lasing.8 Shorter wavelengths are likely possible as the PL envelope extends down to ~700 nm, but efficient lasing at shorter wavelengths may be limited by self-absorption, where it is difficult to maintain high photon densities. We are confident that this was lasing and not a feature resembling lasing for the following reasons. First, the emission had a clear threshold where the intensity rapidly increased. Second, the emission above threshold had a FWHM under 1nm. Third, the emission wavelength depended on the cavity pitch. Fourth, the emission was observed approximately normal to the surface, which was expected of a second order DFB laser. Fifth, the light was strongly polarized. These criteria are discussed in more detail in Figure S2.


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Figure 2. Lasing in 3D and low-D perovskite. a) Shows the PL emission measured from the surface of a single substrate containing multiple DFB gratings. The measurement angle was fixed but the PL wavelength depended on the grating pitch, which was an indicator of lasing. b) Shows the emergence of a lasing peak when excited above the lasing threshold. The insert shows a substrate with gratings of different pitch c) Threshold measurement of the MAPbI3 DFB laser (~7 J/cm2) d) Lasing on a glass grating using low-D perovskite of order i~64. The insert shows that the laser light was polarized. We also demonstrated lasing using low-D perovskites with a minimum FWHM of 0.68 nm (TOC Figure). However, the performance of lasing films was more sensitive to concentrations of NMA than might be expected from ASE measurements. Only relatively high order perovskite films (i ~ 64) were able to demonstrate lasing (Figure 2d). The index of refraction of low-D perovskite films is slightly reduced moving to lower orders,6 but no significant difference from bulk was observed for NMA i~64 films. Lower order films on DFB gratings showed ASE with more narrow emission due to confinement effects from the DFB, but never met all of the criteria for lasing. This laser-like behavior is discussed in Figure S2. At this high order, it is tempting to think that the NMA plays


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little role at all, but looking at the atomic force microscope images the effect of the NMA becomes clear (Figure 3). We can see the average grain size was significantly smaller than the pure 3D. However, the roughness was only slightly less (9.1 nm versus 9.9 nm). We found that the inclusion of dimethyl sulfoxide (DMSO) into precursor solutions made lasing more reproducible but may also have increased the film roughness. The films used for LEDs (Figure 4) were thinner and did not include DMSO, and consequently had a lower roughness (3.9 nm). We also observed that the formation of low-D perovskite films had a significant sensitivity to the substrate. Films deposited on SiO2/Si wafers showed a more significant contribution from low order PL and lower ASE gain when compared to films deposited on glass (Figure S3). For this reason, we fabricated a DFB grating using a glass substrate. We were only able to observe lasing in low-D films using the glass grating. It was difficult to fabricate glass gratings with proper geometry due to slow etching rates. This may partially explain why lasing thresholds were higher in low-D films than what we observed for ASE.


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Figure 3. Atomic force microscopy images (5x5 m) show reduced grain size and surface roughness moving to lower orders of low-D perovskite. a) Film of MAPbI3 prepared using a method suitable for DFB lasers. b) Film of i~64 perovskite prepared using a method suitable for lasing. c) Film of i~10 perovskite using a method suitable for fabrication of LEDs.


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Pure organic materials can demonstrate very efficient performance for blue, green and red emission but typically show limited performance in the near infrared. However inorganic-organic hybrid materials like perovskite are often able to demonstrate high performance in the near infrared regime. Recently there have been many works on lowD perovskite for light emitting diodes, and have been demonstrated with cations such as PEA,14 NMA,15 BA,16 and OA17 to name a few. These materials have demonstrated external quantum efficiencies over 16% for green emission,18 and over 20% for nearinfrared emission.19,20 The highest EQE was achieved from improved out-coupling as a result of a micro-structured perovskite layer. Materials with a high index of refraction like perovskite typically have low out-coupling efficiency and can benefit significantly from improved out-coupling. Just like the ASE results in Figure 1a, we used the material system of NMA2MA(i-1)PbI(3i+1) where i~10 to demonstrate near infrared perovskite LEDs with ~8% EQE (Figure 4a), at an emission wavelength at ~750 nm (Figure 4b). Additionally, we found that these perovskite films used in LEDs will typically increase in efficiency with electrical stress (Figure S4), similar to published works.21 This result is reasonable for inorganic-organic hybrid materials emitting in the near infrared. We include LED performance as a way to show that films are of reasonable quality and can be used as the basis for our study into perovskite lasers.


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Figure 4. Example NMA based low-D perovskite LED (i~10). a) Shows our champion perovskite LED with peak EQE of ~8%. b) Shows the electroluminescent spectra from this device. The inserts are a diagram of the device structure and a photo from an analogous LED. Fabrication conditions for films that work well for lasing are significantly different than those of the optimized LED shown in Figure 4. Films that work well for lasers use processing that seems to favor larger grain sizes, while LEDs perform better when grains are small. Specifically, films used for lasing are annealed at higher temperatures (100 °C), at longer times (~20 min), and benefit from the inclusion of DMSO into the precursor solution. Films that are optimized for LEDs are annealed at lower temperatures (70 °C) and for shorter times (5 min). Consequently, films optimized for lasing show modest performance when used as an LED and operated at low current densities (Figure S4). This may change when operated at high current densities as perovskite materials are well known to increase in PLQY when excited to higher carrier concentrations due to trap filling.13 To get an idea about the operation of LEDs at high current density, we fabricated small


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area LEDs and operated them with short pulses. Similar high current devices have been fabricated with a low lasing threshold organic emitter.22 Both the small area and short pulse length should provide more efficient cooling and allow for higher current densities. Figure 5 shows the current density and EQE of an LED made with a perovskite film of i~10, with an active area of ~0.02 mm2, and pulse length of ~500 ns. We can see that the LED in pulsed operation can operate at substantially higher current (1,000X) than an analogous larger area LED operated with continuous current. The capacitance in this device was significant, therefore we measured the current at an approximately stabilized value of 300 ns. The capacitance was most noticeable during the rise time (100 ns), where we measured a spike in current. Because of equipment constraints, only the relative EQE of this LED was measured. To estimate the value of the EQE during pulsed operation, we measured the EQE of another small area LED from the same substrate in DC. The value of the EQE in pulsed operation was scaled to align with the DC measurement. Use of a smaller area LED improves the relative size of the available heat sink when compared to the 4 mm2 LED presented in figure 4, and allowed for measurement at a higher current density of ~3 A/cm2. A 4 mm2 size LED would typically fail around 500 mA/cm2. This size dependence shows that heating was significant. There is an existing report of small LEDs made of the inorganic CsSnBr3 perovskite sustaining high current densities of ~100’s A/cm2. These inorganic LEDs showed peak EQE at current densities at ~200 A/cm2, similar to the data shown in Figure 5.23


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Figure 5. Pulsed operation of a small area perovskite LED. Pulsed operation limits the increase in temperature and allows us to come closer to the higher current densities needed for electrically driven lasing a) Current profile of a small area (0.024 mm2) lowD (i~10) LED operated from 7 to 40 volts, at 1 kHz frequencies, and ~500 ns pulse length. The LED is able to reach a steady state current after about 300 ns. b) Shows the projected EQE from this low-D LED, where the relative EQE versus current density measured at 300 ns is plotted in the log-log scale. We observed that the device began to fail when current densities were above 200 A/cm2, and completely failed at ~400 A/cm2. The blue lines are from a neighbor LED from the same substrate measured in DC operation using an integrating sphere. The red lines are from an analogous larger area LED (4 mm2) illustrating continuity between the large and small area LEDs operated at different current densities. LEDs operated in both DC and pulsed operation show reduced EQE at higher current densities (roll-off). We can fit a line to the EQE curves in the log-log plot with a slope of approximately -0.4. From this fit, we can estimate an EQE of ~0.05% at current densities of 5kA/cm2. This kind of fit may be reasonable if we can eliminate degradation from


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heating. This gives us an order of magnitude value of the EQE, which we can use to calculate our threshold current. There are a few possible options for the source of roll-off in the LEDs. One recent report concluded that roll-off was likely due to Auger recombination because they observed that PL lifetime and PLQY reduced at higher carrier densities, consistent with Auger recombination.15 They ruled out heating and electric field quenching as possible sources. They observed electric field quenching, but it was only significant at low carrier densities. They reduced roll-off in these devices by increasing the average order of the film, and argued that this decreased the local carrier density within a grain and reduced Auger recombination. Auger recombination is a 3 body nonradiative recombination process and will strongly depend on carrier concentration (N carrier concentration, dN/dt  N3).24 This proposed mechanism is reasonable and agrees with work on quantum dots where reducing size increases Auger recombination rates,25 as well as our recent work showing a near linear increase in ASE threshold as a function of PEA cation.6 We believe that Auger recombination is a likely source for the observed sublinear roll-off presented in Fig. 5b. Although we cannot rule out heating effects, as it still appears to be an issue at high currents. Auger recombination is likely most significant in the lowest orders of the film. As can be seen in Fig. 1c there is a significant portion of i=4 in the film. This portion may make the largest contribution to energy transfer, but is also more susceptible to Auger recombination than higher order emitters. Additionally, carrier balance should be considered, and will likely change when increasing the current density by 4-6 orders of magnitude needed for electrically driven lasing. The devices shown in Figure 5 used the same structure as the LEDs used in low current density operation (Figure 4). It is possible that an imbalance of charge is collecting at one of the interfaces between perovskite and the electron or hole blocking layers, causing an increase in Auger recombination. The observed slope of the roll-off may reflect the rate at which this charge imbalance increases. Further optimization of the device structure is likely needed for high current density operation. We can make an estimate of the carrier density needed for lasing in perovskite films based on optical ASE and lasing thresholds.23. If we used the ASE threshold of 7-19 J/cm2, the corresponding density of carrier pairs per unit time is ~1027 cm-3s-1. In an idealized system, this would correspond to a current density of ~3-6 kA/cm2. If we estimate plausible performance from our LEDs this current could increase to ~4-10 kA/cm2. Depending on the assumed size of the recombination zone this number could be significantly reduced. However, we do not have a direct measurement of this value and will not speculate here. Detailed calculations are provided in the supporting information. We know from optical


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pumping that the relative PLQY of our MAPbI3 films began to roll-off at ~150 J/cm2, corresponding to ~50 kA/cm2.6 This roll-off is at a much higher carrier density than what we see in electrical operation. This provides hope that perovskite should be able to support the current densities needed as long as carriers are uniform, balanced, and pumped for a short time (~ 1ns). At high carrier concentrations, our LEDs may have an imbalanced and non-uniform distribution of carriers leading to significant roll-off, but higher current densities should be possible. There are many solutions that we could potentially implement to get closer to our goal of electrically driven perovskite lasing. In summary, future perovskite laser devices will likely see the most significant benefits from improved light confinement, transport layers that are compatible with the injection of high current, and shorter pulse durations. Confinement represents the aggregate trapping of light within the cavity in x,y, and z directions. In our work, we have confinement perpendicular to the plane (z-axis) resulting from reflection off of the perovskite film’s interfacial surfaces, and confinement from the DFB (x-axis). It is possible to implement a 2D DFB grating that would also confine light in the y-axis. This has been reported to improve the threshold by a factor of 20 in polyfluorene lasers.26 The quality of the “mirrors” can be improved by grating optimization and other techniques. Also, meaningful threshold reductions from improved film quality/crystallinity are likely possible. An in-depth discussion is provided in the supporting information. Within this work, we used low-D perovskite-based on the cation NMA to make LEDs and distributed feedback lasers. We used high order films to fabricate a DFB laser with a FWHM under 1nm. Some possible reasons that DFB lasing using low-D perovskite has not been reported is because of a sensitivity of perovskite growth on the choice of substrate and an increased threshold compared to analogous 3D films. Using NMA based low-D perovskite films, we were able to demonstrate LEDs with an EQE up to ~8%, which is reasonable for an organic material emitting in the near infrared. We demonstrated high current density pulsed operation and proposed that Auger recombination was significant in low-D perovskite LEDs, and is likely exacerbated by charge imbalance. Failure at high current was likely due to local heating and could be improved by changing transport layers and overall LED structure. Device structure needs further optimization before reaching current densities needed for lasing. Based on optical pumping measurements, high-quality 3D and low-D perovskite films, pumped at short times (~1 ns), with a uniform recombination zone should be able to reach current densities


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need for lasing. Low-D perovskite films are more prone to Auger recombination, so increasing the current density in un-optimized LEDs is likely not a viable solution. However, a brute force approach may work for 3D perovskites, as they perform comparatively well at very high carrier densities. Methods Details on preparation and measurement conditions are provided the supplemental information. Abbreviations 1-naphthylmethylamonium - (NMA), Butylammonium - (BA), Octylamine - (OA), Phenylethylammonium - (PEA), Methylammonium - (MA), Formamidinium - (FA), Dimethylsulfoxide - (DMSO), Amplified spontaneous emission - (ASE), Distributed feedback laser - (DFB), External quantum efficiency - (EQE), Photoluminescence (PL), Photoluminescent quantum yield (PLQY) Acknowledgments This work was supported by the Japan Science and Technology Agency (JST), ERATO, Adachi Molecular Exciton Engineering Project, under JST ERATO Grant Number JPMJER1305, Japan, by JSPS KAKENHI grant numbers JP15K14149 and JP16H04192 and The Canon Foundation. We would like to thank Shim Chang-Hoon, Ganbaatar Tumen-Ulzii, Hao Ye, Kazuya Jinnai, Hiroyuki Mieno, and Hiroki Noda, for assistance with equipment. Supporting Information: Transient absorption, criteria for lasing, substrate dependence , LED evolution, lasing film LED, feasibility of electrical lasing, methods section, image of grating, grating LED, X-ray diffraction, absorption spectra, low-D perovskite lasing data, additional discussion.

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