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. 2023 Feb 28;23(5):1637–1644. doi: 10.1021/acs.nanolett.2c02944

Enhancement of Amplified Spontaneous Emission by Electric Field in CsPbBr3 Perovskites

Yang Li †,‡,*, Hang Hu †,‡, Ahmed Farag †,‡, Thomas Feeney †,‡, Isabel Allegro ‡, Uli Lemmer †,‡, Ulrich W Paetzold †,‡,*, Ian A Howard †,‡,*
PMCID: PMC9999453  PMID: 36852434

Abstract

graphic file with name nl2c02944_0004.jpg

Perovskite gain materials can sustain continuous-wave lasing at room-temperature. A first step toward the unachieved goal of electrically excited lasing would be an improvement in gain when electrical stimulation is added to the optical. However, to date, electrical stimulation supplementing optical has reduced gain performance. We find that amplified spontaneous emission (ASE) in a CsPbBr3 perovskite light-emitting diode (LED) held under invariant subthreshold optical excitation can be turned on/off by the addition/removal of an electric field. A positive bias voltage leads to a factor of 3 reduction in the optical ASE threshold, the cause of which can be attributed to an enhancement of the radiative rate. The slow components (10 s time scale) of the modulation in the photoluminescence and ASE when the voltage is changed suggest that the relocation of mobile ions trigger the increased radiative rate and observed lowering of ASE thresholds.

Keywords: amplified spontaneous emission, perovskite, field enhancement, ion migration

Halide perovskite semiconductors are an emerging class of laser gain media1−14 due to their high absorption coefficient, emission color tunability, and low-cost solution processability. Following the milestone of room-temperature continuous-wave (CW) lasing,14 several efforts15−20 on intense electrical driving of light-emitting diodes (LEDs) have been made. One of the most exciting results is an external quantum yield of 1% at 10 kA cm–2 achieved under nanosecond electrical pulses.19 This pushes the electrically injected carrier density to almost the same as the threshold carrier density needed for lasing under picosecond optical pulsed excitation.19 However, electrically pumped lasing is still not realized.

Simultaneous optical and electrical excitation of perovskite gain media is useful to study the transition from optical to electrical lasing. Several studies have been performed on dual optical-electrical excitation,21−25 and devices with different structures have been explored, i.e., vertical LED structures with23,25 and without21 current injection, a horizontal transistor structure,22 and a metal–insulator–semiconductor structure.24 These previous results found that electrical stimulation either did not effect or had an adverse effect on amplified spontaneous emission (ASE) and lasing thresholds, even though photoluminescence (PL) enhancement22,24 upon electrical excitation has been observed (this was attributed to trap filling). In contrast to the above work, we find a reduction (by a factor of 3) in the ASE threshold when a positive voltage over 4 V is applied to CsPbBr3 perovskite LEDs. We demonstrate that optical ASE can be repeatedly switched on/off by modulation of electrical bias. We ascribe the ASE enhancement to an increase in the second-order radiative rate constant and decrease in nonradiative first and second-order rate constants caused by the application of electric field. The slow response of the PL and ASE upon modulation of the bias voltage suggest that mobile ions are responsible for the effect. Ion migration26−28 has been widely observed in perovskite-based optoelectronic devices. For example, it is the main cause of the well-known hysteresis behavior,29 and it has been previously suggested that electric-field-assisted motion of the mobile ions under operating conditions can alter the electric field and carrier distribution within solar cells, LEDs, and photodetectors.30−36

We first develop perovskite LED devices whose device architecture is tuned to minimize the optically pumped ASE threshold. These employ a smooth CsPbBr3 emissive layer fabricated through the previously reported process of annealing a quasi-2D precursor film at a high temperature.37,38 The excited-state population in this layer is known to be dominated by free charge carriers, and this is beneficial for its gain performance.37,38 It is sandwiched between a ZnO electron transporting layer (ETL) and a poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(4-sec-butyl-phenyl)diphenylamine) (TFB) hole transporting layer (HTL). Integrating a CsPbBr3 film into an LED device significantly reduces its ASE performance due to the worse optical confinement in CsPbBr3 layer caused by the change in the dielectric environment (Figure S1a,b). To optimize the ASE performance in an LED structure (opaque device), device engineering was undertaken (Supplementary Note 1 and Figure S1c,d) that resulted in an optimal semitransparent device (Figure 1a) using thin sputtered indium doped tin oxide (ITO) electrodes. To compensate for the thinness of the ITO layers, gold grids (0.2 mm in width) are employed to reduce the series resistance of the LED device. The performance of these devices optimized for low ASE threshold (and their stability) is summarized in Figure S2. Despite the low performance of the LEDs that are unstable at very high (>8 V) voltages needed for significant electroluminescence (EL), they are stable at the sub EL turn-on conditions (≤6 V) used below. The ASE threshold of this LED structure optimized for best optical gain performance is still around two times higher (worse) than that of the pristine CsPbBr3 film (Figure S1) even though the absorption loss due to the electrical contacts and functional layers of the LED has been excluded. This is not an effect of scattering as the CsPbBr3 film in the LED is as smooth (crystal size ∼44 nm, Figure S3) as that of the pristine film.37 Based on such an LED structure optimized for the lowest optically pumped ASE threshold, we report a significant further improvement of the optical ASE threshold upon the application of an electrical bias.

Figure 1.

Figure 1

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Enhancing optically pumped PL and ASE intensity of perovskite LEDs by application of a positive bias voltage. (a) Schematic of the experimental setup exhibiting the collection of perovskite LED emission under dual excitation of a 355 nm laser (1 ns pulse length, 1 kHz) and a DC bias voltage. The pulsed laser is incident from the HTL side and the measurements are conducted in air. (b) Emission spectra of the perovskite LED acquired using subthreshold optical excitation (∼0.8Pth) without (black line) and with (red line) a positive bias of 6 V. ASE is essentially absent without applied electrical bias, but clearly visible with electrical bias (although the optical pumping remains identical). Error bar represents the standard deviation calculated from five off/on cycles. (c) Reproducibility of effect illustrated by the ratio of the peak intensity in the “ASE-region” (green area in (b)) relative to the peak intensity in the PL region (orange area in (b)) over several off/on cycles with sufficient time for the ratio to settle. The optical excitation remains at ∼0.8Pth.

To simultaneously pump the perovskite LED optically and electrically, a 355 nm laser with 1 ns pulse length (1 kHz) was used for optical stimulation alongside a DC bias voltage generated from a Keithley Source Meter Unit. Figure 1a illustrates the experimental setup to record emission spectra with and without the bias voltage. Unless otherwise stated, the pulsed laser is incident from the HTL side of the semitransparent LED, and the above 355 nm pulsed laser is the only optical source for the dual excitation experiments (conducted in air). Figure 1b shows the drastic change in emission spectra upon switching the 6 V bias voltage (stabilizing for 5 s) on and off while the optical excitation from the pulsed laser is held constant at ∼80% of the ASE threshold in the absence of applied bias (0.8Pth, Pth ∼ 600 μJ cm–2). When a bias voltage is applied, the PL is enhanced and the ASE is switched on. This observation is highly reproducible as seen in the small standard deviation of the emission spectra measured over 5 on/off cycles (error bars in Figure 1b), and the almost unchanging ratio between the PL and ASE peaks over subsequent cycles (Figure 1c). Note that the EL of the LED does not contribute to the collected emission because the EL does not turn on yet at 6 V (Figure S2). Therefore, the applied bias must be increasing the emission intensity from the optically generated excited states.

To investigate the influence of the bias voltage on the emission of the optically generated states (i.e., PL and ASE intensity), the emission spectra are collected upon scanning the voltage from −4 to 10 V while the pulsed laser power is still held at 0.8Pth. Figure 2a shows how the emission spectra change as the bias voltage is slowly increased. Broadband PL (peaked at 523 nm) dominates the emission spectra until 3 V. Further increasing the voltage leads to the emergence of ASE (peaked at 538 nm), which is accompanied by a significant PL enhancement. EL is much weaker than PL so does not contribute to this enhancement (Figure S4). A reduction in PL intensity is visible in the negative bias regime (Figure 2b). In the positive bias regime, ASE intensity, PL intensity, and their ratio all increase with the voltage bias and finally remain almost unchanged (over 6 V). A reverse voltage scan (from 10 V to −4 V with 2 V s–1 scan rate, Figure S5) shows significant hysteresis, which often indicates ion migration in perovskite-based devices.26,29 Through scanning the excitation pulse energy of the laser for a sequence of different bias voltages (Figure S6), the ASE threshold as a function of applied bias is derived, as presented in Figure 2c. The exposure time of the sample to high biases and laser intensities was limited to minimize device degradation (see experimental section of Supporting Information). Increasing the positive bias voltage from 0 to 4 V leads to a significant reduction in ASE threshold from ∼600 μJ cm–2 to ∼200 μJ cm–2. Considering the transmission of the ITO/Al2O3/TFB is only 60%, the lowered threshold could be considered to be 120 μJ cm–2 for comparison against the reference perovskite on glass with a threshold of 160 μJ cm–2 (Figure S1). Clearly, the application of bias significantly improves the ASE performance of the material. Further increasing the voltage above 4 V does not further reduce the ASE threshold (nor increase the PL intensity). As discussed further below, the lack of further change beyond a “threshold” bias is consistent with a spatial rearrangement of mobile ions causing the changes in emission. We note that the redistribution of ions depends on the driving force and time, and in that sense, 4 V is not a real threshold voltage but reflects that the bias was applied only for a limited time (1.0 s). Applying lower voltages for longer times could also lead to sufficient rearrangement.

Figure 2.

Figure 2

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Effect of the bias voltage on PL and ASE enhancement. (a) 2D contour plots of emission spectra versus the DC bias voltage. The laser power is fixed at ∼0.8Pth (355 nm, 1 ns duration, 1 kHz), and the voltage is scanned in the forward direction with 1 V step and 2 s stabilizing time for each voltage. (b) Plots of the ASE and PL peak intensity and their ratios versus the DC bias voltage. (c) Plots of the ASE thresholds versus the DC bias voltage. All data suggest that no additional improvement in ASE is achieved with further increase in applied bias over 4 V. (d) Relative PLQYs under pulsed laser excitation versus pump energy densities. The relative PLQYs (solid lines) are calculated through dividing the emission integral by excitation energy (shown in Figure S6), and the red/black dashed lines are the simulated PLQYs using the rate equations (shown in Figure S7). The gray dashed lines indicate the corresponding ASE threshold.

Before examining why field-induced rearrangement of mobile ions is the most likely explanation for the observations, we first consider how the field affects the photoexcited charge carriers’ recombination pathways. The relative PL quantum yield (PLQY) as a function of pulse energy is shown in Figure 2d with and without the 6 V bias. To qualitatively discuss these data, the ABC model for recombination dynamics and PLQY given in eqs 1 and 2 is used.

graphic file with name nl2c02944_m001.jpg 1
graphic file with name nl2c02944_m002.jpg 2

where G(t) is the generation rate, k1 is the trap-assisted recombination coefficient, k2r and k2nonr are radiative and nonradiative second-order rate coefficients, and k3 is the Auger rate coefficient. The origin of k2nonr is not entirely clear but could be related to a nonradiative process involving two free carriers and a trapped one.39−44 As discussed below, inclusion of a k2nonr is necessary to understand the observed changes in PLQY with bias. As seen from eq 2, the PLQY will first increase with carrier density until k1 is negligible compared to (k2 + k2nonr)n. The PLQY will then saturate and decrease once the carrier density becomes so high that k3n(2) becomes comparable with (k2 + k2nonr)n. The PLQY below ASE threshold remains almost constant for both 0 and 6 V data sets (Figure 2d). This indicates that second-order recombination channels dominate, and both first (trap-related) and third-order (Auger) loss processes are negligible in this fluence range. The absence of significant Auger recombination is reasonable considering literature rate constants;37,45 numerical simulations in Figure S7 indicate that the nanosecond pulses of such fluences generate carrier densities less than 1019 cm–3 necessary for Auger processes to become competitive. After the ASE threshold, the PLQY increases significantly for the 0 V case, but hardly for the 6 V case. This indicates that the PLQY for the 6 V case was already very high before the ASE threshold and would barely be increased by the faster radiative processes introduced by ASE. This is also consistent with the saturated PLQY values for 6 V data being 8 times higher than for the 0 V data, which indicates that the fraction of second-order radiative recombination, k2r/(k2r+ k2nonr), has significantly increased after the application of bias. Assuming that the quantum yield is unity for the 6 V data after the ASE threshold, k2r/(k2r+ k2nonr) before threshold can be estimated as 0.09 and 0.74 for the 0 and 6 V data, respectively. Time-resolved PL (TRPL) studies (Figure S8) confirm that k1 decreases and k2r+ k2nonr increases upon the application of bias. Figure S9 presents how parameters for all the rate constants can be chosen to self-consistently explain the PLQY and TRPL data. In summary, the application of bias is found to decrease k1 and k2nonr, both pathways associated with traps. An increase in k2r, perhaps due to faster charge motion, is also suggested.

We now turn to look at how the applied bias might lead to these photophysical changes. As a first check, we replace the DC voltage bias with a CW optical bias (Figure S10). The CW optical bias does not alter the ASE threshold at all. This excludes trap filling as the primary cause of PL and ASE enhancement, as if this was the cause then the CW optical excitation should have a similar effect. As a second check, we can be certain that the accelerated second-order process has nothing to do with the increased carrier densities from the electrical injection. This is because the equilibrium carrier density generated from DC bias voltage is less than 1.6 × 1014 cm–3 (Supplementary Note 2), which is much lower compared to the photogenerated free carriers (>1018 cm–3) by 1 ns pulsed laser. Also, we did not observe light-induced electrical injection as the current density of the LED does not change when the pulsed laser is turned on or off (Figure S11).

To uncover the origin of the bias-assisted enhancement in radiative recombination process, we track the emission spectra on the time scale of seconds upon turning on and off the bias voltage while the optical pump intensity is maintained at ∼0.8Pth. This is presented in Figure 3a and Figure S12. Note that continuous drive LED (at 6 V) for 1 min only leads to a small temperature rise (<5 °C, Figure S13), and hence the Joule heating induced emission change can be neglected.46 After switching on the 4 V bias, the emission intensity slowly rises, followed by the emergence of the ASE signature after ∼10 s. Similarly, upon switching off the bias voltage, the ASE signature also has a slow component (∼10 s) to its decrease (as does the PL). The very slow response of the PL and ASE to the application of field can be slightly accelerated (<5 s) by increasing the bias to a higher positive voltage (4 V vs 6 V) or applying a negative voltage (0 V vs −1 V) when turning off the bias, as shown in Figure 3b. Note that the transients upon switching off the bias also show a fast decay component (Figure 3b right) not present in the buildup. We postulate that the field in the reverse bias direction induced by redistributed ions could assist charge extraction of the photogenerated carriers (this, accompanied by the redistribution of ions back into the device could explain the transient reverse currents observed when the bias is returned to zero as shown in Figure S14). Increasing the operating temperature of the LEDs to 65 °C was attempted (Figure S15), but a statistically significant alteration of the rise/decay dynamics of the PL was not observed. However, slow components on the time scale of second clearly remain even at the elevated temperature. The long time scale of the process and the dependence of the transient on the driving force are strong indications that mobile ions play a significant role in causing the change in PL and ASE characteristics upon the application of an electrical bias.47

Figure 3.

Figure 3

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Effect of ion migration on ASE performance. (a) Transient behavior of emission spectra after switching on and off the 4 V bias. (b) Comparison of the ASE rise (left) and decay (right) transients after switching on and off between different bias voltages. The time resolution for the transients is 0.5 s. (c) Schematic illustration of the increased spatial separation of the photogenerated charge carriers and mobile ions due to redistribution of ions to interfaces caused by the applied field. The circle and square symbols in the orange area indicate the free carriers and mobile ions, respectively. The model also shows that the field generated by the redistributed ions opposes that of the external field.

A schematic of the hypothesized voltage-modulated ion migration is presented in Figure 3c to understand the mobile ions’ effect on the radiative recombination process and ASE transient behavior. Both positive ions (Cs+, Pb2+) and negative ions (Br–) are considered in the schematic, but this discussion is also valid if only one type of mobile ion is present. Considering the material structure and recent studies, Br– ions are expected to dominate.26 At short circuit conditions (0 V), the mobile ions will be evenly distributed across the CsPbBr3 film. The ionic defects throughout the bulk could reduce the radiative recombination rate by scattering and trapping,48 while increasing the trap-related first and second-order nonradiative recombination.39−44

If a positive bias voltage is applied, the external field will drift the negative and positive ions toward the CsPbBr3/HTL and CsPbBr3/ETL interfaces, respectively. This would result in a reduced trap density in the bulk region where most of the photogenerated carriers exist. As a consequence, a decrease in the first and second-order trap-associated rates and an increase in the second-order radiative rate is expected. The faster radiative rate reduces the ASE threshold as it increases the photon density at short times after the pulsed excitation, and therefore the product of the photon and carrier densities. Both the enhanced radiative rate and suppressed trapping increase the PL efficiency, explaining the observations above. Under negative bias, we do not observe an increase in PL or ASE because the reduction in trap density is offset by enhanced nonradiative rates caused by faster carrier extraction. Figure S16 illustrates how the reverse bias causes carriers to drift toward the electrodes at which their extraction is energetically favorable.

The above model also indicates that the modulation of PL/ASE is only related to the direction of bias voltage, not the incident direction of the pulsed laser. As expected, we observe that the PL/ASE is also enhanced (reduced) on the application of positive (negative) voltage when the pulsed laser is incident on the ETL side (Figure S17). As illustrated in Figure S18, we have conducted measurements on electrically insulating devices with blocking layers either directly adjacent to the perovskite layer or between the contacts and carrier transport layers. Our results show that while the former does not exhibit modulation of PL/ASE upon the application of an electric field, the latter does. This confirms that it is the electric field, not current, that is responsible for bias-modulated PL/ASE. We also note that the carrier transport layers seem to play a role in ion migration and recovery under an electric field, as previously reported in literature.49 Further research into this mechanism is currently ongoing. Furthermore, the positive DC bias induced PL enhancement with slow transients in response to the external bias are observed both for CsPbBr3 LED (low optical power excitation, Figure S19) and triple cation perovskite solar cells50 (Figure S20). This further supports our model of ion migration and its effect on emission efficiency, and suggests that this PL enhancement due to ion migration can occur in multiple perovskite-based material compositions.

Our current work reveals that the ASE and PL performance of CsPbBr3 can be significantly enhanced by a positive bias, even though the electrically injected carrier density is far too low to have such a major impact on light emission. Our data suggests that sweeping the mobile ions out from the bulk of the film significantly increases the radiative efficiency and optical gain. In terms of electrical pumping, intense current driven LEDs17−20 adopt brief electrical pulses (ns to μs length), which are too short to redistribute the mobile ions. Therefore, driving such LEDs under simultaneous DC and pulsed electrical excitation may be helpful for realizing the electrical injection lasing.15 Meanwhile, increasing the duty cycle of the pulsed electrical excitation could also be beneficial,15 as the time for sweeping the ions away is increased and the time for their recovery is reduced.

We report a repeatable modulation of PL and ASE in CsPbBr3 based LEDs triggered by switching a positive bias voltage on and off while maintaining a constant subthreshold optical excitation. The applied bias can reduce the ASE threshold by a factor of 3 when applying a positive bias voltage over 4 V. The response of the ASE and PL to a change in the applied bias is very slow, with transients on the order of several seconds. This implies that the ion motion is likely to underlie the effect. PLQY and TRPL measurements as a function of applied bias show that the bimolecular radiative rate is increased, and the first and second-order trap-associated decay is reduced by the application of a positive bias. This explains both the enhanced PL (better competition against nonradiative rates) and the enhanced ASE (faster photon emission leading to a larger product of the photon and charge carrier densities and therefore enhanced gain). This work establishes the importance of mobile ions for optical gain and demonstrates that significant improvement in gain performance can be achieved when ions are extracted from the volume of the material responsible for gain. This has implications for pumping schemes for electrically pumped lasing but should also be considered for optical gain materials and from a fundamental perovskite development perspective.

Acknowledgments

The authors gratefully acknowledge funding from the DFG (PEROLAS, no. 409035484), the Karlsruhe School of Optics & Photonics (KSOP) financed by the Ministry of Science, Research and the Arts of Baden-Wuerttemberg as part of the sustainability financing of the projects of the Excellence Initiative II, the Karlsruhe Nano Micro Facility (KNMF), the Helmholtz Energy Materials Foundry (HEMF), and the Excellence Cluster “3D Matter Made to Order” (EXC-2082/1-390761711). The authors thank the Helmholtz Association MTET program (Materials and Technologies for the Energy Transition) − Topic 1 − Photovoltaics (38.01.05). H.H. would like to thank the financial support from the Chinese Scholarship Council (CSC, no. 201808420221).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.nanolett.2c02944.

  • Additional details of materials and methods, calculation on electrically injected carrier density, additional figures including device engineering toward ASE, EL performance, EL and PL comparison, LED hysteresis, ASE threshold measurement under DC biases or CW optical biases, TRPL, band alignment, voltage-emission characteristics upon laser incident (PDF)

  • Additional video showing the electric-field-assisted modulation of ASE (MP4)

Author Contributions

Y.L., U.W.P, and I.A.H. proposed the idea for this project. Y.L. fabricated the samples and conducted the optical characterization. I.A. assisted with optical characterization. H.H., and T.F. performed and optimized the ALD deposition process for Al2O3 thin films. A.F. performed and optimized the sputtering process for ITO thin films. All the authors contributed to the discussion and data analysis. Y.L., U.W.P., and I.A.H. wrote the manuscript with inputs from all the others. U.L., U.W.P., and I.A.H. supervised this project.

The authors declare no competing financial interest.

Supplementary Material

nl2c02944_si_001.pdf (3.6MB, pdf)
nl2c02944_si_002.mp4 (104.4MB, mp4)

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