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Abstract
The metal halide perovskite nanocrystals (NCs) have attracted much attention because of their excellent optical properties and potential for application in optoelectronic devices. However, their photo- and thermostability are still practical challenges and need further optimization. Here, we have studied the degradation behaviors of CsPbI3 NCs utilized as optical conversion layer in InGaN based blue micro-LEDs in situ. Furthermore, the effects of temperature and light irradiation on perovskite NCs were investigated respectively. The results indicate that both blue light irradiation and high temperature can cause the increased nonradiative recombination rate, resulting in the degradation of perovskite NCs and reduction of the photoluminescence quantum yield (PLQY). Especially in high-temperature condition, both the single-exciton nonradiative recombination rate and the biexciton nonradiative recombination rate are increased, causing the significant reduction of PLQY of perovskite NCs in high temperature environment than blue light irradiation. Our work provides a detailed insight about the correlation between the light irradiation and temperature consequences for CsPbI3 NCs and may help to pave the way toward optoelectronic device applications.
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1. Introduction
All-inorganic cesium lead halide perovskite nanocrystals (NCs) have become particularly attractive fluorescent nanomaterials in recent years due to their nanosecond fluorescence lifetime, continuously adjustable emission wavelength and high photoluminescence quantum yield (PLQY), wide color gamut covering up to 140% of the NTSC color standard [1,2]. By adjusting the size and composition of NCs, the PL emission wavelength of perovskite NCs can cover the whole visible light range [3]. Combining GaN-based blue micro-LED chip and the perovskite NCs, InGaN based color-converted micro-LEDs can be achieved by partially converting blue light into yellow or red emissions, which have the potential to broaden the gamut, improve luminous efficiency, color rendering index and modulation bandwidth, and have significant applications in lighting, display, and visible light communication (VLC) [4,5].
All-inorganic NCs have been successfully demonstrated in optoelectronic devices. For example, the external quantum efficiency (EQE) of perovskite LEDs based on green-emitting perovskite nanocrystals (NCs) have reached 24.5% [6]. The white light-emitting devices can be obtained by combining a UV or blue GaN chip with highly luminescent yellow-red emitting nanocrystals [5,7]. Compared with broad-area LEDs, micro-LEDs can operate at extremely high injection current densities to achieve high modulation bandwidth. Therefore, micro-LEDs are more appropriate light source for VLC. Using blue micro-LED chip and yellow perovskite NCs, the high-bandwidth fluorescent white LEDs have been achieved [8,9]. However, the stability of perovskite NCs remains a practical challenging and needs to be further improved. For organometal mixed-halide perovskite NCs (CH3NH3PbX3, X = Cl, Br, I), the light irradiation induced degradation was ascribed to decomposition of the NCs [10–13]. Recently, the light irradiation induced degradation of CsPbBr3 NCs under high-power LED light was studied, and the reduction of luminescent properties was related to the increase of surface trap states [14]. The PL intensities of Mn-doped CsPbCl3 nanocrystals significantly decreased due to formation of nonradiative defects/traps in nanocrystals during the ultraviolet light irradiation [15]. The photostability and photodegradation processes of all-inorganic perovskite CsPbI3 NCs also have been investigated extensively [16]. The results indicate that light irradiation would cause the detachment of the capping agent, collapse of the nanocrystal surface, and aggregation of surface Pb atoms [17]. But till now, there are few in-situ studies of degradation mechanism for CsPbI3 NCs as conversion layer in GaN-based micro-LEDs, as well as the independent effects of temperature and light irradiation on perovskite NCs.
In this work, the degradation behaviors of CsPbI3 NCs utilized as optical conversion layer in GaN based micro-LED are studied in situ by electroluminescence (EL) and photoluminescence (PL) spectroscopy. Furthermore, the effects of temperature and light irradiation on perovskite NCs are investigated respectively. Based on above analyses, the degradation mechanisms concerning the free carriers and excitons are presented to describe possible physical mechanisms of the degradation processes of perovskite NCs. We believe that this work would help to understand the degradation mechanisms of perovskite NCs in color-converted micro-LEDs and offering a potential route toward high reliability and modulation bandwidth light source for display and visible-light communication.
2. Experiments and results
The preparation processes of all-inorganic cesium lead halide CsPbI3 NCs and InGaN blue LEDs can be referred from our previous works [2]. The PL emission of NCs is centered at λ = 690 nm and has a high PLQY of up to 90%, which are used to coat the surface of the blue LED to realize a hybrid color-converted light emitting device where they are excited by blue light from the LED chip under low current injection conditions, resulting in light emission at distinct wavelengths in the visible spectral range. The EL spectra were measured using the Ocean Optics spectroscopy system in the continuous-wave mode. The different injection currents are supplied by Keithley 2400 source meter. The junction temperatures of the hybrid color-converted LED were measured using the transient thermal analysis. In order to considering the effects of temperature and light illumination on the stability of perovskite NCs, the CsPbI3 NCs were dispersed in methane solvent, and then irradiated with blue light under low temperature environment and stored away from light in high temperature box, respectively. The PL and time-resolved PL (TRPL) spectra measurements were carried out by Edinburgh Instruments FLS980 fluorescence spectrometer with Xenon lamp and 375 nm picosecond pulse laser as the excitation sources, respectively.
The cubic-shaped NCs can be observed via the High-resolution transmission electron microscopy (HRTEM) image in Fig. 1(a). An exposed (200) crystal plane with a clear lattice spacing of 5.8 Å can be easily found in the high-resolution TEM (HRTEM) [18], corresponding to the lattice face of the cubic phase and an average size of ∼11.82 nm [19,20]. The NCs are monodispersed with a lattice space of ∼0.58 nm [21]. In addition, the prepared inorganic perovskite NCs were characterized by X-ray diffraction (XRD). For two-dimensional perovskite NCs, the XRD patterns for the thin films of the FAPbBr3 the dominant peaks at around 15° and 30° are assigned as the (100) and (200) crystal planes, which shows a good crystalline [22]. In addition, Yang Shen et al selected the amino-functionalized ETA to modify the underlying PEDOT: PSS hole-transport layer. The XRD results of the CsPbBr3 perovskite films deposited on various ETA modified PEDOT: PSS substrates shown that the intensity of the dominant diffraction peak at 30.3° assigned to (200) crystal plane is dramatically enhanced with increasing ETA ratios, which indicates the CsPbBr3 perovskite films on the ETA-modified PEDOT: PSS become more oriented with better crystallinity [6]. Figure 1(b) shows the XRD spectra of the CsPbI3 NC films spin-coated on glass slide. The measured XRD patterns display the phase of NCs as cubic corresponding to the standard phase of CsPbI3 (ICSD-181288) [22], it can be seen that there are XRD diffraction peaks of (100) and (200) crystal planes [23], mainly the characteristic peaks of crystal plane (200), indicating the good crystalline of NCs. Then we combine the excellent NCs with the InGaN Micro-LED array, the schematic diagram is shown in Fig. 1(c). Following the fabrication of the LED arrays with diameter d = 30µm, the solution of the mixed NCs in hexane was spin coated on the surface of the LED in a glove box. Then, the device was left in the glove box overnight to fully evaporate the solvent (hexane). Figure 1(d) shows the top-down optical images of color-converted InGaN micro-LED arrays with perovskite NCs. Each array consists of four circular-shaped micro-LED pixels and they share same cathode with individual anodes.
Fig. 1. (a) High resolution TEM (HRTEM) images of a CsPbI3 NCs. (b) Experimental X-ray diffraction (XRD) patterns of the glass slide and CsPbI3 NC film. (c) Schematic diagram of the color-converted InGaN micro-LED arrays with perovskite NCs. (d) Top-down optical images of color-converted InGaN micro-LED arrays with perovskite NCs.
Figure 2(a) shows the EL spectra of blue micro-LED chip for various injection currents (0-100 mA). With the increase of injection current, the EL intensity of the LED chip keeps increasing. Meanwhile, there exists a blue shift of peak wavelength, which is attributed to the Coulomb screening effect of quantum confined stark effect (QCSE). To study the stability of the micro-LED chip, the EL spectra of the same LED chip are measured twice from low injection current to high injection current. The repeatability results of EL peak intensity fluctuating with injection current are shown in Fig. 2(b). It can be observed that the curves of injection current dependent EL peak intensity of the twice measurements are consistent, indicating that the large injection current will not affect the luminous performance of the micro-LED chip.
Fig. 2. (a) El spectra of micro-LED blue chip under different injection current (0-100mA); (b) The injection current dependent EL peak intensity of the chip for two repeated measurements; (c) EL spectra of micro-LED/CsPbI3 hybrid light-emitting device under different injection current (0-100mA); (d) The injection current dependent EL peak intensity of blue LED chip and perovskite NC film for hybrid LED devices.
The color-converted GaN-based LEDs are fabricated by spin-coating the perovskite NCs on the surface of blue micro-LED chip. The EL spectra of color conversion device under the same injection current condition are shown in Fig. 2(c). Among them, the variation trend of injection current dependent EL peak intensity of chip is basically consistent with that of blue micro-LED, but the peak intensity of CsPbI3 NCs initially increases with the increase of injection current, then decreases with the continuous increase of current after reaching saturation, as shown in Fig. 2(d).
According to the ABC model, the recombination dynamics of perovskite NCs can be written as follows [24].
where n is photogenerated carrier concentration. For bulk 3D perovskite, A is the trap-assisted non-radiative monomolecular recombination coefficient, B is the radiative bimolecular recombination coefficient and C is the Auger (non-radiative) recombination coefficient. For strong-confinement perovskite NCs, A consists of non-radiative monomolecular recombination, radiative monomolecular recombination. In perovskite NCs, confining the carriers within NCs yielded high binding energy known as the quantum confinement effect, the photogenerated electrons and holes interact to form excitons in a very short time [25,26]. All discussions below are based on the exciton model. The blue light emitted by the LED chip is used as the excitation light of the perovskite layer. With the increase of the excitation light power by increasing the injection current, the photogenerated excitons concentration n of the perovskite layer gradually increases, enhancing the Auger recombination rate of that. Furthermore, the junction temperature increases with the increase of injection current. Therefore, the decrease of EL intensity of the perovskite NCs with the increase of current may result from the thermal quenching. In addition, the increased SRH recombination also would cause the decrease of EL intensity of the perovskite NCs in color-converted LED. The EL intensity of perovskite layer for the second measurement declines compared with the value for the first measurement, which shows a degradation of PLQY of CsPbI3 NCs rather than thermal quenching process [27]. The degradation mechanisms will be discussed below.The EL spectra of blue micro-LED and hybrid LED were studied at varied operating times and injection currents to further investigate the reliability of perovskite color-converted LED. Firstly, the micro-LED blue chips were continuously lit at 20, 30 and 40 mA for one hour, and electroluminescent characteristics were collected every 10 minutes. As shown in Fig. 3(a) and 3(b), under specific injection current, the EL intensity of the blue LED chip would not change significantly with the increase of lighting time, indicating that the LED blue chip has good reliability.
Fig. 3. (a) The EL spectra of micro-LED blue chip at different times when the LED chip lights up continuously at injection current of 20 mA; (b) The relationship between EL peak intensity of LED chip and operation time under different injection currents; (c) The EL spectra of blue LED/CsPbI3 hybrid LED at different times at injection current of 20 mA; (d) Dependence of the intensity of the NC layer (red) and LED chip (black) emission on the lighting time at injection current of 20 mA.
Subsequently, the EL spectra of perovskite color-converted LED with CsPbI3 as light conversion layer were measured at different radiative times under injection current 20 mA, as shown in Fig. 3(c) and 3(d). It is observed that the EL intensity of blue chip did not change greatly with the increase of lighting time, showing relatively stable and consistent with the above results. However, the PL intensity of perovskite NCs first increases and then decreases with the increase of operation time. When the injection current is constant at 20 mA and the operating time is in the range of 0-50 seconds, the EL intensity of perovskite NCs has an increasing trend. When the time is over 50 seconds, the luminescence intensity decreases obviously, indicating the degradation of luminous efficiency of CsPbI3 NCs. The junction temperature of micro-LED chip under 20mA is close to the room temperature [28]. Therefore, the EL intensity of the perovskite NC layer increases first and then decreases may result from blue light irradiation. Initially, many photo-generated carrier in perovskites NCs transition and transfer to adjacent nanocrystals, promoting the nonradiative energy transfer and resulting the increase of luminous efficiency of perovskite NCs [29,30]. However, the blue light illumination would induce the formation of nonradiative defects/traps in NCs with increasing illumination times, which may cause the decrease of the EL intensity of the perovskite NC layer [15].
It can be seen that the stability of perovskite light conversion layer directly affects the perovskite color-converted LED device. Temperature and light illumination, which are the junction temperature of the LED chip and the blue light emitted by the chip, are two principal parameters that influence the stability of perovskite NCs in hybrid LEDs lighted by applying current. Both temperature and light illumination may attenuate the PLQY of perovskite NCs. Considering that the prepared perovskite NCs have good stability when stored in low temperature environment [2]. In order to further investigate the principal factors affecting the stability of light conversion layer in GaN blue chip/CsPbI3 hybrid LED, the effects of temperature and light illumination on the stability of perovskite NCs were analyzed.
The control sample used in the experiment is perovskite NCs stored in a low-temperature environment (special storage box for chemical laboratory supplies) away from light. The blue light source used in the light illumination experiment is the blue micro-LED used in the perovskite color-converted LED. Then the prepared perovskite NCs were exposed to light illumination at low temperature to avoid the influence of temperature on the experimental results. The separate temperature experiment is to store the prepared perovskite NCs in a high-temperature aging test chamber away from light. The junction temperature (Tj) of the color-converted LED under different injection current were measured using the transient thermal analysis [28]. The Tj at 10 mA (1.4K A/cm2) is close to room temperature, but when the injection current increases to 100 mA (15K A/cm2), the Tj is up to 64℃. The storage temperature is set to 50 ℃ corresponding to the junction temperature of blue micro-LED chip at 70mA, which is the maximum point of PL peak intensity, as shown in Fig. 2(d). Figure 4(a) shows the PL spectra of perovskite NCs after blue light illumination and 50 ℃ respectively. Compared with the control samples, it can be seen that the PL intensities of the samples undergone light illumination and high temperature both decreases. The PL intensity of the perovskite sample stored at 50 ℃ attenuates more seriously than that under blue light illumination. In order to explore the reasons for the degradation of luminescence properties of perovskite NCs, the carrier dynamics have been characterized by PL decays using time-correlated single-photon counting (TCSPC), as shown in Fig. 4(b). The laser source used in TRPL measurements is a 375nm He-Ge laser.
Fig. 4. The PL spectra (a) and TRPL spectra (b) of perovskite CsPbI3 NCs after irradiation under 470 nm blue light (emission wavelength of micro-LED chip) and storage at 50 ℃, respectively.
The fluorescence lifetimes of perovskite NCs are calculated by fitting the TRPL curves using double exponential function, which is written as follows:
Table 1. The lifetimes τ1 and τ2 calculated from the second-order exponential function of perovskite NCs.
The photogenerated exciton recombination processes of perovskite NCs include single exciton recombination and biexciton recombination. The radiative recombination process of single excitons is shown in Fig. 5(a). Most of the energy is emitted in the form of photons, and only a small part is released through electron-phonon interactions. When the recombination process of single excitons is nonradiative recombination. The levels of defect states in the forbidden band can capture free carriers or excitons, and the energy after recombination is released in the form of phonons, as shown in Fig. 5(b). When light excites perovskite NCs to produce multiple excitons, two excitons can form biexcitons with lower energy [32]. Figure 5(c) shows the possible recombination process of biexcitons, such as exciton-exciton annihilation. Exciton-exciton annihilation is the nonradiative recombination when one exciton colliding with another excitons, which energy is dissipated in the form of heat.
Fig. 5. Recombination mechanisms of photogenerated excitons in perovskite NCs. (a) Single exciton radiative recombination; (b) Single exciton nonradiative recombination; (c) Biexciton recombination process.
Compared with the carrier lifetimes of the control sample, when only the light illumination condition is considered, the fast lifetime τ1 of the carrier decreases obviously, and the slow lifetime τ2 has not changed. However, when considering the stress condition of high temperature, the fast lifetime τ1 and slow lifetime τ2 of photogenerated carriers both reduce. The results indicate that light illumination only affects the single exciton recombination process, while high temperature has an effect on the single exciton recombination and biexciton recombination processes of perovskite NCs.
When only blue light illumination is applied to perovskite NCs, the high-energy excitation light makes the surface ligands fall off, exposing the high-energy surface, and resulting in vacancy defects which serve as trap states to capture single excitons, increasing the nonradiative recombination rate, and causing the degradation of PL spectra of perovskite NCs. When the temperature is considered as the stress condition, the fast lifetime and slow lifetime of perovskite NCs both decrease, which shows that not only the nonradiative recombination rate of single exciton increase, but also the nonradiative recombination rate of biexcitons also increase. During the heating process, there may be enhanced phonon-photon interaction, leading to PL quenching. The biexciton emission in PL spectra can be observed under the high excitation intensity of excitation light and explored to study the biexciton quenching of perovskite NCs, [32,33] the detail of which remains further investigation. In addition, the decomposition of quantum dots caused by high temperature will also cause the degradation of PL performance. Therefore, the degradation of luminescence properties for perovskite NCs under temperature stress is greater than that under blue light illumination. For the color-converted LED using the perovskite NCs as the light conversion layer, the perovskite NCs have been embedded into poly-methylmetacrylate (PMMA), yielding composites with excellent optical properties. In addition, when spin coating the PMMA or other encapsulation on the surface of the perovskite NC film followed by UV-curing, the coating layer can isolate moisture in the air and dissipate heat of the LED chip so as to enhance the longevity of perovskite NC films [4,7]. Therefore, the protective layer of the perovskite NCs for color-converted devices is very necessary to protect the perovskites from losing their optical properties and enhance longevity and should be further explored and optimized in future work.
3. Conclusion
In this study, the degradation mechanisms of luminescent properties of CsPbI3 NCs utilized as optical conversion layer in InGaN micro-LEDs have been studied in situ. The EL spectra of color-converted LEDs were measured as a function of injection current. The results reveal that when the operating time of a perovskite color-converted LED increases, the EL intensity of the perovskite NC layer drops. Furthermore, the effects of temperature and light irradiation on perovskite NCs are investigated respectively. Temperature has a greater impact on the PLQY of perovskite NCs than blue light illumination. Blue light irradiation may form vacancy defects on the surface of perovskite NCs, which would capture excitons, causing the increase of nonradiative recombination rate and decrease of the luminous efficiency of perovskite NCs. Meanwhile, at high temperatures, not only the nonradiative recombination rate of single exciton but also the nonradiative recombination rate of biexciton increases, resulting in a more serious degradation of luminescent efficiency of perovskite NCs than the degradation by blue light illumination. As a result, for the applications of perovskite NCs in the field of illumination, display and VLC combined with GaN-based micro-LED, the heat dissipation performance of the device as well as the photostability and thermostability of the perovskite NCs need further improved.
Funding
Natural Science Foundation of Ningxia Province (2022AAC03050, 2022AAC03117); Key Research and Development Program of Ningxia (2021BEB04040, 2022BDE03006); Natural Science Research of Jiangsu Higher Education Institutions of China (21KJB510013); Natural Science Foundation of Jiangsu Province (BK20210036).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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