Stable down-conversion white light-emitting devices based on highly luminescent copper halides synthesized at room temperature

Lin-Tao Wang , Zhuang-Zhuang Ma , Fei Zhang , Meng Wang , Xu Chen *, Di Wu , Yong-Tao Tian , Xin-Jian Li and Zhi-Feng Shi *
Key Laboratory of Materials Physics of Ministry of Education, School of Physics and Microelectronics, Zhengzhou University, Daxue Road 75, Zhengzhou 450052, China. E-mail: shizf@zzu.edu.cn; xchen@zzu.edu.cn; Fax: +86-371-67766629; Tel: +86-150-9333-9165

Received 5th March 2021 , Accepted 9th April 2021

First published on 10th April 2021


Abstract

In recent years, the metal-halide perovskites have attracted intensive attention in the field of white light-emitting devices (WLEDs). However, their further commercial applications are severely hampered by the poor stability and lead toxicity of such materials. In this work, we demonstrate two types of lead-free ternary copper halides (blue-emissive Cs3Cu2I5 and yellow-emissive CsCu2I3), which were synthesized by a convenient supersaturated recrystallization method at room temperature. Both materials have particularly good crystallinity and excellent stability against ultraviolet light, heat, and air oxygen/moisture. Temperature-dependent photoluminescence (PL) and time-resolved PL decay measurements confirm that the highly luminescent and largely Stokes-shifted broadband emission of both materials derived from the self-trapped exciton-related recombination. By using a mixture of Cs3Cu2I5 and CsCu2I3 as the down-conversion phosphors, a high-performance WLED is demonstrated with the color coordinates of (0.324, 0.330), a correlated color temperature of 5877 K, and a color rendering index of 88.4. Also, a high luminous efficiency of 54.6 lm W−1 is realized, which is the highest value among the lead-free perovskite-based WLEDs. More importantly, the studied WLED demonstrates excellent operation stability, and almost no emission degradation occurs after continuously working for 100 h in air ambient. The results suggest that such ternary copper halides may be potentially attractive candidates for the fabrication of efficient, stable, and environmentally friendly WLEDs.


Introduction

In the era of increasing energy shortage, lighting accounts for almost 20% of global electricity consumption, so it is very necessary to develop efficient lighting methods.1 Solid-state lighting in the form of white light-emitting devices (WLEDs) has higher power efficiency than traditional incandescent and fluorescent lamps, so it is the first choice for next-generation lighting equipment.2–5 Currently, the commercial approach for WLED production is to combine blue LEDs with Y3Al15O12:Ge3+ yellow phosphors, because it has the advantages of low cost, simple structure, and high luminous efficiency (LE). However, the intense blue component in the white emission spectrum can damage the retina of humans, which limits the widespread applications of this technique.6–9 Besides, this route is not sufficient to produce a high color rendering index (CRI) because the commonly used phosphors have a narrow excitation band and lack the red and green components in the spectrum. Therefore, a more friendly strategy for WLEDs is to use ultraviolet (UV) LED chips to stimulate a suitable broadband emission phosphor combination, which helps to improve the CRI and reduce the damage to the eyes caused by the undesired blue component.

Recently, metal-halide perovskites have rapidly advanced in down-conversion WLED applications, which benefits from their remarkable optical properties including high color purity, tunable fluorescence emission, high photoluminescence quantum yield (PLQY), and low-cost processing technique.10–21 Since the first report by Snaith et al. in 2016, this field has made great progress.22 However, there is still a lack of major breakthroughs in the process of commercializing WLEDs based on perovskites although numerous efforts have been made. The possible reasons can be summarized as follows.23–27 Firstly, in light of the well-known hypersensitivity of lead-halide perovskites in the air with moisture, oxygen, heat, and UV light, they are easily decomposed and the luminescence will be severely attenuated. Secondly, the large-scale applications of WLEDs based on halide perovskites were limited by the undesirable lead toxicity. Thirdly, for the conventional two-component or multi-component strategy for device preparation, there are still some limitations and disadvantages that need to be taken seriously. For example, due to the self-absorption of photons and the different degradation rates of phosphors, efficiency loss and color rendering changes will be caused after a long-term operation. Therefore, it is extensively urgent to explore non-toxic and highly stable alternatives as phosphors to solve the above issues.

It was firstly proposed to use Sn2+ or Ge2+ with similar diameter and electronic structure to Pb2+ as an on-site replacement in halide perovskites. Unfortunately, the perovskites based on Sn2+ or Ge2+ would be easily oxidized into their tetravalent state (Sn4+, Ge4+) in air.28–32 Similar electronic properties can be obtained by using elements (Bi3+ and Sb3+) adjacent to lead in the periodic table. Unfortunately, the experimental results have confirmed that the obtained PLQY of bismuth/antimony-based perovskites is still not satisfactory.33–41 Compared with the above metal substitutions, copper has attracted the attention of researchers due to its non-toxicity, low cost, and abundant reserves. In our recent studies, blue-emissive Cs3Cu2I5 and yellow-emissive CsCu2I3 were synthesized by solution methods and their applications in electrically driven LEDs were also demonstrated.43 Both copper halides have high emission efficiency and are very stable in air ambient. Moreover, they are all characterized by broadband emission features and large Stokes shift, which brings us hope for fabricating a highly stable WLED because the color instability and photon self-absorption facing current multi-component WLED challenges can be avoided.

In this work, brightly luminescent and stable Cs3Cu2I5 and CsCu2I3 were synthesized by the supersaturated recrystallization method at room temperature (RT). The entire visible light area can be covered because of their broadband emission characteristics. By using a mixture of Cs3Cu2I5 and CsCu2I3 as the down-conversion phosphors, a high-performance WLED was fabricated with a CRI of 88.4 and a LE of 54.6 lm W−1, which is currently the highest value among lead-free perovskite-based WLEDs. More importantly, the unencapsulated WLED in continuous current mode can efficiently sustain for 100 h without any emission decay, demonstrating remarkable operation stability. This research will provide new insights into the design of new white phosphors and diodes for next-generation lighting technologies.

Experimental section

Materials used in this experiment include cesium iodide (CsI, Xi’an Polymer Light Technology Corp., 99.9%), copper (I) iodide (CuI, SIGMA, 99.999%), dimethylformamide (DMF, ≥99.5%), dimethylsulfoxide (DMSO, ≥99.5%), isopropanol and chloroform. Each precursor and solvent was used without further purification.

Synthesis of Cs3Cu2I5 microscale crystals

0.6 mmol CsI and 0.4 mmol CuI were dissolved in 2 mL of DMF and DMSO (1[thin space (1/6-em)]:[thin space (1/6-em)]1) mixed solution. For the synthesis of Cs3Cu2I5 microscale crystals, 0.5 mL of the precursor solution was quickly dropped into chloroform (4 mL) under vigorous stirring. After a few-second reaction, a white precipitate was formed. Then, the solution was centrifuged at 10[thin space (1/6-em)]000 rpm for 10 min to discard the supernatant. Finally, the Cs3Cu2I5 powder was obtained for characterization.

Synthesis of CsCu2I3 microscale crystals

0.2 mmol CsI and 0.4 mmol CuI were dissolved in 2 mL of DMF and DMSO (1[thin space (1/6-em)]:[thin space (1/6-em)]1) mixed solution. For the synthesis of CsCu2I3 microscale crystals, 0.5 mL of the precursor solution was quickly dropped into isopropanol (4 mL) under vigorous stirring. After a few-second reaction, a yellow precipitate was formed. Then, the solution was centrifuged at 10[thin space (1/6-em)]000 rpm for 10 min to discard the supernatant. Finally, the CsCu2I3 powder was obtained for characterization.

Fabrication of WLEDs

The as-prepared blue-emissive Cs3Cu2I5 microscale crystals, yellow-emissive CsCu2I3 microscale crystals and silica gel were mixed in a 20 mL beaker to obtain a homogeneous latex. The WLEDs were fabricated by combining the latex with UV LED chips (310 nm). Then the device was solidified in a vacuum oven at 40 °C for 60 min, and at 110 °C for 1 h sequentially.

Characterizations

The microstructures of the as-synthesized products were characterized using a high-resolution transmission electron microscope (TEM, JEM-3010). The crystallinity characterizations of Cs3Cu2I5 and CsCu2I3 microscale crystals were analyzed by X-ray diffraction (XRD, Panalytical X’ Pert Pro). The morphologies and chemical compositions of the products were analyzed by scanning electron microscope (SEM, JSM-7500F) and energy dispersive X-ray spectroscopy (EDS). The optical properties of the products were measured by using a Shimadzu UV-3150 spectrophotometer and steady-state PL spectra (Horiba; Fluorolog-3) with an excitation line of 290 nm. A closed-cycle helium cryostat (Jannis; CCS-100) was used to carry out the PL measurement at different temperatures. The absolute PLQY of the Cs3Cu2I5 and CsCu2I3 powder were measured by using a fluorescence spectrometer (Horiba; FluoroMax-4) with an integrated sphere (Horiba; Quanta-ϕ) with the excitation wavelength of 290 nm. For high-temperature PL measurement, the perovskite films were placed on a copper heat sink, the temperature of which can be controlled by a heating pane with a heating area of 40 mm × 40 mm. Absorption/PL of Cs3Cu2I5 and CsCu2I3 was conducted with the films by spin-coating microscale crystals on quartz substrates. Excitation power and temperature-dependent PL were conducted with the thin film form by spin-coating the Cs3Cu2I5 and CsCu2I3 microscale crystals on the SiO2/Si substrates.

Results and discussion

In this experiment, a simple RT supersaturated recrystallization approach was employed to synthesize the Cs3Cu2I5 and CsCu2I3 microscale crystals, and the corresponding processing procedures are illustrated in Fig. 1a. First, for the Cs3Cu2I5 microscale crystals, a mixture of CsI and CuI in a molar ratio of 3[thin space (1/6-em)]:[thin space (1/6-em)]2 was dissolved in DMF and DMSO “good” solvent to form a clear precursor solution. Then, the precursor solution was quickly dropped into the chloroform solution, which was the “poor” solvent for these precursor ions. Under vigorous stirring, a highly supersaturated state is immediately triggered and a rapid recrystallization reaction occurs. Finally, the solid powders of Cs3Cu2I5 microscale crystals were purified and obtained by a centrifugation process. It is worth noting that the whole synthesis process is completed at RT under ambient conditions without the requirements of high temperature and inert gas protection, which greatly reduces the preparation cost and complexity. The preparation procedures of the CsCu2I3 microscale crystals are similar to the above processes, except that the molar ratio of CsI and CuI is 1[thin space (1/6-em)]:[thin space (1/6-em)]2 and the “poor” solvent is isopropanol. The right panes of Fig. 1b display the typical optical images of the as-synthesized Cs3Cu2I5 and CsCu2I3 powders under ambient conditions and UV lamp (254 nm) irradiation, respectively. The observed strong and uniform blue and yellow light suggest that the Cs3Cu2I5 and CsCu2I3 powders have excellent luminescence properties, implying their potential as down-conversion phosphors for the construction of optically excited LEDs.
image file: d1tc01037a-f1.tif
Fig. 1 (a) Schematic illustration of the reaction system for the supersaturated recrystallization technique at RT. (b) Photographs of synthesized Cs3Cu2I5 and CsCu2I3 powders under sunlight (left) and 254 nm UV light irradiation (right).

The morphology properties of Cs3Cu2I5 and CsCu2I3 products were characterized by the SEM and TEM measurements. As shown in Fig. 2a and d, the typical Cs3Cu2I5 products exhibit an elongated rod-shaped morphology with a length of ∼7 μm and a width of ∼1 μm, while the CsCu2I3 products show a flat column-shaped morphology with a length of ∼15 μm and a width of ∼2 μm. Fig. 2b and e show their chemical element mapping results measured by EDS, in which the Cs, Cu and I elements are homogeneously distributed for both materials. Fig. S1 and S2 (ESI) show the corresponding EDS spectra, which yield the Cs[thin space (1/6-em)]:[thin space (1/6-em)]Cu[thin space (1/6-em)]:[thin space (1/6-em)]I atomic ratios of 3.00[thin space (1/6-em)]:[thin space (1/6-em)]2.05[thin space (1/6-em)]:[thin space (1/6-em)]5.05 in Cs3Cu2I5 and 1.00[thin space (1/6-em)]:[thin space (1/6-em)]2.03[thin space (1/6-em)]:[thin space (1/6-em)]3.05 in CsCu2I3, close to the stoichiometry of the anticipated composition. Fig. 2d and f present the TEM images of the Cs3Cu2I5 and CsCu2I3 microscale crystals, which all have quite smooth surfaces with uniform diameter along the growth direction. The in-plane structural information of them was revealed by the selected-area electron diffractions (SAED). As depicted in the insets of Fig. 2c and f, the diffraction spots could be assigned to the (130) and (340) crystal planes projecting along the [005] orientation of Cs3Cu2I5 microscale crystals; for the CsCu2I3 microscale crystals, the spots can be indexed to the (020) and (002) crystal planes projecting along the [400] orientation. Note that the high-quality SAED patterns for Cs3Cu2I5 and CsCu2I3 verify their single-crystalline features. XRD measurements were further performed to investigate the structural characteristics of CsCu2I3 and Cs3Cu2I5 microscale crystals. As shown in Fig. 2g and h, the diffraction peaks of the two samples correspond well to the standard diffractions, in which the dominant diffraction peaks at 24.02°, 25.57°, 26.34°, and 30.60° can be assigned to the (122), (320), (222), and (004) planes of orthorhombic Cs3Cu2I5, and the peaks at 10.77°, 13.45°, 21.96°, 26.19°, and 29.39° can be ascribed to the (110), (020), (130), (221), and (002) planes of orthorhombic CsCu2I3. It should be mentioned that no other peaks (e.g., CsI, CuI) were detected above the detection limit, suggesting a high phase purity of two samples. To analyze the optical properties of two samples, UV-visible absorption and steady-state PL spectra were measured. As shown in Fig. 2i, the absorption edge of CsCu2I3 is around 330 nm, whereas that for Cs3Cu2I5 is blue-shifted to approximately 305 nm. In addition, both Cs3Cu2I5 and CsCu2I3 are characterized by large Stokes shifts (CsCu2I3, 228 nm; Cs3Cu2I5, 139 nm) and large linewidths (CsCu2I3, 115 nm; Cs3Cu2I5, 85 nm), which is typical in the case of self-trapped exciton (STE)-related transition,44–48 which will be discussed later. The broadband emission and large Stokes shifts for two copper halides imply that the efficiency losses and color instability issues of WLEDs induced by the photon self-absorption can be effectively suppressed. We then measured the absolute PLQY of the obtained two samples, and the values of 89% for Cs3Cu2I5 and 15% for CsCu2I3 were obtained.


image file: d1tc01037a-f2.tif
Fig. 2 SEM images of (a) Cs3Cu2I5 and (d) CsCu2I3 microscale crystals, and the corresponding EDS elemental mapping of (b) Cs3Cu2I5 and (e) CsCu2I3. TEM images of (c) Cs3Cu2I5 and (f) CsCu2I3 microscale crystals. The insets show the SAED patterns. Experimental XRD patterns of (g) Cs3Cu2I5 and (h) CsCu2I3 microscale crystals. (i) Absorption and PL spectra of the as-prepared CsCu2I3 and Cs3Cu2I5 microscale crystals.

In order to better understand the optical recombination mechanism of the two samples, temperature-dependent PL measurements were carried out with the temperature ranging from 10 to 300 K. It can be observed that the relative PL intensity of both samples exhibits a significant temperature quenching behavior with the increase of temperature, as shown in Fig. 3a and d, which can be assigned to the thermally activated nonradiative recombination process. Then the exciton binding energy (Eb) of Cs3Cu2I5 and CsCu2I3 microscale crystals can be obtained by the following equation:

 
image file: d1tc01037a-t1.tif(1)
where I(T) and I0 are the integrated PL intensities at temperature T and 0 K, respectively. Eb is the exciton binding energy, and kB is the Boltzmann constant. For Cs3Cu2I5 and CsCu2I3 microscale crystals, the Eb were calculated to be 139.3 and 234.4 meV (Fig. 3b and e), respectively, which were much larger than that of traditional perovskite materials.43,49 Such a large Eb can ensure the generation of excitons at RT and promote their efficient radiative recombination, and hence the samples are prone to exhibit strong PL intensity. Moreover, their Huang–Rhys factor (S) and the optical phonon frequency (ħωphonon) were calculated to reveal the exciton–phonon coupling of two materials by the following equation:
 
image file: d1tc01037a-t2.tif(2)


image file: d1tc01037a-f3.tif
Fig. 3 Pseudocolor maps of temperature-dependent PL spectra of (a) Cs3Cu2I5 and (d) CsCu2I3 microscale crystals ranging from 10 to 300 K. Integrated PL intensity of (b) Cs3Cu2I5 and (e) CsCu2I3 microscale crystals as a function of reciprocal temperature. FWHM of (c) Cs3Cu2I5 and (f) CsCu2I3 microscale crystals as a function of reciprocal temperature. Time-resolved PL decay and fitting curves of (g) Cs3Cu2I5 and (h) CsCu2I3 microscale crystals. (i) Configuration coordinate diagram for the STEs’ dynamic mechanism of the Cs3Cu2I5 and CsCu2I3.

Through fitting the temperature-dependent full-width at high-maximum (FWHM) curve, the S factors of Cs3Cu2I5 and CsCu2I3 were extracted as 42.92 and 37.82, and the ħωphonon were 17.54 and 15.29 meV, respectively, as shown in Fig. 3c and f. Note that the obtained S factor is much higher than that of many traditional materials with free exciton recombination features, which indicates the presence of strong electron–phonon coupling in both samples, and facilitates the formation of STEs.50,51 As shown in Fig. S3a and b (ESI), the PL excitation spectra of Cs3Cu2I5 and CsCu2I3 exhibit identical shape and peak position, which suggests that their broadband emission originates from the relaxation of the same excited state.2,50,52 Besides, time-resolved PL measurements of Cs3Cu2I5 and CsCu2I3 were performed, and the obtained results and fitting curves are plotted in Fig. 3g and h. It can be seen that the average lifetimes of Cs3Cu2I5 and CsCu2I3 are 638 ns and 102 ns. These results are similar to other studies on copper-based halides with STE emission features.48–52 In view of the above discussions, the excitation and recombination processes of both samples can be illustrated in the coordinate diagram. As shown in Fig. 3i, upon photoexcitation, electrons are excited from the ground state to the high-energy excitation state, and then are trapped into the intrinsic lower-energy self-trapped states due to the strong electron–phonon coupling effect in Cs3Cu2I5 and CsCu2I3. Consequently, the radiative recombination from self-trapped states generates a highly bright broadband emission with large Stokes shifts and long lifetimes.

It is well known that the stability of lead-halide perovskites has always been a challenge, which is the main obstacle hindering their potential applications significantly. In this study, a comprehensive stability investigation was conducted on two materials by evaluating the influences of heat, UV light irradiation, and long-term storage on their optical and structural characteristics. Firstly, we conducted the thermal cycling PL tests in the temperature range of 20–150 °C, and the relative PL intensities at two representative temperature points (20 °C and 150 °C) are summarized in Fig. 4a and d. One can see that the relative PL intensity of both materials exhibits almost no obvious decay over ten heating/cooling cycles, with the peak intensity, spectral shape and peak position unchanged (Fig. S4, ESI), greatly superior to the previous studies on conventional lead-based halides.10–12 This recoverable emission behavior of Cs3Cu2I5 and CsCu2I3 after high temperature aging is a sign of their good thermal stability. Secondly, the photostability test of two materials was further conducted by continuously illuminating them with a portable UV lamp at a 10 cm distance, and the emission performance was quantitatively studied by monitoring the evolution of PL with time. As shown in Fig. 4b and e, the emission intensity of Cs3Cu2I5 can be almost maintained under continuous UV light irradiation for 8 h. Under the same conditions, a slight emission decay of about 10% occurs for CsCu2I3, which may be caused by the undesired photo-oxidation effect, as observed in conventional lead-based perovskites.12 Fig. S5a and b (ESI) show the corresponding PL spectra of Cs3Cu2I5 and CsCu2I3 recorded at different illumination time intervals. Except for the slight change in peak intensity (only for CsCu2I3), the FWHM and shape of the PL spectra are almost unchanged. Besides, the long-term storage stability of Cs3Cu2I5 and CsCu2I3 microscale crystals, without any protection and encapsulation, was studied by intermittently recording their XRD patterns after different storage periods in air ambient (25 °C, 50–60% humidity). As shown in Fig. 4c and f, both samples show good structural stability, which preserves their structural integrity after 46 day storage without the appearance of additional impurity diffraction peaks. The above results indicate remarkable stability of both Cs3Cu2I5 and CsCu2I3 microscale crystals against heat, UV light, and environment oxygen/moisture, and are also evidence of the reliable copper halides as down-conversion phosphors compatible for WLED applications under harsh conditions.


image file: d1tc01037a-f4.tif
Fig. 4 Relative emission intensity of (a) Cs3Cu2I5 and (d) CsCu2I3 microscale crystals at two representative temperature points (20 °C and 150 °C) over ten thermal cycling measurements. The insets show the emission intensity change of different cycles. Photostability test of the (b) Cs3Cu2I5 and (e) CsCu2I3 microscale crystals under UV light irradiation (365 nm, 8 W). XRD patterns of (c) Cs3Cu2I5 and (f) CsCu2I3 microscale crystals after different storage periods in air ambient (25 °C, 55% humidity).

The combination of low toxicity, high emission efficiency, and excellent stability of two copper halides suggests their promising potential applications in lighting fields. Moreover, the blue-emissive Cs3Cu2I5 and yellow-emissive CsCu2I3 with large Stoke shifts and broadband emission can avoid the photon self-absorption phenomenon of the device to the greatest extent, and thus good color rendering and reliable operation stability can be expected. In the present case, single-color LEDs and WLEDs were constructed by encapsulating Cs3Cu2I5 and CsCu2I3 powders and their mixtures as the down-conversion phosphors on a commercial UV LED chip (310 nm) for blue LED, yellow LED, and WLED fabrication, respectively. Fig. S6 (ESI) shows the schematic structure of the proposed devices, in which the upper phosphor layer is separated from the bottom excitation UV LED. Due to the appreciable distance between the phosphors and LED chip, the heat released from the bottom LED chip is rarely applied to the upper phosphors, so that the efficient light emission can be maintained for a long time. Fig. 5a–c show the typical photographs of three working devices under the same driving current of 5 mA, and strong blue, yellow, and white light can be observed. Fig. 5d–f display the emission spectra of the blue LED, yellow LED, and WLED operated at different currents, and their emission characteristics are well-matched with the PL spectra of Cs3Cu2I5 and CsCu2I3, respectively.


image file: d1tc01037a-f5.tif
Fig. 5 Photographs of UV-pumped (a) blue LED and yellow LED based on Cs3Cu2I5 and CsCu2I3 phosphors, respectively. (c) Photograph of a working WLED by using mixtures of Cs3Cu2I5 and CsCu2I3 as the phosphors. Emission spectra of (d) blue LED, (e) yellow LED, and (f) WLED at different driving currents. (g) CIE color coordinates of the blue LED, yellow LED, and WLED. (h) PL spectra, (i) CRI and CCT of the WLED after different running periods under the driving current of 5 mA, showing its good operation stability.

Another observed phenomenon is that the emission intensity of three devices exhibits a monotonic increase, without premature saturation toward UV light excitation, which also indicates the good color chromatics stability of three devices and the photostability of both phosphors. Fig. 5g plots the Commission International de I’Eclairage (CIE) color coordinates of three devices, in which the blue LED, yellow LED, and WLED are located at (0.161, 0.072), (0.423, 0.533), and (0.324, 0.330), respectively. Besides, other key device parameters for the WLED including the correlated color temperature (CCT), CRI, and LE were also measured to be 5877 K, 88.4, and 54.6 lm W−1, respectively. Note that the high LE (54.6 lm W−1) obtained is a new record for lead-free perovskite-based WLEDs. Table 1 summarizes the device performances of the studied WLEDs and other reported devices constructed by a single or multi-component strategy, and the value of luminous efficiency in our case is the highest among the lead-free perovskite-based WLEDs.

Table 1 Summary of the device performances of the prepared WLEDs
Emitter Lead-free (yes/no) CRI CCT (K) LE (lm W−1) Lifetime (h) Ref.
C3N2H12PbBr4 No 82 4669 18
(EDBE)PbBr4 No 6519 >7 19
α-(DMEN)PbBr4 No 73 7863 20
CsPbBr3/CsPbBr1.2I1.8 No 61.2 10 21
(C6H5C2H4NH3)2PbCl4 No 84 4426 15
CsPbBr3/CsPbBr1.2I1.8 No 82 5853 14.1 16
[PbF][(CH2)4(CO2)2]0.5 No 78 5620 17
Cs3Bi2Br9 Yes 33
(OCTAm)2SnBr4 Yes 89 6530 38
(Cs4N2H14Br)4SnBrxI6−x Yes 84 5632 32.2 >12 41
(C4N2H14Br)4SnBr6 Yes 70 4946 >7 42
Cs3Cu2I5/CsCu2I3 Yes 89.4 5877 54.6 >100 This work


It is generally accepted that the long-term operation stability of perovskite-based optoelectronic devices has always been criticized. To assess the suitability of the studied WLED to practical applications, the preliminary stability study of the device was therefore conducted by tracking the evolution of the emission spectra with running time, in which a continuous current of 5 mA was fixed and the emission spectra were captured intermittently in air ambient (25 °C, 55% humidity). As shown in Fig. 5h, after continuous running for 100 h, there is no change of the emission performance for the studied WLEDs, involving the spectral shape, peak position, and emission intensity, suggesting superior operation stability of the proposed WLED based on Cs3Cu2I5 and CsCu2I3 mixtures. Moreover, the CCT and CRI of the WLEDs were also continuously collected, showing no obvious changes with the running time, as summarized in Fig. 5i. In addition, we found that the CIE of the WLED remained basically unchanged over a long working period, as seen in Fig. S7 (ESI). Together with the environmentally friendly nature and facile processing method, the lead-free ternary copper halides can therefore be regarded as promising candidates as reliable down-conversion phosphors for high-performance WLED applications.

Conclusions

In conclusion, a stable and high-efficiency WLED by using blue-emissive Cs3Cu2I5 and yellow-emissive CsCu2I3 as the down-conversion phosphors was successfully demonstrated, which were synthesized by a facile supersaturated recrystallization method at RT. The broadband emission feature of Cs3Cu2I5 and CsCu2I3 allows their combined spectrum to cover the entire visible light region, and their large Stokes shift effectively avoids the undesired photon self-absorption and color instability issues benefited from the STE-related recombination mechanisms. The device performance of the studied WLED is remarkable in terms of a large CRI of 88.4 and a high LE of 54.6 lm W−1, which is the highest value among the lead-free perovskite-based WLEDs as far as we know. More importantly, the unencapsulated WLED in continuous current mode can efficiently sustain for 100 h without any emission decay, demonstrating remarkable stability. The results obtained highlight the great potential of lead-free Cs3Cu2I5 and CsCu2I3 as environmentally friendly and stable phosphors for high-performance WLEDs compatible with practical applications.

Author contributions

Z. S. and X. C. conceived the idea for detailed experiments. L. W., Z. M. and M. W. performed the material synthesis experiments. F. Z. and M. W. carried out the TEM, SEM, and XRD measurements. L. W. and M. W. performed the PL measurements. D. W., X. C., Y. T. and X. L. analyzed the data. The paper was written by L. W., and Z. S. guided the whole project.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 11774318, 12074347, 12004346, and 61935009) and the Open Fund of the State Key Laboratory of Integrated Optoelectronics (IOSKL2020KF04).

Notes and references

  1. Y. Sun, N. C. Giebink, H. Kanno, B. Ma, M. E. Thompson and S. R. Forrest, Nature, 2006, 440, 908–912 CrossRef CAS PubMed.
  2. R. Lin, Q. Guo, Q. Zhu, Y. Zhu, W. Zheng and F. Huang, Adv. Mater., 2019, 31, 1905079 CrossRef CAS PubMed.
  3. M. K. Choi, J. Yang, K. Kang, D. Kim, C. Choi, C. Park, S. Kim, S. Chae, T. Kim, J. Kim, T. Hyeon and D. H. Kim, Nat. Commun., 2015, 6, 7149 CrossRef CAS PubMed.
  4. Z. Ma, Z. Shi, D. Yang, Y. Li, F. Zhang, L. Wang, X. Chen, D. Wu, Y. Tian, Y. Zhang, L. Zhang, X. Li and C. Shan, Adv. Mater., 2021, 33, 2001367 CrossRef CAS PubMed.
  5. Z. Wang, F. Yuan, X. Li, Y. Li, H. Zhong, L. Fan and S. Yang, Adv. Mater., 2017, 29, 1702910 CrossRef PubMed.
  6. X. Zhuang, H. Zhang, K. Ye, Y. Liu and Y. Wang, ACS Appl. Mater. Interfaces, 2016, 8, 11221–11225 CrossRef CAS PubMed.
  7. T. Lougheed, Environ. Health Perspect., 2014, 122, A81 Search PubMed.
  8. R. Albert and E. Joan, Photobiology, 2012, 88, 1320–1345 CrossRef PubMed.
  9. I. Jaadane, P. Boulenguez, S. Chahory, S. Carre, M. Savoldelli, L. Jonet, F. Behar-Cohen, C. Martinsons and A. Torriglia, Free Radic. Biol. Med., 2015, 84, 373–384 CrossRef CAS PubMed.
  10. L. Wang, Z. Shi, Z. Ma, D. Yang, F. Zhang, X. Ji, M. Wang, X. Chen, G. Na, S. Chen, D. Wu, Y. Zhang, X. Li, L. Zhang and C. Shan, Nano Lett., 2020, 20, 3568–3576 CrossRef CAS PubMed.
  11. L. Protesescu, S. Yakunin, M. I. Bodnarchuk, F. Krieg, R. Caputo, C. H. Hendon, R. X. Yang, A. Walsh and M. V. Kovalenko, Nano Lett., 2015, 15, 3692–3696 CrossRef CAS PubMed.
  12. X. Li, Y. Wu, S. Zhang, B. Cai, Y. Gu, J. Song and H. Zeng, Adv. Funct. Mater., 2016, 26, 2435–2445 CrossRef CAS.
  13. Q. A. Akkerman, V. D'Innocenzo, S. Accornero, A. Scarpellini, A. Petrozza, M. Prato and L. Manna, J. Am. Chem. Soc., 2015, 137, 10276–10281 CrossRef CAS PubMed.
  14. Z. Shi, Y. Li, Y. Zhang, Y. Chen, X. Li, D. Wu, T. Xu, C. Shan and G. Du, Nano Lett., 2017, 17, 313–321 CrossRef CAS PubMed.
  15. K. Thirumal, W. Chong, W. Xie, R. Ganguly, S. K. Muduli, M. Sherburne, M. Asta, S. Mhaisalkar, T. Sum, H. S. Soo and N. Mathews, Chem. Mater., 2017, 29, 3947–3953 CrossRef CAS.
  16. X. Di, J. Jiang, Z. Hu, L. Zhou, P. Li, S. Liu, W. Xiang and X. Liang, Dyes Pigm., 2017, 146, 361–367 CrossRef CAS.
  17. Z. Zhuang, C. Peng, G. Zhang, H. Yang, J. Yin and H. Fei, Angew. Chem., Int. Ed., 2017, 56, 14411–14416 CrossRef CAS PubMed.
  18. E. R. Dohner, E. T. Hoke and H. I. Karunadasa, J. Am. Chem. Soc., 2014, 136, 1718–1721 CrossRef CAS PubMed.
  19. Z. Yuan, C. Zhou, J. Messier, Y. Tian, Y. Shu, J. Wang, Y. Xin and B. Ma, Adv. Opt. Mater., 2016, 4, 2009–2015 CrossRef CAS.
  20. L. Mao, Y. Wu, C. C. Stoumpos, M. R. Wasielewski and M. G. Kanatzidis, J. Am. Chem. Soc., 2017, 139, 5210–5215 CrossRef CAS PubMed.
  21. C. Sun, Y. Zhang, C. Ruan, C. Yin, X. Wang, Y. Wang and W. W. Yu, Adv. Mater., 2016, 28, 10088–10094 CrossRef CAS PubMed.
  22. S. Pathak, N. Sakai, F. W. R. Rivarola, S. D. Stranks, J. Liu, G. Eperon, C. Ducati, K. Wojciechowski, J. T. Griffiths, A. A. Haghighirad, A. Pellaroque, H. Friend and H. J. Snaith, Chem. Mater., 2015, 27, 8066–8075 CrossRef CAS.
  23. X. G. Zhao, J. H. Yang, Y. Fu, D. Yang, Q. Xu, L. Yu, S. H. Wei and L. Zhang, J. Am. Chem. Soc., 2017, 139, 2630–2638 CrossRef CAS PubMed.
  24. F. Zhang, Z. F. Shi, Z. Z. Ma, Y. Li, S. Li, D. Wu, T. T. Xu, X. J. Li, C. X. Shan and G. T. Du, Nanoscale, 2018, 10, 20131–20139 RSC.
  25. M. Leng, Z. Chen, Y. Yang, Z. Li, K. Zeng, K. Li, G. Niu, Y. He, Q. Zhou and J. Tang, Angew. Chem., Int. Ed., 2016, 55, 15012–15016 CrossRef CAS PubMed.
  26. L. Lei, D. Chen, C. Li, F. Huang, J. Zhang and S. Xu, J. Mater. Chem. C, 2018, 6, 5427–5433 RSC.
  27. A. Loiudice, S. Saris, E. Oveisi, D. T. L. Alexander and R. Buonsanti, Angew. Chem., Int. Ed., 2017, 56, 10696–10701 CrossRef CAS PubMed.
  28. A. Babayigit, D. Thanh, A. Ethirajan, J. Manca, M. Muller, H. G. Boyen and B. Conings, Sci. Rep., 2016, 6, 18721 CrossRef CAS PubMed.
  29. S. Shao, J. Liu, G. Portale, H. H. Fang, G. R. Blake, G. H. Brink, L. J. A. Koster and M. A. Loi, Adv. Energy Mater., 2018, 8, 1702019 CrossRef.
  30. Z. Ma, L. Wang, X. Ji, X. Chen and Z. Shi, J. Phys. Chem. Lett., 2020, 11, 5517–5530 CrossRef CAS PubMed.
  31. B. Saparov, J. P. Sun, W. Meng, Z. Xiao, H. S. Duan, O. Gunawan, D. Shin, I. G. Hill, Y. Yan and D. B. Mitzi, Chem. Mater., 2016, 28, 2315–2322 CrossRef CAS.
  32. A. Wang, Y. Guo, Z. Zhou, X. Niu, Y. Wang, F. Muhammad, H. Li, T. Zhang, J. Wang and Z. Deng, Chem. Sci., 2019, 10, 4573–4579 RSC.
  33. M. Leng, Y. Yang, K. Zeng, Z. Chen, Z. Tan, S. Li, J. Li, B. Xu, D. Li, M. P. Hautzinger, Y. Fu, T. Zhai, L. Xu, G. Niu, S. Jin and J. Tang, Adv. Funct. Mater., 2018, 28, 1704446 CrossRef.
  34. H. Shao, X. Bai, H. Cui, G. Pan, P. Jing, S. Qu, J. Zhu, Y. Zhai, B. Dong and H. Song, Nanoscale, 2018, 10, 1023–1029 RSC.
  35. Z. Ma, Z. Shi, L. Wang, F. Zhang, D. Wu, D. Yang, X. Chen, Y. Zhang, C. Shan and X. Li, Nanoscale, 2020, 12, 3637–3645 RSC.
  36. J. Zhang, Y. Yang, H. Deng, U. Farooq, X. Yang, J. Khan, J. Tang and H. Song, ACS Nano, 2017, 11, 9294–9302 CrossRef CAS PubMed.
  37. K. M. McCall, C. C. Stoumpos, S. S. Kostina, M. G. Kanatzidis and B. W. Wessels, Chem. Mater., 2017, 29, 4129–4145 CrossRef CAS.
  38. J. Sun, J. Yang, J. I. Lee, J. H. Cho and M. S. Kang, J. Phys. Chem. Lett., 2018, 9, 1573–1583 CrossRef CAS PubMed.
  39. C. Zuo and L. Ding, Angew. Chem., Int. Ed., 2017, 129, 6628–6632 CrossRef.
  40. Z. Z. Ma, Z. F. Shi, D. Yang, F. Zhang, S. Li, L. Wang, D. Wu, Y. Zhang, G. Na, L. Zhang, X. Li, Y. Zhang and C. X. Shan, ACS Energy Lett., 2020, 5, 385–394 CrossRef CAS.
  41. C. K. Zhou, Y. Tian, Z. Yuan, H. R. Lin, B. H. Chen, R. Clark, T. Dilbeck, Y. Zhou, J. Hurley, J. Neu, T. Besara, T. Siegrist, P. Djurovich and B. W. Ma, ACS Appl. Mater. Interfaces, 2017, 9, 44579–44583 CrossRef CAS PubMed.
  42. C. Zhou, H. Lin, Y. Tian, Z. Yuan, R. Clark, B. Chen, L. J. van de Burgt, J. C. Wang, Y. Zhou, K. Hanson, Q. J. Meisner, J. Neu, T. Besara, T. Siegrist, E. Lambers, P. Djurovich and B. Ma, Chem. Sci., 2018, 9, 586–593 RSC.
  43. Z. Ma, Z. Shi, C. Qin, M. Cui, D. Yang, X. Wang, L. Wang, X. Ji, X. Chen, J. Sun, D. Wu, Y. Zhang, X. Li, L. Zhang and C. Shan, ACS Nano, 2020, 14, 4475–4486 CrossRef CAS PubMed.
  44. Z. Shi, S. Li, Y. Li, H. Ji, X. Li, D. Wu, T. Xu, Y. Chen, Y. Tian, Y. Zhang, C. Shan and G. Du, ACS Nano, 2018, 12, 1462–1472 CrossRef CAS PubMed.
  45. M. A. Halcrow, Chem. Soc. Rev., 2013, 42, 1784–1795 RSC.
  46. B. Murali, S. Dey, A. L. Abdelhady, W. Peng, E. Alarousu, A. R. Kirmani, N. Cho, S. P. Sarmah, M. R. Parida, M. I. Saidaminov, A. A. Zhumekenov, J. Sun, M. S. Alias, E. Yengel, B. S. Ooi, A. Amassian and O. M. Bakr, and O. F. Mohammed, ACS Energy Lett., 2016, 1, 1119–1126 CrossRef CAS.
  47. V. K. Ravi, G. B. Markad and A. Nag, ACS Energy Lett., 2016, 1, 665–671 CrossRef CAS.
  48. Z. Xiao, K. Z. Du, W. Meng, D. B. Mitzi and Y. Yan, Angew. Chem., Int. Ed., 2017, 129, 12275–12279 CrossRef.
  49. J. Luo, X. Wang, S. Li, J. Liu, Y. Guo, G. Niu, L. Yao, Y. Fu, L. Gao, Q. Dong, C. Zhao, M. Leng, F. Ma, W. Liang, L. Wang, S. Jin, J. Han, L. Zhang, J. Etheridge, J. Wang, Y. Yan, E. H. Sargent and J. Tang, Nature, 2018, 563, 541–545 CrossRef CAS PubMed.
  50. Y. E. Kim, J. Kim, J. W. Park, K. Park and Y. Lee, Chem. Commun., 2017, 53, 2858–2861 RSC.
  51. Y. Li, Z. Shi, W. Liang, L. Wang, S. Li, F. Zhang, Z. Ma, Y. Wang, Y. Tian, D. Wu, X. Li, Y. Zhang, C. Shan and X. Fang, Mater. Horiz., 2020, 7, 530–540 RSC.
  52. J. Li, T. Inoshita, T. Ying, A. Ooishi, J. Kim and H. Hosono, Adv. Mater., 2020, 32, 2002945 CrossRef CAS PubMed.

Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/d1tc01037a

This journal is © The Royal Society of Chemistry 2021