- Advanced Photonics
- Vol. 6, Issue 2, 026002 (2024)
Abstract
1 Introduction
Metal halide perovskite light-emitting diodes (PeLEDs) have emerged as prospective candidates for high-resolution display and communication applications because of their tunable bandgaps, high photoluminescence quantum yields (PLQYs), and good color purity.1
Mixed-halide chloride/bromide (Cl/Br) quasi-2D perovskites afford an efficient approach for pure-blue PeLEDs through a simple solution preparation process.13
Here, we propose a multifunctional zwitterionic molecule of 3-(benzyldimethylammonio) propanesulfonate (3-BAS) to promote the photoelectric properties and phase stability of the mixed-halide quasi-2D pure-blue PeLEDs. Theoretical and experimental investigations reveal that the sulfonic group () of 3-BAS can interact with the uncoordinated through Lewis base-acid reaction for passivating the surface defects of the quasi-2D perovskites. In addition, the electropositively quaternary ammonium group () of 3-BAS can interact with the electronegatively halide ions of the perovskites via the formation of electrostatic interactions, thus suppressing the migration of halides. More importantly, the crystallization kinetics of quasi-2D perovskites can be regulated by a 3-BAS molecule, leading to a decreased ratio of small- phase with smoothing energy transfer pathway in mixed quasi-2D perovskites. Based on the optimized perovskites emitter, pure-blue PeLEDs emitting at 466 nm are achieved with a record luminance of , EQE of nearly 10%, and a negligible EL shift under different electrical bias, identifying one of the best performing candidates among pure-blue quasi-2D PeLEDs reported in the literature so far. What is more, the advantageous features of 3-BAS-treated pure-blue PeLEDs, including short response time, high brightness, and stable EL spectra, perfectly match the strict requirements of the light sources for next-generation ultrahigh-speed visible light communication (VLC).
Sign up for Advanced Photonics TOC. Get the latest issue of Advanced Photonics delivered right to you!Sign up now
2 Results and Discussion
To give a first glance of the intermolecular interaction between perovskites and 3-BAS, we adopt density functional theory (DFT) calculations to analyze the relationship among theoretically associated variables. The chemical structure and simulated electrostatic potential (ESP) of the 3-BAS molecule are shown in Fig. 1(a). It is clear that the negative charge (lone electron pairs) of 3-BAS is mainly delocalized on oxygen atoms of the group (light red color). As the electron-rich group tends to coordinate with uncoordinated ions in perovskites through the Lewis base-acid reaction,25,26 the group of 3-BAS can perfectly coordinate with the uncoordinated cations through the formation of Lewis adducts, which provides an efficient way to passivate the trap defects at surface of the perovskites. In contrast, the electron-deficient group of shows positive charge (light blue color), which means that the electrostatic interactions can be easily formed between and negatively uncoordinated halide ions of the perovskites.27
Figure 1.(a) Chemical structure and simulated ESP of 3-BAS molecule. (b) Vacancies formation energies of halide for the perovskites before and after 3-BAS passivation. Calculated PDOS of the perovskites (c) w/o and (d) w/ 3-BAS molecule passivation based on a simplified model. (e) Differential charge density plot of the 3-BAS/perovskite system (isosurface value of
To further understand the defect passivation by incorporating 3-BAS, we conducted projected density of states (PDOS) calculations of the structural configuration of perovskites w/ and w/o 3-BAS. The perovskites with undercoordinated feature conspicuous trap states [Fig. 1(c)], which can capture excited carriers and result in severe nonradiative recombinations with decreased PLQYs.30,31 In contrast, the trap states of the perovskites reduce distinctly after being passivated by 3-BAS [Fig. 1(d)], suggesting that 3-BAS possesses a good capability to improve the optoelectronic performance of perovskites. Figure 1(e) shows the differential charge density plot of the system. It can be found that the group is close to the uncoordinated cation through forming a Pb-O coordinate bond, and the interface charge redistributions on the group and group confirm the formation of Lewis base-acid reaction and electrostatic interactions between 3-BAS and perovskites.32 In addition, the delocalized -electrons on the benzene ring of 3-BAS are beneficial to the charge transfer between individual perovskite crystals, which is imperative for enhancing the performance of photoelectric devices.33
The Cl-Br mixed quasi-2D perovskites thin films were deposited onto indium tin oxide (ITO)/poly(9-vinylcarbazole) (PVK) substrates using a one-step spin-coating method; the details about the film fabrication and precursor recipes are presented in the Supplementary Material. The perovskites with the pure-blue emission at 466 nm can be obtained when the ratio of bromide and chloride is 2:1, as shown in Fig. S2 in the Supplementary Material. To further confirm the chemical interactions between 3-BAS and perovskites, we performed Fourier transform infrared (FT-IR) measurement to analyze the S═O stretching vibration (S═O) of the group and C-N stretching vibration (C-N) of the group for neat 3-BAS and composites. It can be seen that the (S═O) of neat BAS at shifts to a lower wavenumber () for the composites [Fig. 2(a)], indicating the coordination of the group and undercoordinated cations. Meanwhile, the peak at derived from the ν (C-N) for the positively charged group of neat 3-BAS shifts to a lower wavenumber of in the presence of [Fig. 2(b)], suggesting the formation of noncovalent electrostatic interactions between the positively charged group of 3-BAS and the negatively charged halides. Furthermore, X-ray photoelectron spectroscopy (XPS) measurements were carried out to verify the synergistic effect of both the Lewis base-acid reaction and electrostatic interactions formation. In Fig. 2(c), the peaks at 166.5 and 168.4 eV in the targeted perovskite film can be assigned to S and S , respectively, indicating the presence of 3-BAS in targeted perovskites compared to the control film. Figure 2(d) shows the XPS spectra of , where both (136.9 eV) and (141.8 eV) peaks shift to lower binding energy with the incorporation of 3-BAS. Such negative shifts of Pb binding energy can be ascribed to the increased electron cloud density around cations, as explained by the fact that the donates its lone electron pair to the empty orbital of by forming Lewis adducts.34 Meanwhile, negative shifts are also observed in both Br and spectra [Figs. 2(e) and 2(f)], suggesting that Lewis base groups donate electrons to (which reduces their positive charge); thereby the interaction between and halogen ions is altered,20 which is in accordance with the DFT calculation results. Based on the above analysis, we can conclude that the 3-BAS molecule can effectively interact with the quasi-2D perovskites via synergetic effects of both electrostatic interaction formation and the Lewis base-acid reaction, which will remarkably enhance phase stability and minimize nonradiative recombination loss in the quasi-2D perovskites.
Figure 2.(a), (b) FT-IR spectra of S=O and C-N vibration modes of 3-BAS and 3-BAS-incorporated
To modulate the photoelectric properties of the pure-blue perovskite film, the 3-BAS molecule was incorporated with an optimized concentration of 10% (volume fraction). The 3-BAS-processed perovskite film shows the highest PL intensity, with PLQY increased by 100% when compared to the control sample [Fig. 3(a) and Fig. S3 in the Supplementary Material]. The average PL lifetime () of the control perovskite film is 13.61 ns, while the targeted perovskite film exhibits a much longer of 27.88 ns (Fig. S4 and Table S1 in the Supplementary Material), revealing the reduced trap-related nonradiative recombinations in the optimized sample.35 We further fabricated the hole-only device with the structure of to quantify the trap density with the assistance of a space charge-limited current technique (Fig. S5 in the Supplementary Material), which can be achieved by the following equation:21
Figure 3.(a) PL and (b) UV-vis absorption spectra of the pure-blue quasi-2D control and targeted perovskite films. (c), (d) TA spectra of the quasi-2D control and targeted perovskite films with evolution time from 0.21 to 5.16 ps. (e), (f) TA kinetics probed at selected wavelengths for the pure-blue quasi-2D control and targeted perovskite films.
To investigate the ion migration in perovskite, we conducted forward and reverse current–voltage scanning for both control and targeted devices. As shown in Figs. S9(a) and S9(b) in the Supplementary Material, significant hysteresis was observed within the first three scanning cycles for the control device, whereas the targeted device remained stable even after 30 cycles. Therefore, ion migration is effectively suppressed in the 3-BAS treated PeLED as current–voltage hysteresis is known to be associated with ion transport in perovskite devices.25,37 Furthermore, we conducted microscopic PL imaging to support our findings. Specifically, we applied a constant bias (4.0 V) across the perovskite film between two in-plane Ag electrodes [Figs. S9(c) and S9(d) in the Supplementary Material]. For the control film, a noticeable decrease in PL intensity was observed at dark spots, which then gradually extended across the entire perovskite film. In contrast, no significant decrease in PL intensity was observed in the targeted device. All these results collectively indicate that 3-BAS can effectively suppress ion migration in perovskite materials.37
For investigating the phase distribution of the perovskite films before and after incorporation of 3-BAS molecule, we then conducted ultraviolet-visible spectroscopy (UV-vis) measurement. As shown in Fig. 3(b), the four UV-vis shoulder peaks at 383, 410, 435, and 460 nm can be assigned to , 2, 3, and 4 phases of the Cl-Br mixed quasi-2D perovskites, respectively. Compared with the control perovskite film, the absorbance intensities at and 435 nm related to and phases reduce significantly for the targeted perovskite film, suggesting that the perovskite phases are redistributed after incorporation of 3-BAS. We speculate the strong interaction of 3-BAS with and halide ions retards the crystallization kinetics of perovskite films, inhibiting the formation of phase at the early stage of film growth. As a result, more phase octahedrons with the spacer cation of are easily formed because of the low formation energy. At the same time, the phase was inhibited because of the consumption of a large number of in the formation of phase.20 Grazing-incidence wide-angle X-ray scattering patterns were further performed to investigate the effect of the 3-BAS on the perovskites’ crystallization and orientation. As shown in Fig. S10 in the Supplementary Material, the control film exhibits a uniform (110) diffraction ring in the entire plane, originating from the large phase (). In contrast, too short (110) Bragg scattering arcs appear after incorporation of 3-BAS, suggesting the well-oriented distribution of perovskite layers () for the targeted perovskite film.38
Based on the previous analysis, reducing small- phase (especially for ) is vital for promoting energy transfer between quasi-2D perovskite layers toward high-performance PeLEDs.39,41 We then performed ultrafast transient absorption (TA) spectroscopy to investigate the energy transfer dynamics for the control and targeted perovskite films. The pseudo-color TA spectrograms of the samples are presented in Fig. S14 in the Supplementary Material, in which the blue negative parts represent ground-state bleach with the peaks at 383, 410, 435, and 460 nm, corresponding well to , , , and phases, respectively. The photogenerated carriers are primarily formed in small- phases and then transfer to large- phases when prolonging the decay time, suggesting that there is an efficient energy transfer process from small- phases to high- phases. Figures 3(c) and 3(d) show that the bleaching intensity of the phase in the targeted perovskites is lower than that of the pristine sample, indicating that the small- phase of the deep-blue quasi-2D perovskites is suppressed due to the optimized phase distribution after incorporation of the 3-BAS molecule. To further study the energy transfer process, the time traces of , , and phases are extracted respectively, as shown in Figs. 3(e) and 3(f) and Table S2 in the Supplementary Material. According to the fitting parameters, the decay time of , , and phases for the control perovskites is 15.17, 11.03, and 29.22 ps, respectively, which is longer than 8.51, 10.37, and 13.92 ps for the targeted perovskites, suggesting that carrier transformation process from small- phase to large- phase becomes highly efficient and fast in the targeted perovskite film with optimized phase distribution, further improving the PLQY by nearly 100%. As discussed above, both passivating undercoordinated related defects with suppressed halide ion migration and reducing small- phase have jointly contributed to improving the luminescent performance of the targeted perovskite.
Based on the optimized pure-blue quasi-2D perovskite film with improved stability and PLQY, we constructed the PeLEDs with a structure of ITO/PVK (27 nm)/PVP (4 nm)/perovskite (22 nm)/TPBi (40 nm)/LiF (1 nm)/Al (130 nm) [Fig. 4(a)]. The thickness of each layer is further confirmed by the cross-sectional scanning electron microscopy (SEM) image of the device (Fig. S15 in the Supplementary Material). The energy bandgap values of the control and targeted perovskites are calculated by assisting UV photoelectron spectroscopy and optical bandgap estimation. The valence band is estimated to be and for the targeted and control perovskites, respectively (Fig. S16 in the Supplementary Material). The better energy level matching the hole/electron transport layers and the targeted perovskite emitter means that there is a balanced charge injection in PeLED [Fig. 4(b)], and thus the higher device performance can be achieved. The EL spectrum of the targeted PeLEDs exhibits a pure-blue emission at 466 nm with Commission Internationale de l’Eclairage (CIE) coordinates of (0.136, 0.058), which is very near to the Rec. 2020 blue primary color [Fig. 4(c)].42 The EQE and luminance cartograms of the control and targeted perovskites devices are presented in Fig. 4(d). The average EQE and luminance of targeted PeLEDs are 8.63% and , with a relative standard deviation of 7.2% and 6.5%, respectively, which are much better than the 3.58% and of the control perovskites PeLEDs, indicating the good effectiveness and robustness of 3-BAS additive passivation.
Figure 4.(a) Device structure. (b) Energy level alignment. (c) Corresponding CIE coordinates of the targeted pure-blue PeLED. The inset shows the photograph of a bright working targeted device at 5 V. (d) Statistical EQEs and luminance of the control and targeted PeLEDs. (e) J-L-V and (f) J-EQE curves. EL stability of the (g) control and (h) targeted PeLEDs with increasing voltage. (i) Operational stability of the control and targeted PeLEDs.
Current density–luminance–voltage (J-L-V) and current density-EQE (J-EQE) curves of the champion PeLED are given in Figs. 4(e) and 4(f). It can be seen that the targeted PeLED exhibits a significantly higher current density than that of the control device in the whole voltage operation range [Fig. 4(e)], resulting from the reasonable energy bandgap level in the device and easy charge transfer between individual less-defective perovskite crystals neighboring with delocalized -electrons on the benzene ring of 3-BAS. Compared with the low peak EQE of 4.1% and the low maximum luminance of for the control PeLED, the champion device based on the targeted perovskites exhibits a high peak EQE of 9.25% and a very high maximum luminance of . The influence of different perovskite film thicknesses on device performance was also investigated (Fig. S17 in the Supplementary Material). The results showed that the best device performance was achieved under the perovskite layer thickness of about 20 nm. To the best of our knowledge, these performance parameters situate at the forefront of the literature reporting related experiment results (Table S3 in the Supplementary Material), and especially the maximum luminance is higher than the previous record for pure-blue quasi-2D PeLEDs so far, due to effective defect passivation and efficient energy transfer process by dimensional control. The EL spectrum stability is also evaluated at different bias voltages from 3.5 to 6.5 V. It can be seen that there is a large EL peak redshift with huge color-coordinated variation for the control PeLED [Fig. 4(g)], while the EL spectra of the targeted PeLED remain unchanged at around 466 nm with increasing voltage [Fig. 4(h)]. In addition, we analyzed the color purity trend of PeLEDs under various biases (Fig. S18 in the Supplementary Material). The color purity stability of the targeted device is notably superior to that of the control device, which perfectly matches the color purity requirement for display applications. Furthermore, due to the suppressed halide migration and passivated trap defects in the targeted perovskite film, this device shows a fourfold operational lifetime longer than the control device under a constant current density of [Fig. 4(i)], further demonstrating the potential of this innovative method for achieving efficient and stable quasi-2D pure-blue PeLEDs.
The successful realization of stable high-brightness pure-blue PeLEDs can meet the requirements for practical applications well,43
Figure 5.(a), (b) Schematic and actual experimental setup of the VLC system with the targeted pure-blue PeLED as the light source. (c) Transient EL of the targeted pure-blue PeLED at 3.5 V and 20 kHz with a fixed 20% duty cycle. (d) Square-wave response curves of the targeted pure-blue PeLED and commercial LED at 100 kHz. Comparison of (e) input and (f) output signals in the PeLED-assisted VLC audio transmission system. (
3 Conclusion
In summary, an effective and reliable approach of crystallization regulation and defect passivation with multifunctional zwitterionic 3-BAS has been demonstrated for achieving highly efficient and stable pure-blue PeLEDs. The trap-mediated nonradiative recombination losses related to undercoordinated and halide ion migration are greatly suppressed in the pure-blue mixed-halide quasi-2D perovskites, leading to highly improved luminescence efficiency and operational stability with a peak EQE of 9.25% and a spectrally stable emission at 466 nm. Meanwhile, a recorded maximum luminance value of is obtained in the 3-BAS-treated PeLED, which is successfully used as a quick-response low-loss light source for realizing the audio transmission VLC system. This work highlights the crucial role for enhancing the performance of the pure-blue mixed halide quasi-2D PeLEDs and provides an avenue for the study of future low-loss optical communication applications.
Chao Shen received his master’s degree in electronic information from Jinan University, Guangzhou, China, in 2023. He is currently a PhD candidate in the Department of Optoelectronic Engineering, Jinan University, Guangzhou, China. His research focuses mainly on the metal halide perovskite light-emitting diodes and optical fiber sensing.
Jibin Zhang received his PhD from Huazhong University of Science and Technology in 2020. From 2020 to 2022, he was engaged in postdoctoral research in Jinan University and Linköping University. Then, he was appointed a researcher at Zhengzhou University. His main research interests include metal halide perovskite, perovskite nanocrystals, and their application in photoelectric devices.
Yunhan Luo received his PhD from Tianjin University, Tianjin, China, in 2006. He is currently a professor at Jinan University, Guangzhou, China. His research interests include biomedical photonics, optofluidics, micro/nanophotonics, and optical fiber sensing.
Lintao Hou received his PhD in 2006 from South China University of Technology. He then joined Jinan University as a teacher. From 2009 to 2011, he worked in Linköping University as a postdoc. In 2014, he became a full professor at Jinan University. His research focuses on printed optoelectronic devices and optical simulation.
Biographies of the other authors are not available.
Set citation alerts for the article
Please enter your email address