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Abstract
Perovskite light-emitting diodes (PeLEDs) are regarded as exceptional candidates for next-generation high-definition displays. Despite the fact that the all-inorganic perovskites possess an advantage in structural stability among the perovskite family, the electroluminescence (EL) performance of their corresponding PeLEDs are still challenged by the difficulties in depositing smooth, uniform perovskite films and realizing chare balance under working voltages. Here, we report an efficient and stable CsPbBr3 based PeLED, which is enabled by a hole transport layer (HTL) of KBr doped poly(3,4,-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). The KBr dopant can not only improve the charge balance by boosting the hole injection, but also suppress the exciton quenching through passivating halide defects in perovskites. The resulting PeLED exhibits a maximum current efficiency (CE) of 35.09 cd A−1 and a maximum external quantum efficiency (EQE) of 10.02%, which are over 12-fold higher than those of the control device based on undoped HTL, respectively. Our work provides an easy and efficient strategy to boost the EL performance of all-inorganic perovskites.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Metal halide perovskites are promising emitters due to their high photoluminescence quantum yield (PLQY), tunable optical band gap, and high color purity with narrow spectral linewidth [1,2]. Impressively, all-inorganic perovskites exhibit superior structural stability compared to their hybrid counterparts [3,4] and are expected to realize highly efficient and stable light-emitting diodes (LEDs) [5–7]. In spite of these advantages, their electroluminescence (EL) performance remains challenged by the difficulties in depositing smooth, uniform perovskite film and realizing the charge balance in the perovskite LEDs (PeLEDs) [3,8–10]. Previous reports show that the boundaries between perovskite grains are responsible for the non-radiative recombination loss and the ion migration paths, while the existing pinholes can casue the leakage current and thus, the device heating during the operation [11–13]. The unbalanced charge injection would also result in the accumulation of excess charge carriers at the interface, further deteriorating the performance of PeLEDs [14–17].
Considerable efforst have been devoted to addressing the above issues from aspects of perovskite materials and device structures [18–22]. For example, diverse organic ligands were adopted to passivate the defects of perovskite for increasing the PLQY [23,24]. Polymer additives and anti-solvent treatments were employed to control the growth dynamic of perovskite crystals for achieving the high film coverage, and thus, reducing the leakage current [25–30]. Interface engineering were utilized to reduce the loss of nonradiative interfacial recombination, or to improve the charge balance by promoting the injection of holes [31–33]. However, the insulating organic ligands or additives will, to some extent, sacrifice the electrical conductivity of perovskite films, and the employement of interlayer in PeLEDs will not only complicate the fabrication processes but also require higher operating voltages. Therefore, it is highly desired to develop a facile and efficient strategy to improve the charge balance and mitigate the interfacial recombination loss simultaneously for achieving high-performance PeLEDs.
Herein, we demonstrate an efficient and stable PeLED by utilizing the hole transport layer (HTL) of KBr doped poly(3,4,-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), which improves the charge balance through boosting the hole injection, favors the formation of uniform and dense perovskite layer, and suppresses the exciton quenching by passivating the halide defects of perovskites simultaneously. As a result, a green PeLED based on all-inorganic CsPbBr3 perovskite film achieved a maximum external quantum efficiency (EQE) of 10.02%, which is 13.73-fold higher than that of the control device based on undoped HTL. In addition, the operational lifetime of the device was also about 6-fold prolonged.
2. Characterizations
The surface morphology of the films was characterized by field emission scanning electron microscopy (FESEM, JSM-6701F, 5 kV) and atomic force microscopy (AFM, SPI3800N 3AFM instrument). X-ray diffraction (XRD) patterns were obtained using a Bruker D8 advance diffractometer with Cu Kα radiation range from 10° to 40° at a scanning rate of 6° min−1. Absorption spectra were measured using a PerkinElmer Lambda 950 UV-vis-NIR spectrometer. The steady-state photoluminescence (PL) spectra and time-resolved photoluminescence (TRPL) spectra were obtained by using an Edinburgh FLS920 PL spectrometer. The current density-luminance-voltage (J-L-V) characteristics were collected by using a Keithley 2400 source. The EL spectra of the PeLEDs were measured by using a PR-670 Spectra Scan Spectroradiometer. The EQE values were calculated according to the EL emissions of these devices. Ultraviolet photoelectron spectra (UPS) were characterized by a Thermo Scientific Escalab 250Xi, which utilizes a He (I) ultraviolet radiation source (21.22 eV).
3. Experimental details
Materials: Cesium bromide (CsBr, 99.999%) and cesium trifluoroacetate (CsTFA, 99.0%) were purchased from Alfa. Lead bromide (PbBr2, 99.999%) was purchased from Sigma-Aldrich. Potassium bromide (KBr, 99.5%) was purchased from Aladdin Industrial Corp. PEDOT: PSS (Clevios Al 4083) was purchased from Corp. Polyethylene oxide (PEO), 1,3,5-Tris (1-phenyl-2-benzimidazolyl) benzene (TPBI, 99.0%) and lithium fluoride (LiF, 99.0%) were purchased from Lumtec. All these chemicals were directly used without further purification.
Preparation of perovskite films: CsPbBr3 precursor solution was prepared by dissolving 42.5 mg, 73.3 mg, 34.4 mg and 5 mg of CsBr, PbBr2, CsTFA and PEO into 1.5 mL DMSO, which is stirring at 60 °C for 2 hours until all materials are fully dissolved. The solution is then filtered through polytetrafluoroethylene filters with a pore size of 0.45 µm to obtain the final precursor solution. Then, perovskite films were formed on PEDOT: PSS or KBr doped-PEDOT: PSS coated ITO substrates by spin-coating the precursor solutions at 3000 rpm for 60 s and annealing at 70 °C for 7 min [34].
Fabrication of devices: ITO substrates were cleaned by sonication in a detergent water solution, deionized water, acetone and isopropyl alcohol for about 15 min, respectively. The dried substrates were then transferred into oxygen plasma and treated for 15 min. PEDOT: PSS was spin-coated onto the pre-cleaned and oxygen plasma-treated ITO substrates at a speed of 4000 rpm/min for 40 s and then baked at 150 °C for 10 min in air. The PEDOT: PSS: KBr solution was prepared by adding 6 mg KBr powder into 1 mL PEDOT: PSS solution. After the deposition of the perovskite films, the samples were transferred into a vacuum chamber. The 40 nm TPBI, 1 nm LiF and 100 nm Al were sequentially deposited by thermal evaporation in a vacuum thermal evaporation clamber.
4. Results and discussion
The effect of doping KBr in PeDOT:PSS on perovskite film quality is systematically investigated. As shown in Fig. 1, the morphologies of the spin-cast CsPbBr3 films on PEDOT:PSS and PEDOT:PSS:KBr, denoted as PEDOT:PSS/CsPbBr3 film and PEDOT:PSS:KBr/CsPbBr3 film, respectively, have been compared by FE-SEM and AFM. The PEDOT:PSS/CsPbBr3 film presents a rather poor surface coverage with plenty of pinholes and large grains, corresponding to a root mean square (r.m.s) roughness of 5.02 nm (Fig. 1(a), 1(b)), which is also in good agreement with previous reports [34]. In contrast, the PEDOT:PSS:KBr/CsPbBr3 film exhibits a dense and uniform surface coverage, with a reduced roughness of 2.12 nm (Fig. 1(c), 1(d)). Besides, the perovskite grain size for PEDOT:PSS:KBr/CsPbBr3 film is greatly decreased, with an average size of 60 nm and without large grains (100 nm∼140 nm) (Fig. S1, supporting information). This is consistent with XRD data, as shown in Fig. S2 (supporting information), where the broader full-width at half-maximum (FWHM) of the diffraction peaks of PEDOT:PSS:KBr/CsPbBr3 film further confirms the decrease of the perovskite grain size. The size reduction can be ascribed to that the nucleation density is much higher for the CsPbBr3 film deposited on PEDOT:PSS:KBr, resulting in the formation of a large amount of relatively small crystals and a better film coverage. In addition, the contact angle for perovskite precursor solution on PEDOT: PSS: KBr is significantly decreased from 18.8° to 8.6° compared to that on PEDOT: PSS (Fig. S3, supporting information), which reveals the increased wetting capability for the surface of PEDOT: PSS: KBr and is another reason for the enhanced prorovskite film quality.
Fig. 1. Morphologies of PEDOT:PSS/CsPbBr3 and PEDOT:PSS:KBr/CsPbBr3 films. Top-view SEM images of (a) PEDOT:PSS/CsPbBr3 film and (b) the corresponding 2D and 3D AFM images. Top-view SEM images of (c) PEDOT:PSS:KBr/CsPbBr3 film and (d) the corresponding 2D and 3D AFM images.
Figure 2(a) shows the steady-state PL spectra of PEDOT:PSS/CsPbBr3 and PEDOT:PSS:KBr/CsPbBr3 films. The PL peaks are slightly blue shifted from 518 nm for PEDOT:PSS/CsPbBr3 to 517 nm for PEDOT:PSS:KBr/CsPbBr3 films, and the PL intensity is remarkably enhanced for PEDOT:PSS:KBr/CsPbBr3 film, with an increase of PLQY from 36% to 76%, which can also be observed intuitively in the photographs (inset of Fig. 2(a)). Moreover, the FWHM values of the PL spectra are reduced to 15 nm from 19 nm for PEDOT:PSS/CsPbBr3 film. Time-resolved PL (TRPL) measurements are used to further investigate the interaction between KBr and the surface of perovskite (Fig. 2(b)). The PL decay curves were fitted with two-exponential decay function [35,36] and summarized in Table S1 (supporting information). The calculated average lifetimes (τavg) are 3.32 ns for PEDOT:PSS/CsPbBr3 film and 17.57 ns for PEDOT:PSS:KBr/CsPbBr3 film, respectively, which is consistent with the PLQY results. Such dramatic improvments of the PLQY and the PL lifetime indicate that the KBr takes effects in passivating the defects in upper perovskite interface, resulting in an enhanced radiative emission efficiency of the perovskite film [37–39]. Specifically, the K+ cations from KBr can passivate the halide defects, and meanwhile the introduced Br− annions play the passivation role through filling up the halide vacancies in perovskite [40,41]. The elimination of defect states in perovskite crystals substantially inhibits the non-radiactive recombination of excitons, and consequencely, improves their radiative recombination efficiency. This can be expected to facilitate the electroluminescence of the perovskite film under bias voltage according to the previous reports [42,43].
Fig. 2. (a) Steady-state PL spectra and photographs of PEDOT:PSS/CsPbBr3 film (left) and PEDOT:PSS:KBr/CsPbBr3 film (right). (b) Time-resolved PL decay curves of PEDOT:PSS/CsPbBr3 and PEDOT:PSS:KBr/CsPbBr3 films.
In order to evaluate the defect densities in the perovskite films, the space-charge-limited-current (SCLC) measurements were performed on the hole-only devices (HODs) with structures of ITO/PEDOT:PSS/CsPbBr3/MoO3/Al and ITO/PEDOT:PSS:KBr/CsPbBr3/MoO3/Al. The junction point between the Ohmic-type response region (at low bias voltages) and the trap-filling region (at high bias voltages) is defined as the trap-filling limit voltage (VTFL), and its relationship with the trap density (Nt) in hole-only devices can be expressed by the equation
which is proportional to the density of defect states in hole-only device [44,45]. ε of CsPbBr3 stands for the relative dielectric constant (∼22) [4]. As shown in Fig. 3(a), the VTFL for the HOD with KBr is reduced to 0.81 V from 0.93 V for the control HOD without KBr, corresponding to the decrease of the trap density from 5.66 × 1017 to 4.93× 1017 cm−3, revealing the reduced defect densty in the perovskite film deposited on the HTL of KBr doped PEDOT:PSS. Combining with the prolonged PL lifetime (Fig. 2(b)), it is now reasonable to conclude that the defect-induced non-radiative recombination losses for the CsPbBr3 film on PEDOT:PSS:KBr are effectively suppressed. Furthermore, UPS spectra are utilized to analyse the effect of KBr on the work function (WF) of PEDOT: PSS (Fig. 3(b)). The Ecutoff for PEDOT: PSS and PEDOT: PSS: KBr are measured to be 16.17 eV and 16.07 eV, respectively. According to the formula WF = 21.22-Ecutoff [46], the WF values are calculated to be 5.05 eV for PEDOT: PSS and 5.15 eV for PEDOT:PSS: KBr, respectively. The downshift of the WF reduces the energy barrier of holes injected into the perovskites (inset of Fig. 3(b)), which would favor the hole injection and thus better balance charge in PeLEDs, resulting in an improvement in the EL performance of the PeLEDs.Fig. 3. (a) Dark current density-voltage (J-V) curves of hole-only devices with structures of ITO/PEDOT:PSS/CsPbBr3/MoO3/Al (bottom) and ITO/PEDOT:PSS:KBr/CsPbBr3/MoO3/Al (top). (b) UPS spectra of PEDOT: PSS and PEDOT: PSS:KBr films. Inset is the illustration of the corresponding WFs.
Green PeLEDs with structures of ITO/PEDOT:PSS/CsPbBr3/TPBI/LiF/Al (control device) and ITO/PEDOT:PSS:KBr/CsPbBr3/TPBI/LiF/Al (KBr-assisted device) were fabricated and compared (Fig. 4(a)) to further reveal the effect of KBr on the EL performance of perovskites. The corresponding energy level diagram of the devices is shown in Fig. S4 (supporting information). Figure 4(b) presents the EL spectra of the control and KBr-assisted devices, which were both collected under the bias of 5 V. The EL peaks are centered at 519 nm for the control device and 518 nm for the KBr-assisted device, respectively, which are both redshifted by ∼1 nm compared to those of PL spectra (Fig. 2(a)), due to the electric-field-induced Stark effect [34]. Under the same bias voltage, the significant enhanced EL intensity demonstrates an improved radiative recombination efficiency of the exictons in the KBr-assisted device [47]. The inset photograph Fig. 4(b) is an operating KBr-assisted PeLED, showing bright green light emission.
Fig. 4. Device structure and performance characteristics of the control and KBr-assisted PeLEDs. (a) Schematic illustration of the device architecture of ITO/PEDOT:PSS/CsPbBr3/TPBI/LiF/Al. (b) EL spectra of the devices, which were both collected under the bias of 5 V. Inset is a photograph of the KBr-assisted PeLED showing green emission. (c) Current density-voltage-luminance (J-L-V) curves, (d) current efficiency-current density (CE-J) curves, (e) EQE-current density (EQE-J) curves, and (f) operational lifetimes of the control and KBr-assisted PeLEDs.
According to the current density-voltage-luminance (J-V-L) curves in Fig. 4(c), a higher current density was observed for the control device, resulting from the poorer coverage of the perovskite film (Fig. 1), leading to the electrical shunting paths. The turn-on voltage (Von) of 2.6 V for the KBr-assisted device is slightly smaller than that of control device, and the maximum luminance (32,593 cd m−2) of the KBr-assisted PeLED is 4.3-fold higher than that of the control one (7,574 cd m−2). Figure 4(d)-(e) displays the maximum CE (35.09 cd A−1) and EQE (10.02%) of the KBr-assisted PeLED are 12.62-fold and 13.73-fold higher than those of the control PeLED (maximum CE of 2.78 cd A−1 & maximum EQE 0.73%), respectively. Such a prominent performance enhancement can be ascribed to the improved radiative recombination and charge balance in the KBr-assisted device. All the performance parameters are summarized in Table 1. Besides, we also further compared the performance improvement effects of LiBr and NaBr on perovskite devices. As shown in Fig. S5 (supporting information). The detailed parameters of related device performance are summarized in Table S2 (supporting information).
Table 1. Performance summary for the control and KBr-assisted PeLEDs.
Moreover, the stability of these two types of devices are also compared, as displayed in Fig. 4(f). The operational lifetimes were tested in glovebox under the nitrogen atmosphere and without encapsulation. The control PeLED shows a very rapid deterioration from an initial luminance of 100 cd m−2, with a half-lifetime of only 6 min. In contrast, the KBr-assisted PeLED exhibits a greatly improved half-lifetime (T50) of up to 35 min. The dramatical improvement in the operational stability can be attributed to the realization of smooth and dense CsPbBr3 perovskite films, which reduces Joule heating induced by poor surface coverage of the inorganic perovskite emissive layer.
5. Conclusions
In summary, we have demonstrated the highly efficient EL emissions of all-inorganic perovskite films by utilizting the KBr doped PEDOT:PSS as the HTL in the PeLED devices. The multifunctinonal KBr not only deepens the WF of PEDOT:PSS to boost the hole injection, favoring the improvement of charge balance, but also passivates the halide defect states in perovskite films, enhacing the exciton radiative recombination. Moreover, the leakage current and Joule heating of the KBr-assisted PeLED were effectively suppressed, due to the K+ cations enhanced morphology of perovskite film by guiding the growth of smaller perovskite grains. The resulting KBr-assisted PeLED exhibited a significant improvment in EL performance, with a 13.73-fold efficiency enhancement and approximatively 6-fold lifetime extension compared with those of the control PeLED. Our results suggest the KBr doped PEDOT:PSS can serve as a reliable and efficient HTL for PeLEDs and other optoelectronic devices.
Funding
National Natural Science Foundation of China (51675322, 62174104); Science and Technology Commission of Shanghai Municipality (19010500600).
Acknowledgments
The authors would like to thank the financial support from the National Natural Science Foundation of China (Nos. 62174104 and 51675322) and the Shanghai Science and Technology Committee (No. 19010500600).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are available from the corresponding author upon a reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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