Perovskite Light-Emitting Diodes

Abstract

Open AccessCCS ChemistryMINI REVIEW1 Aug 2020Perovskite Light-Emitting Diodes Chenyang Zhao, Dezhong Zhang and Chuanjiang Qin Chenyang Zhao State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022 Google Scholar More articles by this author , Dezhong Zhang State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022 Google Scholar More articles by this author and Chuanjiang Qin *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.020.202000216 SectionsAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Perovskites show exciting potential for photoelectric applications, especially for light-emitting diodes (LEDs), owing to their intrinsically high photoluminescence efficiency and color purity. With efforts made over the last 5 years, the external quantum efficiency (EQE) of lead-halide perovskite-based LEDs has sharply increased beyond 20%, which is comparable to the performance of existing lighting technology. Strategies, including defect passivation, the formation of low-dimensional quantum-well structure perovskites, and a combination of appropriate electron and hole transport materials in electroluminescent devices, not only effectively increases photoluminescence quantum efficiency but also enhances light outcoupling efficiency and carrier transport balance, thus promoting higher efficiency of perovskite LEDs. In this mini review, we will focus on how the EQE can be further increased through diverse perovskite optimization and reasonable device engineering. Furthermore, remaining issues, such as efficiency roll-off at high current density, poor stability, and modest blue perovskite LEDs, are briefly discussed. Perovskite LEDs with high efficiency and long-term stability will accelerate their practical applicability in everyday life. Download figure Download PowerPoint Introduction In recent years, lead-halide perovskite materials with PbX6– octahedral skeletons surrounded by monovalent cations have aroused substantial research interest in versatile photoelectronic applications, including solar cells,1–4 light-emitting diodes (LEDs),5–7 photodetectors, and lasers.8–11 According to different monovalent cations (inorganic or organic), these materials can be broadly divided into all-inorganic and organic–inorganic hybrid perovskite materials. Based on their high absorption coefficient and long charge-carrier diffusion length, perovskite materials are widely utilized as active layers to attain sufficient light absorption and charge separation in solar cells. The highest certified power conversion efficiency of single-junction perovskite solar cells has attained a significant breakthrough of 25.2%, which is comparable to the performance of single-crystal silicon solar cells. Moreover, the competitive advantages of perovskite materials endow them with vast potential for display and lighting applications.12–15 For instance, owing to their high carrier mobility and transparency to visible light, extraordinarily thick (micrometer) 3D perovskite materials, as opposed to common organic transport materials, have been investigated as transport layers in organic LEDs.12 The corresponding devices show inspiring performance without increasing the operational voltage and reducing the electroluminescence (EL) efficiency. Moreover, compared with common organic hosts with wide bandgaps, 3D perovskites with appropriate bandgaps have also been used as hosts in organic LEDs to reduce the carrier injection barrier from transport layers to emitting layers (EMLs).15 Although these findings indicate that perovskite materials show good capability as transport and host materials for high-quality organic LEDs, perovskite materials are more typically employed as emitters because of their advantages in facile color adjustability, high color purity with narrow full widths at half maximum, high photoluminescence quantum efficiency (PLQE), and easy solution processing.16 While 3D perovskite materials show good carrier mobility, their high defect density and small exciton binding energy result in large nonradiative energy loss and restrict both the efficiency and stability of perovskite LEDs. To overcome these issues, in addition to reasonable defect passivation engineering to lower nonradiative energy loss, many strategies, such as constructing perovskite nanocrystals, low-dimensional perovskites, and perovskite quantum-well structures, have been proposed to increase the exciton binding energy and facilitate light radiated by perovskites with relatively low energy.17–20 Along with perovskite optimization, device engineering is an important method for improving the efficiency and stability of perovskite LEDs. The device structure of common p–i–n (holes and electrons are injected from anode and cathode) perovskite LEDs is shown in Figure 1a. As demonstrated in Figure 1b, electrons and holes are injected from the cathode and anode transport via electron and hole transport layers (HTLs), and then they meet in perovskite EMLs for radiative recombination. An appropriate device structure not only facilitates carriers in injecting, transporting, and recombining in perovskite EMLs but also balances the carriers and improves exciton recombination efficiency. Since the crystallization of perovskite films can be strongly influenced by the nature of the bottom substrate, the PLQE can also be improved by choosing the adjacent transport/interface layers.21–24 Over the last 5 years, except for blue perovskite LEDs, which have the best recorded external quantum efficiency (EQE) of 9.5%,7 the best recorded EQEs for green, red, and near-infrared devices are 20.7%, 21.3%, and 21.6%, respectively.25–27 Figure 1 | (a) Common device structure of p–i–n perovskite LEDs. (b) Carrier injection and transport in p–i–n perovskite LEDs. HTL, hole transport layer; EML, emitting layer; ETL, electron transport layer. Download figure Download PowerPoint Generally, the EQE of EL devices refers to the total number of photons emitted from the device to the number of injected electron–hole pairs. According to organic LEDs, the EQE can be ascribed to E Q E = γη r Φ PL η out γ, where is the balance ratio of electrons and holes injected from the electrode, η r is the ratio of the radiative exciton to the total generated exciton in the device, Φ PL is the PLQE of perovskite emitters, and η out is the light outcoupling ratio from the glass. In this mini review, we primarily present points related to η r to reveal why perovskite materials can reach such high EQE values. Then, we will focus on methods for obtaining high EQEs by increasing major parameters ( γ, Φ PL , and η out ) via perovskite optimization and device engineering. Finally, we will discuss issues that need to be resolved and propose solutions that lead to perovskite LEDs with excellent performance. Controlling excitons to approach high η r The η r parameter in the EQE calculation formula depends on the type of emitter. The excitons formed from injected electrons and holes are singlet and triplet excitons, according to the different spin configurations. Owing to the spin parallel characteristic, the lower energy exciton is always a triplet exciton. η r is usually considered to be 0.25 for fluorescent emitters, which can only radiate singlet excitons according to the spin conservation rule. As the exciton relaxation process from the triplet state to the ground state involves spin flip and disobeys this rule, the triplet excitons are always known as “dark excitons” at room temperature. To make the triplet excitons radiate at room temperature, heavy metals, such as Ir, Pt, Pb, and halides, are introduced in organic phosphorescent emitters to accelerate the spin–orbital coupling effect and break the spin forbidden reaction. Thus, dark triplet excitons can efficiently turn into bright excitons, and the η r of organic phosphorescent emitters can be improved to one. Another strategy for obtaining a unit η r is to accelerate the dark triplet excitons up-conversed to singlet excitons. Although the origin of perovskite emitters with high η r values is still not completely understood, several interesting research results have been reported that explain the high efficiency of perovskite LEDs. Although early studies on many inorganic emitters have proven that the lowest excitons are dark, Becker et al.28 found that the lowest excitons of CsPbX3 (X = Cl, Br, I) perovskite nanocrystals are not only triplet excitons but also dipole-allowed and thermally populated at room temperature. As shown in Figure 2a, the singlet energy level lies below the triplet energy level due to the combination of spin and short-range electron–hole exchanges. However, according to theoretical calculations, a large Rashba coefficient can alter the fine structure of CsPbX3 perovskite nanocrystals, resulting in bright triplet excitons with lower energy than that of dark singlet excitons. The ratio of bright triplet excitons to total excitons is as high as 3∶4, indicating a high η r of 0.75 for CsPbX3 perovskite nanocrystals. They can quickly radiate light with a decay time on the order of picoseconds (ps). Another important conclusion was drawn by Qin et al.29 for quasi-two-dimensional (quasi-2D) organic–inorganic hybrid perovskite emitters. Since the management of singlet and triplet energy is critical for achieving high efficiency for organic LEDs, the study hypothesized whether the energy of organic cations is also a key factor in designing high efficiency perovskite LEDs. To confirm this hypothesis, the researchers selected two organic cations, phenylethylammonium (PEA) and 1-naphthylmethylamine (NMA), with different lowest triplet energy levels (T1), to construct quasi-2D FAPbBr3-based green perovskites P2F8 [(PEA)2FA7Pb8Br25] and N2F8 [(NMA)2FA7Pb8Br25]. By analyzing the temperature-dependent transient photoluminescence (PL) decay characteristics, they found that triplet excitons in P2F8 using PEA with a high T1 of 3.3 eV can up-convert to singlet excitons and contribute toward emitting light, while no similar phenomenon occurred in N2F8 using NMA with a relatively low T1 of 2.6 eV. Therefore, P2F8-based devices are able to harvest, through efficient up-conversion, both singlet and triplet excitons, which together lead to an η r of 1, while N2F8-based devices primarily harvest singlet excitons, which account for 0.25 of the generated excitons (Figure 2b). As a result, a high EQE of 12.4% and current efficiency of 52.1 cd/A were attained for the P2F8-based devices, which were more than three times that of the N2F8-based devices. Although the EQE of perovskite LEDs is approaching that of organic LEDs, the origin of photons in perovskite LEDs is still debatable. It is essential to provide more support throught further theoretical calculations and experimental studies. Figure 2 | (a) The fine structure of excitons in CsPbX3 perovskite nanocrystals considering short-range electron–hole exchange and large Rashba effects. (b) Proposed energy transfer mechanism in quasi-2D FAPbBr3-based perovskites, P2F8 and N2F8. Download figure Download PowerPoint Defect passivation and radiative recombination to improve Φ PL For perovskite LEDs, nonradiative energy losses arising from defect-assisted recombination and exciton dissociation are the main reasons for low Φ PL , which results in poor device efficiency. For carriers that can be trapped by defects at the surface and within the bulk of the perovskite, defect passivation is a valid strategy to effectively reduce the defect density and improve the Φ PL of the perovskite EMLs. Stoichiometric control is a fundamental technique for passivating defects.17,30 Compared with the low Φ PL of 3% for MAPbBr3 perovskite films fabricated from MABr∶PbBr2 = 1∶1, the Φ PL increases to 36% by minimizing the luminance quenching from metallic Pb atoms by introducing excess MABr (MABr∶PbBr2 = 1.05∶1), leading to an improved device efficiency from 0.183 to 21.4 cd/A (Figure 3a).17 Other passivating materials, such as Lewis bases/acids, are also common in the literature.31–35 Benefiting from their electron-donating ability, Lewis bases, like phosphine oxide (P=O) derivatives, can passivate the defects originating from halide vacancies through Lewis acid–base interactions. Yang et al.33 reported that the Φ PL of quasi-2D perovskite [PEA2(FAPbBr3)3PbBr4] films significantly increased from 57.3% to 73.8% (Figure 3b) through surface passivation by trioctylphosphine oxide (TOPO). The passivation effect of Lewis acids is due to the hydrogen banding interactions between the ammonium head group and halogen anions. Recently, Xu et al.27 found that hydrogen bonding ability can be changed by introducing O atoms within the alkyl chains of amino-functionalized passivation materials, thus influencing the passivation effect. Since the electrons of the N atoms can be polarized into O atoms owing to their electron-withdrawing inductive effect, the electron-donating ability of the amino group is reduced, resulting in a weaker hydrogen bonding ability and better passivation effect. Based on this finding, a rational passivation material 2,2′-[oxybis(ethylenoxy)]diethylamine (ODEA) was designed. In contrast to the peak Φ PL of only 25% showing strong intensity dependence on the control films, an almost constant Φ PL of 65% was attained for perovskite films passivated by ODEA. As a result, a maximum EQE of 21.6% was obtained for near-infrared perovskite LEDs. Amino acid derivatives with both ammonium and carboxylate ions can passivate the defects as well.36 To further manipulate the defects and obtain better device performance, a deeper understanding of defects, especially deep defects, is essential. Figure 3 | (a) Stoichiometric control using excess MABr to enhance PL intensity. (b) PLQE of perovskite [PEA2(FAPbBr3)3PbBr4] films with and without TOPO passivation. Reprinted with permission from Ref. 17 (Copyright 2015 American Association for the Advancement of Science) and Ref. 33 (Copyright 2018 Springer Nature). Download figure Download PowerPoint In addition to defect passivation, Φ PL can also be promoted by reducing the grain size and lowering the dimensionality of perovskite nanocrystals. As the exciton binding energy of 3D perovskites is comparable or even lower than the thermal energy at room temperature, excitons in 3D perovskites can quickly dissociate to free carriers, leading to substantial luminescence quenching. Benefiting from the spatial restriction of excitons and carriers as well as enhanced exciton binding energy and biomolecular recombination, perovskite nanocrystals with small grain sizes and low dimensionality can effectively promote Φ PL of perovskite films. For example, compared with the films into which the organic small molecule 2,2′,2″-(1,3,5-benzinetriyl)-tris (1-phenyl-1-H-benzimidazole) (TPBi) has not been introduced in the antisolvent chloroform, the average grain size of MAPbBr3 that has been treated by TPBi was significantly smaller at 99.7 nm (Figure 4a and 4b).17 The PL intensity increased by 2.8 fold, resulting in a significant improvement in current efficiency from 21.4 to 42.9 cd/A. Figure 4 | (a), (b) SEM images of perovskite films with and without TPBi introduced in antisolvent. (c) Excitation intensity-dependent PLQE of multiple-quantum-well perovskite films. (d) Schematic of the cascade energy transfer in multiple-quantum-well perovskite films. (e) Domain distribution, exciton transfer, and emitting properties of P2m2 films using mixed spacer molecules of P-PDABr2 and PEABr. (f) Pseudocolor map of PL spectra and excitation dependent PLQE of P2m2 films. Reprinted with permission from Ref. 17 (Copyright 2015 American Association for the Advancement of Science); Ref. 37 (Copyright 2018 Springer Nature); and Ref. 38 (Copyright 2019 Wiley-VCH). Download figure Download PowerPoint In 2018, Zou et al.37 reported perovskite devices based on multiple quantum structures achieving a high Φ PL of approximately 55% at a low excitation intensity of 0.1 mW cm–1 (Figure 4c). The exciton energy can rapidly transfer from quantum wells with small n values to those with larger n values, allowing for efficient radiative recombination in quantum wells with lower energy (Figure 4d). Moreover, Yuan et al.38 combined aromatic polyammonium bromide [1,4-Bis(aminomethyl)ben-zene bromide (P-PDABr2)] with PEABr as a dual spacer to tailor the domain distribution in quasi-2D perovskite films. By suppressing the n1 domain, which causes severe nonradiative recombination and decelerates exciton transfer, the population of the radiative n3 domain was enhanced (Figure 4e). The optimized blue perovskite films delivered ideally independent emission from the n3 domain (peak at 465 nm) with a high Φ PL of 77% (Figure 4f). Strategies for light extraction to enhance η out Due to the mismatched refractive indices (n) between the glass substrate (n = 1.45) and perovskite EMLs (n > 2), as well as the air, the optical losses originating from the waveguide and substrate modes determine that the η out of common perovskite LEDs is low. Strategies that effectively extract the trapped light from perovskite LEDs will help to improve η out , leading to a higher EQE. In fact, recent perovskite LEDs with EQEs over 20% are closely related to enhanced η out . In 2018, Cao et al.39 reported a high efficiency of 20.7% for near-infrared perovskite LEDs based on a spontaneously formed submicrometer-scale structure. By simply introducing amino acid additive 5AVA into the perovskite precursor solution, they found that perovskites with sizes between 100 and 500 nm formed between organic layers. Alternating high-index perovskite and low-index organic layers (n = 1.8) are conducive to extracting wide-angle light trapped in the waveguide modes to enter the low-index organic layer and propagate into the glass substrate (Figure 5a). Therefore, a high value of η out , approximately 30%, was obtained. In addition, given the Φ PL of 80% and EQE of 20.3%, η out is at least 25.3% for perovskite LEDs with quasi-core/shell structures reported by Lin et al.25 In the same year, Zhao et al.40 reported that perovskite LEDs using perovskite–polymer bulk heterostructures (PPBHs) as perovskite EMLs attained a high EQE of 20.1%. Compared to the common organic–inorganic hybrid perovskites with a refractive index of approximately 2.3, they found that the refractive index for PPBH was significantly lower at 1.9, which widened the escape cone of photon emission from the EMLs to 32°. By optical modeling, η out was confirmed up to 21%. Furthermore, according to the researchers, η out could be increased to 25% when the thickness of the perovskites was optimized to further minimize the waveguide mode loss. The importance of the perovskite EML thickness to the EQE was fully investigated by Rand et al.41 in another work. Optical simulation confirmed that the optimized thickness to realize better device efficiency was in the range of 35–40 nm (Figure 5b). When prepared with bulky PEA halide additives, perovskite LEDs with thin perovskite layers not only form high-quality films but also improve light outcoupling. High EQEs of 17.6%, 14.3%, 10.1%, and 11.3% were attained for Cs0.2FA0.8PbI2.8Br0.2, MAPbI3, FAPbI3, and FAPbBr3-based perovskite LEDs, respectively. In addition, Shen et al.42 adopted bioinspired moth-eye nanostructures (MEN) with 2D subwavelength features in CsPbBr3-based perovskite LEDs to suppress the Fresnel reflections and improve the waveguide light. The EQEs of the MEN-modified devices were improved to 20.3%, which was 1.5 times greater than that of the flat devices. Furthermore, by using a half-ball lens to enhance the light trapped in the substrate mode, they eventually succeeded in increasing the EQE to 28.2% (Figure 5c). Recently, Jeon et al.43 increased η out 1.64 times by using a randomly distributed nanohole array embedded in a SiN layer with a high n of 2.02 between the indium tin oxide (ITO) anode and the glass substrate. Without altering the device structure (Figure 5d), the EQEs were enhanced from 8.9% to 14.6% for MAPbI3-based perovskite LEDs. In summary, η out is sensitive to the refractive index and thickness of the functional layers in perovskite LEDs. Optimizing the composition and thickness of perovskite layers to obtain a more matched refraction index and reduce waveguide losses will improve the performance of perovskite LEDs. In addition, low-cost outcoupling technology that can enhance device efficiency without influencing the EL spectra is needed. Figure 5 | (a) Light extraction in devices without/with submicrometer structure. (b) Optical power distribution analysis versus the thickness of MAPbI3-based perovskite EMLs showing an optimized thickness of approximately 40 nm. (c) Performances of CsPbBr3-based perovskite LEDs with and without optical enhancement. (d) Corresponding device structures with nanohole arrays, molecular structure of used transport materials, and scanning electron microscope (SEM) images. Reprinted with permission from Refs. 41–43 (Copyright 2019 Wiley-VCH). Download figure Download PowerPoint Tuning carrier transport characteristics to reach ideal γ Because of the different carrier mobilities of HTLs, electron transport layers (ETLs), perovskites, and barriers at the γ is than in perovskite LEDs. carrier transport characteristics can promote γ to ideal values. Lin et al.25 introduced a MABr additive into a perovskite solution to form a quasi-core/shell structure. They found that the MABr with a larger energy not only effectively the defects but also to reducing electron injection and improving the charge In addition, they used a thin layer of as the between the perovskite and the to balance the charge Thus, a EQE of was attained from In addition, because common with long alkyl chains the carrier et introduced with better electron to organic cations to the The energy level of the device is shown in Figure Compared to the electron mobility of the electron mobility of more charge transport (Figure As a result, perovskite LEDs based on with a higher EQE of while the maximum EQE of the control devices is only Figure | (a) level of the device based on (b) and hole mobilities (c) structure of CsPbBr3-based perovskite quantum using (d) level of FAPbBr3-based devices with different to the of Reprinted with permission from Ref. (Copyright 2019 and Ref. (Copyright 2015 Wiley-VCH). Download figure Download PowerPoint Although perovskite optimization can effectively improve the carrier balance with a high γ, device engineering can more reduce the carrier barrier and realize a carrier balance with an ideal γ by appropriate and materials. The transport layer materials are not only on the barriers between the and the perovskites but also on the mobilities of the transport layer materials. For p–i–n perovskite LEDs, the common organic HTLs, including molecular of approximately of of and of are perovskite EMLs. hole mobilities range from to Owing to the different of and perovskites and et and to construct near-infrared and green perovskite LEDs with better energy level for hole injection and electron respectively. Compared with the of the widely used anode material holes are more injected from to than in of hole transport materials. Thus, is used as the hole layer. than et selected which has lower to form an layer of surface in perovskite devices. As a result, the exciton quenching can be and the hole current can be at this leading to significantly increased current efficiency for FAPbBr3-based perovskite LEDs. However, in contrast to the small barrier between and the barriers between and perovskite especially blue perovskite are always larger than eV. Thus, injected layers and multiple are to attain sufficient hole injection by lowering the barrier by For example, et used the device structure of to CsPbBr3-based perovskite quantum (Figure with the hole injection barrier was reduced, and the electrons were effectively in the perovskite EMLs to with the injected For (holes and electrons are injected from cathode and perovskite LEDs, the used electron transport materials, such as and are The reported perovskite LEDs at room by et used the device structure of for with et reported that high-quality perovskite films were and the of was from to leading to electron injection from Recently, Zhao et the of with to the of to attain more efficient hole injection and improve the charge The corresponding energy level is shown in Figure Thus, compared with the low EQE of obtained using a high EQE of was attained using for near-infrared perovskite LEDs based on with an emission peak of nm. and Although the EQEs of green, red, and near-infrared perovskite LEDs were greater than 20% through the strategies, are still many that be overcome to promote this technology for practical obtaining high performance at high current density corresponding to high is However, substantial efficiency roll-off to poor efficiency at high current in perovskite LEDs. Although et reported that recombination to and are considered to be nonradiative recombination emission quenching. As the efficiency roll-off is closely related to the recombination of perovskite films, et investigated the between recombination and perovskite device by analyzing the