Optical Energy Losses in Organic–Inorganic Hybrid Perovskite Light‐Emitting Diodes

Abstract

easily tunable band gaps. [4][5][6][7] LEDs based on such materials have been attracting increasing attention since the first roomtemperature perovskite LEDs (PeLED) developed in 2014. [13] The development of PeLEDs has progressed very rapidly, realizing a record-high external quantum efficiency (EQE) of 11.7% in 2016. [14] Similar to other LED categories, e.g., organic LEDs (OLEDs) [15] and quantumdot [16,17] LEDs, the EQE of PeLEDs is determined by the internal quantum efficiency (IQE) and light out-coupling efficiency (η), which is expressed asThe IQE is the product of the charge carrier balance (γ), the fraction of excitons capable of radiative decay at room temperature (η S/T ), and the effective radiative quantum yield (q eff ). For 2D perovskite materials with relatively large binding energy, the η S/T could be unity when heavy atoms are involved in the organometallic complex (similar to that in phosphorescent materials [18] ). For 3D perovskite materials with relatively small binding energy, the emission should be bandto-band transitions of free carriers in direct gap semiconductors, and the η S/T term should not be involved in the equation in this case. All in all, this equation shows that the EQE is largely influenced by the η besides the IQE. Thus, studying the limits of η is of great importance for achieving high-EQE PeLEDs.As the fast development of PeLEDs, a conflict appears between the experimental and theoretical results. In general, the η can be estimated according to the ray-optics theory, [19] Light-emitting diodes (LEDs) based on organic-inorganic hybrid perovskites, in particular, 3D and quasi-2D ones, are in the fast development and their external quantum efficiencies (EQEs) have exceeded 10%, making them competitive candidates toward large-area and low-cost light-emitting applications allowing printing techniques. Similar to other LED categories, light out-coupling efficiency is an important parameter determining the EQE of perovskite LEDs (PeLEDs), which, however, is scarcely studied, limiting further efficiency improvement and understanding of PeLEDs. In this work, for the first time, optical energy losses in PeLEDs are investigated through systematic optical simulations, which reveal that the 3D and quasi-2D PeLEDs can achieve theoretically maximum EQEs of ≈25% and ≈20%, respectively, in spite of their high refractive indices. These results are consistent with the reported experimental data. This work presents primary understanding of the optical energy losses in PeLEDs and will spur new developments in the aspects of device engineering and light extraction techniques to boost the EQEs of PeLEDs. Perovskite Light-Emitting DiodesOrganic-inorganic perovskite materials have attracted extensive interest due to their fascinating semiconducting properties, such as high absorption coefficient, long carrier diffusion length, and small exciton binding energy. [1][2][3] Moreover, these materials are easily processed via solution-based techniques, showing great potential applications in low-cost op...

Perovskite Light‐Emitting Diodes with Improved Outcoupling Using a High‐Index Contrast Nanoarray

“…Numerous strategies to improve the EQE of PeLEDs are being actively pursued in order to bring their performance in line with other, more established, LED technologies . However, a disparity of refractive index ( n ) between organic transport layers (typically in the range of 1.6–1.8) and the perovskite emissive layer (≈2.3 near the emission wavelength) holds back performance . Due to the high n of the perovskite layer, the maximum EQE of PeLEDs is limited by outcoupling efficiency and restricted to ≈20%, with the remainder of light being trapped within the thin film and substrate materials, as well as parasitic absorption .…”

“…However, a disparity of refractive index ( n ) between organic transport layers (typically in the range of 1.6–1.8) and the perovskite emissive layer (≈2.3 near the emission wavelength) holds back performance . Due to the high n of the perovskite layer, the maximum EQE of PeLEDs is limited by outcoupling efficiency and restricted to ≈20%, with the remainder of light being trapped within the thin film and substrate materials, as well as parasitic absorption . Therefore, it is necessary to investigate alternative device architectures that are able to enhance outcoupling efficiency and realize direct benefits to EQE.…”

“…The structure and thickness of the organic and perovskite layers have been taken from previously reported highly efficient PeLEDs . A thinner perovskite emissive layer would achieve a higher efficiency owing to lower waveguide losses, however, such ultrathin layers present additional experimental challenges for reproducibility. The only difference in the PeLED with the NHA compared to the reference device is that the NHA is positioned on the substrate between the glass and ITO electrode layers to best extract light from the perovskite EML .…”

Investigating Free Charge‐Carrier Recombination in Organic LEDs Using Open‐Circuit Conditions

“…Extensive research is done in the field of organic LEDs (OLEDs) by incorporating molecules that show thermally activated delayed fluorescence and by exploiting charge‐transfer (CT) states that either emit light or serve as a host system . Besides that, new materials like quantum dots or perovskites are heavily investigated . One major drawback is that in most of the aforementioned fields the device stack design is still mainly done by trial and error, e.g., by testing several material combinations.…”

Efficient single-layer light-emitting diodes based on organic–inorganic lead halide perovskite and tuning luminescence properties

“…Even though numerous studies have been carried out on the use of perovskite structures in light-emitting diodes, the study of the electroluminescence properties of these structures, without the aid of electron and hole transmitter layers, is done, for the first time, in this study. This is due to the higher mobility of charge carriers in perovskite structures [19][20][21][22][23][24]. It has been shown that by sandwiching a perovskite layer between the organic hole layer and the electron blocking layer in perovskite-based light-emitting diodes, a narrow emission spectrum and a high quantum yield can be obtained [25].…”