To improve the brightness, contrast and resolution of the display, while reducing costs and the usage of limited resources, scientists have developed different kinds of light-emitting diodes, such as organic light-emitting diodes (OLEDs), quantum dots LEDs (QLEDs), perovskite-based LEDs (PVSK LEDs) and micro LEDs. Compared to OLEDs, which are widely used in high-end consumer-electronics, the external quantum efficiency of perovskite-based LEDs has rapidly been improved from 0.76 % to 20.7 % in just four years.
Scientists from Cavendish Laboratory, the University of Cambridge have developed the perovskite layer in the LEDs to show close to 100 % internal luminescence efficiency, rivaling that of the best OLEDs. The perovskite-based LEDs can be made at much lower costs, also can be tuned to emit light across the visible and near-infrared spectra with high color purity, which helps open future applications in display, lighting and communications, as well as next generation solar cells.
Limitation of the development of halide perovskite-based LEDs
Perovskite solar cells are currently considered to be the most potential materials compared to conventional Si solar cells. But before achieving the goal, the key to develop higher efficiency and lower costs in solar cells is the comprehensive development of perovskite materials that could one day replace commercial silicon solar cells.
Earlier hybrid perovskite LEDs, first developed by Professor Sir Richard Friend’s group at the University’s Cavendish Laboratory four years ago, were promising, but losses from the perovskite layer, caused by tiny defects in the crystal structure, limited their light-emission efficiency. That’s the reason perovskite-based LEDs have not been nearly as efficient as conventional OLEDs at converting electricity into light.
Perovskite-polymer bulk heterostructure
In November of this year, the Richard Friend’s group and their collaborators have engineered the perovskite-polymer bulk heterostructure LEDs, and it is possible to achieve much higher light-emission efficiencies, close to the theoretical value of membrane OLED efficiency. Their result “High-efficiency perovskite–polymer bulk heterostructure light-emitting diodes” is published in the journal Nature Photonics.
Fig. Basic optical and structural
characterization of PPBH
In this work, the researchers explore the optoelectronic and photophysical properties of perovskite-polymer bulk heterostructure (PPBH). The light-emitting diode emissive layer comprises quasi-two-dimensional and three-dimensional (2D/3D) perovskites and a n insulating polymer (Eg =4.96 eV). The weight ratio of the PPBH precursors (1-naphthylmethylammonium iodide (NMAI), formamidinium iodide (FAI), lead iodide (PbI2) and the poly-HEMA) in the precursor solution is 5:3:8:4 (the weight fraction of the polymer in the precursors is 20%). The volume fractions of the perovskite phase and the polymer phase in the resultant PPBH film are estimated to be 72% and 28%, respectively.
The photoluminescence spectrum of the sample peaks at ~795 nm (~1.56 eV), with a full-width at half-maximum (FWHM) of ~55 nm. Grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements indicate that the perovskite crystallites are is otropically oriented in the PPBH film. High-resolution transmission electron microscopy (HR-TEM) results suggest the presence of quasi-2D/3D crystal structures. From X-ray diffraction (XRD) data, the average crystallite size is estimated to be 30–55 nm based on the FWHM of the diffraction peaks. The average surface roughness of the film is ~3.3 nm.
Fig. LED performance characterization and emissive layer PLQEs
To investigate the electroluminescence properties of the PPBH, the team developed a solution-processed multilayer LED structure. The electroluminescence spectrum of the PPBH LED is nearly identical to that of the steady-state photoluminescence, exhibiting a slightly narrower FWHM of ~49 nm. Photogenerated excitations migrate from quasi-2D to lower-energy sites within 1 ps, followed by radiative bimolecular recombination in the 3D regions. From near-unity external photoluminescence quantum efficiencies and transient kinetics of the emissive layer with and without charge-transport contacts, they find non-radiative recombination pathways to be effectively eliminated, consistent with optical models giving near 100% internal quantum efficiencies. The peak EQE of the best devices reaches 20.1%, a record for perovskite-based LEDs.
“This perovskite-polymer structure effectively eliminates non-emissive losses, the first time this has been achieved in a perovskite-based device,” said Dr Dawei Di from Cambridge’s Cavendish Laboratory, one of the corresponding authors of the paper. “By blending the two, we can basically prevent the electrons and positive charges from recombining via the defects in the perovskite structure.”“The best external quantum efficiencies of these devices are higher than 20% at current densities relevant to display applications, setting a new record for perovskite LEDs, which is a similar efficiency value to the best OLEDs on the market today,” said Baodan Zhao, the paper’s first author.
Understanding the degradation mechanisms of the LEDs is the key to future developments!
While perovskite-based LEDs are beginning to rival OLEDs in terms of efficiency, they still need better stability if they are to be adopted in consumer electronics. When perovskite-based LEDs were first developed, they had a lifetime of just a few seconds. The LEDs developed in the current research have a half-life close to 50 hours, which is a huge improvement in just four years, but still nowhere near the lifetimes required for commercial applications, which will require an extensive industrial development program. “Understanding the degradation mechanisms of the LEDs is a key to future improvements,” said Di.
Challenges of perovskite LEDs measurement
1. Low luminescence and decay very fast. ＜50,000 cd/m2
The luminescence of perovskite LEDs decays more than 20 % within 4 secs. Therefore, accelerating measurement speed is important to get higher conversion efficiency of perovskite-based LEDs.
Enli Tech LQ-100 Luminescence Measurement System provides fast measurement speed with full spectrum range: 350 nm-1000 nm for users getting high efficiency.
2. Narrow FWHM~50 nm
LQE-100 series can record and calculate the error of the photoluminescence spectrum.
3. Non-Lambertian distribution. cd/m2 convert to EQE, the deviation value is higher
With ultra-low light intensity detector: can measure the ultra-low light intensity, below 0.1cd/m2, and get the perfect EQE curve.
4. The emission wavelength cannot be included in luminosity function, and is not able to estimate its efficiency by Lumens (lm) and Luminance (cd/m2)
Baodan Zhao et al. ‘High-efficiency perovskite-polymer bulk heterostructure light-emitting diodes.’ Nature Photonics (2018). DOI: 10.1038/s41566-018-0283-4
University of Cambridge. (2018). New efficiency record set for perovskite LEDs. Retrieved from https://www.cam.ac.uk/research/news/new-efficiency-record-set-for-perovskite-leds (December 4, 2018)