The key elements to measure higher efficiency of perovskite LED devices

Who is the winner in the high efficiency contest of LED devices?

Light-emitting diodes (LEDs) are the main alternative to traditional light sources, and perovskite light-emitting devices plays the key role in the light-emitting field. Although it is not so easy to get into the light-emitting device field; however, LEDs based on metal halide perovskite materials have extraordinary characteristics: narrow emission line widths at high photo-luminescence quantum yield (PLQY), high color purity, and color tunable. It can be made at low cost via facile solution processing and be adopted in flat panel display and solid-state lighting applications. Therefore, perovskite LEDs are able to get into the contest of light-emitting devices with organic LEDs (OLEDs) and inorganic quantum dot LEDs (QLEDs).

However, compared to OLEDs and QLEDs, the external quantum efficiency (EQE) of perovskite LEDs is still well behind them due to its instability. The current highest reported external quantum efficiency of green-light emitting perovskite LEDs is 14.36 %, and the research named” Efficient green light-emitting diodes based on quasi-two-dimensional composition and phase engineered perovskite with surface passivation “is released by Dr. Jingbi You, the Institute of Semiconductors, Chinese Academy of Sciences (CAS). And the highest EQE of red-light emitting perovskite LEDs currently is 11.7 %, and the research named” Perovskite light-emitting diodes based on solution-processed self-organized multiple quantum wells” is released by Dr. Wei Huang and Dr. Jianpu Wang, the Institute of Advanced Materials, Nanjing Tech University. The perovskite LEDs have much room to for improvement in higher efficiency and stability.

Perovskite LEDs with higher EQE exceeding 20 %

■ Dr. Zhanhua Wei, College of Materials Science & Engineering, Huaqiao University

With Dr. Qihua Xiong, Nanyang Technological University, and Dr. Eedward H. Sargent, Toronto University, Dr. Zhanhua Wei released the research “Perovskite light-emitting diodes with external quantum efficiency exceeding 20 percent”.

Fig. Emission properties and formation mechanism of mixture perovskites. (Cited from the research)

The team adopt a new strategy for managing the compositional distribution in the perovskite LED device:
● Mixed a presynthesized CsPbBr3 perovskite with a MABr additive (where MA is CH3NH3), the differing solubilities of which yield sequential crystallization into a CsPbBr3/MABr quasi-core/shell structure.
● Further improved the device charge injection balance by inserting an insulating layer of poly(methyl methacrylate) (PMMA) between the perovskite layer and the electron- transfer layer (ETL), thereby maximizing the device efficiency at 20.3%.
● The half-lifetime (T50)—defined as the time taken for the luminance to decrease to L0/2—was about 10 min. The team estimate this device’s T50 at 100 cdm-2 to be about 100h—to their knowledge, the highest value estimated to date in high-performance perovskite LEDs, and an important step towards practical application. The team also measured the stability of the device under continuous operation with luminance maintained at a constant value of about 100 cdm-2, achieved by tuning the applied current to maintain luminance.

▲Fig. Enhancing the performance of perovskite LEDs by inserting a thin PMMA layer between the perovskite and the ETL. (Cited from the research)


■ Dr. Wei Huang, Institute of Flexible Electronics, Northwest Polytechnical University
With Dr. Jianpu Wang, the Institute of Advanced Materials, Nanjing Tech University, Dr. Wei Huang released the research, “Perovskite light-emitting diodes based on spontaneously formed submicrometre-scale structures”.


▲Fig. Device fabrication and formation of submicrometre structure. (Cited from the research)

The team demonstrated a new method in processing perovskite emitting layer.
●An efficient and high-brightness electroluminescence from solution-processed perovskites that spontaneously form submicrometre-scale structures, which can efficiently extract light from the device and retain wavelength- and viewing-angle-independent electroluminescence. These perovskites are formed simply by introducing amino-acid additives into the perovskite precursor solutions.
●The additives can effectively passivate perovskite surface defects and reduce nonradiative recombination.
●Perovskite LEDs with a peak external quantum efficiency of 20.7 per cent (at a current density of 18 milliamperes per square centimetre) and an energy-conversion efficiency of 12 per cent (at a high current density of 100 milliamperes per square centimetre) can be achieved—values that approach those of the best-performing organic LEDs.


▲Fig. Optoelectronic characteristics of our perovskite LEDs. (Cited from the research)


Do you underestimate the External Quantum Efficiency of your LED devices?
The development of perovskite LED efficiency is rapidly progressing at present, but the most challenging task which most researchers have been facing is how to get the accurate PLQY of perovskite LEDs.
Take PVK LED as an example, the following diagram shows that the luminance of perovskite LED decays more than 20 % within 4 secs.

Therefor, how to get the highest EQE of perovskite LEDs through the right measurement instruments is the key point of perovskite LED devices.

The Challenges of Perovskite LED measurement:
1. Low luminance and decay very fast: <50,000cd/m2
2. Narrow FWHM: PVSK LED~50 nm
3. Non-Lambertian distribution: cd/m2 => EQE, the deviation value is higher
4. High-efficiency NIR extension capability

The target researchers indicate the three key technologies for measuring high-efficiency perovskite LED devices are as follows: 

1. Faster measurement speed, higher conversion efficiency: With Enli Tech LQ-100 automatic exposure time control function, the EQE of perovskite LED can be 20 % higher than the conventional one.

The figure shows: 
A: with LQ-100 automatic exposure time control function (1 ms~1 sec /point)
B: with the conventional luminometer (1 sec/ point)
Conclusion: the EQE of perovskite LED can be 20 % higher than the conventional one.

2. Low Luminance, High-Speed and Accurate Measuring Capabilities
When the luminance measurement (<50 cd/m2) is not accurate, the EQE curve will not be smooth under the low voltage. Therefore, researchers cannot judge the initial voltage of the devices accurately; moreover, they cannot evaluate the highest external quantum efficiency of devices.

3. Record function of Aging Test for LED device
Use constant current and voltage to measure the variation of luminance and efficiency in time.

-->LQ-100 is sited by the Nature Communications Journal.  Learn more information about LQ-100 Luminescence Measurement System.


References:
Perovskite light-emitting diodes with external quantum efficiency exceeding 20 percent, Kebin Lin, Jun Xing, Li Na Quan, F. Pelayo García de Arquer, Xiwen Gong, Jianxun Lu, Liqiang Xie, Weijie Zhao, Di Zhang, Chuanzhong Yan, Wenqiang Li, Xinyi Liu, Yan Lu, Jeffrey Kirman, Edward H. Sargent, Qihua Xiong & Zhanhua Wei
Nature volume 562, pages245–248 (2018)

Perovskite light-emitting diodes based on spontaneously formed submicrometre-scale structures, Yu Cao, Nana Wang, He Tian, Jingshu Guo, Yingqiang Wei, Hong Chen, Yanfeng Miao1, Wei Zou, Kang Pan,  Yarong He, Hui Cao, You Ke, Mengmeng Xu, Ying Wang, Ming Yang, Kai Du, Zewu Fu, Decheng Kong, Daoxin Dai, Yizheng Jin, Gongqiang Li, Hai Li, Qiming Peng, Jianpu Wang & Wei Huang
Nature volume 562, pages249–253 (2018)