Stable perovskite films promise improved LED efficiency

21 Feb 2019

City University of Hong Kong fabrication method boosts performance and operational lifetime.

The development of metal halide perovskite devices able to act as LEDs with acceptable quantum efficiency has been a focus of research for some time, but poor operational stability has proven to be a challenge.

Performance has been limited particularly by the large perovskite grain sizes, which negatively impact the electro-luminescence and encourage so-called trap states at the grain boundaries, where the desired movement of electrons and holes is limited.

A team at City University of Hong Kong has now developed a manufacturing process capable of producing smooth, pinhole-free and small-grained perovskite films, and demonstrated that both quantum efficiency and operational lifetime can benefit. The work was reported in Nature Communications.

The breakthrough relates in particular to perovskites based on inorganic cesium cations, or CsPbX₃, where X can be chlorine, bromine or iodine. These materials exhibit better thermal and chemical stability than other metal halide perovskites, but have proved to have poor electro-luminescence, due to the large perovskite grain sizes.

Using cesium trifluoroacetate (TFA) as the cesium source rather than the more common cesium bromide in a one-step solution coating process proved to be the key, due to its effect on the crystallization rate of the perovskite films. Surface defects in the film were suppressed as a result.

"TFA ions are preferentially bound to the surface Pb²⁺ ions in the CsPbBr₃," noted the team in its published paper. "Compared to the conventional CsBr route, the TFA-derived films show a flatter energy landscape with a more homogeneous energy level distribution for charges, more stable crystal structure, better optical properties, and suppressed ion migration."

Record operational lifetime

Testing the performance of the modified perovskite films as LEDs, the project was able to produce green LEDs demonstrating a current efficiency of 32 cd A⁻¹, corresponding to an external quantum efficiency of 10.5 percent. More importantly, the all-inorganic perovskite LEDs demonstrated a record operational lifetime, with a half-lifetime of over 250 hours at an initial luminance of 100 cd m⁻².

This represents a 17-fold improvement in operational lifetime compared with perovskite LEDs derived from CsBr, according to the University, and could now indicate a route towards inorganic lead-halide perovskite films able to act as highly efficient LEDs.

"Our study suggests that the high color-purity and low-cost all-inorganic lead halide perovskite films can be developed into efficient and stable LEDs via a simple optimization of the grain boundaries," commented Andrey Rogach of City University, a co-author of the paper.

"I foresee significant application potential of such films, as they are easy to fabricate and can be easily deposited by printing to realise various optoelectronic devices."