A research team, led by Prof. Di Dawei from the Zhejiang University College of Optical Science and Engineering, recently discovered that by using germanium (Ge), an environmentally friendly group-IV element, to partially substitute lead in the perovskite, it is possible to create highly luminescent perovskite materials and devices.
To resolve the toxicity problem that arises from the use of lead, an effective method has been the use of tin (Sn) as a partial or full replacement of lead in the perovskite materials. This strategy has been particularly successful for perovskite solar cells. However, tin-based (including tin-lead) perovskite materials are generally very poor light emitters, causing unsatisfactory performance of tin-based perovskite light-emitting devices (LEDs).
A reason for this is that a high density of electronic defects can form during the preparation process of tin-based perovskites, as tin (Sn2+) can oxidize and the crystallization process is not well controlled. Tin-based perovskite LEDs with external quantum efficiencies of 5% were reported in 2020, but these efficiencies were only possible at low brightness (38 cd/m2), well below the requirement for display applications (500-1000 cd/m2).
"In our journey toward low-toxicity perovskite LEDs, we've been searching for alternative environmentally friendly metals to replace lead," said Di. "it was a pleasant surprise that germanium, the element between silicon and tin (on the periodic table) could perform so well as a substitution of lead for LED applications."
Dr. Yang Dexin, a postdoctoral research fellow in Di's group and the first author of the paper, said "we used to study the effect of germanium ions (Ge2+) in germanium telluride alloys, and found that germanium ions may offer some benefits in terms of defects passivation."
The germanium-lead perovskite thin films showed high photoluminescence quantum efficiencies (PLQEs) of up to 71%. This was a relative improvement of about 34% over similarly prepared Pb-based perovskite films. The high luminescence efficiencies came from the enhanced radiative processes and reduced defect densities in the germanium-lead perovskites.
Using these materials, Di and colleagues demonstrated germanium-lead perovskite LEDs for the first time. "In our initial demonstration, we achieved external quantum efficiencies of up to 13.1% at high brightness (~1900 cd/m2) with a maximum brightness of over 10000 cd/m2. This was an efficiency record for reduced-toxicity perovskite LEDs," said Yang.
"While further work is certainly needed for improving the device performance and stability, our results suggest a promising route toward eco-friendly light-emitting technologies based on perovskite semiconductors," said Di.