High Color Purity Lead-Free Perovskite Light-Emitting Diodes via Sn Stabilization.

Hongyan Liang,Bin Sun,Bin Chen,Yunkun Wang,Yunan Gao,Yuan Gao,Congcong Gao,Makhsud I Saidaminov,Fanglong Yuan,Peicheng Li,Mingjian Yuan,Edward H Sargent,Zheng‐Hong Lu,Yitong Dong,Sjoerd Hoogland,Andrew Johnston,Hitarth Choubisa,Golam Bappi,Laxmi Kishore Sagar,Jun Li,Yongchang Liu,Dávid Ma ,Yakun Wang

doi:10.1002/advs.201903213

Abstract

Perovskite‐based light‐emitting diodes (PeLEDs) are now approaching the upper limits of external quantum efficiency (EQE); however, their application is currently limited by reliance on lead and by inadequate color purity. The Rec. 2020 requires Commission Internationale de l'Eclairage coordinates of (0.708, 0.292) for red emitters, but present‐day perovskite devices only achieve (0.71, 0.28). Here, lead‐free PeLEDs are reported with color coordinates of (0.706, 0.294)—the highest purity reported among red PeLEDs. The variation of the emission spectrum is also evaluated as a function of temperature and applied potential, finding that emission redshifts by <3 nm under low temperature and by <0.3 nm V−1 with operating voltage. The prominent oxidation pathway of Sn is identified and this is suppressed with the aid of H3PO2. This strategy prevents the oxidation of the constituent precursors, through both its moderate reducing properties and through its forming complexes with the perovskite that increase the energetic barrier toward Sn oxidation. The H3PO2 additionally seeds crystal growth during film formation, improving film quality. PeLEDs are reported with an EQE of 0.3% and a brightness of 70 cd m−2; this is the record among reported red‐emitting, lead‐free PeLEDs.

Highlights

Tion is currently limited by reliance on lead and by inadequate color purity
This strategy prevents the oxidation of the constituent precursors, through both its moderate reducing properties and through its forming gamut for ultrahigh definition television (UHDTV): it requires each of the primary red, green, blue (RGB) emitters to have a precisely defined wavelength and a narrow emission linewidth (
PeLEDs are reported with an EQE of 0.3% and a brightness of 70 cd m−2; this is the record among reported red-emitting, state-of-art liquid crystal display backlights, crosstalk between color filters reduces the color purity of the RGB primaries.[8]

Summary

Mass loss Final mass

Www.advancedscience.com that, when TGA was carried out in N2, the bulk of the mass loss of PEA2SnI4 and SnI2 occurred at higher temperatures than in air, indicating that the oxidation reaction is responsible for the products that are most readily vaporized. We employed a device architecture based on indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) (≈30 nm)/PEA2SnI4 (≈200 nm)/TPBi (≈60 nm)/ LiF (1 nm)/Al. Poly(2-hydroxyethyl methacrylate) (PHM) was added to improve the film coverage and quality, a method previously reported to help maintain charge balance.[6] We summarize device performance versus PHM additive in Figure S12 in the Supporting Information. LEDs incorporating the Sn-based perovskites show EL emission centered at 633 nm with an FWHM of only 24 nm and CIE coordinates of (0.706, 0.294) These CIE coordinates are the most pure-red among all PeLEDs. The optimized PeLEDs show a maximum luminance of 70 cd m−2 under 5.8 V, giving an EQE of 0.3%, the highest reported brightness and efficiency among red, Pb-free PeLEDs. To avoid oxidation, we identify the chemical pathway of Sn oxidation in 2D-perovskite films and the formation of SnI4 is a key intermediate step.

Experimental Section

Findings

Conflict of Interest