Lead-Free Direct Band Gap Double-Perovskite Nanocrystals with Bright Dual-Color Emission

Abstract

Lead-free double-perovskite nanocrystals (NCs), that is, Cs2AgIn x Bi1–x Cl6 (x = 0, 0.25, 0.5, 0.75, and 0.9), that can be tuned from indirect band gap (x = 0, 0.25, and 0.5) to direct band gap (x = 0.75 and 0.9) are designed. Direct band gap NCs exhibit 3 times greater absorption cross section, lower sub-band gap trap states, and >5 times photoluminescence quantum efficiency (PLQE) compared to those observed for indirect band gap NCs (Cs2AgBiCl6). A PLQE of 36.6% for direct band gap NCs is comparable to those observed for lead perovskite NCs in the violet region. Besides the band edge violet emission, the direct band gap NCs exhibit bright orange (570 nm) emission. Density functional theory calculations suggesting forbidden transition is responsible for the orange emission, which is supported by time-resolved PL and PL excitation spectra. The successful design of lead-free direct band gap perovskite NCs with superior optical properties opens the door for high-performance lead-free perovskite optoelectronic devices.

Tunable Color Temperatures and Efficient White Emission from Cs2Ag1−xNaxIn1−yBiyCl6 Double Perovskite Nanocrystals

“…The lifetime and PLQY have the same evolution trend. The role of Bi 3+ doping contains two possible effects according to the literature's report: one is believed to occupy the Indium vacancies and promote exciton localization, the other is that the Bi 3+ introduction can break the parity forbidden transition of Cs 2 AgInCl 6 by manipulating the crystal symmetry . From our Bi‐doping sample absorption spectra and their strong photoluminescence, the newly introduced absorption peak was obtained ascribed to the direct bismuth s‐p transition and the corresponding emission supported the break of the parity forbidden transition …”

“…But it was usually used as absorber of the solar cell rather than the light‐emitting device . Up to now, several studies have reported the synthesis of colloidal double perovskite NCs including Cs 2 AgBiX 6 (X = Cl, Br, and I), Cs 2 AgSb 1− y Bi y X 6 (X = Br and Cl; 0 ≤ y ≤ 1) and Cs 2 AgIn x Bi 1− x Cl 6 ( x = 0, 0.25, 0.5 0.75, and 0.9) NCs . The reported highest PLQY is ≈36.6% .…”

Halide Perovskite Nanocrystals for Next‐Generation Optoelectronics

“…Hydrogen bond is a weak noncovalent bond, which could be decomposed at high temperature . Hence, by replacing the hydrogen bonds with covalent bonds to form crosslinked PNCs, more promising optoelectronic properties could be gained.…”

“…However, the toxicity of traditional Pb‐based PNCs is a critical problem that makes this technology still not ready for commercialization . This has recently driven an intense research worldwide toward environmentally friendly and chemically robust Pb‐free perovskite bulk materials, as well as nanocrystals . Nevertheless, the reported Pb‐free PNCs perform still poorly when compared to Pb‐based ones .…”

Recent Developments in Lead‐Free Double Perovskites: Structure, Doping, and Applications

“…These results are attributed to the surface defects present on the pristine Cs 2 AgInCl 6 nanocrystal; however, these defects are passivated on the Bi‐doped Cs 2 AgInCl 6 . The possibility of this emission because self‐trapped excitons originate from the Jahn–Teller distortion of the [Ag I Cl 6 ] octahedron in the excited state . Liu et al .…”