Charge-Carrier Dynamics of Lead-Free Halide Perovskite Nanocrystals

Abstract

Conspectus Lead halide perovskite nanocrystals (NCs) have been widely studied for application in optoelectronic devices due to their excellent optical properties and low-cost synthesis. However, the toxicity of lead and the poor stability of the NCs hindered their practical applications. Sn2+-based perovskite with low toxicity was first developed; however, the Sn2+-based perovskite NCs are unstable in air and oxidize easily. Recently, air-stable lead-free perovskite NCs have been developed and received increasing attention. Unfortunately, the optical and optoelectronic properties of these lead-free halide perovskite NCs are generally far worse than those of lead-perovskite NCs. Understanding the charge-carrier dynamics of semiconductors is crucial to improve their optical properties. In this Account, we mainly review our recent research progress on the study of charge-carrier dynamics in air-stable lead-free perovskite NCs. The exciton trapping followed by nonradiative recombination was the major carrier relaxation pathway and resulted in a low photoluminescence quantum efficiency (PLQE). A feasible route for passivating surface traps and tuning the self-trapped excitons from “dark” (nonradiative) to “bright” (radiative) was proposed. Through this strategy, the PLQE could be increased over 100-fold. In addition, we have compared several photophysical properties of lead-free perovskite NCs with that of lead perovskite NCs, such as charge-carrier relaxation, exciton–phonon coupling, and hot-carrier cooling. In 2017, we reported the synthesis, optical properties, and charge-carrier dynamics of Cs3Bi2X9 (X: Cl, Br, I) NCs. The Cs3Bi2Br9 NCs exhibited clear exciton trapping processes with time scales in the range of 2–20 ps. The fast trapping processes could be passivated via the use of surfactants (such as oleic acid), and the PLQE increased over 20-fold (from 0.2% to 4.5%). The low PLQE may be due to the reduced dimensionality of Cs3Bi2Br9 (2D) compared with the 3D cubic perovskite structure of CsPbBr3. We next reported double perovskite Cs2AgSb1–y Bi y X6 (X: Br, Cl; 0 ≤ y ≤ 1) NCs, which exhibited a similar 3D cubic perovskite structure to that of the lead-perovskite NCs. The charge-carrier dynamics indicated that the sub-band-gap exciton trapping processes were dominated by ultrafast (∼1–2 ps) intrinsic self-trapping and trapping at surface defects (∼50–100 ps). While trapping at surface defects can be passivated using surfactants, the self-trapping processes is due to the giant carrier–phonon coupling effect. By designing direct band gap double perovskite NCs to tune the sub-band-gap trapping processes, bright dual-color emission was achieved. Furthermore, the violet PLQE could be improved to 36.6%, which is comparable to that in lead halide perovskite NCs. We hope this Account will deepen the understanding of the charge-carrier dynamics in lead-free perovskite NCs and guide the design of high-performance lead-free perovskites.

Synthesis, Crystal Structure and Photoelectric Response of All‐Inorganic Copper Halide Salts CsCuCl3

“…Unfortunately, we didn't detect the signal of photoluminescence for CsCuCl 3 . According to previous literature,, strong exciton‐phonon coupling induced intrinsic self‐trapping and surface defects trapping mainly dominated the charge‐carrier trapping processes. Thus, we carried out the morphology of CsCuCl 3 single crystal as shown in Figure S1.…”

“…High surface defects trapping are observed. According to previous literature,, passivating surface traps and tuning the self‐trapped excitons from “dark” (nonradiative) to “bright” (radiative) can increase photoluminescence quantum efficiency. We will learn to use this method in our future work.…”

Lead‐Free, Water‐Stable A3Bi2I9 Perovskites: Crystal Growth and Blue‐Emitting Quantum Dots [A=CH3NH3+, Cs+, and (Rb0.05Cs2.95)+]

“…In this regard, Cs 3 Bi 2 X 9 ,R b 3 Bi 2 X 9 ,M A 3 Bi 2 X 9 ,a nd doublep erovskites of Cs 2 AgBiX 6 are the successful examples. [13][14][15][16][17][18][19][20][21][22][23][24][25][26] The key successi st he high stability towards moisture, and low toxicity in comparison to Pb-based perovskites, butt hey exhibit poor PCEs in photovoltaic devices owing to the high effective numbers of charge carriers. Moreover,A 3 Bi 2 X 9 perovskite hast wo metal centers in the crystallographic unit cell with al arger space than that of APbX 3 perovskite and, hence,possible novel properties for variousa pplications with the replacement of ions can be explored.…”

“…Therefore, the development of an ew class of Bi materials in the perovskite family is extremely important.R ecently,c hloride-and bromide-containing Bi perovskites have been sufficiently explored in the literature, including single crystals( SCs) and nanocrystals (NCs) of organic-inorganic, [13,14] all-inorganic, [15][16][17][18][19][20] and double perovskites. [21][22][23][24][25][26][27][28][29] In contrast, iodide-basedB ip erovskites have not been studied much. [14,[17][18][19] In the current study,w eh ave developed SCs and NCs of A 3 Bi 2 I 9 perovskites and investigated their photophysical properties such as band gap (BG), lifetime,a nd photoluminescence quantum yield (PLQY).…”

Synthesis of Mn-doped CsPbClxBr3−x perovskite nanocrystals using ultrasonic irradiation-promoted with decrease of reaction order

“…Doping is a good way of introducing various novel functionalities [8,[11][12][13][14][15][16][17][18]. Charge-carrier dynamics of lead halide perovskite NCs and Mn-doped perovskite NCs have been investigated [19][20][21][22]. There are two methods to introduce impurity into CsPbX 3 NCs.…”