Bright Tail States in Blue-Emitting Ultrasmall Perovskite Quantum Dots

Subscriber access provided by READING UNIV

Letter

Bright Tail States in Blue-Emitting Ultrasmall Perovskite Quantum Dots Jing Li, Lu Gan, Zhishan Fang, Haiping He, and Zhizhen Ye J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02786 • Publication Date (Web): 01 Dec 2017 Downloaded from http://pubs.acs.org on December 2, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry Letters

Bright Tail States in Blue-Emitting Ultrasmall Perovskite Quantum Dots Jing Li, Lu Gan, Zhishan Fang, Haiping He* and Zhizhen Ye* State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China. Corresponding Author *

[email protected] (H. He)

*

[email protected] (Z. Ye)

ACS Paragon Plus Environment

1

The Journal of Physical Chemistry Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 25

ABSTRACT

All-inorganic lead halide perovskite quantum dots (CsPbBr3 QDs) are attracting significant research interests because of their highly efficient light-emitting performance combined with tunable emission wavelength facilely realized by ion exchange. However, blue emission from perovskite QDs with strong quantum confinement is rarely reported and suffers from lower luminescence efficiency. Here we report blue-emitting ultrasmall (~3 nm) CsPbBr3 QDs with photoluminescence (PL) quantum yield as high as 68%. Using time-resolved and steady-state PL spectroscopy, we elucidate the mechanism of the highly efficient PL as recombination of excitons localized in radiative band tail states. Through analyzing the spectral-dependent PL lifetime and the PL lineshape, we obtain a large band tail width of ~80 meV and a high density of state of ~1020 cm-3. The relaxation of photocarriers into the radiative tail states suppresses the capture by nonradiative centers. Our results provide solid evidences for the positive role of band tail states in the optical properties of lead halide perovskites, which can be further tailored for high-performance optoelectronic devices.

TOC GRAPHICS


2

Page 3 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Recently, colloidal lead halide perovskite quantum dots (QDs) have emerged as a class of promising materials for solution-processed light-emitting devices owing to their high (up to 90%) photoluminescence quantum yield (PLQY) and tunable emission wavelength emitting diodes (LEDs) with competitive external quantum efficiency lasing with low threshold

2, 9

7, 8

1-6

. Light-

and optically pumped

using the perovskite QDs have been demonstrated. However, most

studies focused on the green, red, and near infrared emissions, while blue-emitting perovskite QDs were less studied. To reach the blue spectral region, either changing the QDs composition or taking advantage of the quantum confinement effect (QCE) can be effective. For perovskite QDs, so far the former is most frequently adopted because ion exchange has proven facile in this system 1. Lead bromide perovskite is usually used as the starting material for partial anion exchange with Cl-

1, 2

or for cation exchange with Bi3+

10

and M2+ (M=Sn, Cd, Zn)

11

to extend

the emission wavelength from 530 nm to 450 nm or even shorter. On the other hand, studies on reducing the QDs size from ~10 nm (referred as large QDs) down into the strong QCE region, i.e., much smaller than the exciton Bohr radius (~7 nm) of lead bromide perovskites 1, are quite few. Ultrasmall perovskite QDs with strong QCE not only enable the wavelength tuning toward blue emission, but also offer a workbench for studying the unique optical properties of CsPbX3 perovskites in low dimensions. In this regard, it is necessary to grow ultrasmall CsPbX3 QDs. Strongly blue-emitting CsPbBr3 nanowires with width down to the quantum confinement regime has been previously reported

12

. Very recently, blue-emitting MAPbBr3 and CsPbBr3 QDs with

size of 1.8-5.5 nm were obtained by controlling the reaction parameters during colloidal synthesis or confining the QDs in porous template

13-19

. Interestingly, although the ultrasmall

perovskite QDs are expected to show much lower PLQY than the large QDs due to the higher


3


density of surface states, there were some works

15, 16

Page 4 of 25

reporting reasonably high PLQY up to

~70%. However, the mechanism of high PLQY in such small perovskite QDs is not fully understood. The low PLQY of colloidal QDs is usually attributed to trap states in the surface region 21

20,

. In this scenario, photocarriers are trapped by these sub- or mid-gap defect states, in which they

recombine nonradiatively. Accordingly, surface passivation by organic ligands, e.g., oleic acid (OA) or oleylamine (OLA) for perovskite QDs, which eliminates these surface states, is usually invoked to interpret the improved PLQY 6, 22. However, it should be noted that the recombination through trap states is not always nonradiative. A famous example is the compositional fluctuation-induced localized states present in ternary III-V or II-VI semiconductors

23, 24

.

Photocarriers trapped in such localized states can recombine radiatively and escape from being captured by nonradiative defects, which favors high PLQY 25. Similar localized states were also observed in solution-processed lead halide perovskite films

26, 27

. In this regard, it is critical to

understand the nature of trap states as well as the photophysics of photocarriers in ultrasmall perovskite QDs, which is highly instructive for achieving highly efficient blue emission. Here we report ultrasmall (~3 nm) CsPbBr3 colloidal QDs emitting around 460 nm with room temperature PLQY as high as 68%. We provide spectroscopic evidences that the high PLQY is closely allied to the presence of high density but emissive band tail states. The localization of excitons in the tail states also suppresses the possibility of being captured by nonradiative traps, thus eliminating the photo-bleaching of PL. The ultrasmall perovskite QDs were synthesized through a simple one-pot reaction (see the Supporting Information for details). Briefly, Cs2CO3, Pb(CH3COO)2·3H2O, 1-octadecene


4

Page 5 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(ODE), OA, OLA, and 1-Bromododecane (C12H25Br) were mixed and heated to 143 °C in ambient N2. After reaction, the crude solution was centrifuged twice and the products were dispersed in hexane for characterization. Compared with the method commonly used to synthesize CsPbBr3 QDs 1, our method is very convenient, for it is no need to prepare Cs-oleate precursor solution additionally, and the whole experiment took less than half an hour. The X-ray diffraction (XRD) pattern (Fig. 1a) of the sample shows a series of peaks that can be indexed to cubic CsPbBr3 1. As shown in Fig. 1b and 1c, transmission electron microscopy(TEM) observations illustrate that the perovskite QDs have an average diameter of 2.7 nm with narrow size distribution (± 0.7 nm). High-resolution TEM image as well as its FFT pattern (Fig. 1d) reveals well-resolved lattice fringes corresponding to the (220) and (310) planes, respectively, of cubic CsPbBr3. The structural analysis reveals a pure CsPbBr3 phase, and no secondary phase such as elemental Pb was detected in the sample. This is also supported by the chemical states of Pb4f in X-ray photoelectron spectra (Fig. S1), where the signal of metallic Pb at 136.8 eV 28 was not detected.


5


Page 6 of 25

Figure 1. Fundamental characterization of the ultrasmall perovskite QDs. a) XRD pattern. b) TEM image. Scale bar: 10 nm. (c) Histogram of the QDs showing the distributions of size. (d) A typical HRTEM image of the QD. Scale bar: 3 nm. Inset is the corresponding FFT pattern.

Figure 2. (a) Room temperature optical absorption and photoluminescence of the ultrasmall perovskite QDs. Inset shows the photographs of the QDs solution with and without UV light (λ=365 nm) illumination. (b) Logarithm plot of the integrated PL intensity versus excitation density for the ultrasmall perovskite QDs. The data show good power-law dependence with k=1.01. (c) Temperature dependence of the measured PL lifetime (τPL) and extracted radiative lifetime (τr) of the ultrasmall perovskite QDs. The radiative lifetime is constant when T

Bright Tail States in Blue-Emitting Ultrasmall Perovskite Quantum Dots

Recommend Documents