PbS Capped CsPbI3 Nanocrystals for Efficient and Stable Light-Emitting Devices Using p-i-n Structures.
ABSTRACT: Cesium lead halide perovskite nanocrystals (NCs) have unique optical properties such as high color purity and high photoluminescence (PL) efficiency. However, the external quantum efficiency (EQE) of the corresponding light-emitting diodes (LEDs) is low, primarily as a result of the NC surface defects. Here, we report a method to reduce the surface defects by capping CsPbI3 NCs with PbS. This passivation significantly enhanced the PL efficiency, reduced the Stokes shift, narrowed the PL bandwidth, and increased the stability of CsPbI3 NCs. At the same time, CsPbI3 NC films switched from n-type behavior to nearly ambipolar by PbS capping, which allowed us to fabricate electroluminescence LEDs using p-i-n structures. The thus-fabricated LEDs exhibited dramatically improved storage and operation stability, and an EQE of 11.8%. These results suggest that, with a suitable surface passivation strategy, the perovskite NCs are promising for next-generation LED and display applications.
Project description:Cesium lead halide perovskite nanocrystals (NCs) possess excellent optical properties at visible wavelengths with great promise for applications in luminous display fields. We demonstrate a method to modify the surface ligand passivation of perovskite NCs for enhanced colloidal stability and emitting properties by incorporating didodecyl dimethyl ammonium bromide (DDAB). The photoluminescence quantum yield of the NC solution was improved to 96% from 70% and the perovskite film showed fewer trapped sites and enhanced carrier transport ability. The thus fabricated electroluminescent perovskite NC-LEDs exhibited a bright luminance of 11?990 cd m-2, corresponding to 4-times improved external quantum efficiency (EQE), compared to the control device using regular NCs without DDAB.
Project description:Surface engineering has been shown critical for the success of perovskite solar cells by passivating the surface enriched defects and mobile species. The discovery of surface modulators with superior interaction strength to perovskite is of paramount importance since they can retain reliable passivation under various environments. Here, we report a chelation strategy for surface engineering of CsPbI<sub>2</sub>Br perovskite, in which dithiocarbamate molecules can be coordinate to surface Pb sites via strong bidentate chelating bonding. Such chelated CsPbI<sub>2</sub>Br perovskite can realize excellent passivation of surface under-coordinated defects, reaching a champion power conversion efficiency of 17.03% and an open-circuit voltage of 1.37?V of CsPbI<sub>2</sub>Br solar cells. More importantly, our chelation strategy enabled excellent device stability by maintaining 98% of their initial efficiency for over 1400?h in ambient condition. Our findings provide scientific insights on the surface engineering of perovskite that can facilitate the further development and application of perovskite optoelectronics.
Project description:This paper reports a new method to generate stable and high-brightness electroluminescence (EL) by subsequently growing large/small grains at micro/nano scales with the configuration of attaching small grains on the surfaces of large grains in perovskite (MAPbBr3) films by mixing two precursor solutions (PbBr2 + MABr and Pb(Ac)2·3H2O + MABr). Consequently, the small and large grains serve, respectively, as passivation agents and light-emitting centers, enabling self-passivation on the defects located on the surfaces of light-emitting large grains. Furthermore, the light-emitting states become linearly polarized with maximal polarization of 30.8%, demonstrating a very stable light emission (49,119 cd/m2 with EQE = 11.31%) and a lower turn-on bias (1.9 V) than the bandgap (2.25V) in the perovskite LEDs (ITO/PEDOT:PSS/MAPbBr3/TPBi[50 nm]/LiF[0.7 nm]/Ag). Therefore, mixing large/small grains with the configuration of attaching small grains on the surfaces of large grains by mixing two precursor solutions presents a new strategy to develop high-performance perovskite LEDs.
Project description:Caesium lead halide perovskites were recently demonstrated to be a relevant class of semiconductors for photonics and optoelectronics. Unlike CsPbBr<sub>3</sub> and CsPbI<sub>3</sub>, the realization of high-quality thin films of CsPbCl<sub>3</sub>, particularly interesting for highly efficient white LEDs when coupled to converting phosphors, is still a very demanding task. In this work we report the first successful deposition of nanocrystalline CsPbCl<sub>3</sub> thin films (70-150 nm) by radio frequency magnetron sputtering on large-area substrates. We present a detailed investigation of the optical properties by high resolution photoluminescence (PL) spectroscopy, resolved in time and space in the range 10-300 K, providing quantitative information concerning carriers and excitons recombination dynamics. The PL is characterized by a limited inhomogeneous broadening (~15 meV at 10 K) and its origin is discussed from detailed analysis with investigations at the micro-scale. The samples, obtained without any post-growth treatment, show a homogeneous PL emission in spectrum and intensity on large sample areas (several cm<sup>2</sup>). Temperature dependent and time-resolved PL spectra elucidate the role of carrier trapping in determining the PL quenching up to room temperature. Our results open the route for the realization of large-area inorganic halide perovskite films for photonic and optoelectronic devices.
Project description:Abstract 2D transition metal carbides, nitrides, and carbonitrides called MXenes show outstanding performance in many applications due to their superior physical and chemical properties. Herein, a ZnO–MXene mixture with different contents of Ti3C2 is applied as electron transport layers (ETLs) and the influence of the Ti3C2 MXene in all?inorganic metal halide perovskite nanocrystal light?emitting diodes (perovskite NC LEDs) is explored. The addition of Ti3C2 makes more balanced charge carrier transport in LEDs by changing the energy level structure and electron mobility of ETL. Moreover, lower surface roughness is obtained for the ETL, thus guaranteeing uniform distribution of the perovskite NCs layer and further reducing leakage current. As a result, a 17.4% external quantum efficiency (EQE) with low efficiency roll?off is achieved with 10% Ti3C2, which is a 22.5% improvement compared to LEDs without Ti3C2. Adding an appropriate amount of Ti3C2 into the ZnO electron transport layer (ETL) to change the energy level structure and electron mobility of the ETL and further make the carrier transport balanced is explored. The ZnO/TiC ETL?based red emitting perovskite nanocrystals light?emitting diodes exhibit an external quantum efficiency of 17.4%.
Project description:Advances in optoelectronics require materials with novel and engineered characteristics. A class of materials that has garnered tremendous interest is metal-halide perovskites, stimulated by meteoric increases in photovoltaic efficiencies of perovskite solar cells. In addition, recent advances have applied perovskite nanocrystals (NCs) in light-emitting devices. It was found recently that, for cesium lead-halide perovskite NCs, their unusually efficient light emission may be due to a unique excitonic fine structure composed of three bright triplet states that minimally interact with a proximal dark singlet state. To study this fine structure without isolating single NCs, we use multidimensional coherent spectroscopy at cryogenic temperatures to reveal coherences involving triplet states of a CsPbI<sub>3</sub> NC ensemble. Picosecond time scale dephasing times are measured for both triplet and inter-triplet coherences, from which we infer a unique exciton fine structure level ordering composed of a dark state energetically positioned within the bright triplet manifold.
Project description:We show here the first colloidal synthesis of double perovskite Cs<sub>2</sub>AgInCl<sub>6</sub> nanocrystals (NCs) with a control over their size distribution. In our approach, metal carboxylate precursors and ligands (oleylamine and oleic acid) are dissolved in diphenyl ether and reacted at 105 °C with benzoyl chloride. The resulting Cs<sub>2</sub>AgInCl<sub>6</sub> NCs exhibit the expected double perovskite crystal structure, are stable under air, and show a broad spectrum white photoluminescence (PL) with quantum yield of ?1.6 ± 1%. The optical properties of these NCs were improved by synthesizing Mn-doped Cs<sub>2</sub>AgInCl<sub>6</sub> NCs through the simple addition of Mn-acetate to the reaction mixture. The NC products were characterized by the same double perovskite crystal structure, and Mn doping levels up to 1.5%, as confirmed by elemental analyses. The effective incorporation of Mn ions inside Cs<sub>2</sub>AgInCl<sub>6</sub> NCs was also proved by means of electron spin resonance spectroscopy. A bright orange emission characterized our Mn-doped Cs<sub>2</sub>AgInCl<sub>6</sub> NCs with a PL quantum yield as high as ?16 ± 4%.
Project description:Functional CsPbI<sub>3</sub> perovskite phases are not stable at ambient conditions and spontaneously convert to a non-perovskite ? phase, limiting their applications as solar cell materials. We demonstrate the preservation of a black CsPbI<sub>3</sub> perovskite structure to room temperature by subjecting the ? phase to pressures of 0.1 - 0.6?GPa followed by heating and rapid cooling. Synchrotron X-ray diffraction and Raman spectroscopy indicate that this perovskite phase is consistent with orthorhombic ?-CsPbI<sub>3</sub>. Once formed, ?-CsPbI<sub>3</sub> could be then retained after releasing pressure to ambient conditions and shows substantial stability at 35% relative humidity. First-principles density functional theory calculations indicate that compression directs the out-of-phase and in-phase tilt between the [PbI<sub>6</sub>]<sup>4-</sup> octahedra which in turn tune the energy difference between ?- and ?-CsPbI<sub>3</sub>, leading to the preservation of ?-CsPbI<sub>3</sub>. Here, we present a high-pressure strategy for manipulating the (meta)stability of halide perovskites for the synthesis of desirable phases with enhanced materials functionality.
Project description:Lead-free halide perovskite nanocrystals (NCs) have recently attracted attention due to their nontoxicity and stability as alternatives to lead-based perovskite NCs. Here, we report undoped and manganese-doped all-inorganic, lead-free double perovskite (DP) NCs: Cs<sub>2</sub>NaIn <sub><i>x</i></sub> Bi<sub>1-<i>x</i></sub> Cl<sub>6</sub> (0 < <i>x</i> < 1) and Cs<sub>2</sub>NaIn <sub><i>x</i></sub> Bi<sub>1-<i>x</i></sub> Cl<sub>6</sub>:Mn (0 ? <i>x</i> ? 1) NCs. Undoped NCs exhibit blue emission. Through doping Mn<sup>2+</sup> ions into Cs<sub>2</sub>NaIn <sub><i>x</i></sub> Bi<sub>1-<i>x</i></sub> Cl<sub>6</sub> NCs, we can avoid self-trapped exciton emission and realize bright orange red emission of Mn<sup>2+</sup> dopants with the highest photoluminescence quantum efficiency of 44.6%. The photoluminescence (PL) is tunable from yellow emission to orange-red emission corresponding to a red shift from 583 to 614 nm with increasing In content. Interestingly, the PL emission of Mn-doped NCs shows a red shift from the bulk size to the nanoscale. The Mn-doped NCs show good stability in air. In addition, we further prove the process of dark self-trapped state-assisted Mn<sup>2+</sup> emission in DP NCs by ultrafast transient absorption techniques.
Project description:Colloidal lead halide perovskite nanocrystals (NCs) have recently emerged as versatile photonic sources. Their processing and optoelectronic applications are hampered by the loss of colloidal stability and structural integrity due to the facile desorption of surface capping molecules during isolation and purification. To address this issue, herein, we propose a new ligand capping strategy utilizing common and inexpensive long-chain zwitterionic molecules such as 3-(N,N-dimethyloctadecylammonio)propanesulfonate, resulting in much improved chemical durability. In particular, this class of ligands allows for the isolation of clean NCs with high photoluminescence quantum yields (PL QYs) of above 90% after four rounds of precipitation/redispersion along with much higher overall reaction yields of uniform and colloidal dispersible NCs. Densely packed films of these NCs exhibit high PL QY values and effective charge transport. Consequently, they exhibit photoconductivity and low thresholds for amplified spontaneous emission of 2 ?J cm-2 under femtosecond optical excitation and are suited for efficient light-emitting diodes.