We developed a colloidal synthesis of CsPbBr perovskite nanocrystals (NCs) at a relative low temperature (90 °C) for the bright blue emission which differs from the original green emission (∼510 nm) of CsPbBr nanocubes as reported previously. Shapes of the obtained CsPbBr NCs can be systematically engineered into single and lamellar-structured 0D quantum dots, as well as face-to-face stacking 2D nanoplatelets and flat-lying 2D nanosheets via tuning the amounts of oleic acid (OA) and oleylamine (OM). They exhibit sharp excitonic PL emissions at 453, 472, 449, and 452 nm, respectively. The large blue shift relative to the emission of CsPbBr bulk crystal can be ascribed to the strong quantum confinement effects of these various nanoshapes. PL decay lifetimes are measured, ranging from several to tens of nanoseconds, which infers the higher ratio of exciton radiative recombination to the nonradiative trappers in the obtained CsPbBr NCs. These shape-controlled CsPbBr perovskite NCs with the bright blue emission will be widely used in optoelectronic applications, especially in blue LEDs which still lag behind compared to the better developed red and green LEDs.

Because of the superior optical properties and potential applications in display technology, colloidal synthesis of halide perovskite quantum dots has been intensively studied. Although great successes have been made in the fabrication of green emissive CH 3 NH 3 PbBr 3 quantum dots, the fabrication of stable iodide-based CH 3 NH 3 PbI 3 quantum dots remains a great challenge because of their sensitivity to moisture in the open air. Even in a glovebox, the colloidal CH 3 NH 3 PbI 3 quantum dots obtained from N,N-dimethylformamide suffer from instability caused by fast degradation within days to weeks. In this work, we investigated the interactions between perovskite precursors and various polar solvents as well as their influence on the crystallization of CH 3 NH 3 PbI 3 in reprecipitation synthesis. By gaining chemical insight into the coordination effects, we can explain the degradation of CH 3 NH 3 PbI 3 to the defective crystals with coordinated solvents on the surface and/or intrinsic inner iodine vacancies. On the basis of this understanding, we fabricated air-stable CH 3 NH 3 PbI 3 quantum dots with a tunable size from 6.6 to 13.3 nm by selecting noncoordinated acetonitrile as a good solvent through ligand-assisted precipitation synthesis. The fabrication can be processed under ambient conditions, and the resulting CH 3 NH 3 PbI 3 quantum dots exhibit tunable emission with high photoluminescence quantum yields (maximum of ∼46%) as well as good stability. Moreover, the quantum confinement effects in CH 3 NH 3 PbI 3 quantum dots were discussed by correlating the size-dependent photoluminescence properties with theoretical calculations, which can be described by the infinite quantum well approximation model.

decades, heuristic approaches have been successfully used to enhance the performance of OSCs, including synthesis of new donor and acceptor molecules, [5][6][7][8][9][10] optimization of the interface and morphology, [11][12][13][14][15] ternary blending, [16][17][18][19] and device engineering. [20][21][22][23] As a result, power conversion efficiency (PCE) of OSCs has surpassed 15% for single-layer bulkheterojunction systems. [24,25] As the PCE of OSCs is getting closer to the requirements for commercial applications and competing technology, the most efficient OSCs also need to perform consistently or maintain low efficiency loss throughout their lifetimes. [26][27][28] To make organic photovoltaics commercially viable and competitive, researchers have been making efforts on characterizing, understanding, and rationally engineering the long-term stability of OSC devices. [26,[29][30][31][32][33][34][35][36][37][38] Generally, the performance degradation of OSCs comes from the oxidation of electrodes, degradation of the interface layers, and changes in the morphology of the active layer. Among these factors, the oxidation of electrodes and the degradation of the interface layers are attributed to exposure to oxygen and moisture, [39,40] and these drawbacks can be largely prevented by encapsulation. [41,42] However, the intrinsic instability of active layer morphology driven by light, temperature, and thermodynamics cannot be prevented by encapsulation. In-depth studies were carried out to understand the stability of the active layers under multiple stresses. [43] For instance, McGehee and co-workers [29] reported that solar cells based on amorphous materials would suffer from open-circuit voltage (V OC ) burnin degradation resulted from the impact of light-induced traps, and this degradation can be reduced by using materials with high degree of crystallinity. Brabec group [33] demonstrated that the light-induced [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) dimerization would lead to the short-circuit current density (J SC ) loss after aging, while this dimerization can be inhibited by a high degree of polymer-fullerene mixing and it can also be reduced via increasing the crystallization of the fullerene domains. Brabec and co-workers [34] found that burnin degradation driven by low miscibility is the major short-time Long device lifetime is still a missing key requirement in the commercialization of nonfullerene acceptor (NFA) organic solar cell technology. Understanding thermodynamic factors driving morphology degradation or stabilization is correspondingly lacking. In this report, thermodynamics is combined with morphology to elucidate the instability of highly efficient PTB7-Th:IEICO-4F binary solar cells and to rationally use PC 71 BM in ternary solar cells to reduce the loss in the power conversion efficiency from ≈35% to <10% after storage for 90 days and at the same time improve performance. The hypomiscibility observed for IEICO-4F in PTB7-Th (below the percolation threshold) leads to overpurific...

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Delayed electroluminescence measurements are used to probe and differentiate between triplet-triplet-annihilation (TTA) and triplet-polaron-quenching (TPQ) processes and their correlation with efficiency roll-off in fac-tris(2-phenylpyridine) iridium-based phosphorescent organic light emitting devices. Investigations on devices employing 4,4′-bis(9-carbazolyl)-1,1′-biphenyl (CBP) and 4,4′,4″-tris(N-carbazolyl) triphenylamine, two widely used host materials, show that the efficiency roll-off is primarily due to TPQ processes. Guest-guest TTA, on the other hand, is found to play no major role, contrary to speculations, especially at low guest concentrations. Evidence of host-host TTA in certain cases, and its possible contribution to exciton quenching in the case of devices with CBP host, is also reported.