All-Inorganic Quantum-Dot LEDs Based on a Phase-Stabilized α-CsPbI3 Perovskite.

The all-inorganic nature of CsPbI3 perovskites allows to enhance stability in perovskite devices. Research efforts have led to improved stability of the black phase in CsPbI3 films; however, these strategies-including strain and doping-are based on organic-ligand-capped perovskites, which prevent perovskites from forming the close-packed quantum dot (QD) solids necessary to achieve high charge and thermal transport. We developed an inorganic ligand exchange that leads to CsPbI3 QD films with superior phase stability and increased thermal transport. The atomic-ligand-exchanged QD films, once mechanically coupled, exhibit improved phase stability, and we link this to distributing strain across the film. Operando measurements of the temperature of the LEDs indicate that KI-exchanged QD films exhibit increased thermal transport compared to controls that rely on organic ligands. The LEDs exhibit a maximum EQE of 23 % with an electroluminescence emission centered at 640 nm (FWHM: ≈31 nm). These red LEDs provide an operating half-lifetime of 10 h (luminance of 200 cd m-2 ) and an operating stability that is 6× higher than that of control devices.

All-Inorganic Germanium Nanocrystal Films by Cationic Ligand Exchange.

We introduce a new paradigm for group IV nanocrystal surface chemistry based on room temperature surface activation that enables ionic ligand exchange. Germanium nanocrystals synthesized in a gas-phase plasma reactor are functionalized with labile, cationic alkylammonium ligands rather than with traditional covalently bound groups. We employ Fourier transform infrared and (1)H nuclear magnetic resonance spectroscopies to demonstrate the alkylammonium ligands are freely exchanged on the germanium nanocrystal surface with a variety of cationic ligands, including short inorganic ligands such as ammonium and alkali metal cations. This ionic ligand exchange chemistry is used to demonstrate enhanced transport in germanium nanocrystal films following ligand exchange as well as the first photovoltaic device based on an all-inorganic germanium nanocrystal absorber layer cast from solution. This new ligand chemistry should accelerate progress in utilizing germanium and other group IV nanocrystals for optoelectronic applications.

Inorganic Ligand Thiosulfate-Capped Quantum Dots for Efficient Quantum Dot Sensitized Solar Cells.

The insulating nature of organic ligands containing long hydrocarbon tails brings forward serious limitations for presynthesized quantum dots (QDs) in photovoltaic applications. Replacing the initial organic hydrocarbon chain ligands with simple, cheap, and small inorganic ligands is regarded as an efficient strategy for improving the performance of the resulting photovoltaic devices. Herein, thiosulfate (S2O32-), and sulfide (S2-) were employed as ligand-exchange reagents to get access to the inorganic ligand S2O32-- and S2--capped CdSe QDs. The obtained inorganic ligand-capped QDs, together with the initial oleylamine-capped QDs, were used as light-absorbing materials in the construction of quantum dot sensitized solar cells (QDSCs). Photovoltaic results indicate that thiosulfate-capped QDs give excellent power conversion efficiency (PCE) of 6.11% under the illumination of full one sun, which is remarkably higher than those of sulfide- (3.36%) and OAm-capped QDs (0.84%) and is comparable to the state-of-the-art value based on mercaptocarboxylic acid capped QDs. Photoluminescence (PL) decay characterization demonstrates that thiosulfate-based QDSCs have a much-faster electron injection rate from QD to TiO2 substrate in comparison with those of sulfide- and OAm-based QDSCs. Electrochemical impedance spectroscopy (EIS) results indicate that higher charge-recombination resistance between potoanode and eletrolyte interfaces were observed in the thiosulfate-based cells. To the best of our knowledge, this is the first application of thiosulfate-capped QDs in the fabrication of efficient QDSCs. This will lend a new perspective to boosting the performance of QDSCs furthermore.