Jian Qing

Remanagement of Singlet and Triplet Excitons in Single‐Emissive‐Layer Hybrid White Organic Light‐Emitting Devices Using Thermally Activated Delayed Fluorescent Blue Exciplex

A high-performance hybrid white organic light-emitting device (WOLED) is demonstrated based on an efficient novel thermally activated delayed fluorescence (TADF) blue exciplex system. This device shows a low turn-on voltage of 2.5 V and maximum forward-viewing external quantum efficiency of 25.5%, which opens a new avenue for achieving high-performance hybrid WOLEDs with simple structures.

Chlorine Incorporation for Enhanced Performance of Planar Perovskite Solar Cell Based on Lead Acetate Precursor

We show the effects of chlorine incorporation in the crystallization process of perovskite film based on a lead acetate precursor. We demonstrate a fabrication process for fast grain growth with highly preferred {110} orientation upon only 5 min of annealing at 100 °C. By studying the correlation between precursor composition and morphology, the growth dynamic of perovskite film in the current system is discussed. In particular, we found that both lead acetate precursor and Cl incorporation are beneficial to perovskite growth. While lead acetate allows fast crystallization process, Cl improves perovskite crystallinity. Planar perovskite solar cells with optimized parameters deliver a best power conversion efficiency of 15.0% and average efficiency of 14.0% with remarkable reproducibility and good stability.

Pyrite FeS2 microspheres wrapped by reduced graphene oxide as high-performance lithium-ion battery anodes

A composite of pyrite FeS2 microspheres wrapped by reduced graphene oxide (FeS2/rGO) has been synthesized by a facile one-step solvothermal method and applied as an anode in lithium ion batteries (LIBs). Impedance measurements and transmission electron microscopy show that incorporation of rGO significantly decreases the charge transfer resistance and improves the structural stability of the composite. As an anode material for LIBs, the composite exhibits a high capacity of 970 mA h g−1 at a current density of 890 mA g−1 after 300 cycles. Additionally, this composite anode shows impressive performance especially at high current densities. The LIB shows a capacity of 380 mA h g−1 even at a high current density of 8900 mA g−1 (10C) over 2000 cycles, demonstrating its potential for applications in LIBs with long cycling life and high power density.

Spectroscopic study on the impact of methylammonium iodide loading time on the electronic properties in perovskite thin films

Solution processed metal–organic halide perovskite photovoltaic devices have recently drawn tremendous attention due to their simplicity of fabrication and high efficiency. Despite numerous reports on optimizing perovskite films with different fabrication approaches, there is limited understanding on the correlation between sensitive processing conditions and the microstructural and electronic properties of perovskite films. Here we combine several opto-electrical spectroscopy techniques to investigate the methylammonium iodide (MAI) loading time effect on the doping density profile and uncoordinated ions in resulting CH3NH3PbI3 perovskite thin films. We find that even in a very short period of different loading times within two minutes, there is a significant impact on the device power conversion efficiency (PCE) from 2% to over 15%. It is found that the doping density profile is inhomogeneous across the perovskite film with too short MAI loading time, resulting in an S-shape in the current density–voltage (J–V) characteristics. On the other hand, devices with too long loading time have excess uncoordinated ions attributed to the J–V hysteresis. By using combined spectroscopy techniques to pinpoint the electronic properties in perovskite films, this work would shed light on the understanding of the controversial origins of the reported S-shape and hysteresis in perovskite photovoltaic cells.

Bipolar Resistance Switching in Transparent ITO/LaAlO3/SrTiO3 Memristors

We report reversible bipolar resistance switching behaviors in transparent indium-tin oxide (ITO)/LaAlO3/SrTiO3 memristors at room temperature. The memristors exhibit high optical transparency, long retention, and excellent antifatigue characteristics. The high performances are promising for employing ITO/LaAlO3/SrTiO3 memristors in nonvolatile transparent memory and logic devices. The nonvolatile resistance switching behaviors could be attributed to the migration of positively charged oxygen vacancies from the SrTiO3 substrate to the LaAlO3 film, resulting in Poole-Frenkel emission for the low resistance state and thermionic emission for the high resistance state.

The detrimental effect of excess mobile ions in planar CH3NH3PbI3 perovskite solar cells

The origin of the impact of mobile ions in perovskite solar cells (PVSCs) has recently become a hot topic of debate. Here, we investigate systematically the structural effect and various recombination pathways in PVSCs with different ion concentrations. By probing the transient ionic current in PVSCs, we extract mobile ion concentrations in a range of 1016 cm−3 to 1017 cm−3 depending on the processing conditions during a two-step process. The PVSC with the lowest ion concentration has both the highest efficiency over 15% and shelf-life over 1300 hours. Interestingly, in contrast to the commonly adopted models in the literature, we find that the crystal size and the bimolecular and trap-assisted recombination are not responsible for the large difference in photovoltaic performance. Instead, by using transient photocurrent and steady-state photoluminescence approaches, we find that the large reduction of short-circuit current (Jsc) in mobile ion populated devices is ascribed to the slow decay in photocurrent and the increasing amount of non-radiative recombination. In addition, we also find that the excess mobile ions trigger the deformation of perovskite to PbI2, which severely reduces the device lifetime. The results provide valuable information on the understanding of the role of excess mobile ions in the degradation mechanism of PVSCs.

Effects of Small Polar Molecules (MA+ and H2O) on Degradation Processes of Perovskite Solar Cells

Degradation mechanisms of methylammonium lead halide perovskite solar cells (PSCs) have drawn much attention recently. Herein, the bulk and surface degradation processes of the perovskite were differentiated for the first time by employing combinational studies using electrochemical impedance spectroscopy (EIS), capacitance frequency (CF), and X-ray diffraction (XRD) studies with particular attention on the roles of small polar molecules (MA+ and H2O). CF study shows that short-circuit current density of the PSCs is increased by H2O at the beginning of the degradation process coupled with an increased surface capacitance. On the basis of EIS and XRD analysis, we show that the bulk degradation of PSCs involves a lattice expansion process, which facilitates MA+ ion diffusion by creating more efficient channels. These results provide a better understanding of the roles of small polar molecules on degradation processes in the bulk and on the surface of the perovskite film.

Heat Treatment for Regenerating Degraded Low-Dimensional Perovskite Solar Cells

Organolead halide perovskite devices are reported to be susceptible to thermal degradation, which results from heat-induced fast ion diffusion and structural decomposition. In this work, it is found that the performances of degraded low-dimensional perovskite solar cells can be considerably improved (e.g., power conversion efficiency shows ∼10% increase over the fresh device) by a short-time heat treatment (85 °C, 3 min). Capacitance-frequency, X-ray diffraction, and ionic diffusion calculation results suggest that heat treatment can enhance the crystallinity of the degraded low-dimensional perovskite and minimize the detrimental effects caused by water molecules, leading to improved performances. Our results indicate that the heat treatment does not necessarily lead to the accelerated degradation but can also regenerate the degraded low-dimensional perovskite.

Direct Free Carrier Photogeneration in Single Layer and Stacked Organic Photovoltaic Devices

High performance organic photovoltaic devices typically rely on type-II P/N junctions for assisting exciton dissociation. Heremans and co-workers recently reported a high efficiency device with a third organic layer which is spatially separated from the active P/N junction; but still contributes to the carrier generation by passing its energy to the P/N junction via a long-range exciton energy transfer mechanism. In this study the authors show that there is an additional mechanism contributing to the high efficiency. Some bipolar materials (e.g., subnaphthalocyanine chloride (SubNc) and subphthalocyanine chloride (SubPc)) are observed to generate free carriers much more effectively than typical organic semiconductors upon photoexcitation. Single-layer devices with SubNc or SubPc sandwiched between two electrodes can give power conversion efficiencies 30 times higher than those of reported single-layer devices. In addition, internal quantum efficiencies (IQEs) of bilayer devices with opposite stacking sequences (i.e., SubNc/SubPc vs SubPc/SubNc) are found to be the sum of IQEs of single layer devices. These results confirm that SubNc and SubPc can directly generate free carriers upon photoexcitation without assistance from a P/N junction. These allow them to be stacked onto each other with reversible sequence or simply stacking onto another P/N junction and contribute to the photocarrier generation.

Organic Light-Emitting Devices: Remanagement of Singlet and Triplet Excitons in Single-Emissive-Layer Hybrid White Organic Light-Emitting Devices Using Thermally Activated Delayed Fluorescent Blue Exciplex (Adv. Mater. 44/2015)

On page 7079, X.-H. Zhang, C.-S. Lee, and co-workers discuss the drawbacks of single-emitting-layer hybrid white organic light-emitting devices (WOLEDs) based on conventional fluorescent hosts, and demonstrate an effective way to overcome these drawbacks by using the thermally activated delayed fluorescent blue exciplex. Based on this new concept, high-efficiency WOLEDs with a forward-viewing power efficiency of 48.7 lm W(-1) at 100 cd m(-2) are achieved.