Lixia Zhang

High-Performance Hole-Extraction Layer of Sol-Gel-Processed NiO Nanocrystals for Inverted Planar Perovskite Solar Cells**

Hybrid organic/inorganic perovskite solar cells have been rapidly evolving with spectacular successes in both nanostructured and thin-film versions. Herein, we report the use of a simple sol-gel-processed NiO nanocrystal (NC) layer as the hole-transport layer in an inverted perovskite solar cell. The thin NiO NC film with a faceted and corrugated surface enabled the formation of a continuous and compact layer of well-crystallized CH3 NH3 PbI3 in a two-step solution process. The hole-extraction and -transport capabilities of this film interfaced with the CH3 NH3 PbI3 film were higher than those of organic PEDOT:PSS layers. The cell with a NiO NC film with a thickness of 30-40 nm exhibited the best performance, as a thinner layer led to a higher leakage current, whereas a thicker layer resulted in a higher series resistance. With the NiO film, we observed a cell efficiency of 9.11 %, which is by far the highest reported for planar perovskite solar cells based on an inorganic hole-extracting layer.

Cost-efficient clamping solar cells using candle soot for hole extraction from ambipolar perovskites

Ambient-unstable hole transporters and expensive and complicated noble metal electrode deposition are incompatible with the large scale and low-cost production of perovskite solar cells and thus would hamper their commercialization. Herein we report a new modality of perovskite solar cells that do away with the use of conventional hole transporters by directly clamping a selective hole extraction electrode made of candle soot and a deliberately engineered perovskite photoanode. The key soot/perovskite interface, which promotes hole extraction and electron blocking by forming a Schottky junction, was established seamlessly by pre-wetting and reaction embedding the carbon particles. Femtosecond time-resolved photoluminescence revealed a high hole extraction rate at 1.92 ns−1. We have now achieved 11.02% efficiency, making an important step towards roll-to-roll production of perovskite solar cells.

A Quasi-Quantum Well Sensitized Solar Cell with Accelerated Charge Separation and Collection

Semiconductor-sensitized solar cell (SSSC) represents a new generation of device aiming to achieve easy fabrication and cost-effective performance. However, the power of the semiconductor sensitizers has not been fully demonstrated in SSSC, making it actually overshadowed by dye-sensitized solar cell (DSSC). At least part of the problem is related to the inefficient charge separation and severe recombination with the current technologies, which calls on rethinking about how to better engineer the semiconductor sensitizer structure in order to enhance the power conversion efficiency (PCE). Herein we report on using for the first time a quasi-quantum well (QW) structure (ZnSe/CdSe/ZnSe) as the sensitizer, which is quasi-epitaxially deposited on ZnO tetrapods. Such a novel photoanode architecture has attained 6.20% PCE, among the highest reported to date for this type of SSSCs. Impedance spectra have revealed that the ZnSe/CdSe/ZnSe QW structure has a transport resistance only a quarter that of, but a recombination resistance twice that of the ZnSe/CdSe heterojunction (HJ) structure, yielding much longer electron diffusion length, consistent with the resulting higher photovoltage, photocurrent, and fill factor. Time-resolved photoluminescence spectroscopy indicates dramatically reduced electron transfer from ZnO to the QW sensitizer, a feature which is conducive to charge separation and collection. This study together with the impedance spectra and intensity modulated photocurrent spectroscopies supports a core/shell two-channel transport mechanism in this type of solar cells and further suggests that the electron transport along sensitizer can be considerably accelerated by the QW structure employed.

Luminescence signature of free exciton dissociation and liberated electron transfer across the junction of graphene/GaN hybrid structure

Large-area graphene grown on Cu foil with chemical vapor deposition was transferred onto intentionally undoped GaN epilayer to form a graphene/GaN Schottky junction. Optical spectroscopic techniques including steady-state and time-resolved photoluminescence (PL) were employed to investigate the electron transfer between graphene and n-type GaN at different temperatures. By comparing the near-band-edge excitonic emissions before and after the graphene covering, some structures in the excitonic PL spectra are found to show interesting changes. In particular, a distinct “dip” structure is found to develop at the center of the free exciton emission peak as the temperature goes up. A mechanism that the first dissociation of some freely moveable excitons at the interface was followed by transfer of liberated electrons over the junction barrier is proposed to interpret the appearance and development of the “dip” structure. The formation and evolution process of this “dip” structure can be well resolved from the measured time-resolved PL spectra. First-principles simulations provide clear evidence of finite electron transfer at the interface between graphene and GaN.

Solution-Processed, Barrier-Confined, and 1D Nanostructure Supported Quasi-quantum Well with Large Photoluminescence Enhancement

Planar substrate supported semiconductor quantum well (QW) structures are not amenable to manipulation in miniature devices, while free-standing QW nanostructures, e.g., ultrathin nanosheets and nanoribbons, suffer from mechanical and environmental instability. Therefore, it is tempting to fashion high-quality QW structures on anisotropic and mechanically robust supporting nanostructures such as nanowires and nanoplates. Herein, we report a solution quasi-heteroepitaxial route for growing a barrier-confined quasi-QW structure (ZnSe/CdSe/ZnSe) on the supporting arms of ZnO nanotetrapods, which have a 1D nanowire structure, through the combination of ion exchange and successive deposition assembly. This resulted in highly crystalline and highly oriented quasi-QWs along the whole axial direction of the arms of the nanotetrapod because a transition buffer layer (Zn(x)Cd(1-x)Se) was formed and in turn reduced the lattice mismatch and surface defects. Significantly, such a barrier-confined QW emits excitonic light ∼17 times stronger than the heterojunction (HJ)-type structure (ZnSe/CdSe, HJ) at the single-particle level. Time-resolved photoluminescence from ensemble QWs exhibits a lifetime of 10 ns, contrasting sharply with ∼300 ps for the control HJ sample. Single-particle PL and Raman spectra suggest that the barrier layer of QW has completely removed the surface trap states on the HJ and restored or upgraded the photoelectric properties of the semiconductor layer. Therefore, this deliberate heteroepitaxial growth protocol on the supporting nanotetrapod has realized a several micrometer long QW structure with high mechanical robustness and high photoelectric quality. We envision that such QWs integrated on 1D nanostructures will largely improve the performance of solar cells and bioprobes, among others.

Magnetic Field Effect on Photocurrent in Single Layer Organic Semiconductor Devices

We systematically studied the magnetic field effect on photocurrent (MPC) in ITO/N,N′‐di(naphthalene‐1‐yl)‐N,N′‐diphenyl‐benzidine (NPB)/Al sandwich devices as a function of the excitation photon energy and the applied bias voltage. We experimentally identified that the magnetic field induced an increase of intersystem crossing rate between singlet‐ and triplet‐ excitons (ISC‐X) and a decrease of triplet exciton‐polaron quenching rate (TPQ), with ISC‐X was a high magnetic field effect and TPQ a low magnetic field one. It was found that the band alignment and carrier transport within the device played an important role in the MPC behaviors.

One‐dimensional superconductivity of 0.4 nm single‐wall carbon nanotubes

Mono‐sized ultra‐small (0.4 nm in diameter) single‐walled carbon nanotubes (SWNTs) were prepared by pyrolysis of tripropylamine molecules in the channels of porous zeolite AlPO4‐5 (AFI) single crystals. These ultra‐small nanotubes perhaps constitute the best example of one‐dimensional (1D) quantum wires. Because these SWNTs are highly aligned and uniform in size, they show interesting electrical transport properties. Local density functional calculations indicate that when the diameter of the SWNT is smaller than 0.5 nm, strong curvature effects induce strong σ‐π mixing of the unoccupied orbitals. In this regime, metallicity can no longer be predicted by the simple band‐folding picture, and these small‐radius SWNTs generally have finite density of states at the Fermi level. Investigation of the magnetic and transport properties of these SWNTs revealed that at temperatures below 20 K, the 0.4nm tubes exhibit superconducting behavior manifest as an anisotropic Meissner effect, with a superconducting gap and...