Yajuan Li

Lead‐Free, Air‐Stable All‐Inorganic Cesium Bismuth Halide Perovskite Nanocrystals

Lead-based perovskite nanocrystals (NCs) have outstanding optical properties and cheap synthesis conferring them a tremendous potential in the field of optoelectronic devices. However, two critical problems are still unresolved and hindering their commercial applications: one is the fact of being lead-based and the other is the poor stability. Lead-free all-inorganic perovskite Cs3 Bi2 X9 (X=Cl, Br, I) NCs are synthesized with emission wavelength ranging from 400 to 560u2005nm synthesized by a facile room temperature reaction. The ligand-free Cs3 Bi2 Br9 NCs exhibit blue emission with photoluminescence quantum efficiency (PLQE) about 0.2u2009%. The PLQE can be increased to 4.5u2009% when extra surfactant (oleic acid) is added during the synthesis processes. This improvement stems from passivation of the fast trapping process (2-20u2005ps). Notably, the trap states can also be passivated under humid conditions, and the NCs exhibited high stability towards air exposure exceeding 30u2005days.

Low Threshold Two-Photon-Pumped Amplified Spontaneous Emission in CH3NH3PbBr3 Microdisks

Two-photon-pumped amplified spontaneous emission (ASE) of CH3NH3PbBr3 microdisks (MDs) were investigated by using femtosecond laser system. Low threshold at 2.2 mJ cm(-2) was obtained. Also, emission spectral tunability from 500 to 570 nm was demonstrated by synthesis the mixed halide perovskite MDs. The spatial effect of photoluminescence (PL) properties under one-photon and two-photon excitation were also studied by means of two-photon laser scanning microscope (TPLSM) and time-resolved PL spectroscopy. It was found that the band to band emission of near-surface regions and photocarriers diffusion from near-surface regions to interior regions is significant for one-photon excitation. By contrast, reabsorption of emission under two-photon excitation plays a major role in the emission properties of the MDs. These results will give a more comprehensive understanding of the nonlinear effect of CH3NH3PbBr3 single crystals.

Niobium-Doped (001)-Dominated Anatase TiO2 Nanosheets as Photoelectrode for Efficient Dye-Sensitized Solar Cells

TiO2 nanocrystals with different reactive facets have attracted extensive interest since they were first synthesized. The anatase TiO2 nanocrystals with (001) or (100) dominate facets were considered to be excellent electrode materials to enhance the cell performance of dye-sensitized solar cells. However, which reactive facet presents the best surface for benefiting photovoltaic effect is still unknown. We report a systematic study of various anatase TiO2 surfaces interacting with N719 dye by means of density functional theory calculations in combination with microscopic techniques. The (001) surface interacting with N719 would have the lowest work function, leading to the best photovoltaic performances. To further increase the efficiency, Nb dopant was incorporated into the anatase TiO2 nanocrystals. Based on the theoretical prediction, we proposed and demonstrated novel Nb-doped (001)-dominated anatase TiO2 nanosheets as photoelectrode in a dye-sensitized solar cell to further enhance the open-circuit voltage. And a power conversion efficiency of 10% was achieved, which was 22% higher than that of the undoped device (P25 as an electrode).

Limiting Perovskite Solar Cell Performance by Heterogeneous Carrier Extraction

Although the power conversion efficiency of perovskite solar cells has improved rapidly, a rational path for further improvement remains unclear. The effect of large morphological heterogeneity of polycrystalline perovskite films on their device performance by photoluminescence (PL) microscopy has now been studied. Contrary to the common belief on the deleterious effect of morphological heterogeneity on carrier lifetimes and diffusivities, in neat CH3 NH3 PbI3 (Cl) polycrystalline perovskite films, the local (intra-grain) carrier diffusivities in different grains are all surprisingly high (1.5 to 3.3u2005cm2 u2009s-1 ; comparable to bulk single-crystals), and the local carrier lifetimes are long (ca. 200u2005ns) and surprisingly homogenous among grains, and uniform across grain boundary and interior. However, there is a large heterogeneity of carrier extraction efficiency at the perovskite grain-electrode interface. Improving homogeneity at perovskite grain-electrode contacts is thus a promising direction for improving the performance of perovskite thin-film solar cells.

Perovskite CH3NH3PbI3–xBrx Single Crystals with Charge-Carrier Lifetimes Exceeding 260 μs

The long carrier lifetimes in perovskite single crystals have drawn significant attention recently on account of their irreplaceable contribution to high-performance photovoltaic (PV) devices. Herein, the optical and optoelectronic properties of CH3NH3PbI3 and CH3NH3PbI3-xBrx (with five different contents of Br doped) single crystals were investigated. Notably, a superior carrier lifetime of up to 262 μs was observed in the CH3NH3PbI3-xBrx (I/Br = 10:1 in the precursor) single-crystal PV device under 1 sun illumination, which is two times longer than that in the CH3NH3PbI3 single crystal. Further study confirmed that the ultralong carrier lifetime was ascribed to the integrated superiority derived from both the low trap-state density and high charge-injection efficiency of the device interface. On this basis, appropriate incorporation of Br is useful in the design of better PV devices.

Lead-free and stable antimony–silver-halide double perovskite (CH3NH3)2AgSbI6

A mixed metal organic–inorganic perovskite (CH3NH3)2AgSbI6 was developed in which pairs of the Pb(II) atoms in traditional CH3NH3PbI3 perovskite are replaced by Sb(III)/Ag(I) aliovalent units. We used density functional theory (DFT) calculations (the GLLB-SC method) with spin–orbit coupling corrections to predict the optical band gap of the most stable MA2AgSbI6 structure (2.00 eV). The results suggest that it is a promising light absorber. We then synthesized the double perovskite MA2AgSbI6 and confirmed its structure using X-ray diffraction (XRD). The optical band gap (1.93 eV) is in good agreement with the DFT calculations. The band positions of the material (vs. vacuum) are provided for future uses such as achieving the precise energy level alignments for potential photovoltaic applications. This perovskite material exhibits excellent stability in ambient air.

(C6H5C2H4NH3)2GeI4: A Layered Two-Dimensional Perovskite with Potential for Photovoltaic Applications

Recently, two-dimensional organic-inorganic perovskites have attracted increasing attention due to their unique photophysical properties and high stability. Here we report a lead-free, two-dimensional perovskite, (PEA)2GeI4 (PEA = C6H5(CH2)2NH3+). Structural characterization demonstrated that this 2D perovskite structure is formed with inorganic germanium iodide planes separated by organic PEAI layers. (PEA)2GeI4 has a direct band gap of 2.12 eV, in agreement with 2.17 eV obtained by density functional theory (DFT) calculations, implying that it is suitable for a tandem solar cell. (PEA)2GeI4 luminesces at room-temperature with a moderate lifetime, exhibiting good potential for photovoltaic applications. In addition, 2D (PEA)2GeI4 is more stable than 3D CH3NH3GeI3 in air, owing to the presence of a hydrophobic organic long chain. This work provides a direction for the development of 2D Ge-based perovskites with potential for photovoltaic applications.

Combining theory and experiment in the design of a lead-free ((CH3NH3)2AgBiI6) double perovskite

Under the guidance of our theoretical calculations, we synthesized a lead-free double perovskite ((CH3NH3)2AgBiI6) and explored its electronic structure and optical properties using a combination of experiment and density functional theory. (CH3NH3)2AgBiI6 has an indirect bandgap of 1.96 eV, and exhibits high-stability in air.

Extra long electron–hole diffusion lengths in CH3NH3PbI3−xClx perovskite single crystals

Long electron–hole diffusion lengths in organolead trihalide compounds play a key role in achieving the remarkable performance of perovskite photovoltaics. Diffusion lengths in solution-grown CH3NH3PbI3 single crystals have been found to be greater than 175 micrometer (μm). Herein, we report the diffusion lengths in CH3NH3PbI3−xClx single crystals exceeding 380 μm under 1 Sun illumination, which is twice that in CH3NH3PbI3 single crystals. Incorporation of chlorine is found to increase the density of trap-states and reduce the valence band level; these two factors, which dominate the carrier recombination and the charge transfer, respectively, are in a competing relation. As a result, the electron–hole diffusion lengths in a CH3NH3PbI3−xClx single crystal with an optimum Cl proportion (x = 0.005) reach the maximum values. This study provides a strategy for the design of perovskite optoelectronics.

Constructing Sensitive and Fast Lead-Free Single-Crystalline Perovskite Photodetectors

We developed a high-performance photodetector based on (CH3NH3)3Sb2I9 (MA3Sb2I9) microsingle crystals (MSCs). The MA3Sb2I9 single crystals exhibit a low-trap state density of ∼1010 cm-3 and a long carrier diffusion length reaching 3.0 μm, suggesting its great potential for optoelectronic applications. However, the centimeter single crystal (CSC)-based photodetector exhibits low responsivity (10-6 A/W under 1 sun illumination) due to low charge-carrier collection efficiency. By constructing the MSC photodetector with efficient charge-carrier collection, the responsivity can be improved by three orders of magnitude (under 1 sun illumination) and reach 40 A/W with monochromatic light (460 nm). Furthermore, the MSC photodetectors exhibit fast response speed of <1 ms, resulting in a high gain of 108 and a gain-bandwidth product of 105 Hz. These numbers are comparable to the lead-perovskite single-crystal-based photodetectors.