Junjie Si

Interfacial Control Toward Efficient and Low‐Voltage Perovskite Light‐Emitting Diodes

High-performance perovskite light-emitting diodes are achieved by an interfacial engineering approach, leading to the most efficient near-infrared devices produced using solution-processed emitters and efficient green devices at high brightness conditions.

Exciton localization in solution-processed organolead trihalide perovskites

Organolead trihalide perovskites have attracted great attention due to the stunning advances in both photovoltaic and light-emitting devices. However, the photophysical properties, especially the recombination dynamics of photogenerated carriers, of this class of materials are controversial. Here we report that under an excitation level close to the working regime of solar cells, the recombination of photogenerated carriers in solution-processed methylammonium–lead–halide films is dominated by excitons weakly localized in band tail states. This scenario is evidenced by experiments of spectral-dependent luminescence decay, excitation density-dependent luminescence and frequency-dependent terahertz photoconductivity. The exciton localization effect is found to be general for several solution-processed hybrid perovskite films prepared by different methods. Our results provide insights into the charge transport and recombination mechanism in perovskite films and help to unravel their potential for high-performance optoelectronic devices.

Morphology control of perovskite light-emitting diodes by using amino acid self-assembled monolayers

Amino acid self-assembled monolayers are used in the fabrication of light-emitting diodes based on organic-inorganic halide perovskites. The monolayers of amino acids provide modified interfaces by anchoring to the surfaces of ZnO charge-transporting layers using carboxyl groups, leaving the amino groups to facilitate the nucleation of MAPbBr3 perovskite films. This surface-modification strategy, together with chlorobenzene-assisted fast crystallization method, results in good surface coverage and reduced defect density of the perovskite films. These efforts lead to green perovskite light emitting diodes with a low turn-on voltage of 2u2009V and an external quantum efficiency of 0.43% at a brightness of ∼5000u2009cd m−2.

Efficient and High-Color-Purity Light-Emitting Diodes Based on In Situ Grown Films of CsPbX3 (X = Br, I) Nanoplates with Controlled Thicknesses

We report a facile solution-based approach to the in situ growth of perovskite films consisting of monolayers of CsPbBr3 nanoplates passivated by bulky phenylbutylammonium (PBA) cations, that is, two-dimensional layered PBA2(CsPbBr3)n-1PbBr4 perovskites. Optimizing film formation processes leads to layered perovskites with controlled n values in the range of 12-16. The layered perovskite emitters show quantum-confined band gap energies with a narrow distribution, suggesting the formation of thickness-controlled quantum-well (TCQW) structures. The TCQW CsPbBr3 films exhibit smooth surface features, narrow emission line widths, low trap densities, and high room-temperature photoluminance quantum yields, resulting in high-color-purity green light-emitting diodes (LEDs) with remarkably high external quantum efficiencies (EQEs) of up to 10.4%. The solution-based approach is extended to the preparation of TCQW CsPbI3 films for high-color-purity red perovskite LEDs with high EQEs of up to 7.3%.

Iodomethane-Mediated Organometal Halide Perovskite with Record Photoluminescence Lifetime

Organometallic lead halide perovskites are excellent light harvesters for high-efficiency photovoltaic devices. However, as the key component in these devices, a perovskite thin film with good morphology and minimal trap states is still difficult to obtain. Herein we show that by incorporating a low boiling point alkyl halide such as iodomethane (CH3I) into the precursor solution, a perovskite (CH3NH3PbI3-xClx) film with improved grain size and orientation can be easily achieved. More importantly, these films exhibit a significantly reduced amount of trap states. Record photoluminescence lifetimes of more than 4 μs are achieved; these lifetimes are significantly longer than that of pristine CH3NH3PbI3-xClx films. Planar heterojunction solar cells incorporating these CH3I-mediated perovskites have demonstrated a dramatically increased power conversion efficiency compared to the ones using pristine CH3NH3PbI3-xClx. Photoluminescence, transient absorption, and microwave detected photoconductivity measurements all provide consistent evidence that CH3I addition increases the number of excitons generated and their diffusion length, both of which assist efficient carrier transport in the photovoltaic device. The simple incorporation of alkyl halide to enhance perovskite surface passivation introduces an important direction for future progress on high efficiency perovskite optoelectronic devices.

Green light-emitting diodes based on hybrid perovskite films with mixed cesium and methylammonium cations

We report the formation of high-quality Cs0.4MA0.6PbBr3 thin films with nearly full surface coverage and good emission properties upon the introduction of Cs+ into perovskite crystals. The Cs0.4MA0.6PbBr3 thin films were applied as emissive layers in light-emitting diodes. A maximum external quantum efficiency of ~2.0% was achieved for these green-emitting devices.

Ag∕PbTe(111) interface behavior studied by photoemission spectroscopy

We performed investigations on the formation of Ag∕PbTe(111) interface by using photoemission spectroscopy. Upon initial Ag deposition, both Pb 4f and Te 3d core levels and the valence-band edge showed shifts to high binding energy, from which the band bending at Ag∕PbTe(111) is determined to be 0.13eV. Upon further Ag deposition, new components appear in Pb and Te core levels, with intensities remaining constant even at higher Ag thickness. The experimental results, together with ab initio calculation, indicate that both Te and Pb outdiffuse to the outmost surface, and a single Te-terminated Pb–Te bilayer is formed over Ag film on PbTe(111).

Solution-Processed Organic-Inorganic Hybrid Perovskites: A Class of Dream Materials Beyond Photovoltaic Applications

有机-无机杂化钙钛矿材料是可通过溶液工艺低温制备得到的直接带隙半导体晶体薄膜.在众多可溶液加工的半导体材料中,有机-无机杂化钙钛矿薄膜是为数不多的低缺陷密度、双极子传输性能优异的晶体薄膜,同时兼具宽光谱吸收和长载流子扩散距离等特性,是平面异质结太阳能电池的理想选择.另外,作为低缺陷密度的直接带隙半导体晶体材料,杂化钙钛矿薄膜具有优异的发光特性.其发光波长可通过能带工程(在分子水平上改变其组分)进行调节,因此有望在发光二极管和激光等光电器件中得到新应用.总结了钙钛矿材料的优异特性和目前应用研究的进展,并对其未来发展做了展望.Organic-inorganic hybrid perovskite is a class of direct-bandgap semiconductors that can be processed as thin films from solutions by low-temperature methods. Among various solution-processable semiconductor materials, the hybrid perovskites exhibit unique combination of low bulk-trap densities, remarkable ambipolar transport properties, good broadband absorption characteristics and long charge carrier diffusion lengths, making them ideal for photovoltaic applications. Furthermore, as direct bandgap semiconductors with low bulk trap densities, the hybrid perovskite films possess remarkable luminescent properties. The bandgap of the hybrid perovskites can be tuned by crystal engineering, i.e. tuning the composition at molecular levels. These intriguing properties indicate that the hybrid perovskites may also find applications in light-emitting diodes and lasing. This paper reviews the unique properties and current research progresses of this class of dream material and provides our perspective of future directions.