Long Hu

Bio: Long Hu is an academic researcher from University of New South Wales. The author has contributed to research in topics: Perovskite (structure) & Quantum dot. The author has an hindex of 27, co-authored 57 publications receiving 2377 citations. Previous affiliations of Long Hu include Hong Kong University of Science and Technology & Huazhong University of Science and Technology.

Physically Flexible, Rapid‐Response Gas Sensor Based on Colloidal Quantum Dot Solids

Abstract: A gas sensor based on PbS colloidal quantum dots (CQDs) is constructed on a paper substrate, yielding flexible, rapid-response NO₂ gas sensors, fabricated from the solution phase. The devices are highly sensitive and fully recoverable at room temperature, which is attributed to the excellent access of gas molecules to the CQD surface, realized by surface ligand removal, combined with the desirable binding energy of NO₂ with the PbS CQDs.

Strontium-Doped Low-Temperature-Processed CsPbI2Br Perovskite Solar Cells

Abstract: Cesium (Cs) metal halide perovskites for photovoltaics have gained research interest due to their better thermal stability compared to their organic–inorganic counterparts. However, demonstration of highly efficient Cs-based perovskite solar cells requires high annealing temperature, which limits their use in multijunction devices. In this work, low-temperature-processed cesium lead (Pb) halide perovskite solar cells are demonstrated. We have also successfully incorporated the less toxic strontium (Sr) at a low concentration that partially substitutes Pb in CsPb1–xSrxI2Br. The crystallinity, morphology, absorption, photoluminescence, and elemental composition of this low-temperature-processed CsPb1–xSrxI2Br are studied. It is found that the surface of the perovskite film is enriched with Sr, providing a passivating effect. At the optimal concentration (x = 0.02), a mesoscopic perovskite solar cell using CsPb0.98Sr0.02I2Br achieves a stabilized efficiency at 10.8%. This work shows the potential of inorgani...

Gas chromatography-mass spectrometry analyses of encapsulated stable perovskite solar cells.

Abstract: INTRODUCTION Although advances in materials and processing have led to remarkable advancements in the energy conversion efficiency of perovskite solar cells (PSCs), increasing from 3.8% to 25.2% in only 10 years, these solar cells cannot become commercially viable unless their underperforming durability is improved. The instability of perovskites must be addressed if PSCs are to compete with silicon technology, which currently offers a 25-year performance warranty. Previous approaches to this problem include the use of metal oxide barrier layers and butyl rubber sealants. Here, we report a low-cost polymer/glass stack encapsulation scheme that enables PSCs to pass the demanding International Electrotechnical Commission (IEC) 61215:2016 Damp Heat and Humidity Freeze tests. These tests help to determine whether solar cell modules can withstand the effects of outdoor operating conditions by exposing them to repeated temperature cycling (–40° to 85°C) as well as 85% relative humidity. Our airtight encapsulation scheme prevented moisture ingress. It was also effective in suppressing outgassing of decomposition products, which limits decomposition reactions of organic hybrid PSCs by allowing these reactions to come to equilibrium. The gas compositions were verified by gas chromatography–mass spectrometry (GC-MS). RATIONALE In the GC-MS technique, gas chromatography separates the components in a mixture, and the chemical identity of each component is determined with mass spectrometry. We could directly identify with high specificity the decomposition products of multi-cation perovskite precursors, of unencapsulated perovskite test structures, and of encapsulated full cells at elevated temperatures. The results allowed us to identify thermal degradation pathways by determining the outgassing products of mixed-cation perovskites during heating. We then used GC-MS to evaluate the effectiveness of different packaging techniques developed for PSCs. The packaging schemes were a polyisobutylene (PIB)–based polymer blanket encapsulation, a polyolefin-based blanket encapsulation, and a PIB edge seal. These packaging layers were then capped by a glass cover. For the edge seal, the decomposition gases inside the cell were sampled with a syringe. The feasibilities of these packaging techniques were also demonstrated by IEC photovoltaic module standard Damp Heat and Humidity Freeze testing. RESULTS Signature decomposition products such as CH3I, CH3Br, and NH3 were identified and decomposition pathways were proposed for CH3NH3I (MAI), HC(NH2)2I (FAI), CH3NH3Br (MABr), and mixed-cation and mixed-halide (FAI)0.85 + (MABr)0.15 perovskite precursors, including their secondary decomposition reactions at 350°, 140°, and 85°C. The GC-MS results confirmed that the Br-containing precursor was less prone to thermal decomposition than an I-containing precursor. Also, CsFAMA cells were found to outgas one-fifth as much decomposition product as their FAMA counterparts, which indicated that the Cs-containing cells had better thermal stability. Although the decomposition of FAI is reversible, the mixing of MA with FA precursors caused decomposition products to participate in the secondary reaction that was irreversible. This finding confirmed the disadvantage of mixing of MA with FA perovskite through the reduction in chemical stability. The blanket-encapsulated PSCs sustained no efficiency degradation after 1800 hours of Damp Heat testing or 75 cycles of Humidity Freeze testing. CONCLUSION GC-MS identified signature volatile products of the decomposition of organic hybrid perovskites under thermal stress, thereby informing decomposition pathways. The findings are important for developing potential cell-stabilizing strategies, given that cells in the field typically experience high operating temperatures. In addition, results of GC-MS confirm that the low-cost pressure-tight encapsulation we developed is effective in suppressing such outgassing and therefore decomposition reactions of PSCs. This encapsulation scheme is the simplest of all for perovskite cells to pass IEC photovoltaic module standard tests. Our approach can be applied to evaluating the effectiveness of other packaging approaches, as well as testing the effectiveness of coatings and material compositions aimed at limiting light and thermal degradation.

Sequential Deposition of CH3NH3PbI3 on Planar NiO Film for Efficient Planar Perovskite Solar Cells

Abstract: The new emerging organometal trihalide perovskite holds great potential for high-efficiency, low-cost solar cells because of its high solar to electricity conversation efficiency (>16%) achieved within 4 years of research and its low-temperature solution processing. In this Letter we introduce NiO as the hole-collecting and -conducting layer in perovskite solar cells. Through a modified sequential deposition strategy, we successfully fabricated high-quality CH3NH3PbI3 onto a planar NiO layer and built a planar inverted ITO/NiO/CH3NH3PbI3/PCBM/Al photovoltaic device. A device efficiency of 7.6% was achieved with an impressively high open-circuit voltage (Voc) of 1.05 V. Our study demonstrates the potential application of a deep work function NiO layer for perovskite solar cells.

Enhanced performance via partial lead replacement with calcium for a CsPbI3 perovskite solar cell exceeding 13% power conversion efficiency

Abstract: Cesium metal halides are potential light-harvesting materials for use in the top cells of multi-junction devices due to their suitable bandgaps and good thermal stabilities. In particular, CsPbI3 has a bandgap of 1.7 eV, which is suitable for perovskite/Si tandem cells. However, the desirable black phase for CsPbI3 is not stable because Cs is too small to support the PbI6 octahedra. Also, there is room for improvement in terms of cell performance. Herein, we partially replace Pb2+ with Ca2+ in the CsPbI3 precursor, producing multiple benefits. Firstly, more uniform films with larger grains are produced from CsPbI3 with Ca2+, due to the reduction in the size of the colloids in the precursor solution with Ca2+. This morphology improvement provides better contact at the interface between the perovskite and the hole transport layer. In addition, it is found that the surface of the film is modified by the formation of a Ca rich oxide layer, providing a surface passivation effect. Finally, incorporation of Ca increases the band gap, leading to an increase in output voltage. The best CsPbI3 solar cell using 5% Ca2+ substitution in the precursor achieves a stabilised efficiency of 13.3%, and maintains 85% of its initial efficiency for over 2 months with encapsulation.

High-performance photovoltaic perovskite layers fabricated through intramolecular exchange

Abstract: The band gap of formamidinium lead iodide (FAPbI3) perovskites allows broader absorption of the solar spectrum relative to conventional methylammonium lead iodide (MAPbI3). Because the optoelectronic properties of perovskite films are closely related to film quality, deposition of dense and uniform films is crucial for fabricating high-performance perovskite solar cells (PSCs). We report an approach for depositing high-quality FAPbI3 films, involving FAPbI3 crystallization by the direct intramolecular exchange of dimethylsulfoxide (DMSO) molecules intercalated in PbI2 with formamidinium iodide. This process produces FAPbI3 films with (111)-preferred crystallographic orientation, large-grained dense microstructures, and flat surfaces without residual PbI2. Using films prepared by this technique, we fabricated FAPbI3-based PSCs with maximum power conversion efficiency greater than 20%.

Halide Perovskite Photovoltaics: Background, Status, and Future Prospects

Abstract: The photovoltaics of organic–inorganic lead halide perovskite materials have shown rapid improvements in solar cell performance, surpassing the top efficiency of semiconductor compounds such as CdTe and CIGS (copper indium gallium selenide) used in solar cells in just about a decade. Perovskite preparation via simple and inexpensive solution processes demonstrates the immense potential of this thin-film solar cell technology to become a low-cost alternative to the presently commercially available photovoltaic technologies. Significant developments in almost all aspects of perovskite solar cells and discoveries of some fascinating properties of such hybrid perovskites have been made recently. This Review describes the fundamentals, recent research progress, present status, and our views on future prospects of perovskite-based photovoltaics, with discussions focused on strategies to improve both intrinsic and extrinsic (environmental) stabilities of high-efficiency devices. Strategies and challenges regardi...

Organic–inorganic hybrid lead halide perovskites for optoelectronic and electronic applications

Abstract: Organic and inorganic hybrid perovskites (e.g., CH(3)NH(3)PbI(3)), with advantages of facile processing, tunable bandgaps, and superior charge-transfer properties, have emerged as a new class of revolutionary optoelectronic semiconductors promising for various applications. Perovskite solar cells constructed with a variety of configurations have demonstrated unprecedented progress in efficiency, reaching about 20% from multiple groups after only several years of active research. A key to this success is the development of various solution-synthesis and film-deposition techniques for controlling the morphology and composition of hybrid perovskites. The rapid progress in material synthesis and device fabrication has also promoted the development of other optoelectronic applications including light-emitting diodes, photodetectors, and transistors. Both experimental and theoretical investigations on organic-inorganic hybrid perovskites have enabled some critical fundamental understandings of this material system. Recent studies have also demonstrated progress in addressing the potential stability issue, which has been identified as a main challenge for future research on halide perovskites. Here, we review recent progress on hybrid perovskites including basic chemical and crystal structures, chemical synthesis of bulk/nanocrystals and thin films with their chemical and physical properties, device configurations, operation principles for various optoelectronic applications (with a focus on solar cells), and photophysics of charge-carrier dynamics. We also discuss the importance of further understanding of the fundamental properties of hybrid perovskites, especially those related to chemical and structural stabilities.