Jianghui Zheng

Superior Self-Powered Room-Temperature Chemical Sensing with Light-Activated Inorganic Halides Perovskites

Hybrid halide perovskite is one of the promising light absorber and is intensively investigated for many optoelectronic applications. Here, the first prototype of a self-powered inorganic halides perovskite for chemical gas sensing at room temperature under visible-light irradiation is presented. These devices consist of porous network of CsPbBr3 (CPB) and can generate an open-circuit voltage of 0.87 V under visible-light irradiation, which can be used to detect various concentrations of O2 and parts per million concentrations of medically relevant volatile organic compounds such as acetone and ethanol with very quick response and recovery time. It is observed that O2 gas can passivate the surface trap sites in CPB and the ambipolar charge transport in the perovskite layer results in a distinct sensing mechanism compared with established semiconductors with symmetric electrical response to both oxidizing and reducing gases. The platform of CPB-based gas sensor provides new insights for the emerging area of wearable sensors for personalized and preventive medicine.

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

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.

Large area efficient interface layer free monolithic perovskite/homo-junction-silicon tandem solar cell with over 20% efficiency

Monolithic perovskite/silicon tandem solar cells show great promise for further efficiency enhancement for current silicon photovoltaic technology. In general, an interface (tunnelling or recombination) layer is usually required for electrical contact between the top and the bottom cells, which incurs higher fabrication costs and parasitic absorption. Most of the monolithic perovskite/Si tandem cells demonstrated use a hetero-junction silicon (Si) solar cell as the bottom cell, on small areas only. This work is the first to successfully integrate a low temperature processed (≤150 °C) planar CH3NH3PbI3 perovskite solar cell on a homo-junction silicon solar cell to achieve a monolithic tandem without the use of an additional interface layer on large areas (4 and 16 cm2). Solution processed SnO2 has been effective in providing dual functions in the monolithic tandem, serving as an ETL for the perovskite cell and as a recombination contact with the n-type silicon homo-junction solar cell that has a boron doped p-type (p++) front emitter. The SnO2/p++ Si interface is characterised in this work and the dominant transport mechanism is simulated using Sentaurus technology computer-aided design (TCAD) modelling. The champion device on 4 cm2 achieves a power conversion efficiency (PCE) of 21.0% under reverse-scanning with a VOC of 1.68 V, a JSC of 16.1 mA cm−2 and a high FF of 78% yielding a steady-state efficiency of 20.5%. As our monolithic tandem device does not rely on the SnO2 for lateral conduction, which is managed by the p++ emitter, up scaling to large areas becomes relatively straightforward. On a large area of 16 cm2, a reverse scan PCE of 17.6% and a steady-state PCE of 17.1% are achieved. To our knowledge, these are the most efficient perovskite/homo-junction-silicon tandem solar cells that are larger than 1 cm2. Most importantly, our results demonstrate for the first time that monolithic perovskite/silicon tandem solar cells can be achieved with excellent performance without the need for an additional interface layer. This work is relevant to the commercialisation of efficient large-area perovskite/homo-junction silicon tandem solar cells.

Potential for improved transport in core–shell CuInS2 nanoparticle solar cells from an Ag surface termination

A major factor limiting the performance of nanostructured CuInS2 photovoltaic devices is the current density, pointing to poor charge carrier transport in CuInS2 nanoparticle films. In a typical CuInS2 core/shell structure synthesis, ZnS is typically chosen as a shell material for CuInS2 core passivation, which leads to a significant enhancement of the photoluminescence quantum yield from the CuInS2 nanoparticles. While typically a marker for excellent photovoltaic performance, in this case the increased photoluminescence likely signals increased charge carrier confinement and reduced transport through any thin films fabricated from the nanoparticles. Here we show that replacing the typical divalent Zn cation surface termination with a monovalent Ag cation leads to small improvements in charge carrier transport through nanostructured films. This surface termination intentionally introduces lower energy electronic states directly at the surface of the CuInS2 nanoparticles, reducing charge carrier confinement and thus increasing charge carrier mobility between nanoparticles. The study assessed appropriate Ag molar ratios to be used in synthesis, with an 8% Ag : Cu ratio found to be optimal. The passivation offered by Ag surface termination appears comparable to that from Zn with strong photoluminescence observed in both cases. Slight improvements in the performance of all-solid-state nanoparticle CuInS2 photovoltaic devices are obtained, with current densities in the Ag surface terminated case being increased by just under 10%. These findings outline a potential strategy for the synthesis of type II core–shell CuInS2 quantum dot thin film devices with improved charge transport.