抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

Long, balanced electron and hole diffusion lengths greater than 100 nanometers in the polycrystalline organolead trihalide compound CH3NH3PbI3 are critical for highly efficient perovskite solar cells. We found that the diffusion lengths in CH3NH3PbI3 single crystals grown by a solution-growth method can exceed 175 micrometers under 1 sun (100 mW cm(-2)) illumination and exceed 3 millimeters under weak light for both electrons and holes. The internal quantum efficiencies approach 100% in 3-millimeter-thick single-crystal perovskite solar cells under weak light. These long diffusion lengths result from greater carrier mobility, longer lifetime, and much smaller trap densities in the single crystals than in polycrystalline thin films. The long carrier diffusion lengths enabled the use of CH3NH3PbI3 in radiation sensing and energy harvesting through the gammavoltaic effect, with an efficiency of 3.9% measured with an intense cesium-137 source.

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Electron-hole diffusion lengths > 175 μm in solution-grown CH3NH3PbI3 single crystals

Today's best perovskite solar cells use a mixture of formamidinium and methylammonium as the monovalent cations. With the addition of inorganic cesium, the resulting triple cation perovskite compositions are thermally more stable, contain less phase impurities and are less sensitive to processing conditions. This enables more reproducible device performances to reach a stabilized power output of 21.1% and ∼18% after 250 hours under operational conditions. These properties are key for the industrialization of perovskite photovoltaics.

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Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency

In recent years, the power conversion efficiency of perovskite solar cells has increased to reach over 20%. Finding an effective means of defect passivation is thought to be a promising route for bringing further increases in the power conversion efficiency and the open-circuit voltage (VOC) of perovskite solar cells. Here, we report the use of an organic halide salt phenethylammonium iodide (PEAI) on HC(NH2)2–CH3NH3 mixed perovskite films for surface defect passivation. We find that PEAI can form on the perovskite surface and results in higher-efficiency cells by reducing the defects and suppressing non-radiative recombination. As a result, planar perovskite solar cells with a certificated efficiency of 23.32% (quasi-steady state) are obtained. In addition, a VOC as high as 1.18 V is achieved at the absorption threshold of 1.53 eV, which is 94.4% of the Shockley–Queisser limit VOC (1.25 V). Planar perovskite solar cells that have been passivated using the organic halide salt phenethylammonium iodide are shown to have suppressed non-radiative recombination and operate with a certified power conversion efficiency of 23.3%.

Surface passivation of perovskite film for efficient solar cells

State-of-the-art photovoltaics use high-purity, large-area, wafer-scale single-crystalline semiconductors grown by sophisticated, high-temperature crystal growth processes. We demonstrate a solution-based hot-casting technique to grow continuous, pinhole-free thin films of organometallic perovskites with millimeter-scale crystalline grains. We fabricated planar solar cells with efficiencies approaching 18%, with little cell-to-cell variability. The devices show hysteresis-free photovoltaic response, which had been a fundamental bottleneck for the stable operation of perovskite devices. Characterization and modeling attribute the improved performance to reduced bulk defects and improved charge carrier mobility in large-grain devices. We anticipate that this technique will lead the field toward synthesis of wafer-scale crystalline perovskites, necessary for the fabrication of high-efficiency solar cells, and will be applicable to several other material systems plagued by polydispersity, defects, and grain boundary recombination in solution-processed thin films.

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High-efficiency solution-processed perovskite solar cells with millimeter-scale grains

The large photocurrent hysteresis observed in many organometal trihalide perovskite solar cells has become a major hindrance impairing the ultimate performance and stability of these devices, while its origin was unknown. Here we demonstrate the trap states on the surface and grain boundaries of the perovskite materials to be the origin of photocurrent hysteresis and that the fullerene layers deposited on perovskites can effectively passivate these charge trap states and eliminate the notorious photocurrent hysteresis. Fullerenes deposited on the top of the perovskites reduce the trap density by two orders of magnitude and double the power conversion efficiency of CH(3)NH(3)PbI(3) solar cells. The elucidation of the origin of photocurrent hysteresis and its elimination by trap passivation in perovskite solar cells provides important directions for future enhancements to device efficiency.

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Origin and elimination of photocurrent hysteresis by fullerene passivation in CH3NH3PbI3 planar heterojunction solar cells

Solvent-annealing is found to be an effective method to increase the grain size and carrier diffusion lengths of trihalide perovskite materials. The carrier diffusion length of MAPbI3 is increased to over 1 μm. The efficiency remains above 14.5% when the MAPbI3 thickness changes from 250 nm to 1 μm, with the highest efficiency reaching 15.6%.

Solvent annealing of perovskite-induced crystal growth for photovoltaic-device efficiency enhancement.

Organolead trihalide perovskite (OTP) materials are emerging as naturally abundant materials for low-cost, solution-processed and highly efficient solar cells. Here, we show that, in OTP-based photovoltaic devices with vertical and lateral cell configurations, the photocurrent direction can be switched repeatedly by applying a small electric field of <1 V μm(-1). The switchable photocurrent, generally observed in devices based on ferroelectric materials, reached 20.1 mA cm(-2) under one sun illumination in OTP devices with a vertical architecture, which is four orders of magnitude larger than that measured in other ferroelectric photovoltaic devices. This field-switchable photovoltaic effect can be explained by the formation of reversible p-i-n structures induced by ion drift in the perovskite layer. The demonstration of switchable OTP photovoltaics and electric-field-manipulated doping paves the way for innovative solar cell designs and for the exploitation of OTP materials in electrically and optically readable memristors and circuits.

Giant switchable photovoltaic effect in organometal trihalide perovskite devices

Large-aspect-ratio grains are needed in polycrystalline thin-film solar cells for reduced charge recombination at grain boundaries; however, the grain size in organolead trihalide perovskite (OTP) films is generally limited by the film thickness. Here we report the growth of OTP grains with high average aspect ratio of 2.3-7.9 on a wide range of non-wetting hole transport layers (HTLs), which increase nucleus spacing by suppressing heterogeneous nucleation and facilitate grain boundary migration in grain growth by imposing less drag force. The reduced grain boundary area and improved crystallinity dramatically reduce the charge recombination in OTP thin films to the level in OTP single crystals. Combining the high work function of several HTLs, a high stabilized device efficiency of 18.3% in low-temperature-processed planar-heterojunction OTP devices under 1 sun illumination is achieved. This simple method in enhancing OTP morphology paves the way for its application in other optoelectronic devices for enhanced performance.

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Yuchuan Shao

Papers

Electron-hole diffusion lengths > 175 μm in solution-grown CH3NH3PbI3 single crystals

Origin and elimination of photocurrent hysteresis by fullerene passivation in CH3NH3PbI3 planar heterojunction solar cells

Solvent annealing of perovskite-induced crystal growth for photovoltaic-device efficiency enhancement.

Giant switchable photovoltaic effect in organometal trihalide perovskite devices

Non-wetting surface-driven high-aspect-ratio crystalline grain growth for efficient hybrid perovskite solar cells