cells fabricated out of perovskites have al- ready revealed some promise, but in this study, published in the journal ACS Central. Science [Jaffe, et al., ACS Cent.
Materials Today Volume 19, Number 6 July/August 2016
analysed. At 7.23%, the maximum power conversion efficiency is lower than others in the literature, but these cells show none of the current-voltage hysteresis that other perovskite cells suffer from. This makes them considerably more reliable
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over the long term. In addition, across batches of these cells, the performance was consistent (the standard deviation of the efficiencies was just 0.42%), suggesting that their approach to producing large-island thin films is highly reproduc-
ible. Work is ongoing, but the team are confident – they believe that these results could ‘‘pave the way to achieve large-scale production of highly efficient perovskite solar modules.’’ Laurie Winkless
the electronic conductivity of the materials at high pressures was also observed. To explore how pressure affects the way hybrid perovskites react to light, samples of the material were positioned in a diamondanvil cell, a high-pressure device with two opposing diamonds. Each sample was positioned between the diamonds before being squeezed at very high pressures. It was found that, under compression, a sample that is usually orange would become lighter in color, indicating the perovskite was absorbing higher-energy light waves. However, when the pressure was increased, the sample darkened, indicating that lower-energy light was also being absorbed. They tracked the positions of atoms upon compression with X-ray diffraction, which helped to identify how the structure of the materials reacts to pressure. As co-leader Wendy Mao said, ‘‘Our findings suggest that compression can allow us to tailor the wavelength of absorbed light. This compression
may be attained through either mechanical or chemical means.’’ Other studies have pinpointed that hybrid perovskites can efficiently absorb sunlight before converting it to electricity, with some managing to achieve efficiencies of over 20%, which is similar to that of commercially available silicon solar cells. As coleader Hemamala Karundasa also states, ‘‘this work shows that pressure is a tuning knob for improving the properties of perovskite absorbers in a predictable way’’. Some research groups have already produced cheap tandem solar cells fabricated from perovskite that is placed on top of silicon, although achieving the necessary high voltages for high-efficiency tandem cells has not been straightforward. The findings from this new study suggest that pressure could increase the voltages of perovskite solar cells, something that requires further research. Laurie Donaldson
The approach works for nanotubes synthesized by various methods, and film thickness is controllable. The researchers hope to develop computer chips that are bendable as opposed to brittle silicon, although the monodomain films they have produced are ‘‘chiralityenriched’’ and not single-chirality. However, as CNTs grow in batches of random types, they separated the nanotubes by chirality using a simple process to produce enriched films with nanotubes of different types and diameters, before making terahertz polarizers and electronic transistors. They had discovered the filtration technique by serendipitously adding too much water to a nanotube-surfactant suspension, and then feeding it through a filter helped by vacuum. On assessing the resulting film by scanning electron microscope, it was found that, rather than dropping randomly onto the paper, millions of the nanotubes
clumped together in tight and aligned rows. This showed something unusual was happening, provoking them into spending another year and over a 100 films to refine their approach to produce nanotube wafers of up to an inch wide and of any thickness. Each element is significant: the type of filter paper and the vacuum pressure, as well as the concentration of nanotubes and surfactant. To explore why the CNTs line up in this way, they are continuing to look at the mechanics of how the first few nanotubes on the paper combine. With Van der Waals force bringing them together, and they look for their lowest-energy state, that of alignment. As the CNTs vary in length, the overhangs could force other tubes to line up on joining the array. The films can be separated from the paper, and then washed and dried for use, with the final films able to be patterned using lithography. Laurie Donaldson
Researchers at Stanford SLAC National Accelerator Laboratory and Lawrence Berkeley National Laboratory have demonstrated a way to increase the voltages of perovskite solar-cell absorbers just by applying external pressure. Squeezing the solar cells made from artificial crystalline structures called perovskites, a potentially useful and cheap photovoltaic material, helped to improve their performance. Perovskites, which are also benefitting research into new lasers and LEDs, come in many crystalline structures, such as hybrid perovskites that are made of lead, iodine or bromine, and organic compounds. Solar cells fabricated out of perovskites have already revealed some promise, but in this study, published in the journal ACS Central Science [Jaffe, et al., ACS Cent. Sci. (2016), doi:10.1021/acscentsci.6b00055], it was shown that applying pressure can alter the properties of these materials, as well as how they respond to light. A dramatic increase in
Thin films through vacuum filtration A new study by researchers from Rice University and colleagues at Los Alamos National Laboratory in the US has shown how to produce highly aligned, wafer-scale films based on a straightforward filtration process, a breakthrough that could lead to the development of new electronic and photonic devices. The flexible, inch-wide films are of densely packed, chirality-enriched, singlewalled carbon nanotubes (CNTs), cylinders of graphene with its atoms organized in hexagons. It is how these hexagons are turned that specifies the tube’s chirality, thus determining its electronic properties. As presented in Nature Nanotechnology [He, et al., Nat. Nanotechnol. (2016), doi:10.1038/nnano.2016.44], the process depends on the correct solution of CNTs, and under the right conditions. When this happen, millions of the tubes assemble themselves into long rows that are more effectively aligned than achieved previously.
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Pressurizing solar cells into improving
