Impedance Spectroscopy for the Measurement of Thermoelectric Properties

We have published this month a new article reporting the potential of impedance spectroscopy as a method to measure thermoelectric properties. Impedance spectroscopy has proved to be a very useful and reliable technique in a lot of fields and powerful equipments are available in the market and in many research centres. We have proved in this new article that impednce can also become very useful in the field of thermoelectrics since it can provide the usually required thermoelectric parameters.

In a first approach, by measuring a thermoelectric element using Ag paint as contacts, all the thermal properties (thermal conductivity, thermal diffusivity and specific heat) can be measured if the Seebeck coefficient is known. Secondly, we describe that by attaching contacts of known thermal conductivity, such as the ceramics in the thermoelectric modules, a complete characterisation can be achieved, i. e., the Seebeck coefficient, the electrical conductivity and the mentioned thermal properties can potentially be extracted.

In addition, new parameters such as thermoelectric capacitance and thermoelectric resistances can be extracted which give information about the physics of the device. All these results are available in our article in the Journal of Applied Physics and in the seminar I gave at Purdue University few months ago which has been recorded and published at the Nanohub website.

Efficiency of Perovskite Solar Cells above 20%

The Korean Research Institute of Chemical Technology (KRICT) has fabricated the first Perovskite Solar Cell with efficiency higher than 20 %. An exact value of 20.1 % has been certified by the NREL and it now appears in the updated efficiency chart. This new efficiency increase situates Perovskites very close to some of the Crystalline Silicon technologies, CdTe and CIGS solar cells.

New Website of the Cardiff Thermoelectric and Photovoltaic Laboratory

I am pleased to announce the launch of the new website of the Cardiff Thermoelectric and Photovoltaic Laboratory. It was released few weeks ago and there you can look at all the members that form the Laboratory, the funded projects we are involved in, our research lines and a detailed section with all our facilities. This is the website address: www.thermoelectric.engineering.cf.ac.uk

Perovskite Solar Cells Moving On

I would like to highlight a recent article by Michael Grätzel published in Nature Materials which shows in my opinion a very nice view of the emergence, development, and problems to be addressed in the field of Perovskite Solar Cells. The article covers all the relevant aspects of this novel technology in a brief and clear way, which results very helpful for those non-familiar with the topic or willing to contribute to it.

Special emphasis is given to the issues rising in the characterisation due to hysteresis effects, and the problems to be addressed in order for the technology to reach the market. In this respect, stability studies seem to be scarce and the high toxicity of Pb compounds has to be overcome by searching new perovskite materials.

Top 10 Emerging Technologies for 2014

The World Economic Forum has published a relation of the the top 10 most promising technologies for 2014. Within the selected breakthroughs are the ones related to wearable electronics, storage of electricity, screenless displays and brain-computer interfaces. All these technologies offer solutions to important problems of our society and hence could make an important impact in the economic market. To see the whole list and a brief summary of their relevance, click here.

The Art of Scientific Publication

ACS publications have compiled a list of 20 references that cover a wide range of issues and aspects related to scientific publications. It deals with the preparation of graphics, the role of corresponding authors, preparation of cover letters, rejection and acceptance and up to the reply to reviewers. It is an excellent guidance, made from the point of view of the own editors and worth looking at it. The link is here.

The Prospects of Graphene Applications

Graphene has acquire an enormous interest in the scientific community due to its outstanding properties and the large range of applications it can be used. In this month's issue of Nature Nanotechnology, seven experts briefly comment their views in the potential and challenges in the realization of graphene-based products in a range of fields such as displays, inks, energy, corrosion, health and life science, biomolecular analysis and aerospace. The article can be read here.

(Graphene transparent screen from Nature journal)

Remarks About Record SnSe Thermoelectric Material

Few months ago, a new article appeared in Physics Today describing some concerns and comments about the outstanding properties of the new SnSe material which has shown impressive thermoelectric properties recently. Advantages and disadvantages regarding practical considerations are also included in the article along with some comments from different recognised researchers and laboratories which offer very interesting and critical points of view. 

Novel Device for the Heat Flow Control

Since many years ago, we have achieved an excellent control over electronic currents and have created many devices that have produced a huge impact in our society and in the way we life. Transistors, for example, are part of these devices and form the nuclei of our computer, mobiles, etc. However, we are still far from achieving this control over heat flow, that is, over the phonons. The invention of devices able to manipulate heat transport could make an enormous impact, and recently, researchers from the University of California at Berkeley and Georgia State University have reported the first temperature-gated thermal rectifier devices using VO2 beams. 

(Schematic operation and pictures of the novel thermal rectifiers. Picture obtained from the Nano Letters article)

The modification of the heat flux in the beams can be achieved by changing the device temperature, which controls the metallic and insulating nature of the oxide. Maximum heat suppression of 28% is observed below 340 K when mixed metallic and insulating phases coexist in the VO2 beam. Once the devices are heated above 340 K, they become completely metallic and behave as ordinary thermal conductors.

If you want to read more about the potential and recent advances in the control of phonons, I would recommend having a look to a very interesting review article appeared in Nature last year. the article highlights the new opportunities to thermally insulate buildings, reduce environmental noise, transform waste heat into electricity and develop earthquake protection. Sonic and thermal diodes, optomechanical crystals, acoustic and thermal cloaking, hypersonic phononic crystals, thermoelectrics, and thermocrystals could be the devices to create the next technological revolution in phononics.

Thermoelectricity Online Course

For those interested in learning the fundamentals of thermoelectricity from a microscopic point of view, understand the operation of the devices and also learn about measurement of thermoelectric properties in nano-scaled materials and the whole system efficiencies, there is a very interesting course freely available at the nanoHub website.

The course is called "Thermoelectricity: From atoms to systems" and is given by Professors Datta, Lundstrom and Shakouri from Purdue University (USA). It is distributed in 5 weeks and fro the website you have access to the videos of the lectures, slides and quizs and exams. There is also a forum for discussions. From the video above you can have an idea of what the course is about.

Record Efficiency in Thermoelectric Materials

This month has been reported in Nature the thermoelectric material with the highest efficiency (ZT=2.6 at 923 K) so far: SnSe. The main reason for the high ZT is the ultralow thermal conductivity (0.25 W/mK) which is similar to that of polymers. This breakthrough has been achieved at the Mercoury Kanatzidis' Group in Northwestern University (USA).

SnSe is probably the world’s least thermally conductive crystalline material, so it can get hot on one side while keeping cold the other side. In addition, it holds a high Seebeck coefficient (around 0.4 mV/K). These results demonstrate that a high efficiency can be realized in simple layered, anisotropic and anharmonic systems, offering an alternative to the most-followed strategy of nanostructuring in order to improve the efficiency of thermoelectrics.

Solar Thermophotovoltaic: Designing the Sun for the Solar Cell

The way the single-junction solar cells generally work is by directly absorbing the sunlight through a semiconductor layer corresponding to a certain bandgap. However, the solar spectrum is broad. Photons with energy below the bandgap are not absorbed and photons with energy above the bandgap generate electron–hole pairs, but these quickly relax to the band edge losing some of their initial energy. The combination of these two loss mechanisms defines the Shockley–Queisser limit, which states that a single-junction cell cannot have efficiencies exceeding 41%. 

In some way, the Shockley–Queisser analysis indicates that the efficiency limit has more to do with the Sun. If the Sun emitted only photons with energy immediately above the bandgap of the semiconductor, both loss mechanisms in the Shockley–Queisser analysis would be supressed and the same single-junction cell would approach an efficiency of 95%, the highest allowed by the second law of thermodynamics.

(Image of the solar thermophotovoltaic system developed at MIT)

The solar thermophotovoltaic system tries to take advantage of this fact. In this system, an intermediate element is placed between the sunlight and the solar cell. The element includes a solar absorber which can absorb the entire solar spectrum on the side that faces the Sun. On the other side, the element, which becomes very hot, can be designed to generate by thermal emission narrow-band radiation tailored to the bandgap of the solar cell. A significant advantage of this design is that potentially all solar energy could be converted into electricity.

In the past, efficiencies have struggled to reach 1%. However, Evelyn Wang and colleagues from the MIT have reported in Nature Nanotechnology an efficiency of 3.2%. This has been achieved by employing an absorber made of vertically aligned multiwalled carbon nanotubes which provides near-complete absorption over the broad solar spectrum. The emitter includes a one-dimensional photonic crystal made of multilayers of Si and SiO2, optimized to achieve spectral matching between the thermal emission and the bandgap of the InGaAsSb, strongly suppressing sub-bandgap thermal radiation.

This impressive demostration highlights the less usual strategy of controlling the light that reaches the photovoltaic element as a promising line to further develop the current state-of-the-art of the photovoltaic systems.

The Best of 2013 and Things to Come in 2014

We have welcomed these days the new year 2014, and many of us are curious about what will be the things to expect in science in 2014. Nature has made a summary of some of these possible expectations here. On another hand, 2013 has now passed and has left some major breakthroughs which were also highlighted by Nature here, in a top ten list of relevant researchers in 2013.

Both summaries include the high impact achieved by the perovskite solar cells, which reveals as a solid technology to substitute the Si domain in the solar cell market. Henry Snaith in Oxford University has been one of the key scientist responsible for these enormous advances. Let´s see what will be the next to come...