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...

Consolidation of Perovskite Solar Cells

This month a very interesting article in the Journal of Physical Chemistry Letters has been published by Henry Snaith, one of the instigators of the new Perovskite Solar cells that have already consolidated as one of the most promising and feasible low-cost photovoltaic technologies.

(Perovskite solar cells from Swansea Univeristy. Picture by Matt Carnie)

In the article he describes the evolution from the initial liquid dye-sensitised solar cell, to the solid state device using OMeTAD, then the extremely thin absorber (ETA) approach and finally the Perovskite solar cell or also known as Meso-Superstructured Solar Cell (MSSC).

One of the most impressive characteristics of the perovskites is that it groups all the three essential mechanisms for the photovoltaic conversion of energy into a single material, i. e. the perovskite is able to absorb the light, accumulate the separated carriers and transport both electrons and holes to the external contacts.

The most recent improvements in efficiency already reach 15% efficiency, using different configurations, a planar thin-film device (reported by Liu et. al.) and a nanostructured approach (reported by Burschka et al.). Snaith also remarks in the article that efficiencies of 20% or 25% could be also possible in the near future, reaching the most efficient current photovoltaic technologies.

Thermoelectric Organic Semiconductors Reveals New Trends and Improve Efficiency

A recent paper appeared in Nature Materials has revealed new trends in thermoelectricity existing in organic semiconductors which significantly differs from the behaviour widely observed in inorganic semiconductors. The increase of the carrier concentration n in inorganic semiconductors leads to the increase of electrical conductivity σ, but decreases the Seebeck coefficient S. Since the performance of thermoelectric materials is determined by the figure of merit Z=σS^2/κ, where κ is the thermal conductivity, a compromise between all these three parameters needs to be reached. For this reason highly doped semiconductors are the materials that better achieve this trade-off as observed in the classical figure below.

However, Kim et al. at the University of Michigan have proved that this is not the case in organic semiconductors, where a reduction of the dopant concentration leads to an improvement in Z due to the simultaneous increase of S and σ. They attribute this feature to the major influence of the mobility μ rather than the n in the electrical conductivity which is proportional to the product of both (σ=qnμ). In other words, the mobility enhancement (that the authors attribute to a decrease in the tunnelling distances) overwhelms the reduction in carrier concentration. The opposite occurs in inorganic semiconductors where the changes in μ are small relative to changes in n, and S and σ have opposite dependences on n.

This provides a new strategy to optimise the thermoelectric properties of polymer/organic thermoelectrics. Applying this to PEDOT:PSS, a outstanding value of ZT=0.45 at room temperature has been achieved by dedoping the polymer, which represents another significant enhancement in the efficiency of organic thermoelectrics in a short time.

Impressive Electrochromic Device

Llordés et al. at Lawrence Berkeley National Laboratory (USA) have reported these days in Nature an  outstandng electrochromic device capable to block near-infrared and visible light selectively and independently by varying the applied electrochemical voltage over a range of 2.5 volts. This material demonstrates a novel optical switching behaviour that will enable the dynamic control of solar radiation through windows.

The change in transmittance is achieved at different voltages applied to the device (left graph). The right picture shows the device at fully transparent state (pictures from Nature).

They achieve this by incorporating tin-doped indium oxide (ITO) nanocrystals into niobium oxide glass. This synthetic strategy is called "nanocrystal-in-glass" and nanocrystals become covalently bonded into the amorphous material, resulting in a new amorphous structure. The nanocrystal-in-glass film switches progressively between three optical states: 

• Fully transparent (at 4V versus Li)
• Selective blocking of near-infrared (around 2.3 V)
• Simultaneous blocking of visible and near-infrared (1.5 V)

Thus, solar radiation can now be dynamically modulated with spectral selectivity. For example, we can select reducing the infrared part during hot days (reducing the air conditioning consumption). Both infrared and visible can be cut down in very hot days to gain also privacy and reduce heat input further, or we can also leave it transparent during the winter. Additionally the new material is also highly stable to swith on/off cycling.

Perovskite Solar Cells Reach 15% Efficiency

Perovskite Solar Cells experienced last year a significant development, mainly due to their high efficiency and the ability of being processed from solution (See link). This month a significant step forward has been made by achieving a new high efficiency of 15.4%. This has been announced by Oxford Photovoltaics, the spin-off company devoted to the development and scale-up of this novel technology.

 

 (Dr. Henry Snaith from Oxford University holding a perovskite solar cell)

At EMRS meeting in Strassbourg last month, Henry Snaith reported this achievement. Surprisingly the new cell was constructed as a homogeneous thin film structure, i. e. there is no nanostructured material in it. The cell is formed by TiO2 compact layer as blocking layer, the CH3NH3PbI3 perovskite acting as absorber and electron conductor and the organic Spiro-OMeTAD for hole conduction with gold contacts on top. The record cell showed an efficiency of 15.35% (J_sc= 21 mA/cm², V_oc= 1.07 and FF= 0.67) (See link).

This positions these cells as a highly competitive technology able to provide very low-cost devices, which is one of the most demanded requirements for solar energy to be successful. However, very few things have been reported regarding lifetime and stability which is also very relevant for a market success. Let´s see how these very promising technology evolves and if it will be able to meet all the needs of the market, which is not something easy to achieve.

Polymer-based/Organic Thermoelectrics

A lot of attention has been paid recently to the hardly explored use of conducting polymers as thermoelectric (TE) materials. As it is well-known, a good TE material should have a high figure of merit (ZT), that is, high electrical conductivity and Seebeck coefficient and low thermal conductivity. Thermal conductivity of polymers typically lies in the range of 0.1-1 W/mK, around an order of magnitude lower than the best inorganic materials. However, the electrical conductivity of polymer TEs is not as good and usually extends in a very broad range (from 10e-8 S/cm to 104 S/cm). On the other hand, the Seebeck coefficient ranges from 10 μV/K to 1000 μV/K and, as in many materials, it is not easy to achieve very high electrical conductivities without strongly decreasing the Seebeck coefficient.

(Picture of a P3HT/CNT composite TE from Bounioux et al.)

Last year, Xavier Crispin and co-workers from Linköping University in Sweden achieved the polymer-based TE material with the highest ZT (0.25) at the moment. Their results were published in Nature Materials. They were able to provide PEDOT with an electrical conductivity over 1000 S/cm by replacing the PSS anion with tosylate. Additionally, this week another breakthrough has been reported in Nature where it has been shown that the carrier mobility in conducting polymers can be significantly increased (up to 8.5 cm2/Vs) by covering the delocalised π chain with macrocycles, rising another promising opportunity for more improvement of electrical conductivity.

Another approach that is currently being intensively researched is the preparation of polymer-based nanocomposites, where the polymers are complimented with the properties of nanofillers such as carbon nanotubes. One of the main achievements in this strategy has been reported by See et al. who prepared composites of PEDOT:PSS with Te nanorods, which resulted in a figure of merit ZT=0.1.

If you want to gain more insight into this topic, some recent reviews has been published by He et al. and  Bubnova et al.