Archive for the ‘Hot Article’ Category

Photophysical mechanisms for exceeding the Shockley-Queisser limit in solar energy conversion

Researchers are focused on novel solar cells designs with power conversion efficiencies that exceed the Shockely-Queisser limit. Hot carrier solar cells (HCSC) and multi-exciton generation (MEG) technology aim to reduce thermalization and band gap losses, which together account for >55% of the total absorbed solar energy.

Hot carrier equilibration and carrier multiplication in both molecular and nano materials are two photophysical mechanisms discussed in this paper for implementation in third generation photovoltaics.

Carrier-carrier scattering must be ensured to achieve high efficiency HCSC, as well as inefficient carrier-phonon scattering. The photon flux parameter is a challenge that still need to be addressed, but graphene and related two-dimensional materials seem to be promising.

Multi-exciton solar cells can offer an actual implementation especially for singlet fission in organic semiconductors, which have shown exceptional quantum efficiency of 200% and lots of potential for new molecule designs.

Interested in  better understanding this field? Read more in this Perspective article:

Exceeding the Shockley–Queisser limit in solar energy conversion
Cory A. Nelson, Nicholas R. Monahan and X.-Y. Zhu
DOI: 10.1039/C3EE42098A

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This week’s HOT articles

Take a look at this week’s selection! These articles are available free for a limited time:Graphical abstract: Strongly coupled carbon nanofiber–metal oxide coaxial nanocables with enhanced lithium storage properties

Strongly coupled carbon nanofiber–metal oxide coaxial nanocables with enhanced lithium storage properties
Genqiang Zhang, Hao Bin Wu, Harry E. Hoster and Xiong Wen (David) Lou
DOI: 10.1039/C3EE43123A, Communication

Upgrading of lignin-derived bio-oils by catalytic hydrodeoxygenation
Majid Saidi, Fereshteh Samimi, Dornaz Karimipourfard, Tarit Nimmanwudipong, Bruce C. Gates and Mohammad Reza Rahimpour
DOI: 10.1039/C3EE43081B, Review Article

Human hair-derived carbon flakes for electrochemical supercapacitors
Wenjing Qian, Fengxia Sun, Yanhui Xu, Lihua Qiu, Changhai Liu, Suidong Wang and Feng Yan
DOI: 10.1039/C3EE43111H, Paper

Lithium and oxygen vacancies and their role in Li2O2 charge transport in Li–O2 batteries
J. B. Varley, V. Viswanathan, J. K. Nørskov and A. C. Luntz
DOI: 10.1039/C3EE42446D, Paper

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This week’s HOT articles

Take a look at this week’s selection! These articles are available free for a limited time: Graphical abstract: Transparent paper: fabrications, properties, and device applications

Transparent paper: fabrications, properties, and device applications
Hongli Zhu, Zhiqiang Fang, Colin Preston, Yuanyuan Li and Liangbing Hu
DOI: 10.1039/C3EE43024C, Minireview

Stretching and conformal bonding of organic solar cells to hemispherical surfaces
Timothy F. O’Connor, Aliaksandr V. Zaretski, Bijan A. Shiravi, Suchol Savagatrup, Adam D. Printz, Mare Ivana Diaz and Darren J. Lipomi
DOI: 10.1039/C3EE42898B, Paper

Robust production of purified H2 in a stable, self-regulating, and continuously operating solar fuel generator
Miguel A. Modestino, Karl A. Walczak, Alan Berger, Christopher M. Evans, Sophia Haussener, Carl Koval, John S. Newman, Joel W. Ager and Rachel A. Segalman
DOI: 10.1039/C3EE43214A, Communication

Physics and chemistry of CdTe/CdS thin film heterojunction photovoltaic devices: fundamental and critical aspects
S. Girish Kumar and K. S. R. Koteswara Rao
DOI: 10.1039/C3EE41981A, Review Article

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High capacity electrochemical supercapacitors derived from human hair

Researchers from China have demonstrated a simple method to fabricate high-capacity electrode materials from an abundant and low-cost precursor: human hair. The materials display impressive capacitance values of up to 340 Fg-1 – beating commercial devices by a significant margin.

Developing high-performance and low-cost energy storage systems is a key goal in renewable energy research; such devices are required for numerous green-technologies such as electric vehicles and smart grids. In comparison to batteries, supercapacitors have, at the expense of energy density, relatively high power densities, short charging times and long cycle lives. A key goal of supercapacitor research has therefore been to enhance the energy density whilst maintaining these merits, and at the same time minimising cost.

In their recent article, researchers from Soochow University, China, have demonstrated a simple method whereby porous carbon materials can be prepared by high-temperature treatment of an abundant and renewable resource – human hair. When employed as supercapacitor electrodes, the materials displayed excellent performance, with energy densities up to 340 Fg-1 and negligible capacity loss even after 20,000 charge-discharge cycles.

The authors attributed the enhanced performance to the high surface area and porosity of the materials, along with the naturally high abundance of nitrogen and sulphur found in human hair, which dope the resulting carbon after heat-treatment. So will this be the future of energy storage, or is it just a hair-brained idea?

Find out for yourself, read the full article here:

Human Hair-Derived Carbon Flakes for Electrochemical Supercapacitors
Wenjing Qian, Fengxia Sun, Yanhui Xu, Lihua Qiu, Changhai Liu, Suidong Wang, and Feng Yan
Energy Environ. Sci. 2013, Accepted Manuscript
DOI: 10.1039/C3EE43111H, Paper

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This week’s HOT article

Take a look at this week’s selection! This article is available free for a limited time: Graphical abstract: Constructing ionic highway in alkaline polymer electrolytes

Constructing ionic highway in alkaline polymer electrolytes
Jing Pan, Chen Chen, Yao Li, Lei Wang, Lisheng Tan, Guangwei Li, Xun Tang, Li Xiao, Juntao Lu and Lin Zhuang
DOI: 10.1039/C3EE43275K, Paper

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This week’s HOT articles

Take a look at this week’s selection! These articles are available free for a limited time: Graphical abstract: N-doped graphene film-confined nickel nanoparticles as a highly efficient three-dimensional oxygen evolution electrocatalyst

N-doped graphene film-confined nickel nanoparticles as a highly efficient three-dimensional oxygen evolution electrocatalyst
Sheng Chen, Jingjing Duan, Jingrun Ran, Mietek Jaroniec and Shi Zhang Qiao
DOI: 10.1039/C3EE42383B, Paper

Structure–property relationships of oligothiophene–isoindigo polymers for efficient bulk-heterojunction solar cells
Zaifei Ma, Wenjun Sun, Scott Himmelberger, Koen Vandewal, Zheng Tang, Jonas Bergqvist, Alberto Salleo, Jens Wenzel Andreasen, Olle Inganäs, Mats R. Andersson, Christian Müller, Fengling Zhang and Ergang Wang
DOI: 10.1039/C3EE42989J, Paper

Vertically aligned BaTiO3 nanowire arrays for energy harvesting
Aneesh Koka, Zhi Zhou and Henry A. Sodano
DOI: 10.1039/C3EE42540A, Communication

Piezoelectric and ferroelectric materials and structures for energy harvesting applications
C. R. Bowen, H. A. Kim, P. M. Weaver and S. Dunn
DOI: 10.1039/C3EE42454E, Review Article

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Improving solar cell efficiency by optical design

Table of contents imageScientists in the United States have designed a Cu2ZnSn(S, Se)4 (CZTSSe) solar cell with the greatest efficiency to date using an optical-design approach.

The researchers describe a new optical architecture for CZTSSe photovoltaic devices that improves the record power-conversion efficiency for this technology from 11.1 per cent to 12.0 per cent. CZTSSe absorbers are appealing for terawatt-scale thin-film solar deployment because they are composed of earth-abundant, non-toxic metals.

Using analytical modelling, verified by experiments, the team identified the optimal optical design for increasing the amount of light absorbed in the CZTSSe layer. The new design uses thinner CdS and transparent-conducting layers that lie atop the CZTSSe absorber.

The researchers also showed that that the approach typically used for solar-cell photon management – that is minimising the number of photons reflected from the solar cell surface – does not maximise current for this type of device.

Read the full details of this article in Energy & Environmental Science:

Optical designs that improve the efficiency of Cu2ZnSn(S,Se)4 solar cells
Mark T. Winkler, Wei Wang, Oki Gunawan, Harold J. Hovel, Teodor K. Todorov and David B. Mitzi
DOI: 10.1039/C3EE42541J

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Porous Polymer Networks for Industrial CO2 Capture

Efforts towards a less energy and environmentally intensive method for CO2 capture has resulted in a novel porous polymer with noteworthy performance.

Current industrial standards for carbon dioxide scrubbing in coal-fired power plants utilize an energy intensive liquRates of CO2 Adsorptionid amine process to reduce CO2 gas emissions. Ultimately, a technology that can reduce the energy input required to release captured CO2 while maintaining substantial adsorption selectivity and capacity would be ideal. Investigations performed at Texas A&M have demonstrated high adsorption capacities of 1.7 mmol CO2 /g polymer with a gas flow comprised of 15% CO2 / 85% N2 under ambient conditions.

This work is the fruit of grafting NH4 groups to the sulfonate sites on a porous polymer network that was previously published, which leads to a higher selectivity to CO2 over N2 and CH4 gases also present in the exhaust streams of coal-fired power plants. Further improvements in adsorption capacity, while reducing the temperature required to remove the captured gas, can eventually yield a cheaper, more environmentally friendly alternative for greenhouse gas sequestration.

Interested? Read the full communication in Energy and Environmental Science here:

Building multiple adsorption sites in porous polymer networks for carbon capture applications
Weigang Lu, Wolfgang M. Verdegaal, Jiamei Yu, Perla B. Balbuena, Hae-Kwon Jeong and Hong-Cai Zhou
DOI: 10.1039/C3EE42226G

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Save it for later? Solar and Wind Power Storage, From an Energy Cost Perspective

Researchers compare the energy return on energy investment (EROI) ratios of electricity storage versus curtailing electricity production for renewable, but variable, resources.Wind paired with storage energy curtailment rate

Wind and solar power are current leaders in renewable energy resources, but pose a challenge to more widespread use because of their variable, weather dependent nature. Currently, wind and solar energy harvesting is curtailed during times of oversupply, resulting in a forfeiture of energy.  A clear solution to keep from losing all of this energy would be to store it for later use; however, storage can come at its own energetic cost that is often greater than the gain of the energy stored.

In a recent EES paper, Barnhart et al. examined the EROI ratios for methods of energy storage and compared the ratios with those during curtailment of electricity production for solar and wind power generation. Specifically, they explored electrical energy storage through both battery and geologic storage technologies, and compared the electrical cost of those storage methods to curtailment energy losses.

Barnhart et al. found that, depending on the type of energy resource, usage patterns, and storage type, there are cases in which it is more energetically favorable to store excess electricity, and cases in which it is more favorable to curtail the resources. Conventional battery technologies did not perform well in terms of energy cost compared to geologic storage technologies. Based on these findings, the authors recommend focusing on ways to improve battery EROI ratios, such as improving cycle life, as the next step toward developing electricity storage in which the energy cost is less than loss incurred from curtailment of electricity generation.

This paper recognizes that wind and solar energy are becoming more widely used and are proving to be excellent resources, which is great news from the perspective of reducing carbon usage in electricity production. But it is clear from the findings, as well as from a practical standpoint, that energy storage needs to be the next step for an increase in usability of these resources. These power generation technologies continue to grow, but due to variability, production does not always match with consumer usage patterns. Wouldn’t it be great, from the perspectives of both affordability and sustainably, to be able to run your appliances with solar power at night, or with wind power on a still day? Energy storage that is cost-effective from an energy expenditure standpoint is clearly the next step in paving the way for more widespread use of these sustainable, low-carbon energy resources.

Learn more about this analysis in the full EES article here:

The energetic implications of curtailing versus storing solar- and wind-generated electricity
Charles J. Barnhart, Michael Dale, Adam R. Brandt and Sally M. Benson
Energy Environ. Sci., 2012, 5, 8430
DOI: 10.1039/c2ee21581k

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Advancing Solar Energy: High-Efficiency Silicon Solar Microcells

Schematic of silicon solar microcell fabrication process and SEM and optical microscope images of fabricated cells

A) Microcell design B) Microcell fabrication scheme C)SEM of fabricated microcells D) optical image of microcell array

While silicon solar panels corner a large part of the current solar energy market, they are still too expensive to compete directly with fossil fuels.  However, new advances in silicon solar microcells bring increases in process reliability, efficiency and cost, making solar energy more scalable and affordable.

In their recent paper, Yao et. al demonstrate a redesigned silicon solar µ-cell which incorporates a thermal oxide as a robust etching and diffusion mask, which also serves as an anti-reflection and surface passivation coating.  They also put forth design criteria optimizing the spatial distribution of µ-cells to maximize light-trapping and for the integration of backside reflectors and polymer waveguides into devices for optimal performance.

The figures of merit for the champion device are an open-circuit voltage of 0.534 V, a short-circuit current density of 28.7 mA cm-2, a fill-factor of 0.762 and an overall efficiency of 11.7%.  Interestingly, they demonstrate the efficacy of the thermal oxide as a passivation layer by testing a device before and after the oxide is etched off, showing that removing the oxide causes a significant decrease in performance.  They also show that the incorporation of backside reflectors and planarization layers significantly enhances device performance.

An important consideration for any emerging solar energy technology is scalability.  These redesigned silicon solar µ-cells have a peak-power-generation referenced silicon consumption of only 0.4 g Wp-1, substantially lower than the 10 g Wp-1 of commercially available silicon solar cells.  Combined with the scalability of the processing steps used in the µ-cell fabrication, the low amount of silicon required could be a huge step forward in reducing the cost of solar energy.

Excited about new advances in renewable energy?  Read this full article and many more in EES today!

Fabrication and assembly of ultrathin high-efficiencysilicon solar microcells integrating electrical passivation and anti-reflection coatings
Yuan Yao, Eric Brueckner, Lanfang Lib and Ralph Nuzzo
DOI:10.1039/C3EE42230E

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