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Graphene as a front contact for silicon-perovskite tandem solar cells

Team develops elegant process for coating fragile perovskite layers with graphene for the first time

The perovskite film (black, 200-300 nm) is covered by Spiro.OMeTAD, Graphene with gold contact at one edge, a glass substrate and an amorphous/crystalline silicon solar cell.
Credit: F. Lang / HZB


Silicon absorbers primarily convert the red portion of the solar spectrum very effectively into electrical energy, whereas the blue portions are partially lost as heat. To reduce this loss, the silicon cell can be combined with an additional solar cell that primarily converts the blue portions.

Teams at HZB have already acquired extensive experience with these kinds of tandem cells. A particularly effective complement to conventional silicon is the hybrid material called perovskite. It has a band gap of 1.6 electron volts with organic as well as inorganic components. However, it is very difficult to provide the perovskite layer with a transparent front contact. While sputter deposition of indium tin oxide (ITO) is common practice for inorganic silicon solar cells, this technique destroys the organic components of a perovskite cell.

Graphene as transparent front contact:

Now a group headed by Prof. Norbert Nickel has introduced a new solution. Dr. Marc Gluba and PhD student Felix Lang have developed a process to cover the perovskite layer evenly with graphene. Graphene consists of carbon atoms that have arranged themselves into a two-dimensional honeycomb lattice forming an extremely thin film that is highly conductive and highly transparent.

Fishing for graphene:

As a first step, the scientists promote growth of the graphene onto copper foil from a methane atmosphere at about 1000 degrees Celsius. For the subsequent steps, they stabilise the fragile layer with a polymer that protects the graphene from cracking. In the following step, Felix Lang etches away the copper foil. This enables him to transfer the protected graphene film onto the perovskite. "This is normally carried out in water. The graphene film floats on the surface and is fished out by the solar cell, so to speak. However, in this case this technique does not work, because the performance of the perovskite degrades with moisture. Therefore we had to find another liquid that does not attack perovskite, yet is as similar to water as possible," explains Gluba.

Ideal front contact:

Subsequent measurements showed that the graphene layer is an ideal front contact in several respects. Thanks to its high transparency, none of the sunlight's energy is lost in this layer. But the main advantage is that there are no open-circuit voltage losses, that are commonly observed for sputtered ITO layers. This increases the overall conversion efficiency. "This solution is comparatively simple and inexpensive to implement," says Nickel. "For the first time, we have succeeded in implementing graphene in a perovskite solar cell. This enabled us to build a high-efficiency tandem device."

Source by: http://www.sciencedaily.com/releases/2015/10/151002113551.htm

Record High Performance With New Solar Cells

Researchers are reporting record-high efficiency levels for a new generation of solar cells.
Credit: National Renewable Energy Lab

Researchers in China and Switzerland are reporting the highest efficiency ever for a promising new genre of solar cells, which many scientists think offer the best hope for making the sun a mainstay source of energy in the future. The photovoltaic cells, called dye-sensitized solar cells or Grätzel cells, could expand the use of solar energy for homes, businesses, and other practical applications, the scientists say.

The research, conducted by Peng Wang and colleagues — who include Michael Grätzel, inventor of the first dye-sensitized solar cell — involves photovoltaic cells composed of titanium dioxide and powerful light-harvesting dyes. Grätzel cells are less expensive than standard silicon-based solar cells and can be made into flexible sheets or coatings.

Although promising, Grätzel cells until now have had serious drawbacks. They have not been efficient enough at converting light into electricity. And their performance dropped after relatively short exposures to sunlight.

In the new study, researchers describe lab tests of solar cells made with a new type of ruthenium-based dye that helps boost the light-harvesting ability. The new cells showed efficiencies as high as 10 percent, a record for this type of solar cell. The new cells also showed greater stability at high temperatures than previous formulas, retaining more than 90 percent of their initial output after 1,000 hours in full sunlight.

Source by: http://www.sciencedaily.com/releases/2008/11/081103124224.htm

Breakthrough for iron based dyes can lead to cheaper and environmentally friendly solar energy applications

Researchers at Lund University in Sweden have found a new way to capture energy from sunlight -- by using molecules that contain iron. The results are presented in the latest issue of Nature Chemistry. The hope is to develop efficient and environmentally friendly solar energy applications.

Solar energy is an inexhaustible resource that we currently only utilise to a very limited extent. Researchers around the world are therefore trying to find new and more efficient ways to use the energy in sunlight.

The technique the researchers in Lund are working on is solar cells consisting of a thin film of nanostructured titanium dioxide and a dye that captures solar energy. Today, the best solar cells of this type use dyes containing ruthenium metal -- a very rare and expensive element.

"Many researchers have tried to replace ruthenium with iron, but without success. All previous attempts have resulted in molecules that convert light energy into heat instead of electrons, which is required for solar cells to generate electricity," says Villy Sundström, Professor of Chemical Physics at Lund University.

Researchers at the Chemistry Department in Lund, in collaboration with Uppsala University, have now successfully produced an iron-based dye that is capable of converting light into electrons with nearly 100 per cent efficiency.

"The advantage of using iron is that it is a common element in nature. It can provide inexpensive and environmentally friendly applications of solar energy in the future," says Kenneth Wärnmark, Professor of Organic Chemistry at Lund University.

By combining the experiments with advanced computer simulations, the researchers are able to understand in detail required design concepts for the iron molecules to work. This knowledge is now being used for further developing the iron-based dyes. More research is needed before the new solar cell dye can be used in practice, but there are high hopes.

"The results of the study suggest that solar cells based on these materials can be at least as effective as those of today that are based on ruthenium or other rare metals," says Villy Sundström.

The discovery could also advance research on solar fuels in which, like in photosynthesis of plants, water and carbon dioxide are turned into energy-rich molecules -- solar fuel -- with the help of sunlight.

"We envision that the new iron-based molecules could also drive the chemical reactions that create solar fuel," says Kenneth Wärnmark.

The researchers have worked on developing iron-based solar cell dyes for three years and are surprised by how quickly they found a dye that can capture sunlight as efficiently as this.

"Achieving success in research usually takes longer than what we hope for and believe," says Villy Sundström and continues: "For once, it was the opposite!."

Source by: http://www.sciencedaily.com/releases/2015/10/151013112359.htm

Solar vehicle charging at home

An electric vehicle is charged with photovoltaic power from the roof of the house using a charging station.
Credit: © Fraunhofer ISE


Owners of home photovoltaic systems will soon be able to make their households even more sustainable, because PV power is also suitable for charging personal electronic vehicles. A home energy management system created by Fraunhofer researchers incorporates electric vehicles into the household energy network and creates charging itineraries.

The house of the future is environmentally friendly, energy efficient and smart. Its inhabitants can utilize rooftop-generated PV energy not only for household consumption but also to charge their personal electric vehicle. This scenario has already become reality for a collection of row houses built according to the "Passive House" standard in the German city of Fellbach in Baden-Württemberg. The group of new homes was upgraded as part of the "Fellbach ZeroPlus" project to include electromobility enhancements as well as a comprehensive energy management system. The initiative is sponsored by the German Federal Government's "Electric Mobility Showcase" program.

Fast charging stations and home energy management

"The large photovoltaic systems on the rooftops of the houses provide more power than the inhabitants consume over the long term. Surplus power can be fed into the public grid as well as be used for charging the household electric vehicle," explains Dominik Noeren, a scientist at the Fraunhofer Institute for Solar Energy Systems ISE in Freiburg. To efficiently incorporate electromobility enhancements into the daily routines of the households, Noeren and his team designed a 22 kW fast charging station as well as a home energy management system (HEMS) for five of the seven homes. The Java-based HEMS software runs on small computers known as embedded systems. The HEMS collects data from the various electricity meters in the house, including those for the photovoltaic system, the electric vehicle, the heat pump, and general household power. The system displays the various power flows and informs the homeowners about their current power consumption at any time of the day. "They can see how much power is coming from either the public grid or the household solar system, and they can see where it is going -- to the heat pump, household appliances, or the electric vehicle," says Noeren.

Furthermore, the HEMS also forecasts solar intensity over the next 20 hours or so and provides users with information on how much solar power is available. An adaptive algorithm also computes anticipated household power loads for each quarter hour. Using this data, it is possible to determine how much PV power is available for the electric vehicle at any given time. "Electricity from the PV first goes to the house, and power that is not consumed there is stored in the electric vehicle battery. If there is still any electricity left over after that, it is fed into the public electricity grid," explains Noeren.

During two years of field testing, an Android application was created using feedback from the homeowners. The HEMS app provides a visualization of all processes and electricity flows in real time, and gives solar intensity forecast readouts in graphical and numerical form. An adaptive algorithm works to optimize the use of the power generated by each household. Through the app, users can control the charging station as well as view the battery charge level and charging times of the electric vehicle. "These parameters are necessary in order to intelligently charge the electric vehicle," says Noeren.

To create an ideal charging itinerary, the system must know the vehicle's current battery charge level as well as its next planned departure time. The energy management system uses this information together with weather and consumption forecasts to estimate the flows through the household power network. It calculates how much electricity must be topped up, as well as which time periods are ideal for recharging the vehicle using the greatest possible proportion of household-produced solar energy.

"It is more cost effective to consume the self-generated solar electricity than to feed it into the public electricity grid," says Noeren. The HEMS system helps consumers use data on driving times, solar intensity forecasts and current household energy consumption to synchronize electric vehicle charging times with rooftop energy production, so they can maximize the proportion of household-produced energy they use. This not only helps homeowners lower their costs, but it also goes a step closer towards realizing the ideal of low-CO2 homes and personal mobility. Maximizing the proportion of household-produced energy consumed helps unburden the public power grid while reducing household feed-in peaks to the grid.

The HEMS system is based on the Fraunhofer openMUC framework, which supports a wide variety of meters and devices. It offers modular expandability for integrating devices such as wireless Bluetooth or WLAN power outlets that can remotely activate and deactivate household appliances, or for integrating high-consumption items such as heat pumps. Two of the five households in the "Fellbach ZeroPlus" project have been successfully using a car-sharing variant of the system as part of a field test since mid-2014.

Source by: http://www.sciencedaily.com/releases/2015/11/151104130050.htm

Greener' way to assemble materials for solar applications

A surfactant template guides the self-assembly of functional polymer structures in an aqueous solution.
Credit: Image credit: Oak Ridge National Laboratory, U.S. Dept. of Energy; image by Youngkyu Han and Renee Manning.

The efficiency of solar cells depends on precise engineering of polymers that assemble into films 1,000 times thinner than a human hair.

Today, formation of that polymer assembly requires solvents that can harm the environment, but scientists at the Department of Energy's Oak Ridge National Laboratory have found a "greener" way to control the assembly of photovoltaic polymers in water using a surfactant-- a detergent-like molecule--as a template. Their findings are reported in Nanoscale, a journal of the Royal Society of Chemistry.

"Self-assembly of polymers using surfactants provides huge potential in fabricating nanostructures with molecular-level controllability," said senior author Changwoo Do, a researcher at ORNL's Spallation Neutron Source (SNS).

The researchers used three DOE Office of Science User Facilities--the Center for Nanophase Materials Sciences (CNMS) and SNS at ORNL and the Advanced Photon Source (APS) at Argonne National Laboratory--to synthesize and characterize the polymers.

"Scattering of neutrons and X-rays is a perfect method to investigate these structures," said Do.

The study demonstrates the value of tracking molecular dynamics with both neutrons and optical probes.

"We would like to create very specific polymer stacking in solution and translate that into thin films where flawless, defect-free polymer assemblies would enable fast transport of electric charges for photovoltaic applications," said Ilia Ivanov, a researcher at CNMS and a corresponding author with Do. "We demonstrated that this can be accomplished through understanding of kinetic and thermodynamic mechanisms controlling the polymer aggregation."

The accomplishment creates molecular building blocks for the design of optoelectronic and sensory materials. It entailed design of a semiconducting polymer with a hydrophobic ("water-fearing") backbone and hydrophilic ("water-loving") side chains. The water-soluble side-chains could allow "green" processing if the effort produced a polymer that could self-assemble into an organic photovoltaic material. The researchers added the polymer to an aqueous solution containing a surfactant molecule that also has hydrophobic and hydrophilic ends. Depending on temperature and concentration, the surfactant self-assembles into different templates that guide the polymer to pack into different nanoscale shapes--hexagons, spherical micelles and sheets.

In the semiconducting polymer, atoms are organized to share electrons easily. The work provides insight into the different structural phases of the polymer system and the growth of assemblies of repeating shapes to form functional crystals. These crystals form the basis of the photovoltaic thin films that provide power in environments as demanding as deserts and outer space.

"Rationally encoding molecular interactions to rule the molecular geometry and inter-molecular packing order in a solution of conjugated polymers is long desired in optoelectronics and nanotechnology," said the paper's first author, postdoctoral fellow Jiahua Zhu. "The development is essentially hindered by the difficulty of in situ characterization."

In situ, or "on site," measurements are taken while a phenomenon (such as a change in molecular morphology) is occurring. They contrast with measurements taken after isolating the material from the system where the phenomenon was seen or changing the test conditions under which the phenomenon was first observed. The team developed a test chamber that allows them to use optical probes while changes occur.

Neutrons can probe structures in solutions

Expertise and equipment at SNS, which provides the most intense pulsed neutron beams in the world, made it possible to discover that a functional photovoltaic polymer could self-assemble in an environmentally benign solvent. The efficacy of the neutron scattering was enhanced, in turn, by a technique called selective deuteration, in which specific hydrogen atoms in the polymers are replaced by heavier atoms of deuterium--which has the effect of heightening contrasts in the structure. CNMS has a specialty in the latter technique.

"We needed to be able to see what's happening to these molecules as they evolve in time from some solution state to some solid state," author Bobby Sumpter of CNMS said. "This is very difficult to do, but for molecules like polymers and biomolecules, neutrons are some of the best probes you can imagine." The information they provide guides design of advanced materials.

By combining expertise in topics including neutron scattering, high-throughput data analysis, theory, modeling and simulation, the scientists developed a test chamber for monitoring phase transitions as they happened. It tracks molecules under conditions of changing temperature, pressure, humidity, light, solvent composition and the like, allowing researchers to assess how working materials change over time and aiding efforts to improve their performance.

Scientists place a sample in the chamber and transport it to different instruments for measurements. The chamber has a transparent face to allow entry of laser beams to probe materials. Probing modes--including photons, electrical charge, magnetic spin and calculations aided by high-performance computing--can operate simultaneously to characterize matter under a broad range of conditions. The chamber is designed to make it possible, in the future, to use neutrons and X-rays as additional and complementary probes.

"Incorporation of in situ techniques brings information on kinetic and thermodynamic aspects of materials transformations in solutions and thin films in which structure is measured simultaneously with their changing optoelectronic functionality," Ivanov said. "It also opens an opportunity to study fully assembled photovoltaic cells as well as metastable structures, which may lead to unique features of future functional materials."

Whereas the current study examined phase transitions (i.e., metastable states and chemical reactions) at increasing temperatures, the next in situ diagnostics will characterize them at high pressure. Moreover, the researchers will implement neural networks to analyze complex nonlinear processes with multiple feedbacks.

The title of the Nanoscale paper is "Controlling molecular ordering in solution-state conjugated polymers."

Source by: http://www.sciencedaily.com/releases/2015/10/151005163040.htm