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‘Deforming’ solar cells could be clue to improved efficiency

‘Deforming’ solar cells could be clue to improved efficiency

  • Deformations and defects in structures of photoelectric technologies shown to improve their efficiency
  • University of Warwick physicists demonstrate that strain gradient can prevent recombination of photo-excited carriers in solar energy conversion
  • Increasingly important as devices become miniaturised

Solar panelsSolar cells and light-sensing technologies could be made more efficient by taking advantage of an unusual property due to deformations and defects in their structures.

Researchers from the University of Warwick’s Department of Physics have found that the strain gradient (i.e. inhomogeneous strain) in the solar cells, through physical force or induced during the fabrication process, can prevent photo-excited carriers from recombining, leading to an enhanced solar energy conversion efficiency. The results of their experiments have been published in Nature Communications.

The team of scientists used an epitaxial thin film of BiFeO3 grown on LaAlO3 substrate to determine the impact of inhomogenous deformation on the film’s ability to convert light into electricity by examining how its strain gradient affects its ability to separate photo-excited carriers.

Most commercial solar cells are formed of two layers creating at their boundary a junction between two kinds of semiconductors, p-type with positive charge carriers (electron vacancies) and n-type with negative charge carriers (electrons). When light is absorbed, the junction of the two semiconductors sustains an internal field splitting the photo-excited carriers in opposite directions, generating a current and voltage across the junction. Without such junctions the energy cannot be harvested and the photo-excited carriers will simply quickly recombine eliminating any electrical charge.

They found that the strain gradient can help prevent recombination by separating the light-excited electron-holes, enhancing the conversion efficiency of the solar cells. The BiFeO3/LaAlO3 film also exhibited some interesting photoelectric effects, such as persistent photoconductivity (improved electrical conductivity). It has potential applications in UV light sensors, actuators and transducers.

Dr Mingmin Yang from the University of Warwick said: “This work demonstrated the critical role of the strain gradient in mediating local photoelectric properties, which is largely overlooked previously. By engineering photoelectric technologies to take advantage of strain gradient, we may potentially increase the conversion efficiency of solar cells and enhance the sensitivity of light sensors.

“Another factor to consider is the grain boundaries in polycrystalline solar cells. Generally, defects accumulate at the grain boundaries, which would induce photo-carrier recombination, limiting efficiency. However, in some polycrystalline solar cells, such as CdTe solar cells, the grain boundaries would promote the collection of photo-carriers, where the giant strain gradient might play an important role. Therefore, we need to pay attention to the local strain gradient when we study the structure-properties relations in solar cells and light sensor materials.”

Previously, the effect of this strain on efficiency was thought to be negligible. With the increasing miniaturisation of technologies, the effect of strain gradient becomes magnified at smaller sizes. So in reducing the size of a device using one of these films, the magnitude of strain gradient increases dramatically.

Dr Yang adds: “The strain gradient induced effect, such as flexo-photovoltaic effect, ionic migration, etc, would be increasingly important at low dimensions.”

Materials provided by University of Warwick

Floating desalination unit

A unique technique to remove salt from seawater using solar power

Researchers have recently discovered a new convenient nanoscale method of removing salt from sea-water which is likely to benefit 844 Million people globally who don’t have regular access to clean water. The findings have been published in the journal Energy & Environmental Science.

The device completely operates on sunlight and uses a small disc made of super-hydrophilic filter paper laminated with carbon nanotubes for light absorption and is able to remove 100% salt from the original liquid. The new method will turn water into steam using solar energy and solar thermal materials and is based on the traditional way of heating water until it steams, leaving salt and impurities behind. It also maintains a constant rate of water evaporation as the salts are collected and removed to avoid the system to halt because of the covering of salt crystals.

Devices using desalination method powered by sunlight can be used in those places with no access to electricity because it is economical and effective. Chemical engineer Xiwang Zhang, from Monash University in Australia said that the study results progress one step towards solar steam generation technology in seawater desalination, resource recovery from sewage and zero fluid discharge. Moreover, he added that this research can be building brick for future researches in energy-passive methods to supply clean and safe water to millions of people, illuminating environment effects from waste and recovering resource from waste.

A cotton thread of 1-millimetre (0.04 inch) diameter is used to carry saline water to the evaporation disk where clean water is collected, pushing out salt towards the edges. Researchers during testing found out that it makes good use of solar energy that is available with over 94 percent light absorption throughout the entire solar spectrum. Zhang told that this creative method is able to generate 6-8 litres (or 1.6-2.1 gallons) per square metre (10.8 square feet) of surface area of clean water per day. The next target is to increase the production rate.

Last year, a group from US have developed a one more fascinating and powerful desalination method to remove salt from Dead Sea using hydrogel-based, solar powered technique. However, the challenge is to put the systems in place and scaling up the methods so that sufficient amount of water can be processed.

Zhang said that water protection is the greatest challenge globally in the 21st Century as population grows and climate varies, especially for developing and under-developed communities. He also said that one of the permanent and eco-friendly solution for scarcity of clean water is production by utilising solar energy.

Experiments show dramatic increase in solar cell output

Experiments show dramatic increase in solar cell output

In any conventional silicon-based solar cell, there is an absolute limit on overall efficiency, based partly on the fact that each photon of light can only knock loose a single electron, even if that photon carried twice the energy needed to do so. But now, researchers have demonstrated a method for getting high-energy photons striking silicon to kick out two electrons instead of one, opening the door for a new kind of solar cell with greater efficiency than was thought possible.

While conventional silicon cells have an absolute theoretical maximum efficiency of about 29.1 percent conversion of solar energy, the new approach, developed over the last several years by researchers at MIT and elsewhere, could bust through that limit, potentially adding several percentage points to that maximum output. The results are described today in the journal Nature, in a paper by graduate student Markus Einzinger, professor of chemistry Moungi Bawendi, professor of electrical engineering and computer science Marc Baldo, and eight others at MIT and at Princeton University.

The basic concept behind this new technology has been known for decades, and the first demonstration that the principle could work was carried out by some members of this team six years ago. But actually translating the method into a full, operational silicon solar cell took years of hard work, Baldo says.

That initial demonstration “was a good test platform” to show that the idea could work, explains Daniel Congreve PhD ’15, an alumnus now at the Rowland Institute at Harvard, who was the lead author in that prior report and is a co-author of the new paper. Now, with the new results, “we’ve done what we set out to do” in that project, he says.

The original study demonstrated the production of two electrons from one photon, but it did so in an organic photovoltaic cell, which is less efficient than a silicon solar cell. It turned out that transferring the two electrons from a top collecting layer made of tetracene into the silicon cell “was not straightforward,” Baldo says. Troy Van Voorhis, a professor of chemistry at MIT who was part of that original team, points out that the concept was first proposed back in the 1970s, and says wryly that turning that idea into a practical device “only took 40 years.”

The key to splitting the energy of one photon into two electrons lies in a class of materials that possess “excited states” called excitons, Baldo says: In these excitonic materials, “these packets of energy propagate around like the electrons in a circuit,” but with quite different properties than electrons. “You can use them to change energy — you can cut them in half, you can combine them.” In this case, they were going through a process called singlet exciton fission, which is how the light’s energy gets split into two separate, independently moving packets of energy. The material first absorbs a photon, forming an exciton that rapidly undergoes fission into two excited states, each with half the energy of the original state.

But the tricky part was then coupling that energy over into the silicon, a material that is not excitonic. This coupling had never been accomplished before.

As an intermediate step, the team tried coupling the energy from the excitonic layer into a material called quantum dots. “They’re still excitonic, but they’re inorganic,” Baldo says. “That worked; it worked like a charm,” he says. By understanding the mechanism taking place in that material, he says, “we had no reason to think that silicon wouldn’t work.”

What that work showed, Van Voorhis says, is that the key to these energy transfers lies in the very surface of the material, not in its bulk. “So it was clear that the surface chemistry on silicon was going to be important. That was what was going to determine what kinds of surface states there were.” That focus on the surface chemistry may have been what allowed this team to succeed where others had not, he suggests.

The key was in a thin intermediate layer. “It turns out this tiny, tiny strip of material at the interface between these two systems [the silicon solar cell and the tetracene layer with its excitonic properties] ended up defining everything. It’s why other researchers couldn’t get this process to work, and why we finally did.” It was Einzinger “who finally cracked that nut,” he says, by using a layer of a material called hafnium oxynitride.

The layer is only a few atoms thick, or just 8 angstroms (ten-billionths of a meter), but it acted as a “nice bridge” for the excited states, Baldo says. That finally made it possible for the single high-energy photons to trigger the release of two electrons inside the silicon cell. That produces a doubling of the amount of energy produced by a given amount of sunlight in the blue and green part of the spectrum. Overall, that could produce an increase in the power produced by the solar cell — from a theoretical maximum of 29.1 percent, up to a maximum of about 35 percent.

Actual silicon cells are not yet at their maximum, and neither is the new material, so more development needs to be done, but the crucial step of coupling the two materials efficiently has now been proven. “We still need to optimize the silicon cells for this process,” Baldo says. For one thing, with the new system those cells can be thinner than current versions. Work also needs to be done on stabilizing the materials for durability. Overall, commercial applications are probably still a few years off, the team says.

Other approaches to improving the efficiency of solar cells tend to involve adding another kind of cell, such as a perovskite layer, over the silicon. Baldo says “they’re building one cell on top of another. Fundamentally, we’re making one cell — we’re kind of turbocharging the silicon cell. We’re adding more current into the silicon, as opposed to making two cells.”

The researchers have measured one special property of hafnium oxynitride that helps it transfer the excitonic energy. “We know that hafnium oxynitride generates additional charge at the interface, which reduces losses by a process called electric field passivation. If we can establish better control over this phenomenon, efficiencies may climb even higher.” Einzinger says. So far, no other material they’ve tested can match its properties.

Materials provided by Massachusetts Institute of Technology

Researchers pioneer method to purify water using solar energy

Researchers pioneer method to purify water using solar energy

As the global population grows, fresh water supplies are more precious than ever. While scientists and engineers know how to purify water, making those methods sustainable and energy efficient is another question.

One promising approach is solar-driven distillation, or solar steam generation, which can help us get fresh water from wastewater or seawater. Researchers have used this method to successfully distill small batches of purified water, but they are still searching for a way to do this on a large scale.

Researchers at the University of Chicago’s Pritzker School of Molecular Engineering and UChicago-affiliated Argonne National Laboratory were part of a team that developed a pioneering new method of solar steam generation that could help bring this technology into the real world. The materials can be grown on top of wood, fabric or sponges in an easy, one-step process, and show promise for large-scale manufacturing.

“Solar steam generation techniques are still mostly focused on lab use now,” said Zijing Xia, a graduate student at Pritzker Molecular Engineering and lead author of the research. “We want to find an easy way to fabricate solar steam generators at relatively low cost.”

The results of their innovative work were recently published in the journal Advanced Materials Interfaces.

“We want to find an easy way to fabricate solar steam generators at relatively low cost.”

—Graduate student and study lead author Zijing Xia

In the search for solar steam systems, researchers have already tried various materials which convert light to heat, such as carbon materials, plasmonic metals and semiconductors. But many of these options have relatively low efficiency, among other challenges, and so the search continues for a truly transformative method.

A high-performance solar steam generator will ideally combine several characteristics. It should be buoyant on water, be able to absorb a broad spectrum of light, convert light to heat efficiently and be able to transfer that heat to water. Unfortunately, many previously studied methods lack the porous structure needed to facilitate the heat transfer to water.

“Most existing methods cannot be easily engineered to produce steam-generating devices with both arbitrary control over the shape and high photothermal efficiency,” Xia said.

What sets Xia’s method apart is the use of a porphyrin covalent organic framework, or POF. A newly discovered class of materials, POFs can grow uniformly on the surface of a variety of materials with different levels of porosity, and they show high performance for water evaporation. POFs also have unique light-harvesting characteristics beneficial for new applications.

In the lab, POFs successfully grew on the inner and outer surfaces of every tested material. And every template showed favorable photothermal properties, indicating that POF-based materials are promising candidates for solar steam generation. The POF membrane was able to capture more than 95% of light across the majority of the spectrum of sunlight.

The most promising result of the research, Xia said, was the POFs’ ability to grow at the surface of many different kinds of materials, including membranes, fabrics, sponges and wood. The wood showed particularly strong performance, with researchers measuring roughly 80% light-to-steam conversion efficiency.

Solar Steam Illustration

An illustration of water evaporation through the POF‐based materials. (Credit: Zijing Xia et al)

The ability of POFs to grow on many types of materials makes them easily adaptable for use with locally available materials. This versatility, coupled with the easy, one-step fabrication process, could make the method practical for large-scale production.

The POF-based approach proved highly effective in a lab setting, and the research team plans to conduct further experiments outside the lab to observe the practical performance of POFs.

So far, the research suggests POFs could help drive the sustainable water purification systems of the future.

“POF-based interface engineering design shows promise for large-scale purification methods, and it could also be used for desalination, wastewater treatment and beyond,” Xia said.

Other authors of the paper include PME graduate students Ruben Z. Waldman and Chao Zhang, PME professor and Argonne scientist Shrayesh Patel, and Argonne scientist and PME fellow Seth Darling. Additional authors include Zhaowei Chen of Argonne, Hao-Cheng Yang of Sun Yat-sen University and Yusen Zhao of UCLA.

Citation: “Porphyrin Covalent Organic Framework (POF)-Based Interface Engineering for Solar Steam Generation.” Zijing Xia et al. Advanced Materials Interfaces. Doi: 10.1002/admi.201900254

Funding: The Advanced Materials for Energy-Water Systems (AMEWS) Center

Materials provided by the University of Chicago