̽��ֱ�� of Cambridge - exciton

Researchers road-test powerful method for studying singlet fission

tdk25 — Mon, 17 Oct 2016 15:00:00 +0000

Physicists have successfully employed a powerful technique for studying electrons generated through singlet fission, a process which it is believed will be key to more efficient solar energy production in years to come.

Their approach, reported in the journal Nature Physics, employed lasers, microwave radiation and magnetic fields to analyse the spin of excitons, which are energetically excited particles formed in molecular systems.

These are generated as a result of singlet fission, a process that researchers around the world are trying to understand fully in order to use it to better harness energy from the sun. Using materials exhibiting singlet fission in solar cells could make energy production much more efficient in the future, but the process needs to be fully understood in order to optimize the relevant materials and design appropriate technologies to exploit it.

In most existing solar cells, light particles (or photons) are absorbed by a semiconducting material, such as silicon. Each photon stimulates an electron in the material's atomic structure, giving a single electron enough energy to move. This can then potentially be extracted as electrical current.

In some materials, however, the absorption of a single photon initially creates one higher-energy, excited particle, called a spin singlet exciton. This singlet can also share its energy with another molecule, forming two lower-energy excitons, rather than just one. These lower-energy particles are called spin "triplet" excitons. Each triplet can move through the molecular structure of the material and be used to produce charge.

̽��ֱ��splitting process - from one absorbed photon to two energetic triplet excitons - is singlet fission. For scientists studying how to generate more solar power, it represents a potential bargain - a two-for-one offer on the amount of electrical current generated, relative to the amount of light put in. If materials capable of singlet fission can be integrated into solar cells, it will become possible to generate energy more efficiently from sunlight.

But achieving this is far from straightforward. One challenge is that the pairs of triplet excitons only last for a tiny fraction of a second, and must be separated and used before they decay. Their lifespan is connected to their relative "spin", which is a unique property of elementary particles and is an intrinsic angular momentum. Studying and measuring spin through time, from the initial formation of the pairs to their decay, is essential if they are to be harnessed.

In the new study, researchers from the ̽��ֱ�� of Cambridge and the Freie Universität Berlin (FUB) utilised a method that allows the spin properties of materials to be measured through time. ̽��ֱ��approach, called electron spin resonance (ESR) spectroscopy, has been used and improved since its discovery over 50 years ago to better understand how spin impacts on many different natural phenomena.

It involves placing the material being studied within a large electromagnet, and then using laser light to excite molecules within the sample, and microwave radiation to measure how the spin changes over time. This is especially useful when studying triplet states formed by singlet fission as these are difficult to study using most other techniques.

Because the excitons' spin interacts with microwave radiation and magnetic fields, these interactions can be used as an additional way to understand what happens to the triplet pairs after they are formed. In short, the approach allowed the researchers to effectively watch and manipulate the spin state of triplet pairs through time, following formation by singlet fission.

̽��ֱ��study was led by Professor Jan Behrends at the Freie Universität Berlin (FUB), Dr Akshay Rao, a College Research Associate at St John's College, ̽��ֱ�� of Cambridge, and Professor Neil Greenham in the Department of Physics, ̽��ֱ�� of Cambridge.

Leah Weiss, a Gates-Cambridge Scholar and PhD student in Physics based at Trinity College, Cambridge, was the paper's first author. "This research has opened up many new questions," she said. "What makes these excited states either separate and become independent, or stay together as a pair, are questions that we need to answer before we can make use of them."

̽��ֱ��researchers were able to look at the spin states of the triplet excitons in considerable detail. They observed pairs had formed which variously had both weakly and strongly-linked spin states, reflecting the co-existence of pairs that were spatially close and further apart. Intriguingly, the group found that some pairs which they would have expected to decay very quickly, due to their close proximity, actually survived for several microseconds.

"Finding those pairs in particular was completely unexpected," Weiss added. We think that they could be protected by their overall spin state, making it harder for them to decay. Continued research will focus on making devices and examining how these states can be harnessed for use in solar cells."

Professor Behrends added: "This interdisciplinary collaboration nicely demonstrates that bringing together expertise from different fields can provide novel and striking insights. Future studies will need to address how to efficiently split the strongly-coupled states that we observed here, to improve the yield from singlet fission cells."

Beyond trying to improve photovoltaic technologies, the research also has implications for wider efforts to create fast and efficient electronics using spin, so-called "spintronic" devices, which similarly rely on being able to measure and control the spin properties of electrons.

̽��ֱ��research was made possible with support from the UK Engineering and Physical Sciences Research Council (EPSRC) and from the Freie Universität Berlin (FUB). Weiss and colleague Sam Bayliss carried out the spectroscopy experiments within the laboratories of Professor Jan Behrends and Professor Robert Bittl at FUB. ̽��ֱ��work is also part of the Cambridge initiative to connect fundamental physics research with global energy and environmental challenges, backed by the Winton Programme for the Physics of Sustainability.

̽��ֱ��study, Strongly exchange-coupled triplet pairs in an organic semiconductor, is published in Nature Physics. DOI: 10.1038/nphys3908.

In a new study, researchers measure the spin properties of electronic states produced in singlet fission – a process which could have a central role in the future development of solar cells.

Future research will focus on making devices and examining how these states can be harnessed for use in solar cells

Leah Weiss

Spin, an intrinsic property of electrons, is related to the dynamics of electrons excited as a result of singlet fission – a process which could be used to extract energy in future solar cell technologies.

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

Yes

Mirage maker

lw355 — Fri, 30 Oct 2015 11:30:54 +0000

This is a photothermal deflection spectrometer (PDS) and the mirage – only the width of a human hair in distance from the glass – is helping researchers to measure the quality of materials that turn light energy into electricity.

“We can see one defect in a million molecules,” explains Sadhanala, who built the machine while working on his PhD in the lab of Professor Sir Richard Friend. “ ̽��ֱ��PDS technique measures the amount of light absorbed by a material with up to five orders of magnitude more sensitivity than conventional techniques, making it one of the most sensitive absorption spectrometers in the world.”

A mirage is formed as light is bent when it passes through a medium with varying refractive index – a puddle of water seems to appear on the road ahead, for instance, when light meets hotter air radiating from the ground on a sunny day.

Sadhanala’s machine creates a mirage effect when light absorbed by the solar material is released as heat, which passes to a liquid that surrounds the sample. When a laser beam is directed to pass parallel to it, the mirage deflects the beam; the amount of deflection corresponds to the amount of heat absorbed, which in turn corresponds to the amount of light absorbed.

“Before, we would have had to make a whole solar device and spend months and months testing it to find out how efficient the material is,” explains Sadhanala. “Now you can measure a new material in half a day, and you don’t even need to make the whole device – you just need to be able to coat the material onto glass and make a mirage.”

A few weeks ago, Tushita Mukhopadhyay – a chemist at the Indian Institute of Science, Bangalore – carefully packaged up five new materials and flew to the UK to test them on the ‘mirage machine’ in Cambridge, and analyse other properties with researchers at Imperial College London. She had spent months making and characterising the materials – all of them belonging to a group of organic solar cells (OSCs) that can be printed as thin-film sheets.

What connects the two researchers, and indeed many other chemists, physicists and engineers, is the APEX project – an ambitious Anglo-Indian initiative to turn fundamental advances in solar materials into commercial reality.

APEX involves research institutes in Bangalore, Delhi, Hyderabad, Kanpur and Pune in India, and Brunel (which leads the partnership), Cambridge, Edinburgh, Imperial, Swansea and Oxford in the UK, plus solar industries in both countries. It has received almost £6 million funding since 2010 from the Indian Department of Science and Technology and Research Councils UK.

“Solar has always been the eventual solution to our energy problems but it’s always been the day after tomorrow,” explains Friend, who leads the Cambridge component. “Each of the partners in this project has an extensive research programme aimed at developing highly efficient photovoltaic devices, but there is a disconnect between what you can do in the lab and what can be rolled out at huge scale. This project is aimed at moving from established science to a viable technology.”

First though, there is the matter of achieving a major cost reduction and efficiency increase in solar power. ̽��ֱ��APEX team started by focusing on developing a new class of ‘excitonic’ solar cell (which produces electricity from the sun’s energy through the creation of an ‘exciton’ – essentially a free electron). Instead of using the conventional solar material, silicon, the researchers used solar materials made from organic dyes – dye-sensitised solar cells (DSSCs) – which are easy to make, easy to process and cost less.

However, one of the main issues surrounding the search for alternative solar materials to silicon has been their power conversion efficiencies (PCEs) – the amount of the sun’s energy that can be trapped and turned into electricity.

̽��ֱ��PCE for silicon is around 25%, whereas the current state-of-the-art PCE figures for DSSCs and OSCs are a little over 10%. To achieve incremental boosts in these figures, researchers like those in Friend’s group have been analysing what happens at the nanoscale when light hits the material. For instance, they now know that manipulating the ‘spin’ of electrons in solar cells can dramatically improve their performance.

Mukhopadhyay, who travelled to the UK thanks to funding through the UK–India Education and Research Initiative, explains: “My materials have a fast charge transport rate – as we’ve now proved at Cambridge and Imperial – but they have a low PCE. We think that a process called singlet fission, in which one exciton splits into two, is happening. This makes them interesting to look at because if more than one charge carrier is generated then this can increase the PCE.”

Another family of materials the team has high hopes for is a set of perovskite-structure lead halides. Work at the ̽��ֱ�� of Oxford has already achieved a PCE of above 17% for such materials, and Sadhanala has begun using his machine to see the effect of different processing methods on their properties.

As well as developing cheap, high-performing solar materials, the team will scale up towards prototypes that replicate the performance achieved in the research phase.

Although the aim is to provide a technology that can reduce the carbon footprint of electricity generation anywhere in the world, solar energy could fulfil a massive demand for energy in India. India is the fifth largest producer and consumer of electricity, around 70% of which is based on coal, yet around 200 million people are without access to electricity. With a rapidly growing economy and more than 1 billion people, India faces a huge energy challenge to meet the current government’s mission of ‘Power for all, 24x7, by 2019’.

In fact, India could be an ideal place to adopt new solar technologies on a large scale, says Friend: “India is already currently running the largest renewable capacity expansion programme in the world, and there is a sense that the next technology revolutions may well happen in an emerging country like India that hasn’t already built its future renewables-heavy electricity system.”

“India may well leapfrog the UK in taking up radical new approaches to power generation,” he adds. “We want APEX to contribute to the search for such approaches now and in the future. This is a journey, not a day’s outing.”

Aditya Sadhanala wanders over to the wall, turns a pulley, and a wooden box about a metre squared swings up and away. Below it gleams an array of carefully positioned lasers, deflectors and sensors surrounding a piece of glass no bigger than a contact lens. He flips a switch and creates a ‘mirage’.

India may well leapfrog the UK in taking up radical new approaches to power generation

Richard Friend

H. Raab

Venus transits the rising Sun

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

Yes

Licence type:

Attribution-Noncommerical

Scientists move closer to “two for one deal” on solar cell efficiency

tdk25 — Mon, 16 Mar 2015 16:00:54 +0000

̽��ֱ��underlying mechanism behind an enigmatic process called “singlet exciton fission”, which could enable the development of significantly more powerful solar cells, has been identified by scientists in a new study.

̽��ֱ��process is only known to happen in certain materials, and occurs when they absorb light. As the light particles come into contact with electrons within the material, the electrons are excited by the light, and the resulting “excited state” splits into two.

If singlet exciton fission can be controlled and incorporated into solar cells, it has the potential to double the amount of electrical current produced from highly energetic blue and green light, capturing a great deal of energy that would normally be wasted as heat and significantly enhancing the efficiency of solar cells as a source of green energy. Until now, however, scientists have not really understood what causes the process, and this has limited their ability to integrate it into solar devices.

Writing in the journal Nature Physics, a team of researchers shows that there is an unexpected link between the splitting process and the vibration of the molecule that occurs when light comes into contact with the electrons. This vibration is thought to drive the production of two excited electrons, revealing for the first time how singlet exciton fission happens.

̽��ֱ��study was carried out by researchers from the Cavendish Laboratory at the ̽��ֱ�� of Cambridge, and the ̽��ֱ�� of Oxford. As well as solving a hitherto mysterious problem of quantum physics, it potentially provides a basis on which new singlet fission materials could be developed for use in solar cells.

Dr Andrew Musser, a post-doctoral research associate and former PhD student at St John’s College, ̽��ֱ�� of Cambridge, who co-authored the research paper, said: “We tend to characterise singlet exciton fission as a sort of two for the price of one deal on electrons, because you get twice as much electrical current. ̽��ֱ��problem is that if we want to implement this in a solar cell, the material needs to be engineered so that it is compatible with all the other components in the device. That means that we need to design a range of materials that could be used, and to do that, we need to understand more about why and how singlet exciton fission occurs in the first place.”

At its most basic, singlet exciton fission is a product of the fact that when light particles, or photons, come into contact with an electron, the electron is excited by the light and moves. In doing so, it leaves a “hole” in the material’s electronic structure. ̽��ֱ��electron and the hole are still connected, however, by a state of mutual attraction, and the two together are referred to by physicists as an “exciton”.

These excitons come in two very different flavours: spin-singlet and spin-triplet, and in rare circumstances, they can convert from one to the other.

In the natural world, spin-singlet excitons are a part of photosynthesis in plants, because the light absorbed by pigments in the plant generates excitons which then carry energy throughout it. Solar cells imitate this process to generate and drive an electrical current. Conventional solar cells are silicon-based, and the absorption of a single photon leads to the formation of a single, excited electron that can be harvested as electrical current.

In a handful of materials, however, singlet exciton fission occurs instead. Rather than producing just one spin-singlet exciton, two spin-triplets appear when a photon is absorbed. This offers the tantalising prospect of a 100% increase in the amount of electrical current generated.

Researchers attempting to solve the puzzle of why the process happens at all, and why only in certain materials, have typically looked at how the electrons behave when they absorb light. In the new study, however, the team instead focused on the fact when the electrons move in response to the light, the molecule of which they are a part vibrates.

̽��ֱ��team used thin samples of TIPS-pentacene, a semiconducting material in which singlet exciton fission is known to occur. They then fired ultra-fast pulses of laser light at the samples, each pulse lasting just 10 “femtoseconds”, or 10 quadrillionths of a second. ̽��ֱ��miniscule timescale was necessary so that large numbers of molecules could be vibrated synchronously, enabling the researchers to measure the response of the molecule and the resulting effect on the electrons as light hit the material. ̽��ֱ��measurements themselves were made using ultra-fast vibronic spectroscopy.

To the researchers’ surprise, they found that the molecules in the pentacene samples not only vibrated as singlet exciton fission occurred, but also continued to do so afterwards. This implies that the formation of two spin-triplet excitons is stimulated by the vibrations themselves, and the resulting tiny, fast changes in the shape of the molecules.

“We are fairly confident that this underlies all ultrafast singlet fission,” Dr Akshay Rao, a Research Associate at St John’s College, Cambridge, who led the Cambridge team, said. “ ̽��ֱ��picture that emerges is that when they are excited by light, the intrinsic vibrations drive the development of a new electronic state.”

By understanding the fundamentals of singlet exciton fission, the study opens up the possibility of designing new singlet fission materials that would enable the process to be effectively integrated into a new generation of highly efficient solar cells. Future research is already being planned in which the group will examine the precise vibrational states that are required for singlet exciton fission to happen, which will further add to this knowledge.

̽��ֱ��work at Cambridge forms part of a broader initiative to harness high tech knowledge in the physical sciences to tackle global challenges such as climate change and renewable energy. This initiative is backed by the UK Engineering and Physical Sciences Research Council (EPSRC) and the Winton Programme for the Physics of Sustainability.

̽��ֱ��causes of a hitherto mysterious process that could enhance the power of solar cells have been explained in a new study.

If we want to implement this in a solar cell, we need to understand more about why and how singlet exciton fission occurs in the first place.

Andrew Musser

Martin Abegglen on Flickr

"Green Power". While conventional solar cells use silicon, it is possible that other materials could eventually be used that would increase their efficiency.

̽��ֱ��text in this work is licensed under a Creative Commons Licence. If you use this content on your site please link back to this page. For image rights, please see the credits associated with each individual image.

Yes

Licence type:

Attribution-ShareAlike

Hybrid materials could smash the solar efficiency ceiling

sc604 — Thu, 09 Oct 2014 07:00:00 +0000

Researchers have developed a new method for harvesting the energy carried by particles known as ‘dark’ spin-triplet excitons with close to 100% efficiency, clearing the way for hybrid solar cells which could far surpass current efficiency limits.

̽��ֱ��team, from the ̽��ֱ�� of Cambridge, have successfully harvested the energy of triplet excitons, an excited electron state whose energy in harvested in solar cells, and transferred it from organic to inorganic semiconductors. To date, this type of energy transfer had only been shown for spin-singlet excitons. ̽��ֱ��results are published in the journal Nature Materials.

In the natural world, excitons are a key part of photosynthesis: light photons are absorbed by pigments and generate excitons, which then carry the associated energy throughout the plant. ̽��ֱ��same process is at work in a solar cell.

In conventional semiconductors such as silicon, when one photon is absorbed it leads to the formation of one free electron that can be extracted as current. However, in pentacene, a type of organic semiconductor, the absorption of a photon leads to the formation of two electrons. But these electrons are not free and they are difficult to pin down, as they are bound up within ‘dark’ triplet exciton states.

Excitons come in two ‘flavours’: spin-singlet and spin-triplet. Spin-singlet excitons are ‘bright’ and their energy is relatively straightforward to harvest in solar cells. Triplet-spin excitons, in contrast, are ‘dark’, and the way in which the electrons spin makes it difficult to harvest the energy they carry.

“ ̽��ֱ��key to making a better solar cell is to be able to extract the electrons from these dark triplet excitons,” said Maxim Tabachnyk, a Gates Cambridge Scholar at the ̽��ֱ��’s Cavendish Laboratory, and the paper’s lead author. “If we can combine materials like pentacene with conventional semiconductors like silicon, it would allow us to break through the fundamental ceiling on the efficiency of solar cells.”

Using state-of-art femtosecond laser spectroscopy techniques, the team discovered that triplet excitons could be transferred directly into inorganic semiconductors, with a transfer efficiency of more than 95%. Once transferred to the inorganic material, the electrons from the triplets can be easily extracted.

“Combining the advantages of organic semiconductors, which are low cost and easily processable, with highly efficient inorganic semiconductors, could enable us to further push the efficiency of inorganic solar cells, like those made of silicon,” said Dr Akshay Rao, who lead the team behind the work.

̽��ֱ��team is now investigating how the discovered energy transfer of spin-triplet excitons can be extended to other organic/inorganic systems and are developing a cheap organic coating that could be used to boost the power conversion efficiency of silicon solar cells.

A new method for transferring energy from organic to inorganic semiconductors could boost the efficiency of widely used inorganic solar cells.

̽��ֱ��key to making a better solar cell is to be able to extract the electrons from these dark triplet excitons

Maxim Tabachnyk

When light is absorbed in pentacene, the generated singlet excitons rapidly undergo fission into pairs of triplets that can be efficiently transfered onto inorganic nanocrystals.

Yes