"The Quantum Ratchet Concept for Ultra efficient Solar Cells"

Lead Research Organisation: Imperial College London
Department Name: Dept of Physics

Abstract

Solar panel prices are plummeting and they are becoming more widespread, but the impact they can actually make to the carbon problem is ultimately limited by their efficiency. Even if the panels could be made for almost nothing, and if we all covered our roofs with them, at their present working efficiency they could only generate a small fraction of the year-round electricity we have become used to using.

This project aims to develop a radically new way of harvesting solar power that has the potential to improve this conversion efficiency by a factor of 5, and so to put solar power in a position to make a major contribution to the carbon mitigation issue.

The science behind our approach stems from the fact that behind the familiar beauty of the Rainbow lies a vexing problem if you want to the power of sunlight. Solar cells work by absorbing quanta of light, so-called photons, in a way that makes the electrons inside them jump from one energy level to a higher one, like a rung in a ladder. It's these electron jumps that capture the sunlight's energy

The problem with sunlight is that each colour in the rainbow is made from different energy photons. No matter what rung height you decide on, some of the lower energy photons (at the red end of the rainbow) are lost because they can't power the jump. Others (at the blue end) have more energy than the rung spacing, so only part of their energy gets captured. A detailed analysis shows that, no matter what rung size you settle on, the best, the very best you can ever do is the so-called "Shockley-Queisser" efficiency limit, of 31%, and most actual solar cells struggle to reach half of this.

Our research programme sets out to smash this "Shockley-Queisser" limit. We plan to do this by using quantum mechanics to design an energy level structure into the solar cell which is analogous to a ladder with a range of uneven rung spacings. Each rung grabs a different part of the rainbow with high efficiency, and some are designed so that one photon can make an electron jump up two rungs at a time.

To do this we use a revolutionary approach that exploits the sort of nano-technology that gave us the lasers that power computer printers, the internet and DVD drives. Theory indicates that efficiencies up to 89% are possible. We hope and believe that demonstrating even part of this improvement will permanently change the way we design solar cells and dramatically improve the chances of solar power being adapted on a scale that is wide enough to have a genuine positive environmental impact.

This cell also develops much more voltage than present designs, which makes its electrical output easier to use.

As well as determining the rung spacing, we also use the nano-technology to add an extra , and critical twist, a new idea we are calling a "Quantum Ratchet". This can be thought of as, say, a small hollow in each rung, so that if an electron makes it up that far, the likelihood is that it will stay there long enough to absorb another photon and hop up to the next rung, rather than losing its captured energy by sliding back down.

At the moment we are proposing to get as far as demonstrating and optimising the concept, using comparatively expensive test cells and complex laser spectroscopy in a University lab. Even that is a major undertaking though. It will occupy a focussed team of ~ 9 scientists for 4 years, all working towards the same goal if we are to have even a chance of success, but we all believe the results will be worth it.

Planned Impact

At present the remarkable dollar-per-watt cost reductions that have been achieved using crystalline silicon make it the clear winning technology, but at the same time they have changed the very nature of the PV debate, from simply focussing on materials cost, to focussing on absolute efficiency. This is because we have now reached the point where the panel cost is less than their installation costs (rarely less than $100 / m2) and in developed cities, (where the majority of electricity is consumed of course), space is at such a premium that there is an effective financial barrier to installation that can be up to 20 times higher.

It is becoming clear that in the long term, unless we can find new and radical ways of driving the efficiency up, PV systems will simply take up too much space in cities to generate be able a significant fraction of the worlds year-round electricity consumption.

This is an exploratory programme so, for viability, the demonstrator devices will use proven existing materials technology, but we believe that the radical nature of its central "Quantum Ratchet" concept for driving the PV cell efficiency up will mean that it will have manifold impacts.

Once we have established the validity of the idea, it will drive research in other material systems with transition energies that are better suited to the solar spectrum. Currently these materials are harder to work with, but after our programme, research in them will be being driven by a proven concept, and we anticipate a surge of activity from groups who have the skills to develop new, thin-film photovoltaic materials capable of exploiting the quantum ratchet concept in large area devices. Using inorganic thin-films this could include the II-VI and III-N family of semiconductors materials, including graphene, and we have ourselves already theoretically demonstrated an implementation of the idea that uses organic semiconductors.

The solar cell community is well aware of the constraints posed by the so-called Shockley-Queisser limit, that means that conventional crystalline silicon-based PV research aspires to efficiencies that can never exceed 25% (15% in thin film materials). We aim to demonstrate that the Quantum Ratchet concept can raise these aspirations dramatically, in principle up to a new theoretical limit of 89%, marking a complete paradigm shift for photovoltaic device engineering.

The idea is new and our demonstration of a working prototype will represent a landmark achievement which we expect to be publishable in the world's leading journals, (Science , Nature), as well as having a strong draw on the attention of the mainstream popular media.

Along the way this programme will also deliver more immediate benefits including:- substantial advances in basic nanotechnology, ~10 new scientists people trained in the latest branches of nano-photonics, together with wider societal benefits through knowledge dissemination in the media and at a range of science gatherings.
 
Description We have shown that using crystals that are structured at the nanometre scale, i.e. at a wavelength shorter than an electron, we can make solar cells whose potential conversion efficiency is double what was possible before. This is a critical step towards using photovoltaics as a viable form of carbon mitigation.Most recently we have demonstareted a working device, using a semiconductor nanostructure.We realise the Quantum Ratchet concept with a semiconductor nanostructure. It raises the electron lifetime in the metastable trap by ~104 , and gives a STPA channel that increases the photocurrent by a factor of ~50%. This result illustrates a new paradigm for designing ultra-efficient photovoltaic devices.
Exploitation Route We hope they will usher in a new era of quantum design based research for ultra-high efficiency solar cells, with every widespread applications in the long run.
Sectors Aerospace, Defence and Marine,Communities and Social Services/Policy,Construction,Digital/Communication/Information Technologies (including Software),Education,Electronics,Energy,Environment,Healthcare,Leisure Activities, including Sports, Recreation and Tourism,Security and Diplomacy,Transport

URL http://www.imperial.ac.uk/people/chris.phillips
 
Description Sharp Labs Europe 
Organisation Sharp Laboratories of Europe Ltd
Country United Kingdom 
Sector Private 
PI Contribution Jointly supervised a research student, Megumi Yoshida. We did spectroscopy and theory.
Collaborator Contribution Jointly supervised a research student, Megumi Yoshida. They provided funding and samples.
Impact The papers listed under the Photon Ratchet grant.
Start Year 2010
 
Description BBC Newsnight Interview , Live (!) with Jeremy Paxman 11th July 2013 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact I wsa invited to appear live in the Newsnight studio to comment on a "cloaking" story that had Broken that day
Year(s) Of Engagement Activity 2013
URL http://www.bbc.co.uk/iplayer/episode/b02xd173/Newsnight_11_06_2013/
 
Description Presidnt Of Korea Visit to Imperial 2013 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact Soutth Koreas Premier visited us and our reserch was chosen to impress her (above all else going on at Imperial. I set up a demo to make her "dissapper" along with out provost, although challenging lighting conditions, and heavy handed security meant it was as no as impressive as I usually manage.
Year(s) Of Engagement Activity 2013
URL http://www3.imperial.ac.uk/newsandeventspggrp/imperialcollege/newssummary/news_5-11-2013-16-18-47