Enhanced multiple exciton generation in colloidal quantum dots

Lead Research Organisation: University of Manchester
Department Name: Physics and Astronomy

Abstract

Solar power is one of the most promising alternatives to using oil, gas and coal to generate the energy we need. Sunlight is freely available, safe and enough of it reaches the earth from the sun to supply all our energy needs 10,000 times over. It is also clean, releasing none of the carbon dioxide in to the atmosphere that fossil fuels do and which threatens to cause damaging climate change. However, today's solar cells are not yet economic; it is still cheaper to produce power by burning fossil fuels and this is preventing their wide-spread use.

How can we make solar cells economically competitive with fossil fuels? There are two ways: make them more cheaply or make them more efficient, or preferably both! Most of the solar cells you see around today are made from silicon and are up to 20% efficient but are expensive to make. Some newer, different types of cell are beginnning to become available which are a lot cheaper to make but are only 10% efficient at most. The aim of this project is to have the best of both worlds - solar cells that are both cheap and efficient enough to compete with fossil fuels.

The key part of these new cells will be 'quantum dots' - these are tiny crystals of semiconductor that will absorb the sunlight and turn it into electricity. In today's solar cells, about half of the energy from the sun is wasted as heat as soon as the sunlight is absorbed by the cell. In quantum dots, however, something else can happen - the energy that would become waste heat in a normal cell can be used instead to produce extra electricity. This means that solar cells based on quantum dots could be up to 50% more efficient than today's technology.

This is an exciting prospect and could be important but we still need to understand this process better. In this project, we will produce new types of quantum dots which are designed to maximise the effciency with which sunlight is turned into electricity. These dots must also be made from materials which are cheap, abundant and safe. We will use X-rays to study their structure carefully and lasers to study what happens to the light as it is absorbed. Complex computer models will be used to help us better understand what is happening and make the conversion of sunlight to electricity as efficient as it can be. Finally, we will build a prototype solar cell using these new quantum dots which will demonstrate how they can be used to generate electricity safely, cleanly and cheaply.

Planned Impact

Apart from the beneficaries in academia (identified in the 'Academic Beneficaries' section) this work will be of direct relevance to UK manufacturing, and thus to national competitiveness and economic well-being. We have a strong and long-standing relationship with Nanoco Technologies, a UK-based and world leading manufacturer of quantum dots; they are well-placed to exploit the quantum dots designs we will develop for the benefit of the UK economy via, for instance, a joint development agreement with a 3rd generation solar cell manufacturer. Other potential industrial beneficaries include G24i and DyeSol Ltd, which has joint collaborations with Pilkington Glass and Tata Steel to produce building-integrated photovoltaic units, and with BMW to produce automobile integrated units. Internationally, Sony Corporation also have had a 3rd generation photovoltaics development programme for several years. The technology that all these companies are exploiting is the dye-sensitised solar cell; the demonstrator devices we will develop are similar to this but replace the ruthenium dye with quantum dots as the light absorbing element.

The PDRAs employed by the project and the PhD students we will seek to recruit from the Northwest Nanoscience Doctoral Training Centre (jointly run by the universities of Manchester and Lancaster) and at Salford will benefit from training in quantum dot synthesis, characterisation and modelling. They will be equipped with knowledge and skills that will enable them to contribute to the EPSRC Grand Challenges of 'Nanoscale Design in Functional Materials' (Physics) and 'Directed Assembly' (Chemistry), and to areas of nanoscience and nanotechnology generally. The employability of those similarly trained is evidenced by the former postdocs or students of the investigators now working for high tech manufacturers, such as Nanoco, Thales Optronics, Selex Galilao and Sellafield Ltd. The general public will also benefit from a greater understanding of the science associated with solar energy and nanotechnology through our outreach activities, for instance, at the Museum of Science and Industry in Manchester.

Finally, the whole of UK society will benefit from this project from the contribution it makes in the progress towards an affordable, secure and low-carbon energy supply system, both at home and overseas.

Publications

10 25 50
 
Description The key findings to date are:

1. The efficiency of multiple exciton generation can be enhanced by growing core/shell quantum dots
2. Adding a second shell improves photoluminescence quantum yield, but decreases photovoltaic efficiency
3. Treating quantum dots with chloride ions passivates the surface resulting in ~100% photoluminescence quantum yield
4. Dangling bonds on the CQD surface trap holes and provide a sub-nanosecond non-radiative recombination pathway.
5. The efficiency of multiple exciton generation in InP colloidal quantum dots is largely the same after the addition of a CdS shell but other optoelectronics properties that influence solar cell efficiency, such as recombination lifetime, are substantially improved, which promises to lead to overall increase in cell performance.
Exploitation Route To improve the efficiency of quantum-dot based solar cells, and other devices
Sectors Chemicals

 
Description The colloidal quantum dot synthesis know-how and the understanding of the relationship between surface chemistry and optoelectronic performance, particularly of the importance of surface treatments, developed in this project has been transferred to Nanoco Technologies plc, a UK-based and world-leading manufacturer of quantum dots. This was achieved by the recruitment of a PhD student who worked on this project, Robert Page, to Nanoco. We anticipate further non-academic impacts to emerge over the coming period.
First Year Of Impact 2016
Sector Chemicals
Impact Types Economic

 
Description Blue Dot Festival 2017 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Put on a science stall explaining our project at the 'Blue Dot Festival' at Jodrell Bank, which combines science and music
Year(s) Of Engagement Activity 2017
URL https://www.discoverthebluedot.com/
 
Description Jodrell Bank Live 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Demos showing how colloidal quantum dots can be used in solar cells were shown to the general public, promoting interest in science and showing how it can be relevant to societal challenges such as climate change
Year(s) Of Engagement Activity 2013
 
Description National Science and Engineering Week 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Schools
Results and Impact Demos showing how colloidal quantum dots can be used in solar cells were shown to school children, promoting interest in science and showing how it can be relevant to societal challenges such as climate change
Year(s) Of Engagement Activity 2013,2014,2015,2016
 
Description Science Spectacular! 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Demos showing how colloidal quantum dots can be used in solar cells were shown to the general public, promoting interest in science and showing how it can be relevant to societal challenges such as climate change
Year(s) Of Engagement Activity 2013,2014,2015