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Details of Grant 

EPSRC Reference: EP/N026411/1
Title: Engineering energetic barriers to bimolecular recombination in polymer/fullerene solar cells
Principal Investigator: Clarke, Dr T M
Other Investigators:
Researcher Co-Investigators:
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Department: Chemistry
Organisation: UCL
Scheme: EPSRC Fellowship
Starts: 01 June 2016 Ends: 31 May 2021 Value (£): 984,957
EPSRC Research Topic Classifications:
Materials Characterisation Materials Synthesis & Growth
Solar Technology
EPSRC Industrial Sector Classifications:
Related Grants:
Panel History:
Panel DatePanel NameOutcome
19 Apr 2016 EPSRC Physical Sciences Fellowship Interview 19 and 20 April 2016 Announced
18 Feb 2016 EPSRC Physical Sciences Materials - February 2016 Announced
Summary on Grant Application Form
Solar energy is a promising alternative to the non-renewable sources used today. It is also a rapidly growing industry, with solar energy now powering over 50 million homes. Organic photovoltaic (OPV) devices, where the active material is comprised of organic materials such as polymers, offer several advantages over conventional silicon-based solar cells. OPV cells are light, thin, and flexible. Furthermore, OPV materials can be directly printed onto substrates, which means industrial scale production will be fast and cheap.

However, OPV is still in its infancy and although commercialisation has begun, many barriers to its success still exist. This is largely due to relatively low power conversion efficiencies: OPV devices now approach efficiencies of 11%, while inorganic systems can achieve up to 30%. One of the causes of OPV's lower efficiencies is bimolecular recombination, a process in which the charge carriers generated by sunlight recombine back to the ground state (therefore ceasing to exist) before they have a chance to reach the contacts and produce electricity. The aim of this research is to inhibit this important loss mechanism of bimolecular recombination, thereby improving the efficiency of OPV devices.

The aim of reducing bimolecular recombination in OPV devices will be accomplished by introducing energy barriers. All charge carriers tend to follow energy gradients as to reduce their overall energy; it is energetically unfavourable to move to a state of higher energy. This fundamental characteristic will be exploited by deliberately introducing such energy gradients into the solar cell. The morphology of the active layer will be manipulated using concepts such as nucleating agents to create crystalline and amorphous regions that possess different energy levels. The charge carriers will, upon photogeneration, follow their respective energy gradients in order to find regions of the solar cell with the lowest energy levels. As such, they will become spatially separated and it will be energetically unfavourable for them to move back up the gradients in order to encounter one another and recombine: this is the energetic barrier to bimolecular recombination.

The bimolecular recombination will be investigated using time-resolved vibrational spectroscopy. Every molecule possesses a characteristic set of vibrations, whereby the atoms move in relation to one another, that occur at specific frequencies. These vibrations can be monitored using spectroscopy. The particular technique used here, time-resolved Raman, involves generating the charge carriers in the solar cell with a laser pulse, probing the vibrations with a second laser pulse, and then measuring the scattered light that results (which contains the vibrational frequency information) as a function of the time delay between the two laser pulses. Monitoring the evolution over time of vibrational markers for structural moieties - or even specific chemical bonds - of interest provides direct insight into the structural dynamics occurring during bimolecular recombination in solar cells.

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