Engineering energetic barriers to bimolecular recombination in polymer/fullerene solar cells

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.

The initial impact this research will have is the training of future generations of researchers in the form of the requested PDRA. By providing skilled personnel in highly specialised photovoltaic technology, this project will contribute to helping overcome the skills shortage in science and engineering subjects in the UK, particularly in this emerging technology. This research will add to the knowledge base that is so critical for the academic community and for future research, significantly advancing fundamental knowledge of the operating principles and current efficiency limitations of OPV. Future researchers can build on this knowledge base in order to take the photovoltaic field (or others: photonics, spectroscopy, advanced materials, nanotechnology) in new directions.

The aim of this research is to inhibit bimolecular recombination, a major loss mechanism in organic photovoltaics. Success will lead to higher device efficiencies, which will make this technology more feasible for commercialisation. Furthermore, by reducing bimolecular recombination, the devices can be made substantially thicker, allowing cheap industrial-scale fabrication methods such as roll-to-roll printing. If the active layer is too thin, then the efficiency decreases due to lack of film uniformity and the formation of pinholes and other defects. Furthermore, the coating speed also suffers since printing is more difficult to control with thin layers.[1] Inhibiting bimolecular recombination will therefore provide a direct benefit to the photovoltaic industry (including OPV companies such as Heliatek, Solarte, and NanoFlex), and associated industries (glass, engineering, electronics). Production will be faster and cheaper, and the higher efficiencies will encourage higher profits and increase the technology's appeal. The OPV market is forecast to rise to $87 million by 2023, a small fraction of the total photovoltaics market. By tackling OPV's major limitations, whilst maintaining its benefits - such as the ability to be printed on industrial scales onto flexible substrates - this fraction can be substantially improved. Indeed, the magnitude of the overall photovoltaics market would rise, providing a direct benefit to the UK economy. An additional £25 billion is estimated to be injected into the UK economy if a further 20 GW of capacity is installed in the UK by 2030.

Society will be the next beneficiary from this research. The need to replace present energy generation, largely based on fossil fuels, is evident considering both limited reserves and the detrimental effects of climate change through increasing carbon dioxide production. By increasing the supply of renewable energy at a lower cost, the reliance on non-renewable sources can be reduced. This would have a direct impact on health, for instance, by improving the air quality in major cities. If anthropogenic climate change can be successfully tackled, this may lead to a decrease in extreme weather events and desertification, improving quality of life and crop production. According to the International Energy Agency, 1.3 billion people currently lack access to electricity.[2] OPV has been suggested as a method to provide the developing world with a cheap off-grid source of electricity,[3] but this would only be possible by addressing the loss mechanisms such as bimolecular recombination to make the technology more viable for such a purpose. The environment will benefit from this research. Reducing reliance on fossil fuels and its associated pollutants will provider cleaner air, land, and waterways. Furthermore, the need for potentially damaging technologies such as offshore oil platforms and fracking will be greatly reduced.

(1) Gaudiana, R. Journal of Polymer Science Part B: Polymer Physics 2012, 50, 1014.
(2) http://www.iea.org/topics/energypoverty/
(3) Zervos, H; Das, R; Ghaffarzadeh, K. IDTechEx, Organic Photovoltaics (OPV) 2013-2023: Technologies, Markets, Players