Coatings for enhanced light capture in solar cells 
Dr. J. Moghal, Dr. A.A.R. Watt, Dr. J. Best*, Dr. M. Gardener*, Dr. G. Wakefield*
This work focuses on the design and development of anti-reflectance coatings for a variety of solar cell architectures. A solution processed nanoparticle coating has been developed which increases transmission by at least 6% and reduce reflection per surface to <0.5%. We integrate these optical layers into solar cell devices and quantify performance enhancement in terms of reflectance and power conversion efficiency. (*Oxford Advanced Surfaces)

 

Atomistic investigation of hybrid organic-inorganic photovoltaic interfaces 
K. Noori, Dr. A.A.R. Watt, Dr. F. Giustino
Hybrid organic-inorganic solar cells represent a promising avenue toward low-cost solar energy technology. Using first-principles calculations we aim at determining the band alignment and charge injection properties at the interface between the organic polymer and inorganic semiconductor materials. In order to validate our theoretical models, a joint experimental study will be performed in order to fabricate and analyze these nanostructured interfaces.

 

Low cost and toxicity solar cells 
C. Smallwood, L. Droessler, E.X. Zou, Dr. A.A.R. Watt, Professor C.R.M. Grovenor. Dr. C. Blandford*
The leading second generation thin film solar cells are made of copper indium gallium diselenide and cadmium telluride. Significant market penetration of these systems is hampered by their toxicity and use of expensive rare earth metals. There is growing need to develop new second generation thin film photovoltaic materials which are robust, low energy cost, lower toxicity and recyclable. This project develops new visible light absorbing metal oxide thin films using sol-gel chemistry. 

 

Enhancing the efficiency of thin film solar cells using optical confinement 
M. Wincott, A. Powell, Dr. H.E. Assender, Dr. A.A.R. Watt, Dr. J.M. Smith
Thin film solar cells offer an inexpensive means to generate clean energy, but current efficiencies are limited to around five percent, about three times lower than commercial polycrystalline silicon cells. One of the main reasons behind the low efficiency is that a tension exists between the desire to absorb as much as possible of the incident light, in which case the optical path length should be thick (at least several hundred nanometres), and the desire to extract the photogenerated charge carriers efficiently from the cell, in which case the exciton transport path length should be short (no more than a few tens of nanometers). Most attempts to solve this problem involve using a thick cell, and focusing the advanced aspects of cell design on building in some means for ensuring a short transport path length. Here we take the opposite viewpoint; that the optical path can be elongated for a given cell geometry by the use of wave guiding and cavitation, thereby reducing the burden placed on the transport related features of the device. This new project involves the design, fabrication, and testing of devices that explore this theme by employing inexpensive approaches to encourage light to propagate in the plane of the film.

 

Energy harvesting in biomimetic systems 
Dr. B.W. Lovett*, Dr. A.A.R.Watt
Light is converted to chemical energy with extremely high efficiency in photosynthetic systems. Part of the reason for this is that light can create exciton states in protein antenna structures, and these antenae are arranged such that the exciton energy can be transferred to a specific site quickly and efficiently. In my work, I aim to understand this efficiency as the result of an interplay between exciton trasnfer interactions and environmental coupling to phonons. In particular, I aim to design synthetic systems that can mimic biological efficiencies, thus providing a route to optimized solar cells. (*Heriot-Watt University)

 

L. Droessler, Dr. A.A.R. Watt, Dr. H.E. Assender
There is growing need to develop new second generation thin film photovoltaic materials which are robust, low energy cost, low toxicity and recyclable. The project examines visible light absorbing metal oxide semiconductors in all inorganic multi-junction thin film solar cells. A number of vaccum processing methods are being trialled suitable for high-throughput processing including reactive evaporation and sputtering.

 

Advanced optoelectronic characterisation of solar cells 
J. Holder, Dr. A.A.R. Watt, Dr. H.E. Assender
Understanding the optoelectronic nature of solar cells is crucial to optimising fabrication processes and enhancing device efficiency. This project utilises a range of characterisation techniques to extract device parameters such as charge carrier mobility, material interface potential, and trap state density in an effort to better understand the underlying physics of solar cells. A number of techniques are available within the lab including impedance spectroscopy, time of flight, electroabsorption and Hall Effect.

 

Vacuum deposition of polymer photovoltaic devices 
P. Kovacik, Dr. A.A.R. Watt, Dr. H.E. Assender
Conjugated polymers have demonstrated enhanced properties in terms of light absorption and hole-transport, and in combination with fullerene electron-acceptors the highest power conversion efficiency organic solar cells. However, the use of solvents substantially limits the complexity of the devices as the coating solutions interfere with already deposited layers. Vacuum deposition is a solvent-free process, advantageous for its simplicity and ability to evaporate unlimited number of layers with well controlled thickness and composition. Although some polymer materials have been deposited by physical vapour deposition techniques, there have not been any attempts to deposit conjugated polymers in the same way. This project will involves the comparison of evaporated polymer-based photovoltaic devices with those deposited by solution casting, and development of the vacuum deposition processesElectroabsoprtion of nanocomposite photovoltaic materials.

 

Roll-to-roll processing of organic electronics 
Dr G.A.W. Abbas, Z. Ding, Dr. H.E. Assender
Electronics components that can be manufactured using roll-to-roll processing offer the possibility of lower cost devices as well as those that might be mechanically flexible in use.  Roll-to-roll (R2R) processing, using a flexible substrate (typically a polymer film) allows for cheap production of many components very rapidly, with low energy requirements.  Key areas of exploitation of this technology include flexible displays, but there is also a wealth of lower-cost applications. Tagging and tracking of fast moving consumer goods is an example technology that truly exploit the very low-cost nature of the production and in which the manufacturing is closely linked to the manufacturing routes currently exploited for e.g. packaging technologies.  This project seeks to exploit the existing industrialised technology of vacuum R2R processing, widely used for example in the packaging industry, to develop the manufacture of very low cost organic field-effect transistor (OFET)-based devices and circuits.  This manufacturing route, like solvent based systems, is cheap and provides flexible product, and we can exploit high electrical mobility molecular semiconductors. Additional advantages of the solvent-free vacuum processes include: a) likely enhanced web-speed, b) integration with vacuum-based metal deposition for conducting channels, and metal or ceramic deposition for barrier layers and possible interfacial modification, and c) the ability to deposit multiple thin layers to build up device structures without solvent interactions with underlying layers.  The project will exploit our existing R2R web processing facility to explore the principal manufacturing challenges to R2R vacuum production of OFET devices: 1) selection and adaptation of materials to vacuum deposition integrated with design of suitable circuitry, 2) patterning of the semiconductor and insulator layers to allow the formation of circuit connections between devices and 3) reliability of manufacture to be able to produce arrays of multiple transistors for circuits.  It will allow us to explore and develop the deposition of molecular semiconductor and dielectric materials and then the subsequent reliability and thermo-mechanical resilience of the resulting product such that it might need to withstand, for example, during a lamination process.

 

Excitons in semiconductor nano-heterostructures 
E. Tyrrell, S. Fairclough, Dr. A.A.R. Watt, Dr. J.M. Smith 
Semiconductor nanocrystals with strong optical transitions are becoming increasingly important in a wide range of applications in fields as diverse as medicine, renewable energy, and telecommunications. In nanocrystals made from a single constituent material choosing the size also determines other important optical properties, such as the luminescence lifetime and linewidth. However it is also possible to grow heterostructures of different materials, to explore different quantum confinement geometries which modify the behaviour in both quantitative and qualitative fashion. In this project we focus on a particular design of nanocrystal - the 'type II' nano heterostructure - in which electrons and holes are spatially separated in different core and shell regions, all within a few-nanometres diameter crystal! These nanocrystals are particularly attractive for use in solar cells, and in optical amplifiers and lasers. The project involves both experimental and theoretical approaches to determine the design rules and optical properties of these materials.

 

Solution processed transparent conductors 
E.X. Zou, Dr. A.A.R. Watt, Professor C.R.M. Grovenor
Transparent conductive electrodes (TCE) have been developed with a combination of high optical transparency and electrical conductivity. In the field of organic photovoltaics there is a need to develop TCE which are electronically tailored to enhance charge extraction and transport. Development of alternatives to the current leading TCE indium tin oxide is also crucial for electronic, environmental and economic reasons. Aluminium doped zinc oxide (AZO) is one promising alternative due to its abundance, ease of manufacturing and excellent electronic properties. This project uses simple and cost effective sol-gel methods to fabricate doped metal oxide thin films on a variety of substrates. The TCE is then used in bulk heterojunction nanocomposite solar cells.