Charge Carrier Dynamics and Molecular Wiring in Hybrid Optoelectronic Devices

Charge Carrier Dynamics and Molecular Wiring in Hybrid Optoelectronic Devices

In the past decade there has been an explosion of interest in electronic devices using alternative materials to silicon, the traditional 'workhorse' of the semiconductor and photovoltaic industry. These alternative materials such as conducting molecules, polymers, metal oxides and metal sulphides are attractive because they can be deposited in a wide range of shapes and sizes from solution or printed onto flexible substrates. This allows low temperature, cheaper and more versatile manufacturing. They can be used to make devices such as next generation sensitised solar cells (SSCs), hybrid organic/inorganic solar cells and light emitting diodes (LEDs). These promise higher performance/cost ratios than conventional semiconductor devices. However the cost and processing advantages typically come at the expense of poor electrical charge transport due to the nature and processing of the materials used. This can lead to relatively low power conversion efficiencies in solar cells and LEDs based on these materials. Additionally variations in the energy of electrons (workfunction) at the interface between phases such as polycrystalline metal oxides and conducting polymers can cause non-ideal device behaviour leading to poor transfer of charge between the components and a corresponding loss in performance.

My proposal will address these problems by exploiting an interesting phenomenon where electrical charge is transferred between neighbouring molecules in a single continuous layer attached to a surface. This concept, known as two dimensional molecular wiring, has recently been applied in new battery technology which contains components with very poor conductivity. It allows more efficient collection of charge from the interface between the two phases in the electrochemical cell, allowing more rapid charging/discharging and more space for the active ingredients. I will work with the leaders in this field (EPFL in Switzerland) to apply molecular wiring to SSCs and hybrid optoelectronic devices. I have a strong research background related to dye sensitised solar cells (DSSCs, a particular class of SSC), and have recently been involved in a study that demonstrated molecular wiring between dye molecules covering the surface of titanium dioxide nanoparticles in liquid electrolyte DSSCs.

By applying molecular wiring in solid state SSCs and hybrid cells I hope to increase the separation and collection of charge from regions within the devices which are electrically isolated. This would lead to improved photocurrents and power conversion efficiencies. Additionally molecular wiring at oxide electrode interfaces should help to reduce variation in electron energy at interfaces by allowing neighbouring regions to reach equilibrium. This would be very attractive for the plastic electronics industry.

At Imperial College I have developed a unique series of measurements that will allow me to test the effectiveness of different configurations of molecular wiring in the solar cells. I will also work with Prof. Nelson who has expertise in molecular, charge and energy transport modelling to develop a method to screen suitable molecular wiring candidates to incorporate into the devices. I will compare these theoretical calculations with direct measurements of the conductivity through of layers of the molecules using a conducting atomic force microscope coupled with steady state and transient light sources. The results will help to direct researchers at EPFL and elsewhere towards synthesising new functional molecules for these applications. This strategy will be combined with increases in the charge separation efficiency in SSCs leading to easy to manufacture, higher performance devices.

Knowledge
The knowledge generated by this project will contribute to the international research effort to develop low cost solar cells. The broad scope of the research will also help facilitate the transfer of ideas between researchers and add to the UK's research profile. Demonstrating new applications for the fascinating phenomena of hole hopping should generate considerable interest within both the academic and public spheres.

Society
This project will contribute to the development of photovoltaics as a sustainable energy source. There is thus a potential for huge impact in terms of mitigating CO2 emission, energy security, and in particular the development of off-grid communities. In off-grid regions, scalable photovoltaic panels requiring only a modest capital investment can bring major benefits by powering computers and telecommunications. Within the UK there is also a growing demand for photovoltaics to capitalise on recently introduced feed-in tariffs. The Grantham Institute for Climate Change at Imperial College will be able assess advances from the project in terms of their impact as a mitigation technology.

Molecular wiring used in plastic and hybrid electronics has the potential to improve biomedical, sensing and display technologies, all of which could contribute to the quality of life in society.

Economy
The plastic electronics industry in the UK is predicted to grow from £1.25bn today to £330bn by 2027. This industry stands to benefit from improvements in the performance of devices targeted by this proposal. For example homogenised work functions would potentially allow reduced operating voltages and improved efficiencies in OLEDs and OFETs. Imperial College has close links with companies developing materials for the industry such as Merck through its Centre for Plastic Electronics (one of 5 PE centres for excellence in the UK) thus the project is well placed to exploit potential new developments and contribute to wealth generation.

The UK is also a global centre for the commercialisation of DSSCs with G24i Solar having already shipped its first commercial order of cells and Tata Steel Colors investing heavily in the technology (£10m PV accelerator project to coat strip steel with DSSCs). Furthermore companies such as Solar Press (my sponsor Prof. Nelson is a Royal Society Research Fellow working with them) and Eight19 have been founded to commercialise advances in organic photovoltaics. Through the project's collaborations, the UK photovoltaics industry stands to benefit from advances in performance, stability and ease of manufacture resulting from this proposal, as well as through potential spin off companies to develop new advances.

I also anticipate that the research into 2D molecular wiring will have many additional applications in manufactured plastic and hybrid electronic devices, for example with the development of self-assembled monolayer transistors, and optimising the interfaces in organic LEDs.

People
The project will involve training two PhD students who will develop wide-ranging experimental and theoretical skills in what is a rapidly growing and high profile area. Following their studies they will be well positioned not only within the multi-disciplinary research world, but also to satisfy the increasing demand for highly trained individuals by companies in the field.

The multi-disciplinary nature of the project will also expand the academic horizons of the researchers involved in the project for example through strengthened links between the Chemistry and Physics Departments.

As part of the project we will also undertake a modest outreach programme to raise interest in science as a career path. This will include the development of demonstration devices to illustrate the technologies involved for presentations at schools, and offering student summer projects.