High resolution mapping of performance and degradation mechanisms in printable photovoltaic devices

PV materials that can be processed from solution at low temperature offer a route to low cost and low emebedded energy PV modules with potential for integration into buildings and other infrastructure to generate clean electricity on a large scale. Organic PV (OPV) has attracted intense research interest; impressive improvements in efficiency and in fabrication knowhow have been demonstrated. Lead halide perovskites solar cells (PSC) are based on a newly rediscovered active layer material and have shown radical improvements in start-of-life efficiency with recent optimisation of device structure and processing.

However both technology types are challenged by losses in power conversion efficiency under operation, even though they are believed capable of stabilised efficiency of 15-20%. The limited operational stability of such devices inhibits their widespread commercial application. To overcome this there is a need to understand the sources of efficiency loss, both at start-of-life and during ageing in typical operating environments.

Until now, most studies of novel PV device stability have amounted to empirical studies of the evolution of performance parameters for different materials or device structures in different environments, and scientific attention has focussed largely on the oxidative stability of the photoactive layer. Relatively little attention has been paid to the electrodes and interlayers, even though these layers are often the first to fail and additionally they are partly responsible for protecting the active layers. In addition, most performance metrics probe the macroscopic device performance and although imaging methods have been used to observe heterogeneous material properties during ageing mapping techniques have not yet been used to provide detailed insight into the chemical, electrochemical and physical mechanism of current and voltage loss.

This proposal seeks to develop a set of interlinked experimental techniques to probe the basic mechanisms underpinning device degradation and failure in two leading classes of printable photovoltaic (PV) materials, organic photovoltaics (OPV) and organohalide perovskite solar cells (PSCs). Our approach is to develop and adapt two-dimensional mapping techniques that probe the local chemical and electronic state of the materials and combine them with device-scale electrical measurement, structural characterisation and modelling in order to analyse the degradation mechanisms, to identify the local conditions that lead to degradation and to design strategies to inhibit the progression of failure mechanisms. The mapping tools will be developed with the potential to be applied during module manufacture and quality control.

As well as communicating the scientific progress of the research through publication of results in leading journals in material science, chemistry and physics, the team will communicate with a wider audience through public engagement activities and the media.
To access the general public we will use our experience in communicating science. Prof Nelson has ample recent experience in communicating research results through broadcast (for example interviews on Radio 4 'In Our Time' and Australian Broadcast Corporation's 'The Science Show' during 2012) and print (articles in The Guardian and Sublime Magazine during 2012) media. We will use our contacts in the media and with lay organisations to communicate the results of our research.

Lower cost photovoltaic technologies have a high potential for contribution to future power supply, particularly in the developing world and to carbon emissions mitigation. Prof Nelson leads the Mitigation team at The Grantham Institute for Climate Change and the Environment at Imperial College, which runs an active programme of seminars, briefing papers, discussions with policy makers and other stakeholders. During the last year she has presented research results on emerging PV technologies to members of government departments (DECC, DFID), the International Energy Agency, funding bodies, energy economists and science media. In addition Prof Nelson has written a briefing papers for the Grantham Institute on the potential of solar power and on estimation of the mitigation potential of new renewable technologies and has liaised directly with DECC about the subject. This provides a direct route to communicate significant results from this research to a wider energy and policy making community.

The Centre for Plastic Electronics (CPE) at Imperial is one of the five UK centres of excellence in PE and as such has access to the network of affiliated companies (such as Merck, BASF, Bayer, Solvay, hp, Cambridge Display Technology, Plastic Logic, Plextronics, Solar Press), government departments and research institutions that are active in developing devices and materials for organic electronics. Advances will be communicated via this network and other partners in the CPE and we will proactively pursue these relationships to exploit potential new developments and contribute to wealth generation.
Our research programme is directly relevant to companies seeking to develop new experimental capability and new organic photovoltaic devices. For this reason we have engaged Renishaw, Perkin Elmer and Ossila as project partners, with the aim of evaluating the industrial use of our spatially resolved characterization techniques in collaboration with them. Ossila stand to benefit through early access to knowhow that can improve quality control and diagnostic testing of organic photovoltaics during production. In addition, through existing research collaborations at the SPECIFIC IKC where Tsoi and Charbonnean are based we have direct contact with other companies working on commercializing novel photovoltaic technologies (Solar Press, Oxford Photovoltaics, Molecular Photovoltaics,Tata Steel, G24i) and we will exploit these contacts to publicise the potential of the methods being developedin this programme. Finally, Prof Lidzey and Prof Nelson are members of the new EPSRC SuperSolar Hub and as such have access to the UK academic and industrial community involved in solar energy technology.