Hybrids Photovoltaics of the Future: ‘Inorganics-in-Organics’

Renewable energy is one of the key routes to minimize adverse effects of carbon emission from energy conversion. Photovoltaics convert sun’s radiation which is renewable, to electricity directly using photoactive materials. Silicon based photovoltaics dominate 90% of the market, however, the technology is still too expensive for mass uptake.

Organic materials based photovoltaic technologies promise much in this regard, due to their perceived solution based, roll-to-roll processability for inexpensive, high throughput. However, the technology has fundamental limitations resulting in lower efficient photovoltaics compared to inorganic material based devices.

Through the use of carbon nanotubes in an organic system and careful selection of molecular energy levels of the organic materials, the charge carrier mobility can be improved, with the possibility of enhancing the optical absorption simultaneously. Carbon nanotubes can also be used at the photovoltaic device electrodes to enhance the photo-generated charge transfer.

Carbon nanotube-organic PVs are not new, and many groups have used single walled CNTs (SWCNT) to fabricate devices with the SWCNT acting as an electron acceptor when dispersed in a polymer solar cell. Others have tried to extend the hole-accepting electrode in the form of mixed SWCNT-PEDOT:PSS composites, on ITO coated glass substrates. The nature of the CNT allows a significant increase in the conductivity of the PEDOT:PSS layer, while not reducing its transparency, nor the surface morphology. Despite these changes, however, in the case of the above composite materials the power conversion efficiency has remained at least one order of magnitude below that achieved by using fullerene or PCBM as the electron acceptor.

This program of research will only use MWCNTs and not single-wall carbon nanotubes (SWCNTs). Unlike SWCNTs, which are a mixture of metallic and semi-conducting tubes, MWCNTs are invariably metallic owing to their larger diameter and more complex multi-layered structure, offering far more predictable functionality for electronic applications.  Despite these advantages little research effort has focused on the utility of MWCNTs in OPVs. As part of the project we aim to deliver and fulfil the objectives below.

 

• Demonstrate the use of MWCNTs to enhance and extend the spectral range for light harvesting in OPVs.
• Enhance the hole-mobility of the donor phase in OPVs via the incorporation of suitably functionalised MWCNTs, which will extend the top contacts.
• Develop a new class of air stable, solution processable electron extracting electrodes, enabling the realisation of wholly solution processed OPVs.
• Develop new techniques for large-area printing of charge extracting electrodes that are easily scalable.
• Demonstrate enhanced resistance to photo-bleaching in OPVs utilising MWCNTs dispersed within photo-active organic OPVs.
• Demonstrate enhanced power conversion efficiency and extended operational lifetime in OPVs utilising the technologies.
• Explore the integration of the above technologies to realise efficient, low cost hybrid OPVs, with a view to identifying the most commercially viable and environmentally sound approach.