Towards Sinter-free Printing of Photovoltaic Cell Interconnects

Metal thin films are used in a wide variety of technologies, such as solar cells and printed circuit boards for electronics. Inkjet printing has emerged as a practical and low-cost route for manufacturing electrical contacts in these applications. However existing manufacturing technologies use inks that often require a final heat treatment to consolidate or 'sinter' the film. If this last step can be eliminated, by depositing fully dense films, then the inkjet manufacturing process could be applied to temperature sensitive substrates like plastics or vulnerable semiconductor materials.

The purpose of this project is to develop 'sinter-free' inkjet manufacturing processes, by taking ink precursors developed for other thin film processes, and exploiting them to use the significant benefits of inkjet process technology e.g. the direct writing of interconnects or wires. If successful, the project will represent a step-change in the manufacturing methods for this type of film.

Inkjet printing has become a rapidly expanding commercial technology. The total inkjet print market in 2009 was $26.8 billion, 58.2% higher than in 2005. Market forecaster's Pira expect continuing growth, averaging 10.3% CAGR from 2010-15 and anticipates that the market will be worth $46.5billion. In the context of photovoltaics, inkjet printing is used to fabricate contacts and bus bars on the surface of solar cells. Ink jetting has several advantages, including the ability to print narrow features with high resolution. This minimises shadowing and increases the cell efficiency. Direct-write printing also reduces the number of process steps, speeds set-up time, eliminates waste, reduces cost through automation, and enables non-contact printing for thin, sensitive substrates. Thin film PV is currently under 20% of the total global PV production, which is dominated by crystalline silicon. However, thin film PV output which is now dominated by cadmium telluride and is increasing more rapidly than crystalline silicon. The UK has a very strong PV supply chain industry with companies such as Crystalox producing multi-crystalline silicon wafers, Dupont who are a major supplier of conductive pastes and Pilkington NSG which is now the world's largest supplier of transparent conducting oxides (TCOs) for thin film PV.

Inkjet is a significant process technology in printed electronics and includes applications from printed silver interconnects between panels or devices to completely printed computing chips. Printed electronics is moving towards reduced fabrication costs a wide range of devices including displays, sensors, batteries, RFID chips, etc. Inkjet is a mask-less technology that is presently an established tool in plastic electronics R&D and is ideally suited to make certain devices. The primary opportunity is for conductive inks -especially silver inks-but semiconducting and dielectric inks will also be an important part going forward.

The project will impact upon a range of industrial sectors including inkjet printing and processing equipment (Xaar, Renishaw and OpTek Systems); printing inks and precursors (Dupont and SAFC Hitech); end users in photovoltaics (Optic Technium and NSG).