Worldwide research into thin-film PV technology is focusing on devices based on cadmium telluride (CdTe) and copper indium gallium selenide (CIGS), which have traditionally been understood by equating the device to a simple p-n junction.
However, between 1992 and 2001 efficiencies for these cells only improved slightly - from 15.9 per cent to 16.5 per cent. Lately people were very frustrated because their observations didn't agree with solid state theory, said Dr IM Dharmadasa, who leads the semiconductors and devices group at Sheffield Hallam.
Dharmadasa investigated the behaviour of CdTe cells, and noticed that they could be more accurately described by a model that combines a hetero-junction with a large Schottky barrier at the CdTe/metal interface. This model also treated CdTe is an n-type material, rather than p-type.
Applying their model, the researchers built a device structure that uses a Schottky barrier rather than an Ohmic contact to the semiconductor surface, resulting in a cell with 18 per cent efficiency.
These cells produce an open circuit voltage (VOC) of over 600mV, and short-circuit current density (ISC) values over 60mA/cm 2 . The fill factor (the ratio of peak power to ISC x VOC) was over 0.60.
Dharmadasa is currently working on a device that will offer 20 per cent efficiency. Twenty per cent is a nice figure, he said.
The project has received initial funding of £105,000 from the EPSRC, with follow-up from the university worth £140,000.