Linking materials characterisation through to PV device performance PV21 has changed the way that we go about developing new and advanced low cost solar cells.
Firstly, we have the capability to rapidly test new ideas on lab-scale demonstration devices. We do this on our 'device platforms' ’Äì baseline processes where a standard recipe will give well defined device results. A new material can be introduced or novel processing step and results compared with the standard recipe.
Secondly, we recognise that 'low cost' doesn't always mean 'simple' when it comes to materials ’Äì in fact it's just the opposite! In PV21 we tackled this head-on from the outset by using advanced materials characterisation. We have a highly interactive partnership between the consortium members as an approach to materials characterisation, cell making, understanding the results, leading to device improvement.
For example, with thin film polycrystalline solar cells, the challenge is to control the grain size and doping and to use both to create leading PV device structures. To do this we have had to push characterisation tools to the limit, and develop new methods. With cadmium telluride we have shown that the main contribution to harmful resistance in the cells came from grain boundaries -this was found using admittance analysis. Hence we developed a way to increase grain size that was shown to improve cell efficiency from 2 to 11%, and has led to a PV21 patent application.
We also have a new understanding of beneficial doping and harmful deep levels in our cells, which required developing 'thermal admittance spectroscopy' to a leading level. Using this we have been able to compare undoped CdTe cells with arsenic doped cells and relate this to the device performance. We have demonstrated that we can diagnose processing faults in our cells manufacturing and established a direct feedback loop from state-of-the-art characterisation to cell making.
Both PV manufacturers and the scientific instrument industry are keenly interested in this approach. One of our industrial partners Semimetrics Ltd has been very pro-active in developing the advanced characterisation tools that a new industry in thin film PV will demand and working closely with the academic partners to guide this development
