The Solar+Reflectance optical head

An in-line monitoring system that is compatible with copper indium gallium selenide (CIGS) and cadmium telluride solar cell production is available from LayTec, based in Berlin, Germany.

The company claims the technology will accelerate development and reduce processing costs for a range of thin film deposition processes.

‘The future of thin film solar cell production heavily depends on reliable in-line thickness and other solar cell properties that influence effectiveness and production costs,’ says Dr Steffen Uredat, from Laytec’s R&D department.

‘Measurements in thin film photovoltaic (PV) processes are [currently] performed offline and not on each batch, but we have provided PV makers with a tool to control their production in-line and enhance their yield.’

The system, known as Solar+ Reflectance (SolR), is capable of measuring the thickness of all functional layers, providing information on optical properties, surface roughness, and formation of interdiffusion regions between layers.

Copper indium gallium selenide is used in solar cells because it offers a wider bandgap and increases light absorption compared to crystalline silicon cells, traditionally applied in PV technology.

Absorber layers in CIGS, however, are intentionally designed to be rough in order to maximise the internal reflections of sunlight and enhance the efficiency of the cell. Reflectance measurements are usually obtained more easily from smooth layers, but LayTec’s in situ method can establish film-thickness measurements from rough surfaces.

‘We extended our analysis algorithms to handle spectra from ultra-rough layers. From the spectral characteristics data for surface texture, interdiffusion and the conductivity of the transparent conductive oxide can be obtained,’ explains Uredat.

Professor Ken Durose, Director of the Centre for Renewable Energy at Durham University, UK, believes that the technology could have a number of benefits for thin film PV manufacturers. ‘The in-line process will enable manufacturers to have direct feedback into the process variables in a dynamic way. Real-time control of thickness will have huge cost and time saving benefits.’

However Durose stresses that the major drawbacks of CIGS are the high cost and limited supply of indium and gallium. Research is ongoing at Durham to produce solar cells from more common elements, which have comparable PV properties.

The LayTec sensor is not compatible with more commonly used silicon solar cells. The company says that it cannot currently reveal any measures to make it compatible.