Novel techniques for photovoltaics characterisation

Introducing digital technologies and advanced sampling strategies for photovoltaic device testing

The electronic and magnetic materials group at NPL have been developing novel techniques that provide key insights into the performance of photovoltaics and other optoelectronic devices. Accurate measurements help accelerate innovation by providing confidence in the performance and reliability of photovoltaic materials and products, contributing towards worldwide net zero targets. Recently, a key focus of our work has been the development of novel measurement techniques based on the principles of digital light processing (DLP) and compressed sensing. Such techniques can offer specific benefits over other established methods, such increased signal to noise ratio and speed.

Compressed sensing spatial characterisation

Photocurrent mapping or light beam induced current (LBIC) provides a spatial map of the homogeneity of light conversion into electricity. Variations in the measurement map provide critical insight into quality of the photovoltaic device and can be used as a tool for assessing degradation/reliability. However, traditional methods are slow or not suitable for large area cells and modules. NPL has introduced the technique of compressed sensing current mapping for spatial characterisation of PV devices. Instead of applying a raster scan, a series of patterns are projected on the PV sample using DLP, acquiring fewer measurements than the pixels of the final current map. The final reconstruction of the current map is achieved by means of an optimization algorithm, exploiting the compressibility (sparse representation after a transform) of the measured signal. We have demonstrated the signal increase and dynamic range benefits of this technique for samples based on different PV technologies. Having adopted a fast projection-based system, we have moved towards megapixel resolutions for current mapping using this technique. We have expanded such techniques in other application areas, such as imaging of minority charge carrier lifetimes of semiconductors materials and devices.

Fast linearity measurements over wide range of irradiances

Using DLP techniques, we have introduced a new approach for linearity measurements of solar cells which we have demonstrated to offer higher resolution and speed compared to established methods. High accuracy linearity measurements are essential for reference cells and optical sensors, as they ensure the precision of the measured irradiance levels.  

Extracting current-voltage characteristics of individual cells in PV modules

We developed a non-destructive approach based on structured illumination that allows to extract electrical properties of individual cells in PV modules. Such an approach can be potentially utilised for investigation of degradation mechanisms of cells in PV modules, or to provide quantitative validation of other optical imaging techniques such as luminescence imaging and infrared thermography.

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