Spatially resolved measurement of thin film silicon solar modules by laser beam induced current (LBIC) system

摘要

This thesis presents the development of innovative tools to investigate spatially distributed properties of thin film photovoltaic devices. They are required to gain a better understanding of device behaviour driven by how such properties affect the performance of commercial-scale devices. The tools developed for this are a distributed 3D model (D3DM) as simulation software and a laser beam induced current (LBIC) system as a platform for characterisation. The D3DM was developed utilising standard circuit analysis software. It is constructed to simulate realistic device structures and current flows in thin film PV devices. Diode parameters are truly distributed and can be varied independently. The model includes a voltage dependent photocurrent which is a key characteristic of amorphous silicon based solar cells. The D3DM has been used for the investigation of spatial variation in performance due to the distributed nature and non-uniformity of diode parameters and solar cell properties. It is shown that distributed series resistance contributed from the contact layers has a significant impact on solar cell performance and efficiency. The LBIC system is an optical scanning based characterisation tool. Unlike most existing systems, this has been developed specifically for large area, module-size thin film applications. The system provides a detailed photocurrent map which reveals spatial non-uniformity and allows investigation of localised performance variation of the investigated PV devices. System development, components and their characterisation as well as different measurement techniques are described. The model is also applied to LBIC measurements where it is used for a sensitivity analysis of measurement signal with respect to certain cell parameters in cells and modules under different measurement conditions. A new limiting illuminated LBIC (li-LBIC) measurement technique was developed. It is a measurement where the laser-probed cell is brought into limiting condition by means of shading. The signal thus generated is a linear response which was previously unobtainable by typical LBIC measurements. It is unaffected by non-uniform illumination allowing the real properties of investigated cells in a monolithic series connected module to be measured non-destructively.