Determining the junction temperature for STC measurements of thin film solar cells

In expectation of high potentials for cost reduction, many new approaches for thin film solar cell concepts achieved increased attention in the last time. With improving efficiencies, the accurate determination of the solar cell parameters has received growing attention. Calibration laboratories, such as the calibration laboratory of Fraunhofer ISE (ISE CalLab PV Cells), need to develop procedures to determine reliable solar cell parameters of such technologies, in accordance with standard testing conditions (STC, IEC 60904-3). A central requirement is the control of the junction temperature. Many thin film technologies, such as organic solar cells, need to be encapsulated in order to protect the material from environmental influences. This means that the junction temperature cannot be determined by direct probing, and other methods are required. If the solar cell is additionally affected by light soaking, the VOC-method also cannot be performed to determine the junction temperature. Model calculations were performed to predict the spatial and temporal distributions of changes in the temperature of an encapsulated thin film layer. These are confirmed by experiments with specially prepared organic solar cells that also contained a temperature sensor and by infrared recordings. Further experiments to understand the nature of light soaking and investigate the use of the VOC-method were performed.