Dark lock-in thermography is used for monocrystalline or multi-crystalline photovoltaic cells. This method excites the cells with a modulated current signal. The modulation frequency is governed by the algorithm of the lock-in evaluation. This reduces the noise components of the measurement result, enabling identification of even the tiniest of production defects. In order to locate the defects, the thermal radiation from the current signal is analysed. The current density increases at the point of any damage, causing a local rise in temperature. This phenomenon is recorded by a thermography camera. Software is then used to evaluate the thermal images and locate the defect.

Inspection systems which use Illuminated Lock-In thermography excites the charge carriers using a periodically modulated light source. The energy supplied separates the charge at the p-n junction of the cell and forms both a negatively and a positively charged pole. Should a semiconductor between the poles reveal a short circuit or low ohm resistance, then a compensating current flows over it and the solar cell’s efficiency is reduced. With an infrared camera and the IrNDT software for non-destructive testing, the generated current flow is determined using the Lock-in method, and the source of the defect located. Lockin thermography provides rapid readings and can be integrated into production as an inline process control.

Electroluminescence can be used to identify defects such as contacting defects on conductor tracks. The process excites the test object with a current pulse, releasing light emissions within the infrared spectral range. These emissions are plotted using an NIR camera. Defective bonding is then located using software to evaluate the thermal images.

Unlike electroluminescence, photoluminescence uses a light source to generate a photocurrent in the semi conductor, which in turn generates a light emission. With this method, a monochromatic light source is typically used for illumination, its wavelength being below the wavelength of the emission from the semiconductor material.