One-sun illumination is quite intense so there has to be some mechanism to remove the excess heat. Typically the cell is placed on a large metal block and water runs through the block to keep it cool. A thermocouple is inserted in the block and the control system is adjusted to the required 25 °C. The simple arrangement works very well for commercial cells where the entire rear is covered with metal and makes good contact with the test block.
A more sophisticated arrangement is required for cells where some or all of the contacts are on the cell rear, or for bifacial illumination. A common method used is flash testing where light is flashed on the cell and measurements are taken very quickly. While this largely eliminates temperature control problems sophisticated electronics are needed to take measurements quickly and synchronized with the flash. Flash testing is also commonly used for module testing where it is generally not possible to directly control the cell temperature due to the encapsulation.
- Poor temperature control introduces errors in VOC. The error is dependent on the bandgap of material (see Effect of Temperature).