As global environmental and energy problems have become increasingly serious in recent years, photovoltaics have been attracting attention as a new energy source. In particular, organic photovoltaics (OPV) can be manufactured by the roll-to-roll (R2R) method, ensuring low cost and high productivity. As a result, low-price, lightweight, flexible devices can be produced. OPV is expected to be used in a wide range of applications such as window films, mobile devices, and automobiles.
However, the R2R method has a drawback. Since products are manufactured while the sheet is conveyed, it is difficult to monitor the characteristics of the products, making it impossible to conduct production control and in-line tests. To overcome this obstacle to mass production, non-contact sensors are needed.
OPV generates electricity by the following process: (1) light absorption and exciton formation, (2) exciton diffusion, (3) exciton dissociation, (4) charge diffusion, and (5) charge collection at electrodes. In the R2R method, products are manufactured while the sheet is conveyed. For achieving process control and manufacturing OPV at low cost, it is crucial to perform non-contact measurement and testing of the characteristics of products.
By using a laser, Yokogawa is developing two technologies that measure voltage and current, which are indicators of the power generation efficiency of OPV.
The EO sensor is an electric field sensor that uses optical technology. It does not disturb the electric field of measured objects and thus enables floating measurement without needing a ground reference. An optical crystal is used for the antenna element of the sensor. The birefringence of the crystal changes in proportion to the electric field applied. The EO sensor uses this property to measure the applied electric field. By using an algorithm in accordance with the principle of superposing electric fields, the sensor can measure voltage without being affected by the electric field of surrounding cells.
When OPV is irradiated, excitons are formed by light absorption. Then, excitons diffuse and reach the interface, where charges are separated (exciton dissociation), and carriers are formed. Carriers gather at the electrodes, which causes current to flow. Some carriers disappear before reaching the electrodes due to the recombination with excitons or defects in OPV materials. In this case, current cannot be taken out and the efficiency of power generation falls.
Yokogawa’s transient absorption sensor under development can monitor the current generation by OPV. First, the sensor performs laser pulse excitation. The density of carriers changes due to, for example, momentarily formed reaction intermediates, and the sensor can detect this change as a change in light absorption. Since the response time is correlated with current, this detection enables the current generation to be monitored.
Technology for non-contact measurement of organic semiconductors enables non-contact measurement of OPV performance. Previously, voltage and current were measured only by the module, but these can now be measured by the cell. In the R2R method, this technology helps manage the flow process of OPV production. In addition, by unifying quality and process data, the technology can visualize the correlation of various quality factors from multiple angles.
Yokogawa will continue to develop technologies and help customers create added value in production sites.