The Challenge of Measuring Busbarless Solar cells and the Impact on Cell-to-Module Losses
The realistic measurement of solar cells is key for the whole PV industry, as accurate information about cell power is one of the most important aspects in solar cell purchase and PV module design. The omission of busbars introduces new challenges to the current-voltage measurement of solar cells, since contact to every single grid finger has to be established with independent current and voltage contacts. It is not just the shadow correction of the measurement unit that needs to be carried out more laboriously, but also the contacting of the front metal grid, which is more critical because of the high resistivity of the grid fingers. The position of the voltage sensing contact and the number of current contacts can thus have a noticeable impact on the measured performance of busbarless solar cells. Measured efficiencies are highly dependent on the contacting schemes used in different measurement systems, as these vary in contact number and sensing configuration. Two different main approaches for measuring busbarless solar cells have evolved, representing either realistic or idealized application of the cells in the module. The pros and cons of both approaches are discussed in detail in this paper. Realistic measurement conditions lead to efficiencies which best predict module performance, but are hard to realize and require knowledge about the subsequent module design. Although not their primary purpose, the use of idealized measurement conditions can make it easier to achieve record cell efficiencies, but with the disadvantage of limited comparability with busbar-based solar cell concepts. Idealized conditions can moreover lead to hidden losses in performance of the solar cells, related to the application in a module, which in turn causes inflated cell-to-module (CTM) losses. If solar cells are bought in terms of $/Wp and modules are sold likewise, the economic implications arising from the different measurement configurations have to be considered. Whichever approach is used for the measurement of busbarless solar cells, full disclosure of the measurement configuration is absolutely essential.