Assessing current-voltage measurements of busbarless solar cells

For measuring the current-voltage (I–V) characteristics of busbarless solar cells, there is a certain degree of freedom in the choice of the contacting configuration as none has been defined as standard yet. This leads to the question of how the energy conversion efficiency of a busbarless solar cell should be specified when there are different measured values for it and poses the risk of misinterpreting cell measurements: Designing the measurement system for highest cell efficiencies for example may be tempting but can lead to cell results disassociated from later module performance and reduced cell-to-module power factors. The loss in module efficiency can thereby significantly exceed 5 %rel. It is therefore recommended to always adapt the contacting system to the module interconnection to yield meaningful I–V results in cell measurements. As a remedy for non-adapted measurement conditions, it is shown that I–V results can be converted from various cell measurement configurations to the module configuration. An analytical approach for the conversion of I–V results between measurement systems with different numbers of current contact bars as well as different shading properties is proposed. The approach is validated experimentally on busbarless cells over a large grid resistivity range. For three measurement systems with very different contacting schemes, the differences in fill factor and efficiency between the systems before correction clearly exceeded 10 %rel for high grid resistivities. After correction, good agreement in I–V characteristics within typical measurement uncertainties were shown. Limitations to the analytical approach associated with finger interruptions are identified and discussed.