Accurate Measurement of Bifacial Solar Cells with Single- and Both-Sided Illumination
In order to quantify the bifaciality of bifacial solar devices in a standardized way, the IEC technical specification 60904-1-2 defines two different measurement methods that can be conducted in addition to measurements at standard testing conditions. The first method (referred to as bifacial method) is based on simultaneous front and rear side illumination, the second method (referred to as equivalent irradiance (GE) method) comprises front-side illumination with increased irradiance. Intrinsic differences between the two methods can arise from the different photogeneration depth profiles for bifacial solar devices with e.g. injection-dependent recombination or inversion layer shunting. These injection-dependent effects occur as nonlinearity of short-circuit current as a function of irradiance. In this paper, the bifacial and GE methods are compared for bifacial passivated emitter and rear (PERC) solar cells with very strong nonlinearity. To detect differences between the two methods with high accuracy, an advanced measurement setup and an adapted calibration procedure have been used. The measurement uncertainties of the setup have been determined by means of Monte Carlo simulations taking into account correlations between both methods. Differences in IU parameters between the two methods were measured to be below 0.1 %rel, which is clearly within the respective measurement uncertainties. The two methods presented in the IEC technical specification 60904-1-2 can therefore be considered as consistent for bifacial solar cells with respect to nonlinearity. Further measurements and simulations indicate that the consistency also holds for lower front irradiances, which are relevant for energy rating and yield assessment of bifacial devices.