Comprehensive Evaluation of IEC Measurement Procedures for Bifacial Solar Cells and Modules
The IEC technical specification (TS) 60904-1-2 has been published recently in order to establish a standardized way of evaluating bifacial solar cells and modules. Two different methods for indoor measurements of the current-voltage characteristics of bifacial devices have been reported, which are based on simultaneous front and rear illumination (referred to as bifacial method) or on illumination at elevated irradiance levels from front only (referred to as equivalent irradiance (GE) method). In this study, the measurement procedures and input parameters specified by the IEC TS for the bifacial and GE methods are analysed in detail. This particularly applies to the bifaciality coefficients Isc and Pmpp, which are the ratios of rear to front Isc and Pmpp, respectively, and are used for the calculation of the equivalent irradiance GE. It is shown that the bifaciality coefficients have to be selected carefully to accurately determine the bifacial low-light performance with the GE method. Evaluating the bifaciality coefficients as proposed by the IEC TS for nonlinear solar cells can lead to deviations between the bifacial and GE methods of up to several percent in the parameters BiFi, PmppBiFi10% and PmppBiFi20% -standardized measures for the power gain of the bifacial device caused by additional rear irradiance. In addition, differences between bifacial and GE methods can also arise for bifacial solar modules with partial rear shading. By comparing measurements with GE and bifacial methods for these modules, it is shown that following the IEC procedure can lead to errors in BiFi of more than 18 % and in PmppBiFi20% of more than 2 %. An alternative approach of evaluating bifaciality coefficients is therefore proposed in this study: By applying only the bifaciality coefficient Isc, which is physically the most meaningful coefficient, and by evaluating Isc at the actual front irradiance of the measurement, the agreement between the bifacial and GE methods can be considerably improved. A criterion for the applicability of the GE method using Isc is derived by systematically varying the rear shading fraction of bifacial modules. It is thus shown that both methods can be applied in good agreement, if amendments to the IEC TS are made.