Quantifying the rear side contribution in bifacial tandem-photovoltaic devices

Bifaciality is a common approach to increase the power output of photovoltaic devices by capturing a higher portion of the available irradiance. The combination of bifacial and tandem architecture gained significant attention in recent years. However, their accurate characterization remains challenging, hindering device comparisons and the assessment of different development approaches. The procedures for single-junction bifacial devices outlined in IEC TS 60904-1-2 are not fully applicable to two-terminal tandem devices due to current limiting effects. In this study, a combination of simulated and measured data is used to propose new parameters to describe the spectral characteristics of bifacial tandem devices. Those parameters are the short-circuit current and maximum power gain factors σISC and σPMPP as well as the rear side irradiance limit (RIL). Based on these parameters a bifacial perovskite on silicon single-cell module is characterized performing outdoor measurements. Comparing measurements with an open and a covered rear side, a novel approach to quantitatively determine the bifaciality characteristics of bifacial tandem devices is introduced. For the first time, this enables scaling of measured ISC and PMPP values to different rear irradiances. This represents an important step towards a standardized measurement procedure for such devices. Finally, optimization approaches for bifacial tandem devices are proposed based on the presented results.