A Comparison of the Errors in Determining the Conversion Efficiency of Multi-Junction Solar Cells by Various Methods

Methods of accurately measuring the conversion efficiency with respect to a given set of standard reporting conditions have been established for single-junction photovoltaic (PV) devices. Efficiency measurements on two-terminal tandem PV devices with more than one active junction are more difficult since the junction limiting the current and efficiency is determined by the source spectrum. The various methods of determining the illuminated current vs. voltage parameters of two-terminal and multiterminal multijunction solar cells with respect to standard reporting conditions (temperature, spectral irradiance and total irradiance) will be discussed. The methods discussed include a solar simulator and reference cell for each junction, one solar simulator with one reference cell, and one or more solar simulators with the absolute quantum efficiency for each junction of the multijunction cell. All three methods require the measurement of the quantum efficiency of each active junction of the cascade structure. A brief description and error analysis of the various methods will be discussed. Data will be presented comparing the I-V characteristics for several tandems using the various methods.