Extraction of Essential Solar Cell Parameters of Subcells in a Tandem Structure With a Novel Three-Terminal Measurement Technique

Detailed understanding of the device electrical parameters based on the solar cell diode model is important for the optimization of the material and device structure to achieve higher efficiency. This paper discusses a method to extract subcell parameters in a tandem structure in a direct and simultaneous manner. A three-terminal analysis was done on a GaAsP/SiGe tandem solar cell grown on a Si substrate. The method has been proven to work on a monolithically grown cell where a middle contact is possible. The middle contact is used to measure the voltage between the two cells and therefore allowing calculation of the voltage of the individual cells. Circuit simulation demonstrates the validity of the measurement method as well as the calculation formula relating subcells parameters to the overall tandem performance. The resulting current-voltage curves allow calculation of essential solar cell parameters of each subcell, such as dark saturation current, ideality factor, and shunt and series resistances. Moreover, the current limiting cell under different illumination spectra can be accurately identified, which affects the overall power performance depending on the different subcell current-voltage characteristics. This paper focuses on the use of a three-terminal device for characterization purposes.