Designing bottom silicon solar cells for multijunction devices

We report on efforts to design high-efficiency silicon homojunction subcells for use in multijunction stack devices. Both simulation and experimental works have been performed looking at a silicon solar cell under a truncated spectrum below 1.5 eV filtered by the upper layers in the multijunction stack. Good agreement is seen between the modeling and experimental results, identifying different emitter design requirements when the solar cell operates under a full or truncated spectrum. A well-passivated front surface, i.e., with low-interface surface recombination velocity, required a lightly doped emitter profile to maximize open-circuit voltage (V_OC ), while a high-interface recombination surface requires a heavily doped for higher V_OC values. The impact on short-circuit current density (J_SC ) is found to be minimal, even with large variations in the interface recombination and emitter profiles. In a tandem stack, an interface with low- and high-interface recombination velocities would require lightly doped and intermediate-doped emitters, respectively, for maximum conversion efficiency (η).