Simulation of nonuniform irradiance in multijunction IIIV solar cells

Optics for high concentration photovoltaics often delivers a non-uniform irradiance to the cell. This can be a problem for tunnel-junction interconnected (TJIC) IIIV multijunction solar cells if the resulting local photocurrent exceeds the peak tunneling current density. Current spreading in the vicinity of the tunnel junction can mitigate this effect. We use commercial software to simulate current spreading in a simple GaInP/GaInAs cell with a thin GaAs TJIC. We show that for the narrow light beams, the current spreading is fit reasonably well by a Lorentzian with a spreading length on the order of 10 µm. Below some critical irradiance that depends on the width of the light beam, current spreading increases with the local irradiance. At the critical irradiance where the tunnel diode switches to the thermal current state, the current spreading abruptly decreases. Above the critical irradiance the current spreading continues to decrease with increasing irradiance. The effects of other device parameters on current spreading are discussed.