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We analyze the quantum efficiency measurement of a series-connected multijunction solar cell by modeling the cell as an ac resistive-capacitive circuit and studying the complex current as a function of the light biasing. The photocurrent induced by directly absorbed photons is modeled as an independent current source, whereas the luminescent coupling current from higher bandgap to lower bandgap subcells is modeled as a dependent current source in the bottom subcell. We derive expressions for the equivalent impedance and the complex current in measurements of the top and bottom subcells of a two-junction device. High light biasing of the nonlimiting cell drives the magnitude and phase shift of the current toward well-defined limits, but insufficient light biasing yields a composite response that is not fully characteristic of either subcell. We recommend that the experimenter always monitor the phase shift of the signal for signs of insufficient light biasing, and to help identify genuine luminescent coupling effects.