Subtractive method for obtaining the dark current-voltage characteristic and its types for the residual (nongenerating) part of a multi-junction solar cell

The paper is focused on the fundamental loss in the nongenerating (residual) part of multi-junction solar cells. A method for determining the current-voltage characteristic of the residual part of solar cells is suggested and substantiated. The method is a generalization of the technique applicable to single-junction solar photovoltaic converters. The imbalance of photogenerated currents and the luminescence coupling between subcells is taken into account, which makes it possible to apply the suggested method to multi-junction solar cells. The method is applied to single-junction (InGaP, GaAs, Ge) and triple-junction (InGaP/GaAs/Ge) solar cells. Two types of current-voltage characteristics I–V are revealed and empirical laws for these characteristics are found. The first type shows a monotonic superlinearity, $J\propto V^n$, $n\approx$ 1.3–1.4), and is due to spreading resistance. The second type is only observed for triple-junction solar cells and has the form of a double-exponential dependence with a sublinear initial portion, $J\propto[e^{(V/E_1)}-e^{(-V/E_2)}]$, with $E_1\approx$ 0.35 V and $E_2\approx$ 0.15–0.30 V. As a result, it is found that the charge transport in the residual part of multi-junction solar cells is limited not only by the spreading resistance, but also by other factors, e.g., by isotype heterointerfaces.