Photoelectric determination of the series resistance of multijunction solar cells

A method for determining the series resistance $R_s$ of multijunction solar cells is suggested and sub-stantiated. The method uses the presence of a maximum in the dependence of the efficiency on the sunlight concentration ratio $\eta(X)$ or in that of the operating voltage on the photogenerated current, $V_m(J_g)$. The study employs the concept that, in a limited but practically important range of photogenerated currents (up to the maximum $\eta$), the series resistance can be represented by a fixed quantity that is linear and independent of $J_g$. It is analytically substantiated that this resistance can be found from the formula $R_s = (E/J_g)_{\eta=\mathrm{max}}$, where $E = AkT/q$ and $A$ and $J_g$ are local values of the ideality factor and photogenerated current at the maximum $\eta$ (or $V_m$). It is shown that the value of $R_s$, determined by this method, is independent of the spectral composition of the incident light, which was experimentally confirmed in a study of the photovoltaic characteristics of triple-junction InGaP/GaAs/Ge solar cells. The method is suitable for both multi- and single-junction photoelectric converters.