Simulation of the natural characteristics of vertical $a$-Si : H/$\mu c$-Si:H tandem solar cells. 1. General relations

An approach to calculating the characteristics of vertical (series) $a$-Si : H/$\mu c$-Si:H tandem solar cells (SCs) at arbitrary angles of incidence of sunlight is developed. The multiple reflection and refraction of electromagnetic waves at the internal interfaces of a tandem SC, in particular, at the $a$-Si : H/$\mu c$-Si:H interface is taken into account. In the calculation of the ideality factor and saturation current density of the diode component of the I–V characteristic of a tandem SC, as well as in the calculation of all its photovoltaic characteristics on the basis of these parameters at arbitrary sunlight incidence angles, general relations are used that take into account the recombination of excess carriers in both the quasi-neutral regions and in the space charge regions of the investigated structure. Expressions are obtained for determining all the main parameters of the component parts of a tandem SC, which are necessary for calculating the photovoltaic characteristics of the entire tandem SC. The results of calculation for standard AM1.5G (1000 W/m$^2$) illumination conditions are reported.