  We develop a new approach to calculate the obtainable limit of
photoconversion efficiency of tandem solar cells (SCs) and applied it to SCs
with both vertical and lateral designs at AM0 and AM1.5 conditions. To get the
maximum efficiency, only radiative recombination has been considered using
typical radiative recombination parameters of the direct band gap III-V
semiconductors, and explicit energy dependence of light absorption. When
simulating the efficiency, we selfconsistently took into account the fact that
the amount of the heat dissipated by SC decreases as the number of
current-matched sub-cells increases. As the operating SCs temperature decreases
both the open-circuit voltage and the photoconversion efficiency increase. It
is shown that the above effect is especially strong for SCs operating under AM0
conditions. As the number of subcells is increased, narrowing the spectral
range for each subcell, the photocurrent is additionally reduced due to the
energy dependent light absorption, the factor generally ignored in the standard
approaches. Application of our formalism results in a maximum in the
theoretical dependence of the efficiency on the number of subcells, which was
indeed observed experimentally. Besides agreement with experiment, our
theoretical results are also close to other efficiencies calculated using
detailed balance based approaches.
