To realize high efficiency solar cells, new concepts beyond the Shockley-Queisser limit are widely investigated. The
intermediate band solar cell (IBSC) is one of the candidate concepts. From the importance of device physics, we have
developed a device simulator for IBSCs. For device simulation of IBSC, the Poisson equation, carrier continuity
equations of electrons in the conduction band (CB) and the valance band (VB) and balanced equation of IB state
electrons must be solved self-consistently. The simulation methods can clarify the intrinsic device behavior of IBSCs
which cannot be investigated by the detailed balance model. For example, by the existence of electrons trapped in IB
states, electrostatic potential along the depth direction of the solar cells is strongly modified from the equilibrium under
illumination of sunlight. This potential change is strongly related to its absorption property of sunlight. And the doping
to IB region can enhance short circuit current density via IB states. Under larger concentration, this doping effect is
decreased by the photofilling effects in the radiative limit. Absorption coefficients of each band-to-band transition are
decided by the semiconductor materials and fundamental physics. These limitations make the different spectra and
values from ideal treatments and decide the maximum efficiency of the IBSC. In this work, we present the fundamental
properties and suggestions to approach the high efficiency IBSC operations as a device.
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