  Ever science the invention of solar cells, thermodynamics has been used to
assess their performance limits, guiding advances in materials science and
photovoltaic technology to reduce the gap between the practical efficiencies
and the thermodynamic limits to photovoltaic energy conversion. By
systematically addressing the thermodynamic efficiency losses in current
photovoltaic, ultrahigh efficiency photovoltaic can be expected. Currently, the
non-radiative recombination of some ultrahigh efficient solar cells is almost
completely suppressed, and the radiative recombination loss is then the key to
restrict the further improvement of device performance. This work relates the
energy band edge electronic density of states (DOS) of semiconductor absorber
and transport layer, excited/transfer state electronic entropy to
thermodynamically inevitable energy loss during photoelectric conversion in
solar cells. On accounts of the basic limitations of thermodynamic laws on the
energy conversion process, this work reveals a hidden variable that affects the
photovoltaic performance and puts forward the band edge DOS engineering as a
new dimension in performance optimization of solar cell apart from the
traditional material and defect passivation engineering, etc. This work
highlights the great importance of DOS engineering for further improving the
performance of any solar cell devices.
