  We devise an optimised bi-component multi-layered dielectric stack design to
enhance the local irradiance for efficient photovoltaic upconversion materials.
The field intensity profile throughout the photonic structure is numerically
optimized by appropriate tuning of the individual layers' thicknesses. The
optimality of the thus inferred structure is demonstrated by comparison with an
analytically derived upper bound. The optimized local irradiance is found to
increase exponentially with the number of layers, its rate determined by the
permittivity ratio of the two material components. Manufacturing errors which
induce deviations from the optimised design are accounted for statistically,
and set a finite limit to the achievable enhancement. Notwithstanding,
realistic assumptions on manufacturing errors still suggest achievable
irradiances which are significantly larger than those obtained with the
recently proposed Bragg stack structures.
