Up-conversion (UC) is a promising approach to utilize sub-band-gap photons for solar cells (SCs). Due to the non-linear nature of UC, the optimal excitation power regimes between the solar cell semiconductor and the UC material correspond to a difference in solar concentration of more than an order of magnitude. This difference can be bridged with integrated optics by concentrating the photons transmitted through the solar cell to increase the power density and maximize the intensity of UC luminescence. To realize this, dielectric-filled compound parabolic concentrators (CPCs) were used as integrated optics on the rear side of a planar bifacial silicon solar cell together with a 25% Er3+ doped hexagonal sodium yttrium fluoride (beta-NaYF4:Er) UC phosphor. An efficiency increase of 32% from 0.123% to 0.163% under sub-band-gap illumination is quantified by means of the first ever reported I-V characteristics for an up-conversion solar cell (UC-SC) based on c-Si. An enhancement in external quantum efficiency (EQE) is obtained from 133% for the non-concentrating reference UC-SC to 1.80% for a solar cell with integrated optics for an excitation at 1523 nm with an irradiance of 0.024 W/cm(2), corresponding to a normalized EQE of 0.75 W/cm(2). This demonstrates that CPCs are suitable for UC-SC as they increase the concentration in the forwards direction, while maintaining high collection efficiency of the UC emission in the reverse direction. In addition, such an approach enables the optimization of the solar concentration on the UC phosphor independently from the concentration required for the solar cell. (C) 2015 The Authors. Published by Elsevier B.V.