NOVELTY - An energy conversion device comprises a layer of a photon-absorbing semiconductor material (102) having a front photon-receiving surface and a back surface; and an up conversion composite material (130) within cavities. The up conversion material comprises a mixture of greater than or equal to 2 different up conversion materials formed as crystal grains. Each up conversion material is capable of absorbing photons (220-222, 225-227) having wavelengths longer than absorption edge of semiconductor material and emitting photons having wavelengths shorter than the absorption edge.    USE - Energy conversion device used in a photovoltaic solar cell.    ADVANTAGE - The device minimizes losses due to non-absorption and thermalization in solar cells by up converting the energies of incident photons prior to absorption by the semiconductor, and increases the optical coupling between the semiconductor and an up conversion material. The use of up conversion materials overcomes the use of a single up-converter material that only absorbs over a few narrow bands of spectrum.    DETAILED DESCRIPTION - An energy conversion device comprises a layer of a photon-absorbing semiconductor material (102) having a front photon-receiving surface and a back surface; cavities formed in the photon-absorbing semiconductor material extending inwardly from the back surface; and an up conversion composite material (130) disposed within the cavities. The semiconductor material has an absorption edge. The up conversion material comprises a mixture of greater than or equal to 2 different up conversion materials formed as crystal grains dispersed within a dispersion medium comprising an optically transmitting material. Each up conversion material is capable of absorbing photons (220-222, 225-227) having wavelengths longer than the absorption edge of the semiconductor material and emitting photons having wavelengths shorter than the absorption edge, and each up conversion material is different to absorb photons having different wavelengths. The up conversion materials include crystal grains comprising a crystal material doped with dopant atoms capable of absorbing photons having wavelengths longer than the absorption edge of the semiconductor material and emitting photons having wavelengths shorter than the absorption edge. The layer of semiconductor material and the up conversion material disposed within the cavities comprise a photovoltaic solar cell.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic diagram of a conversion device showing light absorption and emission processes.    Photon-absorbing semiconductor material (102)    Emitter (103)    Conversion composite material (130)    Light ray (210)    Photons (220-222, 225-227)