Fluorescent photon down conversion for the improvement of the blue response of ZnO/CdS/Cu(1n,Ga)Se-2 heterojunction solar cells and modules is investigated. Fluorescent dyes of the series Lumogen((R)) F are analyzed by optical transmission and reflection as well as by photoluminescence measurements. A spectral transfer matrix formalism is introduced that allows to predict the suitability of a luminescent dye as a down-converter for a given solar cell from its absorption/emission properties. We find that Lumogen((R)) F Violet 570 and Lumogen((R)) F Yellow 083 as well as a combination of both yields improvements for Cu(In,Ga)Se, solar modules. Particularly, we find that the short circuit current density of a Cu(In,Ga)Se, mini-module is improved by 1.5 mA cm(-2) when applying a varnish with a combination of Lumogen((R)) F Violet and Yellow. About 0.5 mA cm(-2) of this improvement is due to a reduced overall reflectance and an improvement of I mA cm-2 results from the frequency conversion by the dyes. (C) 2007 Elsevier B.V. All rights reserved.