NOVELTY - Perylene diester derivatives (I) are new.    USE - Perylene diester derivatives for use in luminescent dyes for an optical light collection system, a fluorescence-based solar collector, a fluorescence-activated display and/or single-molecule spectroscopy.    ADVANTAGE - The diester derivatives enhance the photostability and photoelectric conversion efficiency of thin film cadmium sulfide/cadmium telluride solar cell, or copper indium gallium diselenide solar cells.    DETAILED DESCRIPTION - Perylene diester derivatives (I) of formulae (A1 or B1) are new.    R1,R11=H, 1-10C alkyl, 3-10C cycloalkyl, 2-10C alkoxyalkyl, 6-18C aryl or 6-20C aralkyl;    m,n=1-5;and    R2,R22=6-18C aryl or 6-20C aralkyl.    INDEPENDENT CLAIMS are also included for the following:    (1) an organic down-shifting luminescent medium comprising an optically transparent polymer matrix and luminescent dye(s), where the medium receives photon(s) having a first wavelength as an input and provides photon(s) having a second wavelength as an output, where the second wavelength is longer than the first wavelength, and the luminescent dye comprises perylene diester derivatives    (2) a photovoltaic module for the conversion of solar light energy into electricity, comprising photovoltaic device(s) or solar cell, and an organic down- shifting luminescent medium, where the photovoltaic device(s) or solar cell is adapted to convert incident solar light energy into electricity and has a light incident surface substrate, and the medium is incorporated on top of, or into, the photovoltaic device(s) or solar cell, such that the incident light passes through the medium prior to reaching an area of the photovoltaic module, where solar light energy is converted into electricity    (3) a method for improving the performance of a photovoltaic device(s) or a solar cell, which involves applying the medium directly onto a light incident side of the photovoltaic device(s) or solar cell.    DESCRIPTION OF DRAWING(S) - The drawing shows a sectional view of a photovoltaic module.    Thin film organic down- shifting luminescent medium (100)    Refractive index matching liquid (101)    Light incident surface substrate (102)    Solar cell (103)