The effect of dye concentration on the efficiency of a luminescent concentrator based on a perylene diimide (PDI) derivative incorporated in poly(methyl methacrylate) films cast on glass substrates is described. The predictions of ray-trace modelling are compared to experimental data from integrating sphere luminescence measurements. Increasing the PDI concentration leads to aggregation of the dye and an associated shift in the emission spectrum. The larger Stokes shift between emission and absorption spectra of the aggregated form reduces luminescence re-absorption and increases collection efficiency. However, the lower fluorescence quantum yield of the PDI aggregates restricts overall absorbed light to luminescence conversion. The relevance of the results to optimize organic molecule based luminescent solar concentrators is discussed. (c) 2012 Elsevier B.V. All rights reserved.