The present work presents some lines of research aimed to contribute to a better performance of multi-junction solar cells at very high concentrations (similar to 1000suns) by minimising the series resistance of these devices. In the first section, a set of results is presented to ascertain the potential of tellurium as a possible n-type dopant to improve the performance of tunnel junctions and/or the top cell emitter. Some anomalies in the incorporation of Te into GaInP are described and their impact on solar cell growth and operation is discussed. In the second section, the contribution of the bottom cell BSF layer to the series resistance is analyzed by comparing three different alternatives, namely p(++) GaAs; pGaInP; and p(++) Al0.2Ga0.8As. BSFs made of moderately doped GaInP are demonstrated to contribute significantly to the series resistance of the device, whilst p(++) GaAs layers are shown to produce lower photocurrents. On the other hand, p(++) Al0.2Ga0.8As layers are shown to unite both high photocurrents and low series resistance, being thus the optimum option. (c) 2006 Elsevier B.V. All rights reserved.