In this work, we investigated the effect of annealing of absorber powder on the conversion efficiency of Cu2ZnSnSe4 (CZTSe) monograin layer solar cells. CZTSe powders were synthesized from binary compounds and elemental Se in the liquid phase of KI in evacuated quartz ampoules at 740 degrees C. In order to study the effect of post-treatments of the absorber material different annealing parameters such as Se and/or SnSe2 vapor pressure, annealing time and temperature were varied with the aim to gain uniform, good quality absorber materials for monograin layer solar cells. The annealing temperature was found to be crucial for the performance of CZTSe monograin layer solar cells. The conversion efficiency of solar cells improved significantly after the heat treatment. The effect can be attributed to the change of the absorber material composition and the crystals surface properties suitable for the effective p-n junction formation. The best CZTSe monograin layer solar cell showed conversation efficiency of 4.4%. (C) 2012 Elsevier B.V. All rights reserved.