Using a modified molecular beam epitaxial (MBE) process called thin-capping-and-regrowth technique we grew quantum dot molecule (QDM) structures having high dot volume density (greater than 10(12) cm(-3)) and thus suitable as an active layer for effective photovoltaic energy conversion. Stacking of QDMs is time consuming and may introduce defects; therefore, optimization of stack number of QDMs is important in terms of achieving high-performance and low-cost solar cells. Samples with 1, 3, 5, 7 and 10 stacks of high-density QDMs are grown, fabricated into solar cells and characterized optically and electrically. It is found that the solar cell performance initially improves with the number of stacks, yet deteriorates once the number exceeds 5 stacks. We attribute the deterioration to defects and these put the optimized number of QDM stacks in our structure to 3-5. (C) 2008 Elsevier B.V. All rights reserved.