Back-contacted, ultrathin (<10 µm), and submillimeter-sized solar cells made with microsystem tools are a new type of cell that has not been optimized for performance. The literature reports efficiencies up to 15% using thicknesses of 14 µm and cell sizes of 250 µm. In this paper, we present the design, conditions, and fabrication parameters necessary to optimize these devices. The optimization was performed using commercial simulation tools from the microsystems arena. A systematic variation of the different parameters that influence the performance of the cell was accomplished. The researched parameters were resistance, Shockley–Read–Hall (SRH) lifetime, contact separation, implant characteristics (size, dosage, energy, and ratio between the species), contact size, substrate thickness, surface recombination, and light concentration. The performance of the cell was measured with efficiency, open-circuit voltage, and short-circuit current. Among all the parameters investigated, surface recombination and SRH lifetime proved to be the most important. Through completing the simulations, an optimized concept solar cell design was introduced for two scenarios: high and low quality materials/passivation. Simulated efficiencies up to 23.4% (1 sun) and 26.7% (100 suns) were attained for 20-µm-thick devices. Copyright © 2012 John Wiley & Sons, Ltd.