Compared with industrial thick crystalline silicon (c-Si) solar cells, thin c-Si cells have unique advantages of greater cost effectiveness and cell flexibility, representing a future technology trend. However, its present efficiency is far behind that of conventional thick c-Si solar cells. To improve the efficiency, it is necessary to consider and implement advanced designs. We report a strategy of optimizing cell designs to significantly improve the efficiency of a 20 μm-thick thin c-Si solar cell. Compared with the reference, the short-circuit current density is increased from 34.3 to 38.2 mA / cm², the open circuit voltage is boosted from 632 to 684 mV, and the fill factor presents an improvement from 76.2% to 80.8%, resulting in an absolute efficiency gain of 4.6% from 16.5% to 21.1%. The experimental results are further explained by the device simulations.