Thin and flexible crystalline silicon (c-Si) heterojunction solar cells are fabricated with very simple processes and demonstrated experimentally based on MoOx/indium tin oxide (ITO) and LiFx/Al as the dopant-free hole- and electron-selective contacts, respectively. With the ITO coating, both hole collection ability and antireflection are greatly improved, leading to significant improvements in the fill factor and short-circuit current density (Jsc). Our 25, 35, and 45 μm thick c-Si solar cells are 87.5, 82.5, and 77.5% thinner than the original 200 μm thick counterpart but are still 71.29, 86.13, and 87.37% efficient compared to the original solar cell, respectively. Their power conversion efficiencies (PCEs) all exceed 10% and are stable for 400 days in air, among the top five highest reported PCEs of <50 μm thick c-Si solar cells with undoped contacts. Without any textures, high Jsc values are still achieved. Especially for the 35 and 45 μm thick c-Si solar cells, the Jsc values are over 80% of their theoretical limits, comparable and even superior to the maximum ratio of the reported cells with undoped contacts. Our thin c-Si solar cells can be bent to a radius as small as 4 mm with minimal PCE degradation, showing excellent mechanical flexibility. Without complex processes involving texturing and doping, our c-Si solar cells with dopant-free contacts are promising to have quite a low cost and, thus, wide applications.