Elevated temperature due to sunshine and self-heating can be detrimental to the performance and reliability of solar cells. In this work, we use theoretical calculations to identify that polydimethylsiloxane (PDMS), which is chemically stable and inexpensive, can be a highly efficient thermal emitter for cooling flexible thin-film solar cells. It is shown that a 200 mu m-thick planar PDMS layer is capable of achieving high emissivity over 0.9 in the whole IR regime of 4-26 mu m. The emissivity can be further increased to near-unity by introducing pyramid structures to the surface in the range of 8-13 mu m, where environment radiation is negligible. According to the calculated radiative cooling rates and realistic power conversion efficiencies, we show that a pyramid-structured PDMS layer can significantly lower the temperature of an organic, a perovskite and a micro-crystalline (mu c)-Si flexible solar cell by 11 degrees C, 12 C and 16 degrees C, respectively. This work may provide important guidance to the design of high performance and reliable flexible solar cells.