Tin-based perovskites have very similar electronic properties to lead-based perovskites and are regarded as the most promising candidates for efficient lead-free perovskite solar cells. The past few years have witnessed rapid development of tinbased perovskite solar cells (Sn-PSCs) on rigid glass, with highest certified power conversion efficiency (PCE) of 12.4%. Development of Sn-PSCs on flexible substrates is interesting for roll-to-roll manufacturing. Currently, there are only a few reports on flexible Sn-PSCs with highest efficiency of around 3%, primarily due to relatively uncontrollable crystallization, and oxidative instability of tin-based perovskites. In this contribution, in-house deposited transparent conducing oxide (TCO) including hydrogenated indium oxide In2O3:H (IO:H), indium zinc oxide (IZO), and commercial indium tin oxide (ITO) on poly(ethylene 2,6-naphthalate (PEN) are used as front electrodes for flexible Sn-PSCs. GeI2 additive is introduced into the perovskite precursor solution to lower the trap densities in tin-perovskites and suppress the easy oxidation of Sn2+ into Sn4+. The device performance of flexible Sn-PSCs with different TCOs are systematically evaluated. We found that the TCO layer is critical to achieving high open-circuit voltage (VOC) and stable device. The underlying mechanism will be elucidated with various characterizations. The microstructure and phase composition of tin perovskites are investigated by scanning electron microscope (SEM), scanning transmission electron microscopy (STEM), and X-ray diffraction (XRD). Time-resolved photoluminescence (TRPL) and thermal admittance spectroscopy (TAS) will also be performed to probe the carrier dynamics and trap density of tin-perovskites.