Where can I download the spectral response function of Sentinel-2 image??
You can download the S2A Spectral Response Function from here. The S2B one will be delivered shortly.
S2 MPC Operations Manager
There is a new version of the Sentinel-2 Spectral Response Functions (S2-SRF) available at:
This spreadsheet contains the measured spectral responses for each band of the Sentinel-2 MultiSpectral Instrument (MSI) for the different satellite units (S2A and S2B).
S2 MPC/CC Operations Manager