Flexible inverted top-illuminated polymer solar cells (IT-PSCs) are fabricated by wetting of polyelectrolyte and designing a microcavity structure by laying an indium-tin-oxide (ITO) interlayer on top of an Ag reflector. The ITO-coated Ag makes the surface hydrophilic, thereby improving wettability of polyethyleneimine (PEIE). This increased wettability of PEIE yields a reflective cathode with low work function of 3.73 eV. The ITO layer also tunes the light absorption spectrum in the active layer. Finite-domain time-difference simulation provides evidence that the ITO layer played a role in both the shift in resonant wavelength in the microcavity and confinement of the electric field to the active layer. Time-dependent simulation suggests that the time to reach steady-state light absorption is longer (6.6 fs) when a microcavity is present than when it is not present (3.8 fs); i.e., the microcavity increases light absorption in the active layer. The designed IT-PSCs show a maximum photo-conversion efficiency of 6.4% on plastic film and 6.1% on opaque copper foil; these are the highest values obtained by top-illuminated PSCs on a metallic substrate. The IT-PSCs have excellent mechanical flexibility and more stable in air than conventional normal structured devices.