An inverted polymer solar cell was fabricated on a polyethylene terephthalate (PET) substrate. The cell structure was ITO/amorphous titanium oxide (TiO x )/[6,6]-phenyl C 61 butyric acid methyl ester (PCBM): regioregular poly(3-hexylthiophene) (P3HT)/poly(3,4-ethylenedioxylenethiophene):poly(4-styrene sulfonic acid) (PEDOT:PSS)/Au (TiO x cell). It was achieved using low boiling point diethyl amine (DEA) as a Ti 4+ complexing agent, and high boiling point 1,2,3,4-tetrahydronaphthalene (Tetralin) containing 2 vol.% 1,8-octanedithiol (ODT) as the solvent for PCBM:P3HT film preparation. With heat treatment at 100 °C, the use of DEA and Tetralin:ODT made it possible to prepare a TiO x electron collection layer and PCBM:P3HT photoactive layer, respectively. Fabricating the TiO x cell on the thin PET film was possible, as PET did not warp at 100 °C. The power conversion efficiency (PCE) of the TiO x cell fabricated on PET was 2.43%, under AM1.5G conditions. When fabricated on glass, the PCE was 2.76%. When [6,6] diphenyl C 62 bis(butyric acid methyl ester) (bis-PCBM) was instead used as the electron acceptor in the photoactive blend layer, the flexible air-stable cell provided a PCE of 2.81%.