Ultra-lightweight solar cells have attracted enormous attention due to their ultra-conformability, flexibility, and compatibility with applications including electronic skin or miniaturized electronics for biological applications. With the latest advancements in printing technologies, printing ultrathin electronics is becoming now a reality. This work offers an easy path to fabricate indium tin oxide (ITO)-free ultra-lightweight organic solar cells through inkjet-printing while preserving high efficiencies. A method consisting of the modification of a poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) ink with a methoxysilane-based cross-linker (3-glycidyloxypropyl)trimethoxysilane (GOPS)) is presented to chemically modify the structure of the electrode layer. Combined with plasma and solvent post-treatments, this approach prevents shunts and ensures precise patterning of solar cells. By using poly(3-hexylthiophene) along rhodanine-benzothiadiazole-coupled indacenodithiophene (P3HT:O-IDTBR), the power conversion efficiency (PCE) of the fully printed solar cells is boosted up to 4.73% and fill factors approaching 65%. All inkjet-printed ultrathin solar cells on a 1.7 µm thick biocompatible parylene substrate are fabricated with PCE reaching up to 3.6% and high power-per-weight values of 6.3 W g−1. After encapsulation, the cells retain their performance after being exposed for 6 h to aqueous environments such as water, seawater, or phosphate buffered saline, paving the way for their integration in more complex circuits for biological systems.