Ultralight photovoltaics are indispensable wherever every gram counts, from self-powered Internet of Things nodes to free-hanging greenhouse covers. In dye-sensitized solar cells (DSCs) the required 450−500 °C sintering of the mesoporous TiO 2 (m-TiO 2) photoanode has so far limited the use of polymer substrates. Here, we replace the furnace step with flash infrared annealing. Near-IR radiation heats the 3.5 μm m-TiO 2 layer to 550 ± 20 °C while keeping a 12.5 μm indium-tin-oxide/polyimide foil below 170 °C with a watercooled heat-sink. We obtain complete removal of organic binders, while the substrate sheet resistance increases modestly from 60.2 to 129.8 Ω sq −1. Flexible DSCs reach power conversion efficiencies of 5.10% under AM 1.5G illumination, a record value for DSCs on sub-25 μm plastics. The finished devices deliver 51× the specific power of equivalent glass devices. A cradle-to-gate life cycle assessment normalized to powerper-mass reveals order-of-magnitude reductions in several categories compared with rigid hot-plate processing. Localized IR sintering thus removes the last processing barrier for truly roll-to-roll printable hybrid solar cells.