One of the key issues concerning the development of efficient polymer solar cell technology is the lack of viable materials which absorb in the near-infrared (NIR) region. This could be resolved by up-converting energy from the NIR into visible using triplet fusion (TF) with an additional layer that is fabricated separately from the solar cell and deposited on top. Theoretically a maximum upconversion (UC) via TF efficiency of 50% could be obtained. Here, it is demonstrated that in a film of commercially available poly(para-phenylene vinylene) copolymer “super yellow” (SY) doped with 4% palladium(meso-tetraphenyl-tetrabenzoporphyrin) (PdTPBP) sensitizer, an UC efficiency of 6% can be achieved. By using femtosecond and nanosecond spectroscopies it is shown that the main UC efficiency loss mechanism is due to triplet quenching in PdTPBP aggregates. The PdTPBP intersystem crossing rate constant is determined to be 1.8 × 1011 s−1 and the triplet energy transfer rate constant from PdTPBP to SY to be 109 s−1. Quenching in PdTPBP aggregates can account for a triplet concentration loss in the range of 76-99%. As such, preventing sensitizer aggregation in NIR-to-visible upconverting films is crucial and may lead to substantial increase of UC efficiencies in films.