Photon Upconversion in a Solid‐State Bilayer Film Using an Anthradithiophene Derivative at Near‐Infrared Wavelengths Above 900 nm

Solid‐state photon upconversion (PUC) operating at the near‐infrared (NIR) region beyond 850 nm has great potential for next‐generation optoelectronic applications. However, conventional triplet–triplet annihilation (TTA) PUC is limited by the availability of sensitizers and the reliance on toxic heavy metals. Here, we demonstrate a heavy‐metal‐free, solid‐state NIR‐to‐visible PUC system utilizing interfacial charge transfer (CT) states, extending the excitation wavelength beyond 850 nm. By employing a series of anthradithiophene (ADT) derivatives as emitters and a nonfullerene acceptor (NFA) as the sensitizer, we systematically investigated the impact of molecular structures on PUC, organic photovoltaic (OPV), and organic light‐emitting diode (OLED) properties. The bilayer film using TES (triethylsilylethynyl)‐ADT bearing two phenyl groups (2PhTES‐ADT) exhibited efficient charge separation owing to its moderate crystallinity, along with distinct TTA‐derived emission. Consequently, the 2PhTES‐ADT in combination with NIR absorbing nonfullerene acceptor successfully performed NIR‐PUC under excitation at 904 nm, which is the longest wavelength reported for CT‐based solid PUC. These findings demonstrate that precise structural tuning to facilitate the smooth progression of charge separation and recombination paves the way for advanced NIR‐PUC materials.