Charge Transfer States in Donor–Acceptor Bulk‐Heterojunctions as Triplet–Triplet Annihilation Sensitizer for Solid‐State Photon Upconversion

Abstract Photon interconversion in semiconductors is of fundamental importance for digital imaging and quantum sensing. Nonlinear processes such as triplet–triplet annihilation (TTA) offer photon upconversion (UC) at yields desired for solid‐state optoelectronic devices. Here, a multilayer molecular system where a near‐infrared (NIR) photosensitizer facilitates robust photon UC is presented. A molecular stack of 2,4‐Bis[4‐(N,N‐diisobutylamino)‐2,6‐dihydroxyphenyl] squaraine (DIB‐SQ): [6,6]‐Phenyl C61 butyric acid methyl ester (PCBM) is optimized for UC by fine‐tuning the PCBM loading to engineer charge transfer states (CT) to amplify triplet generation. This composite photosensitizer layer, at a 1:3 blend ratio in heterojunction with rubrene, drives triplet population density to levels desired for effective TTA. When paired with a fully optimized annihilator layer of tetraphenyldibenzoperiflanthene (DBP) doped rubrene, the sensitizer produces a photon upconversion quantum yield (Φ _UC ) of 1.36% at 690 nm, with a significantly low excitation intensity threshold for the onset of the linear regime of I _th = 60.5 mW cm ⁻² . Time‐resolved photoluminescence, transient absorption spectroscopy, and magnetic field‐dependent photoluminescence measurements reveal a detailed balance of photoexcited states and formation of charge transfer states of triplet character ( ³ CT), which work in tandem with molecular states to sensitize the triplet state (T ₁ ) of rubrene. This approach of harnessing near‐infrared photons presents a promising avenue for advancing solid‐state photon interconversion.