Photon upconversion (UC), the process of converting low energy photons into higher energy photons, holds tremendous potential for various applications including photovoltaics, night vision, photocatalysis, bioimaging, anti-counterfeiting, additive manufacturing, and more. The anti-Stokes shift achieved by short-waveand near-infrared-to-visible UC is instrumental to address the bandgap limitations of silicon and to exploit the positioning of key transparency windows in biological systems. By absorbing sub-bandgap photons in the near infrared (NIR) region, UCassisted photovoltaics have been proposed to achieve higher efficiencies beyond singlejunction solar cells. Additionally, the difficulty in replicating NIR-to-visible anti-Stokes shifts presents an opportunity to employ UC for anti-counterfeiting applications. There have been several reported mechanisms to achieve visible emission from NIR excitation, including excited-state absorption, energy-transfer UC, photon avalanching, collective energy pooling, and triplet-triplet annihilation upconversion (TTA-UC). Various f-block elements have been deployed in nanocrystals and solids to achieve tunable, sharp, and significantly anti-Stokes shifted emission by creating ultraviolet or visible photons from NIR photons. Despite the large anti-Stokes shift achieved by these systems via f-f transitions, they are often limited in applicability due to their need for high intensities of coherent light and narrow absorption bands. UC via TTA shows particular promise for applications because it can occur relatively efficiently under lower intensity irradiation from incoherent and broadband sources of light. In the overall TTA-UC process, two low-energy photons are absorbed (by the sensitizer) and converted into one higher-energy photon (emitted by the annihilator). The mechanism of TTA-UC relies on two species: the sensitizer and the annihilator (see Figure ). The annihilator is an organic semiconductor (often an acene) with energetics such that two triplet-excited annihilator molecules can interact via TTA to generate one singlet-excited and one ground-state annihilator molecule. The singlet-excited annihilator molecule can then relax to the ground state by emitting a high energy photon via photoluminescence (PL). As shown in Figure , sensitizers are necessary to create triplet-excited annihilator molecules, as triplet states in organic semiconductors are typically optically inaccessible. Sensitizer species absorb light (Abs) to create an excited state (typically a singlet), which can undergo intersystem crossing (ISC) to generate a triplet which can transfer its energy to an annihilator via Dexter energy transfer (ET). There are various families of sensitizers for TTA including semiconducting quantum dots (QDs), thermally activated delayed fluorescence molecules, heavy metal porphyrin complexes, and non-fullerene acceptors. Commercial realization of solid-state NIR-to-visible UC demands that this conversion process be highly efficient. Zhou et al. have summarized essential metrics to measure the efficiency of UC systems. Here, we use the external quantum efficiency (EQE) as a performance metric due to its relevance to applications like photovoltaics, night vision, and anti-counterfeiting. EQE is defined as the number of upconverted photons generated by 100 incident low-energy photons. To follow the norms adopted by the field, all EQEs in this article will be reported with a maximum possible 50% (due to the two-photons-toone-photon nature of TTA-UC). Arguably the most well-studied annihilator for NIR-to-visible UC to date is rubrene (see Figure ), a tetracene derivative. DBP, an organic semiconductor (see Figure ), is often doped into rubrene films to improve the resulting photoluminescence quantum yield (PLQY) by acting as a singlet sink to prevent triplet separation. Rubrene has been reported to be sensitized by a wide range of materials, including organic semiconductors (Y6, ITIC-Cl), semiconducting QDs (PbS, PbSe), 2D transition metal dichalcogenides, and heavy metal complexes (PdPc, PtTPTNP, osmium complexes, and more). Despite promising performance from different sensitizers, most NIR-tovisible systems are limited to excitation wavelengths below ~1000 nm.