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Triplet–triplet annihilation‐based photon upconversion (TTA‐UC) can efficiently generate higher energy photons at low relative fluences. Bulk metal halide perovskites have offered promise in efficiently sensitizing molecular triplet states in the solid state, necessary for the integration of TTA‐UC into device‐based applications. Recent work focused on TTA‐UC from a rubrene triplet annihilator sensitized by perovskite thin films has established relatively efficient charge extraction from the perovskite, forming the triplet exciton in rubrene. Yet, the specifics underpinning charge transfer at the perovskite/rubrene interface are not fully elucidated. To improve device performance and study the properties governing charge transfer at the interface, various organic solvents are explored to treat the perovskite surface. Scanning tunneling microscopy and spectroscopy show a difference in the electronic band structure, where both n‐ and p‐type terminated perovskite surfaces are observed depending on the solvent used. Supported by optical spectroscopy, the impact of the perovskite electronic structure is monitored, indicating that n‐type perovskite sensitizers feature higher TTA‐UC efficiencies due to favorable band bending resulting in efficient hole‐mediated triplet formation. Overall, the tuning of the electronic structure of the perovskite sensitizer through solvent treatment is shown to be a key force in tuning the mechanism of efficient triplet generation. [ABSTRACT FROM AUTHOR]

Photon interconversion generally describes processes which change the wavelength of the emitted light upon irradiation, with a gain in photon energy (upconversion), or a splitting of the incident photon energy to create multiple photons (downconversion). The focus of this progress report is placed on halide perovskite materials and their role in upconversion by triplet–triplet annihilation and downconversion via singlet fission in organic semiconductors, and a short perspective is given into quantum cutting and lanthanide‐based upconversion applications. The current advances are highlighted regarding halide perovskites and perovskite‐inspired materials with varying framework and material dimensionalities, as relating to photon interconversion processes. The majority of the literature is based on 3D perovskites, which leaves room for growth for perovskite materials with varying dimensionalities. Gaps in the current field are emphasized, leading to a bright outlook on future possibilities of coupling perovskites to photon interconversion mechanisms, which can open the door for future breakthroughs in this field. [ABSTRACT FROM AUTHOR]

Device fabrication methods for applications in upconversion processes using perovskite thin films have suffered from reproducibility and scalability issues, which prevent the upscaling of this technology. In this contribution, we developed a perovskite‐based upconversion device approach where the triplet annihilator is added in situ to the antisolvent and investigated the effect of the device fabrication procedure on the properties of our device. By comparing the properties of a device based on our new fabrication approach with the existing bilayer procedure, we seek to shed light on the underlying optoelectronic processes influenced by the different fabrication methods, while further advancing possible device architectures for upconversion devices. Device characterization by optical methods, X‐ray diffraction and atomic force microscopy revealed that the in situ fabricated devices match the performance or even outcompete our previously developed bilayer devices while significantly simplifying the device fabrication. In particular, we find that the developed one‐step fabrication technique enables intercalation of the upconverting layer into the perovskite film prior to annealing, resulting in a larger interface thus, more efficient charge extraction. [ABSTRACT FROM AUTHOR]