Perovskite-Sensitized Upconversion under Operando Conditions

Abstract

Perovskite-sensitized upconversion (UC) has resulted in near-infrared-to-visible UC at solar-relevant fluxes. However, the successful implementation of UC devices into operating solar cells will result in exposure to similar environmental stressors as for the commercial photovoltaics (PVs), mainly elevated temperatures, and continuous irradiation. In this article, we investigated the effects of these two stressors, heat and light, on the triplet generation process at the perovskite/rubrene interface. Following exposure to both stressors, local discrepancies across the upconversion device were discovered. The first region showed changes to the morphology, and no detectable upconverted emission was observed. Through the combination of optical microscopy and spectroscopy, crystallization of the organic semiconductor layer, degradation of dibenzotetra­phenyl­periflanthene, and concurrent degradation of the perovskite sensitizer were found. These effects culminate in a reduction in both triplet generation and triplet–triplet annihilation. In the second region, no changes to the morphology were present and visible UC emission was observed following exposure to both stressors. To probe the triplet sensitization process at elevated temperatures, transient absorption spectroscopy was performed. The presence of the excited spin-triplet state of rubrene at 60 °C highlighted successful triplet generation even at elevated temperatures. This work emphasizes the challenges and continued potential for the integration of perovskite-sensitized UC into commercial photovoltaic devices.

Cool carriers: triplet diffusion dominates upconversion yield

“…Data points for MAFArub at temperatures above 300 K are included to ensure the model matches the behavior at all temperatures of interest. 40 The UCPL intensity ( I UC ) is dependent upon the PL QY of the annihilator ( Φ PL ), triplet population ( N A ), overall lifetime of the triplet annihilator ( k T ), and bimolecular rate constant describing TTA k TTA . 52,66,67 …”

“…70,71 Increased temperature will increase triplet diffusion rates based on thermally activated hopping, which results in triplets reaching quenching sites more rapidly and reduces the effective triplet diffusion length, L . 40 As a result, with increasing temperatures, an increasing portion of the initially generated triplet states will be quenched via other pathways rather than the desired bimolecular TTA-UC pathway, leading to a decrease in the available triplet population at time t . 67 Here, k Q is the rate for additional unimolecular triplet quenching pathways.…”

“…To this point, we recently investigated the effect of elevated temperatures (up to 60 °C) on perovskite-sensitized TTA-UC. 40 Under constant exposure to sunlight, solar cells are known to reach temperatures up to 85 °C, even in moderate climates. With the addition of an UC device, slightly lower temperatures are expected under operating conditions due to its location beneath the active photovoltaic.…”

Early-Career and Emerging Researchers in Physical Chemistry Volume 2

“…This investigation illustrates the significant influence of the interfacial structure on spatial charge separation. Using a creative combination of optical spectroscopy and imaging as well as X-ray diffraction, Nienhaus and co-workers investigated the effects of heat and light on perovskite-sensitized upconversion of rubrene under operando conditions. Following exposure to these two stressors, they found two distinct regions across the upconversion device: one with morphology changes and no detectable upconverted emission and the other where no changes to the morphology were found and upconverted emission was still observed.…”

Early-Career and Emerging Researchers in Physical Chemistry Volume 2

“…This investigation illustrates the significant influence of the interfacial structure on spatial charge separation. Using a creative combination of optical spectroscopy and imaging as well as X-ray diffraction, Nienhaus and co-workers 26 investigated the effects of heat and light on perovskitesensitized upconversion of rubrene under operando conditions. Following exposure to these two stressors, they found two distinct regions across the upconversion device: one with morphology changes and no detectable upconverted emission and the other where no changes to the morphology were found and upconverted emission was still observed.…”