Surface Doping Boosts Triplet Generation Yield in Perovskite‐Sensitized Upconversion

Abstract

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.

Recharging upconversion: revealing rubrene's replacement

“…We have previously shown that this solvent treatment increases triplet generation in rubrene due to n-type doping of the perovskite, which increases the hole density at the surface due to interfacial band bending. 55 Once interfaced with the OSCs, additional spectral features corresponding to the T 1 → T n ESA transitions previously shown in Fig. 1 for both OSCs emerge and can be detected on a sub-nanosecond timescale for both RubDBP (Fig.…”

Generating spin-triplet states at the bulk perovskite/organic interface for photon upconversion

“…146,147 While ACN can be classified as type I, it is considered its own category as it fully dissolves MAFA perovskites and partially dissolves the FAMA composition. 144 Based on this solvent classification, the previous report of differing effects in the UC PL intensity when depositing rubrene from CB and TOL can simply be traced back to different effects of the solvent on the perovskite surface. 133…”

“…Expanding on this approach, we have recently investigated in detail the effect of seven different solvents on the perovskite-sensitized UC mechanism: 144 acetonitrile (ACN), ethanol (EtOH), IPA, 1-butanol (ButOH), TOL, CB and ANI. Following Taylor et al , 145 the solvents can be classified into three categories.…”

“…The ACN film is slightly thinner due to the slight perovskite dissolution, hence the reduced T 1 → T n PIA can be (in part) traced to a lower absorbance of the excitation laser. 144…”

“…The surface morphology for the control as measured by scanning tunneling microscopy (STM) is shown in the inset, and no changes are observed with solvent treatment, indicating that the underlying topographical structure is not affected. 144…”