Interfacial Trap‐Assisted Triplet Generation in Lead Halide Perovskite Sensitized Solid‐State Upconversion

Abstract

Photon upconversion via triplet–triplet annihilation (TTA) has promise for overcoming the Shockley–Queisser limit for single‐junction solar cells by allowing the utilization of sub‐bandgap photons. Recently, bulk perovskites have been employed as sensitizers in solid‐state upconversion devices to circumvent poor exciton diffusion in previous nanocrystal (NC)‐sensitized devices. However, an in‐depth understanding of the underlying photophysics of perovskite‐sensitized triplet generation is still lacking due to the difficulty of precisely controlling interfacial properties of fully solution‐processed devices. In this study, interfacial properties of upconversion devices are adjusted by a mild surface solvent treatment, specifically altering perovskite surface properties without perturbing the bulk perovskite. Thermal evaporation of the annihilator precludes further solvent contamination. Counterintuitively, devices with more interfacial traps show brighter upconversion. Approximately an order of magnitude difference in upconversion brightness is observed across different interfacial solvent treatments. Sequential charge transfer and interfacial trap‐assisted triplet sensitization are demonstrated by comparing upconversion performance, transient photoluminescence dynamics, and magnetic field dependence of the devices. Incomplete triplet conversion from transferred charges and consequent triplet‐charge annihilation (TCA) are also observed. The observations highlight the importance of interfacial control and provide guidance for further design and optimization of upconversion devices using perovskites or other semiconductors as sensitizers.

Bulk Metal Halide Perovskites as Triplet Sensitizers: Taking Charge of Upconversion

“…The charge carrier dynamics of perovskites are correlated to grain size, , crystallinity, as well as available trap density, , which are tunable by the perovskite thickness as well as the fabrication methods, e.g., the utilized antisolvent or antisolvent dripping time, , and the illumination time. , It has been observed that the underlying charge carrier dynamics directly influence the UC efficiency, indicating that both bulk and surface trap densities are critical factors in the UC process, despite it being thought of as an interfacial process . Longer perovskite PL lifetimes result in a higher UC QY .…”

“…Three different approaches have been utilized to create the organic layer capable of UC. The organic thin film can be solution cast onto the perovskite film, , the organic molecules can be directly included into the antisolvent, or thermal evaporation can be used. , The local and long-range order of the rubrene molecular arrangement, as well as the rubrene thickness, are factors that play an important role in the UC process, further tunable by the utilized solvent or post-fabrication annealing steps to increase crystallinity.…”

“…To date, all perovskite-sensitized UC devices have been based on a bilayer structure consisting of various compositions of methylammonium and formamidinium lead triiodide perovskites and the organic annihilator rubrene doped with ∼1% dibenzotetraphenyl­periflanthene (DBP). − Rubrene, a tetracene derivative, the current workhorse of visible-to-infrared UC is chosen due to its history of efficient UC in both solution and solid-state UC configurations. ,, A representative device structure is shown in Figure a, where the perovskite thickness can be varied between ∼14 and 380 nm, , and rubrene layers are commonly ∼50–100 nm. Perovskite-based UC devices debuted in 2019 with an internal UC efficiency, η UC , of 3% (normalized to a 100% maximum), which allowed them to immediately rival previous PbS NC-based solid-state UC devices peaking at 7% .…”

Ultrafast Triplet Generation at the Lead Halide Perovskite/Rubrene Interface

“…However, to date, the exact mechanism of triplet sensitization is still unclear. [30][31][32][33] Understanding the fundamental mechanism of triplet sensitization at the perovskite/organic interface will be the key in further improving perovskite-based UC, as it will unlock engineering pathways to further increase the device performance.…”

“…Particularly, it emphasizes that only a small fraction of free charges created upon photoexcitation are successfully extracted and return upconverted photons despite the long-lived nature of the free carriers in the perovskite, which should allow a large number of charges to be extracted within their lifetime. 23,28 Possible causes include localized states mediating electron and hole transfer and limiting the number of generated triplet states 30,31 and short-lived transient states (e.g. hot carriers) being involved in the triplet generation process.…”