Triplet Sensitization by Lead Halide Perovskite Thin Films for Efficient Solid-State Photon Upconversion at Subsolar Fluxes  Sarah Wieghold, Alexander S. Bieber, Zachary A. VanOrman, Lauren Daley, Meghan Leger, Juan-Pablo Correa-Baena, Lea Nienhaus  Matter  DOI: 10.1016/j.matt.2019.05.026 Copyright © 2019 Elsevier Inc. Terms and Conditions

Matter DOI: (10.1016/j.matt.2019.05.026) Copyright © 2019 Elsevier Inc. Terms and Conditions

Figure 1 Thickness-Dependent MAFA Film Morphology (A) Semi-log plot of the MAFA perovskite film thickness dependence on the varying molar precursor concentrations. The measured film thickness for the 1.2 M film is marked in the graph as (B), which results in a 380-nm-thick film. A 0.06 M film yielding a 14-nm-thick film is used as lower limit for the film-thickness approximation and is marked in the graph as “Ref. [43].”43 The film thicknesses of the 0.6, 0.24, and 0.12 M concentrations can be estimated according to the graph, resulting in film thicknesses of 100, 30, and 20 nm, respectively. (B) Cross-sectional SEM image of the 1.2 M film yielding a film thickness of approximately 380 nm. Scale bar, 500 nm. (C–F) AFM images of the 20-nm (C), 30-nm (D), 100-nm (E), and 380-nm (F) MAFA perovskite thin films. Scale bars, 1 μm. Matter DOI: (10.1016/j.matt.2019.05.026) Copyright © 2019 Elsevier Inc. Terms and Conditions

Figure 2 Schematics and Optical Film Characterization (A) Schematic of the UC bilayer device structure excited at a wavelength of 780 nm. For measurement of the MAFA film PL, an 800-nm long-pass filter is used (i.e., NIR MAFA PL). For measurement of the UC properties, a 650-nm short-pass filter is used (i.e., UC PL). (B) Schematic of the proposed rubrene sensitization mechanism. (1) Incident light promotes an electron from the VB (∼5.8 eV) to the CB (∼4.25 eV) of the MAFA perovskite. This excitation can be quenched by several pathways: (2) bimolecular free carrier recombination, (3) defect level trapping and (4) trap-assisted recombination, or (5) carrier extraction to rubrene. The holes can be readily extracted to the HOMO (∼5.4 eV) of rubrene, while the 1 eV mismatch of the perovskite CB and rubrene LUMO blocks direct electron injection into rubrene. However, the bound triplet state T1 of rubrene can be populated. (C) Absorbance spectra of the MAFA thin films (solid lines) and the respective MAFA + rubrene bilayer devices highlighting the additional absorption caused by rubrene/1% DBP in the range of 430–530 nm (dashed lines). The absorption onset of MAFA can be seen at 800 nm for the devices, as expected for a 1.55 eV band gap material. (D) Normalized steady-state PL of the MAFA + rubrene bilayer devices. The inset shows the rubrene/1% DBP PL for all four films. For comparison, spin-coated rubrene/1% DBP on bare glass is shown as a purple dashed line. The excitation wavelength of the laser was set to 405 nm. Matter DOI: (10.1016/j.matt.2019.05.026) Copyright © 2019 Elsevier Inc. Terms and Conditions

Figure 3 Power Dependence under CW Excitation (A–D) Log-log plot of the MAFA + rubrene bilayer device PL intensity (NIR MAFA PL; λ > 800 nm) as a function of the incident excitation power for the 380-nm (A), 100-nm (B), 30-nm (C), and 20-nm (D) films under 780-nm excitation. The dashed lines are fitted curves to extract the slope α, which increases with increasing film thickness. (E–H) Power dependency of the UC PL (λ < 650 nm) for the MAFA + rubrene bilayer devices for the 380-nm (E), 100-nm (F), 30-nm (G), and 20-nm (H) films under 780 nm excitation. The dashed lines are fitted curves to extract the slope β. The TTA threshold (Ith) is marked as a vertical line (purple). The PL slope changes are marked in the graphs as a black (380 nm), blue (100 nm), and green (30 nm) vertical lines. The yellow vertical line indicates the equivalent solar irradiance of 1 sun (i.e., integrated AM1.5G standard spectrum), highlighting the subsolar Ith values of the 30-, 100-, and 380-nm-thick MAFA + rubrene devices. Matter DOI: (10.1016/j.matt.2019.05.026) Copyright © 2019 Elsevier Inc. Terms and Conditions

Figure 4 Time-Resolved PL Spectroscopy of the MAFA Thin Films and MAFA + Rubrene Bilayer Devices (A–C) NIR PL lifetimes of the 380-, 100-, and 30-nm-thick MAFA thin films under varying incident powers or carrier densities at an excitation wavelength of 780 nm. The early time quenching diminishes due to an increased amount of trap filling, while the free carrier lifetimes decrease due to an increase in probability of recombination with carrier density. (D–F) NIR PL lifetimes of the 380-, 100-, and 30-nm-thick MAFA + rubrene bilayer device under the same incident power. (G–I) Extracted difference in the lifetimes of the MAFA films versus MAFA + rubrene devices, resulting in an extracted characteristic time of charge transfer of τCT = 19 ns for the 100-nm device and τCT = 3 ns for the 30-nm device. The samples were excited by a 780 nm laser at a repetition rate of 250 kHz. Matter DOI: (10.1016/j.matt.2019.05.026) Copyright © 2019 Elsevier Inc. Terms and Conditions

Figure 5 UC Dynamics of the MAFA + Rubrene Bilayer Devices (A) UC PL dynamics (< 650 nm) of the 100-nm MAFA + rubrene bilayer device at a repetition rate of 31.25 kHz, showing the characteristic rise and fall expected of TTA-UC. The inset shows the visible emission obtained from the bilayer device under 780 nm excitation. (B) UC PL dynamics (< 650 nm) of the 100-nm MAFA + rubrene bilayer device at a repetition rate of 250 kHz, highlighting the reduction in the characteristic time of diffusion-mediated TTA and the triplet decay when increasing the repetition rate due to a build-up of the triplet population. (C) Extracted difference in the NIR MAFA PL (> 800 nm) for the 380-nm MAFA + rubrene device, overlaid with a simple back-transfer model accounting for exciton recycling. Matter DOI: (10.1016/j.matt.2019.05.026) Copyright © 2019 Elsevier Inc. Terms and Conditions