Dec 3, 2024
8:00pm - 10:00pm
Hynes, Level 1, Hall A
Parvez Akhtar1,Hsiao-Chun Hung2,Henam Sylvia Devi1,Yuh-Renn Wu2,Madhusudan Singh1
Indian Institute of Technology Delhi1,National Taiwan University2
The all-inorganic perovskite material (CsPbBr<sub>3</sub>) is a promising light-emitting material due to its high efficiency, solution processability, and cost-effectiveness. Among the various intrinsic issues in the perovskite thin film, such as low surface coverage, chemical instability, and ion migration, the interfacial layer also impacts the performance of optoelectronic devices. The well-established PEDOT:PSS hole injection layer (HIL)[1], despite its ease of solution processing, high conductivity, and transparency, poses significant challenges due to its highly acidic and hygroscopic nature. These characteristics can cause various instabilities in the perovskite thin film, such as the absorption of oxygen and moisture, leading to hydrolysis, oxidation, and the formation of hydrated complexes with the perovskite material, ultimately contributing to device failure. In this work, we have developed a non-acidic, inorganic, and high-green index vanadium pentaoxide (V<sub>2</sub>O<sub>5</sub>) as a HIL with low surface roughness and a deep work function compared to conventional HIL PEDOT:PSS[2]. Precursors cesium bromide (CsBr) and lead bromide (PbBr<sub>2</sub>) (in DMSO) were heated under vigorous stirring and then combined with the blend of non-expensive PEG:PVP in DMSO and DMF in a 1:1 ratio to form an ink (CsPbBr<sub>3</sub>:PEG:PVP (CPP)) [3]. X-ray diffraction (Rigaku, Cu-Kα 1.54Å) reveals an orthorhombic phase (PDF:96-153-3063) of CPP thin film and field emission scanning electron microscope (FESEM, JSM-7800F Prime, JEOL) image, demonstrating well-packed perovskite crystal grains (101 nm) with almost 100% surface coverage. An intense PL peak is observed at 520 nm with an FWHM of 16nm for CPP thin films (bandgap:2.38eV, CIE-1931 coordinates (0.081,0.762)), indicating a narrow, pure green emission. LED devices (PEDOT:PSS (control) vs. V<sub>2</sub>O<sub>5 </sub>(test)/CPP/TPBi/LiF/Al) were fabricated using a combination of spin coating and thermal evaporation at a pressure 2.3 x 10<sup>-6</sup> Torr (Angstrom Engineering). Test devices show nearly identical maximum current efficiencies (4.23 vs. 4.19 cd/A) and luminous efficacies (2.99 vs. 2.32 lm/W) compared to a control device. The EL spectra of both control and test devices exhibit an identical peak wavelength (518.4 nm, FWHM:17 nm), closely matching the PL measurements. Additionally, the EL spectra show almost no variation in peak position with applied bias, indicating the color stability of the LED device under varying bias conditions. Time-resolved photoluminescence (TRPL, 377nm, FLSP92, Edinburgh Instruments) data reveals an increased radiative and decreased non-radiative component in the test sample compared to the control, suggesting a higher defect density at the PEDOT:PSS/CsPbBr<sub>3</sub> interface, likely due to trap generation from halide vacancies under moisture conditions induced by PEDOT:PSS. The control sample requires a drive current density three times higher than the test device's to achieve maximum brightness. Since most device failures and dark spot formations strongly depend on the drive current density. To further study the mechanism at the interface, we have developed a model based on a physical charge control, including defect-assisted tunneling for hole injection[4]. The model predicts a higher radiative recombination rate in the emissive layer, suggesting a passivation effect at the interface, possibly due to a greater concentration of dipoles radiatively connected to the ground state, leading to a Förster energy transfer mechanism. These findings indicate that solution-processed V<sub>2</sub>O<sub>5</sub> provides better interfacial properties than PEDOT:PSS. In future studies, we will investigate how the interfacial layer impacts the operational stability of perovskite-based devices through lifetime measurements.<br/><br/>[1] Singh, M., Soft Matter 2009, 5(16), 3002-3005.<br/>[2] Devi, H.S., et al. Green Chemistry 2021, 23(20), pp.8200-8211.<br/>[3] Akhtar, P., et al. Thin Solid Films 2023, 787, 140133.<br/>[4] Akhtar, P., et al., Journal of Applied Physics 2024, 135(5), 054501.