Gradient 2D/3D Perovskite Films Prepared by Hot-Casting for Sensitive Photodetectors

Abstract

2D Ruddlesden-Popper perovskites have attracted wide attention recently because of tunable optoelectronic properties and have been used as alternatives to their 3D counterparts in various optoelectronic devices. Here, a series of (PEA)₂(MA) _{n -1}Pb _n I_{3 n +1} perovskite thin films is designed and fabricated by a convenient hot-casting method to obtain gradient n in the films, which leads to the formation of vertical heterojunctions that can enhance charge separation in the films under light illumination. Based on a single gradient perovskite film, a highly sensitive and stable photodetector with a responsivity up to 149 AW^-1 and a specific detectivity of 2 × 10¹² Jones is obtained. This work paves a way to realizing high-performance optoelectronic devices with enhanced charge separation by introducing compositional gradient in a perovskite film.

Keywords: 2D perovskites; hot‐casting; perovskites; photodetectors; vertical heterojunctions.

Figure 1

a) Schematic illustration of the crystal structure of layered perovskite materials (along [110] zone axis) with chemical formula (PEA)₂(MA)_{n −1}Pb_nI_{3 n +1} (n = 1, 2, 3, 4, and ∞). b) Schematic illustration of a newly enhanced hot‐casting method presented with facile one‐step spin‐coating process for vertical 2D/3D perovskite heterojunction fabrication. c) Band energy diagram of (PEA)₂(MA)_{n −1}Pb_nI_{3 n +1} perovskite components with different n numbers. d) Charge transfer diagram of a photodetector based on a gradient perovskite thin film.

Figure 2

Characterization of the surface morphology of five samples by a newly enhanced hot‐casting method, which were fabricated by varying stoichiometric ratios (predefined n‐values = 1, 2, 3, 4, and ∞) in (PEA)₂(MA)_{n −1}Pb_nI_{3 n +1} precursor solution. a) X‐ray diffraction (XRD) patterns. Inset: optical images of the films. b) Absorption spectra of above samples. c) Photoluminescence (PL) spectra of perovskite thin films with n ⁺Si/SiO₂ substrates. Comparative PL spectra of the samples fabricated by predefined n‐values of d) 3 and e) 4. The perovskite thin films are illuminated from the front and back sides (as illustrated in the insets) under 488 nm laser.

Figure 3

Cross‐sectional FIB‐TEM investigations of the sample fabricated by predefined n‐value of 3. a) The cross‐sectional FIB‐STEM image. b) The HRTEM image from middle region of the perovskite layer with the scale bar for 5 nm. The planes with lattice distances of 6.6 and 3.0 Å demonstrate the existences of 2D (blue, close to the back region) and 3D (yellow, close to the front region) perovskites, respectively. The HRTEM images from c) back and d) front regions of the perovskite layer with the scale bars for 5 nm. Two below insets show their SAED patterns, respectively.

Figure 4

Design and performance of the devices based on varying stoichiometric ratios (predefined n = 2, 3, 4, and A74F) in (PEA)₂(MA)_{n −1}Pb_nI_{3 n +1} precursor solution. a) Drain–source current versus time (I _DS –t) curves at varying light intensity of light with 598 nm wavelength. The inset is the schematic illustration of perovskite photodetectors. b) Responsivity versus light intensity (R–E _e) curves of the devices. Drain–source voltage (V _DS) is 4 V and the dotted lines in R–E _e curves are fitting curves with a formula of R ∝ E _e ^β−1.

Figure 5

Photoresponse properties of the device based on a special stoichiometric ratio (predefined n = 3). Plot of a) drain–source current and b) responsivity of the device as functions of V _DS under different illumination power of light with 598 nm wavelength. Plot of responsivity as functions of c) the light intensity under different wavelengths and d) the wavelength under illumination intensity of 10⁻² mW cm⁻². The dotted lines in R–E _e curves are fitting curves with a formula of R ∝ E _e ^β−1.