Ultrahigh sensitivity of methylammonium lead tribromide perovskite single crystals to environmental gases

Abstract

One of the limiting factors to high device performance in photovoltaics is the presence of surface traps. Hence, the understanding and control of carrier recombination at the surface of organic-inorganic hybrid perovskite is critical for the design and optimization of devices with this material as the active layer. We demonstrate that the surface recombination rate (or surface trap state density) in methylammonium lead tribromide (MAPbBr₃) single crystals can be fully and reversibly controlled by the physisorption of oxygen and water molecules, leading to a modulation of the photoluminescence intensity by over two orders of magnitude. We report an unusually low surface recombination velocity of 4 cm/s (corresponding to a surface trap state density of 10⁸ cm^-2) in this material, which is the lowest value ever reported for hybrid perovskites. In addition, a consistent modulation of the transport properties in single crystal devices is evidenced. Our findings highlight the importance of environmental conditions on the investigation and fabrication of high-quality, perovskite-based devices and offer a new potential application of these materials to detect oxygen and water vapor.

Keywords: Organometal trihalide perovskite; gas sensitivity; molecular physisorption; surface passivation; surface recombination velocity.

Fig. 1. XRD and optical properties of MAPbBr₃ single crystals.

(A) hk0 reciprocal lattice plane reconstructed from MAPbBr₃ single crystal XRD data at room temperature. Inset: Image of one of the measured crystals grown from solution. (B) PL spectra in vacuum and air (the PL intensity in vacuum is two orders of magnitude lower than in air). (C) Variation in PL intensity of MAPbBr₃ crystals from air-vacuum-air environments. a.u., arbitrary units. (D) Normalized PL spectra at different times in (C). (E and F) Two-dimensional (2D) pseudocolor plots of TRPL spectra taken in air and vacuum with an excitation power density of 0.71 μJ/cm². (G) Decay of the PL at a wavelength of 560 nm in air and vacuum.

Fig. 2. Effect on the PL intensity of MAPbBr₃ single crystals of exposure to different gaseous environments.

(A) PL intensity as a function of time in vacuum and on exposure to dry N₂, dry CO₂, and dry Ar. (B) PL intensity as a function of time on exposure to air, dry O₂, and moist N₂. In each panel, the blue shaded area indicates vacuum. The crystal was excited with a 400-nm wavelength laser; the laser power was kept constant at 0.71 μJ/cm².

Fig. 3. Two-photon excited fluorescence in MAPbBr₃ crystal and single-photon excited optical properties of cleaved crystal surface.

(A) 2D pseudocolor plot of two-photon (800 nm) excited TRPL measured in air. (B) TRPL dynamics [extracted from measurement reported in (A)]; the fit gives lifetimes of t₁ = 34 ns and t₂ = 4.5 μs. Inset: Image of MAPbBr₃ crystal under two-photon excitation (TPE) with an excitation wavelength of 800 nm. (C) Two-photon excited PL spectra measured in air and vacuum. (D) 2D pseudocolor plot of TRPL of a freshly cleaved crystal in air; the excitation wavelength is 400 nm. The emission peak wavelength as a function of time is indicated by the red line. (E) Calculated photocarrier density profile at various times under 400-nm laser excitation. Inset: Image of MAPbBr₃ crystal under single-photon excitation (SPE) with an excitation wavelength of 400 nm. (F) Calculated PL spectra at various times after excitation.

Fig. 4. Effect of surface recombination on the optical properties of MAPbBr₃ crystal.

(A) Schematic image showing photoexcitation and deep levels within the forbidden gap in proximity to the surface. hv_ex, excitation laser; E_c, conduction band; E_v, valence band. (B) PL lifetime in bulk single crystals as a function of SRV for various carrier diffusion coefficients and bulk lifetimes. (C) PL lifetime in polycrystalline thin film or small crystals for various surface recombination velocities.

Fig. 5. Electronic properties of MAPbBr₃ single-crystal devices.

(A) Structure of the single-crystal device for current-voltage (I-V) measurement. (B and C) Side-view and top-view of a single-crystal device. (D) I-V curves of the MAPbBr₃ single-crystal device under laser illumination in air and vacuum. (E) Dark current of the MAPbBr₃ single-crystal device in air and vacuum.