Composition Engineering of Halide Perovskite Crystals for High-Performance Broadband Photodetectors

Abstract

Halide perovskites have attracted significant attention in recent years owing to their outstanding optoelectronic properties, including high absorption coefficients, long carrier diffusion lengths, and tunable bandgaps. These features make them highly suitable for applications in photodetectors, solar cells, and light-emitting devices. Among these, broadband photodetectors based on halide perovskite single crystals are particularly promising for next-generation optical sensing technologies due to their potential for high sensitivity, fast response, and spectral tunability. This thesis presents a comprehensive study on the compositional engineering of halide perovskite single crystals, aiming to optimize their performance in broadband photodetection. By systematically tuning the A-site organic/inorganic cations (GA⁺, MA⁺, Cs⁺), B-site metal ions (Pb²⁺, Ag⁺, Bi⁺, Fe³⁺), and halide anions (Br⁻, I⁻), we investigated the impact of compositional modifications on crystal quality, optoelectronic behavior, and device performance. Advanced crystal growth methods, such as the slow cooling crystallization technique, were also employed to obtain large-area, high-quality perovskite crystals with improved yield. The main contents and achievements of this study are as follows:

1. Compared to conventional Dion-Jacobson and Ruddlesden-Popper two-dimensional (2D) layered hybrid perovskite compounds, perovskites with alternating cations in the interlayer (ACI) phase exhibit higher crystal symmetry and narrower optical bandgaps, offering great potential for photodetection applications. Herein, we report a high-performance photodetector based on a 2D bilayered hybrid lead halide perovskite single crystal with an ACI phase (GAMA₂Pb₂I₇; GA = C(NH₂)₃⁺ and MA = CH₃NH₃⁺). The single-crystal device exhibits high photoresponsivities of 1.56, 2.54, and 2.60 A/W for incident light wavelengths of 405, 532, and 635 nm under 9.82 nW illumination, respectively, together with correspondingly high detectivity values of 1.86 × 10¹², 3.04 × 10¹², and 3.11 × 10¹² Jones under the same conditions. Furthermore, a high-resolution imaging sensor based on the GAMA₂Pb₂I₇ photodetector confirms the excellent stability and photosensitivity of the system. These results demonstrate that 2D hybrid lead halide perovskites with alternating interlayer cations are highly promising for high-performance visible-light photodetectors and imaging applications.

2. Lead-free double perovskite Cs₂AgBiBr₆ single crystals provide a promising pathway for integrated visible-light and X-ray detection. However, conventional crystal growth methods are often time-consuming and complex, limiting their commercial viability. In this study, we introduce a controlled cooling strategy that enables the rapid synthesis of high-quality Cs₂AgBiBr₆ single crystals, achieving an average size of 2 × 2 × 1.5 mm³ with a yield of 12.66%. Utilizing these high-quality crystals, we fabricated photodetectors that exhibit a responsivity of 1.02 A/W and a detectivity of 8.34 × 10¹¹ Jones under 405 nm laser illumination at –2 V. Moreover, the device demonstrates exceptional X-ray detection sensitivity of 419.05 μC Gyₐᵢᵣ⁻¹ cm⁻² and a detection limit as low as 0.76 μGyₐᵢᵣ s⁻¹ at –10 V. These findings highlight the potential of Cs₂AgBiBr₆ as a single-material platform for dual-band detection of visible light and X-rays, and emphasize a scalable, environmentally friendly growth strategy for next-generation photodetectors.

3. Lead-free double perovskites such as Cs₂AgBiBr₆ have attracted considerable interest as environmentally benign alternatives to lead-based perovskites. However, their relatively wide indirect bandgap (~2.0-2.2 eV) restricts absorption to the visible region, rendering them ineffective for near-infrared (NIR) detection in their pristine form. In this work, we introduce Fe³⁺ doping into Cs₂AgBiBr₆ single crystals to create intermediate electronic states within the bandgap, thereby enabling sub-bandgap absorption and broadband response across the visible and NIR regions. The resulting broadband photodetector based on Cs₂Ag(Bi:Fe)Br₆ crystal exhibit a responsivity of 0.57 A/W and a detectivity of 5.85 × 10¹¹ Jones under 405 nm illumination at –2 V, and a responsivity of 0.025 A/W with a detectivity of 5.15 × 10¹¹ Jones under 808 nm excitation at –2 V. This work demonstrates a sustainable and scalable materials platform for broadband visible-NIR photodetection, combining low toxicity, wide spectral response, and high device performance. Such an approach opens new opportunities for eco-friendly, high-performance optoelectronic devices applicable to imaging, biosensing, and optical communication technologies.

Date of Award	16 Dec 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Johnny Chung Yin HO (Supervisor)