Lead halide perovskites have shown remarkable promise in various optoelectronic applications, including solar cells, LEDs, and detectors, achieving high performance due to their large absorption coefficient, low defect density, long carrier lifetime, and ease of fabrication. However, their thermal properties, specifically low thermal conductivity and small specific heat capacity, have been relatively unexplored. This paper investigates the potential of these thermal properties for use in optoacoustic transducers, devices that convert light into ultrasound pulses. Optoacoustic transducers have wide-ranging applications in biomedical imaging, therapeutic ablation, brain modulation, and non-destructive testing. Unlike traditional piezoelectric transducers, optoacoustic transducers utilize lasers as an excitation source, offering advantages in terms of reduced cabling and electromagnetic interference. Current optoacoustic transducers rely on composite materials, with one component for light absorption (often carbon materials) and another for thermal expansion (typically polydimethylsiloxane or PDMS). This study proposes to replace traditional carbon-based light absorbers with lead halide perovskites to improve the performance of optoacoustic transducers. The unique thermal properties of perovskites—low thermal conductivity and specific heat capacity, coupled with high light absorption—are hypothesized to enhance the optoacoustic conversion efficiency and bandwidth. This research aims to demonstrate the feasibility and advantages of using lead halide perovskites in optoacoustic transducers, paving the way for high-resolution ultrasound imaging applications.

The literature extensively documents the success of lead halide perovskites in solar cells (achieving power conversion efficiencies close to single-crystal silicon), LEDs, and photodetectors. However, research on the thermal properties of these materials, despite their potential applications, remains limited. Studies have highlighted the low thermal conductivity of CH3NH3PbI3, attributed to strong coupling between optical and acoustic phonons resulting in short acoustic phonon lifetimes and inefficient heat dissipation. Existing optoacoustic transducers commonly utilize carbon materials like candle soot particles, carbon nanotubes (CNTs), and carbon nanofibers as light absorbers, combined with PDMS as the thermal expansion layer. While advancements have been made, existing technologies still face challenges in simultaneously achieving broad bandwidths and high acoustic pressure, which are crucial for high-resolution ultrasound imaging. The state-of-the-art optoacoustic transducers employing CNTs/PDMS composites have demonstrated a 6 dB bandwidth of 39.8 MHz and a peak frequency of 28.5 MHz. However, these values lag behind the performance of traditional piezoelectric transducers.

The research involved both experimental and computational methods. The synthesis of methylammonium lead iodide (MAPbI3) perovskite involved a multi-step process, beginning with the synthesis of methylammonium iodide (MAI) from methylamine solution and hydroiodic acid. MAPbI3 perovskite solution was then prepared by dissolving PbI2 and MAI in a mixture of DMF and DMSO. PDMS precursor was prepared by mixing PDMS base and curing agent. Optoacoustic transducers were fabricated by spin-coating PDMS onto glass substrates, followed by spin-coating the MAPbI3 perovskite solution, and finally, another layer of PDMS. The thickness of the PDMS layers was optimized to enhance the optoacoustic performance. For miniaturized devices, MAPbI3 was coated onto optical fibers using a dip-coating method, incorporating polyvinylpyrrolidone (PVP) to improve film uniformity. The optoacoustic performance of the devices was characterized using a pulsed laser (532 nm), a hydrophone to detect the generated ultrasound, and an oscilloscope. The acoustic pressure, bandwidth, and conversion efficiency were measured and compared with existing technologies. Theoretical simulations were performed using COMSOL Multiphysics to model the acoustic field distribution. Density Functional Theory (DFT) calculations were employed to investigate the phonon spectrum of MAPbI3 and understand the origin of its low thermal diffusion coefficient.

The fabricated PDMS/MAPbI3/PDMS optoacoustic transducer achieved remarkable performance metrics: a −6 dB bandwidth of 40.8 MHz, a central frequency of 29.2 MHz, and a high conversion efficiency of 2.97 × 10⁻². These values represent record-high performance for optoacoustic transducers, significantly surpassing those reported for carbon-based absorbers. The high acoustic pressure achieved was 24.89 MPa at a laser energy of 3 mJ pulse⁻¹. The low specific heat capacity of MAPbI3 (~308 J kg⁻¹ K⁻¹) and its low thermal diffusion coefficient (0.145 mm² s⁻¹) were experimentally verified, and these properties played a crucial role in enhancing the optoacoustic conversion efficiency. DFT calculations revealed the overlap of optical and acoustic phonons in MAPbI3, leading to acoustic phonon up-conversion and the observed low thermal diffusion coefficient. Miniaturized devices fabricated on optical fibers (125 µm diameter) successfully demonstrated high-resolution ultrasound imaging of a fish eye, clearly resolving fine structures such as the cornea, iris, and lens surface. The measured cornea thickness (56 µm) was consistent with literature values. The study also optimized the PDMS layer thickness to maximize the superposition of forward and backward acoustic waves, thereby improving the overall performance.

The findings directly address the research question of improving optoacoustic transducer performance by utilizing the unique thermal properties of lead halide perovskites. The superior performance achieved by the perovskite-based transducer demonstrates the significant potential of this material in ultrasound imaging. The high bandwidth contributes to improved spatial resolution, while the high acoustic pressure ensures strong signal detection. The miniaturization on optical fibers opens up possibilities for minimally invasive ultrasound imaging procedures. The DFT calculations provide a fundamental understanding of the material's thermal behavior, explaining the origin of the low thermal diffusion coefficient and its contribution to the enhanced performance. The results have significant implications for advancing optoacoustic technology, pushing it closer to practical applications in diverse fields. This work demonstrates a successful paradigm shift in material selection for optoacoustic transducers, highlighting the versatility of lead halide perovskites beyond traditional optoelectronic applications.

This research successfully demonstrated the superior performance of lead halide perovskite-based optoacoustic transducers, achieving record-high bandwidth, acoustic pressure, and conversion efficiency. The unique thermal properties of MAPbI3, particularly its low thermal conductivity, were found to be key contributors to the improved performance. The fabrication of miniaturized devices on optical fibers showcased the potential for high-resolution, minimally invasive ultrasound imaging. Future research could explore different perovskite compositions to further optimize the optoacoustic properties and investigate the long-term stability of these devices in various environments. Exploring applications in different biological tissues and expanding to three-dimensional imaging are also promising research avenues.

The study primarily focused on the performance of MAPbI3-based transducers, and the long-term stability of perovskite films in aqueous environments needs further investigation. The current study only employed a goldfish eye for high-resolution imaging; further research should validate these findings on a wider range of biological samples. Although the DFT calculations provided insights into the material's thermal behavior, a more comprehensive study might be needed to fully capture the complexity of heat transport mechanisms in these materials.