Perovskite-based devices have gained considerable interest from academia and industry owing to their cost-efficient processing and outstanding optoelectronic properties. The growing use of perovskites in photovoltaic cells has instigated the development of different types of perovskite-based optoelectronic devices, such as photodetectors, lasers, and light-emitting diodes.
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Nanowires have gained considerable attention in photonics and optoelectronics due to their large surface area and unique electrical properties. In particular, the spatially-confined charge transport along the one-dimensional structure of the nanowires makes them especially useful as resonators and waveguides in nanophotonic circuits.
Due to their high transmittance and low sheet resistance, conductive nanowires are highly-suitable for fabricating flexible transparent electrodes in applications such as flexible organic light-emitting diodes, photovoltaic cells, and other optoelectronic devices.
Nanowires as Versatile Building Blocks in Optoelectronics
Traditionally, conductive nanowires have been synthesized from various metals and semiconductors. However, metal nanowires possess inherent disadvantages, including high surface roughness, low adhesion to the support, fast degradation, and relatively short lifespan.
In the past decade, the rapidly advancing field of perovskites has demonstrated the outstanding optoelectronic performance of these materials. Perovskites are a class of materials with a crystal structure similar to calcium titanate. The general chemical formula describing the perovskite materials is ABX3, where A and B are cations, and X is an oxide or halogen anion. There is a wide range of organic and inorganic materials that adopt such structures.
Perovskite-based materials are regarded as important building blocks for novel optoelectronic applications because of their unique physical properties, such as high absorption coefficient, long-range ambipolar charge transport, low exciton binding energy, high dielectric constant, low defect density, long charge carrier lifetime, and short diffusion lengths,
Recent research has demonstrated that single-crystalline one-dimensional perovskite nanowires exhibit a very low number of ionic defects and grain boundaries, resulting in enhanced charge photogeneration and carrier transport properties.
Compared to their thin-film or bulk counterparts, the perovskite nanowires possess numerous advantages that can dramatically improve the performance, reliability, and stability of a wide range of optoelectronic devices.
Strategies for Creating Uniform Perovskite Nanowires
Perovskite nanowires can be synthesized using either solution- or vapor-based approaches. The synthesis in a solution can be performed at low temperatures, providing high yield, excellent aspect ratio control, and facilitating the transfer onto solid substrates for device fabrication. Researchers demonstrated the advantages of the solution-based synthesis of perovskite nanowires for fabricating large-area nanowire arrays.
An important drawback of the method, however, is the presence of non-conductive organic ligands and surfactants in the solution, which can affect unfavorably the physical, optical, and electronic properties of the nanowires.
In contrast, vapor-phase methods, such as chemical vapor deposition, epitaxial growth, and template-directed growth, enable superior control of the size, crystallinity, and orientation of the nanowires while minimizing the defect density within the material.
In recent years, many research groups have successfully employed vapor-phase synthesis to fabricate highly-ordered single-crystal perovskite nanowires.
Applications of Perovskite Nanowires in Photovoltaic Cells
The inherently large surface area of the perovskite nanowires greatly enhances their light-harvesting properties. The spatial confinement of the charge carriers within the highly-crystalline one-dimensional structure further improves charge separation, extraction, and transport. These advantages over the perovskite thin films, together with the flexibility of the nanowires, make them one of the most promising materials for next-generation photovoltaic applications.
Oriented Perovskite Nanowires Increase Photodetector Efficiency
Several research groups worldwide demonstrated photodetectors employing arrays of parallelly-aligned perovskite nanowires with a specially designed structure capable of delaying the recombination of the charge carriers (holes and electrons). Such delayed recombination dramatically increased the generated photocurrent and enhanced the photoconductive gain of the nanowires, thus improving the performance of the device. Besides, the tiny active volume of the nanowires greatly minimized the dark current noise level of the photodetector, compared to conventional semiconductor-based devices.
To meet the demands of future industrial applications, the researchers are refining the nanowire synthesis and deposition techniques to improve the stability of the nanowires and reduce the number of surface defects and grain boundaries within the structure.
High-Performance Perovskite Nanowire Lasers
Nanowire-based lasers benefit from the intrinsic waveguiding along the axial direction of the nanowires, while the two end-facets act as reflecting mirrors for light amplification. Researchers from the University of Wisconsin-Madison and Columbia University in the US demonstrated a single-crystal methylammonium lead halide perovskite nanowire-based laser with a very low lasing threshold, high quality factor, and near-100% quantum yield, thus outperforming previously reported semiconductor nanowire-based lasers.
The researchers also demonstrated that by altering the cation and anion stoichiometry in the perovskite, the emission wavelength of the device can be tuned in the range of 410-820 nm.
All-inorganic perovskite nanowires, such as cesium lead bromide (CsPbBr3), are showing even more promising as they are more robust than their organic-inorganic counterparts. Single-crystal CsPbBr3 nanowires exhibit stable light emission even after 8 hours of continuous operation. Such lasing performance, coupled with a facile solution-phase synthesis of the nanowires that allows emission wavelength tunability in the visible and near-infrared spectral regions, opens up prospects for cheap and efficient miniaturized photonic and optoelectronic devices.
Imaging Applications of Vertically Aligned Nanowires
Recently, a research group from the University of Hong Kong demonstrated a proof-of-concept image sensor containing 1024 photodiode pixels fabricated from an array of vertically aligned methylammonium lead triiodide (MAPbI3) nanowires. Each pixel with dimensions of 200x200 μm2 consisted of approximately 160,000 perovskite nanowires operating as photodetectors. The experimental device demonstrated excellent flexibility under mechanical load with good responsivity and short rise and fall times, making it ideal for biomedical applications in wearable electronics.
Future Developments and Outlook
Perovskite nanowires are among the most promising emerging materials suitable for many photonic and optoelectronic applications as alternatives to thin-film semiconductor devices. The rapid development in the field is likely to result in further optimization of the synthetic methods, ensuring uniform size distribution, controlled nanowire geometries, and increased chemical and thermal stability. Addressing these issues would allow the utilization of the perovskite nanowires in future practical applications.
References and Further Reading
Kumar, G.S., et al. (2022) Perovskite Nanowires for Next-Generation Optoelectronic Devices: Lab to Fab. ACS Applied Energy Materials, 5(2), 1342-1377. Available at: https://doi.org/10.1021/acsaem.1c03284
Liu, M., et al. (2019) Halide Perovskite Nanocrystals for Next-Generation Optoelectronics. Small, 15, 1900801. Available at: https://doi.org/10.1002/smll.201900801
Zhang, J., et al. (2022) A review of geometry-confined perovskite morphologies: From synthesis to efficient optoelectronic applications. Nano Res. Available at: https://doi.org/10.1007/s12274-022-4342-2