Near-infrared photodetectors play increasingly roles in many fields, such as autonomous driving, food safety, medical imaging, machine vision, biometrics, and smart agriculture. However, traditional near-infrared photodetectors are mainly made of inorganic semiconductor materials such as germanium and indium gallium arsenide, which are expensive in the manufacturing process and lack mechanical flexibility and biocompatibility, thus limiting their application range to a certain extent. As a result, detectors based on solution-processable semiconductors are attracting attention, as they offer significant advantages in terms of cost, flexibility, and biocompatibility. Among them, perovskite materials have attracted attention for their excellent optoelectronic properties. However, the toxic element Pb in lead halide perovskites limits their commercialization. Therefore, it is necessary to study lead-free perovskite materials.
It is in this context that the lead-free perovskite Cs2AgBiBr6 has been unveiled, which shows great potential in making non-toxic, solution-processable photodetectors. However, the spectral response of Cs2AgBiBr6 photodetectors is limited to the ultraviolet and visible regions at wavelengths shorter than 560 nm due to the relatively large bandgap. In order to broaden its spectral response to the near-infrared (NIR) region, an innovative approach was adopted in this study. By introducing titanium nitride nanoparticles (TiN NPs) with the assistance of self-assembled polystyrene nanosphere (PS) arrays, a broadband Cs2AgBiBr6 photodetector covering the ultraviolet, visible, and NIR range is demonstrated. In addition, an atomically thick Al2O3 layer is introduced between the Cs2AgBiBr6 film and the TiN NPs, which effectively mitigated the dark current degradation caused by nanoparticle incorporation. As a result, a significant enhancement of the external quantum efficiency (EQE) is achieved beyond the spectral range where Cs2AgBiBr6 absorbs light, with an enhancement factor of up to 2000 in the broadband NIR wavelength range. This enhancement is mainly attributed to the contribution of plasmonic hot holes injected from TiN NPs into Cs2AgBiBr6. This work promotes the development of broadband solution-processable perovskite photodetectors, providing a promising strategy for realizing photodetection in the NIR region. Relevant research results were recently published in Photonics Research, Volume 12, No. 3, 2024. [ Zijian Liu, Yuying Xi, Wenbo Zeng, Ting Ji, Wenyan Wang, Sitong Guo, Linlin Shi, Rong Wen, Yanxia Cui, Guohui Li. Lead-free perovskite Cs2AgBiBr6 photodetector detecting NIR light driven by titanium nitride plasmonic hot holes[J]. Photonics Research, 2024, 12(3): 522 ]
The PS nanosphere self-assembly method together with the reactive-ion etching (RIE) method were utilized to fabricate TiN NP arrays; see Figure 1(a)–1(e). PS nanosphere arrays were prepared as follows. First, a 2 cm × 2 cm glass substrate was treated in the piranha solution (4:1 volume ratio, 98% H2SO4 : 30% H2O2) for 20 min to make its surface hydrophilic. Next, a hydrophilic glass substrate was placed on top of a heating plate at 60°C. Then, the PS nanosphere (average diameter: 100 nm) solution was injected onto the substrate at a rate of 0.5 mL/min in the first half and 0.25 mL/min in the second half by a peristaltic pump. In this way, a single layer of highly ordered closely packed PS nanospheres was formed on the substrate; Next, the RIE was carried out on the closely aligned PS nanosphere layer to reduce the size of the nanospheres. During the RIE process, the oxygen flow rate was 60 sccm, the etching pressure was 5 Pa, and the etching time was 12 min. Consequently, a sparsely distributed PS nanosphere array was obtained in which the diameters of the nanospheres were around 70 nm; The PS nanosphere array was then employed as the mask for the subsequent preparation of TiN NPs. The TiN layer with a thickness of 40 nm was fabricated by the RF magnetron sputtering method. Owing to the mask shadowing effect, TiN NPs in the shape of semi-ellipsoids were acquired, surrounding the PS nanospheres. Finally, the PS nanospheres were removed by acetone solution. The desired TiN NPs with diverse sizes were eventually formed; the three-dimensional (3D) AFM image is shown in Fig. 1(i).
Figure 1 Schematic flow chart illustrating the fabrication process of the plasmonic perovskite Cs2AgBiBr6 PD incorporated with TiN NPs and Al2O3 ultrathin layer. (a) Preparing a hydrophilic glass substrate; (b) forming a single layer of highly ordered closely packed PS nanospheres on glass substrate obtained by self-assembly method; (c) obtaining sparsely distributed PS nanosphere array by the RIE method; (d) depositing a TiN layer with a thickness of 40 nm by the RF magnetron sputtering method; (e) obtaining TiN NP array on the glass substrate after removing PS spheres; (f ) depositing an ultrathin Al2O3 layer on the above formed TiN NPs by ALD; (g) spin coating the Cs2AgBiBr6 thin film on the Al2O3 interfacial layer; and (h) sputtering TiN electrodes with a thickness of 80 nm on Cs2AgBiBr6 using a copper mesh as the mask. In (h), the light is incident on the PD from the glass side. (i) 3D AFM image of the obtained TiN NPs.
The introduced thin layer of Al2O3 improves the quality of the perovskite film and mitigates the dark current deterioration induced by the introduction of TiN NPs, as shown in Fig. 2(a). As a result, the plasmonic Cs2AgBiBr6 photodetector exhibits a significant response enhancement over a broadband wavelength range relative to the Cs2AgBiBr6 photodetector. At long wavelengths beyond the absorption band of Cs2AgBiBr6, it shows an average enhancement factor of more than 2000. as shown in Fig. 2(b). In the intrinsic absorption range of Cs2AgBiBr6, the enhancement originates from the enhanced absorption taking place in perovskite produced by the localized plasmonic resonance of TiN NPs. In contrast, at long wavelengths beyond the optical absorption range of Cs2AgBiBr6, the enhancement is attributed to the generation of plasmonic hot holes within TiN NPs and the collection of those hot holes. As shown in Fig. 2(c), the hot holes produced by TiN NPs enables the photodetector to overcome the wavelength limit imposed by the absorption of Cs2AgBiBr6 perovskite, and realize the detection of optical signals at longer wavelengths, extending to the telecommunication band of 1550 nm. This work provides an alternative strategy for realizing environmentally friendly and broadband perovskite photodetectors.
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Figure 2 (a) I-V curves of Cs2AgBiBr6, Cs2AgBiBr6/TiN NPs, and Cs2AgBiBr6/Al2O3/TiN NP devices measured in the dark. (b) R of the Cs2AgBiBr6 and Cs2AgBiBr6/Al2O3/TiN NP devices at different wavelengths. (c) Working mechanism of the Cs2AgBiBr6/Al2O3/TiN NP device, illustrating hot hole excitation and transfer driven by plasmonic resonances.
