Preparation of High-Performance CsPbIBr₂ Photodetector via Interfacial Seed Layer Modification Strategy

Objective Photodetectors can convert incident light into electric signals and are widely used in many fields such as image sensing, optical communication, environmental monitoring, and biological detection. In recent years, all-inorganic metal halide perovskite CsPbIBr2 has been concerned in photoelectric detection due to its high light absorption coefficient, high charge carrier mobility, and low defect density. On the one hand, CsPbIBr2 film is susceptible to ambient humidity, so it is not usually prepared under an atmospheric environment but in glove boxes by methods such as spin coating. On the other hand, on account of uncontrolled nucleation during crystallization, CsPbIBr2 film has poor morphology and crystallinity, which results in weak photoelectric characteristics and instability of its photodetector. In order to overcome these problems, the morphology, crystallinity, and water/oxygen resistance of CsPbIBr2 thick film can be improved by additives and interface layer strategies. In this study, we employ an interfacial seed layer modification strategy under an atmospheric environment with relative humidity (RH) below 90% to prepare one high-quality CsPbIBr2 thick film with high crystallinity, excellent interfacial contact, and stable structure. We hope that our findings can help fabricate low-cost, highperformance, and long-lasting photodiode-type CsPbIBr2 photodetectors under an atmospheric environment. Methods CsPbIBr2 thick films with controllable thicknesses in the range of 0. 5-100 mu m are prepared under an atmospheric environment with RH below 90% by pneumatic spraying. Prior to spraying, the interfacial seed layers are formed on the substrates by spin-coating followed by annealing. During spin-coating, the density distribution of the interfacial seed layers is realized by controlling the concentration of precursor solution. In this strategy, the interfacial seed layers act as the nucleating points for crystal growth, which improve the crystallization of the thick films in preparation processing. The morphology and the phase structure of the thick films are analyzed by scanning electron microscopy (SEM) and X-ray diffraction. Compared with the thick film without an interfacial seed layer, these films with interfacial seed layers have high crystallinity, excellent interfacial contact, and stable structure. In order to assess the effect of interfacial seed layers on the optical properties of the thick films, these thick films are investigated by absorption, photoluminescence (PL), and time-resolved PL spectra. In order to verify the feasibility of the thick films for photodetection, their photodiode-type photodetectors of Au/ITO/CsPbIBr2/Au are fabricated and measured. The I-V and response time curves of the photodetectors are examined under laser excitation of 405 nm. In order to characterize the long-term stability, tracing measurements on the on-off ratio of the devices are made, and the naked-eye photographs of the corresponding thick films are recorded. Results and Discussions Compared with the control film with small-size crystal grains and a large number of holes on the surface, the modified films by introducing interfacial seed layers exhibit large-size crystal grains and dense morphology [Fig. 1(a)]. From the cross-sectional SEM images, the improved interfacial contacts between the modified films and the substrates lead to columnar growth features [Fig. 1(b)]. The modified films show a preferred orientation on the (110) diffraction plane, especially for 0. 3 mol/L, which is consistent with the SEM results [Fig. 1(c)]. Once seed layers are inserted between the thick films and the substrates, the absorption coefficients and PL peak intensities increase significantly in the whole visible range, and the fluorescence lifetime increases from 0. 95 ns to 4. 49 ns (Fig. 2). The dark current from the control device to the modified devices decreases from 2. 05x10(-7) to 5. 70x10(-10) A, while the on-off ratio significantly increases from 490 to 1. 8x10(4) [Fig. 3(a)]. By fitting the I-V curves under light illumination, it is proved that the modified device of 0. 3 mol/L has a stronger light response (n=0. 87) and larger response range (R-LD=80 dB) than the control device (n= 0. 60 and R-LD=34 dB) [Fig. 3(b)-(e)]. The rising and falling time (t(on) and t(off)) from the control device to the modified devices decreases from 38 mu s to 9 mu s and from 110 mu s to 13 mu s, respectively [Fig. 3(f)]. After lasting 60 days, the modified device of 0. 3 mol/L still retains a high on-off ratio of 1. 5x10(4), which is 83% of the initial on-off ratio, and the naked-eye photographs of the thick films do not change significantly (Fig. 4). Conclusions In this study, under an atmospheric environment with RH below 90%, high-quality CsPbIBr2 thick films are prepared by pneumatic spraying via an interfacial seed layer modification strategy. In this strategy, the interfacial seed layer acts as the nucleating points for crystal growth, which results in the improvement of the crystallinity, interfacial contact, and structural stability of the CsPbIBr2 thick films. Furthermore, the introduction of interfacial seed layers has no significant effect on the optical band gaps of CsPbIBr2 thick films, ranging from 2. 10 eV to 2. 12 eV. It is worth noting that the absorption coefficient of visible light and the PL intensity are enhanced significantly, and meanwhile the fluorescence lifetime is increased (from 0. 95 ns to 4. 49 ns). The photodiode-type CsPbIBr2 photodetector (p-n CsPbIBr2-ITO) shows a low dark current (5. 70x10(-10) A) and possesses high-performance photodetection parameters, namely high on-off ratio (1. 8x10(4)) and microsecond-level response times (9 mu s and 13 mu s). Moreover, the unpackaged CsPbIBr2 photodetector is strongly resistant to water and oxygen under an atmospheric environment with RH below 90%, which is 83% of the initial on-off ratio after lasting 60 days. These results can provide an effective way to prepare low-cost, high-performance, long-lasting, and stable photodiode-type CsPbIBr2 photodetectors under an atmospheric environment.