All-Evaporated, All-Inorganic CsPbI3 Perovskite-Based Devices for Broad-Band Photodetector and Solar Cell Applications

Abstract

Following the rapid increase of organic metal halide perovskites toward commercial application in thin-film solar cells, inorganic alternatives attracted great interest with their potential of longer device lifetime due to the stability improvement under increased temperatures and moisture ingress. Among them, cesium lead iodide (CsPbI₃) has gained significant attention due to similar electronic and optical properties to methylammonium lead iodide (MAPbI₃), with a band gap of 1.7 eV, high absorption coefficient, and large diffusion length, while also offering the advantage of being completely inorganic, providing a higher thermal stability and preventing material degradation. On a device level, however, it seems also essential to replace organic transport layers by inorganic counterparts to further prevent degradation. In addition, devices are mostly fabricated by spin coating, limiting their reproducibility and scalability; in this case, exploring all-evaporated devices allows us to improve the quality of the layers and to increase their reproducibility. In this work, we focus on the deposition of CsPbI₃ by CsI and PbI₂ co-evaporation. We fabricate devices with an all-inorganic, all-evaporated structure, employing NiO and TiO₂ as transport layers, and evaluate these devices for both photodetector and solar cell applications. As a photodetector, low leakage current, high external quantum efficiency (EQE) and detectivity, and fast rise and decay times were obtained, while as a solar cell, acceptable efficiencies were achieved. These all-inorganic, all-evaporated devices represent one step forward toward higher stability and reproducibility while enabling large area compatibility and easier integration with other circuitry and, in future, the possible commercialization of perovskite-based technology.

Figure 1

(a) Photographs of 3 × 3 cm² substrates of the transitions of CsPbI₃ films: the as-deposited film (bottom) exhibits a dark brown color, which transitions to yellow as the film is annealed at 150 °C (middle); finally, when the film is annealed at 335 °C (top), it exhibits a darker, almost black, color. (b) Corresponding absorbance and photoluminescence spectra for each of the phases: the as-deposited film and the film annealed at 335 °C show very similar absorbance and PL spectrum, while the annealed film has a slightly narrower band gap. The film annealed at 150 °C has a notably larger band gap and a broad-band emission spectrum. (c) Comparison of the X-ray diffraction (XRD) spectra for the three CsPbI₃ phases. The as-deposited film has very broad peaks that indicate a nanocrystalline structure and that correspond to the γ-phase. The film annealed at 150 °C also exhibits broad peaks but corresponding to the δ-phase. Finally, the spectrum for the films annealed at 335 °C shows sharper, narrower peaks, indicating a larger crystallite size. The presence of a double peak at ∼15° and again at ∼28° indicates that this film does not belong completely to the α-phase but to a mix of phases.

Figure 2

(a) Values of n and k for the black phase of CsPbI₃. (b) AFM images showing the roughness of the perovskite films, with an average root-mean-square roughness of 22 nm. (c) Band diagram of the CsPbI₃-based device, using NiO and TiO₂ as transport layers. (d) Scanning electron microscopy (SEM) cross section of the device. The interface between the layers is clean and the perovskite is dense and without short circuits.

Figure 3

(a) Device J–V curves in the dark for both forward and reverse sweeps. The device exhibits hysteresis, as commonly happens with perovskite-based devices. Inset shows a photograph of a finished sample. (b) EQE vs wavelength for the device when biased at different voltages. Saturation is reached at a negative bias of −0.5 V. (c) EQE, R, and D* of the device when biased at −0.5 V. (d) Solar cell performance for both forward and reverse sweeps. Efficiency of 7–8% was achieved.

Figure 4

Statistics for several CsPbI₃-based solar cells, for both forward and reverse sweeps.

Figure 5

(a) CsPbI₃ photodetector performance when illuminated with a 530 nm LED at different light intensities. Inset shows the linear behavior of the current density with the light intensity, exhibiting a constant responsivity at −0.5 V. (b) Responsivity vs voltage curve for different light intensities. Responsivity values are constant in the range of 0 to −1.5 V. (c) Photodetector R, EQE, and D* values for different light intensities when illuminated with a 530 nm LED and biased at −0.5 V. Devices are well behaved throughout the whole range of light intensities. (d) Time-dependent photoresponse when the device is biased at −0.5 V. Rise and decay time are in the range of 7–8 μs for a size of 0.025 cm².

Figure 6

Comparison of the performance, in terms of rise time and responsivity, of the CsPbI₃-based photodetector reported here with other CsPbI₃-based photodetectors reported in the literature (references are added to each graph point). As it can be seen, the responsivity is higher than most MSM and photodiodes while the rise time is faster than most of the devices previously reported.