Amorphous In–Ga–Zn–O (a-IGZO) thin-film transistors (TFTs) exhibit high field effect mobility, excellent electrical uniformity, flexibility, and low-temperature process, and thus have been extensively investigated in display panels, flexible electronics, and UV detectors.^[1–8] However, with respect to detections of visible lights, the a-IGZO TFTs often show limited responsivity because of a rather wide bandgap larger than 3 eV in a-IGZO.^[1,2] To achieve significantly improved visible light detections, various light-absorbing materials have been introduced into the devices, including organic semiconductor,^[9–13] two-dimensional materials,^[14,15] inorganic quantum dots (QDs),^[16–19] and halide perovskites.^[20–30] Among them, the halide perovskites are of great interest due to their strong light absorption, long carrier diffusion length, low-cost process, high charge carrier mobility, and tunable bandgaps (∼ 1.7–3.2 eV).^[31–37] Therefore, the concept of a-IGZO/perovskite hybrid-based photodetectors has been proposed in recent years, and presents enhanced photosensitive performance.^[20–24,26]

So far, the reported a-IGZO/perovskite hybrid-based photodetectors include two different device structures, i.e., a-IGZO-covered perovskite channel devices and perovskite-covered a-IGZO channel ones.^[20–24,26] The former, in which perovskite nanowires are sandwiched between a-IGZO and gate dielectric, cannot operate like a transistor because of the gate field shielding effect. The latter with perovskite films or QDs on the back side of the a-IGZO channel can maintain relatively high field-effect mobility, however, their photo-responsiveness is still rather limited.^[20–24,26]

In this work, we propose a hybrid channel consisting of inorganic halide perovskite (IHP) QDs embedded in the a-IGZO film for TFT photodetector applications, which is inspired by the rather good stabilities and high photoluminescence quantum yields of the IHP QDs.^{[33–35,38–43]} Therefore, the fabricated devices exhibit superior photodetecting performance, including high photoresponsivity, excellent detectivity, and so on. In particular, tunable spectrally-selective photodetections of the devices are realized by introducing different bandgap QDs, and the underlying mechanisms are also discussed.^[44,45]

Figure 1(a) shows the TEM image of the representative CsPbBr₃ QDs, exhibiting an average size of around 5 nm and a highly crystalline texture. The XRD patterns of the CsPbBr₃ and CsPbI₃ QDs are presented in Fig. 1(b), indicating the formations of cubic CsPbBr₃ and CsPbI₃ phases.^{[33,35,36,48]} As shown in Fig. 1(c), under an excitation wavelength of 365 nm, the emission peaks of the CsPbBr₃ and CsPbI₃ QDs are centered at 520 nm and 640 nm, respectively. The UV–Vis absorption spectra of the IHP QDs on a quartz substrate are illustrated in Fig. 1(d). The absorbance peaks of the CsPbBr₃ and CsPbI₃ QDs appear at about 490 nm and 630 nm, respectively.^[49]

Fig. 1.

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	Fig. 1. (a) TEM image of the representative CsPbBr3 QDs. (b) XRD patterns, (c) PL (λexc = 365 nm), and (d) optical absorption spectra of the CsPbBr3 and CsPbI3 QDs spin-coated on a quartz substrate.

Figure 2(a) shows the schematic diagram of the fabricated TFT photodetector with CsPbX₃ (X = Br or I) QDs embedded a-IGZO channel (named as CsPbX₃-based hybrid TFTs), in which the highly doped p-type silicon substrate and Al₂O₃ film act as the back gate and dielectric, respectively; and the QDs as a light absorption medium are embedded in the middle of the a-IGZO layer, which can protect the QDs away from the atmosphere containing oxygen and moisture. The properties of the a-IGZO film deposited on the Al₂O₃ dielectrics by using the same RF sputtering method have been elaborately investigated previously, and the a-IGZO film exhibited small surface roughness and only a few oxygen vacancies.^[47] The typical morphology of the QDs layer prepared via spin-coating is shown in Fig. 2(b), exhibiting a uniform and dense distribution of the QDs. Furthermore, the energy band alignment of the CsPbI₃, CsPbBr₃, and a-IGZO is also presented in Fig. 2(c). Since a-IGZO has a bandgap beyond 3.25 eV, it is transparent to visible light.^[1,2] In addition, the conduction band minimums (E_c) of both CsPbI₃ (E_c = –3.6 eV) and CsPbBr₃ (E_c = –3.65 eV) are higher than that of a-IGZO (E_c = –4.3 eV), so the photogenerated electrons from the CsPbI₃ or CsPbBr₃ QDs can transfer to the a-IGZO channel effectively. Furthermore, to demonstrate the superiority of the hybrid channel over the single a-IGZO one, the transfer characteristics of all the devices are compared, especially under different wavelength lights with a fixed power density (P = 0.12 mW/cm²), as shown in Figs. 2(d)–2(f). The a-IGZO TFT demonstrates a very small threshold voltage (V_T) shift (∼ 0.1 V) under the 500 nm light, and almost no V_T shift under the 540 nm light. This reveals that the single a-IGZO channel-based TFT cannot give active responses to the lights with a wavelength ≥ 500 nm. Note that the drain currents for the a-IGZO TFT are extremely small when the gate voltages are between −4 V and −1.5 V, which are recorded as negative values, so that the transfer curves disappear in this region when an exponential coordinate axis is applied. Notably, both the CsPbX₃-based hybrid TFTs exhibit remarkable responses to different wavelength lights. In terms of the CsPbBr₃-based hybrid device, the drain current does not begin to increase gradually until the light wavelength is reduced from 700 nm to 580 nm (see Fig. 2(e)). However, for the CsPbI₃-based hybrid device, the resulting drain current always exhibits a gradual rise with decreasing the light wavelength from 700 nm to 500 nm (see Fig. 2(f) ). Such distinct responses to longer wavelengths (620–700 nm) for the latter should be ascribed to the smaller bandgap of the CsPbI₃ QDs. Briefly speaking, the adoption of the hybrid channel successfully extends the range of detectable light-wavelengths due to the incorporation of the CsPbI₃ or CsPbBr₃ QDs.

Fig. 2.

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Fig. 2. (a) Schematic diagram of the TFT photodetector with CsPbX3 (X = Br or I) QDs embedded in the a-IGZO channel. (b) Typical AFM image of the CsPbBr3 QDs layer. (c) Energy band alignment of CsPbI3, CsPbBr3, and a-IGZO. The transfer curves (ID–VG) of the TFTs under different wavelength lights (P = 0.12 mW/cm2) as well as in the dark for (d) the single a-IGZO channel, (e) the a-IGZO/CsPbBr3 QDs hybrid channel, and (f) the a-IGZO/CsPbI3 QDs hybrid channel.

The transfer curves of both the CsPbX₃-based hybrid TFTs are further measured as a function of power density under 530 nm and 630 nm lights, respectively, as shown in Fig. 3. Under the 530 nm light, the CsPbI₃-based hybrid device exhibits a rapider increase in drain current than the CsPbBr₃-based one with the increase of the power density (i.e., 40 μW/cm² → 120 μW/cm²), as shown in Figs. 3(a) and 3(b). Under the 630 nm light, the drain current as a function of the light power density for the CsPbBr₃-based hybrid device almost does not show any change (see Fig. 3(c)), but the drain current for the CsPbI₃-based hybrid device still exhibits an obvious rise with the increasing power density, as indicated in Fig. 3(d).

Fig. 3.

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	Fig. 3. The transfer curves (ID–VG) of a-IGZO/CsPbBr3 QDs and a-IGZO/CsPbI3 QDs hybrid TFTs under different light intensities with a constant wavelength of (a), (b) 530 nm or (c), (d) 630 nm.

Further, the light power density-dependent threshold voltage (V_T) shifts and field effect mobility (μ) are compared between the CsPbBr₃- and CsPbI₃-based hybrid TFTs, as displayed in Figs. 4(a) and 4(b). Regarding the CsPbBr₃-based device, both the V_T shift and μ are almost stable with the increasing power density up to 120 μW/cm² under the 630 nm light, while they enlarge gradually as a function of the power density under the 530 nm light. With respect to the CsPbI₃-based device, both the V_T shift and μ exhibit a gradual rise with the increasing power density under the 630 and 530 nm lights, respectively, and their change rates under the 530 nm light are much larger than those under the 630 nm light. Furthermore, the light wavelength-dependent V_T shifts and μ are shown in Figs. 4(c) and 4(d), respectively, for both the devices. It is found that the CsPbI₃-based hybrid device shows a gradual decrease in the V_T shift and μ, respectively, with the increasing light wavelength from 500 nm to 700 nm under the same power density. However, the CsPbBr₃-based hybrid device presents a gradual decrease in the V_T shift and μ until the light wavelength increases to 620 nm, followed by unchanged values.

Fig. 4.

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	Fig. 4. Dependence of threshold voltage (VT) shift and mobility on the power density of different wavelength light for (a) the CsPbBr3-based hybrid TFT and (b) the CsPbI3-based one. (c) The VT shifts and (d) mobility of the CsPbBr3- and CsPbI3-based hybrid TFTs as a function of light wavelength, together with those in dark.

In order to well evaluate the photosensitive characteristics of the TFT photodetectors, both photoresponsivity (R) and detectivity (D^*) are introduced, which can be calculated as^[39,50,51]

Fig. 5.

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	Fig. 5. (a), (b) Gate voltage-dependent R and (c), (d) Ilight/Idark ratio under different wavelength lights for the CsPbBr3- and CsPbI3-based hybrid TFTs, respectively. Dependence of R (VG = 5 V) and Ilight/Idark maximum on light wavelength for (e) the CsPbBr3-based hybrid device and (f) the CsPbI3-based hybrid device.

Moreover, the transient response of the a-IGZO/CsPbBr₃ QDs hybrid TFTs to a switching light (λ = 500 nm) was investigated, as shown in Fig. 6(a). The photoswitching curve, including a photocurrent increase (Fig. 6(b)) and reduction (Fig. 6(c)) process, can be fitted with a double-exponential function^[39,50]

Fig. 6.

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	Fig. 6. (a) Photoswitching current curve of the a-IGZO/CsPbBr3 QDs photodetector in dark and under illumination in an on/off cycle. The fitting line of the current during (b) on and (c) off switching.

Finally, the photoresponse mechanisms of the IHP QDs-based hybrid TFTs under different lights are expressed by schematic diagrams of energy band, as shown in Fig. 7. When a positive bias is applied to the gate of the a-IGZO TFT, electrons are accumulated in the a-IGZO channel due to downward bending of the a-IGZO energy band near the interface. Although the a-IGZO TFT is exposed to the 540 nm light, such a light cannot be absorbed by the a-IGZO channel because the energy of the 540 nm light (2.30 eV) is smaller than the bandgap of a-IGZO (E_g = 3.25 eV). Thus, photogenerated carriers cannot be formed in the a-IGZO channel (see Fig. 7(a)), which makes the device be unable to respond to the 540 nm light, as indicated in Fig. 2(d). When the 540 nm light irradiates the CsPbX₃ (X = Br or I)-based hybrid TFTs, it can be absorbed by CsPbBr₃ (E_g ∼ 2.3 eV) or CsPbI₃ (E_g ∼ 1.7 eV). Accordingly, lots of electrons are excited to the conduction band of the IHP QDs, thus producing free photoelectrons and leading to a photogating effect.^[52] These photogenerated electrons further drift to the conduction band of the a-IGZO channel under the external electric filed, as shown in Fig. 7(b). Therefore, the photogating effect results in an increase of electron density in the channel, which reduces the V_T of the device, i.e., the V_T shifts towards a negative bias (see Figs. 2(e) and 2(f)). For a given wavelength light which can be absorbed by the IHP QDs, higher power density means that more photons per unit area can be absorbed, hence resulting in more photogenerated electrons in the channel. This thus gives rise to a gradually increased V_T shift in the direction of a negative bias with enhancing power density, as indicated in Figs. 4(a) (λ = 530 nm) and 4(b) (λ = 530, 630 nm). In addition, it is reported that some surface defects in the form of elemental vacancies usually exist on the IHP QDs. In our cases, the defects could be positive charged and thus act as electron traps in the QDs embedded a-IGZO channels.^[53–55] Under effective illumination, the photogenerated electrons passivate the electron traps, i.e., the positive charged defects, at the interface of IHP QDs/a-IGZO, which is beneficial to improve the channel mobility likely due to the reduced Coulomb scattering effect, as shown in Fig. 4(d). On the other hand, the V_T shift and mobility decrease gradually as the incident light wavelength increases, as shown in Figs. 4(c) and 4(d) for the CsPbI₃-based hybrid TFT. This should be dominated by the change of visible light absorption in the CsPbI₃ QDs, as revealed in Fig. 1(d). When the light wavelength goes up to 620 nm (i.e., 2 eV), which cannot be absorbed by CsPbBr₃ (E_g ∼ 2.3 eV), photoelectrons cannot be easily produced in the CsPbBr₃ QDs (see Fig. 7(c)). In this case, the CsPbBr₃-based hybrid device can not any more give responses to the light with a wavelength ≥ 620 nm, i.e., a wavelength cutoff of photoresponsivity appears, as suggested in Fig. 5(e). However, for the CsPbI₃-based hybrid device, the 620 nm light still can be absorbed by the CsPbI₃ QDs (see Fig. 7(d)), and thus good photoresponsivity is demonstrated till a wavelength of 700 nm, as revealed in Fig. 5(f).

Fig. 7.

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	Fig. 7. Schematic diagrams of energy band for various devices under positive gate voltage: (a) the a-IGZO TFT under 540 nm light; (b) the CsPbBr3- and CsPbI3-based hybrid TFTs under 540 nm light; (c) the CsPbBr3-based hybrid TFT under 620 nm light; (d) the CsPbI3-based hybrid TFT under 620 nm light.

Table 1 compares our CsPbX₃ (X = Br or I)-based hybrid TFT photodetectors with others based on perovskite and a-IGZO. Compared to the other channel constitutions, our CsPbX₃ (X = Br or I) QDs embedded a-IGZO channels can provide much better photo-detecting properties, such as excellent D^* (1.0× 10¹⁶ or 1.7× 10¹⁶) and high R (1400 A/W or 3900 A/W) under 500 nm illumination and a gate bias of 5 V. This should be attributed to a much higher transfer efficiency of the photoelectrons due to a shorter transfer distance between the perovskite QDs and the effective channel region. Moreover, compared with the heterostructure device, the amplifier function of the TFT photodetector also contributes to higher photoresponsivity.

Table 1.

Table 1.

Table 1. Comparison of our CsPbX3 (X = Br or I)-based hybrid TFT photodetectors with reported others based on perovskite and a-IGZO. S/D represents source/drain, and PCBM means[6,6] –phenyl C61-butyric acid methylester. . Device structure Channel constitution (top-down) R/A⋅W−1 D*/Jones Ilight/Idark μ/cm2⋅V−1⋅s−1 Ref. TFT (top S/D) CH3NH3PbI3 film/60 nm a-IGZO 0.025 9.8 × 109 – 12.9 [20] TFT (top S/D) CsPbIxBr3–x film/20 nm a-IGZO 26 8.42 × 1014 8.71 × 106 – [24] Heterostructure 40 nm a-IGZO/CsPbBr3 nanowires 3.794 – 1.2 × 104 – [21] TFT (top S/D) CH3NH3PbI3 film/50 nm a-IGZO 61 9.42 × 1010 3.48 × 106 – [22] TFT (bottom S/D) CH3NH3PbI3 film/PCBM/40 nm a-IGZO 0.01 1.35 × 1012 – – [23] TFT (top S/D) CsPbBr3 QDs/PCBM/40 nm a-IGZO 9.72 2.96 × 1012 106 8.95 [26] TFT (top S/D) 20 nm a-IGZO/CsPbBr3 QDs/20 nm a-IGZO 1.4 × 103 1.0 × 1016 3.7 × 104 2.52 this work TFT (top S/D) 20 nm a-IGZO/CsPbI3 QDs/20 nm a-IGZO 3.9 × 103 1.7 × 1016 1.4 × 105 2.87 this work

Table 1. Comparison of our CsPbX3 (X = Br or I)-based hybrid TFT photodetectors with reported others based on perovskite and a-IGZO. S/D represents source/drain, and PCBM means[6,6] –phenyl C61-butyric acid methylester. .

Photodetectors based on a-IGZO TFT embedded with IHP QDs in the channel were successfully fabricated in this work, which exhibit excellent photosensitive performance with a photoresponsivity over 10³ A⋅W⁻¹, an I_photo/I_dark of 10⁵, and a detectivity in the order of 10¹⁶ to visible light. The enhanced photoresponsivity, as compared with those of previously reported a-IGZO/perovskite hybrid devices, is mainly attributed to a higher transfer efficiency of the photoelectrons from the perovskite QDs to the effective channel region, as well as the amplifier function of the TFT. Moreover, the CsPbBr₃ and CsPbI₃ QDs embedded a-IGZO TFTs exhibit different wavelength-responsive cutoffs of 550 nm and 700 nm, respectively, indicating tunable spectrally selective photodetections. By optimizing the device structure, this work provides a novel strategy for further improving the performance of a-IGZO/perovskite hybrid photodetectors.