Transparent conductive materials (TCMs) are crucial in optoelectronics, serving as key components for transparent electrodes in various devices such as photodetectors, photovoltaic cells, and light-emitting diodes (LEDs)^[1⇓-3]. However, most TCMs are n-type semiconductors, such as doped In₂O₃, ZnO, and TiO₂^[4⇓-6]. In contrast, p-type TCM is still in infancy, and many materials fail to combine preferable optical transparency and conductivity to compete strongly with n-type materials. Transparent electronics depends on the development of a high-performance p-type TCM^[7]. The absence of high-performance p-type TCMs hinders not only the realization of transparent electronics through the combination of p-type and n-type TCMs, but also the advancement of highly efficient heterojunctions based on conventional n-Si technologies.

Copper iodide (CuI) is an emerging p-type wide bandgap semiconductor with high intrinsic Hall mobility, high optical absorption and large exciton binding energy^[8⇓⇓-11]. High-quality and low-cost γ-CuI thin films have been successfully fabricated using various physical and chemical methods, including sputtering, pulsed laser deposition, thermal evaporation, electrochemical deposition, and iodization, etc^{[12⇓⇓⇓⇓⇓⇓⇓-20]}. They have been widely used in various applications such as p-n junctions, transparent electrodes, solar cells, transistors, and thermoelectric devices^[21]. Some heterojunction diodes have been investigated, which involve mixing p-CuI with n-type semiconductors such as AgI, a-IGZO, and ZnO^[22⇓-24]. For example, Yamada et al.^[16] demonstrated the photovoltaic effect under ultraviolet (UV) light in a CuI/IGZO heterojunction, establishing the basis for UV photodetectors using CuI. Zhang et al.^[25] fabricated a heterojunction comprising CsPbBr₃ perovskite and CuI to extend the photodetection region into visible light spectrum by utilizing the relatively narrow bandgap of CsPbBr₃ perovskite (2.23 eV).

Nevertheless, the reported CuI-based photodetectors, including the CuI/Si diodes^[26], mainly detect wavelengths in the UV band. In addition, the difficulty in fabricating high-quality epitaxial thin films of CuI is a drawback to achieve high device performances^[8]. Therefore, developing highly sensitive photovoltaic devices using CuI heterojunctions that have a broad and controllable spectral response remains a challenge. Actually, the CuI/Si heterojunctions have practical significance and show potential for combining the benefits of p-type CuI with the established silicon technology. Heterojunction devices can achieve high performance by utilizing the distinctive characteristics of CuI, such as its wide bandgap and high hole mobility, while also upholding the reliability and durability associated with silicon technology.

In this study, a p⁺n CuI/Si heterojunction photodiode was produced using a facile and low-cost solid-phase iodination method. It exhibited high sensitivity to weak light and switchable response wavelength range. The findings show the significant potential of CuI when integrated with the traditional semiconductor industry.

Fig. 1(a) shows the preparation procedure of CuI/Si heterojunctions. The CuI/Si heterojunctions were fabricated by growing the CuI film on Si (100) single crystal substrate through a facile vapor iodination method. The n-type Si substrate has a resistivity of 1-10 Ω·cm and a thickness of 0.5 mm. First, the Si substrate was covered by a mask with hole size of 1.7 mm×1.7 mm. Then, the Cu thin film with thickness of 20 nm was deposited by thermal evaporation. After that, the sample was placed together with iodine particles (I₂, ≥99.8%, Meryer) into a frosted clear glass vessel, which was heated up to 80 ℃ for the iodization. Similar processes have been previously reported^[19]. The Au electrode with a thickness of 100 nm was employed for ohmic contact.

Fig. 1(b) shows the schematic diagram of CuI/Si photodiode structure. Different monochromatic lasers with wavelengths of 400, 505, 635 and 780 nm were selected for testing the photoresponse, and the effective irradiation area was ~2.4 mm². The I-V characteristic curve of the photodiode was measured using the semiconductor parameter analyzer (Keithley 4200-SCS). The electrical properties of the CuI thin film and the Si substrate were evaluated by Hall effect measurement. The crystalline structure of the samples was analyzed by the X-ray diffraction (XRD, D/MAX-2550, Rigaku Corporation). The photoluminescence (PL) spectra were measured using a Jobin-Yvon LabRAM HR 800 UV micro-Raman instrument with 325 lasers as the excitation source.

Fig. 2(a) shows the XRD θ-2θ scans of the obtained CuI thin films with typical diffraction peaks corresponding to (111), (220), and (311) planes of γ-phase CuI^[27]. This result indicates that the sample is polycrystalline γ-CuI in the zinc-blende crystal structure. Fig. 2(b, c) show the optical properties of the CuI thin films investigated by optical transmission spectroscopy. The pure CuI sample exhibits an average transmittance of ~75% in the visible region with a near-band-edge emission near 410 nm. Fig. 2(d) shows the scanning electron microscope (SEM) image of the CuI film morphology on the Si surface, which demonstrates a relatively compact growth of the CuI film.

Despite of the polycrystalline nature, an ultra-high rectification ratio up to 7.6×10⁴ (±3 V) has been observed for the obtained p⁺n CuI/Si diode, as shown in Fig. 3. This result is almost the best for CuI-based heterojunctions except for the epitaxial diode in our previous report^[8]. The ideal factor of diodes can be deduced from the Shockley equation:

Where $j$ is the current of diode, j_s is the reverse saturation current density, η is the ideal factor, V is the voltage at the diode, ${{K}_{\text{B}}}$ is the Boltzmann constant, $T$ is the absolute temperature of the PN junction, and q is the amount of charge of the electron. In this work, the ideal factor η of ~2.1 indicates the presence of interfacial defects at the heterojunction region, which is similar with most reported heterojunctions made from wide-bandgap semiconductors^[28⇓-30]. In addition to the interfacial defects, other factors may also contribute to a large η, such as coupled defect-level recombination^[31], deep-level assisted tunneling effect^[32], and space-charge-limited conduction^[33]. Herein, the effect of interfacial defects should be dominative concerning the polycrystalline nature of the CuI thin film as well as the large lattice mismatch between CuI and Si. As shown in Fig. 2(b), there is a broad emission peak observed at ~700 nm. This is due to the deep-level defects associated with the iodine vacancies^[34-35].

Interestingly, it seems that these structural defects have little impact on the device performance, since the dark current shows a value within 10^-10-10^-9 A, which is significantly smaller than those of 10^-7-10^-5 A for other Si-based hetero-diodes in the literature^{[36⇓⇓-39]}. Such a high defect tolerance of the CuI/Si diode might be related to the distribution of the depletion region within the heterojunction. The carrier density of the p-CuI thin film (4.4×10¹⁹ cm^-3) is more than 4 orders of magnitude larger than that of the n-Si substrate (2.0×10¹⁵ cm^-3). That means the CuI/Si here forms a p⁺n diode or even a unilateral heterojunction. At zero bias, the Si side depletion depth is estimated to be ~744 nm, whereas the CuI side depletion depth is negligible.

Fig. 4 shows the I-V curves of the p⁺n CuI/Si photodiode under illumination of laser light with different wavelengths. A small incident light power density in the level of μW/cm² is enough to generate a significant photocurrent that is two to three orders of magnitude larger than the dark current, indicating the extremely high weak-light sensitivity of the CuI/Si diode. As a comparison, the detectable light intensity for most reported photodiodes is much higher (in the level of mW/cm²). The overall photocurrent induced by the 400 nm laser is small, which might be related to the insufficient diffusion length of the minority carriers in the polycrystalline CuI thin film.

At low reverse bias voltage within -0.5 V, the photocurrent rises as the incident laser wavelength increases and gets closer to the Si absorption edge (~1100 nm). Under negative bias voltage, the depletion region of the unilateral heterojunction extends to the CuI side, so that the deep-level defects (Fig. 2(b)) in the CuI thin film partially contribute to the absorption of visible light within 600-800 nm. The 635 nm laser produces a larger saturation photocurrent than the 780 nm laser, since shorter wavelength photons have a greater penetration depth.

The weak-light sensibility was further explored by adjusting the incident light power density. Fig. 5 shows the responsivity ${{R}_{\lambda }}$ as a function of light power density^[40].

Where ${{I}_{\lambda }}$ is the photocurrent, ${{P}_{\lambda }}$ is the light power density, and A is the effective contact area of the laser. The responsivity is remarkably high in weak light range (0.5–5 μW/cm²), and the most sensitive light power density is as low as 0.5 μW/cm². Due to equipment restrictions, data for lower light intensities were not gathered. Nevertheless, the existing data already demonstrate the device’s high sensitivity to weak light.

The spectral response of the CuI/Si photodiode is very different at zero or -3 V bias voltage. Under zero-bias voltage (Fig. 5(a)), the device is a unilateral heterojunction, and only visible light can be absorbed at the Si side. This is a typical “Visible” band response for Si. On the other hand, when a bias voltage of -3 V is applied (Fig. 5(b)), the depletion region extends to the CuI side. In this case, the photodiode is switched to a broader “UV-visible” band response mode, and all lasers at 400-780 nm are detectable. Therefore, the switch of different spectral response ranges can be achieved by adjusting the bias voltage.

The depletion regions of both CuI and Si side are extended at -3 V bias voltage, so the maximum responsivity at -3 V (4.7 A/W for 780 nm) is much higher than that at zero-bias voltage condition (1.15 A/W for 780 nm). However, stronger laser power density results in a lower responsivity, which might be related to the saturation of the photoelectric conversion. Defects in CuI are carrier trapping centers, and produce additional carriers due to the persistent photoconductivity effect. Under strong illumination, the defect trapping centers in CuI reach saturation, so no additional carriers are generated. The corresponding detectivity D* and external quantum efficiency (EQE) are directly proportional to the responsivity^[36] (Table 1).

Where e is the elementary charge, I_dark is the dark current, S is the effective contact area of the laser light, h is the Planck constant, c is the speed of light, and $\lambda $ is the wavelength of the laser.

The maximum D* is greater than 10¹⁴ for weak light illumination and ~10¹³ for strong light illumination. These values are record high and more than 100 times greater than other photodetectors made from Cu-based semiconductors, such as CuO photodetectors with D* < 10^11[41-42]. Owing to the weak light sensitivity of the obtained CuI/Si diode, the ultra-high EQE of 1109% for 400 nm laser and ~800% for 505-780 nm lasers are achieved at the “UV-visible” response mode (-3 V bias voltage). At the “Visible” response mode (zero bias voltage), the EQE are also high with a maximum of 175% for 635 nm laser. These responsivity and D* are significantly greater than those of reported Si-based photodiodes (Table 2), demonstrating the high performance of the CuI/Si diode obtained in this work.

In summary, a high-performance photodetector based on CuI/Si unilateral heterojunction has been fabricated using a facile solid-phase iodination method. This device exhibits a high rectification ratio of 7.6×10⁴ and a low dark current of 10^-10 A in spite of the polycrystalline structure of CuI. The unilateral diode structure allows for a dual-band switchable behavior. Under zero-bias voltage, the device is a unilateral heterojunction, and only visible light can be absorbed at the Si side. On the other hand, when a bias voltage of -3 V is applied, the photodiode is switched to a broader “UV-visible” band response mode. Therefore, the spectral response can be easily switched between “Visible” and “UV-visible” bands by adjusting the bias voltage. Moreover, the obtained CuI/Si diode is very sensitive to weak light illumination. The responsivity is remarkably high when the light power density is as low as 0.5 μW/cm². Ultra-high EQE values of 1109% for 400 nm laser and ~800% for 505-780 nm lasers are achieved at the “UV-visible” response mode. At the “Visible” response mode, the EQE values are also high with a maximum of 175% for 635 nm laser. These findings demonstrate the significant potential for applying CuI in combination with the traditional semiconductor industry.