• Journal of Semiconductors
  • Vol. 45, Issue 12, 122402 (2024)
Yifan Yao, Suhao Yao, Jiaqing Yuan, Zeng Liu, Maolin Zhang, Lili Yang*, and Weihua Tang**
Author Affiliations
  • Innovation Center of Gallium Oxide Semiconductor (IC-GAO), College of Integrated Circuit Science and Engineering, Nanjing University of Posts and Telecommunications, Nanjing 210023, China
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    DOI: 10.1088/1674-4926/24050048 Cite this Article
    Yifan Yao, Suhao Yao, Jiaqing Yuan, Zeng Liu, Maolin Zhang, Lili Yang, Weihua Tang. Self-powered PEDOT:PSS/Sn:α-Ga2O3 heterojunction UV photodetector via organic/inorganic hybrid ink engineering[J]. Journal of Semiconductors, 2024, 45(12): 122402 Copy Citation Text show less

    Abstract

    In this work, a PEDOT:PSS/Sn:α-Ga2O3 hybrid heterojunction diode (HJD) photodetector was fabricated by spin-coating highly conductive PEDOT:PSS aqueous solution on the mist chemical vapor deposition (Mist-CVD) grown Sn:α-Ga2O3 film. This approach provides a facile and low-cost p-PEDOT:PSS/n-Sn:α-Ga2O3 spin-coating method that facilitates self-powering performance through p?n junction formation. A typical type-Ⅰ heterojunction is formed at the interface of Sn:α-Ga2O3 film and PEDOT:PSS, and contributes to a significant photovoltaic effect with an open-circuit voltage (Voc) of 0.4 V under the 254 nm ultraviolet (UV) light. When operating in self-powered mode, the HJD exhibits excellent photo-response performance including an outstanding photo-current of 10.9 nA, a rapid rise/decay time of 0.38/0.28 s, and a large on/off ratio of 91.2. Additionally, the HJD also possesses excellent photo-detection performance with a high responsivity of 5.61 mA/W and a good detectivity of 1.15 × 1011 Jones at 0 V bias under 254 nm UV light illumination. Overall, this work may explore the potential range of self-powered and high-performance UV photodetectors.

    1. Introduction

    The ultraviolet C (UV-C) photodetectors (PDs) with operating wavelengths in the range from 220 to 280 nm are widely applied in the fields of industrial inspection, environmental monitoring, biochemical analysis and space exploration owing to their high security, high sensitivity and strong anti-interference capabilities[14]. The self-powered UV-C PDs are also arousing widespread interest among scholars due to their advantages as new energy-saving and green PDs that can be self-driven without applying external power. Gallium oxide (Ga2O3) is a popular and suitable material for UV-C PDs, has received more and more attention because of its ultra-wide bandgap of 4.5 to 5.3 eV, which can be tuned by many means such as doping, alloying, external field control, etc. Typically, Ga2O3 self-powered PDs can be realized by establishing built-in electric field between p−n homojunction interface, heterojunction interface or Schottky junction interface. The difference depends on whether the two sides of the interface are the same semiconductor material, different semiconductor materials, or metal/semiconductor. Ga2O3 homogeneous p−n junction is temporarily difficult to achieve because p-type doping still faces huge challenges. Meanwhile, the types of metals that can form Schottky barriers with Ga2O3 are relatively limited because of the differences in the work function, residual impurities, crystal defects, and chemical reactions forming oxides and eutectics[5]. Therefore, the limited metals such as Pt, Ni, Au, Cu, W, and lr are commonly used to form Schottky barriers with Ga2O3[6]. In contrast, organic semiconductors are more diverse and have richer band structures, offering a potential choice. Ga2O3-based heterojunction usually has an important impact on the generation and separation of carriers due to the different band structures of materials at the interface, which enables better performance in carrier separation, dark current suppression, and light absorption by utilizing the energy band structure of different materials[7, 8]. Zinc oxide (ZnO) and Iridium oxide (Ir2O3) have n-type and p-type conductivity respectively, are often preferred in the forming of n−n and p−n heterojunctions with Ga2O3, but they are costly, have limited performance and require high fabrication processes[9, 10]. Moreover, the problems of lattice matching, thermal expansion coefficient matching and critical heterogeneous film thickness limitation have to be considered during epitaxial growth on Ga2O3 surface. Spin-coating technique is a facile and efficient solution for heterojunction preparation, and it has been reported that Ga2O3 heterojunction PDs have been successfully prepared by this solution with CuSCN, BiFeO3, and CuBil4, but these materials are less stable and more complicated to obtain[1113]. As an alternative material, the p-type PEDOT:PSS with low cost, high conductivity, high stability and good transparency for use as conductive and transmission layers enables the fabrication of heterojunction by spin-coating[1416].

    Among the five different polymorphs of Ga2O3[17], α-Ga2O3 possesses an ultra-wide bandgap of 5.2 eV and the high-quality α-Ga2O3 film is usually prepared by the mist chemical vapor deposition (Mist-CVD) technology[18, 19], which can also be used to effortlessly obtain the element-doped Ga2O3 film by mixing the suitable dopant source into the ink-like Ga-contained aqueous precursor solution. Thus, Sn doping α-Ga2O3 is easily realized by Mist-CVD technology, which facilitates the n-type conductivity, up-shifted Fermi energy and higher carrier transport and injection efficiency[2022]. Furthermore, the p-PEDOT:PSS/Sn:α-Ga2O3 has not yet been reported, which could broaden the potential range of self-powered PDs.

    In this work, a PEDOT:PSS/Sn:α-Ga2O3 hybrid heterojunction diode photodetector (HJD) was fabricated, based on Sn:α-Ga2O3 film deposited by Mist-CVD and PEDOT:PSS aqueous solution. In Mist-CVD system, the crystalline phase of the grown Ga2O3 can be decided by the used substrate, so Sn:α-Ga2O3 films were deposited on inexpensive sapphire (α-Al2O3) substrates and the quality of the thin film was characterized[23]. Afterwards, PEDOT:PSS was spin-coated onto the prepared Sn:α-Ga2O3 thin film to form the hybrid heterojunction, and electrodes were further added to derive the In-PEDOT:PSS/Sn:α-Ga2O3-In HJD. Through measuring the performance of the device, we found it achieves excellent self-powered capabilities. Meanwhile, we fabricated another In-Sn:α-Ga2O3-In PD for performance comparison.

    2. Experiments and characterization

    The Sn:α-Ga2O3 film was deposited on a c-plane α-Al2O3 substrate by a self-assembly Mist-CVD system, using Gallium acetylacetonate (Ga(acac)3) and tin (Ⅱ) chloride dihydrate (SnCl2·2H2O) as the gallium and dopant source respectively. The concentration ratio of SnCl2·2H2O and Ga(acac)3 was set as 4% which were dissolved in DI water and a little hydrochloric acid (HCl) was added to promote dissolution. The experimental and fabrication procedures are shown in Fig. 1, and more details are reported in Ref. [24]. Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)(PEDOT:PSS, Clevios PH1000) aqueous solution as a p-type material, was spin-coated directly onto the prepared Sn:α-Ga2O3 thin film at 3000 rpm, and then annealed for 10 min on a heating table set at 105 °C. Finally, a pair of In electrodes were deposited onto the surface to fabricate the HJD PD. For comparison, the comparative In-Sn:α-Ga2O3-In PD was also prepared using similar methods.

    (Color online) (a) Experimental procedures of the Mist-CVD. (b) Fabrication process of the HJD−PD.

    Figure 1.(Color online) (a) Experimental procedures of the Mist-CVD. (b) Fabrication process of the HJD−PD.

    The light absorbance properties of both materials were acquired by UV−visible (UV−vis) absorbance spectrum, and the bandgap of Sn:α-Ga2O3 was therefore calculated. The crystal quality of Sn:α-Ga2O3 was evaluated by X-ray diffraction (XRD) test. The surface and crossing section morphologies of both materials were characterized via scanning electron microscope (SEM). The elementary composition of the Sn:α-Ga2O3 film was measured and characterized via X-ray photoelectron spectroscopy (XPS). Moreover, the energy band diagram was estimated with the help of ultraviolet photoelectron spectroscopy (UPS). After the preparation and certification of the PEDOT:PSS/Sn:α-Ga2O3 HJD, the optoelectronic properties were analyzed using a 254 nm UV lamp and a Keithley 2450 semiconductor analyzer.

    3. Results and discussions

    The UV−vis absorbance spectrum of PEDOT:PSS is displayed in Fig. 2(a), corresponding to two absorption peaks clearly visible at 200 and 230 nm, which is also consistent with the property of the reported PEDOT:PSS[25]. The UV−vis absorbance spectrum of Sn:α-Ga2O3 is illustrated in Fig. 2(b), from which the optical Eg of the Sn:α-Ga2O3 thin film can be calculated by the Tauc’s relationship[26]:

    (Color online) Optical absorbance spectra of (a) PEDOT:PSS film and (b) Sn:α-Ga2O3 film (the inset calculates the Eg of the Sn:α-Ga2O3 film). (c) XRD pattern of Sn:α-Ga2O3.

    Figure 2.(Color online) Optical absorbance spectra of (a) PEDOT:PSS film and (b) Sn:α-Ga2O3 film (the inset calculates the Eg of the Sn:α-Ga2O3 film). (c) XRD pattern of Sn:α-Ga2O3.

    (αhν)2=C(hνEg).

    PhotodetectorTechnologyDeviceId (A)R (mA/W)D* (Jones)τr/τd (s)Ref.
    PEDOT:PSS/Sn:α-Ga2O3Mist-CVDHJD@0 V7.06 × 10−115.61.14 × 10110.36/0.09this work
    PEDOT:PSS/Ga2O3NWs/n-SiCVDHJD@0 V1 × 10−1026.81.4 × 10120.017/0.038[46]
    Ga2O3/Bi2WO6MOCVDHJD@0 V6.3 × 10−152.210.132/0.069[47]
    Spiro/Ga2O3/SiMOCVDHJD@0 V4.330.03/0.196[48]
    β-Ga2O3 nanoflakes/p-SiHYDROTHERMALHJD@0 V2.33 × 10−90.16[49]
    MoS2/β-Ga2O3HJD@0 V9 × 10−132.051.21 × 1011[50]
    PEDOT: PSS/Ga2O3 microwireEFGHJD@0 V39.82.4 × 10125.3 × 10−4/6.75 × 10−3[25]
    PEDOT:PSS/ Ga2O3MOCVDHJD@0 V3.7 × 10−1237.49.2 × 10123.3 × 10−6/7.12 × 10−5[51]

    Table 1. Performance comparison of Ga2O3-based heterojunction PDs.

    where h is Planck's constant and ν is the frequency of incident light, and represents the photon energy of the incident light. By extrapolating the function of (αhν)2 and , Eg of the Sn:α-Ga2O3 thin film can be evaluated as 5.29 eV, demonstrated in the Fig. 2(b) inset, which is a typical value for α-Ga2O3. Furthermore, by analyzing the XRD pattern of the Sn:α-Ga2O3 thin film shown in Fig. 2(c), it was found that a sharp peak corresponding to α-Ga2O3 appeared at 40.26°, thus determining the crystalline phase of this thin film. Meanwhile, there is no peak at 39.02° which belongs to β phase, indicating the high purity of α-Ga2O3 film. The α-Ga2O3 diffraction peak, with a Laue edge around 40.3°, is higher than this work[27]. This indicates that the representative diffraction peaks of α-Ga2O3 are shifted to a smaller 2θ angle as Sn doping occurs, implying that the substitutional doping of Sn in α-Ga2O3 increases the lattice constant of α-Ga2O3[28]. Full width at half maximum (FHWM) was also calculated by Gaussian fitting to evaluate the crystal quality, which was determined to be 163 arcsec. Such a result was relatively high compared with other results due to the existence of Sn dopants. The surface SEM photograph of the Sn:α-Ga2O3 thin film is illustrated in Fig. 3(a). The surface of Sn:α-Ga2O3 is not smooth and micro voids can be found, which may be due to the vacuum-free growth conditions. The interface SEM photograph of the PEDOT:PSS/Sn:α-Ga2O3 was illustrated in Fig. 3(b), the interface line can be viewed clearly, indicating PEDOT:PSS was successfully coated on the Sn:α-Ga2O3. Even though, as illustrated in Fig. 3(c), the cross-section SEM photograph reveals that there is no obvious variation in the thickness of the Sn:α-Ga2O3, which is extracted as 467 nm. Meanwhile the thickness of the PEDOT:PSS is extracted as 100 nm.

    (Color online) (a) Surface of the Sn:α-Ga2O3 thin film and (b) interface of Sn:α-Ga2O3 and PEDOT:PSS. (c) Cross-section SEM photograph at the interface of PEDOT:PSS and Sn:α-Ga2O3.

    Figure 3.(Color online) (a) Surface of the Sn:α-Ga2O3 thin film and (b) interface of Sn:α-Ga2O3 and PEDOT:PSS. (c) Cross-section SEM photograph at the interface of PEDOT:PSS and Sn:α-Ga2O3.

    To further investigate the elemental composition of the Sn:α-Ga2O3 thin film, the XPS spectrum of the film was shown in Fig. 4(a). According to the result, the Sn 3d peak does not appear probably because the Sn doping concentration is below the XPS minimum detection limit. Furthermore, X-ray fluorescence (XRF) spectroscopy was used to determine the Sn doping concentration and the results showed that the atomic ratio of Ga to Sn was 2921 : 1. The binding energies of Ga 2p1/2, Ga 2p3/2, and O 1s are 1144.85, 1118.25, and 530.83 eV, as shown in Figs. 4(b) and 4(c), respectively. The O 1s peak can be fitted into O (lattice oxygen) and O (oxygen vacancies), and the ratio of O and (O + O) is ~38.6%, higher than the regular α-Ga2O3 thin film and is possibly due to donor doping[29]. Moreover, the energy band diagram of Sn:α-Ga2O3 was further identified by measuring the energy distribution of the electrons by UPS, and the result is displayed in Fig. 4(d). The calculated work function of Sn:α-Ga2O3 is 21.22 − 18.07 = 3.15 eV, where 21.22 is the energy value of helium discharge lamp photons. Additionally, the valence band edge value (relative to the Fermi level) is extracted to be 5.15 eV[30]. Meanwhile, PEDOT:PSS is widely considered as a p-type organic material[31], indicating a p−n heterojunction is formed.

    (Color online) (a) The XPS spectrum of the sample of the Sn:α-Ga2O3 film. (b) The high-resolution XPS spectrum of the Ga 2p1/2 and Ga 2p3/2. (c) The high-resolution XPS spectrum of the O 1s. (d) The UPS spectrum of the Sn:α-Ga2O3 film.

    Figure 4.(Color online) (a) The XPS spectrum of the sample of the Sn:α-Ga2O3 film. (b) The high-resolution XPS spectrum of the Ga 2p1/2 and Ga 2p3/2. (c) The high-resolution XPS spectrum of the O 1s. (d) The UPS spectrum of the Sn:α-Ga2O3 film.

    To research the photoelectric properties of the PEDOT:PSS/Sn:α-Ga2O3 HJD PD and the contradistinctive Sn:α-Ga2O3 PD, the curves of the voltage−current with variable light power density under 254 nm light are shown in Fig. 5. The logarithmic IV curve of the HJD is illustrated in Fig. 5(a). Under the 254 nm light, as the light intensity of the UV lamp increases, the photocurrent also increases gradually and reaches a maximum value of 1.29 μA when the light intensity reaches 266.3 μW/cm2. The dark current of the HJD is high, reaching 70 pA at 0 V. The approximately linear coordinate curve of the IV was also illustrated in Fig. 5(a), which displays that the HJD demonstrates an excellent photovoltaic effect and has a high open-circuit voltage (Voc) of 0.4 V, suggesting the self-powered mode of this HJD[32]. To explain clearly the unique properties of HJD, the comparable Sn:α-Ga2O3 PD is also researched. As illustrated in Fig. 5(b), the IV curve of Sn:α-Ga2O3 PD exhibits approximately linear, thus demonstrating a typical Ohmic contact between the Sn:α-Ga2O3 film and the electrode. Compared with the dark situation, the photocurrent displays a conspicuous increase with variable light power density under 254 nm light, displaying the intrinsic properties of Sn:α-Ga2O3 thin films[33, 34]. It can also be found that the dark current of the PEDOT:PSS/Sn:α-Ga2O3 HJD is lower than that of the Sn:α-Ga2O3 PD, which indicates the intrinsic properties of heterojunction photodetectors.

    (Color online) Logarithmic coordinate I−V curve of (a) HJD (the inset is the linear coordinate I−V curve) and (b) Sn:α-Ga2O3 PD. (c) R, (d) EQE, (e) D*, and (f) the functional relationship between the Iph and light intensity (the inset is the relationships at different devices and bias).

    Figure 5.(Color online) Logarithmic coordinate I−V curve of (a) HJD (the inset is the linear coordinate I−V curve) and (b) Sn:α-Ga2O3 PD. (c) R, (d) EQE, (e) D*, and (f) the functional relationship between the Iph and light intensity (the inset is the relationships at different devices and bias).

    To further investigate the photoelectric performance of the HJD in self-powered mode, key parameters such as the responsivity (R), external quantum efficiency (EQE), detectivity (D*), and the functional relationship between the Iph and light intensity were calculated and shown in Figs. 5(c)−5(f). The formulas are displayed in the Eqs. (2)−(5)[35, 36] respectively.

    R=IphIdSP,

    EQE=hcReλ,

    D*=RS2eId,

    IphPθ.

    Here Iph represents the photocurrent, while Id stands for the dark current. P denotes the light intensity, and S refers to the effective illumination area. Constants include h for Planck's constant, c for the speed of light, and e for the electron charge. λ represents the wavelength of the incident light, and θ represents the influencing factor. The diameter of the electrodes and the distance of the electrodes are all set as 1 mm, thus the illumination area S is considered as 1 mm2. As shown in Figs. 5(c)−5(e), the R, EQE, and D* decrease with increasing light intensity, with maximum values of 5.61 mA/W, 1.9%, and 1.15 × 1011 Jones at a light intensity of 32.2 μW/cm2 with 0 V bias, respectively. This phenomenon can be explained by increased carrier scattering and higher electron−hole recombination probability due to increased light intensity[37, 38], which can also be confirmed by the linear relationship between Iph with the illumination intensity with an influence factor θ of about 0.94. On the other hand, the greater the carrier concentration, the lower the mobility, which further contributes to this phenomenon. The results demonstrated the hybrid heterojunction diode possessed a nearly linear response over a wide range of light intensities. However, while using the Mist-CVD to grow the Sn:α-Ga2O3 thin film, elements such as the non-vacuum nature of the growth environment, inappropriate growth temperature, and incomplete reaction of the precursors may lead to the generation of oxygen vacancies, which results in high dark current[39]. Although Sn dopant has similar atomic radii to Ga, the high-concentration doping will lead to a large number of defects that need to be finely tuned[40], so the performance of the HJD can be further enhanced by improving film growth and solution doping process to reduce the dark current. However, at lower Sn doping concentrations, the improvement in the electrical conductivity of Ga2O3 can also be significant. By further comparing the performance between different devices in Fig. 5 inset, they all exhibit similar properties to the self-powered state, and the R, EQE, D*, and photocurrent of the Sn:α-Ga2O3 PD are found to be higher than HJD through comparison. This may be due to the lack of sophistication in the preparation process as well as defects in the interface between the Sn:α-Ga2O3 thin film and the PEDOT:PSS, which affects the carrier mobility and lifetime, and consequently reduces the performance of the HJD[41].

    The transient response of the PEDOT:PSS/Sn:α-Ga2O3 HJD and the Sn:α-Ga2O3 PD at varying bias with illumination intensities ranging from 32.2 to 266.3 μW/cm2 are displayed in Fig. 6. Under four cycles with a period of 20 s at varying light intensities, both the HJD and Sn:α-Ga2O3 PDs immediately respond to the switching transition and generate the corresponding current, highlighting the exceptional sensitivity and uniformity of the PDs. The Iph increases with the bias voltage due to the change in the average internal electric field in response to the external bias voltage, which provides the drift energy for the carriers[32]. The Fig. 6(a) shows the transient response of HJD at 0 V. As illustrated in Figs. 6(b) and 6(c), the Iph of HJD are about −0.23 and 0.38 μA with the light intensity of 266.3 μW/cm2 at −5 and 5 V bias, respectively. However, as demonstrated in Fig. 6(d), the photocurrent of Sn:α-Ga2O3 PD could reach 0.91 μA under the same conditions. The reason for the discrepancy is similar to those mentioned earlier, because the formation of heterojunctions introduces interfacial states that may affect carrier transport and charge injection, thus reducing the photocurrent of the device[41]. The switching ratio (Ron/off) is another important parameter of the photodetector, which is defined as the ratio of device's on-state current to its off-state current for one complete switching cycle. The Ion and Ioff of the HJD under a light intensity of 266.3 μW/cm2 at 0 V were 11.4 and 0.125 nA, so the Ron/off can be calculated as 91.2. The great ratio switched between the on and off states of the HJD demonstrated excellent reproducibility and rapid speed. It was also found that Ron/off is smaller than PDCR, which is because the Id does not drop to the previous level immediately after UV exposure and may recover slowly and increase over successive It measurements. This phenomenon is largely attributed to the effects of the persistent photoconductivity effect (PPC)[42]. The long waiting time for the test leads to this discrepancy, and as previously stated, the Sn doping increases the dark current, causing this phenomenon to be more pronounced.

    (Color online) I−t curves of the HJD under different light intensities and different biases, (a) 0 V, (b) −5 V, and (c) 5 V. (d) I−t curves of the Sn:α-Ga2O3 PD under different light intensities and 5 V bias.

    Figure 6.(Color online) I−t curves of the HJD under different light intensities and different biases, (a) 0 V, (b) −5 V, and (c) 5 V. (d) I−t curves of the Sn:α-Ga2O3 PD under different light intensities and 5 V bias.

    To further investigate the transient characteristics of PD, the rise time (10%–90% of the maximum current level) and decay time (90%–10% of the maximum current level), as the key parameters for PD, could be fitted by the biexponential relaxation equation[43]:

    I=I0+Aet/τ1+Bet/τ2.

    Here I0 refers to the saturation Iph, A and B are the constants, t is the transient time, and τ1 and τ2 are the relaxation time constants. As illustrated in Fig. 7, the data for the fitting curve was obtained under varying bias conditions. At 0 V bias, the rise and decay time of the HJD are 0.38 and 0.28 s, as shown in Fig. 7(a). At −5 and 5 V bias, the rise and decay time of the HJD are 0.14 and 0.30 s, 0.32 and 0.32 s, respectively, as shown in Figs. 7(b) and 7(c). Meanwhile, as a comparison, the rise time and decay time of the Sn:α-Ga2O3 PD are 0.45 and 0.36 s, as shown in Fig. 7(d). By comparison, it can be seen that the rise and decay times of HJD are shorter than those of Sn:α-Ga2O3 PD, demonstrating that the realization of HJD's rapid response on account of the advanced p−n junction formed at the interface[44]. Through comparing the performances of the above, the HJD has lower dark current and faster response time, and can facilitate efficient separation of photogenerated carriers and improve charge collection efficiency at metal−semiconductor interfaces by exploiting energy band shifts and electric field effects between materials[45]. So the HJD PD is of more importance for self-powered applications than Sn:α-Ga2O3 PD. A comprehensive performance comparison between the PEDOT:PSS/Sn:α-Ga2O3 PD and other recently reported Ga2O3-based heterojunction PDs is summarized in Table 1.

    (Color online) Fitted curve of the current rise and decay processes responding to 254 nm light of HJD under (a) 0 V, (b) −5 V, and (c) 5 V. (d) Sn:α-Ga2O3 PD under 5 V bias.

    Figure 7.(Color online) Fitted curve of the current rise and decay processes responding to 254 nm light of HJD under (a) 0 V, (b) −5 V, and (c) 5 V. (d) Sn:α-Ga2O3 PD under 5 V bias.

    To illustrate the operation of this HJD, the PEDOT:PSS/Sn:α-Ga2O3 hybrid heterojunction energy band schematic is shown in Fig. 8. The lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of PEDOT:PSS are reckoned to be 5.2 and 3.3 eV, respectively[52]. The χ is the electron affinity energy of Sn:α-Ga2O3, which can be defined as the energy difference between the vacuum energy level and the bottom of the conduction band, here calculated as 3.01 eV. Fig. 8(a) shows the energy band alignment of the PEDOT:PSS and Sn:α-Ga2O3 before contact. Where Ec, Ev, Eimp, EF, and W denote the conduction band, valence band, impurity energy level, Fermi level, and work function, respectively. The W is the energy difference between the vacuum energy level and the Fermi energy level, and is 3.15 eV. As the heterojunction forms, the EF at the interface undergoes realignment, which leads to the establishment of a classic type-Ⅰ heterojunction in the PEDOT:PSS/Sn:α-Ga2O3 system as presented in Fig. 8(b). To form such type-Ⅰ heterojunction, the energy bands of p-PEDOT:PSS should be shifted upwards and bent downwards at the interface, while the energy bands of n-Sn:α-Ga2O3 should be shifted downwards and bent upwards at the interface. When p-PEDOT:PSS contacts n-Sn:α-Ga2O3, holes in the p region diffuse to the n region, while electrons diffuse in the opposite direction, resulting in positively charged donor ions in the n region and negatively charged acceptor ions in the p region. This diffusion process forms a depletion region without free carriers near the interface of the p−n junction. Due to the presence of fixed charged ions, a built-in electric field is also generated to keep the balance of the charge carrier drift and diffusion. Because of the existence of the built-in electric field, the electrons have additional potential energy at various points in the depletion layer, which bends the energy bands. The total amount of bending qVD is defined as the difference in Fermi energy levels, counted as 1.62 eV. Compared with intrinsic Ga2O3, the large amount of Sn will introduce an impurity level, which provides additional conducting paths and trap states. When p-PEDOT:PSS contacts n-Sn:α-Ga2O3, holes in the p region diffuse to the n region, while electrons diffuse in the opposite direction, resulting in a depletion region without free carriers near the interface of the p−n junction. Due to the presence of fixed charged ions, a built-in electric field in the direction of Sn:α-Ga2O3 pointing towards the PEDOT:PSS is also generated to keep the balance of the charge carrier drift and diffusion, and electrons are gradually captured during drift from PEDOT:PSS to Sn:α-Ga2O3. The impurity level can also increase the photo-generated carriers and improve the electrical conductivity[53]. Also for self-powered photodetectors, the barrier height is a significant factor. Due to the higher concentration of free carriers, the Fermi level of Sn-doped α-Ga2O3 is closer to the conduction band than that of intrinsic α-Ga2O3, and therefore the barrier height of the junction is also larger[54]. Under 254 nm light, the incident photons are absorbed to produce a large amount of electron−hole pairs, which are rapidly separated by the built-in electric field, as shown in Fig. 8(c). If this heterojunction is connected to a circuit, the separated electron−hole pairs move towards the external circuit, forming a photogenerated current[45]. Thus, the photogenerated current can then support this HJD to be able to operate in self-powered mode.

    (Color online) Schematic energy band diagrams of the PEDOT:PSS/Sn:α-Ga2O3 hybrid heterojunction under (a) before contact, (b) dark, and (c) ultraviolet illumination at 0 V bias.

    Figure 8.(Color online) Schematic energy band diagrams of the PEDOT:PSS/Sn:α-Ga2O3 hybrid heterojunction under (a) before contact, (b) dark, and (c) ultraviolet illumination at 0 V bias.

    4. Conclusions

    In summary, by vacuum-free Mist-CVD technology, highly sensitive α-Ga2O3 thin film is acquired, and based on it, a low-cost p-PEDOT:PSS/n-Sn:α-Ga2O3 spin-coating method is provided. UV-C PD based on such wholly ink engineering exhibited competed performances compared with those based on vacuum system, exhibiting high R, EQE, and D* of 5.61 mA/W, 1.9%, and 1.15 × 1011 Jones in self-powered mode, respectively. Moreover, HJD owns a quick rise/decay time of 0.38/0.28 s and a high Ron/off of 91.2, which demonstrates excellent photodetection performance. Additionally, The HJD has a high Voc of 0.4 V and good stability in ambient air. This work extends the potential of UWBG Ga2O3, and the device parameters can be further investigated to improve performance.

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    Yifan Yao, Suhao Yao, Jiaqing Yuan, Zeng Liu, Maolin Zhang, Lili Yang, Weihua Tang. Self-powered PEDOT:PSS/Sn:α-Ga2O3 heterojunction UV photodetector via organic/inorganic hybrid ink engineering[J]. Journal of Semiconductors, 2024, 45(12): 122402
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