A fast-response self-powered UV–Vis–NIR broadband photodetector based on a AgIn5Se8/t-Se heterojunction

Kang Li(李康), Lei Xu(许磊), Qidong Lu(陆启东), and Peng Hu(胡鹏)

School of Physics,Northwest University,Xi’an 710127,China

Keywords: AgIn5Se8/t-Se heterojunction,self-power broadband photodetector

1.Introduction

Based on the photoelectric effect, a photodetector, an electromagnetic detection energy sensor,converts optical signals into electrical signals, and has a wide range of applications in the industry and military fields,[1–7]such as missile guidance and infrared imaging.[8–10]Conventional photodetectors generally require a bias voltage,which results in the expansion and complexity of the photoelectrical device and also limits application in some special environments.[9,11]Furthermore,broadband detection(UV–Vis–NIR)is an effective way to minimize the volume of the photodetection system.[6,12–17]Therefore,self-powered broadband photodetectors are becoming an important development direction for future photodetectors.One way to achieve the self-powered photoresponse is to construct heterojunctions,[18–21]which lead to the rapid separation of the photogenerated electron–hole pairs by the built-in electric field at the heterojunction interface.[19–24]To realize broadband detection, one feasible way is to find narrow bandgap materials,[25–28]which have the ability to absorb light with a wide spectrum.AgIn5Se8(AIS),an n-type ternary semiconductor,[29]has a direct bandgap of 1.25 eV,[30]which enables photons to be excited from the valence band to the conduction band without the assistance of phonons.[31–35]Furthermore,AIS shows a high absorption coefficient,high optical conductivity and excellent light stability, which indicates that AIS demonstrates photodetection performance.[36–38]In our previous study, a AgIn5Se8/FePSe3heterojunction photodetector exhibited a fast photoresponse under broad spectral irradiation from 365 nm to 1020 nm.[36]However, the rough surface and the uncontrollable thickness of the FePSe3film resulted in poor quality of the heterojunction interfaces, which resulted in low responsivity and detectivity.

Selenide (Se), a p-type semiconductor, possesses a bandgap of about 1.7 eV with high electron mobility (∼0.14 cm2·V-1·s-1) and high carrier concentration (∼9.35×1016cm-3), and also shows significant conductivity change upon illumination.[39–45]Furthermore, due to chain-like molecules,Se tends to form low-dimensional structures,such as microtubules,nanoribbons and nanowires,which can effectively increase the contact area of heterojunctions and further increase the photodetection performance.[46,47]

In this work, continuous and dense trigonal-Se (t-Se)films were prepared by electrochemical deposition[48]and the AIS/t-Se heterojunction was further fabricated.The AIS/t-Se photodetector based on the heterojunction shows good selfpowered broadband photodetection and fast photoresponse performance atVbias=0 V.Compared with AIS/FPS heterojunction photodetectors,[36]theRandD*values of the AIS/t-Se heterojunction photodetectors increased 9 and 4 times, respectively.The better performance is mainly due to the formation of the t-Se nanoribbons and the further increased contact area of the heterojunction interface, which optimized the carriers’ transport path and increased the self-powered photoresponse.The results indicate that the AIS/t-Se heterojunction photodetector is a promising candidate for ultrafast,selfpowered and broadband photodetection.

2.Experiment details

2.1.Material preparation

The preparation of AIS was carried out according to our previous work.[36]The t-Se films were grownin situby electrochemical deposition[48]which was carried out in a three-electrode system.An ITO plate (1×2 cm2), Pt sheet(1×1 cm2) and Ag/AgCl were used as the working, counter and reference electrodes, respectively.A Na2SeO3solution(0.01 mol/L) was used as the electrolyte.The pH and the temperature for deposition were 2◦C and 80◦C,respectively.The electrochemical deposition potential was determined to be-0.7 V by cyclic voltammetry(CV),as shown in Fig.S1.

2.2.Device fabrication

The t-Se film was immersed in DI water for 10 min to remove the Na2SeO3on the surface,and 100 mg AIS flakes were dispersed in 500µL DI water via 30 min ultrasonic treatment as an AIS precursor.Then, the dispersed AIS precursor was drop-cast on the t-Se film.The AIS/t-Se heterojunction photodetector was obtained after drying for 30 min at 60◦C under vacuum.Silver was spot-coated on the surface of the AIS film and ITO glass as the electrodes,and the effective illumination area of the heterojunction photodetector was 5.8×10-3cm2(Fig.S2).

2.3.Characterization

The phases and morphology of the materials were analyzed using x-ray diffraction (XRD, DX-2700, Hao Yuan,China), scanning electron microscopy (SEM, FEI Apreo S)and high-resolution transmission electron microscopy (TEM,FEI Tecnai G2 F20).The elemental composition and atomic chemical states of the materials were analyzed using x-ray photoelectron spectroscopy (XPS, ESCALAB Xi+).Diffuse reflectance spectra and the optical bandgap of the materials were obtained using a UV–Vis–NIR spectrophotometer (PerkinElmer Lambda 950).Steady-state photoelectrical properties were measured using a KEYSIGHT B2901A source instrument and light-emitting diode lamps of different wavelengths,and a DHC GCI-73 electronic timer and a DHC GCI 7103M-B shutter were used to control light-switching.

3.Results and discussion

3.1.Characterization of AIS/t-Se

The phase purity of AIS was characterized by XRD, as shown in Fig.1(a).The strong diffraction peaks at 26.55◦,44.20◦, 52.29◦, 64.14◦, 70.56◦and 80.97◦correspond to the(112), (204), (116), (008), (316) and (228) lattice planes,[36]respectively, and no other peaks were observed, which indicated that the sample has a pure tetragonal phase,according to the JCPDS #191149.The SEM image (Fig.1(b)) shows that AIS has a multilayer stacked structure.The elemental energy dispersive x-ray(EDAX)mapping(Fig.1(c))analysis verifies the uniform distribution of Ag, In and Se in AIS.The TEM image (Fig.1(d)) also exhibits the multilayer structure of the AIS flakes.The HRTEM image (Fig.1(e)) shows the crystal lattice spacing of 0.331 nm,corresponding to the(112)plane of tetragonal AIS crystals.

The t-Se films were prepared by electrochemical deposition.Figures 2(a)–2(c) show the SEM images of t-Se films obtained at an electrodeposition voltage of-0.7 V for 1.5 h,which exhibited a hemisphere morphology (Fig.2(b)).With the deposition time increased, the edge and the center part of the t-Se hemisphere start to bulge and collapse(Fig.S3).Furthermore, more nanoribbons were formed to cover and connect the hemisphere (Fig.2(b)).The thickness of the t-Se films was increased with the deposition time,which is 1.3µm for the sample deposited for 1.5 h (Fig.2(c)).The XRD results(Fig.2(d))indicated that the t-Se films had a pure phase with a preferable growth crystal plane of (011).The star (*)peaks around 30◦–40◦in Fig.2(d) are attributed to ITO.The t-Se exhibits strong light absorption from 200 nm to 1200 nm(Fig.2(e)).The optical bandgap can be calculated using the conventional Tauc equation

whereαis the absorption coefficient,hνis the photon energy,Ais the constant,n=2 for a directly allowed transition andn=4 for an indirectly allowed transition.Then=2 was used to calculate the optical bandgap of t-Se because it is a direct bandgap semiconductor.[49]The corresponding transformed reflectance spectra are shown in the inset of Fig.2(e),

where the intersection of the horizontal axis and the tangent line is the bandgap energy.Consequently,the calculated optical bandgap of t-Se is about 1.68 eV.

The elementary composition and oxidation states of AIS and t-Se are identified by XPS.Two peaks are located at 374.1 eV and 368.1 eV,corresponding to Ag 3d3/2and 3d5/2,respectively (Fig.3(a)).In addition, the main peaks centered at 452.7 eV and 445.1 eV belong to 3d3/2and 3d5/2of In3+(Fig.3(b)), while two peaks at 55.2 eV and 54.1 eV can be assigned to Se 3d3/2and Se 3d5/2of Se2-(Fig.3(c)).Figure 3(d) shows the high-resolution XPS spectra of Se 3d in t-Se.The Se 3d5/2and Se 3d3/2are located at 54.8 eV and 55.7 eV,respectively,representing the t-Se.Due to the different valence states, the position of the peak is different from that of the Se 3d peak in AIS.[50,51]

3.2.Photodetection performances of photodetectors

A schematic diagram of the device structure of the AIS/t-Se heterojunction photodetector is shown in Fig.4(a).The electrical and optical detection properties of the heterojunction devices were examined using a two-probe method under ambient conditions.The electrodeposition time dramatically affects the optoelectronic performance of the photodetector (Fig.S4).The device showed the largest photocurrent when the deposition time was 1.5 h at a deposition voltage of-0.7 V.The AIS/t-Se heterojunction photodetector exhibits a photocurrent of 80 nA at a bias voltage of-5 V under 780 nm illumination and a broadband photoresponse from 365 nm to 1200 nm (Fig.4(b)).The short circuit current and open circuit voltage are 3.46 nA and-0.36 V,respectively,indicating the broadband self-powered characteristics of the devices(inset of Fig.4(b)).The self-powered behavior of the AIS/t-Se heterojunction photodetector is mainly due to the separation of photocarriers by the built-in electric field of the heterojunction.The time-dependent photoresponse of the heterojunction device at different wavelengths at 0 V is shown in Fig.4(c),which also indicates the broadband self-powered performance with the best photoresponse at 780 nm.

The single period time-resolved photocurrent on/off behavior (Fig.4(d)) shows a clear on-state and off-state under light illumination and in the dark, respectively.The rise time(τr)and decay time(τd)are defined as the time difference between the photocurrent rising from 10% to 90% and falling from 90%to 10%,respectively.The rise time and decay time(τr/τd) of the AIS/t-Se devices are estimated to be 0.2 s and 0.04 s, respectively.The multi-periodI–tcurves (Fig.4(e))of the photodetector under 780 nm illumination indicate good stability and reproducibility with light switching.TheI–tcurves of the heterojunction photodetector under 780 nm illumination with different optical power densities atVbias=0 V are shown in Fig.4(f).The photocurrent and the on/off ratio increase to 2 nA and 260 at an optical power density of 9.9 mW/cm2.The relationship between the optical power density and the photocurrent is shown in Fig.4(g), which can be fitted by the function ofIph=Pα.The parameterαis calculated to be 0.61,indicating the existence of defects and/or trap states inside the photodetector.

Responsivity (R) and detectivity (D*) are two important parameters that are used to evaluate the performance of a photodetector,which can be calculated using the following equations:

whereIphis the photocurrent,Pis the optical power density,Sis the effective illumination area, andeis the fundamental charge value.Figure 4(h)shows theRandD*of the photodetector under different wavelengths of light at a bias voltage of 0 V.The dark current is 0.78×10-11A and the photocurrent is 1.81×10-9A, respectively.The device exhibited maximumRandD*values of 32µA/W and 1.8×109Jones under 780 nm illumination,respectively,which showed an increment of about 9 and 4 times, respectively, compared with those of the AIS/FePSe3devices.TheRandD*are not only related to the photocurrent,but also to the optical power density.Therefore,the trends in the photocurrent with different wavelengths are different from the trends in theRandD*.Detailed values of optical power densities at different wavelengths are shown in Table S1.Compared with other typical thin-film heterojunction self-powered photodetectors, the optoelectronic performances are comparable to or better than most of the previous results(Table S2),which could be useful in fast broadband photodetection.The performance improvement is attributed to the formation of the t-Se nanoribbons and the relatively smooth t-Se film at the interface.Thus, the contact area of the heterojunction interface is increased,which optimizes the carriers’transport path and increases the photoresponse.

As shown in Figs.4(i)and S5,the performance of the heterojunction photodetector was tested by controlling the on/off speed of the optical switch atVbias=0 V with different frequencies of incident light.When the on/off speed is 10 s(0.1 Hz), 1 s (1 Hz) and 0.1 s (10 Hz), the device exhibits a significant photoresponse to 780 nm light.In addition,different frequencies of incident light affect the magnitude of the photocurrent.With different frequencies of 0.1 Hz,1 Hz,and 10 Hz, our photodetector showed a fast switching response and good reproducibility.The photocurrent of the device decreased by only 6% at 1 Hz compared to 0.1 Hz, which is the result of recombination due to trap densities in AIS thin film.[52]The results indicated that the AIS/t-Se heterojunction photodetectors have the ability to detect flickering light signals.

To understand the self-powered mechanism of the AIS/t-Se photodetector,the energy band diagram of the AIS/t-Se heterojunction is shown in Fig.5.The work function, valence band position and the bandgap of AIS are 4.43 eV, 1.22 eV and 1.34 eV, respectively.[36]The work function of t-Se is 5.07 eV, determined by UPS measurements (Fig.S6(a)), and the valence band position is 0.28 eV(Fig.S6(b)),by XPS.The energy band arrangement of the AIS/t-Se heterojunction photodetector is obtained by considering the above values and the bandgap of t-Se.Since the Fermi energy level of AIS is higher than that of t-Se,the electrons drift from AIS to t-Se after contact until the Fermi energy level reaches the same level.At the same time, a built-in electric field is formed at the interface of the heterojunction with the direction from AIS to t-Se by the diffusion of carriers.When the energy of the light is larger than the bandgap of the AIS,the photogenerated electron–hole pairs separate rapidly due to the presence of the built-in electric field.[53]With the electrons moving toward the conduction band of the AIS and the holes moving toward the valence band of t-Se,a photovoltage is formed at the interface of the heterojunction,resulting in a self-powered optical response.

4.Conclusion

In summary, continuous and dense t-Se thin films were prepared via an electrochemical deposition method using 0.01 mol/L Na2SeO3solution as the electrolyte.When the electrochemical was deposited at-0.7 V for 1.5 hours,a large number of t-Se nanoribbons were formed, which resulted in the best photoelectrical performance of the AIS/t-Se heterojunction device.The AIS/t-Se photodetector demonstrates a self-powered performance under broad spectral irradiation from 365 nm to 1020 nm with the bestRandD*values of 32 µA/W and 1.8×109Jones, respectively, under 780 nm at 0 V.This result indicates that the t-Se nanoribbons significantly enhance the performance of the heterojunction photodetector.The AIS/t-Se heterojunction device demonstrates better optoelectronic performance than the AIS/FPS heterojunction device, showing about 9 and 4 times increases inRandD*,respectively.This work provides a potential approach to realize ultrafast and self-powered optoelectronic devices based on AIS.

Acknowledgments

Project supported by the National Natural Science Foundation of China (Grant No.51803168), the Key Research and Development Program of Shaanxi Province(Grant No.2022GY-356),and the Youth Innovation Team of Shaanxi Universities.