Enhanced photoresponse in a Ag2S/In2Se3 heterojunction based visible light photodetector

Abstract

Ag₂S and In₂Se₃, two prominent functional materials, have recently gained extensive research attention. Ag₂S shows good chemical stability with excellent photoconducting ability as a direct, narrow band-gap semiconductor. In₂Se₃ compounds have been widely used in optoelectronics, photodetectors, and gas sensors. In this work, we develop a visible light photodetector by combining these two materials from Ag₂S/In₂Se₃ heterostructure films at room temperature. The Ag₂S/In₂Se₃ bilayer films were annealed at different temperatures, and their photodetection parameters were compared to different annealed films. The bilayer structure and the interdiffusion of Ag₂S into the In₂Se₃ layer were confirmed through a cross-sectional FESEM view. The 250 °C annealed sample shows better photoconductivity with a maximum responsivity of 2.01 × 10^-1 A/W and 7.32 × 10⁹ Jones of detectivity than the other films. The current increase from nA to mA upon annealing significantly increased the photo response of the 250 °C annealed films. The hydrophilic properties improved with annealing, as confirmed by contact angle measurement. It is also further verified by the increased porosity observed from the FESEM images of the surface morphology. The transition from amorphous to polycrystalline was confirmed through XRD. The interdiffusion by annealing resulted in the formation of ternary phases like AgInS₂ and AgInSe_2, as seen from the XRD and HRTEM data. The crystallite size that increased upon annealing reduced the dislocation density from 3.29 × 10¹⁵ to 2.73 × 10¹⁵ m^-2. The optical bandgap, density, extinction coefficient, and skin depth showed changes as probed by UV-visible spectroscopy. Upon annealing, the band gap was enhanced by 0.157 eV from its as-prepared state. The observed changes in optical parameters and photoconductivity make the film a suitable candidate for visible light photodetection.

Scheme 1. Bilayer film structure and thermal annealing setup.

Scheme 2. Setup for measuring photo response.

Fig. 1. Structural data by XRD for Ag₂S/In₂Se₃ films.

Fig. 2. View of cross-sectional FESEM for bilayer (a) Ag₂S/In₂Se_3, and (b) 250 °C Ag₂S/In₂Se₃ annealed film.

Fig. 3. FESEM image of different Ag₂Se/In₂Se₃ films at 1 μm scale.

Fig. 4. (a–e) Elemental mapping and (f) EDX spectra of 250 °C annealed Ag₂S/In₂Se₃ thin film.

Fig. 5. (a and b) TEM picture, (c) HRTEM picture, and (d) SAED pattern of 250 °C annealed Ag₂Se/In₂Se₃ thin film.

Fig. 6. XPS (a)survey, core level (b) Ag 3d, (c) S 2p, (d) In 3d and (e) Se 3d spectra of 250 °C annealed Ag₂Se/In₂Se₃ thin film.

Fig. 7. (a) Transmittance change (b) skin depth (c) optical density (d) bandgap of the films.

Fig. 8. (a–e) Contact angle pictures of different film samples.

Fig. 9. I–V characteristics plots of as-prepared and annealed Ag₂S/In₂Se₃ thin films plotted in logarithmic scale for both dark and light conditions (a) asp, (b) 100 °C, (c) 150 °C, (d) 200 °C, (e) 250 °C, combined I–V plots of all the samples (f) in dark condition and (g) in light condition.

Fig. 10. Temporal photo response curve showing the variation between current and time for different biasing voltages of 2.5, 5, 7.5, and 10 V for different annealed thin films (a) 150 °C, (b) 200 °C, and (c) 250 °C, respectively. Variation in the rise and fall time of photocurrent for different annealed thin films (d) 150 °C, (e) 200 °C, and (f) 250 °C, respectively.