Charge carrier dynamics modulation of Ag_xInS₂ thin films for high-performance photodetection

Abstract

Chalcogenide semiconductors have attracted significant attention in the field of photodetection due to their excellent optoelectronic properties. Among them, solution-processed AgInS₂ materials show promising application potential, but the understanding of their optoelectronic behaviors at the microscopic scale remains insufficient. In this study, by adjusting the Ag content in the AgInS₂ precursor solution to optimize the film-forming process, combined with transient absorption spectroscopy, it was found that Ag_0.8InS₂ exhibits more excellent carrier dynamics. Furthermore, we revealed the interaction mechanism between Ag_0.8InS₂ and different transport layers, clarifying the design principles of device structures more conducive to photodetection. By introducing semi-transparent electrodes, we investigated the carrier dynamics in the complete device and finally fabricated photodetectors with great application potential in the field of ultraviolet communication. This work provides a microscopic theoretical basis for the optimization of the design of AgInS₂-based photodetectors and is of great significance for promoting their practical applications.

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1. Introduction

With the continuous advancement of optoelectronic technology, the photodetection field has put forward new demands for the functional characteristics and operational performance of photodetector devices.^1–4 In particular, chalcogenide semiconductors have emerged as ideal candidates for photodetector devices due to their excellent optoelectronic properties, such as high absorption coefficients, tunable bandgaps, and good chemical stability.^5–12 For example, chalcogenide materials like Cu₂ZnSn(S,Se)₄ (CZTSSe),^13,14 AgBiS₂^15–17 and Bi₂S₃,^18,19 which can be prepared via solution methods, exhibit broadband response capabilities in the visible to near-infrared range, providing a new pathway for the development of high-performance photodetectors.^20–23 Chen et al. constructed self-powered photodetectors based on CZTSSe thin films, which feature high sensitivity, broadband response, and fast response speed, meeting various application needs in multiple scenarios.²⁴ Huang and coauthors fabricated near-infrared photodetectors with excellent detection performance using colloidal AgBiS₂ nanocrystals.²⁵ Yang et al. reported a Sb_2−xBi_xS₃ precursor-based sol–gel method for fabricating high-quality thick films, then developed X-ray detectors with low dark current, ultra-fast response, high X-ray sensitivity, and high spatial resolution.²³ However, the range of applications for these materials remains limited owing to their high density of defect states and poor carrier transport capability. In recent years, indium-based semiconductor materials have found widespread application in optoelectronic technology owing to their excellent carrier transport capability.^26–30 Among these, AgInS₂, as a representative example, has gradually been employed in the fabrication of photodetectors. Jia et al. developed a flexible and simple solution precursor method for preparing AgInS₂ thin films with high carrier mobility.³¹ Subsequently, using this process, they fabricated and optimized photodiode-type photodetectors with good detection capability, which have demonstrated great potential for multi-application scenarios.³¹ Chen and coauthors prepared AgInS₂ thin films via ultrasonic spray pyrolysis and further enhanced film quality through post-selenization, thus fabricating photodetectors with high responsivity.³² In addition, Li et al. constructed core–shell structured AgInS₂@AgGaS₂ quantum dots and used these to fabricate photodetectors with excellent image sensing capabilities.³³

Although the application of solution-processed AgInS₂ in photodetectors has achieved preliminary results, there is still a lack of key insights into the optoelectronic behaviors of the material at the microscopic level. Transient absorption (TA) spectroscopy, based on the pump–probe principle, where short pump pulses excite the material to generate non-equilibrium states, and probe pulses with a time delay line detect the transient changes in optical absorption induced by the pump.³⁴ This technique can capture key information such as excited-state lifetimes, carrier recombination pathways, charge transfer dynamics, and energy relaxation processes, with high temporal resolution (from femtoseconds to nanoseconds), and thus serves as a powerful tool for studying the optoelectronic properties of semiconductor materials.^35–39 For example, Yang et al. investigated charge transfer at the interface of p-type gallium-indium phosphide (p-GaInP₂) using the transient reflection method.⁴⁰ Kastl and coauthors studied the excited carrier dynamics in prototypical metal–organic chalcogenides [AgSePh]_∞ via low-temperature transient absorption spectroscopy, unraveling three exciton resonances.⁴¹ Kaur et al. revealed instantaneous electron transfer from Sb₂Se₃ to CdS in heterostructures through transient absorption.⁴² Kharbanda and coauthors explored plasmon-induced charge transfer dynamics on an ultrafast time scale.⁴³ Extensive applications have demonstrated their practicality. Therefore, the study of the optoelectronic properties of materials using transient absorption spectroscopy is conducive to theoretically guiding the construction and fabrication of novel optoelectronic devices, thus providing a theoretical basis for their development.

To further advance attempts and research on the construction methods and theories of AgInS₂-based semiconductor devices, in this work, we first investigate the effect of Ag content on AgInS₂-based semiconductor materials and devices by adjusting the Ag content in the precursor solution. Furthermore, through transient absorption spectroscopy analysis, we clarify the carrier transport mechanism between the thin film and each transport layer, providing theoretical guidance for the fabrication of high-performance optoelectronic devices. Furthermore, by combining the introduction of semitransparent electrodes, we investigate the carrier dynamics in complete AgInS₂ devices. On this basis, the devices fabricated by us exhibit excellent optoelectronic detection capability. Furthermore, we adopted the Ag/LiF/Ag optical resonant cavity structure as the electrode, which exhibits excellent selective transmittance for the ultraviolet (UV) band. This endows the device with outstanding UV detection performance, demonstrating great application value in the field of UV optical communication. Meanwhile, when light is incident from different ports of the device, the device can exhibit a broad-spectrum response in the visible light band and a narrow-band response in the UV band, respectively, which lays a solid foundation for future diversified applications.

2. Results and discussion

Benefiting from our previously reported method for preparing AgInS₂ thin films and devices via a solution process (as shown in Fig. 1a), compositional engineering can be used to effectively tune the properties of the materials.³¹ By dissolving precursor components with different contents in 2-methoxyethanol (2-MOE) to form the corresponding precursor solutions, high quality Ag_xInS₂ thin films can then be prepared via the spin-coating method. Notably, a decrease in the Ag content induces progressive deterioration of the thin film morphology. Films with Ag content below 0.8 suffer from macroscopic structural flaws, including discontinuous coverage, pinholes, and large particle agglomerates that are devastating to device processing. To guarantee the structural integrity required for subsequent optoelectronic device assembly, we determined that the minimum feasible Ag ratio for systematic investigations in this work is 0.8. Fig. 1b compares the absorbance spectra of various thin films, and their absorption coefficients, transmittance and reflectance are shown in Fig. S1. The steep absorption edge of the material, which indicates a narrow distribution of its band gap and high compositional uniformity, is beneficial for enhancing the response sharpness of optoelectronic devices to specific wavelengths. Moreover, with the increase of the Ag content, the absorbance increases in the non-absorption region while exhibiting an overall absorption redshift characteristic, which suggests that the increase of Ag content may lead to more internal defects in the thin films. Furthermore, Fig. 1c shows the comparison of Raman spectra of Ag_xInS₂ thin films. Characteristic vibrational peaks at specific positions are in good agreement with those of the AgInS₂ samples and the Raman spectra of materials with different Ag contents are largely consistent, confirming the purity of the AgInS₂ as the main phase.

Fig. 1 (a) Schematic diagram of Ag_xInS₂ precursor solution and thin film preparation process. (b) Absorbance spectra and (c) Raman spectra of the Ag_xInS₂ thin films.

Having a good understanding of their basic material properties, we are now focusing on the optoelectronic properties of the materials. To better investigate the ultrafast dynamical processes of photoexcited carriers in the materials, transient absorption spectroscopy, whose schematic diagram is shown in Fig. 2a, was employed. As depicted in Fig. 2b, taking Ag_0.8InS₂ as an example, the transient transmission spectrum exhibits a very prominent ground-state bleaching signal. This phenomenon can be attributed to its unique electronic structure and excited-state dynamic properties. Specifically, within the detection energy range, the energy is insufficient to induce further absorption of the excited state, rendering the excited-state absorption (ESA) negligible within this energy interval. Furthermore, the material exhibits no stimulated emission (SE) properties under experimental conditions, which further suppresses the generation of interference signals. Thus, in the transient transmission spectrum, only the bleaching signal arising from the reduction in intrinsic absorption caused by the presence of the excited state dominates. When the time delay is fixed at 10 ps and the transient transmission spectra of various samples are studied via time slicing, as shown in Fig. 2c, it is found that Ag_0.8InS₂ exhibits the strongest signal and the narrowest full width at half maximum (FWHM) of the signal, showing excellent semiconductor properties. With the decrease in the Ag content, the intensity of the bleaching signal increases. This indicates a significant increase in the number of excited-state carriers generated by the material upon photon absorption per unit time, thereby revealing its enhanced photoresponsive capability. Furthermore, a triple exponential decay was used to fit the carrier dynamics spectrum of Ag_xInS₂. The fitting results are summarized in Fig. 2d and Fig. S2 and Table S1. The decay components with time constants of approximately 10 picoseconds (ps) (τ₁), 100 ps (τ₂), and 2 nanoseconds (ns) (τ₃) can be attributed to carrier trapping processes associated with shallow traps, deep traps, and exciton recombination, respectively. With the decrease in the Ag content, the proportion of the exciton recombination component increases, which indicates a reduction in the number of carriers trapped by defects. Meanwhile, its long carrier lifetime further confirms that Ag_0.8InS₂ is more conducive to the fabrication of optoelectronic devices. In addition, the photoconductivity and dark conductivity of these materials were investigated to further compare the differences in their performance, whose details are shown in Fig. 2e–i. With increasing Ag content, the dark conductivity of the material gradually increases. Thus, it is self-evident that samples with a low Ag content are more conducive to photodetector fabrication.

Fig. 2 (a) Schematic diagram of the transient absorption system. (b) Transient transmission spectrum of Ag_0.8InS₂. (c) Profiles of the transient absorption spectrum of Ag_xInS₂ at a time delay of 10 ps. (d) The fitted time constant (τ₁, τ₂, τ₃) and corresponding proportion of τ₃. Photocurrent and dark current curves of photoconductive devices based on the (e) Ag_0.8InS₂, (f) Ag_0.9InS₂, (g) Ag_1.0InS₂, (h) Ag_1.1InS₂ and (i) Ag_1.2InS₂, with the illumination being 100 mW cm⁻² white light.

After discussing the influence of the Ag content on material properties, we are now further directing our research focus to the Ag-deficient samples. A typical linear absorption spectrum (red squares) and a TA spectrum (blue circles) of the Ag_0.8InS₂ thin film are provided in Fig. 3a. Modeling and analysis of the steady-state absorption spectrum reveal that the optical band gap (E_g) of the sample is 2.06 eV, and the exciton binding energy is 5.01 meV. The typical features of the TA spectrum are relatively prominent: a photobleaching peak with a slight blue shift relative to the excitonic transition peak and a positive photoinduced absorption signal. The photobleaching near the bandgap originates from the state-filling effect. Its prominence directly reflects the carrier filling efficiency and separation capability in the material, and provides a critical structural basis for the material to achieve high responsivity in photodetector devices. The photoinduced absorption signal above the bandgap is often attributed to the activation of forbidden exciton transitions in the material. This characteristic indirectly indicates that the carrier lifetime of the material can be prolonged through exciton dissociation pathways, and this process helps reduce the performance loss of the material. Furthermore, to better track the hot carrier cooling dynamics, we conducted pump fluence-dependent TA experiments, and the TA spectra are summarized in Fig. S3. The strong linear relationship between the bleach amplitude and pump intensity exhibited in Fig. 3b indicates that within the pump fluence range of this experiment, the generation efficiency of photogenerated carriers in the material is stable and not significantly affected by the enhanced carrier recombination or trap state saturation. Meanwhile, the variation of bleach center energy reflects a distinct band filling effect. Fig. 3c presents the normalized carrier dynamics characteristics of the Ag_0.8InS₂ thin film under various pump fluences. As the pump fluence gradually increases, carrier–carrier scattering accelerates the relaxation process of carriers to low-energy states and enhances the coupling efficiency between carriers and lattice phonons, ultimately leading to a shortening of the effective carrier lifetime. Notably, the lifetime decay dominated by scattering that does not affect filling efficiency reflects that the material possesses both stable linear photoresponse and controllable carrier relaxation characteristics within this range, which provides a key basis for balancing the linear response range and carrier lifetime in the subsequent device design. For the subsequent device fabrication, we investigated the TA spectra of the material combined with the transport layer. Fig. S4 compares the steady-state absorption spectra of materials with various transport layers, and the steady-state transmittance and reflectance spectra of samples with various transport layers are also shown in Fig. S5. As shown in Fig. S6, upon the addition of various transport layers, the spectral characteristics remain unchanged. However, the peak value of the bleaching signal decreases after their addition, which is shown in Fig. 3d. Simultaneously, Fig. 3e compares the normalized kinetics of the photobleaching feature of different structures. When the material was combined with the electron transport layer (ETL), both the carrier lifetime and photobleaching signal were significantly decreased. In contrast, when combined with the hole transport layer (HTL), these two parameters showed almost no change. This distinct difference not only directly reflects the carrier transport properties of the material but also provides strong experimental evidence that electrons are the majority carriers. Thus, for this material, it is necessary to prioritize optimizing the interface contact and energy level alignment of the ETL to reduce carrier recombination losses, while the selection of the HTL can focus more on stability. Furthermore, through the introduction of the semi-transparent electrode, the TA spectrum of the device was successfully collected, as shown in Fig. S7a. Fig. S7b shows that the carrier lifetime shortens under high light intensity. Finally, we investigated how different composite structures affect the carrier cooling curves through monitoring the carrier temperature as a function of time, as shown in Fig. 3f. The hot carrier temperature exhibits a two-step process, which essentially reflects that the hot carrier undergoes two distinct energy dissipation mechanisms in the material. After the introduction of the heterojunction interface, the carrier cooling rate in the absorber layer is significantly accelerated. This further confirms the efficient carrier extraction by the device structure, and also reveals the intrinsic origin of the device performance.

Fig. 3 (a) Steady-state and TA spectra of a typical Ag_0.8InS₂ thin film. (b) Pump fluence-dependent bleach amplitude and bleach center energy for Ag_0.8InS₂ samples. (c) Normalized pump fluence-dependent kinetics of bleaching feature for Ag_0.8InS₂ samples. (d) TA spectral profiles taken at 10 ps and (e) normalized kinetics of photobleaching feature for Ag_0.8InS₂ samples with transport layers. (f) Carrier cooling curves for Ag_0.8InS₂ samples.

Having systematically understood the origin of the excellent optoelectronic performance within the device, we now turn our focus to the device performance. Inspired by transparent electrodes, we fabricated devices with an Ag(30 nm)/LiF(60 nm)/Ag(30 nm) optical resonant cavity structure serving as the electrode, which is referred to as the Fabry–Pérot cavity. As shown in Fig. S8, the structure can effectively block the visible light region while transmitting the ultraviolet region. Then, we characterized the device performance in detail. As shown in Fig. 4a, the response current of the device exhibits good adaptability under different light intensities. Meanwhile, we collected the noise of the photodetector under different bias voltages, as displayed in Fig. 4b. Obviously, the noise of the device at 0 V coincides with the intrinsic noise of the instrument itself. Therefore, the responsivity presented in Fig. 4c exhibits a certain degree of underestimation. Meanwhile, by recording the steady-state current values of the photodetector under different light intensities, the linear response range of this photodetector was obtained and shown in Fig. 4d, which demonstrates its excellent detection performance. Additionally, the specifically selected electrode structure enables the different responses of the same device, which undoubtedly expands the functionality of the photodetector. As displayed in Fig. 4e, when light is incident from the ITO side, the device exhibits a broadband response in the visible light region, but when incident from the Ag electrode side, the device shows a response to ultraviolet light. Meanwhile, light incidence from the Ag side eliminates the absorption effect of the glass substrate, resulting in the device also exhibiting a good response to 254 nm ultraviolet light, as shown in Fig. S9. Furthermore, we evaluated the response time of the device via ultrafast pulser excitation and detection. It can be determined that the device has a fast response capability of 325.4 ns, as displayed in Fig. 4f. Overall, its fast response speed and excellent ultraviolet light response characteristics grant it significant advantages in ultraviolet communication applications. Therefore, we tested the application potential of the Ag_0.8InS₂ based photodetector in ultraviolet communication using the testing method described in the inset of Fig. 4g. As depicted in Fig. 4h, the Ag_0.8InS₂ based photodetector can respond to a relatively large data rate of 10⁴ bps. To sum up, this innovative device design and application attempt for ultraviolet communication not only breaks the limitations of traditional schemes with its unique performance, but also provides a new perspective for technological breakthroughs in this field, highlighting its pioneering significance in advancing the development of ultraviolet communication.

Fig. 4 (a) Light intensity dependent J–V curves, (b) reverse bias dependent noise density, (c) specific detectivity and (d) linear dynamic range of Ag_0.8InS₂-based photodiode. (e) Normalized responsivity with light incident on the device from different directions. (f) Ultrafast transient photocurrent response of Ag_0.8InS₂-based photodiode. (g) Schematic illustration of the ultraviolet communication data rate test. (h) The recorded signal output at different transmission rates.

3. Conclusions

In conclusion, by adjusting the Ag content in the AgInS₂ precursor solution, we optimized its film-forming process and systematically investigated its carrier dynamics using transient absorption spectroscopy. It was found that Ag_0.8InS₂ exhibits more excellent semiconductor properties, making it more favorable for the fabrication of photodetectors. Subsequently, we explored the interaction mechanisms between Ag_0.8InS₂ and different transport layers via transient absorption spectroscopy and discussed the device structure design that is more conducive to photodetection. Furthermore, by introducing semi-transparent electrodes, we studied the carrier dynamics in the complete device combined with transient absorption spectroscopy, and on this basis, fabricated high-performance photodetectors with tunable response spectra from ultraviolet to visible light. In particular, their ultrafast response speed of ∼325 ns endows them with great application potential in the field of ultraviolet communication.

Author contributions

Zhenglin Jia: data curation, formal analysis, investigation, visualization, and writing – original draft. Yuke Zhao: formal analysis. Qianqian Lin: conceptualization, methodology, funding acquisition, supervision, and writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

Data will be made available on request.

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: notes, figures, table, TA spectra and further experimental details. See DOI: https://doi.org/10.1039/d5tc04269k.

Acknowledgements

This work was financially supported by the Guangdong Basic and Applied Basic Research Foundation (Grant No. 2025A1515010734), the Wuhan Science and Technology Bureau (Grant No. 2022010801010108) and the National Natural Science Foundation of China (Grant No. 61875154). We thank the Core Facility of Wuhan University for access to analytical equipment.

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