1. Introduction

In the field of photoelectronics, researchers have made significant efforts to develop high-performance and cost-effective materials and devices, aiming to drive technological innovation and practical applications [1,2]. Within this context, heterojunctions based on cadmium sulfide (CdS) have gained considerable interest due to their unique photoelectric properties and potential applications in lasers [3], light-emitting diodes (LEDs) [4], solar cells [5,6], and photodetectors (PDs) [7–10]. However, there have been very few reports on their application in position sensitive detectors (PSDs). PSD is one kind of non-contact sensors that utilize the lateral photovoltaic effect (LPE) as its working principle. Unlike the conventional PDs, PSD generates a lateral photovoltage (LPV) that shows a high linear correlation with the spot position under non-uniform illumination [11,12]. The discovery of the LPE dates back to 1930 [13], and the concept of precise point position detection utilizing this effect was first proposed in 1957 [14]. Since then, extensive research has been conducted on PSDs, with a focus on developing high-performance self-powered PSDs for accurate position detection. Despite the substantial progress in both theoretical and experimental investigations on PSDs, the sensitivity limitation remains a primary obstacle. There is a growing demand for lower power consumption, high precision signal detection, and long working distance capabilities in PSDs. Therefore, it is urgently needed to explore and introduce novel modulation methods and technologies to address these challenges.

CdS is a direct band gap semiconductor belonging to the II-VI group. It possesses several advantageous properties of low work function, high refractive index, and excellent carrier transport characteristic. Exploiting these properties, researchers have designed and fabricated various CdS-based heterostructures, including CdS/molybdenum disulfide (MoS₂) [15], CdS/zinc oxide (ZnO) [16], CdS/perovskite [1,7], and CdS/silicon (Si) [8,17]. One key advantage of these heterostructures is the presence of a built-in electric field. When illuminated, the interface of these heterostructures generates electron-hole pairs, which are then separated by the built-in electric field, leading to the generation of the photovoltaic effect [18–20]. This effect allows CdS-based heterostructure devices to operate autonomously, eliminating the need for an external power supply. Among these heterostructures, the combination of CdS and Si has attracted significant interest due to well-established processing techniques, cost-effectiveness, and comprehensive properties. Additionally, the wurtzite structure of CdS exhibits a pronounced pyroelectric property due to its non-centrosymmetric structure [8,9,21]. The pyroelectric effect can be efficiently coupled with the photovoltaic effect within the CdS/Si heterojunction through the pyro-phototronic effect [20,22–24]. This effect allows for the modulation of the photoresponse by controlling the temperature of the device. Therefore, the CdS/Si heterojunction, along with the pyro-phototronic effect, holds great potential for the development of high-sensitivity PSD. However, the theoretical and experimental understanding of the pyro-phototronic effect on the performance of PSDs, particularly under varying ambient temperatures and electrode contact distances, remains unknown.

In this work, we successfully fabricated an ITO/CdS/Si heterojunction with a vertically oriented CdS thin film, which was developed into a high-performance PSD. The PSD exhibited an exceptional response based on the LPE across a wide range of wavalengths from 405 to 1064 nm, achieving a position sensitivity (PS) of 655.1 mV/mm. To further enhance the performance of the PSD, the pyro-phototronic effect was introduced. This effect utilized the pyroelectric field induced by instant temperature changes in the CdS film to facilitate the separation and transport of carriers, resulting in an improved PS of 1061.3 mV/mm with an enhancement of 160.5%, and an ultrafast response time of 0.01/0.04 ms. Moreover, the integration of the pyro-phototronic effect extended the response wavelength to 1550 nm, enabling it a wider spectrum range. Besides, we also investigated the impact of ambient temperature and electrode distances on the pyro-phototronic effect in the PSD. The results indicated that the device demonstrated remarkable environmental stability and remained operational at temperatures up to 150 °C. Furthermore, even at a large working distance of 11 mm, the PSD still maintained a PS of 128.99 mV/mm and sustained a linearity of <13%. This work provides valuable insights into the influence of the pyro-phototronic effect on the LPE response in the ITO/CdS/Si heterojunction PSD.

2. Experimental methods

2.1. Preparation process of ITO/CdS/Si heterojunction PSD

A p-type Si wafer (100) with a resistivity of 1-10 Ω cm and a thickness of 500 µm was used. Firstly, the wafer was placed in a mixed solution of isopropanol (5 vol%) and potassium hydroxide (KOH) (5 wt%) at ∼80 °C for about 30 minutes. This process resulted in the formation of a pyramid-like structure on the wafer surface through anisotropic etching, as shown in Supplement 1, Fig. S1. After the etching, the wafer was removed from the solution and thoroughly washed with acetone, alcohol, and deionized water for 15 minutes to eliminate any residual KOH. The CdS was then fabricated using the chemical batch deposition (CBD) method. The etched Si wafers were submerged in a solution containing 0.0015 M cadmium nitrate tetrahydrate (Cd(NO₃)₂•4H₂O), 0.0375 M thiourea (CH₄N₂S), and 1.125 M ammonia (NH₃•H₂O). The solution was rapidly heated to 70 °C and kept stationary for 50 mininuts to allow the growth of the CdS film. As a result, a uniform yellow CdS layer with a thickness of 150 nm was successfully deposited on the Si substrate, with the cross-sectional SEM morphology shown in Fig. S2. Subsequently, the CdS film was thoroughly rinsed with deionized water to remove any residual chemicals, and annealed at different temperatures ranging from 100 to 300 °C for 10 minutes. Finally, a 30 nm-thick ITO film was deposited as the transparent conductive layer, and two dot-like Ag films were prepared as electrodes using a magnetron sputtering system. The photograph of one device can be found in Fig. S3. The distance between these electrodes was varied to investigate its effect on the performance of the PSD.

2.2. Characterization and measurement

The crystal structure of the CdS film was analyzed using X-ray diffraction (XRD) (Bruker, D8 Advance). The current-voltage (I-V) curves were measured using a Keithley 4200 source meter. To obtain the position-dependent LPV curves, a home-built measurement system was utilized. This system consisted of a Keithley 2700 voltmeter and an electric motion stage. The LPV curves were collected under irradiation of lasers with various wavelengths (405, 450, 532, 671, 780, 1064, and 1550 nm), and the laser beam was focused prior to reaching the device to ensure a beam size of smaller than 100 µm. A schematic diagram of the LPV testing system has been given in Supplement 1, Fig. S4. The LPV versus time (LPV-t) curves were obtained using an oscilloscope (Tektronix 3 series). The curves were collected by subjecting the PSD to pulsed laser illumination at various frequencies ranging from 0 to 400 Hz.

3. Results and discussion

In Fig. 1(a), the I-V curves of the ITO/CdS/Si heterojunctions, annealed at varying temperatures, exhibit linear characteristics. This suggests that there is a stable Ohmic contact established between the ITO film and the two electrodes. It indicates that the silver electrodes do not have a significant effect on the overall contact behavior. Figure 1(b) illustrates the longitudinal I-V characteristics of the ITO/CdS/Si heterojunction device typically annealed at 200 °C, under both dark and illumination conditions. These results reveal pronounced reverse rectification characteristics of a standard diode. Additionally, an increase in photocurrent is observed with increasing laser power, indicating the superior photoresponse performance of the ITO/CdS/Si heterojunction. The inset of Fig. 1(c) illustrates the LPE measurements for the PSD of the ITO/CdS/Si heterojunction. When laser irradiation is non-uniform, light is absorbed by the CdS and Si layers, generating electron-hole pairs. The built-in electric field then separates these charge carriers, with electrons being transported to the ITO layer, while holes are transported to the Si layer. The difference in carrier concentration between the illuminated and non-illuminated regions further causes these electrons to diffuse horizontally within the ITO layer, resulting in their collection by the electrodes and an output of LPV. As the laser spot moves between the two electrodes, the LPV curves demonstrate notable linearity (Fig. 1(c)). The optimal LPE response is observed when the CdS layer in the ITO/CdS/Si heterojunction devices is annealed at a temperature of 200 °C. This has been undoubtedly testified by the LPV results under varying wavelengths and laser powers, as shown in Supplement 1, Fig. S5 to S9. The enhanced response can be attributed to the superior crystal quality and increased light absorption of the CdS thin film annealed at this temperature [7,25], as shown in Fig. S10. The formation of wurtzite CdS is confirmed by the presence of a pronounced (002) diffraction peak at 26.5° in the XRD results [26].

 

Fig. 1. (a) Lateral I-V curves of the ITO/CdS/Si heterojunction PSDs annealed at different temperatures, with the inset showing the measurement diagram. (b) Longitudinal I-V curves of the PSD annealed at 200 °C under different illumination powers of 780 nm laser, with the inset showing the measurement diagram. (c) Laser position-dependent LPV curves of the PSD annealed at different temperatures, with the inset showing the measurement diagram. (d) Laser position-dependent LPV curves of the PSD annealed at 200 °C under the illumination of 780 nm laser at different powers, with the inset showing LPV values extracted at x = 0.3 mm. (e) Laser position-dependent LPV curves of the annealed at 200 °C under different lasers at 15mW, with the inset showing the extracted maximum LPV value. (f) Laser power-dependent PSs of the PSD annealed at 200 °C under different wavelengths at various laser powers.

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Then, the LPE response of the 200 °C-annealed device was well investigated. Figure 1(d) shows the LPV curves under a 780 nm laser irradiation of different powers ranging from 0.05 to 15 mW. The LPV curves all exhibit a clear linearity, and LPE response improves notably as the laser power increases. To analyze the LPE trend, the maximum LPV value at a specific position of x = 0.3 mm is presented in Fig. 1(d). At low powers, the LPV rises considerably with increasing laser power, then gradually slows and approaches saturation at high powers. This behavior is determined by complex processes of carrier generation, separation, and recombination in this heterojunction [27,28]. With increasing laser power, there is a rapid increase in the number of photogenerated carriers, leading to enhanced separation and an increase in LPV. However, as the carrier density increases, their separation becomes gradually challenging, and carrier recombination intensifies, causing a nearly static increase in the number of separated carriers. To determine the response spectrum of the PSD, various lasers with wavelengths of 405, 450, 532, 671, 780, and 1064 nm were used to irradiate the heterojunction. The results, shown in Fig. 1(e) and Supplement 1, S9, indicate that this heterojunction PSD exhibits a broadband LPE response within the range of 405 to 1064 nm, with the optimal response observed at 780 nm. In LPE devices, position sensitivity (PS), which is evaluated by linear fitting, is a crucial parameter. Figure 1(f) presents the laser power-dependent PS values for various laser wavelengths. The PS rises quickly, and then slowly, and eventually reaching saturation. The maximum PS of this heterojunction is observed to be 655.1 mV/mm at 780 nm, surpassing these of previously reported heterostructures, as summarized in Table 1. This demonstrates the significant potential of the ITO/CdS/Si heterojunction in visible to near-infrared high sensitivity PSDs.

Table 1. LPE Performance Comparisons of Different Heterojunction PSDs.

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The CdS material used in the ITO/CdS/Si heterojunction can exhibit pronounced pyroelectric characteristics due to its wurtzite crystal structure. In this heterojunction, the pyroelectric effect and photovoltaic effect are coupled when subjected to pulsed laser irradiation, resulting in the pyro-phototronic effect. To gain a comprehensive understanding of the mechanism behind the pyroelectric effect in CdS and its regulation of the LPE response in the heterojunction, the LPV-t curve of the ITO/CdS/Si heterojunction PSD was examined, with a typical result under 1064 nm laser irradiation shown in Fig. 2(a). The LPV-t curve exhibits a distinct four-stage behavior, which is a significant characteristic of the pyro-phototronic effect. This multi-stage LPV response reveals the different internal physical mechanisms of the device under varying illumination stages. Figure 2(b) presents an enlarged depiction of the LPV behavior across these four stages (labeled I, II, III, and IV), with the working mechanisms presented in Fig. 2(c) [29,30]. During stage I, in the dark, owing to the II-type band alignment of the heterojunction, a depletion region is formed at the interface, resulting in a built-in electric field (E_b). Without external excitation, the device remains at zero output voltage (LPV_dark). Stage II begins with the sudden illumination of light. The photoexcitation generates numerous electron-hole pairs at the interface. The E_b effectively separates these charge carriers, resulting in an output voltage (LPV_ph) due to the photovoltaic effect. Simultaneously, the transient temperature rise (dT/dt > 0) in the CdS film induced by illumination generates a pyroelectric field (E_py). The E_py further facilitates the separation of these carriers, resulting in a total voltage of LPV_ph + py. In stage III, with continuous illumination, the temperature of the PSD stabilizes (dT/dt = 0), and the E_py quickly diminishes, leaving only a stable photovoltaic response (LPV_ph). Stage IV begins when the laser is turned off. In this stage, the absence of photogenerated carriers leads to a quick reduction in the LPV_ph. However, a reverse pyroelectric electric field is induced in the CdS film due to rapid temperature decline (dT/dt < 0), opposite to the direction of the E_b. As a result, a reverse pyroelectric voltage (LPV_py’) is generated. As the temperature slowly equalizes with the ambient environment, the inverse pyroelectric field and the pyroelectric voltage gradually dissipate, returning the output voltage to its original dark state (LPV_dark).

 

Fig. 2. (a) Transient LPV-t curves of the x = 0.3 mm under the illumination of 1064 nm laser at 15mW with a periodic chopper frequency of 200 Hz. (b) Typical cycle of the four-stage LPV-t curve. (c) Schematic illustration of the energy band diagrams of the PSD and the working principle of the pyro-phototronic effect.

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To investigate the influence of the pyroelectric effect on the spectral range of the ITO/CdS/Si heterojunction PSD, the LPV-t curves were measured under illumination of different lasers with wavelengths of 405, 450, 532, 671, 780, 1064 and 1550 nm, as shown in Fig. 3(a). During the transient laser switching stages, the LPV-t curves exhibit periodic peaks of pyroelectric voltage in the entire wavelength range of 405-1550 nm, indicating that the pyroelectric effect has remarkable spectral broadening characteristics in this heterojunction PSD. Particularly noteworthy is the response at the 1550 nm, which outsides the bandgaps of ITO, CdS, and Si. Besides, similar to the photovoltaic response, the pyroelectric response is also highly dependent on the laser wavelength. However, there is more or less difference for their changing tendency with the laser wavelength, which can be illustrated from the specific working mechanisms, as shown in Fig. 3(b)-(e). At laser wavelengths of 405 and 450 nm, absorption mainly occurs within the CdS layer, while the Si layer exhibits relatively minor absorption. Consequently, the generated LPV_ph and LPV_py(/LPV_py’) are mainly attributable to the CdS layer, as illustrated in Fig. 3(b). However, when the laser wavelength extends to 532 nm, the main absorption shifts from the CdS layer to the Si substrate. This change in absorption behavior leads to a decrease in the photogeneration efficiency, resulting in a decend in the LPV_ph. Despite the lower efficiency, as the absorption is now closer to the interface of the heterojunction compared to other wavelengths, the photogenerated carriers can be more easily facilitated by the pyroelectric field (Fig. 3(c)). Therefore, moderate LPV_py and LPV_py’ values are observed at this wavelength. Furthermore, at wavelengths of 671, 780, and 1064 nm lasers, the Si layer begins to significantly absorb photon energy, and the photovoltaic and pyroelectric responses show the same wavelength dependence (Fig. 3(d)). When the laser wavelength reaches 1550 nm, surpassing the absorption region of the heterojunction and ceasing the generation of photoexcited carriers, but the pyroelectric field can still be generated by the pyroelectric effect, reserving the instantaneous output of LPV_py(/LPV_py’), as illustrated in Fig. 3(e). These experimental findings demonstrate that the introduction of the pyroelectric effect has significantly broadened the light response spectrum of the ITO/CdS/Si heterostructure PSD.

 

Fig. 3. (a) Transient LPV-t curves of the ITO/CdS/Si heterojunction PSD under the irradiation of different lasers. Schematic band diagrams of the ITO/CdS/Si heterojunction PSD for (b) 405/450 nm, (c) 532 nm, (d) 671/780/1064 nm, and (e) 1550 nm laser irradiations to illustrate the working mechanisms of the pyro-phototronic effect.

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In the study of PSDs utilizing the pyro-phototronic effect, it is crucial to consider the instantaneous temperature change on the heterojunction caused by the laser irradiation. Subsequently, the pyro-phototronic effect was studied under illumination of different laser powers. Figure 4(a) shows the LPV-t characteristic curves obtained under 780 nm laser illumination with powers ranging from 0.05 to 15 mW at a pulse frequency of 200 Hz. The laser irradiation was fixed at the position of x = 0.047 mm. The curves exhibit a four-stage photovoltage response across the entire power range, confirming the wide power response range of the pyro-phototronic effect in the ITO/CdS/Si heterojunction PSD. Figure 4(b) shows the extracted values of LPV_ph, LPV_ph + py, and LPV_ph + py−LPV_py′ at different laser powers. It is evident that all the output volatges increase as laser power increases. However, the increments in LPV_ph + py, and LPV_ph + py−LPV_py′ are relatively larger compared to the increase in LPV_ph. This observation further supports the idea that incorporating the pyroelectric effect into the PSD significantly enhances its LPE response. Figure 4(c) showcases the calculated position sensitivities of PS_ph, PS_ph + py, and PS_ph + py+ PS_py′ at different laser powers. The maximum values for PS_ph + py and PS_ph + py+ PS_py′ reach 779 and 825 mV/mm, respectively, at a laser power of 15 mW. Figure 4(d) gives the enhancement ratios of PS_ph + py/PS_ph and (PS_ph + py+ PS_py′)/PS_ph. These enhancement ratios gradually increase with escalating laser power, reaching maximum ratios of 123% and 131% at 15 mW, respectively. As the laser power increases, the number of photogenerated carriers in the ITO/CdS/Si heterojunction PSD also increases, leading to an increase in LPV_ph. However, in general, the carrier separation efficiency induced by the built-in field slightly decreases with an increase of laser power. Consequently, the increase in the LPV_ph becomes gradually slower, as illustrated in Fig. 4(b). Unlike the photovoltaic effect, in the pyroelectric effect, the increase in laser power is expected to result in a linear rise in the transient temperature change (dT/dt), causing the pyroelectric potential to continuously increase. As a result, the LPV_py(/LPV_py′) shows a larger increasing rate compared to the LPV_ph, which explains why the enhancement ratios increase with increasing laser power.

 

Fig. 4. (a) Transient LPV-t curves under the illumination of 780 nm laser at different powers. Extracted (b) LPVs, (c) PSs, and (d) PS enhancement ratios as a function of the laser power. (e) The maximum PSs and PS enhancement ratios as a function of the laser wavelength.

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In addition to the LPV-t curves under 780 nm laser irradiation, LPV-t curves were also measured at various powers under different laser wavelengths, as shown in Supplement 1, Fig. S11-S15. Similar increasing trends can be observed across the entire range of laser wavelengths. Figure 4(e) summarizes the maximum PS values and PS enhancement ratios of different laser wavelengths at 15 mW. With 405 nm laser irradiation, the maximum values of PS_ph + py and PS_ph + py+ PS_py′ are 588.9 and 627.8 mV/mm, respectively, with increases of 108% and 115% compared to PS_ph (541.3 mV/mm). For 450 nm laser irradiation, the maximum values for PS_ph + py and PS_ph + py+ PS_py′ are 571.5 and 628.8 mV/mm, respectively, showing increases of 116% and 127% (PS_ph of 491.8 mV/mm). At 532 nm laser wavelength, the maximum values reach 365 and 425 mV/mm, with increases of 140% and 163% (PS_ph of 259.7 mV/mm). For the 671 nm laser, the maximum values are 779.6 and 838.4 mV/mm, indicating increases of 119% and 128% (PS_ph of 650.3 mV/mm). Under 1064 nm laser irradiation, the maximum values are 192.9 and 246.4 mV/mm, with increases of 118% and 162% (PS_ph of 162.4 mV/mm). These results demonstrate that the ITO/CdS/Si heterojunction PSD exhibits significant performance enhancement across a broad spectral range induced by the pyro-phototronic effect.

The turn on/off time of the laser also has a significant influence on the transient temperature variation, which in turn affects the pyroelectric effect. Figure 5(a) displays the LPV-t results under pulsed laser irradiation at frequencies ranging from 10 to 400 Hz, with a wavelength of 780 nm and a power of 15 mW. In the entire frequency range, the LPV values during the laser’s turn-on or turn-off stages remain almost constant, as shown in Fig. 5(b). This indicates that the heterojunction PSD exhibits excellent stability and a robust ability for fast optical signal detection. As the pulse frequency increases, there is a gradual enhancement of transient LPV peaks (LPV_py and LPV_py’) observed in the instant turn-on and turn-off stages of the laser. This enhancement is attributed to the different turn-on and turn-off times. The times decrease as the frequency increases, resulting in a faster temperature variation rate. Consequently, the pyroelectric potential, along with the pyroelectric response, improves gradually. Figures 5(b) and 5(c) show the LPV and PS results corresponding to the different frequencies. When the frequency escalates from 10 to 400 Hz, The LPV_ph keeps at 29.8 mV. While the LPV_ph + py and LPV_ph + py−LPV_py′ increase from 29.8 and 30 mV to 40 and 48 mV, respectively. Under the influence of the pyroelectric effect, the maximun PS_ph + py of 897.1 mV/mm and PS_ph + py+ PS_py’ of 1061.3 mV/mm are observed. Furthermore, the frequency-dependent enhancement ratios of (PS_ph + py-PS_ph)/PS_ph and (PS_ph + py+ PS_py’-PS_ph)/PS_ph are calculated, as illustrated in Fig. 5(d). At lower frequencies, specifically 10 and 20 Hz, the enhancement ratio is negligible. However, as the frequency increases, the ratios begin to rise markedly. Upon reaching 400 Hz, the maximum enhancement ratios of (PS_ph + py-PS_ph)/PS_ph and (PS_ph + py+ PS_py’-PS_ph)/PS_ph improve to 132.3% and 160.5%, respectively.

 

Fig. 5. (a) Transient LPV-t response curves under the illumination of 780 nm laser at 15mW with different frequencies ranging from 10 to 400 Hz. Extracted (b) LPVs, (c) PSs, and (d) PS enhancement ratios as a function of the laser frequency. (e) The maximum PSs and PS enhancement ratios as a function of the laser frequency. Amplified LPV-t curves of (f) 10Hz and (g) 400 Hz to determine the response times. (h) The extracted rise time and fall time as a function of the laser frequency.

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The transient LPE responses of the PSD were investigated at various pulse frequencies under illumination of different lasers (405, 450, 532, 671, and 1064 nm). The corresponding LPV-t curves, LPV values, and PS values are illustrated in Fig. S16 to S20. The LPE responses at these wavelengths, including LPV_ph, LPV_ph + py, LPV_ph + py-LPV_py’, PS_ph, PS_ph + py, PS_ph + py+ PS_py’ and the enhancement ratios, exhibits a similar frequency variation characteristic, with the best results of each wavelength summarized in Fig. 5(e). To ascertain the response times of the heterojunction PSD, the typical LPV-t curves at 10 and 400 Hz are enlarged in Fig. 5(f) and 5(g), respectively. As the pulse frequency increases, the response time of the PSD significantly decreases, as clearly shown in Fig. 5(h). Specifically, when the LPV expands from 10% to 90% of the peak value, the measured rise time decreases from 1.11 to 0.01 ms. Conversely, when the LPV reduces from 90% to 10% of the peak value, the fall time decreases from 1.2 to 0.04 ms. The significant improvement in the PS(/LPV) and response speed clearly demonstrates that the pyroelectric field induced by transient temperature variations in the CdS layer plays a crucial role in enhancing the separation and transport of carriers in the heterojunction.

The effects of different ambient temperatures ranging from 25 to 150 °C on the pyroelectric effect were also investigated. The results are shown in Fig. 6(a). The LPE response of the PSD exhibits a four-stage behavior, and the photovoltaic response (LPV_ph) remains nearly constant across the temperature range. However, the pyroelectric voltage peaks gradually decrease with increasing the temperature and nearly disappear at 150 °C. The LPV values of LPV_ph, LPV_ph + py, and LPV_ph + py−LPV_py′ at different temperatures are extracted and plotted in Fig. 6(b). There is an obvious decrease in the LPV_ph + py and LPV_ph + py−LPV_py′, especially at higher ambient temperatures. This can be attributed to the weakened light-induced temperature variation (dT/dt) in the heterojunction caused by the ambient temperature [22]. This result demonstrates that the transient LPV peaks are indeed a result from the pyroelectric effect. Additionally, the corresponding enhancement ratios are calculated to well illustrate the impact of ambient temperature on the pyroelectric effect, as depicted in Fig. 6(c). As the ambient temperature increases from 25 to 150 °C, the enhancement ratios of LPV_ph + py/LPV_ph and (LPV_ph + py−LPV_py′)/LPV_ph decline quickly from 642.89% and 415.19% to 106.62% and 106.02%, respectively. Nonetheless, the ITO/CdS/Si heterojunction PSD demonstrates outstanding photovoltaic effect-induced LPE responses at various ambient temperatures, even up to 150 °C, showcasing its substantial potential in harsh environments.

 

Fig. 6. (a) Transient LPV-t response curves of the ITO/CdS/Si heterojunction PSD under 450 nm illumination with different background environmental temperatures ranging from 25 to 150 °C. Extracted (b) LPVs, and (c) LPV enhancement ratios as a function of background environmental temperatures.

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In the design and application of commercial PSDs, the working distance is also a key performance parameter. Therefore, the LPE responses of the PSD were studied at various contact distances, ranging from 0.9 mm to 11 mm. Figure 7(a) displays the typical results of laser position-dependent LPV curves for different contact distances under a 780 nm laser irradiation at 15 mW. Obviously, the PSD works well across the entire contact distance, and the working distance is much larger than what has been reported in other material systems. As the contact distance increases from 0.9 to 11 mm, the PS decreases from 345.27 to 33.99 mV/mm, as shown in Fig. 7(b). Although the nonlinearity slightly increases at larger contact distances, it remains within the permissible threshold of <15% [31]. Furthermore, the impact of contact distance on the pyroelectric effect was also investigated. The LPV-t curves of different contact distances are illustrated in Fig. 7(c). The four-stage behaviors can be found in all these curves, and both the photovoltaic and pyroelectric responses decrease gradually with an increase in contact distance. The LPV and PS values of different contact distances are extracted and summarized in Fig. 7(d) and 7(e), respectively. Notably, the LPV_ph values of various contact distances are consistent with these in the laser position-dependent LPV curves at the corresponding positions (Fig. 7(a)), confirming the stable LPE responses in the PSD. Moreover, although there is a decrease in LPV_ph + py and LPV_ph + py−LPV_py′ at larger contact distances, the clear improvement of the LPE response induced by the pyroelectric effect can still be observed compared to the LPV_ph value. At a working distance of 11 mm, the LPV_ph + py and LPV_ph + py−LPV_py′ are 3.22 and 6.06 mV, respectively, all of which are much larger than the LPV_ph value of 1.50 mV. Meanwhile, the corresponding PS_ph + py of 68.59 mV/mm and PS_ph + py+ PS_py’ of 128.99 mV/mm are observed, which are improved by 213.8% and 402.1%, respectively, compared to the PS_ph of 32.08 mV/mm. This result demonstrates that the ITO/CdS/Si heterojunction, in combination with the pyroelectric effect, shows promising potential in developing high-sensitivity large-working distance PSDs.

 

Fig. 7. (a) Laser position-dependent LPV curves of the ITO/CdS/Si heterojunction PSD for various electrode distances under the illumination of 780 nm laser at 15 mW. (b) Extracted PSs and nonlinearity results as a function of electrode distances. (c) Transient LPV-t response curves of different electrode distances at 200 Hz. Extracted (e) LPVs and (f) PSs as a function of electrode distances.

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4. Conclusions

In conclusion, we successfully fabricate an ITO/CdS/Si heterojunction PSD with excellent linearity and a wide response range. The integration of the pyroelectric effect significantly enhances the LPE responses, with improvements depending on laser wavelength, power, and pulse frequency. With 15 mW illumination at 780 nm and a pulse frequency of 400 Hz, the PSD exhibits a maximum PS of 1061.3 mV/mm, which surpasses previous results. The response time is also ultrafast, with rise and fall times of 0.01 and 0.04 ms. Furthermore, the PSD’s response range extends to the near-infrared wavelength of 1550 nm, breaking the absorption limit of the ITO/CdS/Si heterojunction. Additionally, the impact of ambient temperature and working distance on the pyroelectric effect in the PSD was studied. The PSD demonstrates stable LPE response at ambient temperatures up to 150 °C, and it maintains a high PS of 128.99 mV/mm even at a large working distance of 11 mm. This work not only highlights the significant potential of the ITO/CdS/Si heterojunction in developing high-sensitivity, broadband, large working distance, and ultrafast PSDs, but also provides insights for designing high-performance PSDs using other heterostructures.