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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Fabrication and Characterization of a Capacitive Photodetector Comprising a ZnS/Cu Particle/Poly(vinyl butyral) Composite Sungwoo Jun,†,‡,∥ Su Bin Choi,§,∥ Chul Jong Han,† Yeon-Tae Yu,§ Cheul-Ro Lee,§ Byeong-Kwon Ju,*,‡ and Jong-Woong Kim*,§ †
Display Materials & Components Research Center, Korea Electronics Technology Institute, Seongnam 463-816, Republic of Korea Display and Nanosystem Laboratory, College of Engineering, Korea University, Seoul 136-713, Republic of Korea § School of Advanced Materials Engineering, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju 54896, Republic of Korea ACS Appl. Mater. Interfaces Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/19/19. For personal use only.
‡
S Supporting Information *
ABSTRACT: Most photodetectors developed to date essentially measure photocurrents induced by the generation and separation of electron−hole pairs in semiconductors during irradiation. Although the above light detection method is well established, highly sensitive, and applicable to a broad range of semiconductor materials, it requires the presence of a stable and direct contact between the semiconductor and the electrode for accurate photocurrent measurements. In turn, this prerequisite necessitates the use of various costly processes for device fabrication (e.g., photolithography and vacuum deposition of semiconductors/metals) and complicates the development of flexible devices. Herein, inspired by the fact that the dielectric properties of certain materials can be changed by light irradiation, we dispersed ZnS/Cu semiconducting particles in poly(vinyl butyral) to prepare a free-standing composite film and formed two layers of Ag nanowire electrodes on both sides of the cured composite to fabricate a photodetector of a completely new type. The developed device exhibited a capacitance very sensitive to irradiation with light of a specific wavelength and additionally featured the advantages of simple structure/operation mechanism, mechanical flexibility, and transparency, not showing any signs of performance deterioration even after severe damage. KEYWORDS: photodetector, capacitive photosensing, polymer composite, electroluminescence, Ag nanowires
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INTRODUCTION The fabrication of wearable optoelectronic devices, patch-type surveillance systems, and image sensors for surgical robots requires the development of flexible, semitransparent, and skinlike photodetectors (PDs).1 Generally, PDs convert incident photons into an electrical signal that is then supplied to the integrated circuits of various devices and adjusted to suit the needs of a particular application. Today’s PDs utilize a broad range of semiconductors (e.g., Si, GaP/SiC, ZnO, InGaAs/Ge, quantum dots, organic polymers, and organometal halide perovskites) exhibiting different band-gap energies, spectral responses, and carrier mobilities/diffusion lengths, which allow for variations of responsivity, detectivity, and response time to light irradiation.1−12 The operation of semiconductor-based PDs relies on the creation of electron−hole pairs upon illumination with light. In particular, when a semiconductor is irradiated by photons with an energy higher than or equal to that of its band gap, valenceband electrons are excited to the higher-lying conduction band, whereas positively charged holes are retained in the valence band. The above separation of electron−hole pairs gives rise to a photocurrent, the intensity of which at a given wavelength increases with increasing irradiation intensity.1−8 Therefore, to © XXXX American Chemical Society
use the photocurrent as a sensory parameter, semiconductorbased PDs should comprise a current measurement system directly connected to the semiconductor material through electrodes. Although the above principle appears simple, its implementation in flexible devices is hindered by the rigidity of most high-efficiency semiconductors and the difficulty of establishing reliable semiconductor−electrode contacts. The intrinsic rigidity of the semiconductors implies that flexible devices could ideally utilize them in the form of a percolated structure comprising particles or wires. Semiconductor materials in the form of layers are inevitably subject to mechanical stresses since they are deformed or bent upon absorbing stresses applied to the device surface. Therefore, if the semiconductor material features a relatively low-density percolated structure, a significant amount of the stress applied to the device surface may be absorbed by the deformation of the exposed substrate, allowing the semiconductor itself to function without significant damage despite its high brittleness. In fact, a number of PD structures using these types of Received: November 15, 2018 Accepted: January 7, 2019
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DOI: 10.1021/acsami.8b20136 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 1. Schematic fabrication of capacitive PDs.
materials have already been reported.13−17 However, the above approach has its inherent drawbacks, for example, it necessitates the establishment of direct contact between electrodes and semiconductor materials for photocurrent measurements. Considering the fact that the electrode− semiconductor contact area may be very small in this case, the above requirement can lead to issues such as poor current flow in either direction and instable electrode−semiconductor bonding. According to recent studies, the dielectric response of composites comprising semiconductor particles dispersed in a polymer changes upon irradiation with light, which has been explained by interfacial polarization and the Maxwell− Wagner−Sillars effect at particle−polymer interfaces.18−20 The latter effect is known to originate from the chargeaccumulation-induced formation of large dipoles at particle− polymer interfaces. When a composite of the above type is irradiated with high-energy photons, the amount of charge accumulated at the particle−polymer interface increases, which results in enhanced dielectric permittivity.18 Inspired by this fact, we herein propose a PD of a totally new type based on the photodielectric effect and develop a device architecture with excellent mechanical flexibility/stability. To achieve this, we fabricated a semiconductor-polymer composite by dispersing copper-doped zinc sulfide (ZnS/Cu) microparticles, a wellknown phosphorescent material, into a flexible, free-standing polymer because they are easy to obtain, low in cost, and easy to disperse due to low specific surface area. The operation of the developed PD comprising the ZnS/Cu particles and polymer matrix relies on the abovementioned enhancement of dielectric permittivity in the composite upon irradiation with light of a specific wavelength. In particular, the dielectric permittivity change increases the capacitance formed in the composite film sandwiched between parallel transparent electrodes, that is, the developed architecture does not require the semiconductor material and the electrode to be in direct contact. Furthermore, the formation of a sensory layer by mixing semiconductor particles with a polymer material that can be processed into a film after the curing reaction allows one to fabricate a PD that is highly flexible and mechanically stable without using a separate substrate. Finally, the fabrication process is very simple, and the measurement
method and principle are highly intuitive, which allow the developed semitransparent and mechanically flexible capacitive PD to be viewed as a prototype of other next-generation devices.
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EXPERIMENTAL SECTION
Materials. Poly(vinyl butyral) (PVB; Butvar B-98, Mn ≈ 36 000 g/ mol) with a poly(vinyl alcohol) hydroxyl content of 18% was purchased from Victorchem, Korea. Hexamethylene diisocyanate (HDI) and N,N-dimethylformamide (DMF) were obtained from Tokyo Chemical Industry, Japan. All chemicals were used as received without purification. A 0.5 wt % dispersion of silver nanowires (AgNWs; average diameter and length = 24 nm and 20 μm, respectively) in isopropanol was purchased from Nanopyxis Ltd. (Korea) and was used as received without removing the poly(vinyl pyrrolidone) deposited on nanowires. Spherical ZnS/Cu (copper content: 9 atom %) microparticles (average diameter ≈ 20 μm) were purchased from National EL Technology, Korea. Fabrication of PDs. The procedure used to fabricate a ZnS/Cu− PVB-based PD is schematically described in Figure 1. A glass slide cleaned by sequential rinsing with a detergent solution, deionized water, acetone, and isopropanol was treated with oxygen plasma to form surface hydroxyl groups. The oxygen flow rate, pressure, power, and treatment time were controlled at 30 mL/min, 15 Pa, 50 W, and 40 s, respectively. The glass substrate was spin-coated with the AgNW dispersion and dried under infrared light for 5 min. To prepare the ZnS/Cu−PVB composite, PVB (2 g) was dissolved in DMF (20 mL) upon 5 min sonication, and the obtained solution was treated with HDI (0.3 g). Subsequently, ZnS/Cu microparticles were dispersed in the obtained mixture (to achieve loadings of 20, 35, 45, 50, and 55 wt %) upon 10 min sonication, and the resulting dispersion was spincoated onto the AgNW-coated glass. Polymer cross-linking by overnight curing at 80 °C afforded a film with a thickness of 30 μm, and the cured ZnS/Cu−PVB film was spin-coated with another layer of AgNWs and dried under UV irradiation for 5 min. The thusprepared samples were exposed to 12 rounds of irradiation with intense pulsed light (IPL; intensity is equal to 0.5 J/cm2) at 5 s intervals using a photonic sintering system (Sinteron 2000, Polytec Ltd.) and soaked in water at 25 °C for 10 min to induce the hygroscopic swelling of the composite film for its safe peel-off from the preliminary glass substrate. Characterization. Field-emission scanning electron microscopy (FESEM; JSM6700F, JEOL Ltd., Japan) was used to investigate the sample microstructure. Film optical transmission and absorbance were measured using a UV−visible spectrophotometer (V-560, Jasco, Japan), and sheet resistance (Rs) was measured using a noncontact B
DOI: 10.1021/acsami.8b20136 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 2. FESEM micrographs of (a) ZnS/Cu particles, (b) cross section of a device comprising the ZnS/Cu−PVB composite sandwiched by two AgNW electrodes. (c) Bottom-view and (d) top-view FESEM images of the fabricated device. Expanded views of NWs on bottom (e) and top (f) surfaces of the device. measurement system (EC-80P, Napson Corporation, Japan). The electrode surface morphology was investigated by atomic force microscopy (AFM; XE-100TM, Park Systems). An automatic bendtesting machine (Bending Tester, Jaeil Optical System, Korea) was used to measure long-term device reliability under repeated bending at a rate of 1.2 cycles/min. An alternating current (AC) power source (6600 series, Extech Electronics, Taiwan) was used to power the emitting devices, and luminance was measured by a luminance meter (LS-100, Konica Minolta, Japan). An inductance−capacitance− resistance meter (HP4284A, Agilent) was used to measure the sample capacitance. Most parameters were measured for more than 10 sample replicates.
of a percolated network of AgNWs on the surface of the plasma-treated glass was followed by spin-coating of the ZnS/ Cu−polymer composite. PVB was selected as a polymer matrix in view of its high transparency, optical clarity, strong binding ability, flexibility, and good adhesion to a wide range of surfaces.21−23 Furthermore, PVB exhibited a free-standing nature after sufficient cross-linking with HDI, which implied that the ZnS/Cu−PVB composite did not need any support such as glass or other polymeric films after cross-linkage formation. Since the strain formed on the surface of a film is equal to its thickness divided by twice the bending radius, the lack of a separate substrate originating from the free-standing nature of the composite greatly contributed to the mechanical stability of the resulting device during bending sequences.24 After cross-linking, another layer of AgNWs was formed on the opposite side of the composite to afford a complete sandwich structure comprising two AgNW layers and a ZnS/Cu−PVB composite layer. When the completed structure was peeled off from the glass support, the AgNW layer on the bottom side of the composite (initially deposited on glass) was expected to be fully embedded just underneath the composite surface.25−27 The above structure is usually implemented by inverted layer processing, which features the initial deposition of an electrode material such as AgNWs or carbon nanotubes onto a
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RESULTS AND DISCUSSION The dielectric permittivity enhancement of semiconductormaterial-containing polymer composites upon irradiation with light of a specific wavelength is expected to change the capacitance formed in the composite sandwiched by two conductors. To verify this hypothesis, we designed and fabricated a free-standing composite film comprising ZnS/Cu semiconducting particles dispersed in a dielectric polymer matrix and formed a AgNW electrode on both surfaces of the composite film, as shown in Figure 1. First, a glass substrate was treated with oxygen plasma to form surface hydroxyl groups and enable the uniform deposition of AgNWs and their stable adhesion to the glass surface. The subsequent deposition C
DOI: 10.1021/acsami.8b20136 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 3. ZnS/Cu particle-content-dependent (a) transmittance and (b) haziness of devices with two layers of AgNW electrodes. Inset of (a) shows an image of the fabricated device held over a paper with “KETI” emblems.
square-wave excitation, the applied frequency was expected to be positively correlated with EL brightness.32 Figure 2b presents a cross-sectional micrograph of the fabricated device (film thickness = total thickness of the fabricated device ≈ 30 μm), showing that ZnS/Cu particles were uniformly distributed within the PVB matrix. Here, since the specific surface area of the spherical ZnS/Cu particles was much smaller than those of most nanosized particles (nanoparticles), the aggregation between the particles was not observed. Furthermore, separation of the ZnS/Cu particles from the PVB matrix or tearing of the film caused by the stress singularity at the particles did not occur even after tens of times of tape testing or bending, confirming that the ZnS/Cu particles were stably bonded to the PVB. Figure 2c and d reveals the presence of AgNWs on the bottom and top surfaces of the composite, respectively, additionally demonstrating the presence of particle-shaped objects and thus confirming that ZnS/Cu particles were embedded into the PVB matrix. Interestingly, the nanowires look slightly fuzzy in the bottom electrode image (Figure 2c), whereas the top electrode NWs in Figure 2d appear sharper. Figure 2e presents an expanded tilt-view image of the fabricated device, demonstrating that in the case of the bottom electrode, NWs embedded underneath the composite surface featured a significantly flattened shape. However, in the case of the top electrode (Figure 2f), the NWs seem to have been partially fused by the underlying polymer without being completely impregnated beneath the surface. This difference was ascribed to the instant increase of AgNW temperature by the highly intense pulse of photonic energy delivered within a few hundred microseconds.33 The heat generated at NWs increased the temperature of the surrounding polymer, inducing its partial melting and fusion around the NWs (Figure S1), whereas the subsequent rapid cooling of the AgNW−PVB interface due to the high thermal conductivity of Ag afforded the partially embedded structure shown in Figure 2f. Figure S2 shows AFM micrographs of both electrodes, revealing that their roughness was significantly lower than that of AgNWs directly deposited onto glass or polymer surfaces (typically, Rpv > 100 nm, RRMS > 20 nm). Here, the low roughness values of the bottom electrode are attributed to the high smoothness of the glass surface initially coated with AgNWs and PVB and demonstrate that the AgNW network is completely submerged directly under the surface of the PVB. On the contrary, AgNWs of the top electrode were
temporary substrate such as glass followed by application of a solution-processable polymer material and substrate removal. Since the electrode material was impregnated directly under the surface of the cured polymer, the contact area between the polymer and the electrode material was maximized, which resulted in improved mechanical stability, that is, film flexibility. However, the mechanical stability of the electrode applied to the opposite side of the composite was a subject of concern since the nanowires were simply placed on the composite surface without any chemical interaction and no stable bond formation between the two materials could therefore be expected. Herein, we employed photoinduced heating using an IPL system to resolve this issue in view of the well-known ability of IPL irradiation to dramatically increase the temperature of AgNWs and the nearby region, in line with the propensity of nanoscale metallic structures to generate heat under optical illumination.23,28 Notably, the above ability is strongly enhanced by plasmon resonance when the frequency of incident light matches that of the collective resonance of the fabricated nanostructures,24,29,30 which allows one to very rapidly increase the temperature of AgNWs and stimulate a wide range of physical processes and chemical reactions with unprecedented spatial and temporal control.28,31 The most noteworthy aspect of this type of heating with subsequent cooling is that it is very rapid and selective, which implies that areas located far from NWs remain unheated and is particularly important in view of the fact that heating may damage other device constituents. Therefore, this approach was expected to result in the incorporation of top-side AgNWs into the composite surface. Peel-off (step 6 in Figure 1) afforded a device in which the top electrode was partially fused into the PVB surface, whereas the bottom electrode was fully embedded just underneath the PVB surface. After repeated tape tests (up to 20), performed by applying commercial adhesive tape onto both electrodes, Rs did not increase by more than 3%, which indicated that the extent of electrode adhesion to the composite was sufficient for use in flexible devices. As shown in Figure 2a, ZnS/Cu particles with an average diameter of ∼20 μm were employed as a photoresponsive semiconductor material exhibiting AC-driven electroluminescence (EL). Notably, since the above particles emit bright blue-green (455 nm) light upon voltage switching when driven with a 100−200 V AC supply in the 0.1−100 kHz range using D
DOI: 10.1021/acsami.8b20136 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 4. (a) Luminance of fabricated devices as a function of applied voltage and ZnS/Cu particle content in the composite. (b−d) Images of light-emitting devices operated under the conditions marked in (a). (e) Image of a bent light-emitting device demonstrating its flexibility and emission at both sides.
The thickness of the composite could be precisely controlled by the maximum rotation speed of the spin-coating process. As shown in Figure S6, the maximum luminance of the device (driven by application of 300 V at a fixed frequency of 500 Hz) decreased with increasing thickness of the composite, mainly due to the formation of a larger electric field in the emissive layer of thinner thickness. Therefore, to obtain the highest EL performance, we set the thickness to the minimum possible thickness of 30 μm considering the phosphor particle size. The emission intensity of films with varying ZnS/Cu particle loadings was also measured as a function of applied voltage at a fixed frequency of 500 Hz (Figure 4a) and increased with increasing applied voltage and the ZnS/Cu particle content. The increment of EL intensity with increasing voltage was ascribed to the fact that the probability of electrons to be accelerated to a given energy and subsequently excite luminescent centers steeply increases beyond a certain bias voltage, whereas the higher density of lighting sources (ZnS/ Cu particles) also contributes to EL intensity enhancement, as shown in Figure 4a−d.31,34 The high transparency of PVB and the well-designed structure of the fabricated device allowed us to obtain a record high maximum luminance of ∼89 cd/m2. In addition, since both the top and bottom electrodes were transparent, the device emitted light on both sides with equal EL intensity (Figure 4e). Given that this study primarily aimed to develop a PD with a novel structure and sensory mechanism, we tried to measure the band-gap energy of the ZnS/Cu-particle-containing composite film. In doing so, we employed the Tauc bandgap determination method,35−37 which assumes that the optical absorption of a material depends on the difference between the photon energy and the band gap as follows38
applied on the surface of the PVB, so the initial roughness was high, but it decreased by the post-treatment with the IPL. However, in this case, only a part of the contact portion of the AgNWs with the PVB surface was impregnated and showed somewhat higher roughness values than those of the bottom electrode, which is completely embedded. Figure 3a and b shows the transmittance and haziness (i.e., the ratio of diffused and total transmittances), respectively, of the fabricated films including the top and bottom electrodes (Rs of both electrodes was determined as ∼9 Ω/sq) for various ZnS/Cu loadings, with the corresponding data for films without electrodes given in Figure S3. Films with a ZnS/Cu loading of 20 wt % featured high transmittance over the entire visible range, for example, a transmittance of ∼84.9% was observed at 550 nm. Notably, the above films transmitted >50% of light with a wavelength of 350 nm, which cannot be achieved for aromatic polymers such as colorless polyimide, in which case the strong absorption of light by aromatic moieties and the formation of charge transfer complexes in highly conjugated molecular structures result in poor transmittance in the wavelength range of 350−450 nm.31 Considering the fact that the fabricated films were intended for use in PDs, the high transmittance of the entire assembly originating from the excellent transparency of PVB was very important (Figure S4). For samples with a ZnS/Cu loading of 55 wt %, the transmittance at 550 nm still exceeded 60%, and the letters under the film were clearly visible, as shown in the inset of Figure 3a. The somewhat yellowish color of the film was ascribed to the illumination-induced strong surface plasmon resonance due to the strong coherent oscillation of free surface electrons in NWs.31 Actually, the transmittance of the AgNWfree ZnS/Cu−PVB film was found to be higher in the lowwavelength region (Figure S3a). Figure 3b shows that film haziness at 550 nm ranged from 18 to 35% depending on the ZnS/Cu content. Considering the two-layer NW electrode architecture and the considerable loading of ZnS/Cu particles, the above values were considered to be fairly good and suitable for the fabrication of various invisible devices. The first feature implemented in the developed device (Figure S5) corresponded to AC-driven EL. To optimize the structure of the device prior to evaluation of the EL performance, the influence of the thickness of the ZnS/Cu− PVB composite on the EL intensity was measured (Figure S6).
(αhν)1/ n = A(hν − Eg )
(1)
where α is the absorption coefficient, h is Planck’s constant, ν is the photon frequency, Eg is the band-gap energy, and A is a proportionality constant.39 Since ZnS-based semiconductors are well known to exhibit a direct allowed transition, a value of n = 0.5 was used. Notably, the Tauc model is based on the fact that absorbed photons induce electronic transitions from the valence band to conduction bands35−37 and implies that photons with energies lower than that of the band gap are not E
DOI: 10.1021/acsami.8b20136 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 5. (a) Absorption spectra and (b) Tauc plots of ZnS/Cu−PVB composites with variable ZnS/Cu particle contents.
Figure 6. Photoswitching characteristics of as-prepared devices upon dark−light (420 nm, 0.666 mW/cm2) cycling determined for variable (a) ZnS/Cu particle loadings and (b) bending conditions. (c) Capacitance of the composite under dark and light conditions and device sensitivity as a function of bending cycle number. (d) Photoswitching characteristics of uncut and cut devices.
Based on the position of the absorption peak and the composite band-gap energy, a lighting source with a wavelength of 420 nm and a power of 0.666 mW/cm2 was employed to test the fabricated PDs. The photoresponsive capacitance was measured under ambient conditions using a two-probe method and a 50 kHz signal. The ZnS/Cu-contentdependent photoresponsivities of the above PDs are demonstrated in Figure 6a, which shows that the areal capacitance of the composite layer showed high photosensitivity and low photoresponse time. Here, the thickness of the composite was also set to 30 μm because the highest sensitivity was measured at this thickness, just like the EL
absorbed (i.e., the irradiated material is transparent to such light), whereas the absorption of light with near-band-gap photon energy gets stronger. Figure 5a shows the absorption spectra of composite films with varying ZnS/Cu contents at room temperature, demonstrating that the absorption peak observed at 423 nm in all cases gained intensity with increasing ZnS/Cu content. Figure 5b presents the Tauc plots for spectra displayed in Figure 5a, with the apparent linear behaviors indicating the occurrence of a direct transition. Extrapolation of the linear regions of (αhν)2 vs hν plots showed that the band gap of composite films equaled 2.93 eV and was independent of the ZnS/Cu content. F
DOI: 10.1021/acsami.8b20136 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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CONCLUSIONS Herein, inspired by the fact that the dielectric permittivity of a given semiconductor increases upon its irradiation by photons with an energy equal to or higher than the semiconductor band-gap energy, we successfully developed a capacitive PD by dispersing ZnS/Cu particles in dielectric PVB. The strong hydrogen-bonding interactions between PVB chains and their entanglement allowed for the formation of a free-standing and mechanically robust ZnS/Cu−PVB composite, obviating the need for a separate substrate. A percolated layer of AgNWs was embedded just underneath the composite surface by inverted layer processing, whereas another layer of AgNWs deposited onto the opposite side of the composite was subjected to photoinduced embedding. The simple fabrication and structure of the device enabled the realization of two independent functions, namely, (1) AC-driven EL and (2) capacitive photosensing. Moreover, an equally high EL intensity of ∼89 cd/m2 was measured on both sides of the device owing to its symmetrical structure and the high transmittance of PVB. Thus, the fabricated device could function as a capacitive PD with high sensitivity, fast response time, mechanical flexibility, optical transparency, and wavelength selectivity. The variation of dielectric characteristics of the ZnS/Cu−PVB composite upon irradiation was reflected in the corresponding capacitance changes. Moreover, the operation of the capacitive sensing mechanism obviated the need for ZnS/Cu particles to be in direct contact with the electrode. In view of this uniqueness, the above device was mechanically flexible and stable, and no functionality deterioration was observed even after severe damage such as cutting. The most significant advantage of the produced PD is the ease of implementing various functionalities such as stretchability or self-healability by a simple change of polymer in the composite. In view of the simplicity of the involved fabrication and operation procedures, the present findings are expected to provide a practical guideline for the fabrication of various forms of next-generation capacitive PDs.
intensity (Figure S6). Importantly, the photoresponsivity of the fabricated PDs was very selective, that is, no capacitance change was observed upon irradiation by light with a wavelength of 550 nm. The above result implies that sensitizers of different wavelengths can be fabricated using semiconductor particles with different band-gap energies as fillers. Finally, the photoresponsivity remained high irrespective of the ZnS/Cu particle content, whereas the observation of a slight sensitivity enhancement with increasing particle content indicated the existence of a trade-off between the transparency and sensitivity of as-fabricated PDs. This behavior was ascribed to the fact that the photodielectric effects in ZnS/Cu particles represent the dielectric response of photoinduced dipole moments comprising significantly localized photoexcited carriers.40 Since the composite was mechanically flexible, we also measured the areal capacitances of as-fabricated PDs (ZnS/Cu content = 55 wt %) in the bent state (Figure 6b). Although these measurements were carried out without accounting for the diffused incidence angle and the areal issues of light originating from PD bending, the effect of curvature radius on sensitivity appeared to be negligible. Additionally, the capacitance of PDs was recorded as a function of bending cycle number under dark and light conditions (Figure 6c). In these measurements, a bending radius of 500 μm was used, corresponding to the generation and dissipation of an approximately 3% strain on the PD surface. Both dark- and light-mode capacitances slightly increased with increasing number of bending cycles possibly owing to the compressive plastic strain accumulated in the composite layer during repeated bending and releasing, and a 12.8% decrease of sensitivity was observed after 5000 cycles. Considering the resolution of capacitance measurement systems typically used in today’s capacitive touch sensors (∼1 pF), such sensitivity decreases should not be a matter of big concern in practice. In contrast, this insignificant sensitivity reduction highlights the excellent flexibility and mechanical stability of the developed PDs that can be attributed to (1) the high strength (44.8 MPa) and elongation at break (23.3%) of cross-linked PVB originating from the strong hydrogen-bonding interactions and entanglement of PVB chains,21 (2) the mechanical stability of transparent electrodes partially or fully embedded underneath the composite surface, and (3) the reduction of PD thickness enabled by substrate elimination. Owing to factor (3), a strain of only 3% was formed on the device surface by a near-folding sequence. Furthermore, we did not need to employ any costly processes such as thin-film deposition under vacuum or specific pattern generation by photolithography, relying only on the hybridization of polymer and semiconductor particles followed by the formation of two layers of transparent electrodes. Figure 6d also shows that the areal capacitance remained virtually unchanged after the complete cutting of PDs with a pair of scissors, which was ascribed to the simplified device structure. Notably, each piece of the cut device maintained its functionality without any change in sensitivity, which is an advantageous feature that cannot be implemented in devices based on existing photocurrent-based PDs. Since the core of the developed device is based on the polymer−semiconductor particle hybrid, it allows for the implementation of completely new functions via the replacement of PVB with other polymers such as those exhibiting stretchability or self-healing ability.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b20136.
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Additional experimental data, AgNW embedding sequence by photoinduced fusion of the PVB surface, AFM micrographs of bottom and top electrodes shown in Figure 2b, graphs of transmittance and haziness of AgNW-free devices with variable ZnS/Cu particle loadings as well as of a bare PVB film, photograph of a fabricated device for measurements of EL and PD characteristics, thickness effect of ZnS/Cu−PVB composite on the EL intensity (luminance) and photosensitivity (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. Tel: +82 2 3290 3237 (B.-K.J.). *E-mail:
[email protected]. Tel: +82 63 270 2292 (J.-W.K.). ORCID
Yeon-Tae Yu: 0000-0001-5003-0660 Cheul-Ro Lee: 0000-0003-2082-8828 Byeong-Kwon Ju: 0000-0002-5117-2887 G
DOI: 10.1021/acsami.8b20136 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
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Jong-Woong Kim: 0000-0003-4010-056X Author Contributions ∥
S.J. and S.B.C. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by a National Research Foundation of Korea (NRF) grant (Numbers 2015R1A4A1042417, 2018R1D1A1B07047386, and 2016M3A7B4910) funded by the Korean government (MSIP).
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ABBREVIATIONS PD, photodetector PVB, poly(vinyl butyral) HDI, hexamethylene diisocyanate DMF, N,N-dimethylformamide AgNW, silver nanowire ZnS/Cu, Cu-doped ZnS IPL, intense pulsed light FESEM, field-emission scanning electron microscopy Rs, sheet resistance AFM, atomic force microscopy EL, electroluminescence
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DOI: 10.1021/acsami.8b20136 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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DOI: 10.1021/acsami.8b20136 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
