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An Omnidirectionally Stretchable Photodetector Based on Organic-Inorganic Heterojunctions Tran Quang Trung, Vinh Quang Dang, Han-Byeol Lee, Doil Kim, Sungjin Moon, Nae-Eung Lee, and Hoen Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09411 • Publication Date (Web): 26 Sep 2017 Downloaded from http://pubs.acs.org on September 29, 2017

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ACS Applied Materials & Interfaces

An

Omnidirectionally

Stretchable

Photodetector

Based

on

Organic-Inorganic

Heterojunctions

Tran Quang Trung,‡,† Vinh Quang Dang,±,† Han-Byeol Lee,‡ Do-Il Kim,‡ Sungjin Moon,± NaeEung Lee,‡,#,§,* and Hoen Lee±,*

‡

School of Advanced Materials Science & Engineering, Sungkyunkwan University, Suwon,

Kyunggi-do16419, Republic of Korea #

SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon,

Kyunggi-do16419, Republic of Korea §

Samsung Advanced Institute for Health Sciences & Technology (SAIHST), Sungkyunkwan

University, Suwon, Kyunggi-do16419, Republic of Korea ±

Department of Materials Science and Engineering, Korea University, Seongbuk-gu, Anam-ro

145, Seoul 02841, Republic of Korea *

e-mail: [email protected].*e-mail: [email protected]

†

These authors contributed equally to this work

Keywords: Omnidirectional stretchability, photodetector, organic-inorganic heterojunction, ZnO nanorods, stretchable optoelectronics

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ABSTRACT

Omnidirectionally stretchable photodetectors are limited by difficulties in designing material and fabrication processes that enable stretchability in multiaxial directions. Here, we propose a new approach involving an organic-inorganic p-n heterojunction photodetector comprised of free-standing ZnO nanorods (NRs) grown on a poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate transport layer coated on a three-dimensionally micro-patterned stretchable substrate containing bumps and valleys. This structure allows for efficient absorption of stretching strain. This approach allows the device to accommodate large tensile strain in all directions. The device behaves as a photogated p-n heterojunction photodetector, in which current modulation was obtained by sensing mechanisms that rely on photovoltage and photogating effects. The device exhibits a high photoresponse to UV light and reliable electrical performance under applied stretching in uniaxial and omni-axial directions. Furthermore, the device can be easily and conformally attached to a human wrist. This allowed us to investigate the response of the device to UV light during human activity.


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INTRODUCTION Enormous efforts have been made to realize stretchable electronics; these include a myriad of developments such as new materials,1-3 novel structural engineering,4-6 and smart fabrication processes.7 Stretchable electronics that provide morphological adaptability and mechanical properties that match those of human skin and organs can be conformally attached to a human body to monitor physiological activity and communicate with external electronics, thereby playing an important role in personalized medicine and healthcare.8-11 Many stretchable sensor-integrated platforms have been developed that have the ability to transform human activities into discernible electrical signals and transfer those electrical signals to external electronics for further analysis in order to obtain insight into user activity level and/or health.12-15 Among these platforms, some stretchable optoelectronic devices such as stretchable light-emitting diodes (LEDs),16,

17

stretchable solar cells,18 and stretchable

photodetectors19-21 have been demonstrated. Stretchable photodetectors that can be conformally attached to the human body or any arbitrary surface are promising for a variety of innovative applications. For example, they can be mounted on the hemispherical lens of the human eye to enlarge the field of view, tune focal lengths to improve imaging capabilities, and help blind people regain their visual senses.19, 21 Moreover, stretchable photodetectors can be used to develop wearable biological systems such as wearable optoelectronic sensors for medical applications, wearable pulse oximetry for physiological monitoring,22 electronic eyes in cameras for special curvilinear shapes,19 and infrared cameras for night vision.23 Stretchable UV photodetectors have been fabricated via two approaches: (1) transferring UV photodetectors on pre-stretched elastomeric substrates, followed by strain release to generate wavy,24 crumpled,25 or nanopatterned26 structures and (2) embedding semiconductor NWs such as ZnO NWs20 and Zn2SnO4 NWs27 into an elastomeric substrate. However, these approaches have some limitations. For example, complicated structural engineering processes make the processes complex, costly, and difficult to control. 3 ACS Paragon Plus Environment


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Furthermore, it is difficult to obtain NW materials (such as conductors, semiconductors, and insulators) that can be embedded in an elastomer matrix. Additionally, most of the reported stretchable UV photodetectors can be stretched only in the uniaxial direction, and they have low mechanical durability (see Supporting Table 1). In this respect, it is reasonable to consider an alternative approach to develop stretchable UV photodetectors with omnidirectional stretchability and high mechanical durability based on conventional facile processes. Zinc oxide nanorods (ZnO NRs) have the potential for use in ultraviolet (UV) photodetectors because of their wide direct bandgap (3.37 eV), large exciton binding energy (60 meV), and high surface-to-volume ratio.28 ZnO NR-based UV photodetectors can be fabricated using four different conventional structures: p-n photodiodes,29,

30

Schottky

photodiodes,31, 32 metal-semiconductor-metal photoconductors,33 and hybrid photodetectors.34, 35

Most of these are rigid or flexible UV photodetectors. Developing stretchable UV

photodetectors based on brittle ZnO NRs is a challenge, and no studies have investigated stretchable ZnO NR-based UV photodetectors. Therefore, to achieve stretchable UV photodetectors based on brittle ZnO NRs, it is necessary to design a new device structure and stress-absorbing substrate to realize stretchable optoelectronics. Herein, we propose a new approach for the fabrication of omnidirectionally stretchable UV photodetectors based on a p-n heterojunction of free-standing ZnO NRs and a poly(3,4ethylenedioxythiophene)-polystyrene sulfonate (PEDOT:PSS) transport layer formed on a three-dimensionally-patterned (3D-patterned) polydimethylsiloxane (PDMS) substrate with a micro-scale mogul-like pattern that can absorb tensile strain in multiple directions. The use of a 3D-patterned stretchable substrate allows the electrodes and transport layer of the device to be coated and patterned successfully, and this design is capable of sustaining tensile strain in all directions.36 The key aspect of the design of this device is the growth of free-standing ZnO NRs (as a UV absorbing material) on an organic material (PEDOT:PSS) as a transport layer to 4 ACS Paragon Plus Environment

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generate a p-n heterojunction structure that has good mechanical compliance and can be easily formed on any arbitrary surface. Based on these characteristics, the heterojunction device can be combined with a stretchable stress-adaptable structure, such as a 3D-patterned elastomeric substrate, to create a stretchable UV photodetector with free-standing brittle ZnO NRs that are free of stretching deformation. Furthermore, the formation of the p-n heterojunction in PEDOT:PSS-ZnO NRs also provides highly sensitive detection capabilities for stretchable UV photodetectors via simultaneous photovoltage37 and photogating38 effects. Based on these sensing mechanisms, the device operates as a photogated heterojunction photodetector.37, 38 The fabricated device exhibits high sensitivity to UV and reliable electrical performance under applied stretching in uniaxial and multiaxial directions. Furthermore, the device can be easily and conformally attached to a human wrist so that the response of the device to UV light during human activities can be investigated. Based on these features, stretchable UV photodetectors show promise for applications in future stretchable and wearable optoelectronics.

RESULTS AND DISCUSSION Omnidirectionally stretchable UV photodetector architecture. A 3D-patterned PDMS substrate was fabricated by replication of a mogul-patterned mold on a glass substrate, which was first generated via double photolithography. A more detailed description of the fabrication process and the characteristics of the 3D-patterned PDMS substrate were presented in a previous report.36 The bumps and valleys on the surface of the 3D-patterned PDMS substrate were regularly positioned in a hexagonal closed packed structure, on which the fabricated UV photodetectors were capable of sustaining tensile strains in all directions. The stretchable UV sensor could be conformally attached to the wrist of a human body to continuously detect UV light during human movement (Figure 1a).



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The fabrication process and structure of the stretchable UV photodetector are presented in Figure 1b. The Cr/Au electrodes and p-n heterojunction of PEDOT:PSS/ZnO NRs in the device were continuous, uniform, and formed directly on the 3D-patterned PDMS substrate. In the heterojunction structure, free-standing ZnO NRs (as a UV absorbing material) were grown on the PEDOT:PSS transport layer to create a p-n heterojunction with UV responsivity. The field-emission scanning electron microscopy (FE-SEM) image in Figure 1c illustrates the morphology of the ZnO NRs grown on the PEDOT:PSS film, which was directly spin-coated onto the stretchable 3D-patterned PDMS substrate. The ZnO NRs were grown at a high density to fully and uniformly cover the 3D-patterned PDMS substrate. The average length and diameter of the rods were around 2 µm and 90 nm, respectively. The optical absorption properties of the PEDOT:PSS/ZnO NRs heterojunction were also investigated by measuring the UV-VIS absorption spectrum from 300 nm to 800 nm. It is obvious that the peak absorption was located in the UV region (365 nm), which confirms the material’s usefulness as a UV sensor (see Figure S1). The UV absorption of the heterojunction structure was attributed to the wide band gap of the ZnO NRs (~3.37 eV).39 Therefore, free-standing ZnO NRs were used as the sensing material and were free from any stretching damage induced in the heterojunction structure. Photoresponse and sensing mechanisms. To obtain a better understanding of the photodetection operation, we investigated the current-voltage (ID-VD) characteristics and the time-dependent response of the stretchable UV photodetector under UV illumination at λ = 365 nm (i.e., we used a UV LED supplied with a constant power). The symmetrical and linear ID-VD output curves measured under dark conditions indicate the Ohmic contact between the Cr/Au electrodes and PEDOT:PSS channel (Figure 2a). The large dark current of the stretchable UV photodetector was attributed to the high conductivity of the PEDOT:PSS layer. PEDOT:PSS was chosen as a transport layer in the p-n heterojunction structure because it was suitable for fabrication of the transport layer in a stretchable a p-n heterojunction UV 6 ACS Paragon Plus Environment

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photodetector. For example, the aqueous solution of PEDOT:PSS can be spin-coated directly on 3D-patterned PDMS substrate with high uniformity without affecting PDMS characteristics (organic solvent can cause PDMS to swell). In addition, the electrical and optical properties of PEDOT:PSS were safe in the hydrothermal reaction during growth of ZnO NRs on the PEDOT:PSS layer. The time-dependent responses of the photodetector were measured at a UV intensity (P) of 2.32 mW/cm2 and a fixed voltage (VD) of 1 V (Figure 2b). Our device exhibited a high photocurrent under exposure to UV light. The increase in current shown in Figures 2a,b under UV illumination can be explained in terms of the following sensing mechanisms. In the proposed device structure, the growth of ZnO NRs (photoresponsive layer) on the electronic transmission channel of PEDOT:PSS resulted in the formation of a localized p-n junction at the interface between ZnO NRs and PEDOT:PSS. This created a charge depletion layer at the interface, and a built-in potential (∆E0) formed inside the depletion layer (Figure 2c). Simultaneously, oxygen molecules from the air can be adsorbed on the surfaces of ZnO NRs under dark conditions to form a negative ionic charge, O2- (Figure 2d).35, 40 It should be noted that the width of the deletion layer affecting the conductance of the electronic transmission channel can be modulated using UV light. ∆E0 serves as a driving force to separate out hole-electron pairs generated inside the deletion layer under illumination with UV light. Therefore, electron-hole pairs are generated in the depletion layer and are separated by ∆E0 when the device is exposed to UV light. Holes move toward the PEDOT:PSS side, while electrons move toward the ZnO NRs side, reducing the built-in potential from ∆E0 to ∆EP (Figures 2e,f). At the same time, the width of the depletion layer decreases, and the electronic transmission channel is broadened, resulting in an increase in the conductance of the PEDOT:PSS channel (Figure 2f). This phenomenon is referred to as the photovoltage effect. Under UV light illumination, the electrons inside the ZnO NRs were lifted from the valence band to the conduction band, resulting in generation of electron-hole pairs inside the 7 ACS Paragon Plus Environment


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ZnO NRs. The holes can easily migrate to the surface of ZnO NRs and discharge the O2- ions that formed on the surfaces of ZnO NRs under dark conditions. Meanwhile, electrons were trapped inside the ZnO NRs, leading to the formation of a negative potential near the depletion layer, which seems to have a gating effect on the PEDOT:PSS layer (Figure 2f). As a result, the additional negative potential inside the ZnO NRs effectively gated the PEDOT:PSS channel through capacitive coupling. This means that holes accumulated in the PEDOT:PSS channel, where they increased the photocurrent (i.e., a photogating effect). Based on the two sensing mechanisms mentioned above, we assumed that the stretchable UV photodetector operates as a combination of photovoltage and photogating transistors, where the ZnO NRs serves as a photo-responsive gate, the depletion layer acts as a dielectric, the PEDOT:PSS serves as the channel, and the Cr/Au electrodes act as source-drain electrodes. A schematic illustration of the device concept (taking the form of a vertical transistor structure) under dark conditions is presented in Figure 2g. Under UV exposure, a photovoltage was created at the PEDOT:PSS/ZnO NR interface to control the electrostatics of the depletion layer, and the photogating effect was generated inside the ZnO NRs to gate the PEDOT:PSS channel. Therefore, the conductivity of the PEDOT:PSS channel was modulated (Figure 2h). To further confirm that the UV photodetector operated as a photogated heterojunction photodetector, we investigated the output characteristics of the device under biasing via UV exposure, where the power (P) ranged from 0.063 to 2.32 mWcm-2. Figure S2 shows the photoresponse of the PEDOT:PSS channel current as a function of P, where an increase in the output current was observed when photo-biasing increased. This phenomenon can be seen more clearly in the photocurrent change (∆Iph) in the device versus VD under varying UV power, as presented in Figure 3a. Here, ∆Iph =ID,ph- ID,0, where ID,ph and ID,0 are the light and dark currents, respectively. The data in Figure 3b show the transfer characteristics of the device, indicating the obtained photocurrent at a VD of 4 V (from Figure 3a) versus P. The device exhibited a photocurrent as high as 0.793 mA at an incident P of 2.32 mWcm-2. 8 ACS Paragon Plus Environment

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Figures 3c,d show the time-dependent ∆Iph and photoresponse (∆Iph/ID,0) of the device, respectively, when UV light with different values of P was exposed to the photodetector under a VD=1 V. It can be seen clearly that the values of ∆Iph and ∆Iph/ID,0 increase when P increases. Additionally, ∆Iph/ID,0 reached 30.4% at an incident P of 2.32 mWcm-2. The values of ∆Iph/ID,0 are plotted as function of P in Figure S3, demonstrating a nearly linear dependence on P, with a slope of 11.6% per mWcm-2. To determine the intrinsic photosensitivity of the photodetector, two crucial parameters, responsivity (R) and photoconductive gain (G), were calculated from ∆Iph under UV illumination. R is defined as the ∆Iph generated per unit intensity of incident light on the effective area of a photodetector,41 and it is expressed using a conventional model:

R=

∆I ph 35, 40 . Figure 3e presents the R values of the UV photodetector versus light radiation. P

R decreases with an increase in P, showing a nonlinear relationship. Interestingly, the nonlinear relationship between R and P (Figure3e) fit very well to the reciprocal function R=

a 1 + (bP )

n

, where a and b are constants, and n is a phenomenological fitting parameter (n

≈1). This coincides with a theory that the responsivity is dependent on the light intensity as

R=

eλ  T0  hc  Tt

  

1 P 1 +   P0

  

n

,35, 42 where T0 is the carrier lifetime at low P, P0 is the excitation

intensity at which the surface states are fully filled, and Tt is the carrier transit time. The maximum responsivity (Rmax) was estimated to be 86 AW-1 from the fitting curve (P→0). The definition of G for a photodetector is the number of carriers in the channel generated by each absorbed photon per unit time.43 It can be calculated by the formula G =

UV energy is

hc

λ

∆I hc . The incident P eλ

, where h is Planck’s constant, c is the speed of light, and λ is the



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wavelength. Figure 3f shows G values as a function of P. The G values drastically decreased at low excitation intensity and reached a saturated state at high excitation intensity. Under low excitation intensity, the photogenerated holes inside the p-n heterojunction moved toward the PEDOT:PSS and contributed to the photocurrent. Photogenerated holes inside ZnO NRs migrated to the surface of the ZnO NRs to occupy and discharge adsorbed O2-. As the excitation intensity increased, more photogenerated electron-hole pairs were produced until all densities of states (inside p-n heterojunction) and surface states (inside ZnO NRs) were filled at a certain excitation intensity. When the intensity was increased further, a depletion of available electron-hole pairs occurred inside the p-n heterojunction, and photogenerated electron–hole pairs inside ZnO NRs recombined immediately. Therefore, these processes do not contribute to the charge transfer process leading to the situation where the G value became nearly saturated at high excitation intensity.35, 40, 44 Based on the results of Figure 3f, the G values can be fitted to a reciprocal function, G =

c , where c and d are constants, and n n 1 + (dP )

T  is a phenomenological fitting parameter (n ≈1). G can be expressed as G =  0   Tt 

1 P 1 +    P0 

n

,

where T0/Tt is the ratio of carrier lifetime to carrier transit time, which is a usual expression for the gain, and the

1 P 1 +    P0 

n

term represents trap saturation wherein the density of states and

surface states are completely filled under high excitation intensity.44 From the fitting curve (Figure 3f), the highest photoconductive gain (Gmax) under UV excitation (365 nm) was calculated to be 291 at a very low excitation intensity (P → 0), indicating that as many as 291 photogated carriers were generated by each absorbed photon. To evaluate the performance of the photodetector, detectivity (D*) values are also presented and discussed in more detail in

Figure S4. 10 ACS Paragon Plus Environment

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Photoresponse under applied mechanical deformation. To demonstrate the performance of the UV photodetector under mechanical stretching, we investigated the photoresponse of the device during applied uniaxial and multiaxial strain. For uniaxial stretching, the device was stretched in one direction (see the insert in Figure 4a).However, in the case of multiaxial strain, the photodetector was stretched in multiple directions (see the insert in Figure 4b). The device can sustain a maximum tensile strain of 30 % based on the features of the 3D-patterned elastomeric substrate.36 Therefore, the ∆Iph/ID,0 value of the device was evaluated under applied uniaxial strains of 5 %, 10 %, 20 %, and 30 % and multiaxial strains of 15 % and 30 %. The photoresponse data in Figures 4a,b show the excellent electrical stability under stretching up to uniaxial and multiaxial strains of 30 %, respectively. Strikingly, the values of ∆Iph/ID,0, response time, and relaxation time were almost unchanged under static uniaxial and multiaxial stretching conditions. A more detailed discussion on the response and recovery times of the device is presented in Supporting Information I1. The reliability of the UV photodetector was also evaluated by measuring the ∆Iph of the fabricated device after uniaxial cyclic stretching. Figure 4c exhibits the ∆Iph values of the device after cyclic stretching for 0 to 20,000 cycles at a strain of 30 %. We can see that the ∆Iph values of the device after cyclic stretching gradually decreased between 100 and 5,000 stretching cycles, stabilized from 5,000 to 15,000 cycles (∆Iph nearly did not change), and then quickly decreased at 20,000 cycles (Figure4d). The device still works after 20,000 repeated stretching cycles. The photocurrent data of the device after cyclic stretching for 20,000 cycles is presented in Figure 4c, and these data were enlarged and are shown in

Figure S5 for clarity. We measured the current level of the device without UV illumination after cyclic stretching in order to understand the mechanism of the response change of the device to UV after cyclic stretching. The current changes of the device after cyclic stretching are presented in Figure S6. The tendency of the current changes is nearly the same as that of the 11 ACS Paragon Plus Environment


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photocurrent changes. These results indicate that the change in photocurrent with increasing number of stretching cycles was mainly due to changes in device current. In our device structure, the device current was mainly from the PEDOT:PSS transport layer. Therefore, we investigated the surface morphology of the PEDOT:PSS layer on a 3D-patterned PDMS substrate before and after cyclic stretching by FE-SEM. The results are presented in Figures

S7a,b,c,d,e. Some wrinkles and cracks appeared on the PEDOT:PSS surface on 3D-patterned PDMS after cyclic stretching, which caused an increase in charge-carrier scattering in the PEDOT:PSS layer and, in turn, led to current changes in the device. Even though the 3Dpatterned substrate was very effective in reducing the strain level in the PEDOT:PSS layer during cyclic stretching, the layer can be subject to plastic deformation and/or delamination due to repetitive deformation during cyclic stretching. Furthermore, we also investigated the surface morphology of the PEDOT:PSS layer on a planar PDMS substrate for comparison with that of the PEDOT:PSS layer on a 3D-patterned PDMS substrate. More severe cracking was observed on the PEDOT:PSS layer on planar PDMS compared to the PEDOT:PSS on 3D-patterned PDMS (Figure S7f), which leads to a much larger current change in the PEDOT:PSS on the planar PDMS substrate (Figure S8). To improve the device stability further, the elasticity of the PEDOT:PSS layer needed to be increased by, for example, mixing PEDOT:PSS with an ionic liquid or plasticizer.45, 46 To demonstrate the wearability and the photodetection capability on the human body, the stretchable UV photodetector was conformally attached to the wrist of a human to detect UV light during normal human activities (Figure 5). The ∆Iph/ID,0 of the device with no movement in the wrist is shown in Figure 5a. The ∆Iph/ID,0 of the device was investigated during typing, moving the hand up and down, and twisting the hand. The results in Figures

5b,c,d for typing, moving the hand up and down, and twisting the hand, respectively, demonstrate that the current change induced by strain during human activities was much smaller than the photocurrent generated by UV light illumination. This clearly indicates that 12 ACS Paragon Plus Environment

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the strain induced by human activities when the device was attached to a wrist did not significantly affect the sensing signal of the stretchable UV photodetector. The omnidirectionally stretchable UV photodetector showed the ability to be wrapped around the human wrist to detect UV light (i.e., harmful light) while experiencing body motion during daily activities.

CONCLUSIONS We presented an omnidirectionally stretchable UV photodetector based on a stretchable 3Dpatterned PDMS substrate, on which curvilinear 3D-patterns enabled the effective absorption of external strain up to 30%. This substrate also helps the organic-inorganic heterojunction UV photodetector sustain tensile strains in all directions. A stretchable UV photodetector with ZnO NRs/PEDOT:PSS in the form of a photogated vertical p-n heterojunction structure was demonstrated by growing free-standing ZnO NRs on a PEDOT:PSS channel layer and combining the strain-absorbing structure with a stretchable substrate. The sensing mechanism of the device under UV illumination can be explained by two mechanisms: photovoltage and photogating effects. Additionally, the stretchable UV photodetector can be assumed to operate as a “photogated organic-inorganic heterojunction photodetector.” The stretchable UV photodetectors exhibited high responsivity to UV, good stability when subjected to uniaxial and multiaxial strains up to 30%, and robustness to 15,000 stretching cycles at 30% strain. Interestingly, the fabricated device could also be conformally attached to the human body and was shown to detect UV light during human activities with a significant change in sensing signal without exhibiting any strain-induced damage. The concept used to design these omnidirectionally-stretchable photodetectors that can detect UV light can be extended for photodetection in other wavelength ranges. These results demonstrate that our omnidirectionally stretchable UV photodetector has great potential in a variety of new



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stretchable optoelectronics and represents a new platform for the development of stretchable photodetectors for use in other visible and infrared light ranges.

EXPERIMENTAL SECTION Fabrication. A UV photodetector in the form of a vertical p-n heterojunction structure with PEDOT:PSS-ZnO NRs was directly fabricated on a 3D-patterned PDMS substrate, as shown schematically in Figure 1b. The channel length and width of the device were 400 and 8000 µm, respectively, and source-drain electrodes of Cr/Au (5/60 nm) were formed by e-beam evaporation. Afterward, the 90 nm PEDOT:PSS layer was spin-coated onto the substrate, and the effect of layer thickness on the device performance is presented in Figure S9. To pattern the PEDOT:PSS layer, a thin film of Ni (5 nm) was deposited as an etching mask through a shadow mask onto the PEDOT:PSS film by e-beam evaporation. Then, the sample was exposed to oxygen microwave plasma in a plasma chamber system at 640 mTorr. A microwave power of 50 W was used to etch the unmasked PEDOT:PSS, and the Ni mask patterns were then removed by an HNO3 acid solution. Next, a 5 wt% dispersion of ZnO nanoparticles (ZnO NPs) was spin-cast onto the PEDOT:PSS channel to obtain high-density ZnO NRs. Finally, ZnO NRs were grown in aqueous solution containing 20 mM zinc nitrate hexahydrate (Zn(NO)3·6H2O), hexamethylenetetramine (C6H12N4, HMTA), and 200 mL of deionized (DI) water at 90 °C for 3 h. To study the effect of the ZnO NR growth process on the electrical conductance, device currents were measured using PEDOT:PSS only, PEDOT:PSS in the presence of a hydrothermal reaction(in this case, the PEDOT:PSS surface without the ZnO NPs seed layer faces the bottom of the beaker so the ZnO NRs cannot grow on the PEDOT:PSS surface), and PEDOT:PSS/ZnO NRs. The results are presented in Figure

S10. Characterization. The surface morphology and density of ZnO NRs that were grown on the 3D-patteredPDMS substrate were analyzed by field-emission scanning electron microscopy 14 ACS Paragon Plus Environment

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(FE-SEM) (JEOL, Model JSM-6500F). A UV light-emitting diode (LED) was used for illumination during sensor measurements. Electrical characterization of the stretchable UV photodetectors was carried out using a semiconductor parameter analyzer (Agilent, Model 4145B) under dark and illuminated conditions. The stretching system in Figure S11 was used to measure the stability of the device for stretching and cyclic stretching. All photodetection experiments from the device attached to a human body were carried out on a voluntary basis with the informed consent of the subject (age 34, one male) and all authors. According to the standard, this study did not require approval from a committee or departments. No harm came to the subject during the experiments.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Additional information, including a UV-VIS spectrum of the PEDOT:PSS/ZnO NRs, an IDVD plot of the stretchable UV photodetector, the response of the stretchable UV photodetector, detectivity of the device, surface morphology and electrical properties of PEDOT:PSS on 3Dpatterned PDMS after cyclic stretching, effect of PEDOT:PSS thickness on device performance,

discussion of the response and recovery times of the device, and a table

comparing the performance of stretchable PEDOT:PSS/ZnO NRs heterojunction UV photodetectors with other reported stretchable UV photodetectors.

AUTHOR INFORMATION Corresponding Authors *

(N-E Lee) email: [email protected]

*

(H. Lee) email: [email protected] 15 ACS Paragon Plus Environment


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Author Contributions †

T. Q. Trung and V. Q. Dang contributed equally to this work

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This

research

was

supported

(2016R1D1A1B03934709)

and

by the

the Korea

Basic

Science

Research

Research Fellowship

Program Program

(2015H1D3A1062350) through the National Research Foundation of Korea funded by the Ministry of Science and ICT.

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a)

b)

c)

Figure 1. The architecture and fabrication of the device, and the FE-SEM characterization of ZnO NRs. a. Schematic of the stretchable UV photodetector attached to the wrist of a human for UV detection. b. Fabrication process of the UV photodetector on a stretchable 3D-patterned PDMS substrate. c. FE-SEM image of ZnO NRs grown on a PEDOT:PSS layer that was directly spin-coated and patterned onto the stretchable 3Dpatterned PDMS substrate.


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6

a)

b) Off

Dark current UV light current

0.9 3

Current, I (mA)

Current, ID (mA)

0

-3

0.8

On 0.7 2

4

0

20

40

60

80

ZnO NRs

d) ZnO NRs

LUMO 3.3 eV ∆E0 4.8 eV

EC Depletion layer

EF

HOMO

Drain

5.3 eV

Source

Depletion layer

O2- O2- O2- O2O2-

c) PEDOT:PSS

100

Time, (sec)

O2O2-

0

Voltage, VD (V)

O2-

-2

O2-

-4

O2-

-6

7.8 eV

EV

Dark current (ID,0) PEDOT:PSS

Electrons f) Holes

ZnO NRs

ZnO NRs

LUMO

hν

3.3 eV ∆EP

EC

EF

HOMO

Depletion layer

5.3 eV

Source

4.8 eV

O2- O2- O2- O2-

PEDOT:PSS

O2-

Depletion layer

O2- O2- O2- O2- O2-

e)

Drain

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Light current (ID,ph)

7.8 eV EV

PEDOT:PSS



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

g)

h)

Figure 2. Response to UV light and the sensing mechanism of the fabricated device. a. ID-VD characteristics of the stretchable UV photodetector under 365 nm UV light (P = 2.32 mW/cm2) and under dark conditions. b. Photoresponse at VD= 1 V when the stretchable UV photodetector was illuminated with a 365 nm UV source (P = 2.32 mW/cm2). c. Schematic energy band structure of the PEDOT:PSS/ZnO NRs heterojunction under dark conditions. The built-in potential (∆E0) was induced in the depletion layer. d. Schematic diagram of the UV photodetector with the PEDOT:PSS/ZnO NRs heterojunction under dark conditions. Oxygen molecules were adsorbed on the ZnO NRs to form negative oxygen ions O2- on the surface. e. Schematic illustration of the energy band structure of the PEDOT:PSS/ZnO NRs heterojunction, and the electron-hole generation process and transmission under UV illumination. The built-in potential (∆Ep) under UV illumination decreased due to the ∆E0 generated in the depletion region. f. Schematic diagram of the UV photodetector with the PEDOT:PSS/ZnO NR heterojunction under UV illumination. In the depletion layer, the photogenerated holes moved toward the PEDOT:PSS side, and the photogenerated electrons moved toward the ZnO NRs side (i.e., the photovoltage effect was observed). Inside the ZnO NRs, the photogenerated holes migrated to the surface of the ZnO NRs and discharged O2-, while the photogenerated electrons were trapped inside the ZnO NRs. This resulted in an additional negative potential near the depletion layer, leading to the accumulation of holes in the PEDOT:PSS transport layer (i.e., the photogating effect was 22 ACS Paragon Plus Environment

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observed). g.h. Schematic diagrams of the photodetector in the vertical heterojunction structure under dark conditions and UV illumination, respectively.



b)

Photocurrent, ∆Iph=ID,ph- ID,0 (mA)

1.0

2

0 mW/cm 2 P=0.063 mW/cm 2 P=0.201 mW/cm 2 P=0.534 mW/cm 2 P=1.42 mW/cm 2 P=2.32 mW/cm

0.8 0.6 0.4 0.2 0.0

0

1

2

3

0.20

0.6 0.4 0.2 0.0 0.0

1.0

1.5

2.0

2.5

d) 2

2

2

P=0.534 mW/cm 2 P=1.42 mW/cm 2 P=2.32 mW/cm

P=0.011 mW/cm 2 P=0.015 mW/cm 2 P=0.063 mW/cm 2 P=0.201 mW/cm

0.5

UV intensity, P (mWcm-2)

Response, ∆ Iph/ID,0 (%)

0.25

0.8

4

Voltage, VD (V)

c)

0.15 0.10 0.05

30

2

P=0.011 mW/cm 2 P=0.015 mW/cm 2 P=0.063 mW/cm 2 P=0.201 mW/cm

P=0.534 mW/cm 2 P=1.42 mW/cm 2 P=2.32 mW/cm

20

10

0

0.00 0

20

40

60

80

100

0

Time (sec)

e)

20

40

60

200

Experiment ReciprocalMod Fit

40

20

0

80

100

Time (sec)

f)

60

Photoconductive gain, G

Photocurrent, ∆Iph = ID,ph- ID,0 (mA)

Photocurrent, ∆ Iph = ID,ph- ID,0 (mA)

a)

Responsivity, R (AW-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Experiment ReciprocalMod Fit

150

100

50

0 0.0

0.5

1.0

1.5

2.0

2.5

UV Intensity, P (mWcm-2)

0.0

0.5

1.0

1.5

2.0

2.5

UV Intensity, P (mWcm-2)

Figure 3. Device characterization of a stretchable UV photodetector. a. The output characteristics of the photocurrent, ∆Iph, versus the drain voltage, VD, when various UV intensities were applied to the stretchable UV photodetector. b. The transfer characteristics showing the dependence of photocurrent and ∆Iph, on UV radiation intensity under a VD of 4 V. c.d. The time dependence of photocurrent, ∆Iph, and photoresponse, ∆Iph/ID,0, respectively,


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at VD = 1 V under different UV intensities. e. f. Responsivity, R, and photoconductive gain, G, versus UV illumination



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a)

b)

c)

d)

Figure 4. Device performance under applied stretching strain. a. The time dependence of the photoresponse, ∆Iph/ID,0, at VD = 1V and P = 1.42 mW/cm2 under uniaxial stretching while straining from 5 % to 30 %. b. The time dependence of the photoresponse, ∆Iph/ID,0, at VD = 1V and P = 1.42 mW/cm2 under multiaxial stretching while straining from 15 % to 30 %. c. The time dependence of photocurrent, ∆Iph, at VD = 4 V and P = 0.534 mW/cm2after cyclic stretching for 100 to 20,000 cycles at a strain of 30 %. d. Relationship between photocurrent, ∆I, and number of stretching cycles.


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b) Photocurrent, ∆ Iph = ID,ph - ID,0(mA)

Off

Without movement 0.10 P = 1.42 mW/cm2 Off Off 0.08

0.06 0.04 0.02

On

On On

0.00 0

200

c) 0.06 0.05

400

600

800

0.04 0.03 0.02 On

0.01 0.00 -0.01 100

0.08 0.06 0.04 0.02

On

0.00 -0.02 0

50

100

150

200

250

300

Time (sec) During twisting

During moving up and down 2 P = 1.42 mW/cm Off

50

0.10

During typing 2 P = 1.42 mW/cm Off

d)

Time (sec)

0

0.12

1000

Photocurrent, ∆ Iph = ID,ph - ID,0(mA)


a)


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150

0.10 P = 1.42 mW/cm2 Off

0.08 0.06 0.04 0.02

On

0.00 -0.02 0

200

50

100

150

200

Time (sec)

Time (sec)

Figure 5. Detection capability of a stretchable UV photodetector attached to the wrist of a human during activity. The stretchable UV photodetector was conformally attached to the wrist of a human to measure the photocurrent, ∆Iph, generated by UV light at VD = 1 V during human activities: (a) without movement, (b) typing, (c) moving up and down, and (d) twisting.



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An Omnidirectionally Stretchable Photodetector ... - ACS Publications

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