Boosting Responsivity of OrganicâMetal Oxynitride Hybrid

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Boosting Responsivity of Organic-Metal Oxynitride Hybrid Heterointerface Phototransistor You Seung Rim, Kyung-Chul Ok, Yang (Michael) Yang, Huajun Chen, Sang-Hoon Bae, Chen Wang, Yu Huang, Jin-Seong Park, and Yang Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02814 • Publication Date (Web): 19 May 2016 Downloaded from http://pubs.acs.org on May 24, 2016

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ACS Applied Materials & Interfaces

Boosting Responsivity of Organic-Metal Oxynitride Hybrid Heterointerface Phototransistor You Seung Rim,†,‡,# Kyung-Chul Ok,§,# Yang (Michael) Yang,†,‡,# Huajun Chen,†,‡ SangHoon Bae,†,‡ Chen Wang,† Yu Huang,† Jin-Seong Park,*,§ and Yang Yang*,†,‡

†

Department of Materials Science and Engineering, ‡California NanoSystems Institute,

University of California, Los Angeles, Los Angeles, California 90095, United States, and §

The Division of Materials Science and Engineering, Hanyang University, Seoul 04763,

Korea. #These authors contributed equally.

KEYWORDS Phototransistor, Bulk-hetero junction, Zinc oxynitride, Oxide semiconductor, Heterointerface, Thin-film transistor

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ABSTRACT Amorphous metal oxides are attractive materials for various sensor applications owing to high electrical performance and easy processing. However, low absorption coefficient, slow photo-response, and persistent photoconductivity of amorphous metal oxide films from the origin of deep-level defects, are obstacles to their use as photonic applications. Here, we demonstrate ultrahigh photoresponsivity of organic-inorganic hybrid phototransistors featuring bulk heterojunction polymers and low-bandgap zinc oxynitride. Spontaneous formation of ultrathin zinc oxide on the surface of zinc oxynitride films could make an effective band-alignment for electron transfer from the dissociation of excitons in bulk-hetero junction, while holes were blocked by the deep highest occupied molecular orbital level of zinc oxide. These hybrid structure-based phototransistors are ultrasensitive to broad-bandwidth photons in ultraviolet to near-infrared regions. The detectivity and a linear dynamic range exceeded 1012 Jones and 122.3 dB, respectively.

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INTRODUCTION Amorphous oxide semiconductors (AOSs) have been widely researched for their high electron mobility, transparency in the visible range, and easy deposition for optoelectronic and sensor applications.1-4 Their unique advantages, when used as channel materials in thin-film transistors (TFTs), are their high mobility and stability in largescale deposition, and low-temperature processing.5,6 Representative candidates, such as InGaZnO, InZnSnO, and ZnO, have focused on improving mobility and stability.3 However, the elimination of oxygen vacancies (Vo) in deep donor levels to improve the stability measured in illumination stress test remains a challenge. The photo-transition from deep-donor-like states (Vo) near the valence-band maximum (VBM) to shallowdonor states (Vo+ or Vo2+ formed by the photonic ionization of Vo) near the conductionband minimum (CBM) produces an unexpected instability in illuminated devices.7,8 To reduce this photon-accelerated instability, high-pressure annealing,7,9 oxygen plasma treatment,10 and addition of dopants (i.e., Hf or Zr)11,12 were previously proposed. More recently, following the general strategy of deactivating oxygen vacancies via the positioning of deep level Vo region upper the oxygen 2p orbital, the formation of new orbital state by a non-oxide anion yielded a narrow bandgap (~1.6 eV).13 The proposed TFTs, based on zinc oxynitride (ZnON), not only have a high mobility (> 50 cm2 V-1 s-1) but also suppress the high persistent photocurrents (PPC).13-15 In particular, photocarrier relaxation time in ZnON is as short as 10 ps, owing to negligible Auger recombination, and nanosecond-time- scale relaxation.16 These features produce a high photoresponsivity and negligible PPC, as desired for optoelectronic applications.

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The photosensitivity of large-bandgap AOSs to short-wavelength photons can be beneficial, e.g., for ultraviolet (UV) detectors. However, large PPC and the slow response of the deep oxygen-vacancy defects are also critical problems. On the other hand, ZnON has a narrow bandgap and can absorb photons from the UV to near-infrared (NIR) regions, making it potentially applicable to phototransistors and image sensors. Here, we report bulk heterojunction (BHJ) polymers and ZnON hybrid phototransistors in a broad wavelength range (380–940 nm). BHJ structures involving a narrow-bandgap polymer and PC71BM effectively achieve the charge separation of excitons, allowing electrons to transfer to the spontaneous ZnO and ZnON semiconductor surface. As a result, the photosensitivity in hybrid structures is dramatically improved relative to ZnON phototransistors.

Phototransistors have been widely used in interactive interfaces, control systems, and biological health systems.17-28 In this perspective, photosensitivity and detectivity are critical figures of merit for evaluating photonic devices.29,30 Charge separation of electrons and holes in BHJ is accompanied by dissociating excitons.31 Proposed polymers are described in Figure 1a. The BHJ consists of the narrow-bandgap conjugated polymer poly[2,6′-4,8-di(5-ethylhexylthienyl)benzo[1d,2-b;3,4-b]dithiophene-alt-5-dibutyloctyl3,6-bis(5-bromothiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4-dione] (PBDTT-DPP) and [6,6]phenyl C71 butyric acid methyl ester (PC71BM). Homogeneous PBDTT-DPP:PC71BM has high absorption spectra in the UV to NIR regions and displays strong absorption between 650 and 850 nm, as shown in Figure 1c.32 ZnON also absorbs over a broad range from 400 to 900 nm. The ZnON bandgap was previously measured as ~1.3 eV,14,15 less than

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our present measurement (~1.5 eV) (Figure 1b). To find evidence of the extended bandgap in ZnON, we investigated the ion distribution in ZnON films by Auger depth profiling (Figure 1d). Interestingly, the region near the surface (5-10 nm) was oxygenrich and nitrogen-deficient compared to the bulk region. That is, the surface of ZnON film spontaneously form ZnO layer, thereby contributing to the bandgap widening. It could be attributed to the formation of a spontaneous passivation layer on metastable ZnON surface. However, the exact mechanism is still uncertain and requires further study. The spontaneous band alignment in the ZnON significantly matched the BHJ polymers. This facilitates the transfer of electrons and the blocking of holes by forming ZnO layer, as shown in Figure 1e; the narrow bandgap of ZnON without ZnO buffer layer could not block the holes, so that both electron and hole generated inside the BHJ can transfer to ZnON and recombine. On the other hand, the spontaneous formation of the ZnO layer can easily transfer electron from BHJ to the ZnON, while the hole is blocked. Figure 1f shows the photoluminescence (PL) intensity of the PC71BM and PC71BM/ZnON. The PL intensity of PC71BM/ZnON was considerably quenched at the interface compared to only PC71BM, which could an evidence of the electron transfer from PC71BM to ZnO/ZnON. Thus, the electrons transfer from PC71BM to ZnO/ZnON could be expected to change the potential of ZnON and ultimately leads to the negative shift of the turn-on voltage of the device. Figure 2 shows the photosensitivity characteristics of the ZnON phototransistors with the different structures. All devices showed similar field-effect mobilities (48–51 cm2/Vs), regardless with polymer types. The electrical variations in the devices was examined for illumination wavelengths λ = 380–940 nm (power density = 1 mW cm-2). The threshold

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voltage (VTH) of the ZnON phototransistors shifted negatively with decreasing wavelength (Figure 2a). The narrow bandgap of ZnON varied with photonic excitation and VTH changed accordingly (Detail variations of devices were listed in Table S1). To confirm the effect of organic materials on top of the ZnON phototransistors, we examined PBDTT-DPP, PC71BM, and BHJ PBDTT-DPP:PC71BM. We confirmed that only the PBDTT-DPP:PC71BM/ZnON phototransistors were more photoresponsive than the ZnON phototransistors. The PBDTT-DPP- and PC71BM-based ZnON phototransistors were less photosensitive than ZnON (Figs. 2b and c). PBDTT-DPP or PC71BM shows strong exciton binding energy, so that photogenerated excitons does not separate into holes and electrons.33 It is also difficult to separate electron and hole near the ZnON/ZnO surface due to the limited exciton diffusion length (~ 10 nm) of the polymer.34 On the other hand, BHJ PBDTT-DPP:PC71BM can generate electrons and holes through efficient exciton dissociation and increased diffusion lengths. In other words, photo-induced carriers in PBDTT-DPP:PC71BM contribute to the change in VTH and to an off-current (Figure 2d). To confirm the phototransistor performance, the responsivity R, detectivity D*, linear dynamic range LDR, and noise equivalent power NEP were investigated. R represents the ratio of the photocurrent Iph to the photo-power density Llight, and reflects the detector efficiency:35,36 ܴ=௅

ூ೛೓ ೗೔೒೓೟

(1)

D* is expressed by ‫= ∗ܦ‬

ሺ஺௙ሻభ/మ ሺ௜೙ /ோሻ

(2)

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where A is the active area of the phototransistor, f the electrical bandwidth, and in the noise current. It was measured from the noise current using a lock-in amplifier, giving 0.045 pA Hz-1/2 at 1 Hz (Figure S1). The calculated detectivity, for various wavelengths and gate voltages (VGS), is shown in Figures 3(a) and (b). Values in ZnON phototransistors had over 1011 Jones within the broad range of 380–940 nm. Unlike conventional ZnO-based phototransistors (Eg > 3.0 eV), used as UV detectors, the narrow bandgap of ZnON favors a high photosensitivity over a broad wavelength range (Figure 3a). On the other hand, ZnON phototransistors with embedded PBDTT-DPP were less photosensitive than ZnON, because photo-excited electron/hole pairs cannot be generated, owing to the strong exciton binding energy of a single polymer (> 0.3 eV). Thus, most excitons do not dissociate at only one interface. ZnON phototransistors with embedded PC71BM showed higher detectivity, compared with ZnON phototransistors owing to well-matched CB and electron transfer into the ZnON surface. In the case of BHJ PBDTT-DPP:PC71BM, electrons and holes can be efficiently separated at the donoracceptor interface, which contributes to a large increase in free electrons. In particular, a spontaneously formed ZnO layer transports electrons and blocks holes, thereby helping to reduce electron-hole recombination within the ZnON. Significantly, the detectivity of BHJ PBDTT-DPP:PC71BM-embedded ZnON phototransistors attained 1.00 1012 Jones compared with ZnON (D* = 1.82 1011) at 630 nm (VGS = -10 V). Additionally, we investigated detectivity dependence with VGS variations, as shown in Figure 3b. PBDTTDPP:PC71BM-embedded ZnON phototransistors showed a higher detectivity for VGS values ranging from -40 to 0 V, compared with other structures. At VGS = -40 V, the detectivity of PBDTT-DPP:PC71BM-embedded ZnON phototransistors was more than

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103 times higher than that of ZnON only. The effective quantum efficiency (EQE) is given by R E 100%, where E is the incident photon energy.37 The photoresponsivities of ZnON and PBDTT-DPP:PC71BM/ZnON photodetectors at 630 nm were 0.031 A W-1 and 0.16 A W-1, respectively (Figure 3c). EQE values for PBDTTDPP:PC71BM/ZnON photodetectors at the same wavelength were approximately five times higher than those of ZnON photodetectors. The NEP is the performance of weak light detection for the photonic device. In order to obtain exact NEP value, we calculated it using measured noise current, which is given by 29

ܰ‫= ܲܧ‬

௜೙

(3)

ோ

LDR represents the linearity of the photosensitivity at various incident photo-power intensities and is given by38 ௉

ೞೌ೟ ‫ = ܴܦܮ‬10 log ቀோ௉ ቁ

(4)

where ܲ௦௔௧ is the saturation power density, measured at the photo-power intensity of the maximum photocurrent linearity. The LDR of the PBDTT-DPP:PC71BM/ZnON phototransistors, under a photo-power density showed 1 × 10-4–5 mW cm-2 (calculated photon flux density range from 3.2 1011 to 1.6 1016 numbers-1 cm-2) at 630 nm (Figure 4a). The calculated LDR for the device was 122.3 dB, much higher than for conventional photodetectors made from, e.g., Si (120 dB) or InGaAs (66 dB).36 Figure 4b shows the photoresponsivity and EQE of PBDTT-DPP:PC71BM/ZnON phototransistors as functions of the photo-power density. These values are significantly improved compared to those of ZnON phototransistors and attained, respectively, 168 A W-1 and 3.3 104 % (at VG = -10 V; light intensity = 1 10-4 mW cm-2). Recently, some hybrid-

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structure-based photodetectors showed very high photoresponsivities (~107 A W-1) under very low incident light intensities (~ 1 pW cm-2).39 To compare these values, we plotted the photoresponsivity with different light intensities (Figure S2).37 Although we could measure the photocurrent up to 100 nW cm-2, the photoresponsivity showed linearity. It is expected to realize high-gain hybrid photodetectors. Figure 4c depicts the photoswitching of PBDTT-DPP:PC71BM/ZnON phototransistors at various frequencies from 1 to 100 Hz and a wavelength of 630 nm. Devices displayed fast photoswitching. For drain voltages of 2–10 V, the photogain changed significantly and the photocurrent showed fast saturation/recovery (Figure 4d). Larger drain voltages effectively amplify electron transfer throughout suppress recombination at the PBDTT-DPP:PC71BM and ZnON interface.

CONCLUSION In summary, we developed narrow-bandgap ZnON semiconductor-based phototransistors involving BHJ polymers for boosting photosensitivity, for use as high-performance photodetectors. The spontaneous band alignment of ZnON with an oxygen-rich surface was well matched to the BHJ polymers to facilitate electron transfer electrons and hole blocking. This can improve the electron charge transport over the conduction band under photon illumination. BHJ PBDTT-DPP:PC71BM contributed to improving the photodetectivity over the IR and NIR regions, and the photodetectivity and LDR of hybrid structures increased significantly to over 1012 Jones and 122.3 dB, respectively. Calculated photoresponsivity values reached 1.7 102 A W-1 at 100 nW cm-2. We found that hybrid ZnON phototransistors not only maintain high field-effect mobility but also

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improve the photosensitivity. These approaches are promising avenues for further developing simple, affordable, and high-performance photosensing technologies.

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METHODS Materials. For the BHJ layer, Poly[2,60′-4,8-bis(5-ethylhexylthienyl)benzo[1,2-b;3,4b]dithiophene-alt-5-dibutyloctyl-3,6-bis(5-bromothiophen-2-yl)pyrrolo[3,4-c]-

pyrrole-

1,4-dione] (PBDTT-DPP) was used as a light-sensitive polymer, previously developed in our group.32 [6,6]-Phenyl C71-butyric acid methyl ester was purchased from Nano-C (Westwood, MA, USA). We dissolved 16 mg of PBDTT-DPP:PC71BM (1:2) in 2 ml of 1,2-dichlorobenzene (DCB, Aldrich, 99%). Device fabrication. A ZnON channel layer was deposited onto SiO2 (100 nm)/p++Si by direct current (DC) reactive sputtering with a metallic Zn target.15 An Ar/O2/N2 ratio of 5/1.2/40 was used for the deposition and the working pressure was fixed at 5 mTorr. The channel thickness of the ZnON film was 30 nm, and the channel region was defined using a shadow mask. For the source/drain (S/D) electrodes, indium-tin-oxide (ITO) was deposited by sputtering over a shadow mask. The channel region of ZnON TFTs was defined with a width (W) and a length (L) of 800 and 200 µm, respectively. Devices were annealed in air at 250°C for 5 hours. Subsequently, PBDTT-DPP, PC71BM, and PBDTTDPP:PC71BM were spin-coated onto ZnON TFTs at 5000 rpm for 60 s in a nitrogen-filled glove box. Overall polymer thickness had about 100 nm. Film and device characterization. The optical transmittance was measured using a UVvisible spectrophotometer (U-4100, Hitachi) and the optical bandgap was extrapolated from a Tauc plot.40 The work function and Fermi level of the ZnON film were examined by X-ray photoelectron spectroscopy (XPS, Omicron, and supplementary Figure S8) and ultraviolet photoelectron spectroscopy (UPS, Omicron). Auger depth profiling was performed to measure the vertical element distribution of the ZnON film. The

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microstructures and morphologies of the films were investigated using atomic force microscopy (AFM, Dimension Icon system, Bruker) in tapping mode and a scanning electron microscope (SEM). Micro-photoluminescence (PL) was performed using a Horiba LabRAM HR Evolution confocal Raman system with an Ar ion laser (488 nm) excitation (100x objective; 100 µW power). The PL intensity of the ZnON film was shown in Figure S3. The Agilent 4155C Semiconductor Parameter Analyzer was used to measure the devices in the dark. The inducing VGS was set in the range -40 and 40 V, and VDS to 10 V. Detailed device parameters of devices are listed in Table 1. Monochromatic light-emitting diodes (LEDs, 380–940 nm) were used as the light source to measure the photoresponse. The light intensity and frequency were controlled using a function generator (AFG3252, Techtronix). The actual light intensity was tuned using a power meter (Model 1830-C, Newport).

ASSOCIATED CONTENT Supporting Information. Included are dark current noise measurements through the lock-in amplifier, expected linear dynamic range results, and the PL spectra of the ZnON film. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors E-mail: [email protected] E-mail: [email protected] Author Contributions 12


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#

Y.S.R., K.-C.O. and Y.(M.).Y. contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by a grant from the National Science Foundation (Grant No. DMR-1210893, Program Director Andrew J. Lovinger; DMR is a program under Division of Materials Research), the Office of Naval Research (Program Manager Dr. Paul Armistead; Grant Number: N000141410648), and UCLA internal funds. This research was also supported by Global Frontier Program through the Global Frontier Hybrid Interface Materials (GFHIM) of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2013M3A6B1078870) and done by the MOTIE (Ministry of Trade, Industry & Energy (Grant No: 10051403) and KDRC (Korea Display Research Corporation) support program for the development of future devices technology for display industry.

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Figure 1. Hybrid BHJ polymers-ZnON phototransistors. (a) Chemical structures of PBDTT-DPP and PC71BM and schematic of the PBDTT-DPP:PC71BM/ZnON phototransistors with the bottom-gate and the top contact structure. (b-c) Absorption spectra of ZnON (Eg = 1.50 eV) and PBDTT-DPP:PC71BM/ZnON films. (d) Auger depth profiling of a ZnON film for the tracing in Zn, O, and N. (e) Energy-band alignment of a spontaneous ZnO buffer layer formed at the PC71BM and ZnON interface. (f) PL spectra of ZnON and PC71BM/ZnON films. Photo-excited carriers were quenched at the interface between PC71BM and ZnON.

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Figure 2. Photoresponse of different structures of ZnON phototransistors. (a) ZnON only, (b) PBDTT-DPP/ZnON, (c) PC71BM/ZnON, and (d) PBDTT-DPP:PC71BM/ZnON under illumination by wavelengths in the range 380–940 nm.

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Figure 3. Detectivity, responsivity, and EQE of phototransistors. Detectivity of ZnON (black, ), PC71BM/ZnON (red, ), PBDTT-DPP/ZnON (green, ), and PBDTT:PC71BM/ZnON (blue, ) phototransistors (a) under illumination by wavelengths in the range 380–940 nm and (b) for varying VGS. (c) Responsivity and EQE of ZnON and PBDTT-DPP:PC71BM/ZnON phototransistors under illumination by wavelengths in the range 380–940 nm.

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Figure 4. LDR and photoresponse of phototransistors. (a) Linear dynamic range of PBDTT-DPP:PC71BM/ZnON phototransistors under a photo-power density ranging from 1 10-4 to 5 mW cm-2. The linear dynamic range of the devices exceeded 120 dB. (b) Responsivity and EQE of PBDTT-DPP:PC71BM/ZnON phototransistors at 630 nm wavelength under a photo-power density ranging from 1 10-4 to 5 mW cm-2. (c) Transient photocurrent response of PBDTT-DPP:PC71BM/ZnON phototransistors at a pulse frequency ranging from 1 to 100 Hz. (d) Amplified photocurrent response of PBDTT-DPP:PC71BM/ZnON phototransistors with VDS set to 2, 5, or 10 V, at a 1 Hz pulse frequency.

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Table 1. Device performance of ZnON phototransistors with the different structures in the dark.

Structure

Mobility (cm2 V-1 s-1)

VTH (V)

S.S (V dec-1)

On/off ratio

ZnON

51.94±2.46

-6.19±0.98

0.71±0.08

108

PC71BM/ZnON

50.98±3.11

-7.41±1.87

1.12±0.14

108

PBDTT-DPP/ZnON

48.49±1.79

-4.26±1.33

1.26±0.19

108

PBDTT-DPP:PC71BM/ZnON

48.57±3.35

-2.72±1.66

1.27±0.16

108

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Boosting Responsivity of OrganicâMetal Oxynitride Hybrid

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