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thought to build an exciton-dissociation mechanism similar to planar heterojunction. b) The plasmonic carrier injection adds the number of collectable...
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Achieving Weak Light Response with Plasmonic Nanogold Decorated Organic Phototransistors Xiao Luo, Lili Du, Yuanlong Liang, Feiyu Zhao, Wenli Lv, Kun Xu, Ying Wang, and Yingquan Peng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03732 • Publication Date (Web): 24 Apr 2018 Downloaded from http://pubs.acs.org on April 24, 2018

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Achieving Weak Light Response with Plasmonic Nanogold Decorated Organic Phototransistors

Xiao Luo1,2,*, Lili Du1, Yuanlong Liang1, Feiyu Zhao1, Wenli Lv3, Kun Xu1, Ying Wang4 and Yingquan Peng1,3,*

Author affiliations 1

Institute of Microelectronics, School of Physical Science and Technology, Lanzhou

University, 222 South Tianshui Road, Lanzhou 730000, China 2

State Key Laboratory of Molecular Reaction Dynamics and Collaborative Innovation Center

of Chemistry for Energy Materials (iChEM), Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China 3

Institute of Microelectronics, College of Optical and Electronic Technology, China Jiliang

University, 258 Xueyuan Street, Hangzhou 310018, China 4

College of Information Engineering, China Jiliang University, 258 Xueyuan Street,

Hangzhou 310018, China

*To whom correspondence and requests for materials should be addressed: [email protected]; [email protected].

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Abstract Weak light response of organic photodetectors has fascinating potentials in fields of modern science and technology. However, their photoresponsivity is hindered by poor photo-carrier excitation and transport. Decorating active-layer surface with plasmonic nanometals is considered a viable strategy to address this issue. Here, we demonstrate a plasmonic nanogold decorated organic phototransistor achieving remarkable enhancement of photoresponsivity. Meanwhile, the photoresponsive range is broadened by four orders of magnitude. The proposed design is substantiated by a schematic energy level model combined with theoretical simulation analysis, enabling the development of the advanced optoelectronics. Keywords: weak light; photoresponse; plasmon; organic phototransistor; nanogold decoration.

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In the past decades, intensive efforts have been devoted to the investigation of organic optoelectronic devices due to their potential applications for low-cost, flexible, and wearable electronics1-3. Motivated by the wide interest above, the development of weak-light detectable organic phototransistor is highly fascinating, as such an integrated platform offers a simple solution for optical signal detection and amplification followed by output as photocurrent4. However, these organic devices generally suffer from low carrier mobility, short exciton diffusion length, and weak photo absorption duo to limited film thickness, which essentially contributes to the fact that weak intermolecular forces (Van der Waals forces) induce hopping transport of localized carriers of organic semiconductors5. Therefore, the weak light response of organic phototransistors is limited by insufficient absorption and inefficient carrier extraction. To overcome this limitation, using nanometal decoration of organic photoactive layer that supports both electrical doping and photon trapping offers an exciting and practical approach to simultaneously increase the carrier-collection and light-harvesting efficiency2, 6-8, which is considered a viable solution for achieving highly responsive photodetectors. Here we demonstrate a proof-of-concept organic phototransistor based on the design of decorating active-layer surface with plasmonic gold nanoparticles (AuNPs), featuring notably enhanced carrier transport and light-matter interaction with ultrahigh photoresponsivity. This methodology affords a route to ultra-weak light response of organic photodetectors. Organic field-effect transistor is a gate-controlled device that uses an electric field to control the conductivity of the channel, capable of achieving output current amplification9. Thus, it can be usually utilized as a photodetector (known as phototransistor) to amplify weak

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photosignal, enabling effective photoelectric conversion3. Based on our previous publication10, we propose a device design of nanogold decorated organic transistor that proves direct detection and efficient amplification of ultraweak light. (Fig. 1a). The working process is described as follows: p-type organic photoactive layer is used here to form a hole transport channel (HTC) near the insulator/semiconductor interface; AuNPs as surface dopants induce an electron transport channel (ETC) due to additional channel effect of gap-state assisted electron transport10; meanwhile, AuNPs are also plasmon excitation centers (optical antenna) to achieve energy transform under illumination, and the absorption spectrum can be tailored by the nanostructure design8. Compared with those junction-based devices3, the proposed phototransistor has several advantages as a sensing platform such as high photoresponsivity from the amplification mechanism, and functionalization capability of integration on semiconductor chips due to its electrically isolated AuNPs. Here, copper phthalocyanine (CuPc) is used as p-type organic photoactive layer because it’s a good model system to investigate the electrical and optical properties of as-design devices (see section 2 of SI for details). Firstly, UV/visible spectrometer is used to measure the film absorption. Generally, CuPc will achieve B band π→π* and Q band n→π* transitions as two main absorption bands upon UV and Vis illuminations, respectively11. The localized surface plasmon resonance (LSPR) absorption of AuNPs is in the visible wavelength around 500-700 nm12. As shown in Fig. 1b, 1-nm thick nanogold (see Fig. S2 for details) shows an absorption peak at ~560 nm, which keeps in consonance with literatures13. With AuNPs decoration, the hybrid film absorption was greatly enhanced over the UV-Vis and NIR spectral regions due to the LSPR and

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scattering effect of AuNPs7. The net absorption enhancement is calculated and extracted in Fig. 1c. It is clearly seen that the maximum absorption enhancement is characterized in redNIR spectral regions with consistency of Q band absorption of CuPc. This indicates a high level of plasmonic effect of energy transform due to light-matter interaction8. Thus, a 655-nm red laser is utilized here to characterize the light responsive properties of as-design devices. To certify the device design, we carried out a set of optoelectrical characteristics using two different samples: pristine device (reference) and plasmonic device with nanogold decoration (see experimental method of SI for details). It can be seen from Fig. S3 that as-prepared devices show typical p-channel field-effect characteristics. The plasmonic device exhibits larger drain output current as compared with the pristine device, which is contributed to additional channel effect proposed by our previous publication10. Output characteristics of as-prepared phototransistors under 655 nm illumination are presented in Fig. 2. The plasmonic device illustrates obvious photocurrent under no matter off-state (Vg= 0 V, Fig. 2a) or on-state (Vg= -100 V, Fig. 2b), which represent two different operation modes relating to photovoltaic effect and photoconductive effect in phototransistor9. Note that the plasmonic device achieves a comparable photocurrent (~200 nA) response to a weak incident intensity (0.01 µWcm-2), even up to ~3600 nA (250 mWcm-2) under on-state condition. The result indicates the plasmonic device enables a very wide detectable range of light intensity with evident signal output, which is crucial to photoelectronic integrated applications3. In contrast, the pristine device shows a diminished photoresponse (Fig. 2c and Fig. S4). Through a detailed comparison (Fig. 2d vs. e), it can be easily found that the pristine device features an absence of capacity for weak-light response, while the plasmonic device is

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responsive. Further, we measure the transfer characteristics of the as-prepared devices under different incident intensities, as shown in Fig. 3. The right vertical axe of Fig. 3 (Sqrt(Id)-Vg) presents the shift of threshold voltage to positive direction with light intensity increasing as a result of photovoltaic effect. Correspondingly, the left vertical axe of Fig. 3 (Id-Vg) is the half-logarithmic plot of transfer curves under illumination, also exhibiting high resolution of weak-light response (Fig. 3b vs. 3a). The length of vertical arrow stands for the maximum ratio between the photo and the dark current up to ~103. It can be clearly observed that the plasmonic device (Fig. 3b) enables distinguished response for weak light as compared with the pristine one (Fig. 3a). The time-dependent photoresponse over 7 minutes with a periodic laser on/off operation is measured under fixed light intensity and applied voltage. As shown in Fig. S5, a stable and repeatable electric-switching behavior between two states is presented clearly, revealing the comparable retention time for photodetection applications. Based on the definitions of figure-of-merits for phototransistors (see section 3 of SI for details), the photocurrent (Iph) and dark current (Idark) are extracted from I-V curves (Fig. 2) to calculate photoresponsivity (R), photosensitivity (P), external quantum efficiency (EQE) and detectivity (D*)3. The photoresponsivity dependence of incident light intensity (Pinc) is examined as shown in Fig. 4, illustrating a linear decrease due to a reduced proportion of photogenerated carriers available for extraction under high Pinc14. It is worth noting that the plasmonic device achieves an extremely high R up to 23359 A/W under a weak photoexcitation, while the pristine device leads to inadequate photoresponse (< 1A/W) owing to absence of weak-light detection. The fact reveals that the plasmonic nanogold decorated device greatly broadens its photoresponsive range by four orders of magnitude, which will

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significantly favor to some fascinating applications, such as night surveillance, environmental monitoring and spectral photometry15. In addition, Fig. S6 exhibits the R as a function of laser wavelength, demonstrating that the nanogold decorated phototransistors have significant performance enhancement, which is consistent with the absorption of as-prepared photoactive films. We further summarize some other figure-of-merits (Pmax, D* and EQE), as listed in Table 1. It is intuitively seen that all performance parameters of the plasmonic device are notably higher than those of the pristine device, apart from a slightly decrease of Pmax as a result of increased dark current. The results above sufficiently demonstrate the fact that nanogold decoration of photoactive layer enables remarkable enhancement of photoresponse in organic transistors. The potential photophysical mechanisms can be understood as following three aspects: a) The nanogold decoration enhances the photoabsorption of CuPc and the efficiency of exciton dissociation16. As shown in Fig. 1b, the photo-induced LSPR of AuNPs will couple with neighboring CuPc molecules, leading to a larger absorption cross section by light trapping (far-field scattering effect) which increases film absorbance and then photoelectric conversion of devices8. In addition, we further reveal the LSPR effect through a simplified near-field simulation (Fig. 5a). It is clearly seen that AuNPs induce ∼109 V/m strong near-field (Fig. 5b), favoring to high exciton generation and dissociation (the exciton-dissociation efficiency is estimated to be 21% based on our previous method)17-18. Comparatively, the exciton dissociation of the pristine device generally occurs near the source electrode and is limited by the diffusion length (~10 nm)19, resulting in a very low dissociation efficiency. In this regard, a universal strategy is to introduce planar or hybrid

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heterojunction and then promote exciton dissociation20. As for the plasmonic device, it can be thought to build an exciton-dissociation mechanism similar to planar heterojunction. b) The plasmonic carrier injection adds the number of collectable photogenerated charges21. As shown in Fig. 5c, AuNPs will produce more hot carriers at the wavelength of plasmonic absorption in addition to photogenerated carriers of CuPc, which can be collected by effective transport channels. A recent work has demonstrated that a 10-nm AuNP affords the generation of about 70 hot electrons22. Thus, the plasmonic carrier injection of as-prepared device is comparative and viable which can be interpreted as the effect of semiconductor doping with nanometals resulting in increased photocurrent6. The suitable energy level alignment of AuNP and CuPc satisfies the requirement for plasmon-induced hot electron transfer to the CuPc layer (dashed arrow, Fig. 5d). c) Plasmon-induced resonance energy transfer (PIRET) leads to photon energy amplification and transfer to adjacent semiconductor23. The efficiency mainly depends on the overlap between the LSPR absorption and the optical transitions in nearby CuPc (Fig. 1b). In principle, photon trapping causes energy amplification and then induces carrier transitions according to plasmonic dipole-dipole coupling and near-field enhancement, which is supposed to be primary reason of achieving weak light response (wavy arrow, Fig. 5d). In summary, we have successfully demonstrated a photodetector design of plasmonic nanogold-decorated organic transistor that proves direct response and efficient amplification of weak light. The as-prepared device manifests extremely high photoresponsivity (up to 23359 A/W) with weak light detection limit of ~10 nWcm-2. Meanwhile, the design enables the plasmonic device to broaden its photoresponsive range by four orders of magnitude,

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which would be useful in many photoelectronic applications such as night surveillance, environmental monitoring and spectral photometry. The present work exhibits a meaningful design toward understanding how plasmonic nanometals can be used as decoration of photoactive layer to achieve ultra-weak light response in phototransistors.

ASSOCIATED CONTENT Supporting Information: The Supporting Information (SI) is available free of charge on the ACS Publications website. Experimental method, phthalocyanine and its optical absorption, some important parameters for evaluating the performances of phototransistors, and Fig. S1-6.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]. Author contributions X.L. and Y.P. conceived the idea and designed the experiments. X.L. and L.D. fabricated and characterized the devices, and wrote the paper. Y.L., F.Z. and K.X. carried out the optical and electrical measurements. W.L. and Y.W. directed the study. All authors provided valuable feedback. Notes: The authors declare no competing financial interests.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China Grant No. 10974074, the Research Fund for the Doctoral Program of Higher Education of China Grant No. 20110211110005 and the National Key R&D Program of China Grant No. 2016YFF0203605.

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REFERENCES 1. Ostroverkhova, O., Organic Optoelectronic Materials: Mechanisms and Applications. Chemical Reviews 2016, 116 (22), 13279-13412. 2. Lee, Y. H.; Lee, T. K.; Song, I.; Yu, H.; Lee, J.; Ko, H.; Kwak, S. K.; Oh, J. H., Boosting the Performance of Organic Optoelectronic Devices Using Multiple-Patterned Plasmonic Nanostructures. Advanced materials 2016, 28 (25), 4976-4982. 3. Baeg, K. J.; Binda, M.; Natali, D.; Caironi, M.; Noh, Y. Y., Organic Light Detectors: Photodiodes and Phototransistors. Advanced materials 2013, 25 (31), 4267-4295. 4. Zhang, Y.; Yuan, Y.; Huang, J., Detecting 100 fWcm-2 Light with Trapped Electron Gated Organic Phototransistors. Advanced materials 2017, 29 (5), 1603969. 5. Brütting, W.; Adachi, C., Physics of Organic Semiconductors, Second Edition. Wiley-VCH: 2012. 6. Kolwas, K.; Derkachova, A., Modification of Solar Energy Harvesting in Photovoltaic Materials by Plasmonic Nanospheres: New Absorption Bands in Perovskite Composite Film. The Journal of Physical Chemistry C 2017, 121 (8), 4524-4539. 7. Jung, J. H.; Yoon, M. J.; Lim, J. W.; Lee, Y. H.; Lee, K. E.; Kim, D. H.; Oh, J. H., High-Performance UV-Vis-NIR Phototransistors Based on Single-Crystalline Organic Semiconductor-Gold Hybrid Nanomaterials. Advanced Functional Materials 2017, 27 (6), 1604528. 8. Atwater, H. A.; Polman, A., Plasmonics for Improved Photovoltaic Devices. Nature materials 2010, 9 (3), 205. 9. Lucas, B.; Trigaud, T.; Videlot-Ackermann, C., Organic Transistors and Phototransistors Based on Small Molecules. Polymer International 2012, 61 (3), 374-389. 10. Luo, X.; Li, Y.; Lv, W.; Zhao, F.; Sun, L.; Peng, Y.; Wen, Z.; Zhong, J.; Zhang, J., Position-Dependent Performance of Copper Phthalocyanine Based Field-Effect Transistors by Gold Nanoparticles Modification. Nanotechnology 2015, 26 (3), 035201. 11. Djurišić, A. B.; Kwong, C. Y.; Lau, T. W.; Guo, W. L.; Li, E. H.; Liu, Z. T.; Kwok, H. S.; Lam, L. S. M.; Chan, W. K., Optical Properties of Copper Phthalocyanine. Optics Communications 2002, 205 (1), 155-162. 12. Link, S.; El-Sayed, M. A., Size and Temperature Dependence of the Plasmon Absorption of Colloidal Gold Nanoparticles. The Journal of Physical Chemistry B 1999, 103 (21), 4212-4217. 13. Balamurugan, M.; Kaushik, S.; Saravanan, S., Green Synthesis of Gold Nanoparticles by Using Peltophorum Pterocarpum Flower Extracts. Nano Biomed Eng 2016, 8 (4), 213-218. 14. Bube, R. H., Photoelectronic Properties of Semiconductors. Cambridge University Press: 1992. 15. Haus, J., Optical Sensors. Wiley Online Library: 2010. 16. Ni, Z.; Ma, L.; Du, S.; Xu, Y.; Yuan, M.; Fang, H.; Wang, Z.; Xu, M.; Li, D.; Yang, J., Plasmonic Silicon Quantum Dots Enabled High-Sensitivity Ultrabroadband Photodetection of Graphene-Based Hybrid Phototransistors. ACS nano 2017, 11 (10), 9854-9862. 17. Sun, L.; Li, Y.; Ren, Q.; Lv, W.; Zhang, J.; Luo, X.; Zhao, F.; Chen, Z.; Wen, Z.; Zhong, J., Toward Ultrahigh Red Light Responsive Organic FETs Utilizing Neodymium

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Phthalocyanine as Light Sensitive Material. IEEE Transactions on Electron Devices 2016, 63 (1), 452-458. 18. Zhang, P.; Wang, T.; Gong, J., Mechanistic Understanding of the Plasmonic Enhancement for Solar Water Splitting. Advanced materials 2015, 27 (36), 5328-5342. 19. Terao, Y.; Sasabe, H.; Adachi, C., Correlation of Hole Mobility, Exciton Diffusion Length, and Solar Cell Characteristics in Phthalocyanine/Fullerene Organic Solar Cells. Applied Physics Letters 2007, 90 (10), 103515. 20. Peng, Y.; Lv, W.; Yao, B.; Fan, G.; Chen, D.; Gao, P.; Zhou, M.; Wang, Y., High Performance Near Infrared Photosensitive Organic Field-Effect Transistors Realized by an Organic Hybrid Planar-Bulk Heterojunction. Organic Electronics 2013, 14 (4), 1045-1051. 21. Lin, J.; Li, H.; Zhang, H.; Chen, W., Plasmonic Enhancement of Photocurrent in MoS2 Field-Effect-Transistor. Applied Physics Letters 2013, 102 (20), 203109. 22. Shokri Kojori, H.; Yun, J.-H.; Paik, Y.; Kim, J.; Anderson, W. A.; Kim, S. J., Plasmon Field Effect Transistor for Plasmon to Electric Conversion and Amplification. Nano letters 2015, 16 (1), 250-254. 23. Li, J.; Cushing, S. K.; Meng, F.; Senty, T. R.; Bristow, A. D.; Wu, N., Plasmon-Induced Resonance Energy Transfer for Solar Energy Conversion. Nature Photonics 2015, 9 (9), 601-607.

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Figure Legends

Figure 1. (a) Schematic design for achieving a plasmonic nanogold decorated organic phototransistor; p-type organic active layer forms a hole transport channel near the insulator/semiconductor interface, and AuNPs induce an electron transport channel. (b) Absorption spectra of 1 nm nanogold (AuNPs), 20 nm CuPc and corresponding CuPc/AuNPs thin films. (c) Absorption enhancement of AuNPs decorated CuPc films.

Figure 2. Output characteristics of as-prepared organic phototransistors under 655 nm laser illumination with different radiation intensity. (a) and (b) are the photoresponse

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characteristics of plasmonic device under off state (Vg = 0 V) and on state (Vg = -100 V), respectively. (c) is the photoresponse characteristics of pristine device. (d) and (e) show a comparation of weak light response between pristine and plasmonic devices.

Figure 3. Transfer characteristics of the pristine (a) and plasmonic (b) phototransistor in the dark and under 655 nm laser illumination with different radiation intensity.

Figure 4. A comparation of photoresponsivity between pristine and plasmonic devices; the parameters were calculated by extracting current values at Vg=-100 V and Vd=-50 V.

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Figure 5. (a) 3-D model for FDTD simulation of the plasmonic nanogold decorated organic phototransistor; (b) near-field distribution under 655 nm light excitation. (c) Sketch and (d) schematic energy level diagram of photoexciton generation, dissociation and transport for as-prepared plasmonic devices; the mechanisms involve AuNPs LSPR and CuPc absorption, plasmon-induced hot electron injection and resonance energy transfer (PIRET); Egap denotes AuNPs dopant-induced gap state. Table Table 1. Comparison of figures-of-merits between pristine and plasmonic devicesa. As-prepared

Pinc,minb

Rmax c

devices

(µWcm-2)

(A/W)

Pristine device

106.7

0.37±0.02

(2.5±0.13)×103

(4.0±0.05)×108

0.71±0.04

Plasmonic device

0.01

(2.3±0.14)×104

(2.0±0.11)×103

(1.0±0.29)×1013

(4.4±0.27)×104

a

Pmax d

D* c

EQE c

(Jones)

All parameters are average values obtained from four devices; Pinc,min denotes detectable minimum incident light intensity;

b c

The parameters are calculated by extracting current values at Vg=-100 V and Vd=-50 V under minimum incident light intensity; d

The parameter is obtained at maximum incident light intensity (~250 mWcm-2).

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Table Of Contents (TOC) graphic

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