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Improved Photoresponse Performance of Self-Powered ZnO/Spiro-MeOTAD Heterojunction Ultraviolet Photodetector by Piezo-Phototronic Effect Yanwei Shen, Xiaoqin Yan, Haonan Si, Pei Lin, Yichong Liu, Yihui Sun, and Yue Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12870 • Publication Date (Web): 12 Feb 2016 Downloaded from http://pubs.acs.org on February 16, 2016

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ACS Applied Materials & Interfaces

Improved Photoresponse Performance of SelfPowered ZnO/Spiro-MeOTAD Heterojunction Ultraviolet Photodetector by Piezo-Phototronic Effect Yanwei Shena, Xiaoqin Yana,*, Haonan Sia, Pei Lina, Yichong Liua, Yihui Suna, Yue Zhanga,b,* a

State Key Laboratory for Advanced Metals and Materials, School of Materials Science and

Engineering, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China. b

Beijing Municipal Key Laboratory of New Energy Materials and Technologies, University of

Science and Technology Beijing, Beijing 100083, People’s Republic of China. E-mail: [email protected]; [email protected] KEYWORDS: piezo-phototronic effect, ZnO/Spiro-MeOTAD heterojunction, ultraviolet photodetector, self-powered, flexible

ABSTRACT

ACS Paragon Plus Environment

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Strain-induced piezoelectric potential (piezopotential) within wurtzite-structured ZnO can engineer the energy-band structure at a contact or a junction, and thus enhance the performance of corresponding optoelectronic devices by effectively tuning the charge carriers’ separation and transport. Here we report the fabrication of a flexible self-powered ZnO/Spiro-MeOTAD hybrid heterojunction ultraviolet photodetector (UV PD). The obtained device has a fast and stable response to the UV light illumination at zero bias. Together with responsivity and detectivity, the photocurrent can be increased about one times upon applying a 0.753% tensile strain. The enhanced performance can be attributed to more efficient separation and transport of photogenerated electron-hole pairs, which is favored by the positive piezopotential modulated energyband structure at the ZnO-Spiro-MeOTAD interface. This study demonstrates a promising approach to optimize the performance of a photodetector made of piezoelectric semiconductor materials through straining.

Introduction Due to the wide direct-bandgap (~3.37 eV), high exciton binding energy (~60 meV), high carrier mobility (>100 cm2V-1s-1 at RT), high internal photoconductive gain and high resistance to irradiation,1-2 ZnO has attracted much more attention for developing high-performance optoelectronics devices, such as UV laser diodes (LDs)3, light emitting diodes (LEDs)4-5, photodetectors (PDs)6-8, solar cells9-10, and water photo-splitting devices11-12. Among them, ZnO based UV PDs, especially the self-powered ones, have drawn researchers’ great interest, since self-powered PDs can operate in some harsh conditions without external power source and have a wide range of potential applications in environmental monitoring, biological and chemical analysis, flame warning, optical communications, and astronomical research.13-14 These novel


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self-powered PDs driven by the photovoltaic effect have three structure types in terms of interface: Schottky junction type15-16, p-n junction type17-20 and photoelectrochemical (PEC) type21-22. Compared with other two types, p-n junction PDs are more ideal structure due to some special advantages of low applied fields, high sensitivity, fast response time, and high stability.14 Up to now, several kinds of ZnO based p-n junction PDs have been reported17-20, in which the organic/inorganic hybrid PDs have gained tremendous research interest because of their unique properties combining high flexibility, low cost and easy fabrication at low temperature of organic polymers with high stability of inorganic materials. Among various organic materials, 2,2’,7,7’tetrakis (N,N-di-p-methoxyphenylamine) 9,9’-spirobifluorene (Spiro-MeOTAD) is a nonhazardous solid hole conductor and exhibits efficient hole regeneration capability and high conductivity, thus may be a good choice in UV PDs23. However, ZnO/Spiro-MeOTAD hybrid PDs have seldom been studied, especially flexible PDs which have high potential for use in the monitoring of human beings and their surrounding environment due to several advantages over the rigid devices, including facile processing, low cost, transparent, lightweight, shock resistance, and flexibility.24 Recently, piezo-phototronic effect, which is referred to use the strain-induced piezopotential in wurtzite materials to modulate the charge carriers’ separation and transport behaviors across a Schottky or p−n junction interface,25-26 has been utilized to enhance the performance of optoelectronic devices, such as LEDs27-28, PDs15-16, 29-30 and solar cells31-32. It is noteworthy that in addition to one-dimensional (1D) ZnO nanowires/rods, radio-frequency (RF) sputtered ZnO thin films have been demonstrated to exhibit the coupled piezoelectric, optoelectronic and semiconducting properties.33-34 From a technology point of view, ZnO films surpass 1D ZnO nanostructure materials, because 1D ZnO nanostructure materials still suffer from problems of


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lacking uniformity in dimensions, morphologies and doping levels, which have hindered corresponding devices’ application, however, these piezotronic ZnO films can take full advantage of the state-of-art microfabrication technologies which can guarantee well-controlled material properties and scalable production. To the best of our knowledge, till now, piezophototronic devices based on piezotronic ZnO films are seldom reported and developed. Wen et al.33 first reported a piezotronic ZnO film based UV PD which exhibited an improved UV sensing capability upon externally applying tensile strains, however, the UV sensing performance was still rather poor - the UV sensitivity was only 112.5% and the recovery time as long as 337 s at a bias of 5 V upon a 0.48% tensile strain. In this paper, a flexible self-powered organic/inorganic hybrid heterojunction UV PD fabricated using n-type RF-sputtered ZnO films and p-type Spiro-MeOTAD was demonstrated. The obtained PD had a fast and stable response to the UV light illumination at zero bias. Furthermore, the photoresponse properties of the device under varying tensile and compressive strains at zero bias were systematically investigated. The underlying strain-modulated mechanism was interpreted in terms of the energy-band diagrams of ZnO-Spiro-MeOTAD heterojunction with and without strain. 1. Experimental details Commercial flexible ITO/PET substrate (sheet resistance is 35 ohm/sq and the thickness of PET is 125 µm) was used to deposit ZnO film in a home-made RF sputtering system. Prior to deposition, firstly peel off the protecting layer, and then clean the ITO/PET substrate by acetone, ethanol and DI water in sequence and dry it by N2, finally, shield a part of the substrate with PI Kapton tapes to serve as bottom electrode and avoid current leakage during spin-coating the Spiro-MeOTAD solution. ZnO (purity, 5N) was selected as the target material. The base vacuum pressure was evacuated to 3 × 10-6 Torr. The RF power and Argon flow rate for sputtering were


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kept at 80 W and 30 SCCM, respectively. The sputtering pressure was maintained at 7.5 × 10-3 Torr. The thickness of the deposited ZnO film was about 1 µm, which was read from thin film deposition controller (FTC-2800, Kert J. Lesker). In order to prevent overheating the substrate and avoid the substrate deformation, it is necessary to select small RF power and suspend deposition every 30 minutes for a 15-minute period. Schematic diagram of the device structure is shown in Fig. 1. The as-grown ZnO sample was cut into 5 mm × 10 mm pieces, which were then glued on the center of polystyrene substrates with dimensions of 40 mm × 10 mm ×0.2 mm. Next, the Spiro-MeOTAD layer was fabricated by spin coating at 3000 r.p.m. for 30 s. The solution formulation of Spiro-MeOTAD was prepared using the method reported previously.35 Au electrode was thermally evaporated onto Spiro-MeOTAD film as a top electrode, in which a simple shadow mask was applied, and ITO acted as bottom electrode. Highly flexible thin wires were secured on the Au electrode and ITO electrode using Ag paste. Finally, the whole device was completely and seamlessly sealed with a thin layer of poly(dimethylsiloxane) (PDMS). The morphologies of the as-deposited ZnO film were characterized using a Field emission scanning electron microscope (FESEM, FEI Quanta 3D) and atomic force microscope (AFM) operating in the tapping mode. The crystalline properties of the ZnO sample were investigated by X-ray diffraction (XRD) (Rigaku DMAX-RB with Cu Kα radiation l = 0.15406 nm). UV-vis absorption spectra of the ZnO/Spiro-MeOTAD films were measured using a UV-vis-NIR spectrometer (Varian Cary 5000). The photoelectric properties of the obtained UV PDs were characterized using a Semiconductor Characterization System (Keithley 4200). The photocurrent was measured when the device was illuminated by a handheld 365 nm UV light source.


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2. Results and discussion The cross-sectional and top view scanning electron microscope (SEM) morphologies of the ZnO film sputtered on a preferential Si substrate are presented in Fig. 2a. It can be clearly observed that the as-grown ZnO film is composed of multiple vertical-oriented columnar grains and is about 1 µm thick. The inset of Fig. 2a shows that the diameter of columnar grains is in the range of 100-300 nm. Fig. 2b exhibits the three-dimensional surface morphology of the sputtered ZnO film on PET substrate. According to the AFM analysis, we can obtain the surface root mean square roughness of about 160 nm, which shows that the surface of the sputtered ZnO is relatively smooth. X-ray diffraction (XRD) pattern in Fig. 2c illustrates that a dominant (002) plane diffraction peak of wurtzite ZnO centers at 2θ=34.4°. Furthermore, the weak (101) diffraction peak resulting from non-equilibrium growth can be observed within the range of (30°, 40°). The above result demonstrates that the RF sputtered ZnO film has a preferred c-axis growth orientation along the , indicating that the film exhibits piezoelectric property. The optical absorption spectra of ZnO and ZnO/Spiro-MeOTAD heterostructure are shown in Fig. 2d, which clearly shows that both of them have an intensive absorption below a wavelength of 400 nm, and the ZnO/Spiro-MeOTAD heterostructure shows much more favorable UV absorption than ZnO. In addition, the bandgap of ZnO can be determined to be about 3.23 eV from the plot of (αhυ)2 vs. hυ (α and hυ are the absorption coefficient and photon energy, respectively), as shown in the inset of Fig. 2d. To further investigate the electronic properties of the as-fabricated ZnO/Spiro-MeOTAD heterojunction, we tested the current-voltage (I–V) characteristics in dark and under 365 nm UV light illumination with a power density of 1.0 mW/cm2. As shown in Fig. 3a, the dark I-V curve of the device displays a typical diode-like rectifying behavior with a fairly low turn-on voltage of


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0.8 V and a high rectification ratio of 7.69 × 102 at ±1 V. For ZnO based organic-inorganic hybrid p-n junction diode, the rectification ratio value of 769 is almost an order of magnitude greater than the previously reported value,36-37 which could be ascribed to well-aligned energyband structure and a low leakage current at the Spiro-MeOTAD-ITO interface. Using the diode equation,38 the ideality factor of the ZnO/Spiro-MeOTAD heterojunction can be derived from the slope of the dark logI vs V curve plotted in the inset of Fig. 3a. The value is about 4.09, which is higher than the ideal value 1. The high ideality factor may be attributed to the presence of multiple current pathways associated with the surface states at ZnO/Spiro-MeOTAD interface.23 Furthermore, the I-V curve under UV light illumination demonstrates the fabricated device exhibits an obvious photoresponse both under forward and reverse bias. It can be clearly seen from Fig. 3b, an enlarged I-V curve of Fig. 3a near zero bias, that a measurable photovoltaic effect existed under UV light illumination, which demonstrated the feasibility of the fabricated ZnO/Spiro-MeOTAD UV PDs operating in self-powered mode (i.e. at zero bias). The photocurrent response property of the ZnO/Spiro-MeOTAD device at zero bias was further studied as shown in Fig 3c. More than four repeat cycles of switching the 1.0 mW/cm2 UV source on (15 s) and off (15 s) were recorded. The photocurrent response under each cycle was rapid, consistent, repeatable and had no noticeable decay. Interestingly, for each cycle the photocurrent change has two stages. In the first stage, a sharp current peak emerged when the UV light was switched on. Then in the second stage, the photocurrent decreased quickly and reached a steady plateau. This two-stage phenomenon had also been observed in previous report, and the sharp peaks may stemmed from the synergy of photovoltaic effect and pyroelectric effect under UV illumination, during the second stage, the pyroelectric potential disappeared quickly since the temperature kept constant by retaining the UV illumination, so the photocurrent


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reached a steady value.39 It is worth noting that the photocurrent value derived from the first stage is much larger than that from the second stage, whose values are observed as 1.06 × 10-7 and 8.41 × 10-8 A, respectively, indicating 120% enhancement on photocurrent by pyroelectric effect. The response time is a key parameter of photodetector for practical applications. The rise time and decay time (defined as the time required for the photocurrent to increase from 10% to 90% and drop from 90% to 10%, respectively) were found to be 0.16 s and 0.20 s, respectively, which can be obtained from the enlarged rising and decaying edges of the photoresponse curve shown in Fig. 3d, indicating a fast photoresponse behavior. Moreover, taking the time resolution of our test instrument into consideration, the rise time and decay time should be shorter, which are far less than the values published previously.33 Utilizing piezo-phototronic effect to modulate the interface energy-band structure via staininduced piezopotential within wurtzite-structured ZnO is very attractive, which has been demonstrated to be an effective and promising way to enhance the performance of corresponding optoelectronic devices. The photoresponse properties of the obtained ZnO/Spiro-MeOTAD heterojunction UV PD under different strains were systematically studied, as shown in Fig. 4. The cross-sectional schematic of strain-dependent electrical measurement system is illustrated in the inset of Fig. 4a. The different strains ε could be introduced and calculated using the local radius R and the thickness of polystyrene substrate D via applying different bending curvatures, as 40 ε = ± D / 2R

(1)

Fig. 4a shows that the I-V characteristics of the device under different strains all exhibit good diode-like rectifying behavior, more importantly, the threshold voltage increases when a 0.753%


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tensile strain is applied to the device, indicating that the barrier height of ZnO/Spiro-MeOTAD heterojunction increases, on the contrary, compressive strain decreases the barrier height. The photocurrent response curves of the device under varying tensile and compressive strains at zero bias are presented in Fig. 4b. During each test, the power density of 365 nm UV source was kept at 1 mW cm-2. It can be clearly observed that the photocurrent increases step by step when the applied tensile strain increases gradually, while in contrast, the photocurrent decreases when compressive strain increases. Meanwhile, the response time and recover time remain almost unchanged. It should be noted that one-fold increase of photocurrent could be achieved upon applying a 0.753% tensile strain and the maximum reaches about 1.64 × 10-7 A, which is twice as much as that reported for self-powered p-NiO/n-ZnO heterojunction UV PDs.41 To further study the strain modulation on the photoresponse performance of ZnO/Spiro-MeOTAD heterojunction UV PDs, we plotted the curves of the responsivity and detectivity versus strains in Fig. 3c, respectively. Here, the responsivity (R in mA/W) is defined as the ratio of photocurrent to incident-light intensity; the detectivity (D* in cm Hz1/2/W) denotes the smallest detectable signal in view of the dark current and can be expressed as42 D* = R / (2qJd )1/2

(2)

Where R is the responsivity, q is the absolute value of electron charge and Jd is the measured dark current density. As can be seen from Fig.4c, both the responsivity and detectivity increase monotonically with the increase of tensile strain, and a nearly one-fold increase of responsivity and detectivity can be obtained upon applying a 0.753% tensile strain, whose maximum value can reach 0.8 mA/W and 4.2 ×109 cm Hz1/2/W, respectively. On the contrary, both the responsivity and detectivity decrease monotonically with the increase of compressive strain.


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The potential mechanism of the strain-modulated photoresponse performance of the fabricated device could be understood by piezo-phototronics theory,26 which could be interpreted using the band diagram of the ZnO/Spiro-MeOTAD heterojunction in the presence of strain, as shown in Fig.5. Since the c-axis of RF sputtered ZnO thin film grown on ITO/PET substrate is pointing from ITO/PET substrate to ZnO, once the ZnO film suffers a tensile strain, permanent positive piezoelectric polarization charges will be induced at the ZnO/Spiro-MeOTAD interface. This positive piezopotential at the interface can lower the local conduction band level of ZnO, thus increasing the barrier height from ∆E0 to ∆E+, as shown in Fig. 5a. The increased barrier height will strengthen the built-in electric field, therefore, the separation efficiency of photogenerated electron-hole pairs is enhanced, leading to the improved photocurrent and responsivity. On the contrary, when the RF sputtered ZnO film suffers a compressive strain, the induced negative piezoelectric polarization charges at the interface will lift the local conduction band level of ZnO, thus lowering the barrier height from ∆E0 to ∆E-, as shown in Fig. 5b. As a result, the strength of build-in electric field is weakened, thus the charge separation efficiency is jeopardized, resulting in the decreased photocurrent and responsivity. 3. Conclusions In summary, a flexible, low-cost and self-powered UV photodetector composed of RF sputtered ZnO films and p-type Spiro-MeOTAD was developed, which exhibited fast and stable response to UV light illumination at zero bias. The response time is shorter than 0.2 s. Meanwhile, the modulation effect of strain-induced piezopotential on the performance of the ZnO-SpiroMeOTAD heterojunction PD was investigated. A one-fold increase of photocurrent, together with nearly one-fold increase of responsivity and detectivity, could be achieved upon applying a 0.753% tensile strain. The enhancement mechanism could be attributed to more favorable


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separation and transport of photo-generated electron-hole pairs, which is a result of the modification effect of the positive piezopotential on the band structure at the ZnO-SpiroMeOTAD interface. This study provides a guideline to optimize the performance of an organicinorganic hybrid heterojunction self-powered UV PD via strain engineering. ACKNOWLEDGMENT This work was supported by the National Major Research Program of China (No. 2013CB932602), the Program of Introducing Talents of Discipline to Universities (B14003), National Natural Science Foundation of China (No. 51527802, 51232001 and 51372023), Beijing Municipal Science & Technology Commission, and the Fundamental Research Funds for Central Universities. REFERENCES 1. Kind, H.; Yan, H.; Messer, B.; Law, M.; Yang, P. Nanowire Ultraviolet Photodetectors and Optical Switches. Adv. Mater. 2002, 14, 158-160. 2. Özgür, U.; Alivov, Y. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; Doğan, S.; Avrutin, V.; Cho, S. J.; Morkoç, H. A Comprehensive Review of ZnO Materials and Devices. J. App. Phys. 2005, 98, 041301. 3. Chu, S.; Wang, G.; Zhou, W.; Lin, Y.; Chernyak, L.; Zhao, J.; Kong, J.; Li, L.; Ren, J.; Liu, J. Electrically Pumped Waveguide Lasing from ZnO Nanowires. Nat. Nanotechnology 2011, 6, 506-510. 4. Zhang, X.-M.; Lu, M.-Y.; Zhang, Y.; Chen, L.-J.; Wang, Z. L. Fabrication of a HighBrightness Blue-Light-Emitting Diode Using a ZnO-Nanowire Array Grown on p-GaN Thin Film. Adv. Mater. 2009, 21, 2767-2770. 5. Pan, C.; Dong, L.; Zhu, G.; Niu, S.; Yu, R.; Yang, Q.; Liu, Y.; Wang, Z. L. HighResolution Electroluminescent Imaging of Pressure Distribution Using a Piezoelectric Nanowire LED Array. Nat. Photonics 2013, 7, 752-758. 6. Yang, Y.; Guo, W.; Qi, J.; Zhao, J.; Zhang, Y. Self-Powered Ultraviolet Photodetector Based on a Single Sb-doped ZnO Nanobelt. Appl. Phys. Lett. 2010, 97, 223113. 7. Tian, W.; Zhang, C.; Zhai, T.; Li, S.-L.; Wang, X.; Liu, J.; Jie, X.; Liu, D.; Liao, M.; Koide, Y.; Golberg, D.; Bando, Y. Flexible Ultraviolet Photodetectors with Broad Photoresponse Based on Branched ZnS-ZnO Heterostructure Nanofilms. Adv. Mater. 2014, 26, 3088-3093. 8. Chen, H.-Y.; Liu, K.-W.; Chen, X.; Zhang, Z.-Z.; Fan, M.-M.; Jiang, M.-M.; Xie, X.-H.; Zhao, H.-F.; Shen, D.-Z. Realization of a Self-Powered ZnO MSM UV Photodetector with High Responsivity Using an Asymmetric Pair of Au Electrodes. J. Mater. Chem. C 2014, 2, 96899694.


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26. Wang, Z. L. Progress in Piezotronics and Piezo-Phototronics. Adv. Mater. 2012, 24, 4632-4646. 27. Yang, Q.; Liu, Y.; Pan, C.; Chen, J.; Wen, X.; Wang, Z. L. Largely Enhanced Efficiency in ZnO Nanowire/p-Polymer Hybridized Inorganic/Organic Ultraviolet Light-Emitting Diode by Piezo-Phototronic Effect. Nano Lett. 2013, 13, 607-613. 28. Wang, C.; Bao, R.; Zhao, K.; Zhang, T.; Dong, L.; Pan, C. Enhanced Emission Intensity of Vertical Aligned Flexible ZnO Nanowire/p-Polymer Hybridized LED Array by PiezoPhototronic Effect. Nano Energy 2015, 14, 364-371. 29. Yang, Q.; Guo, X.; Wang, W.; Zhang, Y.; Xu, S.; Lien, D. H.; Wang, Z. L. Enhancing Sensitivity of a Single ZnO Micro-/Nanowire Photodetector by Piezo-Phototronic Effect. ACS Nano 2010, 4, 6285-6291. 30. Zhang, F.; Niu, S.; Guo, W.; Zhu, G.; Liu, Y.; Zhang, X.; Wang, Z. L. Piezo-Phototronic Effect Enhanced Visible/UV Photodetector of a Carbon-Fiber/ZnO-CdS Double-Shell Microwire. ACS Nano 2013, 7, 4537-4544. 31. Yang, Y.; Guo, W.; Zhang, Y.; Ding, Y.; Wang, X.; Wang, Z. L. Piezotronic Effect on The Output Voltage of P3HT/ZnO Micro/Nanowire Heterojunction Solar Cells. Nano Lett. 2011, 11, 4812-4817. 32. Shi, J.; Zhao, P.; Wang, X. Piezoelectric-Polarization-Enhanced Photovoltaic Performance in Depleted-Heterojunction Quantum-Dot Solar Cells. Adv. Mater. 2013, 25, 916921. 33. Wen, X.; Wu, W.; Ding, Y.; Wang, Z. L. Piezotronic Effect in Flexible Thin-Film Based Devices. Adv. Mater. 2013, 25, 3371-3379. 34. Zhang, Y.; Yan, X.; Yang, Y.; Huang, Y.; Liao, Q.; Qi, J. Scanning Probe Study on the Piezotronic Effect in ZnO Nanomaterials and Nanodevices. Adv. Mater. 2012, 24, 4647-4655. 35. Burschka, J.; Pellet, N.; Moon, S. J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Gratzel, M. Sequential Deposition as a Route to High-Performance Perovskite-Sensitized Solar Cells. Nature 2013, 499, 316-319. 36. Wang, L.; Zhao, D.; Su, Z.; Fang, F.; Li, B.; Zhang, Z.; Shen, D.; Wang, X. High Spectrum Selectivity Organic/Inorganic Hybrid Visible-Blind Ultraviolet Photodetector Based on ZnO Nanorods. Org. Electron. 2010, 11, 1318-1322. 37. Mridha, S.; Basak, D. ZnO/Polyaniline Based Inorganic/Organic Hybrid Structure: Electrical and Photoconductivity Properties. Appl. Phys. Lett. 2008, 92, 142111. 38. Shah, J. M.; Li, Y. L.; Gessmann, T.; Schubert, E. F. Experimental Analysis and Theoretical Model for Anomalously High Ideality Factors (n≫2.0) in AlGaN/GaN p-n Junction Diodes. J. Appl. Phys. 2003, 94, 2627. 39. Wang, Z.; Yu, R.; Pan, C.; Li, Z.; Yang, J.; Yi, F.; Wang, Z. L. Light-Induced Pyroelectric Effect as An Effective Approach for Ultrafast Ultraviolet Nanosensing. Nat. Commun. 2015, 6, 8401-8407. 40. Yang, R.; Qin, Y.; Dai, L.; Wang, Z. L. Power Generation with Laterally Packaged Piezoelectric Fine Wires. Nat. Nanotechnol. 2009, 4, 34-39. 41. Hasan, M. R.; Xie, T.; Barron, S. C.; Liu, G.; Nguyen, N. V.; Motayed, A.; Rao, M. V.; Debnath, R. Self-Powered p-NiO/n-ZnO Heterojunction Ultraviolet Photodetectors Fabricated on Plastic Substrates. APL Mater. 2015, 3, 106101. 42. Gong, X.; Tong, M.; Xia, Y.; Cai, W.; Moon, J. S.; Cao, Y.; Yu, G.; Shieh, C. L.; Nilsson, B.; Heeger, A. J. High-Detectivity Polymer Photodetectors with Spectral Response from 300 nm to 1450 nm. Science 2009, 325, 1665-1667.


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Figures

Figure 1. Schematic diagram of the device structure


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Figure 2. (a)The cross-sectional SEM morphologies of the ZnO film sputtered on a preferential Si substrate, and the top view is shown in the inset. (b) Three-dimensional surface morphology of the sputtered ZnO film on PET substrate imaged by AFM in a tapping mode. (c) X-ray diffraction spectrum of the as-deposited ZnO film. (d) Optical absorption spectrum of ZnO film and ZnO/Spiro-MeOTAD heterostructure, the inset shows a plot of (αhυ)2 vs. hυ


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Figure 3. (a) I-V characteristics of the as-fabricated ZnO/Spiro-MeOTAD heterojunction UV PD in dark and under 365 nm UV light illumination with a power density of 1.0 mW/cm2, the inset is the I-V curve in dark plotted on the semi-logarithmic scale. (b) The enlarged I-V curve of (a) near zero bias. (c) The photocurrent response property of the device at zero bias. More than four repeat cycles of switching the 1 mW/cm2 UV source on (15 s) and off (15 s) were recorded. (d) The enlarged rising and decaying edges of the photocurrent response curve (c).


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Figure 4. (a) I-V characteristics of the obtained device under varying tensile and compressive strains. The inset is the schematic of strain-dependent electrical measurement system. (b) The photocurrent response curves of the device under varying tensile and compressive strains at zero bias. (c) The curves of calculated responsivity and detectivity as a function of applied strains, respectively.


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Figure 5. Schematic band diagram of ZnO/Spiro-MeOTAD heterojunction with and without the presence of (a) tensile and (b) compressive strain, shown in red and black curves, respectively.


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Figure 1. Schematic diagram of the device structure 99x57mm (300 x 300 DPI)


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Figure 2. (a)The cross-sectional SEM morphologies of the ZnO film sputtered on a preferential Si substrate, and the top view is shown in the inset. (b) Three-dimensional surface morphology of the sputtered ZnO film on PET substrate imaged by AFM in a tapping mode. (c) X-ray diffraction spectrum of the as-deposited ZnO film. (d) Optical absorption spectrum of ZnO film and ZnO/Spiro-MeOTAD heterostructure, the inset shows a plot of (αhυ)2 vs. hυ 130x99mm (300 x 300 DPI)



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Figure 3. (a) I-V characteristics of the as-fabricated ZnO/Spiro-MeOTAD heterojunction UV PD in dark and under 365 nm UV light illumination with a power density of 1.0 mW/cm2, the inset is the I-V curve in dark plotted on the semi-logarithmic scale. (b) The enlarged I-V curve of (a) near zero bias. (c) The photocurrent response property of the device at zero bias. More than four repeat cycles of switching the 1 mW/cm2 UV source on (15 s) and off (15 s) were recorded. (d) The enlarged rising and decaying edges of the photocurrent response curve (c). 130x99mm (300 x 300 DPI)


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Figure 4. (a) I-V characteristics of the obtained device under varying tensile and compressive strains. The inset is the schematic of strain-dependent electrical measurement system. (b) The photocurrent response curves of the device under varying tensile and compressive strains at zero bias. (c) The curves of calculated responsivity and detectivity as a function of applied strains, respectively. 91x70mm (300 x 300 DPI)



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Figure 5. Schematic band diagram of ZnO/Spiro-MeOTAD heterojunction with and without the presence of (a) tensile and (b) compressive strain, shown in red and black curves, respectively. 91x70mm (300 x 300 DPI)


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Figure 5. Schematic band diagram of ZnO/Spiro-MeOTAD heterojunction with and without the presence of (a) tensile and (b) compressive strain, shown in red and black curves, respectively. 91x70mm (300 x 300 DPI)



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A flexible, low-cost and self-powered UV photodetector composed of RF sputtered ZnO films and p-type Spiro-MeOTAD has been developed. The photoresponse properties of the fabricated device under varying tensile and compressive strains at zero bias are systematically investigated. 70x29mm (300 x 300 DPI)


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Spiro

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