Multilevel Nonvolatile Organic Photomemory Based on Vanadyl

Sep 13, 2017 - Molecular template growth has been developed to produce high performance organic devices based on high-quality organic semiconductor th...
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Multi-Level Nonvolatile Organic Photomemory based on Vanadyl-Phthalocyanine/para-Sexiphenyl Heterojunctions Chuan Qian, Jia Sun, Ling-An Kong, Ying Fu, Yang Chen, Juxiang Wang, Shitan Wang, Haipeng Xie, Han Huang, Junliang Yang, and Yongli Gao ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00898 • Publication Date (Web): 13 Sep 2017 Downloaded from http://pubs.acs.org on September 15, 2017

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Multi-Level Nonvolatile Organic Photomemory based on Vanadyl-Phthalocyanine/para-Sexiphenyl Heterojunctions Chuan Qian,† Jia Sun,*,† Ling-an Kong,† Ying Fu,† Yang Chen,† Juxiang Wang,† Shitan Wang,† Haipeng Xie,† Han Huang,† Junliang Yang,† and Yongli Gao*,†, ‡ †

Hunan Key Laboratory for Super Microstructure and Ultrafast Process, School of Physics and Electronics, Central South

University, Changsha, Hunan 410083, P. R. China ‡

Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627, USA

Corresponding Author *E-mail: [email protected] (J. Sun), [email protected] (Y.L. Gao)

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ABSTRACT Organic

photomemory

based

on

heterojunction

phototransistor

has

been

fabricated

utilizing

vanadyl-phthalocyanine (VOPc) on para-sexiphenyl (p-6P) thin films. Under 365 nm ultraviolet light irradiation, the ratio of photocurrent and dark current (Iph/Idark) and photoresponsivity of phototransistors are about 1.5×105 and 87 A/W, respectively. Such devices can transduce the input light signals into electrical signals and the output signals can be stored for recording the light simulation. After applying a light pulse (4.2 mW/cm2, 100 ms) on the device, the stored current level lasted for ~5000 s with only a 20% decrease, indicating a good photomemory behavior. Importantly, the photomemory behavior is effectively modulated by gate voltage. Multi-level photomemory behaviors are observed by modulating light pulse duration and light power intensity. Because of the construction of type-I heterojunction, the superior photomemory characteristics are mainly originated from efficient charge trapping at VOPc/p-6P interface. In-situ current sensitive atomic force microscopy (CSAFM) is used for monitoring surface current of the VOPc/p-6P heterojucntions. A change of conductivity in grains is observed upon 365 nm light illumination. After turning off the light, the current of grains did not rapidly decrease, but displayed the behavior of photomemory. This study provides a guide for designing high-performance organic photomemory devices.

Keyword: organic heterojunction; molecular template growth; nonvolatile organic photomemory; multi-level photocurrent storage; current sensitive atomic force microscopy.

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Organic electronics devices have been intensively studied and applied in displays, sensors, integrated circuits, and so on, because of the low cost, flexibility, and ease of processing.1-6 In particular, organic optoelectronic applications have been regarded promising due to the fascinating photodetecting and photoconducting characteristics in a wide wavelength region.7-11 In addition to the charge carriers generated by the gate modulation, organic phototransistor is a three-terminal optoelectronic device with incident light as the additional control terminal to induce photocarriers. Phototransistors convert incident optical signals to electrical signals with amplification properties, and photomemory devices convert and also store the light information as electrical signal, which is a building block for optical signal processing and photonic neuromorphic circuits. The high performance of organic photomemory based on phototransistor can be realized from the effective control of the gate voltage and the external light terminal. Up to now, the research of photomemory is seldom reported,12-16 especially lack of the direct results of nonvolatile photocurrent as stored light information. Recently, Prof. Samorì and his group have demonstrated the fabrication of flexible photomemory by using P3HT and photochromic diarylethene with an 8-bit light storage.17 Two-dimensional materials/perovskite materials based nonvolatile photomemory devices with a floating gate transistor were also demonstrated.18-21 Because of the charge trapping in the floating gate layer, high performance photomemory behaviors were observed. Molecular template growth has been developed to produce high performance organic devices based on high-quality organic semiconductor thin film with controllable morphologies, interface, photoelectric properties, etc.22,23 With strong light absorption in the visible range, excellent thermal and chemical stabilities, metal phthalocyanine is a kind of very popular organic photoelectronic materials.24 Metal phthalocyanines have been widely utilized for high-performance organic field-effect transistors. Prof. Yan and his group have demonstrated the fabrication of highly ordered organic thin film with a high mobility by using weak epitaxy growth.25,26 Moreover, with the heterojunction of template layer and organic active layer, a long relaxation time of the high photocurrent in the channel can been observed.27 The organic ACS Paragon Plus Environment

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heterojunction phototransistors could be very useful for nonvolatile photomemory.28 In this study, we study on the photomemory behavior in VOPc/p-6P phototransistors fabricated by molecular template growth. The VOPc thin film forms an effective heterojunction with the p-6P thin film, which is very useful for enhancing the light absorption and electron trapping capability at the interface. Under 365 nm ultraviolet (UV) light irradiation, the ratio of photocurrent and dark current (Iph/Idark) and photoresponsivity of VOPc/p-6P heterojunction phototransistors are highly improved. More importantly, such devices can be used for transforming and storing the light information with a long time of larger than 5000 s by applying a light pulse. We also achieved multi-level storage of photomemory by manipulating the photo-pulse duration and light power intensity. These results reported here may open up new possibilities for high performance photoelectric devices by using organic heterojunctions.

RESULTS AND DISCUSSION

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Figure 1. (a) Schematic illustration of the VOPc/p-6P photomemory device structure in this study. (b) Topographic AFM image (10 µm × 10 µm) of VOPc thin film. (c~d) Morphology of the VOPc/p-6P thin film. (c) 10 µm × 10 µm and (d) 2 µm × 2 µm.

Figure 1a shows a schematic illustration of the VOPc/p-6P photomemory device. In Figure 1b, the AFM morphology of 10 nm VOPc thin film shows completely disorderly arrangement of small grains with a large density of boundaries. Compared with VOPc films grown on bare SiO2/Si substrate, the VOPc/p-6P thin film shows a typical island growth behavior in Figure 1c and the size of crystal grains is larger. Therefore fewer boundaries in VOPc/p-6P thin film will affect the transport of the carriers.29 Figure 1d shows the enlarged topographic image of VOPc/p-6P thin film with a smaller scan area of 2 µm × 2 µm. The lamellar VOPc crystal grains display an obviously pyramid-like structure.

Figure 2. The typical (a) output and (b) transfer characteristic curve of VOPc/p-6P phototransistor in the dark. (c) Transfer characteristics of VOPC/p-6P phototransistor measured in the dark or under UV (365 nm) with various light intensities at Vds = - 50 V. (d) The ∆VT and Ιph/ Ιdark of VOPc/p-6P phototransistor as a function of light power intensity. (e) The photoresponsivity of VOPc/p-6P phototransistor as a function of power intensity at Von. (f) UV-vis absorption spectra of VOPc/p-6P, VOPc and p-6P thin films on glass substrate. (g) UPS spectra of the cut-off region and HOMO region for VOPc/p-6P and p-6P thin films. (h) UPS measured energy levels diagram at the VOPc/p-6P heterojunction interface. ACS Paragon Plus Environment

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The VOPc/p-6P phototransistor exhibits good field-effect modulation characteristics, as shown in Figure 2a and b. The threshold voltage (VT) of VOPc/p-6P phototransistors in the dark is about 3.0 V and the turn-on voltage (Von) is about 32 V. At Von, the gate electrode do not electrically induce any holes in channel. The mobility of the device is calculated as about 0.08 cm2/Vs and the current on/off ratio in the dark is about 106. In Figure 2b, the transfer curve displays an obvious anti-clockwise hysteresis loop as the voltage swept between the forward and reverse, which is mainly due to the charge carriers trapping and detrapping at the VOPc and p-6P interface.19,30 At the forward gate voltage sweep, many mobile electrons are easy to be trapped at the interfaces. Subsequently, with the reverse sweeping from negative gate bias, stored electrons are completely detrapped and quickly back to an “off” state. The memory window of hysteresis loop became enhanced with an increase in the Vg sweep range, also indicating that charge trapping occurred upon the application of Vg (Figure S1). With increasing light power intensity, photogenerated carrier concentration dramatically increases and more carriers are trapped with an enlarged hysteresis window (Figure S2). With removing the light, the transfer curve is almost back to initial state. The transfer characteristics of VOPc/p-6P phototransistors are shown in Figure 2c, which are measured in the dark or under UV (365 nm) with various light intensities at Vds = - 50 V. The figure clearly shows the increase of Ids with the irradiation of UV light. The photoelectric properties of VOPc/p-6P phototransistors with different p-6P layer thickness or VOPc layer thickness are displayed in Figure S3 and the summarized photoelectrical performances are presented in Table S1. The device with 10 nm VOPc/7.5 nm p-6P has the best performance. The difference between the VT (∆VT) and Ιph/ Ιdark of 10nm VOPc/7.5nm p-6P phototransistor as a function of light power intensity is shown in Figure 2d. The obvious positive shift in VT with increasing power intensity is indicating that light can be used as an effective control terminal to photogenerate charge carriers. When the devices are irradiated with 4.2 mW/cm2 UV light, the VT of VOPc/p-6P phototransistors positively shift to be about 42 V and the ∆VT is 39 V. The large positive shift in VT results from the trapping of photogenerated electrons near the source electrode and the transport of photogenerated holes.7,31 The Iph/Idark is an important parameter to ACS Paragon Plus Environment

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evaluate the performance of phototransistors to control the carriers in the channel by light. In order to make sure that the increased holes in channel are photogenerated, the Iph/Idark is obtained at Von =32 V. The Iph/Idark dramatically increases with increasing the incident light power. The maximum value is 1.5×105 under 4.2 mW/cm2 UV light, which is much larger than that of previous work.7,27 Photoresponsivity (R) is an important parameter of phototransistors, which indicates the capability of photoelectric conversion. In Figure 2e, with the power intensity increased from 4.2 µW/cm2 to 925 µW/cm2, the photoresponsivity is also increased at Von = 32 V. The maximum photoresponsivity is 87 A/W, which is better than that of mostly other organic phototransistors based on metal phthalocyanine materials.27 When the light power reached 4.2 mW/cm2, the photocurrent begins to saturate and the photoresponsivity decreases a bit. The p-6P layer played an important role on the performance of VOPc/p-6P phototransistors. The p-6P layer could form an effective heterojunction with VOPc layer, which was greatly helpful to enhance the light absorption and photo-generated carriers. The highest performance of 10 nm VOPc/7.5 nm p-6P heterojunction phototransistors in UV region is attributed to the improved light absorption by inserting a p-6P layer. UV-vis absorption spectra of VOPc/p-6P, VOPc and p-6P thin films are shown in Figure 2f. The absorption intensity in the p-6P spectrum continually increases from 400 nm to 320 nm. The spectroscopic feature associated with optical absorption in 10 nm VOPc thin film is the absorption of Soret band (B-band) around 365 nm.32 As the p-6P layer inserted, the absorption intensity of VOPc/p-6P thin film is obviously improved, especially at the wavelength of about 365nm. The VOPc/p-6P heterojunction interface acts as an effective holes transport channel. Figure 2g shows the UPS spectra of the cut-off and highest occupied molecular orbital (HOMO) region for VOPc/p-6P and p-6P thin films. The work function (WF) is the energy difference between the secondary cut-off and the Fermi energy (EF) of the system (21.218 eV). The WF of VOPc/p-6P and p-6P thin film is 3.97 eV and 4.02 eV, respectively. The HOMO of p-6P thin film is located at 1.6 eV, which is larger than the value of 0.97 eV for VOPc/p-6P thin film. Figure 2h illustrate energy levels diagram at VOPc/p-6P heterojunction interface. ACS Paragon Plus Environment

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The location of lowest unoccupied molecular orbital (LUMO) is determined by existing literature.33,34 Because of the construction of type-I heterojunction, in the electric field of gate voltage, the photo-generated holes in p-6P layer can transfer from p-6P layer into VOPc layer. The electrons in VOPc layer are hard to cross the interface and easy to be trapped with a 1.02 eV potential barrier. The high-level electron trapping capability at the interface is essential to operate the behavior of photomemory.

Figure 3. Retention time tests of the photomemory devices based on the VOPc/p-6P heterojuction phototransistor at (a) Vg = 8 V and (b) Vg = - 25 V. Energy diagrams of VOPc/p-6P photomemory (c) under light and after photo-pulse at (d) Vg = 8 V

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and (e) Vg = - 25 V. (f) Retention time tests of this photomemory at Vg = 0 V. A1 is defined as the photocurrent under the light pulse and A2 is defined as the residual photocurrent after 100 s. (g) The photomemory factor A2/A1 as a function of gate voltage with light density of 4.2 mW/cm2 or 26 mW/cm2.

The excellent photomemory properties originated from efficient and reversible charge trapping and detraping.18 The traps density can be modulated by the voltage applied on the gate electrode. Retention time tests of the photomemory devices based on the VOPc/p-6P heterojuction phototransistor are shown in Figure 3a and b. At the gate voltage of 8.0 V, the device has a long relaxation time of the photocurrent and could be used as a nonvolatile photomemory. The devices operated under a 100 ms light pulse with the intensity of 4.2 mW/cm2 and could store the light information on the device in the form of a persistent current with assistance of the gate voltage. The current level of photomemory maintained a certain value within the first 100 seconds and then just decreased about 20% after about 5000 s. Energy diagrams of VOPc/p-6P photomemory are shown in Figure 3c and d. Under illumination, absorbed photons produce photogenerated electrons and holes, the photogenerated electrons are easily and quickly trapped in the interface or the bulk regions. In this case, hole transport is dominated in the devices. A high local electric field will be form near the source electrode by the accumulated electrons. Under an 8.0 V gate electric field, even after the light pulse is switched off, photogenerated electrons in channel are still trapped into the trap sites and the excess holes also retain. Because of the high trapping energy barrier, a long-term and stable photomemory behavior is observed. Different applied gate-source bias could modulate high- and low-conducting state of channel. At gate voltage of -25 V, the light pulse induced current is much larger than that at gate voltage of 0 V, but the photocurrent is rapidly disappearing in 8 seconds. As shown in Figure 3e, the voltage of -25 V facilitated detrapping process. The large negative bias applied on gate electrode is to draw the trapped electrons out of the thin film and make the device fast return to the initial state. At a moderate gate voltage of 0 V, switching the light off does not remove the photocurrent after 500 seconds later. As displayed in Figure 3f, A1 and A2 are defined as the photocurrent under the incident light ACS Paragon Plus Environment

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pulse and 100 s after turning off the light. The photomemory factor A2/A1 is dependent on the density of trap and the quantity of traps tuned by the gate voltage. The photomemory factor A2/A1 vs gate voltage with light power density of 4.2 mW/cm2 or 26 mW/cm2 is shown in Figure 3g. The processes of charging and discharging traps can be controlled by the gate electric field. The A2/A1 is decreasing with the gate voltage varying from 5 V to -10 V. When the gate voltage is smaller than -10 V, the photomemory factor is equal to zero, which means that the current is back to initial state without any photocurrent in 100 s. Under the modulation of different gate voltage, the photomemory device displays different behaviors. When the current is back to initial state in a certain period of time, the function of automatic back to off state is achieved and the length of time is adjustable. In additional, we can also apply the reset electric pulse to redistribute the trap centers and bring the current back to its initial state (Figure 4c). The photomemory factor also can be tuned by adjusting the light power density. When the light density is 26 mW/cm2, some photo-generated carriers are captured by traps, but the photogenerated carriers are much more than trap sites and current decreases rapidly after light pulse. Under the same condition, the needed time back to initial is shorten than that with the light density of 4.2 mW/cm2 and A2/A1 is smaller. The phenomenon consistently supports that the photomemory behavior is dominated by illumination parameters and gate voltage.

Figure 4. Photomemory functionalities of the VOPc/p-6P photomemory device controlled by (a) photo-pulse duration and (b) light power intensity. (c) Cycling tests for the VOPc/p-6P photomemory, light pulses are 0.3s 18.5mW/cm2 UV light and resetting electrical pulses are 0.5s -40V on gate electrode.

In addition, because the density of photogenerated carriers depend on the intensity of the incident light, ACS Paragon Plus Environment

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the current levels in photomemory could be controlled by varying the photo-pulse duration and light power intensity. In Figure 4a and b, as photo-pulse duration is applied from 100 ms to 300 ms or light power intensity is applied from 4.2 mW/cm2 to 5.4 mW/cm2, keeping both Vds and Vg at fixed values, the current almost increased stepwise, which present the multilevel signal storage characteristics in the photomemory. Cycling tests for the VOPc/p-6P photomemory are shown in Figure 4c. The photocurrent induced by light pulse can be completely erased by applying a -40V (5 ms) resetting electrical pulses on gate electrode. This process is repeatable without attenuation and the photomemory level is stable, even after multi-erasing.

Figure 5. (a) Topographic image of VOPc/p-6P thin film. (b) The change of current detected by the tip as function of time under dark or under light. The corresponding current images of VOPc/p-6P thin film (c) under dark and (d) under light. The scan area is 5 µm × 5 µm.

Photocurrent mapping is performed using in-situ CSAFM in contact mode to obtain quantitative information of spatial variations.35 Topographic image and corresponding surface conductivity image of VOPc/p-6P thin film are shown in Figure 5a and c. Because of the signal interference from the extra electric ACS Paragon Plus Environment

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field, the morphology of lamellar crystal grains is a little blurred, compared with the topographic image in Figure 1d. In surface conductivity image, the highest absolute value of current is shown as black and lowest absolute value of current is presented as white. The cantilever is kept at virtual ground while a -2.0 V bias voltage is applied to the sample. The current image shows the same track to its morphology and the current is most prominent in lamellar crystal grains. Because the regions in lamellar grain have the single-crystal-like characteristics and the traps are located in the boundaries, more conductive region evidently exists in grains. When a weak power density of 365 nm light was used to irradiate VOPc/p-6P thin film, the magnitude of the current is increased, but the increased photocurrent is still confined in the vicinity of the lamellar grains, as shown in Figure 5d. The change of current detected by the tip as function of time under dark or under light is shown in Figure 5b. Because of high contact resistance between the tip and semiconductor and lower power density of UV light, the current is much smaller than that in Figure 3. The detected area by the tips is in the grains, in which the conductance is able to control by light. The applied bias voltage on the sample is -1.0 V and the gate voltage is 0 V. Before the time of 54 s, the film is under dark and the current is about -63 pA. When the UV light is turned on, the magnitude of current is generated in channel and increased to about -200 pA. After turning off the light, the current of grains did not rapidly decrease, but displayed the behavior of photomemory: the electrons are trapped and stay for a long time, the excess holes contribute to the persistent photocurrent in the lamellar grain.

CONCLUSION In summary, we demonstrated high performance VOPc/p-6P heterojunction phototransistors and its adjunctive photomemory behavior. Under 365 nm ultraviolet light irradiation, the Iph/Idark and photoresponsivity of VOPc/p-6P heterojunction phototransistors are about 1.5×105 and 87 A/W at Von of 32 V, respectively. 80% photocurrent under the gate voltage of 8.0 V lasted for about 5000 s after 100 ms light pulse with the intensity of 4.2 mW/cm2. The photomemory could be modulated by gate voltage and the illumination parameters and multi-level photomemory behaviors are also observed by modulating light pulse ACS Paragon Plus Environment

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duration and light power intensity. A change of conductivity in grains is observed by in-situ CSAFM upon 365nm light illumination. The trapped charges play the important role for memory behavior in the grains. There results described here provide a new possibility for designing high-performance organic photomemory devices. EXPERIMENTAL SECTION The heavily doped Si acted as the back gate electrode and about 200 nm-thick SiO2 layer was the gate insulator layer. The VOPc and p-6P organic materials were obtained from Jilin OLED Material Tech Company. The VOPc, p-6P and VOPc/p-6P thin films were grown with a deposition rate of ~ 1 nm/min and the substrate temperature was keep at 150 oC. Thermally deposited Au electrodes through a micrometer sized carbon fiber shadow mask define a channel with a width of 200 µm and a length of 8 µm (Figure S4). The vacuum during these depositions was 10-4 ~ 10-5 Pa. The electrical measurements of the phototransistors and photomemory devices were measured by a semiconductor characterization system (Keithley 4200 SCS) connected with a probe station in air and at room temperature. The light source used to irradiate the sample was a 365 nm UV LED lamp. The absorption spectra were measured by an ultraviolet–visible spectrophotometer (UV–vis, Puxi, T9, China). The ultraviolet photoelectron spectroscopy (UPS) experiments were performance in an ultrahigh vacuum system with a SPECS Microwave UV Light Source (He I=21.218 eV, Specs, Germany). The morphology and current line profiles were simultaneously performed by Single-Pass Contact-mode of Agilent Technologies 5500 AFM/SPM System (USA). The electrically conductive, Platinum-Iridium coated tips were the SCM-PIC probe (Bruker Nano Inc). The gate voltage was applied by using a Keithley 2400 sourcemeter. The 980 nm wavelength of laser light used for the AFM beam deflection was outside the absorption range of the VOPc/p-6P thin film. ASSOCIATED CONTENT Supporting Information. Transfer and output characteristic curves of VOPc/p-6P phototransistors. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Funding Sources

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This work was supported by the National Natural Science Foundation of China (61306085, 11334014). Y. G. acknowledges support by National Science Foundation CBET-1437656.

Notes The authors declare no competing financial interest.

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For Table of Contents Use Only Multi-Level Nonvolatile Organic Photomemory based on Vanadyl-Phthalocyanine/para-Sexiphenyl Heterojunctions Chuan Qian, Jia Sun,* Ling-an Kong, Ying Fu, Yang Chen, Juxiang Wang, Shitan Wang, Haipeng Xie, Han Huang, Junliang Yang, and Yongli Gao*

Table of Contents/Abstract Graphic：Organic photomemory based on heterojunction phototransistor was presented, which can transduce the input light signals into electrical signals and the output signals can be stored for recording the light simulation. After applying a light pulse on the device, the stored current level lasted for ~5000 s with only a 20% decrease.

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