ZTO Interface on Optoelectronics

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Interplay of Nanoscale, Hybrid P3HT/ZTO Interface on Optoelectronics and Photovoltaic Cells Jian-Jhong Lai, Yu-Hsun Li, Bo-Rui Feng, Shiow-Jing Tang, WenBin Jian, Chuan-Min Fu, Jiun-Tai Chen, Xu Wang, and Pooi-See Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06135 • Publication Date (Web): 05 Sep 2017 Downloaded from http://pubs.acs.org on September 6, 2017

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ACS Applied Materials & Interfaces

Interplay of Nanoscale, Hybrid P3HT/ZTO Interface on Optoelectronics and Photovoltaic Cells Jian-Jhong Lai,1 Yu-Hsun Li,1 Bo-Rui Feng,1 Shiow-Jing Tang,1Wen-Bin Jian,1,* Chuan-Min Fu,2 Jiun-Tai Chen,2 Xu Wang,3 Pooi-See Lee3 1

Department of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan, ROC

2

Department of Applied Chemistry, National Chiao Tung University, Hsinchu 300, Taiwan,

ROC 3

School of Materials Science and Engineering, Nanyang Technological University, Singapore

639798, Singapore Keywords: P3HT Nanofiber, Zinc Tin Oxide Nanowire, Gatable Diode, Photodetector, Photovoltaic Cell

Abstract:

Photovoltaic effects in poly(3-hexylthiophene-2,5-diyl) (P3HT) attract much attention recently. Here natively p-type doped P3HT nanofibers and n-type doped zinc tin oxide (ZTO) nanowires are used for making not only field-effect transistors but also p-n nanoscale diodes. The hybrid P3HT/ZTO p-n heterojunction shows applications in many directions and it also facilitates the investigation of photoelectrons and photovoltaic effects at the nanoscale. As for applications, the

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heterojunction device shows simultaneously high on/off ratio of n- and p-type field-effect transistors, gatable p-n junction diodes, tri-state buffer device, gatable photodetectors, and gatable solar cells. On the other hand, P3HT nanofibers are taken as a photoactive layer and the role of p-n heterojunction playing on the photoelectric and photovoltaic effects is investigated. It is found that the hybrid P3HT/ZTO p-n heterojunction assists to increase photocurrents and to enhance photovoltaic effects. Through the controllable gating of the heterojunction, we can discuss the background mechanisms of photocurrent generation and photovoltaic energy harvest.

1 Introduction Organic molecules and macromolecules of polymers have been studied for more than five decades due to possible substitution for channel materials of semiconductor technology. Moreover, organics carry additional advantages such as flexibility and easy processing in a large scale even without using any vacuum systems. At the earliest stage, the crystalline films of anthracene molecules were prepared either in solution or using sublimation and, more excitingly, electroluminescence at a voltage of several tens of volt has been observed.1,2 It was followed by investigation of charge transport in the luminescence process.2 Actually, charge transport in organics such as macro molecules of conjugated polymers like polyacetylene3 was always an important issue that attracted intensive studies. In addition, efforts have been made in organic electroluminescence thus, two or three decades ago, light emission was observed at an applied voltage from as low as 2 or 3 V.4,5 On the other hand, the problem due to disorder and charge carrier scattering has gradually been resolved, giving gradually increased mobility in polymers. Such as poly(3-hexylthiophene-2,5-diyl) (P3HT) field-effect transistors (FET), the mobility of charge carriers approaches 0.1 cm2 V-1 s-1.6 Moreover, it was demonstrated to emit multi-color light from organic light-emitting diodes7 that lays the foundation for the applications to large


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panel displays. Recently, other attempts have been made for organic electronics either to integrate it with inorganic semiconductors thus forming hybrid devices, or to exploit it in other technical fields such as solar cells.8-10 P3HT polymer has been intensely investigated and employed in device applications in the past three decades. Since the discovery of a high mobility,6 several attempts have been made in the investigation of electron transport and of regioregular nature of P3HT.11-14 P3HT was taken as a bulk in the beginning and the electron transport could be expressed by either the space charge limited current model11,12 or Mott’s three-dimensional variable range hopping model.13 Later, electron transport in the naturally formed nanofiber of P3HT was explored and the thermal activation transport was observed in isolated nanofibers.14 On the other hand, P3HT is a photoactive material and it can be applied not only in organic light-emitting diodes6 but also in many other fields like photodetectors,15 organic thin film transistors,16 photodiodes,17 and hybrid solar cells.18-20 P3HT was sometimes blended with other organic or inorganic materials to make hybrid solar cells.19 Due to its naturally fibrous structure, P3HT was used for making bulkheterojunction devices. In recent studies, bulk-heterojunction solar cells based on P3HT materials did demonstrate high power conversion efficiency up to 6% and high stability in ambient conditions.21,22 The ternary oxide material, zinc tin oxide (ZTO), has two different structural phases of Zn2SnO4 and ZnSnO3.23 The phase of Zn2SnO4 is cubic spinel structure. It is relatively easy to be synthesized and has been characterized considerably. Recently, ZTO of both two structural phases have been converted to form nanowires.24,25 Electrical property characterizations and device applications of ZTO nanowires were demonstrated. Inorganic ZTO nanowires are natively n-type whereas organic P3HT nanofibers are natively p-type semiconductors. The nanoscale


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interface forms when P3HT nanofibers are placed on ZTO nanowires. The nanoscale interface as well as the nanojunction always play important roles on electron transport and the device performance.26,27 Although the using of organic and inorganic bulk-heterojunction for making hybrid solar cells, FETs and photodetectors has been presented, the investigation of optoelectronic and photovoltaic effects on the microscopic scale of the interface between organic nanofibers and inorganic nanowires has not been carried out yet. In addition, the FET devices were fabricated but the electric-field gating effects on the photodetectors and the photovoltaic cells had not been explored yet. In this study, we make a multifunctional device on single ZTO nanowire with P3HT nanofibers crossover on the surface. The device will consist of n- and ptype FETs, a p-n junction diode, a tri-state buffer device, a photodetector, and a photovoltaic cell. Moreover, the diode, the photodetector, and the solar cell are tunable using a back-gating electric field. The functions of gatable devices can be completely switched off for the purpose of device protection and, additionally, the functions can be adjusted to optimal efficiencies. Thus, the gatable devices are essential for extensive applications.

2 Experimental ZTO nanowires were synthesized on Si wafers. The detail of ZTO nanowire synthesis was described elsewhere.25 ZTO nanowires were transferred on a heavily p-type doped Si substrate which was capped with a 300-nm thick SiO2 and was pre-patterned with large contact pads and wide current leads. The as-transferred ZTO nanowires and the substrate were annealed in a high vacuum of 10-5 torr at 500oC for 24 h. A scanning electron microscope (SEM, JEOL JSM-IT300) equipped with an electron-beam controller (Raith ELPHY Quantum) was used for nanolithography and nanofabrication. Usually two pairs of Ti/Au (10/200 nm in thickness) electrodes were deposited using a standard lithography followed by a thermal evaporation. The


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first pair (ZTO pair) of electrodes were deposited on top of ZTO nanowires as terminals for electrical measurements. The second pair (P3HT pair) of electrodes were placed on neighboring areas beside the ZTO nanowire that were used latter as electrical contacts for P3HT nanofibers. Another electron-beam lithography process was carried out to open a window at which the P3HT nanofibers were going to be attached (see Fig. S1 in Supporting Information). P3HT nanofibers were prepared by the whisker method. The details of synthesis and characterizations were reported in our previous report.28 P3HT nanofibers were spin-coated on the substrate. Some nanofibers were deposited in the window area and placed across the ZTO nanowires and on the top of the P3HT pair of electrodes. Experimental parameters of several typical P3HT-ZTO devices (HPZ-01 to HPZ-04) are listed in Table S1 and S2 in Supporting Information. An atomic force microscope (AFM, Seiko Instruments Inc. SPA-300HV) was employed to inspect the dimensions of ZTO nanowires and P3HT nanofibers. The as-fabricated hybrid devices were loaded in a probe station (Lake Shore Cryotronics TTPX) for electrical characterizations. The transfer characteristics of the hybrid devices were obtained using electrometers of Keithley 6430 and 6517. Light exposure was done by using a He-Ne laser with a wavelength of 633 nm and an average intensity of 36.7 mW cm-2.

3 Results and discussion An AFM image of HPZ-01 device is shown in Fig. 1(a), displaying the dimensions of a typical hybrid P3HT/ZTO and p-n heterojunction device. The AFM image clearly reveals a crossover heterojunction structure of P3HT nanofibers on a ZTO nanowire. The ZTO nanowire is of ~69 nm in diameter and ~16 µm in channel length (see Fig. S1 in Supporting Information). In addition, P3HT nanofibers possess a rectangular cross section that has been discussed previously.28 Figure 1(b) shows an average height of 3.7 nm with a standard deviation of 0.74


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nm which is estimated using AFM images. The width of P3HT nanofibers is ~25 nm. A complete scheme of the hybrid P3HT/ZTO device is presented in Fig. 1(c). The separation of the P3HT pair electrodes is 4 µm. The coverage of P3HT nanofibers on the ZTO nanowire is estimated using high-resolution AFM images (Fig. S1(b) in Supporting Information). There are ~40 P3HT nanofibers crossing over the ZTO nanowire. We prepared four devices (HPZ-01 to HPZ-04) of the same device structure shown in Fig. 1(c) whereas we carried out measurements on the p-n junction, the tri-state buffer, the photodetector and the solar cell effects on all our devices. The behaviors between different devices reveal similar and consistent results. Typical results are presented as follows. As a first step, taking the hybrid P3HT/ZTO device as transistors, we characterize its electrical behaviors at room temperature and display data in Figs. 1(d) and 1(e). Both ZTO nanowire and P3HT nanofibers exhibit linear current-voltage (I-V) behaviors in a wide voltage range from -2 to 2 V. At zero gating voltage, the resistances of the ZTO nanowire and the P3HT nanofibers are about 2.1×1010 and 5.0×1010 Ω, respectively, corresponding to resistivity of 5.0×102 and 4.5×103 Ω cm (see Fig. S2 and descriptions in Supporting Information). If we look at the conduction channel from one electrode contacting on ZTO to another electrode on P3HT, we will obtain a diode behavior shown in Fig. 1(d). The diode of the P3HT/ZTO p-n heterojunction has a threshold voltage of +2.5 V applying on the P3HT nanofibers. Notably, the P3HT nanofibers can also be used as a top gating electrode on the ZTO nanowire if the voltage is lower than the threshold value. By adjusting the back gating voltages from -50 to 50 V, transfer characteristics of the ZTO nanowire, the P3HT nanofibers, and the p-n heterojunction are all presented in Fig. 1(e). The ZTO nanowire exhibits a transfer characteristic of n-type semiconductor. Its on-off ratio is ~104 and the mobility of electrons is ~0.18 cm2 / V s (Section 2 in Supporting Information). The P3HT


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nanofibers exhibit a typical p-type gating behavior. Their on-off ratio is ~102 and their hole mobility is ~2.6×10-4 cm2 / V s (Section 2 in Supporting Information). It is noted that the mobility of P3HT nanofibers is very close to that of P3HT nanofibers without annealing.28 The P3HT/ZTO p-n junction shows a distinct, gating feature that can be turned on at back gating voltages in the range from -35 to 25 V. Moreover, the p-n junction diode can be completely turned off at gating voltages either higher than 25 or lower than -35 V. The on/off ratio of the P3HT/ZTO p-n junction diode is ~60. The gating behaviors are explained by energy band diagrams shown in Figs. 1 (f)-(i). Without any bias voltage applied, the Fermi level alignment on the interface between p-type P3HT nanofibers and n-type ZTO nanowire is presented in Fig. 1(f). When the junction is forward biased, the band diagram shall be tilted as shown in Figs. 1(g)-(i). Under a low gating voltages such as the configuration shown in Fig. 1(g), the forward biased junction is turned on with a current from recombined electron-hole pairs or space charges.29 When the junction is negatively gated (see Fig. 1(h)), the electron channel is switched off and the hole channel is blocked by a potential barrier in the heterojunction. At positive gating voltages as shown in Fig. 1(i), the hole channel is off and the electron channel is blocked by a potential barrier in the junction.


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Figure 1. (a) AFM image of a hybrid P3HT/ZTO, p-n heterojunction of HPZ-01 device where P3HT nanowires look like crossover cables on a ZTO nanowire. (b) Statistical distribution with a Gaussian curve fitting of the height of P3HT nanowires, estimated from the AFM image. The average height and the standard deviation are about 3.7 nm and 0.74 nm, respectively. (c) Scheme of the hybrid P3HT/ZTO nanowire device with a connection circuit indicated. (d) Characteristic current-voltage (I-V) curves for charge carriers flowing through ZTO (black closed circles), P3HT (red closed squares), and P3HT/ZTO p-n heterojunction (blue closed triangles). (e) Transfer characteristics of charge carriers flowing through ZTO (black closed circles), P3HT (red closed squares), and the P3HT/ZTO p-n heterojunction (blue closed triangles)


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at the same bias voltage of 3 V. Transfer characteristics of the ZTO nanowire device is shown as black open circles in a full current range according to the y-coordinate axis on the right. (f) Band diagram of the P3HT/ZTO p-n heterojunction at zero bias voltage. Band diagrams of the junction under a high forward bias at three different gating conditions: (g) neutral gating voltages (−35 V < V < 25 V, (h) negative gating voltages (V < −35 V), and (i) positive gating voltages (V > 25 V). We demonstrate another electronic device application in Fig. 2. Here the P3HT nanofibers are used as a top gate at a voltage lower than the threshold voltage (+2.5 V) of the p-n heterojunction diode. It is used to control ZTO nanowire channel. Different I-V behaviors of the ZTO nanowire at different voltages on top and back gates are displayed in Fig. 2(a). When the ZTO nanowire is switched on at a back gating voltage of 50 V, the current in the ZTO nanowire can be further altered by applying ±2V on the P3HT top gate. On the other hand, the ZTO nanowire channel is completely off at a back gating voltage of 0 V. The current in the ZTO nanowire channel gated by P3HT is displayed in the inset to Fig. 2(a). The on-off ratio is only about 3 possibly owing to the small cross-sectional areas of P3HT nanofibers and the small interfacial areas of the p-n junction diode. The inset to Fig. 2(a) shows an off current of ~0.07 nA in the ZTO nanowire under the P3HT top gating control. In contrast, the channel current can be completely switched off by back gate. The difference between top and back gating control possibly originates from different decaying electric-field behaviors between wires (P3HT top gate) and planes (back gate). The P3HT top gate and the back gate form a dual-gate device structure that can be operated as a tri-state buffer device.30 A circuit diagram of the tri-state buffer device is given in Fig. 2(b). The ZTO nanowire is set at a bias voltage of +3 V. The P3HT top gating voltage of -2 and +2 V


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is denoted as input logic signals of 0 and 1. The output is the state of either high or low current. The tri-state buffer device can be further turned off and on, denoted as logic states of 0 and 1, by the back gating voltages of -50 and +50 V, respectively. Integrating both top and back gates, we denote (0, 1) in Fig. 2(c) as logic states of 0 for P3HT top gate and of 1 for back gate. Figure 2(c) presents a time dependent operation of the tri-state buffer device and the output current. When the device is switched off at a back-gate logic state of 0, the output current is lower than 0.2 nA, marked as a Z state. No matter the input (top-gate) state is in 0 or 1, the output always gives the Z state. In contrast, when the device is switched on at a back-gate logic state of 1, the current will be either 2.1 or 3.5 nA corresponding to the input state of 0 or 1. We denote the output currents of 2.1 and 3.5 nA as output logic states of 0 and 1, respectively. The operation truth table is listed in Table 1.

Figure 2. Electrical operation of the ZTO nanowire FET of HPZ-02 device in which P3HT nanofibers and the Si wafer substrate are used as top- and back-gate electrodes, respectively. (a) Current-voltage behaviors of ZTO nanowires at various conditions of top- and back-gate


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voltages. The inset exhibits current as a function of top-gate voltage. (b) Schematic diagram symbol of the tri-state buffer device where the back-gate voltage is used to turn on/off of the device. (c) The operation of the tri-state buffer device. The device state is controlled by two digital numbers where the first and the second digital states are operated using the back- and topgate voltages, respectively.

Table 1. Truth table of a tri-state buffer device. The source-drain voltage (VZTO) of 3 V is applied in the ZTO nanowire channel.

Input

Vbg

Output

1 (Vtg = 2 V)

1 (Vbg = 50 V)

1

0 (Vtg = -2 V)

1 (Vbg = 50 V)

0

1 (Vtg = 2V)

0 (Vbg = 0 V)

Z

0 (Vtg = -2V)

0 (Vbg = 0 V)

Z

P3HT is a photoactive material so the P3HT/ZTO hybrid device may be used as a visible light photodetector. A He-Ne laser with a wavelength of 633 nm is used to excite photocurrent. Additionally, the surface area of the P3HT nanofibers between the P3HT pair electrodes are used in the following estimation of light energy absorption (see Section 3 in Supporting Information). Figure 3(a) indicates the photoactive role played by P3HT nanofibers (Fig. S3 and descriptions in Supporting Information) and, in particular, we will focus on the photosensitivity of the


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P3HT/ZTO p-n heterojunction. Before the characterization of the heterojunction, we measure the responsivity (R) and the external quantum efficiency (EQE) of P3HT nanofibers at a bias voltage of 3 V. The responsivity R is evaluated as the photocurrent dividing by the power of the light shining on the P3HT nanofibers and the EQE is estimated by

, where ℎ is the Planck constant,

is light velocity, is electron charge, and is the wavelength of the He-Ne laser. Figure 3(b) displays experimental results of R and EQE of P3HT nanofibers as a function of back gating voltages. The photocurrent is the current under light illumination subtracted by the dark current. To compare Fig. 3(b) with Fig. 1(e), the P3HT in dark is completely switched off while, under light illumination, it still possesses photocurrent at positive gating voltages from 0 to 50 V. The R and EQE of P3HT nanofibers keep a constant value at back gating voltages V < 8 V while they undergo a fast decreasing process at back gating voltages higher than 8 V. The I-V behaviors of the P3HT/ZTO p-n heterojunction in dark and under light illumination are shown in Fig. 3(c). At the forward biased voltage of 3 V on the junction, the current in dark is ~0.28 nA and the illuminated current increases up to ~0.4 nA. At the backward biased voltage of -3 V, the illuminated current is ~0.036 nA that cannot be switched off. This result implies light-induced barrier variation in the diode. The p-n heterojunction diode is then set at a forward bias voltage of 2 V and the back gating behavior is investigated as shown in Fig. 3(d). The dark current of the p-n heterojunction diode is switched on at back gating voltages from -25 to 25 V (similar behaviors shown in Fig. 1(e)). The illuminated p-n heterojunction diode is however switched on in a wider gating voltage range between -25 and 50 V with a doubled peak current. The peak in dark current (Fig. 3(d)) is at a gating voltage of 10 V and it in illuminated current is shifted to 25 V. All the results corroborate that the main photocurrents originate from the photoactive layer of P3HT nanofibers. Figure 3(e) presents R and EQE of the P3HT/ZTO p-n


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heterojunction diode that are all six times higher as compared with that of P3HT nanofibers shown in Fig. 3(b). The electron-hole pairs are generated in P3HT nanofibers while the p-n heterojunction diode enhances the separation of electron-hole pairs. Our results not only show the gatable feature of the p-n heterojunction photodetector but also uncover the high photosensitivity enhanced by the p-n interface.

Figure 3. Characteristics of photodetectors of P3HT nanofibers and hybrid P3HT/ZTO p-n heterojunctions of HPZ-03 device. (a) Scheme of P3HT nanofiber, the active layer of photodetectors, and P3HT/ZTO heterojunctions. (b) Photoresponsivity and external quantum efficiency of P3HT nanofibers as a function of back-gate voltage under light illumination. (c) Current-voltage behaviors of hybrid P3HT/ZTO heterojunctions in dark and under laser light illumination. (d) Transfer characteristics of P3HT/ZTO heterojunctions in dark and under light illumination, and measured at a bias voltage of either 2 or -2 V. The gray dashed curve is guide


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to eyes. (e) Photoresponsivity and external quantum efficiency of the P3HT/ZTO heterojunction under light illumination.

The P3HT/ZTO p-n heterojunction consists of a nanoscale interface in which an interfacial dipole layer exists, thus the unique structure integrated with a photoactive layer of P3HT nanofibers shall show benefits in photovoltaic effects. Here a He-Ne laser of 633 nm in wavelength is used to explore photovoltaic effects. In Fig. 4(a), we emphasize a microscopic interfacial area between the ZTO nanowire and the P3HT nanofiber. Electron-hole pairs are generated in the photoactive P3HT nanofibers and separated across the interfacial dipole layer by an internal electric field. A scheme of the mechanism and a short circuit current ( ) combined with a circuit diagram for measurements is given in Fig. 4(a) as well. The P3HT/ZTO interfacial areas are used in the evaluation of the (see Section 4 in Supporting Information). Figure 4(b) presents current density-voltage behaviors of the p-n heterojunction with/without laser light shining. A negative current at a bias voltage in the range between 0 and 0.12 V is apparent, signifying an open circuit voltage of 0.12 V. The zero bias current density is about 0.16 mA cm-2. Power density of the solar cell is calculated and the maximum power density is determined (Fig. S5(b) in Supporting Information). The maximum power density is used to estimate the fill factor (FF) of 0.33 and the power conversion efficiency (η) of 0.018%. The power conversion efficiency is low since the solar cell is operated at zero gating voltage rather than at the gating voltage of its optima performance. On the other hand, the He-Ne laser of 633 nm in wavelength is not an efficient light source for P3HT. According to absorption spectra of P3HT nanofibers,31 the maximum absorption occurs at a light wavelength of ~500 nm and the


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absorption decays to one fifth at the wavelength of 633 nm. Therefore, the power conversion efficiency shall be higher under normal sunlight exposure. The P3HT/ZTO p-n heterojunction solar cell is particularly interesting since it is tunable by back gating voltage. The gatable solar cell exhibits similar phenomena as the gatable diode and photodetectors presented in Fig. 1(e) and Fig. 3(e), respectively. In Fig. 4(c), characteristic variations of the solar cell (HPZ-03 device) as a function of back gating voltage are revealed. The current density to voltage of HPZ-03 device is shown in the inset to Fig. 4(c). It is interesting that all photovoltaic parameters including power conversion efficiencies, short circuit currents, and open circuit voltages give similar dependencies on back gating voltages and reach maximum values at ~15 V. The maximum power conversion efficiency at the back gating voltage of 15 V is about three times larger than that at zero gating voltage. The maximum occurs at the same gating voltage as that of reverse biased (-2 V) photodetector presented in Fig. 3(d). In contrast, the forward biased (2 V) photodetector shown in Fig. 3(d) exhibits a maximum shifted to the gating voltage of 25 V. On the other hand, the solar cell functions are completely switched off at gating voltages less than -25 V or higher than +25 V. The switchable solar cell device gives one more freedom to look into the nanoscale interface of the P3HT/ZTO p-n heterojunctions. In comparison with gatable photodetectors, the photovoltaic effects of the junction diode shown in Fig. 4(c) for HPZ-03 device are switched off at back gating voltages from +25 to +50 V but the photodetector function is still turned on, portrayed in Fig. 3(e) for the same device. It indicates that the on/off switching of the solar cell at the interface of the p-n heterojunctions is different from the photodetector functions. The photodetector function is dominated by the photoactive layer of P3HT with enhanced electron hole pair separation at the


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p-n heterojunctions. In contrast, the photovoltaic effect is directly occurring in the p-n heterojunction. Band diagrams are presented again in Figs. 4(d) and 4(e) to illustrate different mechanisms between photodetectors and photovoltaic effects. Figure 4(d) delineates the band diagram of forward biased photodetectors. Electron-hole pairs are generated in P3HT nanofibers and holes in P3HT are driven by the forward bias voltage to recombine with electrons from ZTO nanowires at the interface. In this situation, the interface states could help to trap electrons and holes and to increase the efficiency of recombination. On the other hand, Fig. 4(e) draws the band diagram of the solar cell which is the same as that of zero or reverse biased photodetectors. Electron-hole pairs are generated in P3HT and electrons in P3HT are driven to ZTO by the buildin electric field ( ! ). In this situation, the interface states scarcely affect the photovoltaic effects.


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Figure 4. (a) Scheme of P3HT/ZTO heterojunction solar cells. The lower panel shows the generation and separation of electron-hole pairs in the interface of the heterojunction. (b) Current density to voltage behaviors of the P3HT/ZTO heterojunction solar cell of HPZ-04 device at zero back-gate voltage, measured in dark and under light illumination. (c) Energy conversion efficiency (top panel), short-circuit current density (center panel), and open-circuit voltage (bottom panel) as a function of back-gate voltage for P3HT/ZTO heterojunction solar cells of HPZ-03 device. The gray dashed lines are guide to eyes. The inset to top panel shows current density to voltage behaviors of the P3HT/ZTO heterojunction solar cell of HPZ-03 device at zero back-gate voltage. (d) Band diagram of the photodetector of the forward biased P3HT/ZTO


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heterojunction under light illumination. (e) Band diagram of the solar cell of the zero-biased P3HT/ZTO p-n heterojunction under light illumination.

4

Conclusions The crossover device structure of P3HT nanofibers on the ZTO nanowire with a back gating

electrode is fabricated. P3HT nanofiber and ZTO nanowire devices present p- and n-type FETs, respectively. In addition, the device of P3HT/ZTO p-n heterojunction shows a diode I-V behavior which is gatable and can be switched off. At a bias voltage lower than the threshold voltage of the P3HT/ZTO p-n heterojunction diode, P3HT nanofibers are used as top gating electrodes and some logic operations of tri-state buffer devices are illustrated. On the other hand, P3HT nanofibers are a highly photoactive material. Photodetectors of P3HT nanofibers and that of P3HT/ZTO p-n heterojunction diodes are compared. We discover six times higher photosensitivity in the p-n heterojunction diode in comparison with that of P3HT nanofiber device, indicating enhanced separation of electron hole pairs generated in P3HT nanofibers. The photodetectors are controllable and can be switched off by back gate. Finally, we inspect the photovoltaic effects in the P3HT/ZTO p-n heterojunction. The power conversion efficiency of the heterojunction solar cell is also controllable by back gating. It shows an optimum power conversion efficiency at nonzero back gating voltages. The efficiency is about three times higher than that at zero gating voltage. We look into the mechanisms at the nanoscale interface of the P3HT/ZTO p-n heterojunction from simultaneous observations of gatable photovoltaic and optoelectronic effects. The photovoltaic effect is dominated in the p-n heterojunctions whereas the photodetector function, coming from the photoactive layer of P3HT, is enhanced by p-n heterojunctions.


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Acknowledgements This work was supported by Taiwan Ministry of Science and Technology under Grant Number MOST 103-2628-M-009-004-MY3 and MOST 104-2119-M-009-009-MY3, and by the MOE ATU Program. Electronic Supplementary Material: Supplementary material (device parameters including resistivity and on/off ratio, and estimations of field-effect mobility, photo responsivity, and power conversion efficiency) is available. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: A top view of the whole device structure; information of the devices used in presentation; estimation of resistivity and field-effect mobility; estimation of photo responsivity and external quantum efficiency; evaluation of fill factor and power conversion efficiency AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Wen-Bin Jian: 0000-0002-1898-9641 Jiun-Tai Chen: 0000-0002-0662-782X Pooi-See Lee: 0000-0003-1383-1623


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Table of Contents


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ZTO Interface on Optoelectronics

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