Interface Engineering To Boost Photoresponse Performance of Self

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Interface Engineering To Boost Photoresponse Performance of SelfPowered, Broad-Bandwidth PEDOT:PSS/Si Heterojunction Photodetector Zhimin Liang,† Pingyang Zeng,† Pengyi Liu,† Chuanxi Zhao,*,† Weiguang Xie,*,†,# and Wenjie Mai*,†,# †

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Siyuan Laboratory, Guangzhou Key Laboratory of Vacuum Coating Technologies and New Energy Materials, Department of Physics, Jinan University, Guangzhou 510632, People’s Republic of China # Guangdong Provincial Key Laboratory of Optical Fiber Sensing and Communications, Jinan University, Guangzhou 510632, People’s Republic of China S Supporting Information *

ABSTRACT: Organic−inorganic hybrid heterojunctions are poised to push toward novel optoelectronics applications, such as photodetectors, but significant challenges complicating practical use remain. Although all organic based photodetectors have been reported with great success, their potential in highspeed, broadband, self-powered photodetectors have not been fully explored. Herein, a self-powered, broad bandwidth of photodetector based on PEDOT:PSS/Si heterojunction is built by a facial low temperature spin-coating method. By interface engineering of heterojunction with optimal band alignment and heteromicrostructures, enhanced photoresponse performances are obtained. The bandwidth of the hybrid photodetector could be broadened by 10 kHz after interfacial passivation by a methyl group. Further manipulating the geometrical structure of the hybrid heterojunction with silicon nanowire, a broad spectrum response from 300 to 1100 nm, with bandwidth as high as 40.6 kHz, fast response speed of 2.03 μs and high detection of 4.1 × 1011 Jones under zero bias was achieved. Meanwhile, the close dependence between the photoresponse performance of heterojunctions and Si nanowire length is observed in the topcoverage configuration. Finally, a coverage effects model is proposed based on the competition of Si bulk and surface recombination, which is also confirmed by the designed bottom-coverage experiment. The mechanisms behind the enhanced photoresponse of the hybrid photodetector is attributed to the optimum band alignment, as well as the optimum balance of carrier dissociation and recombination of heterojunction. The scalable and low temperature method would be of great convenience for large-scale fabrication of the PEDOT:PSS/Si hybrid photodetector. KEYWORDS: hybrid photodetector, high-speed, self-powered, bandwidth, interface engineering

1. INTRODUCTION Organic photodetectors (PDs) have been extensively studied in the past decades due to their merits of large-area and low-cost fabrication, compatible with flexible substrates, as well as wide selection of materials.1 The potential applications range from remote control, chemical/biological sensing, medical instruments, and optical communication et al.2 High performance PDs have been reported, such as high external quantum efficiency,3 full-color,4 and wide spectral response.5 X. Gong et al. demonstrate the broad-band (UV to IR) and high detectivity of PDDTT photodetectors that are comparable or even better than that of inorganic materials.5 However, the poor carrier mobility of most organic materials (usually 10−3 cm−2V−1S−1) is far below than that of inorganic materials (∼104 cm−2V−1S−1), which induces longer response time and poor frequency response.6 Additionally, inorganic materials exhibit more stability than that of organic in atmosphere, especially in © 2016 American Chemical Society

moist environment. To develop highly efficient photodetector, it is of great urgency to design device architectures with novel configuration, and choose proper optical/electrical materials to realize the optimal photoelectric conversion. One solution to achieve this goal is to combine the advantageous characteristic of inorganic and organic materials together to form hybrid photodetector. Among the numerous conducting polymers that have been investigated, poly(3,4ethylene-dioxy thiophene) (PEDOT) has been one of the most successful materials, from both fundamental and practical perspective. It exhibits the excellent property of a moderate band gap, as well as high conductivity and broad adsorption from 1.5 eV to infrared.6 Feng’s group reported ultrathin Received: May 26, 2016 Accepted: July 8, 2016 Published: July 8, 2016 19158

DOI: 10.1021/acsami.6b06301 ACS Appl. Mater. Interfaces 2016, 8, 19158−19167

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ACS Applied Materials & Interfaces

substrates were immersed into a solution consisting of 4.8 M hydrofluoric acid (HF) and 0.02 M silver nitrate (AgNO3) at 50 °C. The length of the nanowires could be controlled by the etching time. Finally, the substrate was cleaned with deionized water and dipped into a HNO3 solution (30% w/w) for 30 min to completely remove all the residual silver particles between Si nanowires. The methyl groupterminated samples were followed by chlorination and alkylation method. In order to broaden the interspace of nanowires to investigate the coverage effect, we kept the concentration of PCl5/CB solution and the following alkylation the same, and only extend the chlorination period from 1 to 3 h. 2.2. Interfacial Passivation. The surface with hydrogen passivation was obtained by soaking the Si nanowires in an HF solution (5 M) for 10 min, and then the H-terminated samples were transferred into a glovebox. The methyl group-terminated samples was followed by chlorination and alkylation method.20 The process of the CH3-terminated surface was conducted in a glovebox wherein the sample is first passivated by hydrogen and then immersed in a chlorobenzene (CB, 2 M) solution of superfluous phosphorus pentachloride (PCl5) at 100 °C for 1 h. This process is conducted under nitrogen in order to convert the SiH bonds into SiCl ones. Then the sample was cleaned with CB and tetrahydrofuran (THF) and immersed in a solution of CH3MgCl (1M) in THF at 80 °C for 8 h. The methyl-terminated Si NW arrays were formed after the sample was rinsed with THF under high purified nitrogen atmosphere. 2.3. Device Fabrication. Aluminum of 200 nm thickness was deposited on the back side of the sample as a rear electrode by thermal evaporation. The highly conductive PEDOT:PSS (Clevios, PH1000, with 5 wt % dimethyl sulfoxide (DMSO)) solution was spin-coated onto the nanostructured silicon at a speed of 4000 r/min, followed by annealing at 125 °C for 30 min in nitrogen atmosphere. The conductivity of the as-prepared thin film is around 102 S/cm.21 Silver grids with thicknesses of 100 nm were deposited on the surface of PEDOT:PSS by thermal evaporation and act as the top electrode through a fishbone-like shadow mask. 2.4. Transient Photoresponse Characterization. The photovoltaic J−V characteristics were tested by a Keithley 2400 source meter at 100 mW/cm2 irradiation intensity under the condition of AM 1.5G using a solar simulator (Abet Technologies). A home-built measurement system that combined a laser diode (650 nm), oscilloscope, and pulse generator was used to investigate the photoresponse properties of the device under different pulsed light illumination. For transient response measurement, the laser diode is powered by square pulses from a universal waveform generator or the spectral response was studied by standard QE-R system (Enli tech’s QE-R system) which is composed of a xenon lamp (75 W), a monochromator, and two lockin amplifier (SR830). The responsivity of the photodetector for multispectrum is measured by QE-R system with a light chopper. The laser was turned on/off regularly by a function generator with a square voltage pulse. The device’s current output was read by Keithley 2601A sourcemeter. The response speeds of these photodetectors were evaluated by combining a pulse laser and a digital oscilloscope (Tektronix TDS 2012C, 200 MHz).

organic photodetectors with hybrid structure of graphene/ PEDOT:PSS, with a responsivity of 0.16 A/W at 500 nm.7 Zhang’s group has reported a deep ultraviolet photodetector with a PEDOT:PSS anode, with detectivity of 1.1 × 1012 jones under −12 V.8 Friedel et al. reported the optimal thickness is 70 nm and the annealing temperature is 250 °C in organic photodetectors with a PEDOT:PSS electrode.9 Nakano and coworkers have reported UV photodetectors based on a transparent conducting PEDOT:PSS layer.10 Zhang’s group has reported the self-powered photodetector based on PEDOT:PSS/ZnO with the long photoresponse time of 0.1 s.11 However, technical challenges remain for polymer-based photodetectors to compete with silicon photodetectors in terms of high responsivity, self-driven, low dark current, high stability, and fast response speed. These above detrimental drawbacks strongly restrict their applications in high-speed and low-power PDs. Therefore, it is of great demand for developing novel architecture of photodetector with fast response and selfpowered characteristic. Until now, substantial research efforts on interfacial passivation have been devoted to enhancing the photodetecting performance. Miao et al. reported that doping graphene with TFSA could greatly enhance the power conversion efficiency of graphene/n-Si Schottky junctions.12 Chen’s group has reported surface functionalization by the MoO3 layer of a Gr/Si self-powered photodetector with remarkable performance enhancement.13 Recently, Zhang’s group also reported another novel method of energy band engineering, namely, the piezo-phototronic effect, which can remarkably improve the photoresponse performance of ZnO/ Spiro-MeOTAD heterojunction ultraviolet photodetectors.14 In-depth investigation and comprehensive understanding of the interfacial, structure dependent photoresponse property relationships of organic/inorganic photodetectors should guide further development of polymer-based photodetectors. We choose n-type silicon as the inorganic material for it merits of high electron conductivity, high thermal conductivity, and light radiation stability, which has already shown excellent photoresponse performance in our previous studies of MoO3−x/Si heterojunction photodetector.15 Although the hybrid heterojunction has been applied to solar cells with great success,16−18 its potential application in high-speed, broadband photodetectors are still not completely explored. Herein, we demonstrate the self-powered, high-speed, and broad-band response of hybrid photodetector based on PEDOT:PSS/n-Si. This facile, low-temperature solution method by spin coating of PEDOT:PSS onto n-Si to form a Schottky junction can replace conventional costly fabrication processes. Finally, optimum interfacial passivation and novel heterostructures have been proposed for further improving the photodetecting performance, as well as the enhanced environmental stability, along with a discussion of the underlying mechanisms.

3. RESULTS AND DISCUSSION 3.1. PEDOT:PSS/n-Si Heterojunction Fabrication and Characterization. In contrast to inorganic semiconductors, photoexcitation of organic semiconductor generates strong bound excitons rather than free charge carriers. Donor and acceptor heterojunction is the typical approach to dissociate the bound excitons efficiently. The organic/inorganic hybrid solar cell is the successful configuration of excitons dissociation. As shown in Figure 1(a), the p-type semiconductor organic layer is PEDOT:PSS, which resulting from combination with styrene sulfonic acid. It acts as hole selective contact with n-type silicon and forms a heterojunction. The vertical top/down structure contributes to enhancing the overall light absorption for photodetecting. Figure 1(b) shows the scheme of as fabricated

2. EXPERIMENTAL SECTION 2.1. Si Nanowires Synthesis. All the devices were fabricated on 400-μm thick, double-side polished, n-type (100)-oriented single crystal Si wafer with a resistivity of 3−4.8 Ω·cm. Samples were sequentially ultrasonically cleaned in acetone, ethanol, and deionized water in sequence for 15 min, respectively. Following, the substrate was cleaned with piranha solution (3:1 H2SO4/H2O2) at 80 °C for 30 min and rinsed with deionized water for several times to ensure surface without any residuals. Then, the cleaned Si wafers were flowed by dry N2 stream. The growth of silicon nanowires was synthesized by a metal-assisted chemical etching method.19 Briefly, the as-prepared Si 19159


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Figure 1. (a) Structure of PEDOT:PSS, (b) device structure of Ag grids/PEDOT:PSS/n-Si hybrid photodetector, and (c, d) titled/crosssectional SEM images of PEDOT:PSS/Si heterojunction, respectively. Figure 2. (a) Typical J−V curves of PEDOT:PSS/H−Si heterojunction in the dark and varied intensity. (b) Time-dependent photoresponse of devices with hydrogen and methyl passivation under pulsed 650 nm laser illuminations. (c) Energy band diagram of the PEDOT:PSS/Si heterojunction under light illumination. (d) Photovoltage of the CH3 terminated planar heterojunction as a function of light intensity. The inset shows the photocurrent varied with light intensity at 0 V and can be fitted according to power law.

devices, top silver grids collects holes, while the bottom Al electrode collects electrons. Details of the device fabrication process are given in the Experimental Section. Figure 1(c,d) gives the inclination of a 45° view and cross-sectional views of PEDOT:PSS/Si hybrid heterojunction, respectively. The mean thickness of spin coating of PEDOT:PSS layer for planar silicon is determined to be about 47 nm. 3.2. Self-Powered Properties of PEDOT:PSS/Si Heterojunction and Band Diagram Analysis. Interfacial passivation plays an important role in Si-based heterojunction photodetectors. It was reported that hydrogenated Si arrays shows poor stability and were susceptible to the surrounding environments, while methyl-terminated Si exhibits much more stable photoresponse even after long-term exposure to air. The effects of interfacial passivation by H and CH3 on the photodetecting performance of heterojunctions have been investigated. Figure 2(a) shows typical current−voltage (J−V) characteristics of hydrogen and methyl passivated heterojunction which measured in the dark and light illumination condition (100 mW/cm2). It clearly showed that both heterojunctions exhibit remarkable photovoltaic characteristic. The photocurrent density for H and CH3 passivated heterojunction is 24.6 and 26.7 mA/cm2, respectively. Due to the obvious photovoltaic property of PEDOT:PSS/Si heterojunction, both hybrid photodetectors can operate without external power. Namely, PEDOT:PSS/Si heterojunction shows excellent self-powered characteristics, could be promising candidate for future devices aiming at reduced size and weight. Inset of Figure 2(a) gives a typical semilog current density versus voltage (J−V) under dark conditions, which clearly exhibit a rectifying characteristic. In Figure 2(b), more distinctive photoresponse curves of PEDOT:PSS/Si heterojunctions have been shown. It revealed that the hybrid photodetector can be reversibly switched when the light is on and off repeatedly at 0 V. The ratio of photocurrent (Iph) to dark current (Id) for hydrogenate and methyl passivated Si surface is on the order of 105−106. The sharp rise and decay edges is attributed to the rapid separation of photoinduced electron−hole pairs near the PEDOT:PSS/Si interface. Additionally, the photodetector possesses an excellent stability and reproducibility to pulsed light with about 20 s per cycle. It indicates that interface passivation by methyl (CH3) possesses a little higher photocurrent than that of hydrogenated type. In Figure 2(c), the photoresponse characteristic can be

further understood from the energy-band diagram illustrated. Refering to our previous report of PEDOT:PSS/Si, the energylevel alignment at the interface would be constructed due to the difference in Fermi level of PEDOT:PSS and Si. The maximum photovoltage depends on the interfacial energy-level alignment, which can be determined by (EF−EHOMO−ΔES). The maximum value observed in our experiment is 0.53 V, which is close to the previously reported optimized value of 0.6 V.22 Therefore, the energy levels near PEODT:PSS will bend upward, whereas the energy near the Si surface will bend downward, and eventually the Fermi levels of PEDOT:PSS and Si align in the same level. Once the light is on, electrons would be easily excited to the conduction band of Si, due to its narrow band characteristic, and then it dissociates to free carriers. Electrons will tend to move to the Si side, whereas holes will move to PEODT:PSS layer, due to its high hole-selective conductivity. Since the excitons of PEDOT:PSS bind strongly, few electron− hole pairs would be formed in PEDOT:PSS layers. In this perspective, the photocarrier separation efficiency of the PEDOT:PSS/Si heterojunction is largely dependent on the Si side. Figure 2(d) depicts the dependence of detection performance with light intensity. By tuning the light intensity of 650 nm laser from 25 W/m2 to 250 W/m2, the photovoltage first increases rapidly from 0.34 to 0.435 V. However, a further increase of the light intensity leads to saturation of the photovoltage. Inset of Figure 2(c) shows the dependence of photocurrent on light intensity at zero external bias voltage. The curve can be well fitted with a power law, I = APθ, where A is a constant for a certain wavelength, and θ (0.5 < θ < 1) determines the response of the photocurrent to light intensity. The fitting gives an almost straight line with θ = 0.95, which is very close to the ideal value 1, implying very low trap states exists in the band gap. Next, we also investigate the linear dynamic range of the PEDOT:PSS/Si hybrid photodetector, which can be calculated using the following equation: LDR(dB) = 20 log10(Pmax/Pmin), where Pmax is the highest incident light 19160


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ACS Applied Materials & Interfaces power, and Pmin is the lowest incident light power of the range in which the photodetector response is linear with the incident power. The measured LDR extended 42 dB, which is smaller than that of NiO quantum dots modified PbS/ZnO heterostructure photodetectors.23 3.3. Effects of Interfacial Passivation on the Photoresponse Property of PEDOT:PSS/Si Heterojunctions. Response speed is one of the key factors to determine the capability of a photodetector. Figure 3(a) shows the schematic

However, when the frequency increases to 1000 Hz, a remarkable decrease of photovoltage was observed in a hydrogen passivated planar heterojunction. With the further increase of frequency, more distinctive gaps between the photoresponse curve and the zero voltage were observed. The photovoltage curve would be severely distorted under a frequency of 5000 Hz. On the contrary, a more stable photoresponse was detected in the methyl passivated heterojunction. The relative balance of the planar hybrid photodetector could be largely enhanced for the methyl passivated Si surface, even when the switched frequency extended to 5000 Hz. The high-frequency photoresponse performance corresponds to the process of photocarrier extraction or recombination. It has been reported that Si surface passivation by hydrogen would induce downward bending of the interfacial band structure, and an upward bending for the methyl terminated Si surface.24 This indicates more efficient dissociation of photoinduced carrier for the methyl terminate Si surface, but some carrier would be easily accumulated in the interface for the hydrogen passivated Si surface. To elucidate the reasons accounting for the effects of interfacial-passivation on high-frequency photodetecting performance, H and CH3 types of planar heterojunctions were characterized by impedance spectroscopy. As shown in Figure 4(a), the impedance spectra were obtained in the frequency

Figure 3. (a) Schematic illustration of the setup for studying the transient photoresponse of the PEDOT:PSS/Si photodetector. (b) Comparison of the −3 dB bandwidth of hybrid photodetectors with H and CH3 interfacial passivation, respectively. (c,d) Typical photoresponse curves of PEDOT:PSS/Si heterojunction with H/CH3 interfacial passivation to pulsed 650 nm light with frequency from 100 to 5000 Hz.

illustration of the setup for studying the time response of the photodetector. It consists of a laser diode (650 nm), oscilloscope (Tektronix, TDS2012C), and a pulse generator. The variation of the photovoltage of H and CH3 terminated Si planar heterojunction photodetectors is monitored and recorded by an oscilloscope. As shown in Figure 3(b), the normalized photoresponse of two heterojunctions to pulse light is analyzed. The −3 dB bandwidth of H- and CH3-terminated Si planar heterojunction is determined to be 7585 and 17 680 Hz, respectively. It indicates that interfacial passivation by CH3Si could greatly enhance the high frequency photoresponse region of hybrid photodetector. The relative balance of H-terminated Si planar heterojunction shows much faster decay rate in the same frequency region. Given to the same Si structure, the improved bandwidth of the heterojunction is greatly related to the interfacial passivation. Figure 3(c,d) shows the detailed photoresponse curves of the hybrid PEDOT:PSS/Si planar photodetector with interface H and CH3 passivation under pulsed light (650 nm), respectively. Under the switched frequency of 100 Hz, both types of photodetectors show a regular and repeatable rectangle curves.

Figure 4. (a) Electrochemical impedance spectroscopy of HSi, CH3Si planar, and CH3Si NW based PEDOT:PSS/Si heterojunctions. (b) Mott−Schottky plots of the hybrid heterojunction with different interfacial passivation. (c) EQE spectrum and total optical reflectance of hybrid heterojunctions with different Si structures. (d) Comparison of the bandwidth of the hybrid photodetector with different interfacial passivation and Si microstructure.

range of 0.1 Hz to 100 kHz at room temperature. The nearly semicircular shape of the impedance spectra of both types of devices indicates that the Schottky junction can be expressed using an equivalent circuit model which is composed of a combination of resistance and capacitance (RC) networks.25 It can be seen that the radius of the semicircles of H terminated Si device is much larger than that of the CH3 one, which indicates that the total impedance of the CH3 passivated device is smaller than that of hydrogen. Since the series resistance accounts for the contact and bulk resistance of the device. The inset of Figure 4(a) depicts the equivalent circuit model, and the 19161


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Figure 5. Typical cross-setional view SEM images of heterojunctions and corresponding schematicl illustration: (a−d) top coverage (TC) and (e−h) bottom coverage (BC).

Figure 6. Enlarged photoresponse curves of the rise/decay edges for top coverage (TC) PEDOT:PSS/Si hybrid photodetectors to pulsed light with a frequency of 1000 Hz, respectively: (a,f) planar, (b,g) 375 nm, (c,h) 820 nm, (d,i) 1000 nm, and (e,j) 2000 nm nanowire.

semicircular shape impedance plots were fit to a parallel RC circuit in series with a series resistance, Rs, the value is determined to be 13.41 and 64.41 Ω for CH3 and H terminated Si, respectively. The smaller the contact resistance, the higher the efficiency of the electron transfer property in the heterojunction becomes. Due to the higher efficiency of the electron transfer property, the device with the CH3 interface passivation would exhibit a higher frequency response character. The surface band diagram of HSi is bending downward, and the electron would be easily trapped at the interface. Due to the limited electron conductivity, charges would be accumulated at the junction and repel the following separated electrons. Namely, the separation efficiency would be largely limited by the contact resistance. This is the main reason accounting for the broader bandwidth of the heterojunction by CH3 interfacial passivation. The broader bandwidth is also in line with the resulting complex impedance which varied with that of frequency (Figure S1 of the Supporting Information).

Capacitance versus voltage measurements have been done to investigate the barrier height, charge carrier density, and depletion width of the hybrid. Moreover, to further understand the change in the Voc with respect to the surface configuration, we measured Mott−Schottky (MS) curves of H- and CH3terminated planar heterojunctions. As shown in Figure 4(b), the extrapolated intercept in the potential coordinate axis of the two-electrode system corresponds approximately to the Schottky-barrier height. The figure clearly indicates that the Schottky-barrier height for H- and CH3-terminated planar heterojunction is ∼0.61 and 0.48 V, respectively. Our result is close to a previous study reported by Lewis’s group.26 It was reported that barrier heights for CH3-terminated n-Si/PEDOT contacts showing 0.12 V higher than that of H-terminated contacts. As shown in Figure 4(c), the heterojunction with the nanowire structure exhibits a higher external quantum efficiency in the region of 500 nm when compared with that of the planar 19162


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shows little increase, but the decay time remained almost the same. It it proposed that the photoresponse speed is largely limited by the photocarrier’s mobility and dissociation, while the decay time is mainly dependent on the photocarrier recombination. It indicates that there exists competitive contribution between the Si nanowire and Si bulk to the observed photoresponse. The dependency between the Si nanowire length and the photoresponse characteristics could be illustrated as follows: Considering the planar and tiny nanowire Si structures, the bulk would absorb most of the light and generate a great amount of electron−holes. Then it would diffuse into the depletion region of the heterojunction and split into free carriers. The extract effeciency of the electrons in the heterojunction determines the rise time. Namely, the built-in potential difference determines the dissociation of photocarrier. When the carrier diffusion length in Si is larger than that of nanowire length. The photoresponse speed is indistinguishable for planar and nanowire structures. However, a further increase of the nanowire length would extend the diffusion time of the photocarriers from bulk to the heterojunction. And the light adsorption contribution of nanowires would become stronger in the photodectection. Besides the carrier diffusion length, carrier mobility is another important factor that affects the rise time. With the increase of nanowire length, the mobility would be restricted due to greater scattering, which mainly results from surface defects in longer nanowires. Next, the decay time could be greatly improved by modifying the heterojunction with nanowire structure. Since the decay time corresponds to the recombination process of photocarriers. It was reported that surface recombination would be enhanced with the increase of silicon nanowire length.27 The longer the Si nanowire, the greater the density of surface traps related recombination centers becomes. The carrier diffusion length in silicon is determined by the surface traps, which could well explain the shorter decay time of Si NWs than that of the planar structure. With the increase of the NW’s length, the decay time first decreases and then becomes stable. When the balance of carrier diffusion and recombination is reached, an optimal decay time of 55.4 μs is obtained. Therefore, the suggested length for optimized hybrid PEDOT:PSS/Si heterojunction is proposed to be 820 nm. 3.5. Mechanism of Coverage Effects and Optimized Photoresponse Characteristic. As illustrated in Figure 7, the coverage effects of PEDOT:PSS on Si NWs could be discussed from two aspects: top coveage and bottom coverage. The

structure. Meanwhile, the heterojunction with methyl passivation of the planar Si also exhibits better EQE (Figure S3). The excellent EQE efficiency is proposed to originate from the lower reflection of the nanowire structure. This would greatly improve the light adsorption and contribute to the photocurrent density. The superior geometry structure of the heterojunction was also confirmed by the bandwidth measurement. Figure 4(d) shows the −3 dB cutoff frequency (f 3 dB) of the HSi,CH3Si planar, and CH3Si nanowire based heterojunctions, which indicates that the nanowire structure could further extend photodetectors to a broader frequency up to 40.6 kHz. The high frequency photoresponse is very important for high performance photodetectors, especially for obtaining more reliable parameters. Therefore, we choose methyl passivation and Si nanowire structure in the following comparison of various types of hybrid heterojunctions. The remarkable enhancement by interface engineering can be found in Tables S1 and S2. 3.4. Transient Photoresponse Properties of Heterojunction with Varied Si NW Length. Next, we focus on investigating the coverage effect of PEDOT:PSS and Si nanowire length on the transient photoresponse characteristic of hybrid heterojunctions. As shown in Figure 5, the coverage effect of PEDOT:PSS and Si nanowire lengths on their photoresponse property is investigated in detail. First, nanowires with lengths of 375, 820, 1000, and 2000 nm could be obtained by controlling the etching time (Figure 5(a−c). Figure 5(a) shows the typical cross-sectional SEM image of the PEDOT:PSS/Si heterojunctions with 375 nm Si nanowire. It can be seen that the PEDOT:PSS layer was mainly coated on the surface of the Si NWs, with the increase of Si nanowire length, only part of the PEDOT:PSS permeated along the Si NWs into the inside, and leave the root of Si NWs uncovered. Figure 5(d) depicts the corresponding schematic diagram of top coverage. However, if we want to study the bottomcoverage effect, then a longer chlorination period was employed (see the Experimental Section). As shown in Figure 5(e−g), due to the weak etching effect of PCl5, the interspace of the Si nanowire will be broadened. The PEDOT:PSS layer would permeate into the interspace of Si nanowire arrays and covered the bottom (named as bottom coverage). Our previous study indicates that both types of heterojunctions can be well formed after the PEDOT:PSS was spin coated on the Si nanowires,19 the difference lies in the interface effects which will be disccussed later. Figure 6(a−j) shows the effects of nanowire length on the transient photoreponse of hybrid photodetector. The enlarged temporal photoresponse curves and the response times were obtained under pulsed light with a frequency of 1000 Hz. The rise and decay times are determined from the time interval for the response to rise (decay) from 10% (90%) to 90% (10%) of its peak value, respectively. It can be seen that the rise and decay times become longer with the increase of Si nanowire length. For planar Si-based heterojunctions, the rise and decay time is estimated to be 2.05 and 175.2 μs at zero external bias voltage, respectively. However, when the Si structure was changed to nanowire, the photoresponse speed showed a clear enhancement. The rise time can be further decreased to 2.03 μs, and the decay time is also largely decreased to 75.6 μs. However, with a further increase of the Si nanowire length to 820 nm, the rise time shows a clear increase to 3.17 μs, and the decay time shows a distinctive reduction to 55.4 μs. When the nanowire length extends from 1000 to 2000 nm, the rise time

Figure 7. Schematic illustration of PEDOT:PSS/Si heterojunctions with top-coverage configuration, planar (a), short/long nanowire (b,c) and bottom-coverage (d), respectively. 19163


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Figure 8. Bottom-coverage (BC) of PEDOT:PSS/Si heterojunction with varied length of Si nanowire: (a,e) ∼50 nm, (b,f) 200 nm, (c,g) 600 nm, and (d,h) 1600 nm.

To verify the above analysis of geomerty-dependent photoresponse, bottom-coverage of hybrid heterojunctions with varied length of Si NWs are presented. In Figure 8(a− d), it reveals that the rise time shows more sensitive dependence to that of Si NW length, while the decay time is almost on the same order. The results give solid evidence for the above analysis of coverage dependent on photoresponse performance, namely, the top or bottom coverage of PEDOT:PSS. Therefore, we can conclude that the photoresponse speed could be largely optimized by manipulating the heterojunction geometry, such as the distribution density and aspect ratio of the nanowires. The photoresponsivity is an important parameter for evaluating the photodetecting efficiency. As mentioned above, we choose hybrid heterojunction with top coverage configuration for it better photodetecting performance. The photoresponsivity (R) could be given by the equation of R = (Iph − Id)/Pin, where Iph, Id, and Pin are the photocurrent, dark current, and illumination power on the active area of the photodetector, respectively. As shown in Figure 9(a), the photoresponsivity first increases with wavelength from 300 to 920 nm and then becomes decreased in the infrared region. The maximum photoresponsivity is estimated to be 37.8 mA/W. To the best of our knowledge, this is the first report about PEDOT:PSS based self-powered photodetector with such broad photoresponse, high speed, and high responsivity. Addtionally, detectivity could also be determined by the equation, D* = (Iph/Llight)/(2qId)1/2 = (A1/2R)/(2qIdark)1/2, where Llight is the incident light intensity, q is the elementary charge, and A is the active area of the device. Evidently, in order to obtain better D*, the dark current should be as low as possible to distinguish weak optical signals. By assuming that the shot noise from the dark current is the major contribution, the detectivity can be estimated to be 4.1 × 1011 cmHz1/2W1−. The outstanding photodetecting performance of the as-fabricated photodetector is also compared with previous organic-based photodetectors (Table 1). It indicates that our hybrid phototector shows comparable performance to that of similar types, especially in

photodetecting performance of both heterojunctions corresponding to the processes of carrier generation, diffusion, and recombination. In Figure 7(a), considering the planar Si heterojunction, the photocarriers would be largely generated in Si bulks and then diffuse to the deplection region. The rise time corresponding to the photocarrier generation and diffusion process. And the photoresponse speed of the planar heterojunction is limited by the carrier mobility in n-type silicon. The decay time is largely dependent on the photocarrier recombination rate, which could be divided into bulk and surface recombinations, as will be discussed later. Since surface traps would be largely passivated by the PEDOT:PSS layer, the decay time is therefore dominated by the bulk recombination rate. It has to be mentioned that the introduction of Si NWs in the heterojunction could help us to differentiate the recombination process of the surface and bulk. In Figure 7(b), for the shorter nanowire based heterojunction, rise time could be maintained for the limited carrier diffusion distance. However, the decay time could be improved by the enhanced surface recombination rates, resulting from the exposed nanowires surface traps. Meanwhile, it has to be kept in mind that the carrier mobility would be limited by surface traps or defects in longer Si NW. Therefore, it is reasonable to believe that there exists a balance between the length of Si NWs and the photoresponse performance, a detailed discussion can be found in Figure S2. To optimize the photodetection of PEDOT:PSS/Si heterojunction, a bottom-coverage configuration of PEDOT:PSS/n-Si is proposed (Figure 7(d)). Due to the perfect coverage (core− shell) of Si nanowires, the decay time is expected to be longer, while the rise time could be shorter for longer lengths. The unique core−shell configuration of the heterojunction which benefits photocarriers diffusing to the depletion area, which will then be collected and extracted out at once and contributed to photoresponse speed. However, the surface traps would be largly depressed, which will expand the lifetime of photocarriers and accounts for the longer decay time. 19164


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istic is largely dependent on the Schottky junction between PEDOT:PSS and Si, which can be observed from the evolution process of the J−V curves. We chose planar heterojunctions for the environmental stability measurements. In Figure 9(b), it can be seen that the J−V curves could kept stable when the air duration extended to 3 h, however, further duration could induce the origin of “S-kink”. This process could probably be related to the surface oxidation of Si nanowires.28 Figure 9(c) shows the comparison of as-fabricated and stored devices with varied air duration. It clearly indicates that the our fabricated hybrid photodetector exhibits excellent ON/OFF rectification characteristics even after 18 days of exposure to air, although the absolute photovoltage becomes smaller. As shown in Figure 9(d,e), the rise time exhibits relative stability around 2.5−2.8 μs, while the decay time is a little longer with increase of air duration. The above preliminary stability studies reveal that the photocurrent curves of the device without encapsulation showed good stability with no obvious degradation after long-term exposure in air. Although these parameters are still inferior to all inorganic detectors, it is believed that our hybrid PDs have not been optimized, such as that in complete epoxy encapsulation, reducing the active area of the devices (see Figure S5).

4. CONCLUSIONS To summarize, novel types of PEDOT:PSS/n-Si heterojunction photodetectors were systematically fabricated by facile spin coating of a PEDOT:PSS layer on n-type Si. It was found that the hybrid photodetectors could operate without external bias, and the ON/OFF ratio could be as high as 105∼106, along with a wide response spectrum from 300 to 1100 nm. Remarkable photodetecting enhancement of the hybrid heterojunction was demonstrated by interfacial engineering. After interfacial passivation by a methyl group, the bandwidth of the planar photodetector could be largely broadened from 7.59 to 17.68 kHz. Further introduction of Si nanowire in the heterojunction structure, the bandwidth could be further extended to 40.6 kHz, which is the best result obtained for PEDOT based selfpowered photodetectors. Additionally, the photodetector exhibits a high detectivity of 4.1 × 1011 Jones and a fast response speed of 2.03 μs. Finally, coverage effects on the photoresponse of PEDOT:PSS/n-Si photodetector have been investigated. For the top coverage case, it is proposed that the

Figure 9. (a) Responsivity of the TC heterojunction with nanowire length of 820 nm under different wavelength light illumination. (b) Photocurrent density versus voltage (J−V) characteristics of hybrid planar heterojunction with different air duration. (c) Stability of the photoresponse curves for as-fabricated and air-exposed devices for varied duration (1000 Hz, 650 nm). (d,e) Enlarged rise and decay edges with different air duration.

the infrared region. Additionally, the high photodetecting performance is aquired at zero bias voltage and in contrast to high external bias in previous studies. The excellent selfpowered performance provides a broad avenue for potential application in the active optoelectronic devices. As shown in Figure 9(b−e), we also investigated the environmental stability of the hybrid photodetector with interfacial passivation of CH3. Since the self-powered character-

Table 1. Comparison of Detection Performance for Organic Based Heterojunction Photodetector Reported in Previous Literature heterojunction type PTT:PCBM PS/TPD/PFCB/PD DIT:PC60BM/C60 PBD/NPB/PEDOT PEDOT/ZnO BCP/C60/C-TPD/P EDOT:PSS PEDOT:PSS/MAP bI3/C60/BCP PEDOT:PSS/DPP- TIIG/BCP PEDOT:PSS/Si

measurement condition (λ,V) near infrared 300−1450 nm V = −0.1 V 248−370 nm V = −12 V 325 nm V = 0 V 300−700 nm V = −6 V 350, 530, 740 nm V = −1 V 300−1200 nm V = −0.1 V 300−1100 nm V=0V

responsivity (mA/W)

detection (cmHz1/2W−1)

rise time/decay time (μs) ∼100∼100 nm

ref

130 (at 850 nm) 170 (at 800 nm)

2.3 × 1013

1 5

180 (at 280 nm)

1.1 × 1012

8

3.63 × 1011 gain (43 ± 7.5)

84, 203, 242

∼10 000 ∼50 10 ± 0.8 46.7 ± 4.2

1.4 × 1013

20.8 (basic) 159 (optimized) (at 800 nm) 37.8 (at 920 nm)

4.1 × 1011

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3.17 55.4

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rise time is largely limited by the carrier diffusion distance, which shows the reciprocal ratio with the Si nanowire length. However, the decay time seems much more complicated and could be understood from the competition of bulk/surface recombination processes. The above assumptions have been confirmed by the photoresponse measurements of bottom coverage heterojunctions. The position matching between heterojunction and the adsorption area is the key factor that influences the photocarrier generation, dissociation, extraction, and recombination processes. The scalable and low temperature solution method for flexible, tunable band gap PEDOT:PSS thin films would be great convenient and competitive for large-scale fabrication of hybrid photodetectors.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b06301. Complex impedance measurements, qualitative analysis of photocarrier process, EQE spectra, photodetecting performance of hybrid heterojunctions with varied interfacial engineering (passivation by hydrogen or CH3, manipulating Si nanowire), bandwidth measurements, and photoresponse speed of hybrid heterojunctions with varied device areas (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.Z.). *E-mail: [email protected] (W.X.). *E-mail: [email protected] (W.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support for the project from the National Natural Science Foundation of China (Grant Nos. 21376104 and 11574119), the Fundamental Research Funds for the Central Universities (Grant No. 21615309), and Natural Science Foundation of Guangdong Province (Grant Nos. 2014A030310302, 2014A030313381, and 2014A030306010).

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