Anomalous photocurrent characteristics in fullerene C60 thin film-based organic field-effect transistors under illumination

Anomalous photocurrent characteristics in fullerene C60 thin film-based organic field-effect transistors under illumination

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Journal Pre-proofs Research paper Anomalous photocurrent characteristics in fullerene C60 thin film-based organic field-effect transistors under illumination Qinyong Dai, Sunan Xu, Yingquan Peng, Wenli Lv, Lei Sun, Yi Wei PII: DOI: Reference:

S0009-2614(20)30048-8 https://doi.org/10.1016/j.cplett.2020.137133 CPLETT 137133

To appear in:

Chemical Physics Letters

Received Date: Revised Date: Accepted Date:

7 December 2019 17 January 2020 19 January 2020

Please cite this article as: Q. Dai, S. Xu, Y. Peng, W. Lv, L. Sun, Y. Wei, Anomalous photocurrent characteristics in fullerene C60 thin film-based organic field-effect transistors under illumination, Chemical Physics Letters (2020), doi: https://doi.org/10.1016/j.cplett.2020.137133

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© 2020 Published by Elsevier B.V.

Anomalous photocurrent characteristics in fullerene C60 thin film-based organic field-effect transistors under illumination Qinyong Dai1, Sunan Xu2, 1*, Yingquan Peng1, 2, Wenli Lv1, Lei Sun1, Yi Wei1 1 College

of Optical and Electronic Technology, Institute of Microelectronics, China Jiliang University, Hangzhou 310018, 2 School of Physical Science and Technology, Institute of Microelectronics, Lanzhou University, Lanzhou 730000, China *E-mail:

[email protected]

Abstract We investigated anomalous photocurrent behavior in organic field-effect transistors(OFETs) based on C60 thin film. With increasing incident optical power, it was interesting to observe the photocurrent decreases.The inverse photocurrent was mainly dependent on intensity of the light, not the wavelength. Meanwhile, this paper revealed that the main sources of the negative photocurrent were not the electrodes, channel and the interface effect but a distinct optoelectronic property of the C60 material itself. Such mechanism of photocurrent may find potential applications in the future. Keywords: Fullerene, Anomalous photocurrent, organic field-effect transistors.

1. Introduction Fullerene C60 has a significant spherically symmetric structure because the carbon atoms are arranged in a closed shell to form polyhedra with 60 vertices (32 faces), including 12 pentagonal faces and 20 hexagon faces [1-5]. The special physicochemical properties of C60 are determined by their structural characteristic which is a semiconductor and photoconductive material and is also a good electron acceptor [6-9]. During the last decade, C60 has received great attention in the research of optical applications, such as photovoltaics [10-14], photodetectors [15-16] and solar cells [17-20], using advantage of the strong absorption of ultraviolet light and high electron mobility [9]. For example, X. H. Zhang et al. reported photodetectors based on single-layer C60 film fabricated by physical vapor deposition, showing a high electron mobility greater than C60-based devices fabricated by conventional process [21]. Won-Ik Jeong er al. demonstrated that the photoconductivity of C60 bulk-ionizing is an origin of the linear dependence of the photocurrent in organic photovoltaics based on CuPc/C60 heterojunction under reverse-bias and small forward-bias conditions through experiment and theoretical model [22]. More recently, Shuchao Qin et al. has developed ultraviolet photodetector array based on Graphene/C60 heterojunction, resulting in an UV photoresponsivity of ~107 A/W, due to extremely high light guide gain resulted from effective exciton dissociation and enhanced ultraviolet light absorption at the graphene/C60 heterojunction interface. [23]. However, widely reports focus on the positive photocurrent range of the C60, leaving the potential of C60 in the inverse photocurrent characteristics unexplored on to the best of our knowledge. Inverse photocurrent in organic semiconductor devices is a rare phenomenon, since photocurrent is normally enhanced which attributed to photoexcitation of charge carriers [24-26]. In recent years, anomalous photocurrent characteristics have aroused great interest from researchers. For instance, Eunhye Baek et al. firstly report the transition of negative and positive photoconductance regimes observed in n- and p-doped silicon nanowire FETS and derive this transition is determined by the competition between the light induced interfacial

trapping and the mobile carriers recombination [27]. The surface effect caused by the high surface-to-volume ratio is also an important reason for the material to reveal anomalous photocurrent characteristics, such as some low dimensional materials( nanoparticles, thin film and nanowires) [28-33]. Although anomalous photocurrent characteristics has attracted substantial attention, there are still few reports about this phenomenon occurring in organic semiconductor materials, and the theory of it is not deep enough. In this regard, anomalous photocurrent characteristics studies of devices based on organic material will be a key issue, not only for the in-depth understanding of the optoelectronic properties of organic materials, but also for making it possible to open new applications research. In this work, we report the observation of anomalous photocurrent characteristics in C60 single layer organic field-effect transistors under different wavelength illumination. The photocurrent characteristics were investigated depending on the intensity of light, different wavelength, channel length as well as different electrodes. Noteworthy, the decreasion of the photocurrent is only observed under slightly larger light intensity, which indicates that, the existence of two mechanisms affecting photocurrent. Moreover, we can find that the phenomenon of anomalous photocurrent characteristics is all observed in devices with different electrodes as well as under different wavelength illumination. Finally, we think that our observation of anomalous photocurrent characteristics can be considered, as an inherent characteristic of the C60 film itself.

2. Experimental C60 powder with 99.5% purity was purchased from Tci. and used as received. The device structure and the chemical structure of C60 are shown in Fig. 1. Heavily doped n-type silicon (n+-Si) (capacitance per unit area, Cox=3.18nF/ cm2) wafers with an 1um thermally grown SiO2 were used as the gate and gate dielectric. C60 was used as the channel layer. Prior to deposition of the channel layer, the substrates were ultrasonically cleaned in acetone, ethanol, and deionized water and dried by blowing high-pure N2. Next, the thin films of C60 (50nm) were firstly deposited on the top of the substrates. After that, Au (denoted as Au-device) source-drain electrodes, Al (denoted as Al-device) source-drain electrodes, Cu (denoted as Cudevice) source-drain electrodes or Ag (denoted as Ag-device) source-drain electrodes were deposited through a shadow mask which defined a channel length (L)/width (W) of 25 μm (50μm and 80μm)/3 mm. During deposition the chamber pressure was kept at 4×10-4 Pa and the evaporation rate at 0.10–0.15 Å/s, monitored by a quartz crystal oscillator. For optical absorption measurements, thin films of C60 (50 nm) were deposited on cleaned quartz substrates. TU-1901 spectrometer was used for the measurements of absorption spectra. After device fabrication, the samples were immediately transferred into a vacuum chamber (vacuum level ~10 Pa) and measured by using an organic semiconductor characterization system. The transfer characteristics were measured at saturation region (Vd =150 V) and linear region (Vd =10 V). For the measurements of photo effects, six commercially available lasers with a central wavelength of 405 nm (28.616 mW/cm2), 532 nm (33.545 mW/cm2), 365 nm (42.3 mw/cm2), 450 nm (50.3 mw/cm2), 650 nm (46.3 mw/cm2) and 780 nm (32.3 mw/cm2) were used as the light source separately. It is worth noting that we used a more powerful 405nm laser (100mW/cm2) and inserted narrow band filters with a wavelength of 365nm before the laser diode to obtain 365nm UV light. The variation of incident optical power density was realized by inserting neutral filters before the laser diode. Crystal structure of C60 films were determined by X-ray diffraction (XRD) (Rigaku D/max-2400). Topography of organic channel layers was investigated by atomic force microscopy (AFM) (AFM, Agilent 5500) in the tapping mode.

Fig.1. Schematic configuration and illustration of as-fabricated C60 single layer photosensitive organic field-effect transistors. The inset is the molecular structure of C60

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Fig. 2. Absorption spectrum of C60 thin film (50nm) on quartz glass. The inset is AFM 2D images of C60 films deposited on SiO2 substrates.

Fig. 3. XRD patterns of C60 (50 nm) films deposited on the n+-Si/SiO2 substrate respectively.

3. Results and discussion 3.1 Material and device structural considerations For this study, we fabricate a single layer organic field-effect transistor to disentangle photocurrent characteristics from the interface behavior in multilayer devices. The absorptions of C60 on quartz glass are shown in Fig 2. Owing to wideband gap (2.6 eV) of C60 molecules, its optical absorption is mainly in the ultraviolet region [34-35]. As can be seen from the spectra, an absorption peak at 378 nm and an appreciable absorption at 405 nm were observed which demonstrating that C60 has an obvious absorption in the ultraviolet region. As is observed from Fig.3, diffraction peaks on the crystal surfaces of (311), (422), (440) and (731) were observed on the C60 films with a thickness of 50nm on SiO2. Their corresponding diffraction angles were 20.66 ○ , 30.94 ○ , 35.9 ○ and 51 ○ , respectively. The above experimental data are consistent with the peak positions of C60 films reported in other literatures [27]. In addition, the AFM image of a 50 nm thick C60 film deposited on SiO2 was measured to observe the quality of the film (see the inset of Fig. 2). A good crystallinity of the C60 film fabricated by our experiment has been observed from this image. In summary, the C60 film we fabricated is conventional, and the anomalous experimental phenomenon in our work is believable rather than occasional. As is known to us, fullerene (C60), a zero-dimensional (0D) carbon allotrope has been widely employed in various optoelectronic applications because of numerous exciting chemical and physical properties. The strong and tunable absorption of UV light by C60 molecules opens up the potential for fabricating high-performance photodetectors. So far, much research on C60 photodetectors or materials is focused on positive photocurrent characteristics. But inverse photocurrent characteristics are observed for the first time in C60 single layer OFETs in our work.

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Fig. 4. Transfer characteristics of the C60 single layer Cu-device. (a) is the photosensitive characteristics in saturation region (Vd = 150V) under 405 nm laser illumination with different radiation power and (b) is in linear region (Vd = 5V). (c) is the photosensitive characteristics at Vd = 150V and Vg = 150V. (d) is the photosensitive characteristics at Vd = 150V and Vg = 5V.

3.2 The anomalous photocurrent characteristics Fig.4a shows transfer characteristics of the Cu-device under 405 nm illumination in saturation region (Vd = 150V). It can be seen from Fig. 4, the drain current firstly increase and then decrease with the incident light intensity increasing and reach maximums at a incident optical power density of 1.056 mW/cm2. We analyzed dominant photoresponse mechanisms for the phototransistors in terms of different incident light power densities. Under an illumination with optical power density of 1.056 mW/cm2, the phototransistors exhibit positive photoresponse owing to the density of photo generated charge carriers is dominant. In contrast, upon the optical power density of 1.056 mW/cm2 – 28.616 mW/cm2, it is interesting to note that the phototransistors show negative photoresponse. So, there is no denying that the existence of two mechanisms which affect photocurrent, one that decreases the photocurrent and the other increases the photocurrent. At weak optical power density (1.056 mW/cm2), the rate of photocurrent increases with the light intensity is very high due to the low density of optical carriers, occupying only shallow traps, and the trapped charge density is also low [36]. As a result, the devices exhibit positive photoresponse because positive mechanism is stronger than negative mechanism. However, the rate of increase in photocurrent with light intensity will be affected as the light intensity increases, because the density of photocharge carriers increases with the light intensity increases, and deep traps are occupied, resulting in a limited conduction of the trapped charge [37-38]. So, negative mechanism is stronger than positive mechanism which results the phototransistors show negative photoresponse. From Fig. 4b, we find that the phenomenon of anomalous photocurrent characteristics is still observed and more pronounced when the devices are operating in a linear region (Vd = 5V). This is well understood because there are fewer free carriers and more traps in the linear region which results in photoelectric effect and photoconductivity reduction in phototransistors.

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Fig. 5. The photosensitive characteristics of the devices with different electrodes and different channel lengths under 405 nm laser illumination with different radiation power. We also investigated whether the phenomenon of anomalous photocurrent characteristics will be affected by the electrodes and channel lengths. Fig. 5 shows the photosensitive characteristics of the devices with different electrodes and channel lengths under a 405 nm laser illumination with different radiation power. We can find that the phenomenon of anomalous photocurrent characteristics is all observed in Au-devices, Al-devices and Ag-devices. In addition, the phenomenon is also observed in devices with different channel lengths, which indicates that the main sources of the anomalous photocurrent characteristics phenomenon is not the electrodes and channel lengths but a distinct optoelectronic property of the C60 film itself.

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Fig. 6. Transfer characteristics of the C60 single layer Ag-device. (a) is the photosensitive characteristics in saturation region (Vd = 150V) under 365 nm laser illumination with different radiation power and (b) is in linear region (Vd = 5V). (c) is statistical analysis of the photosensitive characteristics at Vd = 150V and Vg = 150V. (d) is statistical analysis of the photosensitive characteristics at Vd = 150V and Vg = 5V. (e) is the photosensitive characteristics at Vd = 150V and Vg = 150V under different wavelength illumination. (f) is the photosensitive characteristics at Vd = 150V and Vg = 5V under different wavelength illumination. (g) The dependence of R and absorption on the wavelength under different incident light power Because there is no strong band at the investigated wavelength of 405nm, the photosensitive characteristics of Ag-device under 365 nm laser illumination with different radiation power is investigated (see Fig. 6). It is found that the phenomenon of anomalous photocurrent characteristics is also observed under 365 nm illumination, which indicates that the anomalous phenomenon also exists at wavelengths with strong bands. In addition, statistical analysis of the photosensitive characteristics of Ag-device is showed in Fig. 6c and Fig. 6d. It can be seen that the standard deviation is too small to affect the experimental results which prove that each data point is stable and reliable. A particular attention is paid to the phenomenon of anomalous photocurrent characteristics is all observed under different wavelength illumination (see Fig .6e), indicating the main sources of the anomalous photocurrent characteristics phenomenon is not the wavelength but a distinct optoelectronic property of the C60 film itself. Photoresponsivity (R) is an important parameter of photodiodes, which is expressed as follows: 𝑹=

𝑰𝒑𝒉 𝑷𝒐𝒑𝒕

=

𝑰𝒊𝒍𝒍 ― 𝑰𝒅𝒂𝒓𝒌 𝑷𝒊𝒏𝒕.𝑨

where Iill is the current under illumination, Iph is the photocurrent, which equals the difference between Iill and Idark. Popt is the incident optical power on the device, Pint is the incident optical intensity, and A is the effective irradiated area of the device. The power spectrum compared with the absorption spectrum is shown in Fig. 6g to understand the experimentally observed phenomenon. The maximum responsivity of 8.31 A/W was obtained under 365 nm illumination at 0.821 mW/cm2 incident light power density, and then decreased with the irradiation wavelength increasing until reaches a minimum value of -0.7065 A/W under 780 nm monochromatic light. The responsivity increases with decreasing wavelength, because the higher excitation energy provided by higher photon energies can produce more excitations. It is worth noting that the devices show negative photoresponsivity when subjected to 532nm-780nm laser illumination due to the weak absorption of the C60 films in the range which make negative photocurrent mechanism occupy the dominant position. As expected, the decrease of R with incident light intensity is observed and is attributed to trapped charge limited current conduction at high charge carrier concentrations, the exciton-exciton quenching at high exciton densities and anomalous photocurrent characteristics in C60 films. The phenomenon of anomalous photocurrent characteristics is explained by photoinduced denaturation and photothermal effect. It is well-known that C60 has a significant spherically symmetric structure because the carbon atoms are arranged in a closed shell to form polyhedra with 60 vertices (32 faces), including 12 pentagonal faces and 20 hexagon faces. In most cases,

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the photochemical changes drive a reconstructive crystal-to-crystal phase transition that leads to crystal fragmentation and disintegration [39-43]. Therefore, we think that the photoinduced shape change and dangling bond cleavage of C60 molecules increase the number of traps in the film which influence the photogeneration exciton dissociation and charge carrier transport efficiency resulting anomalous photocurrent characteristics. In addition, the heat generated by the strong light illumination results in enhanced phonon scattering, reduced carrier mobility, and consequently negative photocurrent.

4. Conclusions In summary, we report anomalous photocurrent characteristics in C60 single layer OFETS. Study of parameter are performed with different electrodes, wavelength and intensity of the light. The properties of the devices indicate that the C60 material itself leads to diminish photocurrent with increasing incident optical power. Furthemore, by analyzing experimental phenomena and applying Stockmann model, two mechanisms are discerned to work together to interpret the present photoelectric response. One mechanism is caused by photothermal effect, which leads to enhanced phonon scatering. The other mechanism is related to the structural disorder under illumination. Although we cannot give a particular description of the relevant molecular energy levels, this work provides the important supplement for other researchers working with C60 based photodetectors, photovoltaics and solar cells, and may find potential applications such as radiation measurement and imaging systems in future.

Acknowledgements The authors gratefully acknowledge the Natural Science Foundation of Zhejiang Province Grant No. LY18F050009, LY18F040007 and National Key R&D Program of China Grant No. 2016YFF0203605 for financial support.

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Graphical Abstract 900 800 700

Id (nA)

600 500

Vd = 5V

400

Vg = 150V

Cu-device Ag-device Al-device Au-device

300 200 100

dark

0.203

0.264

0.791

1.828

3.075

7.154

incident optical power(uw)

Figure 1

Figure 2

A simple device topology is presented in Fig. 1. The curve in Fig. 2 show that photocurrent tend to increase first, and then decrease with the increase of incident optical power. Furthermore, we confirm that the phenomenon of anomalous photocurrent is observed in devices with Au, Cu, Al and Ag electrode respectively, which indicates that the main sources of the anomalous photocurrent characteristics phenomenon is not the electrodes but a distinct optoelectronic property of the C60 material itself. We clarify the phenomenon of anomalous photocurrent resulting from photoinduced denaturation. This mechanism of photocurrent make it possible to open new applications research based on C60 thin film.

Highlights 1. 2. 3. 4. 5.

We research systematacially the characteristics of anomalous photocurrent behaviors in organic field effect transistors (OFETs) based on C60 thin film under illumination. We fabricate a single layer organic field-effect transistor to disentangle this photocurrent characteristics from the interface behavior in multilayer devices. This paper shows that the phenomenon of anomalous photocurrent characteristics is independent from device with different electrodes (Al, Au, Ag and Cu) and different channel length (25um, 50um and 80um). This paper shows that the phenomenon of anomalous photocurrent characteristics is independent from device under different wavelength illumination. This work clarify the phenomenon of anomalous photocurrent resulting from photoinduced denaturation.

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Qinyong Dai: Writing- Original draft preparation, Software and Formal analysis. Sunan Xu: Conceptualization, Methodology, Writing- Reviewing and Editing. Yingquan Peng: Supervision. Wenli Lv: Data curation. Lei Sun: Investigation. Yi Wei: Validation. Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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