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Abstract
The emerged demand for high-performance systems promotes the development of two-dimensional (2D) graphene-based photodetectors. However, these graphene-based photodetectors are usually fabricated by an expensive photolithography and complicated transferred process. Here, a semi-transparent reduced graphene oxide (rGO) photodetector on a polyethylene terephthalate (PET) substrate with ultra-low power operation by simple processes is developed. The photodetector has achieved a transmittance about 60%, a superior responsivity of 375 mA/W and a high detectivity of 1012 Jones at a bias of -1.5 V. Even the photodetector is worked at zero bias, the photodetector exhibits a superior on/off ratio of 12. Moreover, the photoresponse of such photodetector displays little reduction after hundred times bending, revealing that the photodetector is reliable and robust. The proposed fabrication strategy of the photodetector will be beneficial to the integration of semi-transparent and low-power wearable devices in the future.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Two-dimensional graphene consists of hexagonal structured carbon atoms with properties of high carrier mobility [1], high conductivity [2], high transmittance [3], fast carrier dynamics [4,5], and flexible structure [6], which enable the realization of numerous photonic and optoelectronic applications such as photodetectors [7,8], lasers [9], modulators [7], solar cells [10,11], and electrodes [12,13]. Several methods have been developed for the graphene synthesis, such as mechanical exfoliation, chemical vapor deposition, epitaxial growth, and chemical synthesis. Although fabrication methods in the literature produce low-defect graphene, the complicated and expensive process as well as the limited size cannot meet the requirement of industrial applications.
To overcome these problems, cost-effective and scalable methods for fabricating reduced graphene oxide (rGO) have been presented [14–16]. Compare rGO to graphene, rGO has several advantages, such as low cost [17–19], excellent dispersibility in various solvents, and compatibility with various substrates [20], making rGO a potential material for optical, electrical, and bio-fields applications. For example, rGO-based photodetectors with easily modified characteristics have been widely and continuously developed. As these photodetectors absorb incident light, photons are converted into electrical signals that may be used in optical communications, wearable devices, autonomous driving systems, surveillance, and chemical/biological sensing [21–26]. However, some defects exist in rGO sheets, leading to an inefficient collection of photogenerated electron-hole pairs. As a result, such photodetector usually works under a high bias voltage and exhibits a low on/off ratio. In addition, in order to apply rGO based photodetectors in wearable or bio systems, flexible photodetectors have been widely reported [27–29], however, an issue of a high-power consumption persists.
Our focus, therefore, is to propose a cost-effective fabrication method for a flexible, semi-transparent and self-powered photodetector based on Ag/rGO/indium tin oxide (ITO)/polyethylene terephthalate (PET) structure, which allows for insights into spectral engineering that has not been previously integrated on a flexible substrate. With this design, the distribution of rGO film is modified, resulting in a low dark current and a great photoresponse. Even when the rGO photodetector operates at a low bias of -1.5 V, it exhibits an outstanding responsivity of 375 mA/W, revealing it has great potential for the application in next generation low-power wearable devices.
2. Structure and design
Effective modulation of optical properties by tuning the structural formation of photodetectors may appreciably expand the detection range and performance. Theoretically, characteristics of rGO-based photodetectors are determined by the transfer efficiency from photons to electrical signals. One method to enhance this efficiency is by controlling the rGO film and engineering the absorbing property to maximize the optical absorbance and the electron transfer length. To reduce the power consumption of the device, we integrate rGO films in a vertical asymmetric metal-rGO-metal structure, therefore, the incident light can be absorbed, and carriers will transfer to electrodes with short path length.
First, a PET substrate was rinsed with deionized (DI) water to remove particles, and 300 nm-thick ITO was deposited on a PET by sputtering. To detect the incident light, rGO flakes dispersed in ethanol were coated on an ITO/PET substrate using a spin coater. To obtain the rGO solution, GO was first synthesized by a modified Hummers’ method. The obtained GO was treated by a high temperature (>600 °C) and mixed acid reduction method to break oxygen bonds. The rGO flakes were then formed and the conductivity of rGO was improved (compared to GO) due to a formation of a planar sp2 hybridization. These rGO flakes were further modified by C, N, and O block copolymer to improve the dispersion of rGO flakes. Then, 1 wt% rGO, 0.05 wt% dispersant, and ethanol were mixed to form an absorbing material for a photodetector. To engineer the absorption spectra and improve the photoresponse of photodetectors, morphology and distribution of rGO films were precisely controlled by optimizing the first and the second spin rate of a spin coater. The first spin rate of sample 1, sample 2, sample 3, and sample 4 was 1000, 2000, 3000, and 4000 rpm, respectively; the second spin rate of sample 1, sample 2, sample 3, and sample 4 was 500, 1500, 2500, and 3500 rpm, respectively. Low-cost shadow masks were then placed on rGO/PET samples to define contact electrodes. Finally, 120 nm-thick silver finger electrodes were deposited via sputtering. The length and width of the rGO photodetector were 15 and 2 mm, respectively.
Figure 1(a) shows the schematic view of a vertical Ag/rGO/ITO/PET photodetector and Fig. 1(b) presents photographs of flexible photodetectors. It is evident that the photodetector is semi-transparent and can be bent in multi directions. In Figs. 1(c)∼1(f), the morphology of solution-processed rGO films on ITO/PET substrates was investigated by the scanning electron microscopy (SEM) for four spectra-engineered samples. The edge of one rGO flake (about 1∼5 µm in size) was obviously in contact with neighboring flakes, forming a continuous and interlinked rGO film. The wrinkled-sheet structures at edges of rGO are originated from the oxygen functional group and structural defects [30]. After spin parameters are modified, wrinkle structures and roughness of rGO film are evidently reduced [Figs. 1(e) and 1(f)], indicating a better carrier transportation and a lower dark current in photodetectors can be achieved.
Fig. 1. (a) Schematic cross-sectional view of a flexible and semi-transparent rGO photodetector. (b) Images of rGO photodetectors bent in two directions. The morphologies of rGO films of (c) sample 1, (d) sample 2, (e) sample 3, and (f) sample 4 performed by SEM. The spectra characteristics can be engineered by modifying the morphology and distribution of rGO films.
3. Experiments and discussion
We analyze Raman spectra to investigate the structural characteristics of carbon-based materials. Figure 2(a) shows the Raman characteristics of sample 1, sample 2, sample 3 and sample 4 on ITO/PET substrates. Peak at ∼1723 cm−1 of all samples are resulted from the carbonyl (C5O) stretching vibration of PET substrates [31,32]; all samples have two distinct peaks, the D band and the G band. The D band can be attributed to the breathing mode of k-point phonons of A1g symmetry, and the intensity of D band is related to the formation of sp2 domains. The G band is caused by the first-order scattering of the E2g phonons of carbon atoms, and the intensity of G band is related to the size of in-plane sp2 domain. The position of the G band helps to determine the layer of the film because the G band position is sensitive to the number of layers. The peak of the D band for all sample locates at around 1353 cm−1. The peak of the G band for sample 1, sample 2, sample 3, and sample 4 is at 1576, 1576, 1578, and 1578 cm−1, respectively. Compared with sample 1, the G peak position of sample 4 shifts from 1576 to 1578 cm−1, which is attributed to the lower accumulation of rGO flakes. As the thickness of rGO is increased, the position of G peak shifts to the lower wavenumber. The ID/IG intensity ratio is affected by the disorder or the oxidation degree [33], which is inversely proportional to the average sp2 cluster size. As spin rates are controlled (sample 3 and sample 4), the accumulations and edge wrinkles of rGO films can be minimized. Moreover, the reduced ID/IG demonstrates that the enlargement of sp2 carbon domain. Defects in rGO films are therefore reduced, resulting in a low ID/IG ratio.
Fig. 2. (a) Raman spectra of samples 1, sample 2, sample 3, and sample 4. All samples have two obvious D and G peaks. (b) XPS spectrum of block copolymer modified rGO flakes. High resolution XPS spectra of (c) C1s region and (d) O1s region of the rGO film. (e) Transmittance, (f) reflectance, and (g) absorbance of sample 1, sample 2, sample 3, and sample 4.
X-ray photoelectron spectroscopy (XPS) as shown in Fig. 2(b) was used to study the elemental composition and the environment of elements present in rGO flakes. Peaks at 285, 400, and 532 eV are corresponding to C1s, N1s, and O1s, indicating that the block copolymer modified rGO flakes are well coated on the substrate. The C1s XPS spectrum of rGO as shown in Fig. 2(c) clearly reveals that carbon bondings in different functional groups: C = C (284.6 eV), C-C (285.1 eV), C-N (285.9 eV), and C = O (287.6 eV) are presented in the rGO film. The C-N peak might be attributed to the mixture of rGO flakes and N-contained dispersion solution. The O1s (Fig. 2(d)) spectrum can also be deconvoluted into two chemical components, including C = O and C-O located at 531.0 eV and 532.6 eV, respectively [34,35].
To demonstrate the spectra engineering phenomena, transmittance, reflectance, and absorbance spectra of ITO/PET-supported rGO films for sample 1, sample 2, sample 3, and sample 4 were analyzed. In Fig. 2(e), transmittance values of these samples are ranged from 50 to 60% at a wavelength of 550 nm. When the spin rate increases, the thickness of rGO film reduces, leading to a higher transmittance. Figure 2(f) displays reflectance spectra for four samples. Then we consider the relationship between spin rates and optical properties, sample 1 shows the lowest reflectance value at a wavelength of 600 nm which is resulted from the lower spin rate and more textured rGO surface. The absorption ability and the spectra distribution of rGO films will critically affect the performance of photodetectors. Therefore, the absorbance was deeply investigated as shown in Fig. 2(g). The absorption curves for all samples are ranged from 25 to 40% in the 400 to 1600 nm wavelength region, indicating that the electron π-conjugation exists in the graphene film and the absorption is extended to the NIR region.
To explore photoelectric characteristics of rGO photodetectors, linear and logarithm I–V relationships [ Figs. 3(a)∼3(d)] of spectra-engineered samples were analyzed under 532 nm laser illumination at room temperature. The power densities of light source include 7.33, 11.75, 16.10, and 20.60 W/cm2. We use asymmetric ITO and Ag electrodes to construct semi-transparent and self-powered devices. The I-V curves don’t obviously exhibit rectifying trends because the work function between ITO and Ag is not large enough. In the future, we will modify these two electrodes to further improve transparent and self-powered characteristics of photodetectors. It is worth noting that all samples can operate at a low bias due to a precisely control of rGO films. For example, as sample 1, sample 2, sample 3, and sample 4 are worked under 20.60 W/cm2 illumination with a -0.5 V bias, they exhibit excellent light currents of 17.21, 13.72, 12.95, and 4.97mA, respectively. Sample 1 with the lowest spin rate exhibits the highest light current, however, the high dark current may be an issue for the performance of the photodetector. This is because the rough surface of sample 1 results in the high dark current and limits the responsivity and detectivity. From these experimental results, it is found that the uniformity, roughness and absorption of a rGO film simultaneously affect the performance of the photodetector. By controlling the distribution and quality of the rGO film, the photocurrent and dark current of the rGO photodetector can be effectively tuned. Compared with sample 1, the dark current of sample 4 is significantly reduced from -35.8 to -3.4 mA at the bias of -1.5 V, demonstrating that the distribution of rGO film is modified and therefore the resistance of the rGO film is reduced. To recognize the short-circuit current for sample 1, sample 2, sample 3, and sample 4, the current under 20.6 W/cm2 illumination at voltage ranging from -6 mV to 6 mV is displayed in Fig. 3(e). Due to the asymmetric electrodes and the photothermoelectric effect, samples all have non-zero short-circuit currents without an external bias. It is noticed that the short-circuit current of sample 3 displays the largest value of 131 μA due to the proper distribution or the rGO film.
Fig. 3. Current-voltage characteristics of (a) sample 1, (b) sample 2, (c) sample 3, and (d) sample 4 under dark, 7.33, 11.75, 16.10, and 20.60 W/cm2 light illuminations. (e) Current for sample 1, sample 2, sample 3, and sample 4 under 20.6 mW/cm2 illumination at voltage ranging from -6 mV to 6 mV. Photocurrent as a function of light intensity at bias of 0, -0.5, -1, and -1.5 V for (f) sample 1, (g) sample 2, (h) sample 3, and (i) sample 4.
Figures 3(f)∼(i) display photocurrent current as a function of light power density with different bias voltages for sample 1, sample 2, sample 3, and sample 4, respectively. The photocurrent can be defined by ${I_p} = {I_L} - {I_D}$, where ${I_p}$ is the photocurrent, ${I_L}$ is the light current, and ${I_D}$ is the dark current. It is noticed that even these photodetectors are operated under a low bias of -1.0 V and a 20.60 W/cm2 illumination, sample 1, sample 2, sample 3, and sample 4 exhibits an excellent photocurrent current of 13.93, 14.85, 25.02, and 10.88 mA, respectively. The relationship between the light intensity and photocurrent can be expressed as
where Iph is the photocurrent, c is a constant, P is the incident power density, and k is an empirical value [36]. The nonlinearity of the relationship reflects that complex charge transport mechanisms involving electron–hole generation, trapping, and recombination occur. Within the rGO band, the trap states can capture the photogenerated carriers and enhance the conductivity of rGO at a low light intensity. As the intensity of light power is increased, the photocurrent of most samples increases linearly with the light intensity. In contrary, the photocurrent of sample 4 achieves a saturation under a strong illumination, which is associated with the saturated filling of sensitizing centers in photoconductors [37,38]. Consequently, as the device is illuminated under a strong light intensity, the traps are all effectively filled and leads to a saturation photocurrent.To study the transportation mechanism in a photodetector, the band diagram of rGO and the transfer mechanism is descripted in Fig. 4(a). The bandgap of rGO can be tuned from ∼1.00 to 1.69 eV depending on the degree of reduction, hence, rGO behaves like a semiconductor or semi-metal. The optoelectronic characteristic of rGO with a combination of sp2 and sp3 bondings are mainly controlled by $\pi $ and ${\pi ^\ast }$ states of sp2 locations. The disorder-induced states may lie in the band tail of $\pi $ and ${\pi ^\ast }$ gap or present deep inside the gap. The disordered structure might create additional sp2 clusters which generate isolated states and facilitate the charge carrier transition by hopping [39,40]. Typically, the mechanism of generated photocurrent for layered photodetectors may contain photovoltaic effect, photoconductive effect, and photothermoelectric effect. Once the photodetector is illuminated by the light source, a rGO film absorbs photons, generates electron-hole pairs and these carriers are driven out by the external bias voltage until the photogenerated carriers recombine. During the drifting procedure, carriers may be trapped by the defect states within the bandgap or rGO/electrodes interface. If these trapped carriers are affected by the higher energy photons, they can de-trap and transport to the perspective bands. Meanwhile, as the photodetector absorbs photons, a temperature difference is constructed, which drives the diffused charge carriers from the hot to the cold region, generating a voltage difference. This charge carrier diffusion is resulted from the carrier energy or concentration gradient induced by the temperature gradient. Consequently, photodetectors operated by the photothermoelectric effect do not need an external bias. Additionally, the channel length of this photodetector is longer than the cooling length of hot carriers, the hot carrier assisted photothermoelectric effect can be neglected and the diffused carrier is caused by the temperature difference in the lattice.
Fig. 4. (a) Band diagram and carrier transfer mechanism of rGO photodetectors. Responsivity as a function of bias voltage under 7.33, 11.75, 16.10, and 20.60 W/cm2 light illumination of (b) sample 1, (c) sample 2, (d) sample 3, and (e) sample 4. (f) On/off ratio versus bias voltage for samples 1, sample 2, sample 3, and sample 4.
The responsivity (R) is defined as the ratio of the photocurrent to the power of the light incident to the detector at a certain wavelength. The responsivity is given by
where Iph is the photocurrent, P is the power density of the laser, and A is the geometrical area illuminated by the light source. Because the size of photodetectors is larger than the area the light spot, the total beam area of 1.13 mm2 is used to calculate the responsivity for all samples. Responsivity as a function of bias voltage for sample 1, sample 2, sample 3, and sample 4 are also evaluated in Figs. 4(b)∼(e), respectively. For samples 1 and 2, a responsivity increases as a light intensity is increased. In contrary, sample 3 and sample 4 with thinner rGO films have better performance under low light illumination. Through fabrication and spectra optimizations, sample 3 with a low dark current exhibits maximal responsivity of 375 mA/W under 7.33 W/cm2 illumination at a bias of -1.5 V. Compared with other flexible photodetectors fabricated by solution processes, the performance of our photodetectors is significantly improved from tens mA/W to 375 mA/W [41,42]. On/off ratios of photodetectors operated under 20.60 W/cm2 illumination with different bias voltages and zero bias (self-powered mode) are plotted in Fig. 4(f). Sample 3 with a low dark current possesses the highest on/off ratio of 12 at a zero bias, offering extendable availabilities for low-power and green device applications.The detectivity (D*) is an essential characteristic to assess the distinctiveness of photodetectors which is given by [43]
where A is the detecting area, R is the responsivity, q is the electron charge, and ${I_D}$ is the dark current. Figures 5(a)∼(d) display the calculated detectivity under 7.33, 11.76, 16.10, and 20.60 W/cm2 illumination for sample 1, sample 2, sample 3, and sample 4, respectively. Detectivities of different samples ranges from 109 to 1012 Jones and sample 3 shows the best detectivity of 1012 Jones under 7.33 W/cm2 illumination at a bias of -1.5 V. When the light power increases, detectivities may saturate owing to the saturation of the photocurrent. By minimizing wrinkles and defects in a rGO film, sample 3 with the lowest dark current exhibits the highest detectivity, which is important in the detection of a photodetector.Fig. 5. Detectivity versus applied voltages of sample 1, sample 2, sample 3, and sample 4 for (a) 7.33, (b) 11.76, (c) 16.10, and (d) 20.60 W/cm2 light illumination. Sample 3 shows the best detectivity of 1012 Jones under 7.33 W/cm2 illumination at a bias of -1.5 V.
To investigate the spectral response of the photodetector, external quantum efficiency (EQE) versus wavelength of sample 3 operated under -1.5, 1.0 and 0.5 V were taken. The EQE is defined by the number of extracted electrons per incident photon which can be expressed by the following equation [44]
where ${I_{ph}}$ is the photocurrent, ${I_d}$ is the dark current, h is Planck’s constant, c is the velocity of light, e is the electron charge, $\lambda $ is the excited wavelength, and P is the light power density. In Fig. 6(a), EQE is generally below 100% under a low bias voltage because the photogenerated charges mainly contribute to the EQE response. In contrary, if the photodetector is worked at a high bias voltage, a strong charge injection from electrodes or the trap-induced secondary injection leads the EQE increasing above 100% [45,46]. Additionally, these EQE values are stable in the wavelength region from 400 to 800 nm and the EQE trend is consistent with the absorbance spectra in Fig. 2(d). Consequently, it is demonstrated that the spectral response of the photodetector can be effectively engineered by modifying the rGO film and these photodetectors can be applied for a visible light detection in the indoor or outdoor environment. To further study the performance of the photodetector after several bending times, normalized light current as a function of bending times operated at -0.5, -1.0 and -1.5 V are executed as shown in Fig. 6(b). The light current of the photodetector with 5 and 10 bending times behaves little reduction in the current intensity. After 500 times bending, the light current of the photodetector increases first and then decreases. This change of current intensity could be processed through the back-end program design. The dynamic response of the photodetector (sample 3) illuminated by a 532 nm laer and a low power of 83 mW/cm2 was measured to ascertain the resolution and speed of the photodetector. From the figure, the response time of the photodetector is ∼1.5s and it is evident that the photodetector can detect low-power incident light with different bias voltages. In the future, we will put our effort to improve the bandwidth of the flexible photodetectors by futher modifications of shot noise, photocurrent, and dark current.Fig. 6. (a) EQE spectra of the photodetector (sample 3) operated at different voltages. (b) Normalized light current versus bending times. (c) Dynamic response for sample 3 under a 532 nm and a low power of 83 mW/cm2 illumination with different bias voltages.
4. Conclusions
In summary, we demonstrate a self-powered and semi-transparent rGO-based photodetector, featuring a low dark current and a high responsivity. Subject to a fabrication optimization, a transmittance can be well-controlled to 60% at the wavelength of 550 nm and the dark current can be effectively reduced from 35.8 to 3.4 mA at a bias of -1.5 V. It is also observed that, even when the photodetector is operated at zero bias, an on/off ratio exhibits an outstanding value of 12. Importantly, the simple-processed photodetector exhibits extraordinary performance, including a superior responsivity of 375 mA/W and a high detectivity of 1012 Jones under 7.33 W/cm2 illumination at a bias of -1.5 V. Additionally, the photoresponse of the photodetector bent 100 times displays little reduction in current intensity. This rGO photodetector enables improved optical performance that potentially leads to a new pathway for large-area, semi-transparent and low-power systems in the future.
Funding
Ministry of Science and Technology, Taiwan (MOST 107-2218-E-006-061-MY2, MOST 109-2221-E-006 -212).
Acknowledgement
We acknowledge Center for Micro/Nano Science and Technology, National Cheng Kung University and Mr. Jui-Chin Lee of Instrument Center, National Cheng Kung University for experimental support.
Disclosures
The authors declare no conflicts of interest.
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