Transparent Graphene Electrodes Research Articles

Influence of deposition temperature and hydrogen on sustainable and transfer-free graphene transparent electrode for organic solar cells

The transfer-free graphene transparent conducting electrode (TCE) is a promising alternative to indium tin oxide (ITO) for organic solar cells (OSCs). In the present work, a comprehensive investigation on how deposition temperature and H2 flow rates affect the growth, structural, optical, and electrical properties of graphene produced by RF plasma enhanced chemical vapor deposition using sustainable sources was conducted. Inverted-geometry OSCs with P3HT: PCBM photoactive layer were fabricated on transfer-free graphene TCEs developed under different conditions. Moreover, the coupling of silver nanowires (AgNWs) with different graphene films was studied for hybrid graphene-AgNWs TCEs for OSCs. Devices based on graphene TCEs prepared at low or zero H2 flow have shown better performances than those at high flow of H2. Similarly, graphene TCEs prepared at high temperature (>700 °C, on quartz) led to a deteriorated device performance due to the highly increased growth of vertically oriented graphene nanosheets, which dramatically reduced film transmittance and increased surface roughness. The present work provides solid understanding of the growth mechanism of RF-PECVD graphene on glass from a sustainable carbon source. More importantly, the sustainable, ecofriendly, cost- and time-effective production of scalable transfer-free graphene TCEs for OSCs is optimized which paves the way towards ITO-free optoelectronics.

Stably doped graphene transparent electrode with improved light-extraction for efficient flexible organic light-emitting diodes

Stably doped graphene transparent electrode with improved light-extraction for efficient flexible organic light-emitting diodes

Transparent shape memory polyimide enables OLED for smart deformation

An innovative process for producing flexible transparent graphene electrodes has been proposed. The electrochemically polished copper-foil was used as the metal substrate to prepare high-quality monolayer graphene. Monolayer graphene was transferred from copper-foil to transparent shape memory polyimide (TSMPI) intactly by novel flow-casting transfer technology, and the smart, ultra-clean and smooth flexible graphene electrode (pG@TSMPI) was fabricated. The light transmittance of pG@TSMPI at 550 nm is 86%, the average sheet resistance is 310 Ω/sq, and the surface roughness (Ra) is 18 nm. Smart white OLED was manufactured using pG@TSMPI as transparent anode, and its luminescence performance was comparable to that of ITO-based device. Depending on the shape memory effect of TSMPI substrate, the smart OLED has the superiorities of variable stiffness and reconfigurable shape, which can transform from 2D structure into complex 3D structure.

Carrier Transport Regulation of Pixel Graphene Transparent Electrodes for Active-Matrix Organic Light-Emitting Diode Display.

Integrating a graphene transparent electrode (TE) matrix with driving circuits is essential for the practical use of graphene in optoelectronics such as active-matrix organic light-emitting diode (OLED) display, however it is disabled by the transport of carriers between graphene pixels after deposition of a semiconductor functional layer caused by the atomic thickness of graphene. Here, the carrier transport regulation of a graphene TE matrix by using an insulating polyethyleneimine (PEIE) layer isreported. The PEIE forms an ultrathin uniform film (≤10nm) to fill the gap of the graphene matrix, blocking horizontal electron transport between graphene pixels. Meanwhile, it can reduce the work function of graphene, improving the vertical electron injection through electron tunneling. This enables the fabrication of inverted OLED pixels with record high current and power efficiencies of 90.7cd A-1 and 89.1lm W-1 , respectively. By integrating these inverted OLED pixels with a carbon nanotube-based thin-film transistor (CNT-TFT)-driven circuit, an inch-size flexible active-matrix OLED display isdemonstrated, in which all OLED pixels are independently controlled by CNT-TFTs. This research paves a way for the application of graphene-like atomically thin TE pixels in flexible optoelectronics such as displays, smart wearables, and free-form surface lighting.

Semitransparent Solar Cells Employing n-Type Graphene on LaVO3.

To effectively utilize solar energy, semitransparent solar cells are essential in various fields such as building-integrated solar power generation and portable solar chargers. We report triethylenetetramine (TETA)-doped graphene (Gr) transparent conductive electrode (TCE)-based LaVO3 semitransparent solar cells. To optimize the Gr TCE, we varied the TETA molar concentration (nD) from 0.1 to 0.3 mM. TETA-doped Gr (TETA-Gr)/LaVO3 semitransparent solar cells exhibit the highest 1.45% efficiency and 62% average visible transmittance at nD = 0.2 mM. These results indicate that the TETA-Gr/LaVO3 structure not only harvests solar energy in the ultraviolet-visible region but also exhibits translucency, thanks to the thin film. Thanks to its translucent properties, we improved the power conversion efficiency (PCE) to 1.99% by adding an Al reflective mirror to the semitransparent cells. Finally, the device's PCE loss is only within 3% for 3000 h in air, suggesting good durability.

Highly Sensitive Ultraviolet Photodetector Based on an AlGaN/GaN HEMT with Graphene‐On‐p‐GaN Mesa Structure

AbstractThe advantageous role of 2D electron gas presence at the AlGaN/GaN interface attracts huge interest in the field of GaN‐based ultraviolet photodetector technology. However, the presence of high dark current deteriorates the photodetector performance by diminishing several figures of merit. In this work, enhanced figures of merit are demonstrated by employing interdigitated p‐GaN finger structure on the top of the AlGaN/GaN heterostructure. The commonly present high dark current in p‐GaN/AlGaN/GaN planar photodetector is largely reduced (from ≈µA to few pA) by etching the p‐GaN, excluding the electrode region. Furthermore, by using a graphene transparent electrode along with the p‐GaN interdigitated fingers on AlGaN/GaN heterostructure, ultraviolet photodetectors with superior sensitivity (3.55 × 106) and ultrahigh detectivity (1.91 × 1014 cm Hz1/2 W−1) are realized at 360 nm. A comparison of graphene/p‐GaN and Ni/Au/p‐GaN interdigitated fingers and planar p‐GaN (with interdigitated graphene contacts) all on AlGaN/GaN heterostructure allows to understand the dominant roles of electrode transparency and the heterojunction structure. The simple and high electron mobility transistor‐compatible fabrication process of UV detectors provides a unique application in the field of UV sensing technology.

High-performance self-powered ultraviolet photodetector in SnO2 microwire/p-GaN heterojunction using graphene as charge collection medium

• Using a facile vapor transport deposition method, the samples of SnO 2 microwires were synthesized individually self-catalyzed by a fast and simple CVD method. By combining p-type GaN substrate, a single SnO 2 microwire heterojunction photodetector working in the ultraviolet wavelengths was constructed. • By replacing the top electrodes from ITO to monolayer graphene, enhanced ultraviolet photoresponses were obtained, which exhibiting a peak responsivity 223.7 mA/W, detectivity ∼ 6.9 × 10 12 Jones, high EQE of 77%, and a fast response speed (rising/decaying times ∼ 18/580 μs) at 360 nm light illumination when the device is operated in self-driven mode. Our results suggest that the graphene transparent electrodes can not only allow the majority of the incident light to reach the contact area but also offer an easy carrier transport channel when the electron-holes were separated. • In particular, the microscopic photoresponse mechanism of the fabricated n-SnO 2 microwire/p-GaN heterojunction photodetector that using monolayer graphene as charge collection medium was studied. The Schottky-potential is formed by the built-in electric-field from graphene to the SnO 2 wire. The built-in field is in the same direction as the original SnO 2 /GaN built-in electric field, which was similar to the reverse voltage applied to the heterojunction. Therefore, the net built-in field toward the SnO 2 /GaN heterointerface can be increased, resulting in the increment of the carrier collection efficiency through graphene as a charge collection medium. Graphene monolayer has been extensively applied as a transparency electrode material in photoelectronic devices due to its high transmittance, high carrier mobility, and ultrafast carrier dynamics. In this study, a high-performance self-powered photodetector, which is made of a SnO 2 microwire, p-type GaN film, and monolayer graphene transparent electrode, was proposed and fabricated. The detector is sensitive to ultraviolet light signals and illustrates pronounced detection performances, including a peak responsivity ∼ 223.7 mA W –1 , a detectivity ∼ 6.9 × 10 12 Jones, fast response speed (rising/decaying times ∼ 18/580 μs), and excellent external quantum efficiency ∼ 77% at 360 nm light illumination without external power supply. Compared with the pristine SnO 2 /GaN photodetector using ITO electrode, the device performances of responsivity and detectivity are significantly increased over 6 × 10 3 % and 3 × 10 3 %, respectively. The performance-enhanced characteristics are mainly attributed to the high-quality heterointerface of n-SnO 2 /p-GaN, the highly conductive capacity, and the unique transparency of graphene electrodes. Particularly, the built-in potential formed at the SnO 2 /GaN heterojunction interface could be strengthened by the Schottky potential barrier derived from the graphene electrode and SnO 2 wire, enhancing the carrier collection efficiency through graphene as a charge collection medium. This work is of great importance and significance to developing excellent-performance ultraviolet photodetectors for photovoltaic and optoelectronic applications in a self-powered operation manner.

Photovoltaics literature survey (no. 182)

Photovoltaics literature survey (no. 182)

Graphene-based microfluidic perforated microelectrode arrays for retinal electrophysiological studies.

Perforated microelectrode arrays (pMEAs) have become essential tools for ex vivo retinal electrophysiological studies. pMEAs increase the nutrient supply to the explant and alleviate the accentuated curvature of the retina, allowing for long-term culture and intimate contacts between the retina and electrodes for electrophysiological measurements. However, commercial pMEAs are not compatible with in situ high-resolution optical imaging and lack the capability of controlling the local microenvironment, which are highly desirable features for relating function to anatomy and probing physiological and pathological mechanisms in retina. Here we report on microfluidic pMEAs (μpMEAs) that combine transparent graphene electrodes and the capability of locally delivering chemical stimulation. We demonstrate the potential of μpMEAs by measuring the electrical response of ganglion cells to locally delivered high K+ stimulation under controlled microenvironments. Importantly, the capability for high-resolution confocal imaging of the retina tissue on top of the graphene electrodes allows for further analyses of the electrical signal source. The new capabilities provided by μpMEAs could allow for retinal electrophysiology assays to address key questions in retinal circuitry studies.

Plasmon-enhanced Graphene/4H–SiC /graphene metal-semiconductor-metal ultraviolet photodetector: Concept and optimization

In this paper, a hybrid approach that combines a light trapping formalism and a metaheuristic optimization approach is suggested to enhance speed and sensing capabilities of a thin film/4H–SiC photodetector (PD). To overcome the shadowing problem, interdigitated electrodes made of graphene are employed. Besides, Ag nanoparticles are inserted to enhance photon absorption in the 4H–SiC active area. To avoid the complexity of considering all randomly deposited nanoparticles, the Maxwell-Garnet theory-based dielectric method is employed. The impact of plasmonic engineering factors on the photodetector characteristics is studied. By using embedded Ag nanoparticles and highly transparent graphene electrodes, the proposed device exhibits a maximum responsivity of 269.6 mA/W, a high photocurrent to dark current ratio (PDCR) of 1.7 × 106, a linear-dynamic-range (LDR) of 272 dB, a response time τ = 13.45 μs, and a detectivity of 5.4 × 1013 Jones. In addition, the suggested model is regarded as a fitness function to be used for a Multi Objective Genetic Algorithm (MOGA) optimization method which optimizes the sensing ability of the device and its speed. The optimized design discloses optimum performances in terms of responsivity (564.5 mA/W), PDCR (3.75 × 106), LDR (301 dB), and detectivity (1.25 × 1014 Jones). Also, it balances the compromise between sensing performance and response time (τ = 4.7 μs). Our results show that using an efficient light trapping formalism and a metaheuristic optimization technique, it is possible to notably improve the photodetector sensitivity.

Efficient optical phase modulator based on an III–V metal-oxide-semiconductor structure with a doped graphene transparent electrode

We propose a III–V metal-oxide-semiconductor (MOS) optical modulator with a graphene gate electrode along with the analysis of the modulation properties. With p-type doped graphene used as a transparent gate electrode, we can fully utilize the electron-induced refractive index change in an n-type InGaAsP waveguide with the reduction of the hole-induced optical absorption observed in a III–V/Si hybrid MOS optical modulator. Numerical analysis displays that up to the phase modulation efficiency of 0.82 V·cm and 0.22 dB optical loss for π phase shift can be achieved when the gate oxide thickness is 100 nm. With the elimination of the unnecessary parasitic capacitance found in the overlapping of graphene on the slab part of the waveguide, in conjunction with the high electron mobility in InGaAsP, the device also enables a modulation bandwidth of greater than 200 GHz.

Boosting Phototransistor Performance in Monolayer TMDs via Multiple Reflections from DBR.

Transition-metal dichalcogenides (TMDs) are intensively studied for high-performance phototransistors. However, the device performance is limited by the single photoexcitation. Here, we show a unique strategy in which phototransistor performance can be boosted by fabricating the device on top of a distributed Bragg reflector (DBR). Monolayer molybdenum disulfide (MoS2) and tungsten disulfide (WS2) phototransistors were fabricated on DBR and SiO2 substrates for comparison. Furthermore, phototransistor performances including photocurrent, responsivity, photoinduced mobility, and subthreshold swing highlight 582 times enhancement in photoresponsivity ratio and 350 times enhancement in photocurrent ratio in the DBR sample using transparent graphene electrode and hBN encapsulation.

Analysis of the Effect of Graphene, Metal, and Metal Oxide Transparent Electrodes on the Performance of Organic Optoelectronic Devices.

Transparent electrodes (TEs) are important components in organic optoelectronic devices. ITO is the mostly applied TE material, which is costly and inferior in mechanical performance, and could not satisfy the versatile need for the next generation of transparent optoelectronic devices. Recently, many new TE materials emerged to try to overcome the deficiency of ITO, including graphene, ultrathin metal, and oxide-metal-oxide structure. By finely control of the fabrication techniques, the main properties of conductivity, transmittance, and mechanical stability, have been studied in the literatures, and their applicability in the potential optoelectronic devices has been reported. Herein, in this work, we summarized the recent progress of the TE materials applied in optoelectronic devices by focusing on the fabrication, properties, such as Graphene, ultra-thin metal film, and metal oxide and performance. The advantages and insufficiencies of these materials as TEs have been summarized and the future development aspects have been pointed out to guide the design and fabrication TE materials in the next generation of transparent optoelectronic devices.

Single-step, catalyst-free, and green synthesis of graphene transparent electrode for organic photovoltaics

Graphene is a promising transparent conducting electrode (TCE) for organic photovoltaics (OPVs) due to its outstanding optical and electrical properties. However, the high cost and complexity of graphene synthesis on transparent substrates as well as the use of hazardous carbon sources limits the large-scale application of graphene TCEs. This work reports a single-step synthesis of graphene TCEs on glass substrates for OPVs from plant extract by Radio frequency plasma enhanced chemical vapor deposition. This approach provides fast, cheap, and catalyst-free production of graphene from a sustainable and environmentally-friendly carbon resource.The as-prepared graphene films reveal high quality structure with promising optical and electrical properties. Inverted OPV devices built on the as-synthesized graphene TCEs exhibit 80% of the power conversion efficiency of their indium tin oxide counterparts, and higher short circuit current density and open circuit voltage.

Graphene/PtSe2/Pyramid Si van Der Waals Schottky Junction for Room-Temperature Broadband Infrared Light Detection

Uncooled infrared (IR) photodetection has attracted increasing research interest due to its important applications in civil and military fields. Recently, platinum diselenide (PtSe2), a newly discovered group-10 two-dimensional (2D) noble metal dichalcogenide (NMD) member, has emerged as an attractive candidate for highly sensitive IR photodetection due to its layer-dependent bandgap transition from semiconductor to semimetal, wide optical absorption, and high carrier mobility. Here, we demonstrate the successful assembly of PtSe2/pyramid Si mixed-dimensional van der Waals (vdW) Schottky junction with a graphene transparent electrode. Due to the novel vertical device structure with a graphene top contact, the photodetector achieves an appealing device performance, including an ultrabroadband response up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10.6~\mu \text{m}$ </tex-math></inline-formula> , a high responsivity of 0.528 A/W, a large specific detectivity of ~1012 Jones, and a fast response time of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$8.2~\mu \text{s}$ </tex-math></inline-formula> at zero bias voltage. More importantly, these findings have enabled the realization of an excellent room-temperature long-wave IR (LWIR) imaging capability and its utilization as an optical receiver in optical IR communication. Our work demonstrates a reliable approach to the construction of a high-performance Schottky junction device for room-temperature broadband IR photodetection.

A System of Rapidly Detecting Escherichia Coli in Food Based on a Nanoprobe and Improved ATP Bioluminescence Technology.

Bacterial contamination is an important factor causing food security issues. Among the bacteria, Escherichia coli is one of the main pathogens of food-borne microorganisms. However, traditional bacterial detection approaches cannot meet the requirements of real-time and on-site detection. Thus, it is of great significance to develop a rapid and accurate detection of bacteria in food to ensure food safety and safeguard human health. The pathogen heat-treatment module was designed in this paper based on the techniques including nanoprobe, pathogen heat-treatment, graphene transparent electrode (GTE), and adenosine triphosphate (ATP) bioluminescence technology. The system mainly consists of two parts: one is the optical detection unit; the other is the data processing unit. And it can quickly and automatically detect the number of bacterial colonies in food such as milk etc. The system uses not only the probe to capture and enrich E. coli by antigen-antibody interaction but also the heat treatment to increase the amount of ATP released from bacterial cells within five minutes. To enhance the detecting accuracy and sensitivity, the electric field generated by GTE is adopted in the system to enrich ATP. Compared to the other conventional methods, the linear correlation coefficient of the system can be reached 0.975, and the system meets the design requirements. Under the optimal experimental conditions, the detection can be completed within 25 min, and the detectable concentration of bacteria is in the range of 3.1 × 101–106 CFU/mL. This system satisfies the demands of a fast and on-site inspection.

Life cycle assessment of organic solar cells and perovskite solar cells with graphene transparent electrodes

Indium Tin Oxide as the transparent electrode in solar cells has shown a bottleneck due to the use of scarce mental. The graphene transparent electrode (GTE) opens a sustainable route for third-generation solar cells. This work investigates the environmental performance of flexible organic solar cells and perovskite solar cells with GTEs by life cycle assessment. The manufacturing process of solar cells is developed including detailed production procedures of GTEs. The comprehensive life cycle inventories (LCIs) are provided for two kinds of solar cells. According to LCIs, life cycle impact assessment is carried on to unmask various impact categories in their full life cycles. Sensitivity analyses using the Monte Carlo method are conducted to quantify the uncertainty of final environmental impacts. The results prove that perovskite solar cells with GTEs have the shortest Energy Payback Time. A longer lifespan of both solar cells should be pursued to meet the requirement of sustainable manufacture.

Electrochemical Transparency of Graphene.

In the present study, we used the electrochemical transparency of graphene to show that the direct intercalation of alkali-metal cations is not a prerequisite for the redox reaction of Prussian blue (PB). PB thin films passivated with monolayer graphene still underwent electrochemical redox reactions in the presence of alkali-metal ions (K+ or Na+) despite the inability of the cations to penetrate the graphene and be incorporated into the PB. Graphene passivation not only preserved the electrochemical activity of the PB but also substantially enhanced the stability of the PB. As a proof of concept, we showed that a transparent graphene electrode covering PB can be used as an excellent hydrogen peroxide transducer, thereby demonstrating the possibility of realizing an electrochemical sensor capable of long-term measurements.

Enhancing the efficiency and stability of Organic/Silicon solar cells using graphene electrode and Double-layer Anti-reflection coating

Silicon (Si) based organic/inorganic hybrid solar cells have attracted much attentions due to their low-temperature solution process and high power conversion efficiency (PCE). However, the use of metallic grids electrode impedes the efficient light absorption, and the hybrid solar cells still suffer from poor air stability. Here, we demonstrate the fabrication of high-efficiency, air stable organic/Si heterojunction solar cells by using a graphene transparent electrode as well as a double-layer anti-reflection coating (DL-ARC). The use of highly transparent graphene film could avoid the strong light blocking effect of conventional metallic grids electrode and ensure the effective carrier collection. Meantime, the light-harvesting of Si wafer is remarkably enhanced by the DL-ARC based on polymethyl methacrylate (PMMA) and Poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) stacking layer. The assembled organic/Si heterojunction devices present outstanding photovoltaic performance with a PCE as high as 13.01% under AM 1.5G irradiation. More importantly, thanks to the protective effect of graphene/PMMA layer, the devices exhibited an excellent stability, and 94% of the pristine efficiency could be maintained after storing in air for 4 months without any encapsulation. Our work paves the way towards the fabrication of hybrid solar cells with high performance and stability.

Ultrathin, transparent, flexible, and dual-side white light-responsive two-dimensional molybdenum disulfide quantum disk light-emitting diodes

Ultrathin, transparent, and flexible devices with multifunctionalities are indispensable for emerging technologies. However, the related reports are very few, and the efficiency is rather limited owing to material restriction and complicated fabrication processes. In this work, a light-responsive white light-emitting diode (LED) with ultrathin, transparency, dual-side emission, and flexible characteristics based on graphene-insulator-semiconductor (GIS) structure is demonstrated. The GIS light-responsive white LED consists of highly luminescent histidine-doped two-dimensional molybdenum disulfide (MoS2) quantum disks, an Al2O3 insulating layer, and a graphene transparent electrode. By applying an external bias, it enables to generate white light emission with a high emission efficiency of around 9%. The underlying mechanism can be well understood by quantum tunneling. In addition, the GIS structure also shows good durability under a bending test of 200 cycles. Unlike conventional white LEDs, which always required multiple emissive layers, our GIS light-responsive LED is only based on a single emissive material of histidine-doped MoS2 quantum disks, which possess a broadband emission property and low toxicity. Therefore, with the superior multiple functionalities, our study shown here is very useful for the future development of next-generation optoelectronic devices.