Graphene Transparent Conductive Electrodes Research Articles

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.

High uniformity and stability of graphene transparent conducting electrodes by dual-side doping

Chemical doping is an efficient method to tailor the electrical properties of graphene transparent conductive electrodes. In general, chemically doped graphene by single-side exhibits a drawback of high conductivity but inferior uniformity and stability after exposure to chemical solvent or annealing process. Here, we report a highly uniform and stable graphene transparent conducting electrodes doped by dual-side with macro- and small molecular organic dopants such as Nafion on the top and benzimidazole (BI) at the bottom. The electrical properties, optical properties, and stability were compared depending on the top-side dopants. Dual-side doping showed a higher work function (>5 eV), and a uniform low sheet resistance (less than 200 Ω sq−1) compared to the single-side doping. The Dual-N exhibited a relatively higher figure of merit (FoM, σDC/σop ∼ 62.38), a smoother surface (Rrms ∼ 0.54 nm), and a superior thermal/chemical stability than the Dual-A, showing the potential possibility as alternative electrodes for next-generation flexible electronic devices.

Photostable electron-transport-layer-free flexible graphene quantum dots/perovskite solar cells by employing bathocuproine interlayer

Electron transport layer (ETL)-free perovskite solar cells (PSCs) are very useful for the low-temperature process that can reduce the instability caused by the ETL such as TiO2 layer. There are several publications on ETL-free PSCs, but most of them are based on transparent conductive oxides, not suitable for flexible applications. Here, we first report photostable ETL-free PSCs of maximum 15.72% power conversion efficiency (PCE) by employing an insulating interlayer of bathocuproine (BCP) between the graphene (GR) transparent conductive electrode (TCE) and the graphene quantum dots-added perovskite layer. The BCP interlayer effectively enhances the electron carrier injection by reducing the recombination centers at the active layer/GR TCE interface. Only 8 or 17% of the original PCE is lost under ambient conditions or under light soaking, respectively for 500 h. A flexible-type of the PSC shows a PCE comparable to that of the rigid-type counterpart, and 83% of the initial PCE is maintained even after 1000 repeated bending tests at a radius of 4 mm. These results suggest that the ETL-free PSCs exhibit excellent long-term and mechanical stabilities.

Porous silicon solar cells with 13.66% efficiency achieved by employing graphene-quantum-dots interfacial layer, doped-graphene electrode, and bathocuproine back-surface passivation layer

Formation of Porous Si (PSi) enhances the light absorption of Si thanks to the increased surface-to-volume ratio, useful for improving the power conversion efficiency (PCE) of Si-based solar cells. Here, we first employ graphene quantum dots (GQDs) as an interfacial layer for better band alignment at the graphene (GR) transparent conductive electrode (TCE)/PSi interface. The PCE exhibits a maximum of 13.66%, highest ever achieved in GR/PSi solar cells, by doping the GR TCE and passivating the Si back surface in addition to the GQDs interfacial layer. The use of the interfacial and the passivating layers results in the reduction of the carrier recombination at the GR TCE/PSi and Si/cathode interfaces, respectively. The cell also shows excellent long-term stability by maintaining 84% of its original PCE even in air for 30 days.

Enhancement of efficiency and stability in organic solar cells by employing MoS2 transport layer, graphene electrode, and graphene quantum dots-added active layer

For enhancing efficiency and flexibility of organic solar cells, new constituent materials are highly required. Here, we first report flexible organic solar cells (FOSCs) by employing MoS2 hole transport layer (HTL), bis(trifluoromethanesulfonyl)-amide-doped graphene (TFSA-GR) transparent conductive electrode (TCE), and GR quantum dots (GQDs)-added active layer. Power conversion efficiency (PCE) of the FOSCs without GQDs strongly depends on number of layers (Ln) of MoS2 as well as on doping concentration (nD) of TFSA-GR, thereby showing maximum PCE of 3.56% at Ln = 2 and nD = 20 mM, resulting from the lowest resistance at the TCE/MoS2/active layer interfaces. The long-term stability of the FOSCs is almost two times better than that of their counterparts with conventional organic HTL. By adding GQDs in the active layer, the PCE is further enhanced to 4.23% and is maintained at 89/82% of the original value after inner/outer repeated bending tests for 1000 cycles, respectively, indicating outstanding mechanical stability. These results suggest that GQDs are very useful for the active layer of FOSCs with better PCE and flexibility.

Extremely efficient flexible organic solar cells with a graphene transparent anode: Dependence on number of layers and doping of graphene

Graphene has shown tremendous potential as a transparent conductive electrode (TCE) for flexible organic solar cells (OSCs). However, the trade-off between electrical conductance and transparency as well as surface roughness of the graphene TCE with increasing layer number limits power conversion efficiency (PCE) enhancement and its use for large-area OSCs. Here, we use a 300 nm-thick poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c]-[1,2,5]thiadiazole)]:[6,6]-phenyl-C71-butyric acid methyl ester blend as the photoactive layer and a benzimidazole (BI)-doped graphene as the transparent anode to demonstrate efficient OSCs with good flexibility. It is found that 3 layer (L) graphene had the best balance between sheet resistance, optical transmittance and surface roughness for optimized cell design. A 0.2 cm2 cell with a 3L BI-doped graphene anode had a PCE of 6.85%, which is one of the highest PCE values reported so far for flexible graphene anode-based OSCs. The flexible cells were mechanically robust, showing only a small performance degradation during up to 250 flexing cycles. Moreover, the combination of the thick photoactive layer with the optimized 3L BI-doped graphene TCE enabled production of 1.6 cm2 flexible OSCs with a PCE of 1.8%. Our work illustrates the importance of graphene TCE development for flexible OSCs as well as other wearable optoelectronic devices.

Performance enhancement of graphene/porous Si solar cells by employing layer-controlled MoS2

Porous Si (PSi) is attractive due to its high light absorption originating from the large surface to volume ratio and additional benefits such as surface passivation and texturization effects, thereby possibly improving Si-based solar cells. Here, we first employ (trifluoromethanesulfonyl)-amide (TFSA)-doped graphene (TFSA-GR) transparent conductive electrode and MoS2 interlayer in GR/PSi solar cells. The photovoltaic parameters of the cell strongly depend on the doping concentration (nD) of the TFSA- GR and the number of layers (Ln) of the MoS2. In particular, the MoS2 interlayer well adjusts the band alignment between the TFSA-GR and PSi, thereby facilitating the separation/collection of carriers, resulting in the enhancement of the photovoltaic parameters. TiOx passivation is also used for reducing the recombination loss at the back surface. The TFSA-GR/MoS2/PSi/Si/TiOx solar cells show maximum power conversion efficiency (PCE) of 13.18% at nD = 20 mM and Ln = 4. While the PCE is monitored under 20–22 °C and 40% humidity for 30 days, it shows only a 15% loss, excellent long-term stability.

Enhancing carrier transport and carrier capture with a good current spreading characteristic via graphene transparent conductive electrodes in InGaN/GaN multiple-quantum-well light emitting diodes

In this work, InGaN/GaN multiple-quantum-wells light-emitting diodes with and without graphene transparent conductive electrodes are studied with current-voltage, electroluminescence, and time-resolved electroluminescence (TREL) measurements. The results demonstrate that the applications of graphene electrodes on LED devices will spread injection carriers more uniformly into the active region and therefore result in a larger current density, broader luminescence area, and stronger EL intensity. In addition, the TREL data will be further analyzed by employing a 2-N theoretical model of carrier transport, capture, and escape processes. The combined experimental and theoretical results clearly indicate that those LEDs with graphene transparent conductive electrodes at p-junctions will have a shorter hole transport time along the lateral direction and thus a more efficient current spreading and a larger luminescence area. In addition, a shorter hole transport time will also expedite hole capture processes and result in a shorter capture time and better light emitting efficiency. Furthermore, as more carrier injected into the active regions of LEDs, thanks to graphene transparent conductive electrodes, excessive carriers need more time to proceed carrier recombination processes in QWs and result in a longer carrier recombination time. In short, the LED samples, with the help of graphene electrodes, are shown to have a better carrier transport efficiency, better carrier capture efficiency, and more electron-hole recombination. These research results provide important information for the carrier transport, carrier capture, and recombination processes in InGaN/GaN MQW LEDs with graphene transparent conductive electrodes.

Hot-Electron Photodetection Based on Graphene Transparent Conductive Electrode

Excited in metal nanoantennas by surface plasmons, the hot electrons with high energy can pass through the Schottky barrier between metal and semiconductor to form photocurrent. Because the energy required to excite hot-electrons is lower than the bandgap of the semiconductor of silicon, the photodetectors can obtain a wider detection spectrum, including the communication wavelengths. In recent years, the two-dimensional material of graphene has attracted wide attention due to its excellent optical transmissivity and electrical conductivity. In this paper, hot-electron photodetectors based on graphene transparent conductive electrode are proposed. Compared with the device utilizing Indium Tin Oxide (ITO) as transparent conductive electrode, the responsivity of the proposed device is greatly enhanced, which achieves 27.5 nA/mW and 17.4 nA/mW for wavelength of 1520 nm and 1550 nm respectively. This study shows that the proposed hot-electron photodetector with graphene transparent conductive electrode possesses great potential in optical communication.

Self-powered and flexible perovskite photodiode/solar cell bifunctional devices with MoS2 hole transport layer

Hybrid organic-inorganic perovskites are highly attractive for the use in optoelectronic devices, but their instabilities should be solved before the practical applications. As one of the solutions, it is important to find a transport layer that can improve the stability and durability of the devices. Here, we first employ MoS2 for a hole transport layer (HTL) in high-performance flexible p-i-n-type perovskite photodiode (PD)/solar cell bifunctional devices (PPSBs) with co-doped graphene transparent conductive electrodes. The current of the PPSB increases by up to 106 times by illumination even at 0 V, meaning “self-powered”. The PPSB exhibits high responsivity and on/off ratio in a broad spectral range of ultraviolet to visible light at a PD mode and good photovoltaic properties at a solar cell mode. The photoresponse shows only 38% degradation during 30 days, and the photo-stability is almost perfect under continuous light soaking for 100 h. Flexible PPSB exhibits excellent mechanical properties by maintaining ~57% of its initial photocurrent even after 3000 bending cycles at a curvature radius of 4 mm. These results suggest that MoS2 films can be successfully used as a HTL in perovskites-based rigid/flexible optoelectronic devices.

Enhancement of Stability of Inverted Flexible Perovskite Solar Cells by Employing Graphene-Quantum-Dots Hole Transport Layer and Graphene Transparent Electrode Codoped with Gold Nanoparticles and Bis(trifluoromethanesulfonyl)amide

We first report p-i-n-type perovskite solar cells (PSCs) using graphene quantum dots (GQDs) hole transport layer (HTL) and graphene transparent conductive electrode codoped with gold nanoparticles ...

High-detectivity and -stability multilayer-graphene/Si-quantum-dot photodetectors with TiOx back-surface passivation layer

We first report Si quantum dots (SQDs)-embedded SiO2 (SQDs:SiO2) multilayers (MLs)/Si photodetectors (PDs) using multilayer graphene (MLG) transparent conductive electrode (TCE) and TiOx back-surface passivation layer. With increasing number of graphene layer (Ln) to 5, the sheet resistance of the MLG TCE sharply decreases to ∼273 Ω/sq whilst the work function gradually increases to ∼4.71 eV, indicating p-type doping, useful for collection of photo-excited carriers at the MLG/SQDs:SiO2 MLs interface. At Ln = 4, the PD without TiOx exhibits best performance of 0.340 AW-1 responsivity (R) and 3.21 × 109 cm Hz1/2 W−1 specific detectivity (D*) at the peak wavelength. With TiOx on the back side of Si wafer (Ln = 4), the R slightly increases to 0.351 AW-1, but the D* is greatly enhanced to 9.42 × 1011 cm Hz1/2 W−1, resulting from the sharp reduction of the dark current by the suppression of the carrier recombination at the Si/TiOx interface. The PD performance is almost consistent for 1000 h in air, irrespective of the use of the TiOx layer. These behaviors are much better than ever achieved in graphene/Si wafer PDs.

N-i-p-type perovskite solar cells employing n-type graphene transparent conductive electrodes

Despite intensive studies on perovskite solar cells (PSCs) by employing graphene transparent conductive electrodes (GR TCEs), no use of n-type GR as a TCE has been reported for PSCs. It is known to be technologically more easy to prepare p-type GR TCEs than n-type ones because even pristine GR usually exhibits p-type characteristics in air. P-type GR can be employed in p-i-n-type PSCs as a TCE, but not in n-i-p-type ones. In addition, most of the p-i-n-type structures are not suitable for flexible substrates due to the inherent high-temperature annealing process. Here, n-type (Ag nanowires-doped) GR TCEs are first used for n-i-p CH3NH3PbI3 (MAPbI3) PSCs. With increasing the doping concentration (nA) of Ag nanowires to 0.3 wt%, the sheet resistance (Rs) and transmittance (T) of the TCE monotonically decrease to ∼52 Ω/sq and ∼90%, respectively. Due to the nA-dependent trade-off relation between the Rs and the T, the DC conductivity/optical conductivity ratio is the largest (∼62) at nA = 0.1 wt%, resulting in maximum power conversion efficiencies (PCEs) of 15.80 and 13.45% for the PSCs on rigid and flexible substrates, respectively, indicating PCE enhancements more than 50 and 30% by using the n-type GR TCEs.

High-Performance n-i-p-Type Perovskite Photodetectors Employing Graphene-Transparent Conductive Electrodes N-Type Doped with Amine Group Molecules

We first report high-performance n-i-p-type perovskite photodetectors (PDs) by employing n-type graphene (GR) transparent conductive electrodes (TCEs). The n-type GR is prepared by doping with ethylene diamine, diethylene triamine, and triethylene tetramine (TETA) containing two, three, and four amine groups, respectively. With increasing the number of amine groups (An) to four, the sheet resistance of the doped GR TCE is reduced to ∼205 Ω/sq with only an ∼1% decrease of the transmittance at 550 nm while the work function is gradually enhanced to ∼4.46 eV, meaning n-type doping. The PDs optimized at An = 4 exhibit 106 photocurrent/dark current ratio, 0.343 AW–1 responsivity (R), 5.82 × 109 cm Hz1/2-W-1 specific detectivity, 108 dB linear dynamic range, and 1.02 μs response time, comparable to the performances of the previously reported indium tin oxide-based perovskite PDs. In addition, flexible perovskite PDs with TFSA-doped GR TCEs show excellent bending stability, maintaining approximately 70% of the o...

Significantly-enhanced Stabilities in Flexible Hybrid Organic-Inorganic Perovskite Resistive Random Access Memories by Employing Multilayer Graphene Transparent Conductive Electrodes

Despite several reports on perovskite resistive random access memories (RRAMs), most of them are based on transparent conductive oxide electrodes, not suitable for high flexibility. We first employ multilayer-graphene (MLG) transparent conductive electrodes (TCEs) for flexible hybrid organic-inorganic perovskite RRAMs, showing reversible bipolar resistive switching behaviors with about 0.68 and −0.5 V as set and reset bias voltages, respectively. The low-resistance state (LRS) and high-resistance state (HRS) of the RRAMs are almost constant even by data retention, switching, and bending for >104 s, > 500 cycles, and > 1000 cycles, respectively. The I-V curve at the HRS during the set process is consistent with the Ohm’s law for small voltages, but is well described by the space-charge-limited conduction mechanism for large voltages. The Ohmic conduction is also observed at the LRS during the set process. These behaviors are similarly repeated during the reset process, and well explained based on defect migration and charge trapping. These results suggest that perovskite RRAMs can be remarkably stable enough for practical applications by employing MLG TCEs.

Remarkable enhancement of stability in high-efficiency Si-quantum-dot heterojunction solar cells by employing bis(trifluoromethanesulfonyl)-amide as a dopant for graphene transparent conductive electrodes

The efficiency and long-term stability of graphene (GR)/Si quantum dots-embedded SiO2 heterojunction solar cells are remarkably enhanced by employing bis(trifluoromethane sulfonyl)–amide (TFSA) as a dopant for GR. With increasing the doping concentration (nD) to 30 mM, the sheet resistance of the TFSA-doped GR transparent conductive electrode sharply decreases to ∼191 Ω/sq with only 1% reduction in its transmittance at 550 nm. The DC conductivity/optical conductivity ratio saturates to ∼62.5 at nD = 20 mM, resulting in maximum power conversion efficiency (PCE) of 16.61% and almost no loss of the PCE under 25 °C and 40% humidity atmosphere for 700 h. The solar cell also maintains ∼94% (absolutely from 16.61 to 15.57%) of its initial PCE even under continuous light soaking of 1 Sun at 60 °C and 30% humidity for 1000 h.

Graphene-Based Semiconductor Heterostructures for Photodetectors.

Graphene transparent conductive electrodes are highly attractive for photodetector (PD) applications due to their excellent electrical and optical properties. The emergence of graphene/semiconductor hybrid heterostructures provides a platform useful for fabricating high-performance optoelectronic devices, thereby overcoming the inherent limitations of graphene. Here, we review the studies of PDs based on graphene/semiconductor hybrid heterostructures, including device physics/design, performance, and process technologies for the optimization of PDs. In the last section, existing technologies and future challenges for PD applications of graphene/semiconductor hybrid heterostructures are discussed.

Use of Graphene for Solar Cells

In the last decade, graphene has been spotlighted as one of the novel materials for transparent conductive electrodes (TCEs) of solar cells. This paper provides an overview of recent progress for the application of graphene TCEs in solar cells employing representative active materials. This review focuses especially on the structure and characteristics of solar cells employing three majorgroup materials: Si-based materials (crystalline Si, porous Si, Si nanowire, and Si quantum dots), compound (CdTe and GaAs) thin films; organic and perovskite materials. The graphene TCEs are very promising for producing high-efficiency solar cells, but their stabilities are a key issue to overcome for the practical applications. The significance and outlook of the graphene-TCE-based solar cells are finally summarized.

Effect of layer number on flexible perovskite solar cells employing multiple layers of graphene as transparent conductive electrodes

Recently, excellent performance of perovskite solar cells has led to increasing interest in their flexible/portable power applications. Providing reliable flexible transparent conductive electrodes (TCEs) is believed to be crucial for maximizing the performance of flexible perovskite solar cells. Here, we first report efficient and reliable ultra-flexible p-i-n-type CH3NH3PbI3 perovskite solar cells employing multiple layers of graphene as anode TCEs. Through the variation of the layer number (Ln) of graphene TCEs, the solar cells are shown to be optimized at Ln = 2, where the power-conversion efficiency (PCE) exhibits 13.35 and 13.94% for forward and reverse scans, respectively with almost no hysteresis in the current density-voltage curves. These results originate from the Ln–dependent trade-off correlation between the structural, optical, and electrical properties of the solar cells, resulting in largest external quantum efficiency at Ln = 2. The PCE is maintained at ∼90% of its original value after 1000 bending cycles even at a bending radius of 2 mm, overwhelming bending stability against the bending deformation.

Graphene transparent conductive electrodes doped with graphene quantum dots-mixed silver nanowires for highly-flexible organic solar cells

Recent active studies on flexible photovoltaic cells strongly call for matchable flexible transparent electrodes. Graphene (GR) is one of the promising candidates as transparent conductive electrodes (TCEs) for flexible photovoltaic cells, but high sheet resistance of GR limits the efficiency of the cells. Here, we first fabricate GR TCEs doped with graphene quantum dots (GQDs)-mixed silver nanowires (Ag NWs) on polyethylene terephthalate substrates for highly-flexible organic solar cells (OSCs). With increasing doping concentration of GQDs to 0.03 g/L, the sheet resistance of the Ag NWs/GR TCE decreases to ∼92 Ω/sq whilst its work function increases to ∼4.53 eV, resulting in 3.66% power-conversion efficiency (PCE). In addition, the GQDs enhance the bending flexibility of the Ag NWs/GR TCEs, thereby maintaining the initial PCE of the OSCs over 90% even after 1000 bending cycles at a curvature radius of 4 mm.