Graphene is a kind of allotrope of carbon, and it is also a two-dimensional thin film material, with unique conductive properties and mechanical bending properties, which can be used as a flexible electrode for solar cells and organic light-emitting devices.
In addition, graphene and organic polymer materials can form a large donor-acceptor interface, which is conducive to the improvement of diffusion rate and carrier mobility, and can be used as an acceptor material for solar cells.
It has a one-dimensional sharp knife edge, has a large electric field enhancement coefficient, and can be used as an electron conduction and electric field emission material in field-induced emission devices due to its own good conductivity.
Graphene in the application of optoelectronic devices, is a very promising new research field, today let's talk about this topic.
Properties of graphene
Graphene is a two-dimensional thin film material with a honeycomb lattice structure composed of a single layer of carbon atoms in the carbon family, and the carbon atoms form a hexagonal lattice structure with sp2 hybrid orbitals
The conduction band and valence band of graphene exhibit linear dispersion relations and intersect with the Fermi surface composed of hexagonal vertices in the Brillouin zone, showing metallic or zero-bandgap semiconductor properties, so it has excellent mechanical, optical, thermal and electrical properties.
In general, the thickness of the single layer of graphene is 0.335 nm, the length of the carbon-carbon bond is only 0.142 nm, and the flexible connection between the carbon atoms makes the graphene maintain a stable lattice structure when the atomic plane is subjected to external forces.
The adjacent carbon atom Pz orbital electron cloud in graphene collides to form a large π conjugate plane, and the resulting delocalized π electrons can move freely, so graphene has good electrical conductivity and has very high electron mobility at room temperature.
The theoretical surface area of single-atomic layer graphene materials can reach 2630 m2/g, and the zero-bandgap two-dimensional graphene materials have high carrier mobility, good electron transport capacity, large specific surface area and high light transmission, which can be used in many fields.
Graphene and solar cells
2.1 Graphene battery electrode
Graphene has high mobility, high light transmittance, high conductivity, and low carrier concentration makes the reflectivity low, easier to pass through a larger wavelength range of light, compared with ITO, FTO, AZO and other oxide transparent conductive materials, can pass through most of the infrared.
8-layer graphene was prepared by P-type doping of graphene film prepared by CVD chemical vapor deposition by HNO3, and the square resistance of the film was 90Ω/sq and the light transmittance was 80%, which was similar to the traditional light-transmitting electrode.
Lee et al. doped graphene prepared from CVD with fluoropolymers to obtain high-performance flexible light-transmitting electrodes, so graphene has become a new alternative material for transparent conductive electrodes in solar cells.
Graphene is used in dye-sensitized solar cell (DSSC) electrodes. In dye-sensitized batteries, there is a good physical adsorption and charge transport interaction between graphene and TiO2, which can reduce the recombination of photoelectrons and be used as a photoanode counter electrode to replace platinum.
As the counter electrode of dye-sensitized solar cells, the transmittance of PSS composite material in visible light is 80%, and the photoelectric energy energy conversion efficiency reaches 4.5%, which is comparable with the 6.3% photoelectric energy conversion efficiency of platinum electrode as counterelectrode under the same conditions.
Graphene prepared by non-covalent modification of PBASE organic molecules is used as an anode for solar cells with Glass/graphene/PEDOT: PSS/P3HT: PCBM/LiF/Al structure, with a photoelectric energy conversion efficiency of 1.71%......
2.2 Graphene is applied to the buffer layer of solar cells
PSS is a traditional hole transport layer, which is acidic and will corrode the ITO electrode, causing indium to migrate into the active layer and degrade the battery performance. The alternative use of graphene can improve the stability and life of battery devices.
Using graphene oxide and PEDOT:PSS composites as buffer layers for organic solar cells of P3HT:PCBM system, the photoelectric energy conversion efficiency reached 3.8%.
The cathode buffer layer material of polymer solar cells was obtained by connecting Cs2CO3 with COOH on GO, which was used in solar cells with positive structure ITO/PEDOT:PSS/P3HT:PCBM/GOCs/Al, and the photoelectric energy conversion efficiency reached 3.08%.
2.3 Graphene is applied to the active layer of solar cells
The composite of honeycomb graphene and organic polymer materials can form a large donor acceptor interface, which is conducive to the improvement of the diffusion rate and carrier mobility of excitons in the battery, and eliminates the secondary aggregation caused by the destruction of the charge transport path, so graphene will be a good choice for electronic acceptor materials for organic solar cells.
Using the monolayer graphene (SPF Graphene) functionalized by phenyl isocyanate as the acceptor and P3HT as the donor, it was found that electrons had a strong effect at the interface of the acceptor, resulting in strong energy transfer, and the device obtained an open-circuit voltage of 0.72 V, a short-circuit current density of 4.0 mA/cm2, and a photoelectric energy conversion rate of 1.1%.
Graphene is used as acceptor, polythiophene is used as an electron donor, and the two materials are blended to make an activation layer of solar cells, and the photoelectric energy conversion efficiency is 1.4%...
Graphene quantum dots are used in solar cells
Defect-free graphene is a zero-bandgap semiconductor material, but it is generally necessary to introduce a bandgap for applications in the optoelectronic field. In recent years, research has been carried out in the conversion of graphene two-dimensional materials into zero-dimensional graphene quantum dots.
Graphene quantum dots have edge effects and quantum effects, and their electronic and photoelectric properties vary depending on the size and functional groups of GQDs. This property of graphene quantum makes it also have relevant applications in solar cells.
GQDs and Cs2CO3 hybrid materials are used as buffer layers for solar cell devices, which are used for ITO/GQDs-Cs2CO3/P3HT:PCBM/V2O5/Au structure, and the photoelectric energy conversion efficiency reaches 3.23%.
Using the unique band structure of graphene quantum dots and the characteristics that photogenerated electron-hole pairs can effectively form separation on the surface of the junction, solar cells based on c-Si/GQDs heterojunctions were developed, and the photoelectric energy conversion efficiency reached 6.63%.
GQD was prepared by electrolytic graphite, and GQD was used as the acceptor material combined with donor material P3HT as the active layer of organic solar energy, which was applied to ITO/PEDOT:PSS/P3HT:GQDs/Al structure, and the photoelectric energy conversion efficiency was 1.28%.
GQD was obtained by reducing graphene oxide, and GQD was added to PTB7/PC71BM as the active layer of solar cells, and the photoelectric energy conversion efficiency reached 7.6%.
Double-walled carbon nanotubes were used to cut to obtain uniformly distributed graphene quantum dot DGQs, and P3HT:PCBM:GQDs ternary polymer solar cells were produced, and the photoelectric energy conversion efficiency reached 5.24%.
Since the unique band structure of GQD is conducive to charge transport of blended structured films, the photoelectric energy conversion efficiency of solar cells is improved.
Application of graphene in organic light-emitting diodes
The high transmittance of graphene makes it suitable as a transparent conductive electrode for organic light-emitting diodes (OLEDs). The fewer graphene layers, the higher the transmittance, but the higher the surface resistance, the square resistance of a single layer of graphene is as high as 600Ω/sq, and the work function is only 4.30 eV.
By mixing graphene with inorganic dielectric materials or polymer semiconductor materials, graphene can obtain carriers from dielectric materials, thereby reducing surface resistivity and making graphene possible as a transparent conductive electrode for display devices.
By adding titanium oxide and PEDOT:PSS mixture to the graphene surface, the block resistance of graphene is reduced to 86Ω/sq and the work function is increased to 5.12 eV, and the current efficiency of OLED devices based on this composite transparent conductive electrode is almost the same as that of OLED devices with ITO as electrodes.
A functional graphene water-soluble liquid with a thickness of 7 nm was spun coated on quartz glass, and after heat treatment, it was used as the anode of the OLED device to make an OLED device with graphene/PEDOT:PSS/NPD/Alq3/LiF/Al structure, with an opening voltage of 4.5 V, and its brightness was 300 cd/m2 when the voltage reached 11.7 V.
MoO3 metal oxide was prepared by thermal evaporation on the graphene surface as the electrode of OLED, and 4 layers of doped graphene were obtained, and the square resistance was 30Ω/sq.
The graphene/MoO3/CBP sequential structure is conducive to the effective injection of anode charge into OLED devices, while the low light absorption of graphene layer reduces the electrode light absorption compared with ITO film, thereby improving the current efficiency of the device.
If the graphene film is prepared by transferring CVD chemical vapor deposition on PET substrate, 80 nm PEDOT:PSS is sprayed on the graphene film treated with nitric acid, and the graphene/PEDOT:PSS composite conductive film is obtained after heat treatment at 150°C, and the square resistance is 350Ω/sq.
The flexible orange-yellow organic light-emitting diode device is made by the thin film electrode, and the maximum current efficiency of the device is 0.9 cd/A when the voltage is 12 V, and the luminous brightness of the device does not change significantly after bending 100 times under the radius of bending curvature of 10 mm.
Graphene, magnetron sputtered silver and aluminum-doped zinc oxide were prepared by CVD chemical vapor deposition method, and a flexible electrode with graphene/silver/aluminum-doped zinc oxide composite structure was made on PET substrate, and it was found that the green organic light-emitting diode device obtained a high current efficiency of 1.46 cd/A, and still had stable luminescence performance after 10 mm bending test.
The organic light-emitting device made by evaporation doping with Alq3 as the light-emitting layer as the electron transport layer has a luminous brightness of 1.2 times that of the doped-free device, and the current efficiency is 1 times higher than that of the doped-free device.
Application of graphene in field-induced emission devices
Graphene has the characteristics of large aspect ratio, high conductivity and special sheet structure, especially graphene nanosheets have one-dimensional sharp knife edges, and the electric field enhancement coefficient is large.
Therefore, graphene is also used in field-induced emission (FED) devices based on quantum tunneling effect as a material for electron conduction and electric field emission.
Graphene and tin dioxide composites were used as field emission cathodes, and the results showed that graphene improved the field emission characteristics of tin dioxide nanomaterials.
A mixture of graphene nanosheets and carbon nanotubes was used as the field emission material to obtain stable field emission performance, and the emission current density was greater than 20 mA/cm2.
Four-needle zinc oxide coated with graphene presents a lower working voltage, better stability and more uniform emission than pure four-needle zinc oxide, and the introduction of graphene oxide improves the mechanical connection and electronic conduction between zinc oxide and electrodes, thus improving field emission performance.
Using the suspended graphene film as the field emission device for surface conduction, the suspended graphene parallel to the gate electric field leads to a strong electric field at the edge of the graphene, and the device obtains a high emission efficiency of 30% at a low operating voltage of less than 150 V by depositing the magnesium oxide film on the four-needle zinc oxide nanostructure.
conclusion
In general, graphene has broad application prospects in optoelectronic devices due to its high transmittance, high electron mobility, large specific surface area and other characteristics, and there have been a large number of application studies.
Graphene has unique advantages over other materials, but with the further understanding of the characteristics of graphene itself, some shortcomings in optoelectronic applications have also emerged.
For example, in OLED applications, the problem of mismatch with the work function of organic materials, etc., these problems need to be studied more vigorously, and more technical solutions are proposed from different angles such as materials and devices to solve.
In general, the combination of graphene and optoelectronic devices is expected to break through the bottleneck of optoelectronic technology, which is a promising new research field. What are your different views on this matter?
References: "Application of Graphene in Optoelectronic Devices", "Research and Application of Graphene"