WO2011141717A1 - Device comprising graphene oxide film - Google Patents

Device comprising graphene oxide film Download PDF

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Publication number
WO2011141717A1
WO2011141717A1 PCT/GB2011/000740 GB2011000740W WO2011141717A1 WO 2011141717 A1 WO2011141717 A1 WO 2011141717A1 GB 2011000740 W GB2011000740 W GB 2011000740W WO 2011141717 A1 WO2011141717 A1 WO 2011141717A1
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Prior art keywords
anode
cathode
active layer
graphene oxide
oxide film
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PCT/GB2011/000740
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French (fr)
Inventor
Manish Chhowalla
Chun-Wei Chen
Shao-Sian Li
Kun-Hua Tu
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Imperial Innovations Limited
National Taiwan University
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Publication of WO2011141717A1 publication Critical patent/WO2011141717A1/en

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    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K30/00—Organic devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation
    • H10K30/30—Organic devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation comprising bulk heterojunctions, e.g. interpenetrating networks of donor and acceptor material domains
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K30/00—Organic devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation
    • H10K30/80—Constructional details
    • H10K30/81—Electrodes
    • H10K30/82—Transparent electrodes, e.g. indium tin oxide [ITO] electrodes
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K2102/00—Constructional details relating to the organic devices covered by this subclass
    • H10K2102/10—Transparent electrodes, e.g. using graphene
    • H10K2102/101—Transparent electrodes, e.g. using graphene comprising transparent conductive oxides [TCO]
    • H10K2102/103—Transparent electrodes, e.g. using graphene comprising transparent conductive oxides [TCO] comprising indium oxides, e.g. ITO
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K30/00—Organic devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation
    • H10K30/50—Photovoltaic [PV] devices
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K85/00—Organic materials used in the body or electrodes of devices covered by this subclass
    • H10K85/10—Organic polymers or oligomers
    • H10K85/111—Organic polymers or oligomers comprising aromatic, heteroaromatic, or aryl chains, e.g. polyaniline, polyphenylene or polyphenylene vinylene
    • H10K85/113—Heteroaromatic compounds comprising sulfur or selene, e.g. polythiophene
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/549—Organic PV cells

Definitions

  • the present invention relates to utilization of a graphene oxide thin film as a hole transport layer in electronic or optoelectronic devices.
  • HTL hole transport layer
  • Organic photovoltaics comprise a photoactive layer (an active layer) between a cathode and an anode.
  • the active layer comprises two types of functional material: an electron accepting material (acceptor phase) and an electron donating material (donor phase).
  • acceptor phase an electron accepting material
  • donor phase an electron donating material
  • To-date, the most promising OPVs are based on a random dispersion of acceptor phase in the form of organic or inorganic nanostructures within a conjugated polymer matrix, which acts as the donor phase.
  • the interface of the acceptor phase nanostructures and the donor phase host polymer matrix create bulk heteroj unctions (BHJs).
  • the BHJs provide a large number of sites for charge separation and bi- continuous pathways for efficient charge carrier transport.
  • HTL hole transport layer and electron blocking layer
  • a HTL must be formed from a wide band gap p-type material, so that it allows hole transport whilst substantially blocking electron transport.
  • inorganic materials such as V 2 0 5 , NiO and M0O3 have been reported as being useful to form a HTL, with NiO being the most effective, yielding efficiencies greater than 5%.
  • inorganic HTLs are deposited using vacuum deposition techniques that are incompatible with solution processing. The ability to form solution processable printable electronics, for example on flexible plastic substrates, is a key advantage of organic devices.
  • PEDOT:PSS semiconducting poly(3,4- ethylenedioxythiophene)-polystyrenesulfonate
  • PEDOT:PSS has the advantage that it can be deposited from solution.
  • the anode is a transparent and conducting indium tin oxide (ITO) electrode
  • ITO indium tin oxide
  • a PEDOT:PSS HTL can serve to minimize the detrimental effects of ITO roughness as well as to align the work functions of P3HT and ITO to give more efficient collection of holes.
  • PEDOT:PSS is usually deposited from highly acidic (pH ⁇ l) aqueous suspensions that are known to corrode ITO at elevated temperatures and can also introduce water into the active layer, diminishing device performance. Similar detrimental effects are likely with other oxide based electrode materials.
  • Chemically derived graphene oxide (GO) has been incorporated into OPVs as a replacement for an ITO transparent and conducting electrode.
  • graphene oxide in its reduced form (rGO) has been incorporated into OPVs as the acceptor phase bulk heteroj unction component in place of PCBM.
  • rGO BHJ devices yield promising device efficiencies of ⁇ 1.4%. These devices do not utilise GO as an HTL material.
  • the invention is based on the determination that a graphene oxide (GO) film is a suitable solution processable material, compatible with OPV materials and fabrication techniques, that can be used as the HTL in an electronic or optoelectronic devices to overcome limitations of other HTL materials such as PEDOT:PSS.
  • GO graphene oxide
  • the invention provides a device comprising: an anode;
  • HTL hole transport layer
  • the components of the device are preferably present in series as follows: anode; HTL; active layer; and cathode.
  • the device also comprises a substrate.
  • the components in such a device are present in series as follows: substrate; anode; HTL; active layer; and cathode.
  • the active layer is positioned between the anode and the cathode, with the HTL being an interlayer positioned between the active layer and the anode.
  • the HTL comprises a graphene oxide film which is formed from non-reduced graphene oxide.
  • the device may additionally comprise an electron transport layer, which is an interlayer positioned between the active layer and the cathode.
  • the electron transport layer is capable of transporting electrons and blocking holes (partially or completely).
  • the electron transport layer is a LiF layer or a ZnO layer.
  • the anode is preferably a transparent and conducting electrode; which may be formed from a layer of transparent conductive oxide, such as indium tin oxide, or from a graphene layer.
  • the cathode may be a metal cathode, such as an aluminium cathode. Numerous anode and cathode combinations are known, however, and are possible for use in a device of the invention.
  • the device is a device in which the cathode is a transparent layer and a metal electrode acts as the anode. This type of device can be referred to as an "inverted" device.
  • the anode may be a silver (Ag) anode.
  • the cathode may be an ITO layer.
  • the substrate may be formed from any suitable material such as glass or plastic, for example PET (polyethylene terephthalate).
  • PET polyethylene terephthalate
  • a plastic substrate is preferably a flexible plastic substrate.
  • the device may be an electronic or optoelectronic device, such as a photovoltaic cell or light emitting diode.
  • the device may be an organic device.
  • the organic device may be an organic optoelectronic device, for example an organic photovoltaic device (OPV) or an organic light-emitting diode (OLED).
  • OOV organic photovoltaic device
  • OLED organic light-emitting diode
  • the active layer may comprise an electron accepting material, such as a fullerene material, and an electron donating material, such as a conjugated polymer.
  • the device may be a bulk heteroj unction device, in which the active layer is formed from a blend of an electron accepting material and an electron donating material.
  • the active layer may be a P3HT:PCBM active layer, formed from a blend of poly(3- hexylthiophene) and phenyl-(61 -butyric acid methyl ester.
  • the active layer may be a MEH-PPV (poly[2-methoxy-5(2'-ethyl- hexyloxy)-l ,4-phenylene vinylene) active layer.
  • the device may be an optoelectronic device which is a photovoltaic device.
  • the photovoltaic device (preferably an OPV), in addition to the HTL comprising a graphene oxide film, may comprise one, more, or all of the following:
  • ITO indium tin oxide
  • an active layer comprising a blend of poly(3-hexylthiophene) (P3HT) and phenyl-C61 -butyric acid methyl ester (PCBM); and
  • the HTL may comprise a polymeric material such as PEDOT:PSS, a macrocyclic compound such as copper phthalocyanine (CuPC) or an inorganic materials such as V 2 0 5 , NiO and Mo0 3 .
  • the use of a GO film as the HTL avoids the need for any additional HTL.
  • the sole hole transport layer in the device may be a GO film.
  • the optoelectronic device is an organic light-emitting diode (OLED).
  • An OLED of the invention in addition to the HTL comprising a graphene oxide film, may comprise one, more, or all of the following:
  • a metal cathode such as an aluminium cathode.
  • the thickness of the graphene oxide film is less than 50nm, preferably less than 20nm, more preferably less than lOnm, even more preferably less than 3.5nm.
  • the film thickness may be 0.5nm to 20nm, preferably 0.5nm to lOnm, more preferably 0.5nm to 3.5nm, even more preferably lnm to 3nm, even more preferably about 2nm.
  • the GO film may be the only HTL present. Accordingly, in a preferred embodiment of any of the embodiments describes above, no additional HTL is present.
  • the invention provides a process for producing a device comprising, in series:
  • a hole transport layer comprising a graphene oxide film
  • the graphene oxide film may be deposited on the first electrode by spin coating, vacuum filtration, Langmuir Blodgett, spray casting, dip coating, roll to roll, screen printing or inkjet printing.
  • GO can be deposited from a neutral solvent.
  • the solvent may be water, an aqueous solution, a water miscible solvent (such as methanol), or a mixture thereof.
  • Spin coating with a suspension of GO sheets in water, an aqueous solution, a water miscible solvent (such as methanol), or a mixture thereof is preferably useed.
  • the spin coating speed is preferably from 600-8000 RPM.
  • the concentration of the suspension is preferably 0.05 to 1 mg/ml.
  • the invention provides a hole transport layer (HTL) comprising a graphene oxide film.
  • HTL hole transport layer
  • the invention provides use of a graphene oxide film as a hole transport layer (HTL).
  • the use may be in an electronic device or an opto-electronic device.
  • the device is an organic device. More preferably, the device is an organic opto-electronic device.
  • Figure 1 shows (a) a transmission electron microscopy (TEM) image of a GO sheet on a lacey carbon support; (b) the corresponding selected area electron diffraction pattern (SAED) with diffraction spots labelled with Miller-Bravais indices; and (c) the relative intensity profile obtained from the diffraction pattern in (b).
  • Figure 2 shows atomic force microscope (AFM) topography images of GO thin films, with thicknesses of approximately (a) 2nm, (b) 4nm and (c) lOnm, respectively.
  • TEM transmission electron microscopy
  • SAED selected area electron diffraction pattern
  • Figure 2(d) shows the corresponding optical transmission spectra of the three GO films deposited on ITO/glass substrates.
  • the inset shows the Tauc plot of the 2nm thick GO thin film.
  • Figure 3(a) shows a schematic of the structure of a photovoltaic device consisting of ITO/GO/P3HT:PCBM/Al.
  • Figure 3(b) shows energy level diagrams of the bottom electrode ITO, interlayer materials (PEDOT:PSS, GO), P3HT (donor) and PCBM (acceptor), and the top electrode Al.
  • PEDOT:PSS interlayer materials
  • P3HT donor
  • PCBM acceptor
  • Figure 4(a) shows current-voltage characteristics of three photovoltaic devices one with no hole transport layer (curve labelled as ITO), one with a 30nm PEDOT:PSS layer, and one with a 2nm thick GO film.
  • Figure 4(b) shows current- voltage characteristics of ITO/GO/P3HT:PCBM/Al devices with GO thickness of approximately 2nm, 4nm and 10 nm. All the measurements were under simulated A.M. 1.5 illumination at 100 mW/cm 2 .
  • Figure 5 shows a plot of charge recombination rate constant k rec versus light intensities obtained using TOCVD measurements for ITO-only, PEDOT:PSS, and 2 nm GO thin film devices.
  • the inset shows the transient photovoltage decay curves of the corresponding devices measured at lOOmW/cm .
  • Figure 6(a) shows a schematic of an inverted-type polymer solar cell.
  • Figure 6(b) shows a current-voltage plot for this cell.
  • Figure 7(a) shows a flexible LED comprising a MEH-PPV active layer and schematic thereof.
  • Figure 7(b) shows a current-voltage plot for this device and also includes traces for a device with a PEDOT:PSS HTL and a device with no HTL.
  • graphite oxide is formed by the oxidation of graphite.
  • Monomolecular sheets of graphite oxide are known as graphene oxide sheets.
  • graphene oxide (GO) is a graphene sheet functionalized with oxygen in the form of epoxy and hydroxyl groups on the basal plane and various other groups at the edges (such as phenol groups).
  • Graphene sheets can be arranged to cover a surface, thereby forming a graphene oxide film.
  • the thickness of a graphene oxide film is determined as a function of the GO sheet monolayer, i.e. the extent to which the GO sheets overlap.
  • Thickness of a graphene oxide film can be measured by Raman spectroscopy, atomic force microscopy profilometry or electron diffraction.
  • Raman spectroscopy measurement measures film thickness by monitoring the shape, position and width of the 2D phonon zone boundary peak (G Eda et al, Nat. Nanotech. 3, 270-274, 2008).
  • a Raman map is created, thickness is measured at multiple points within the map and an average film thickness value can be calculated.
  • thickness values referred to herein are average thickness values.
  • Opto-electronics is the study and application of devices that source, detect and control light, usually considered a sub-field of photonics.
  • An optoelectronic device converts electrical energy to optical energy (or vice versa), or uses such conversion in its operation.
  • An organic device such as an organic optoelectronic device, is a device comprising conductive organic polymeric material or small molecules. It will be appreciated that whilst at least one component of an organic optoelectronic device comprises conductive organic polymeric material or small molecules, the device may also comprise inorganic components.
  • a 'hole transport layer' is commonly known in the art to be a layer of material that is capable of transporting holes and blocking the passage of electrons.
  • electron blocking may be partial or complete.
  • the HTL will give as close to complete electron blocking as possible.
  • a 'hole transport layer' can be referred to as a 'hole transport and electron blocking layer'.
  • 'approximately' can refer to a variation of ⁇ 10%.
  • fullerene material encompasses fullerenes and fullerene derivatives such as PCBM.
  • Thin films of GO can be deposited from neutral aqueous solutions.
  • P3HT photoactive poly(3-hexylthiophene)
  • PCBM phenyl-C61- butyric acid methyl ester
  • ITO indium tin oxide
  • GO provides a simple solution processable alternative to PEDOT:PSS as the effective hole transport layer in OPV and light emitting-diode devices.
  • Graphene oxide is a graphene sheet functionalized with oxygen in the form of epoxy and hydroxyl groups on the basal plane and various other types at the edges.
  • the C-0 bonds within graphene oxide are covalent and thus disrupt the sp conjugation of the hexagonal graphene lattice, making GO an insulator.
  • Material to form a HTL should be as insulating as possible and should be a wide band gap p-type semiconductor.
  • a large energy barrier for electron injection into the HTL can be introduced by placing the conduction band as far away energetically from the valence band as possible (i.e. having a large band gap material). Reduction of GO leads to the formation of conducting pathways.
  • reduced graphene oxide would allow both types of charge carriers (electrons and holes) to conduct and would not function as an HTL as required by the invention.
  • UV-Visible absorption spectra were obtained using a Jasco V-570 UV/Vis NIR Spectrophotometer.
  • Current-voltage characteristics (Keithley 2410 source meter) were obtained by using a solar simulator (Newport Inc.) with the A.M.1.5 filter under irradiation intensity of 100 mW/cm 2 .
  • TOCVD transient open-circuit voltage decay
  • the transient decay signals were recorded by a digital oscilloscope (Tetronix TDS5052B).
  • the samples were mounted in a vacuum chamber under 10 "3 torr during measurement.
  • the work function was measured by scanning Kelvin probe microscopy (SKPM) (Innova, Vecco Inc.). For all measurements, Pt/Ir coated cantilevers were used.
  • Non-reduced GO was obtained from purified natural graphite powder (SP-1 , Bay Carbon) using the modified Hummers method (Hirata, M.et al; Thin-Film Particles of Graphite Oxide 1 : High- Yield Synthesis and Flexibility of The Particles. Carbon 2004, 42, 2929-293 and G Eda et al, Nat. Nanotech. 3, 270-274, 2008).
  • the modified Hummers method is routine in the art. In summary, the method involves oxidation of graphite to form graphite oxide. A graphite and NaN0 3 mixture is cooled in an ice bath, 3 ⁇ 4S0 4 is added and stirred until homogenized. KMn0 4 is gradually added while stirring.
  • a viscous slurry is obtained.
  • 5wt% H 2 S0 4 is added gradually while stirring, followed by 3 ⁇ 40 2 .
  • the mixture can be purified by dispersing and precipitating in an aqueous solution of 3wt% H 2 S0 and 0.5wt% H 2 0 2 .
  • the resulting slurry is then dispersed in deionized water by ultrasonication to yield a suspension of exfoliated individualized GO sheets.
  • a concentration of 8mg/ml of GO sheets in water was prepared for deposition of GO thin films.
  • TEM transmission electron microscopy
  • TEM Transmission electron microscopy
  • SAED selected area electron diffraction pattern
  • a suspension with a concentration of 0.05 to 1 mg/ml of GO in an aqueous solution or water miscible solvent e.g. water or methanol
  • spin coating speed can vary from 600 - 8000 RPM. Adjustment of the suspension and spin coating parameters allows tailoring of the film thickness.
  • a film thickness of ⁇ 2nm was achieved by spin coating 60ml of a 0.4 mg/ml suspension on an ITO substrate with a spin coating speed of 6000 RPM. Film thicknesses of ⁇ 4nm and ⁇ 10 nm were achieved keeping the volume and concentration constant, but reducing the spin coating speeds to 4000RPM and 800RPM, respectively.
  • the GO thin films used had lateral resistivity values in excess of 10 5 ⁇ /cm. It must be clarified that the vertical resistivity of non-reduced GO is an order of magnitude lower ( ⁇ 10 3 - 10 4 ⁇ /cm) than the lateral resistivity. This is attributed to the fact that the sp 2 clusters are isolated laterally but are in contact with the electrodes in a sandwich metal/GO/metal structure. Thus, charge carriers (both holes and electrons) can be injected and transported via the isolated sp clusters with relative ease in a sandwich device structure. However since the density of the isolated sp clusters is sufficiently low in non-reduced GO, the vertical resistivity is substantial and transport is dictated by the conduction and valence bands of the sp 3 sites.
  • Atomic force microscope (AFM) topography images of GO thin films of three different thicknesses on ITO are shown in Figure 2(a), (b) and (c).
  • the lateral dimensions of GO sheets ranged from 1 - 10 ⁇ .
  • GO thin films with three different thicknesses were deposited.
  • the average thicknesses of the three samples were estimated to be ⁇ 2nm, ⁇ 4 nm and -10 nm with corresponding RMS roughness values of 0.70, 0.97 and 1.40 nm, respectively. These values are lower than the 3 nm RMS roughness of bare glass/ITO substrate, indicating that the deposition of GO layers serves to planarize the anode surface.
  • the optical transmission spectra of GO layers with various thicknesses deposited on ITO/glass substrates shown in Figure 2(d) reveal that although the transmittance decreases slightly with thickness, the GO thin films do not significantly alter the transparency of ITO.
  • the heterogeneous sp 2 /sp 3 structure of GO makes it difficult to assign a specific energy for the band gap. Since conduction in GO and reduced GO occurs via tunneling between sp 2 sites, the band gap represents potential barriers for transport formed by covalent sp bonds. That is, in as-synthesized GO the sp fraction can be as high as 60% so that the sp 2 domains are non-percolating, which prevents conduction.
  • the optical gap of GO can be
  • NiO which can be used as a highly efficient hole transport layer.
  • a LiF layer below the Al cathode could also be incorporated. This can be done just before the metal evaporation for the top electrode.
  • FIG. 3(a) An example photovoltaic device structure consisting of ITO/GO/P3HT:PCBM/Al layers is shown in Figure 3(a).
  • the device comprises an ITO anode, a GO film, a P3HT/PCBM active layer and an Al cathode, which would be deposited sequentially on a substrate.
  • the corresponding energy levels of each component shown in Figure 3(a) are shown in Figure 3(b).
  • the band energies of the ITO, P3HT:PCBM, and Al layers are well known.
  • the work function of GO thin films was determined by scanning Kelvin probe microscopy. The average work function values obtained from measurements on ten different GO thin film samples was found to be 4.9 eV, slightly higher than typical values (4.6 eV) obtained for pristine graphene. The higher values for GO are most likely due to the larger electronegativity of O atoms, which produce surface C 8+ -0 5 ⁇ dipoles via extraction of ⁇ electrons from graphene
  • ITO-only control devices consisting of ITO/P3HT:PCBM/Al structure; and conventional OPVs incorporating 30 nm PEDOT:PSS as the hole transport layer with the following structure: ITO/PEDOT:PSS/P3HT:PCBM/Al.
  • the photovoltaic characteristics of the fabricated devices were characterized under simulated A.M. 1.5 illumination at 100 mW/cm 2 .
  • the current-voltage plots of the devices are shown in Figure 4(a).
  • the average short circuit current density (J sc ), open circuit voltage (V oc ), fill factor (FF), and power conversion efficiency values for each set of devices are summarized in Table I. It can be seen that the ITO-only device exhibits average power conversion efficiency ( ⁇ ) of 1.8 ⁇ 0.2%.
  • the insertion of a 2nm thick GO thin film between ITO and P3HT:PCBM results in a substantial increase in J sc , V oc , and FF, leading to an enhancement in the power conversion efficiency ( ⁇ ) to 3.5 ⁇ 0.3%.
  • typical device performance of ITO/PEDOT:PSS/P3HT:PCBM/Al with efficiency values of around 3.6 ⁇ 0.2% is also shown in Figure 4(a).
  • Table I Summary of typical photovoltaic parameters of the control and GO HTL devices.
  • TOCVD transient open-circuit voltage decay
  • the photovoltage decay curves of the three devices under 100 mW/cm illumination are shown in the inset.
  • the decay lifetimes for ITO-only, PEDOT:PSS HTL, and thin GO HTL devices were found to be 8.1 , 9.6, and 1 1.6 ⁇ , respectively.
  • the longer recombination lifetimes in GO indicate lower recombination rates, suggesting effective suppression of leakage current and separation of carriers via efficient transport of holes to ITO and blocking of electrons.
  • the hole transport mechanism in GO from the band diagram ( Figure 3b), it can be seen that the low injection barrier between the GO and P3HT allows holes to be readily injected into the GO for collection by the ITO electrode.
  • the large band gap ( ⁇ 3.6 eV) of GO hinders transport of electrons from the PCBM LUMO to the ITO anode, acting as an effective electron blocking layer.
  • non-reduced GO is insulating (or at least a wide band semiconductor) and in contrast to NiO where transfer of holes to ITO is facilitated by injection into Ni 2+ defect levels close to the valence band, such p-type behaviour is not observed in clean GO in our transport measurements.
  • the presence of isolated sp clusters near the Fermi level should facilitate the transfer of both electrons and holes to ITO and therefore does not contribute to the enhancement in efficiency.
  • the energy level diagram in Figure 3(b) suggests that injection of holes into the valence of GO is favourable but the exact configuration of the band alignment requires further investigation.
  • a photovoltaic (solar) cell was prepared.
  • a schematic of this cell is shown in Figure 6(a).
  • the cell comprises an Ag anode, a graphene oxide HTL, a P3HT:PCBM active layer, a ZnO film (an electron transport layer) and an ITO cathode.
  • the transparent layer (ITO) becomes a cathode, rather than the anode as is conventional and the metal electrode becomes the anode.
  • the use of an Ag anode gives the advantage of stability because of use of the noble metal Ag.
  • Figure 6(b) shows the current-voltage plot for this device. This device exhibited a power conversion efficiency of 3.4%. It is noted that the GO layer in this device has an optimised thickness of 2nm.
  • An LED was prepared to comprise a flexible PET substrate, a graphene anode, a graphene oxide HTL, a MEH-PPV active layer and an aluminium cathode.
  • This device is shown in Figure 7(a).
  • the current-voltage plot for this device is illustrated in Figure 7(b), together with plots for a device comprising a PEDOT:PSS HTL an a device without any HTL present.
  • the device with a graphene oxide HTL showed similar performance compared to one using a PEDOT.:PSS HTL and the device without any HTL showed a large leakage current.
  • This LED is both flexible and thin, with the thickness of the graphene anode and the graphene oxide HTL being lOnm, compared to 150nm thickness for conventional ITO/PEDOT materials. It will be appreciated to a skilled person reading this disclosure that various changes in form and detail can be made without departing from the true spirit and scope of the invention and such changes are encompassed by the invention.

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Abstract

The present invention relates to utilisation of a graphene oxide thin film as a hole transport layer in electronic or octoelectronic devices. Provided herein is a device comprising an anode, a hole transport layer comprising a graphene oxide film, an active layer and a cathode.

Description

Device Comprising Graphene Oxide Film
The present invention relates to utilization of a graphene oxide thin film as a hole transport layer in electronic or optoelectronic devices.
The performance of many electronic or optoelectronic devices can be improved by incorporation of a hole transport layer (HTL). Examples of such devices are photovoltaic cells and light emitting diodes.
Organic devices, such as organic photovoltaic cells and organic light-emitting diodes, hold tremendous potential as solution processable, inexpensive devices. Organic photovoltaics (OPVs) comprise a photoactive layer (an active layer) between a cathode and an anode. The active layer comprises two types of functional material: an electron accepting material (acceptor phase) and an electron donating material (donor phase). To-date, the most promising OPVs are based on a random dispersion of acceptor phase in the form of organic or inorganic nanostructures within a conjugated polymer matrix, which acts as the donor phase. The interface of the acceptor phase nanostructures and the donor phase host polymer matrix create bulk heteroj unctions (BHJs). The BHJs provide a large number of sites for charge separation and bi- continuous pathways for efficient charge carrier transport.
Although numerous combinations of acceptor phase and donor phase materials have been reported, the most popular BHJ OPVs utilise blends of poly(3-hexylthiophene) (P3HT) and the fullerene derivative, phenyl-C61 -butyric acid methyl ester (PCBM) as the active layer. The mechanisms for achieving high photovoltaic efficiencies in excess of 6% in P3HT:PCBM BHJ devices are well established. However, several key components remain unresolved and must be addressed if the theoretical efficiencies of ~ 10% are to be realized. For example, in a simple BHJ device both the donor and acceptor phases are in direct electrical contact with the cathode and anode electrodes, leading to recombination of charge carriers (electrons and holes) and current leakage. To minimize such detrimental effects, a hole transport layer and electron blocking layer (HTL) can be deposited between the active layer and the anode. A HTL must be formed from a wide band gap p-type material, so that it allows hole transport whilst substantially blocking electron transport. Several inorganic materials such as V205, NiO and M0O3 have been reported as being useful to form a HTL, with NiO being the most effective, yielding efficiencies greater than 5%. However, inorganic HTLs are deposited using vacuum deposition techniques that are incompatible with solution processing. The ability to form solution processable printable electronics, for example on flexible plastic substrates, is a key advantage of organic devices.
The most commonly employed HTL in organic devices is a semiconducting poly(3,4- ethylenedioxythiophene)-polystyrenesulfonate (PEDOT:PSS) layer. PEDOT:PSS has the advantage that it can be deposited from solution. In addition, where the anode is a transparent and conducting indium tin oxide (ITO) electrode, a PEDOT:PSS HTL can serve to minimize the detrimental effects of ITO roughness as well as to align the work functions of P3HT and ITO to give more efficient collection of holes. However, PEDOT:PSS is usually deposited from highly acidic (pH~l) aqueous suspensions that are known to corrode ITO at elevated temperatures and can also introduce water into the active layer, diminishing device performance. Similar detrimental effects are likely with other oxide based electrode materials. Chemically derived graphene oxide (GO) has been incorporated into OPVs as a replacement for an ITO transparent and conducting electrode. In addition, graphene oxide in its reduced form (rGO) has been incorporated into OPVs as the acceptor phase bulk heteroj unction component in place of PCBM. rGO BHJ devices yield promising device efficiencies of ~ 1.4%. These devices do not utilise GO as an HTL material.
The invention is based on the determination that a graphene oxide (GO) film is a suitable solution processable material, compatible with OPV materials and fabrication techniques, that can be used as the HTL in an electronic or optoelectronic devices to overcome limitations of other HTL materials such as PEDOT:PSS.
In a first aspect, the invention provides a device comprising: an anode;
a hole transport layer (HTL) comprising a graphene oxide film;
an active layer; and
a cathode.
The components of the device are preferably present in series as follows: anode; HTL; active layer; and cathode. Preferably, the device also comprises a substrate. The components in such a device are present in series as follows: substrate; anode; HTL; active layer; and cathode.
The active layer is positioned between the anode and the cathode, with the HTL being an interlayer positioned between the active layer and the anode. The HTL comprises a graphene oxide film which is formed from non-reduced graphene oxide. The device may additionally comprise an electron transport layer, which is an interlayer positioned between the active layer and the cathode. The electron transport layer is capable of transporting electrons and blocking holes (partially or completely). Preferably, the electron transport layer is a LiF layer or a ZnO layer. The anode is preferably a transparent and conducting electrode; which may be formed from a layer of transparent conductive oxide, such as indium tin oxide, or from a graphene layer. The cathode may be a metal cathode, such as an aluminium cathode. Numerous anode and cathode combinations are known, however, and are possible for use in a device of the invention. Alternatively, the device is a device in which the cathode is a transparent layer and a metal electrode acts as the anode. This type of device can be referred to as an "inverted" device. In such a device the anode may be a silver (Ag) anode. The cathode may be an ITO layer.
The substrate may be formed from any suitable material such as glass or plastic, for example PET (polyethylene terephthalate). A plastic substrate is preferably a flexible plastic substrate. The device may be an electronic or optoelectronic device, such as a photovoltaic cell or light emitting diode. In particular, the device may be an organic device. The organic device may be an organic optoelectronic device, for example an organic photovoltaic device (OPV) or an organic light-emitting diode (OLED).
The active layer may comprise an electron accepting material, such as a fullerene material, and an electron donating material, such as a conjugated polymer. The device may be a bulk heteroj unction device, in which the active layer is formed from a blend of an electron accepting material and an electron donating material. For example, the active layer may be a P3HT:PCBM active layer, formed from a blend of poly(3- hexylthiophene) and phenyl-(61 -butyric acid methyl ester. In an alternative embodiment, the active layer may be a MEH-PPV (poly[2-methoxy-5(2'-ethyl- hexyloxy)-l ,4-phenylene vinylene) active layer. The device may be an optoelectronic device which is a photovoltaic device.
The photovoltaic device (preferably an OPV), in addition to the HTL comprising a graphene oxide film, may comprise one, more, or all of the following:
a glass or plastic substrate;
an indium tin oxide (ITO) anode;
an active layer comprising a blend of poly(3-hexylthiophene) (P3HT) and phenyl-C61 -butyric acid methyl ester (PCBM); and
a metal cathode, such as an aluminium cathode. In conventional OPVs, where a HTL is present between the photoactive layer and the anode, the HTL may comprise a polymeric material such as PEDOT:PSS, a macrocyclic compound such as copper phthalocyanine (CuPC) or an inorganic materials such as V205, NiO and Mo03. In a device of the invention, the use of a GO film as the HTL avoids the need for any additional HTL. Thus, the sole hole transport layer in the device may be a GO film. In another embodiment, the optoelectronic device is an organic light-emitting diode (OLED). An OLED of the invention, in addition to the HTL comprising a graphene oxide film, may comprise one, more, or all of the following:
a plastic substrate
a graphene anode
a MEH-PPV active layer
a metal cathode, such as an aluminium cathode.
In certain embodiments of the invention, the thickness of the graphene oxide film is less than 50nm, preferably less than 20nm, more preferably less than lOnm, even more preferably less than 3.5nm. The film thickness may be 0.5nm to 20nm, preferably 0.5nm to lOnm, more preferably 0.5nm to 3.5nm, even more preferably lnm to 3nm, even more preferably about 2nm. These thicknesses can apply to any of the embodiments of the invention described herein.
In a device of the invention the GO film may be the only HTL present. Accordingly, in a preferred embodiment of any of the embodiments describes above, no additional HTL is present. In a second aspect, the invention provides a process for producing a device comprising, in series:
a substrate;
a transparent anode;
a hole transport layer comprising a graphene oxide film;
an active layer; and
a cathode;
wherein the process comprises the steps of
(a) providing a transparent anode on a substrate;
(b) depositing a graphene oxide film on the first electrode;
(c) applying an active layer on the graphene oxide film;
(d) applying a cathode on the active layer. The graphene oxide film may be deposited on the first electrode by spin coating, vacuum filtration, Langmuir Blodgett, spray casting, dip coating, roll to roll, screen printing or inkjet printing. GO can be deposited from a neutral solvent. The solvent may be water, an aqueous solution, a water miscible solvent (such as methanol), or a mixture thereof. Spin coating with a suspension of GO sheets in water, an aqueous solution, a water miscible solvent (such as methanol), or a mixture thereof is preferably useed. The spin coating speed is preferably from 600-8000 RPM. The concentration of the suspension is preferably 0.05 to 1 mg/ml. It will be appreciated that all features described for the device of the first aspect of the invention also apply to a device as produced by the process of the second aspect of the invention.
In a third aspect the invention provides a hole transport layer (HTL) comprising a graphene oxide film.
In a fourth aspect, the invention provides use of a graphene oxide film as a hole transport layer (HTL). The use may be in an electronic device or an opto-electronic device. Preferably, the device is an organic device. More preferably, the device is an organic opto-electronic device.
All features described for the HTL of the device of the first aspect of the invention also apply to the HTL of the third and fourth aspects of the invention. Non-limiting examples of the invention will be described with reference to the accompanying drawings, in which:
Figure 1 shows (a) a transmission electron microscopy (TEM) image of a GO sheet on a lacey carbon support; (b) the corresponding selected area electron diffraction pattern (SAED) with diffraction spots labelled with Miller-Bravais indices; and (c) the relative intensity profile obtained from the diffraction pattern in (b). Figure 2 shows atomic force microscope (AFM) topography images of GO thin films, with thicknesses of approximately (a) 2nm, (b) 4nm and (c) lOnm, respectively.
Figure 2(d) shows the corresponding optical transmission spectra of the three GO films deposited on ITO/glass substrates. The inset shows the Tauc plot of the 2nm thick GO thin film.
Figure 3(a) shows a schematic of the structure of a photovoltaic device consisting of ITO/GO/P3HT:PCBM/Al. Figure 3(b) shows energy level diagrams of the bottom electrode ITO, interlayer materials (PEDOT:PSS, GO), P3HT (donor) and PCBM (acceptor), and the top electrode Al.
Figure 4(a) shows current-voltage characteristics of three photovoltaic devices one with no hole transport layer (curve labelled as ITO), one with a 30nm PEDOT:PSS layer, and one with a 2nm thick GO film. Figure 4(b) shows current- voltage characteristics of ITO/GO/P3HT:PCBM/Al devices with GO thickness of approximately 2nm, 4nm and 10 nm. All the measurements were under simulated A.M. 1.5 illumination at 100 mW/cm2. Figure 5 shows a plot of charge recombination rate constant krec versus light intensities obtained using TOCVD measurements for ITO-only, PEDOT:PSS, and 2 nm GO thin film devices. The inset shows the transient photovoltage decay curves of the corresponding devices measured at lOOmW/cm . Figure 6(a) shows a schematic of an inverted-type polymer solar cell. Figure 6(b) shows a current-voltage plot for this cell.
Figure 7(a) shows a flexible LED comprising a MEH-PPV active layer and schematic thereof. Figure 7(b) shows a current-voltage plot for this device and also includes traces for a device with a PEDOT:PSS HTL and a device with no HTL.
Meanings of terms used herein are explained below. Graphite oxide is formed by the oxidation of graphite. Monomolecular sheets of graphite oxide are known as graphene oxide sheets. Thus, graphene oxide (GO) is a graphene sheet functionalized with oxygen in the form of epoxy and hydroxyl groups on the basal plane and various other groups at the edges (such as phenol groups). Graphene sheets can be arranged to cover a surface, thereby forming a graphene oxide film. The thickness of a graphene oxide film is determined as a function of the GO sheet monolayer, i.e. the extent to which the GO sheets overlap. Thickness of a graphene oxide film can be measured by Raman spectroscopy, atomic force microscopy profilometry or electron diffraction. Raman spectroscopy measurement measures film thickness by monitoring the shape, position and width of the 2D phonon zone boundary peak (G Eda et al, Nat. Nanotech. 3, 270-274, 2008). A Raman map is created, thickness is measured at multiple points within the map and an average film thickness value can be calculated. Thus, thickness values referred to herein are average thickness values.
Opto-electronics is the study and application of devices that source, detect and control light, usually considered a sub-field of photonics. An optoelectronic device converts electrical energy to optical energy (or vice versa), or uses such conversion in its operation.
An organic device, such as an organic optoelectronic device, is a device comprising conductive organic polymeric material or small molecules. It will be appreciated that whilst at least one component of an organic optoelectronic device comprises conductive organic polymeric material or small molecules, the device may also comprise inorganic components.
A 'hole transport layer' is commonly known in the art to be a layer of material that is capable of transporting holes and blocking the passage of electrons. In a HTL, electron blocking may be partial or complete. Ideally the HTL will give as close to complete electron blocking as possible. In particular, in use a HTL facilitates hole transport between an active layer and an anode, whilst reducing current leakage by blocking electron passage. Thus, a 'hole transport layer' can be referred to as a 'hole transport and electron blocking layer'.
The term 'approximately' can refer to a variation of ± 10%.
It will be appreciated that in the context of this invention a "fullerene material" encompasses fullerenes and fullerene derivatives such as PCBM.
Thin films of GO can be deposited from neutral aqueous solutions. The incorporation of a GO film between the photoactive poly(3-hexylthiophene) (P3HT): phenyl-C61- butyric acid methyl ester (PCBM) layer and a transparent and conducting indium tin oxide (ITO) anode in an OPV leads to a decrease in recombination of electrons and holes and leakage currents. This results in a dramatic increase in the OPV efficiency, giving values that are comparable to devices fabricated with PEDOT:PSS as the hole transport layer. Thus, GO provides a simple solution processable alternative to PEDOT:PSS as the effective hole transport layer in OPV and light emitting-diode devices.
Graphene oxide (GO) is a graphene sheet functionalized with oxygen in the form of epoxy and hydroxyl groups on the basal plane and various other types at the edges. The C-0 bonds within graphene oxide are covalent and thus disrupt the sp conjugation of the hexagonal graphene lattice, making GO an insulator. The
2 3 electronic structure of GO is heterogeneous due to presence of mixed sp and sp hybridizations and therefore cannot be readily explained by traditional valence and conduction band states. Rather, lateral transport occurs by hopping between localized states (sp2 sites) at the Fermi level. The density of such localized states can be increased by removing oxygen using a variety of chemical and thermal reduction treatments, which facilitate the transport of charge carriers. Thus, in contrast to non- reduced GO, reduced GO is a semi-metal.
Material to form a HTL should be as insulating as possible and should be a wide band gap p-type semiconductor. To prevent electron transport, a large energy barrier for electron injection into the HTL can be introduced by placing the conduction band as far away energetically from the valence band as possible (i.e. having a large band gap material). Reduction of GO leads to the formation of conducting pathways. Thus, reduced graphene oxide would allow both types of charge carriers (electrons and holes) to conduct and would not function as an HTL as required by the invention.
Experimental setup
GO sheets and OPVs were prepared and tested as described below. UV-Visible absorption spectra were obtained using a Jasco V-570 UV/Vis NIR Spectrophotometer. Current-voltage characteristics (Keithley 2410 source meter) were obtained by using a solar simulator (Newport Inc.) with the A.M.1.5 filter under irradiation intensity of 100 mW/cm2. For the transient open-circuit voltage decay (TOCVD) measurements, the devices were measured at open circuit condition under illumination from the solar simulator with adjustable intensities. A small perturbation generated by a pulse from a frequency-double Nd:YAG pulsed laser ( A = 532ww , repetition rate 10Hz, a duration- 5 ns) was used. The transient decay signals were recorded by a digital oscilloscope (Tetronix TDS5052B). The samples were mounted in a vacuum chamber under 10"3 torr during measurement. The work function was measured by scanning Kelvin probe microscopy (SKPM) (Innova, Vecco Inc.). For all measurements, Pt/Ir coated cantilevers were used.
Preparation of GO Sheets
Non-reduced GO was obtained from purified natural graphite powder (SP-1 , Bay Carbon) using the modified Hummers method (Hirata, M.et al; Thin-Film Particles of Graphite Oxide 1 : High- Yield Synthesis and Flexibility of The Particles. Carbon 2004, 42, 2929-293 and G Eda et al, Nat. Nanotech. 3, 270-274, 2008). The modified Hummers method is routine in the art. In summary, the method involves oxidation of graphite to form graphite oxide. A graphite and NaN03 mixture is cooled in an ice bath, ¾S04 is added and stirred until homogenized. KMn04 is gradually added while stirring. After further stirring for five days a viscous slurry is obtained. To this 5wt% H2S04 is added gradually while stirring, followed by ¾02. The mixture can be purified by dispersing and precipitating in an aqueous solution of 3wt% H2S0 and 0.5wt% H202. The resulting slurry is then dispersed in deionized water by ultrasonication to yield a suspension of exfoliated individualized GO sheets. A concentration of 8mg/ml of GO sheets in water was prepared for deposition of GO thin films. For transmission electron microscopy (TEM) study a drop of the suspension was deposited on the TEM grid. Transmission electron microscopy (TEM) and diffraction results confirm the structure of GO sheets. A TEM image of a GO sheet and the corresponding selected area electron diffraction pattern (SAED) are shown in Figures 1 (a) and 1(b). The 6-fold symmetry in the diffraction pattern is consistent with the hexagonal structure and the relative intensity of the inner ι I 00 - type and outer 2i I 0-type reflection shown in Figure 1 (c) is consistent with that of a monolayer. Deposition on ITO coated glass
To investigate the hole transport properties of GO in OPVs, uniform thin GO films were deposited on ITO coated glass by spin coating. To create a GO film deposited on a substrate a suspension with a concentration of 0.05 to 1 mg/ml of GO in an aqueous solution or water miscible solvent (e.g. water or methanol) can be spin coated onto a substrate. Spin coating speed can vary from 600 - 8000 RPM. Adjustment of the suspension and spin coating parameters allows tailoring of the film thickness. A film thickness of ~2nm was achieved by spin coating 60ml of a 0.4 mg/ml suspension on an ITO substrate with a spin coating speed of 6000 RPM. Film thicknesses of ~4nm and ~10 nm were achieved keeping the volume and concentration constant, but reducing the spin coating speeds to 4000RPM and 800RPM, respectively.
The GO thin films used had lateral resistivity values in excess of 105 Ω/cm. It must be clarified that the vertical resistivity of non-reduced GO is an order of magnitude lower (~103 - 104 Ω/cm) than the lateral resistivity. This is attributed to the fact that the sp2 clusters are isolated laterally but are in contact with the electrodes in a sandwich metal/GO/metal structure. Thus, charge carriers (both holes and electrons) can be injected and transported via the isolated sp clusters with relative ease in a sandwich device structure. However since the density of the isolated sp clusters is sufficiently low in non-reduced GO, the vertical resistivity is substantial and transport is dictated by the conduction and valence bands of the sp3 sites.
Atomic force microscope (AFM) topography images of GO thin films of three different thicknesses on ITO are shown in Figure 2(a), (b) and (c). The lateral dimensions of GO sheets ranged from 1 - 10 μηι. GO thin films with three different thicknesses were deposited. The average thicknesses of the three samples were estimated to be ~ 2nm, ~ 4 nm and -10 nm with corresponding RMS roughness values of 0.70, 0.97 and 1.40 nm, respectively. These values are lower than the 3 nm RMS roughness of bare glass/ITO substrate, indicating that the deposition of GO layers serves to planarize the anode surface. The optical transmission spectra of GO layers with various thicknesses deposited on ITO/glass substrates shown in Figure 2(d) reveal that although the transmittance decreases slightly with thickness, the GO thin films do not significantly alter the transparency of ITO.
The heterogeneous sp2/sp3 structure of GO makes it difficult to assign a specific energy for the band gap. Since conduction in GO and reduced GO occurs via tunneling between sp2 sites, the band gap represents potential barriers for transport formed by covalent sp bonds. That is, in as-synthesized GO the sp fraction can be as high as 60% so that the sp2 domains are non-percolating, which prevents conduction.
To obtain information about the sp3 potential barriers, the optical gap of GO can be
I
obtained from the Tauc plot using the relation ahv∞ (hv - Eg)2 , where a is the absorption coefficient and hv is the photon energy and E is the optical gap. The Tauc plot for the 2nm thick GO films shown in the inset of Figure 2(d) indicates that Eg is ~ 3.6 eV. This value is comparable to the experimental band gap energy of
NiO, which can be used as a highly efficient hole transport layer. Fabrication of OPV devices For the fabrication of OPV devices (area = 0.1 cm2), the ITO substrates coated with GO thin films were moved into a nitrogen-purged glove box for deposition of the organic active layer and top electrode (Al cathode). The photoactive layers were deposited on top of the GO thin films by spin coating using a 1 :0.8 weight ratio blend of P3HT:PCBM dissolved in chlorobenzene. The film thickness of the photoactive layer was maintained at 200 nm. Al cathodes were then deposited onto the blend layer by thermal evaporation at a pressure of 2x10"6 Torr.
A LiF layer below the Al cathode could also be incorporated. This can be done just before the metal evaporation for the top electrode.
An example photovoltaic device structure consisting of ITO/GO/P3HT:PCBM/Al layers is shown in Figure 3(a). The device comprises an ITO anode, a GO film, a P3HT/PCBM active layer and an Al cathode, which would be deposited sequentially on a substrate. The corresponding energy levels of each component shown in Figure 3(a) are shown in Figure 3(b). The band energies of the ITO, P3HT:PCBM, and Al layers are well known. The work function of GO thin films was determined by scanning Kelvin probe microscopy. The average work function values obtained from measurements on ten different GO thin film samples was found to be 4.9 eV, slightly higher than typical values (4.6 eV) obtained for pristine graphene. The higher values for GO are most likely due to the larger electronegativity of O atoms, which produce surface C8+-05~ dipoles via extraction of π electrons from graphene .
In addition to GO devices, a set of two other devices was fabricated for comparison. ITO-only control devices consisting of ITO/P3HT:PCBM/Al structure; and conventional OPVs incorporating 30 nm PEDOT:PSS as the hole transport layer with the following structure: ITO/PEDOT:PSS/P3HT:PCBM/Al.
The photovoltaic characteristics of the fabricated devices were characterized under simulated A.M. 1.5 illumination at 100 mW/cm2. The current-voltage plots of the devices are shown in Figure 4(a). The average short circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF), and power conversion efficiency values for each set of devices are summarized in Table I. It can be seen that the ITO-only device exhibits average power conversion efficiency (η) of 1.8 ± 0.2%. The insertion of a 2nm thick GO thin film between ITO and P3HT:PCBM results in a substantial increase in Jsc, Voc, and FF, leading to an enhancement in the power conversion efficiency (η) to 3.5 ± 0.3%. For comparison, typical device performance of ITO/PEDOT:PSS/P3HT:PCBM/Al with efficiency values of around 3.6 ± 0.2% is also shown in Figure 4(a).
The influence of GO film thickness on OPV characteristics is shown in Figure 4(b). A clear trend of decreasing power conversion efficiency with increasing GO film thickness can be observed. The thinnest film yielded the best results, most likely due to the increase in serial resistance resulting in lower Jsc and FF and the slightly lower transmittance of the films with thickness. An improvement over ITO only devices was seen for all GO film thicknesses. The corresponding device characteristics are also summarized in Table I.
Table I: Summary of typical photovoltaic parameters of the control and GO HTL devices.
Voc (V) Jsc (mA-cm"2) FF (%) η (%)
ITO only 0.45 9.84 41.5 1.8 ± 0.2
PEDOT:PSS 0.58 11.15 56.9 3.6 ± 0.2
GO(2nm) 0.57 11.40 54.3 3.5 ± 0.3
GO(4nm) 0.57 10.22 33.9 2.0 ± 0.2
GO(lOnm) 0.59 7.84 18.8 0.9 ± 0.2 The overall efficiencies of devices incorporating GO and PEDOT:PSS are within the measurement errors and also variability in device fabrication procedure. The above results clearly indicate that GO thin films are very promising as the hole transport layers in OPVs.
To obtain insight into the recombination rate and mechanisms in the GO HTL devices, transient open-circuit voltage decay (TOCVD) measurements were performed. In TOCVD measurements, devices are illuminated by white light and operated at the steady-state Voc condition, which can be adjusted by varying the intensity of the bias light. Since no charge is collected at Voc, carriers generated by the short laser pulse have finite lifetimes, τ, before recombining as indicated by the transient decay of the photovoltage. The recombination rate, krec , is therefore proportional to 1 /r . The charge recombination rate constants versus illumination intensities for the three types of devices tested are shown in Figure 5(a). The photovoltage decay curves of the three devices under 100 mW/cm illumination are shown in the inset. The decay lifetimes for ITO-only, PEDOT:PSS HTL, and thin GO HTL devices were found to be 8.1 , 9.6, and 1 1.6 ≤, respectively. The longer recombination lifetimes in GO indicate lower recombination rates, suggesting effective suppression of leakage current and separation of carriers via efficient transport of holes to ITO and blocking of electrons. Considering the hole transport mechanism in GO, from the band diagram (Figure 3b), it can be seen that the low injection barrier between the GO and P3HT allows holes to be readily injected into the GO for collection by the ITO electrode. It can be further surmised that the large band gap (~3.6 eV) of GO hinders transport of electrons from the PCBM LUMO to the ITO anode, acting as an effective electron blocking layer. Unlike PEDOT:PSS, non-reduced GO is insulating (or at least a wide band semiconductor) and in contrast to NiO where transfer of holes to ITO is facilitated by injection into Ni2+ defect levels close to the valence band, such p-type behaviour is not observed in clean GO in our transport measurements. The presence of isolated sp clusters near the Fermi level should facilitate the transfer of both electrons and holes to ITO and therefore does not contribute to the enhancement in efficiency. The energy level diagram in Figure 3(b) suggests that injection of holes into the valence of GO is favourable but the exact configuration of the band alignment requires further investigation.
Band alignment energies obtained from optical gap and work function measurements of the GO thin films demonstrate that hole transport and electron blocking is facilitated.
"Inverted" polymer photovoltaic cell
A photovoltaic (solar) cell was prepared. A schematic of this cell is shown in Figure 6(a). The cell comprises an Ag anode, a graphene oxide HTL, a P3HT:PCBM active layer, a ZnO film (an electron transport layer) and an ITO cathode. In this "inverted" cell the transparent layer (ITO) becomes a cathode, rather than the anode as is conventional and the metal electrode becomes the anode. The use of an Ag anode gives the advantage of stability because of use of the noble metal Ag.
Conventionally, aluminium is used which can be subject to oxidation. Thus, this device shows good stability when stored in air.
Figure 6(b) shows the current-voltage plot for this device. This device exhibited a power conversion efficiency of 3.4%. It is noted that the GO layer in this device has an optimised thickness of 2nm.
Flexible Organic LED
An LED was prepared to comprise a flexible PET substrate, a graphene anode, a graphene oxide HTL, a MEH-PPV active layer and an aluminium cathode. This device is shown in Figure 7(a). The current-voltage plot for this device is illustrated in Figure 7(b), together with plots for a device comprising a PEDOT:PSS HTL an a device without any HTL present. The device with a graphene oxide HTL showed similar performance compared to one using a PEDOT.:PSS HTL and the device without any HTL showed a large leakage current. This LED is both flexible and thin, with the thickness of the graphene anode and the graphene oxide HTL being lOnm, compared to 150nm thickness for conventional ITO/PEDOT materials. It will be appreciated to a skilled person reading this disclosure that various changes in form and detail can be made without departing from the true spirit and scope of the invention and such changes are encompassed by the invention.

Claims

1. A device comprising:
an anode;
a hole transport layer comprising a graphene oxide film;
an active layer; and
a cathode.
2. The device of claim 1, wherein the device further comprises a substrate.
0
3. The device of claim 2, wherein the substrate is a glass or plastic substrate.
4. The device of claim 1 or 2, wherein the device comprises an electron transport layer positioned between the active layer and the cathode.
5
5. The device of claim 4, wherein the electron transport layer comprises LiF or ZnO.
6. The device of any preceding claim , wherein the anode is a transparent0 conductive anode.
7. The device of claim 6, wherein the transparent conductive anode is formed from a layer of a transparent conductive oxide. 5
8. The device of claim 7, wherein the oxide is indium tin oxide.
9. The device of claim 8, wherein the transparent conductive anode is a graphene anode. 0
10. The device of claim 9, comprising:
an Ag anode;
a hole transport layer comprising a graphene oxide film;
2078176v1 an active layer;
an ITO cathode.
1 1. The device of any one of claims 1 to 9, wherein the cathode is a metal cathode.
12. The device of claim 11 , wherein the cathode is an aluminium cathode.
13. The device of any preceding claim, wherein the active layer comprises a blend of an electron accepting material and an electron donating material.
14. The device of any preceding claim, wherein the device is an optoelectronic device.
15. The device of claim 14, wherein the device is an organic optoelectronic device.
16. The device of claim 15, wherein the active layer comprises a blend of an electron accepting material and an electron donating material and the electron accepting material is a fullerene or fullerene derivative and the electron donating material is a conjugated polymer.
17. The device of any preceding claim, wherein the device is a photovoltaic device.
18. The device of any preceding claim wherein at least one of the following features is present:
the device comprises a glass or plastic substrate;
the anode is an indium tin oxide (ITO) anode;
the active layer is an active layer comprising a blend of poly(3- hexylthiophene) (P3HT) and phenyl-C61 -butyric acid methyl ester (PCBM) or the active layer is a MEH-PPV layer; and
the cathode is an aluminium cathode.
19. The device of claim 18, comprising:
an ITO anode;
a P3HT-.PCBM active layer;
a hole transport layer comprising a graphene oxide film; and
an aluminium cathode.
20. The device of any preceding claim, wherein the thickness of the graphene oxide film is less than 50nm, preferably less than 20nm, more preferably less than lOnm, even more preferably less than 3.5nm.
21. The device of any preceding claim wherein the GO film thickness is 0.5nm to 20nm, preferably 0.5nm to lOnm, more preferably 0.5nm to 3.5nm, even more preferably lnm to 3nm.
22. The device of any one of claims 1 to 16, 18 and 19, wherein the device is an organic light-emitting diode.
23. The device of claim 22, comprising:
a plastic substrate (for example PET);
a graphene anode;
a hole transport layer comprising a graphene oxide film;
an active layer (for example a MEH-PPV layer); and
a metal cathode (for example an aluminium cathode).
24. A process for producing a device comprising, in series:
a substrate;
a transparent anode;
a hole transport layer comprising a graphene oxide film;
an active layer; and
a cathode;
wherein the process comprises the steps of (e) providing a transparent anode on a substrate;
(f) depositing a graphene oxide film on the first electrode;
(g) applying an active layer on the graphene oxide film;
(h) applying a cathode on the active layer.
25. A device as produced by the method of claim 24.
26. A hole transport layer (HTL) comprising a graphene oxide film.
27. Use of a graphene oxide film as a hole transport layer (HTL).
28. A device or process substantially as described herein with reference to one or more of the examples and/or figures.
2078176v1
PCT/GB2011/000740 2010-05-14 2011-05-13 Device comprising graphene oxide film WO2011141717A1 (en)

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CN103426634A (en) * 2012-05-25 2013-12-04 海洋王照明科技股份有限公司 Flexible current collector, preparation method thereof and application
KR20140136698A (en) * 2013-05-21 2014-12-01 한국화학연구원 Organic light emitting diode using graphene oxide, and the fabrication method threrof
KR101650705B1 (en) * 2013-05-21 2016-08-24 한국화학연구원 Organic light emitting diode using graphene oxide, and the fabrication method threrof
WO2015175765A1 (en) * 2014-05-15 2015-11-19 Elwha Llc Applications of graphene grids in vacuum electronics
CN105280824A (en) * 2014-06-13 2016-01-27 中国科学院苏州纳米技术与纳米仿生研究所 Carbon functional material and preparation method thereof, and solar cell and making method thereof
WO2016134595A1 (en) * 2015-02-25 2016-09-01 Boe Technology Group Co., Ltd. Organic light-emitting diode display device, manufacturing method thereof, and display apparatus
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WO2017108882A1 (en) * 2015-12-23 2017-06-29 Commissariat A L'energie Atomique Et Aux Energies Alternatives Organic optoelectronic device, array of such devices and method for producing such arrays
US10186677B2 (en) 2016-04-20 2019-01-22 Boe Technology Group Co., Ltd. Electroluminescent device and manufacturing method thereof and display device
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