WO2017106811A1 - Polymer passivated metal oxide surfaces and organic electronic devices therefrom - Google Patents

Polymer passivated metal oxide surfaces and organic electronic devices therefrom Download PDF

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WO2017106811A1
WO2017106811A1 PCT/US2016/067432 US2016067432W WO2017106811A1 WO 2017106811 A1 WO2017106811 A1 WO 2017106811A1 US 2016067432 W US2016067432 W US 2016067432W WO 2017106811 A1 WO2017106811 A1 WO 2017106811A1
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layer
nio
metal oxide
electronic device
pvp
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French (fr)
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Franky So
Shuyi LIU
Hyeonggeuyn YU
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University Of Florida Research Foundation, Inc.
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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
    • H10K30/80—Constructional details
    • H10K30/81—Electrodes
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K50/00—Organic light-emitting devices
    • H10K50/80—Constructional details
    • H10K50/805—Electrodes
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K10/00—Organic devices specially adapted for rectifying, amplifying, oscillating or switching; Organic capacitors or resistors having potential barriers
    • H10K10/40—Organic transistors
    • H10K10/46—Field-effect transistors, e.g. organic thin-film transistors [OTFT]
    • H10K10/462—Insulated gate field-effect transistors [IGFETs]
    • H10K10/468—Insulated gate field-effect transistors [IGFETs] characterised by the gate dielectrics
    • H10K10/471—Insulated gate field-effect transistors [IGFETs] characterised by the gate dielectrics the gate dielectric comprising only organic materials
    • 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
    • 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
    • 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

  • Metal oxides have been used as functional layers in organic light-emitting diodes (OLED), organic photovoltaics (OPV), photodetectors (PD), and Field-Effect Transistors (FET). Based on their different transport mechanisms and energy band alignments, they function as hole transport layers (HTL), hole injection layers (HIL), electron transport layers (ETL), electron injection layers (EIL), or dielectric insulators. Most of these metal oxides can be prepared using simple and inexpensive solution processes that are compatible with roll-to-roll fabrication.
  • metal oxide interfaces due to the large amount of trap states or the strong dipoles formed at the metal oxide interface, light quenching phenomena or hysteretic device performance are observed at these metal oxide interfaces, which is detrimental to the OLED, OPV, PD, and FET performance.
  • nickel oxide one of the few p-type metal oxides, displays a strong quenching effect at its surface, which limits its use as a HTL in high efficiency OLEDs and OPVs.
  • the light quenching effect are also observed at the interface between the organic layers and the vanadium oxide (VO x ) or molybdenum oxide (MoO x ) layers, which have been widely used as hole injection layers in OLEDs and hole extraction layers in OPVs.
  • VO x vanadium oxide
  • MoO x molybdenum oxide
  • the light quenching effect is alleviate by inserting a blocking layer between the quenching surface and the active light-emitting/light absorbing layer, based on the effective quenching mechanism, suppression by a blocking layer is not necessarily an effective approach.
  • Most state of the art blocking layers are thermal evaporated small molecules; their use is not compatible in solution-processed devices.
  • Polymers such as poly[N,N'-6 «(4- butylphenyl)-N,N'-bis(phenyl)-benzidine] (Poly-TPD) or poly[2,7-(9,9-di-n-octylfluorene)- co-(l,4-phenylene[(4-sec-butylphenyl)imino]-l,4-phenylene)] (TFB) can be used as an insoluble passivation layer, however, they remain quenchers for the phosphorescent green and blue dopants in OLED due to their lower triplet energy.
  • Polyvinylpyrrolidone has a large band gap and blocks the excitons, but is of little use in OLEDs, OPVs and PDs because of its insulating properties.
  • Embodiments of the invention are directed to the use of polymer having carbonylate functional group based as the binding agent to passivate the surface defects of different types of metal oxides.
  • the organic light-emitting diodes (OLEDs) and organic photovoltaics (OPVs) devices incorporating the metal oxides functional layers show significantly suppressed luminescence quenching with enhanced efficiency.
  • the organic field effect transistors (OFETs) incorporating the metal oxides dielectric insulators show decreased gate leakage current, improved dielectric breakdown strength and significantly decreased hysteresis in the cyclic transfer curve.
  • the infrared photodetectors (PDs) incorporating metal oxide functional layers show enhanced EQE, responsivity and detectivity.
  • FIG. 1 is a schematic of formation of a carbonyl comprising polymer passivation coating on a metal oxide hole injection layer (HIL), according to an embodiment of the invention.
  • HIL metal oxide hole injection layer
  • FIGS. 2A-2C show (FIG. 2 A) a portion of an OLED with a HIL and plots of the photoluminescence of emitting layer (EML) on NiO x (FIG 2B) and VO x (FIG. 2C) HIL where quenching is effectively suppressed after PVP passivation, according to an embodiment of the invention.
  • EML emitting layer
  • FIGS. 3A-3C show (FIG. 3 A) an OLED with a NiO x HIL and plots of current density vs. voltage (FIG. 3B) and current efficiency (FIG. 3C) vs luminescence, where the advantage of the carbonyl comprising polymer layer, according to an embodiment of the invention, provides superior performance.
  • FIGS. 4A-B show (FIG. 4A) a solar cell OPV device geometry with a NiO x HIL and
  • FIGS. 5A-5C show (FIG. 5A) a field-effect transistor where a Hf0 2 gate dielectric and plots showing that (FIG. 5B) the gate leakage current and (FIG. 5C) dielectric breakdown strength of transistors without and with passivated by PVP, according to an embodiment of the invention, is improved by the passivation layer.
  • FIGS. 6A-6B show photoluminescence plots verses wavelength where FIG. 6A is that of a 20 nm thick TCTA: 5 wt% Ir(ppy) 3 emitting layer deposited onto a 40 nm thick HTL of TAPC (top curve), as-prepared NiO x (bottom curve), PVP-passivated NiO x before (2 nd from top curve) and after UV-O3 treatment, according to an embodiment of the invention; and FIG.
  • 6B shows photoluminescence plots for verses wavelength for as-prepared NiO x (bottom curve) and with 10 nm, 20 nm, and 30 nm TAPC exciton blocking layers (assending curves) where the thickest TAPC layer on NiO x displays PL intensity similar to that of the emitter on TAPC without a NiO x layer (top curve).
  • FIGS. 7A-7D show structure and characteristics where FIG. 7 A shows a device structure with energy levels with respect to vacuum level for thermal-evaporated OLEDs;
  • FIG. 7B is a composite plot of the devices J-V characteristics;
  • FIG. 7C is a composite plot of L-V characteristics; and
  • FIG. 7A-7D show structure and characteristics where FIG. 7 A shows a device structure with energy levels with respect to vacuum level for thermal-evaporated OLEDs;
  • FIG. 7B is a composite plot of the devices J-V characteristics;
  • FIG. 7C is a composite plot of L-V characteristics; and
  • FIG. 7D shows current efficiency curves of OLEDs with as- prepared NiO x (open square), UV-ozone treated NiO x (UVO-NiO x , solid square), PVP passivated NiO x (P-NiO x , open circle), PVP passivated NiO x followed by UV-ozone treatment (P-UVO-NiO x , solid circle) , according to an embodiment of the invention (solid circle), and reference TAPC (dashed line) HTLs.
  • FIGS. 8A-8F show an atomic force micrograph (AFM) image where FIG. 8 A is the topography of a 40 nm as-prepared NiOx HTL, FIG. 8B is the topography image after deposition of PVP to P-NiOx HTL, FIG. 8C is the topography image after UV-ozone treatment to P-UVO-NiOx HTL, FIG. 8D is a phase image of 40 nm as-prepared NiOx HTL, FIG. 8E shows a phase image of an AFM phase image after deposition of PVP to P-NiOx HTL where the insert is the AFM phase image of P-NiO x at a different scale bar, and FIG. 8F is a phase image of an AFM phase image after UV-ozone treatment to P-UVO-NiOx HTL.
  • AFM atomic force micrograph
  • FIGS. 9A-9D show high-resolution X-ray photoelectron spectroscopy (XPS) acquisition of (FIG. 9A) C 1 s of P-NiOx (insert: the molecular structure of PVP with labeled CI, C2, C3 and C4), (FIG. 9B) O Is of P-NiO x , (FIG. 9C) is of C Is of P-UVO-NiO x , and (FIG. 9D) is of O Is of P-UVO-NiO x where all of the XPS measurements were carried out at a takeoff angle of 45°.
  • FIGS. 9A C 1 s of P-NiOx (insert: the molecular structure of PVP with labeled CI, C2, C3 and C4)
  • FIGS. 9B O Is of P-NiO x
  • FIG. 9C is of C Is of P-UVO-NiO x
  • FIG. 10A shows the photo luminescence plot of a 20 nm thick TCTA:5 wt % Ir(ppy) 3 emitting layer deposited onto a 40 nm thick as-prepared VO x (bottom curve), PVP-passivated VO x after U V-0 3 treatment (middle curve), and as-prepared VO x with 10 nm thick TAPC exciton blocking layer (top curve), where the baseline (dash) is the photoluminescence intensity on 40 nm thick TAPC HTL and FIG. 10B shows a plot of the J-V-L characteristics, and FIG. 10C shows current efficiency curves of the OLEDs with as-prepared VO x (square) and P- UVO-VO x (circle) HILs.
  • FIGS. 11A-11B show the structure and characterization for perovskite solar cells, where FIG. 11A shows the J-V curves of the perovskite solar cells incorporating PEDOT:PSS (top dash), as-prepared NiO x HTL (second from bottom at OV), UVO-NiO x HTL (top solid), and P-UVO-NiO x HTL (bottom) under AM 1.5 G one sun illumination, and FIG. 11B shows the EQE spectrum of the perovskite solar cells incorporating P-UVO-NiO x HTLs, where the inset shows the device architecture of the perovskite solar cells, according to an embodiment of the invention.
  • FIGS. 12A-12D show the characterization of photodetectors where FIG. 12A shows EQE curves, FIG. 12B shows responsivities, FIG. 12C shows detectivity, and FIG. 12D shows EQE spectral responses of the PbS quantum dots based IR photodetectors incorporating as prepared copper oxide (CuO x ) HTL (black) and PVP passivated CuO x (blue), where the response IR wavelength detected for the plots of FIGs. 12A-C is 1010 nm and the operating bias for measurement of the spectra of FIG. 12D is -1 V.
  • CuO x copper oxide
  • HTL black
  • PVP passivated CuO x blue
  • Embodiments of the invention are directed to organic electronic devices employing a metal oxides hole transport layer or hole injection layer with a thin layer of a carbonyl functional group based polymer for passivation of the metal oxides surface defects to improve device performance in OLED, OPV, PD, or FET.
  • the passivation process is shown in FIG. 1.
  • the metal oxides could work as charge transport layers, charge injection/extraction layers, or gate insulating layers applied to OLED, OPV, PD, or FET.
  • the metal oxides can be nickel oxide, vanadium oxide, molybdenum oxide, zinc oxide, tungsten oxide, titanium oxide, hafnium oxide, aluminum oxide, copper oxide, or any other metal oxide.
  • the polymer is dissolved into the organic solvent with a certain concentration and spin-coated onto the metal oxide.
  • the polymer can be applied by any other means including spray- coating, roll-coating, or any other method of coating.
  • the pristine solvent is spun cast onto the substrate to remove some residue polymer to yield an ultra-thin capping polymer on top of the metal oxide, for example a film of about 1 to about 50 nm.
  • the device being formed with a polymer-capped metal oxide layer is transferred to a UV-ozone cleaner chamber to trigger the chemical reaction between the polymer and metal oxide surface species and to further remove residue polymer.
  • the capping polymers form a rigid binding with the metal oxides.
  • the key parameter here is to optimizing the polymer concentration and spin-speed according to different adsorption abilities of the metal oxides, as well as the UV-ozone treatment time.
  • the UV-ozone treatment oxidizes the residual polymer film and permit bonding with the metal oxide surface. Insufficient UV-ozone exposure will not completely convert all the polymers or remove a sufficient amount of insulating polymer, which results in poor device performance due to the large amount of charge carriers trapped at the metal oxide/capping polymer interface. Excess UV-ozone exposure will fully remove the binding capping polymer and result in insufficient passivation.
  • Metal oxides that are synthesized in air are known to be rich of hydroxyl species that are exciton quenching sites that effect device performance.
  • Efficiency degradation or "roll- off can be very significant when NiO x is used as an HTL. This efficiency roll-off has been attributed to poor charge balance. This evaluation has been determined to be incorrect by the inventors who have discovered that the roll-off results from strong quenching at the NiO x HIL/HTL interface.
  • common strategies included changing the carrier profile in the active layers by modifying the injection layers and inserting an exciton blocking layer to spatially separate the active layer and the metal oxide layer. These strategies do not address the quenching problem directly, but focus on keeping the exciton- forming zone away from the metal oxide surface.
  • SAMs self-assembly monolayers
  • PVP polyvinylpyrrolidone
  • PVP has: good complexion ability with transitional metal ions; good solubility in both organic and polar solvents due to its amphipathic properties; and large band gap energy (about 5.6 eV).
  • PVP is an insulating polymer prohibiting carrier transport and injection/extraction. The insulating layer is inappropriate for optoelectronic device applications.
  • PVP is deposited as a passivation layer on top of metal oxides to suppress exciton quenching for efficient OLED, solar cell, and other electronic devices.
  • UV-ozone UV-ozone
  • nickel oxide NiO x is used as an HTL and vanadium oxide (VO x ) is used as an HIL where PVP is applied as a layer to effectively passivate the metal oxide surfaces and suppress exciton quenching.
  • PVP vanadium oxide
  • Facilitation of charge injection is achieved with the PVP-passivated metal oxides by treatment with UV-03. Upon treatment, strong chemical binding between PVP and the metal oxides surface occurs.
  • polyvinylpyrrolidone (PVP) capping polymer is employed, however other polymer can be used that give similar or superior results.
  • PVP polyvinylpyrrolidone
  • FIG. 3 A shows the OLED architecture with NiO x as hole transport layers and the corresponding device performance.
  • the device with TAPC as a non-quenching HTL was set as a reference.
  • FIG. 4A shows a solar cell, OPV, architecture with NiO x as the HTL and plots of device performance (FIG. 4B), with a device with PEDOT:PSS was set as a reference HTL for comparison of devices with as-prepared NiO x UV-ozone treated NiO x , and PVP- passivated NiO x , which shows the highest open circuit voltage (V oc ), short circuit current (J sc ), fill factor (FF) and external quantum efficiency (EQE).
  • V oc open circuit voltage
  • J sc short circuit current
  • FF fill factor
  • EQE external quantum efficiency
  • FIG. 5A shows an exemplary gate dielectric with PVP passivation used for formation of a field effect transistor (FET), according to an embodiment of the invention.
  • FET field effect transistor
  • FIG. 5B gate leakage current is decreased by at least one order of magnitude
  • FIG. 5C dielectric breakdown strength is increased from 3MV/cm to 5MV/cm, and the hysteresis in the cyclic transfer curve is decreased by 72% in voltage, while keeping the on/off ratio consistent as 105.
  • Figs. 12A-D show the performance a IR photodetector, where the IR absorbing material is lead sulfide quantum dots and the hole transport layer is copper oxide.
  • the carbonyl comprising polymers are oxidized to form carboxylate groups for binding with metal oxides, while retaining the ability to passivate the quenching metal oxide surface without sacrificing the transport and injection/extraction of charge carriers.
  • the devices with the UV-ozone treated polymer-passivated metal oxides have significantly higher efficiencies.
  • metal oxide gate dielectrics with a polymer containing the carbonyl group improves FET performance as well.
  • As-deposited metal oxide gate dielectrics fabricated by solution process or vacuum deposition inherently have hydroxyl groups on the surface that function as charge-trapping sites, causing a hysteresis in the transfer curve.
  • gate leakage current is decreased, dielectric breakdown strength is improved, and the hysteresis in the cyclic transfer curve is significantly decreased.
  • other polymers and oligomers can be employed to passivate the metal oxide.
  • the polymers and oligomers can be copolymers of vinylpyrrolidone and acrylic acid, methacrylic acid, or acid derivatives thereof, such as active esters and halocarbonyls, where the amide/acid ratio can be 1 to 10.
  • Oligomers of vinylpyrrolidone can be on average a dimer through decamer and can be end capped by carboxylate groups by employing chain transfer agents, such as, 2-mercaptoethanol or 2- isopropoxyethanol, to the radical polymerization of the monomer or monomer mixture followed by oxidation of the terminal alcohol to the carboxylic acid.
  • chain transfer agents such as, 2-mercaptoethanol or 2- isopropoxyethanol
  • Nickel acetate tetrahydrate (Ni-(CH 3 COO) 2 4H 2 0) was dissolved in ethanol with mono-ethanolamine (NH 2 CH 2 CH 2 0H) (0.1-0.2 mol L "1 ) at a mole ratio of 1 : 1 to yield the NiO x solution precursor.
  • the VO x solution precursor was synthesized by mixing vanadium oxytriisopropoxide (VO(OCH(CH 3 ) 2 ) 3 ) with isopropanol at a volume ratio of 1 :50.
  • the metal oxide thin films were prepared by spin-coating the precursor solution onto the appropriate substrate. For optical measurements, quartz substrates were used to prevent UV absorption.
  • NiO x film is a p-type semiconductor with a polycrystalline structure confirmed by X-ray diffraction, and the as-prepared VO x film is an n-type amorphous semiconductor.
  • polyvinylpyrrolidone was dissolved in chloroform and spin-cast onto the metal oxide surface.
  • Emitting layers composed of a 20 nm-thick tra(4-carbozoyl-9-ylphenyl)- amine (TCTA) doped with 5 wt% c-tr ⁇ (2-phenylpyridine)-iridium (Ir(ppy)3) deposited on different samples were excited with a monochromatic excitation wavelength of 350 nm.
  • PVP passivation polymer for suppressing the exciton quenching of metal oxides was revealed by comparing the PL intensities of the deposited EMLs to that of a reference sample, which is prepared by depositing the EML on top of a 4,4'-cyclohexylidene-£/s[TSi,N-&w(4- methylphenyl)benzenamine] (TAPC) thin film. Because of its large E g and high triplet energy ( ⁇ ), TAPC can effectively block the singlet and triplet excitons, forming a non- quenching interface with the phosphorescent emitter. To investigate the exciton quenching effects in OLED performance, phosphorescent green OLEDs were fabricated on different samples.
  • a dual -EML was used for easy tuning of the charge balance and confining of the emitting zone and was composed of a 20 nm thick layer of 5 wt % Ir(ppy)3 doped 4,4'-bis(N- carbazolyl)-l,l '-biphenyl (CBP) and a 20 nm thick layer of 5 wt% Ir(ppy) 3 doped TCTA.
  • Jm[3-(3-pyridyl)-mesityl]-borane (3TPYMB) and LiF/Al were vacuum deposited as the ETL and cathode, respectively.
  • FIG. 6A shows the PL spectra with the data normalized to the PL intensity of the TAPC control sample.
  • the as-prepared NiO x sample shows the lowest PL intensity, indicating the strong luminescence quenching nature of the metal oxide.
  • Nickel oxy-hydroxide is a known strong luminescence quencher, and the dipolar NiOOH species on NiO x surface is present in this layer.
  • the strong NiOOH dipoles can facilitate non-radiative decay and the high exciton quenching rate at long distances.
  • the thin PVP passivation polymer appears to: suppress long-range exciton quenching of NiO x by passivating the NiOOH species; and suppress short-range exciton quenching of NiO x by passivating its other surface defects.
  • PL of the emitter on NiO x effectively suppresses exiton quenching using a thin PVP passivation layer.
  • OLED devices were fabricated with five different HTLs: a 40 nm thick as-prepared NiO x HTL, a 40 nm thick as-prepared NiO x HTL followed by UV-0 3 treatment (UVO- NiOx), a 40 nm thick PVP-passivated NiO x HTL (P-NiO x ), a 40 nm thick PVP-passivated NiO x HTL followed by UV-0 3 treatment (P-UVO-NiO x ), and a reference 40 nm thick TAPC HTL.
  • the energy band diagram is shown in FIG.
  • FIGs. 7B-D respectively, and device performances are summarized in Table 1, below.
  • Devices with the as-prepared NiO x HTL, UVO-NiO x HTL, and P-UVO-NiO x HTL have similar luminescence turn-on voltage (V on of about 2.7 eV), similar to the control device with a TAPC HTL, which indicates good energy level alignment between the HTL and EML. Because of superior hole transport properties of NiO x , the three above-mentioned devices show higher current densities than the control device.
  • the device with the as-prepared NiO x HTL shows a very strong efficiency roll-off, which can be attributed to exciton quenching of the NiO x HTL.
  • the emitting zone extends toward the HTL/EML interface, resulting in a stronger EL quenching.
  • the UVO-NiO x HTL device shows higher current densities and a maximum current efficiency that is shifted to higher EL intensity.
  • PVP is a good passivation layer
  • devices with P-NiO x HTL show a very high V on of about 5.0 V and a current efficiency less than 1 Cd/A because of the insulating PVP polymer on the surface of the P-NiOx HTL that inhibits hole injection from the NiO x HTL into the EML.
  • a P-UVO- NiO x device shows significantly improved performance with a maximum current efficiency of 90.8 ⁇ 2.1 Cd/A, which is about 20% higher than the as-prepared NiO x device, along with significantly reduced efficiency roll-off.
  • the current efficiency curve of the P-UVO-NiO x device is also comparable to that of the TAPC control device due to the effectively suppressed exciton quenching.
  • the UV-0 3 treatment plays a significant role in improving the hole injection ability of the PVP-passivated NiO x HTL.
  • AFM was used to investigate the phases present on the surface of PVP-passivated NiO x before and after UV-O3 treatment. To minimize surface contamination, all samples were stored in a vacuum chamber before measurements. AFM topography images of 40 nm thick as-prepared NiO x , P-NiO x and P-UVO-NiO x films on top of indium-tin oxide (ITO) coated substrates are shown in FIGs. 8A-C.
  • the as-prepared NiO x film shows a root-mean- square roughness 280 (RMS) of 1.8 nm with a maximum height variation of 10.4 nm.
  • RMS root-mean- square roughness 280
  • the film is flattened with an RMS of 0.8 nm and a maximum height variation of 5.1 nm.
  • RMS 0.8 nm
  • UV-O3 treatment film roughness increases slightly with an RMS of 1.1 nm and a maximum height variation of 5.9 nm.
  • FIGs. 8D-F AFM phase images are shown in FIGs. 8D-F.
  • the phase image of the as-prepared NiO x film shows a poly crystalline texture with an average grain size of 30 nm.
  • the bright color in the phase image corresponds to the "hard” metal oxide surface.
  • With PVP passivation the NiOx texture completely disappears and the phase image is homogeneous.
  • the dark color in the AFM image corresponds to the "soft" PVP surface.
  • UV-O3 treatment a heterogeneous phase appears with both bright and dark regions.
  • Ni 2p 3/2 , Ols, and C Is were carried out to measure the binding energies of Ni 2p 3/2 , Ols, and C Is.
  • the Ni 2p 3 /2 signal of the PVP-passivated NiOx film remains the same after the UV-O3 treatment Contradictory to UV-O3 treatment effects on the as-prepared NiO x film, treatment with a thin PVP passivation layer does not result in higher oxidization states and enhancement in hole injection of PVP-passivated NiO x is not due to the introduction of more dipolar NiOOH species as previously found in UV-O3 treated NiO x .
  • a detail XPS study of the C Is and O Is signals indicates the surface chemical changes of the PVP-passivated NiO x before and after UV-O3 treatment.
  • FIG. 9A shows a C Is spectrum of the P-NiO x film.
  • the C Is spectrum is de-convoluted into four carbon peaks corresponding to the signal associated with: adventitious carbon with a binding energy (BE) of 285.0 eV (CI); the carbon linked to the carbonyl group with a BE of 285.4 eV (C2); the carbon-nitrogen bond with a BE of 286.2 eV (C3); and the carbonyl bond with a BE of 287.8 eV (C4).
  • BE binding energy
  • FIG. 9B shows an O 1 s spectrum of the P-NiO x film that is composed of a NiO x main peak at a BE of 529.5 eV, a NiO x defect peak at a BE of 531.2 eV, a carbonyl peak at a BE of 532.0 eV, and a surface absorbent peak at a BE of 533.2 eV.
  • FIG. 9C shows the C Is spectrum of the P-UVO-NiO x film. After UV-O3 treatment, changes in the XPS spectrum occurs at high binding energies due to the presence of an ester group with a BE of 288.9 eV.
  • FIG. 9D shows the O Is spectrum of the P-UVO-NiO x film where change to higher binding energies is due to the presence of the ester functional group with a BE of 532.9 eV.
  • the carbon atoms from the carbonyl and ester groups makes up 20.9% and 14.8% of the C Is spectrum, respectively, which are significantly larger than the corresponding values (11.8% and 8.4%) determined from the O Is spectrum and the C/O atomic ratio.
  • the discrepancy in the carbon composition determined from the C Is and O Is signals indicates the presence of additional oxygen atoms due to the formation of ester groups as a result of the chemical reaction between PVP and NiO x , indicating that NiO x shares its oxygen atoms with PVP to form carbonyl/ester groups after UV-O3 treatment.
  • Ni/O/C/N 0.27:1 :3:0.5
  • Ni:0:C:N 0.26:1: 1.6:0.3
  • P-UVO- NiOx 0.27:1 :3:0.5
  • 3 ⁇ 4umbers outside the parentheses indicate the binding energies (eV) of different components and the number inside the parentheses indicate the component ratio.
  • the asterisks indicate the corresponding atoms.
  • VO x shows a PVP passivation effect on their quenching mechanism.
  • PL intensities were normalized to that of the TAPC sample as shown in FIG. 10A.
  • VO x is an n-type HIL with a deep electron affinity; excitons are directly quenched at the VO x /EML interface.
  • FIG. 10B shows J-V-L curves of the devices incorporating the as-prepared VO x and PVP-passivated VO x followed by the UV-0 3 treatment (P-UVO-VO x ), and FIG. IOC shows the current efficiency data.
  • the device with P-UVO-VO x shows improved current efficiency starting from high EL intensities (>2000 Cd m ⁇ 2 ). However, the device does not show an enhanced current efficiency at low luminescence intensities. This appears to be due to absence of long-range exciton quenching in the VO x devices. The EL is thus only quenched when the emitting zone is close to the VO x /EML interface, which occurs at relatively high luminescence.
  • PVP passivation is beneficial to metal oxides for other optoelectronic applications, for example, iodine perovskite methyl ammonium lead iodide (MAPbI 3 ) solar 399 cells with PHJ structures.
  • MAPbI 3 iodine perovskite methyl ammonium lead iodide
  • the active layer of MAPbI 3 was synthesized by dipping a 150 nm thick Pbl 2 film into the MAI solution.
  • the J-V curves (under AM 1.5G 1 sun illumination) of the PHJ perovskite solar cells incorporating the as-prepared NiO x HTL, UVO-NiO x HTL, P-UVO- NiO x HTL and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PE DOT:PSS) as a control device are shown in FIG. 1 1 A.
  • the external quantum efficiency (EQE) spectrum of the PHJ perovskite solar cell incorporating P-UVO-NiO x HTL is shown in FIG. 11B.
  • the device architecture is shown as an insert in FIG. 1 1B.
  • the device performance data are summarized in Table 3, below.
  • the integrated J sc from the EQE spectrum is 20.2 mA 41 1 cm , which is consistent with the measured value of 20.3 mA cm for the device with P- UVO-NiO x HTL.
  • the PHJ perovskite solar cell with P-UVO-NiO x HTL shows the highest open circuit voltage (V oc ) of 1.04 ⁇ 0.02 V, the highest short circuit current density (Jsc) of 20.1 ⁇ 0.4 mA cnT 2 , and the highest PCE of 10.9 ⁇ 0.3% among all of the four samples, indicating the passivation technique is also beneficial for metal oxides used in solar cells.

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Abstract

An electronic device is constructed with a passivation layer on a metal oxide surface of the device. The metal oxide can be on an electrode as a hole transport layer (HTL), hole injection layer (HIL), electron transport layer (ETL), or electron injection layer (EIL). The passivation layer is a polymer or oligomer comprising carboxylate groups that can be deposited on the metal oxide surface distal to the electrode from solution and optionally oxidized. The oxidation can be carried out by treatment of the deposited polymer with ultraviolet radiation in the presence of ozone.

Description

DESCRIPTION
POLYMER PASSIVATED METAL OXIDE SURFACES AND ORGANIC ELECTRONIC DEVICES THEREFROM
CROSS-REFERENCE TO A RELATED APPLICATION This application claims the benefit of U.S. Provisional Application Serial No. 62/268,806, filed December 17, 2015, the disclosure of which is hereby incorporated by reference in its entirety, including all figures, tables and drawings.
BACKGROUND OF INVENTION
Metal oxides have been used as functional layers in organic light-emitting diodes (OLED), organic photovoltaics (OPV), photodetectors (PD), and Field-Effect Transistors (FET). Based on their different transport mechanisms and energy band alignments, they function as hole transport layers (HTL), hole injection layers (HIL), electron transport layers (ETL), electron injection layers (EIL), or dielectric insulators. Most of these metal oxides can be prepared using simple and inexpensive solution processes that are compatible with roll-to-roll fabrication. However, due to the large amount of trap states or the strong dipoles formed at the metal oxide interface, light quenching phenomena or hysteretic device performance are observed at these metal oxide interfaces, which is detrimental to the OLED, OPV, PD, and FET performance. For example, nickel oxide, one of the few p-type metal oxides, displays a strong quenching effect at its surface, which limits its use as a HTL in high efficiency OLEDs and OPVs. The light quenching effect are also observed at the interface between the organic layers and the vanadium oxide (VOx) or molybdenum oxide (MoOx) layers, which have been widely used as hole injection layers in OLEDs and hole extraction layers in OPVs.
The light quenching effect is alleviate by inserting a blocking layer between the quenching surface and the active light-emitting/light absorbing layer, based on the effective quenching mechanism, suppression by a blocking layer is not necessarily an effective approach. Most state of the art blocking layers are thermal evaporated small molecules; their use is not compatible in solution-processed devices. Polymers such as poly[N,N'-6«(4- butylphenyl)-N,N'-bis(phenyl)-benzidine] (Poly-TPD) or poly[2,7-(9,9-di-n-octylfluorene)- co-(l,4-phenylene[(4-sec-butylphenyl)imino]-l,4-phenylene)] (TFB) can be used as an insoluble passivation layer, however, they remain quenchers for the phosphorescent green and blue dopants in OLED due to their lower triplet energy. Polyvinylpyrrolidone) has a large band gap and blocks the excitons, but is of little use in OLEDs, OPVs and PDs because of its insulating properties.
BRIEF SUMMARY
Embodiments of the invention are directed to the use of polymer having carbonylate functional group based as the binding agent to passivate the surface defects of different types of metal oxides. With the passivation polymer, the organic light-emitting diodes (OLEDs) and organic photovoltaics (OPVs) devices incorporating the metal oxides functional layers show significantly suppressed luminescence quenching with enhanced efficiency. The organic field effect transistors (OFETs) incorporating the metal oxides dielectric insulators show decreased gate leakage current, improved dielectric breakdown strength and significantly decreased hysteresis in the cyclic transfer curve. The infrared photodetectors (PDs) incorporating metal oxide functional layers show enhanced EQE, responsivity and detectivity.
BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a schematic of formation of a carbonyl comprising polymer passivation coating on a metal oxide hole injection layer (HIL), according to an embodiment of the invention.
FIGS. 2A-2C show (FIG. 2 A) a portion of an OLED with a HIL and plots of the photoluminescence of emitting layer (EML) on NiOx (FIG 2B) and VOx (FIG. 2C) HIL where quenching is effectively suppressed after PVP passivation, according to an embodiment of the invention.
FIGS. 3A-3C show (FIG. 3 A) an OLED with a NiOx HIL and plots of current density vs. voltage (FIG. 3B) and current efficiency (FIG. 3C) vs luminescence, where the advantage of the carbonyl comprising polymer layer, according to an embodiment of the invention, provides superior performance.
FIGS. 4A-B show (FIG. 4A) a solar cell OPV device geometry with a NiOx HIL and
(FIG. 4B) current density voltage plots of a PEDOT:PSS reference HIL and that of the NiOx HIL, as deposited, after UV-ozone treatment, and after deposition of a PVP passivation layer followed by UV-ozone treatment, according to an embodiment of the invention. FIGS. 5A-5C show (FIG. 5A) a field-effect transistor where a Hf02 gate dielectric and plots showing that (FIG. 5B) the gate leakage current and (FIG. 5C) dielectric breakdown strength of transistors without and with passivated by PVP, according to an embodiment of the invention, is improved by the passivation layer.
FIGS. 6A-6B show photoluminescence plots verses wavelength where FIG. 6A is that of a 20 nm thick TCTA: 5 wt% Ir(ppy)3 emitting layer deposited onto a 40 nm thick HTL of TAPC (top curve), as-prepared NiOx (bottom curve), PVP-passivated NiOx before (2nd from top curve) and after UV-O3 treatment, according to an embodiment of the invention; and FIG. 6B shows photoluminescence plots for verses wavelength for as-prepared NiOx (bottom curve) and with 10 nm, 20 nm, and 30 nm TAPC exciton blocking layers (assending curves) where the thickest TAPC layer on NiOx displays PL intensity similar to that of the emitter on TAPC without a NiOx layer (top curve).
FIGS. 7A-7D show structure and characteristics where FIG. 7 A shows a device structure with energy levels with respect to vacuum level for thermal-evaporated OLEDs; FIG. 7B is a composite plot of the devices J-V characteristics; FIG. 7C is a composite plot of L-V characteristics; and FIG. 7D shows current efficiency curves of OLEDs with as- prepared NiOx (open square), UV-ozone treated NiOx (UVO-NiOx, solid square), PVP passivated NiOx (P-NiOx, open circle), PVP passivated NiOx followed by UV-ozone treatment (P-UVO-NiOx, solid circle) , according to an embodiment of the invention (solid circle), and reference TAPC (dashed line) HTLs.
FIGS. 8A-8F show an atomic force micrograph (AFM) image where FIG. 8 A is the topography of a 40 nm as-prepared NiOx HTL, FIG. 8B is the topography image after deposition of PVP to P-NiOx HTL, FIG. 8C is the topography image after UV-ozone treatment to P-UVO-NiOx HTL, FIG. 8D is a phase image of 40 nm as-prepared NiOx HTL, FIG. 8E shows a phase image of an AFM phase image after deposition of PVP to P-NiOx HTL where the insert is the AFM phase image of P-NiOx at a different scale bar, and FIG. 8F is a phase image of an AFM phase image after UV-ozone treatment to P-UVO-NiOx HTL.
FIGS. 9A-9D show high-resolution X-ray photoelectron spectroscopy (XPS) acquisition of (FIG. 9A) C 1 s of P-NiOx (insert: the molecular structure of PVP with labeled CI, C2, C3 and C4), (FIG. 9B) O Is of P-NiOx, (FIG. 9C) is of C Is of P-UVO-NiOx, and (FIG. 9D) is of O Is of P-UVO-NiOx where all of the XPS measurements were carried out at a takeoff angle of 45°. FIGS. 1 OA- IOC show characterization plots for VOx HIL comprising layers where FIG. 10A shows the photo luminescence plot of a 20 nm thick TCTA:5 wt % Ir(ppy)3 emitting layer deposited onto a 40 nm thick as-prepared VOx (bottom curve), PVP-passivated VOx after U V-03 treatment (middle curve), and as-prepared VOx with 10 nm thick TAPC exciton blocking layer (top curve), where the baseline (dash) is the photoluminescence intensity on 40 nm thick TAPC HTL and FIG. 10B shows a plot of the J-V-L characteristics, and FIG. 10C shows current efficiency curves of the OLEDs with as-prepared VOx (square) and P- UVO-VOx (circle) HILs.
FIGS. 11A-11B show the structure and characterization for perovskite solar cells, where FIG. 11A shows the J-V curves of the perovskite solar cells incorporating PEDOT:PSS (top dash), as-prepared NiOx HTL (second from bottom at OV), UVO-NiOx HTL (top solid), and P-UVO-NiOx HTL (bottom) under AM 1.5 G one sun illumination, and FIG. 11B shows the EQE spectrum of the perovskite solar cells incorporating P-UVO-NiOx HTLs, where the inset shows the device architecture of the perovskite solar cells, according to an embodiment of the invention.
FIGS. 12A-12D show the characterization of photodetectors where FIG. 12A shows EQE curves, FIG. 12B shows responsivities, FIG. 12C shows detectivity, and FIG. 12D shows EQE spectral responses of the PbS quantum dots based IR photodetectors incorporating as prepared copper oxide (CuOx) HTL (black) and PVP passivated CuOx (blue), where the response IR wavelength detected for the plots of FIGs. 12A-C is 1010 nm and the operating bias for measurement of the spectra of FIG. 12D is -1 V.
DETAILED DISCLOSURE
Embodiments of the invention are directed to organic electronic devices employing a metal oxides hole transport layer or hole injection layer with a thin layer of a carbonyl functional group based polymer for passivation of the metal oxides surface defects to improve device performance in OLED, OPV, PD, or FET. The passivation process is shown in FIG. 1. The metal oxides could work as charge transport layers, charge injection/extraction layers, or gate insulating layers applied to OLED, OPV, PD, or FET. The metal oxides can be nickel oxide, vanadium oxide, molybdenum oxide, zinc oxide, tungsten oxide, titanium oxide, hafnium oxide, aluminum oxide, copper oxide, or any other metal oxide. The polymer is dissolved into the organic solvent with a certain concentration and spin-coated onto the metal oxide. Alternately the polymer can be applied by any other means including spray- coating, roll-coating, or any other method of coating. After drying at an appropriate temperature for a few minutes, for example 1-20 minutes at room temperature to about 120 °C, the pristine solvent is spun cast onto the substrate to remove some residue polymer to yield an ultra-thin capping polymer on top of the metal oxide, for example a film of about 1 to about 50 nm. After drying, the device being formed with a polymer-capped metal oxide layer is transferred to a UV-ozone cleaner chamber to trigger the chemical reaction between the polymer and metal oxide surface species and to further remove residue polymer. After UV-ozone exposure, the capping polymers form a rigid binding with the metal oxides. The key parameter here is to optimizing the polymer concentration and spin-speed according to different adsorption abilities of the metal oxides, as well as the UV-ozone treatment time. The UV-ozone treatment oxidizes the residual polymer film and permit bonding with the metal oxide surface. Insufficient UV-ozone exposure will not completely convert all the polymers or remove a sufficient amount of insulating polymer, which results in poor device performance due to the large amount of charge carriers trapped at the metal oxide/capping polymer interface. Excess UV-ozone exposure will fully remove the binding capping polymer and result in insufficient passivation.
Metal oxides that are synthesized in air are known to be rich of hydroxyl species that are exciton quenching sites that effect device performance. Efficiency degradation or "roll- off can be very significant when NiOx is used as an HTL. This efficiency roll-off has been attributed to poor charge balance. This evaluation has been determined to be incorrect by the inventors who have discovered that the roll-off results from strong quenching at the NiOx HIL/HTL interface. To alleviate exiton quenching, common strategies included changing the carrier profile in the active layers by modifying the injection layers and inserting an exciton blocking layer to spatially separate the active layer and the metal oxide layer. These strategies do not address the quenching problem directly, but focus on keeping the exciton- forming zone away from the metal oxide surface. These approaches complicate device architecture and add limitation to the device's application. According to embodiments of the invention, suppression of exciton quenching is achieved by passivation of the surface of the metal oxides. To such an end, self-assembly monolayers (SAMs) have been constructed on top of the oxides. However, most SAMs are not designed for device applications, and they are especially unfavorable for solution-processed optoelectronic devices due to the hydrophobic surface of the SAM layer that is problematic for the wetting process. Therefore, to passivate the metal oxide surface for the solution process, a hydrophilic polymer is required and polyvinylpyrrolidone (PVP) is a potential candidate.
PVP has: good complexion ability with transitional metal ions; good solubility in both organic and polar solvents due to its amphipathic properties; and large band gap energy (about 5.6 eV). However, PVP is an insulating polymer prohibiting carrier transport and injection/extraction. The insulating layer is inappropriate for optoelectronic device applications. In an embodiment of the invention, PVP is deposited as a passivation layer on top of metal oxides to suppress exciton quenching for efficient OLED, solar cell, and other electronic devices. By treating the PVP passivation layer with UV-ozone (UV-O3), carrier injection through the PVP passivation layer is drastically improved allowing substantially enhanced device performance absent the PVP layer or without treatment of the PVP layer.
In exemplary embodiments of the invention, nickel oxide NiOx is used as an HTL and vanadium oxide (VOx) is used as an HIL where PVP is applied as a layer to effectively passivate the metal oxide surfaces and suppress exciton quenching. Facilitation of charge injection is achieved with the PVP-passivated metal oxides by treatment with UV-03. Upon treatment, strong chemical binding between PVP and the metal oxides surface occurs. This treatment was shown to enhance a phosphorescence green OLED incorporating the PVP- passivated NiOx HTL upon subsequent UV-O3, where upon treatment a high current efficiency of 90.8 ± 2.1 Cd A-1, which is among the highest efficiency, if not the highest efficiency, for all OLEDs incorporating solution-processed metal oxides as a carrier transport layer. OLEDs according to an embodiment of the invention, using NiOx as an HTL or VOx as an HIL show significantly reduced efficiency roll-off when a treated PVP passivation layer is present. In an embodiment of the invention, passivation perovskite solar cells incorporating solution-processed NiOx HTLs, display significantly enhanced power conversion efficiency (PCE). The passivation technique is fully compatible with solution processing of optoelectronic devices.
In an exemplary embodiment of the invention, polyvinylpyrrolidone (PVP) capping polymer is employed, however other polymer can be used that give similar or superior results. After polymer passivation and UV-ozone treatment, the light quenching effect is suppressed, for example, on NiOx as a HTL and VOx as a HIL, as revealed by the photoluminescence (PL) measurement in FIG. 2B where a non-quenching HTL, TAPC, is shown as a reference. FIG. 3 A shows the OLED architecture with NiOx as hole transport layers and the corresponding device performance. The device with TAPC as a non-quenching HTL was set as a reference. Compared to the as-prepared NiOx and UV-ozone treated NiOx, the PVP-passivated NiOx shows highest current efficiency (FIG. 3C) with same turn-on voltage (FIG. 3B). FIG. 4A shows a solar cell, OPV, architecture with NiOx as the HTL and plots of device performance (FIG. 4B), with a device with PEDOT:PSS was set as a reference HTL for comparison of devices with as-prepared NiOx UV-ozone treated NiOx, and PVP- passivated NiOx, which shows the highest open circuit voltage (Voc), short circuit current (Jsc), fill factor (FF) and external quantum efficiency (EQE). FIG. 5A shows an exemplary gate dielectric with PVP passivation used for formation of a field effect transistor (FET), according to an embodiment of the invention. After PVP passivation, (FIG. 5B) gate leakage current is decreased by at least one order of magnitude, (FIG. 5C) dielectric breakdown strength is increased from 3MV/cm to 5MV/cm, and the hysteresis in the cyclic transfer curve is decreased by 72% in voltage, while keeping the on/off ratio consistent as 105. Figs. 12A-D show the performance a IR photodetector, where the IR absorbing material is lead sulfide quantum dots and the hole transport layer is copper oxide. After PVP passivation on the copper oxide, due to the suppression of the strong interfacial dipole, which is detrimental to hole extraction, the holes generated in PbS quantum dots films are more efficiently extracted, leading to a higher EQE (Fig. 12 A), responsivity (Fig. 12B), and detectivity (Fig. 12C) of the device.
Upon UV-ozone exposure, the carbonyl comprising polymers, according to embodiments of the invention, are oxidized to form carboxylate groups for binding with metal oxides, while retaining the ability to passivate the quenching metal oxide surface without sacrificing the transport and injection/extraction of charge carriers. Compared to the OLED and OPV devices incorporating either as-prepared or UV-ozone treated metal oxides, the devices with the UV-ozone treated polymer-passivated metal oxides have significantly higher efficiencies.
Surface passivation of metal oxide gate dielectrics with a polymer containing the carbonyl group improves FET performance as well. As-deposited metal oxide gate dielectrics fabricated by solution process or vacuum deposition inherently have hydroxyl groups on the surface that function as charge-trapping sites, causing a hysteresis in the transfer curve. In this work, after passivating the surface of metal oxides such as Hf02 or AI2O3 films with the polymer containing the carbonyl group, gate leakage current is decreased, dielectric breakdown strength is improved, and the hysteresis in the cyclic transfer curve is significantly decreased. In embodiments of the invention, other polymers and oligomers can be employed to passivate the metal oxide. The polymers and oligomers can be copolymers of vinylpyrrolidone and acrylic acid, methacrylic acid, or acid derivatives thereof, such as active esters and halocarbonyls, where the amide/acid ratio can be 1 to 10. Oligomers of vinylpyrrolidone can be on average a dimer through decamer and can be end capped by carboxylate groups by employing chain transfer agents, such as, 2-mercaptoethanol or 2- isopropoxyethanol, to the radical polymerization of the monomer or monomer mixture followed by oxidation of the terminal alcohol to the carboxylic acid. By controlling the proportion of the carboxylic acid in the oligomer or polymer, passivation of the metal oxides with conversion of the MOOH to MO(OC=0-R) can be prepared without a UV-ozone degradation of the polymer.
METHODS AND MATERIALS
Nickel acetate tetrahydrate (Ni-(CH3COO)2 4H20) was dissolved in ethanol with mono-ethanolamine (NH2CH2CH20H) (0.1-0.2 mol L"1) at a mole ratio of 1 : 1 to yield the NiOx solution precursor. The VOx solution precursor was synthesized by mixing vanadium oxytriisopropoxide (VO(OCH(CH3)2)3) with isopropanol at a volume ratio of 1 :50. The metal oxide thin films were prepared by spin-coating the precursor solution onto the appropriate substrate. For optical measurements, quartz substrates were used to prevent UV absorption. For X-ray photoelectron spectroscopy (XPS) measurements, substrates were single crystal Si wafers. Other measurements employed pre-cleaned indium tin oxide (ITO) substrates. After spin-coating, samples were heated to 500 °C for NiOx and 150 °C for VOx. The as-prepared NiOx film is a p-type semiconductor with a polycrystalline structure confirmed by X-ray diffraction, and the as-prepared VOx film is an n-type amorphous semiconductor. To passivate the metal oxide surface, polyvinylpyrrolidone was dissolved in chloroform and spin-cast onto the metal oxide surface. Samples were spin-rinsed with chloroform, yielding an ultrathin (<5 nm) PVP passivation polymer. A short UV-ozone treatment (5-10 min) was subsequently carried out on the PVP-passivated NiOx or VOx surface. No significant change of the film transmittance was observed after PVP passivation and UV-ozone treatment.
Emitting layers (EMLs) composed of a 20 nm-thick tra(4-carbozoyl-9-ylphenyl)- amine (TCTA) doped with 5 wt% c-tr^(2-phenylpyridine)-iridium (Ir(ppy)3) deposited on different samples were excited with a monochromatic excitation wavelength of 350 nm. The role of PVP passivation polymer for suppressing the exciton quenching of metal oxides was revealed by comparing the PL intensities of the deposited EMLs to that of a reference sample, which is prepared by depositing the EML on top of a 4,4'-cyclohexylidene-£/s[TSi,N-&w(4- methylphenyl)benzenamine] (TAPC) thin film. Because of its large Eg and high triplet energy (Ετ), TAPC can effectively block the singlet and triplet excitons, forming a non- quenching interface with the phosphorescent emitter. To investigate the exciton quenching effects in OLED performance, phosphorescent green OLEDs were fabricated on different samples. A dual -EML was used for easy tuning of the charge balance and confining of the emitting zone and was composed of a 20 nm thick layer of 5 wt % Ir(ppy)3 doped 4,4'-bis(N- carbazolyl)-l,l '-biphenyl (CBP) and a 20 nm thick layer of 5 wt% Ir(ppy)3 doped TCTA. Jm[3-(3-pyridyl)-mesityl]-borane (3TPYMB) and LiF/Al were vacuum deposited as the ETL and cathode, respectively. To investigate the role of UV-03 treatment on PVP- passivated metal oxides, atomic force microscopy (AFM) was used to examine the phase presence on PVP-passivated NiOx and X-ray photoemission spectroscopy (XPS) measurements were carried out to study the surface chemistry. To demonstrate that the PVP passivation is beneficial to metal oxides for other optoelectronic applications, perovskite solar cells with planar heteroj unction (PHJ) structure were prepared on different NiOx samples.
To study the exciton quenching effect of NiOx HTLs, EMLs composed of a 20 nm- thick TCTA doped with 5 wt% iridium (Ir(ppy)3) were deposited onto as-prepared NiOx, PVP-passivated NiOx (before and after UV-03 treatment), and TAPC (as a control HTL). FIG. 6A shows the PL spectra with the data normalized to the PL intensity of the TAPC control sample. The as-prepared NiOx sample shows the lowest PL intensity, indicating the strong luminescence quenching nature of the metal oxide. With a thin PVP passivation layer, this quenching effect is effectively suppressed as evidenced by the similar PL intensity compared with the control sample. With subsequent UV-03 treatment on PVP-passivated NiOx, the PL intensity is also very strong, indicating that the suppression of luminescence quenching is not significantly affected by the treatment. There are two possible exciton quenching mechanisms due to NiOx: deep trap states located at the NiOx surface act as the non-radiative recombination sites; and energy/charge transfer from the emitter to the NiOx surface species. Since exciton quenching via deep trap states only takes place when the excitons reach the NiOx surface, inserting a thin exciton blocking layer between the NiOx and EML should efficiently suppress this quenching effect. Samples with 10, 20, and 30 nm thick TAPC films as exciton blocking layers between the NiOx film and the EML were constructed and the PL intensities of these samples, as shown in FIG. 6B, indicated that luminescence quenching remained strong until a thick 30 nm TAPC layer was inserted, indicating a long-range interaction between the quenching species at the NiOx surface and the excitons with a quenching distance of about 30 nm. Nickel oxy-hydroxide (NiOOH) is a known strong luminescence quencher, and the dipolar NiOOH species on NiOx surface is present in this layer. The strong NiOOH dipoles can facilitate non-radiative decay and the high exciton quenching rate at long distances. The thin PVP passivation polymer appears to: suppress long-range exciton quenching of NiOx by passivating the NiOOH species; and suppress short-range exciton quenching of NiOx by passivating its other surface defects. PL of the emitter on NiOx effectively suppresses exiton quenching using a thin PVP passivation layer.
OLED devices were fabricated with five different HTLs: a 40 nm thick as-prepared NiOx HTL, a 40 nm thick as-prepared NiOx HTL followed by UV-03 treatment (UVO- NiOx), a 40 nm thick PVP-passivated NiOx HTL (P-NiOx), a 40 nm thick PVP-passivated NiOx HTL followed by UV-03 treatment (P-UVO-NiOx), and a reference 40 nm thick TAPC HTL. The energy band diagram is shown in FIG. 7A and current density-voltage (J-V) curves, corresponding luminescence-voltage (L-V) and current efficiency curves are shown in FIGs. 7B-D, respectively, and device performances are summarized in Table 1, below. Devices with the as-prepared NiOx HTL, UVO-NiOx HTL, and P-UVO-NiOx HTL have similar luminescence turn-on voltage (Von of about 2.7 eV), similar to the control device with a TAPC HTL, which indicates good energy level alignment between the HTL and EML. Because of superior hole transport properties of NiOx, the three above-mentioned devices show higher current densities than the control device. The device with the as-prepared NiOx HTL shows a very strong efficiency roll-off, which can be attributed to exciton quenching of the NiOx HTL. By increasing bias, the emitting zone extends toward the HTL/EML interface, resulting in a stronger EL quenching. The UVO-NiOx HTL device shows higher current densities and a maximum current efficiency that is shifted to higher EL intensity.
Table 1 OLED Performance Incorporating As-Prepared NiOx, UVO-NiOx, P-NiOx, P-UVO- NiOx, and TAPC HTLs
Device's HTL Von (V) V at 10 max CE CE at l0J Cd CE at l04 Cd
Cd m~2 (V) (Cd A"1) rn^ CCd A"1) m^ fCd A'1) a-prepared NiOx 2.71 ± 0.03 5.67 ± 0.07 74.2 ± 1.3 41.4 ± 1.6 18.1 ± 1.9
UVO-NiOx 2.65 ± 0.02 4.29 ± 0.10 71.0 ± 2.2 66.5 ± 2.0 36.9 ± 2.1
P-NiOx 5.26 ± 0.29 0.15 ± 0.05
P-UVO-NiOx 2.66 ± 0.55 5.56 ± 0.13 90.8 ± 2.1 76.8 ± 1.9 56.5± ±2.3
TAPC (ref) 2.66 ± 0.01 6.35 ± 0.08 92.0 ± 0.9 80.2 ± 1.5 61.5 ± 2.1 These results are consistent with UV-03 treatment introducing additional NiOOH dipolar species on the NiOx surface with enhanced hole injection due to the emitting zone extending further away from the NiOx surface and resulting in a reduction in efficiency roll- off. However, because of the formation of more NiOOH species, UV-ozone treatment slightly aggravated the quenching effect of NiOx resulting in a lower maximum current efficiency. Although PVP is a good passivation layer, devices with P-NiOx HTL show a very high Von of about 5.0 V and a current efficiency less than 1 Cd/A because of the insulating PVP polymer on the surface of the P-NiOx HTL that inhibits hole injection from the NiOx HTL into the EML. In contrast, according to an embodiment of the invention, a P-UVO- NiOx device shows significantly improved performance with a maximum current efficiency of 90.8 ± 2.1 Cd/A, which is about 20% higher than the as-prepared NiOx device, along with significantly reduced efficiency roll-off. The current efficiency curve of the P-UVO-NiOx device is also comparable to that of the TAPC control device due to the effectively suppressed exciton quenching. The UV-03 treatment plays a significant role in improving the hole injection ability of the PVP-passivated NiOx HTL.
AFM was used to investigate the phases present on the surface of PVP-passivated NiOx before and after UV-O3 treatment. To minimize surface contamination, all samples were stored in a vacuum chamber before measurements. AFM topography images of 40 nm thick as-prepared NiOx, P-NiOx and P-UVO-NiOx films on top of indium-tin oxide (ITO) coated substrates are shown in FIGs. 8A-C. The as-prepared NiOx film shows a root-mean- square roughness 280 (RMS) of 1.8 nm with a maximum height variation of 10.4 nm. After PVP passivation, the film is flattened with an RMS of 0.8 nm and a maximum height variation of 5.1 nm. For the PVP-passivated NiOx film followed by 5 min UV-O3 treatment film roughness increases slightly with an RMS of 1.1 nm and a maximum height variation of 5.9 nm.
AFM phase images are shown in FIGs. 8D-F. The phase image of the as-prepared NiOx film shows a poly crystalline texture with an average grain size of 30 nm. The bright color in the phase image corresponds to the "hard" metal oxide surface. With PVP passivation, the NiOx texture completely disappears and the phase image is homogeneous. The dark color in the AFM image corresponds to the "soft" PVP surface. After UV-O3 treatment, a heterogeneous phase appears with both bright and dark regions. These results indicate that PVP may react with the NiOx surface species upon UV-O3 treatment, yielding NiOx-rich domains (bright) and PVP -rich domains (dark). XPS was carried out to measure the binding energies of Ni 2p3/2, Ols, and C Is. The Ni 2p3/2 signal of the PVP-passivated NiOx film remains the same after the UV-O3 treatment Contradictory to UV-O3 treatment effects on the as-prepared NiOx film, treatment with a thin PVP passivation layer does not result in higher oxidization states and enhancement in hole injection of PVP-passivated NiOx is not due to the introduction of more dipolar NiOOH species as previously found in UV-O3 treated NiOx. A detail XPS study of the C Is and O Is signals indicates the surface chemical changes of the PVP-passivated NiOx before and after UV-O3 treatment.
FIG. 9A shows a C Is spectrum of the P-NiOx film. On the basis of the carbon configuration in PVP, the C Is spectrum is de-convoluted into four carbon peaks corresponding to the signal associated with: adventitious carbon with a binding energy (BE) of 285.0 eV (CI); the carbon linked to the carbonyl group with a BE of 285.4 eV (C2); the carbon-nitrogen bond with a BE of 286.2 eV (C3); and the carbonyl bond with a BE of 287.8 eV (C4). The ratio of the four carbons is consistent with the composition of the repeating unit of PVP (CI :C2:C3 :C4 = 2: 1 :2.T). FIG. 9B shows an O 1 s spectrum of the P-NiOx film that is composed of a NiOx main peak at a BE of 529.5 eV, a NiOx defect peak at a BE of 531.2 eV, a carbonyl peak at a BE of 532.0 eV, and a surface absorbent peak at a BE of 533.2 eV. From the C Is spectrum, the C=0 double bond makes up 16.6% of the whole C Is spectrum, which is consistent with the value determined from the O Is spectrum and the C/O atomic ratio, and the C=0 signals in the O Is spectrum is entirely attributed to the carbonyl groups in the PVP polymer. These results indicate that there is no chemical reaction between the as-prepared NiOx and PVP. FIG. 9C shows the C Is spectrum of the P-UVO-NiOx film. After UV-O3 treatment, changes in the XPS spectrum occurs at high binding energies due to the presence of an ester group with a BE of 288.9 eV. FIG. 9D shows the O Is spectrum of the P-UVO-NiOx film where change to higher binding energies is due to the presence of the ester functional group with a BE of 532.9 eV. The carbon atoms from the carbonyl and ester groups makes up 20.9% and 14.8% of the C Is spectrum, respectively, which are significantly larger than the corresponding values (11.8% and 8.4%) determined from the O Is spectrum and the C/O atomic ratio. The discrepancy in the carbon composition determined from the C Is and O Is signals indicates the presence of additional oxygen atoms due to the formation of ester groups as a result of the chemical reaction between PVP and NiOx, indicating that NiOx shares its oxygen atoms with PVP to form carbonyl/ester groups after UV-O3 treatment. The resulting chemical binding between PVP and the NiOx surface species facilitates hole injection from P-UVO-NiOx into the adjacent EMLs, yielding devices with a higher efficiency. The resolved details of the C Is, O Is, and Ni 2p3/2 XPS signals of the PVP passivated NiOx surface before (P-NiOx) and after (P-UVO-NiOx) UV-03 treatment are summarized in Table 2, below.
Table 2 Resolved XPS data of C I s, O Is, and Ni 2p3 2 Signals of PVP Passivated NiOx Surface before and after UV-03 Treatment^
C 1 s signal CI (adv.) C2 C3 (C-N) C4 (C=0) C5 (0-C=Q)
P-NiOx 285.0 285.4 286.2 287.8
(33.0%) (17.8%) (32.7%) (16.6%)
P-UVO- 285.0 285.4 286.2 287.8 288.9
NiOx (25.5%) (13.3%) (25.6%) (20.9%) (14.8%)
O ls signal NiO (main) NiO (shldr) C=0* 0=C-0* H20, 02 (rsd)
P-NiOx 529.5 531.2 532.0 533.3 (4.9%)
(28.5%) (16.7%) (49.8%)
P-UVO- 529.5 531.1 532.0 532.9 533.4 (7.3%)
NiOx (26.9%) (19.9%) (32.4%) (13.5%)
Ni 2p3/2 signal NiO (main) NiO (shoulder) NiO (shakeup)
as-prepared NiOx 854.1 856.0 (29.2%) 861.2 (50.3%)
(20.6%)
UVO-NiOx 854.1 855.9 (34.2%) 861.2 (49.5%)
(16.3%)
P-NiOx 854.1 856.0 (26.6%) 861.2 (48.8%)
(24.6%)
P-UVO-NiOx 854.1 856.0 (28.3%) 861.2 (46.3%)
(25.4%)
atomic ratio Ni/O/C/N = 0.27:1 :3:0.5 (P-NiOx) Ni:0:C:N = 0.26:1: 1.6:0.3 (P-UVO- NiOx)
¾umbers outside the parentheses indicate the binding energies (eV) of different components and the number inside the parentheses indicate the component ratio. The asterisks indicate the corresponding atoms.
Other metal oxides, such as VOx show a PVP passivation effect on their quenching mechanism. The photoluminescence of a 20 nm thick TCTA:5 wt % Ir(ppy)3 film on VOx with and without a 10 nm thick of TAPC exciton blocking layer, and PVP-passivated VOx film followed by UV-O3 treatment. PL intensities were normalized to that of the TAPC sample as shown in FIG. 10A. Unlike NiOx, VOx is an n-type HIL with a deep electron affinity; excitons are directly quenched at the VOx/EML interface. However, because of the absence of long-distance (>10 nm) quenching process, a 10 nm thick TAPC layer on top of VOx is sufficient to suppress the luminescence quenching. The lower PL intensity from the sample with a PVP-passivated VOx layer is due to the thinner PVP passivation layer (<5 nm), which cannot sufficiently suppress exciton quenching near the VOx surface based on a quenching mechanism different from that of NiOx. To study the passivation effect on OLEDs, devices were fabricated with the VOx HILs. FIG. 10B shows J-V-L curves of the devices incorporating the as-prepared VOx and PVP-passivated VOx followed by the UV-03 treatment (P-UVO-VOx), and FIG. IOC shows the current efficiency data. The device with P-UVO-VOx shows improved current efficiency starting from high EL intensities (>2000 Cd m~2). However, the device does not show an enhanced current efficiency at low luminescence intensities. This appears to be due to absence of long-range exciton quenching in the VOx devices. The EL is thus only quenched when the emitting zone is close to the VOx/EML interface, which occurs at relatively high luminescence.
PVP passivation is beneficial to metal oxides for other optoelectronic applications, for example, iodine perovskite methyl ammonium lead iodide (MAPbI3) solar 399 cells with PHJ structures. The active layer of MAPbI3 was synthesized by dipping a 150 nm thick Pbl2 film into the MAI solution. The J-V curves (under AM 1.5G 1 sun illumination) of the PHJ perovskite solar cells incorporating the as-prepared NiOx HTL, UVO-NiOx HTL, P-UVO- NiOx HTL and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PE DOT:PSS) as a control device are shown in FIG. 1 1 A. The external quantum efficiency (EQE) spectrum of the PHJ perovskite solar cell incorporating P-UVO-NiOx HTL is shown in FIG. 11B. The device architecture is shown as an insert in FIG. 1 1B. The device performance data are summarized in Table 3, below. The integrated Jsc from the EQE spectrum is 20.2 mA 41 1 cm , which is consistent with the measured value of 20.3 mA cm for the device with P- UVO-NiOx HTL. The PHJ perovskite solar cell with P-UVO-NiOx HTL shows the highest open circuit voltage (Voc) of 1.04 ± 0.02 V, the highest short circuit current density (Jsc) of 20.1 ± 0.4 mA cnT2, and the highest PCE of 10.9 ± 0.3% among all of the four samples, indicating the passivation technique is also beneficial for metal oxides used in solar cells. Similar results were also found in organic bulk heteroj unction (BHJ) solar cells incorporating NiOx HTLs. Using poly(N-9-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',l ',3'- benzothidiazole) (PCDTBT) and [6,6]-phenyl-C71 butyric acid methyl ester (PC70BM) as an active layer in the polymer solar cell, where the device power conversion efficiency is enhanced by more than 60% due to passivation of the NiOx layer. Table 3 Performance of MAPbI3 Perovskite Solar Cells Incorporating PEDOT:PSS, As-Prepared NiOx, UVO-NiOx, and P-UVO-NiOx HTLs
devices with diff. Voc (V) Jsc (mA cm 2) FF (%) PCE (%) HTLs
PEDOT:PSS 0.89 ± 0.03 8.9 ± 0.5 53.7 ± 1.1 4.3 ± 0.4 as-prepared NiOx 0.92 ± 0.02 18.0 ± 0.4 49.9 ± 0.8 8.3 ± 0.3
UVO-NiOx 1.00 ± 0.03 14.4 ± 0.7 46.5 ± 0.8 6.6 ± 0.4
P-UVO-NiOx 1.04 ± 0.02 20.1 ± 0.4 52.1 ± 0.9 10.9 ± 0.3
Exciton quenching of solution-processed NiOx HTL and VOx HIL were investigated with PL measurements. A long-range quenching effect is evidenced at the NiOx/organic interface while a short-range quenching effect is evidenced at the VOx/organic interface. With a thin PVP passivation polymer, the luminescence quenching of the phosphorescent green emitter on both NiOx HTL and VOx HIL is efficiently suppressed. A short UV-03 treatment on the PVP-passivated metal oxide surface is required to yield high-performance OLEDs and solar cell devices. XPS analysis revealed that there are chemical bindings between the PVP polymer and the surface species of metal oxides after the UV-03 treatment, resulting in enhanced charge injection and extraction in OLEDs and solar cell devices, respectively.
It should be understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application.

Claims

CLAIMS We claim:
1. An electronic device, comprising a metal oxide layer having a passivation layer situated on the metal oxide layer distal to an electrode layer, wherein the passivation layer comprises a polymer or oligomer with carboxylate functionality.
2. The electronic device according to claim 1, wherein the metal oxide comprises nickel oxide, vanadium oxide, molybdenum oxide, copper oxide, zinc oxide, tungsten oxide, titanium oxide, hafnium oxide, or aluminum oxide.
3. The electronic device according to claim 1, wherein the passivation layer comprises oligovinylpyrrolidone or polyvinylpyrrolidone having carboxylate groups associated with the metal oxide layer.
4. The electronic device according to claim 3, wherein the oligovinylpyrrolidone or polyvinylpyrrolidone having carboxylate groups associated with the metal oxide layer is an ultraviolet ozone degraded polyvinylpyrrolidone.
5. The electronic device according to claim 3, wherein the oligovinylpyrrolidone or polyvinylpyrrolidone having carboxylate groups are cooligomers or copolymers of vinylpyrrolidone and acrylic acid or methacrylic acid.
6. The electronic device according to claim 3, wherein the passivation layer comprises oligovinylpyrrolidone terminated with carboxylic acid groups.
7. The electronic device according to claim 1, wherein the metal oxide is a hole transport layer (HTL), hole injection layer (HIL), electron transport layer (ETL), or electron injection layer (EIL).
8. The electronic device according to claim 1, further comprising an emitting layer (EML) wherein the device is an organic light emitting diode (OLED).
9. The electronic device according to claim 8, wherein the metal oxide layer is a hole transport layer (HTL) and/or hole injection layer (HIL) and wherein the passivation layer resides between the metal oxide layer and the emitting layer.
10. The electronic device according to claim 1, further comprising a photoactive layer wherein the device is an organic photovoltaic cell (OPV) or a photodetector (PD).
11. The electronic device according to claim 10, wherein the metal oxide layer is a hole transport layer (HTL) and/or hole injection layer (HIL) and wherein the passivation layer resides between the metal oxide layer and the active layer.
12. The electronic device according to claim 1, wherein the active layer is an organic comprising perovskite having a planar heteroj unction (PHJ) or an organic active layer comprising a PHJ or a bulk heteroj unction (BHJ).
13. The electronic device according to claim 1, wherein the metal oxide layer is a gate dielectric of a field effect transistor (FET).
14. A method of preparing a device according to claim 1, comprising:
providing a substrate comprising an electrode;
depositing a metal oxide layer from solution or suspension on the electrode; and depositing a passivation layer on the metal oxide layer from solution.
15. The method of claim 14, further comprising treating the passivation layer with ultraviolet radiation and ozone to form carboxylate groups.
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