ORGANIC PHOTODETECTORS AND PRODUCTION METHOD THEREOF

Abstract

An organic photodetector for detecting infrared, visible and ultraviolet radiation is provided with a tunable spectral response to achieve a high responsivity at different design wavelengths. The organic photodetector comprises at least a substrate, a first electrode, a second electrode and at least one organic material, which is arranged between the first and the second electrodes, wherein a Schottky barrier is formed at the interface between the first electrode and the organic material and/or at the interface between the second electrode and the organic material. The tunability in the responsivity of the organic photodetector is achieved by structuring at least one electrode so that it comprises nano-apertures for exciting surface plasmon resonances.

Claims

1. An organic photodetector for detecting infrared, visible and ultraviolet radiation, comprising: a substrate; at least a first electrode and a second electrode wherein at least one electrode of the first electrode and the second electrode has a surface exposed to incident radiation; a charge transport layer arranged between the first electrode and the second electrode, the charge transport layer comprising at least one organic material; wherein a Schottky barrier is formed at an interface between the first electrode and the at least one organic material and/or at an interface between the second electrode and the at least one organic material; wherein the at least one electrode having a surface exposed to incident radiation, and with a Schottky barrier formed between the at least one electrode and the at least one organic material, comprises nano-apertures for exciting surface plasmon resonances, wherein hot carriers generated by surface plasmon decay contribute to a photocurrent; and wherein the nano-apertures are configured to selectively detect the incident radiation at a design wavelength.

2. The organic photodetector as claimed in claim 1, wherein the at least one organic material has a bandgap larger than an energy corresponding to the design wavelength.

3. The organic photodetector as claimed in claim 1, wherein the first electrode is formed on the substrate on which the at least one organic material is interposed between the first and the second electrode.

4. The organic photodetector as claimed in claim 1, wherein the at least one organic material is formed on the substrate, on which material the first and the second electrodes are disposed to be laterally spaced apart from each other.

5. The organic photodetector as claimed in claim 1, wherein a Fabry-P?rot cavity is formed between the first electrode and the second electrode and/or between the first electrode and the substrate by the at least one organic material being transparent at least in a range of the design wavelength of the nano-apertures, and a thickness of a charge transport layer comprising at least one organic material is selected to provide cavity resonance at the design wavelength.

6. The organic photodetector as claimed in claim 1, wherein the substrate comprises a dielectric layer and an electrode layer to form a third electrode which is provided as a gate electrode of an organic transistor.

7. The organic photodetector as claimed in claim 1, wherein the nano-apertures are provided in form of an array having a periodic arrangement.

8. The organic photodetector as claimed in claim 1, wherein the at least one electrode which comprises nano-apertures is transparent, semi-transparent or non-transparent to the incident radiation.

9. A method of producing an organic photodetector as claimed in claim 1, the method comprising: providing a first electrode, a second electrode and at least one organic material on a substrate, where the at least one organic material is arranged between the first electrode and the second electrode; and structuring at least one of the electrodes to form nano-apertures for exciting surface plasmon resonances; wherein the at least one organic material is made of organic small molecules or a polymer to form a charge transport layer and to form a Schottky barrier between the first electrode and the at least one organic material and/or between the second electrode and the at least one organic material; wherein the geometry and arrangement of the nano-apertures are adjusted to selectively detect incident radiation at a design wavelength for providing a tunable spectral response.

10. The method of producing an organic photodetector as claimed in claim 9, wherein the at least one organic material is chosen to have a bandgap larger than an energy corresponding to the design wavelength of the organic photodetector.

11. The method of producing an organic photodetector as claimed in claim 9, wherein the first electrode is deposited on the substrate, the at least one organic material is deposited on the first electrode, and the second electrode is deposited on the at least one organic material to form a vertical configuration.

12. The method of producing an organic photodetector as claimed in claim 9, wherein the at least one organic material is deposited on the substrate, the first and the second electrode are provided on the at least one organic material, wherein the first electrode is spaced laterally apart from the second electrode to form a lateral configuration.

13. The method of producing an organic photodetector as claimed in claim 9, wherein the at least one organic material is transparent at least in a range of the design wavelength of the nano-apertures, and a thickness of the at least one organic material is selected to form a Fabry-P?rot cavity between the first and the second electrode and/or between the first electrode and the substrate for providing cavity resonance at the design wavelength.

14. The method of producing an organic photodetector as claimed in claim 9, wherein at least one of the electrodes is structured to form an array of nano-apertures having a periodic arrangement.

15. The method of producing an organic photodetector as claimed in claims 9, wherein an oxide layer and/or a dopant layer is deposited between the first electrode and the at least one organic material and/or between the second electrode and the at least one organic material to adjust the Schottky barrier.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Exemplary embodiments of the invention will now be described with reference to the drawings. In the drawings:

[0045] FIGS. 1a and 1b are schematic sectional views of a photodetector in a vertical configuration;

[0046] FIGS. 2a and 2b are schematic sectional views of a photodetector in a lateral configuration;

[0047] FIG. 3 is a schematic top view of an electrode with nano-holes;

[0048] FIG. 4 shows a SEM image of an electrode with nano-apertures;

[0049] FIG. 5 is a plot of curves of transmittance versus wavelength for the first electrode;

[0050] FIG. 6 is a plot of curves of absorptance versus wavelength with and without Fabry-P?rot cavity;

[0051] FIG. 7a is a plot of the measured absorptance spectrum of an organic semiconductor film;

[0052] FIG. 7b is a comparison of the responsivity spectra of a photodetector according to the invention and a reference device, and

[0053] FIG. 8 is a comparison of the responsivity spectra of organic photodetectors according to the invention with structured electrodes of different thickness.

DETAILED DESCRIPTION OF THE DRAWINGS

[0054] An exemplary photodetector in a vertical configuration is illustrated in FIG. 1a. The photodetector comprises a substrate 1, a first photoactive electrode 2, an organic material 3 as the charge transport layer and a second electrode 4 to form a sandwich structure. The organic material 3, for example small molecules or a polymer, has a wider band gap than the design wavelength of the photodetector, which provides a good Schottky barrier for low leakage current and exhibits compatibility with large-area deposition processes.

[0055] Substrate 1 is made of transparent materials such as glass or plastics for mechanical support. Under bottom illumination, incident light 5 may transmit through substrate 1 and impinge on the surface of first electrode 2.

[0056] First electrode 2 preferably comprises nano-apertures such as nano-apertures for exciting plasmon resonance on its surface faced to the organic material and is also used as a light absorbing and charge conducting electrode at the same time. A Fabry-P?rot cavity is formed between both electrodes 2, 4 by tuning the thickness of the organic material 3 to provide cavity resonance at a design wavelength of interest. The second electrode 4 may work as a mirror to reflect light for collecting photons more efficiently. The photodetector may be operated with a bias voltage 6 between two electrodes to enhance the responsivity thereof. This vertical configuration may provide an easy integration with further organic devices.

[0057] FIG. 1b shows another exemplary photodetector. Substrate 1 may comprise a transparent oxide layer 11 to form a gate dielectric of an organic transistor and an electrode layer 10 to form a gate electrode thereof. In this case, the organic transistor can work as photodetector.

[0058] FIGS. 2a and 2b show further exemplary photodetectors in the lateral configuration which is a simple and versatile alternative to devices built in sandwich configuration. Both electrodes 2, 4 are arranged on top of the transparent organic material 3 spaced apart from each other. The substrate 1 serves as mechanical support (FIG. 2a) or as gate with a dielectric layer (FIG. 2b) as described above. Both embodiments may be operated with a bias voltage 6.

[0059] FIG. 3 shows an electrode, e.g. first electrode 2, having nano-apertures which are provided in form of hexagonally-ordered nano-holes 21 in metallic material 2. The wavelength of surface plasmon resonance ?SPR can be determined by the following equation:

[00001]${?}_{\mathrm{SPR}}=\frac{a_0}{\sqrt{i^2+j^2+\mathrm{ij}}}.Math.\sqrt{\frac{3}{4}.Math.\frac{{\mathrm{.Math.}}_m.Math.{\mathrm{.Math.}}_d}{{\mathrm{.Math.}}_m+{\mathrm{.Math.}}_d}},$

where a.sub.0 is the distance between two holes, (i,j) the Bragg resonance orders, ?.sub.m the dielectric function of the electrode and ?.sub.d the dielectric function of the organic material. In this way, the design wavelength may be tailored by the nano-apertures.

[0060] FIG. 4 shows a SEM image of an electrode structured by colloidal lithography. For fabricating such nano-apertures, polymer particles such as polystyrene nanoparticles are packed into a hexagonal arrangement and placed onto a clean glass substrate. The nanoparticles are etched by oxygen plasma using RIE (reactive ion etching) to reduce the diameter of the nanoparticles, which also defines the size of the holes on the first electrode. In this manner, nano-apertures having sub-wavelength dimension can easily be produced. The nanoparticles serve as a lithographic mask. After the mask is formed with desired dimension and pattern, a thin metal layer is deposited on the mask, e.g. by thermal evaporation in vacuum, spin-coating, or various printing techniques etc. The nanoparticles are dissolved and removed by a lift-off process to form a continuous perforated metal electrode. FIG. 4 is an SEM image of the first electrode which is made of a 30 nm thick silver film. Nanoparticles were etched by oxygen plasma for 1 minute. The electrode has nano-apertures of an array of sub-wavelength holes with a hole-diameter of 337 nm and a period distance of 608 nm.

[0061] FIG. 5 shows experimental results of transmission of a silver film with and without nano-holes in comparison with simulation results. The simulation was carried out using parameters of an electrode shown in FIG. 4. Curve 100 is the transmittance of 30 nm thick silver film without nano-holes and can be regarded as a reference line. Light at a wavelength of about 350 nm can transmit through the closed silver film with a transmittance of about 70%. The transmittance drops dramatically as the wavelength increases. As shown by curve 101 which represents the measured transmittance of the electrode having nano-holes with hole diameters of 337 nm and a period of 608 nm. The silver film has another transmittance peak at a longer wavelength. The transmittance peak at a wavelength of about 1000 nm is caused by the surface plasmon resonance effect using nano-hole structures. Curve 102 represents simulation results, which show a good agreement with the experimental results. This indicates that the fabricated electrode has highly ordered nano-holes and is plasmonically active.

[0062] FIG. 6 shows the absorption enhanced by a Fabry-P?rot cavity, which is modeled by finite-difference time-domain simulations (FDTD). Curve 200 is the absorptance of a structured electrode made of a 30 nm thick silver film having nano-apertures as shown in FIG. 4, but without the second electrode as a mirror, i.e., without Fabry-P?rot cavity. Curve 200 can be regarded as a reference line. Curve 201 is the absorptance with a Fabry-P?rot cavity which is formed by a stack of 30 nm Ag/air/100 nm Al. With the help of the cavity, more efficient light collection is expected.

[0063] FIG. 7a shows the measured absorptance spectrum 300 of intrinsic 2,2,7,7-Tetrakis-(N,N-di-p-methylphenylamino)-9,9-spirobifluorene (spiro-TTB). It has a wide energy gap of approximately 3.4 eV and therefore shows only very weak absorption of photons in the visible region of the spectrum, which ranges approximately from 1.6 eV to 3.3 eV.

[0064] FIG. 7b show a comparison of the measured responsivity spectrum 401 of a photodetector according to the invention with a nano-hole structured electrode facing the incident radiation and the responsivity spectrum 400 of a reference device with a planar electrode facing the incident radiation. A tungsten-halogen lamp dispersed by a grating monochromator is used to illuminate the devices. When the lamp is turned on, the devices can be illuminated from the bottom side thereof with unpolarized, monochromatic light.

[0065] For the measurement of curve 401, a photodetector as described in FIG. 1 with bias voltage was fabricated using a spiro-linked compound as organic semiconductor. For the example, a 30 nm thick silver film was deposited on the substrate as the first electrode and structured using colloidal lithography to have nano-hole structures as shown in FIG. 4. A 500 nm thick 2,2,7,7-Tetrakis-(N,N-di-p-methylphenylamino)-9,9-spirobifluorene (spiro-TTB) layer was deposited on the first electrode and a 100 nm thick aluminum film on the organic semiconductor layer as the second electrode. The device used for measuring curve 400 features the same design with an unstructured silver film.

[0066] As displayed by curve 400, within the optical gap of spiro-TTB, the responsivity of the device with the planar silver film is low and increases with increasing photon energy, as expected for internal photoemission across the silver/spiro-TTB interface. The peaks located at photon energies of 1.29 eV, 1.92 eV, 2.46 eV and 2.88 eV correspond to resonant orders of the vertical Fabry-P?rot cavity formed by the device.

[0067] In contrast to curve 400, curve 401 shows a significantly improved responsivity, featuring a continuous band between 1 and 2 eV with a peak at approximately 1.5 eV, corresponding to a wavelength of approximately 830 nm. The detection mechanism can be summarized as follows: Photons are transmitted through the substrate and reach the interface between the electrode and the organic material. Around the surface plasmon resonance wavelength, photons couple with the nano-hole electrode and induce charge density oscillations, allowing a strong absorption in the electrode. The absorbed photons create surface plasmons, which decay non-radiatively into hot electrons. By applying an electric field between two electrodes, hot electrons can be injected into the organic materials and result in a detectable photocurrent.

[0068] The exemplary photodetector according to the invention demonstrates an enhanced sub-bandgap response in the near-infrared region. The contribution of the fundamental internal photoemission to the photocurrent still exists as a background signal, but is comparably small, and the spectrum is clearly dominated by the plasmon-induced signal. This photodetector has experimentally shown a detection peak at a wavelength of about 830 nm and may be used as an organic infrared sensor. The detection peak may be further tuned by changing the diameter and periodicity of nano-holes. The photocurrent may be increased by increasing the bias voltage, which results in improved responsivity.

[0069] Another advantageous feature of the organic photodetector according to the invention displayed in curve 401 is that the organic photodetector is optically inactive in the region of energies between 1.77 eV and 3 eV. By using a transparent spiro-TTB layer and the silver nano-hole structured electrode for plasmon excitation in the near-infrared region, a transparent window for higher energy photons can be opened up, which implies that the organic photodetector for the near-infrared spectrum can be stacked with a detector for the visible spectrum, by matter of example.

[0070] FIG. 8 shows a comparison of the responsivity spectrum 500 of an organic photodetector according to the invention as displayed in FIG. 1 with a nano-hole structured electrode of thin silver with a thickness of 30 nm, and the responsivity spectrum 501 of an organic photodetector according to the invention as displayed in FIG. 1 with a nano-hole structured electrode of thick silver with a thickness of 100 nm. Both curves 500, 501 display a pronounced peak in the responsivity around a wavelength of 830 nm. This demonstrates that optical transmission through the silver electrode is not essential for the working principle of the organic photodetector, as the organic semiconductor layer is not an active layer and does not need to absorb photons. The responsivity 500 of the thin electrode is still higher than that of the thick electrode because, to a certain extent, the thick silver layer impedes the progress of hot carriers to the Schottky junction.

LIST OF REFERENCE NUMERALS

[0071] 1 Substrate

[0072] 2 First electrode

[0073] 3 Organic material

[0074] 4 Second electrode

[0075] 5 Incident light

[0076] 6 Bias voltage

[0077] 10 Electrode layer

[0078] 11 Dielectric layer

[0079] 2 Metallic material

[0080] 21 Nano-hole

[0081] 100 Transmittance curve

[0082] 101 Transmittance curve

[0083] 102 Transmittance curve

[0084] 200 Absorptance curve

[0085] 201 Absorptance curve

[0086] 300 Absorptance curve

[0087] 400 Responsivity curve

[0088] 401 Responsivity curve

[0089] 500 Responsivity curve

[0090] 501 Responsivity curve