GB2558233A

GB2558233A - Photodetector

Info

Publication number: GB2558233A
Application number: GB1622032.9A
Authority: GB
Inventors: Newsome Christopher
Original assignee: Sumitomo Chemical Co Ltd
Current assignee: Sumitomo Chemical Co Ltd
Priority date: 2016-12-22
Filing date: 2016-12-22
Publication date: 2018-07-11
Also published as: GB201622032D0

Abstract

A method of forming a photodetector comprises: forming at least one layer of a photoactive zone 105 over a first electrode 103; forming multiple charge transporting regions in a laterally spaced patterned layer 107 over the photoactive layer; and forming a patterned second electrode 109 selectively on the plurality of charge transporting regions. The patterned layers may be formed by printing. Colloidal metal ink may be used to print the second electrode. Optionally, the first electrode is an anode and the second is a cathode which may be formed from silver. The transport layer may be formed from a metal oxide such as zinc oxide, or a metal carbonate, to transport electrons. There may also be a hole transporting layer (111, Fig. 2) between the anode and photoactive zone. The photoactive layer may be a mixture of an electron acceptor and donor. These may be formed from organic materials. There is also a sensor comprising an array of light emitters (201, Fig. 4) and photodetector, where the first electrode is supported on an additional substrate 101.

Description

(71) Applicant(s):

Sumitomo Chemical Company Limited (Incorporated in Japan)

27-1, Shinkawa 2-chome, Chuo-ku, Tokyo 104-8260, Japan (72) Inventor(s):

Christopher Newsome (56) Documents Cited:

WO 2014/006812 A1 US 20100148072 A1 US 20040094721 A1

WO 2013/019077 A2 US 20080087835 A1 (58) Field of Search:

INT CL G01T, H01L

Other: WPI, EPODOC, Patent Fulltext (74) Agent and/or Address for Service:

Venner Shipley LLP

Byron House, Cambridge Business Park,

Cowley Road, Cambridge, CB4 0WZ, United Kingdom (54) Title of the Invention: Photodetector

Abstract Title: Method of forming a high resolution photodetector with patterned electrodes (57) A method of forming a photodetector comprises: forming at least one layer of a photoactive zone 105 over a first electrode 103; forming multiple charge transporting regions in a laterally spaced patterned layer 107 over the photoactive layer; and forming a patterned second electrode 109 selectively on the plurality of charge transporting regions. The patterned layers may be formed by printing. Colloidal metal ink may be used to print the second electrode. Optionally, the first electrode is an anode and the second is a cathode which may be formed from silver. The transport layer may be formed from a metal oxide such as zinc oxide, or a metal carbonate, to transport electrons. There may also be a hole transporting layer (111, Fig. 2) between the anode and photoactive zone. The photoactive layer may be a mixture of an electron acceptor and donor. These may be formed from organic materials. There is also a sensor comprising an array of light emitters (201, Fig. 4) and photodetector, where the first electrode is supported on an additional substrate 101.

FIGURE lA

FIGURE IB

1/3

FIGURE ιΑ

FIGURE IB

2/3

FIGURE 2

FIGURE 3

301

3/3

FIGURE 4

Photodetector

Field of the Invention

The present invention relates to photodetectors and methods of making the same.

Background of the Invention

A range of organic electronic devices comprising organic semiconductor materials are known including organic light-emitting devices, organic field effect transistors, organic photovoltaic devices and organic photodetectors (OPDs).

OPDs comprise an electron acceptor and an electron donor between two electrodes. US 2003/066950 discloses a photoresponsive device comprising a blend of semiconductive polymers.

J.J. van Franeker, “All-solution processed regular organic solar cells using a new inkjetprintable cathode” http://alexandria.tue.nl/extra2/afstversl/st/754846.pdf discloses a solar cell having a zinc oxide electron transport layer processed from a nanoparticle dispersion and a silver cathode.

OPDs may be used in photosensors in which the OPD is configured to detect light from a light source.

It is an object to provide a photodetector suitable for use in high resolution photosensors.

Summary of the Invention

The present inventors have found that a patterned charge-transporting layer may be used to form a corresponding patterned electrode layer of a photodetector. The photodetector may be suitable for use in a high resolution photosensor.

Accordingly, in a first aspect the invention provides a method of forming a photodetector, the method comprising the steps of:

forming a photoactive zone comprising at least one layer over a first electrode;

forming a patterned charge-transporting layer over the photoactive zone wherein the patterned charge-transporting layer comprises a plurality of charge-transporting regions laterally spaced apart from one another; and forming a patterned second electrode selectively on the plurality of charge-transporting regions.

In a second aspect the invention provides a photodetector obtainable by a method according to the first aspect.

In a third aspect the invention provides a photodetector comprising a first electrode supported on a substrate; a second electrode; a photoactive zone comprising at least one layer between the first and second electrodes; and a charge-transporting layer between the photoactive zone and the second electrode and adjacent to the second electrode, wherein the chargetransporting layer and the second electrode are in a pattern of a plurality of regions spaced apart from one another.

In a fourth aspect the invention provides a sensor comprising at least one light source and a photodetector according to the second or third aspect wherein the photodetector is configured to detect light emission from the at least one light source.

Description of the Drawings

The invention will now be described in more detail with reference to the drawings in which:

Figure 1A is a schematic illustration of a cross-section of a photodetector according to an embodiment of the invention having a patterned electron-transporting layer and cathode;

Figure IB is a schematic illustration of a plan view of the photodetector according to Figure 1A;

Figure 2 is a schematic illustration of a cross-section of a photodetector according to an embodiment of the invention, having a structure as illustrated in Figure 1A and further comprising a hole-transporting layer;

Figure 3 is a is a schematic illustration of a cross-section of a photodetector according to an embodiment of the invention, having a patterned hole-transporting layer and anode; and

Figure 4 is a schematic illustration of a photosensor comprising a light array and a photodetector according to an embodiment of the invention.

Detailed Description of the Invention

Figure 1A, which is not drawn to any scale, illustrates a photodetector according to an embodiment of the invention. The photodetector comprises an anode 103 supported by a substrate 101, a cathode 109, a photoactive layer 105 between the anode and the cathode forming a photoactive zone of the device and an electron-transporting layer 107 between the photoactive layer 105 and the cathode 109.

In operation of the device, photons incident on the device are separated in the photoactive zone into holes and electrons. The photoactive zone of Figure 1A is a bulk heterojunction layer 105 comprising or consisting of an electron acceptor material and an electron donor material, however in other embodiments (not shown) it will be appreciated that the photoactive zone may be two layers consisting of an electron-accepting layer comprising or consisting of an electron acceptor and a second, adjacent electron donor layer comprising or consisting of an electron donor material. It will be understood that a bulk heterojunction photoactive layer as described anywhere herein may be replaced with these two layers.

The electron acceptor and electron donor may each be selected from organic and inorganic materials. Preferably, both the electron acceptor and the electron donor of photodetectors as described herein are organic materials and the photodetectors as described herein are organic photodetectors.

At least one of the anode 103 and cathode 107 is transparent so that light incident on the device may reach the photoactive layer. A transparent electrode as described herein preferably has a transmittance of at least 70%, optionally at least 80%, to wavelengths in the range of 400-1500 nm, optionally 400-1100 nm.

Preferably, the anode is transparent. Optionally, the anode comprises a layer of transparent conducting oxide, preferably indium tin oxide or indium zinc oxide. The anode may or may not be patterned. Preferably, the anode is unpatterned. The anode preferably has a thickness in the range of about 10-200 nm, preferably 25-150nm.

Preferably, the cathode comprises or consists of a layer of a reflective metal.

In another embodiment, the cathode may be transparent and anode may be transparent or reflective. If the anode and cathode are transparent then a reflective layer, optionally a metal layer, may be provided between the anode and the substrate.

The substrate 101 may be, without limitation, a glass or plastic substrate. The substrate is transparent if, in use, incident light is to be transmitted through the substrate and the first electrode supported by the substrate. The first electrode may be in direct contact with the substrate or may be separated from the substrate by one or more intervening layers, for example a reflective layer.

The substrate supporting one of the anode and cathode may or may not be transparent if, in use, incident light is to be transmitted through the other of the anode and cathode.

One or more further layers may be provided between the anode and the cathode.

The bulk heterojunction layer optionally has a thickness in the range of about 10-2000 nm.

The electron-transporting layer 107 and the cathode 109 are in a pattern of a plurality of regions that are spaced apart from one another, each region comprising the electrontransporting material and the or each cathode material.

With reference to Figure IB, each region has a surface area A.

Each region of the electron-transporting layer as illustrated in Figures 1A and IB is in the shape of a square although it will be appreciated that the regions may have any shape, and regions of a photodetector may have the same or different shapes and / or the same or different areas. The shape and area of the regions and the spacing between regions may be selected according to the intended application of the photodetector.

The regions of the device defined by each patterned region may each function as an independent photodetector. The very small (micrometre) lateral scale at which the patterns may be formed enables formation of a photodetector suitable for use in high resolution emitter - detector arrays. Each patterned region may have a size of at least 50 microns. By “size” of a patterned region as used herein is meant the largest distance between two points on the perimeter of a surface of the patterned region, for example the diagonal of a square pattern or the diameter of a circular pattern. Optionally, each patterned region has a size of up to about 10 millimetres.

The spacing between patterned regions may be at least 5 microns or at least 10 microns. Optionally, the spacing between patterned regions is up to 5 mm. Preferably, the spacings between regions are gaps in which no material is present during manufacture of the device.

The anode 103 and at least two, preferably each, of the patterned cathode regions 109 is connected to circuitry (not shown) configured to measure photocurrent. Conversion of light incident on an area A of the device into electrical current may be detected in reverse bias mode. The or each area of the photodetector that light is incident on may thereby be individually identifiable.

To form the device, the patterned electron-transporting layer is formed directly on photoactive layer 105 (or on the electron-accepting layer in the case where the photoactive zone comprises an electron accepting layer and an electron donating layer).

The electron-transporting layer may be deposited and then patterned, however it is preferred that the electron-transporting layer is deposited in the form of the pattern using a selective deposition method.

Suitable selective deposition methods include, without limitation, printing methods and evaporation through a shadow mask. Suitable evaporation methods include, without limitation, electron beam evaporation and thermal evaporation. Suitable printing methods include, without limitation, inkjet printing, deposition through a stencil, for example screen printing, gravure printing and flexographic printing.

In the case where the electron-transporting layer is deposited from an ink comprising an electron-transporting material and one or more solvents, the contact angle of the ink on the underlying photoactive layer (or, in the case where the photoactive zone comprises two layers, the electron acceptor layer) is preferably in the range of 10-50°.

Contact angles as described anywhere herein may be as measured with an optical contact angle goniometer.

The one or more solvents of the ink comprising the electron-transporting material are preferably selected to avoid dissolution of the underlying layer. Preferably, the solvents comprise or consist of solvents selected from the group consisting of water and polar solvents. Preferred polar solvents are protic solvents, more preferably alcohols, optionally propanol.

Electron-transporting materials which may be used in the electron-transporting layer include, without limitation, organic electron-transporting materials; metal carbonates for example caesium carbonate; and metal oxides, for example titanium oxide and zinc oxide. Preferably, the electron-transporting layer comprises or consists of a metal oxide, more preferably zinc oxide.

The electron-transporting layer preferably has a transmittance of at least 70%, optionally at least 80%, to wavelengths in the range of 400-1500 nm, optionally 400-1100 nm.

The electron-transporting layer preferably comprises an electron-transporting material having a LUMO that is deeper, preferably at least 0.5 eV deeper, than that of the electron donor or electron acceptor material. Optionally, the electron-transporting material has a LUMO level in the range of 4.0-5.0 eV, optionally 4.5-5.0 eV.

The electron-transporting layer preferably has a LUMO level that is within 0.5 eV of the work function of a conductive material of the cathode.

The electron-transporting layer preferably has a thickness in the range of 10-100 nm.

The cathode may be formed by printing an ink comprising the material or materials of the cathode onto the electron-transporting layer and one or more liquid compounds. The ink may comprise particulate metal, preferably nanoparticulate metal, for example a colloidal metal ink. Preferred metals have a work function above 4.0 eV, more preferably silver. The ink may comprise water and / or one or more polar solvents, optionally one or more protic polar solvents. Exemplary inks comprise water and an alcohol, optionally a glycerol.

Work functions of elemental metals are as given in the CRC Handbook of Chemistry and Physics, 87^th Edition, 12-114. For any given element, the first work function value applies if more than one work function value is listed.

The electron-transporting layer is as a template for printing of the cathode. The surface of the electron-transporting layer preferably has a lower contact angle, preferably at least 20° or 30° lower, for the ink from which the cathode is deposited than the surface of the underlying photoactive layer (or, in the case where the photoactive zone comprises two layers, the electron acceptor layer). Consequently, the ink may be confined on the surface of the patterned electron-transporting layer, with little or no ink flowing onto the surface of the underlying photoactive layer.

Following formation of the cathode, the anode and at least two, optionally each, of the patterned regions may be connected to circuitry for detection of a photocurrent and the device may be encapsulated. The encapsulant may be transparent or opaque, and may be selected depending on whether the cathode is opaque or transparent and whether light incident on a transparent cathode is to be detected, or light incident on a transparent anode is to be detected in the case of a device as described with reference to Figure 3.

One or more further layers may be provided between the anode and the cathode. A holetransporting layer may be provided between the anode and the bulk heterojunction layer.

Figure 2, which is not drawn to any scale, illustrates a photodetector according to another embodiment of the invention. The photodetector is as described with reference to Figures 1A and IB except that a hole-transporting layer 111 is provided between the anode 103 and the photoactive layer 105.

Exemplary hole-transporting materials are doped polythiophenes poly(ethylene dioxythiophene) (PEDT) or poly(thienothiophene)s including, without limitation, PEDT doped with a charge-balancing polyacid such as polystyrene sulfonate (PSS), polyacrylic acid or a fluorinated sulfonic acid, for example Nafion ®. Optionally, the hole-transporting layer has a thickness in the region of 20 to lOOnm.

In the devices of Figures 1 and 2 the anode is the electrode on, or closest to, the substrate and the electron-transporting layer and cathode are patterned.

Figure 3, which is not drawn to any scale, illustrates a photodetector according to another embodiment of the invention and comprises a cathode 309 supported on a substrate 301, an anode 303, a photoactive layer 305 between the anode and the cathode forming a photoactive zone of the device and a hole-transporting layer 311 between the photoactive layer 305 and the anode 303. In the device of Figure 3 the cathode is the electrode on, or closest to, the substrate and the hole-transporting layer 311 and anode 303 are patterned. The materials of the device and methods of forming the device of Figure 3 may be as described anywhere herein.

One or more further layers may be provided between the anode and cathode of the device of Figure 3. In a yet further embodiment, the photodetector is as described with reference to Figure 3 except that an electron-transporting layer is provided between the cathode 309 and the photoactive layer 305.

Electron donor and electron acceptor

The electron donor and electron acceptor may each be an inorganic or organic material. An exemplary inorganic material is a perovskite. Preferably, both the electron donor and electron acceptor are organic materials.

It will be understood that the electron donor may be a single electron donor material or a mixture of two or more electron donor materials, and the electron acceptor may consist of a single electron acceptor material or may be a mixture of two or more electron acceptor materials.

The electron acceptor and the electron donor may each independently be a polymeric material or a non-polymeric organic material.

Preferably, the electron donor is a polymer. Electron donor polymers are optionally selected from conjugated hydrocarbon or heterocyclic polymers including polyacene, polyaniline, polyazulene, polybenzofuran, polyfluorene, polyfuran, polyindenofluorene, polyindole, polyphenylene, polypyrazoline, polypyrene, polypyridazine, polypyridine, polytriarylamine, poly(phenylene vinylene), poly(3-substituted thiophene), poly(3,4-bisubstituted thiophene), polyselenophene, poly(3-substituted selenophene), poly(3,4-bisubstituted selenophene), poly(bisthiophene), poly(terthiophene), poly(bisselenophene), poly(terselenophene), polythieno [2,3 -b] thiophene, polythieno [3,2-b] thiophene, polybenzothiophene, polybenzo[l,2-b:4,5- b']dithiophene, polyisothianaphthene, poly(monosubstituted pyrrole), poly(3,4-bisubstituted pyrrole), poly-l,3,4-oxadiazoles, polyisothianaphthene, derivatives and co-polymers thereof. Preferred examples electron-donor polymers are copolymers of polyfluorenes and polythiophenes, each of which may be substituted, and polymers comprising benzothiadiazole-based and thiophene-based repeating units, each of which may be substituted. An electron-accepting polymer or an electron-donating polymer as described herein may have a polystyrene-equivalent number-average molecular weight (Mn) measured by gel permeation chromatography in the range of about 1x10 to 1x10 , and preferably lxlO³ to 5xl0⁶. The polystyrene-equivalent weight-average molecular weight (Mw) of the polymers described herein may be 1x10 to 1x10, and preferably 1x10 to 1x10

Preferably, the electron acceptor is a non-polymeric organic compound, more preferably a fullerene.

The fullerene may be a C6o, C70, C76, C78 and Cs4 fullerene or a derivative thereof including, without limitation, PCBM-type fullerene derivatives (including phenyl-C61-butyric acid methyl ester (CeoPCBM), TCBM-type fullerene derivatives (e.g. tolyl-C61-butyric acid methyl ester (C60TCBM)), and ThCBM-type fullerene derivatives (e.g. thienyl-C61-butyric acid methyl ester (CeoThCBM).

The or each layer of the photoactive zone may be formed by any process including, without limitation, thermal evaporation and solution deposition methods.

Preferably, the or each layer of the photoactive zone is formed by depositing a formulation comprising the acceptor material and the electron donor material dissolved or dispersed in a solvent or a mixture of two or more solvents. The formulation may be deposited by any coating or printing method including, without limitation, spin-coating, dip-coating, rollcoating, spray coating, doctor blade coating, slit coating, ink jet printing, screen printing, gravure printing and flexographic printing.

The one or more solvents of the formulation may optionally comprise or consist of benzene substituted with one or more substituents selected from chlorine, Cmo alkyl and Cmo alkoxy wherein two or more substituents may be linked to form a ring which may be unsubstituted or substituted with one or more Ci-6 alkyl groups, optionally toluene, xylenes, trimethylbenzenes, tetramethylbenzenes, anisole, indane and its alkyl-substituted derivatives, and tetralin and its alkyl-substituted derivatives.

The formulation may comprise a mixture of two or more solvents, preferably a mixture comprising at least one benzene substituted with one or more substituents as described above and one or more further solvents. The one or more further solvents may be selected from esters, optionally alkyl or aryl esters of alkyl or aryl carboxylic acids, optionally a Cno alkyl benzoate or benzyl benzoate.

The formulation may comprise further components in addition to the electron acceptor, the electron donor and the one or more solvents. As examples of such components, adhesive agents, defoaming agents, deaerators, viscosity enhancers, diluents, auxiliaries, flow improvers colourants, dyes or pigments, sensitizers, stabilizers, nanoparticles, surface-active compounds, lubricating agents, wetting agents, dispersing agents and inhibitors may be mentioned.

Applications

The photodetector as described herein may be used in a wide range of applications including, without limitation, detecting the presence and / or brightness of ambient light and in a sensor comprising the photodetector and a light source. The photodetector may be configured such that light emitted from the light source is incident on the photodetector and changes in wavelength and / or brightness of the light may be detected. The sensor may be, without limitation, a gas sensor, a biosensor, an X-ray imaging device, a motion sensor (for example for use in security applications) a proximity sensor or a fingerprint sensor.

With reference to Figure 4, one or more devices as described herein may be used as part of an emitter - detector array. Each patterned region may be aligned with an emitter 201 of an emitter array 200.

The emitters and the or each detector of the array may be on opposing sides of a fluid flow channel, optionally a channel of a microfluidic device.

In other embodiments the photodetector may be configured to detect light from a single light source, optionally ambient light.

Examples

A device having the following structure may be formed:

ITO (45 nm) / HTL (40 nm) / BHL / ETL (40 nm) / Cathode wherein ITO is an indium-tin oxide anode; HTL is a hole-transporting layer; BHL is a bulk heterojunction layer; and ETL is an electron-transporting layer.

The ITO may be preformed on the substrate or may be formed by deposition such as sputtering under a reduced atmospheric pressure. If the anode is to be patterned this may be conducted through a chemical wet etching process, whereby the regions that are required to remain on the substrate are coated with a protective photoresist. The photoresist acts as a protection for any ITO removal. Once the etch process is completed the photoresist is then subsequently removed through a resist removal process.

The hole-transporting layer, such as PEDOT:PSS, may be deposited from solution to a thickness of about 40 nm.

The bulk heterojunction layer may be deposited from a solution in which an electron-acceptor and an electron donor material are dissolved.

An exemplary composition of the bulk heterojunction layer is 1 part donor to 2 parts acceptor (by mass). To form the solution, the solvent, such as 1,2,4-tri-methylbenzene, may be added to the pre-mixed solidsfollowed by heated at 80°C for at least 10 hours to ensure complete dissolution of the solids in the solvent.

The acceptor material may be C60 or C70 PCBM

The donor material may be a polymer of formula:

The bulk heterojunction layer may be coated to a thickness of at least 150nm. This can be achieved using spin coating for example, which allows for a uniform thickness over a large area. Once coated to a desired wet film thickness (e.g. spin coating at lOOOrpm for 30 seconds), solvent removal is required. This may be achieved by using a contact type hotplate set at 80°C on which the substrate is heated for at least 1 minute.

Once the photoactive layer is cooled, a zinc oxide electron-transporting layer can be deposited and patterned in one step for example via a stencil coating process or by a printing process such as gravure printing that includes patterning with regions defined by the printing cylinder. The film thickness may be 40nm.

Finally the cathode layer may be deposited from solution using a method that does not require a patterned template or stencil. Examples include inkjet printing, or dispense printing, whereby the cathode ink is directed to the predetermined patterns of the zinc oxide regions. The contrast in the surface energies of the zinc oxide and the photoactive films is such that the cathode ink will be self-directed to the zinc oxide regions as this will be energetically favourable. The deposited cathode ink may be died at a temperature in the range of 100 to 150°C.

Finally an electrical contact pinning and encapsulation process is conducted. Encapsulation may be achieved using a counter substrate which may contain a getter material to trap any moisture in the device.

Although the present invention has been described in terms of specific exemplary embodiments, it will be appreciated that various modifications, alterations and/or combinations of features disclosed herein will be apparent to those skilled in the art without departing from the scope of the invention as set forth in the following claims.

Claims

1. A method of forming a photodetector, the method comprising the steps of:

forming a patterned charge-transporting layer over the photoactive zone wherein the patterned charge-transporting layer comprises a plurality of charge-transporting regions laterally spaced apart from one another; and forming a patterned second electrode selectively on the plurality of chargetransporting regions.

2. A method according to claim 1 wherein the first electrode is an anode; the second electrode is a cathode; and the charge-transporting layer is an electron-transporting layer.

3. A method according to claim 2 wherein the cathode comprises silver.

4. A method according to any one of claims 2 or 3 wherein the electron-transporting layer comprises a metal oxide or a metal carbonate.

5. A method according to claim 4 wherein the electron-transporting layer comprises zinc oxide.

6. A method according to any one of claims 1-5 wherein the photodetector further comprises a hole-transporting layer between the anode and the photoactive zone.

7. A method according to claim 1 wherein the first electrode is a cathode; the second electrode is an anode; and the charge-transporting layer is a hole-transporting layer.

8. A method according to any one of the preceding claims wherein the photoactive zone comprises a layer comprising a mixture of an electron-acceptor and an electron donor.

9. A method according to any one of the preceding claims wherein the photodetector is an organic photodetector comprising an organic electron acceptor and an organic electron donor.

10. A method according to any one of the preceding claims wherein the patterned chargetransporting layer is formed by a printing method.

11. A method according to any one of the preceding claims wherein the second electrode is formed by a printing method.

12. A method according to claim 11 wherein the second electrode is formed by printing an ink comprising a colloidal metal.

13. A photodetector obtainable by a method according to any one of the preceding claims.

14. A photodetector comprising a first electrode supported on a substrate; a second electrode; a photoactive zone comprising at least one layer between the first and second electrodes; and a charge-transporting layer between the photoactive zone and the second electrode and adjacent to the second electrode, wherein the chargetransporting layer and the second electrode are in a pattern of a plurality of regions laterally spaced apart from one another.

15. A sensor comprising at least one light source and a photodetector according to any one of claims 13 and 14 wherein the photodetector is configured to detect light emission from the at least one light source.

Intellectual

Property

Office

Application No: GB1622032.9 Examiner: Guy Cooper