GB2572573A

GB2572573A - Organic photodetector

Info

Publication number: GB2572573A
Application number: GB1805469.2A
Authority: GB
Inventors: Yaacobi-Gross Nir; Bovo Gianluca; Burroughes Jeremy
Original assignee: Sumitomo Chemical Co Ltd
Current assignee: Sumitomo Chemical Co Ltd
Priority date: 2018-04-03
Filing date: 2018-04-03
Publication date: 2019-10-09
Also published as: GB201805469D0; WO2019193331A3; JP7473477B2; JP2021520069A; GB2572659A; WO2019193331A2; CN112243538A; GB201815231D0

Abstract

An organic photodetector comprising: a first electrode (107); a second electrode (103); and a photosensitive organic layer (105) positioned between the electrodes, wherein the photosensitive organic layer comprises a donor compound and an acceptor compound, wherein the acceptor compound does not comprise a fullerene group and wherein the LUMO energy level of the acceptor compound is equal to or deeper than the LUMO energy level of fullerene derivative C7oIPH. Also shown is a sensor and method of detecting light using the organic photodetector.

Description

Organic Photodetector

Field

The disclosure relates to photoactive compounds and their use in organic electronic devices, in particular organic photodetectors.

Background

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).

US 6,972,431 discloses organic photodetectors having a reduced dark current.

WO 2017/117477 discloses a-substituted PDI derivatives as small molecular and polymerized electron acceptors in organic photovoltaic cells.

US 2017/0057962 discloses non-fullerene electron acceptors for highly efficient OPVs.

WO 2017/191468 discloses non-fullerene electron acceptors which maybe used in organic optical or electronic devices.

CN106025073 discloses organic solar cells.

WO 2013/182847 discloses novel organic compounds for use as electron acceptors.

US 2015/0270497 discloses efficient organic photosensitive devices.

US 7,893,428 discloses photosensitive organic semiconductor compositions.

Baran et al., Energy Environ. Sci., 2016, 3783-3793 discloses using non25 fullerene acceptors in organic solar cells.

- 2 Susarova et al., Sol. Energy Mater Sol. Cells, 2010, 803-811 discloses novel perylene diimide Py-PDI and naphthalene diimide Py-NDI possessing chelating pyridyl groups.

Yao et al., Organic Electronics, 2015, 305-313 discloses 2,1,3-benzothiadiazole5 5,6-dicarboxylic imide based low-bandgap polymers for solution processed photodiode applications.

Hu et al., Polym. Chem., 2017, 528-536 discloses dark current reduction strategies using edge-on aligned donor polymers for high detectivity and responsivity organic photodetectors.

US 8,853,679 relates generally to organic semiconductors and in particular to organic semiconductors for forming part of a thin film transistor.

Zhao et al., J. Am. Chem. Soc., 2017, 7148-7151 discloses that a new polymer donor (PBDB-T-SF) and a new small molecule acceptor (IT-4F) for fullerenefree organic solar cells (OSCs) were designed and synthesised.

Lin et al., Adv. Mater., 2015,1170-1174 discloses the design and synthesis of a novel electron acceptor (ITIC) based on a bulky seven-ring fused core (indacenodithieno[3,2-b]thiophene, IT), end-capped with 2-(3-oxo-2,3dihydroinden-i-ylidene)malononitrile (INCN) groups, and with four 4hexylphenyl groups substituted on it.

WO 2017/125719 discloses organic photodiodes for use as photodetectors. It shows the use of fullerene derivatives in order to reduce dark currents in organic photodiodes.

A drawback with OPDs is the presence of dark current, i.e. current flowing through the device in the absence of any photons incident on the device, which 25 may affect the limit of detection of the device.

It is therefore an object of the invention to provide an OPD having low dark current.

-3It is a further object of the invention to provide an OPD having low dark current and good external quantum efficiency.

Summary

According to a first aspect of the invention, there is provided an organic photodetector comprising: a first electrode; a second electrode; and a photosensitive organic layer positioned between the electrodes, wherein the photosensitive organic layer comprises a donor compound and an acceptor compound, wherein the acceptor compound does not comprise a fullerene group and wherein the LUMO energy level of the acceptor compound is equal to 10 or deeper than the LUMO energy level of fullerene derivative C70IPH.

In some embodiments, the OPD is connected to a voltage source such that a reverse bias maybe applied to it in operation.

In some embodiments, the acceptor compound is represented by General Formula (V):

General Formula (V)

N=rR²⁰ ²¹R^Y^^r23

R²² wherein each of R¹¹, R¹², R¹'^!, Ru, R15 are each independently selected from one of: H, a halogen or either of the following formulae (VIII) and (IX):

(VI11) ; and wherein R¹⁶ and R¹⁷ are each independently selected from the group consisting of: H; branched, linear or cyclic C1-20 alkyl wherein one or more non5 adjacent, non-terminal C atoms may be replaced with O, S, CO or COO and one or more H atoms may be replaced with F; and aryl or heteroaryl which is substituted or unsubstituted with one or more substituents.

In some embodiments, R^1;‘>, R²⁰> R²¹, R²² and R²3 are H or a halogen selected 10 from the group consisting of: Cl, Br, I, F, and each of R¹⁶ and R¹⁷ is -(CH₂)_nCH₃, where n is an integer selected from 1 to 20.

In some embodiments, R¹¹, R¹², R^ and R^ are formula (VIII), where n is 5.

In some embodiments, R¹¹, R¹², R^1! and R¹4 are formula (IX), where n is 5.

In some embodiments, the acceptor compound is ITIC or ITIC-Th.

In some embodiments, the acceptor compound is represented by General

Formula (I):

-5R2 _R2 _R2 _R2

General Formula I wherein each R¹ is independently selected from the group consisting of: H; branched, linear or cyclic C1-20 alkyl wherein one or more non-adjacent, nonterminal C atoms may be replaced with O, S, CO or COO and one or more H atoms may be replaced with F; and aryl or heteroaryl which is substituted or unsubstituted with one or more substituents;

wherein each of R² and R3 are independently selected from the group consisting of: H; branched, linear or cyclic C1-20 alkyl wherein one or more non-adjacent, non-terminal C atoms may be replaced with 0, S, CO or COO and one or more H atoms maybe replaced with F; aryl or heteroaryl which is substituted or unsubstituted with one or more substituents; and a group having the following formula (II):

(II)

-6; and wherein each of R4 and Rs are independently selected from the group consisting of: H; branched, linear or cyclic C1-20 alkyl wherein one or more nonadjacent, non-terminal C atoms may be replaced with O, S, CO or COO and one or more H atoms may be replaced with F; and aryl or heteroaryl which is substituted or unsubstituted with one or more substituents.

In some embodiments, R¹ and R4 are each independently selected from a group having the following formula (IV):

R¹⁰ (IV) wherein each of R9 and R¹⁰ are independently selected from the group consisting of: -CH₃ and -(CH₂)_nCH₃, wherein n is an integer selected from 120.

In some embodiments, the acceptor compound has the following formula (XII):

(xii)

-7 wherein R¹⁸ has the following formula (IV):

R⁹

(IV) ; and wherein each of R9 and R¹⁰ are independently selected from the group 5 consisting of: -CH₃ and -(CH₂)_nCH₃, wherein n is an integer selected from 120. In some embodiments, n is 4 or 5.

In some embodiments, the donor compound is a semiconducting polymer.

In some embodiments, the first electrode is an anode and the second electrode is a cathode.

In some embodiments, the weight ratio of the donor compound to the acceptor compound is from about 1:0.5 to about 1:1.2.

According to a second aspect of the invention, there is provided a sensor comprising a light source and an organic photodetector as claimed in any one of claims 1 to 13, wherein the organic photodetector is configured to receive light from the light source.

According to a third aspect of the invention, there is provided a method of detecting light comprising measurement of a photocurrent generated by light incident on an organic photodetector of the first aspect.

-8In some embodiments, the method of detecting light comprises measurement of the photocurrent generated by light incident on the organic photodetector and emitted from the light source of the sensor according to the second aspect.

According to a fourth aspect of the invention, there is provided a use of a compound that does not comprise a fullerene group having a LUMO energy level and is deeper than the LUMO energy level of fullerene derivative C70IPH in a photosensitive layer of an organic photodetector to reduce dark current.

Description of Drawings

Figure 1 illustrates an organic photodetector according to an embodiment of the invention;

Figures 2 and 3 are graphs of current density vs applied voltage for devices in dark conditions according to embodiments and a comparative device; and

Figures 4 and 5 are graphs of EQE vs wavelength of devices according to embodiments and a comparative device.

Detailed Description

Figure 1 illustrates an OPD according to an embodiment of the invention. The

OPD comprises a cathode 103 supported by a substrate 101, an anode 107 and a bulk heterojunction layer 105 located between the anode and the cathode comprising a mixture of an electron acceptor and an electron donor. Optionally, the bulk heterojunction layer consists of the electron acceptor and the electron donor. In the embodiment illustrated in Figure 1, the OPD comprises a layer of material 106 which modifies the work function of the cathode 103. In other embodiments, this layer may or may not be present.

The OPD may comprise other layers not shown in Figure 1. For example, the device may comprise a hole transport layer (HTL) located between the anode

107 and the heterojunction layer 105.

In use, the photodetectors as described in this disclosure may be connected to a voltage source for applying a reverse bias to the device and a device configured

-9to measure photocurrent. In some embodiments, the photodetectors are part of a system comprising a plurality of photodetectors. For example, the photodetectors may be part of an array in an image sensor of a camera. The voltage applied to the photodetectors maybe varied. In some embodiments, the 5 photodetectors may be continuously biased when in use.

The OPD maybe incorporated into a sensor comprising a light source and the OPD may be configured to receive light emitted from the light source.

In some embodiments, the light from the light source may or may not be changed before reaching the light source. For example, the light maybe filtered, down-converted or up-converted before it reaches the light source.

A high dark current in photodetectors limits the detectible optical input signal 15 due to a low signal-to-noise ratio.

The inventors have surprisingly found that incorporating acceptor compounds having a deep LUMO energy level relative to the fullerene derivative C70IPH into OPDs can reduce dark current as compared to an OPD containing C70IPH 20 as the acceptor.

Preferably, the electron acceptor compounds described herein do not comprise a fullerene group, and are described hereinafter as “non-fullerene acceptors”.

The organic photodetector comprises:

a first electrode, which maybe the anode or the cathode;

a second electrode, which may be the other of the anode or the cathode; and a photosensitive organic layer positioned between the electrodes, wherein the photosensitive organic layer comprises a donor compound and an acceptor compound, wherein the acceptor compound does not comprise a fullerene group and wherein the LUMO energy level of the acceptor compound

- 10 is equal to or deeper than the LUMO energy level of fullerene derivative C70IPH.

Reference to a “deeper” LUMO energy level as used herein means further from vacuum level and reference to a “shallower” LUMO energy level as used herein means closer to vacuum level. It will therefore be understood that the LUMO energy level of an acceptor compound which does not comprise a fullerene group as described herein is further from vacuum level than the LUMO energy level of the fullerene derivative C70IPH.

Preferably, the LUMO energy of the acceptor compound is at least 3.65, 3.66, 3.67, 3.68, 3.69, 3.70, 3.71, 3.72, 3.73, 3.74 or 3.75 eV deeper than the vacuum level as measured by square wave voltammetry.

The non-fullerene acceptor compounds described herein may be small molecule non-fullerene acceptors (SM-NFAs).

The non-fullerene acceptor compound maybe a compound of General Formula (I):

R² R² R² R²

Ο_χ /0 ¹R-N /⁼\'^N_R1

O/—\ /—\O

R² R² R3 R²

General Formula I wherein:

each R¹ is independently selected from the group consisting of: H; branched, linear or cyclic C1-20 alkyl wherein one or more non-adjacent, non-terminal C atoms maybe replaced with 0, S, CO or COO and one or more H atoms maybe

- 11 replaced with F; and aryl or heteroaryl which is substituted or unsubstituted with one or more substituents;

each of R² and R3 are independently selected from the group consisting of: H;

branched, linear or cyclic C1-20 alkyl wherein one or more non-adjacent, nonterminal C atoms may be replaced with O, S, CO or COO and one or more H atoms maybe replaced with F; aryl or heteroaryl which is substituted or unsubstituted with one or more substituents; and a group having one of the following formulae (II) or (III):

(Π)

; and wherein R4, R5, R6, R7 and R⁸ are each independently selected from the group consisting of: H; branched, linear or cyclic C1-20 alkyl wherein one or more non-adjacent, non-terminal C atoms maybe replaced with O, S, CO or COO and 5 one or more H atoms may be replaced with F; and aryl or heteroaryl which is substituted or unsubstituted with one or more substituents.

Preferably, each R¹, R4 and R⁶ is independently selected from a branched, linear or cyclic C1-20 alkyl group.

Preferably, each R¹, R4 and R⁶, is independently selected from a group having the following formula (IV):

R⁹

(IV)

-13wherein each of R⁹ and R¹⁰ are independently selected from the group consisting of: H; branched, linear or cyclic C1-20 alkyl wherein one or more nonadjacent, non-terminal C atoms may be replaced with O, S, CO or COO and one or more H atoms may be replaced with F; and aryl or heteroaryl which is substituted or unsubstituted with one or more substituents.

Preferably, each of R⁹ and R¹⁰ are independently selected the group consisting of: -CH₃ and -(CH₂)_nCH₃, wherein n is an integer selected from 1-20.

Preferably, each of R⁹ and R¹⁰ are independently selected the group consisting of: -CH3, -(CH2)4CH3 and -(CH2)5CH3. Preferably, each of R⁹ and R¹⁰ is -(CH2)₄CH₃ or -(CH₂)₅CH₃.

Preferably, each R² is independently selected from the group consisting of: H;

branched, linear or cyclic Ci-₂o alkyl.

In preferred embodiments, each R² is H.

Preferably, R³ is selected from an aryl or heteroaryl which is substituted or unsubstituted with one or more substituents.

In preferred embodiments, R³ is selected from a group according to formula (II) or (III). Preferably, R³ is formula (II).

Preferably, each R4 is independently selected from a branched, linear or cyclic Ci-₂₀ alkyl group.

Preferably, each R4 is independently selected from a group having the following formula (IV):

-14R⁹

(IV) wherein each of R9 and R¹⁰ are independently selected from the group consisting of: H; branched, linear or cyclic C1-20 alkyl wherein one or more nonadjacent, non-terminal C atoms may be replaced with O, S, CO or COO and one or more H atoms may be replaced with F; and aryl or heteroaryl which is substituted or unsubstituted with one or more substituents.

Preferably, each of R9 and R¹⁰ are independently selected the group consisting of: -CH₃ and -(CH₂)_nCH₃, wherein n is an integer selected from 1-20.

In preferred embodiments, each of R9 and R¹⁰ are independently selected the group consisting of: -CH3, -(CH2)4CH3 and -(CH2)5CH3. Preferably, each of R⁹and Rio is -(CH2)₄CH₃ or -(CH₂)₅CH₃.

Preferably, each Rs is independently selected from the group consisting of: H; branched, linear or cyclic Ci-₂o alkyl.

In preferred embodiments, each Rs is H.

Preferably, each R⁶ is independently selected from a branched, linear or cyclic Ci-₂₀ alkyl group.

Preferably, each R⁶ is independently selected from a group having the following formula (IV):

-15R⁹

(IV) wherein each of R⁹ and R¹⁰ are independently selected from the group consisting of: H; branched, linear or cyclic C1-20 alkyl wherein one or more nonadjacent, non-terminal C atoms may be replaced with O, S, CO or COO and one or more H atoms may be replaced with F; and aryl or heteroaryl which is substituted or unsubstituted with one or more substituents.

Preferably, each of R⁹ and R¹⁰ are independently selected the group consisting of: -CH3, -(CH2)4CH3 and -(CH2)5CH3. Preferably, each of R⁹ and R¹⁰ is (CH2)₄CH₃ or -(CH₂)₅CH₃.

Preferably, each of R? and R⁸ are independently selected from the group consisting of: H; branched, linear or cyclic Ci-₂o alkyl.

In preferred embodiments, each of R~ and R⁸ are H.

In some embodiments, the non-fullerene acceptor compound maybe a compound of General Formula (V):

-16r

wherein each of R¹¹, R¹², R¹·³, R¹^ R¹, R²⁰, R²¹, R²² and R²³ are independently selected from the group consisting of: H; a halogen (including F, Cl, Br, I); branched, linear or cyclic C1-20 alkyl wherein one or more non-adjacent, non5 terminal C atoms may be replaced with O, S, CO or COO and one or more H atoms maybe replaced with F; and aryl or heteroaryl which is substituted or unsubstituted with one or more substituents.

In some embodiments, each of R¹¹, R¹², R¹³, R^1zf R¹5, Rao, R21, R22 _anj R23 _are independently selected from one of either the following formulae (VI) and (VII):

(VI) (VID

In some embodiments, each of R¹¹, R¹², R¹³, R^1zf R¹^ R20, R21, R22 _anj R23 _are independently selected from one of either formulae (VI) and (VII) wherein one or more H atoms of each of formula (VI) and (VII) are independently replaced with a substituent selected from the group consisting of: branched, linear or cyclic C1-20 alkyl wherein one or more non-adjacent, non-terminal C atoms may be replaced with 0, S, CO or COO and one or more H atoms maybe replaced with F; and aryl or heteroaryl which is substituted or unsubstituted with one or more substituents.

-17In some embodiments, each of R¹¹, R¹², R¹·³, R^ are independently selected from the group consisting of: H; a halogen (including F, Cl, Br, I); formulae (VI) and (VII); wherein one or more H atoms of each of formula (VI) and (VII) are independently replaced with a substituent selected from the group consisting of:

branched, linear or cyclic C1-20 alkyl wherein one or more non-adjacent, nonterminal C atoms may be replaced with O, S, CO or COO and one or more H atoms maybe replaced with F; and aryl or heteroaryl which is substituted or unsubstituted with one or more substituents; and R¹', R²⁰, R²¹, R²² and R²³ are selected from the group consisting of: H; F ; branched, linear or cyclic C1-20 alkyl wherein one or more non-adjacent, non-terminal C atoms maybe replaced with 0, S, CO or COO and one or more H atoms maybe replaced with F; and aryl or heteroaryl which is substituted or unsubstituted with one or more substituents.

In preferred embodiments, each of R¹¹, R¹², R¹³, Ru are independently selected from either formulae (VI) and (VII), wherein one or more H atoms of each of formula (VI) and (VII) are independently replaced with a substituent selected from the group consisting of: branched, linear or cyclic C1-20 alkyl wherein one or more non-adjacent, non-terminal C atoms may be replaced with 0, S, CO or COO and one or more H atoms maybe replaced with F; and aryl or heteroaryl which is substituted or unsubstituted with one or more substituents, R¹⁵ is H and each of R²⁰, R²¹, R²² and R²³ is H.

In preferred embodiments, R¹¹, R¹², R¹³, R¹4 are selected from either formulae (VI) or (VII), wherein one or more H atoms of each of formula (VI) and (VII) are independently replaced with a substituent selected from the group consisting of:

branched, linear or cyclic C1-20 alkyl wherein one or more non-adjacent, nonterminal C atoms may be replaced with 0, S, CO or COO and one or more H atoms maybe replaced with F; and aryl or heteroaryl which is substituted or unsubstituted with one or more substituents, R¹' is H and each of R²⁰, R²¹, R²² and R²³ is a halogen selected from F, Cl, I, Br.

-18Preferably, R¹¹, R¹², R¹³, R¹⁴ are selected from either of the following formulae (VIII) and (IX), R¹³ is H and each of R²⁰, R²¹, R²² and R²³ is H or a halogen selected from F, Cl, I, Br:

(VIII)

R¹⁷ (IX) wherein R¹⁶ and R¹⁷ are each independently selected the group consisting of: CH₃ and -(CH₂)_nCH₃, wherein n is an integer selected from 1-20.

ΙΟ

Preferably, R¹¹, R¹², R¹³, R¹⁴ are selected from either of the following formulae (X) and (XI), R¹³ is H and each of R²⁰, R²¹, R²² and R²³ is independently selected from H or a halogen selected from F, Cl, I, Br:

CH₃

Preferably, R¹¹, R¹², R¹³, R¹⁴ are selected from either of the following formulae (X) and (XI), R¹³ is H and each of R²⁰, R²¹, R²² and R²³ is H:

ch₃

Preferably, R¹¹, R¹², R¹³, R¹⁴ are selected from either of the following formulae (X) and (XI), R¹³ is H and each of R²⁰, R²¹, R²² and R²³ is independently selected from a halogen selected from: F, Cl, I, Br:

Preferably, R¹¹, R¹², R¹³, Ru are selected from either of the following formulae (X) and (XI), R¹' is H, three of R²⁰, R²¹, R²² and R²3 is H and one of R²⁰, R²¹, R²² and R²3 is independently selected from a halogen selected from: F, Cl, I, Br:

Preferably, R¹¹, R¹², R¹·³, Ru are selected from either of the following formulae ίο (X) and (XI), R¹' is H, two of R²⁰, R²¹, R²² and R²³ is H and two of R²⁰, R²¹, R²² and R²³ are independently selected from a halogen selected from: F, Cl, I, Br:

Preferably, R¹¹, R¹², R¹³, R^ are selected from either of the following formulae (X) and (XI), R¹' is H, R²⁰ and R²³ are H and R²¹ and R²² are independently selected from a halogen selected from: F, Cl, I, Br:

Preferably, Ru is H and each of R²⁰, R²¹, R²² and R²3 is independently selected from H or a halogen selected from: F, Cl, I, Br.

Preferably, the non-fullerene acceptor compound has an external quantum efficiency of at least 10%, optionally at least 15% or at least 20%, as measured in a device as described in Device Example 1.

A non-exhaustive list of compounds that are suitable for use as the acceptor compound in the devices according to the present disclosure, as well as a comparison between the LUMO energy levels of these acceptor compounds and the LUMO energy level of reference compound C70IPH, is shown below in Table

1.

- 21 Table ι

Material	Structure	LUMO level (eV)
C70IPH* (reference)	t'N-··. ·; -:.	-3-75
di-PDI*	h₃c ch₃ v wyy^yy _rJ / ^N )—\ /=\ ^N \ H3C /—' //—\\ //—\\ /)—'—\ pH₃\—/ _o V—7 '_o \—/ h₃c '—\ /)—e y-/ /—' ch₃ Vn yy yy n—ς /—' 0 '—' '—' 0 '— h₃c ch₃	-3-87
me*	,n 0 [Γ Jj K_ \v r¹¹,r¹²,r¹³,r¹⁴= O CeHia	-3-83
ITIC-Th*	f L H w R¹¹,R¹²,R¹³,R¹⁴= ch₃	-3-84

*Purchased from ι-Material Inc.

- 22 The donor compound (p-type) is not particularly limited and maybe appropriately selected from electron donating materials that are known to the person skilled in the art, including organic polymers, oligomers and small molecules.

The donor compound can be a semiconducting polymer.

In a preferred embodiment the p-type donor compound comprises an organic conjugated polymer, which can be a homopolymer or copolymer including alternating, random or block copolymers. Preferred are non-crystalline or semicrystalline conjugated organic polymers. Further preferably the p-type organic semiconductor is a conjugated organic polymer with a low bandgap, typically between 2.5 eV and 1.5 eV, preferably between 2.3 eV and 1.8 eV. As exemplary p- type donor polymers, polymers 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[i ,2-b:4,5-b'jdithiophene, polyisothianaphthene, poly(monosubstituted pyrrole), poly(3,4-bisubstituted pyrrole), poly-i ,3,425 oxadiazoles, polyisothianaphthene, derivatives and co-polymers thereof may be mentioned. Preferred examples of p-type donors 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 maybe substituted. It is understood that the p-type donor may also consist of a mixture of a plurality of electron donating materials.

-23In some embodiments, the weight ratio of the donor compound to the acceptor compound is about 1:0.5,1:0.6,1:0.7,1:0.8,1:0.9,1:1, 1:1.1,1:1.2,1:1.3,1:1.4, 1:1.5,1:1-6,1:1.7,1:1-8,1:1.9 or 1:2.

In some embodiments, the weight of the donor compound to the acceptor compound is from about 1:0.5 to about 1:2.

Preferably, the weight ratio of the donor compound to the acceptor compound is about 1:1 or about 1:1.5.

At least one of the first and second electrodes is transparent so that light incident on the device may reach the bulk heterojunction layer. In some embodiments, both of the first and second electrodes are transparent.

Each transparent electrode preferably has a transmittance of at least 70%, optionally at least 80%, to wavelengths in the range of 300-900 nm.

In some embodiments, one electrode is transparent and the other electrode is reflective.

Optionally, the transparent electrode comprises or consists of a layer of transparent conducting oxide, preferably indium tin oxide or indium zinc oxide.

In preferred embodiments, the electrode may comprise poly 3,4ethylenedioxythiophene (PEDOT). In other preferred embodiments, the electrode may comprise a mixture of PEDOT and polystyrene sulfonate (PSS). The electrode may consist of a layer of PEDOT:PSS.

Optionally, the reflective electrode may comprise a layer of a reflective metal.

The layer of reflective material may be aluminium or silver or gold. In some embodiments, a bi-layer electrode maybe used. For example, the electrode may be an indium tin oxide (ITO)/silver bi-layer, an ITO/aluminium bi-layer or an ITO/gold bi-layer.

-24The device may be formed by forming the bulk heterojunction layer over one of the anode and cathode supported by a substrate and depositing the other of the anode or cathode over the bulk heterojunction layer.

The area of the OPD maybe less than about 3 cm², less than about 2 cm², less than about 1 cm², less than about 0.75 cm², less than about 0.5 cm² or less than about 0.25 cm².The substrate maybe, without limitation, a glass or plastic substrate. The substrate can be described as an inorganic semiconductor. In some embodiments, the substrate maybe silicon. For example, the substrate can be a wafer of silicon. The substrate is transparent if, in use, incident light is 10 to be transmitted through the substrate and the electrode supported by the substrate.

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.

The bulk heterojunction layer may be formed by any process including, without limitation, thermal evaporation and solution deposition methods.

Preferably, the bulk heterojunction layer 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 20 may be deposited by any coating or printing method including, without limitation, spin-coating, dip-coating, roll-coating, spray coating, doctor blade coating, wire bar coating, slit coating, inkjet printing, screen printing, gravure printing and flexographic printing.

The one or more solvents of the formulation may optionally comprise or consist 25 of benzene substituted with one or more substituents selected from chlorine, Ci10 alkyl and C1-10 alkoxy wherein two or more substituents may be linked to form a ring which may be unsubstituted or substituted with one or more C1-6 alkyl groups, optionally toluene, xylenes, trimethylbenzenes, tetramethylbenzenes, anisole, indane and its alkyl-substituted derivatives, and tetralin and its alkyl30 substituted derivatives.

-25The 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 maybe selected from esters, optionally alkyl or aryl esters 5 of alkyl or aryl carboxylic acids, optionally a Ci-io alkyl benzoate, benzyl benzoate or dimethoxybenzene. In preferred embodiments, a mixture of trimethylbenzene and benzyl benzoate is used as the solvent. In other preferred embodiments, a mixture of trimethylbenzene and dimethoxybenzene is used as the solvent.

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 maybe mentioned.

The organic 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 organic photodetector and a light source. The photodetector maybe configured such 20 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 maybe, 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. The photodetector maybe form part of 25 a 1D or 2D array in an image sensor. For example, the photodetector may be part of an array of photodetectors in a camera image sensor.

Examples

Comparative Device 1

A device having the following structure was prepared:

Cathode / Donor : Acceptor layer / Anode

- 26 A glass substrate coated with a patterned layer of indium-tin oxide (ITO) was treated with polyethyleneimine (PEIE) to modify the work function of the ITO.

A bulk heterojunction layer of a mixture of Donor Polymer 1 and acceptor compound C70IPH was deposited over the modified ITO layer by bar coating 5 from 1,2,4-trimethylbenzene : benzyl benzoate in a donor : acceptor mass ratio of 1:1.7.

An anode (Clevios HIL-E100) available from Heraeus was formed over the donor / acceptor mixture layer by spin-coating.

Donor Polymer 1 has the structure:

Example 1

Devices were formed as described for Comparative Device 1 except that a bulk heterojunction layer of a mixture of Donor Polymer 1 and either di-pdi, ITIC or ITIC-Th as the acceptor compound was deposited over the modified ITO layer 15 by spin-coating from 1,2,4-trimethylbenzene: benzyl benzoate in a MASS donor : acceptor ratio of 1:1.

With reference to Figure 2 and Figure 3 , it can be seen that dark current is considerably higher for Comparative Device 1 compared with devices of Example 1 that comprise di-pdi,, ITIC or ITIC-Th as the acceptor compound.

External quantum efficiencies (EQE) of the devices made according to Example 1 were measured in reverse bias (2V). With reference to Figure 4 and Figure 5 , the EQE for the device comprising ITIC is above 40% over almost all of the region and is very close to the EQE achieved for the comparative device

-27comprising C70IPH in the green region of the spectrum (i.e. between 495 nm and 570 nm). This makes the device particularly suitable for use in X-ray imaging applications.

When the dark current and EQE measurements for Example 1 are considered together, it will be appreciated that, overall, the devices of Example 1 exhibit an improved signal-to-noise ratio compared with Comparative Device 1.

Method Used to Determine LUMP Energy Levels

The LUMO energy levels reported herein were determined using square wave voltammetry (SWV) at room temperature in solution. In Squarewave

Voltammetry, the current at a working electrode is measured while the potential between the working electrode and a reference electrode is swept linearly in time. The difference current between a forward and reverse pulse is plotted as a function of potential to yield a voltammogram. The apparatus to measure HOMO or LUMO energy levels by SWV may comprise a cell containing tertiary butyl ammonium perchlorate or tertiary butyl ammonium hexafluorophosphate in acetonitrile; a glassy carbon working electrode; a platinum counter electrode and a leak free Ag/AgCI reference electrode.

Ferrocene is added directly to the existing cell at the end of the experiment for calculation purposes where the potentials are determined for the oxidation and reduction of ferrocene versus Ag/AgCI using cyclic voltammetry (CV).

Apparatus:

CHI 660D Potentiostat.

3mm diameter glassy carbon working electrode Leak free Ag/AgCI reference electrode Pt wire auxiliary or counter electrode.

0.1 M tetrabutylammonium hexafluorophosphate in acetonitrile.

- 28 Method:

The sample is dissolved in Toluene (3mg/ml) and spun at sooorpm directly on to the glassy carbon working electrode.

LUMO = 4.8-E ferrocene (peak to peak average) - E reduction of sample (peak 5 maximum).

HOMO = 4.8-E ferrocene (peak to peak average) + E oxidation of sample (peak maximum).

A typical SWV experiment runs at 15Hz frequency; 25mV amplitude and 0.004V increment steps. Results are calculated from 3 freshly spun film samples for both the HOMO and LUMO data.

All experiments are run under an Argon gas purge.

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 15 those skilled in the art without departing from the scope of the invention as set forth in the following claims.

Claims

Claims

1. An organic photodetector comprising:

a first electrode;

5 a second electrode; and a photosensitive organic layer positioned between the electrodes, wherein the photosensitive organic layer comprises a donor compound and an acceptor compound, wherein the acceptor compound does not comprise a fullerene group and wherein the LUMO energy level of the acceptor compound io is equal to or deeper than the LUMO energy level of fullerene derivative

C70IPH.
2. An organic photodetector as claimed in claim 1, wherein the acceptor compound is represented by General Formula (V):

wherein each of R¹¹, R¹², R¹·³, R^, Rv are each independently selected from one of: H, a halogen or either of the following formulae (VIII) and (IX):

(VI11) ; and wherein R¹⁶ and R¹⁷ are each independently selected from the group consisting of: H; branched, linear or cyclic C1-20 alkyl wherein one or more non5 adjacent, non-terminal C atoms may be replaced with 0, S, CO or COO and one or more H atoms may be replaced with F; and aryl or heteroaryl which is substituted or unsubstituted with one or more substituents.
3. An organic photodetector as claimed in claim 2, wherein R¹⁷>, R²⁰> R²¹, R²²10 and R²3 are H or a halogen selected from the group consisting of: Cl, Br, I, F, and each of R¹⁶ and R¹⁷ is -(CH₂)_nCH₃, where n is an integer selected from 1 to 20.
4. An organic photodetector as claimed in claim 3, wherein R¹¹, R¹², R¹s and

15 R¹4 are formula (VIII), where n is 5.
5. An organic photodetector as claimed in claim 3, wherein R¹¹, R¹², R^1! and R!4 are formula (IX), where n is 5.

20
6. An organic photodetector as claimed in any one of the preceding claims, wherein the acceptor compound is ITIC or ITIC-Th.
7. An organic photodetector as claimed in claim 1, wherein the acceptor compound is represented by General Formula (I):

-31R2 _R2 _R2 _R2

General Formula I wherein each R¹ is independently selected from the group consisting of: H; branched, linear or cyclic C1-20 alkyl wherein one or more non-adjacent, nonterminal C atoms may be replaced with O, S, CO or COO and one or more H

5 atoms may be replaced with F; and aryl or heteroaryl which is substituted or unsubstituted with one or more substituents;

wherein each of R² and R3 are independently selected from the group consisting of: H; branched, linear or cyclic C1-20 alkyl wherein one or more non-adjacent,

10 non-terminal C atoms may be replaced with 0, S, CO or COO and one or more H atoms maybe replaced with F; aryl or heteroaryl which is substituted or unsubstituted with one or more substituents; and a group having the following formula (II):

(II)

-32; and wherein each of R4 and Rs are independently selected from the group consisting of: H; branched, linear or cyclic C1-20 alkyl wherein one or more nonadjacent, non-terminal C atoms may be replaced with O, S, CO or COO and one 5 or more H atoms may be replaced with F; and aryl or heteroaryl which is substituted or unsubstituted with one or more substituents.
8. An organic photodetector as claimed in claim 7, wherein R¹ and R4 are each independently selected from a group having the following formula (IV):

(IV) wherein each of R
9 and R¹⁰ are independently selected from the group consisting of: -CH₃ and -(CH₂)_nCH₃, wherein n is an integer selected from 120.

15 9. An organic photodetector as claimed in either of claims 7 or 8, wherein the acceptor compound has the following formula (XII):

wherein R¹⁸ has the following formula (IV):

R⁹

(IV) ; and wherein each of R9 and R¹⁰ are independently selected from the group consisting of: -CH₃ and -(CH₂)_nCH₃, wherein n is an integer selected from 120.
10 io. An organic photodetector as claimed in either one of claims 8 or 9, wherein n is 4 or 5.
11. An organic photodetector as claimed in any one of the preceding claims, wherein the donor compound is a semiconducting polymer.
12. An organic photodetector as claimed in any one of the preceding claims, wherein the first electrode is an anode and the second electrode is a cathode.
13. An organic photodetector as claimed in any one of the preceding claims,

5 wherein the weight ratio of the donor compound to the acceptor compound is from about 1:0.5 to about 1:1.2.
14. A sensor comprising a light source and an organic photodetector as claimed in any one of claims 1 to 13, wherein the organic photodetector is

10 configured to receive light emitted from the light source.
15. A method of detecting light comprising measurement of a photocurrent generated by light incident on an organic photodetector as claimed in any one of claims 1 to 13.
16. A method of detecting light as claimed in claim 15, wherein the method comprises measurement of the photocurrent generated by light incident on the organic photodetector and emitted from the light source of the sensor as claimed in claim 14.
17. Use of a compound that does not comprise a fullerene group having a LUMO energy level and is deeper than the LUMO energy level of fullerene derivative C70IPH in a photosensitive layer of an organic photodetector to reduce dark current.