WO2024115882A1

WO2024115882A1 - Device

Info

Publication number: WO2024115882A1
Application number: PCT/GB2023/053037
Authority: WO
Inventors: Pascal CACHELIN; Matthew Stevenson
Original assignee: Sumitomo Chemical Co., Ltd; Cambridge Display Technology Limited
Priority date: 2022-11-28
Filing date: 2023-11-21
Publication date: 2024-06-06
Also published as: GB2624713A; GB202217841D0

Abstract

A near infrared, NIR organic photodiode (1) is described which includes an anode (2), a cathode (3) and a photoactive layer (4) disposed between the anode (2) and cathode (3). The NIR organic photodiode (1) also includes a hole-blocking layer (5) separating the cathode (3) and the photoactive layer (4). The hole blocking layer (5) includes an organic hole-blocking material and inorganic hole-blocking nanoparticles.

Description

DEVICE

BACKGROUND

Two key parameters for the performance of photodiodes are the sensitivity and the dark current. It is desirable to maximise the former and minimise the latter.

Krebsbach et al, “Inkjet-Printed Tin Oxide Hole-Blocking Layers for Organic Photodiodes”, https://doi.org/10.1021/acsaelm.lc00760, ACS Appl. Electron. Mater. 2021, 3, 11, 4959-4966, October 20, 2021, describes the fabrication of inkjet-printed SnO2 hole-blocking layers (HBLs) for organic photodiodes (OPDs).

Kadam et al, “Optimization of ZnO:PEIE as an Electron Transport Layer for Flexible Organic Solar Cells”, https://doi.org/10.1021/acs.energyfuels.lc00639, Energy Fuels 2021, 35, 15, 12416-12424, July 21, 2021, describes flexible organic solar cells including an electron transport layer (ETL) fabricated by making a nanocomposite from zinc oxide (ZnO) and polyethyleneimine ethoxylated (PEIE).

Li et al, “Carrier Blocking Layer Materials and Application in Organic Photodetectors”, https://doi.org/10.3390/nanol l061404, Nanomaterials 2021, 11(6), 1404, May 2021, reviews charge carrier blocking layers for organic photodetectors.

CN 111162173A describes an organic photoelectric detector having a doped electron transport layer. The organic photoelectric detector is sequentially provided with a substrate, a transparent conductive ITO cathode, an electron transport layer, a photoactive layer, a hole transport layer and a metal anode from bottom to top. The electron transport layer is formed by mixing a ZnO nanoparticle solution and a doped PCBM, and the thickness of the electron transport layer is 40 to 50 nm.

Shen et al, “High-efficiency polymer solar cells with low temperature solution-processed SnO2/PFN as a dual-function electron transporting layer” J. Mater. Chem. A, 2018,6, 17401-17408, https://doi.org/10.1039/C8TA06378H, describe polymer solar cells PSCs with low temperature solution-processed tin dioxide (SnCL) nanocrystals and a poly-[(9,9-bis(3'- (N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) stacked structure as an electron transporting layer. Su Jin In et al, “Reduced interface energy loss in non-fullerene organic solar cells using room temperature- synthesized SnO2 quantum dots”, Journal of Materials Science & Technology, Volume 52, 2020, Pages 12-19, ISSN 1005-0302, https://doi.Org/10.1016/j.jmst.2020.02.054, describes using 3 to 4 nm SnCh quantum dots as an electron transporting layer in non-fullerene organic solar cells.

SUMMARY

According to a first aspect of the invention there is provided a near infrared (NIR) organic photodiode including an anode, a cathode and a photoactive layer disposed between the anode and cathode. The NIR organic photodiode also includes a hole blocking layer separating the cathode and the photoactive layer. The hole-blocking layer includes an organic hole-blocking material and inorganic hole-blocking nanoparticles.

A material/nanop article may be hole-blocking if the highest occupied molecular orbital (HOMO) of that material/nanoparticle is deeper (further from vacuum) than that of the electron donor(s) and the electron acceptor(s) of the photoactive layer.

The inorganic hole-blocking nanoparticles may be distributed substantially homogenously within the hole-blocking layer. Substantially homogenously may mean that there is no more than 20% variation between a volume fraction of inorganic hole-blocking nanoparticles between an interface facing the cathode and an interface facing the photoactive layer.

The organic hole-blocking material may be conductive. The organic hole-blocking material may be semi-conductive. The organic hole-blocking material may be insulating.

A hole blocking layer may alternatively be referred to as an electron transport layer.

An external quantum efficiency of the NIR organic photodiode may peak at a wavelength exceeding 800 nm. An external quantum efficiency of the NIR organic photodiode may peak at a wavelength between 800 nm and 1100 nm. An average external quantum efficiency of the NIR organic photodiode across the range from 800 nm to 1100 nm may be greater than or equal to 20%. An average external quantum efficiency of the NIR organic photodiode across the range from 800 nm to 1100 nm may be greater than or equal to 25%. An average external quantum efficiency of the NIR organic photodiode across the range from 800 nm to 1100 nm may be greater than or equal to 30%. An average external quantum efficiency of the NIR organic photodiode across the range from 800 nm to 1100 nm may be greater than or equal to 40%.

The photoactive layer may have sensitivity at wavelengths exceeding 900 nm. The photoactive layer of the NIR organic photodiode may be considered to have sensitivity at a given wavelength provided the external quantum efficiency (EQE) at the given wavelength is no less than one tenth of a peak EQE. The sensitivity of the NIR organic photodiode may have a peak at a wavelength greater than or equal to 1100 nm. The peak sensitivity may be between 1100 and 1600 nm. The peak sensitivity may be at a wavelength greater than or equal to 1400 nm. The peak sensitivity may be between 890 and 920 nm. The peak sensitivity may be 905 nm. The peak sensitivity may be between 1540 and 1560 nm. The peak sensitivity may be 1550 nm.

The NIR organic photodiode may also include an electron-blocking layer separating the anode and the photoactive layer. A material/nanoparticle may be electron-blocking if the lowest unoccupied molecular orbital (LUMO) of that material/nanoparticle is shallower (closer to vacuum) than that of the electron donor(s) and the electron acceptor(s) of the photoactive layer. An electron-blocking layer may alternatively be referred to as a hole transport layer.

The organic hole-blocking material may include, or take the form of, a polymer material.

The organic hole-blocking material may include, or take the form of, a small molecule material.

The organic hole-blocking material may include, or take the form of, an organometallic material.

The organic hole-blocking material may include, or take the form of ethoxylated polyethyleneimine (PEIE). The organic hole-blocking material may include, or take the form of, N,N'-Bis(N,N-dimethylpropan-l -amine oxide)perylene-3,4,9,10-tetracarboxylic diimide (PDINO). The organic hole-blocking material may include, or take the form of, N,N'-Bis{3- [3-(Dimethylamino)propylamino]propyl}perylene-3,4,9,10-tetracarboxylic diimide (PDINN). The organic hole-blocking material may include, or take the form of, a melamine polymer. The organic hole-blocking material may include, or take the form of polyimide (PI).

The inorganic hole-blocking nanoparticles may include one or more of tin oxide, SnCh nanoparticles, zinc oxide, ZnO nanoparticles, sodium fluoride, NaF nanoparticles, barium hydroxide, Ba(0H)2 nanoparticles, and/or aluminium doped zinc oxide, ZnO nanoparticles.

The inorganic hole -blocking nanoparticles may have a mean average size between 6 nm and 8 nm.

The inorganic hole -blocking nanoparticles may have particle sizes within a range spanning between 5 nm and 50 nm. Particle sizes (approximating diameters) of the inorganic holeblocking nanoparticles may be measured by dynamic light scattering (DLS). Particle size may correspond to hydrodynamic diameter. In DLS measurements, hydrodynamic diameter corresponds to the diameter of a sphere that has the same translational diffusion coefficient as the particle(s) being measured.

The photoactive layer may include a non-fullerene acceptor. The photoactive layer may include no fullerene acceptors.

The hole blocking layer may be formed from a combination of ethoxylated polyethyleneimine (PEIE) and tin oxide, SnCh nanoparticles. The hole blocking layer may be formed from a combination of ethoxylated polyethyleneimine (PEIE) and zinc oxide, ZnO nanoparticles. The hole blocking layer may be formed from a combination of ethoxylated polyethyleneimine (PEIE) and sodium fluoride, NaF nanoparticles. The hole blocking layer may be formed from a combination of ethoxylated polyethyleneimine (PEIE) and barium hydroxide, Ba(OH)2 nanoparticles. The hole blocking layer may be formed from a combination of ethoxylated polyethyleneimine (PEIE) and aluminium doped zinc oxide, ZnO nanoparticles.

The hole blocking layer may be formed from a combination of N,N'-Bis(N,N-dimethylpropan- 1-amine oxide)perylene-3,4,9,10-tetracarboxylic diimide (PDINO) and tin oxide, SnO2 nanoparticles. The hole blocking layer may be formed from a combination of N,N'-Bis(N,N- dimethylpropan- 1 -amine oxide)perylene-3,4,9,10-tetracarboxylic diimide (PDINO) and zinc oxide, ZnO nanoparticles. The hole blocking layer may be formed from a combination of N,N'- Bis(N,N-dimethylpropan-l-amine oxide)perylene-3, 4, 9, 10-tetracarboxylic diimide (PDINO) and sodium fluoride, NaF nanoparticles. The hole blocking layer may be formed from a combination of N,N'-Bis(N,N-dimethylpropan-l-amine oxide)perylene-3,4,9,10- tetracarboxylic diimide (PDINO) and barium hydroxide, Ba(OH)2 nanoparticles. The hole blocking layer may be formed from a combination of N,N'-Bis(N,N-dimethylpropan-l-amine oxide)perylene-3,4,9,10-tetracarboxylic diimide (PDINO) and aluminium doped zinc oxide, ZnO nanoparticles.

The hole blocking layer may be formed from a combination of N,N'-Bis{3-[3- (Dimethylamino)propylamino] propyl}perylene-3,4,9,10-tetracarboxylic diimide (PDINN) and tin oxide, SnO2 nanoparticles. The hole blocking layer may be formed from a combination of N,N'-Bis{3-[3-(Dimethylamino)propylamino] propyl}perylene-3,4,9,10-tetracarboxylic diimide (PDINN) and zinc oxide, ZnO nanoparticles. The hole blocking layer may be formed from a combination of N,N'-Bis{3-[3-(Dimethylamino) propylamino] propyl Jperylene- 3,4,9, 10-tetracarboxylic diimide (PDINN) and sodium fluoride, NaF nanoparticles. The hole blocking layer may be formed from a combination of N,N'-Bis{3-[3- (Dimethylamino)propylamino] propyl Jperylene- 3, 4, 9, 10-tetracarboxylic diimide (PDINN) and barium hydroxide, Ba(OH)2 nanoparticles. The hole blocking layer may be formed from a combination of N,N'-Bis{3-[3-(Dimethylamino)propylamino] propyl Jperylene- 3, 4, 9, 10- tetracarboxylic diimide (PDINN) and aluminium doped zinc oxide, ZnO nanoparticles.

The hole blocking layer may be formed from a combination of a melamine polymer and tin oxide, SnO2 nanoparticles. The hole blocking layer may be formed from a combination of a melamine polymer and zinc oxide, ZnO nanoparticles. The hole blocking layer may be formed from a combination of a melamine polymer and sodium fluoride, NaF nanoparticles. The hole blocking layer may be formed from a combination of a melamine polymer and barium hydroxide, Ba(OH)2 nanoparticles. The hole blocking layer may be formed from a combination of a melamine polymer and aluminium doped zinc oxide, ZnO nanoparticles.

The hole blocking layer may be formed from a combination of polyimide (PI) and tin oxide, SnO2 nanoparticles. The hole blocking layer may be formed from a combination of polyimide (PI) and zinc oxide, ZnO nanoparticles. The hole blocking layer may be formed from a combination of polyimide (PI) and sodium fluoride, NaF nanoparticles. The hole blocking layer may be formed from a combination of polyimide (PI) and barium hydroxide, Ba(OH)2 nanoparticles. The hole blocking layer may be formed from a combination of polyimide (PI) and aluminium doped zinc oxide, ZnO nanoparticles.

Apparatus may include the NIR organic photodiode, and an amplifier arranged to amplify a photocurrent output by the NIR organic photodiode.

The NIR organic photodiode may be configured for zero-bias operation. Alternatively, the NIR organic photodiode may be configured for reverse bias operation. The configuration of the NIR organic photodiode for reverse-bias operation may include, or take the form of, including contacts enabling application of a reverse-bias to the photo-diode.

The reverse bias may preferably be low. A low reverse bias may be a bias less than a breakdown voltage of the NIR organic photodiode. A low reverse bias may be less than or equal to 5.5 V, less than or equal to 5 V, less than or equal to 3.3 V, or less than or equal to 3 V. A low reverse bias may be calibrated to correspond to a dark-current of the NIR organic photodiode remaining below a dark-current threshold. The dark-current threshold may be a current density of Ip A. cm^-2 (the relevant area being the area of the NIR organic photodiode junction).

A sensor may include the NIR organic photodiode or the apparatus.

According to a second aspect of the invention, there is provided a method of fabricating a NIR organic photodiode comprising an anode, a cathode and a photoactive layer disposed between the anode and cathode. The method includes forming a hole blocking layer separating the cathode and the photoactive layer. The hole blocking layer includes an organic hole-blocking material and inorganic hole-blocking nanoparticles.

The method may include features corresponding to any features of the NIR organic photodiode and/or an apparatus/sensor including the NIR organic photodiode. Definitions applicable to the NIR organic photodiode (and/or features thereof) and/or an apparatus/sensor including the NIR organic photodiode (and/or features thereof) may be equally applicable to the method (and/or features/steps thereof). An external quantum efficiency of the NIR organic photodiode may peak at a wavelength exceeding 800 nm. An external quantum efficiency of the NIR organic photodiode may peak at a wavelength between 800 nm and 1100 nm. An average external quantum efficiency of the NIR organic photodiode across the range from 800 nm to 1100 nm may be greater than or equal to 20%. An average external quantum efficiency of the NIR organic photodiode across the range from 800 nm to 1100 nm may be greater than or equal to 25%. An average external quantum efficiency of the NIR organic photodiode across the range from 800 nm to 1100 nm may be greater than or equal to 30%. An average external quantum efficiency of the NIR organic photodiode across the range from 800 nm to 1100 nm may be greater than or equal to 40%.

The photoactive layer may have sensitivity at wavelengths exceeding 900 nm.

Forming the hole blocking layer may include distributing the inorganic hole-blocking nanoparticles substantially homogenously within the hole-blocking layer. Substantially homogenously may mean that there is no more than 20% variation between a volume fraction of inorganic hole-blocking nanoparticles between an interface facing the cathode and an interface facing the photoactive layer.

The method may also include forming an electron-blocking layer separating the anode and the photoactive layer.

The inorganic hole-blocking nanoparticles may include one or more of tin oxide, SnCh nanoparticles, zinc oxide, ZnO nanoparticles, sodium fluoride, NaF nanoparticles, barium hydroxide, Ba(OH)2 nanoparticles and aluminium doped zinc oxide, ZnO particles. The inorganic hole -blocking nanoparticles may have a mean average size between 6 nm and 8 nm.

DESCRIPTION OF DRAWINGS

The disclosed technology and accompanying figures describe some implementations of the disclosed technology.

Figure 1 is a schematic cross-section of an organic photodiode;

Figure 2 is a circuit diagram of an apparatus for amplifying photocurrents from a photodiode;

Figure 3 plots experimental values of dark current density corresponding to four exemplary types of NIR organic photodiodes which were fabricated;

Figure 4 plots summary statistics of dark current density for the four exemplary types of NIR organic photodiode at a fixed bias voltage of 3 V; and

Figure 5 plots external quantum efficiency (EQE) as a function of wavelength for the four exemplary types of NIR organic photodiode.

The drawings are not drawn to scale and have various viewpoints and perspectives. The drawings are some implementations and examples. Additionally, some components and/or operations may be separated into different blocks or combined into a single block for the purposes of discussion of some of the embodiments of the disclosed technology. Moreover, while the technology is amenable to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and are described in detail below. The intention, however, is not to limit the technology to the particular implementations described. On the contrary, the technology is intended to cover all modifications, equivalents, and alternatives falling within the scope of the technology as defined by the appended claims.

DETAILED DESCRIPTION Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise," "comprising," and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to." Additionally, the words "herein," "above," "below," and words of similar import, when used in this application, refer to this application as a whole and not to any particular portions of this application. Where the context permits, words in the Detailed Description using the singular or plural number may also include the plural or singular number respectively. The word "or," in reference to a list of two or more items, covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list. References to a layer “over” another layer when used in this application means that the layers may be in direct contact or one or more intervening layers may be present. References to a layer “on” another layer when used in this application means that the layers are in direct contact. References to a specific atom include any isotope of that atom unless specifically stated otherwise.

The teachings of the technology provided herein can be applied to other systems, not necessarily the system described below. The elements and acts of the various examples described below can be combined to provide further implementations of the technology. Some alternative implementations of the technology may include not only additional elements to those implementations noted below, but also may include fewer elements.

These and other changes can be made to the technology in light of the following detailed description. While the description describes certain examples of the technology, and describes the best mode contemplated, no matter how detailed the description appears, the technology can be practiced in many ways. As noted above, particular terminology used when describing certain features or aspects of the technology should not be taken to imply that the terminology is being redefined herein to be restricted to any specific characteristics, features, or aspects of the technology with which that terminology is associated. In general, the terms used in the following claims should not be construed to limit the technology to the specific examples disclosed in the specification, unless the Detailed Description section explicitly defines such terms. Accordingly, the actual scope of the technology encompasses not only the disclosed examples, but also all equivalent ways of practicing or implementing the technology under the claims. To reduce the number of claims, certain aspects of the technology are presented below in certain claim forms, but the applicant contemplates the various aspects of the technology in any number of claim forms.

In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of implementations of the disclosed technology. It will be apparent, however, to one skilled in the art that embodiments of the disclosed technology may be practiced without some of these specific details.

One approach to fabricating organic photodiodes is to blend two materials, a donor polymer and a small molecule acceptor, to produce a bulk heterojunction for the photoactive layer. In this specification, the impact of hole-blocking layers (HBL) including inorganic nanoparticles is discussed. The aim of this layer is to reduce charge injection from the cathode into the active layer and thereby reduce the dark current density (JD). AS discussed hereinafter (see Figures 3 and 4) there was found to be a limited impact of hole blocking layers including inorganic nanoparticles on dark current density JD. However, it was observed (see Figure 5) that the use of a hole blocking layer including inorganic nanoparticles resulted in significantly reduced distortion in the mid- absorption wavelength region of the organic photodiodes compared to control devices (Example 1 hereinafter) having a purely organic hole blocking layer. This results in increased detectivity in the band-edge region. Furthermore, a hybrid system containing a blend of organic hole blocking material (HBM) and inorganic hole blocking nanoparticles (Example 3 hereinafter) was observed (see Figure 5) to result in a further increase in detectivity. The hybrid system (Example 3 hereinafter) was also observed to have improved performance over a multi-layer having an inorganic nanoparticle layer stacked over an organic HBM layer (Example 4 hereinafter). Examples 3 and 4 are not directly comparable because the organic component of Example 3 was polymeric, whereas Example 4 used a small molecule organic component. Herein, detectivity refers to specific detectivity, equal to the reciprocal of noise-equivalent power (NEP), normalized per square root of the sensor's area and frequency bandwidth (reciprocal of twice the integration time).

Near-infrared (NIR) photodetection, spanning wavelengths 750 to 1400 nm, is of interest for many applications including medical imaging and machine vision. Organic photodetectors (OPDs) according to the present specification offer an opportunity for low cost, solutionprocess compatible NIR photodetectors. Referring to Figure 1, an organic photodiode (OPD) 1 is shown in schematic cross-section.

The OPD 1 includes an anode 2, a cathode 3, a photoactive layer 4 disposed between the anode 2 and cathode 3, and a hole blocking layer 5 separating the cathode 3 and the photoactive layer 4. The hole blocking layer 5 includes an organic hole-blocking material and inorganic holeblocking nanoparticles (not shown separately from the hole blocking layer 5). The organic hole-blocking material and inorganic hole-blocking nanoparticles are blended together to form a composite material, rather than being segregated into two separate or substantially separate layers.

Herein, reference to a material and/or nanoparticle being hole-blocking refers to the highest occupied molecular orbital (HOMO) of that material/nanoparticle being deeper (further from vacuum) than that of the electron donor(s) and the electron acceptor(s) of the photoactive layer. A hole blocking layer 5 may alternatively be referred to as an electron transport layer. Preferably, the organic hole-blocking material used for the hole blocking layer 5 is conductive or semi-conductive. However, in some examples the organic hole -blocking material used for the hole blocking layer 5 may be insulating.

The organic hole-blocking material of the hole blocking layer 5 may include one or more of a polymer material, a small molecule material and/or an organometallic material. For example, the organic hole-blocking material may include, or take the form of, ethoxylated polyethyleneimine (PEIE) or polyimide (PI). In another example the organic hole blocking material may include, or take the form of, N,N'-Bis(N,N-dimethylpropan-l-amine oxide)perylene-3,4,9,10-tetracarboxylic diimide (PDINO). In a still further example, the organic hole blocking material may include, or take the form of, N,N'-Bis{3-[3- (Dimethylamino)propylamino]propyl} perylene-3,4,9,10-tetracarboxylic diimide (PDINN). In other examples, the organic hole-blocking material may include, or take the form of, a melamine polymer, for example:

The inorganic hole-blocking nanoparticles providing the second component of the blended/composite hole blocking layer 5 are formed from one or more of tin oxide, SnCh, zinc oxide, ZnO, sodium fluoride, NaF, and barium hydroxide, Ba(0H)2, and aluminium doped zinc oxide, ZnO.

A blend of ethoxylated polyethyleneimine (PEIE) and tin oxide, SnO2 has been observed to provide good improvements in EQE (see Figure 5). A blend of ethoxylated polyethyleneimine (PEIE) and zinc oxide, ZnO or aluminium doped zinc oxide, ZnO is expected to perform similarly due to similarities in band-gap and other electrical characteristics. Polyimide (PI) is expected to provide similar performance to ethoxylated polyethyleneimine (PEIE) as the organic hole blocking component.

The inorganic hole -blocking nanoparticles may have a mean average size between 6 nm and 8 nm. Herein, the “size” of a nanoparticle refers to the largest dimension of that particle. For a roughly spherical nanoparticle the size would correspond to a diameter, for a roughly ellipsoidal nanoparticle the size would correspond to twice the semi-major axis and so forth. The range of largest dimensions for the inorganic hole-blocking nanoparticles may span a range from about 5 nm up to about 50 nm. In some examples, the size of the nanoparticles may preferably be less than a total thickness of the hole blocking layer 5, with the intention of reducing a surface roughness of the hole blocking layer 5. The sizes of the inorganic holeblocking nanoparticles may be measured by dynamic light scattering (DLS). When using DLS to characterize particle sizes, the measured particle size corresponds to a hydrodynamic diameter. The total thickness of the hole blocking layer 5 is typically up to about 50 nm or less.

The OPD 1 generates a photocurrent IP in response to illumination with incident light 6. Preferably, the photoactive layer 4 has sensitivity at wavelengths exceeding 900 nm, i.e. into the short-wav elength infrared SWIR or infrared (IR) bands. The photoactive layer 4 of the OPD 1 may be considered to have sensitivity at a given wavelength provided the external quantum efficiency (EQE) at the given wavelength is no less than one tenth of a peak EQE.

The sensitivity of the photoactive layer 4 of the OPD 1 may have a peak at a wavelength greater than or equal to 1100 nm, for example between 1100 and 1600 nm (inclusive of endpoints). The peak sensitivity may be at, or around, a wavelength commonly used for IR communications and/or for applications such as, for example, laser detection and range finding (LIDAR), machine vision, hyperspectral imaging, free-space optical communication (FSO), and so forth. For example, the peak sensitivity may occur around 905 nm or around 1550 nm. Sensitivity of the photoactive layer 4 of the OPD 1 is controlled through selecting the materials forming the photo-active layer 4.

The photoactive layer 4 may include a non-fullerene acceptor. Preferred non-fullerene acceptors are compounds of formula ADA or ADA’DA wherein A is a monovalent electronaccepting unit; D is a divalent electron-donating unit; and A’ is a divalent electron-accepting unit, wherein each A may be directly bound to D or spaced apart therefrom by a bridging unit, for example thiophene. Exemplary compounds of formula ADA are the following compounds disclosed in WO2022/129137, the contents of which are incorporated herein by reference:

Exemplary compound of formula ADA’DA include:

An exemplary method for forming ADA’ DA compounds is:

A preferred electron-donating material is a donor-acceptor (DA) polymer, for example:

The bulk heterojunction layer may consist of a single electron-donating material and a single electron-accepting material or it may comprise one or more further materials. In a preferred embodiment, the bulk heterojunction layer comprises an electron-donating material, a nonfullerene acceptor and a fullerene acceptor.

However, the photoactive layer 4 is not particularly limited, and any combination of donor and acceptor materials suitable for forming a bulk heterojunction sensitive to a desired wavelength range may be used.

The OPD 1 may optionally include an electron-blocking layer 7 (EBL) separating the anode 2 and the photoactive layer 4. However, an electron-blocking layer 7 is not essential (see for example the experimental results presented hereinafter and obtained without electron-blocking layers 7). Herein, a material/nanoparticle may be electron-blocking if the lowest un-occupied molecular orbital (LUMO) of that material/nanoparticle is shallower (closer to vacuum) than that of the electron donor(s) and the electron acceptor(s) of the photoactive layer 4. An electron-blocking layer 7 may alternatively be referred to as a hole transport layer. The material used for the electron-blocking layer 7 is not particularly limited, and materials known for electron-blocking/hole transport in other organic electronic device may be used. For example, without limitation, one of 4, 4-N,N’ -dicarbazole- 1,1 ’-biphenyl (CBP); N,N’-bis(3- methylphenyl)-N,N’ -diphenylbenzidine (TPD); 4,4',4"-tris[2-naphthyl(phenyl) amino] triphenylamine (2-TNATA); l,l-bis[(di-4-tolylamino) phenyl]cyclohexane (TAPC); N,N'-Bis [4-(diphenylamino)phenyl]-N,N'-di (1-naphthyl) benzidine (NPD-DPA) or di( 1 -naphthyl) - N,N’ -diphenyl-( 1,1’ -biphenyl)-4,4’ -diamine) (NPB) .

The cathode 3 may optionally be supported on a substrate 8. Depending on the material and thickness of the cathode 3, mechanical support from a substrate 8 may be unnecessary. A substrate 8 may also provide environmental encapsulation, which may be completed by an encapsulating layer 9 formed over the anode 2. The encapsulation may protect the photoactive layer 4 and/or other organic containing layers such as the hole blocking layer 5 from exposure to moisture and/or air.

Referring also to Figure 2, an apparatus 10 for amplifying a photocurrent output by an organic photodiode 1 is shown.

The apparatus 10 includes the organic photodiode 1 and an amplifier 11 arranged to amplify a photocurrent Ip output by the organic photodiode 1. The organic photodiode 1 may be configured for zero-bias operation by setting the switch SW to connect to system ground potential, or for reverse bias by setting the switch SW to a reverse bias potential -VB- The reverse bias -VB is preferably low, for example less than or equal to 5.5 V. Alternatively, the reverse bias - VB may be calibrated to correspond to a dark-current of the organic photodiode 1 remaining below a dark-current threshold, for example a dark current density of Ip A. cm^-2 (the relevant area being the area of the organic photodiode junction).

The apparatus 10 may be incorporated into a sensor (not shown).

In the example illustrated in Figure 2, the amplifier 11 takes the form of a transimpedance amplifier in combination with a feedback network having capacitance CF and resistance RF. However, in the general case, any amplifier 11 suitable for amplifying the photocurrent Ip may be used.

Fabrication

The OPD 1 may be fabricated using solution processing techniques used in the production of organic photodiodes and/or organic light emitting diodes. The hole blocking layer 5 formed from a blend of the organic hole-blocking material and inorganic hole-blocking nanoparticles may be incorporated by using a solution of suitable composition. For example, the inorganic hole-blocking nanoparticles may be dispersed in a solution comprising a solvent having the organic hole-blocking material dissolved therein, and the inorganic hole-blocking nanoparticles suspended therein. Alternatively, the organic hole-blocking material may be placed into a solution containing the inorganic hole-blocking nanoparticles. The hole-blocking layer 5 may then be deposited using any suitable solution processing technique such as, for example spin-coating, doctor blading, printing and so forth.

Experimental examples

All experimental organic photodiodes 1 were constructed in the same way, with the exception of using different materials to form the hole blocking layer 5 of each. The organic photodiodes 1 had cathodes 3 formed from an ITO substrate (150nm thick, sputtered) and then subsequently coated with the relevant hole-blocking layer 5 for each example (i.e. the cathode 3 and substrate 8 were provided bya single element). The photoactive layer 4 was deposited by blade coating the photoactive material blend in each case from a solution of 15 mg/mL in 1,2,4- trimethylbenzene (TMB) : 1,2-dimethoxybenzene (DMOB) (95:5 v/v) to a film thickness of nominally 500 nm. The anode 2 was 10 nm evaporated MoOs capped by 50 nm sputtered ITO. Encapsulating layers 9 were formed from glass.

Example 1:

In a first example, the hole blocking layer 5 was formed from ethoxylated polyethyleneimine (PEIE) of nominal thickness <5 nm. The photoactive layer 4 was formed of Donor Polymer 1, NFA-1 and fullerene CsoPCBM in a Donor Polymer 1 : NFA-1 : CsoPCBM ratio of ratio of 1 : 0.7 : 0.3, and having a nominal thickness of 500 nm.

NFA-1 (TEICO-4CN)

Example 2:

In a second example, the hole blocking layer 5 was formed from tin oxide, SnCh, nanoparticles having mean size (largest dimension) of 7 nm, and a range of sizes from 5 nm to 50 nm. The total thickness of the tin oxide nanoparticle hole blocking layer 5 was between 30 and 40 nm (measured using a stylus surface profilometer). In the same way as the Example 1 devices, the photoactive layer 4 was formed of Donor Polymer 1, NFA-1 and CeoPCBM of nominal thickness 500 nm.

Example 3:

In the third example, the hole blocking layer 5 was formed from a blend of ethoxylated polyethyleneimine (PEIE) and the same tin oxide, SnCh, nanoparticles used in Example 2. The total thickness of the blended hole blocking layer 5 was difficult to measure, and is expected to be of the same order as that of Example 2, namely between 30 and 40 nm. In the same way as the Example 1 devices, the photoactive layer 4 was formed of Donor Polymer 1, NFA-1 and CeoPCBM of nominal thickness 500 nm.

Example 4:

In the third example, the hole blocking layer 5 was formed as a dual-layer structure with a first layer formed of the same tin oxide, SnCE, nanoparticles used in Examples 2 and 3, to a nominal thickness of 15 to 20 nm. The second layer of the hole blocking layer 5 was formed of N,N'- Bis(N,N-dimethylpropan-l-amine oxide)perylene-3,4,9,10-tetracarboxylic diimide (PDINO) to a thickness of 15 to 20 nm. In the same way as the Example 1 devices, the photoactive layer 4 was formed of Donor Polymer 1, NFA-1 and CsoPCBM of nominal thickness 500 nm.

A total of eight devices were produced for each of the Examples 1 to 4, and each device included three separate organic photodiodes. In total, twenty-four organic photodiodes (8 times 3) were produced for each of Examples 1 to 4, and the dark current characteristics of each were measured.

Referring also to Figure 3, the median dark current density JD for each of Examples 1 to 4 is plotted against biasing voltage VB-

For the different compositions of hole blocking layer 5 of Examples 1 to 4, no clear impact on the dark current density JD was observed under reverse or forward bias.

Referring also to Figure 4, summary statistics in the form of box and whisker plots are shown corresponding to the fabricated organic photodiodes 1 at a bias voltage of 3 V.

In common with Figure 3, the summary statistics of Figure 4 are observed to exhibit no significant difference in dark current density JD with the different compositions of hole blocking layer 5 used in Examples 1 to 4.

Referring also to Figure 5, EQE spectra are shown for organic photodiodes 1 of each of Examples 1 to 4. Each series is the mean of spectra corresponding to a pair of organic photodiodes.

A clear change may be observed in the behaviour of the organic photodiode 1 in the mid-band (approximately 800 to 1100 nm), with substantially decreased EQE in Example 1 (decreased compared to what would be expected for a device containing only the p-type donor) in the wavelengths associated with the p-type donor used in the photoactive layer 4. The increase in EQE in the mid-band (relative to Example 1) observed for the Examples 2, 3 and 4 results in increased detectivity in the long-wavelength region. The largest increase was observed for the blended nanoparticle and organic hole blocking layer 5 corresponding to the Example 3 devices.

Claims

1. A near infrared, NIR, organic photodiode comprising: an anode; a cathode; a photoactive layer disposed between the anode and cathode; a hole-blocking layer separating the cathode and the photoactive layer, the hole blocking layer comprising an organic hole-blocking material and inorganic hole-blocking nanoparticles.

2. The NIR organic photodiode of claim 1, wherein an external quantum efficiency of the NIR organic photodiode peaks at a wavelength exceeding 800 nm.

3. The NIR organic photodiode of claim 1 or claim 2, wherein the inorganic hole-blocking nanoparticles are distributed substantially homogenously within the hole-blocking layer.

4. The NIR organic photodiode of any one of claims 1 to 3, further comprising an electron- blocking layer separating the anode and the photoactive layer.

5. The NIR organic photodiode of any one of claims 1 to 4, wherein the organic hole- blocking material comprises a polymer material.

6. The NIR organic photodiode of any one of claims 1 to 5, wherein the organic hole- blocking material comprises a small molecule material.

7. The NIR organic photodiode of any one of claims 1 to 6, wherein the organic hole- blocking material comprises an organometallic material.

8. The NIR organic photodiode of any one of claims 1 to 7, wherein the inorganic holeblocking nanoparticles comprise one or more of tin oxide, SnCh nanoparticles, zinc oxide, ZnO nanoparticles, sodium fluoride, NaF nanoparticles, barium hydroxide, Ba(OH)2 nanoparticles, and aluminium doped zinc oxide, ZnO nanoparticles.

9. The NIR organic photodiode of any one of claims 1 to 8, wherein the inorganic holeblocking nanoparticles have a mean average size between 6 nm and 8 nm.

10. The NIR organic photodiode of any one of claims 1 to 9, wherein the photoactive layer comprises a non-fullerene acceptor.

11. Apparatus comprising the NIR organic photodiode of any one of claims 1 to 10, and an amplifier arranged to amplify a photocurrent output by the NIR organic photodiode.

12. The apparatus of claim 11, wherein the NIR organic photodiode is configured for reverse bias operation.

13. A sensor comprising the NIR organic photodiode of any one of claims 1 to 10 or the apparatus of claims 11 or 12.

14. A method of fabricating a NIR organic photodiode comprising an anode, a cathode and a photoactive layer disposed between the anode and cathode, the method comprising: forming a hole blocking layer separating the cathode and the photoactive layer, the hole blocking layer comprising a NIR organic hole-blocking material and inorganic hole-blocking nanoparticles.

15. The method of claim 14, wherein an external quantum efficiency of the NIR organic photodiode peaks at a wavelength exceeding 800 nm.

16. The method of claim 14 or claim 15, wherein forming the hole blocking layer comprises distributing the inorganic hole-blocking nanoparticles substantially homogenously within the hole-blocking layer.

17. The method of any one of claims 14 to 16, further comprising forming an electronblocking layer separating the anode and the photoactive layer.

18. method of any one of claims 14 to 17, wherein the organic hole-blocking material comprises a polymer material.

19. The method of any one of claims 14 to 18 wherein the organic hole-blocking material comprises a small molecule material.

20. The method of any one of claims 14 to 19, wherein the organic hole-blocking material comprises an organometallic material.

21. The method of any one of claims 14 to 20, wherein the inorganic hole-blocking nanoparticles comprise one or more of tin oxide, SnCh nanoparticles, zinc oxide, ZnO nanoparticles, sodium fluoride, NaF nanoparticles, barium hydroxide, Ba(OH)2 nanoparticles, and aluminium doped zinc oxide, ZnO nanoparticles.

22. The method of any one of claims 14 to 21, wherein the inorganic hole-blocking nanoparticles have a mean average size between 6 nm and 8 nm.

23. The method of any one of claims 14 to 22, wherein the photoactive layer comprises a non-fullerene acceptor.