photodetector

Abstract

A photodetector is provided having a layered structure comprising: a first electrode, a second electrode, a photoactive layer configured to absorb infrared light and located between the first electrode and the second electrode, and a transport layer. The transport layer is located between the first electrode and the photoactive layer and between the second electrode and the photoactive layer. The transport layer comprises at least a first sublayer formed of a first material and a second sublayer formed from a second material, wherein the second material is distinct from the first material.

Description

[0001] PHOTODETECTOR

[0002] Field

[0003] The present application relates to a photodetector.

[0004] Background

[0005] Photodetectors are devices used to detect electromagnetic radiation, typically in the ultraviolet (UV), optical and / or infrared (IR) wavelength ranges. In the present application, the term “light” is used to indicate electromagnetic radiation in the above wavelength ranges, while the terms “optical light” and “visible light” are used to indicate electromagnetic radiation typically falling within the wavelength sensitivity range of human eyesight. One example of a photodetector is an image sensor, which forms an image from light which is incoming (incident) onto the photodetector. Photodetectors generally incorporate a photoactive material for detecting (sensing) incoming light. The photoactive material absorbs the incident light to produce energy (typically electrical) within the photodetector. This energy is then detected to sense the presence of the incoming radiation. There exist several types of photodetectors, classified by the type of device architecture. The most common types are photoconductors, photodiodes (e.g. p-n, p-i-n, Schottky, and avalanche photodiodes), phototransistors, metal-semiconductor-metal, and quantum-well photodetectors.

[0006] One example of a photoactive material comprises quantum dots, which are nanometre-scale semiconductor particles having optical and electronic properties “that differ from those of larger particles as a result of quantum mechanics” - see https: / / en.wikipedia.org / wiki / Quantum_dot . Quantum dots are sometimes referred to as nanocrystals, nanoparticles or similar, and they may be used to form image sensors or other types of photodetector. In some photodetectors, incoming light is directly incident onto, and absorbed by, a photoactive layer of quantum dots. The photoactive layer may produce an electrical signal in response to the incoming light, and this electrical signal is typically transferred to a silicon-based electronic device for conversion into an image or other form of analog or digital output.

[0007] Quantum dots may be formed from various materials and this in turn affects their physical properties, such as the wavelengths at which they absorb light. A photodetector comprising quantum dots may operate across a (contiguous) range of wavelengths comprising UV, optical light and / or IR radiation. For example, some quantum dots for use in a photodetector are sensitive to a range of short wavelength IR (SWIR). As used herein, the term “SWIR” typically encompasses infrared electromagnetic radiation in the approximate wavelength range 0.8 to 2.5 pm. A given SWIR device may operate across the entirety or just a portion of this range. In some cases, a SWIR device may further provide sensitivity outside the SWIR range, for example, a SWIR device may also respond across the visible-SWIR range of 300-2500 nm.

[0008] One known device for detecting IR radiation is a conventional silicon image sensor, i.e. one which does not incorporate quantum dots but may, for example, be a charge-coupled device (CCD) or a CMOS detector having an IR sensitivity. However, a silicon detector tends to have lower external quantum efficiency (EQE) when operating at longer wavelengths in the IR, for example, at a wavelength of 940 nm compared with operating at 850 nm. Furthermore, silicon-based photodetectors are generally insensitive to SWIR wavelengths beyond 1100 nm.

[0009] One material used in existing SWIR detectors is indium gallium arsenide (InGaAs). Other alternative material candidates include HgCdTe (MCT), InSb, InAs / lnAsSb, and InGaAs / GaAs. Current SWIR image sensors, especially those operating in the range 1000-1700nm, incorporate InGaAs thin films which are grown by molecular beam epitaxy. The integration of epitaxially grown InGaAs onto an image sensor typically involves a time-consuming process called hybridization. As a consequence, the manufacturing costs for InGaAs thin film sensors are relatively high and hence such sensors are generally only suitable for niche applications which are less sensitive to pricing levels.

[0010] Among quantum dot materials, PbS, PbSe, InAs, InSb, HgTe are currently considered to be the most useful / promising semiconductors for SWIR sensing applications. They have a major advantage compared to conventional epitaxial semiconductors in that a thin film of the material can be deposited directly onto a CMOS wafer and thus monolithically integrated during manufacture.

[0011] Further background information about various types of photodetector, especially for use in the infrared, can be found in the following documents, which are provided by way of background example only:

[0012] 1) CN115295645A - this relates to a colloidal quantum dot photoconductive infrared detector based on a capture mode and a preparation method and application thereof.

[0013] 2) KR20230076113A - this discloses a quantum dot thin film-based optical amplification sensor and a manufacturing method thereof. A lower electrode is formed on a substrate; an electron transport layer is formed on the lower electrode; a photoactive layer is formed on the electron transport layer and includes first quantum dots substituted with a first ligand; a hole transport layer is formed on the photoactive layer and includes second quantum dots substituted with a second ligand; and an upper electrode is formed on the hole transport layer

[0014] 3) US2008142075A1 - this relates to photovoltaic devices or solar cells having one or more photoactive layers where at least one of the photoactive layers comprises a sublayer made of photoactive nanoparticles that differ in size, composition or both.

[0015] 4) US2015014627A1 - this relates to two-terminal devices which include a first electrode residing on a substrate; an l-layer comprising an inorganic insulating or broadband semiconducting material residing on top of the first electrode; a semiconductor layer preferably comprising a p-type semiconductor residing over the l-layer; and a second electrode residing over the semiconductor layer.

[0016] 5) US2019140130A1 - this relates to a photosensitive field-effect transistor configured to provide an electrical response when illuminated by electromagnetic radiation incident on the transistor. The photosensitive field-effect transistor comprises a layer of two-dimensional material which forms a horizontal transistor channel configured to transport current and a horizontal semiconducting layer in contact with the transistor channel. 6) US2019221686A1 - this relates to a photosensitive device that includes a conductive electrode, a dielectric layer, a sensing electrode composed of a two-dimensional layered material, and a photoactive layer which can be configured to absorb electromagnetic radiation.

[0017] 7) US2022102570A1 - this relates to a photodetector which comprises a measurement layer and at least a first photoactive layer which covers the measurement layer. The measurement layer may be a transistor channel or a charge accumulation electrode. The conductivity type of the measurement layer is n-type, p-type or ambipolar and the first photoactive layer exhibits intrinsic semi-conductivity.

[0018] 8) US2023015790A1 - this relates to an infrared photodiode which includes a first electrode including a reflective layer, a second electrode facing the first electrode, and a photoelectric conversion layer between the first electrode and the second electrode. The photoelectric conversion layer includes an infrared absorbing material. A maximum absorption wavelength of the infrared absorbing material in a solution state is greater than about 700 nm and less than or equal to about 950 nm.

[0019] 9) US2023098450A1 - this discloses photosensitive semiconducting devices, such as bipolar junction transistors (BJTs), which can be built up over a substrate that may include a read-out integrated circuit (ROIC). Semiconducting layers can be deposited over the substrate and bottom electrodes that are on or at the substrate's top surface. The bottom electrodes may be the input pads of the ROIC. A top electrode is deposited over the semiconducting layers. The semiconducting layers can form BJTs between the bottom electrodes and the top electrode. The top electrode and the bottom electrodes are the BJTs collectors and emitters. The semiconducting layers include a P-type quantum dot layer and an N-type metal oxide layer. The quantum dots act as light sensors for the ROIC because photons absorbed in a semiconducting layer can produce a BJT base current.

[0020] 10) US2023129045A1 - this relates to a photodiode which comprises a substrate; a first electrode arranged on the substrate; a first transport layer arranged on the first electrode; and a photoactive layer arranged on the first transport layer, the photoactive layer including a P-type semiconductor layer and an N-type semiconductor layer.

[0021] 11) Thin-Film Quantum Dot Photodiode for Monolithic Infrared Image Sensors; Malinowski, P.E et al. 2017, 17, 2867 - this describes a CMOS-compatible pixel stack based on lead sulfide quantum dots (PbS QD) with a tunable absorption peak. A photodiode with a 150-nm thick absorber in an inverted architecture shows dark current of 10-6A / cm2at -2 V reverse bias and EQE above 20% at 1440 nm wavelength. This stack can be integrated on a CMOS ROIC, enabling cost reduction for infrared sensors.

[0022] 12) Solution-Processed Metal Oxide Layers for Use in Infrared Photodetectors Manuel Antonio Gomes Filipe de Matos. Faculty of Sciences and Technology, NOVA

[0023] University of Lisbon. 2019 - this relates to the optimization of a PbS QD-based photodiode stack, with a focus on solution-processed metal oxide electron and hole transport layers. Optimization was done first on glass substrates and then transferred to silicon-based substrates. Glass substrate devices showed a 37% external quantum efficiency (EQE) at 1450 nm wavelength and Si substrate devices above 20% EQE for the same wavelength. A 200-nm thick photodiode stack was achieved, showing dark currents around 102mA / cm2at -3 V for both glass and silicon substrates.

[0024] In many industries, such as smartphones, automotive machinery, robotics, unmanned aerial vehicle (UAV), manufacturing and surveillance, sensor technology is used to provide devices with better information about their environment, thereby enabling more advanced operation and behaviour. SWIR image sensors (and other SWIR photodetectors) achieve image detection that goes significantly beyond human vision and produces images that are not readily available with conventional cameras. Accordingly, devices including SWIR imaging capabilities have potential in many fields of technology, such as (by way of example, not limitation):

[0025] - Smartphones: ToF (time-of-fight) sensors; enhancing security for biometric facial recognition, low light / dark conditions, 3D-photography, skin detection

[0026] - Machine Vision: quality control; detecting defective products by seeing through packaging, plastic sorting, and so on

[0027] - Automotive: safety of driver-assistance systems; improving vision at night and / or in adverse weather.

[0028] Accordingly, there is an ongoing interest in enhancing the properties of infrared photodetectors to support and exploit these new market and technology opportunities.

[0029] Summary

[0030] The invention is defined in the appended claims.

[0031] As disclosed herein, a photodetector is provided having a layered structure comprising: a first electrode, a second electrode, a photoactive layer configured to absorb infrared light and located between the first electrode and the second electrode, and a transport layer. The transport layer is located between the first electrode and the photoactive layer and between the second electrode and the photoactive layer. The transport layer comprises at least a first sublayer formed of a first material and a second sublayer formed of a second material, wherein the second material is distinct from the first material.

[0032] Brief Description of the Figures

[0033] Various implementations of the claimed invention will now be described by way of example only with reference to the following drawings.

[0034] Figure 1 is a schematic diagram of an example of an SWIR image sensor.

[0035] Figure 2 is a schematic diagram of a photodetector as disclosed herein.

[0036] Figure 3 is a schematic diagram illustrating the general operation of a photodiode, such as the photodetector of Figure 2.

[0037] Figure 4 is a schematic energy diagram providing further information about a photodetector such as shown in Figure 2.

[0038] Figure 5 provides two graphical plots relating to a photodetector such as disclosed herein, for example with reference to the photodetector of Figure 2. The lower plot (b) shows the EQE of the photodetector. The upper plot (a) shows the photocurrent produced by the photodetector in response to various intensities of incoming radiation. Figure 6 shows two graphical plots relating to a photodetector such as disclosed herein (the same photodetector as used for Figure 5) and illustrates the electrical current density as a function of bias voltage, at the dark and light conditions, for two different materials (configurations).

[0039] Detailed Description

[0040] Infrared imaging is of great importance across many areas of technology, including surveillance, safety, industrial space, agricultural business, and so on. Existing InGaAs photodetectors may be approaching their limit in terms of production volumes and price. A new generation of narrow bandgap semiconductors may be formed using colloidal quantum dots (CQDs) assembled from organic solutions. This new generation of photodetectors based on CQDs may, for example, be able to fill existing market and technology gaps in which conventional epitaxially grown materials struggle to meet growing market demands.

[0041] The operational wavelengths of particular interest for SWIR vary across the intended type of device and associated application. Thus for machine vision applications, all SWIR wavelengths from 800 to 2500 nm are generally of interest. For automotive applications, wavelengths of (or in the range between) 1200 nm and 1550 nm are of particular interest. For other applications such as eyetracking, there is interest in specific wavelengths, such as a waveband centred on 940 nm; a waveband centred on 1200 nm; a waveband corresponding to or including 1300-1450 nm; and a waveband corresponding to or including 1750-2500 nm. These wavebands may represent regions of the infrared spectrum in which the earth’s atmosphere has relatively high absorption. Over extended distances, such as corresponding to the full depth of the atmosphere, there is significant attenuation of light at these wavelengths. As a result, comparatively little solar radiation in these wavebands passes through the atmosphere to ground level, and so there is little interference from background solar radiation at these wavelengths.

[0042] The quantum dots used in making SWIR photodetectors are generally nanoparticles of semiconductor materials which typically range in size from 1 nm-20nm (sometimes larger). These quantum dots may be dispersed in colloidal liquid form using various solvents which allows the quantum dots to be deposited directly onto a silicon CMOS readout integrated circuit (ROIC) via technologies such as spin-coating / printing / photolithography, thereby creating (for example) an image sensor as described in more detail below. Sometimes this method is called a monolithic integration as opposed to hybridization process or heterogeneous integration often used for manufacturing InGaAs devices. There is also interest in using quantum dots for making MWIR photodetectors.

[0043] This approach based on CQDs is convenient and affordable for accurately depositing the quantum dots onto the silicon substrate to satisfy any suitable form or dimensions, from small nanodroplets to larger device areas. Further, this approach is compatible with the mainstream manufacture and processing associated with large numbers of conventional (existing) silicon circuits - thereby avoiding more complicated and bespoke manufacturing processes. The benefits of using colloidal QD technology include high manufacturing scalability - e.g. up to 1 billion sensor units per / year; good affordability - e.g. a price typically in the range $5-500 per sensor device; and production of sensors with a large image size, for example in the range 2-10 megapixels, to support high resolution and / or a wide field of view.

[0044] Figure 1 is a schematic diagram of an example of a SWIR image sensor. In particular, Figure 1 depicts two different image sensors 120A, 120B, whereby image sensor 120B includes a pixellated array 201 B, but image sensor 120A does not have such a pixellated array.

[0045] The image sensors 120A, 120B both comprise a layered (stack) structure which can be formed using standard CMOS lithographic techniques, including colloidal deposition. Each image sensor 120A, 120B has a silicon substrate which is used to provide a readout integrated circuit (ROIC) 260 for the image sensor. In effect, the silicon ROIC provides the image data to an external device for processing and analysis. The image sensors 120A, 120B further comprise several thin layers deposited on top of the silicon ROIC 260. Progressing from the bottom up, the device (image sensor) structure includes a bottom electrode 240, a hole transport layer 230A, 230B, an optically sensitive (photoactive) layer of colloidal quantum dots 225A, 225B, an electron transport layer 220A, 220B, and a transparent top electrode 210A, 210B. Note that the top of the image sensors 120A, 120B, as provided by the top electrode 210A, 210B, may also be regarded as the front end of the image sensing device, i.e. as the portion of the image sensing device 120A, 120B which directly receives incoming light (photons), with the silicon ROIC 260 then being regarded as the back end of the device.

[0046] Accordingly, the image sensors 120A, 120B have bottom electrodes 240 formed on the top of the silicon ROIC 260, a hole transport layer 230A, 230B formed on top of the bottom electrodes 240, a layer of quantum dots 225A, 225B formed on top of the hole transport layer 230A, 230B, and an electron transport layer 220A, 220B formed on top of the quantum dot layer 225A, 225B. The image sensors 120A, 120B, are both further provided with a transparent top layer comprising a top electrode 210A, 210B which is formed directly on the corresponding electron transport layer 220A, 220B. (Note that the hole transport layer may be referred to generically as 230 while the electron transport layer may be referred to generically as 220).

[0047] It will be appreciated that the configurations shown in Figure 1 are provided as non-limiting examples, and other configurations may be used. For example, in other implementations, the hole transport layer and the electron transport layer may be swapped in position. Furthermore, in some implementations, glass or another material may be used to form the ROIC instead of silicon. In some implementations, the quantum dot layers 225A, 225B may comprise two or more (sub) layers.

[0048] In operational terms, the incoming electromagnetic radiation (light) passes through the transparent top electrode 210A, 210B and also through the electron transport layer 220A, 220B to interact with and be absorbed by the quantum dots in layer 225A, 225B to generate holes and electrons. The electrons formed (liberated) by the interactions between the incoming photons and the quantum dots are attracted towards the electron transport layer 220A, 220B; analogously, the holes formed (liberated) by the interactions between the incoming photons and the quantum dots are attracted towards the hole transport layer 230A, 230B. The holes and electrons are then transferred to the bottom electrode 240A, 240B and the top electrode 210A, 210B respectively via the hole transport layer 230A, 230B and the electron transport layer 220A, 220B. The holes and electrons then travel further from the electrode to the silicon ROIC 260 (the path from the top electrode 21 OA, 21 OB to the silicon ROIC 260 is not shown in Figure 1).

[0049] Since the quantum dots 225A, 225B provide the active light detection layer, the layers above the quantum dots 225A, 225B are generally transparent to incoming radiation so that such radiation is able to progress to and hence be detected by the quantum dots in layers 225A, 225B. Note that this transparency applies in particular to the operational wavelengths used by a light source and the light detector 120A, 120B to perform detection, e.g. imaging; the layers above the quantum dots 225A, 225B may therefore be opaque (or only partially transparent) at wavelengths outside the set of wavelengths used by the image detectors 120A, 120B to perform imaging.

[0050] As mentioned above, the imaging device 120A has a different structure from the imaging device 120B, in that the latter incorporates a pixelated array 201 B, in which all the layers stacked above the ROIC 260 have a pixelated structure. Thus the top electrode 21 OB, the electronic transport layer 220B, the quantum dot layer 225B, the hole transport layer 230B and the bottom electrode layer 240 share the same pixel structure, which therefore extends from top to bottom (front to back) through the imaging detector 120B. Typically, the spacings which extend vertically between adjacent pixels are produced using one or more etching techniques and may be filled with material that prevents cross-talk or interference between adjacent pixels, thereby helping to provide a high spatial resolution (low point spread function). Accordingly, the material between pixels is typically reflective or opaque, but generally not transparent for the light wavelengths of interest to the sensing device 120B.

[0051] In contrast, the imaging device 120A has a structure in which the bottom electrode 240 is pixelated (as for the imaging device 120B), but the higher layers in the device do not have a pixelated structure. Such a configuration for the imaging device 120A may be easier to manufacture than the pixelated array structure 201 B of imaging device 120B, but the spatial resolution of the resulting image data may be slightly lower because the lack of isolation between different pixels could potentially lead to a higher level of cross-talk. In practice, the level of cross-talk in imaging device 120A may generally be maintained at an acceptable level due to the voltage differential between the top and bottom electrodes 210A, 240 which generally causes the electrons and holes to move primarily in a vertical direction (for the orientation shown in Figure 1). In addition, the layers shown in Figure 1 are also relatively thin and electrically resistive, which further constrains the amount of crosstalk within device 120A.

[0052] The layer of quantum dots 225A, 225B has a thickness typically in the range 50-1000 nm, for example in the range 200-500 nm and serves as a photoactive material. A thinner layer of quantum dots (<200 nm) may allow certain photons to pass through the photoactive layer without being absorbed. A thicker layer of quantum dots (>500 nm) may make it harder for all interactions with incoming photons to be picked up by the electron and hole transport layers 220A, 220B, 230A, 230B. Accordingly, having a thickness in particular in the range 200-500 nm can help to maximise the quantum efficiency of image sensors 120A, 120B.

[0053] In operation of the image sensors 120A, 120B, SWIR light is incident upon the quantum dot layer 225A, 225B and generates electron-hole pairs (charges) in the quantum dot layer. As discussed above, these charges are collected in the corresponding transport layers: holes in hole transport layer 230A, 230B and electrons in the electron transport layer 220A, 220B. The greater the intensity of light received by the quantum dot layer 225A, 225B, the greater the generation of holes and electrons. The electrons and holes in effect form a current for each pixel and the silicon ROIC 260 determines the output of this current for each individual pixel such that the output corresponds to the image intensity at that pixel. The image sensor 120A, 120B outputs an image comprising an intensity value for each pixel in the set of pixels provided in the image sensor 120A, 120B. In many applications, the image sensor may be configured to provides a time series of images (such as a video).

[0054] The quantum dots in layer 225A, 225B may be formed from various materials. By way of example (and without limitation) quantum dots active in the SWIR range include PbS, PbSe, PbTe, HgTe, HgCdTe, HgSe, HgS, InAs, InAsP, InGaAs, TllnAs, InSb, InAsSb, InGaSb, InSbP, TllnSb, Cu2S, Cu2Se, Ag2S and / or Ag2Se, or core / shell quantum dots with any of the preceding constituents. Examples of core / shell QDs are PbSe / PbS, PbS / CdS, InAs / ZnS, InAs / ZnSe HgTe / CdSe and HgTe / CdS.

[0055] The choice of material for the quantum dots affects the optical properties of the quantum dots, including the wavelengths at which they transmit or absorb light. For example, InAs quantum dots are a promising alternative to those containing PbS or other heavy metals to produce lead-free quantum dots for use in an image sensor for infrared wavelengths. InAs quantum dots generally have a lower EQE than PbS quantum dots, for example around >40% at a wavelength of 940nm and >20% at a wavelength of 1400 nm. InAs quantum dots do have a relatively fast response time which makes them attractive for applications in which this fast response time is beneficial, such as high-speed video and / or light detection and ranging (LIDAR).

[0056] The energy levels, doping polarity, and charge carrier density in quantum dot semiconductors may be modified by suitably chosen capping ligands, which donate charges to the nanocrystal core and introduce localized dipoles on their surface. Alternatively, the work function and other semiconducting characteristics may be controlled by chemical doping of the quantum dot core, for example with other material such as Zn, Al, Ga, Tl etc. The capping ligands on the above quantum dots may comprise organic molecules or inorganic molecules, or a combination of both. Organic ligands may include, but are not limited to, aryl or alkyl thiols, such as 1 ,4-benzenethiol, 1 ,2- ethanedithiol, 3-mercaptopropionic acid, and so on. Organic ligands may further include N-hetero- cycles or amines, such as pyridine, 1 ,2-ethylenediamine, and so on. Inorganic ligands may include chalcogens (S, Se), pseudo halogens (SON), or atomic halogens (I, Br, Cl), chalcogens (S, Se), or more complex metal halides (Pbl2, PbBr2, I n I3, lnBr3, Snl2, SnBr2) or metal chalcogenides.

[0057] To enhance performance of image sensors 120A, 120B, specialised layers, in particular charge carrier selective layers, are introduced at the interface with the quantum dot layers. These specialised layers decrease or increase the energy barrier for a certain charge carrier type. For example, an electron transport layer (ETL), also referred to as a hole blocking layer (HBL), favour electron transport. Conversely, a hole transport layer (HTL), (also known as an electron blocking layer (EBL)), favour the transport of positively charged carriers, also known as holes. The specialised layers may also be referred to as electron / hole transport material (ETM / HTM) or an electron / hole injection layer (El L / H IL) , or electron / hole injection material (EIM / HIM). The ETL layers may comprise inorganic materials such as ZnO, TiO2, SnO2, SrTiO3, Zn2SnO4, lrO2, PbS, InAs, InSb, etc., or organic small molecules such as C60, TPBi, NPB, BCP, PCBM, PTCDA, BPhen, Alq3, etc. The HTL layers may comprise inorganic materials such as NiO, Mo03, Cui, Cu2O, CuSCN, PbS, InAs, InSb, etc., or organic small molecules such as 2TNATA, m-MTDATA, Spiro-OMeTAD, NPNPB, TPB, NPB, etc., or organic polymers such as PEDOT:PSS, P3HT, PTAA, Poly-TPD, MEH-PPV, PVK, etc. The carrier selective layer may be a conducting or semiconducting layer having a thickness in the range of 5-200 nm. In some implementations, the introduction of suitable HTL and ETL into a photodiode architecture may result in a decrease of dark leakage current from 1 mA / cm2to 1 pA / cm2; in some implementations from 10 pA / cm2to 10 nA / cm2. The introduction of suitable HTL and ETL layers may also increase the EQE figure of merit from 5% to 40%, in some implementations from 40% to 80%.

[0058] The bottom and top electrodes of a detector such as an image sensor or photodiode may comprise a material selected from the following group of metals: Au, Ni, Ag, Pd, Cu, Al, etc., or from the following group of transparent conductive oxides: In - SnO2 (ITO), In -ZnO (IZO), Al - ZnO (AZO), Al - SnO2 (ATO), F - SnO2 (FTO), and so on. The work function of electrodes may be suitably chosen to favour the charge carrier transport through the interfaces.

[0059] The SWIR image sensors 120A, 120B of Figure 1 based on colloidal quantum dots may be produced using known (widespread) CMOS technology that can be scaled up to a large number of units per year on large CMOS ROIC wafers with diameters 20 or 30 cm. Each individual wafer may contain from 100 to 50,000 sensors depending on the resolution of the sensors and fill factor. This scaling is significant in reducing the cost of SWIR quantum dot sensors per unit which in turn makes it financially viable to utilise such a quantum dot SWIR sensor in many different types of device and application (in contrast to existing SWIR sensors which may be too expensive for use across all devices and applications).

[0060] Figure 2 is a schematic diagram of a photodetector as disclosed herein. The device shown in Figure 2 is similar in many respects to the image sensor 120A, 120B of Figure 1 and comprises a photodetector 320 having a layer structure. In order from top (front) to bottom (back), the photodetector comprises a transparent top electrode 210A, a second electron transport (sub)layer (ETL) 220A2, a first electron transport (sub)layer 220A1 , a radiation absorber 225A comprising a photoactive material, a second hole transport (sub) layer (HTL) 230A2, a first hole transport (sub)layer 230A2, a bottom electrode 240, and a substrate 260 which may implement or support a silicon readout integrated circuit. Typically, the substrate may be formed from a silicon substrate, a silicon CMOS ROIC, a glass substrate, or a plastic substrate. Note that the stack shown in Figure 2 may be patterned to form an isolated photodetector device, or an array of photodiodes such as for use in an image sensor.

[0061] The implementation (such as materials etc) and operation of the different layers of the photodetector 320 in Figure 2 may be generally the same as or similar to the image sensor 120A, 120B of Figure 1 . For example, the formation and operation of the electrodes 210A, 240 shown in Figure 2 may generally follow the formation and operation of top and bottom electrodes 210A, 240 shown in Figure 1 ; likewise, the formation and operation of the first and second ETL sublayers 220A1 , 220A2 shown in Figure 2 are similar to the formation and operation of the ETL 220A layer shown in Figure 1 (but with some significant differences as discussed below); the formation and operation of the photoactive layer 225A shown in Figure 2 may generally follow the formation and operation of the photoactive layer 225A shown in Figure 1 ; the formation and operation of the first and second HTL sublayers 230A1 , 230A2 shown in Figure 2 are similar to the formation and operation of the HTL 230A layer shown in Figure 1 (but with some significant differences as discussed below); and the formation and operation of the substrate 260 shown in Figure 2 may generally match the formation and operation of the substrate 260 shown in Figure 1 . Accordingly, the present description will focus on the differences between the photodetector 320 and the image sensor 120A, 120B, rather than repeating shared information regarding the formation and operation of the photodetector 320 and the image sensor 120A, 120B.

[0062] The photodetector 320 of Figure 2 comprises a single pixel device (rather than a multi-pixel device such as the image sensors 120A, 120B as shown in Figure 1). However, the single pixel shown in Figure 2 may be replicated, for example across a silicon chip or wafer, to provide a multipixel image sensor based on the approach of Figure 2. Note that such a multi-pixel image sensor derived by extending or replicating the single pixel structure of device 320 may be formed with or without creating a pixelated array 201 B such as shown in Figure 1 for image sensor 120B.

[0063] Figure 2 also shows to the right a circuit representation of the photodetector 320 corresponding to a photodiode 360. In accordance with a standard representation, the photodiode 360 is shown in Figure 2 as a conventional diode in conjunction with arrows 370 which indicate light which is incident on the photodiode 360. As described in relation to Figures 1 and 2, the photodiode 360 includes a photoactive layer 225A which absorbs this incident electromagnetic radiation (photons) 370 to create charge carriers, namely electrons and holes. Background information about the operation of a photodiode can be found in https: / / learnabout- electronics.org / Downloads / Semiconductors_module_02.pdf

[0064] In Figure 2, the transparent top electrode 21 OA is connected to a cathode as indicated by the -ve (negative) connection; in contrast, the bottom electrode 240 is connected to an anode as indicated by the +ve connection. In operation, the photodiode 360 is typically reversed biased, such that the cathode 21 OA is more positive than the anode 240. The top and bottom electrodes 21 OA, 240 create and define a voltage (electric field) that decreases from the cathode to the anode (according to the reverse bias). This voltage moves the charge carriers created by the photoactive layer so that the holes move from the photoactive layer 225A towards the bottom electrode 240 via the HTL while the electrons move in the opposite direction from the photoactive layer 225A towards the top electrode 21 OA via the ETL. This movement of the charge carriers creates an electrical current in the direction from the cathode to the anode (due to the reverse bias) for as long as there is incident light 370 on the photoactive layer to sustain a supply of charge carriers.

[0065] Figure 3 is a schematic diagram illustrating the general operation of a photodiode, such as photodiode 360 from Figure 2. Note that Figure 3 only shows the central layers of the stack for photodiode 360 (rather than the whole stack such as shown in Figures 1 and 2). In addition, the photodiode 360 of Figure 3 is shown at a different orientation from the device shown in Figures 1 and 2, with the layers being stacked from one side to another rather than from top to bottom (it will be appreciated that this is just a change in view, it does not indicate any change in the design and configuration of the photodiode 360).

[0066] The photodiode 360 includes a p-i-n configuration comprising a p-type layer 427 doped with holes (p), an intrinsic (i) or quasi-intrinsic layer 425 formed without doping, and an n-type layer 423 doped with electrons (n). In some implementations, the intrinsic layer 425 may be formed using an undoped material (such as silicon), or else it may be formed as a depletion zone at the interface of the p-type layer 427 and the n-type layer 423. The p-type layer 427 is connected to the anode and the n- type layer 423 is connected to the cathode, but with a reverse bias applied, the cathode is positive compared with the anode. The p-type layer 427 acts as a hole transport layer (HTL), while the n-type layer 423 acts as an electron transport layer (ETL).

[0067] The p-type layer 427 contains an excess of positive charge carriers, namely holes, while the n-type layer 423 contains an excess of negative charge carriers, namely electrons; the excess holes and electrons represent charge carriers which are available to support a flow of electrical current through the photodiode 360. The intrinsic material 425 has very few charge carriers available in general and hence supports only a low level of electric current. In particular, for the intrinsic layer 425, the valence band generally does not contain free charge carriers, while the conduction band is separated from the valence band by a bandgap (energy gap), so transition from the latter to the former is difficult. However, if the intrinsic layer 425 is subject to incident radiation 370 from a light source, for example optical light or SWIR, an incoming photon from the light source can in effect split a hole-electron pair, promoting the electron into the conduction layer as a first charge carrier and also creating a free (unpaired) hole in the valence band as a second charge carrier. These first and second charge carriers are then attracted to their respective electrodes: the electron (first charge carrier) moves via the conduction band towards the positively biased cathode, while the hole (second charge carrier) moves through the valence band towards the negatively biased anode. Accordingly, the creation of the first and second charge carriers by the radiation 370 results in a current flow through the photodiode.

[0068] If a photon interacts with a hole-electron pair in the p-type layer 427, this again creates an electron and a hole as free charge carriers, but the electron then interacts with one of the existing hole charge carriers in the p-type layer 427 (which is doped to contain many such hole charge carriers) to create a new electron-hole pair. As a result, the photon impact in the p-type layer does not increase the number of available (free) charge careers, and hence the electric current is not increased. An analogous outcome applies if a photon interacts with a hole-electron pair in the n-type layer 423 - this creates an electron and a hole as free charge carriers. However, the hole then interacts with one of the existing electron charge carriers in the n-type layer 423, which is doped to contain many (an excess of) such electron charge carriers so as to create a new electron-hole pair. As a result, a photon impact in the n-type layer 423 does not increase the number of available charge carriers, and hence the electric current is again not increased.

[0069] The operation of the photodiode 360 is governed by a number of parameters. One important parameter is the level of dark current, i.e. the amount of current that flows even if there is no incident radiation 370. This dark current reduces the linearity of the relationship between (i) the level of incoming light radiation 370 and (ii) the level of current passing through the photodiode. In particular, such a dark current makes it more difficult to measure a weak signal - i.e. a signal caused by a low level of incident radiation 370. Another important parameter is the external quantum efficiency (EQE) - this is the number of charge carriers created by the photodiode 360 in the photoactive layer 225A and subsequently detected in the ROIC per external light photon 370 incident on the photoactive layer 225A. The higher the EQE of a photodiode 360, the better the sensitivity and linearity of the output electrical current due to the light incident on the photoactive layer 225A.

[0070] It is known to use an electron transport layer (ETL) and / or a hole transport layer (HTL) such as shown in Figure 1 to help improve the performance of a photodetector such as the image sensor 120A, 120B of Figure 1. For example, the ETL and / or the HTL may help to reduce the level of the dark current and / or improve the level of the EQE.

[0071] Although a conventional ETL 220A and / or HTL 230A such as shown in Figure 1 is reasonably effective for helping to improve the performance of a photodetector, as disclosed herein, it has been found that a further improvement of performance can be obtained by forming the ETL and / or the HTL with multiple sublayers (also referred to herein as a compound ETL / HTL). One example of this is shown in Figure 2, in which the ETL is formed of two contiguous sublayers, namely the first electron transport sublayer 220A1 and the second electron transport sublayer 220A2. Similarly, the HTL is formed from two contiguous sublayers, namely the first hole transport sublayer 230A1 and the second hole transport sublayer 230A2.

[0072] More generally, a photodetector 320 may be formed having an ETL comprising multiple (two or more) sublayers. The photodetector may further have an HTL formed of a single layer or a compound HTL which is also formed of multiple sublayers (or may have no HTL at all). A photodetector may also be formed having an HTL comprising multiple (two or more) sublayers. The photodetector may further have no ETL, an ETL formed of a single layer, or a compound ETL which is also formed from multiple sublayers.

[0073] For an HTL / ETL layer having multiple sublayers, the sublayers may be formed of different materials. Typically, each sublayer in a given layer is formed of a different material. For example, an HTL / ETL may comprise three sublayers, with the first, second and third sublayers comprising first, second and third materials respectively, the first, second and third materials all being distinct from one another. However other configurations are also contemplated. For example, an HTL / ETL may comprise three sublayers, with the first, second and third sublayers comprising the first, second and first materials respectively (the first and second materials being distinct from one another).

[0074] Generally, in photodetector 320, the layers and sublayers shown as adjacent to one another (such as the transparent node 210A and the second electron sublayer 220A2, likewise the photoactive material 225A and the first hole sublayer 230A2) interface directly with one another. In other words, the surfaces of such adjacent layers and sublayers physically contact each other. However, in some implementations, the photodetector may potentially have one or more intervening layers to provide additional functionality. (In this context, the sublayers are considered as part of the ETL layer / HTL layer as appropriate, rather than being considered as additional intervening layers). Providing sublayers within the HTL and / or ETL helps to solve the problem of excessive carrier recombination and re-injection of charges from electrodes - this presents as large reverse-bias dark current values which are often seen in devices built from quasi-amorphous imperfect semiconductors obtained using solution processing. Consequently, providing sublayers within the HTL and / or ETL helps to integrate colloidal quantum dots into a high-performance infrared photodetector device. Such a device may have a performance which is at least competitive with existing PbS CQD photodetectors and state-of-the-art epitaxially grown InGaAs, InSb, HgCdTe photodiodes (which are more costly to produce).

[0075] In contrast, the performance of existing CQD-based SWIR and mid-wave infrared (MWIR) photodetectors (such as photodiodes) is typically low compared with traditional detectors currently available in the market, such as InGaAs, HgCdTe and type-ll superlattices. Current CQD devices tend to suffer from limited performance associated with their disordered semiconductor nature, including poor electrical transport at interfaces, and high density of defects. As a result, an accurate charge balance in the stack (such as illustrated in Figures 1 and 2) and a rationalised design of specialised layers within the stack represent significant factors for controlling photodetector characteristics and developing enhanced photodetectors.

[0076] In some implementations described herein, colloidal quantum dots are used as an i-type layer in a p-i-n configuration. Existing photodiodes based on CQDs may be susceptible to excessive recombination rates and, as a result, to high dark current levels. Such devices are therefore inherently limited to small, disordered energy gaps with highly probable defects and trap states. Poor interfaces with p-type and n-type layers and the overall charge balance in such devices are often not ideal due to lack of material maturity and uncertainties associated with the energy landscape; such issues contribute further to high dark current, especially reverse saturation current, large noise levels, and low EQE. As a result, only a relatively small fraction of photoexcited charge may be extracted, thus significantly limiting performance and quantum efficiency.

[0077] A p-i-n photodiode architecture as disclosed herein may be based on semiconductor nanocrystals (also referred to as colloidal quantum dots). This architecture offers improved characteristics such as higher quantum efficiency up to 80-90%, lower dark current below 100 nA / cm2, and better noise performance down to < 1014A / Hz05. This architecture includes an ETL and / or an HTL having multiple sublayers. A photodiode stack structure having such an architecture is suitable for incorporating colloidal quantum dot materials to provide (for example) broadband light sensitivity from 400nm to 2500nm. The CQD photodiode 360 provides IR detectors at lower cost (compared to other IR detectors of similar sensitivity, such as epitaxially grown devices). Accordingly, such photodiodes 360 enable more affordable manufacture using monolithic integration on CMOS platforms and support room temperature operation. Note that although Figure 2 shows a photodetector comprising a photodiode, the same approach based on multiple sublayers for the ETL / HTL may also be implemented on other types of photodetector, such as an image sensor formed from an array of photodetector elements (pixels).

[0078] Figure 4 is a schematic diagram providing further information about a photodetector such as shown in Figure 2. In particular, Figure 4 has an upper section labelled (a) and a lower section labelled (b). Each section shows a photodetector (e.g. photodiode) having an ordered stack comprising an anode 240, an HTL 230A, a photoactive layer 225A, an ETL 220A and a cathode 210A. The HTL comprises two sublayers 230A1 and 230A2 and the ETL comprises two sublayers 220A1 and 220A2. The ordering of the sublayers varies from section (a) to section (b). Thus in section (a), the HTL sublayer 230A2 is between the HTL sublayer 230A1 and the photoactive layer 225A, whereas in section (b) the HTL sublayer 230A1 is between the sublayer HTL 230A2 and the photoactive layer 225A. There is also an analogous change in ordering of the ETL sublayers between section (a) and section (b).

[0079] The sublayers within the HTL are shown with a different thickness - sublayer 230A1 is thicker than sublayer 230A2. This differentiation indicates that sublayers 230A1 and 230A2 represent distinct materials with different physical properties (such as supporting a different level of dark current). An analogous differentiation also applies to the ETL sublayers 220A1 , 220A2.

[0080] The layers in Figure 4 also show an energy level (relative to vacuum) within each layer according to a horizontal black line. This schematic diagram shows the relative energy potentials of the layers before they are brought into contact. A certain degree of energy band bending is expected upon contact (this is not depicted in Figure 4). In operation, the cathode 210A has a positive voltage (from the reverse bias) and so has the highest (most positive) voltage and therefore receives the free electrons created within the CQD (photoactive) layer 225A. In a similar fashion, the anode 320 has the lowest (most negative) volage and therefore receives the free holes created within the CQD layer 225A.

[0081] The charge balance and recombination rates at both p-type and n-type contacts can be controlled effectively by adopting a multilayer HTL 230 and ETL 220 structure such as illustrated in Figures 2 and 4. Such a configuration may help to reduce or minimise dark leakage current of narrow-bandgap quantum dot layers by suppressing interfacial recombination between the photoactive layer 225A and a transport layer 220 / 230, thereby helping to increase or maximise the built-in voltage across the photodiode 320. Such a configuration also helps to increase the effective light capture cross-section within the photodiode, thereby helping to increase the quantum efficiency (EQE) of the photodiode 320, which in turn may lead to greater sensitivity and an overall improvement in performance. The charge carrier density and energy levels of HTL and ETL can also be controlled, thereby adding additional flexibility in design and implementation to enhance photodiode performance. p-i-n photodiodes such as disclosed herein, e.g. where the p-type layer 427 is a compound HTL and / or where the n-type layer 423 is a compound ETL, may be configured to provide depletion- current-limited operation. This offers the possibility to detect diffraction- and background-limited weak light signals at temperatures above 0 degrees C for most spectral bands in the infrared (IR) region. Such advantages arise from the elimination (or significant reduction) of parasitic diffusion currents and / or the efficient exploitation of long Shockley-Read (SR) lifetimes. Such p-i-n photodetectors may be fabricated using colloidal quantum dots (CQDs) as described herein and demonstrate competitive performance levels across a wide wavelength range encompassing UV, visible light, SWIR, midwavelength (MW)IR, and long wavelength (LW)IR. In a typical implementation of a photodetector described herein (for example, a photodiode or image sensor), the device includes a hole transport layer (HTL) 230, also referred to as an electronblocking layer (EBL), which serves as a charge carrier (hole / electron) selective medium at the p-side of the junction with the photoactive layer 225A. The HTL is connected to the anode side 240 of the device (e.g. photodiode). This HTL layer is a p-type semiconductor hybrid structure having at least two sublayers 230A1 , 230A1 which are adjacent to (interface with) one another. The two sublayers comprise two distinct (different) respective materials. For example, one of the materials may have an energy bandgap larger than the energy bandgap of the second material, and / or the charge carrier density of one sublayer may be greater than the charge carrier density of the second sublayer. The energy levels of band edges relative to vacuum may also be different for sub-layers of the HTL.

[0082] In a similar fashion, the device may include an electron transport layer (ETL) 220, also referred to as a hole-blocking layer (HBL), which serves as a charge carrier (hole / electron) selective medium at the n-side of the junction with the photoactive layer 225A. The ETL is connected to the cathode side 210A of the device. This layer is an n-type semiconductor hybrid structure having at least two sublayers 220A1 , 220A2 which are adjacent to (interface with) one another. The two sublayers of the ETL comprise two distinct (different) materials. For example, one of the materials may have an energy bandgap larger than the energy bandgap of the other material, such as by more than 0.5 eV, and / or the charge carrier density of one sublayer may be greater than the charge carrier density of a second sublayer.

[0083] The photosensitive layer 225A in the photodetector (e.g. photodevice 360) comprises a material that absorbs electromagnetic radiation. The photoactive layer 225A is typically a quasi- intrinsic (i-type) semiconductor layer but may be formed differently. This layer comprises at least one type of semiconductor nanocrystal (e.g. colloidal quantum dot). It may also comprise two adjacent layers with n-type and p-type materials having different energy bandgaps, or a blend of two or more materials forming a heterojunction.

[0084] As discussed above, for a given photodetector, the HTL 230 may be formed from multiple sublayers and / or the ETL may be formed from multiple sublayers according to the circumstances of any particular implementation. The photodetector may be formed as or incorporated into a device which includes electrode layers, namely anode 240 and cathode 210A, which provide an interface to a readout CMOS integrated circuit. The photodetector may comprise, for example, a continuous optically transparent conducting layer functioning as a global shutter or grounding contact, or a dense array of electrically isolated electrodes such as for use in a high-resolution spatial imaging device (camera). In some cases, the photodetector device 320 may comprise an array of photodetectors grouped in proximity with each other. The photodetectors may share p-i-n layers and at least one common electrode. It is also possible for the junction to be inverted such that the cathode can be regarded as the first electrode and anode can be regarded as the second electrode; in this latter case, the configuration of the photodetector becomes n-i-p type.

[0085] Conventional architectures for CQD photodiodes generally rely on a single layer HTL and a single layer ETL. However, a detector such as disclosed herein may be formed, for example, as a photodiode 360 based on a p-i-n architecture. The photodiode may integrate multiple sublayers into the carrier selective HTL and / or ETL semiconducting layers. The bandgap of a first sublayer may, for example, exceed the bandgap of a second sublayer by more than 0.5 eV. Providing two adjacent sublayers in this manner forms a complex (compound) HTL / ETL with an enhanced capability for blocking charge carriers (electrons / holes) having an opposite polarity charge. For example, an HTL layer 230 having multiple sublayers 230A1 , 230A2 may block the path of electrons, and an ETL layer 220 having multiple sublayers 220A1 , 220A2 may block the path of holes.

[0086] A typical p-i-n photodiode includes the following features, (i) operation as reverse bias or zero-bias photodetector, (ii) the energy equilibrium is upset upon illumination (a photoactive response), (iii) electron-hole pairs and excitons (bound electron-hole pairs) are generated, (iv) the electrons are attracted to the n-side and the holes are attracted to the p-side, so photo-generated current flows from the n-side to the p-side, (v) photocurrent is further facilitated by an external electric field, (vi) the photon-induced current or open-circuit photovoltage is measured as an electrical signal.

[0087] The use of such a p-i-n junction architecture for the photoactive layer 225A, such as discussed above, is generally advantageous when compared to other implementations due to reduced dark current and larger absorption cross-section area; this leads in turn to improved photodiode characteristics. A p-i-n junction architecture may be particularly suited for devices made of colloidal quantum dots and other devices using imperfect nanocrystal (quantum dot) based semiconductors.

[0088] The typical components of the p-i-n junction photodiode comprise an HTL 230, ETL 220, photoactive layer 225A, and positive and negative electrodes (anode 240 and cathode 210A respectively). The HTL 230 is formed from p-type semiconductor 427. The functional role of the HTL 230 is to provide hole transport in a carrier-selective fashion (the HTL may also be termed an electron blocking medium). This HTL layer 230 comprises a wide-bandgap semiconductor which predominantly transmits holes and blocks electrons. The energy bandgap for the HTL between the valence band edge and the conduction band edge can typically vary between 1 eV and 4 eV. By way of example, a conduction band energy level for the HTL, based on the lowest unoccupied molecular orbit (LUMO), may lie in the range 2.0-4.0 eV; the valence band energy level for the HTL, based on the highest occupied molecular orbit (HOMO), may lie in the range 4.0-6.0 eV; the charge carrier density for the HTL may lie in the range 1016— 1020cm3; and the thickness for the HTL may lie in the range 5-200 nm. These features and values generally apply to both a single layer HTL or a compound (multilayer) HTL.

[0089] The ETL 220 is formed from n-type semiconductor 423. The functional role of the ETL 220 is to provide electron transport in a carrier-selective fashion (the ETL may also be termed a hole blocking medium). The ETL layer 220 is a wide-bandgap semiconductor which predominantly transmits electrons and blocks holes. The energy bandgap for the ETL between the valence band and the conduction band can typically vary between 1 eV and 4 eV. By way of example, a conduction band energy level for the ETL, based on LUMO, may lie in the range 3.0-5.0 eV; the valence band energy level for the ETL, based on the HOMO, may lie in the range 5.0-8.0 eV; the charge carrier density for the ETL may lie in the range 1016— 1020cm3; and the thickness for the ETL may lie in the range 5-200 nm. Again, these features and values generally apply to both a single layer HTL or a compound (multilayer) HTL.

[0090] The photoactive layer 225A may be implemented as an intrinsic (undoped) or quasi-intrinsic (neutralised charges) semiconductor to provide absorption of light 370. An intrinsic semiconductor does not contain any impurity, or the concentrations of both types of carriers are ideally equal to one another. In particular, the layer 225A is responsible for the absorption of light, which results in the generation and separation of photoexcited carriers upon illumination by light. This photoactive layer 225A is generally (nearly / quasi) intrinsic with eliminated minority charge carriers. The photoactive layer may also (alternatively) be implemented as a combination of p-type 427 and n-type 423 layers counterbalancing the charge state of each other. The carrier density in the intrinsic portion of layer 225A may lie in the range 1014— 1018cm3; the energy bandgap between the valence band and the conduction band for the intrinsic layer may lie in the range 0.01-2.5 eV; the light absorption wavelength for the layer 225A may lie in the range 400-2500nm (optical plus SWIR), or in the range 400-5000nm (optical plus SWIR plus MWIR); and the thickness for the layer 225A may lie in the range 50-5000 nm. Note that the layer 225A is typically thicker than the HTL and the ETL, but with a lower charge density and potentially a smaller energy bandgap between the valence band and the conduction band.

[0091] The electrodes (anode 240 and cathode 210A) provide electrical contact to the stack. In a typical implementation, at least one of the electrodes may be built into the CMOS circuitry which interfaces between the signal readout electronics (not shown in Figure 2) and photosensitive device 225A. Such an electrode may be metallic, providing high electrical conductivity and a suitable work function (for example, in the range 4.5-6.5 eV for the anode 240 and in the range 3.5-5.5 eV for the cathode 250). In addition, one electrode (the anode 240 in the example of Figure 2) has high optical reflectivity and the other electrode (the cathode in the example of Figure 2) has high optical transmittance within the wavelength range of interest for the photodetector 320. The transparency of one electrode allows incoming light to pass through the electrode en route to interacting with the photoactive layer 225A which then leads to detection of the incoming light. The reflectance of the other electrode may help to return incoming light back towards the photoactive layer 225A and so can help to enhance EQE. The electrodes are typically formed with a thickness in the range 5-500nm. Each electrode may be configured as a single electrical element forming part of the photodiode 360. Alternatively, at least one electrode may be divided into an array of multiple parallel contacts to form an image (pixel) sensor, as illustrated for the bottom electrode (anode 240) as shown in Figure 1 .

[0092] As illustrated in Figure 4, the device is formed as a multi-layer device including multiple sublayers in the HTL and / or ETL layers. The multiple sublayers may comprise alternating p and n layers. Within the compound HTL / ETL, the bandgap of one sublayer is typically larger than the band gap of a neighbouring (immediately adjacent) sublayer. This approach may improve the carrier booking ability by introducing additional energy barriers for the charges of opposite polarity.

[0093] In some implementations, the sublayer formed from a larger bandgap material may be in direct contact with the i-type absorber layer 225A, or may be positioned next to the adjacent electrode instead (the anode 240 for the HTL 230, or the cathode 210A for the ETL 220). The charge carrier density may differ between a first sublayer (e.g. 230A2) having a larger bandgap and a second sublayer (e.g. 230A1) adjacent to the first sublayer and having a smaller bandgap. Alternatively, the first and second sublayers within the HTL (or within the ETL) may have the same or a similar bandgap.

[0094] The valence band energy level (HOMO) of a first HTL sublayer 230A2 immediately adjacent to intrinsic material 425 of the photoactive layer 225A may be aligned as closely as possible (within 0.01 - 0.5 eV) to the valence band level of the i-type absorber material 425. This helps to minimise the energy barrier for carriers at both ends of the junction, hence providing more efficient charge transport of photo excited carriers. In some implementations, one or more further sublayers may also have a valence band energy level aligned with the valence band level of the i-type absorber material 425 (and hence also aligned with the valence band level of the first HTL sublayer 230A2). The electron workfunction of a first electrode (anode 240) may be aligned as closely as possible (within 0.01 - 0.5 eV) to the valence band energy level (HOMO) of at least a second sublayer 230A1 of the HTL 230 which is immediately adjacent to the first electrode. The first and second HTL sublayers 230A2, 230A1 may be provided with a wide bandgap or a narrow bandgap (likewise for any further sublayers within the HTL 230).

[0095] The conduction band energy level (LUMO) of a first ETL sublayer 220A1 immediately adjacent to intrinsic material 425 of the photoactive layer 225A may be aligned as closely as possible (within 0.01 - 0.5 eV) to the conduction band level of the i-type absorber material 425. The electron workfunction of the second electrode (cathode 210A) is aligned as closely as possible (within 0.01 - 0.5 eV) to the conduction band energy level (LUMO) of at least one sublayer 220A2 of the ETL 220 which is immediately adjacent to the second electrode. The first and second ETL sublayers 220A1 , 220A2 may be provided with a wide bandgap or a narrow bandgap (likewise for any further sublayers within the ETL 220).

[0096] The specialised layers within the photodetector stack 320, in particular the sublayers within the ETL / HTL, are carrier selective and may be used to decrease or increase the energy barrier for a certain charge carrier type. For example, an electron transport layer 220 favours electron transport while a hole transport layer 230 favours the transport of positively charged carriers (holes). These specialised layers may also be called electron / hole transport material (ETM / HTM), an electron / hole injection layer (El L / H IL) , electron / hole injection material (EIM / HIM) or an electron / hole blocking layer (EBL / HBL).

[0097] The carrier selective (sublayers of the HT / ETL may be formed as conducting or semiconducting layers having a thickness in the range of 5-200 nm. In some implementations, the introduction of suitable HTL / ETL (sub)layers into a photodiode architecture such as shown in Figure 2 may result in a decrease of dark leakage current, for example, from 1 mA / cm2 to 1 pA / cm2, and in some cases from 1 pA / cm2 to 1 nA / cm2. The introduction of suitable HTL / ETL (sub) layers may increase the EQE figure of merit from 5% to 50%, in some cases from 50% to 90%.

[0098] More generally, there are various strategies available for improving the performance of a compound HTL / ETL by suitable manipulation of various properties (composition, etc) of the sublayers. For example, the sublayers may be configured to reduce the reverse saturation current (the main component of dark current), to reduce parasitic currents, and / or to minimise energy barriers for the corresponding charge carrier types so as to favour higher EQE (charge carrier extraction efficiency).

[0099] The photodetector 320 / photodiode 360 may comprise additional layers of optical enhancers and / or light focussing lenses which may be added to the stack (such as shown in Figures 1 and 2). Additionally (or alternatively), encapsulation layers may be included in the device structure to protect the device from ingress of environmental molecules.

[0100] Fabrication methods for a photodetector 320 (such as photodiode 360) may include solution processes such as spin-coating, printing, slot-die coating, blade coating, dip coating, drop casting, and so on. Fabrication methods may also include any vacuum deposition techniques such as CVD, ALD, PVD, thermal evaporation, magnetron RF or DC sputtering, and so on. The photodetector device may be patterned on wafers using any available lithography technique.

[0101] The following table provides information about the materials for different layers within the photodetector. These materials are identified by way of example rather than by way of limitation. Please note that the materials and / or fabrication methods discussed above in relation to Figure 1 may also be applied where appropriate to a photodetector 320 such as described herein.

[0102] Table 1

[0103] Various properties of a photodetector 320 such as disclosed herein (for example with reference to the photodetector of Figure 2) are illustrated in Figures 5 and 6, each of which comprises two graphical plots (graphs), namely plot (a) (top)and plot (b) (bottom). Note that all four graphs (5a, 5b, 6a and 6b) relate to the same photodetector 320 incorporating the same photoactive (photo- absorptive) material. In particular, the results from Figures 5 and 6 were obtained from a SWIR sensitive photodiode based on narrow- bandgap colloidal quantum dots integrated in the p-i-n configuration using PbS quantum dot photodiodes. This photodiode features hybrid bandgap carrier- selective sublayers as disclosed herein. Other potential materials with suitable features include InAs and HgTe. The data for these plots was acquired at 293K.

[0104] With reference to Figure 5, the lower plot (b) shows the EQE of a photodetector for different wavelengths. It can be seen that the photodetector has a peak of good photo-absorption (and hence good EQE) for IR radiation having a wavelength around 1450 nm. The EQE also rises for shorter wavelengths (towards the optical region). The upper plot (a) of Figure 5 shows the variation in photocurrent (Amps) output from the photodetector as a function of the irradiance at 1450 nm (Watts per square metre) by light which is absorbed to produce the photocurrent. It can be seen that this relationship is linear over a relatively large range (>104), thereby providing a reliable indication of irradiance from the measured photocurrent.

[0105] A photoelectric device 320 such as disclosed herein therefore exhibits a strong rectifying behaviour of l-V characteristics, with clearly distinguished regions for (i) forward bias region and (ii) reverse bias, wherein the current for reverse bias increases upon illumination. The response speed of such a photodetector is primarily limited by a carrier drift time and junction capacitance / resistance, which depends on the layer thicknesses, electrical mobility, charge carrier density, permittivity, and internal / external electric field strength. The minimisation of a dark leakage current is important to achieve low noise and hence a high signal-to-noise ratio. The EQE characteristics depend on the quality of semiconductor layers, their light absorption behaviour, carrier transport properties of specialised (sub)layers as described herein, and series resistance of contact electrodes.

[0106] With reference to Figure 6, this shows the photocurrent (amps) as a function of bias voltage. The upper graph (a) relates to a device with a conventional HTL / ETL configuration, while the lower graph (b) relates to a photodiode with a compound HTL / ETL configuration which is optimised to display lower dark currents and to have better noise performance. Each graph plots two lines, one line corresponding to dark conditions (no external illumination), while the second (grey) line is measured under illumination at 1450 nm.

[0107] The right-hand portion of each plot of Figure 6 corresponds to a bias voltage >0, in other words positive bias. For such a positive bias, the current flow is primarily driven by the operation of a standard diode (with little or no sensitivity to the level of incident light). In contrast, for the left-hand portion of the plots of Figure 6, the photodetector 320 is negatively biased. It can be seen that in this negative bias region, the current flow through the photodetector increases with the level of incident light 370 (in conformity with the expected operation of a photodiode).

[0108] The upper graph (a) was obtained using a single HTL / ETL photodiode. The HTL is formed from PbS nanocrystals with thiol ligand and the ETL is formed from metal oxide nanocrystals. The lower graph (b) was obtained using a modified HTL / ETL according to the approach described herein. The HTL contains two sublayers with the materials taken from Table 1 above and the ETL contains two sublayers with the materials taken from Table 1 above. At room temperature, the device (a) (top) has a dark current of around 1 ,000 nA / cm2at -0.5V bias and the device (b) (bottom) has the a current of around 100 nA / cm2at -0.5V bias. Such an improvement (reduction) in the dark current is attributed to the presence of the compound HTL and ETL layers. The device (b) can be utilized at room temperature conditions.

[0109] ***

[0110] In conclusion, while various implementations and examples have been described herein, they are provided by way of illustration, and many potential modifications will be apparent to the skilled person having regard to the specifics of any given implementation. Accordingly, the scope of the present case should be determined from the appended claims and their equivalents. Furthermore, unless the context clearly indicates to the contrary, it is specifically disclosed herein that the features of any independent claim and / or its associated dependent claims may be combined with the features of any other independent claim and / or its associated dependent claims (irrespective of whether such a combination is explicitly claimed, since the claims are used to determine the scope of protection, not the overall disclosure of the application).

Claims

Claims1 . A photodetector having a layered structure comprising: a first electrode; a second electrode; a photoactive layer configured to absorb infrared light located between the first electrode and the second electrode; and a transport layer located between the first electrode and the photoactive layer and between the second electrode and the photoactive layer, wherein the transport layer comprises at least a first sublayer formed of a first material and a second sublayer formed from a second material, wherein the second material is distinct from the first material.

2. The photodetector of claim 1 , wherein the transport layer is an electron transport layer or a hole transport layer.

3. The photodetector of claim 2, wherein said transport layer is a first transport layer, and the photodetector further comprises a second transport layer, wherein if the first transport layer is an electron transport layer, the second transport layer is a hole transport layer, and if the first transport layer is a hole transport layer, the second transport layer is an electron transport layer.

4. The photodetector of claim 3, wherein the second transport layer comprises at least a first sublayer formed of a third material and a second sublayer formed from a fourth material, wherein the fourth material is distinct from the third material.

5. The photodetector of claim 4, wherein the third and fourth materials are different from the first and second materials.

6. The photodetector of any preceding claim, wherein the photodetector comprises a p-i-n junction.

7. The photodetector of claim 6, wherein the p-i-n junction includes an / region comprising colloidal quantum dots.

8. The photodetector of any of claims 1 to 6, wherein the photoactive layer comprises quantum dots.

9. The photodetector of claim 8, wherein the quantum dots are deposited onto the photodetector in colloidal liquid form.

10. The photodetector of claim 8 or 9, wherein the quantum dots are core / shell quantum dots, optionally comprising PbSe / PbS, PbS / CdS, InAs / ZnS, InAs / ZnSe, HgTe / CdSe and HgTe / CdS.11 . The photodetector of any preceding claim, wherein the photoactive layer is sensitive to infrared radiation across some or all of the visible (400 - 900 nm), the SWIR (900 - 2500 nm) and / or the MWIR (2500 - 5000 nm) wavelength range.

12. The photodetector of any preceding claim, wherein the transport layer comprises a hole transport layer having one or more of any of the following properties:(i) an energy bandgap between the valence band and the conduction band between 1 eV and 4 eV;(ii) a conduction band energy level, based on the lowest unoccupied molecular orbit (LUMO), in the range 2.0-4.0 eV;(iii) a valence band energy level, based on the highest occupied molecular orbit (HOMO), in the range 4.0-6.0 eV;(iv) a charge carrier density in the range 1016— 1020cm3; and(v) a thickness in the range 5-200 nm.

13. The photodetector of any preceding claim, wherein the transport layer comprises an electron transport layer having one or more of any of the following properties:(i) an energy bandgap between the valence band and the conduction band between 1 eV and 4 eV;(ii) a conduction band energy level, based on LUMO, in the range 3.0-5.0 eV;(iii) a valence band energy level, based on HOMO, in the range 5.0-8.0 eV;(iv) a charge carrier density in the range 1016— 1020cm3; and(v) a thickness in the range 5-200 nm.

14. The photodetector of any preceding claim, wherein the bandgap of the first sublayer exceeds the bandgap of the second sublayer by more than 0.5 eV.

15. The photodetector of any preceding claim, wherein the first sublayer and the second sublayer differ in respect of bandgap and / or charge carrier density.

16. The photodetector of any preceding claim, wherein the transport layer is an HTL layer having the first sublayer immediately adjacent to the photoactive layer, wherein the valence band energy level (HOMO) of the first sublayer is aligned with the valence band energy level of an i-type absorber material within the photoactive layer.

17. The photodetector of any preceding claim, wherein the transport layer is an HTL layer having the second sublayer immediately adjacent to the first electrode, wherein the electron workfunction of the first electrode is aligned with the valence band energy level (HOMO) of the second sublayer.

18. The photodetector of any preceding claim, wherein the transport layer is an ETL layer having a first sublayer immediately adjacent to the photoactive layer, wherein the conduction band energy level (LUMO) of the first sublayer is aligned with the conduction band level of an i-type absorber material within the photoactive layer.

19. The photodetector of any preceding claim, wherein the transport layer is an ETL layer having a second sublayer immediately adjacent to the second electrode, wherein the electron workfunction of the second electrode is aligned with the conduction band energy level (LUMO) of the second sublayer.

20. The photodetector of any preceding claim, wherein the first and second sublayers have a thickness in the range of 5-200 nm.21 . The photodetector of any preceding claim, wherein the photodetector is positioned atop a substrate comprising a silicon substrate, a silicon CMOS ROIC, a glass substrate, or a plastic substrate.

22. The photodetector of any preceding claim, wherein the photodetector is configured to operate at room temperature or in a thermoelectrical cooled (TEC) environment or in a cryogenic cooled environment.

23. The photodetector of any preceding claim, wherein the photodetector is located on a silicon CMOS ROIC substrate and is patterned into an array of pixels to provide an image sensor.

24. A device comprising photodetectors for machine vision, spectroscopy, mobile phones, wearable devices, wherein said device incorporates: the photodetector of any claims 1-23; and packaging.

25. A device comprising machine vision camera, camera for robotics, camera for UAV, nightvision camera, mixed reality goggles, a smartphone, an automotive camera, an eye tracking camera, or a set of contact lenses, wherein said device incorporates: the photodetector of any claims 1-23; a detector board; a processor board, a lens; and housing.

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