GB2582982A

GB2582982A - Radiation detector

Info

Publication number: GB2582982A
Application number: GB1905234.9A
Authority: GB
Inventors: John Bevan Adrian; Kreouzis Theo; Gillin William; Timis Cozmin; Liu Xiaoqi
Original assignee: Queen Mary University of London
Current assignee: Queen Mary University of London
Priority date: 2019-04-12
Filing date: 2019-04-12
Publication date: 2020-10-14
Also published as: GB201905234D0

Abstract

A radiation detector comprising a semiconductor device formed of one or more organic semiconductor materials having dispersed neutron sensitiser element therein, such as Boron, Lithium, Gadolinium or Cadmium. The detector device may be a diode or transistor (e.g. Field Effect Transistor). The organic semiconductor materials may comprise a donor organic semiconductor material and an acceptor organic semiconductor material or a combination of organic and inorganic donor-acceptor materials. Dispersion of particles such as microparticles or Nanoparticles, or thin layers is described.

Description

RADIATION DETECTOR

Field of the Invention

[0001] The present invention relates to radiation detectors, in particular to neutron detectors using organic semiconductor materials and to methods of manufacture thereof.

Background

[0002] The most basic form of an organic radiation detector is a Shockley diode, in which the semiconductor is sandwiched between anodes and cathodes of different materials (with different workfunctions). Reverse biasing such a device will allow charge to flow when incident radiation deposits energy in it. There are many ways to improve this structure, including making layers of intrinsic and/or doped semiconductor sandwiched between anode and cathode (e.g. pn and pin diodes, etc.) and blending different types of organic semiconductor in order to effectively create a donor/acceptor organic matrix to improve sensitivity.

[0003] The electrodes of such a radiation detector can be arranged as pads, strips or pixels to make single channel or multichannel (position sensitive) devices. These structures can be made as thick or thin 2D devices, or stacked to create 3D devices that may be read out as single or multi-channel sensors. Such devices can be operated in low voltage mode (a few tens of volts reverse bias), or with high voltage (avalanche mode).

[0004] To detect neutrons, thin layers of boron are applied next to the sensitive region of semiconductor. It has also been proposed to construct columns or pillars of boron, for example see [Ref I]. Boron neutron capture releases alpha particles, with a lithium recoil that is the source of secondary radiation that makes these devices efficient. There is a limitation on efficiency provided by the limited range of an alpha particle in material. For silicon this is a few microns, and a similar range is found for organic material. An efficiency of 35% is considered state of the art for solid state detectors of this type, where towers of boron are embedded into the semiconductor matrix in a complicated fabrication process.

Summary

[0005] There is therefore a need for improved neutron detectors, in particular detectors with increased efficiency and which can be manufactured at lower cost. -2 -

[0006] According to the present invention, there is provided a radiation detector comprising a semiconductor device formed of one or more organic semiconductor materials having a neutron sensitiser element dispersed therein.

[0007] The sensitiser element may be one or more elements selected from the group consisting of boron, cadmium, lithium and gadolinium.

[0008] The sensitiser element may be provided in an amount of between 1 x 1018 atoms/m5 and 2.5 x 1021 atoms/m3 within the organic semiconductor material.

[mos] The sensitiser element may be provided in the form of particles dispersed in the organic semiconductor material.

coon] The sensitiser element may be contained in an organic compound.

[0011] The sensitiser element may be provided in the form of a plurality of layers embedded in the organic semiconductor material.

[0012] The organic semiconductor material desirably has a charge carrier mobility of at least 10-6 C1112V-15-1.

[0013] The organic semiconductor materials may comprise a donor organic semiconductor material and an acceptor organic semiconductor material or a combination of organic and inorganic donor-acccptor materials.

[0014] The donor organic semiconductor component may be electron-rich (has a smaller electron affinity) compared to the acceptor component.

[0015] The acceptor component may be electron-deficient (has a larger electron affinity) compared to the donor, for example fullerene, fullerene derivative, pi-conjugated polymer, small molecule or perovskite.

[0016] The semiconductor device may have a thickness in the range of from 1µm to 500 [0017] The semiconductor device may comprise a diode, a transistor (e.g. a Field Effect Transistor) or a non-rectifying, symmetric, semiconductor device.

[0018] The radiation detector may further comprise a bias voltage source configured to apply a potential difference in the range 1 V to 1 kV.

[0019] The radiation detector may contain sensitiser components additional to the neutron sensitiser, for example X-ray absorbers dispersed in the organic semiconductor material, the X-ray absorber comprising an element having an atomic number greater than 20.

[0020] According to the invention, there is also provided an object detector comprising a radiation detector as described above and a neutron source. -3 -

[0021] Thus, the present invention can provide a neutron detector which has a sufficient efficiency and can be manufactured in a variety of different forms at low cost.

Brief Description of the Figures

[0022] Exemplary embodiments of the invention are described below with reference to the accompanying figures, in which: [0023] Figure I depicts in cross-section a diode forming part of a radiation detector according to an embodiment of the invention; [0024] Figure 2 depicts energy levels in the diode of the embodiment of Figure 1, [0025] Figure 3 depicts the relative detection efficiency versus detector area for a range of organic semiconductor device efficiencies.

[0026] Figures 4(a) and 4(b) present transient a particle signals obtained using a P3HT based diode in avalanche operation; [0027] Figure 5 depicts in cross-section a FET forming part of a radiation detector according to an embodiment of the invention; and [0028] Figure 6 depicts an object detector according to an embodiment of the invention. [0029] In the various figures, like parts are denoted by like references.

Detailed Description of Embodiments

[0030] Figure 1 depicts a diode 1 which may form the radiation sensitive part of a neutron detector according to an embodiment of the invention. Diode 1 comprises a substrate 10 on which is formed a first electrode (e.g. anode) 11, an organic semiconductor layer 12 and a second electrode (e.g. cathode) 13. The organic semiconductor layer 12 comprises two components: an acceptor 12a and a donor 12b. Either or both of the acceptor 12a and donor 12b have a sensitiser element dispersed therein. The sensitiser element can be provided in a sensitiser compound, e.g. boron nitride (BN) (given the natural abundance of the 10B isotope, no enrichment is necessary, but is possible if desired). Other suitable sensitiser elements include cadmium, lithium and gadolinium, which can be provided in the form of suitable sensitiser compounds. Suitable techniques for formation of the acceptor 12a and donor 12b are described below. The first electrode can be the cathode and second the anode. In the following description, the term "sensitiser" is used to refer to either a sensitiser element or a sensitiser compound.

[0031] In use, the diode can be biased by applying a potential difference between the first and second electrodes 11, 13. The diode can be either forward or reverse biased. The diode -4 -can also be intrinsically biased by the use of electrodes of different materials. It will be appreciated that the semiconductor need not be a diode but can instead be a FET or non-rectifying, symmetric, semiconductor device.

[0032] When energy is deposited in the semiconductor layer, e.g. by incident radiation, electron-hole pairs are produced across the semiconductor bandgap. Negative charge carriers (electrons) 14a collect in the acceptor 12a and positive charge carriers (holes) 14b collect in the donor 12b. Current can therefore flow until the charge carriers recombine with each other or in the electrodes. Thus, incident radiation can be detected as an increased current through the diode.

[0033] Detection of neutrons is achieved through the presence of the nuclei of the sensitiser element (e.g. boron) dispersed within the bulk of the organic semiconductor layer. In the event that a neutron is captured by a boron nucleus, an alpha particle is emitted and a lithium ion recoils: 2,5H [0034] The ionising radiation (e.g. alpha particle, recoil daughter nucleus) emitted post neutron capture by the sensitiser element is used to increase the mobile charge carrier density within the semiconducting components of a device and cause an increase in the device current. The ionising radiation energy is lost to the semiconducting components, exciting electron-hole pairs across the bandgap, the dissociation of which can be aided by the use of donor-acceptor (D-A) interfaces (see Figure 2).

[0035] The increase in device current can be detected in the steady state (e.g. by the use of a suitable shutter mechanism and/or phase locked amplification) or detected in the transient response of a device (using charge sensitive and/or voltage pre-amplification). In all cases the device drive conditions are chosen to maximise the signal to noise ratio. Embodiments can employ avalanche mode detection or low voltage detection. It will be appreciated that any property of the semiconductor device that changes observably in the presence of neutron radiation can be measured to detect radiation. The device may be used in a mode which simply detects the presence of a neutron flux (greater than a threshold) or may be calibrated to measure the magnitude of the flux.

[0036] The sensitiser, e.g. boron, can be dispersed or embedded in the bulk of the organic semiconductor in any convenient way, for example in thin layers, particulates (e.g. ion -5 -implanted or mixed in nanoparticles or microscopic powders), or via boron containing organic molecules. In embodiments of the invention, the integrated active volume of the detector is maximised, and the whole of the device can potentially be used to detect a neutron incident on a detector. An ideal organic device with embedded boron, relative to a planar device with boron on the surface, has a ratio of active volumes varying between 1 and 20 for a 5 to 7 NIeV alpha particle in a device of thickness between a few pm and 100 pm. Desirably, the thickness of the organic semiconductor (OSC) layer is of the order of the Bragg peak position e.g. between 1 and 100 pm.

[0037] Known detectors have a high intrinsic efficiency, but small area and are expensive to manufacture. Embodiments of the invention can have a large area and be cost effective even if there is a small intrinsic efficiency as the relative detection efficiency for a source depends on the product of intrinsic efficiency times area of detector. Comparing existing silicon detector devices to material costs for organic devices of the same area a factor of 50 cost saving can be expected. An ideal silicon DSNISND of [Ref1] has a value of 35% intrinsic efficiency so that an organic semiconductor device according to an embodiment of the invention can be made large enough to have the same relative efficiency as a given silicon device cost effectively even if the semiconductor device has an intrinsic efficiency as low as 1%. This is illustrated in Figure 3, where the relative device efficiency versus detector area is plotted for devices of varying (intrinsic) efficiency. The horizontal line illustrates how equal relative efficiency can be achieved using lower efficiency devices by increasing the active area.

[0038] The amount of boron as sensitiser element in the organic semiconductor layer is desirably greater than or equal to about 5 x 1018 atoms/m3, more desirably greater than or equal to 1 x 1019 atoms/m3. These figures apply to naturally occurring boron, if the proportion of mB is enriched, the concentration may be correspondingly reduced. For Li as the sensitiser element the concentration is desirably at least 8 x 1019 atoms/m3, more desirably greater than or equal to 1.5 x 1020 atoms/m3. For Gd as the sensitiser element the concentration is desirably at least 1 x 1018 atoms/m3, more desirably greater than or equal to 2 x 1018 atoms/m3. For Cd as the sensitiser element the concentration is desirably at least 2 x 1018 atoms/m3, more desirably greater than or equal to 5 x 1018 atoms/m3. If the amount of sensitiser is too low, the detector efficiency may be too low.

[0039] The amount of boron as sensitiser element in the organic semiconductor layer is desirably no more than about 2 x 1020 atoms/m3, more desirably no more than about t x 1020 atoms/m3. For Li as sensitiser element in the organic semiconductor layer is desirably no -6 -more than about 5 x 10" atoms/m3, more desirably no more than about 2.5 x 1031 atoms/m3. For Gd as sensitiser element in the organic semiconductor layer is desirably no more than about 7 x 1019 atoms/m3, more desirably no more than about 3.2 x 1019 atoms/m3. For Cd as sensitiser element in the organic semiconductor layer is desirably no more than about 2 x 1020 atoms/m3, more desirably no more than about 8 x 10'9 atoms/m3. If the amount of sensitiser element is too high it may affect the properties of the organic semiconductor in an undesirable manner. Although detector efficiency improves with increased amount of sensitiser element, if the amount of sensitiser element is too large there is no further increase in detector efficiency because the organic semiconductor layer becomes opaque to neutrons. The inventors have determined that amounts of sensitiser elements in the above ranges are not detrimental to the functioning of the organic semiconductor device.

[0040] The sensitiser can be dispersed within a bulk heterojunction or within the polymer layer of a polymer perovskite layered device or can be constrained within one or more layers in a multilayer device. The sensitiser component is not expected to form any percolation (charge) pathway at such low concentrations. The sensitiser may act as a barrier or trap for different charge polarities and may lead to essentially unipolar or ambipolar devices (barriers can be circumvented at low concentrations, whereas traps cannot). Trapping one polarity of carrier can lead to current gain effects which may be desirable.

[0041] In an embodiment of the invention, the first and second electrodes (anode and cathode) can be made of the same material, in which case no intrinsic bias is created and an external bias is used to drive the device which does not display rectification. Alternatively, an embodiment of the invention can have electrodes made with materials with different work functions to ensure that an in built bias exists. Any conductive material can be used for the electrodes as is convenient for manufacturing, such as indium tin oxide (ITO), Au, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT-PSS) or aluminium.

[0042] Embodiments of the invention allow high efficiency neutron detectors to be made with a commercially cheap process. In addition to high efficiency neutron detection, it is possible to make large area detectors that can be applied to scientific applications (particle and nuclear physics experiments), radiation dosimetry in laboratories, and radiation monitoring at commercial and government nuclear facilities such as operating and decommissioned reactors. Because it allows large scale detectors to be made economically, the present invention is particularly useful in security applications where large scale detectors can quickly scan people and/or freight. -7 -

[0043] As a proof of concept, test devices were fabricated by dropcasting 0.5 mL of 1:1 mass ratio solution of P3HT:PCBM in Dichlorobenzene onto pre-patterned Indium Tin Oxide (ITO) coated glass substrates with concentrations varying between 10 and 40 mg/mL A neutron sensitiser component consisting of 1 ± 0.5 wt% BN nanoparticles (70-90 nm diameter) was incorporated by suspending the nanoparticles in the organic semiconductor solution prior to deposition. After drying, a -100 nm Al cathode was deposited by vacuum evaporation (typically 10-6 mbar base pressure) at -2 nm The resulting typical individual diode area was -4 mm2. A schematic sample structure is shown in Figure 1 where the BN nanoparticles were included throughout the organic semiconductor donor-acceptor layer. All fabrication was carried out in a nitrogen filled glovebox. The samples were transferred into the relevant sample chambers under nitrogen and all subsequent measurements carried out under vacuum (typically 10-5 mbar base pressure). After all measurements were completed, the individual sample thickness (typically 5 -38 pin) was measured using a Veeko DekTak profilometer.

[0044] To test functioning as a radiation detector, the test devices were exposed to a particles by mounting them with the Al cathode facing an 241Am source at a distance of 7 mm separated by a moveable shutter. The measured a particle flux under these conditions was -625 mm-2 s-1 at the sample position. Current-Voltage (1-V) measurements were carried out between ±20 V bias using a Keithley 4200 source-measure unit with the shutter alternatively open and closed and repeated 16 times. In the absence of exposure the diode displayed reasonable rectifying behaviour, as expected from the work function difference between anode and cathode (see Figure 2). The current increase under exposure was clearly observable under both forward and reverse bias at modest drive conditions, between 10 and 20 V applied across the sample i.e. at small applied electric fields, < -1 V itm-1.

[0045] The test devices demonstrated steady state a detection using P3HT:PCBM based diode devices (with OSC layer thickness between 5 and 38 ttm) using low bias (< 20 V) with useful reproducible and repeatable sensitivities.

[0046] The device particle detection sensitivity (and associated gain efficiency product) is optimised by choosing the OSC layer thickness to correspond to the Bragg peak position for the 5.49 MeV particles used (obtained by modelling). For OSC layers smaller than the Bragg peak position, devices are charge generation limited, whereas for those larger than the Bragg peak position, devices appear charge collection limited. Transient photoconduction measurements confirm electron trapping and return hole mobility lifetime values consistent with Hecht equation fitting of the particle detection sensitivity. -8 -

[0047] On the basis of demonstrated a particle detection, it can be expected that neutron detection can be achieved by detecting a particles emitted on the capture of thermal neutrons by boron nuclei dispersed in the organic semiconductor layer. We have also carried out transient response a detection on P3HT based diodes under high bias (500V). Figure 4a is a single a particle signal and Figure 4b shows a series of such signals. The variable signal height observed in Figure 4b provides indication that the device operates in avalanche mode. [0048] Examples of organic semiconductors that can be used in the invention include Pi conjugated organic semiconductors (OSCs), which may include inorganic components (e.g solution processable perovskites and nanoparticle sensitisers) for blends and multi-layer devices. The OSCs can be polymeric or small molecule based.

[0049] Desirably, the semiconducting components possess suitable bandgaps (of order eV) and selected electron affinities and ionisation potentials (Highest Occupied Molecular orbital, HOMO, and Lowest Unoccupied Molecular Orbital, LUMO, level positions or Valence and Conduction bands in the inorganic case). The HOMO and LUMO levels are desirably suitable for constructing structures, e.g. diodes and Field Effect transistors (FETs). Where appropriate, charge carrier injection can occur from one or two electrodes if desired. [0050] In the case of Donor-Acceptor (D-A) systems, the energetics are desirably tailored for electron transfer from the donor HOMO to the acceptor LUMO. The OSCs and/or inorganic components desirably possess reasonable charge carrier mobilities (at least le cm2V1s1, desirably 10' cm2V1s1 for at least one type of carrier) and ranges (of at least one type of carrier, this is desirably of the order of the Bragg peak position i.e. between 1 and 100 pm depending on the alpha energy post neutron capture).

[0051] The organic and inorganic components are desirably suitable for deposition such that the neutron capture sensitiser can be included as well as forming a controlled amount of D-A interface (or electron transport layer-hole transport layer interface) by microsegregation or layering. The sensitiser itself can consist of inorganic nanoparticles, such as BN or B4C, or may be included as part of a metalo-organic complex, such as a Ga substituted Phthalocyanine.

[0052] The semiconductor component or components may be either fully organic or organic:inorganic hybrids e.g polymer:fullerene:BN nanoparticle or polymer:polymer:nanoparticle or polymer:nanoparticle:perovskite, or polymer:metalo-organic complex. Individual components can perform more than one function (e.g. an electron acceptor or donor may also contain the sensitiser). -9 -

[0053] Diode devices according to embodiments of the invention can consist of sensitised dispersed bulk heterojunctions (e.g. D-A microsegregated blends, such as polymer:fullerene:BN) or of layered devices (e.g. organic hole transport layer -perovskite electron transport/charge generation layer devices, e.g. polymer:BN nanoparticle:perovskite).

The diodes can be defined by a high work function anode and a low work function cathode and may include additional charge injection layers between the electrodes and the semiconducting components. Multi-layer devices with dedicated hole and electron transport layers as well as charge generation layers are also possible. The diode thickness is desirably of the order of the Bragg peak position for the ionising radiation emitted post neutron capture.

Current changes can be measured either in the steady state or in transient response.

[0054] Figure 5 depicts Field Effect Transistor (FET) device according to an embodiment of the invention. The FET detector la is a thin-film transition formed on substrate 10. Gate 16 is an electrode and is covered by insulator 17. A semiconductor layer 18 forms the channel between the source 18a and drain 18b electrodes. The organic semiconductor forming the channel may itself be sensitised, alternatively a layer, 18c (adjacent to the channel) may comprise of a sensitised organic semiconducting component. Thus the sensitiser may be incorporated in or adjaccnt to the transistor channel. The channel semiconductor may be formed of an organic semiconductor, hybrid or other (e.g. graphene) material. The operating principle is that free charges generated in the sensitised organic component can migrate to the gate dielectric interface under the effect of the gate bias and affect the source-drain transistor current. The radiation can be detected by suitable changes in either the output or transfer behaviour of the transistor and can be measured either in the steady state or in transient response.

[0055] A variety of single and multiple detector architectures are possible. Individual detectors of arbitrary area can be manufactured. In the case of large area devices, individual detectors may be segregated (into quadrants, pixels or stripes, for example) to form multi-pixel detectors. The pixels may consist of separate diodes or FETs or both. Vertical integration (e.g. "tandem" or "stacked" detectors) is also possible. For example, a three dimensional pixelated and stacked all organic architecture could be used as a "phantom" for medical neutron beam applications.

(0056) Embodiments of the invention can also be employed in object detection e.g. mine detection. A mine detection device embodying the invention is depicted in Figure 6. A neutron source 2 directs neutrons into the ground and a detector 1 detects the back-scattered -10 -neutrons. Differences in the back-scattering between earth and a mine 3 enable the mine to be detectable.

[0057] An organic semiconductor radiation detector according to the invention will naturally detect a particles and 13 particles and 7 radiation in addition to neutrons. If desired, the radiation detector can be made selective to neutrons and desired radiation by suitable encapsulation to exclude radiation types that are not of interest. In addition, the radiation detector can be made sensitive to x-rays by inclusion of an element having an atomic number greater than 20.

[0058] Manufacturing techniques that can be used in the invention include both solution processing techniques and non-solution processing techniques. Examples of suitable solution processing techniques include: drop-casting, spin coating, inkjet printing, screen printing, roll to roll printing. Examples of suitable non-solution processing techniques include: vacuum deposition and co-evaporation, solid state (high temperature and/or pressure) processing. [0059] Having described embodiments of the invention, it will be appreciated that variations can be made to the described embodiments, which are intended to be illustrative not prescriptive. The invention is defined by the appended claims.

References [Ref 1] R.G. Fronk, S.L. Bellinger, L.C. Henson, T.R. Ochs, C.T. Smith, J.K. Shultis, D.S. McGregor, "Dual-Sided Microstructured Semiconductor Neutron Detectors (DSMSNDs)," Nucl.

Instrum. Meth., A804 (2015) 201-206.

Claims

CLAIMS1. A radiation detector comprising a semiconductor device formed of one or more organic semiconductor materials having a neutron sensitiser element dispersed therein.
2. A radiation detector according to claim 1 wherein the sensitiser element is one or more elements selected from the group consisting of boron, cadmium, lithium and gadolinium.
3. A radiation detector according to claim 1 or 2 wherein the sensitiser element is provided in an amount of between 1 x 1018 atoms/m3 and 2.5 x 1021 atoms/m3 within the organic semiconductor material.
4. A radiation detector according to any one of the preceding claims wherein the sensitiser element is provided in the form of particles dispersed in the organic semiconductor material.
5. A radiation detector according to any one of the preceding claims wherein the sensitiser element is contained in an organic compound.
6. A radiation detector according to any one of the preceding claims wherein the sensitiser element is provided in the form of a plurality of layers embedded in the organic semiconductor material.
7. A radiation detector according to any one of the preceding claims wherein the organic semiconductor material has a charge carrier mobility of at least 1 0-6uric 2v -1 s -1
8. A radiation detector according to any one of the preceding claims wherein the organic semiconductor materials comprise a donor organic semiconductor material and an acceptor organic semiconductor material or a combination of organic and inorganic donor-acceptor materials.
9. A radiation detector according to claim 8 wherein the donor organic semiconductor component is electron-rich (has a smaller electron affinity) compared to the acceptor component.-12 -
10. A radiation detector according to claim 8 or 9 wherein the acceptor component is electron-deficient (has a larger electron affinity) compared to the donor, for example fullerene, fullerene derivative, pi-conjugated polymer, small molecule or perovskite.
11. A radiation detector according to any one of the preceding claims wherein the semiconductor device has a thickness in the range of from 1µm to 500 pm.
12. A radiation detector according to any one of the preceding claims wherein the semiconductor device comprises a diode, a transistor (e.g. a Field Effect Transistor) or a non-rectifying, symmetric, semiconductor device.
13. A radiation detector according to any one of the preceding claims further comprising a bias voltage source configured to apply a potential difference in the range 1 V to 1 kV.
14. A radiation detector according to any one of the preceding claims containing sensitiser components additional to the neutron sensitiser, for example X-ray absorbers dispersed in the organic semiconductor material, the X-ray absorber comprising an element having an atomic number greater than 20.
15. An object detector comprising a radiation detector according to any one of the preceding claims and a neutron source.