EP2898530A1 - Self-assembling molecular photo-detector device - Google Patents

Self-assembling molecular photo-detector device

Info

Publication number
EP2898530A1
EP2898530A1 EP13801772.8A EP13801772A EP2898530A1 EP 2898530 A1 EP2898530 A1 EP 2898530A1 EP 13801772 A EP13801772 A EP 13801772A EP 2898530 A1 EP2898530 A1 EP 2898530A1
Authority
EP
European Patent Office
Prior art keywords
layer
photo
electrode layer
dye
detector device
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP13801772.8A
Other languages
German (de)
French (fr)
Inventor
Mario Caironi
Annamaria PETROZZA
Lorenzo CARANZI
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Fondazione Istituto Italiano di Tecnologia
Original Assignee
Fondazione Istituto Italiano di Tecnologia
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Fondazione Istituto Italiano di Tecnologia filed Critical Fondazione Istituto Italiano di Tecnologia
Publication of EP2898530A1 publication Critical patent/EP2898530A1/en
Withdrawn legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K39/00—Integrated devices, or assemblies of multiple devices, comprising at least one organic radiation-sensitive element covered by group H10K30/00
    • H10K39/30—Devices controlled by radiation
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K10/00—Organic devices specially adapted for rectifying, amplifying, oscillating or switching; Organic capacitors or resistors having potential barriers
    • H10K10/701—Organic molecular electronic devices
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K85/00—Organic materials used in the body or electrodes of devices covered by this subclass
    • H10K85/30—Coordination compounds
    • H10K85/341—Transition metal complexes, e.g. Ru(II)polypyridine complexes
    • H10K85/344—Transition metal complexes, e.g. Ru(II)polypyridine complexes comprising ruthenium
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K85/00—Organic materials used in the body or electrodes of devices covered by this subclass
    • H10K85/60—Organic compounds having low molecular weight
    • H10K85/631—Amine compounds having at least two aryl rest on at least one amine-nitrogen atom, e.g. triphenylamine
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/549—Organic PV cells

Definitions

  • the present invention relates generally to electronic devices, and more specifically to a photo-detector device according to the preamble to Claim 1.
  • a photo-detector device is a device capable of detecting incident optical radiation, supplying an electrical signal at the output having a current intensity or a potential difference proportional to the intensity of the radiation detected.
  • the fabrication of a molecular electronic circuit requires the fabrication of devices based on junctions in which a functional molecular layer is included between a pair of conducting electrodes connected to the rest of the circuit.
  • organic photo-detector devices are not known in the literature in which an exci- ton may be separated into free charge carriers which may be injected into an electrical circuit, when a functional layer composed of a single molecular layer is interposed directly between two electrodes.
  • Such a structure has for example been used in the fabrication of rectifier and switching devices.
  • These compounds possess two distinct isomers, of conjugated and non-conjugated form, respectively, which exhibit electrical conductivities different by 1-2 orders of magnitude.
  • the conductance of the device can be con- trolled optically, in a completely reversible manner, taking advantage of the photo- isomerization properties of the compound.
  • This device does not possess photo-detecting properties per se, since it is not capable of supplying an electrical signal at the output having a current intensity proportional to the intensity of an incident optical radiation.
  • the property of varying the electrical conductivity depends on the form that the molecule assumes at a specific range of wavelengths, but in the device no generation of charge carriers takes place at the operating wavelength.
  • the charge carriers which pass through the device under the conduction operating condition are injected from an external circuit.
  • organic photovoltaic cells are also known (DSSC, dye-sensitized solar cells, also known as Graetzel cells), in which organic dye substances capture photons of optical radiation generating excitons which dissociate at the interface with an oxide layer, releasing by diffusion an electron which is injected into the associated anode electrode contact via a charge transport mechanism, and in which an electrolyte including a redox pair regenerates the dye releasing to it its own electron, that in turn it receives from a cathode electrode.
  • DSSC dye-sensitized solar cells, also known as Graetzel cells
  • Organic photovoltaic cells are devices on the mesoscopic scale, whose complex structure is difficult to integrate with a fabrication technology for integrated electronic circuits.
  • the process of fabrication of organic photovoltaic cells requires high temperatures, which involves the selection of substrates compatible with such temperatures, limiting the level of applicability and integration.
  • a further drawback of organic photovoltaic cells comes from the intrinsic slowness of the electrical response, associated with the charge transport phenomena within the bulk of the device.
  • the aim of the present invention is thus to provide a molecular photo-detector device capable of supplying an electrical signal proportional to the intensity of an incident optical radiation.
  • the invention furthermore aims to provide a photo-detector device having a non- complex layered structure that can be integrated with a fabrication technology for integrated electronic circuits.
  • a further subject of the invention is a photo-detection arrangement as claimed.
  • the present invention is based on the principle of fabricating a photo-detector device by means of a functional layer of conjugated organic molecules, for example comprising a donor group and an acceptor group, which is included between a pair of electrical conductors, of which at least one is a (semi)-transparent electrode, arranged for connecting to downstream electronic signal processing means.
  • the functional layer of organic molecules is preferably a self-assembling molecular monolayer of an organic dye, bonded to a conducting electrode including a layer of bonds having anchoring sites capable of promoting a chemical bond with anchoring groups of the dye molecules, for example a thin layer of native oxide of the electrode metal, capable of bonding itself with -COOH groups of the dye.
  • Spectral measurements of a photo-current in the device have demonstrated that the rise time of the photo-current corresponds to the photonic absorption time of the molecules, whereby the photo-generation mechanism is substantially correlated to the opto-electronic properties of the molecular layer.
  • the photo-detector device may advantageously be scaled proportionally and integrated into a complex electronic circuit since its fabrication requires simple processing steps for the deposition of the materials.
  • the possibility of operating in photovoltaic mode without a polarizing electric field applied to the electrodes allows the device, subject of the invention, to detect optical signals with a high signal/noise ratio with respect to solid state photo-detection devices.
  • Figure 1 shows schematically a cross section of a structure of a photo-detector device according to the invention
  • Figure 2 shows schematically a cross section of a portion of the structure of a photo- detector device according to the invention, highlighting the photo-detection mechanism
  • Figures 3a, 3b, 3c and 3d are chemical structures of four dye substances preferred as functional molecular layer for a photo-detector device according to the invention
  • Figures 4a-4c are diagrams of I-V characteristic curves of a photo-detector device according to the invention.
  • Figure 5 is a diagram representing the behaviour of a photo-current over time as a function of the illumination conditions for a photo-detector device according to the invention
  • Figure 6 is a comparison diagram between the I-V characteristic curves of a photo- detector device according to the invention with various dye substances for the functional layer, as a function of the illumination conditions;
  • Figure 7 shows a schematic photo-detection circuit arrangement comprising a photo- detector device according to the invention.
  • Figure 1 shows one embodiment of a photo-detector device, subject of the invention, indicated overall with 10.
  • the embodiment in Figure 1 is substantially based on a configuration of a molecular junction with a large surface area, analogous to that provided in the prior art and adopted by A. J. Kronemeijer et al. in "Reversible Conductance Switching in Molecular Devices", Adv. Mater. 2008, Vol. 20, pp. 1467-1473 for the fabrication of molecular switches based on the photochromic properties of some organic molecules.
  • the device comprises a lower electrode 20 of aluminium deposited on a substrate (not shown), on which is grown a thin native oxide layer (A10 x ) 22 able to form anchoring sites for a functional molecular layer.
  • the oxide layer is grown, for example, by thermal evaporation of the surface layers of the electrode and exposure to air.
  • the functional (active) molecular layer is a single molecular layer (monolayer) of a photo-active organic substance, such as an organic dye substance, preferably self-assembling, comprising at least one conjugated part which includes a donor group and an acceptor group responsible for the absorption of an optical radiation, and an anchoring part which includes a bonding group such as for example a thiol, a phosphonic acid, a carboxyl group which promote the bonding of the molecule onto specific surfaces (amongst which are oxides, but also metals, semiconductors, etc.).
  • a photo-active organic substance such as an organic dye substance, preferably self-assembling, comprising at least one conjugated part which includes a donor group and an acceptor group responsible for the absorption of an optical radiation, and an anchoring part which includes a bonding group such as for example a thiol, a phosphonic acid, a carboxyl group which promote the bonding of the molecule onto specific surfaces (amongst which are oxides, but also metal
  • the functional molecular layer 24 comprises a dye substance with -COOH anchoring group, capable of implementing a self- assembly of the molecules on the native oxide layer of aluminium 22 following a deposition process from a liquid phase or from a vapour phase.
  • the functional molecular layer 24 is bounded laterally by a thicker inert polymer layer 26, preferably a layer of photoresist.
  • the functional layer 24 is conventionally formed by deposition of the molecular substance into a cavity obtained via a photolithographic process of the inert polymer layer 26 previously deposited, by which a volume of photoresist is removed from a predefined area by means of masking while the subsisting photoresist is chemically stabilized.
  • the cavity is preferably of circular shape and with a diameter in the range preferably between 1 micron and 1 millimetre, depending on the circuit requirements.
  • a second electrode (or upper electrode) 28 is formed from (semi-)transparent conducting polymer, preferably PEDOT:PSS, over one area of which a metal contact 30 is deposited, for example gold, in order to reduce the resistivity of the polymer electrode.
  • Figure 2 shows schematically a region of the device in which a single molecule of the functional monolayer 24 is shown, in order to highlight the photo-detection mechanism.
  • the molecule is the commercial dye D102, whose basic formula is C 37 H 3 oN 2 0 3 S 2 , marketed by ⁇ UK LIMITED.
  • D and A respectively indicate the donor group and the acceptor group of the molecule and, in conjunction with these, the transfer of a hole (h + ) and of an electron (e " ) to the respective electrodes are furthermore shown.
  • the acceptor group A is bonded to a carboxyl anchoring group and is disposed in contact with the lower electrode 20 made of aluminium, and the donor group D is disposed in contact with the upper polymer electrode 28 by virtue of the rigidity of the molecule itself.
  • the molecule When the molecule absorbs a photon (hv) through the (semi-)transparent electrode 28, it assumes a charge transfer state in which an exciton is generated on the conjugated part of the molecule, in other words a weakly-coupled electron-hole pair, which respectively resides on the acceptor group A and on the donor group D.
  • the donor group and the acceptor group are disposed within the molecule in such a manner that the excitation induced by an incident optical radiation generates an intra-chain charge transfer state and the charge carriers are easily separated, whereby the electron is rapidly injected into the lower electrode 20 through the anchoring layer 22 (in a time typically less than lOOps), whereas the hole is rapidly injected into the upper electrode 28.
  • the charge carriers thus give rise to an electrical current flow in an external circuit connectable across the photo-detector device, supplying the detection information of the optical radiation.
  • the current-voltage (I-V) characteristic curves have been measured under dark conditions (curve D) and under conditions of illumination (curve L) for a photo-detector device with a functional layer comprising the dye D102, clearly detecting a photo-current, as shown in Figures 4a-4c.
  • Figure 4a shows the behaviour of the current- voltage (I-V) curves by means of a comparison between the dark condition and condition of illumination over a range of bias voltages between -IV and +1V.
  • Figure 4b shows an enlargement of the behaviour of the current-voltage (I-V) curves in Figure 4a over a range of bias voltages around 0V.
  • the characteristic curves of Figure 4a are shown with a semi-logarithmic current scale.
  • a weak current is measured determined by the injection of holes into the lower electrode, this being a condition inhibited by the topology of the device, in particular by the arrangement of the donor and acceptor groups of the molecules of the functional layer with respect to the electrodes.
  • Figure 5 shows the time response of the device, subject of the invention, to a series of light pulses.
  • the device responds promptly generating a detection current, whose rising edge shown in the figure is limited exclusively by the integration time of the measurement.
  • the structure of the photo-detector device of the invention opens up the way to the production of ultra-small-scale photo-detection devices, which devices may be easily integrated, by virtue of the auto-assembling properties of the organic layer that does not require particular processing phases, into photo-detection circuit arrangements and, more generally, into opto-electronic systems (lab-on-chip where the local detection of optical signals is re ⁇ quired, for example emissions of markers associated with analyses) fabricated with fabrication technologies for conventional solid state electronic circuits.
  • a photo-detector device may be integrated into a photo- detection circuit arrangement illustrated in Figure 7.
  • elements or components identical or functionally equivalent to those illustrated in Figure 1 have been indicated with the same references already used in the description of such preceding figures.
  • a biasing potential difference is applied to the electrodes of the molecular photo-detector device 10 between a first biasing terminal Bl and a second biasing terminal B2.
  • Signal terminals SI and S2 are associated with these same electrodes of the device for the connection of the device to the rest of an electronic circuit, for example to a complex signal processing electronic circuit P, downstream of the photo-detector device.

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Abstract

A photo-detector device is described including an active layer adapted to absorb an optical radiation and to generate in a corresponding way pairs of electrical charge carriers, comprised between a first and a second electrode layer including a respective electrically conductive material, at least one of which is a layer of optically transparent material, arranged to be connected to an external electrical signal processing circuit. The active layer includes a self-assembling monolayer of molecules comprising a donor group facing the first electrode layer and an acceptor group facing the second electrode layer, the molecules being adapted to assume a charge transfer state resulting from the absorption of the optical radiation, whereby the charge carriers generated reside separately on the donor group and on the acceptor group and are transferred therefrom to the adjacent electrode layers, in such a manner as to determine a flow of a detection electrical current in the signal processing circuit.

Description

Self-assembling molecular photo-detector device
The present invention relates generally to electronic devices, and more specifically to a photo-detector device according to the preamble to Claim 1.
A photo-detector device is a device capable of detecting incident optical radiation, supplying an electrical signal at the output having a current intensity or a potential difference proportional to the intensity of the radiation detected.
Recently, the use of organic molecules which exhibit charge transfer properties as active elements in electronic devices has attracted the attention of researchers, who have experimentally demonstrated the electron transport properties, and thus the electrical conductivity on the macroscopic scale of various organic compounds, mainly semiconductor polymers.
The fabrication of a molecular electronic circuit requires the fabrication of devices based on junctions in which a functional molecular layer is included between a pair of conducting electrodes connected to the rest of the circuit.
In general, organic photo-detector devices are not known in the literature in which an exci- ton may be separated into free charge carriers which may be injected into an electrical circuit, when a functional layer composed of a single molecular layer is interposed directly between two electrodes.
Such a structure has for example been used in the fabrication of rectifier and switching devices. A. J. Kronemeijer et al. in "Reversible Conductance Switching in Molecular Devices", published in Adv. Mater. 2008, Vol. 20, pp. 1467-1473, have demonstrated the properties of switching between a conducting state and a non-conducting state of a molecular monolayer of photochromic diaryl ethene, forming a molecular switch. These compounds possess two distinct isomers, of conjugated and non-conjugated form, respectively, which exhibit electrical conductivities different by 1-2 orders of magnitude. By means of the irradiation at various specific wavelengths, the conductance of the device can be con- trolled optically, in a completely reversible manner, taking advantage of the photo- isomerization properties of the compound.
This device does not possess photo-detecting properties per se, since it is not capable of supplying an electrical signal at the output having a current intensity proportional to the intensity of an incident optical radiation. The property of varying the electrical conductivity depends on the form that the molecule assumes at a specific range of wavelengths, but in the device no generation of charge carriers takes place at the operating wavelength. The charge carriers which pass through the device under the conduction operating condition are injected from an external circuit.
In the field of the production of electrical power, which is completely different from the field of signal generation, amongst the photo-electrochemical cells, organic photovoltaic cells are also known (DSSC, dye-sensitized solar cells, also known as Graetzel cells), in which organic dye substances capture photons of optical radiation generating excitons which dissociate at the interface with an oxide layer, releasing by diffusion an electron which is injected into the associated anode electrode contact via a charge transport mechanism, and in which an electrolyte including a redox pair regenerates the dye releasing to it its own electron, that in turn it receives from a cathode electrode.
Organic photovoltaic cells are devices on the mesoscopic scale, whose complex structure is difficult to integrate with a fabrication technology for integrated electronic circuits. The process of fabrication of organic photovoltaic cells requires high temperatures, which involves the selection of substrates compatible with such temperatures, limiting the level of applicability and integration.
A further drawback of organic photovoltaic cells comes from the intrinsic slowness of the electrical response, associated with the charge transport phenomena within the bulk of the device.
The aim of the present invention is thus to provide a molecular photo-detector device capable of supplying an electrical signal proportional to the intensity of an incident optical radiation. The invention furthermore aims to provide a photo-detector device having a non- complex layered structure that can be integrated with a fabrication technology for integrated electronic circuits.
According to the present invention such an object is met with a photo-detector device having the features claimed in the Claim 1.
Specific embodiments form the subject of the dependent claims, whose content is to be understood as an integral part of the present description.
A further subject of the invention is a photo-detection arrangement as claimed.
In summary, the present invention is based on the principle of fabricating a photo-detector device by means of a functional layer of conjugated organic molecules, for example comprising a donor group and an acceptor group, which is included between a pair of electrical conductors, of which at least one is a (semi)-transparent electrode, arranged for connecting to downstream electronic signal processing means. The functional layer of organic molecules is preferably a self-assembling molecular monolayer of an organic dye, bonded to a conducting electrode including a layer of bonds having anchoring sites capable of promoting a chemical bond with anchoring groups of the dye molecules, for example a thin layer of native oxide of the electrode metal, capable of bonding itself with -COOH groups of the dye.
In operation, when a photon is absorbed by the functional molecular layer, this assumes a charge transfer state and the charge carriers are immediately collected at opposite electrodes without a charge transport phenomenon occurring within a bulk layer. Consequently, the time response of the device is limited exclusively by the formation of the charge transfer layer in the molecules (from a few femtoseconds to a few picoseconds).
Spectral measurements of a photo-current in the device have demonstrated that the rise time of the photo-current corresponds to the photonic absorption time of the molecules, whereby the photo-generation mechanism is substantially correlated to the opto-electronic properties of the molecular layer.
The photo-detector device, subject of the invention, may advantageously be scaled proportionally and integrated into a complex electronic circuit since its fabrication requires simple processing steps for the deposition of the materials.
Despite the high transmission losses due to the limited number of photons that can be absorbed by a single molecular layer, the possibility of operating in photovoltaic mode without a polarizing electric field applied to the electrodes allows the device, subject of the invention, to detect optical signals with a high signal/noise ratio with respect to solid state photo-detection devices.
Further features and advantages of the invention will be presented in the following detailed description of one of its embodiments, given by way of non-limiting example, with reference to the appended drawings, in which:
Figure 1 shows schematically a cross section of a structure of a photo-detector device according to the invention;
Figure 2 shows schematically a cross section of a portion of the structure of a photo- detector device according to the invention, highlighting the photo-detection mechanism;
Figures 3a, 3b, 3c and 3d are chemical structures of four dye substances preferred as functional molecular layer for a photo-detector device according to the invention;
Figures 4a-4c are diagrams of I-V characteristic curves of a photo-detector device according to the invention;
Figure 5 is a diagram representing the behaviour of a photo-current over time as a function of the illumination conditions for a photo-detector device according to the invention;
Figure 6 is a comparison diagram between the I-V characteristic curves of a photo- detector device according to the invention with various dye substances for the functional layer, as a function of the illumination conditions; and
Figure 7 shows a schematic photo-detection circuit arrangement comprising a photo- detector device according to the invention. Figure 1 shows one embodiment of a photo-detector device, subject of the invention, indicated overall with 10.
The embodiment in Figure 1 is substantially based on a configuration of a molecular junction with a large surface area, analogous to that provided in the prior art and adopted by A. J. Kronemeijer et al. in "Reversible Conductance Switching in Molecular Devices", Adv. Mater. 2008, Vol. 20, pp. 1467-1473 for the fabrication of molecular switches based on the photochromic properties of some organic molecules.
The device comprises a lower electrode 20 of aluminium deposited on a substrate (not shown), on which is grown a thin native oxide layer (A10x) 22 able to form anchoring sites for a functional molecular layer. The oxide layer is grown, for example, by thermal evaporation of the surface layers of the electrode and exposure to air.
The functional (active) molecular layer, indicated with 24, is a single molecular layer (monolayer) of a photo-active organic substance, such as an organic dye substance, preferably self-assembling, comprising at least one conjugated part which includes a donor group and an acceptor group responsible for the absorption of an optical radiation, and an anchoring part which includes a bonding group such as for example a thiol, a phosphonic acid, a carboxyl group which promote the bonding of the molecule onto specific surfaces (amongst which are oxides, but also metals, semiconductors, etc.).
In one currently preferred embodiment of the invention, the functional molecular layer 24 comprises a dye substance with -COOH anchoring group, capable of implementing a self- assembly of the molecules on the native oxide layer of aluminium 22 following a deposition process from a liquid phase or from a vapour phase.
The functional molecular layer 24 is bounded laterally by a thicker inert polymer layer 26, preferably a layer of photoresist. The functional layer 24 is conventionally formed by deposition of the molecular substance into a cavity obtained via a photolithographic process of the inert polymer layer 26 previously deposited, by which a volume of photoresist is removed from a predefined area by means of masking while the subsisting photoresist is chemically stabilized. The cavity is preferably of circular shape and with a diameter in the range preferably between 1 micron and 1 millimetre, depending on the circuit requirements.
On the functional molecular layer 24, a second electrode (or upper electrode) 28 is formed from (semi-)transparent conducting polymer, preferably PEDOT:PSS, over one area of which a metal contact 30 is deposited, for example gold, in order to reduce the resistivity of the polymer electrode.
Figure 2 shows schematically a region of the device in which a single molecule of the functional monolayer 24 is shown, in order to highlight the photo-detection mechanism.
The molecule is the commercial dye D102, whose basic formula is C37H3oN203S2, marketed by ΓΝΑΒΑΤ UK LIMITED. D and A respectively indicate the donor group and the acceptor group of the molecule and, in conjunction with these, the transfer of a hole (h+) and of an electron (e") to the respective electrodes are furthermore shown.
In the molecule structure, the acceptor group A is bonded to a carboxyl anchoring group and is disposed in contact with the lower electrode 20 made of aluminium, and the donor group D is disposed in contact with the upper polymer electrode 28 by virtue of the rigidity of the molecule itself.
When the molecule absorbs a photon (hv) through the (semi-)transparent electrode 28, it assumes a charge transfer state in which an exciton is generated on the conjugated part of the molecule, in other words a weakly-coupled electron-hole pair, which respectively resides on the acceptor group A and on the donor group D.
The donor group and the acceptor group are disposed within the molecule in such a manner that the excitation induced by an incident optical radiation generates an intra-chain charge transfer state and the charge carriers are easily separated, whereby the electron is rapidly injected into the lower electrode 20 through the anchoring layer 22 (in a time typically less than lOOps), whereas the hole is rapidly injected into the upper electrode 28. The charge carriers thus give rise to an electrical current flow in an external circuit connectable across the photo-detector device, supplying the detection information of the optical radiation.
In Figures 3a, 3b, 3c and 3d, the structures of four molecules are shown that are currently preferred for the implementation of a photo-detector device according to the invention, the dyes D102 (C37H3oN203S2), D131 (C35H2gN202) and D205 (C48H47N304S3), respectively, which dyes are entirely organic, and the dye N719 a dye based on Ruthenium, whose absorption spectra differ from one another in terms of spectral range and absorption coefficient, but which have the same -carboxyl anchoring group.
The possibility of extracting a photo-current from a device comprising a self-assembling photo-active molecular monolayer via a pair of opposing electrodes contacts has been demonstrated by the inventors.
In order to provide a demonstration of the possibility of detecting an optical signal with the device of the invention, the current-voltage (I-V) characteristic curves have been measured under dark conditions (curve D) and under conditions of illumination (curve L) for a photo-detector device with a functional layer comprising the dye D102, clearly detecting a photo-current, as shown in Figures 4a-4c. Figure 4a shows the behaviour of the current- voltage (I-V) curves by means of a comparison between the dark condition and condition of illumination over a range of bias voltages between -IV and +1V. Figure 4b shows an enlargement of the behaviour of the current-voltage (I-V) curves in Figure 4a over a range of bias voltages around 0V. Finally, in Figure 4c, the characteristic curves of Figure 4a are shown with a semi-logarithmic current scale.
From an examination of the abovementioned curves, it can be appreciated that, when the lower electrode 20 is biased positively with respect to the upper electrode 28, a net increase in the current is observed when going from the dark condition to the condition of illumination. This condition corresponds to the injection of electrons into the lower electrode 20 and of holes into the upper electrode 28 according to the mechanism illustrated in Figure 2.
When the lower electrode is biased negatively with respect to the upper electrode, a weak current is measured determined by the injection of holes into the lower electrode, this being a condition inhibited by the topology of the device, in particular by the arrangement of the donor and acceptor groups of the molecules of the functional layer with respect to the electrodes.
In Figure 4b, an enlargement is shown of the I-V characteristic curves in the neighbourhood of the bias voltage 0V, which demonstrates the possibility of the device for operating in photovoltaic mode. By biasing the device around 0V, the ratio between photo-current and dark current is very high and thus the signal/noise ratio of the device is increased.
Figure 5 shows the time response of the device, subject of the invention, to a series of light pulses. In the presence of illumination, the device responds promptly generating a detection current, whose rising edge shown in the figure is limited exclusively by the integration time of the measurement.
Finally, in Figure 6 a comparison is shown between the I-V characteristic curves of two devices, with the dye D102 (curve A) and N719 (curve B) as a molecular layer, respectively, for which the dark currents are shown with a dashed line and the currents under a condition of illumination with a continuous line. It will be noted that the device with the dye D102 works best, which may be explained by the greater optical cross section of the organic dye.
The structure of the photo-detector device of the invention opens up the way to the production of ultra-small-scale photo-detection devices, which devices may be easily integrated, by virtue of the auto-assembling properties of the organic layer that does not require particular processing phases, into photo-detection circuit arrangements and, more generally, into opto-electronic systems (lab-on-chip where the local detection of optical signals is re¬ quired, for example emissions of markers associated with analyses) fabricated with fabrication technologies for conventional solid state electronic circuits.
A photo-detector device according to the invention may be integrated into a photo- detection circuit arrangement illustrated in Figure 7. In the figure, elements or components identical or functionally equivalent to those illustrated in Figure 1 have been indicated with the same references already used in the description of such preceding figures. A biasing potential difference is applied to the electrodes of the molecular photo-detector device 10 between a first biasing terminal Bl and a second biasing terminal B2. Signal terminals SI and S2 are associated with these same electrodes of the device for the connection of the device to the rest of an electronic circuit, for example to a complex signal processing electronic circuit P, downstream of the photo-detector device.
It goes without saying that, remaining within the principle of the invention, the embodiments and the implementation details could be substantially varied with respect to what has been described and illustrated purely by way of non-limiting examples, without however straying from the scope of protection of the invention defined by the appended claims.

Claims

1. Photo-detector device including an active layer adapted to absorb an optical radiation and to generate in a corresponding way pairs of electrical charge carriers, comprised between a first and a second electrode layer including a respective electrically conductive material, at least one of which is a layer of optically transparent material, arranged to be connected to an external electrical signal processing circuit,
characterized in that the said active layer includes a self-assembling monolayer of molecules comprising a donor group facing the first electrode layer and an acceptor group facing the second electrode layer, the said molecules being adapted to assume a charge transfer state resulting from the absorption of the optical radiation whereby the charge carriers generated reside separately on the donor group and on the acceptor group and are transferred therefrom to the adjacent electrode layers, in such a manner as to determine a flow of a detection electrical current in the signal processing circuit.
2. Device according to Claim 1, wherein at least one electrode layer carries associated bonding sites adapted to retain the molecules of the said active layer.
3. Device according to Claim 2, wherein said bonding sites comprise a native oxide layer of the material forming the said electrode layer.
4. Device according to Claim 1, 2 or 3, wherein said active layer is a mono-layer of an organic dye substance having a carboxylic group for molecular anchoring to said bonding sites of at least one electrode layer.
5. Device according to Claim 4, wherein said dye substance is dye D102, having a formula C37H30N2O3S2, dye D131, having a formula C35H28N202, or dye D205, having a formula C48H47N3C>4S3.
6. Device according to Claim 4, wherein said dye substance is dye N719, having a formula C58H86N808RuS2.
7. Device according to any one of the preceding claims, wherein said first electrode layer is a layer of PEDOT.PSS.
8. Device according to any one of the preceding claims, wherein said second electrode layer is an aluminium layer.
9. Device according to any one of the preceding claims, wherein said active layer is confined within a region bound by a photoresist.
10. Photo-detection circuit arrangement comprising a photo-detector device according to any one of Claims 1 to 9, a circuit for biasing the photo-detector device in a working point and means for connecting to an external signal processing circuit, said means being adapted to allow the flow of a photoconduction current generated by the device to said circuit.
EP13801772.8A 2012-09-24 2013-09-23 Self-assembling molecular photo-detector device Withdrawn EP2898530A1 (en)

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IT000826A ITTO20120826A1 (en) 2012-09-24 2012-09-24 AUTO-ASSEMBLY MOLECULAR PHOTO-DETECTOR DEVICE.
PCT/IB2013/058776 WO2014045256A1 (en) 2012-09-24 2013-09-23 Self-assembling molecular photo-detector device

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US9786855B2 (en) 2014-12-30 2017-10-10 Indian Institute Of Technology Bombay Micro electro mechanical system (MEMS) based wide-band polymer photo-detector

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