EP2115788A2

EP2115788A2 - Semiconductor device structure

Info

Publication number: EP2115788A2
Application number: EP08715844A
Authority: EP
Inventors: Michael Graetzel; Henry James Snaith
Original assignee: ETeCH AG
Current assignee: ETeCH AG
Priority date: 2007-02-16
Filing date: 2008-02-18
Publication date: 2009-11-11
Also published as: WO2008098797A3; WO2008098797A2

Abstract

The semiconductor device structure demonstrates a novel organic device concept, capitalizing on knowledge from both the photovoltaic cell and the field effect transistor. This hybrid 'photo gated' detector comprises an inorganic capacitive layer (15), a molecular sensitizing layer (14) and an organic charge transport layer (10), caped with in-plane gold electrodes (11, 12) defining an active channel. Under illumination, the sensitizer injects an electron in the capacitive layer, constituted by a wide band gap inorganic oxide of mesoscopic morphology, while the resulting positive charge is transferred to the hole- transporter. The increased hole density results in vastly enhanced film conductivity and charge carrier mobility. Strikingly high light/dark current ratios of up to 106 at low temperature and 104 at room temperature are observed corresponding to mobility enhancements of up to 103 This not only presents a new method for estimating mobility in organic materials but also offers a broad range of sensing and memory applications.

Description

Semiconductor device structure

Technical field of the invention

The invention relates to a novel detector device structure having an organic device concept, capitalizing on knowledge from both the photovoltaic cell and the field effect transistor. This hybrid "photo gated" detector comprises of an inorganic capacitive layer, a molecular sensitizing layer and an organic charge transport layer, caped with in- plane gold electrodes defining an active channel. Under illumination, the sensitizer injects an electron in the capacitive layer, constituted by a wide band gap inorganic oxide of mesoscopic morphology, while the resulting positive charge is transferred to the hole-transporter. The increased hole density results in vastly enhanced film conductivity and charge carrier mobility. Strikingly high light/dark current ratios of up to 10⁶ at low temperature and 10⁴ at room temperature are observed corresponding to mobility enhancements of up to 10³. This not only presents a new method for estimating mobility in organic materials but also offers a broad range of sensing and memory applications.

Technical background of the invention

The recent interest in organic optoelectronics has provoked an explosion in research activity. There are many distinctly different devices incorporating organic semiconductors with the most widely researched being the light emitting diode (LED), photovoltaic cell (PV) and the field effect transistor (FET), with the recent realization of light emitting field effect transistors (LFET), and organic phototransistors (OPT). The effect of light and photoconductivity on field effect transistors has been extensively studied for inorganic materials. FET's which exhibit strong photoconductivity are generally termed "phototransistors" and have been found to act as useful light sensors with a broad range of applications. With organic semiconductors, there is a large binding energy to the exciton due to strong electron phonon coupling. This results in relatively low photoconductive properties since, in the absence of a strong electric field, excitons can not be ionized in the bulk of an organic semiconductor. For this reason, there have been very few studies upon the photoconductive properties of organic FET's, though it is attracting increasing interest. Exciton ionization is facilitated at the junction between two organic semiconductors if the energy levels are suitably offset rendering charge generation energetically favourable. This is the principle upon which "bulk heterojunction" organic based solar cells operate.

Summary of the invention The system presented here avoids the problem of exciton diffusion and dissociation altogether by employing a molecular sensitizer layer as the photoactive material, which is located at the interface between a capacitive "electron reservoir" and a hole transporting material (HTM). Upon light excitation the sensitizer injects an electron in to the capacitive component and is subsequently regenerated with hole transfer to the HTM, both events are completed within nanoseconds. The increased hole density in the hole-transporter results in up to 10⁶ enhancement in conductivity at low temperature and 10⁴ at room temperature. It is shown by the inventors that this is be primarily due to an enhanced mobility and find that at room temperature the hole mobility in the HTM is increased from 2.0 x 10^"6 to 1.2 x 10^"3 cm²/Vs when increasing the incident illumination intensity from dark to 100 mWcm^" .

The device demonstrates a novel organic device concept, capitalizing on knowledge from both the photovoltaic cell and the field effect transistor. This hybrid "photo gated" detector comprises an inorganic capacitive layer, a molecular sensitizing layer and an organic charge transport layer, caped with in-plane gold electrodes defining an active channel. Under illumination, the sensitizer injects an electron in the capacitive layer, constituted by a wide band gap inorganic oxide of mesoscopic morphology, while the resulting positive charge is transferred to the hole-transporter. The increased hole density results in vastly enhanced film conductivity and charge carrier mobility. Strikingly high light/dark current ratios of up to 10⁶ at low temperature and 10⁴ at room temperature are observed corresponding to mobility enhancements of up to 10³. This not only presents a new method for estimating mobility in organic materials but also offers a broad range of sensing and memory applications.

Short description of the Figures:

Fig. Ia Chemical structure of the sensitizing dye (termed K68) and the molecular hole-transporter. Fig. Ib Schematic illustration of the photo gated detector. Fig. 2a Conductivity of the film versus white light illumination intensity (I) at

296 K (solid squares) and 215 K (open circles).

Fig. 2b Normalized incident-photon to collected electron action spectra for the same PGD as measured in (a) (open circles) and absorption spectrum of a 1 μm thick film of nanoporous TiO₂ sensitized with "K68" dye (solid- squares).

Fig. 2c The calculated charge carrier mobility as a function of illumination intensity (bottom axis) and charge density (top axis). The inset shows the charge recombination rate constant for a diode fabricated with an identical active layer to that of the PGD, measured by a transient open- circuit voltage decay technique.

Fig. 2d Transient response of the PGD to a light pulse.

Fig. 3 Scheme for the charge generation and "trap-filing" processes occurring at the dye-sensitized heterojunction. Fig. 4 Arrhenius plot of conductivity versus temperature for the photo gated detector over a range of illumination intensity.

Fig. 5 A schematic view of a device structure according to an embodiment of the invention.

Fig. 6 A schematic view of a device structure according to another embodiment of the invention.

Detailed description of the preferred embodiments of the invention

The active layer is similar to that in the solid-state dye-sensitized solar cell (SDSC), as disclosed in U. Bach, D. Lupo, P. Comte, et al., Nature 395, 583 (1998). Such a solid- state DSC is known from US 6,335,480. It comprises of a glass slide coated with a 1 μm thick mesoporous film of TiO₂ nanoparticles (a capacitive component). This film is coated with dye molecules to act as an optical sensitizer, and infiltrated with a molecular hole transporting material (HTM), 4,4":4"',4""-ter-N,N-diphenyl[4- (methoxymethyl)phenyl] amine (ter-DPMMPA) (Avecia). Gold electrodes are then contacted by shadow mask evaporation upon the surface of the hole-transporter, defining a channel width of approximately 50 μm. The chemical structures of the materials used in this study and a schematic of the device architecture are shown in Fig. 1. Fig. Ia shows the chemical structure of the sensitizing dye (termed K68) and the molecular hole-transporter. Fig. Ib shows schematic illustration of the photo gated detector. Light is absorbed in the dye molecules with subsequent electron transfer to the TiO₂ and hole transfer to the HTM. The TiO₂ acts as a capacitive layer allowing a large charge density to build-up within the HTM. The hole density in the HTM controls the charge mobility. We note that the diagram is not to scale. The average TiO₂ particle size is 19 nm and the nanoporous film has a 60 % porosity.

The conductivity between the electrodes in the dark is very low, the material is essentially insulating in the ground state. However, it increases dramatically under optical illumination. In Fig. 2a we present data for the film conductivity versus illumination intensity (white light diode source: Lumiled Model LXHL-NWE8 whitestar) at room temperature and at 215 K. We note that the points at an intensity of 0.02 mWcm^"2 are taken in the dark and placed at this point to fit on the logarithmic axis). The light was incident from the electrode side (top), though we note that the illumination direction made negligible difference to the measured currents. We observe approximately four orders of magnitude increased conductivity at room temperature over the intensity range studied, demonstrating a highly sensitive response to light. At low temperature this response becomes even more exaggerated with a phenomenal enhancement of 10⁶. This corresponds to a photosensitivity of approximately 0.1 A/W with 15 V between the electrodes. The response will scale linearly with voltage between the electrodes and in a squared manner with reducing the channel length (resistance drops linearly and absolute incident illumination reduces linearly with L). Therefore, purely by reducing the channel length form 50 to 5 μm the sensitivity should be increased to 10 A/W. The reason for the exaggerated response at low temperature will be discussed later. The spectral response of the photo gated detector, which we will abbreviate to PGD, with 15 V bias between the electrodes, is shown in Fig. 2b along with the absorption spectrum of a TiO₂ film sensitized with the dye molecules. The photovoltaic action spectrum closely follows the absorption of the dye, demonstrating that it is light absorption in the dye molecules which results in the enhanced conductivity. We note that we have used no gating electrode to generate a large electric field. The presence of a gate electrode is necessary for organic phototransistors to operate, as has been demonstrated by Narayan et al. where a light/dark current ratio of only 1.4 was observed with poly-3-octylthiophene-2,5-diyl on a quartz substrate. This compared with 100 when a gating electrode was present. If the enhancement in conductivity was purely due to the steady drift of photo-generated holes through the HTM, then we would expect the conductivity to scale with the charge density in the film. Since we observe a super-linear response to light intensity over a large range of intensity (I^{1 4} at 296 K), an increase in the charge mobility is also likely to contribute to the enhanced conductivity. With knowledge of the charge density (p), we can estimate the mobility (μ) in the film from the conductivity (σ) using σ = pqμ. Under steady state conditions the rate of change of charge density,

^ = G -k_recp = 0 , (1)

where G is the charge generation rate and k_rec is the recombination rate constant. Since only the HTM is electronically contacted, the situation in the active layer of the PGD is identical to that in a photovoltaic diode at open-circuit, hi order to estimate k^ in the PGD, we have fabricated solid-state dye-sensitized solar cells (sandwich cell configuration) ¹³ incorporating an identical active layer to that employed here. We have then estimated the charge recombination rate constant as a function of illumination intensity by employing open-circuit voltage decay measurements: A small square wave pulsed red light perturbation is superimposed upon a large white light bias with the transient photo-voltage response recorded on an oscilloscope, the decay of which is proportional to the decay of the charge in the film. As the inset to Fig. 2c we show k_rec versus bias illumination intensity (I). We find for this system that the recombination increases rapidly with intensity as I^{0 78}. Assuming a charge generation efficiency from absorbed photons of unity, we have calculated an upper estimate of the charge generation rate as a function of illumination intensity by taking the overlap integral of the light source with the dye absorption. Thus, assuming that the dark charge density (po) « the photoinjected charge density, the charge density and the mobility have been calculated as a function of illumination intensity. This is presented for the measurements at room temperature in Fig. 2c. We observe only a five fold increase in charge density as the illumination intensity is increased by three orders of magnitude. However, there is a striking enhancement in mobility by three orders of magnitude from 2 x 10^"6 to 1.2 x 10³ cm²/Vs. To the best of our knowledge, this presents a new method for estimating charge carrier mobility in hybrid and organic composites, directly applicable to analyzing materials for photovoltaic applications. Fig. 2d shows the response of the device to light pulses from a white light diode source. The device has been biased with a 9 V battery and connected in series with a 1 MΩ resistor, with the voltage across the resistor recorded on an oscilloscope. To check the rise and fall time of the diode light source, a fast silicon diode was exposed to the pulse simultaneously, with the voltage generated also recorded on the oscilloscope. The rise and fall time (time to rise/fall to l/exp(l) of the final/initial value) of the light source is approximately 140 μs. The rise and fall times for the PGD signal are 230 and 210 μs respectively. This gives a response of the PGD to a square wave light pulse in the range of 70 to 90 μs (calculated as the rise/fall time of the light source subtracted from the rise/fall time of the PGD signal). The PGD was exposed to light pulses from a white light diode source. The voltage drop across the resistor was measured on an oscilloscope, from which the current was calculated. The inset shows the response to a series of light pulses.

The mechanism which we propose for the device operation is largely based upon the photovoltaic process in the solid-state dye-sensitized solar cell and the current knowledge of charge transport in disordered organic semiconductors. Light is absorbed in the sensitizing molecules with subsequent rapid (femtosecond) electron transfer in to the conduction band (CB) of the TiO₂ nanoparticles. The hole is then transferred to the hole-transporter (on a nanosecond to microsecond timescale).¹³ For the solid-state dye- sensitized solar cell (not the PGD), following charge generation the hole migrates to and is collected at the cathode and the electron to the anode. We have verified that the material combination used in this study does function in a standard SDSC, with a reasonable power conversion efficiency of around 1 %, when tested under standard Air Mass 1.5 illumination conditions. For the device studied herein, we have no anode and no electronic contact to the TiO₂, thus there will be a large build-up of electrons within the TiO₂ nanoparticles and holes in the HTM, similar to the conditions at open-circuit in the solar cell. We can therefore consider there to be a filling of the lower energy sites in the hole-transporter by photo-injected charges. It is understood that the charge mobility in disordered organic semiconductors increases considerably with increasing charge density, by reducing the activation energy required for charge hopping. Thus here, the charge mobility is moderated by the photo-injected carriers. This process is represented diagrammatically in Fig. 3. The energy levels for the materials used are also shown there (from vacuum). The highest occupied molecular orbital (HOMO) energy level of the hole-transporter is approximately -5.0 eV with respect to vacuum (measured by cyclic voltammetry). The density-of states (DOS) in the HOMO level is depicted as a Gaussian, the energetic spread has been exaggerated for clarity. Electrons and holes are represented by filled and empty circles respectively and the dashes in the HOMO level represent available sites for holes. E₁ is the energy region in which charge transport can easily occur due to an abundance of states.

In order to further investigate the operational mechanism we have performed temperature dependent conductivity measurements over a range of light intensities, these are presented in Fig. 4. The legend gives the incident illumination intensity in mW/cm^"2, the lines are exponential fits to the dark and the highest illumination intensity data, giving activation energies of 195 meV and -190 meV respectively.

First considering the dark conductivity measurements (bottom curve), we observe thermally activated charge transport. This is consistent with a charge hopping mechanism within a disordered organic semiconductor. The activation energy calculated from the Arrhenius plot is 195 meV. When we expose the device to high intensity white light (97 mWcm^"2, top-curve) we observe a completely different behaviour with the device exhibiting "negative activation", that is the conductivity increases with reducing temperature. At first sight this appears to suggest that we are observing "band-like" charge transport, with the lowering of the temperature freezing out scattering from phonons. It is perfectly reasonable to postulate that photo-generated holes, injected from the dye molecules into the HTM, fill up the lower energy sites in the tail of the DOS of the HTM. This will reduce the activation necessary for hole transport, and in principle could result in an activation-less transport if all the states are filled up to the transport level (E_t). However, we must also consider the effect temperature has upon the charge density within the composite, specifically the influence upon the charge recombination mechanism at the TiO₂/dye/HTM interface. From studies on solid-state dye-sensitized solar cells, and on liquid electrolyte dye-sensitized solar cells, we know that the charge recombination is thermally activated with the lifetime increasing as the temperature is reduced. This implies that at any given light intensity the charge density within the film will increase as the temperature is reduced. It is this increased charge density, facilitating increased charge mobility, which is the likely origin of the negative activation at higher light intensities.

The sensitivity of the device will depend strongly upon the nature of the density-of- states (DOS) in the hole-transporter, with a narrower tail to the DOS giving a larger light/dark current ratio. The sensitivity will also be directly related to the charge recombination lifetime, with a larger steady state charge density possible if the recombination is suppressed. This can be envisioned to be achieved by having further capacitive layers in contact with the TiO₂ which are "down hill" in energy for the electrons. A charge "cascade" will then further separate the electrons and holes, reducing recombination in a similar way to that employed in natural photosynthesis. We do note that this will slow down the response of the device, however if the charges are suitably separated such that the device remains "on" for a sufficiently long period of time, then this device concept functions as a light driven short-term memory application.

A charge density dependent mobility has been difficult to observe experimentally in space-charge limited diodes due to the filed dependence and the charge density dependence simultaneously increasing with applied bias. Field effect transistors generally operate at much higher charge densities, of greater than 10¹⁸ cm^"3, due to the charges being confined to a thin accumulation layer, making it difficult to study the low charge density regime. Electrochemical doping of materials adds further complications due to the added dopant impurities. To the best of our knowledge, the PGD presents a new and "clean" method for estimating charge carrier mobility in hybrid and organic composites, directly applicable to analyzing materials for photovoltaic applications. Our findings clearly demonstrate that a charge density dependent mobility should be considered when analyzing organic materials under photovoltaic operation.

In summary we have demonstrated that this hybrid dye-sensitized photo gated detector can perform as a very sensitive and fast photodetector. The dye-sensitized functionality enables the responsive region of the spectrum to be finely tuned by choosing the desired dye. Further to having applications for sensors in the ultra-violet, visible and near infrared region of the spectrum, in principle this PGD can be made sensitive to x-rays by using heavy metal nanoparticles as the sensitizing component. The PDG is therefore useful in applications of low cost large area medical imaging. It can also be used as a memory device, by introducing further capacitive layers to significantly enhance the "on time". Further to the broad range of applications, this device presents a very "clean" system to study the effects of photo-injected charges upon the charge transport in organic semiconductors, and a novel method for estimating charge carrier mobility in such materials.

The basic structure of an embodiment of the device of the invention is shown in Fig. 5. Such a structure comprises a capacitive layer 15, a charge transport layer 10 and a sensitizer layer 14, the latter being situated at the interface of the two formers. The capacitive layer 15 may be made of an n-type material and the charge transporter of a p- type material, or vice versa, respectively. Incoming photons (not shown) will separate an electron from the sensitizer 14 and eject it to the n-type layer, with a hole being generated simultaneously in the p-type layer. The charge generated in the charge transport layer 10 will modify the conductivity of the same, thus giving rise to the device applications mentioned in the claims.

An "in-plane" device structure according to the embodiment shown in Fig. 6 comprises electrodes 11, 12 which inject and collect charge which are symetrically contacting a charge transport layer 10, which is in contact with a sensitizer layer 14 and a capacitive layer 15, said sensitizer layer 14 being located at the interface between the capacitive layer 15 and the charge transport layer 10. Said electrodes 11, 12 can be fabricated from metal, conducting metal oxides, conducting semiconductor or conducting polymers.

Such a device having non mesoscopic junctions i.e. planar device structures, is illustrated in Fig 6. Therefore said material layers (capacitive, sensitizer and hole- transporter) comprise of flat, planar material layers.

The charge transport layer can be an organic charge transport layer 10, especially comprising a molecular, oligomeric or polymeric charge transport material. Preferably it comprises an organic hole or electron transporting material. Organic hole or electron transporting materials may be selected from acetylene, benzene, napthaline, indene, fluorene, phenantrene, anthracene, triphenylene, pyrene, pentalene, indene, azulene, heptalene, biphenylene, indacene, phenalene, acenaphtene, fluoranthene, pyridine, pyrimidine, pyridazine, perylene, quinolizidine, quinoline, isoquinoline, quinoxaline, phtalazine, naphthyridine, quinazoline, fullerene, phtalocyanine, cinnoline, pteridine, indolizine, indole, isoindole, carbazole, carboline, acridine, phenanthridine, 1,10- phenanthroline, thiophene, thianthrene, oxanthrene, vinylene, optionally substituted derivatives of these materials and combinations comprising two or more of these. Most of the listed organic charge transport materials exhibit both e- and h+ transport with comparable mobilities. Therefore, organic charge transport material can generally be used in an n-type or p-type mode depending upon which carrier is injected (see: Chua Nature 2005, Zaumseil Nat Matter 2006).

The capacitive layer 10 may comprise an organic, capacitive material. Preferably, the capacitive layer comprises materials selected from organic electron or hole storing materials (n-layer or p-layer). For example, the capacitive layer comprises materials selected from acetylene, benzol, napthaline, indene, fluorene, phenantrene, anthracene, triphenylene, pyrene, pentalene, indene, azulene, heptalene, biphenylene, indacene, phenalene, phthalocyanine, acenaphtene, fluoranthene, fullerene, pyrimidine, pyridazine, perylene, quinolizidine, quinoline, isoquinoline, quinoxaline, phtalazine, naphthyridine, quinazoline, cinnoline, pteridine, indolizine, indole, isoindole, carbazole, carboline, acridine, phenanthridine, 1,10-phenanthroline, thiophene, thianthrene, oxanthrene, vinylene, optionally substituted derivatives thereof, and combinations of two or more of these materials.

The capacitive layer 10 may comprise an inorganic, capacitive material. Preferably, the capacitive layer comprises materials selected from inorganic electron storing materials (n-layer), such as Si, TiO₂, SnO₂, Fe₂O₃, WO₃, ZnO, Nb₂O₅, CdS, CdSe, CdTe, ZnS, PbS, PbSe, Bi₂S₃, GaP, InP, GaAs, CuInS₂, and/or CuInSe₂ or from hole storing materials (p-layer), such as SiC, CdTe, CuI, CuSCN.

When the charge transporting layer 10 is an electron transporting layer (n-layer), the capacitive layer 15 is an p-layer, or, when the charge transporting layer 10 is an hole transporting layer (p-layer), the capacitive layer 15 is an n-layer. Preferably, the charge transporting layer is a p-layer and the capacitive layer is an n-layer. The device according to the invention allows to provide a method for estimating or studying charge carrier mobility in organic semiconducters and in particular hybrid and organic composites. Such a method then comprises the steps of:

Estimating the charge density in the device structure according to the invention as a function of incident light intensity;

Measuring the conductivity through the device structure according to the invention as a function of light intensity; and/or,

Estimating the charge mobility in the device structure according to the invention as a function of light intensity.

The device structure according to the invention can also be applied for analyzing materials for photo-voltaic devices, or suitable for light driven short-term memory applications, or having application in low cost large area medical imaging.

The device structure according to the invention can have the charge transport layer (i) or the capacitive layer (ii) acting as the sensitizer, said sensitizer thus not being separate from (i) or (ii), but being integrated in one of them. For example, light absorbing polymers, acting as capacitive layer 15 or charge transport layer 10 can at the same time absorb light and inject charges into the the capacitive 15 and/or charge transport layer 10. Therefore the wording has be chosen that said sensitizer layer 14 is located at the interface between the capacitive layer 15 and the charge transport layer 10, which also may encompass the situation of incorporating one or the other layer as intregal part.

Molecular sensitizers can be dyes, metal organic complexes, organic sensitizers, incorporating a surface attachment group such as carboxilic acid, or phosphonic acid, for example. Molecular metal complex sensitizers can be found in US 5,463,057. Polymeric sensitizers comprise sensitizer made of poly phenylene vinylene, poly thiothene, poly fluorene, poly acetylene, poly perylene, and derivatives thereof. Quantum dot sensitizer, for example PbS or CdSe nanoparticles as well as inorganic sensitizers are described in "Solid state heterojunction and solid state sensitized photovoltaic cell" US 2002/017656. A thin layer of a semicondutor, for example Si, CuO, CdTe or PbSe capable of injecting one charge carrier in the capacitve charge storage layer and the oppositely charged carrier in the charge transport layer, can also be used as sensitizer as well as ruthenium sensitizers. Finally it is clear to someone skilled in the art to use combinations comprising two or more selected from all of the above.

References The following references are entirely incorporated herein by reference:

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Claims

What we claim is:

1. A device structure comprising two electrodes (11, 12) provided in a predetermined distance (13) on a charge transport layer (10), wherein a capacitive layer (15) and a sensitizer layer (14) are provided on the charge transport layer (10) on a side opposite to the electrodes (11, 12), said sensitizer layer (14) being located at the interface between the capacitive layer (15) and the charge transport layer (10).

2. The device structure according to claim 1, wherein the capacitive layer (15) is made of an n-type material and the charge transport layer (10) is made of a p-type material, or vice versa, respectively.

3. The device structure according to claim 1 or 2, wherein the charge transport layer is an organic charge transport layer (10), especially comprising a molecular, oligomeric or polymeric charge transport material, more preferably comprising an organic hole or electron transporting material.

4. The device structure according to claim 1 or 2, wherein the charge transport layer is an inorganic charge transport layer (10), preferably comprising an inorganic hole transporting material, which may be selected from SiC, CdTe, CuI, CuSCN, or an inorganic electron transporting material, which may be selected from Si, TiO₂, SnO₂, Fe₂O₃, WO₃, ZnO, Nb₂O₅, CdS, CdSe, CdTe, ZnS, PbS, PbSe, Bi₂S₃, GaP, InP, GaAs, CuInS₂, and/or CuInSe₂, wherein in each case the charge transport layer (10) may comprise combinations of two or more respective materials.

5. The device structure according to one of claims 1 to 4, wherein the charge transport layer (10) is an electron transporting layer (n-layer) or a hole transporting layer (p-layer), preferably a hole transporting layer (p-layer).

6. The device structure according to one of claims 1 to 5, wherein the sensitizer layer (14) is made of a material selected from: molecular sensitizers, such as dyes, metal organic complexes, organic sensitizers, incorporating a surface attachment group such as carboxilic acid, or phosphonic acid, for example; polymeric sensitizers, such as poly phenylene vinylene, poly thiothene, poly fluorene, poly acetylene, poly perylene, and derivatives thereof; quantum dots, for example PbS or CdSe nanoparticles; a thin layer of a semicondutor, for example Si, CuO, CdTe or PbSe capable of injecting one charge carrier in the capacitve charge storage layer and the oppositely charged carrier in the charge transport layer; combinations comprising two or more selected from all of the above.

7. The device structure according to one of claims 1 to 6, wherein the capacitive layer (10) comprises an organic, capacitive material.

8. The device structure according to one of claims 1 to 6, wherein the capacitive layer (10) comprises an inorganic, capacitive material, which may be selected from materials selected from inorganic electron storing materials (n-layer), such as Si, TiO₂, SnO₂, Fe₂O₃, WO₃, ZnO, Nb₂O₅, CdS, CdSe, CdTe, ZnS, PbS, PbSe, Bi₂S₃, GaP, InP, GaAs, CuInS₂, and/or CuInSe₂ or from hole storing materials (p-layer), such as SiC, CdTe, CuI, CuSCN.

9. The device structure according to one of claims 1 to 8, wherein, when the charge transporting layer (10) is an electron transporting layer (n-layer), the capacitive layer

(15) is an p-layer, or, when the charge transporting layer (10) is an hole transporting layer (p-layer), the capacitive layer (15) is an n-layer.

10. The device structure according to one of claims 1 to 9, which is a hybrid photo- gated device structure.

11. The device structure according to one of claims 1 to 10, wherein said capacitive layer (15) is an inorganic capacitive layer comprising an inorganic oxide of mesoscopic morphology, preferably a wide band gap inorganic oxide.

12. The device structure according to one of claims 1 to 11, wherein said sensitizer layer (14) is provided to inject under illumination an electron or an electron charge in said capacitive layer (15), while the resulting positive charge is transferred to the charge transport layer (10).

13. The device structure according to one of claims 1 to 12, wherein said capacitive layer (15) comprises a glass slide (16) coated with a mesoporous film of TiO₂ nanoparticles.

14. The device structure according to claim 13, wherein said mesoporous film of TiO₂ is approximately 0.5 to 3 μm thick.

15. The device structure according to claim 13 or 14, wherein said film is coated with dye molecules to act as an optical sensitizer, and infiltrated with an organic charge transporter.

16. The device structure according to one of claims 1 to 15, wherein the electrodes (11, 12) upon the surface of the charge transport layer (10) are metal electrodes, separated by 3 - 500 μm, preferably 5-200 μm and most preferably 10 - 100 μm, defining a channel (13) of said width.

17. The device structure according to one of claims 1 to 16, wherein the charge transport layer (10) comprises an organic charge transport material, preferably 4,4' ' :4" ',4" "-ter-NΛ-diphenyl[4-(methoxymethyl)phenyl] amine (ter-DPMMPA).

18. The device structure according to one of claims 1 to 17, wherein a charge mobility and/or conductivity of the device, and in particular of the charge transport layer (10), is moderated or modulated, preferably increased, by light, preferably by visible, ultra-violet, and/or near infrared light.

19. The device structure according to claim 1 and/or 18, wherein a charge mobility and/or conductivity is moderated or modulated, preferably increased, by photo-injected carriers.

20. The device structure according to any one of claims 1 to 19, which is a detector device structure, in particular a photo-gated detector device structure.

21. A transistor, preferably a photo-gated transistor, comprising the device structure of any of claims 1 to 20.

22. A memory device, preferably a short-term memory device, comprising the device structure of any of claims 1 -20.

23. An imaging device comprising the device structure of any of claims 1 to 20.