HK1111519B - Organic photosensitive devices - Google Patents

Description

Organic photosensitive device

Technical Field

The present invention relates generally to organic photosensitive optoelectronic devices. And more particularly, to organic photosensitive optoelectronic devices having nanoparticles.

Background

Optoelectronic devices rely on the optical and electronic properties of materials to electronically generate or detect electromagnetic radiation or to generate electricity from ambient electromagnetic radiation.

Photosensitive optoelectronic devices convert electromagnetic radiation into electricity. Solar cells, also known as Photovoltaic (PV) devices, are one type of photosensitive optoelectronic device that is particularly used to generate electricity. PV devices that can generate electrical energy from light sources other than sunlight can be used to drive electrical consumers to provide, for example, lighting, heating, or to power electronic circuitry or devices such as calculators, radios, computers, or remote monitoring or communication equipment. These power generation applications also often involve the charging of batteries or other energy storage devices so that operation can continue when direct illumination from the sun or other light source is unavailable, or to balance the power output of the PV device with the requirements of the specific application. As used herein, the term "resistive load" refers to any power consuming or storing circuit, device, equipment, or system.

Another type of photosensitive optoelectronic device is a photoconductor cell. In this function, the signal detection circuitry monitors the resistance of the device to detect changes due to absorption of light.

Another type of photosensitive optoelectronic device is a photodetector. In operation, the photodetector is used in conjunction with a current detection circuit that measures the current generated when the photodetector is exposed to electromagnetic radiation and possibly with an applied bias voltage. The detection circuit described herein is capable of providing a bias voltage to the photodetector and measuring the electronic response of the photodetector to electromagnetic radiation.

These three classes of photosensitive optoelectronic devices can be characterized according to the presence or absence of a rectifying junction as defined below and according to whether the device operates using an applied voltage, also referred to as a bias voltage or bias voltage. The photoconductor cells do not have rectifying junctions and typically operate using a bias voltage. The PV device has at least one rectifying junction and operates without a bias voltage. The photodetector has at least one rectifying junction and typically, but not always, operates using a bias voltage. As a general rule, photovoltaic cells provide power to a circuit, device or equipment, but do not provide a signal or current to control the detection circuitry, or the output of information from the detection circuitry. Conversely, a photodetector or photoconductor provides a signal or current that controls the detection circuitry, or the output of information from the detection circuitry, but does not provide power to the circuitry, device or equipment.

Traditionally, photosensitive optoelectronic devices have been constructed from a number of inorganic semiconductors such as crystalline, polycrystalline and amorphous silicon, gallium arsenide, cadmium telluride, and the like. The term "semiconductor" herein denotes a material that can conduct electricity when charge carriers are induced by thermal or electromagnetic excitation. The term "photoconductive" generally refers to the process by which electromagnetic radiant energy is absorbed and thereby converted to excitation energy of charge carriers so that the carriers can conduct, i.e., transport, the charge in a material. The terms "photoconductor" and "photoconductive material" are used herein to refer to semiconductor materials that are selected for their property of absorbing electromagnetic radiation to generate charge carriers.

PV devices can be characterized by their efficiency in converting incident solar energy into useful electrical power. Devices utilizing crystalline or amorphous silicon dominate commercial applications, and some have achieved efficiencies of 23% or greater. However, efficient crystalline-based devices, particularly with large surface areas, are difficult and expensive to produce due to the problems inherent in producing large crystals without significant efficiency-reducing defects. On the other hand, high efficiency amorphous silicon devices still suffer from stability problems. Currently commercially available amorphous silicon cells have a stable efficiency of 4-8%. Recent efforts have focused on the use of organic photovoltaic cells to obtain acceptable photovoltaic conversion efficiency at economical production costs.

The PV device may be a standard illumination condition (i.e., a standard test condition, i.e., 1000W/m)²AM 1.5 spectral illumination) is optimized for the maximum product of photocurrent and photovoltage. The power conversion efficiency of such cells under standard illumination conditions depends on the following three parameters: (1) current at zero bias, i.e. short-circuit current I_SC(2) photovoltage under open circuit conditions, i.e. open circuit voltage V_OCAnd (3) duty cycle ^.

When PV devices are connected across a load and illuminated by light, they produce a photo-generated current. When illuminated under an infinite load, a PV device generates its maximum possible voltage, Vopen circuit voltage or Vopen circuit voltage_OC. When illuminated and its electrical contacts shorted, the PV device produces its maximum possible current, Ishort or Ishort_SC. When actually used to generate electricity, PV devices are connected to a finite resistive load and the power output is given by the product of current and voltage, I × V. The maximum total power generated by the PV device cannot per se exceed the product I_SC×V_OC. When the load value is optimized for maximum power extraction, the current and voltage each have a value of I_maxAnd V_max。

The quality factor of the PV device is the duty cycle, defined as:

ff＝{I_maxV_max}/{I_SCV_OC}(1)

where ff is always less than 1, since I_SCAnd V_OCIn actual use, the two are never obtained simultaneously. However, as ff approaches 1, the device has a smaller series or internal resistance, thereby biasing I to an optimum condition_SCAnd V_OCA larger percentage of the product of (a) is delivered to the load. P_incIs the power incident on the device, the power efficiency of the device eta_PCan be calculated from:

η_P＝ff*(I_SC*V_OCC)/P_inc

when electromagnetic radiation of appropriate energy is incident on a semiconducting organic material, such as an Organic Molecular Crystal (OMC) material or a polymer, a photon may be absorbed to produce an excited molecular state. This is symbolized by S₀+hv＝·So^*. Here So and So^＊Representing the ground state and the excited molecular state, respectively. This energy absorption is coupled with an electron from a binding state in the HOMO level, which may be a pi bond, to a possible pi^＊The increase in the LUMO level of a bond, or equivalently, the increase in holes from the LUMO level to the HOMO level, is correlated. In organic thin film photoconductors, productionIs generally considered to be an exciton, i.e., an electron-hole pair in a bound state that is transported as an quasi-particle. Excitons may have an appropriate lifetime before they recombine in pairs, which refers to the process by which an original electron and hole recombine with each other, as opposed to recombining with a hole or electron from another pair. To generate a photocurrent, electron-hole pairs typically become separated at the donor-acceptor interface between two different contacting organic thin films. If the charges are not separated, they can recombine radiatively, in a pair recombination process, also known as quenching, by the emission of light of lower energy than the incident light, or non-radiatively, by the generation of heat. Either of these efforts is undesirable in photosensitive optoelectronic devices.

The electric field or inhomogeneity at the contact may cause the exciton to quench, rather than dissociate at the donor-acceptor interface, resulting in no net contribution to current flow. Therefore, it is desirable to keep the photogenerated excitons away from the contacts. This has the effect of limiting the diffusion of excitons to regions near the junction so that the associated electric field has an increased chance of separating charge carriers released by the separation of excitons near the junction.

In order to generate an internally generated electric field occupying a basic volume, it is common practice to juxtapose two layers of material having appropriately selected conductive properties, in particular with respect to their molecular quantum energy state distribution. The interface of these two materials is known as a photovoltaic heterojunction. In conventional semiconductor theory, the materials forming the PV heterojunction are often denoted as n-or p-type. Where n-type means that the majority carrier type is electron. This can be viewed as a material having many electrons in relatively free energy states. p-type means that the majority carrier type is holes. This material has many holes in the relative free energy states. The background type, i.e. the non-photogenerated majority carrier concentration, depends mainly on the unintentional doping of defects or impurities. The type and concentration of impurities determine the gap between the Highest Occupied Molecular Orbital (HOMO) level and the Lowest Unoccupied Molecular Orbital (LUMO) level, referred to as the Fermi energy value or level within the HOMO-LUMO gap. The Fermi energy characterizes the statistical occupancy of molecular quantum energy states represented by energy values with an occupancy probability equal to a. The Fermi energy near the LUMO level indicates that electrons are the dominant carrier. Fermi energy near the HOMO level indicates that holes are the dominant carriers. Hence, Fermi energy is a major characterizing property of conventional semiconductors, and the prototype PV heterojunction is traditionally a p-n interface.

The term "rectifying" especially means that the interface has an asymmetric conductive property, i.e. the interface supports charge transport preferably in one direction. Rectification is typically associated with an internal electric field occurring at a heterojunction between appropriately selected materials.

As used herein and generally understood by those skilled in the art, a first "highest occupied molecular orbital" (HOMO) or "lowest unoccupied molecular orbital" (LUMO) energy level is "greater than" or "higher than" a second HOMO or LUMO energy level if the first energy level is closer to the vacuum energy level. Since the Ionization Potential (IP) is measured as negative energy relative to the vacuum level, a higher HOMO level corresponds to an EP with a smaller absolute value (a smaller negative EP). Similarly, a higher LUMO energy level corresponds to an Electron Affinity (EA) having a smaller absolute value (a smaller negative EA). On a conventional energy level diagram with the vacuum level at the top, the LUMO energy level of a material is higher than the HOMO energy level of the same material. The "higher" HOMO or LUMO energy level appears closer to the top of this figure than the "lower" HOMO or LUMO energy level.

In the context of organic materials, the terms "donor" and "acceptor" refer to the relative positions of the HOMO and LUMO energy levels of two contacting but different organic materials. This is in contrast to the use of these terms in the inorganic context, where "donor" and "acceptor" may refer to the types of dopants that may be used to create inorganic n-and p-type layers, respectively. In the organic range, a material is an acceptor if the LUMO level of the material in contact with another material is low. Otherwise it is a donor. In the absence of an external bias, this is energetically favorable for electrons at the donor-acceptor junction to move into the acceptor material, and holes to move into the donor material.

An important property in organic semiconductors is carrier mobility. Mobility measures how easily charge carriers can move through a conductive material in response to an electric field. In the context of organic photosensitive devices, a layer comprising a material that conducts preferentially by electrons due to high electron mobility may be referred to as an electron transport layer or ETL. A layer including a material that conducts preferentially by holes due to high hole mobility may be referred to as a hole transport layer or HTL. Preferably, but not necessarily, the acceptor material is an ETL and the donor material is an HTL.

Conventional inorganic semiconductor PV cells use p-n junctions to create internal fields. Early organic thin film units, such as reported by Tang, appl. Phys lett.48, 183(1986), contained heterojunctions similar to those used in conventional inorganic PV units. However, it should now be appreciated that in addition to the establishment of the p-n type junction, the energy level shift of the heterojunction plays an important role.

The energy level shift of the organic D-a heterojunction is considered important for the operation of organic PV devices due to the fundamental nature of the photogeneration process in organic materials. When the organic material is photoexcited, local Frenkel or charge transport excitons are generated. In order for electrical detection or current generation to occur, the bound excitons must dissociate into their constituent electrons and holes. This process can be initiated by an internal electric field, but is typically found in organic devices (F10)⁶V/cm) is low. The most efficient exciton dissociation in organic materials occurs at the donor-acceptor (D-a) interface. At such an interface, a donor material having a low ionization potential forms a heterojunction with an acceptor material having a high electron affinity. Depending on the alignment of the energy levels of the donor and acceptor materials, the separation of excitons may become energetically favorable at such interfaces, resulting in free electron polarons in the acceptor material and free hole polarons in the donor material.

Organic PV cells have many possible advantages when compared to conventional silicon-based devices. Organic PV cells are lightweight, save in material usage, and can be deposited on low cost substrates such as flexible plastic foils. However, some organic PV devices typically have relatively low external quantum efficiencies, on the order of 1% or less. This is believed to be due in part to the second order nature of the intrinsic photoconductive process. That is, carrier generation requires exciton generation, diffusion, and ionization or collection. There is an efficiency η associated with each of these processes. Subscripts may be used as follows: p is power efficiency, EXT is external quantum efficiency, a is photon absorption, ED is exciton diffusion, CC is charge collection, and ENT is internal quantum efficiency. Using this notation:

η_P～η_EXT＝η_A*η_ED*η_CC

η_EXT＝η_A*η_iNT

diffusion length (L) of excitons_D) Typically the length of optical absorptionMuch smallerThere is a trade-off between using a thick and thus resistive cell with multiple or highly corrugated interfaces and a thin cell with low optical absorption efficiency.

Typically, when light is absorbed to form excitons in an organic thin film, singlet excitons are formed. By the mechanism of intersystem crossing, a single exciton may decay into a triplet exciton. In this process, energy is lost, which results in lower efficiency of the device. The use of materials that generate triplet excitons would be desirable if there were no energy loss due to intersystem crossing, as triplet excitons typically have a longer lifetime and thus a longer diffusion length than singlet excitons.

By using an organometallic material in the photo-active voltage, the device of the present invention can effectively utilize triplet excitons. It is believed that the mono-triplet mixing may be so strong for the organometallic compound that the absorption involves excitation from the singlet ground state directly to the triplet excited state, eliminating the losses associated with the transition from the singlet excited state to the triplet excited state. The longer lifetime and diffusion length of triplet excitons, as compared to single excitons, may allow the use of thicker photoactive regions, since triplet excitons may diffuse a greater distance to reach the donor-acceptor heterojunction without sacrificing device efficiency.

Disclosure of Invention

The present invention relates generally to organic photosensitive optoelectronic devices. And more particularly, to organic photosensitive optoelectronic devices having a photoactive organic region containing encapsulated nanoparticles exhibiting plasmon resonances. The enhancement of the incident optical field is achieved via surface plasmon polariton resonance. This enhancement increases the absorption of incident light, resulting in a more efficient device.

Drawings

Fig. 1 shows an organic PV device.

Fig. 2 shows a schematic and transmission electron micrograph of a cross section of a cascaded organic photovoltaic cell.

FIG. 3 shows the real number (. epsilon.) of Ag calculated as a function of photon energy₁) And an imaginary number (. epsilon.)₂) Dielectric function.

FIG. 4 shows the dielectric function ε as embedding medium_mSimulated Surface Plasmon Polariton (SPP) resonance wavelength of 5nm spherical Ag particles.

Fig. 5 shows the axial ratio of simulated SPP resonance wavelength versus Ag particles in vacuum.

Fig. 6 shows the absorbance spectra of 1nm Ag deposited on a quartz substrate (point curve), 7nm CuPc (short-dashed curve) and 7nm CuPc thin film on 1nm Ag (solid curve).

Fig. 7 shows the calculated intensity enhancement (I/I) of a series of Ag particles with a diameter 2R of 5nm and a center-to-center distance d of 10nm at λ 690nm (I/I)₀) Is/are as followsA contour map.

FIG. 8 shows the average calculated intensity enhancement (I/I) on the surface of 5nm diameter Ag particles as a function of wavelength for different embedding media₀)。

FIG. 9 shows simulated absorption (dashed line) and average intensity enhancement (I/I) on the surface of 5nm diameter spherical and ellipsoidal particles (axial ratio of 0.5)₀) (solid line).

FIG. 10 shows (a) maximum calculated intensity enhancement (I/I) at the center of the ID chain of microparticles₀) Comparing delta; and (b) simulated Surface Plasmon Polariton (SPP) peak wavelength as a function of the surface spacing S of the ID chains of 5nm diameter spherical (solid line) and elliptical particles (dashed line).

FIG. 11 shows the calculated intensity enhancement (I/I) at the axis of the ID chain of microparticles embedded in n 2+0.5I media₀) The wavelength is compared.

Fig. 12 shows the measured absorbance a for varying thicknesses of CuPc with (triangular) and without (square) 10A Ag cluster layers on quartz at wavelength λ 690 nm. The fit of the data (solid curve) is described in the text.

Fig. 13 shows the measured difference in absorbance (AA) versus CuPc thickness t for CuPc films with and without Ag layers.

Fig. 14 shows the effective reinforcement lengths calculated for ID chains of 5nm diameter spherical (solid line) and elliptical (axial ratio 0.5) particles (dashed line) embedded in n-2 +0.5i dielectric as a function of the surface spacing of the particles in the chain.

FIG. 15 shows the calculated external quantum efficiency (. eta.) of CuPc/PTCBI tandem PV cells in the presence and absence of Ag clusters (a) and (b)_EQE) Spectrum of light.

Detailed Description

An organic photosensitive optoelectronic device is provided. The organic devices of embodiments of the present invention may be used, for example, to generate usable current from incident electromagnetic radiation (e.g., PV devices) or may be used to detect incident electromagnetic radiation. Embodiments of the invention may include an anode, a cathode, and a photoactive region between the anode and the cathode. The photoactive region is a portion of a photosensitive device that absorbs electromagnetic radiation to generate excitons that may dissociate in order to generate an electrical current. The organic photosensitive optoelectronic device may also include at least one transparent electrode to allow incident radiation to be absorbed by the device. Several PV device materials and configurations are described in U.S. patent nos. 6,657,378, 6,580,027 and 6,352,777, the entire contents of which are incorporated herein by reference.

FIG. 1 shows an organic photosensitive optoelectronic device 100. The figures are not necessarily to scale. Device 100 may include a substrate 110, an anode 115, an anode smoothing layer 120, a donor layer 125, an acceptor layer 130, a barrier layer 135, and a cathode 140. Cathode 140 may be a composite cathode having a first conductive layer and a second conductive layer. The device 100 may be fabricated by depositing the described layers in sequence. Charge separation may occur primarily at the organic heterojunction between the donor layer 125 and the acceptor layer 130. The internal potential at the heterojunction is determined by the HOMO-LUMO energy level difference between the two materials that are in contact to form the heterojunction. The HOMO-LUMO gap offset between the donor and acceptor materials creates an electric field at the donor/acceptor interface that facilitates charge separation, so that excitons are generated within the exciton diffusion length at the interface.

The particular arrangement of layers illustrated in fig. 1 is merely exemplary and not intended to be limiting. For example, some layers (e.g., barrier layers) may be omitted. Additional layers (e.g., reflective layers or additional acceptor and donor layers) may be added. The order of the layers may be changed. Schemes other than those specifically described may be used.

The substrate may be any suitable substrate that provides the desired structural properties. The substrate may be flexible or rigid, planar or non-planar. The substrate may be transparent, translucent, or opaque. Plastic and glass are examples of preferred rigid substrate materials. Plastic and metal foils are examples of preferred flexible substrate materials. The material and thickness of the substrate may be selected to obtain desired structural and optical properties.

U.S. Pat. No. 6,352,777, incorporated herein by reference, provides examples of electrodes or contacts that may be used in photosensitive optoelectronic devices. As used herein, the terms "electrode" and "contact" refer to a layer that provides a medium for carrying photogenerated current to an external circuit or for providing a bias to the device. That is, the electrodes or contacts provide an interface between the active region of the organic photosensitive optoelectronic device and a wire, lead, trace or other means of transporting charge carriers to or from an external circuit. In photosensitive optoelectronic devices, it is desirable to allow a maximum amount of ambient electromagnetic radiation from outside the device into the photoconductively activated interior region. That is, the electromagnetic radiation must reach the photoconductive layer where it can be converted to electricity by photoconductive absorption. This often requires that at least one of the electrical contacts should minimally absorb and minimally reflect incident electromagnetic radiation. That is, such contacts should be substantially transparent. The counter electrode may be a reflective material such that light that has passed through the cell without being absorbed is reflected back through the cell. As used herein, a layer of material or a series of layers of different materials is referred to as "transparent" when one or more of the layers allows at least 50% of ambient electromagnetic radiation within the wavelength of interest to be transmitted through the one or more layers. Similarly, a layer that allows transmission of some, but less than 50%, of the ambient electromagnetic radiation within the relevant wavelengths is referred to as "semi-transparent".

As used herein, "top" means furthest from the substrate, and "bottom" means closest to the substrate. For example, for a device with two electrodes, the bottom electrode is the electrode closest to the substrate, and is typically the first electrode fabricated. The bottom electrode has two surfaces, a bottom surface closest to the substrate and a top surface further from the substrate. Where a first layer is described as being "deposited on" a second layer, the first layer is deposited farther away from the substrate. There may be other layers between the first and second layers unless it is specified that the first layer is "in physical contact with" the second layer. For example, a cathode may be described as "deposited over" an anode, even though various organic layers are present therebetween.

The electrodes preferably comprise a metal or "metal substitute". The term "metal" is used herein to include materials composed of elemental pure metals, such as Mg, as well as metal alloys, i.e., materials composed of two or more elemental pure metals, such as Mg and Ag together, denoted as Mg: Ag. Here, the term "metal substitute" refers to a material that is not a metal within the standard definition, but has metal-like properties that are desirable in certain suitable applications. Commonly used metal alternatives for the electrodes and charge transport layers will include doped wide bandgap semiconductors such as transparent conducting oxides, for example Indium Tin Oxide (ITO), Gallium Indium Tin Oxide (GITO) and Zinc Indium Tin Oxide (ZITO). In particular, ITO is a highly doped degenerate n + semiconductor with an optical bandgap of about 3.2eV such that it is suitable for use with semiconductor devices having optical bandgaps greater than about 3.2eVIs transparent. Another suitable metal substitute is the transparent conductive polymer Polyaniline (PANI) and its chemical derivatives. The metal substitute can be further selected from a wide range of non-metallic materials, wherein the term "non-metallic" is intended to include a wide range of materials, so long as the material does not contain metal in its un-compounded form. When a metal is present in its unbound form, alone or in combination with one or more other metals as an alloy, the metal may alternatively be referred to as being present in its metallic form or as "free metal". Thus, the metal substitute electrode of the present invention can sometimes be referred to as "metal-free," where the term "metal-free" is expressly intended to include materials that do not contain metal in their unbound form. Free metals typically have the form of metallic bonds throughout the metal lattice created by a large number of valence electrons that are able to move freely in the electron conduction band. While metal substitutes may contain a metal component, they are "non-metallic" in several primary components. They are not pure free metals, nor are they alloys of free metals. When metals are present in their metallic form, the electron conduction band tends to provide, among other properties of the metal, high electrical conductivity and high reflectivity of optical radiation.

Embodiments of the present invention may include a highly transparent, non-metallic, low resistance cathode, such as disclosed in U.S. Pat. No. 6,420,031 to Parthasarathy et al ("Parthasarathy '031"), or a high efficiency, low resistance metal/non-metallic composite cathode, such as disclosed in U.S. Pat. No. 5,703,436 to Forrest et al ("Forrest' 436"), both of which are incorporated herein by reference in their entirety, as one or more of the transparent electrodes of a photosensitive optoelectronic device. Each type of cathode is preferably prepared in a manufacturing process that includes the step of sputter depositing an ITO layer onto an organic material, such as copper phthalocyanine (CuPc), to form a highly transparent, non-metallic, low resistance cathode, or onto a thin Mg: Ag layer to form a high efficiency, low resistance metal/non-metallic composite cathode.

Here, the term "cathode" is used in the following manner. Electrons move from the photoconductive material to the cathode in a non-stacked PV device or in a single unit of a stacked PV device, such as a PV device, under ambient illumination and connected to a resistive load and without an applied voltage. Similarly, the term "anode" is used herein such that in a PV device under illumination, holes move from the photoconductive material to the anode, which is equivalent to electrons moving in the opposite manner. It should be noted that the anode and cathode may be electrodes or charge transport layers, as the term is used herein.

An organic photosensitive device will include at least one photoactive region where light is absorbed to form an excited state, or "exciton" which may subsequently separate into an electron and a hole. The separation of excitons will typically occur at a heterojunction formed by the juxtaposition of an acceptor layer and a donor layer. For example, in the device of fig. 1, a "photoactive region" may include a donor layer 125 and an acceptor layer 130.

Acceptor materials may include, for example, perylene, naphthalene, fullerene, or nanotubes. An example of an acceptor material is 3,4, 9, 10-perylenetetracarboxylic dibenzoimidazole (PTCBI). Alternatively, the acceptor layer may comprise a fullerene material, as described in U.S. Pat. No. 6,580,027, which is incorporated herein by reference in its entirety. Adjacent to the acceptor layer is a layer of organic donor-type material. The boundaries of the acceptor and donor layers form a heterojunction that may generate an internally generated electric field. The material of the donor layer may be phthalocyanine or porphyrin, or derivatives or transition metal complexes thereof, such as copper phthalocyanine (CuPc). Other suitable acceptor and donor materials may be used.

In a preferred embodiment of the present invention, the stacked organic layers include one or more Exciton Blocking Layers (EBLs), as described in U.S. Pat. No. 6,097,147 to Peumans et al, applied Physics Letters 2000, 76, 2650-52, and co-pending application Ser. No. 09/449,801 filed 11/26 1999, both of which are incorporated herein by reference. Higher internal and external quantum efficiencies have been achieved by including EBLs to confine photogenerated excitons to regions near the separation interface and to prevent quenching of parasitic excitons at the photosensitive organic/electrode interface. In addition to limiting the volume in which excitons may diffuse, EBLs can also serve as diffusion barriers for species introduced during electrode deposition. In some cases, the EBL may be made thick enough to fill pinholes or shorting defects that might otherwise cause the organic PV device to fail. The EBL can help protect fragile organic layers from damage that occurs when electrodes are deposited onto the organic material.

It is believed that EBLs derive their exciton-blocking properties from neighboring organic semiconductors whose LUMO-HOMO gap is substantially larger than the exciton is blocked. Therefore, the presence of confined excitons in the EBL is prohibited due to energy considerations. While it is desirable for the EBL to block excitons, it is undesirable for the EBL to block all charges. However, EBLs may block charge carriers of one sign because of the nature of adjacent energy levels. By design, an EBL will exist between two other layers, typically an organic photosensitive semiconductor layer and an electrode or charge transport layer. The adjacent electrode or charge transport layer will be a cathode or an anode in the context. Thus, the material of the EBL at a given location in the device is chosen such that carriers of the desired sign will not be hindered in transport to the electrodes or charge transport layers. Proper energy level alignment ensures that no potential barrier to charge transport exists, preventing an increase in series resistance. For example, it is desirable for the material used as the cathode side EBL to have a LUMO energy level that closely matches the LUMO energy level of the adjacent ETL material so that any undesirable barriers to electrons are minimized.

It is understood that the exciton blocking properties of a material are not intrinsic to its HOMO-LUMO energy gap. Whether a given material will function as an exciton blocker depends on the relative HOMO and LUMO energy levels of the adjacent organic photoactive materials. Therefore, it is not possible to identify a class of compounds as exciton blockers in isolation regardless of the device environment in which they may be used. However, using the teachings herein, one skilled in the art can identify whether a given material will function as an exciton blocking layer when used with a selected set of materials to construct an organic PV device.

In a preferred embodiment of the invention, the EBL is located between the acceptor layer and the cathode. Preferred materials for the EBL include 2, 9-dimethyl-4, 7-diphenyl-1, 10-phenanthroline (also known as bathocuproine or BCP), or bis (2-methyl-8-hydroxyquinoline) -aluminum (III) phenol (Alq), believed to have a LUMO-HOMO level separation of about 3.5eV₂OPH). BCP is an effective exciton blocker that can readily transport electrons from the acceptor layer to the cathode.

The EBL layer may be doped with a suitable dopant including, but not limited to, 3,4, 9, 10-perylenetetracarboxylic dianhydride (PTCDA), 3,4, 9, 10-perylenetetracarboxylic diimide (PTCDI), 3,4, 9, 10-perylenetetracarboxylic bisbenzimidazole (PTCBI), 1, 4, 5, 8-naphthalene tetracarboxylic dianhydride (NTCDA), and derivatives thereof. The BCP deposited in the present device should be considered amorphous. It is evident that the amorphous BCP exciton blocking layer can exhibit thin film recrystallization, which is particularly fast at high light intensities. The resulting morphological changes of the polycrystalline material result in a lower quality film with possible defects such as shorts, voids or electrode material intrusion. Thus, it has been found that the EBL structure can be stabilized against the morphological changes of reduced performance by doping some EBL materials, such as BCP, exhibiting this effect with appropriate, relatively large and stable molecules. It will also be appreciated that doping the EBL as a transport electron in a given device with a material having a LUMO energy level close to the EBL will help ensure that electron traps are not formed which could create space charge build-up and reduce performance. In addition, it is understood that the relatively low doping density minimizes exciton generation at isolated doping sites. This absorption reduces the device photoconversion efficiency because such excitons are effectively prevented from diffusing by the surrounding EBL material.

Exemplary embodiments may also include a transparent charge transport layer or charge recombination layer. As described herein, charge transport layers differ from acceptor and donor layers in that charge transport layers are often, but not necessarily, inorganic (often metallic) and may be selected to be not photoconductively active. The term "charge transport layer" is used herein to refer to a layer similar to, but different from, an electrode, except that the charge transport layer transports charge carriers from only one subsection of the optoelectronic device to an adjacent subsection. The term "charge recombination layer" is used herein to refer to a layer similar to but different from an electrode, except that the charge recombination layer allows recombination of electrons and holes between cascaded photosensitive devices, and may also enhance the internal optical field strength near one or more active layers. The charge recombination layer may be composed of semitransparent metal nanoclusters, nanoparticles, or nanorods, as described in U.S. patent No. 6,657,378, which is incorporated herein by reference in its entirety.

In another preferred embodiment of the invention, an anode smoothing layer is located between the anode and the donor layer. Preferred materials for this layer include 3, 4-polyethylene dioxythiophene: poly (p-phenylethenesulfonic acid) (PEDOT: PSS) film. Between the anode (ITO) and the donor layer (CuPc) PEDOT is introduced: the PSS layer may result in greatly improved manufacturing yield. This is due to the spin-coated PEDOT: the ability of PSS films to planarize ITO, otherwise its rough surface may cause short circuits through the thin molecular layer.

In another embodiment of the invention, one or more layers may be treated with a plasma prior to depositing the next layer. For example, the layer may be treated with a mild argon or oxygen plasma. This process is beneficial because it reduces the series resistance. PEDOT was allowed to: it is particularly advantageous that the PSS layer is subjected to a mild plasma treatment.

The simple layered structure illustrated in fig. 1 is provided as a non-limiting example, and it should be understood that embodiments of the present invention may be used in conjunction with a wide variety of other structures. The specific materials and structures described are exemplary in nature, and other materials and structures may be used. Functional devices may be implemented by combining various layers described in different ways, or layers may be omitted entirely based on design, performance, and cost factors. Other layers not specifically described may also be included. Materials other than those specifically described may be used. While many of the examples provided herein describe various layers as comprising a single material, it should be understood that combinations of materials may be used, such as mixtures of a host and a dopant, or more generally mixtures. Also, the layer may have various sub-layers. The names given to the various layers herein are not intended to be strictly limiting. Organic layers that are not part of the photoactive region, i.e., organic layers that do not generally absorb photons that contribute significantly to photocurrent, may be referred to as "non-photoactive layers". Examples of the non-photoactive layer include an EBL and an anode smoothing layer. Other types of non-photoactive layers may also be used.

Preferred organic materials for use in the photoactive layer of the photosensitive device include cyclic organometallic metal compounds. The term "organometallic" as used herein is as generally understood by those skilled in the art and as given, for example, by gary.l. miesler and Donald a in "inorganic chemistry" (second edition), Tarr, prence Hall (1998). Thus, the term organometallic refers to a compound having an organic functional group bonded to a metal through a carbon-metal bond. This class does not include coordination compounds per se, which are substances having only donor bonds from heteroatoms, such as metal complexes of amines, halides, pseudohalides (CN, etc.), and the like. In practice, the organometallic compound typically includes one or more donor bonds from the heteroatom, in addition to one or more carbon-metal bonds to the organic species. Carbon-metal bonds to organic species refer to direct bonds between the metal and carbon atoms of organic functional groups such as phenyl, alkyl, alkenyl, etc., but do not refer to metal bonds to "inorganic carbons," such as the carbons of CN or CO. The term cyclic metal refers to a compound that includes a bidentate organometallic ligand such that, when bound to a metal, a ring structure is formed that includes the metal as one of the ring elements.

The organic layer may be fabricated using vacuum deposition, spin coating, organic vapor deposition, ink jet printing, and other methods known in the art.

The organic photosensitive optoelectronic devices of embodiments of the present invention may be used as PVs, photodetectors, or photoconductors. Whenever the organic photosensitive optoelectronic devices of the present invention are used as PV devices, the materials used in the photoconductive organic layers and their thicknesses may be selected, for example, to optimize the external quantum efficiency of the device. Whenever the organic photosensitive optoelectronic devices of the present invention are used as photodetectors or photoconductors, the materials used in the photoconductive organic layers and their thicknesses may be selected, for example, to maximize the sensitivity of the device to the desired spectral region.

This result can be achieved by considering several criteria that may be used in the selection of layer thicknesses. Exciton diffusion length L_DGreater than or comparable to the layer thickness L is desirable because it is believed that most exciton dissociation will occur at the interface. If L is_DLess than L, many excitons may recombine before disassociating. In addition, it is desirable that the total photoconductive layer thickness be of the order of 1/α (where α is the absorption coefficient) of the electromagnetic radiation absorption length, such that nearly all radiation incident on the PV device is absorbed to generate excitons. Furthermore, the photoconductive layer thickness should be as thin as possible to avoid excessive series resistance due to the high bulk resistivity of the organic semiconductor.

Thus, these competing criteria essentially require tradeoffs in selecting the thickness of the photoconductive organic layer of the photosensitive optoelectronic cell. Thus, on the one hand, a thickness comparable to or larger than the absorption length is desirable (for a single cell device) in order to absorb the maximum amount of incident radiation. On the other hand, as the photoconductive layer thickness increases, two undesirable effects increase. One is that because of the high series resistance of the organic semiconductor, increasing the thickness of the organic layer increases the device resistance and reduces efficiency. Another undesirable effect is that increasing the photoconductive layer thickness increases the likelihood that excitons will be generated away from the effective field at the charge separation interface, resulting in an increased likelihood of pair-wise recombination, and again reducing efficiency. Therefore, a device configuration that balances between these competing effects in a manner that yields high external quantum efficiency for the entire device is desirable.

The organic photosensitive optoelectronic devices of the present invention can be used as photodetectors. In this embodiment, the device may be a multilayer organic device, such as described in U.S. application serial No. 10/723,953 filed on 26.11.2003, which is incorporated herein by reference in its entirety. In this case, an external electric field may be generally applied to facilitate extraction of the separated charges.

A concentrator or trap configuration may be used to increase the efficiency of the organic photosensitive optoelectronic device, in which photons are forced to pass through a thin absorbing region multiple times. U.S. Pat. Nos. 6,333,458 and 6,440,769, the entire contents of which are incorporated herein by reference, address this problem by using structural designs that optimize the optical geometry for high absorption and use with optical concentrators that increase collection efficiency to enhance the light conversion efficiency of photosensitive optoelectronic devices. This geometry of the photosensitive device substantially increases the optical path through the material by trapping incident radiation within a reflective cavity or waveguide structure, thereby recycling light by multiple reflections through the photoresponsive material. The geometries disclosed in U.S. Pat. nos. 6,333,458 and 6,440,769 therefore enhance the external quantum efficiency of the device without causing a substantial increase in bulk resistance. Included in the geometry of such a device is a first reflective layer; a transparent insulating layer that should be longer in all dimensions than the optical coherence length of the incident light to prevent optical microcavity interference effects; a transparent first electrode layer adjacent to the transparent insulating layer; a photosensitive heterostructure adjacent to the transparent electrode; and a second electrode that is also reflective.

The coating may be used to concentrate the light energy in a desired area of the device. U.S. patent application No. 10/857,747, the entire contents of which are incorporated herein by reference, provides an example of such a coating.

In tandem bilayer solar cells, each subcell can be thin enough to allow a large percentage of exciton dissociation, while the device is thick enough to achieve high absorption efficiency. Fig. 2 shows a schematic diagram 200 of a cross-section of a cascaded organic PV cell and a high resolution transmission electron micrograph 290. The two cells 210 and 220 are contacted by an Indium Tin Oxide (ITO) anode 230 and an Ag cathode 240, and separated by a layer of Ag nanoparticles 250. As used herein, the term "nanoparticle" refers to a particle located within and/or between organic layers of an organic device. The preferred nanoparticle size is aboutOr smaller, although the nanoparticles may be encapsulated in other materials that may increase this size. Marking the enhancement distance and diffusion length L of the donor (D) and acceptor (A) layers of each device_D ^DAnd L_D ^A. Ag clusters are visible in the micrograph and are shown in the schematic (filled circles). The schematic shows a representation of the current generation in a cascaded cell. When light is absorbed, excitons are formed in photovoltaic subcells 210 and 220. After separation at DA interfaces 270 and 280, holes in PV subcell 210 and electrons in PV subcell 220 are collected at adjacent electrodes 230 and 240. To prevent the build-up of charge within the cell, electrons in PV subcell 210 and holes in PV subcell 220 diffuse to metal nanoparticle layer 250 where they recombine. The attraction of the initial charge to the nanoparticles is primarily a result of the image charge effect. Once the metal particles are individually charged, coulomb attraction of the free counter charge results in rapid recombination at the Ag surface 250.

This series cascaded cell structure is advantageous because it results in an open circuit voltage V compared to the single double layer cell case_OCIs increased. Suppose V_P＝I_SCV_CCFF/P_inc(wherein I)_SCIs short circuit current density, FF is duty cycle, and P_incIs the incident optical power density) provided that other parameters remain unchanged, this may be doneResult in eta_PIs increased. Thus, the challenge in implementing a cascaded cell is to balance the photocurrent from each cell, since the current in the device is limited by the smaller of the two currents generated in PV subcell 210 or PV subcell 220. This can be achieved by varying the thickness or material composition of the various device layers, but is complicated by optical interference effects. The serial concatenation unit may also include a plurality of sub-units, including more than two sub-units, electrically connected, wherein each sub-unit includes an acceptor layer and a donor layer. Other schemes of subunits may be used as will be apparent to those skilled in the art.

In addition to acting as an effective carrier recombination layer to prevent cell charging, the nanoparticles can also enhance the incident electric field, which in turn can increase absorption in nearby organic thin films. The shaded area 260 in the diagram of fig. 2 represents the region where the electric field is affected by the Ag nanoparticles 250. The electric field enhancement is generated by Surface Plasmon Polariton (SPP) resonance that is optically excited on the surface of the nanoparticles. As used herein, and as generally understood by those skilled in the art, "surface plasmon polariton resonance" refers to the coupling of plasmon oscillations of incident photons to the surface of a particle, where "plasmon oscillations" refers to the collective excitation of conducting electrons in the particle. SPP resonance arises from the displacement of negatively conducting electrons from a positively charged background due to an applied electric field. This creates a polarized charge on the surface of the nanoparticles that results in a restoring force and thus a resonant eigenfrequency. This property of metal nanoparticles can also be applied to Schottky and dye-sensitized PV cells, where the photoactive region is in contact with the nanoparticle layer.

The SPP resonance location of the nanoparticles or the aggregation of the nanoparticles may be affected by irregular particle shapes, different embedded dielectric and substrate effects, and inter-particle coupling. Taking advantage of these various effects, the resonance of a nanoparticle or nanoparticle array can be tuned to wavelengths within the visible and infrared spectrum.

Because SPP resonance enhances the local electromagnetic field, the nanoparticles do not need to be in direct contact with the photoactive region to achieve the benefits of SPP resonance. In one embodiment of the invention, the encapsulated nanoparticles are dispersed within an active organic region located between two electrodes. The nanoparticles may be randomly or uniformly distributed throughout the region. Other arrangements of nanoparticles are also possible and may be advantageous for specific applications. In a preferred embodiment of the invention, the photoactive region comprises one or more PV cells. In this embodiment, the encapsulated nanoparticles may be located in a planar layer between adjacent PV cells. The photoactive region may comprise other suitable organic materials, including dye sensitising materials. Dispersing the nanoparticles in the photoactive region enhances the electric field incident on the surrounding area due to the resonance of the SPPs on the particle surface. The nanoparticles preferably comprise a metal, with Ag, Cu and Au being particularly preferred. The use of these materials provides SPP resonances that result in enhanced absorption of visible wavelengths. The nanoparticles may also include doped degenerate semiconductors or other semiconductor materials.

The resonant wavelength occurs when the following expression is minimized:

[ε₁(ω)+2ε_m(ω)]²+ε₂constant (ω)

Wherein epsilon₁(omega) and ε₂(omega) for metals, and ε_m(ω) for embedded media. This can be simplified to:

ε₁(ω)＝-2ε_m(ω)

let ε be₂(ω) or d ε₂The/d ω is small, which is typically true for e.g. Ag in the resonance range of 3.0-3.5 eV. Fig. 3 shows the real 310 and imaginary 320 dielectric functions of Ag as photon energy. The bulk Ag is shown as a solid line and also shows Ag clusters of 10nm (dashed line) and 5nm (dotted line) diameter. Fig. 4 shows the effect of an embedded media of 2R ═ 5nm Ag nanoparticles on SPP resonance, where the variation of the dielectric function has been taken into account. The dashed line indicates the resonance wavelength of the fine particles having an axial ratio b/a of 0.6. The insert shows the simulated geometry.

The shape of the nanoparticles may particularly influence the SPAnother factor of P resonance. For example, for an ellipsoidal nanoparticle, the SPP may be split into two modes, one corresponding to the major axis a of the ellipsoid and the other corresponding to the minor axis b. In fig. 5, the SPP peak position of the elliptical nanoparticles in vacuum is shown. As used herein, the term "axial ratio" refers to the ratio of the shortest axis to the longest axis, i.e., b/a. For small values of axial ratio, the wavelength spacing between two formants reaches a value of 300nm, and for b/a 1, the SPP position corresponds to λ in vacuum_PPosition of the spherical nanoparticles at 338 nm. For example, the dashed line in FIG. 5 shows that an axial ratio of 0.6 results in λ_b334 and λ_aSPP mode at 360 nm. This division of the dipole oscillation mode can be generalized to the case of any non-spherical particle shape due to the resulting distribution of charges of the asymmetric nanoparticles. In a preferred embodiment of the invention, the nanoparticles have a particle size of no greater than aboutAnd an axial ratio of not less than about 0.1. For more spherical particles (i.e., those having an axial ratio of about 1), it is preferred that the average surface spacing not be more than about. Larger particle sizes and/or smaller average spacing reduce the amount of organic material available for absorption, which may reduce the enhancement of the incident optical field due to SPP resonance. However, in some instances, other dimensions than those specifically described may be used. Further preferably, the nanoparticles are non-spherical and the longest axis lies parallel to the interface. It is believed that this approach increases the enhancement of the incident optical field generated by the dipole interaction and SPP resonance of the nanoparticles. For non-spherical particles (those with an axial ratio less than 1), inter-particle coupling may have less effect on local field enhancement. Thus, preferably, the average surface spacing of the nanoparticles is no more than about. Other arrangements and arrangementsSpacers may be used for some purposes. In some cases, the encapsulated nanoparticles can include a significant percentage of the active region volume.

For PV unit applications, it is advantageous to introduce field enhancement over the full range of the solar spectrum covering the absorption spectrum of the photoactive material. The spectral dependence of the absorbance will now be discussed.

Fig. 6 shows typical measured absorption spectra of three thin films on quartz with and without nanoparticles. The nanoparticles in a 1nm thick Ag layer have an average diameter of about 2R-5 nm, and a center-to-center distance of about d-10 nm.Curve 610 of the thick Ag island film has a wavelength of λ due to surface plasmon activation of the nanoparticles_PA wavelength of 440nm as the central 100nm peak (full width at half maximum). The peak position and intensity represent the distribution of particle shapes and sizes, as well as the dipole coupling between nanoparticles that broaden the optical response with decreasing particle spacing. The absorption of a 7nm thick film of CuPc (curve 620) and a 7nm film of CuPc deposited on a 1nm blanket of Ag island film (curve 630) is also shown. The plasmon peak of the Ag nanoparticle layer is red-shifted by 30nm to λ due to the presence of the surrounding CuPc dielectric_P470nm, although λ_cThe position of the CuPc peaks at 625nm and 690nm is unchanged. However, the most significant feature is the increase in CuPc absorption at wavelengths λ > 470 nm. This broadband, non-resonant enhancement can result in an approximately 15% increase in efficiency of the cascaded PV cells beyond this expectation simply by combining the efficiencies of several stacked CuPc/PTCBI bilayers.

Enhancement possible at the surface plasmon frequency ω_PThe following occurs. At omega_PIn the following, the collection of randomly distributed nanoparticles may create "hot spots" in the electric field due to the dipole-to-dipole interactions between the particles, whereas the absorption of the nanoparticle film is due to the dipole plasmon modes formed on the particle surface.

Fig. 7 shows a typical field distribution for a planar array of Ag cylinders with a diameter 2R of 5nm and a uniform surface spacing d of 5nm on a quartz substrate surrounded by a CuPc dielectric. The particles are located on a quartz substrate (n 1.46, z 0) and embedded in a dielectric (CuPc). The contour marks represent the calculated intensity enhancement and are spaced 0.5 apart. The polarization vector is represented by an arrow and propagates in the + z direction. The field distribution is for an excitation wavelength of λ 690nm and the polarization is parallel to the nanoparticle chains. The contour lines represent the intensity enhancement (I/I) of the electric field₀) Wherein I is the local field strength, and I₀Is the incident field strength. These intensities are respectively associated with | E²And | E₀|²Proportional, where E is the local field amplitude, and E₀Is the incident field amplitude. Twelve-fold strength enhancement can be found in the gaps of the cylinders. The dipole behavior of the field strength is significant, with field decay appearing in the "shadow" of the sphere.

The effect of the embedding medium on the location of SPP resonances and the enhanced spectral bandwidth is particularly important for solar cell applications where enhancing a broad range of wavelengths is a concern. Fig. 8 shows the intensity enhancement of the incident field that is concentrated on the surface of a single 2R ═ 5nm spherical particle. The resonance peak is red-shifted as a result of the increased dielectric constant of the embedding medium. As n increases from 1 to 2, the resonance peak becomes stronger, and the range of the enhancement step on the long wavelength side of the SPP peak decreases. The particles are embedded in a material with n 2+0.5i, which strongly absorbs typical values of organic thin films, so that the dipole SPP peak is suppressed by the amplitude level compared to the non-absorbing dielectric.

Fig. 9 shows the spectra of spherical (2R-5 nm) nanoparticles embedded in a dielectric with n-2 +0.5i and b/a-0.5 elliptical nanoparticles with an area equal to that of the spherical nanoparticles. The two particles have the same area and are embedded in a dielectric with n-2 +0.5 i. The absorption (dotted line) peak of the ellipsoidal nanoparticles is located at-470 nm and is red-shifted from the peak of the spherical nanoparticles at 392 nm. The polarization of the incident light is parallel to the long axis of the elliptical particles, thus exciting the mode. The elliptical shaped microparticles have red-shifted enhancement tails that extend beyond the absorption of most organic PV materials, making this shape of the microparticles better suited for use in organic PV cells.

The charge recombination layer in the cascaded organic PV cell may include a thermally evaporated, random array of nanoparticles of various sizes, shapes and spacings. Fig. 10 shows the intensity enhancement at the center of the array of spherical Ag nanoparticles 1010, 1020 and ellipsoidal nanoparticles 1030, 1040 in a medium with n-2 +0.5 i. For δ > 10nm, the enhancement decreases monotonically with rapidly increasing spacing for δ < 10nm due to the nonlinear increase in dipole coupling between adjacent nanoparticles. The SPP resonance location is red-shifted for δ ≦ 10nm, while for larger δ, the SPP resonance converges to a single particle wavelength.

Fig. 11 shows the spectral response for spherical (solid line) arrays 1110, 1130 and 1150, and elliptical (dotted line) arrays 1120, 1140 and 1160, δ being 10nm 1110 and 1120, 5nm1130 and 1140, and 2.5nm 1150 and 1160. The solid line represents an array of 5nm diameter clusters, while the dotted line represents elliptical particles with an axial ratio of 0.5 of the same area. Surface spacings of 10nm (open squares), 5nm (filled circles) and 2.5nm (open triangles) are shown. In each case, the elliptical array has a greater maximum enhancement than the spherical case. As δ decreases, the coupling effect is stronger than the shape effect. The enhancement step of these structures is wide due to inter-particle coupling. Also, there is an attenuation region at wavelengths just below the SPP resonance. The solar spectral intensity of λ < 350-400nm is weak and therefore does not significantly affect device performance compared to the enhancement that occurs at long wavelengths.

It is also of interest that there is an enhanced distance from the recombination layer of the cascaded organic PV cells. FIG. 12 shows the wavelength λ 690nm with (triangle) and without (square) on quartzMeasured absorbance a of varying thickness of CuPc of Ag cluster layer. Measured absorbance values for varying thickness (t) CuPc films deposited directly on quartz substrates and on Ag island films for an off-resonance wavelength of λ 690nm are shown in fig. 12, respectively. For this wavelength, a direct comparison of the change in CuPc absorption is provided, since the absorption of Ag nanoparticles is negligible. When t is 10nm or less, the absorption increases more rapidly for a CuPc film absorbed onto the Ag islands 1210 than for the pure film 1220. For large t, the absorption is no longer enhanced. Fig. 13 shows the measured difference in absorbance (AA) versus CuPc thickness t for CuPc films with and without Ag layers.

Nano-sized Ag nanoparticle films have near zero scattering and reflection efficiency. The scattering loss due to the dipole oscillation mode may only become larger than the absorption loss of particles with 2R ≧ 30 nm.

Fig. 14 shows the effective thickness of a thin film dielectric region surrounding an array of particles, n 2+0.5i, which is located within the "enhanced region" of the array, including the regions within the array of particles. For very small δ, the increase in nanoparticle intensity is strong, although it is mainly limited to this small region. The enhancement of the spherical array 1410 and the elliptical array 1420 peaks at approximately 25nm, extending to distances of approximately 7 and 9nm, respectively.

A cascaded PV cell comprising two CuPc/PTCBI DA heterojunctions stacked in series and separated by a thin Ag nanoparticle recombination layer has a power efficiency η of about (2.5 + -0.1)%_PHowever η of a single CuPc/PTCBI subunit_PIt was (1.1. + -. 0.1)%, under 1 sun (100mW/cm2) simulated AM 1.5G (air mass 1.5 global) irradiation. V of cascaded units_OCAbout twice the value of a single cell. I is_SCMay result in η_PAn increase of about 15% to 2.5%. I is_SCObtained using the following formula:

where S (λ) is the simulated AM 1.5G solar irradiance spectrum, q is the electron charge, c is the speed of light, and h is the Planck constant.

Fig. 15 shows for the cascade structure: eta calculated with 150nm ITO/10nm CuPc/13nmPTCBI/1nm Ag/13nm CuPc/30nm PTCBI/100nm Ag, with Ag nanoparticle layer 1510 and without Ag nanoparticle layer 1520_EQE(lambda). Eta of the hollow circle display front cell (PV 1 nearest to the anode)_EQEAnd η of the solid square display back cell (PV 2 nearest the cathode)_EQE. It is also shown for PV1 (solid curve) and PV2 (dashed curve) due to the η -pair of the CuPc and PTCBI layers_EQEThe contribution of (c). The rear cell is thicker than the front cell to compensate for the reduction in field strength due to absorption in the front cell and due to parasitic optical interference effects. Eta of PV1 (open circle) and PV2 (solid square) in structures without Ag nanoparticles 1520_EQE(λ) are similar in shape, although PV1 is due to the higher η across most of the photoactive region_EQEBut has a larger I_SC. In PV2, the current imbalance is I_SCLimited to a small current. For PV1 and PV2, p eta_EQEThe major contribution of (λ) comes from the CuPc layer because of the diffusion length of CuPcDiffusion length greater than PTCBI. For the enhanced case, the short circuit current density is balanced, although η of PV1 and PV2 is balanced_EQE(λ) have different shapes. Because of the field enhancement of the nanoparticles, there is a pair of η from the PTCBI layer for PV1 and from the CuPc layer for PV2_EQEIs largeA contribution.

Small L of these materials in the CuPc/PTCBI architecture_DAllowing the deposition of thin layers in the front and back cells so that the DA interface is located in the enhancement region. For having a large L_DOf materials, e.g. C₆₀If the layer thickness is about L_DCurrent architectures do not allow significant enhancement at the DA interface, as for optimal layer thicknesses of bilayer organic PV units. For this material, it is possible to fabricate tandem devices from co-evaporated thin films of D and A materials, where exciton dissociation is not subject to L_DAnd (4) limiting. In this case, the PV subcells can be kept thin to maintain high FF, although the absorption in the cell is increased due to the enhancement of the nanoparticle charge recombination layer.

The intensity of the optical field in the near field of a series of metal nanoparticles can be increased by up to a hundred times compared to the intensity of the incident light. The enhancement covers a wide spectral range and may extend up toAllowing for increased absorption in the organic thin film in contact with or near the nanoparticles. The enhancement may result in higher power efficiency in the cascaded bilayer organic PV cell.

The relatively small diffusion length in the CuPc/PTCBI PV cell allows for a thin layer of enhanced absorption at the current generating DA interface. For theThe exciton quenching at Ag nanoparticles may limit the potential for efficiency enhancement via increased absorption. One possible approach to prevent exciton quenching from competing with efficiency gains is to encapsulate the metal nanoparticles in a thin insulating layer. These encapsulated nanoparticles can then be dispersed throughout the organic film, enhancing absorption without reducing the electrical efficiency of the cell. The encapsulated nanoparticles may comprise a significant percentage of the volume of the organic thin film.

Encapsulated nanoparticles can be made using layer-by-layer self-assembly as described in Ung et al j.phys.chem.b 2001, 105, 3441-52 and Salgueirino-Maccira et al j.phys.chem.b 2003, 107, 10990-. Other methods of making and sealing nanoparticles may be used, as will be understood by those skilled in the art.

The use of these encapsulated nanoparticles may allow for the tuning of particle-to-particle coupling effects, microscopic properties of the matrix material, and other effects. In one embodiment of the invention, the nanoparticles are encapsulated in an insulating material. In a preferred embodiment of the invention, the nanoparticles are encapsulated in an oxide. Insulating layer not less than aboutAnd not greater than aboutParticularly preferred. Less than aboutThe quantum effect may become less trivial, but greater than aboutThe separation of the nanoparticles may begin to damp the SPP resonance effect. The nanoparticles may not need to be in physical contact with the organic photoactive region. In another embodiment of the invention, the nanoparticles may be distributed throughout the "active region". As used herein, an "active region" is a region slightly larger than a "photoactive region". In particular, the "active region" is the region where the nanoparticles can have a significant positive effect on absorption in the photoactive region. Generally, the "active region" includes the organic material comprising the photoactive region, and about the photoactive regionThe organic material in (b). The active region may comprise a non-photoactive material and may typically comprise, for example, a blocking layer adjacent to the photoactive region.

Once fabricated according to any of a variety of methods, the encapsulated nanoparticles may be incorporated into the device by any suitable method. In a preferred embodiment, the nanoparticles are included in the solution deposited organic layer by suspension in a solution prior to deposition. Other methods may also be used, such as co-depositing encapsulated particles with an organic layer deposited by evaporation. The orientation of such nanoparticles (in the case of non-spherical particles) can be controlled by mechanical means such as spin coating and/or by the application of fields such as magnetic and electric fields during deposition. In some embodiments, the encapsulated nanoparticles may be fabricated in situ.

While the invention has been described with respect to specific examples and preferred embodiments, it will be understood that the invention is not limited to these examples and embodiments. Thus, the claimed invention may include variations from the specific examples and preferred embodiments described herein, as would be apparent to one skilled in the art.

Claims (29)

1. An optoelectronic device comprising:

a first electrode;

a second electrode;

a photoactive region comprising an organic material located between and electrically connected to the first electrode and the second electrode; and

a plurality of encapsulated nanoparticles located within the photoactive region, wherein the nanoparticles have a plasmon resonance.

2. An optoelectronic device according to claim 1 wherein the nanoparticles are comprised of a metal.

3. An optoelectronic device according to claim 1 wherein the nanoparticles are encapsulated in an oxide.

4. An optoelectronic device according to claim 1 wherein the nanoparticles are encapsulated in an insulating material.

5. An optoelectronic device according to claim 1 wherein the nanoparticles are distributed throughout the photoactive region.

6. An optoelectronic device according to claim 1 wherein the photoactive region comprises a first sub-unit, the first sub-unit further comprising:

a first donor layer; and

a first acceptor layer in direct physical contact with the first donor layer.

7. An optoelectronic device according to claim 6 wherein the photoactive region further comprises a second sub-unit, the second sub-unit further comprising:

a second donor layer; and

a second acceptor layer in indirect physical contact with the first donor layer, wherein the second subunit is located between the first subunit and the second electrode.

8. An optoelectronic device according to claim 6 wherein the nanoparticles are located in the first acceptor layer and the first donor layer.

9. An optoelectronic device according to claim 7 wherein the nanoparticles are located between the first and second subunits.

10. An optoelectronic device according to claim 1 wherein the nanoparticles are non-spherical.

11. An optoelectronic device according to claim 10 wherein the photoactive region is planar and the longest axis of each nanoparticle is substantially parallel to the plane of the photoactive region.

12. An optoelectronic device according to claim 10 wherein the nanoparticles are elliptical, the axial ratio of each nanoparticle being not less than about 0.1, the axial ratio being the ratio of the shortest axis to the longest axis.

13. An optoelectronic device according to claim 1 wherein the average surface spacing between the nanoparticles is no greater than about

14. An optoelectronic device according to claim 1 wherein the smallest axis of each nanoparticle is no greater than about

15. An optoelectronic device according to claim 4 wherein the thickness of the insulating material is not less than about

16. An optoelectronic device according to claim 4 wherein the thickness of the insulating material is not greater than about

17. An optoelectronic device according to claim 2 wherein the nanoparticles comprise Ag.

18. An optoelectronic device according to claim 2 wherein the nanoparticles comprise Au.

19. An optoelectronic device according to claim 2 wherein the nanoparticles comprise Cu.

20. An optoelectronic device according to claim 1 wherein the photoactive region comprises a bulk heterojunction.

21. An optoelectronic device according to claim 1 wherein the photoactive region comprises a dye sensitizing material.

22. An optoelectronic device according to claim 1 wherein the nanoparticles comprise a conductive material.

23. An optoelectronic device according to claim 1 wherein the nanoparticles comprise a semiconductor material.

24. An optoelectronic device according to claim 1 wherein the nanoparticles comprise a doped degenerate semiconductor.

25. An optoelectronic device comprising:

a first electrode;

a second electrode;

an active region positioned between and electrically connected to the first and second electrodes, the active region further comprising:

a photoactive region within the active region and between and electrically connected to the first and second electrodes; and in the optically active regionAn organic material within; and

a plurality of encapsulated nanoparticles located within the active region, wherein the nanoparticles have a plasmon resonance.

26. An optoelectronic device according to claim 25 wherein the active region further comprises an organic exciton blocking layer adjacent to the photoactive region.

27. A method of fabricating an optoelectronic device, comprising:

obtaining encapsulated nanoparticles;

manufacturing a first electrode;

fabricating an organic photoactive region, wherein the encapsulated nanoparticles are located within the photoactive region; and

a second electrode is fabricated.

28. The method of claim 27, further comprising a method of depositing the photoactive region from a solution process, wherein the nanoparticles are dispersed in a solution comprising a photoactive material.

29. The method of claim 27, wherein the encapsulated particles are co-deposited with an organic layer deposited by evaporation.