Patent Application
20090308456
The present invention relates to the field of organic optoelectronics. More particularly, the present invention relates to photovoltaic structures and to methods to produce the same.
Recently, the power conversion efficiency of organic photovoltaics has improved rapidly, offering new potential as low-cost renewable energy sources, driven primarily by the development of new materials, device architectures, and processing techniques. Since the absorption of a photon in an organic semiconductor results in the creation of a bound electron-hole pair, also called an exciton, the device performance relies on the ability of excitons to diffuse to a donor-acceptor interface in order to dissociate into free charge carriers.
The major bottleneck for achieving highly efficient organic solar cells is balancing the low diffusion length (LD) inherent to current organic semiconductors while achieving sufficiently thick layers to absorb most of the incident light. In fact, the bulk heterojunction (BHJ) device concept, in which the donor and acceptor materials are deposited simultaneously to form an interpenetrating donor-acceptor interface, was created in order to sidestep this trade-off.
However, it is difficult to achieve a very fine phase separation between the electron donor material and the electron acceptor material, of the order of a few LD, while at the same time preserving the good charge transport required for low electrical resistance, efficient cells. This is possibly one reason for the large research effort focused on new processing techniques to obtain a high level of morphological control over the blend.
In a first aspect, the present embodiment relates to a photovoltaic structure comprising:
wherein the lowest excited singlet state of the guest is lower (i.e. has a lower energy) than the lowest excited singlet state of the host, wherein the fluorescence emission spectrum of the host (at least partially) overlaps with the absorption spectrum of the guest and wherein the lowest excited triplet state of the guest is higher (i.e. has a higher energy) than the lowest excited triplet state of the host.
For example, one embodiment of the invention is a photovoltaic structure comprising:
As an optional feature, the highest occupied molecular orbital of the guest may be greater in energy (i.e. farther away from the vacuum energy level) than that of the host. This can prevent charge trapping on the guest molecule and allow for efficient charge transport on the host molecule, enabling the host:guest system to have transport properties similar to that of the pure host for a charge carrier.
As another optional feature, the electron acceptor material and the host in the electron donor material may be selected so that an electron transfer occurs between said electron acceptor material and said host upon illumination of the electron donor material at a wavelength of absorption of the host (for instance, the electron acceptor material and the host may be selected so that an electron transfer occurs between said electron acceptor material and said host upon illumination of the electron donor material (at least) at the wavelength of maximal absorption of the host).
As another optional feature, the host may comprise an organic semiconductor having at least a phenylene unit in its chemical structure (for instance, the host may comprise an organic semiconductor having a fluorenylene unit in its chemical structure). As another optional feature, the host may comprise an organic semiconductor having at least a thiophenylene unit in its chemical structure. As another optional feature, the host may comprise an organic semiconductor having at least a selenophenylene unit in its chemical structure.
As another optional feature, the host may comprise an organic semiconductor having an acene, an acenyl or acenylene unit in its chemical structure.
As another optional feature, the host may comprise an organic semiconductor having a porphyrin unit in its chemical structure. As another optional feature, the host may comprise an organic semiconductor having a phthalocyanine unit in its chemical structure. In some embodiments, carbene units can be present in the chemical structure of the organic semiconductor.
As another optional feature, the host may be selected form the group consisting of poly(para-phenylene vinylene) derivatives, poly(para-phenylene) derivatives, biphenyl derivatives, polythiophene derivatives, polyselenophene derivatives and aromatic tertiary amine derivatives.
As an embodiment of this optional feature, the semi-conducting electron donor material may comprise a poly(para-phenylene vinylene) derivatives having one of the following general formula:
wherein R is an alkyl chain such as e.g. a C1-C12 alkyl chain, wherein x, y and z are such that the molecular weight (Mw) is from 10000 to 80000 g/mol. In a specific embodiment R is 3,7-dimethyl-octyl and x/(y+z) is ˜1. In an embodiment, x/(y+z) is from 0.8 to 1.2. In one embodiment, y>z.
n is such that the molecular weight Mw is from 10000 to 80000 g/mol. In one embodiment, C10 is 3,7-dimethyloctyl.
As another optional feature, the guest may be selected from the group consisting of lanthanide metal chelates, heavy metal porphyrins, heavy metal phthalocyanines and iridium metal chelates. These guests, or dopants, can have efficient intersystem crossing from singlet to triplet excitons due to spin-orbit coupling originating from the presence of a heavy metal atom. Examples of suitable heavy metal atoms include but are not limited to Os, Ir and Pt amongst others.
As another optional feature, the guest may be selected from the group consisting of tris[2-(2-pyridinyl)phenyl-C,N]-iridium (Ir(Ppy)3), Iridium (III) bis[4,6-di-fluorophenyl-pyridinato-N,C2]picolinate [CAS Registry Number 376367-93-0] (Firpic) and platinum octaethylporphyrin (PtOEP).
As another optional feature, the host may be an aromatic tertiary amine compound and the guest may be Ir(ppy)3. In an embodiment of this optional feature, said aromatic tertiary amine compound may be selected from the group consisting of N,N′-diphenyl benzidine (α-NPD) and N,N′-bis(3-methylphenyl)-N,N′-bis(phenyl)-benzidine (TPD).
As another optional feature, the host may be a polyparaphenylene derivative and the guest may be Ir(Ppy)3.
In an embodiment of this optional feature, said polyparaphenylene derivative may be selected from the group consisting of polyfluorene derivatives, indenofluorene derivatives and ladder polyparaphenylene derivatives.
As another optional feature, the guest material may be platinum octaethylporphyrin (PtOEP).
As another optional feature, the guest material may be a diketone derivative such as but not limited to benzil (i.e. 1,2-diphenylethanedione) or 4,4′-bis(dimethylamino)benzyl.
As another optional feature, the host material may be 4,4′-N,N′-dicarbazole-biphenyl (CBP) and the guest material may be bis[4,6-di-fluorophenyl-pyridinato-N, C2]picolinate (Firpic).
As another optional feature, the guest/host weight ratio may be from 0.001 to 0.20 or from 0.01 to 0.10 or from 0.02 to 0.09 or from 0.03 to 0.07. This range of weight ratios between guest and host allows for efficient quenching of host singlet excitons to the guest, without providing an excess of guest molecules which could disrupt charge and exciton transport on the host material.
As another optional feature, the semiconductor electron acceptor material (e.g. as a layer) and the semiconductor electron donor material (e.g. as a layer) may be adjacent and do not form a bulk heterojunction. For example, the electron acceptor material and the electron donor material may form essentially a planar heterojunction.
As another optional feature, the electron acceptor material and the electron donor material may be mixed to form a bulk heterojunction.
As another optional feature, the photovoltaic structure according to any embodiments of the first aspect may further comprise conductive material layers (e.g. as electrodes). For instance at least two conductive layers may be used.
Preferably, one conductive layer is provided at the side of the photovoltaic structure closer or adjacent to the electron acceptor material and another conductive layer is provided at the side of the photovoltaic structure closer or adjacent to the electron donor material. In an embodiment, one or both of the conductive layers can take the form of a conductive substrate as defined elsewhere in this text.
In a second aspect, the present embodiment relates to a method for manufacturing a photovoltaic device. In an embodiment, the method may comprise:
In one embodiment, the method may comprise:
wherein the lowest excited singlet state of the guest is lower (i.e. has a smaller energy) than the lowest excited state of the host, wherein the fluorescence emission energy spectrum of the host overlaps (at least in part) with the absorption energy spectrum of the guest and wherein the lowest excited triplet state of the guest is higher (i.e. has a larger energy) than the lowest excited triplet state of the host.
In another embodiment, the method may comprise:
wherein the lowest excited singlet state of the guest is lower (i.e. has a smaller energy) than the energy of the lowest excited singlet state of the host, wherein the fluorescence emission energy spectrum of the host overlaps (at least in part) with the absorption energy spectrum of the guest and wherein the lowest excited triplet state of the guest is higher (i.e. has a larger energy) than the energy of the lowest excited triplet state of the host.
As an optional feature, a conductive layer may be applied on top of the photovoltaic structure, i.e. on the side opposite to the substrate.
As an optional feature to any embodiments of the second aspect of the present embodiment, the method may further comprise providing electrical contacts (e.g. two contacts, on each conductive layer) to the photovoltaic structure. The contacts fulfill the purpose of extracting current generated in the photovoltaic structure. Contacts are preferably provided on the conductive layer(s).
In an embodiment of the second aspect, the present embodiment relates to a method for manufacturing a photovoltaic structure comprising:
wherein the energy of the lowest excited singlet state of the guest is smaller than the energy of the lowest excited singlet state of the host, wherein the fluorescence emission energy spectrum of the host overlaps with the absorption spectrum of the guest and wherein the energy of the lowest excited triplet state of the guest is larger than the energy of the lowest excited triplet state of the host, and
Preferably, in any embodiment involving two conducting layers (one on each side of the photovoltaic structure), at least one conducting layer may be at least partly transparent to at least a wavelength absorbed by the host of the electron donating material.
An embodiment of the second aspect of the present embodiment, relates to a method comprising:
wherein said first and second conductive layers are chosen so that at least one is at least partially transparent.
In any embodiments, the conductive layers may be structured.
In a third aspect, the present embodiment relates to a method for operating a photovoltaic structure comprising:
In certain embodiments of the third aspect, the conducting contacts comprise conductive layers. For instance, a conductive layer can be applied on a first side of the device (e.g. the electron acceptor side of the device in the case of a planar heterojunction) and/or a conductive layer can be applied on the other side of the device (e.g. the electron donor side of the device in the case of a planar heterojunction). In one embodiment, one or both of the conductive layers can take the form of a conductive substrate as defined elsewhere in this description.
In one embodiment, the load can be an electrical device to which electrical power is delivered such as a storage device (e.g. a battery or a capacitor), or an electrical device using the current produced to perform work or provide energy (e.g. a motor or a heater).
In one embodiment of the third aspect it relates to a method for operating a photovoltaic structure comprising:
wherein the energy of the lowest excited singlet state of the guest is smaller than the energy of the lowest excited singlet state of the host, wherein the fluorescence emission energy spectrum of the host overlaps with the absorption spectrum of the guest and wherein the energy of the lowest excited triplet state of the guest is larger than the energy of the lowest excited triplet state of the host
Another embodiment relates to a method of operation of a photovoltaic cell comprising:
A further aspect of the present embodiment relates to the use of photovoltaic structure, according to any embodiment of the first aspect, for the fabrication of a photovoltaic cell. Accordingly, one embodiment a photovoltaic cell comprising a photovoltaic structure as described herein.
In certain aspects and embodiments, the present can provide good photovoltaic configurations or structures having an organic semiconducting electron donor material capable of efficient exciton transport to a donor-acceptor interface.
Certain aspects and embodiments have the advantages that relatively large LDs can be achieved for the photo-generated excitons when compared to prior art organic photovoltaic devices. Prior art organic photovoltaic devices are usually based on the light induced generation of singlet excitons. The exciton diffusion length, LD, is typically of the order of 3-10 nm for singlet excitons. One embodiment permits longer-lived triplet excitons to be generated (in the electron donor material), which have longer LDs than singlet excitons. Their generation allows for the harvesting of excitons generated further away from a donor-acceptor interface than was previously possible. With LD increased, the ultimate device efficiency is also increased, both for planar heterojunction solar cells and for bulk heterojunction solar cells. In planar heterojunction organic solar cells, LD values can be increased to values closer to the absorption length (i.e. absorption depth) than was previously possible. This drives the efficiency of a simple bilayer device architecture comprising a photovoltaic structure forming a planar heterojunction according to embodiments of the present toward values approaching that of a BHJ cell of the prior art. This embodiment is advantageous as it considerably simplifies device design and fabrication. In photovoltaic structures wherein the semi-conducting electron acceptor material and the semi-conducting electron donor material are mixed to form a bulk heterojunction according to embodiments, use is made of the longer LD of triplet excitons, allowing to relax the scale of separation between the donor and acceptor materials: a nano-morphology between donor and acceptor materials in a bulk heterojunction organic solar cell has usually a granularity of the order of LD, and thus an increase in LD leads to the possibility to use a coarser nano-morphology. This has implications on properties such as the mobility of charge carriers in the blend film and the amount of bulk recombination of electron and hole charge carriers leads to an increased photocurrent generation. Certain embodiments relate to a method to convert absorbed photons into triplets in an absorbing organic semiconductor electron donor material. Triplets are virtually non-existent in optically excited fluorescent materials, as both direct generation by light as well as intersystem crossing (ISC) from singlets are inefficient processes. Some embodiments relate to a process of sensitized phosphorescence or sensitized triplet formation, whereby a properly chosen phosphorescent compound (e.g. a phosphorescent dye) converts initially generated host singlet excitons into triplet excitons. Certain embodiments are advantageous as they may demonstrate enhanced photocurrent in a fluorescent material.
Particular and preferred aspects of the embodiments are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims and with features of other dependent claims as appropriate and not merely as explicitly set out in the claims.
Although there has been constant improvement, change and evolution of devices in this field, the present concepts are believed to represent substantial new and novel improvements, including departures from prior practices, resulting in the provision of more efficient, stable and reliable devices of this nature.
The above and other characteristics, features and advantages of the present embodiments will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. This description is given for the sake of example only, without limiting the scope of the invention. The reference figures quoted below refer to the attached drawings.
The present disclosure will be described with respect to particular embodiments and with reference to certain drawings but is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions and practice.
Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequence, either temporally, spatially, in ranking or in any other manner. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments described herein are capable of operation in other sequences than described or illustrated herein.
Moreover, the terms top, bottom, over, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments described herein are capable of operation in other orientations than described or illustrated herein.
It is to be recognized that the term “comprising”, used in the claims, should not be interpreted as being restricted to the elements or steps listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression “a device comprising means A and B” should not be limited to devices consisting only of components A and B.
Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present disclosed. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.
Similarly it should be appreciated that in the description of exemplary embodiments, various features are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment.
Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.
In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.
The following terms are provided solely to aid in the understanding of the embodiments.
As used herein and unless provided otherwise, the term “host” relates to a material representing more than 50% of the weight of a material mixture (typically a host:guest system).
As used herein and unless provided otherwise, the term “guest” relates to a material representing less than 50% of the weight of a material mixture (typically a host:guest system)
As used herein and unless provided otherwise, the term “small molecule” refers to a non-polymeric organic or organometallic molecule.
As used herein and unless provided otherwise, the terms “sensitized phosphorescence” and “sensitized triplet formation” relate to a process wherein triplet excitons are created in a host material by operating a singlet energy transfer from the host to the guest, an intersystem crossing within the guest and a triplet energy transfer from the guest to the host.
As used herein and unless stated otherwise, the use of the phrasing “material A: material B” relates to a host-guest system wherein A is the host and B is the Guest.
As used herein and unless provided otherwise, the term “bulk heterojunction” relates to donor and acceptor materials which are deposited simultaneously and/or are mixed to form an interpenetrating donor-acceptor interface wherein the donor and the acceptor are mixed together in the bulk and contain a relatively high surface area of dispersed interfaces when compared to planar heterojunction.
As used herein and unless provided otherwise, the term “planar heterojunction” relates to donor and acceptor materials which are deposited one after the other to form a donor-acceptor interface wherein the donor and the acceptor are not substantially mixed although the interface need not itself be planar in a macroscopic sense, i.e. the interface can be structured in order, for instance, to increase the surface area.
As used herein and unless provided otherwise, the term “vacuum level” relates to the energy at which an electron becomes free from its ion. In
As used herein and unless provided otherwise, the term “acene” relates to a chemical group having the following formula wherein n is 0 to 5, preferably 1 to 3.
As used herein and unless provided otherwise, the term “phenylene” relates to a benzene ring which is at least di-substituted, preferably in position 1 and 4. Fluorene, indenofluorene and ladder polyparaphenylene are examples of structures comprising one or more phenylene moieties.
As used herein and unless provided otherwise, the term “derivative” when applied to a chemical compound relates to a substituted or unsubstituted version of this compound.
For instance, the term “PPV derivative” relates to PPV itself or to substituted PPV, e.g. to 2,5-substituted PPV, 2-substituted PPV or 5-substituted PPV. Typical examples of substituents on the phenylene rings for PPV derivatives are C1l-C12 alkoxy chains. PPV derivatives also comprises PPV substituted on the vinylene (e.g. CN-PPV).
As another example, the term “polyparaphenylene (PPP) derivative” relates to PPP itself and to substituted PPP. Examples of substituted PPP are C1-12 alkyl substituted PPP, polyfluorene derivatives, polyindenofluorene derivatives, and ladder PPP derivatives. Polyfluorene, polyindenofluorene and ladder PPP can for instance be substituted on the methyl bridge (9,9′ positions in fluorene) by C1-C12 alkyl chains, or an aryl or substituted aryl group such as phenyl, aniline, phenol, phenoxy, triarylamine, among others.
The description will now be described by a detailed description of several embodiments. It is clear that other embodiments of the invention can be configured according to the knowledge of persons skilled in the art without departing from the true spirit or technical teaching of the invention, the invention being limited only by the terms of the appended claims.
In a first aspect, the present embodiment relates to a photovoltaic configuration comprising an electron acceptor material and an electron donor material.
The electron acceptor material may comprise any electron acceptor material known by the person skilled in the art to be usable in an organic photovoltaic structure. For instance, the electron acceptor material may comprise perylene derivatives (e.g. perylene tetracarboxidiimide, perylene tetracarboxidiimidazole such as 3,4,9,10-perylenetetracarboxylic bis-benzimidazole and the like), naphthalene derivatives (e.g. naphthalene tetracarboxidiimidazole, and the like), fullerene derivatives (e.g. buckminsterfullerene (C60), C70, C84 and the like), nanotubule derivatives, anthraquinone acridone pigment, polycyclic quinone, CN— or CF3-substituted poly(phenylenevinylene), fluorenone derivatives (polyfluorenone homo- and co-polymers) amongst others. Mixtures of two or more of such electron acceptor materials can also be used. Preferably, the electron acceptor material comprises buckminsterfullerene (C60).
In certain embodiments, the electron acceptor material can comprise a mixture of two or more materials chosen in such a way that after excitation of a singlet exciton on the first material, the excitation energy can be transferred to the second material. In this embodiment, this would require that the lowest singlet excitation energy level of the second material is smaller than for the first material. In this embodiment, the second component is preferably chosen in such a way that singlet excitons transferred to it, can become triplet excitons with a probability of at least 50% via intersystem crossing. Such electron accepting (and electron harvesting) material can for instance be as described in DE 102005010978 A1 which is hereby incorporated herein by reference in its entirety.
In some embodiments, the electron acceptor material can be present as a layer, a dispersed phase (e.g. in electron donor material) or a matrix phase (a phase in which a dispersed phase is dispersed, e.g. in which an electron donor material is dispersed).
The electron donor material comprises a host material and a guest material. Optionally, more than one host material and/or more than one guest material may be used in combination in the same electron donor material. For the sake of clarity, the rest of this description will mainly describe embodiments wherein one host and one guest material are composing the electron donor material but it should be well understood that embodiments wherein more than one host material and/or more than one guest material are used in combination in the same electron donor material are also.
In certain embodiments, use is made of an electron donor material with host:guest system, preferably in the form of an electron donor layer, wherein the host (e.g. a fluorescent host material) accounts for most of the light absorption. In certain embodiments, use is made of an electron donor material with a host:guest system, preferably in the form of an electron donor layer, wherein the host accounts for most of the charge transport. In certain embodiments, use is made for the electron donating material of a host:guest system wherein the host operates most of the exciton transport. In certain embodiments, the guest serves mainly (e.g. only) to generate triplets in the host. In the absence of a heterojunction with an acceptor material, the process of triplet generation in the host by use of a phosphorescent guest is named sensitized phosphorescence. In the presence of an acceptor material, the host may not emit phosphorescent light. Therefore, in more general terms the function of the guest may be called sensitized singlet to triplet conversion. Furthermore, the relatively low concentration of the guest molecules in the host matrix makes transport (e.g. of charges or excitons) and absorption by the guest molecules more difficult than in the host matrix. In embodiments, the guest molecules participate to small or negligible degree in direct absorption or direct charge transport.
In some embodiments, the weight ratio guest/host is 0.001 or more, preferably 0.005 or more, more preferably 0.01 or more, still more preferably 0.02 or more, most preferably 0.03 or more. In certain embodiments, the weight ratio guest host may be 0.20 or less, preferably 0.15 or less, more preferably 0.10 or less, still more preferably 0.09 or less, most preferably 0.07 or less. Any embodiments defining lower limits for the weight ratio guest/host can be combined with any embodiments defining higher limits for the weight ratio guest/host.
The host material in the donor material is preferably an organic hole transporting material.
The host can be a polymer (such as e.g. a semi-conductive polymer) or a small molecule.
Non-limiting examples of polymers suitable as host material in the donor material in the embodiments are hole transporting PPV derivatives (such as Super Yellow obtained from Merck OLED Materials GmbH (see formula (I) wherein R is 3,7-dimethyl-octyl, wherein x/(y+z) is 1 and wherein y>z)), Green (see formula (IV)), MDMO (see formula (V)) or MEH (see formula (VI)) amongst others), polyparaphenylene derivatives (such as polyfluorene derivatives, polyindenofluorene derivatives, ladder polyparaphenylene derivatives (such as methyl-substituted ladder-type poly(para-phenylene) (MeLPPP, see formula VII), and the like), polythiophene derivatives (e.g. a fluorene-thiophene alternating copolymer) and polyselenophene derivatives.
Non-limiting examples of small molecules suitable as host in the donor material in embodiments of the present invention are biphenyl derivatives such as 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl; 4,4′-bis [N-(p-tolyl)-N-phenyl-amino]biphenyl, 4,4′-N,N′-dicarbazole-biphenyl (CBP), phthalocyanines, porphyrins and polyacenes (including tetracene and pentacene as well as polyacene derivatives such as e.g. anthradithiophene) amongst others.
The guest material in the donor material is preferably a phosphorescent dopant such as e.g. a lanthanide metal chelate, a metal phthalocyanines (e.g. copper phthalocyanine or platinum phthalocyanine), a metal porphyrin such as but not limited to platinium porphyrin (e.g. PtOEP) or zinc-tetra (4-carboxyphenyl)porphyrin, an iridium metal chelate (e.g. tris[2-(2-pyridinyl)phenyl-C,N]-iridium (Ir(Ppy)3) or iridium (III) bis[4,6-di-fluorophenyl-pyridinato-N, C2] picolinate (Firpic)), amongst others. Other examples of possible dopant are diketone derivatives such as but not limited to Benzil (i.e. 1,2-diphenylethanedione) or 4,4′-Bis(dimethylamino)benzil. Examples of diketone derivatives usable as dopant in embodiments of the present invention may have the following general formula:
wherein R and R′ are independently selected from the group consisting of H, CH3, ethyl, propyl, —O—R″ and NR″2 wherein R″ is selected from the group consisting of H, CH3, ethyl and propyl. Preferably, R and R′ are both H or N(CH3)2.
It is most preferred that the host material and the guest material in the donor material are chosen such that (i) the lowest excited singlet energy level of the guest is lower in energy (i.e. has a smaller energy) than the lowest excited singlet energy level of the host in the donor material (S1,guest<S1,host, see e.g.
It is also preferred, when selecting a guest-host couple for use in the electron donor material, that the highest occupied molecular orbital (HOMO) of the guest is greater in energy than that of the host. This condition is further explained in
It is also preferred, when selecting a couple of guest material and host material for use in the electron donor material, that the lowest unoccupied molecular orbital (LUMO) of the guest is higher in energy than the lowest unoccupied molecular orbital (LUMO) of the host. This is advantageous as it can prevent trapping of electrons on guest sites as they move in the host matrix.
Examples of host:guest couples that are suitable for use in the donor material of the photovoltaic device according to embodiments of the present invention include aromatic tertiary amine compound:Ir(ppy)3 such as for instance N,N′-diphenyl benzidine (□-NPD): Ir(ppy)3 or N,N′-bis(3-methylphenyl)-N,N′-bis(phenyl)-benzidine (TPD): Ir(ppy)3 amongst others.
Examples of host:guest couples that are suitable for use in the donor material of the photovoltaic device according to embodiments of the present invention include polyparaphenylene derivative: Ir(ppy)3 such as but not limited to polyfluorene derivatives: Ir(ppy)3 or MeLPPP: Ir(ppy)3 amongst others.
Examples of host:guest couples that are suitable for use in the donor material of the photovoltaic device according to embodiments of the present invention include polyparaphenylene derivative:diketone derivatives such as but not limited to MeLPPP:benzil amongst others.
Examples of host:guest couples suitable for use in the donor material of the photovoltaic device according to embodiments of the present invention include but are not limited to PPV derivative:PtOEP such as but not limited to SY:PtOEP, Green PPV:PtOEP, MDMO PPV:PtOEP, MEH PPV:PtOEP amongst others.
Other example of host:guest couples suitable for use in the donor material of the photovoltaic device according to embodiments of the present invention include but are not limited to biphenyl derivative: Firpic such as but not limited to CBP:Firpic.
In some embodiments of the present invention, it is preferred that the electron acceptor material and the host in the donor material are selected so that an electron transfer occurs between said electron acceptor material and said host upon illumination of the electron donor material at least one wavelength (e.g. at the wavelength of maximal absorption) of the host absorption spectrum.
Preferably, the materials are carefully chosen so that efficient charge transport of photogenerated holes can occur through the host material in the donor material (e.g. by choosing a guest having its highest occupied molecular orbital (HOMO) farther from the vacuum level than that of the host.
In some embodiments, the electron donor material can be present as a layer, a dispersed phase (e.g. in electron acceptor material matrix) or a matrix phase (a phase in which a dispersed phase is dispersed, e.g. in which an electron acceptor material is dispersed).
Host:guest systems can be prepared by any manner known to the person skilled in the art. For instance, the host and the guest can be co-deposited from a solution comprising both the host and the guest (by e.g. spin coating, dip coating or blade coating among others). If the host and the guest are sufficiently volatile, e.g. when they are both small molecules, they can be co-deposited from the gas phase (e.g via CVD).
A bulk heterojunction can be prepared by any method well known to the person skilled in the art. Typically, it can be fabricated by coating (e.g. spin coating) a solution comprising a mixture of the donor and acceptor materials. During (spin) coating and solvent evaporation, the donor and acceptor materials can phase separate, creating an intricate interpenetrating network with a large interfacial area between the two phases. The morphology of the resulting structure can be controlled by e.g. changing the spin conditions, solvents and relative material concentrations.
In a second aspect, the present embodiment relates to a method for manufacturing a photovoltaic device.
In one embodiment, the method may comprise:
In one embodiment, the method may comprise:
wherein the lowest excited singlet state of the guest is lower (i.e. has a smaller energy) than the lowest excited state of the host, wherein the fluorescence emission energy spectrum of the host overlaps (at least in part) with the absorption energy spectrum of the guest and wherein the lowest excited triplet state of the guest is higher (i.e. has a larger energy) than the lowest excited triplet state of the host. This embodiment of the second aspect relates to the fabrication of a component or structure usable to form a photovoltaic cell.
In another embodiment, the method comprises:
wherein the lowest excited singlet state of the guest is lower (i.e. has a smaller energy) than the energy of the lowest excited singlet state of the host, wherein the fluorescence emission energy spectrum of the host overlaps (at least in part) with the absorption energy spectrum of the guest and wherein the lowest excited triplet state of the guest is higher (i.e. has a larger energy) than the energy of the lowest excited triplet state of the host. In order to build a fully operational photovoltaic cell, two electrodes may be added to the photovoltaic structure, at least one of which has to be at least partly transparent for a part of the spectrum absorbed by at least one of the active semiconductor materials, the donor or the acceptor. Preferably, one electrode is on the electron donor side of the structure and the other electrode is on the electron acceptor side of the structure.
In another embodiment of the second aspect, the present disclosure relates to a method of manufacturing a photovoltaic device comprising:
In this embodiment, the first and second electrodes are preferably chosen so that at least one is at least partially transparent.
Embodiments of the first or second aspect of the present disclosure not comprising two conductive layers and/or contacts can, for example, be used as components in photovoltaic cells comprising such conductive layers or contacts.
The conductive substrate can be formed of one material or of more than one material. For instance, the conductive substrate can comprise a conductive material optionally coated with one or more conductive layers or a non-conductive material coated with one or more conductive layers. The conductive substrate is preferably a transparent conductive substrate such as but not limited to ITO on glass. The conductive substrates and/or layers may be patterned. The substrate (e.g. a conductive substrate as defined above) can be flexible, rigid, planar, non-planar, transparent, opaque or translucent. It can be of any thicknesses.
In certain embodiments, a conductive layer may be applied on one or both sides of the photovoltaic structure (e.g. one on the substrate and one on the top of the photovoltaic structure). Preferably, one conductive layer plays the role of a cathode and the other conductive layer plays the role of an anode. For instance, a cathode can be placed adjacent to the electron acceptor material and opposite to the electron donor material in the case of a planar heterojunction. The selection of an appropriate cathode is well within the skills of the person skilled in the art. Examples of suitable cathodes are silver, aluminum, lithium, calcium and barium and their alloys amongst others.
The conductive layers may then be connected to an external load or circuit via a contact such as a wire, lead, or other means for transporting charges from the device to or from the load or circuit.
In some embodiments, one or more intermediate layers may be applied between the conductive substrate (e.g. a substrate having a conductive layer) and the electron donor material (or the mixture electron donor material/electron acceptor material in the case of a bulk heterojunction). The purpose of those layers can be to serve as hole injecting layer, hole transporting layer and/or electron blocking layer. Usable hole injecting layers (or hole transporting layer or electron blocking layer) are well known to the person skilled in the art and include for instance polyaniline (“PANI”), poly(3,4-ethylenedioxythiophene) (“PEDOT”), polypyrrole, organic charge transfer compounds (such as e.g. tetrathiafulvalene tetracyanoquinodimethane (“TTF-TCNQ”)), as well as high work function metal oxides such as molybdenum oxide, vanadium oxide, and tungsten oxide, amongst others. The assembly of a conductive substrate (e.g. substrate having a conductive layer such as ITO on glass) and of said one or more intermediate (conductive or semi-conductive) layers is to be understood as forming a conductive substrate in the broadest sense.
In some embodiments, one or more intermediate layers may be applied adjacent to the electron acceptor material and opposite to the electron donor material (e.g. between the electron acceptor material and the conductive layer). The purpose of those layers can be to serve as electron-injecting layer and/or electron transport layer and/or hole-blocking layer. Usable electron injecting layers (or electron transport layer or hole-blocking layer) are well known to the person skilled in the art and include for instance a metal-chelated oxinoid compound (e.g., Alq3 or aluminum(III)bis(2-methyl-8-quinolinato)4-phenylphenolate (“BAlq”)); a phenanthroline-based compound (e.g., 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (“DDPA”) or 9,10-diphenylanthracence (“DPA”)); an azole compound (e.g., 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole (“PBD”) or 3-(4-biphenyl)-4-phenyl-5-(4-t-butylphenyl)-1,2,4-triazole (“TAZ”); an electron transport material as described in Kido; a diphenylanthracene derivative; a dinaphthylanthracene derivative; 4,4-bis(2,2-diphenyl-ethen-1-yl)-biphenyl (“DPVBI”); 9,10-di-beta-naphthylanthracene; 9,10-di-(naphenthyl)anthracene; 9,10-di-(2-naphthyl)anthracene (“ADN”); 4,4′-bis(carbazol-9-yl)biphenyl (“CBP”); 9,10-bis-[4-(2,2-diphenylvinyl)-phenyl]-anthracene (“BDPVPA”); anthracene, N-arylbenzimidazoles (such as “TPBI”); 1,4-bis[2-(9-ethyl-3-carbazoyl)vinylenyl]benzene; 4,4′-bis[2-(9-ethyl-3-carbazoyl)vinylenyl]-1,1′-biphenyl; 9,10-bis[2,2-(9,9-fluorenylene)vinylenyl]anthracene; 1,4-bis[2,2-(9,9-fluorenylene)vinylenyl]benzene; 4,4′-bis[2,2-(9,9-fluorenylene)vinylenyl]-1,1′-biphenyl; perylene, substituted perylenes; tetra-tert-butylperylene (“TBPe”); bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium III (“F(Ir)Pic”); a pyrene, a substituted pyrene; a styrylamine; a fluorinated phenylene; oxidazole; 1,8-naphthalimide; a polyquinoline; one or more carbon nanotubes within PPV, as well as low work function metal oxides such as titanium oxide and zinc oxide, amongst others.
There are different ways to assemble the electron acceptor material and the electron donor material.
In certain embodiments, the electron acceptor material forms a layer adjacent to the electron donor material. In view of the longer LD achieved by using electron donor material according to embodiments of the present disclosure, such a simple layered structure is usable. One embodiment therefore relates to a photovoltaic structure comprising an electron acceptor layer, and an electron donor layer, wherein said electron donor layer is as described in any embodiments of the first aspect of the present disclosure and wherein the electron acceptor layer and the electron donor layer are adjacent and do not intermix significantly. In this embodiment, some degree of intermixing at the interface between both layers is permissible.
In another embodiment, the electron acceptor layer and the electron donor layer can form a bulk heterojunction (BHJ).
As an example the phenyl-substituted poly(p-phenylene vinylene) (PPV) donor polymer Super Yellow (SY) obtainable from Merck OLED materials GmbH (see formula (I)) doped with the phosphorescent molecule platinum octaethylporphyrin (PtOEP) (see formula II) is used.
Furthermore, the effect of doping with the Pt-free analogue of (II) octaethylporphyrin (OEP) (III) was investigated to demonstrate the opposite effect: a dopant that allows singlet energy transfer (SET) but not triplet energy transfer (TET) is expected to actually reduce the photocurrent.
In this example, it is demonstrated how an embodiment (via a new process of sensitized phosphorescence or sensitized triplet formation in an electron donor material), can be used to increase the photocurrent in an absorbing layer.
For the demonstration of sensitized phosphorescence in a solar cell, the following device structure was used: indium-tin-oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT) (30 nm)/SY (15 nm)/C60 (30 nm)/bathocuproine(BCP) (8 nm)/Al (80 nm). Here, ITO/PEDOT served as the anode, SY as the donor layer, C60 as the acceptor layer, BCP as an exciton blocking layer, and Al as the cathode. The schematic energy diagram of this device is shown in the inset to
With an increase in PtOEP concentration, JSC increases by approximately 10%, pealdng at a dopant concentration of 5%, beyond which the increase in JSC is reduced. In contrast, the addition of OEP to the SY matrix always results in a decrease of JSC, which appears to saturate at a reduction of approximately 15-18%. This can be understood by considering again the results of
In conclusion, this example experimentally demonstrates an organic solar cell with a donor layer enhanced by the process of sensitized phosphorescence. By converting optically excited singlet excitons to triplet excitons efficiently, LD was more than doubled, from 4 to 9 nm. In contrast, a similar dopant which lacks the ability to convert host singlet excitons to triplet excitons was shown to reduce the photocurrent from the host material. The ability to increase LD, ideally toward distances equal to the absorption length in organic materials can allow simplified bilayer architectures to possess efficiencies approaching that of BHJ devices, or can at least relax the restrictions placed on the phase separation of blends.
The ITO coated glass substrates (Merck Display Technologies, ITO thickness 100 nm, sheet resistance <20 Ω/square) were solvent cleaned followed by UV/O3 treatment for 10 min. The PEDOT (H.C. Starck, Baytron P VPAI4083) solution was spincast at 3000 rpm for 60 s to yield a 30 nm thick film, followed by baking at 120° C. to remove water. The SY was dissolved in toluene at a concentration of 1.7 mg/ml, with doping of PtOEP or OEP by weight, followed by spincasting at 2000 rpm for 60 s. The samples were transferred to an ultrahigh vacuum chamber (base pressure <5×10−9 Torr) for deposition of C60 and BCP (deposition rate ˜1.0 Å/s, followed by Al deposition (deposition rate ˜2-3 Å/s). The samples were then loaded into a concealed measurement unit in an N2 atmosphere for testing. The device area was measured with an optical microscope, with an average area of 3.3 mm2. Films for absorption and emission were deposited directly onto clean quartz substrates. The absorption spectra were measured using a Shimadzu UV-1601PC spectrophotometer, while emission was measured with a Shimadzu RF-5301PC spectrofluorophotometer, with an excitation λexe=385 nm. The J-V characteristics were measured using an Agilent 4156C parameter analyzer with illumination from a LOT-Oriel 1000 W Xe arc lamp fitted with AM 1.5D filters. Calibration was performed by a KG3 band pass filter and a calibrated Si photodetector. For the measurement of λEQE, light from a Xe arc lamp (Lot-Oriel 300 W O3-free) was coupled into a monochromator (Jobin-Yvon H25). The monochromatic light intensity was calibrated under normal incidence with a Si photodiode (Newport 818-UV). The light incident on the device was chopped by a square-wave at 300 Hz (Stanford Research 540) and the modulated current signal was detected with a current-voltage amplifier (Stanford Research 570) and lock-in amplifier (Stanford Research 810). During measurement, the device was kept in a vacuum of <10−4 Torr.
It is to be understood that although preferred embodiments, specific constructions and configurations, as well as materials, have been discussed herein for devices according to the present disclosure, various changes or modifications in form and detail may be made without departing from the scope and spirit of this disclosure. Steps may be added or deleted to methods described within the scope of the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| EP 08167746.0 | Oct 2008 | EP | regional |
This application claims priority to U.S. Provisional Application Ser. No. 60/061,451 filed Jun. 13, 2008 and European Patent Application Serial No. 08167746.0 filed Oct. 28, 2008, the contents of each of which are incorporated by reference herein in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61061451 | Jun 2008 | US |
