HK1190505A - Organic photovoltaic cell incorporating electron conducting exciton blocking layers - Google Patents

Description

Organic photovoltaic cells comprising electron-conducting exciton blocking layers

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. provisional application No. 61/444,899 filed on day 21/2/2011 and U.S. provisional application No. 61/479,237 filed on day 26/4/2011, both of which are incorporated herein by reference in their entireties.

Statement regarding federally sponsored research

This invention was made with U.S. government support under both DE-SC00000957 and DE-SC0001013 awarded by the U.S. department of energy. The united states government has certain rights in this invention.

Joint research protocol

The claimed invention is made by, in accordance with a university-corporation research agreement, one or more of the following parties, on behalf of and/or in cooperation with one or more of the following parties: university of Michigan (University of Michigan) and Global photovoltaic Energy Corporation. The agreement was in effect on and before the date the invention was made, and the claimed invention was made precisely because of the actions taken within the scope of the agreement.

Technical Field

The present disclosure relates generally to photosensitive optoelectronic devices comprising at least one blocking layer, e.g., electron conducting, exciton blocking layer. The present disclosure also relates to methods of improving power conversion efficiency in photosensitive optoelectronic devices using at least one blocking layer described herein. The electron conducting, exciton blocking layers of the devices disclosed herein may provide improved performance characteristics, such as increased open circuit voltage, short circuit current, fill factor, or power conversion efficiency.

Background

Opto-electronic 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 a class of photosensitive optoelectronic devices that are particularly useful for generating electrical power. PV devices can generate electrical energy from light sources other than sunlight, and can be used to drive loads that consume electrical power to provide, for example, lighting, heating, or to provide electrical power to electronic circuits or devices such as calculators, radios, computers, or remote monitoring or communication equipment. These power generation applications also typically involve charging batteries or other energy storage devices to enable continued operation when direct illumination from the sun or other light source is not available, or to balance the power output of the PV device as required by the particular application. As used herein, the term "resistive load" refers to any circuit, device, apparatus, or system that consumes or stores power.

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

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 rectifying junctions defined below, and also according to whether the device is operating under an applied voltage, also referred to as a bias voltage or bias voltage. Photoconductor cells do not have rectifying junctions and typically operate under bias. The PV device has at least one rectifying junction and does not operate under bias. The photodetector has at least one rectifying junction and is typically, but not always, operated under bias. As a general rule, photovoltaic cells provide power to a circuit, device or apparatus, but do not provide a signal or current to control or output information from a detection circuit. Instead, the photodetector or photoconductor provides a signal or current to control or output information from the detection circuit, but does not provide power to the circuit, device or apparatus.

Photosensitive optoelectronic devices have traditionally been constructed from a wide variety of inorganic semiconductors such as crystalline silicon, polycrystalline silicon and amorphous silicon, gallium arsenide, cadmium telluride and the like. As used herein, the term "semiconductor" refers to a material that is capable of conducting electricity when thermally or electromagnetically excited to induce the generation of charge carriers. The term "photoconductive" generally refers to a process in which electromagnetic radiation energy is absorbed and thereby converted to excitation energy of charge carriers, such that the carriers can conduct, i.e., transport, 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 power to useful electrical power. Devices utilizing crystalline or amorphous silicon dominate commercial applications, some of which have achieved efficiencies above 23%. However, due to problems inherent in producing large crystals without significant efficiency-reducing defects, it is difficult and expensive to produce efficient devices based on crystals, particularly devices with large surface areas. On the other hand, high efficiency amorphous silicon devices still suffer from stability problems. Currently commercially available amorphous silicon cells have a stable efficiency between 4% and 8%. More recent efforts have focused on the use of organic photovoltaic cells to achieve acceptable photoelectric conversion efficiencies and economical production costs.

The PV device may be optimized for standard lighting conditions (i.e., standard test conditions: 1000W/m)²AM1.5 spectral illumination) to obtain a maximum product of photocurrent and photovoltage. The power conversion efficiency of such cells under standard lighting conditions depends on three parameters: (1) current at zero bias, i.e. short-circuit current I_SCIn amperes, (2) photovoltage under open circuit conditions, i.e. open circuit voltage V_OCIn volts, and (3) a fill factor FF.

When PV devices are connected across a load and illuminated by light, they generate a photo-generated current. When illuminated under an infinite load condition, a PV device produces its maximum possible voltage, V_{Open circuit}Or V_OC. When illuminated with short-circuiting of its electrical contacts, the PV device produces its maximum possible current, I_{Short circuit}Or I_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 must not exceed I_SC×V_OCThe product is obtained. When the load value is optimized for maximum power extraction, the current and voltage each have an I_maxAnd V_maxThe value is obtained.

The figure of merit of a PV device is the fill factor FF, which is defined as:

FF={I_maxV_max}/{I_SCV_OC} （1）

where FF is always less than 1, since in practical applications I can never be obtained simultaneously_SCAnd V_OC. Nonetheless, when FF is near 1 under optimal conditions, the device has less series or internal resistance, thus delivering a higher percentage of I to the load_SCAnd V_OCThe product is obtained. If P is_incIs the power incident on the device, the power efficiency η of said device_PCan be calculated according to the following equation:

η_P=FF*（I_SC*V_OC）/P_inc

when electromagnetic radiation of appropriate energy is incident on an organic semiconductor material, such as an Organic Molecular Crystal (OMC) material or a polymer, photons can be absorbed to produce an excited molecular state. This is symbolized by S₀+hvΨS₀*. Here, S₀And S₀Refers to the ground and excited states of the molecule, respectively. This energy absorption and promotion of electrons from a bound state (which may be a B-bond) at the Highest Occupied Molecular Orbital (HOMO) level to the lowest unoccupied molecular orbital(s)The tunnel (LUMO) level, which may be a B-bond, or equivalently, is associated with promoting holes from the LUMO level to the HOMO level. In organic thin film photoconductors, the molecular states generated are generally considered to be excitons, i.e. electron-hole pairs in a bound state that are transported as quasi-particles. Excitons may have a considerable lifetime before they recombine in pairs, which refers to the process by which the 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 are separated, typically at the donor-acceptor interface between two different contacting organic thin films. If the charges are not separated, they can recombine radiatively by emitting light of lower energy than the incident light, or non-radiatively by generating heat in a pair-wise recombination process, also known as quenching. In photosensitive optoelectronic devices, either of these results is undesirable.

The electric field or inhomogeneity at the contact causes the exciton to quench at the donor-acceptor interface rather than dissociate, resulting in no net contribution to the current. It is therefore desirable to keep the photogenerated excitons away from the contacts. This has the effect of limiting the diffusion of excitons to the region near the junction so that the associated electric field has a greater opportunity to separate charge carriers released by dissociation of excitons near the junction.

In order to generate an endogenous electric field that occupies a large volume, a common approach is to juxtapose two layers of material having a suitably chosen conductivity, in particular with respect to the quantum state distribution of its molecules. The interface of these two materials is known as a photovoltaic heterojunction. In conventional semiconductor theory, the materials used to form PV heterojunctions are generally referred to as n-type or p-type materials. Here, n-type means that most of carrier types are electrons. This can be seen as the material having many electrons in relatively free energy states. p-type means that the majority carrier type is holes. Such materials have many holes in relatively free energy states. The background type, i.e. the non-photogenerated majority carrier concentration, is mainly dependent on unintentional doping by defects or impurities. The type and concentration of impurities determine the value of the fermi energy or fermi energy level within the energy gap between the Highest Occupied Molecular Orbital (HOMO) level and the Lowest Unoccupied Molecular Orbital (LUMO) level, known as the HOMO-LUMO energy gap. Fermi energy characterizes the statistical occupancy of the quantum states of a molecule represented by the energy values at which the probability of occupancy equals 1/2. The fermi energy near the LUMO energy level indicates that electrons are the predominant carrier. The fermi energy near the HOMO energy level indicates that holes are the predominant carrier. 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 asymmetric conduction characteristics, i.e. that the interface preferably supports electron charge transport in one direction. Rectification is typically associated with a built-in electric field that occurs at a heterojunction between appropriately selected materials.

As used herein, and as is 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 IP with a smaller absolute value (less negative IP). Similarly, a higher LUMO energy level corresponds to an Electron Affinity (EA) with a smaller absolute value (EA that is less negative). On a conventional energy level diagram, the vacuum level is at the top and the LUMO level of a material is higher than the HOMO 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 case 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 different from using 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-type and p-type layers, respectively. In the organic case, 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, electrons at the donor-acceptor junction move into the acceptor material, and holes move into the donor material, energetically favorable.

A significant property in organic semiconductors is carrier mobility. Mobility measures the ease with which charge carriers can move through a conductive material in response to an electric field. In the case of organic photosensitive devices, the layer containing 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 containing 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 ETL and the donor material is HTL.

Conventional inorganic semiconductor PV cells employ a p-n junction to establish an internal electric field. Early organic thin film cells, such as that reported by Tang in appl.phys.lett.48, 183 (1986), contained heterojunctions similar to those employed in conventional inorganic PV cells. However, it has now been realized that in addition to establishing a p-n type junction, the energy level shift of the heterojunction plays an important role.

It is believed that the shift in energy levels at the organic D-a heterojunction is important for the operation of the organic PV device due to the fundamental nature of the photogeneration process in organic materials. Upon photoexcitation of the organic material, localized frenzel (Frenkel) or charge transfer excitons are generated. In order to conduct electrical detection or generate an electrical current, the bound excitons must be dissociated into their constituent electrons and holes. Built-in electric fields can induce such processes, but the electric fields that are typically present in organic devices (F10)⁶V/cm), the efficiency 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 hetero-phase with an acceptor material having a high electron affinityAnd (6) knotting. Depending on the energy level arrangement of the donor and acceptor materials, exciton dissociation at such an interface may become energetically favorable, leading to the generation of free electron polarons in the acceptor material and free hole polarons in the donor material.

Organic PV cells have many potential advantages over traditional silicon-based devices. Organic PV cells are lightweight, economical in material usage, and can be deposited on low cost substrates such as flexible plastic foils. However, organic PV devices typically have relatively low external quantum efficiencies (conversion efficiency of electromagnetic radiation to electricity), on the order of 1% or less. This is believed to be due in part to the inherent secondary nature of the photoconductive process. That is, the generation of carriers requires the generation, diffusion, and ionization or collection of excitons. Each of these processes has an efficiency η associated with it. The subscripts are used as follows: p refers to power efficiency, EXT refers to external quantum efficiency, a refers to photon absorption, ED refers to diffusion, CC refers to collection, and INT refers to internal quantum efficiency. The following notation was used:

η_P～η_EXT=η_A*η_ED*η_CC

η_EXT=η_A*η_INT

diffusion length (L) of excitons_D）（L_D5 nm) is typically much smaller than the light absorption length (50 nm), which requires a trade-off between using thick layers where the excitons produced are too far apart to dissociate at the heterojunction, or thin cells where the light absorption efficiency is low.

The power conversion efficiency can be expressed as

Wherein V_OCIs the open circuit voltage, FF is the fill factor, J_scIs a short-circuit current, and P₀Is the input optical power. Improvement of eta_pA method forFormula (II) is obtained by increasing V_ocSaid V is_ocStill 3-4 times less than the photon energy typically absorbed in most organic PC cells. Dark current and V_ocThe relationship between can be derived from the following equation:

wherein J is the total current, J_sIs the inverse dark saturation current, n is the ideality factor, R_sIs a series resistance, R_pIs a parallel resistance, V is a bias voltage, and J_phIs the photocurrent (Rand et al, Phys. Rev. B, vol.75,115327 (2007)). Setting J = 0:

wherein J_ph/J_s>>1，V_OCAnd In (J)_ph/J_s) Proportional, indicating a large dark current J_sResult in V_OCAnd (4) descending.

Exciton blocking layers that also act as electron blocking layers have been developed for use in polymer Bulk Heterojunction (BHJ) PV cells (Hains et al, appl.phys.lett., vol.92,023504 (2008)). In polymer BHJ PV cells, a blended polymer of donor and acceptor materials is used as the active region. These blends may have a region of donor or acceptor material extending from one electrode to the other. Thus, there may be electron or hole conduction paths between the electrodes through one type of polymer molecule.

In addition to polymer BHJ PV cells, other structures including planar PV devices are at Δ E_LOr Δ E_HAlso showed significant donor cross-over in hoursElectron or hole leakage current from/acceptor heterojunctions, even though these films may not have a single material (donor or acceptor) path between the two electrodes.

The present disclosure relates to a photosensitive optoelectronic device comprising a composite blocking layer between an acceptor material and a negative electrode, the composite blocking layer comprising: at least one electron conducting material and at least one wide-gap electron conducting exciton blocking layer. This combination of materials acts as an efficient electron conductor, resulting in improved fill factor and improved power conversion efficiency compared to similar devices using conventional barrier layers.

Disclosure of Invention

The present invention discloses an organic photosensitive optoelectronic device comprising: two electrodes comprising a positive electrode and a negative electrode in a stacked relationship; at least one donor material and at least one acceptor material, wherein the donor material and the acceptor material form a photoactive region between the two electrodes; and a composite barrier layer between the acceptor material and the negative electrode.

In one embodiment, the composite barrier layer comprises: at least one electron conducting material; and a wide-gap electron-conducting exciton blocking layer.

In another embodiment, the at least one acceptor material has a lowest unoccupied molecular orbital energy (LUMO-1) and the at least one electron-conducting exciton blocking layer has a lowest unoccupied molecular orbital energy (LUMO-2), wherein LUMO-1 and LUMO-2 are aligned to allow direct electron transport from the acceptor material to the anode. As used herein, a LUMO level aligned to allow direct transport from the acceptor material to the negative electrode means having a band gap between a first lowest unoccupied molecular orbital energy and a second lowest unoccupied molecular orbital energy of no greater than 0.5eV, such as no greater than 0.3eV, or even less than 0.2 eV.

In one embodiment, the at least one donor material is selected from Squaraine (SQ), boron subphthalocyanine chloride (SubPc), copper phthalocyanine (CuPc), chloro-aluminum phthalocyanine (clarpc), poly-3-hexylthiophene (P3 HT), tin phthalocyanine (SnPc), pentacene, tetracene, diindenoperylene (diindenoperylene) (DIP), and combinations thereof.

The at least one acceptor material is selected from C₆₀、C₇₀Fullerene, 3,4,9, 10-perylenetetracarboxylic dianhydride (PTCDA), perfluoro-copper phthalocyanine (F)₁₆-CuPc）、PCBM、PC₇₀BM and combinations thereof.

In one embodiment, the at least one electron conducting material comprises 3,4,9, 10-perylenetetracarboxylic bisbenzimidazole (PTCBI).

In another embodiment, the at least one wide-gap electron-conducting exciton blocking layer comprises 1,4,5, 8-naphthalene tetracarboxylic dianhydride (NTCDA).

Another embodiment uses a combination of one electron conducting material comprising 3,4,9, 10-perylenetetracarboxylic bisbenzimidazole (PTCBI) and at least one wide-gap electron conducting exciton blocking layer comprising 1,4,5, 8-naphthalene tetracarboxylic dianhydride (NTCDA).

The various layers disclosed herein have the following thicknesses: the thickness of the composite blocking layer is in the range of 10-100nm, the thickness of the at least one electron conducting material is in the range of 2-10nm, and the thickness of the at least one wide-gap electron conducting exciton blocking layer is in the range of 5-100 nm.

It is to be understood that the organic photosensitive optoelectronic device is an organic photodetector such as an organic solar cell. In one embodiment, the organic solar cell exhibits at least one of the following properties:

-a fill factor greater than 0.62;

-a spectrally corrected power conversion efficiency of at least 5.0% at 1 sun am1.5g illumination; or

Short-circuit electricityThe flow is at least 7.5mA/cm²。

In one embodiment, at least one electrode may comprise a transparent conductive oxide, a thin metal layer, or a transparent conductive polymer. Non-limiting examples of the conductive oxide include Indium Tin Oxide (ITO), Tin Oxide (TO), Gallium Indium Tin Oxide (GITO), Zinc Oxide (ZO), and Zinc Indium Tin Oxide (ZITO), the thin metal layer is composed of Ag, Al, Au, or a combination thereof, and the transparent conductive polymer includes Polyaniline (PANI) and 3, 4-polyethylenedioxythiophene: polystyrene sulfonate (PEDOT: PSS).

Non-limiting examples of the at least one electrode include a metal substitute, a non-metal material, or a metal material selected from Ag, Au, Ti, Sn, and Al.

In one embodiment, the at least one donor material comprises squaraine and the at least one acceptor material comprises C₆₀The at least one electron conducting material comprises 3,4,9, 10-perylenetetracarboxylic bisbenzimidazole (PTCBI), and the at least one wide-gap electron conducting exciton blocking layer comprises 1,4,5, 8-naphthalene tetracarboxylic dianhydride (NTCDA).

The present invention also discloses a method of making an organic photosensitive optoelectronic device, said method comprising depositing onto a substrate: at least one electrode comprising a positive electrode and a negative electrode in a stacked relationship; at least one donor material and at least one acceptor material, wherein the donor material and the acceptor material form a photoactive region between the two electrodes; and depositing a composite barrier layer between the acceptor material and the anode, the composite barrier layer comprising: at least one electron conducting material and at least one wide-gap electron conducting exciton blocking layer.

In another embodiment, the method includes depositing disclosed materials, such as at least one acceptor material having a lowest unoccupied molecular orbital energy (LUMO-1) and at least one electron-conducting exciton blocking layer having a lowest unoccupied molecular orbital energy (LUMO-2), wherein LUMO-1 and LUMO-2 are aligned to allow direct electron transport from the acceptor material to the anode.

In one embodiment, the at least one donor material used in the disclosed method is selected from Squaraine (SQ), boron chloride subphthalocyanine (SubPc), copper phthalocyanine (CuPc), chloro-aluminum phthalocyanine (ClAlPc), poly-3-hexylthiophene (P3 HT), tin phthalocyanine (SnPc), pentacene, tetracene, Diindenoperylene (DIP), and combinations thereof.

The at least one acceptor material used in the disclosed method is selected from C₆₀、C₇₀Fullerene, 3,4,9, 10-perylenetetracarboxylic dianhydride (PTCDA), perfluoro-copper phthalocyanine (F)₁₆-CuPc）、PCBM、PC₇₀BM and combinations thereof.

In one embodiment, the at least one electron conducting material used in the disclosed method comprises 3,4,9, 10-perylenetetracarboxylic bisbenzimidazole (PTCBI).

In another embodiment, the at least one wide-gap electron-conducting exciton blocking layer used in the disclosed methods comprises 1,4,5, 8-naphthalene tetracarboxylic dianhydride (NTCDA).

Another embodiment of the disclosed method uses a combination of an electron conducting material such as 3,4,9, 10-perylenetetracarboxylic bisbenzimidazole (PTCBI) and at least one wide-gap electron conducting exciton blocking layer such as 1,4,5, 8-naphthalene tetracarboxylic dianhydride (NTCDA).

The methods disclosed herein include depositing layers of various thicknesses, such as a composite blocking layer having a thickness in the range of 10-100nm, at least one electron conducting material having a thickness in the range of 2-10nm, and at least one wide-gap electron conducting exciton blocking layer having a thickness in the range of 5-100 nm.

The disclosed method further includes depositing at least one electrode, which may comprise a transparent conductive oxide, a thin metal layer, or a transparent conductive polymer. Non-limiting examples of the conductive oxide include Indium Tin Oxide (ITO), Tin Oxide (TO), Gallium Indium Tin Oxide (GITO), Zinc Oxide (ZO), and Zinc Indium Tin Oxide (ZITO), the thin metal layer is made of Ag, Al, Au, or a combination thereof, and the transparent conductive polymer includes Polyaniline (PANI) and 3, 4-polyethylenedioxythiophene: polystyrene sulfonate (PEDOT: PSS).

Non-limiting examples of the at least one electrode include a metal substitute, a non-metal material, or a metal material selected from Ag, Au, Ti, Sn, and Al.

In one embodiment, the method comprises depositing at least one donor material comprising a squaraine, comprising C₆₀Comprises at least one electron-conducting material of 3,4,9, 10-perylenetetracarboxylic bisbenzimidazole (PTCBI) and at least one wide-gap electron-conducting exciton-blocking layer comprising 1,4,5, 8-naphthalene tetracarboxylic dianhydride (NTCDA).

The present disclosure also relates to methods of increasing the power conversion efficiency of a photosensitive optoelectronic device comprising including the composite blocking layer into the device.

In addition to the subject matter discussed above, the present disclosure also includes a number of other exemplary features such as those explained below. It is to be understood that both the foregoing description and the following description are exemplary only.

Drawings

The accompanying drawings are incorporated in and constitute a part of this specification.

Fig. 1 shows an energy level diagram of an exciton blocking layer transporting charges via the following a), b) and c): a) damage induced trap state (trap state), b) electron-hole recombination, and c) electron transport through the lowest unoccupied molecular orbital.

Fig. 2 shows the Fill Factor (FF) as a function of thickness for devices with BCP buffer layer (square), PTCBI (circle), NTCDA (triangle) and composite NTCDA/PTCBI (star) under spectrally corrected 1 sun am1.5g illumination. The straight line is the guide of the eye. Illustration is shown: 1-NPSQ molecular Structure.

FIG. 3 shows the spectrally corrected short-circuit current (J) as a function of thickness for a device with a BCP buffer layer (square), PTCBI (circular), NTCDA (triangle) and composite NTCDA/PTCBI (star) under 1 sun AM1.5G illumination_sc). The solid line is the guidance of the eye. The dotted line is J based on light intensity modeling in the device under NTCDA/PTCBI buffer conditions_sc。

Figure 4 shows the spectrally corrected current density versus voltage characteristics in the fourth quadrant under 1 sun am1.5g illumination for an optimized device without buffer (diamond), with 5nm BCP (square), 10nm PTCBI (circle), 10nm NTCDA (triangle), and composite 15nm NTCDA/5nm PTCBI buffer (star).

Detailed Description

Significant progress has been made over the last 25 years in improving the efficiency of Organic Photovoltaic (OPV) cells. An important milestone in increasing efficiency is the introduction of a buffer layer interposed between the acceptor layer and the negative contact, forming a so-called "double heterojunction" solar cell. Ideal buffering serves multiple purposes: protecting underlying receptor material (e.g. C)₆₀) Protection from damage due to evaporation of hot cathode metal atoms; providing efficient electron transport to the cathode; acts as an Exciton Blocking Layer (EBL) to prevent quenching of excitons generated in the acceptor at the negative electrode; and act as spacers to maximize the optical field at the active donor-acceptor heterojunction.

The most commonly used EBL is a wide-gap (and therefore transparent) semiconductor such as Bathocuproine (BCP) that transports carriers via negative metal deposition-induced damage that results in a high density of conductive well states (fig. 1 a). However, since this layer is only conductive in the presence of the well, the thickness is limited by the depth of damage (< 10 nm), which is not optimal for obtaining maximum optical field intensity in the active region of the device.

One possible approach to using a thicker wide-gap EBL is to dope the film to increase its conductivity. Based on tris- (acetylacetonato) ruthenium (III) (Ru (acac)₃) And related compounds with small Highest Occupied Molecular Orbital (HOMO) energies, a second type of EBL is introduced. In this case, holes from the negative electrode are along Ru (acac)₃And recombine with electrons at the acceptor/EBL interface, as shown in fig. 1 b.

The inventors have discovered a third type of EBL, in which the alignment of the lowest occupied molecular orbital (LUMO) with the acceptor allows electrons to be transported directly from the acceptor to the negative electrode with low resistance. It was shown that 3,4,9, 10-perylenetetracarboxylic bisbenzimidazole (PTCBI) acts as an efficient electron conductor and forms a low energy blocking contact with Ag cathodes. This results in an increase in fill factor from FF =0.60 typical of similar BCP-based devices to FF = 0.70. In addition, 1,4,5, 8-naphthalene tetracarboxylic dianhydride (NTCDA) is shown to act as a wide-gap electron-conducting EBL. By using both NTCDA and PTCBI in the composite barrier layer structure as shown in fig. 1c, an optimal optical separation is obtained, resulting in an increase of photocurrent. This results in a spectrally corrected power conversion efficiency η under 1 sun am1.5g simulated solar illumination_p=5.1 ± 0.1%, improvement compared to a conventional device with BCP blocking>25%。

C₆₀The HOMO and LUMO energies of (b) were 6.2 and 3.7eV, respectively, while the corresponding energies of BCP were 6.4 and 1.7eV, as shown in fig. 1. Although the low LUMO energy of BCP indicates a large barrier to electron extraction at the negative electrode, transport in BCP occurs through damage-induced trap states generated by evaporation of hot metal atoms onto the BCP surface. Since the LUMO of PTCBI and NTCDA is approximately identical to C₆₀So that electron transport can occur between these materials without damage.

In at least one embodiment, the PV cell is a planar heterojunction cell. In another embodiment, the PV cell is a planar hybrid heterojunction cell. In other embodiments of the present disclosure, the PV cell is non-planar. For example, the photoactive region may form at least one of a mixed heterojunction, a planar heterojunction, a bulk heterojunction, a nanocrystalline bulk heterojunction, and a hybrid planar mixed heterojunction.

Regardless of the type of cell, the organic photosensitive optoelectronic devices disclosed herein comprise at least one photoactive region in which light is absorbed to form an excited state, i.e., an "exciton," which can subsequently dissociate into an electron and a hole. Because dissociation of excitons typically occurs at a heterojunction containing a photoactive region formed by juxtaposition of an acceptor layer and a donor layer, it is generally desirable for the exciton blocking layer to prevent quenching of excitons generated in the acceptor at the negative electrode.

The devices disclosed herein comprise two electrodes including a positive electrode and a negative electrode. The electrodes or contacts are typically metals or "metal substitutes". In this context, the term metal is used to encompass both materials composed of an element-pure metal, such as Al, as well as metal alloys, which are materials composed of two or more element-pure metals. The term "metal substitute" herein refers to a material that is not a metal in its normal definition, but which has certain metal-like properties desirable for suitable applications. Commonly used metal alternatives for the electrodes and charge transport layers include doped wide bandgap semiconductors such as transparent conducting oxides like 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 having an optical bandgap of about 3.2eV such that the ITO pair is greater than aboutIs transparent to the wavelength of (b).

Another suitable metal substitute material is the transparent conducting polymer Polyaniline (PANI) and its chemically related substances. The metal substitute may also be selected from a wide range of non-metallic materials, wherein the term "non-metallic" is meant to encompass a wide range of materials, provided that the material does not contain the metal in chemically uncombined form. When a metal is present in its chemically uncombined 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". As such, the metal substitute electrodes of the present disclosure may sometimes be referred to as "metal-free," where the term "metal-free" expressly refers to a material that is free of metal in chemically unbound form. Free metals typically have the form of metallic bonds, which are considered to be one type of chemical bond, caused by a large number of valence electrons across the metal lattice. Although metal substitutes may contain metal components, they are "non-metallic" on some basis. They are neither pure free metals nor alloys of free metals. When metals are present in their metallic form, electron conduction bands tend to provide other metallic properties such as high electrical conductivity and high reflectivity to light radiation.

Herein, the term "anode" is used in the following manner. In a non-stacked PV device or individual units of a stacked PV device, such as a solar cell, under ambient illumination and connected to a resistive load and without an externally applied voltage, electrons move from the adjacent photoconductive material to the negative electrode. Similarly, the term "positive electrode" is used herein such that in a solar cell under illumination, holes move from the adjacent photoconductive material to the positive electrode, which is equivalent to electrons moving in the opposite manner. It should be noted that the terms positive and negative electrode as used herein may be either electrodes or charge transfer regions.

When the PV cell is operated under illumination, an output photocurrent is formed by collecting the photo-generated electrons at the negative electrode and the photo-generated holes at the positive electrode. Due to the induced potential drop and the electric field, the dark current flows in the opposite direction. Electrons and holes are injected from the cathode and anode, respectively, and if no significant energy blocking is encountered, they are able to move to the opposite electrode. They can also recombine at the interface to form a recombination current. Electrons and holes thermally generated inside the active region also contribute to dark current. Although this persistent component is dominant when the solar cell is reverse biased, it is negligible under forward bias conditions.

Dark current for operating PV cells comes primarily from the following sources: (1) generation/recombination current I due to electron-hole recombination at the donor/acceptor interface_gr(ii) a (2) Electron leakage current I due to electrons passing from the negative electrode to the positive electrode through the donor/acceptor interface_e(ii) a And (3) hole leakage current I due to holes passing from the positive electrode to the negative electrode through the donor/acceptor interface_h. In operation, the solar cell has no externally applied bias. The magnitude of these current components depends on the energy level. I is_grEnergy gap with interface Delta E_gIs increased. I is_eWith Delta E_LIs decreased and increased, Δ E_LIs the difference in the Lowest Unoccupied Molecular Orbital (LUMO) energies of the donor and acceptor. I is_hWith Delta E_HIs decreased and increased, Δ E_HIs the difference in the Highest Occupied Molecular Orbital (HOMO) energies of the donor and acceptor. Any of these three current components may be the dominant dark current, depending on the energy levels of the donor and acceptor materials.

In one embodiment, the photoactive region forms at least one of a mixed heterojunction, a bulk heterojunction, a nanocrystalline bulk heterojunction, and a hybrid planar mixed heterojunction.

Stacked organic photosensitive optoelectronic devices are also contemplated herein. The stacked device of the present disclosure may comprise a plurality of photosensitive photovoltaic subcells, wherein at least one subcell comprises: two electrodes comprising a positive electrode and a negative electrode in a stacked relationship; a donor region between the two electrodes, the donor region being formed of a first photoconductive organic semiconductor material; an acceptor region between the two electrodes and adjacent to the donor region, the acceptor region being formed of a second photoconductive organic semiconductor material; and at least one of an electron blocking layer and a hole blocking layer between the two electrodes and adjacent to at least one of the donor region and the acceptor region. Such stacked devices may be constructed in accordance with the present disclosure to achieve high internal and external quantum efficiencies.

When the term "subcell" is used hereinafter, it refers to an organic photoactive optoelectronic configuration that can include at least one of the electron blocking EBL and the hole blocking EBL of the present disclosure. When used alone as a photosensitive optoelectronic device, a subcell typically includes a complete set of electrodes, i.e., a positive electrode and a negative electrode. As disclosed herein, in certain stacked configurations, adjacent subcells may utilize a common, i.e., shared, electrode, charge transfer region, or charge recombination region. In other cases, adjacent subcells do not share a common electrode or charge transfer region. The term "subcell" as disclosed herein includes a subcell configuration regardless of whether each subcell has its own unique electrode or shares an electrode or charge transfer region with an adjacent subcell. As used herein, the terms "cell," "subcell," "cell," "subunit," "section," and "subsection" are used interchangeably to refer to a region or group of regions that are photoconductive and an adjacent electrode or charge transfer region. As used herein, the terms "stacked", "multi-part", and "multi-cell" refer to any optoelectronic device having multiple regions of photoconductive material separated by one or more electrodes or charge transfer regions.

Since the stacked sub-cells of a solar cell can be fabricated using vacuum deposition techniques that allow external electrical connection to the electrodes separating the sub-cells, each sub-cell in the device can be electrically connected in parallel or in series, depending on whether the power and/or voltage generated by the PV cell is maximized. The improved external quantum efficiency that can be achieved for the stacked PV cell embodiments of the present disclosure can also be attributed to the fact that: the sub-cells of the stacked PV cells can be electrically connected in parallel, since the parallel electrical configuration allows for a significantly higher fill factor than when the sub-cells are connected in series.

In the case of a device where the PV cells are comprised of sub-cells electrically connected in series to generate higher voltages, the stacked PV cells can be fabricated such that each sub-cell generates approximately the same current to reduce failure. For example, if the incident radiation passes in only one direction, the stacked sub-cells may have a gradually increasing thickness with the outermost sub-cell being the thinnest, which is most directly exposed to the incident radiation. Alternatively, if the subcells are stacked on a reflective surface, the thickness of each subcell can be adjusted according to the total combined radiation from the primary and reflective directions entering each cell.

Furthermore, it may be desirable to have a dc power supply capable of producing a large number of different voltages. For such applications, external connections to the intervening electrodes may have significant utility. Thus, in addition to being able to provide the maximum voltage generated across the entire set of sub-cells, exemplary embodiments of stacked PV cells of the present disclosure may also be used to provide multiple voltages from a single power source by tapping selected voltages from selected sub-cell subsets.

Representative embodiments of the present disclosure may also include a transparent charge transfer region. As described herein, the charge transfer layer is significantly different from the acceptor and donor regions/materials due to the fact that: the charge transfer regions are often, but not necessarily, inorganic and they are typically selected to be not photoconductive active.

The organic photosensitive optoelectronic devices disclosed herein can be used in a variety of photovoltaic applications. In at least one embodiment, the device is an organic photodetector. In at least one embodiment, the device is an organic solar cell.

Detailed Description

The present disclosure may be understood more readily by reference to the following detailed description of exemplary embodiments and working examples. It is understood that other embodiments will become apparent to those skilled in the art from consideration of the specification and examples disclosed herein.

Example 1

The devices were grown on a 150nm thick Indium Tin Oxide (ITO) layer pre-coated on a glass substrate. Before deposition, the ITO surface was cleaned in surfactant and a series of solvents, then exposed to uv-ozone for 10 minutes, and then loaded into a high vacuum chamber (base pressure)<10^-7In torr), in the high vacuum chamber, MoO is caused to flow₃Thermal evaporation at-0.1 nm/s. Then, the substrate is transferred to N₂In a glove box, 2, 4-bis [4- (N-phenyl-1-naphthylamino) -2, 6-dihydroxyphenyl is spin-coated from a heated 6.5mg/ml solution in 1, 2-dichlorobenzene]Squaraine (1-NPSQ, see molecular structural formula in inset of fig. 2) films and thermally annealed on a hot plate at 110 ℃ for 5 minutes to promote growth of the nanocrystalline morphology.

The substrate was again transferred to a high vacuum chamber to deposit the purified organics at 0.1nm/s followed by deposition of a 100nm thick Ag negative electrode at 0.1nm/s through a shadow mask having an array of 1mm diameter openings. In the dark and under simulated AM1.5G solar illumination from filtered 150W Xe lamps, at ultra-pure N₂Current density versus voltage (J-V) characteristics were measured in the environment. A neutral density filter is used to vary the intensity of the lamp. A Si detector calibrated with NREL was used as a reference for light intensity and the photocurrent measurements were corrected for spectral mismatches. The errors shown correspond to the deviation of the mean values of three or more devices on the same substrate.

A device having the following structure was fabricated: glass/150 nm ITO/8nm MoO₃/15nm1-NPSQ/40nm C₆₀Buffer/100 nm Ag. The open circuit voltage depends on the interfacial energy gap between the donor and acceptor, which is found to be V_oc=0.90 to 0.96 ± 0.01V, irrespective of the composition of the buffer layer.

FIG. 2 shows FF as a function of buffer layer thickness x for BCP, PTCBI, NTCDA and a composite buffer consisting of (x-5) nm NTCDA/5 nmPTBI. The best performance of devices with BCP occurs at 5nm thickness, FF =0.60 ± 0.01, beyond which thicknessLater, the efficiency drops dramatically due to the limited depth that the damage-induced transport state extends from the surface into the film. In contrast, the device with PTCBI showed FF =0.70 ± 0.01, with only a small drop when x → 50nm, confirming low resistance transport in this material. The optimal thickness of PTCBI is 10nm, with η of thicker films_pPhotocurrent (J) due to short circuit_sc) Is reduced because the absorption of PTCBI overlaps with the absorption of the active acceptor and donor layers. Devices with NTCDA buffer layers showed FF =0.62 ± 0.01. In contrast, the device with the composite 15nm ntcda/5nm PTCBI buffer has FF =0.68 ± 0.01, which is similar to the FF of PTCBI alone.

The composite NTCDA/PTCBI buffer layer results in J compared to PTCBI alone_scAnd (4) improving. Unlike PTCBI, wide energy gap NTCDA is transparent in the visible spectrum. Thus, PTCBI was kept thin enough (5 nm) to provide a low blocking negative contact without introducing excessive light absorption. At the same time, the thickness of NTCDA is adjusted to maximize the optical field at the donor-acceptor junction without increasing the series resistance, as opposed to having BCP. J as a function of the buffer layer_scThe trend of (c) is consistent with optical modeling using the transfer-matrix method, shown by the dashed line in fig. 3. Optimized device implementation J using composite buffering_sc=8.0±0.1mA/cm²In contrast, BCP was 7.2. + -. 0.1mA/cm²PTCBI of 7.1 +/-0.1 mA/cm²As shown in fig. 3. For unbuffered devices and devices with BCP, PTCBI, NTCDA and PTCBI/NTCDA buffering, the following values were measured, respectively: eta_p=2.8 ± 0.1, 4.0 ± 0.1, 4.6 ± 0.1, 3.2 ± 0.1 and 5.1 ± 0.1%. These results are summarized in table 1.

To understand the FF differences between the several buffer layer combinations explored, we describe the current density using the ideal diode equation:

J＝J_s{exp[q(V_a-JR_s)/nk_bT]-1}-qη_PPd(V_a)J_X, (1)

wherein J_sIs a reverse saturation current, n is an ideality factor, V_aIs an applied voltage, R_sIs the series resistance, T is the temperature, q is the electronic charge, η_PPd(V_a) Is the field-dependent polaron pair dissociation efficiency, and J_XIs the exciton current to the heterojunction. For optimized BCP, PTCBI and NTCDA/PTCBI buffer layer devices, R_s<10Ω-cm²Indicating efficient electron transport to the cathode. At layer thicknesses of up to 50nm, PTCBI and R of the composite buffer_sThere was no significant change, whereas for BCP, x =50nm, R_sIs increased to>10kΩ-cm². The device with NTCDA buffer only has R for all thicknesses_s>100Ω-cm²This is because an electron extraction barrier is formed at the NTCDA/Ag interface, which is reported to be>1 eV. However, NTCDA-based devices including 5nm thick PTCBI layers have R similar to PTCBI-alone devices_sThis is in good agreement with previous reports of 0.1eV blocking at the PTCBI/Ag interface. One possible mechanism for FF difference of devices with BCP or PTCBI is that the trapped charge has an effect on the internal electric field. The current density of the device under illumination is given by η according to equation 1_PPdIs determined. Because electron transport in BCP occurs through damage-induced wells, their residence in these deep energy levels induces charge transfer from V_aThe induced field opposes the electric field, resulting in increased recombination at the heterojunction. This appears as an increase in slope at zero bias in the J-V characteristic (see fig. 4), resulting in a decrease in FF.

The performance of all devices is summarized in table 1. Measured for V under standard AM1.5G sun illumination of one sun_OC、J_scFill Factor (FF) and power conversion efficiency (eta)_p) The value of (c).

Table 1 performance of devices with different buffer layers under simulated 1 sun (mismatch corrected) am1.5g illumination.

As shown, the inventors have demonstrated the use of electron conducting EBLs in OPVs. Here, electrons are directly transported from the acceptor to the negative electrode via the LUMO state. By using PTCBI as a buffer layer, FF =0.70 ± 0.01, in contrast to FF =0.60 ± 0.01 for conventional BCP-based devices. The addition of NTCDA electron conducting EBL in combination with PTCBI allows optimization of the optical spacing and efficient exciton blocking, resulting in η_pRelative to an analogous squaraine/C₆₀Improvement of/BCP OPV>25 percent. The higher stability of PTCBI compared to BCP can also potentially extend the operational lifetime of OPVs using barrier layers.

It is intended that the specification and examples disclosed herein be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.

Other than in the examples, or where otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, analytical measurements and so forth used in the specification and claims are to be understood as being modified in all instances by the term "about". Accordingly, unless indicated to the contrary, the numerical parameters set forth in the specification and claims are approximations that may vary depending upon the desired properties sought to be obtained by the present disclosure. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the disclosure are approximations, the numerical values set forth in the specific examples are reported as precisely as possible unless otherwise indicated. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements.

Claims (34)

1. An organic photosensitive optoelectronic device, comprising:

two electrodes comprising a positive electrode and a negative electrode in a stacked relationship;

a photoactive region between the two electrodes; and

a blocking region that conducts electrons and blocks excitons, wherein the blocking region comprises at least one organic material located between the photoactive region and the anode, the organic blocking region comprising at least one electron-conducting material.

2. The device of claim 1, wherein the photoactive region comprises at least one donor material and at least one acceptor material.

3. The device of claim 2, wherein at least one acceptor has a lowest unoccupied molecular orbital energy (LUMO-1) and at least one electron-conducting exciton blocking layer has a lowest unoccupied molecular orbital energy (LUMO-2), wherein LUMO-1 and LUMO-2 are aligned to allow direct electron transport from the acceptor material to the anode.

4. The device of claim 3, wherein an energy gap between the first lowest unoccupied molecular orbital energy and the second lowest unoccupied molecular orbital energy is no greater than 0.3 eV.

5. The device of claim 2, wherein the at least one donor material comprises Squaraine (SQ), boron subphthalocyanine chloride (SubPc), copper phthalocyanine (CuPc), chloro-aluminum phthalocyanine (ClAlPc), poly-3-hexylthiophene (P3 HT), tin phthalocyanine (SnPc), pentacene, tetracene, Diindenoperylene (DIP), and combinations thereof.

6. The device of claim 2, wherein the at least one acceptor material is C₆₀、C₇₀Fullerene, 3,4,9, 10-perylenetetracarboxylic dianhydride (PTCDA), perfluoro-copper phthalocyanine (F)₁₆-CuPc）、PCBM、PC₇₀BM and combinations thereof.

7. The device of claim 1, wherein the at least one electron conducting material comprises 3,4,9, 10-perylenetetracarboxylic bisbenzimidazole (PTCBI).

8. The device of claim 1, wherein the blocking region further comprises at least one wide-gap electron conducting exciton blocking material.

9. The device of claim 8, wherein the at least one wide-gap electron-conducting exciton blocking material comprises 1,4,5, 8-naphthalene tetracarboxylic dianhydride (NTCDA).

10. The device of claim 1, wherein the thickness of the barrier region is in the range of 10-100 nm.

11. The device of claim 1, wherein the at least one electron conducting material has a thickness in the range of 2-10 nm.

12. The device of claim 8, wherein the at least one wide-gap electron conducting exciton blocking material has a thickness in the range of 5-100 nm.

13. The device of claim 1, wherein the blocking region comprises an electron conducting material comprising 3,4,9, 10-perylenetetracarboxylic bisbenzimidazole (PTCBI) and a wide-gap electron conducting exciton blocking material comprising 1,4,5, 8-naphthalene tetracarboxylic dianhydride (NTCDA).

14. The device of claim 13, wherein the thickness of NTCDA is in the range of 5-100nm and the thickness of PTCBI is up to 5 nm.

15. The device of claim 1, wherein the device is an organic photodetector.

16. The device of claim 15, wherein the organic photodetector is an organic solar cell exhibiting at least one of the following properties:

-a fill factor greater than 0.62;

-a spectrally corrected power conversion efficiency of at least 5.0% at 1 sun am1.5g illumination; or

Short-circuit current of at least 7.5mA/cm²。

17. The device of claim 1, wherein at least one electrode comprises a transparent conductive oxide, a thin metal layer, or a transparent conductive polymer.

18. The device of claim 17, wherein the conductive oxide is selected from the group consisting of Indium Tin Oxide (ITO), Tin Oxide (TO), Gallium Indium Tin Oxide (GITO), Zinc Oxide (ZO), and Zinc Indium Tin Oxide (ZITO), the thin metal layer is comprised of Ag, Al, Au, or combinations thereof, and the transparent conductive polymer comprises Polyaniline (PANI) and 3, 4-polyethylenedioxythiophene polystyrene sulfonate (PEDOT: PSS).

19. The device of claim 1, wherein at least one electrode comprises a metal substitute, a non-metallic material, or a metallic material selected from Ag, Au, Ti, Sn, and Al.

20. A method of making an organic photosensitive optoelectronic device, the method comprising depositing onto a substrate:

at least one electrode comprising a positive electrode and a negative electrode in a stacked relationship;

a photoactive region between two electrodes; and

a blocking region that conducts electrons and blocks excitons, wherein the blocking region comprises at least one organic material located between the photoactive region and the anode, the organic blocking region comprising at least one electron-conducting material.

21. The method of claim 20, wherein said photoactive region comprises at least one donor material and at least one acceptor material.

22. The method of claim 21, wherein the at least one donor material is selected from Squaraine (SQ), boron subphthalocyanine chloride (SubPc), copper phthalocyanine (CuPc), chloro-aluminum phthalocyanine (ClAlPc), poly-3-hexylthiophene (P3 HT), tin phthalocyanine (SnPc), pentacene, tetracene, Diindenoperylene (DIP), and combinations thereof.

23. The method of claim 21, wherein the at least one acceptor material is selected from C₆₀、C₇₀Fullerene, 3,4,9, 10-perylenetetracarboxylic dianhydride (PTCDA), perfluoro-copper phthalocyanine (F)₁₆-CuPc）、PCBM、PC₇₀BM and combinations thereof.

24. The method of claim 20, wherein the at least one acceptor has a lowest unoccupied molecular orbital energy (LUMO-1) and the electron conducting material has a lowest unoccupied molecular orbital energy (LUMO-2), wherein LUMO-1 and LUMO-2 are aligned to allow direct electron transport from the photoactive region to the anode.

25. The method of claim 24, wherein the energy gap between the first lowest unoccupied molecular orbital energy and the second lowest unoccupied molecular orbital energy is no greater than 0.3 eV.

26. The method of claim 20, wherein the blocking region further comprises at least one wide-gap electron conducting exciton blocking material.

27. The method of claim 26, wherein the at least one wide-gap electron-conducting exciton blocking material comprises 1,4,5, 8-naphthalene tetracarboxylic dianhydride (NTCDA).

28. The method of claim 20, wherein the thickness of the barrier region is in the range of 10-100 nm.

29. The method of claim 20, wherein the at least one electron conducting material has a thickness in the range of 2-10 nm.

30. The method of claim 26, wherein the at least one wide-gap electron conducting exciton blocking material has a thickness in the range of 5-100 nm.

31. The method of claim 20, wherein the blocking region comprises an electron conducting material comprising 3,4,9, 10-perylenetetracarboxylic bisbenzimidazole (PTCBI) and a wide-gap electron conducting exciton blocking material comprising 1,4,5, 8-naphthalene tetracarboxylic dianhydride (NTCDA).

32. The method of claim 31, wherein the thickness of NTCDA is in the range of 5-100nm and the thickness of PTCBI is up to 5 nm.

32. The method of claim 20, wherein at least one electrode comprises a transparent conductive oxide, a thin metal layer, or a transparent conductive polymer.

33. The method of claim 32, wherein the conductive oxide is selected from the group consisting of Indium Tin Oxide (ITO), Tin Oxide (TO), Gallium Indium Tin Oxide (GITO), Zinc Oxide (ZO), and Zinc Indium Tin Oxide (ZITO), the thin metal layer is composed of Ag, Al, Au, or a combination thereof, and the transparent conductive polymer comprises Polyaniline (PANI) and 3, 4-polyethylenedioxythiophene polystyrene sulfonate (PEDOT: PSS).

34. The method of claim 33, wherein at least one electrode comprises a metal substitute, a non-metallic material, or a metallic material selected from Ag, Au, Ti, Sn, and Al.