EP3170212A2 - Stable organic photosensitive devices with exciton-blocking charge carrier filters utilizing high glass transition temperature materials - Google Patents
Stable organic photosensitive devices with exciton-blocking charge carrier filters utilizing high glass transition temperature materialsInfo
- Publication number
- EP3170212A2 EP3170212A2 EP15745674.0A EP15745674A EP3170212A2 EP 3170212 A2 EP3170212 A2 EP 3170212A2 EP 15745674 A EP15745674 A EP 15745674A EP 3170212 A2 EP3170212 A2 EP 3170212A2
- Authority
- EP
- European Patent Office
- Prior art keywords
- layer
- energy gap
- exciton
- bis
- buffer
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
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- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
- H10K30/00—Organic devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation
- H10K30/30—Organic devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation comprising bulk heterojunctions, e.g. interpenetrating networks of donor and acceptor material domains
- H10K30/353—Organic devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation comprising bulk heterojunctions, e.g. interpenetrating networks of donor and acceptor material domains comprising blocking layers, e.g. exciton blocking layers
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- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
- H10K2101/00—Properties of the organic materials covered by group H10K85/00
- H10K2101/30—Highest occupied molecular orbital [HOMO], lowest unoccupied molecular orbital [LUMO] or Fermi energy values
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E10/00—Energy generation through renewable energy sources
- Y02E10/50—Photovoltaic [PV] energy
- Y02E10/549—Organic PV cells
Definitions
- the present invention generally relates to electrically active, optically active, solar, and semiconductor devices and, in particular, to organic photosensitive optoelectronic devices comprising at least one exciton-blocking charge carrier filter comprising a wide energy gap material that is morphologically stable at the operating temperature of the device. Also disclosed herein are methods of preparing the same.
- Optoelectronic devices rely on the optical and electronic properties of materials to either produce or detect electromagnetic radiation electronically or to generate electricity from ambient electromagnetic radiation.
- Photosensitive optoelectronic devices convert electromagnetic radiation into electricity.
- Solar cells also called photovoltaic (PV) devices
- PV devices which may generate electrical energy from light sources other than sunlight, can be used to drive power consuming loads to provide, for example, lighting, heating, or to power electronic circuitry or devices such as calculators, radios, computers or remote monitoring or communications equipment.
- power generation applications also often involve the charging of batteries or other energy storage devices so that operation may continue when direct illumination from the sun or other light sources is not available, or to balance the power output of the PV device with a specific application's requirements.
- the term "resistive load” refers to any power consuming or storing circuit, device, equipment or system.
- Another type of photosensitive optoelectronic device is a
- signal detection circuitry monitors the resistance of the device to detect changes due to the absorption of light.
- a photodetector In operation, a photodetector is used in conjunction with a current detecting circuit which measures the current generated when the photodetector is exposed to electromagnetic radiation and may have an applied bias voltage.
- a detecting circuit as described herein is capable of providing a bias voltage to a photodetector and measuring the electronic response of the photodetector to electromagnetic radiation.
- These three classes of photosensitive optoelectronic devices may be characterized according to whether a rectifying junction as defined below is present and also according to whether the device is operated with an external applied voltage, also known as a bias or bias voltage.
- a photoconductor cell does not have a rectifying junction and is normally operated with a bias.
- a PV device has at least one rectifying junction and is operated with no bias.
- a photodetector has at least one rectifying junction and is usually but not always operated with a bias.
- a photovoltaic cell provides power to a circuit, device or equipment, but does not provide a signal or current to control detection circuitry, or the output of information from the detection circuitry.
- photoconductor provides a signal or current to control detection circuitry, or the output of information from the detection circuitry but does not provide power to the circuitry, device or equipment.
- photosensitive optoelectronic devices have been constructed of a number of inorganic semiconductors, e.g., crystalline, polycrystalline and amorphous silicon, gallium arsenide, cadmium telluride and others.
- semiconductor denotes materials which can conduct electricity when charge carriers are induced by thermal or electromagnetic excitation.
- photoconductive generally relates to the process in which electromagnetic radiant energy is absorbed and thereby converted to excitation energy of electric charge carriers so that the carriers can conduct, i.e., transport, electric charge in a material.
- photoconductor and “photoconductive material” are used herein to refer to semiconductor materials which are chosen for their property of absorbing electromagnetic radiation to generate electric charge carriers.
- PV devices may be characterized by the efficiency with which they can convert incident solar power to useful electric power.
- Devices utilizing crystalline or amorphous silicon dominate commercial applications, and some have achieved efficiencies of 23% or greater.
- efficient crystalline-based devices, especially of large surface area are difficult and expensive to produce due to the problems inherent in producing large crystals without significant efficiency-degrading defects.
- high efficiency amorphous silicon devices still suffer from problems with stability. More recent efforts have focused on the use of organic photovoltaic cells to achieve acceptable photovoltaic conversion efficiencies with economical production costs.
- PV devices may be optimized for maximum electrical power generation under standard illumination conditions (i.e., Standard Test Conditions which are 1000 W/m 2 , AM1 .5 spectral illumination), for the maximum product of photocurrent times photovoltage.
- standard illumination conditions i.e., Standard Test Conditions which are 1000 W/m 2 , AM1 .5 spectral illumination
- the power conversion efficiency of such a cell under standard illumination conditions depends on the following three parameters: (1 ) the current under zero bias, i.e., the short-circuit current kc, in Amperes (2) the photovoltage under open circuit conditions, i.e., the open circuit voltage Voc, in Volts and (3) the fill factor, FF.
- PV devices produce a photo-generated current when they are connected across a load and are irradiated by light.
- a PV device When irradiated under infinite load, a PV device generates its maximum possible voltage, V open-circuit, or Voc- When irradiated with its electrical contacts shorted, a PV device generates its maximum possible current, I short-circuit, or Isc- When actually used to generate power, a PV device is connected to a finite resistive load and the power output is given by the product of the current and voltage, I * V.
- the maximum total power generated by a PV device is inherently incapable of exceeding the product, Isc x Voc- When the load value is optimized for maximum power extraction, the current and voltage have the values, Lax and V max , respectively.
- a figure of merit for PV devices is the fill factor, FF, defined as: (1 )
- the power efficiency of the device, ⁇ may be calculated by:
- n-type denotes that the majority carrier type is the electron. This could be viewed as the material having many electrons in relatively free energy states.
- p-type denotes that the majority carrier type is the hole. Such material has many holes in relatively free energy states.
- the type of the background, i.e., not photo-generated, majority carrier concentration depends primarily on unintentional doping by defects or impurities.
- the type and concentration of impurities determine the value of the Fermi energy, or level, within the gap between the conduction band minimum and valance band maximum energies, also known as the HOMO-LUMO gap.
- the Fermi energy characterizes the statistical occupation of molecular quantum energy states denoted by the value of energy for which the probability of occupation is equal to 1 ⁇ 2.
- a Fermi energy near the conduction band minimum (LUMO) energy indicates that electrons are the predominant carrier.
- a Fermi energy near the valence band maximum (HOMO) energy indicates that holes are the predominant carrier. Accordingly, the Fermi energy is a primary characterizing property of traditional semiconductors and the prototypical PV junction has traditionally been the p-n interface.
- rectifying denotes, inter alia, that an interface has an asymmetric conduction characteristic, i.e., the interface supports electronic charge transport preferably in one direction. Rectification is associated normally with a built- in electric field which occurs at the junction between appropriately selected materials.
- a significant property in organic semiconductors is carrier mobility. Mobility measures the ease with which a charge carrier can move through a conducting material in response to an electric field.
- a layer including a material that conducts preferentially by electrons due to a high electron mobility may be referred to as an electron transport layer, or ETL.
- a layer including a material that conducts preferentially by holes due to a high hole mobility may be referred to as a hole transport layer, or HTL.
- an acceptor material may be an ETL and a donor material may be an HTL.
- the donor material with a low ionization potential forms a heterojunction with an acceptor material with a high electron affinity.
- the dissociation of the exciton can become energetically favorable at such an interface, leading to a free electron polaron in the acceptor material and a free hole polaron in the donor material.
- Carrier generation requires exciton generation, diffusion, and ionization or collection. There is an efficiency ⁇ associated with each of these processes. Subscripts may be used as follows: P for power efficiency, EXT for external quantum efficiency, A for photon absorption, ED for diffusion, CC for collection, and INT for internal quantum efficiency. Using this notation:
- the diffusion length (I_D) of an exciton is typically much less (I_D ⁇ 50
- optical absorption length (-500 A) than the optical absorption length (-500 A), requiring a tradeoff between using a thick, and therefore resistive, cell with multiple or highly folded interfaces, or a thin cell with a low optical absorption efficiency.
- Organic PV cells have many potential advantages when compared to traditional silicon-based devices.
- Organic PV cells are light weight, economical in materials use, and can be deposited on low cost substrates, such as flexible plastic foils. For commercialization, however, device efficiencies must further improve via new material and device design approaches.
- Wide energy gap materials such as BCP and BPhen, have been employed as buffers. These materials function by blocking the transport of excitons due to their wide HOMO-LUMO energy gap while transporting electrons through defect states induced by the deposition of the cathode.
- a second function of these wide gap buffers is to space the optically absorbing layers further from the reflective cathode at the optimal position in the optical field.
- these buffers are limited to extremely thin films ( ⁇ 10 nm) by the penetration depth of the defect states accrued during deposition and they are highly resistive.
- a third type of buffer has been developed based on materials with LUMO energies aligned to that of the acceptor, such as PTCBI and NTCDA.
- the alignment of the LUMO levels allows efficient conduction of electrons from the acceptor to the cathode.
- These materials can also function to block excitons if their HOMO/LUMO gap is sufficiently large.
- these materials can hinder device performance if they absorb in the same spectral region as the active layer materials. Improvements to these device architectures must be made to increase conversion efficiencies of organic PV cells.
- the present inventors have developed a new type of buffer, disclosed herein as exciton-blocking charge carrier filters.
- These novel buffers comprise a mixture of at least one wide energy gap material and at least one electron or hole conducting material.
- the filters are optimized depending on their location in the device. That is, an exciton-blocking hole conducting filter is disposed between the photoactive region and the anode in order to block excitons and conduct holes to the anode.
- an exciton-blocking electron conducting filter is disposed between the photoactive region and the cathode in order to block excitons and conduct electrons to the cathode.
- the exciton-blocking electron filter electrons are transported by the electron conducting material via an impurity-band like mechanism. Simultaneously, excitons are blocked by a combination of an energetic barrier caused by the wide energy gap material and a statistical barrier caused by a reduction in the number of states available for transfer to the electron conductor.
- a problem with many buffers such as BCP or BPhen is that they are highly resistive and rely on damage induced transport states which limit the practical layer thickness to -10 nm.
- a wide energy gap material e.g. BCP
- a material with good transport properties e.g. C-6o
- the overall conductivity can be improved by using an impurity band-like transport.
- a second problem with buffers such as BCP or Bphen is that they can become morphologically unstable at operating temperatures that begin to approach or, in some cases, exceed their respective glass transition temperatures (T g ), which can contribute to performance degradation over time, significantly reducing the operational lifetime of the device. Under such conditions, the buffers can crystallize and degrade.
- T g glass transition temperatures
- the present inventors discovered that by mixing the wide energy gap material with an electron conductor (such as a fullerene) or a hole conductor, the buffer layer has increased stability, akin to "doped pinning" of the morphology.
- the operational lifetime of the devices can be significantly extended.
- the exciton-blocking charge carrier filters described herein also function to prevent a buildup of charges in the active layer which helps to reduce exciton-polaron quenching of excitons, increasing the short circuit current and fill factor of devices.
- an organic photosensitive optoelectronic device comprises two electrodes in superposed relation comprising an anode and a cathode; a photoactive region comprising at least one donor material and at least one acceptor material disposed between the two electrodes to form a donor-acceptor heterojunction, wherein the at least one acceptor material has a Lowest Unoccupied Molecular Orbital energy level (LU MOA CC ) and a Highest Occupied Molecular Orbital energy level (HOMOA CC ), and the at least one donor material has a Lowest Unoccupied Molecular Orbital energy level (LUMO d on) and a Highest Occupied Molecular Orbital energy level (HOMO don ); and an exciton- blocking electron filter disposed between the cathode and the at least one acceptor material, wherein the electron filter comprises a mixture comprising at least one cathode-side wide energy gap material and at least one electron conducting material, and wherein the at least one
- LU MOCS-WG Lowest Unoccupied Molecular Orbital energy level
- HOMOCS-WG Highest Occupied Molecular Orbital energy level
- the at least one cathode-side wide energy gap material has a glass transition temperature equal to or greater than 85° C.
- an organic photosensitive optoelectronic device comprises two electrodes in superposed relation comprising an anode and a cathode; a photoactive region comprising at least one donor material and at least one acceptor material disposed between two electrodes to form a donor-acceptor heterojunction, wherein the at least one donor material has a Lowest Unoccupied Molecular Orbital energy level (LUMOoon) and a Highest Occupied Molecular Orbital energy level (HOMOoon); and an exciton-blocking hole filter disposed between the anode and the at least one donor material, wherein the hole filter comprises a mixture comprising at least one anode-side wide energy gap material and at least one hole conducting material, and wherein the at least one anode-side wide energy gap material has:
- HOMO A S-WG Highest Occupied Molecular Orbital energy level
- LU MOAS-WG Lowest Unoccupied Molecular Orbital energy level
- the at least one anode-side wide energy gap material has a glass transition temperature equal to or greater than 85° C.
- Figure 1 shows schematics of exemplary organic photosensitive optoelectronic devices in accordance with the present disclosure.
- Device B comprises an exciton-blocking electron filter or an exciton-blocking hole filter
- Device B comprises an exciton-blocking electron filter and an exciton-blocking hole filter
- Figure 2 shows extinction spectra of C6o:BCP films with volume doping ratios of 1 :0 ⁇ t), 3:1 ( ⁇ ), 1 : 1 ( ⁇ ), 1 :2 ( ⁇ ), and 0:1 ( ⁇ ) calculated from k determined by spectroscopic ellipsometry. Inset: Decay of extinction as a function of C 60 fraction. 450 nm ( ⁇ ), 360 nm ( ⁇ ).
- Figure 3 shows J-V curves of devices under one sun AM1 .5G illumination on top with top inset showing characteristics of the devices and a plot of external quantum efficiency on bottom with bottom inset showing device structure.
- A:B 1 :0 (D1 ), 2:1 (D2), 1 :1 (D3), and 1 :2 (D4).
- Figure 6 shows J-V curves of devices under one sun AM1 .5G illumination on top, and external quantum efficiency on the bottom, with the bottom inset showing the device structure.
- Figure 7 shows J-V curves of devices under one sun AM1 .5G illumination on top, and external quantum efficiency on the bottom, with the bottom inset showing the device structure.
- Figure 8 shows the external quantum efficiency under applied bias (+0.5 V dashed, -1 V solid) normalized to EQE at zero bias for various buffer layers.
- FIG. 9 shows Monte Carlo simulation of the diffusion of excitons into a mixed layer of BCP:C 60 on top of a neat C 60 active layer, based solely on the reduced number of C6o molecules in the mixed films available for exciton transfer.
- Excitons are randomly generated in the active layer. They move randomly for a set number of steps, then their final position is recorded. They are assumed to diffuse solely by nearest-neighbor hopping.
- the probability for hopping between layers is scaled by the relative number of molecules of C 60 in each layer.
- Figure 10 on top shows normalized extinction spectra of C 7 o capped with different buffer layers and on bottom shows emission spectra of C 7 o (excited at 450 nm) capped with quenching (NPD), blocking (BCP), and mixed buffer layers on the bottom.
- Figure 11 shows EQE spectra of devices capped with various buffer layers (top) and J-V curves of devices under 0.8 sun AM1 .5G illumination.
- Figure 12 shows J-V curves of devices under one sun AM1 .5G illumination on top, and a plot of external quantum efficiency on the bottom, with bottom inset showing the device structure. Buffers: 10 nm BCP (D1 1 ), 10 nm C 60 :BCP (D12), 10 nm PTCBI (D13), 10 nm C 60 :BCP/5 nm PTCBI (D14), 10 nm BCP:C 60 /5 nm BCP (D15).
- Figure 13 on top shows EQE at -1 V normalized to 0 V EQE for the devices of Figure 12 with various buffer layers, and on bottom shows responsivity as a function of illumination intensity for the devices.
- Buffers 10 nm BCP (D1 1 ), 10 nm BCP:C 60 (D12), 10 nm PTCBI (D13), 10 nm BCP:C 60 /5 nm PTCBI (D14), 10 nm BCP:C 60 /5 nm BCP (D15).
- Figure 15 shows J-V curves under one sun AM1 .5G illumination on top and on the bottom a plot of the external quantum efficiency of devices with various buffer layers.
- Figure 16 shows J-V curves under one sun AM1 .5G illumination on top and on the bottom a plot of the external quantum efficiency of devices with various buffer layers containing various ratios of Ceo to BCP.
- Figure 17 shows J-V for illumination at 1 -sun simulated AM1 .5G illumination (upper left) and EQE (upper right) and extracted efficiency parameters (lower) for planar-mixed OPV cells with active layers comprising a relatively thick 1 :8 volume ratio of DBP and a neat layer of C 7 o.
- the thickness and volume ratio of the BPhen:C 7 o for each buffer is given in the table.
- Figure 18 shows J-V for illumination at 0.7-sun simulated AM 1 .5G illumination (upper left) and EQE (upper right) and extracted efficiency parameters (lower) for planar-mixed OPV cells with active layers comprising a relatively thin 1 :8 volume ratio of DBP and a neat layer of C 7 o and various buffers.
- the thickness and volume ratio of the BCP:C 7 o for each buffer is given in the table.
- Figure 19 shows the EQE and J-V curves for dilution with BPhen, CBP, and UGH2.
- Figure 20 shows spectrally-corrected current density v. voltage (J- V) characteristics under simulated AM 1 .5G, 1 sun illumination for DBP:C 7 o mixed-HJ OPV cells. The shaded region emphasizes the difference in fill factors, and hence maximum power output, of the two cells.
- Inset shows schematic of the device structure, and (b) shows external quantum efficiency (EQE) spectra for the cells in (a).
- Insert shows schematic diagrams of energy levels at the DBP:C70/buffer interface (left: neat BPhen buffer; right: BPhen:C60 compound buffer).
- Figure 21 shows responsivity v. light intensity for the mixed-HJ control cell and the compound buffer cell with linear fits according to bimolecular
- Figure 22 shows charge extraction time v. electric field for various layer thicknesses calculated using 3-D Monte-Carlo simulations.
- the inset shows cell series resistance (Rs) v. layer thickness with a linear fit (dashed line) to the data obtained from the OPV cells (squares) (Error bars in the inset are smaller than data points), and
- Figure 23 (a) shows spectrally-corrected current density v. voltage (J- V) characteristics under simulated AM 1 .5G, 1 sun illumination and (b) shows external quantum efficiency spectra for DBP:C 7 o PM-HJ OPV cells with various buffer layers.
- Figure 24 shows calculated absorption spectra and internal quantum efficiency for the control cell and the cell with a BPhen:C6o/BPhen buffer.
- Figure 25 shows responsivity (solid square) and power conversion efficiency (hollow square) v. light intensity for the control cell and the cell with a BPhen:C 60 /BPhen buffer.
- Figure 26 (a) shows spectrally-corrected J-V characteristics under simulated AM 1 .5G, 1 sun illumination, and (b) shows external quantum efficiency spectra as a function of thickness of BPhen:C6o mixed layer in the cells with a BPhen:C 60 /BPhen buffer.
- Figure 27 shows modeled median extraction time v. electric field as a function of BPhen:C6o mixed layer thickness, and the inset shows series resistance v. mixed layer thickness with a fitting.
- Figure 28 (a) shows J-V for illumination under 1 -sun simulated AM 1 .5G illumination with inset showing NPD, and (b) shows the external quantum efficiency for OPV cells with active layers comprised of DBP and C 60 and various buffers.
- Figure 29 shows schematics of exemplary tandem organic photosensitive optoelectronic devices in accordance with the present disclosure.
- Device A comprises an exciton-blocking electron filter or an exciton-blocking hole filter
- Device B comprises an exciton-blocking electron filter and an exciton- blocking hole filter.
- Figure 30 shows a DBP:C 7 o device having a mixed BPhen:C6o buffer layer (10 nm) with a thin BPhen cap layer (5 nm) between the mixed buffer layer and the Al electrode, or, alternatively, a mixed TPBi:C 7 o buffer layer (10 nm) with a thin TPBi cap layer (3 nm) between the mixed buffer layer and the Al electrode.
- Figures 31 A to 31 C show normalized responsivity, fill factor, Voc, and PCE over time at 50° C, 60° C, and 80° C, respectively, for the device of Figure 30 having the mixed Bphen:C6o buffer layer with the thin BPhen cap layer.
- Figures 32A to 32D show normalized responsivity, fill factor, Voc, and PCE over time at 50° C, 80° C, and 105° C, and 130° C, respectively, for the device of Figure 30 having the mixed TPBi:C6o buffer layer within the thin TPBi cap layer.
- Figure 33 shows the normalized power conversion efficiency over time of a DBP:C 7 o mixed heterojunction with various buffers.
- Figures 34 to 38 show molecular structures of exemplary wide energy gap materials BAIq, TPBi, Alq 3, BP4mPy, and 3TPYMB, respectively.
- Figures 39-41 show normalized responsivity, fill factor, Voc, and PCE at 55° C over time for devices having mixed buffers containing a mixture of C 7 o and one of TPBi, 3TPYMB, and BAIq, respectively.
- Figure 42 shows a DBP:C 70 device having a mixed 3TPYMB:C 60 buffer layer.
- Figures 43-46 show normalized responsivity, fill factor, Voc, and PCE over time for the device of Figure 42 at 55° C, 70° C, 85° C, and 100° C,
- organic includes polymeric materials as well as small molecule organic materials that may be used to fabricate organic photosensitive devices.
- Small molecule refers to any organic material that is not a polymer, and "small molecules” may actually be quite large. Small molecules may include repeat units in some circumstances. For example, using a long chain alkyl group as a substituent does not remove a molecule from the "small molecule” class. Small molecules may also be incorporated into polymers, for example as a pendent group on a polymer backbone or as a part of the backbone.
- the terms "donor” and “acceptor” refer to the relative positions of the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) energy levels of two contacting but different organic materials. If the LUMO energy level of one material in contact with another is further from the vacuum level, then that material is an acceptor. Otherwise it is a donor. It is energetically favorable, in the absence of an external bias, for electrons at a donor-acceptor junction to move into the acceptor material, and for holes to move into the donor material.
- HOMO Highest Occupied Molecular Orbital
- LUMO Lowest Unoccupied Molecular Orbital
- the term "cathode” is used in the following manner. In a non- stacked PV device or a single unit of a stacked PV device under ambient irradiation and connected with a resistive load and with no externally applied voltage, e.g., a solar cell, electrons move to the cathode from the adjacent photoconducting material.
- the term “anode” is used herein such that in a solar cell under illumination, holes move to the anode from the adjacent photoconducting material, which is equivalent to electrons moving in the opposite manner. It is noted that the “anode” and “cathode” electrodes may be charge transfer regions or recombination zones, such as those used in tandem photovoltaic devices. In a photosensitive optoelectronic device, it may be desirable to allow the maximum amount of ambient electromagnetic radiation from the device exterior to be admitted to the
- the electromagnetic radiation must reach a photoconductive layer(s), where it can be converted to electricity by photoconductive absorption.
- a photoconductive layer(s) where it can be converted to electricity by photoconductive absorption.
- at least one of the electrical contacts should be minimally absorbing and minimally reflecting of the incident electromagnetic radiation. In some cases, such a contact should be transparent or at least semi-transparent.
- An electrode is said to be “transparent” when it permits at least 50% of the ambient electromagnetic radiation in relevant wavelengths to be transmitted through it.
- An electrode is said to be “semi-transparent” when it permits some, but less that 50% transmission of ambient electromagnetic radiation in relevant wavelengths.
- the opposing electrode may be a reflective material so that light which has passed through the cell without being absorbed is reflected back through the cell.
- a photoactive region refers to a region of the device that absorbs electromagnetic radiation to generate excitons.
- a layer is “photoactive” if it absorbs electromagnetic radiation to generate excitons. The excitons may dissociate into an electron and a hole in order to generate an electrical current.
- a "layer” refers to a member or component of a photosensitive device whose primary dimension is X-Y, i.e., along its length and width. It should be understood that the term layer is not necessarily limited to single layers or sheets of materials. In addition, it should be understood that the surfaces of certain layers, including the interface(s) of such layers with other material(s) or layers(s), may be imperfect, wherein said surfaces represent an interpenetrating, entangled or convoluted network with other material(s) or layer(s). Similarly, it should also be understood that a layer may be discontinuous, such that the continuity of said layer along the X-Y dimension may be disturbed or otherwise interrupted by other layer(s) or material(s).
- a first HOMO or LUMO energy level is "smaller than” a second HOMO or LUMO energy level if the first HOMO or LUMO energy level is closer to the vacuum level than the second HOMO or LUMO energy level.
- a first HOMO or LUMO energy level is "larger than” a second HOMO or LUMO energy level if the first HOMO or LUMO energy level is further from the vacuum level than the second HOMO or LUMO energy level.
- two orbital energy levels are "equal" to one another as that term is used herein if their energies match to the tenth decimal place. For example, a LUMO energy of -3.70 eV would be considered “equal" to a LUMO energy of -3.79 eV for purposes of the present disclosure.
- LUMO Acc and ⁇ ⁇ represent the Lowest Unoccupied Molecular Orbital energy level and the Highest Occupied Molecular Orbital energy level, respectively, of the at least one acceptor material.
- LUMO Don and HOMO Don represent the Lowest Unoccupied Molecular Orbital energy level and the Highest Occupied Molecular Orbital energy level, respectively, of the at least one donor material.
- LU MOCS-WG and HOMOCS-WG represent the Lowest Unoccupied Molecular Orbital energy level and the Highest Occupied Molecular Orbital energy level, respectively, of the at least one cathode-side wide energy gap material.
- LU MOAS-WG and HOMOAS-WG represent the Lowest Unoccupied Molecular Orbital energy level and the Highest Occupied Molecular Orbital energy level, respectively, of the at least one anode-side wide energy gap material.
- LUMO E c and HOMO E c represent the Lowest Unoccupied Molecular Orbital energy level and the Highest Occupied Molecular Orbital energy level, respectively, of the at least one electron conducting material.
- LU MOHC and HOMOHC represent the Lowest Unoccupied Molecular Orbital energy level and the Highest Occupied Molecular Orbital energy level, respectively, of the at least one hole conducting material.
- a HOMO-LUMO energy gap is the energy difference between the HOMO and LUMO of a material.
- Electrode 110 comprises an anode or cathode.
- Electrode 140 comprises an anode when electrode 110 comprises a cathode.
- Electrode 140 comprises a cathode when electrode 110 comprises an anode.
- a photoactive region comprises donor/acceptor organic layers 120 and 130 to form a donor-acceptor heterojunction as described herein.
- the photoactive region may comprise additional donor and/or acceptor layers to form, e.g., a hybrid planar-mixed heterojunction.
- Organic layer 120 comprises at least one donor material or at least one acceptor material.
- Organic layer 130 comprises at least one donor material when layer 120 comprises at least one acceptor material.
- Organic layer 130 comprises at least one acceptor material when layer 120 comprises at least one donor material. It is noted that the
- donor/acceptor layers in Figure 1 need not be planar. That is, the present disclosure contemplates all types of donor-acceptor heterojunctions known in the art for organic photovoltaic devices, including those specifically described herein.
- layer 115 is an exciton-blocking electron filter when electrode 1 10 comprises a cathode, organic layer 120 comprises at least one acceptor material, organic layer 130 comprises at least one donor material, and electrode 140 comprises an anode.
- Layer 115 is an exciton-blocking hole filter when electrode 110 comprises an anode, organic layer 120 comprises at least one donor material, organic layer 130 comprises at least one acceptor material, and electrode 140 comprises a cathode.
- the device includes both an exciton-blocking electron filter and an exciton-blocking hole filter.
- Layer 115 is an exciton-blocking electron filter when electrode 110 comprises a cathode, organic layer 120 comprises at least one acceptor material, organic layer 130 comprises at least one donor material, layer 135 is an exciton-blocking hole filter, and electrode 140 comprises an anode.
- Layer 115 is an exciton-blocking hole filter when electrode 110 comprises an anode, organic layer 120 comprises at least one donor material, organic layer 130 comprises at least one acceptor material, layer 135 is an exciton- blocking electron filter, and electrode 140 comprises a cathode.
- devices A and B may comprise additional buffer layers or cap layers located between the exciton-blocking electron/hole filters and the nearest electrode.
- the exciton-blocking electron filter is disposed between the cathode and the at least one acceptor material and comprises a mixture comprising at least one cathode-side wide energy gap material and at least one electron conducting material.
- the at least one cathode-side wide energy gap material has:
- LU MOCS-WG Lowest Unoccupied Molecular Orbital energy level
- HOMOCS-WG Highest Occupied Molecular Orbital energy level
- the at least one electron conducting material has a Lowest
- Unoccupied Molecular Orbital energy level (LUMOEC) larger than, equal to, or within 0.3 eV smaller than the LU MOA CC , such as within 0.2 eV smaller.
- some embodiments of the invention utilize a cathode-side wide energy gap material having a glass transition temperature that is sufficiently high, e.g., higher than the temperature or temperature range at which the device typically operates, higher than a highest operating temperature of the device, higher than a threshold temperature value, etc.
- the HOMOCS-WG is larger than the HOMOACC, such as at least 0.2 eV larger, at least 0.3 eV larger, at least 0.5 eV larger, at least 1 eV larger, at least 1 .5 eV larger, or at least 2 eV larger, and the LU MOCS-WG is smaller than the LU MOA CC , such as at least 0.2 eV smaller, at least 0.3 eV smaller, at least 0.5 eV smaller, at least 1 eV smaller, at least 1 .5 eV smaller, or at least 2 eV smaller.
- the LU MOEC is equal to the LU MOA CC -
- the LU MOEC is larger than the LU MOA CC , such as within 0.5 eV larger, within 0.4 eV larger, within 0.3 eV larger, or within 0.2 eV larger.
- the LU MOEC is no more than 0.1 eV smaller or larger than the LU MOA CC -
- the LU MOCS-WG is smaller than the LU MOEC, such as at least 0.2 eV smaller, at least 0.3 eV smaller, at least 0.5 eV smaller, at least 1 eV smaller, at least 1 .5 eV smaller, or at least 2 eV smaller.
- the LU MOCS-WG is more than 0.2 eV smaller than the LU MOA CC , such as more than 0.3 eV smaller, more than 0.5 eV smaller, more than 1 eV smaller, more than 1 .5 eV smaller, or more than 2 eV smaller.
- the at least one cathode-side wide energy gap material comprises a material chosen from bathocuproine (BCP),
- BPhen bathophenanthroline
- UH-2 p-Bis(triphenylsilyl)benzene
- CBP (4,4'- ⁇ , ⁇ '- dicarbazole)biphenyl
- mCP N,N'-dicarbazolyl-3,5-benzene
- PVK poly(vinylcarbazole)
- phenanthrene and alkyl and/or aryl substituted phenanthrenes alkyl and/or aryl substituted derivatives of benzene, triphenylene and alkyl and/or aryl substituted triphenylenes, aza-substituted triphenylenes, oxidiazoles, triazoles, aryl-benzimidazoles, adamantane and alkyl and/or aryl substituted adamantanes, tetraarylmethane and its derivatives, 9,9-dialkyl-fluorene and its oligomers, 9,9-diaryl-fluorene and its oligomers, spiro-biphenyl and substituted derivatives, corannulene and its alkyl and/or aryl substituted derivatives, and derivatives thereof.
- PVK poly(vinylcarbazole)
- the operational lifetime of the devices may be increased by utilizing a cathode-side wide energy gap material that has a sufficiently high T g , e.g., higher than the temperature or temperature range at which the device typically operates, higher than a highest operating temperature of the device, higher than a threshold temperature value, etc.
- T g sufficiently high temperature
- the performance over time (i.e., normalized responsivity, fill factor, Voc, and PCE) of an exciton-blocking charge carrier filter comprising a mixture of Bphen:C6o (T g of Bphen ⁇ 62° C) degrades faster as the operating temperature increases from 50° C to 80°C than an exciton-blocking charge carrier filter comprising a mixture of TPBi:C 7 o (T g of TPBi ⁇ 122° C) as the operating temperature increases from 50° C to 130° C. That is, the performance of the device utilizing TPBi:C 7 o degrades more slowly over time even at higher operating temperatures than the device utilizing BPhen.
- the efficiency and operational lifetime of a device can be improved by, for example, replacing Bphen and blocking materials with similar T g values with a cathode-side wide energy gap material having a higher T g , such as TPBi.
- the at least one cathode-side wide energy gap material comprises a material having a sufficiently high T g , e.g., higher than the temperature or temperature range at which the device typically operates, higher than a highest operating temperature of the device, higher than a threshold temperature value, etc.
- the at least one cathode-side wide energy gap material comprises a material chosen from 3,3',5,5'-Tetra[(m-pyridyl)- phen-3-yl]biphenyl (BP4mPy), 2,2',2"-(1 ,3,5-Benzinetriyl)-tris(1 -phenyl-1 -H- benzimidazole) (TPBi), Bis(2-methyl-8-quinolinolate)-4-(phenylphenolato)aluminium (BAIq), Tris(8-hydroxy-quinolinato)aluminium (Alq3), Tris(2,4,6-trimethyl-3-(pyridin-3- yl)phenyl)borane (3TPYMB), 4,40-(1 ,3-phenylene) bis(2,6-dip-tolylpyridine-3,5- dicarbonitrile) (m-MPyCN), 4,40-(1 ,3-phenylene)
- the glass transition temperature of the cathode- side wide energy gap material is equal to or greater than 85° C, equal to or greater than 95° C, equal to or greater than 105° C, equal to or greater than 1 15° C, equal to or greater than 125° C, equal to or greater than 135° C, equal to or greater than 145° C, equal to or greater than 155° C, equal to or greater than 165° C, equal to or greater than 175° C, equal to or greater than 185° C, equal to or greater than 195° C, equal to or greater than 200° C, equal to or greater than 225° C, or equal to or greater than 250° C.
- the glass transition temperature of the cathode- side wide energy gap material is between 85-200° C, such as between 90-195° C, between 95-190° C, between 100-185° C, between 105-180° C, between 1 10-175° C, between 1 15-170° C, or between 120-165° C. It is noted that materials having glass transition temperatures greater than 200° C may be used, such as between 200-300° C, 200-275° C, 200-250° C, or 200-225° C.
- the operating temperature of the device may vary and depend on a number of factors, such as, for example, ambient conditions (e.g., temperature, light intensity, etc.) and whether enhancement mechanisms (e.g., solar concentrators) are used in conjunction with the device.
- ambient conditions e.g., temperature, light intensity, etc.
- enhancement mechanisms e.g., solar concentrators
- the ambient temperature may vary depending on the geographic location of the device, time of year, time of day, etc.
- light intensity may also depend on the geographic location, time of year, and time of day, as well as the amount of cloud cover, angle of incidence, and other factors.
- the cathode-side wide energy gap material has a sufficiently high T g , e.g., higher than the temperature or temperature range at which the device typically operates (e.g., at normal ambient conditions), higher than the highest operating temperature of the device under normal ambient conditions, etc.
- a solar concentrator may be integrated with or used in conjunction with the device to increase, magnify, or otherwise enhance light directed at the device.
- the use of concentrators and/or other enhancement mechanisms may raise the operating temperature of the device beyond that experienced by the device under normal ambient conditions. Accordingly, to increase the stability and operational lifetime of the device, the cathode-side wide energy gap material may have a T g that is higher than the highest operating temperature experienced by the device under enhanced lighting conditions.
- the at least one acceptor material comprises a material chosen from subphthalocyanines, subnaphthalocyanines, dipyrrin complexes, such as zinc dipyrhn complexes, BODIPY complexes, perylenes, naphthalenes, fullerenes and fullerene derivatives (e.g., PCBMs, ICBA, ICMA, etc.), and polymers, such as carbonyl substituted polythiophenes, cyano-substituted polythiophenes, polyphenylenevinylenes, or polymers containing electron deficient monomers, such as perylene diimide, benzothiadiazole or fullerene polymers.
- subphthalocyanines such as zinc dipyrhn complexes, BODIPY complexes
- perylenes such as zinc dipyrhn complexes, BODIPY complexes
- naphthalenes naphthalenes
- the at least one electron conducting material comprises a material chosen from subphthalocyanines, subnaphthalocyanines, dipyrhn complexes, such as zinc dipyrhn complexes and BODIPY complexes, perylenes, naphthalenes, fullerenes and fullerene derivatives (e.g., PCBMs, ICBA, ICMA, etc.), and polymers, such as carbonyl substituted polythiophenes, cyano- substituted polythiophenes, polyphenylenevinylenes, or polymers containing electron deficient monomers, such as perylene diimide, benzothiadiazole or fullerene polymers.
- subphthalocyanines such as zinc dipyrhn complexes and BODIPY complexes
- perylenes naphthalenes, fullerenes and fullerene derivatives
- polymers such as carbonyl substituted polythiophenes, cyano- substituted polythiophene
- the at least one acceptor material comprises a material chosen from fullerenes and functionalized fullerene derivatives.
- the at least one electron conducting material comprises a material chosen from fullerenes and functionalized fullerene derivatives.
- Fullerenes are of particular interest for use as the at least one electron conducting material.
- C-60 for example, has an absorption spectrum in solution that is dominated by two features with peaks at wavelengths of 260 nm and 340 nm that are attributed to allowed electronic transitions resulting in Frenkel-type (i.e., monomolecular) excited states, while the absorption at longer wavelengths is due to a symmetry-forbidden transition.
- CT intermolecular charge transfer
- fullerenes when mixed with a cathode-side wide energy gap material can be used as a good electron conducting material, with diminished absorptivity (even at moderate dilutions, such as 70% C 60 and 30% wide gap material), so as not to generate excitons in the fullerene electron conducting material that would otherwise fail to contribute to the device's efficiency.
- the at least one electron conducting material comprises a material chosen from C 60 and C 70 .
- the at least one acceptor material and the at least one electron conducting material comprise the same material.
- the same material is a fullerene or functionalized fullerene derivative.
- the same material is C 60 or C 70 .
- the at least one acceptor material and the at least one electron conducting material comprise different materials.
- the at least one acceptor material and the at least one electron conducting material are chosen from different fullerenes and functionalized fullerene derivatives.
- the mixture comprises the at least one cathode- side wide energy gap material and the at least one electron conducting material at a ratio ranging from about 10:1 to 1 :10 by volume, such as about 8:1 to 1 :8 by volume, about 6:1 to 1 :6 by volume, about 4:1 to 1 :4 by volume, or about 2:1 to 1 :2 by volume. In certain embodiments, the ratio is about 1 : 1 . It should be understood that the identified ratios include integer and non-integer values.
- the donor-acceptor heterojunction is chosen from a mixed heterojunction, bulk heterojunction, planar heterojunction, and hybrid planar-mixed heterojunction.
- the donor-acceptor heterojunction is a hybrid planar-mixed heterojunction (PM-HJ).
- PM-HJ hybrid planar-mixed heterojunction
- ⁇ the Langevin recombination constant
- n (p) the free electron (hole) density.
- a second significant loss is due to exciton-polaron quenching in the neat acceptor layer.
- the electron-polaron build-up has been observed at the neat acceptor/blocking layer interface that results in quenching and, therefore, a reduction of internal quantum efficiency (IQE).
- IQE internal quantum efficiency
- exciton-polaron quenching follows a similar relationship to bimolecular recombination, as both exciton and polaron concentrations are proportional to intensity. Both mechanisms can result in a loss in photocurrent under forward bias that increases the slope of current density-voltage (J-V) characteristics in the fourth quadrant, ultimately decreasing both FF and PCE.
- the exciton-blocking electron filter disposed between the photoactive region and the cathode can increase the efficiency of bilayer OPV cells.
- the electron conducting material efficiently conducts electron-polarons and the wide energy gap material blocks excitons.
- Exciton-polaron quenching can be significantly reduced in bilayer cells employing the electron filter due to its ability to spatially separate excitons and polarons at the blocking interface. In turn, this can lead to a significant increase in Jsc, while Voc and FF remained unchanged.
- PM-HJ cells additionally suffer from bimolecular recombination in the mixed photoactive layer.
- the filters (mixed layers) of the present disclosure result in a reduced interfacial field with the active layer due to its increased conductivity compared to a neat, conventional blocking buffer layer.
- the resulting increase in field across the photosensitive region leads to more rapid charge extraction. This, in turn, leads to reduced bimolecular recombination in the cell.
- the device further comprises at least one additional buffer layer or cap layer disposed between the exciton-blocking electron filter and the cathode.
- the at least one cap layer has a LU MO energy level larger than, equal to, or within 0.3 eV smaller than the LUMOEC, such as within 0.2 eV smaller, in order to conduct electrons to the cathode.
- the LU MO energy level of the cap layer is within 0.5 eV larger than the LU MOEC, such as within 0.4 eV larger, within 0.3 eV larger, or within 0.2 eV larger.
- the cap layer has a LU MO energy level no more than 0.1 eV smaller or larger than the LU MOEC-
- the at least one cap layer is chosen from fullerenes and functionalized fullerene derivatives.
- the at least one cap layer comprises PTCBI.
- the cap layer comprises a material having a LUMO energy level that does not facilitate conduction of electrons to the cathode.
- the cap layer may be sufficiently thin so as to transport electrons through damaged induced states.
- the at least one cap layer comprises a material chosen from BCP, BPhen, UGH-2, and CBP.
- the at least one cap layer and the at least one electron conducting material comprise the same material. In some embodiments, the at least one cap layer, the at least one electron conducting material, and the at least one acceptor material comprise the same material.
- the at least one cap layer and the at least one cathode-side wide energy gap material comprise the same material.
- the exciton-blocking hole filter is disposed between the anode and the at least one donor material and comprises a mixture comprising at least one anode- side wide energy gap material and at least one electron conducting material.
- the at least one anode-side wide energy gap material has:
- HOMOAS-WG Highest Occupied Molecular Orbital energy level
- LUMOAS-WG Lowest Unoccupied Molecular Orbital energy level
- the at least one hole conducting material has a Highest Occupied Molecular Orbital energy level (HOMOHC) smaller (closer to the vacuum) than, equal to, or within 0.2 eV larger (further from the vacuum) than the HOMODon-
- some embodiments of the invention utilize an anode-side wide energy gap material having a glass transition temperature that is sufficiently high, e.g., higher than the temperature or temperature range at which the device typically operates, higher than a highest operating temperature of the device, higher than a threshold temperature value, etc.
- the HOMOAS-WG is larger than the HOMO DON , such as at least 0.2 eV larger, at least 0.3 eV larger, at least 0.5 eV larger, at least 1 eV larger, at least 1 .5 eV larger, or at least 2 eV larger, and the LU MOAS-WG is smaller than the LUMOoon, such as at least 0.2 eV smaller, at least 0.3 eV smaller, at least 0.5 eV smaller, at least 1 eV smaller, at least 1 .5 eV smaller, or at least 2 eV smaller.
- the HOMOHC is equal to the HOMO DON - [125] In some embodiments, the HOMOHC is smaller than the HOMOoon, such as within 0.5 eV smaller, within 0.4 eV smaller, within 0.3 eV smaller, or within
- the HOMOHC is no more than 0.1 eV smaller or larger than the HOMO DON -
- the HOMOAS-WG is larger than the HOMOHC, such as at least 0.2 eV larger, at least 0.3 eV larger, at least 0.5 eV larger, at least 1 eV larger, at least 1 .5 eV larger, or at least 2 eV larger.
- the HOMOAS-WG is more than 0.2 eV larger than the HOMOoon, such as more than 0.3 eV larger, more than 0.5 eV larger, more than 1 eV larger, more than 1 .5 eV larger, or more than 2 eV larger.
- the at least one anode-side wide energy gap material comprises a material chosen from tetraaryl-benzindines, such as ⁇ , ⁇ '- diphenyl-N,N'-bis(1 -naphthyl)-1 -1 'biphenyl-4,4'diamine (NPD) and N,N'-bis-(3- methylphenyl)-N,N'-bis-(phenyl)-benzidine (TPD), triaryl amines, 5,10-disubstituted anthracenes, oligothiophenes, 9,9-dialkyl-fluorene and oligomers thereof, 9,9-diaryl- fluorene and oligomers thereof, oligophenylenes, spiro-biphenyl and substituted derivatives thereof, and derivatives thereof.
- tetraaryl-benzindines such as ⁇ , ⁇ '- diphenyl-N,N'-bis(1 -na
- the operational lifetime of the devices may be increased by utilizing an anode-side wide energy gap material that has a sufficiently high T g , e.g., higher than the temperature or temperature range at which the device typically operates, higher than a highest operating temperature of the device, higher than a threshold temperature value, etc.
- the at least one anode- side wide energy gap material comprises a material chosen from 3,3',5,5'-Tetra[(m- pyridyl)-phen-3-yl]biphenyl (BP4mPy), 2,2',2"-(1 ,3,5-Benzinetriyl)-tris(1 -phenyl-1 -H- benzimidazole) (TPBi), Bis(2-methyl-8-quinolinolate)-4-(phenylphenolato)aluminium (BAIq), Tris(8-hydroxy-quinolinato)aluminium (Alq3), Tris(2,4,6-trimethyl-3-(pyridin-3- yl)phenyl)borane (3TPYMB), 4,40-(1 ,3-phenylene) bis(2,6-dip-tolylpyridine-3,5- dicarbonitrile) (m-MPyCN), 4,40-(1 ,3-phenylene) bis
- the glass transition temperature of the anode- side wide energy gap material is equal to or greater than 85° C, equal to or greater than 95° C, equal to or greater than 105° C, equal to or greater than 1 15° C, equal to or greater than 125° C, equal to or greater than 135° C, equal to or greater than 145° C, equal to or greater than 155° C, equal to or greater than 165° C, equal to or greater than 175° C, equal to or greater than 185° C, equal to or greater than 195° C, equal to or greater than 200° C, equal to or greater than 225° C, or equal to or greater than 250° C.
- the glass transition temperature of the anode- side wide energy gap material is between 85-200° C, between 90-195° C, between 95-190° C, between 100-185° C, between 105-180° C, between 1 10-175° C, between 1 15-170° C, between 120-165° C. It is noted that materials having glass transition temperatures greater than 200° C may be used, such as between 200- 300° C, 200-275° C, 200-250° C, or 200-225° C.
- the operating temperature of the device may vary and depend on a number of factors, such as, for example, ambient conditions (e.g., temperature, light intensity, etc.) and whether enhancement mechanisms (e.g., solar concentrators) are used in conjunction with the device.
- ambient conditions e.g., temperature, light intensity, etc.
- enhancement mechanisms e.g., solar concentrators
- the ambient temperature may vary depending on the geographic location of the device, time of year, time of day, etc.
- light intensity may also depend on the geographic location, time of year, and time of day, as well as the amount of cloud cover, angle of incidence, and other factors.
- the anode-side wide energy gap material has a sufficiently high T g , e.g., higher than the temperature or temperature range at which the device typically operates (e.g., at normal ambient conditions), higher than the highest operating temperature of the device under normal ambient conditions, etc.
- a solar concentrator may be integrated with or used in conjunction with the device to increase, magnify, or otherwise enhance light directed at the device.
- the use of concentrators and/or other enhancement mechanisms may raise the operating temperature of the device beyond that experienced by the device under normal ambient conditions. Accordingly, to increase the stability and operational lifetime of the device, the anode-side wide energy gap material may have a T g that is higher than the highest operating temperature experienced by the device under enhanced lighting conditions.
- the at least one donor material comprises a material chosen from phthalocyanines, such as copper phthalocyanine(CuPc), chloroaluminium phthalocyanine (CIAIPc), tin phthalocyanine (SnPc), zinc phthalocyanine (ZnPc), and other modified phthalocyanines, subphthalocyanines, such as boron subphthalocyanine (SubPc), naphthalocyanines, merocyanine dyes, boron-dipyrromethene (BODIPY) dyes, thiophenes, such as poly(3-hexylthiophene) (P3HT), low band-gap polymers, polyacenes, such as pentacene and tetracene, diindenoperylene (DIP), squaraine (SQ) dyes, tetraphenyldibenzoperiflanthene (DBP), and derivatives thereof.
- phthalocyanines such as copper phthal
- squaraine donor materials include but are not limited to 2,4-bis [4-(N,N-dipropylamino)-2,6-dihydroxyphenyl] squaraine, 2,4- bis[4-(N,Ndiisobutylamino)-2,6-dihydroxyphenyl] squaraine, 2,4-bis[4-(N,N- diphenylamino)-2,6-dihydroxyphenyl] squaraine (DPSQ).
- the at least one hole conducting material comprises a material chosen from phthalocyanines, such as copper
- subphthalocyanines such as boron subphthalocyanine (SubPc), naphthalocyanines, merocyanine dyes, boron-dipyrromethene (BODIPY) dyes, thiophenes, such as poly(3-hexylthiophene) (P3HT), low band-gap polymers, polyacenes, such as pentacene and tetracene, diindenoperylene (DIP), squaraine (SQ) dyes,
- squaraine donor materials include but are not limited to 2,4-bis [4-(N,N- dipropylamino)-2,6-dihydroxyphenyl] squaraine, 2,4-bis[4-(N,Ndiisobutylamino)-2,6- dihydroxyphenyl] squaraine, 2,4-bis[4-(N,N-diphenylamino)-2,6-dihydroxyphenyl] squaraine (DPSQ).
- the at least one donor material and the at least one hole conducting material comprise the same material. In some embodiments, the at least one donor material and the at least one hole conducting material comprise different materials.
- the mixture comprises the at least one anode- side wide energy gap material and the at least one hole conducting material at a ratio ranging from about 10:1 to 1 :10 by volume, such as about 8:1 to 1 :8 by volume, about 6:1 to 1 :6 by volume, about 4:1 to 1 :4 by volume, or about 2:1 to 1 :2 by volume. In certain embodiments, the ratio is about 1 : 1 . It should be understood that the identified ratios include integer and non-integer values.
- the device further comprises at least one additional buffer layer or cap layer disposed between the exciton-blocking hole filter and the anode.
- the organic photosensitive optoelectronic devices disclosed herein can be grown or placed on any substrate that provides desired structural properties.
- the device further comprises a substrate.
- the substrate may be flexible or rigid, planar or non-planar.
- the substrate may be transparent, translucent or opaque.
- the substrate may be reflective.
- Plastic, glass, metal, and quartz are examples of rigid substrate materials.
- Plastic and metal foils and thin glass are examples of flexible substrate materials.
- the material and thickness of the substrate may be chosen to obtain the desired structural and optical properties.
- Organic photosensitive optoelectronic devices of the present disclosure may function, for example, as PV devices, such as solar cells, photodetectors, or photoconductors.
- the materials used in the photoconductive organic layers and the thicknesses thereof may be selected, for example, to optimize the external quantum efficiency of the device. For example, appropriate thicknesses can be selected to achieve the desired optical spacing in the device and/or reduce resistance in the device.
- the materials used in the photoconductive organic layers and the thicknesses thereof may be selected, for example, to maximize the sensitivity of the device to desired spectral regions.
- the devices may further comprise at least one smoothing layer.
- a smoothing layer may be located, for example, between the photoactive layers and either or both of the electrodes.
- PEDOT polyethylenedioxythiophene:polystyrenesulfonate
- PSS polystyrenesulfonate
- the organic photosensitive optoelectronic devices of the present disclosure may exist as a tandem device comprising two or more subcells.
- a subcell as used herein, means a component of the device which comprises at least one donor-acceptor heterojunction.
- a subcell When a subcell is used individually as a photosensitive optoelectronic device, it typically includes a complete set of electrodes.
- a tandem device may comprise charge transfer material, electrodes, or charge recombination material or a tunnel junction between the tandem donor- acceptor heterojunctions.
- the subcells may be electrically connected in parallel or in series.
- the recombination layer may be chosen from Al, Ag, Au, M0O 3 , Li, LiF, Sn, Ti, W03, indium tin oxide (ITO), tin oxide (TO), gallium indium tin oxide (GITO), zinc oxide (ZO), or zinc indium tin oxide (ZITO).
- the charge transfer layer or charge recombination layer may be comprised of metal nanoclusters, nanoparticles, or nanorods.
- the devices of the present disclosure may comprise an exciton-blocking charge carrier filter, as described herein, disposed between a first subcell and a second subcell of a device that comprises two or more subcells.
- an exciton-blocking charge carrier filter as described herein, disposed between a first subcell and a second subcell of a device that comprises two or more subcells.
- Electrode 110 comprises an anode or cathode.
- Electrode 140 comprises an anode when electrode 110 comprises a cathode.
- Electrode 140 comprises a cathode when electrode 110 comprises an anode.
- the tandem devices include two photoactive regions 150 and 160. Each of these regions may comprise donor and acceptor organic materials to form donor-acceptor heterojunctions as described herein.
- layer 115 may be an exciton-blocking electron filter as described herein. In some embodiments, layer 115 may be an exciton-blocking hole filter as described herein. The layer 115 is disposed between the photoactive regions 150 and 160 of the subcells. In still other embodiments, as depicted by device B of Figure 29, a tandem device may include additional exciton- blocking charge carrier filters. For example, the tandem photosensitive device may include two charge carrier filters disposed between the subcells. In this
- the tandem device may also further include exciton blocking charge carrier filters disposed between a photoactive region and an electrode, such as between the photoactive region 150 and the electrode 140.
- Devices A and B may further comprise an additional separating layer disposed between the subcells.
- the separating layer may comprise at least one electrode, at least one charge transfer layer, or at least one charge recombination layer.
- the separating layer may be disposed between exciton blocking charge filter 115 and either photoactive region 150 or photoactive region 160, and is optionally adjacent to the exciton blocking charge filter 115.
- the separating layer is a charge recombination layer as known in the art or otherwise described herein.
- the separating layer may be disposed between exciton blocking charge filter 115 and exciton blocking charge filter 135, and is optionally adjacent to either or both of layers 115 and 135.
- the separating layer is a charge recombination layer as known in the art or otherwise described herein.
- the layer 115 may be an exciton-blocking hole filter that blocks excitons and conducts holes from the photoactive region 160 to the charge recombination layer.
- the layer 115 may be an exciton-blocking electron filter that blocks excitons and conducts electrons from the photoactive region 160 to the charge recombination layer.
- a tandem device could also include two or more exciton-blocking charge filters disposed between the subcells, as depicted by device B of Figure 29.
- a charge recombination layer is disposed between layers 115 and 135, when the electrode 110 comprises a cathode, the layer 135 may be an exciton-blocking hole filter that blocks excitons and conduct holes from the photoactive region 160 to the charge recombination layer.
- the layer 115 may be an exciton-blocking electron filter that blocks excitons and conducts electrons from the photoactive region 150 to the charge recombination layer.
- the layer 135 may be an exciton-blocking electron filter that blocks excitons and conducts electrons from the photoactive region 160 to the charge recombination layer.
- the layer 115 may be an exciton-blocking hole filter that blocks excitons and conducts holes from the photoactive region 150 to the charge recombination layer.
- an advantage of the present disclosure is that the exciton-blocking charge carrier filters may be transparent because they utilize wide energy gap materials. By mixing a transparent wide energy gap material with an electron or hole conducting material, the electron or hole conducting material may be sufficiently diluted, rendering the exciton-blocking charge carrier filter transparent at relevant wavelengths.
- An exciton-blocking charge carrier filter is said to be "transparent" when it permits at least 50% of the incident electromagnetic radiation at relevant wavelengths to be transmitted through it.
- the filters permit at least 60%, at least 70%, at least 80%, at least 90%, or approximately 100% of incident electromagnetic radiation at relevant wavelengths to be transmitted through it.
- the charge carrier filters are said to be optically lossless when they absorb very little ( ⁇ 1 %) electromagnetic radiation in relevant wavelengths.
- the devices may employ one or more buffer layers that are preferably transparent throughout the visible spectrum.
- Charge collecting/transporting buffer layers may be between the photoactive regions and the corresponding electrodes.
- Charge collecting/transporting buffer layers may also be disposed, for example, between the separating layer and a photoactive region.
- the charge collecting/transporting layers comprise a material chosen from metal oxides.
- the metal oxides are chosen from M0O 3 , V2O5, ZnO, and Ti0 2 .
- Layers and materials may be deposited using techniques known in the art.
- the layers and materials described herein can be deposited or co- deposited from a solution, vapor, or a combination of both.
- organic materials or organic layers can be deposited or co-deposited via solution processing, such as by one or more techniques chosen from spin-coating, spin- casting, spray coating, dip coating, doctor-blading, inkjet printing, or transfer printing.
- organic materials may be deposited or co- deposited using vacuum evaporation, such as vacuum thermal evaporation, organic vapor phase deposition, or organic vapor-jet printing.
- the exciton-blocking charge carrier filters of the present disclosure which comprise a mixture of materials, may be fabricated by varying deposition conditions.
- concentration of each material in the mixture can be controlled by varying the deposition rate of each material.
- BCP bathocuproine
- Ceo and bathocuproine (BCP) were mixed at various concentrations to form an exciton-blocking electron filter.
- BCP is a wide-energy gap material with higher singlet (3.17 eV) and triplet (2.62 eV) energies than C6o (1 .86 eV singlet, 1 .55 eV triplet) and a LUMO (-1 .6 eV), making BCP an inert dopant and preventing both energy and electron transfer from C 60 -
- the doped C6 0 :BCP film effectively blocks excitons, while still conducting electrons. Based on these properties, the doped films were applied as a buffer layer/filter resulting in improved device performance compared to devices with other buffers .
- UVOCS T10X10/OES for 10 min immediately before loading into the high vacuum chamber.
- Deposition rates for layers of neat materials were as follows: MoO x (0.02 nm/s), NPD (0.1 nm/s), C 60 (0.1 nm/s), BCP (0.1 nm/s) and Al (0.2 nm/s).
- Deposition rates for doped films were as follows: C6o:BCP (2: 1 ) - codeposition C 60 (0.08 nm/s) : BCP (0.04 nm/s); C 60 :BCP (1 :1 ) - codeposition C 60 (0.06 nm/s) : BCP (0.06 nm/s); C 60 :BCP (1 :2) - codeposition C 60 (0.04 nm/s) : BCP (0.08 nm/s).
- masks with 2 mm stripe width were placed on substrates under N 2 , and 100 nm of Al cathode was deposited. The area of the devices was 4 mm 2 .
- Table 1 Device performance data.
- Table 2 provides performance data for this device.
- Table 2 Device performance data.
- FIG. 5 A device, shown in Figure 5, was fabricated according to the fabrication method disclosed in Example 2.
- Table 3 provides performance data for this device.
- Table 3 Device performance data.
- OPV devices as shown in Figure 6 (bottom inset), were fabricated.
- OPVs containing a 10 nm thick BCP:C6o layer sandwiched between two ⁇ layers (one is x ⁇ 35nm thick and the other is [40 nm - x] thick) were fabricated with the red absorbing donor (2,4-bis[4-(N,N-diphenylamino)-2,6-dihydroxyphenyl] squaraine) (DPSQ).
- DPSQ red absorbing donor
- the total thicknesses of neat C 60 and BCP:C6o films were 50 nm.
- the Jsc decreased from 6.2 ⁇ 0.3 mA/cm 2 to 4.1 ⁇ 0.2 mA/cm 2 as the BCP:C 60 layer moved toward the D/A interface (i.e. as x is decreased). This trend was also apparent in the EQE spectra where the response from C 60 decreased as the thickness of the neat ⁇ layer adjacent to the D/A interface decreased (D20 to D23 in Table 4). These data suggested that BCP:C6o prevents excitons generated in the ⁇ film adjacent to the metal electrode from diffusing to the D/A interface where dissociation into free charges can occur.
- FF 0.72 ⁇ 0.01
- V oc 0.94 ⁇ 0.01 V.
- Increasing the thickness of the C & o layer adjacent to the D/A interface from x 5 nm to 35 nm increased the power conversion efficiency under 1 sun, AM 1 .5G illumination from 2.7 ⁇ 0.1 % to 4.1 ⁇ 0.1 %.
- Table 4 Device performance data.
- FIG. 7 A device, shown in Figure 7, was fabricated according to the fabrication method disclosed in Example 2.
- Figure 7 shows J-V curves of devices under one sun AM1 .5G illumination with the inset showing characteristics of the devices, and external quantum efficiency as a function of wavelength with the inset showing the device structure. These devices compared the performance of the mixed buffer layer capped with an additional layer to enhance charge collection to a single neat PTCBI buffer layer.
- FIG. 8 A device, as shown in Figure 8, was fabricated according to the fabrication method disclosed in Example 2.
- Figure 8 shows the external quantum efficiency under applied bias (+0.5 V dashed, -1 V solid) normalized to EQE at zero bias for various buffer layers.
- Example 8 The mechanism by which the mixed layer blocks excitons can be thought of in a statistical manner, where a reduction in the density of states available in the doped layer reduces the rate of exciton transfer. In the mixed layer there are a significantly reduced number of states which the energy can transfer into, effectively blocking their progress.
- the effect of the change in the density of states was modeled by a Monte Carlo simulation, the results of which can be seen in Figure 9.
- excitons were randomly generated in a neat film adjacent to a mixed film. To simulate diffusion, the excitons then moved via a random walk for a set number of steps and their final position was recorded. The excitons were assumed to transfer solely through nearest-neighbor hopping.
- the probability for hopping between layers was scaled by the relative number of available sites in each layer.
- the model predicted that for a junction between two materials with equal site densities, 50% of the excitons diffused into the buffer. In the case of a 50 % reduction in sites in the buffer, which corresponded to the case of a Frenkel exciton of C6o approaching a buffer of 1 :1 C6o:BCP, only 20% of excitons were transferred. At 80% site reduction, simulating the case of a CT exciton approaching a buffer of 1 :1 C 60 :BCP, less than 5% of excitons were transferred. These simulations demonstrated that the doped buffer blocked excitons quite well even when only considering statistical means.
- the V oc of the devices remained constant at 0.95 ⁇ 0.01 V, independent of the buffer.
- the other buffers with thicknesses of 10 nm, BCP (D1 1 ) and C 60 :BCP (D12) did not absorb, resulting in an increase in Jsc to 7.5 ⁇ 0.1 mA/cm 2 and 7.6 ⁇ 0.1 mA/cm 2 , respectively.
- the buffer comprised of only C6o:BCP (D12) had a slightly better FF of 0.66 ⁇ 0.01 .
- Table 5 Device performance data.
- the doped layer In contrast to its exciton blocking function, the doped layer exhibited good charge conductivity as the FF of the devices remained constant around 0.72 ⁇ 0.01 . The Voc also remained constant at 0.94 ⁇ 0.01 . Overall, increasing the thickness of the neat ⁇ layer adjacent to the D/A interface from 5 nm to 35 nm increased ⁇ from 2.7 ⁇ 0.1 % to 4.1 ⁇ 0.1 %.
- Table 6 Device performance data.
- Figure 15 shows J-V curves under one sun AM1 .5G illumination on top and on the bottom a plot of the external quantum efficiency of devices with various buffer layers as a function of wavelength. These devices demonstrated the performance enhancement due to inclusion of the mixed buffer layer.
- Figure 16 shows J-V curves under one sun AM1 .5G illumination on top and on the bottom a plot of the external quantum efficiency of devices with various buffer layers containing various ratios of C 60 to BCP. These devices demonstrated an optimum blending ratio of 1 :1 by volume.
- Figure 17 shows J-V for illumination at 1 -sun simulated AM1 .5G illumination (upper left) and EQE (upper right) and extracted efficiency parameters (lower) for planar-mixed OPV cells with active layers comprising a relatively thick 1 :8 volume ratio of DBP and a neat layer of C 7 o.
- the thickness and volume ratio of the BPhen:C 7 o for each buffer is given in the table. The best fill factor and efficiency was for devices with the mixed buffer with an additional BPhen or PTCBI layer between the mixed region and the contact demonstrating that this contact also improved mixed or bulk heterojunction devices.
- Figure 18 shows J-V for illumination at 0.7-sun simulated AM1 .5G illumination (upper left) and EQE (upper right) and extracted efficiency parameters (lower) for planar-mixed OPV cells with active layers comprising a relatively thin 1 :8 volume ratio of DBP and a neat layer of C 7 o and various buffers.
- the thickness and volume ratio of the BCP:C 7 o for each buffer is given in the table.
- the best fill factor and efficiency were for devices with the mixed buffer with an additional BPhen or PTCBI layer between the mixed region and the contact demonstrating that this contact also improved mixed or bulk heterojunction devices and could yield exceptional fill factors in mixed-layer devices.
- the device in Figure 19(b) had the structure ITO/Mo0 3 /DPSQ/C60/C 60 :BCP(x)/BPhen/AI.
- the device in Figure 19(c) had the structure ITO/Mo0 3 /DPSQ/C60/C 60 :UGH2(x)/BPhen/AI.
- Figure 19 shows the EQE and J-V curve for dilution with BPhen, CBP, and UGH2. An enhancement was observed in all the cases, confirming that reducing the buildup of charge at the C Buffer interface resulted in enhancement.
- OPV cells were grown by vacuum thermal evaporation (VTE) at a base pressure of 2x10 "7 torr on glass substrates pre-coated with indium tin oxide (ITO, sheet resistance: 15 ⁇ /D). Prior to deposition, the substrates were cleaned in diluted Tergitol® (Type NP-10), deionized water, acetone and isopropyl alcohol, and then exposed to ultraviolet-ozone for 10 min.
- M0O3 was obtained from Acros Organics, C60 from Materials and Electrochemical Research Corp., BPhen and DBP from Luminescence Technology Corp., and C70 from SES Research. DBP, C 60 and C70 were purified once via thermal gradient sublimation.
- the M0O 3 and BPhen layers were grown at a rate of 0.1 nm/s, DBP and C70 were co-deposited using a DBP deposition rate of 0.02 nm/s and a C70 deposition rate of 0.16 nm/s to achieve a 1 :8 ratio.
- the BPhen:C6o mixed buffer was grown by co-depositing BPhen and C 60 each at a rate of 0.05 nm/s creating a 1 :1 blend.
- a 100 nm-thick Ag cathode was subsequently deposited through a shadow mask defining an array of 15, 1 mm diameter devices (device area of 0.008 cm 2 ).
- the same intercept for both cells at zero light intensity suggested that both OPV cells had the same responsivities as > 0 in the absence of bimolecular recombination.
- ?for the control was four times larger than that for the cell with the compound buffer.
- the smaller ?for the compound buffer cell suggested that bimolecular recombination was only 25% of that of the control cell, indicating that the electron and hole concentrations had each decreased, on average, by 50% due to the increased electric field in the mixed region.
- This larger internal field across the heterojunction in the compound buffer cell compared to that of the control for a given external bias resulted in improved charge extraction and, therefore, higher FF.
- the mobility of the layer was then calculated from the relationship between extraction time and electric field, normalized by setting the zero-field mobility of electrons in neat C&o layer to the experimental value of 5.1 * 10 "2 cm 2 /V-s.
- the model predicted an effective mobility of 4.7x10 "3 cm 2 /V-s, only one order of magnitude lower than that of neat C&o-
- the neat BPhen film had a significantly lower electron mobility of 1 .9* 10 "5 cm 2 /V-s, leading to charge pile-up at the buffer interface that promoted quenching.
- the PL intensity of the mixed buffer was nearly identical to the intensity found for the blocking reference (see Figure 22(b)), demonstrating that BPhen:C6o mixed layer could efficiently block excitons. Since BPhen:C6o mixture had a relatively high electron mobility, the mixed buffer layer could spatially separate excitons and polarons acting as an effective filter, leading to a reduction of exciton-polaron quenching within the neat fullerene layer.
- OPV cells with a hybrid planar-mixed heterojunction were fabricated according to the experiment disclosed in Example 18. DBP and C 7 o were employed as donor and acceptor, respectively in OPV cells.
- the OPV cells had a device structure of indium tin oxide (ITO)/ M0O3 (10 nm)/DBP: C 7 o (54 nm, 1 :8 ratio by vol.)/C 7 o (9 nm)/Buffer/Ag (100 nm).
- Table 8 Device performances of OPV cells with mixed buffer layer for different thicknesses.
- Figure 28(a) shows J-V for illumination under 1 -sun simulated AM1 .5G illumination with inset showing NPD, and (b) shows the external quantum efficiency for OPV cells with active layers comprised of DBP and C 60 and various buffers.
- Table 9 Measure Efficiency Parameters for OPV Cells with DBP:C6o active layer and various buffers.
- the device shown in Figure 30 was fabricated as illustrated.
- the full structure was glass substrate/ITO/Mo0 3 (10nm)/DBP:C 70 (1 :8) (54 nm)/C 70 (9 nm)/ Buffer/AI (100 nm).
- This device was grown on a commercially purchased pre- patterened ITO substrate.
- the substrate was cleaned using solvents and UV-ozone system.
- the organic layers, metal oxide, and cathode were deposited in a thermal evaporator at a rate of 0.1 nm/second.
- the device was then encapsulated by putting an epoxy layer around the device, and pressing a glass slide on top.
- the epoxy was UV-cured.
- the buffer layer was a mixed layer of BPhen:C6o (1 :1 ) (10 nm) with a BPhen cap layer (5 nm).
- the buffer layer was a mixed layer of TPBI:C 70 (1 :1 ) (10 nm) with a TPBi cap layer (3 nm). It is noted that other wide energy gap materials, such as those listed in Table 10 below and others discussed above, may be used.
- the device shown in Figure 33 was fabricated as illustrated and using the same techniques described in example 21 .
- the full structure was glass substrate/ITO/MoO 3 (10nm)/C 70 :DBP(8: 1 ) (54 nm)/C 70 (9 nm)/ Buffer (15 nm)/AI (100 nm) with various buffer layers as shown, the buffer layers having a mixed layer of a wide energy gap material and an electron conducting material and including a cap layer of the wide energy gap material. It is noted that other wide energy gap materials, such as those discussed above, may be used.
- Figure 33 shows the relative normalized power conversion efficiency at 55°C of devices using BPhen in the buffer layer compared to three devices that each use a different wide energy gap material having a higher T g than BPhen, namely BAIq, 3TPYMB, and TPBi.
- Figures 39-41 show normalized responsivity, fill factor, Voc, and PCE at 55° C over time for devices having mixed buffers containing a mixture of C 7 o and one of TPBi, 3TPYMB, and BAIq, respectively. As shown, the normalized
- performance values e.g., based on PCE, V 0 c, FF, and R
- performance values of devices comprising one of TPBi, 3TPYMB, and BAIq remained significantly higher over time as compared to the performance of devices comprising BPhen (performance of BPhen devices shown in Figures 31A-C and Figure 33).
- the device shown in Figure 42 was fabricated as illustrated and using the same techniques described in example 21 .
- the full structure was glass substrate/ITO (160 nm)/Mo0 3 (10nm)/C 70 :DBP(8:1 ) (54 nm)/C 70 (9 nm)/ 3TPYMB:C 60 (1 :1 ) (10 nm)/AI (100 nm).
- the 10 nm layer of 3TPYMN:C 60 was the buffer layer comprising the wide energy gap material 3TPYMB (T g of 3TPYMB ⁇ 106° C). The lifetime of the device at elevated temperatures was tested using the same
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