Patent Publication Number: US-2011049367-A1

Title: Organic thin films for infrared detection

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application No. 61/070,093, filed Mar. 19, 2008, and U.S. Provisional Application No. 61/053,289, filed May 15, 2008, the contents of which are incorporated herein by reference in their entireties. 
    
    
     UNITED STATES GOVERNMENT RIGHTS 
     This invention was made with U.S. Government support under grant DAAB07-01-D-G602, Army Night Vision and Electronic Sensors Directorate. The government has certain rights in this invention. 
    
    
     JOINT RESEARCH AGREEMENT 
     The claimed invention was made by, on behalf of, and/or in connection with one or more of the following parties to a joint university-corporation research agreement: Princeton University, the University of Michigan, the University of Southern California, and Universal Display Corporation. The agreement was in effect on and before the date the claimed invention was made, and the claimed invention was made as a result of activities undertaken within the scope of the agreement. 
     FIELD OF THE INVENTION 
     The present invention generally relates to organic photosensitive devices. More specifically, it is directed to organic photosensitive devices having sensitivity to infrared radiation. 
     BACKGROUND 
     Photodetectors are photosensitive devices that convert incident electromagnetic radiation into a measurable electrical signal. They are typically used in conjunction with current-detecting circuits, which measure the photocurrent generated when the photodetector is exposed to electromagnetic radiation. 
     Photodetectors may be classified 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. Photoconductor (or photoresistor) cells respond to incident radiation with a change in conductance; they do not have a rectifying junction and are normally operated with a bias. Photodiodes and photovoltaic devices respond to incident radiation with the generation of excitons. They have at least one rectifying junction; photodiodes are usually but not always operated with a bias. 
     As used herein, the term “rectifying” denotes, inter alia, that an interface has an asymmetric conduction characteristic, i.e., the interface supports electronic charge transport preferentially in one direction. The term “semiconductor” denotes materials which can conduct electricity when charge carriers are induced by thermal or electromagnetic excitation. The term “photoconductive” generally relates to the process in which electromagnetic radiant energy is absorbed and converted to excitation energy of electric charge carriers, so that the carriers can conduct (i.e., transport) electric charge in a material. The term “photoconductive material” refers to semiconductor materials which are utilized for their capacity to generate electric charge carriers upon absorption of electromagnetic radiation. As used herein, the term “heterojunction” refers to a donor/acceptor semiconductor heterojunction, unless otherwise characterized. 
     Solid state electronic devices are most often constructed by deposition of various materials, in patterned layers, on a solid substrate. As used herein, “top” means furthest away from the substrate, while “bottom” means closest to the substrate. There may be intervening layers (for example, if a first layer is “on” or “over” a second layer), unless it is specified that the first layer is “in physical contact with” or “directly on” the second layer; the latter description does not preclude intervening surface treatments (e.g., exposure of the first layer to oxygen or hydrogen plasma, or ion implantation.) 
     When electromagnetic radiation of an appropriate energy is incident upon an organic semiconductor material, a photon can be absorbed to produce an excited molecular state. In organic photoconductive materials, the excited molecular state is generally believed to be an “exciton,” an electron-hole pair in a bound state which is transported as a quasi-particle. An exciton can have an appreciable lifetime before geminate recombination (“quenching”), in which the original electron and hole recombine with each other (as opposed to recombination with holes or electrons from other pairs). To produce a photocurrent, the electron-hole pair forming the exciton) are typically separated at a rectifying junction. 
     In the case of photosensitive devices, the rectifying junction is a heterojunction. Types of organic photovoltaic heterojunctions include donor-acceptor heterojunctions formed at an interface of a donor material and an acceptor material, and Schottky barrier heterojunctions formed at the interface of a photoconductive material and a metal. 
     The broad absorption spectra and high absorption coefficients (˜10 5 cm −1 ) of organic materials, as well as the chemists&#39; ability to tailor the electronic and photonic properties to suit a particular application or spectral range, make organic materials of particular interest for application to photodetection and solar energy conversion. P. Peumans, A. Yakimov, and S. R. Forrest,  J. Appl. Phys.  93, 3693 (2003); C. W. Tang,  Appl. Phys. Lett.  48, 183 (1986); A. Shah, P. Torres, R. Tscharner, N. Wyrsch, and H. Keppner,  Science  285, 692 (1999); P. Peumans and S. R. Forrest,  Appl. Phys. Lett.  79, 126 (2001). The low indices of refraction of the organic materials and their corresponding transparent substrates (glass or plastics) also allow for efficient light coupling into devices, leading to potentially high quantum efficiencies. 
     A significant property of 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. In the context of organic photosensitive devices, a material that conducts preferentially by electrons due to high electron mobility may be referred to as an electron transport material. A material that conducts preferentially by holes due to high hole mobility may be referred to as a hole transport material. A layer that conducts preferentially by electrons, due to mobility and/or the position of the layer in the device, may be referred to as an electron transport layer (“ETL”). A layer that conducts preferentially by holes, due to mobility and/or the position of the layer in the device, may be referred to as a hole transport layer (“HTL”). 
     For additional background explanation and description of the state of the art for organic photosensitive devices, including their general construction, characteristics, materials, and features, U.S. Pat. No. 6,972,431 to Forrest and Xue, U.S. Pat. No. 6,657,378 to Forrest et al., U.S. Pat. No. 6,580,027 to Forrest et al., and U.S. Pat. No. 6,352,777 to Bulovic et al. are incorporated herein by reference in their entirety. For a review, see P. Peumans, V. Bulovic, and S. R. Forrest,  Appl. Phys. Lett.  76, 3855 (2000). 
     Photodetectors sensitive to infrared radiation are known in the art, and widely used in remote sensing, military and astronomy applications. Where sensitivity is paramount, cryogenic infrared photodetectors, typically made of small bandgap (about 0.1-0.2 eV) semiconductors such as HgCdTe, are commonly employed. The manufacturing cost of these detectors, especially in the form of imaging arrays, is high, the circuit designs are complex, and cryogenic temperatures are required, which adds to the complexity of the devices. In applications where the sensitivity of cryogenic detectors is not needed, less costly uncooled thermal detectors are available. These devices (e.g. pyrometers and bolometers) rely on the initial conversion of received infrared photons into heat. There is a need for infrared detectors that are sensitive, yet relatively simple and inexpensive. 
     Organic semiconductors are inexpensive to manufacture and lend themselves to mass production. However, in the field of organic photosensitive devices, there remains a need for devices having sensitivity to infrared radiation. Typical organic semiconductor band gaps are &gt;1.5 eV, which is too large to permit excitation by infrared radiation, which carries at most about 0.3 eV. The present invention provides organic semiconductor devices capable of the detection of near- and mid-IR radiation. 
     SUMMARY OF THE INVENTION 
     The present application provides a method of detecting infrared radiation. The method of the invention comprises irradiating a photoconductive organic material with a pump light which has a photon energy suitable to generate in the photoconductive organic material one or more first excitons of energy E, where E is lower than minimum excitation energy E min , for generating charge carriers in the photoconductive organic material by an energy difference corresponding to the infrared region of the electromagnetic spectrum, exposing the photoconductive organic material to infrared radiation, and detecting charge carriers generated by absorption of the infrared radiation by the first excitons. The method of the present invention can be used to detect near- or mid-infrared radiation having an energy in the range of about 0.1 to about 1.7 eV. 
     The pump light wavelength is determined based on the photoconductive organic material used, and is generally in the visible and ultraviolet region of the electromagnetic spectrum. The intensity of the pump light is preferably chosen such that a sufficient population of the first excitons is generated to allow sufficient charge generation by the IR for accurate charge detection. 
     The pump light can be in any suitable temporal form. In a preferred embodiment, the pump light is pulsed light. 
     In one embodiment, the first excitons are first singlet excitons. The first singlet excitons can be generated by the pump light in the photoconductive organic material by direct singlet-singlet transition. In another embodiment, the pump light generates in the photoconductive organic material one or more higher lying singlet excitons which subsequently convert to the one or more first singlet excitons. 
     The photoconductive organic material can be a pure material or a mixed material. In a preferred embodiment, the photoconductive organic material comprises a host material and a dopant material. In one embodiment, the host material and dopant material can be selected such that the dopant material absorbs the pump light to generate the first exciton. In another embodiment, the host material and dopant material can be selected such that the host material absorbs the pump light to generate an exciton in the host material which then generates the first exciton in the dopant material. 
     In another embodiment, the photoconductive organic material comprises a host material and a dopant material which serves to promote charge separation. In this embodiment, the absorption of the infrared radiation by the first exciton results in generation of a second exciton which dissociates into charge carriers by relaxation to the LUMO of the dopant material. 
     In another embodiment, the first excitons are first triplet excitons. Infrared absorption by these triplet excitons results in the generation of higher lying triplet excitons which dissociate into charge carriers. In one embodiment, the pump light generates in the photoconductive organic material one or more singlet excitons which subsequently convert to the one or more first triplet excitons. The first triplet excitons can also be generated by the pump light in the photoconductive organic material by a direct singlet-triplet transition. 
     In embodiments employing first triplet excitons, the photoconductive organic material can comprise a host material and a phosphorescent dopant material. In a preferred embodiment, the pump light generates in the phosphorescent material one or more singlet excitons which convert to the first triplet excitons in the phosphorescent dopant material, e.g., via intersystem crossing. In another preferred embodiment, the pump light generates in the host material one or more singlet excitons which convert to the first triplet excitons in the phosphorescent dopant material. The conversion can be achieved via direct intersystem crossing from the singlet excitons to the first triplet excitons. The conversion can also be achieved via energy transfer to a singlet exciton in the phosphorescent dopant material followed by intersystem crossing from the singlet excitons in the phosphorescent dopant to the first triplet excitons. Preferably, the singlet excitons convert to the first triplet excitons on a time scale less than the time scale for charge separation. In a specific embodiment, the singlet excitons convert to the first triplet excitons on a time scale less than about 1 picosecond. Such a fast singlet to triplet conversion allows utilizing singlet excitons lying above E min , to generate triplet excitons of the appropriate energy. 
     In another preferred embodiment, a sensitizer is used to enhance the production of the first triplet excitons. In one embodiment, the host material is selected to have singlet and triplet exciton levels nested in singlet and triplet exciton levels of the phosphorescent dopant material. Such a host material therefore serves as the sensitizer. 
     In another embodiment, the photoconductive organic material comprises a host material, a phosphorescent dopant material and a sensitizer material. The pump light generates in the sensitizer material one or more singlet excitons which convert to the first triplet excitons in the phosphorescent dopant material. Preferably, the sensitizer material has singlet and triplet exciton levels nested in singlet and triplet exciton levels of said phosphorescent dopant material. 
     The method of the present invention can be carried out using a photoconductive device, whereby carriers generated by the IR are detected by detecting the changes in conductivity or resistance in the photoconductive material. 
     The method of the present invention can also be carried out using a photovoltaic device including a rectifying junction, whereby carriers generated by the IR are detected by detecting the voltage across the rectifying junction. The rectifying junction can be but is not limited to a planar rectifying junction, a mixed heterojunction, or a bulk heterojunction. In one embodiment, the photoconductive organic material is a donor material, which is used in contact with an acceptor material to form a rectifying junction. In such an embodiment, is preferably no less than the LUMO energy of said acceptor material. More preferably, E min  is about 50 mV higher than the LUMO energy of said acceptor material. 
     In another embodiment, the photoconductive organic material is an acceptor material, which is used in contact with a donor material to form a rectifying junction. In such an embodiment, E min  is preferably no higher than the HOMO energy of said acceptor material. More preferably, E min  is about 50 mV lower than the HOMO energy of said acceptor material. 
     The present invention also provides an organic photosensitive optoelectronic device for detecting infrared radiation, comprising a first electrode and a second electrode; and a photoconductive organic material of the present invention disposed between the first electrode and the second electrode. The photoconductive organic material can be any of those described above in connection with the method of the invention. 
     The organic photosensitive optoelectronic device of present invention may further comprise a source of visible and/or ultraviolet light disposed so as to illuminate the organic photoconductive material. The intensity of visible and/or ultraviolet light can be selected in a similar manner as described in connection with the method of the invention. The organic photosensitive optoelectronic device of present invention may further comprise at least one exciton blocking layer between the two electrodes and adjacent to at least one of the two electrodes. 
     The present invention also provides an organic material for use in an organic photosensitive optoelectronic device for detection of infrared radiation. The organic material can be any of those described above in connection with the method of the invention. 
     The present invention also provides an imager for imaging infrared radiation, comprising a plurality of the organic photosensitive optoelectronic devices of the present invention. The imager of the invention preferably has at least 10, 100, 10000, 10 6  or more organic photosensitive optoelectronic devices. The organic photosensitive optoelectronic devices in the imager of the present invention can be 1 μm×1 μm or less in size. Preferably, the plurality of organic photosensitive optoelectronic devices in the imager of the invention form an individually addressable array. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The component structures in the devices shown in the following figures are not necessarily drawn to scale: 
         FIG. 1  presents energy level diagrams showing two-photon excitation via S 1  singlet ( 1 A and  1 D) and T 1  triplet ( 1 B and  1 C) excited states of a dopant. 
         FIG. 2  is an energy level diagram illustrating photoinduced exciton formation in either component of a donor-acceptor heterojunction, and dissociation of the exciton into separated charge carriers at the junction. 
         FIG. 3  illustrates photoinduced exciton formation in a donor, and dissociation of the exciton at the planar heterojunction of a donor-acceptor bilayer. 
         FIG. 4  illustrates photoinduced exciton formation and dissociation in a mixed heterojunction, sandwiched between a donor layer and an acceptor layer. 
         FIG. 5  illustrates photoinduced exciton formation, and dissociation in a bulk (non-planar) heterojunction. 
         FIG. 6  illustrates an organic photosensitive device with an integral light source. 
         FIG. 7  shows the structures of representative triplet sensitizers. 
         FIG. 8  shows an energy level diagram illustrating of triplet exciton generation via a sensitizer. 
     
    
    
     DETAILED DESCRIPTION 
     As used herein, a first HOMO or 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. A higher HOMO energy level corresponds to an ionization potential (“IP”) having a smaller absolute energy relative to a vacuum level. Similarly, a higher LUMO energy level corresponds to an electron affinity (“EA”) having a smaller absolute energy relative to vacuum level. On a conventional energy level diagram, with the vacuum level at the top, the LUMO energy level of a material is higher than the HOMO energy level of the same material. 
     In the context of organic materials, 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. Where one material is in contact with another, the material having the lower energy LUMO is the acceptor and the material having the higher-energy LUMO is the donor. It is energetically favorable, in the absence of an external bias, for electrons promoted to the donor LUMO to move into the acceptor material at the donor-acceptor junction. The result is dissociation of the exciton into separated charge carriers, which can be conducted to external circuitry for detection. 
       FIG. 2  is an energy-level schematic representation of a donor-acceptor heterojunction. The absorption of a photon  6  in the donor  152  or the acceptor  154  creates an exciton  8 , which dissociates at the rectifying interface. The donor  152  transports the hole (open circle) and the acceptor  154  transports the electron (dark circle). 
       FIG. 3  illustrates the photogeneration of an exciton in the donor layer of a bilayer heterojunction, the dissociation of the exciton into charge carriers at the planar junction, and conduction of the charge carriers away from the junction. 
       FIG. 4  illustrates the same process taking place in a photoactive planar mixed heterojunction layer  153 . In a planar mixed heterojunction, the donor and acceptor materials are blended at or close to the molecular level, on the order of a few nanometers or less, and in the case of the hybrid heterojunction shown in  FIG. 4  the mixed layer is sandwiched between donor and acceptor layers. The concentrations of the components in a mixed layer may increase along a gradient toward the corresponding donor or acceptor layer or electrode. 
       FIG. 5  illustrates the process taking place in a bulk heterojunction, where the photoactive layer is a blend of interpenetrating organic donor and acceptor phases. The phase regions may range in size from a few nanometers to about 100 nanometers in a bulk heterojunction. The heterojunction may be a disordered mixture, or an ordered interpenetrating network of donor and acceptor phases. 
     As used herein, the term “organic” includes polymeric materials as well as small molecule organic materials that may be used to fabricate organic opto-electronic 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. Small molecules may also serve as the core moiety of a dendrimer, which consists of a series of chemical shells built on the core moiety. The core moiety of a dendrimer may be a fluorescent or phosphorescent small molecule emitter. A dendrimer may be a “small molecule.” In general, a small molecule has a defined chemical formula with a molecular weight that is the same from molecule to molecule, whereas a polymer has defined element ratios with a molecular weight that may vary from molecule to molecule. As used herein, “organic” includes metal complexes of hydrocarbyl and heteroatom-substituted hydrocarbyl ligands. 
     As used herein, “near- and mid-IR radiation” refers to radiation having energies between 0.1 and 1.7 eV. 
     The present application provides a method of detecting infrared radiation. In the method of the invention, a photoconductive organic material is irradiated with a pump light which has a photon energy suitable to generate in the photoconductive organic material one or more first excitons of energy E, where E is lower than minimum excitation energy conducive for generating charge carriers in the photoconductive organic material by an energy difference in the infrared region of the electromagnetic spectrum. If ΔE represents the energy difference, and E min  represents the minimum excitation energy conducive for generating charge carriers in the photoconductive organic material, then ΔE=E min −E. The photoconductive organic material is exposed to infrared radiation. If the infrared radiation is present, absorption of the infrared radiation by the first excitons results in generation of charge carriers, which are detected. The method of present invention can be used to detect near- or mid-infrared radiation, i.e., ΔE is in the range of about 0.1 to about 1.7 eV. 
     The pump light wavelength is determined based on the photoconductive organic material used, and is generally visible and ultraviolet region of the electromagnetic spectrum. The intensity of the pump light is preferably chosen such that a sufficient population of the first excitons is generated. Various factors may affect the selection of pump intensity, including one or more of (a) absorption coefficient of the excited state to the wavelength to be detected, (b) the lifetime of the excited state, (c) the thickness of the device active region (which in turn is dependent on the charge collection efficiency), and (d) expected repetition rate of the incident light to be detected. In one embodiment, the intensity of the pump light is determined based on Equations (1) and (2) below. The number of excitons N generated by a pump power P is given by: 
     
       
         
           
             
               
                 
                   N 
                   = 
                   
                     
                       
                         P 
                          
                         
                             
                         
                          
                         τ 
                       
                       E 
                     
                      
                     
                       α 
                       
                         R 
                          
                         
                           ( 
                           
                             1 
                             - 
                             
                               exp 
                                
                               
                                 ( 
                                 
                                   
                                     - 
                                     α 
                                   
                                    
                                   
                                       
                                   
                                    
                                   d 
                                 
                                 ) 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     where τ=excited state lifetime
 
α=absorption coefficient of the ground to excited state transition
 
R=reflectivity of the layer
 
d=layer thickness
 
E=photon energy
 
The signal current is then
 
     
       
         
           
             
               
                 
                   
                     J 
                     SIG 
                   
                   = 
                   
                     
                       qP 
                       SIG 
                     
                      
                     
                       
                         R 
                          
                         
                           ( 
                           
                             1 
                             - 
                             
                               exp 
                                
                               
                                 ( 
                                 
                                   
                                     - 
                                     
                                       α 
                                       SIG 
                                     
                                   
                                    
                                   d 
                                 
                                 ) 
                               
                             
                           
                           ) 
                         
                       
                       
                         E 
                         SIG 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     where the meanings of the symbols are the same as above except that they refer to the signal pulse, of duration t, as opposed to the pump, and the absorption coefficient at the signal wavelength is α SIG =α SIG (N). Therefore, for a given signal current detection method, the desirable pump power P can be determined according to Equations (1) and (2) based on the desired sensitivity of detecting J SIG . 
     The pump light can be in any suitable temporal form. In a preferred embodiment, the pump light is pulsed light. 
     The first excitons can be any excitons in the photoconductive organic material which can be generated by the pump light and absorbs the infrared radiation to generate carriers with sufficient efficiencies. The first excitons can be generated by the pump light in the photoconductive organic material by direct transition. The first excitons can also be generated indirectly by the pump light via internal conversion, where absorption of the pump light generates an exciton which converts to the first exciton. 
     In one embodiment, the first excitons are first singlet excitons. The first singlet excitons can be generated by the pump light in the photoconductive organic material by direct singlet-singlet transition (see e.g.,  FIG. 1A ). In another embodiment, the pump light generates in the photoconductive organic material one or more higher lying singlet excitons which subsequently convert to the one or more first singlet excitons (see e.g.,  FIG. 1D ). 
     Singlet materials suitable for use in this invention include singlet based materials that have high molar absorptivity of visible or UV, a long singlet exciton lifetime and a good transition probability for IR absorption to a higher lying state. Acenes are good candidates for this, with acenes that are sufficiently sterically bulky preferred to prevent aggregation and self quenching. Rubrene is a good example of such a compound. Phenyl or tert-butyl substituted pyrene or perylene are also good candidates. 
     The photoconductive organic material can be a pure material or a mixed material. In a preferred embodiment, the photoconductive organic material comprises a host material and a dopant material. In one embodiment, the host material and dopant material can be selected such that the dopant material absorbs the pump light to generate the first exciton. In another embodiment, the host material and dopant material can be selected such that the host material absorbs the pump light to generate an exciton in the host material which then generates the first exciton in the dopant material. 
     In another embodiment, the photoconductive organic material comprises a host material and a dopant material which serves to promote charge separation. In this embodiment, the absorption of the infrared radiation by the first exciton, e.g., in the host material, results in generation of a second exciton which dissociates into charge carriers by relaxation to the LUMO of the dopant material. 
     In embodiments utilizing a host material and a dopant material, examples of preferred host materials include but are not limited to carbazole containing materials, arylamines, tetraarylphenylenediames, conjugated semiconducting polymers or polymer blends such as poly(phenylene vinylene); phthalocyanines, porphyrins, and transition metal complexes thereof such as CoPc, F16CoPc, F16ZnPc, NiTPP, and the like; fullerenes such as C o ; polycyclic aromatics such as PTCDA, NTCDA, TCTA, 3,4,9,10-perylenetetracarboxylic acid bis-benzimidazole, and the like; and transition metal, actinide, and lanthanide complexes. 
     Examples of preferred dopant materials include but are not limited to metallophthalocyamines, metalloporphyrins, aceces, such as anthracene, tetracene and pentacene, substitutes acenes, such as rubrene, and common laser dyes, such as coumarins, fluorescence, rhodamines and others. 
     The method of the present invention can be carried out using a photoconductive device, whereby carriers generated by the IR are detected by detecting the changes in conductivity or resistance in the photoconductive material. 
     The method of the present invention can also be carried out using a photovoltaic device including a rectifying junction, whereby carriers generated by the IR are detected by detecting the voltage across the rectifying junction. The rectifying junction can be but is not limited to a planar rectifying junction, a mixed heterojunction, or a bulk heterojunction. In one embodiment, the photoconductive organic material is a donor material, which is used in contact with an acceptor material to form a rectifying junction. In such an embodiment, E min  is preferably no less than the LUMO energy of said acceptor material. More preferably, E min  is about 50 mV higher than the LUMO energy of said acceptor material. 
     In another embodiment, the photoconductive organic material is an acceptor material, which is used in contact with a donor material to form a rectifying junction. In such an embodiment, E min  is preferably is no higher than the HOMO energy of said acceptor material. More preferably, E min  is about 50 mV lower than the HOMO energy of said acceptor material. 
     In embodiments employing a rectifying junction, suitable donor material include but are not limited to carbazole containing materials, arylamines, tetraarylphenylenediames, oligothiophenes, phenylenevinylenes, oligopyrroles, tetrathiafulvalene, its derivatives and related fulvalene materials, conjugated semiconducting polymers or polymer blends such as poly(phenylene vinylene); phthalocyanines, porphyrins, acenes and substituted acenes, carbon nanotubes, as well as coordination compounds and organometallic complexes with HOMO levels above those of the acceptor materials used in the device, such that they are effective electron donors. 
     Suitable acceptor material include but are not limited to F16CoPc, F16ZnPc, NiTPP, and the like, tetracyanoethylene, tetracyanoquinuclidine and its derivatives, C 60 , C 70  and other fullerenes; polycyclic aromatics such as PTCDA, NTCDA, TCTA, 3,4,9,10-perylenetetracarboxylic acid bis-benzimidazole, as well as coordination compounds and organometallic complexes with LUMO levels below those of the donor, such that they are effective electron acceptors. 
     In another embodiment, the first excitons are first triplet excitons. Infrared absorption by these triplet excitons results in generation of higher lying triplet excitons which dissociate into charge carriers. A triplet excited state has a lifetime orders of magnitude longer than a singlet excited state, markedly increasing the likelihood of absorption of an IR photon by the triplet excited state. Metalloporphyrins are good examples of the materials that can be used. Preferably, the porphyrin is sterically substituted to prevent aggregation and the self quenching of the triplet state. Porphyrins also have a high molar absorbance for visible light into both the Soret and Q-bands. Preferred examples of porphyrins that can be used in the present invention are Pt and Pd complexes of tetraphenylporphyrin, tetraphenyltetrabenzoporphyrin and related sterically substituted metalloporphyrins. These materials give high ISC efficiency and have triplet lifetimes over 100 microseconds. 
     Another family of materials that can be used in the present invention is heavy metal containing complexes, particularly those of platinum and iridium. Many of these complexes can be sterically substituted to prevent self quenching, even at very high concentrations. These complexes also have highly efficient ISC. Three Pt complexes are listed in Example 1, which have long triplet lifetimes, even as neat solids. These materials have long triplet lifetimes, in both lightly and heavily doped films (see Table 1 of Example 1). The T 1  energy here is based on the phosphorescence energy for thin films. Excitation for these compounds can be accomplished with UV to blue light. 
     In one embodiment, the pump light generates in the photoconductive organic material one or more singlet excitons which subsequently convert to the one or more first triplet excitons (see e.g.,  FIG. 1B ). The first triplet excitons can also be generated by the pump light in the photoconductive organic material by a direct singlet-triplet transition (see e.g.,  FIG. 1C ). 
     In embodiments employing first triplet excitons, the photoconductive organic material can comprise a host material and a triplet-generating dopant material such as a phosphorescent dopant material. The triplet-generating material can be selected, in part, on the basis that the energy of triplet-triplet absorption commensurate with the energy of the radiation to be detected, a high molar absorptivity, efficient intersystem crossing, a long triplet lifetime, and efficient charge separation in the higher lying triplet state. It will be appreciated that for use in the present invention, efficient light emission from the triplet-generating dopant material is not required. Suitable triplet-generating dopant materials include but are not limited to phosphorescent materials. In the present application, for simplicity reasons, embodiments employing triplet-generating dopant material are often disclosed with reference to phosphorescent materials. However, it is contemplated that such disclosures are equally applicable to other triplet-generating materials. 
     In a preferred embodiment, the pump light generates in the phosphorescent material one or more singlet excitons which convert to the first triplet excitons in the phosphorescent dopant material, e.g., via intersystem crossing. In another preferred embodiment, the pump light generates in the host material one or more singlet excitons which convert to the first triplet excitons in the phosphorescent dopant material. The conversion can be achieved via direct intersystem crossing from the singlet excitons to the first triplet excitons. In this embodiment, the conversion can also be achieved via energy transfer to a singlet exciton in the phosphorescent dopant material followed by intersystem crossing from the singlet excitons in the phosphorescent dopant to the first triplet excitons. Preferably, the singlet excitons convert to the first triplet excitons on a time scale less than the time scale for charge separation. In a specific embodiment, the singlet excitons convert to the first triplet excitons on a time scale less than about 1 picosecond. Such a fast singlet to triplet conversion allows utilizing singlet excitons lying above E min , to generate triplet excitons of the appropriate energy. 
     Preferred host materials include but are not limited to carbazole containing materials, arylamines, tetraarylphenylenediames, oligothiophenes, phenylenevinylenes, oligopyrroles, tetrathiafulvalene, its derivatives and related fulvalene materials, conjugated semiconducting polymers or polymer blends such as poly(phenylene vinylene); phthalocyanines, porphyrins, acenes and substituted acenes, carbon nanotubes, as well as coordination compounds and organometallic complexes with HOMO levels above those of the acceptor materials used in the device, such that they are effective electron donors. 
     Preferred phosphorescent dopant materials include but are not limited to metalloporphyrins and phthalocyanines with heavy metal ions, whose atomic number is greater than 40, preferably Pd, Pt, Au; organometallic complexes whose metal ion atomic number is greater than 40, preferably Re, Os, Ir, Pt, Pb, Th, Bi; metal complexes that exhibit strong intersystem crossing that have atomic number less than 40, such as Cu and Mn complexes, organic materials that have intersystem crossing efficiencies greater than 25%, such as ketones and heterocyclic materials. In a more preferred embodiment, an organometallic complexes of Ir or Pt is used as the phosphorescent dopant material. 
     In another preferred embodiment, a sensitizer is used to enhance the production of the first triplet excitons. In order to have a high optical density of the triplet excited states, a high density of triplet excitons in the solid film is preferable. Forming a triplet excited state in an organic material can be enhanced by triplet sensitization. This process employs a sensitizer material that gives efficient intersystem crossing to a triplet state that is higher in energy than the IR absorbing organic material&#39;s triplet. The sensitizer&#39;s triplet is transferred to the IR absorbing organic molecule. This process is illustrated in  FIG. 8 . In this embodiment, a key aspect is to select the sensitizer having the appropriate energies. Preferably, the singlet and triplet energies of the sensitizer are between (“nested”) the singlet and triplet energies of the IR absorbing organic material. This way the sensitizer can be selectively excited (for example by a tuned laser) to a singlet state, e.g., the S I , which will rapidly intersystem cross to the triplet, which will energy transfer to the triplet levels in the IR absorbing organic material. This energy flow process was reported in a paper published in J. Am. Chem. Soc. (Sudhakar Madhusoodhanan, et al,  JACS,  2003, 125(26), 7796-7797). In this paper, the energy transfer processes for (ĈN) 2 Ir(acac) transferring energy to a trimer of fluorine is disclosed. The energy transfer process between (ĈN) 2 Ir(acac) and polyfluorene is also examined. 
     Examples of sensitizers for this process include heavy metal containing complexes, e.g., of the type that have been developed for OLED emitters. They have small singlet-triplet gaps and can be prepared with almost any triplet energy desired. In a specific embodiment, (1 np)Pt(dpm) or (btp)Pt(dpm) is used as the sensitizer for a rubrene IR absorbing material. Energy transfer to triplet levels in organics has been demonstrated to be very efficient (see the above referenced JACS paper). The organic material (IR absorber) can be chosen to have a long lived triplet in the solid state, with energy levels matched to the donor or acceptor to make a heterojunction that charge separates form higher lying triplet level(s), i.e. T n . A second example is the combination of (ppy) 2 Ir(acac) as a sensitizer for a fluorine oligomer or polymer. The triplet level in the oligomer or polymer has a long lifetime, making it an excellent IR absorber. Oligo and polyfluorenees as well as acenes, such as tetracene or pentacene, can also be used, preferably they being substituted to prevent self quenching. 
     The sensitizer can also be chosen from organic materials that are known to those of skill in the art to be triplet sensitizers. In one embodiment, organic materials of this type have small energy spacings between the singlet and triplet, and have both π-π* and n-π* transitions. Examples of triplet sensitizing organic materials include ketones, such as benzoin, benzophenone and thioxanthone. These materials are also excellent sensitizers for the formation of triplet states on oligo and polyfluorene. 
     In one embodiment, the host material is selected to have singlet and triplet exciton levels nested in singlet and triplet exciton levels of said phosphorescent dopant material. Such a host material therefore serves as the sensitizer. In a specific embodiment, the photoconductive organic material comprises a host material of (1 np)Pt(dpm) or (btp)Pt(dpm), and a phosphorescent dopant material of rubrene. 
     In another embodiment, the photoconductive organic material comprises a host material, a phosphorescent dopant material and a sensitizer material. The pump light generates in the sensitizer material one or more singlet excitons which convert to the first triplet excitons in the phosphorescent dopant material. Preferably, the sensitizer material has singlet and triplet exciton levels nested in singlet and triplet exciton levels of said phosphorescent dopant material. 
     The invention also provides an organic photosensitive optoelectronic device for detecting infrared radiation, comprising: a first electrode and a second electrode and a photoconductive organic material of the invention disposed between the first electrode and the second electrode. The device can also comprise a source of visible and/or ultraviolet light disposed so as to illuminate the organic photoconductive material. In one embodiment, the device also comprises at least one exciton blocking layer between the two electrodes and adjacent to at least one of the two electrodes. 
     An organic photosensitive device comprises at least one photoactive region in which light is absorbed to form an exciton, which may subsequently dissociate into an electron and a hole. In  FIG. 2 , the photoactive region comprises a donor material  152  and an acceptor material  154 . Organic materials for use in the photoactive region may include organometallic compounds, including cyclometallated organometallic compounds. The term “organometallic” as used herein is as generally understood by one of ordinary skill in the art and as given, for example, in “Inorganic Chemistry” (2nd Edition) by Gary L. Miessler and Donald A. Tarr, Prentice Hall (1998). Thus, the term organometallic refers to compounds which have an organic group bonded to a metal through a carbon-metal bond. This class does not include per se coordination compounds, which are substances having only donor bonds from heteroatoms, such as metal complexes of amines, halides, pseudohalides (CN, etc.), and the like. In practice organometallic compounds generally comprise, in addition to one or more carbon-metal bonds to an organic species, one or more donor bonds from a heteroatom. The carbon-metal bond to an organic species refers to a direct bond between a metal and a carbon atom of an organic group, such as phenyl, alkyl, alkenyl, etc., but does not refer to a metal bond to an “inorganic carbon,” such as the carbon of CN or CO. Preferably the metal is a transition metal, actinide, or lanthanide. Heavy metals are particularly preferred, i.e. those in periods 5-7 of the periodic table. There are a large number of suitable organometallic complexes known in the art which have triplet energy levels of suitable energies for use in the present invention. 
     The terms “electrode” and “contact” are used interchangeably herein to refer to a layer that provides a medium for delivering photo-generated current to an external circuit or providing a bias current or voltage to the device. Electrodes may be composed of metals or “metal substitutes.” Herein the term “metal” is used to embrace both materials composed of an elementally pure metal, and also metal alloys which are materials composed of two or more elementally pure metals. The term “metal substitute” refers to a material that is not a metal within the normal definition, but which has the metal-like properties such as conductivity, such as doped wide-bandgap semiconductors, degenerate semiconductors, conducting oxides, and conductive polymers. Electrodes may comprise a single layer or multiple layers (a “compound” electrode), may be transparent, semi-transparent, or opaque. Examples of electrodes and electrode materials include those disclosed in U.S. Pat. No. 6,352,777 to Bulovic et al., and U.S. Pat. No. 6,420,031, to Parthasarathy, et al., each incorporated herein by reference for disclosure of these respective features. As used herein, a layer is said to be “transparent” to a given wavelength of electromagnetic radiation if it transmits at least 50% of electromagnetic radiation having that wavelength, and “partially transparent” if it transmits between 20% and 50% of that radiation. 
     The substrate may be any suitable substrate that provides desired structural properties. The substrate may be flexible or rigid, planar or non-planar. The substrate may be transparent, translucent or opaque. Rigid plastics and glass are examples of preferred rigid substrate materials. Flexible plastics and metal foils are examples of preferred flexible substrate materials. 
     An anode-smoothing layer may be situated between the anode layer and the donor layer. Anode-smoothing layers are described in U.S. Pat. No. 6,657,378 to Forrest et al., incorporated herein by reference for its disclosure related to this feature. 
     Unless otherwise specified, any of the layers of the various embodiments may be deposited by any suitable method. For the organic layers, preferred methods include thermal evaporation, ink-jet, such as described in U.S. Pat. Nos. 6,013,982 and 6,087,196, which are incorporated by reference in their entireties, organic vapor phase deposition (OVPD), such as described in U.S. Pat. No. 6,337,102 to Forrest et al., which is incorporated by reference in its entirety, and deposition by organic vapor jet printing (OVJP), such as described in U.S. patent application Ser. No. 10/233,470, which is incorporated by reference in its entirety. Other suitable deposition methods include spin coating and other solution based processes. Solution based processes are preferably carried out in nitrogen or an inert atmosphere. For the other layers, preferred methods include thermal evaporation. Preferred patterning methods include deposition through a mask, cold welding such as described in U.S. Pat. Nos. 6,294,398 and 6,468,819, which are incorporated by reference in their entireties, and patterning associated with some of the deposition methods such as ink-jet and OVJD. Other methods may also be used. The materials to be deposited may be modified to make them compatible with a particular deposition method. For example, substituents such as alkyl and aryl groups, branched or unbranched, and preferably containing at least 3 carbons, may be used in small molecules to enhance their ability to undergo solution processing. Substituents having 20 carbons or more may be used, but 3-20 carbons is the preferred range. Materials with asymmetric structures may have better solution processability than those having symmetric structures, because asymmetric materials may have a lower tendency to recrystallize. Dendrimer substituents may be used to enhance the ability of small molecules to undergo solution processing. 
     How to pair two organic photoconductive materials to serve as a donor and an acceptor in a photovoltaic heterojunction based upon carrier mobilities and relative HOMO and LUMO levels is well-known in the art, and is not addressed here. 
     Examples of various types of donor-acceptor heterojunctions are shown in  FIGS. 3-5 .  FIG. 3  illustrates a donor-acceptor bilayer forming a planar heterojunction.  FIG. 4  illustrates a hybrid heterojunction including a mixed heterojunction  153  comprising a mixture of donor and acceptor materials arranged between the donor material  152  and the acceptor material  154 .  FIG. 5  illustrates an idealized “bulk” heterojunction. A bulk heterojunction, in the ideal photocurrent case, has a single continuous interface between the donor material  252  and the acceptor material  254 , although multiple interfaces typically exist in actual devices. Mixed and bulk heterojunctions can have multiple donor-acceptor interfaces as a result of having plural domains of material. Domains that are surrounded by the opposite-type material (e.g., a domain of donor material surrounded by acceptor material) may be electrically isolated, such that these domains do not contribute to photocurrent. Other domains may be connected by percolation pathways (continuous photocurrent pathways), such that these other domains may contribute to photocurrent. The distinction between a mixed and a bulk heterojunction lies in degrees of phase separation between donor and acceptor materials. In a mixed heterojunction, there is very little or no phase separation (the domains are very small, e.g., less than a few nanometers), whereas in a bulk heterojunction, there is significant phase separation (e.g., forming domains with sizes of a few nanometers to 100 nm). 
     Small-molecule mixed heterojunctions may be formed, for example, by co-deposition of the donor and acceptor materials using vacuum deposition or vapor deposition. Small-molecule bulk heterojunctions may be formed, for example, by controlled growth, co-deposition with post-deposition annealing, or solution processing. Polymer mixed or bulk heterojunctions may be fowled, for example, by solution processing of polymer blends of donor and acceptor materials. 
     If a photoactive region includes a mixed layer ( 153 ) or bulk layers ( 252 ,  254 ) and one or both of the donor ( 152 ) and acceptor layers ( 154 ), the photoactive region is said to include a “hybrid” heterojunction. The arrangement of layers in  FIG. 4  is an example. For additional explanation of hybrid heterojunctions, published U.S. Patent Application 2005/0224113, entitled “High efficiency organic photovoltaic cells employing hybridized mixed-planar heterojunctions” by Jiangeng Xue et al., published Oct. 13, 2005, is hereby incorporated by reference. 
     In general, planar heterojunctions have good carrier conduction, but poor exciton dissociation; a mixed layer has poor carrier conduction and good exciton dissociation, and a bulk heterojunction has good carrier conduction and good exciton dissociation, but may experience charge build-up at the end of the material “cul-de-sacs,” lowering efficiency. Unless otherwise stated, planar, mixed, bulk, and hybrid heterojunctions may be used interchangeably as donor-acceptor heterojunctions throughout the embodiments disclosed herein. 
     The donor material has an ionization potential that is smaller than that of the acceptor material. Further, the ionization potential HOMO/LUMO gap of the donor layer must be smaller than that of the acceptor layer. Generally, the materials comprising the donor or acceptor layers should have long exciton diffusion length, and thus are preferably those materials which lend themselves to ordered stacking of the molecules, such as planar, aromatic molecules. 
     The acceptor material may comprise, for example, perylenes, naphthalenes, fullerenes or nanotubules. A suitable acceptor material is 3,4,9,10-perylenetetracarboxylic bis-benzimidazole (PTCBI). Alternatively, the acceptor layer may be comprised of a fullerene material as described in the U.S. Pat. No. 6,580,027, which is incorporated herein by reference in its entirety. 
     Adjacent to the acceptor layer, is a layer of organic donor-type material. The boundary of the acceptor layer and the donor layer forms a heterojunction which may produce an internally generated electric field. The photoconductive organic materials of the present invention may be any of the materials known in the art to generate excitons upon irradiation with visible or ultraviolet light. Suitable materials for the donor and acceptor component include, but are not limited to, conjugated semiconducting polymers or polymer blends such as poly(phenylene vinylene); phthalocyanines, porphyrins, and transition metal complexes thereof such as CoPc, F16CoPc, F16ZnPc, NiTPP, and the like; C60; polycyclic aromatics such as PTCDA, NTCDA, TCTA, 3,4,9,10-perylenetetracarboxylic acid bis-benzimidazole, and the like, and transition metal, actinide, and lanthanide complexes, e.g., Ru(bpy) 3   2+  complexes and the like. 
     In certain embodiments, the active region comprises multiple subcells, i.e. it is an alternating donor-acceptor multilayer stack. It is preferred that the alternating donor layers and acceptor layers be thin layers in order to allow charge tunneling. It will be understood that all photoactive layers will preferably have a thickness that is at least on the order of the IR radiation to be detected, more preferably several or many wavelengths thick. 
     The devices, as illustrated in  FIG. 6 , may be connected to an element  607 , which is a current- or voltage-detecting circuit which measures the current or voltage generated when the photodetector is exposed to light, and which may apply a bias to the device (as described for example in Published U.S. Patent Application 2005/0110007, published May 26, 2005 to Forrest et al.). If the rectifying junction is eliminated from the device (e.g., using a single photoconductive material as the photoactive region), the resulting structures may be used as a photoconductor cell, in which case the element  607  is a signal detection circuit to monitor changes in resistance across the device due to the absorption of light. Unless otherwise stated, each of these arrangements and modifications may be used for the devices in each of the drawings and embodiments disclosed herein. 
     An organic photosensitive device may also comprise transparent charge transfer layers, electrodes, or charge recombination zones. A charge transfer layer may be organic or inorganic, and may or may not be photoconductively active. A charge transfer layer is similar to an electrode, but does not have an electrical connection external to the device and only delivers charge carriers from one subsection of a device to the adjacent subsection. A charge recombination zone is similar to a charge transfer layer, but allows for the recombination of electrons and holes between adjacent subsections of tandem photosensitive devices. A charge recombination zone may include semi-transparent metal or metal substitute recombination centers comprising nanoclusters, nanoparticles, and/or nanorods, as described for example in U.S. Pat. No. 6,657,378 to Forrest et al.; Published U.S. Patent Application 2006-0032529 A1, entitled “Organic Photosensitive Devices” by Rand et al., published Feb. 16, 2006; and Published U.S. Patent Application 2006-0027802 A1, entitled “Stacked Organic Photosensitive Devices” by Forrest et al., published Feb. 9, 2006; each incorporated herein by reference for its disclosure of recombination zone materials and structures. A charge recombination zone may or may not include a transparent matrix layer in which the recombination centers are embedded. A charge transfer layer, electrode, or charge recombination zone may serve as a cathode and/or an anode of subsections of the device. An electrode or charge transfer layer may serve as a Schottky contact. 
     Examples of electrodes used in organic photosensitive devices are shown in U.S. Pat. No. 6,352,777, which is incorporated herein by reference. When used herein, the term “electrode” refers to a conductive layer that provides a medium for delivering photogenerated charge carriers to an external circuit, and for providing any bias voltage to the device. That is, an electrode provides the interface between the photoactive regions of the device and a wire, lead, trace or other means for transporting the charge carriers to or from the external circuit. 
     In a photosensitive device, it is desirable to allow the maximum amount of ambient electromagnetic radiation from the device exterior to be admitted to the photoactive interior region. That is, the electromagnetic radiation must reach a photoactive layer, where it can generate the excitons that give rise to charge carriers. This often dictates that preferably, at least one of the electrodes should be minimally absorbing and minimally reflecting of the incident electromagnetic radiation. That is, such an electrode should be substantially transparent. The opposing electrode may be transparent as well, or it may be reflective so that light which has passed through the cell without being absorbed is reflected back through the cell. As used herein, a layer of material or a sequence of layers of different materials is said to be “transparent” when the layer or layers permit at least 50% of the ambient electromagnetic radiation in relevant wavelengths to be transmitted through the layer or layers. 
     Embodiments of the present invention may include, as one or more of the transparent electrodes of the photosensitive device, a highly transparent, non-metallic, low resistance cathode such as disclosed in U.S. patent application Ser. No. 09/054,707 to Parthasarathy et al., or a highly efficient, low resistance metallic/non-metallic compound cathode such as disclosed in U.S. Pat. No. 5,703,436 to Forrest et al. (“Forrest &#39; 436 ”). Each type of cathode is preferably prepared in a fabrication process that includes the step of sputter depositing an ITO layer onto either an organic material, such as CuPc, to form a highly transparent, non-metallic, low resistance cathode or onto a thin Mg:Ag layer to form a highly efficient, low resistance metallic/non-metallic compound cathode. Parthasarathy et al. disclose that an ITO layer onto which an organic layer had been deposited, instead of an organic layer onto which the ITO layer had been deposited, does not function as an efficient cathode. 
     In each of the devices described above, layers may be added, such as reflective layers or additional photoactive regions. The order of layers may be altered or inverted. A concentrator or trapping configuration may be employed to increase efficiency, as disclosed, for example in U.S. Pat. No. 6,333,458 to Forrest et al. and U.S. Pat. No. 6,440,769 to Peumans et al., which are incorporated herein by reference. Coatings may be used to focus optical energy into desired regions of a device, as disclosed, for example in published US Patent Application No. 2005/0266218 of Peumans et al., published Dec. 1, 2005, which is incorporated herein by reference. In the tandem devices, transparent insulative layers may be formed between cells, with the electrical connection between the cells being provided via electrodes. Also in the tandem devices, one or more of the photoactive regions may be a Schottky-barrier heterojunction instead of a donor-acceptor heterojunction. Arrangements other than those specifically described, such as for example recombination zones deployed between the cells, may be used. 
     The photoactive regions of the detectors of the present invention comprise a photoconductive organic material. In operation, this photoactive material is first excited with a pulse of high-energy radiation (e.g. from a flash lamp or diode laser), having sufficient energy to produce excitons in an S 1  state. During the lifetime of the excitons, the photoactive region is then exposed to the near- or mid-IR radiation which is to be detected. It will be appreciated that the lifetime of singlet excitons is typically very short, and that suitably short pulses and fast circuitry will be employed in operating these detectors. While not energetic enough to excite the ground-state photoconductive material to the S 1  state, the mid- or near-IR radiation is sufficiently energetic to raise S 1  excitons to a higher but more closely-spaced level, typically an S 2  or LUMO state. 
     Absorption of infrared radiation typically excites vibrational modes in organic materials. Electronic excitation is rare because the energy gap between HOMO and LUMO in organic materials is typically large, and formation of an exciton requires a photon in the visible or ultraviolet region of the spectrum to induce an S 0  to S 1  transition. The detectors of the present invention take advantage of the electronic energy levels between excited states in a semiconductor or dopant, which are more closely spaced than the S 0 -S 1  gap. A molecule is pumped to an excited state by an initial UV or visible light pulse, and the excited state, because of the closely spaced excited states, serves as the IR-absorbing species. In preferred embodiments, the IR radiation is near- and mid-IR radiation. 
     Detection of charge carriers derived from the dissociation of the resulting higher-energy excitons then provides a measurement of the IR intensity. Preferably, the photoconductive organic host material is doped so as to provide a route for the relaxation and dissociation of the S 2  or LUMO excitons into their constituent charges, and to enable collection and detection of the charges. The photoconductive organic host material can be chosen, in part, on the basis that the accessible S 2  or LUMO levels are separated from the S 1  state by a gap corresponding to the energy of the radiation to be detected. The dopant can be selected, in part, on the basis that the energy level of the dopant LUMO is lower than the energy level of the host material S 2  or LUMO excitons. Suitable materials include, but are not limited to, rubrene and phenyl or tert-butyl substituted pyrene, perylene, and acenes (e.g. tetracene and pentacene). 
     In another alternative embodiment, irradiation of the photoconductive material generates a singlet state excitons of higher energy than the IR absorbing singlet state exciton, and energy transfers from the singlet state exciton of higher energy to the IR absorbing singlet state exciton. Irradiation of the IR absorbing singlet state exciton with the near- or mid-infrared radiation to be detected converts at least one of these excitons to singlet state exciton having sufficient energy to dissociate into charges (see e.g.,  FIG. 1D ). The higher-energy singlet state exciton then dissociates, and the charges generated are detected as described above. 
     In a preferred embodiment, inconveniences arising from the short lifetime of the singlet state excitons are reduced by employing a triplet-generating material. After irradiation by the visible or UV pulse, intersystem crossing from the singlet state to a longer-lived triplet state (e.g., T 1 ) in the triplet-generating material takes place. During the lifetime of the triplet state excitons, they are exposed to the near- or mid-IR radiation to be detected, which excites them to a higher triplet (e.g., T 2 ) state. As with the singlet embodiments described above, these higher-energy triplet excitons relax by dissociation, e.g., at a heterojunction, with subsequent detection of the charges thereby produced. The triplet-generating material can be selected, in part, on the basis that the higher lying triplet energy levels from the energy level of the triplet state exciton generated by the pump light by a gap commensurate with the energy of the radiation to be detected. Preferred materials will have a high molar absorptivity, efficient intersystem crossing, a long triplet lifetime, and efficient charge separation in the higher lying triplet state. Suitable materials include, but are not limited to, sterically hindered metalloporphyrins, particularly Pd and Pt complexes of tetraphenylporphyrin, tetraphenyltetrabenzoporphyrin, and the like. 
     Suitable are cyclometallated Pt and Ir diketonate complexes; representative examples of which are shown in  FIG. 7 . 
       FIG. 6  illustrates, by way of example, a device according to the present invention, which incorporates a solid-state high-energy light source  601  as an optical pump, which may be, for example, a light-emitting diode or a laser diode. Particularly preferred as a light source are injection laser diodes. The light source will be selected so as to adequately match the emitted radiation to the absorbance spectrum of the host material or dopant for the desired initial S 0  to S 1  transition. In this embodiment, light from the source  601  passes through a transparent insulating layer  602  and transparent electrode  603  (which may be an anode or cathode) and into the doped semiconductor layer  604 , where the photons are absorbed and S 1  state excitons are created. These S 1  state excitons transfer energy to the dopant, creating an exciton which undergoes intersystem crossing to create a relatively long-lived T 1  exciton. Interaction with infrared radiation, if any is present to be detected, promotes the dopant T 1  excitons to a T 2  (or higher) state, which is higher in energy than the LUMO of the acceptor component of the heterojunction. Migration of the triplet exciton to the heterojunction with layer  605  is followed by charge separation at the junction, and, by means well-known in the art, the charges are then conducted via electrodes  603  and  606  to the detector circuit  607 . In other embodiments, layers  604  and  605  are combined into a mixed or bulk heterojunction. Electrode  606  may be transparent to infrared radiation, and serve as the detector window. 
     External circuitry may be employed to pulse the source  601 , and to sample output from detector  607 , at a frequency appropriate to the lifetimes of the relevant excited states. 
     In alternative embodiments, layer  605  may be doped with the same or a different dopant, or multiple dopants may be employed in either or both layers in order to take advantage of a broader range of incident wavelengths. 
     The photoconductive organic material contains a dopant into which the host excitons may transfer. Any of the dopants known in the art to be suitable for use with the photoconductive material may be employed. Suitable materials include, but are not limited to, Zn(BTZ) 2 , DPVBi, BCzVBi, NPAFN, indeno[1,2,3-cd]perylene, and the like. The amount of dopant will typically be less than about 15% relative to the host material, on a weight basis. Preferably the dopant is present at a level of 2-10%. 
     In preferred embodiments, the dopant is a material which, upon irradiation with visible or ultraviolet light, or upon energy transfer from a singlet exciton, undergoes intersystem crossing to yield a triplet T 1  state. Particularly preferred are metalloporphyrins and organometallic iridium complexes, such as are known in the art to be useful as emitters in organic light-emitting diodes (OLEDs). Because of the utility of phosphorescent dopants in the manufacture of OLEDs, many such materials are known to those of skill in the art. See, for example, Bald° et al., “Highly Efficient Phosphorescent Emission from Organic Electroluminescent Devices,”  Nature , vol. 395, 151-154, 1998; and Baldo et al., “Very high-efficiency green organic light-emitting devices based on electrophosphorescence,”  Appl. Phys. Lett ., vol. 75, No. 1, 4-6 (1999), which are incorporated by reference in their entireties. Other representative phosphorescent dopants are disclosed in U.S. Pat. Nos. 6,303,238, 6,310,360, 6,830,828, 6,939,624, and 6,835,469; and in Patent Application Publication Nos. WO 02/074015 and US 2008/0061681, all of which are incorporated herein by reference in their entireties; see also “Organic Light-Emitting Devices: Synthesis, Properties and Applications” K. Mullen and U. Scherf, eds. (Wiley-VCH, Weinheim, Germany, 2006); and R. C. Evans, P. Douglas and C. J. Winscom,  Coordination Chem. Rev., vol.  250(15-16), 2093-2126 (2006), incorporated herein by reference. It will be appreciated that for use in the present invention, efficient light emission is not a criterion, and preferred dopants will be those which more efficiently undergo dissociation of the triplet excitons into component charges. Suitable examples include, but are not limited to, Ir(ppy) 3 , Ir(bim) 2 acac, Ir(btb) 2 acac, Ir(piq) 2 (acac), and the like. 
     In alternative embodiments, a triplet sensitizer is employed to convert the pumping radiation into a triplet state exciton, which upon energy transfer to the host material (or to an added dopant) directly produces a long-lived, IR-absorbing T 1  state. The sensitizer may have singlet and triplet energies between the singlet and triplet energies of the host material, so that it can be selectively pumped to the S 1  state. Intersystem crossing then yields the T 1  triplet state. Suitable sensitizers include, but are not limited to, aryl ketones such as benzoin, benzophenone, thioxanthone, and the like; heavy metal complexes such as the cyclometallated iridium complexes FP, PPY, BT, PQ, BTP, and the like; and platinum complexes such as (1-np)Pt(dpm) and (btp)Pt(dpm). (See  FIG. 7 .) The organic host or dopant to which the sensitizer T 1  state transfers energy can be chosen to have a long lived triplet. The energy levels can be matched to the heterojunction donor-acceptor pair so that charge separation takes place from the higher T n  levels produced by absorption of IR radiation. Suitable materials include oligo- and poly-(9,9-dialkylfluorenyl-2,7-diyl), rubrene, tetracene, pentacene, hexacene, and the like. Suitable sensitizers and methods for selecting them are disclosed in P. Djurovich et al.,  Dalton Trans.,  2007, 3763-3770 and M. Sudhakar et al.,  J. Am. Chem. Soc.  2003, 125:7796-7797, which are incorporated herein by reference in their entireties. 
     The IR-sensitive photodetector devices of the present invention may be manufactured on small scale, e.g., 1 μm×1 μm or less in size, for use in focal plane arrays suitable for infrared imaging applications. Therefore, the present invention provides an imager for imaging infrared radiation, comprising a plurality of the organic photosensitive optoelectronic devices as described above. The plurality of organic photosensitive optoelectronic devices can comprise at least 10, 100, 10000 or 10 6  or more of the organic photosensitive optoelectronic devices. In a preferred embodiment, the plurality of organic photosensitive optoelectronic devices forms an array. Preferably, each device in the array is individually addressable. By way of example, the imager of the present invention may be incorporated into satellite- and aircraft-borne reconnaissance and weapons-targeting cameras, night vision goggles, gunsights, videocameras, security cameras, amateur telescopes, digital cameras, manufacturing line monitors, medical imaging, remote sensing equipment, and night vision systems for automobiles, trucks, and buses. In addition to imaging applications, the detectors of the invention are expected to be stable and amenable to calibration, and may be employed in instruments such as pyrometers, infrared thermometers, and infrared line scanning systems. 
     EXAMPLES 
     The following examples are presented to illustrate the methods and devices of the present invention, and are not intended to limit the invention in any way. 
     Example 1 
     Detection of Infrared Radiation Using a Singlet Based Material 
     This example demonstrates detection of infrared radiation in rubrene. Rubrene exhibits high molar absorptivity in the blue region of visible around 2.5 eV, a long S 1  lifetime and a good transition probability for NPR conversion to the S 2  state around 4 eV. It is also sufficiently sterically bulky which prevents aggregation and self quenching. 
     Photoconductive cells containing an ITO cathode layer, a LiF:Al anode layer and a thin film of rubrene are fabricated on ITO coated glass substrate by depositing via thermal evaporation a layer of 50 nm of rubrene, followed by depositing a 5 nm layer of LiF and 100 nm Al. 
     The absorption of the photoconductive cells is measured. The photoconductive cell is illuminated from the glass side by two independent radiation sources, a visible pump light tuned to 450 nm, and a near-infrared radiation of wavelength of 750 nm. The relative timing of the visible and IR pulses is adjustable, allowing studying the effect of the delay between the IR and the visible on IR absorption and charge generation. No absorption of the infrared radiation is detected when the pump light is switched off, confirming that the infrared radiation is not sufficient to excite excitons in these materials. When the visible pump light is turned on, absorption of the infrared radiation is detected when the visible and IR overlap in time, but is not detected when the delay between the IR and the visible is longer than 100 nanoseconds. The results demonstrate IR absorption via S 1  state of rubrene. 
     Electrical resistance of the film is also measured. A decrease in resistance is observed to correlate with IR absorption. 
     Similar measurements are performed with the photoconductive cells containing a layer of poly(phenylene vinylene) (PPV) doped with 10% of rubrene instead of neat rubrene layer. Similar results are obtained. 
     Photoconductive cells containing an ITO cathode layer, C 60  (50 nm), a thin film of poly(phenylene vinylene) (PPV) doped with 10% of rubrene (50 nm), and a LiF:Al anode ( 5  nm LiF:100 nm Al) layer is fabricated on glass substrates. The voltage across the electrodes and its IR absorption are measured are measured as a function of the delay between the IR and the visible. 
     Example 2 
     Detection of Infrared Radiation Using a Triplet Based Material 
     This example demonstrates detection of infrared radiation in a triplet based material. 
     Photoconductive cells containing an ITO cathode layer, a LiF:Al anode layer and a thin film of poly(phenylene vinylene) (PPV) doped with about 10% of Pt tetraphenylporphyrin (PtTPP) are fabricated on ITO coated glass substrate by thermal co-evaporation of the precursor for PPV and PtTPP in a ultrahigh vacuum to produce a layer of 50 nm of PPV: PtTPP on ITO, followed by deposition of a 5 nm layer of LiF and 100 nm Al. 
     The absorption of the photoconductive cells is first measured. The photoconductive cell is illuminated from the glass side by two independent radiation sources, a visible pump light tuned to 400 nm, and a near-infrared radiation of wavelength of 750 nm. The relative timing of the visible and IR pulses is adjustable, allowing studying the effect of the delay between the IR and the visible on IR absorption and charge generation. No absorption of the infrared radiation is detected when the pump light is switched off, confirming that the infrared radiation is not sufficient to excite excitons in these materials. When visible pump light is turned on, and the visible and IR overlap in time, absorption of the infrared radiation is detected. The effect of time delay between the pump light and IR on IR absorption is also investigated. It is observed that IR absorption decreases as the delay between the IR and visible increases with a half-life of about 100 μs. 
     Electrical resistance of the film is also measured. A decrease in resistance is observed to correlate with IR absorption. 
     Similar measurements are performed with photoconductive cells containing PPV doped with about 10% of the following three Pt complexes (dpm=dipivolylmethane (bis-tBu-acac, which have very long triplet lifetimes, even as neat solids). These three Pt complexes have long triplet lifetimes, in both lightly and heavily doped films. The T 1  energy here is based on the phosphorescence energy for thin films. Excitation for these compounds can be accomplished with UV to blue light. 
     
       
         
         
             
             
         
       
     
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Excited state lifetimes for thin films 
               
            
           
           
               
               
               
               
            
               
                   
                 Cpd 
                 τ at RT (μs) 
                 T 1  energy (nm) 
               
               
                   
                   
               
               
                   
                 (ppy)Pt(dpm) 
                 6.1 
                 477 
               
               
                   
                 (1np)Pt(dpm) 
                 4.9 
                 570 
               
               
                   
                 (btp)Pt(dpm) 
                 8.7 
                 600 
               
               
                   
                   
               
            
           
         
       
     
     The results are consistent with those obtained with PtTPP-doped film. 
     Example 3 
     Detection of Infrared Radiation Using a Triplet Based Material and a Sensitizer 
     Photoconductive cells containing electrodes and a thin film of poly(phenylene vinylene) (PPV) doped with about 10% of fluorine oligomer F 3  with or without about 10% of (ppy) 2 Ir(acac) as a sensitizer. The cells are fabricated by thermal co-evaporation of the precursor for PPV, F 3  with or without (ppy) 2 Ir(acac) to form a layer of 50 nm of PPV:F 3  or PPV:(ppy) 2 Ir(acac):F 3  on ITO, followed by depositing a 5 nm layer of LiF and 100 nm Al. F 3  is estimated to have a triplet energy roughly about 540-580 nm. In addition, F 3  has negligible absorption in the region where the phosphors absorb strongly (i.e., 435 nm), allowing the direct excitation of the phosphor. 
     The absorption of the photoconductive cells is first measured. The photoconductive cell is illuminated from the glass side by two independent radiation sources, a visible pump light tuned to 435 nm, and a near-infrared radiation of wavelength of 750 nm. The relative timing of the visible and IR pulses is adjustable, allowing studying the effect of the delay between the IR and the visible on IR absorption and charge generation. No absorption of the infrared radiation is detected when the pump light is switched off, confirming that the infrared radiation is not sufficient to excite excitons in these materials. When visible pump light is turned on, and the visible and IR overlap in time, absorption of the infrared radiation is detected. The IR absorption is higher in films with (ppy) 2 Ir(acac) as compared to film without (ppy) 2 Ir(acac), demonstrating enhancement of intersystem crossing as a result of the inclusion of the sensitizer. 
     Electrical resistance of the film is also measured. A decrease in resistance is observed to correlate with IR absorption.