Abstract:
An apparatus for suppressing electron/hole recombination includes a photonic device that generates electron/hole pairs responsive to a light beam interacting with the photonic device. An orbital angular momentum (OAM) generation device is located to impart an orbital angular momentum to a light beam before the light beam interacts with the photonic device. The electron/hole pair recombination generated from an OAM imparted light beam is less than electron/hole pair recombination of a non-OAM imparted light beam.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims benefit of U.S. Provisional App. No. 62/035,212, filed on Aug. 8, 2014, entitled SUPPRESSION OF ELECTRON-HOLE RECOMBINATION USING ORBITAL ANGULAR MOMENTUM FOR NEXT GENERATION ORGANIC-BASED SEMICONDUCTOR DEVICES (Atty. Dkt. No. NXGN-32312), which is incorporated by reference herein in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to suppression of electron hole recombinations in photonic devices, and more particularly, to the use of orbital angular momentum to inhibit electron hole recombinations in photonic devices. 
       BACKGROUND 
       [0003]    Within photonic devices, light energy is used for exciting electrons to higher energy levels and creating electrical energy or currents responsive to the light energy. The recombination of photo generated electrons and holes is a major source of energy loss within photonic devices. Examples of photonic devices include photovoltaic diodes, biological light harvesting complexes (LHCs), organic light emitting diodes (OLEDs) and organic photovoltaic cells (OPVs). Biological light harvesting complexes prevent recombination via the use of cascade structures which lead to spatial separation of charge carriers. Organic photovoltaic cells use nanoscale morphology to provide a high rate of electron hole encounters which results in the formation of spin triplet excitons. OPVs would have poor quantum efficiencies if every encounter led to recombination, but state-of-the-art OPVs demonstrate better quantum efficiency. This suppression of recombination between electrons and holes has been engineered to the interplay of spin, orbital angular momentum and delocalization of electron excitations in organic semiconductors. Time resolve spectroscopy can be used to study different engineered models having high efficiency polymer-fullerene systems in which the lowest lying molecular triplet exciton (T 1 ) lies below the intermolecular charge transfer state (CT). Encounters of spin-uncorrelated electrons and holes generate CT states with both spin singlet ( 1 CT) and spin triplet ( 3 CT) characters. Triplet exciton formation can be the major loss mechanism in OPVs. The CT energy lies below T 1  making relaxation from  3 CT to T 1  energetically favored. However, the more efficient 1:3 blend no triplet formation is possible at room temperature, but at low temperatures, bi-molecular triplet formation can be observed in this blend. This suggests that there is a thermally activated process that competes with relaxation to T 1 . This process is disassociation of  3 CT back to free charges. However, even when energetically favored, the relaxation of  3 CT spin triplet and CT back to T 1  can be strongly suppressed via control over wave function delocalization, allowing for the disassociation of  3 CT back to free charges (FC). This reduces recombination and enhances device performance. Thus, some manner for increasing the suppression of electron/hole recombination would further improve the efficiencies of these types of photonic devices. 
       SUMMARY 
       [0004]    The present invention, as disclosed and described herein, in one aspect thereof, comprises an apparatus for suppressing electron/hole recombination includes a photonic device that generates electron/hole pairs responsive to a light beam interacting with the photonic device. An orbital angular momentum (OAM) generation device is located to impart an orbital angular momentum to the light beam before the light beam interacts with the photonic device. The electron/hole pair recombination generated from an OAM imparted light beam is less than electron/hole pair recombination of a non-OAM imparted light beam. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    For a more complete understanding, reference is now made to the following description taken in conjunction with the accompanying Drawings in which: 
           [0006]      FIG. 1  illustrates a light beam; 
           [0007]      FIG. 2  illustrates a plane wave front; 
           [0008]      FIG. 3  illustrates a helical wave front; 
           [0009]      FIG. 4  illustrates the use of light harvesting complexes (LHCs) for generating electron-hole pairs; 
           [0010]      FIG. 5  illustrates the use of organic photovoltaic cells (OPVs) for generating electron-hole pairs; 
           [0011]      FIG. 6  illustrates the effect of spin and decentralization on electron-hole recombination; 
           [0012]      FIG. 7  illustrates the effect of an OAM twisted beam on electron-hole recombination; 
           [0013]      FIG. 8  illustrates a functional block diagram of one manner for improving the suppression of electron-hole recombination in a photonic circuit; 
           [0014]      FIG. 9  illustrates the manner in which electrons move between different states within an organic photovoltaic cell; and 
           [0015]      FIG. 10  is a flow diagram illustrating the process for controlling electron-hole recombination using the application of orbital angular momentum to a light beam. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Referring now to the drawings, wherein like reference numbers are used herein to designate like elements throughout, the various views and embodiments of s suppression of electron-hole recombination using orbital angular momentum semiconductor devices are illustrated and described, and other possible embodiments are described. The figures are not necessarily drawn to scale, and in some instances the drawings have been exaggerated and/or simplified in places for illustrative purposes only. One of ordinary skill in the art will appreciate the many possible applications and variations based on the following examples of possible embodiments. 
         [0017]    Referring now to the drawings, and more particularly to  FIG. 1 , there is illustrated one embodiment of a beam for use with the system. A light beam  104  consists of a stream of photons  102  within the light beam  104 . Each photon has an energy ±ω and a linear momentum of ±k which is directed along the light beam axis  106  perpendicular to the wavefront. Independent of the frequency, each photon  102  within the light beam  104  has a spin angular momentum  108  of ± aligned parallel or antiparallel to the direction of light beam propagation. Alignment of all of the photons  102  spins gives rise to a circularly polarized light beam. In addition to the circular polarization, the light beams also may be processed to carry an orbital angular momentum  110  which does not depend on the circular polarization and thus is not related to photon spin. 
         [0018]    Referring now to  FIGS. 2 and 3 , there are illustrated plane wavefronts and helical wavefronts. Ordinarily, laser beams with plane wavefronts  202  are characterized in terms of Hermite-Gaussian modes. These modes have a rectangular symmetry and are described by two mode indices m  204  and n  206 . There are m nodes in the x direction and n nodes in the y direction. Together, the combined modes in the x and y direction are labeled HG mn    208 . In contrast, as shown in  FIG. 3  beams with helical wavefronts  302  are best characterized in terms of Laguerre-Gaussian modes which are described by indices I  303 , the number of intertwined helices  304 , and p, the number of radial nodes  306 . The Laguerre-Gaussian modes are labeled LG mn    610 . For l≠0, the phase singularity on a light beam  204  results in 0 on axis intensity. When a light beam  204  with a helical wavefront is also circularly polarized, the angular momentum has orbital and spin components, and the total angular momentum of the light beam is (l±) per photon. 
         [0019]    Existing techniques for improving electron hole recombination have varied depending upon the particular type of device being utilized. For example as illustrated in  FIG. 4 , within biological light harvesting complexes (LHCs), a light beam  402  is applied to the LHC such that photonic absorption  404  creates molecular excited states. These molecular excited states comprise excitons  406  that are funneled to reaction centers  408  where they are disassociated into electrons  410  and holes  412 . The electrons  410  and holes  412  are separated further via a series of cascading charge transfer steps. 
         [0020]    As illustrated in  FIG. 5 , organic photovoltaic cells (OPVs) receive a light beam  502  and use photo absorption  504  to create the excitons  506  that are applied to a single donor-acceptor heterojunction  508  formed within a de-mixed blend of a donor and acceptor semiconductor to generate the electrons  510  and holes  512 . The most efficient OPV systems comprise nanoscale (less than symbol 5 nm) domains of pure fullerene acceptor and domains of fullerene intimately mixed with a polymer donor. These length scales are smaller than the Coulomb capture radius (CCR) in organic semiconductors (kT=e 2 4πε 0 εr) which is estimated to be as large as 16 nm at room temperature due to the low dielectric constant of the material (approximately 3-4). Thus, in contrast to LHCs, electrons and holes diffusing through an OPV may encounter one another before they reach the electrodes. This is similar to standard inorganic solar cells, where bimolecular electron-hole recombination (BR) determines solar cell performance. 
         [0021]    The rate of electron hole encounters that produce Coulombically bound states is given by R=γn(p), where n(p) is the electron (hole) population density and γ is the Langevin recombination constant given by γ=q&lt;μ&gt;/ε, where q equals the electric charge, &lt;μ&gt; equals the effective electron/hole mobility and ε equals the dielectric constant. This model successfully describes the principal operating mechanism of OLEDs, where charges injected through the electrodes capture one another to form strongly bound excitons. 
         [0022]    In empirically optimized OPV&#39;s, the recombination rate is suppressed by up to three orders of magnitude compared to the Langevin rate, allowing external quantum efficiencies as high as 80 percent. The recombination of bound states formed via electron/hole encounters is mediated not only by energetics, but also by spin and delocalization, allowing for free charges to be reformed from these bound states thus suppressing recombination. This is more particularly illustrated in  FIG. 6  which illustrates when a light beam  602  has spin and delocalization characteristics  604  applied thereto, the light beam including spin characteristics  606  may be applied to a photonic device  608 . Results have shown that the photonic device  608  will have an improved suppression of the recombination rate due to the spin and delocalization characteristics. 
         [0023]    Referring now also to  FIG. 7 , a similar suppression of recombinations between electrons and holes may be achieved using twisted photons that carry orbital angular momentum. The light beam  702  has an orbital angular momentum applied thereto by for example a passive hologram, SLM (spatial light modulator), phase plate, etc. The generated twisted beam  706  may then result in new quantum states which slow down the rate of recombination when applied to a photonic device  708  providing further improved recombination rates. 
         [0024]    A new design for an artificial photo-conversion system uses circuitry, such as a phase mask holograms, to apply orbital angular momentum (OAM) to the light signal and enable the suppression of electron hole recombination by avoiding the formation of triplet states in enhancing fluorescence efficiency. By placing OAM circuitry within the path of a photon, the orbital angular momentum generated by the photon can be transferred to an electron and a new quantum state created where suppression of electron-hole recombination is supported. This suppression is due to the change in total angular momentum of the electron (spin+orbital) using a device that twists the protons with a prescribed topological charge using a variety of methods (passively using a hologram or actively using other methods). The amount of orbital angular momentum applied results in a specific topological charge that can control the rate of recombination. Thus, by controlling the amount of applied OAM, the rate of suppression of recombination may be controlled. 
         [0025]    Thus, referring now to  FIG. 8 , there is illustrated a functional block diagram of one manner for improving the suppression of electron hole recombination in a photonic circuit. A light beam  802  is passed through an OAM device  804  in order to generate an OAM twisted light beam  806 . The OAM device  804  may comprise a passive hologram, a spatial light module (SLM), a spatial plate, an amplitude mask, phase mask or any other device capable of applying an orbital angular momentum to the light beam. The amount of OAM twist applied to a light beam may be controlled via an OAM control circuit  805 . The OAM control circuit  805  may control the level of OAM twist based upon a desired level of electron-hole recombination within a photonic device. The OAM twisted light beam  806  is applied to some type of photonic circuit  808  that energizes electrons responsive to the OAM light beam  806  to higher states that will decompose into a ground state. The photonic circuit may comprise optical LEDs, organic photovoltaic cells, biological light harvesting complexes, organic semiconductors, PN junctions, semiconductor, solid state devices, solar cells or any other light affected device. 
         [0026]    Referring now to  FIG. 9 , there is illustrated the manner in which electrons move from different states between various quantum states within an organic photovoltaic cell. Conversions between excited state species are shown at  902  while recombination channels are shown at  904 . State S 1    906  and state T 1    908  are the lowest lying singlet excitons  906  and triplet excitons  908 , respectively. CT is the charge transfer state. Photoexcitation goes from the ground state S 0    910  to a singlet excitation S 1    906  at  912 . The singlet exciton S 1    906  is ionized at a heterojunction leading to the formation of  1 CT states which separates into free charges (FC)  902 ,  904  with high efficiency at  914 . Biomolecular recombination of electrons and holes leads to the formation of  1 CT state at  916  and  3 CT states at  918  in a 1:3 ratio as mandated by spin statistics. The  1 CT can recombine slowly to the ground state as shown at  920 . The  3 CT state recombination to the ground state S 0    910  is spin forbidden but relaxation of the T 1  state  908  at  922  is energetically favorable. Once formed triplet excitons  3 CT can return to ground state via an efficient triplet charge annihilation channel at  924 . Under favorable conditions, the time required for CT states to reorganize to free charges at  918  is less than the time required for relaxation to T 1  at  922 . Thus, CT states are recycled back to free charges leading to a suppression of recombination. 
         [0027]    In order to probe the dynamics of these bound states, we can first consider the initial dissociation of the photo generated singlet exciton, S 1    906  at the D-A interface. The first step of this process is charge transfer across the D-A interface, which can lead to either long-range charge separation or the formation of bound interfacial charge transfer (CT) states. Such bound charge pairs then decay to the ground state S 0    910  via geminate recombination (GR). It is important to note that spin must be taken into account when considering CT states as they can have either singlet ( 1 CT) or triplet ( 3 CT) spin character which are almost degenerate in energy. Disassociation of photo generated singlet excitons leads to the formation of only  1 CT states  904  due to spin conservation. In contrast, recombination of spin-uncorrelated charges leads to the formation of  1 CT and  3 CT states in a 1:3 ratio based on spin statistics.  1 CT states can either dissociate or recombine to the ground state either via luminescence which is slow for this intermolecular D-A process or non-radioactive decay. For  3 CT states, decay to ground states is spin forbidden and hence both radiative and non-radiative processes are very slow. However, if the energy of the lowest lying molecular triplet exciton (T 1 ) lies below the  3 CT energy, then  3 CT can relax to T 1 . 
         [0028]    The model for recombination in the importance of spin statistics are well-established in OLEDs where the formation of non-luminescent triplet excitons is a major loss mechanism. Efforts to overcome this problem have focused on the use of metal organic complexes to induce spin orbiting coupling and recently on the use of low exchange energy materials that can promote inner system crossing from T 1  to S 1 . 
         [0029]    Thin films using transient absorption (TA) spectroscopy have been used in the past. In this technique a pump pulse generates photoexcitations within the film. At some later time, the system is interrogated using a broadband probe pulse. Although TA has been widely employed to study the photophysics of OPV blends previous measurements have been severally limited by three factors. The first limitation has an insufficient temporal range, typically a maximum of 2 ns delay between pump and probe. A second limitation has a limited spectral range and lack of broadband probes, which hinders the observation of dynamic interactions between excitations. And lastly, insufficient sensitivity, which mandates the use of high fluence pump pulses to create large signals. 
         [0030]    These issues have been resolved recently using broad temporal (up to 1 ms) and spectral windows (out to 1500 nm) and high sensitivity (better than 5×10−6). This temporal window is created by using an electrically delayed pump-pulse and allows for the study of long-lived charges and triplet excitons. In conjugated polymers local geometrical relaxation around charges (polaron formation) causes rearrangement of energy levels, bringing states into the semiconductor gap and giving rise to strong optical transitions 700 nm-1500 nm. The absorption bands of singlet and triplet excitons are also found to lie in the near IR making a broadband spectral window necessary to track the evolution of the excited state species. The high sensitivity of the experiment is essential as it allows one to probe the dynamics of systems when the excitation densities are similar to solar illumination conditions (10 16 -10 17  excitations/cm 3 ). At higher excitation densities bimolecular exciton-exciton and exciton-charge annihilation processes can dominate, creating artifacts, making such measurements unreliable indicators of device operation. One can further combine these measurements with advanced numerical techniques that allow one to resolve the spectral signatures of the overlapping excited state features and track their kinetics. 
         [0031]    The overlapping spectrum of the excited states makes the analysis of their kinetics difficult. In order to overcome this problem one can use a genetic algorithm (GA), which enables us to extract the individual spectra and kinetics from the data set. Within this approach a linear combination of two or more spectra and associated kinetics can be taken and ‘evolved’ until they best fit the experimental data. 
         [0032]    The extracted kinetics can demonstrate that triplets may grow as charges decay. One can consider that the primary decay channel for triplets is triplet-charge annihilation, due to the high charge densities present, and model the time evolution of the system with the Langevin equation given below: 
         [0000]    
       
         
           
             
               
                  
                 
                   N 
                   T 
                 
               
               
                  
                 t 
               
             
             = 
             
               
                 
                   - 
                   a 
                 
                  
                 
                   
                      
                     p 
                   
                   
                      
                     t 
                   
                 
               
               - 
               
                 
                   β 
                    
                   
                     [ 
                     
                       N 
                       T 
                     
                     ] 
                   
                 
                  
                 
                   [ 
                   p 
                   ] 
                 
               
             
           
         
       
     
         [0000]    where:
       p: Charge concentration;   N T : Triplet concentration;   α: is the fraction of decaying charges that form triplets;   β: is the rate constant for triplet-charge annihilation.       
 
         [0037]    We now turn to the question of whether the time taken for relaxation from  3 CT to T 1 , process  922  shown in  FIG. 9  with an associated timescale τ 4 , is fast and if not, whether there are the competing processes for the decay of  3 CT. As noted earlier the CT energy lies above T 1 , making relaxation from  3 CT to T 1  energetically favored. However, for the more efficient 1:3 blend, no triplet formation is possible at room temperature. But at low temperatures (&lt;240K), bimolecular triplet formation can be observed in this blend. This suggests that there is a thermally activated process that competes with relaxation to T 1 . We consider this process to be the dissociation of  3 CT back to free charges. Thus at high temperatures (&gt;240K) the dissociation of  3 CT back to free charges, process  918  shown in  FIG. 9  with an associated timescale τ 3 , out competes relaxation of  3 CT to T 1  i.e. τ 4 &gt;τ 3 . Hence one of the two channels for recombination (the other being recombination through  1 CT) is suppressed, allowing for high EQEs. At lower temperatures this dissociation process is suppressed, such that τ 4 &lt;τ 3 , leading to a buildup of triplet excitons. 
         [0038]    Referring now to  FIG. 10 , there is illustrated a flow diagram describing one manner for utilizing the application of orbital angular momentum to a light beam to suppress electron-hole conversion within a photonic device. Initially at step  1010 , the desired OAM level is selected for application to the light beam. The selected OAM level will provide a desired suppression of the electron-hole recombination rate. The OAM generating device is placed in the path between the light beam and the photonic device at step  1012 . The light beam is allowed to pass at step  1014  through the OAM generating device and the OAM twist is applied to the light beam. The OAM twisted light beam then interacts with the photonic device to generate energy at step  1016  such that the electron-hole recombination within the photonic device is affected by the OAM twisted beam. 
         [0039]    It will be appreciated by those skilled in the art having the benefit of this disclosure that this suppression of electron-hole recombination using orbital angular momentum semiconductor devices provides an improved manner for suppressing the number of electron-hole recombinations within a photonic device. It should be understood that the drawings and detailed description herein are to be regarded in an illustrative rather than a restrictive manner, and are not intended to be limiting to the particular forms and examples disclosed. On the contrary, included are any further modifications, changes, rearrangements, substitutions, alternatives, design choices, and embodiments apparent to those of ordinary skill in the art, without departing from the spirit and scope hereof, as defined by the following claims. Thus, it is intended that the following claims be interpreted to embrace all such further modifications, changes, rearrangements, substitutions, alternatives, design choices, and embodiments.