Abstract:
An optically controlled switch includes first and second electrodes, a channel extending between the electrodes, and a light source positioned to illuminate the channel. The light source produces a wavelength capable of changing the material&#39;s conductivity. The channel includes a photosensitive organic material and is configured to operate as a light controlled switch.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to photosensitive electrical devices. 
     2. Discussion of the Related Art 
     Many complex systems use electrical control circuits to operate other devices. Some such electrical control circuits use photosensitive materials to control the currents or voltages therein. The photosensitive materials include semiconductors such as gallium arsenide (GaAs). 
     In a semiconductor, light of an appropriate wavelength optically excites mobile carriers. The optical generation of mobile carriers reduces the resistance of a channel made of the semiconductor. The optically induced change in channel resistance has been used as a trigger for such electrical control circuits. 
     SUMMARY OF THE INVENTION 
     When a conventional semiconductor is not illuminated, the material still has a significant conductivity. Thus, a channel made from a conventional semiconductor typically supports a significant leakage current when not illuminated. Due to the high leakage current, a conventional semiconductor channel does not function like optically controlled switch. 
     Various embodiments according to principles of the invention provide a photosensitive switch. The photosensitive switch has a conducting state in which the switch supports a substantial current and an insulating state in which the switch supports, at most, a low leakage current. The photosensitive switch goes rapidly from the insulating state to conducting state when illuminated by light of an appropriate wavelength. The photosensitive switch is advantageous as a regulator for a high voltage source, because the switch passes, at most, a low leakage current when not illuminated. 
     One optically controlled switch according to principles of the invention includes first and second electrodes, a channel extending between the electrodes, and a light source. The channel includes a photosensitive organic material. The light source is capable of illuminating the entire length of the channel and of changing the channel from an insulating state to a conducting state. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a cross-sectional view of an optically controlled switch; 
     FIG. 2 shows a control circuit based on the optically controlled switch of FIG. 1; 
     FIG. 3 is a flow chart for a method of operating the optically-based control circuit of FIG. 2; and 
     FIG. 4 is an oblique view of a micro-electromechanical (MEM) device that uses the optically-based control circuit of FIG.  2 . 
    
    
     In the Figures, like reference numbers refer to functionally equivalent elements or features. 
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     FIG. 1 shows an optically controlled switch  10 . The switch  10  includes a photosensitive switch  12  and a light source  14 . The photosensitive switch  12  is based on a planar structure. The planar structure includes an insulating substrate  16 , two electrodes  18 ,  20  located on the substrate  16 , and a photosensitive layer  22  that overlays both electrodes  18 ,  20  and the substrate  16 . The light source  14  produces light with a wavelength that is adapted to change the resistivity of the material in the photosensitive layer  22 . 
     In the planar topology, the thickness of photosensitive layer  22  is less than the length of channel region  26 . Also, light source  14  transmits light in a direction transverse to the conduction direction, L, in channel region  26 . Thus, the light is able to penetrate the entire length of the channel region  26  even if the channel region  26  is long. 
     For switch-like behavior, the ratio of the resistance of channel region  26  when illuminated, i.e., bright state, to the resistance of the channel region  26  when not illuminated, i.e., dark state, must be at least 10 4 , preferably is at least 10 6 , and more preferably is 10 8  or more. To obtain such a high ratio of resistances, the entire length of the channel region  26  must illuminated by light source  14  in the bright state. If a small transverse section along the channel region  26  remains insulating in the bright state, the resistance of that portion will dominate the entire channel resistance, because the resistivity of the channel material is orders of magnitude larger in the insulating state than in the conducting state. Thus, if a small section along the length of the channel region  26  remains non-illuminated, the ratio of the dark-state resistance to the bright-state resistance will not have the larger values characteristic of switch behavior. 
     This should be contrasted with a stacked topology common to solar cells (not shown). In a stacked topology, incident light propagates along the direction of current flow in the channel region. The length of the channel region must be short if light is to penetrate the entire length of the channel region. 
     In the planar topology, channel region  26  may be as long as desired without interfering with the ability of light source  14  to illuminate the entire channel region  26 . In contrast with the stacked topology, the planar topology enables the channel length to be long enough to provide a high channel breakdown voltage without interfering with the need for the whole channel region  26  to be conducting in the bright state. Exemplary breakdown voltages for channel region  26  are at least 50 volts, preferably at least 100 volts and more preferably at least 300 volts. 
     The planar topology also allows channel region  26  to have a dark-state electrical resistance characteristic of switch behavior, i.e., due to the long channel length. Exemplary channel regions  26  have dark-state resistances of at least 10 7  ohms, preferably at least 10 8  ohms, and more preferably 10 9  ohms or more. These large resistances insure that photosensitive switch  12  has a very low leakage current in the dark state. 
     In FIG. 1, the electrodes  18 ,  20  are made of gold (Au), aluminum (Al), indium-tin-oxide, titanium nitride (TiN), heavily doped silicon, or other conductors. In preferred embodiments, both electrodes  18 ,  20  are made from the same conductor so that illumination does not photovoltaically produce a voltage across channel region  26 . 
     The material of photosensitive layer  22  has a resistivity that responds to light in a preselected wavelength range. When not illuminated, the photosensitive layer  22  is a good insulator, and when illuminated, the photosensitive layer  22  is a fairly good conductor. For channel region  26 , the ratio of the resistance in the dark state to the resistance in the light state is significantly higher than for inorganic semiconductors. 
     The photosensitive layer  22  includes an organic matrix that is doped with an appropriate electron donor or acceptor to produce a material that conducts when suitably illuminated. 
     Exemplary organic materials for photosensitive layer  22 , include conjugated organic oligomers and polymers such as derivatives of oligomers and polymers containing aromatic units such as phenylenevinylenes, fluorenes, thiophenes, and pyrroles. Exemplary oligomers and polymers of phenylenevinylenes have substitutions of alkoxyl or cyano groups off the main chains. Some matrices include copolymers and blends of one or more of the above-described conjugated organic oligomers and polymers. 
     Preferred organic materials are fully conjugated oligomers and/or polymers that are molecularly aligned to increase the conductivity between electrodes  18 ,  20  when suitably illuminated. The preferred alignments increase inter-molecular overlaps to provide higher charge mobilities when suitably illuminated, e.g., mobilities of about 10 −6  cm 2 /volt-second or more. The matrix molecules may be aligned by stretching a matrix film prior to deposition, quenching the matrix to a liquid crystal state from a liquid state, or depositing the matrix on an alignment layer. 
     Exemplary dopants for organic matrices include organic oligomers and polymers, inorganic nanocrystals, and organo-metallic complexes. The dopants are either miscible in the organic matrix or chemically bound to the matrix molecules. Upon illumination, the dopants function as either electron donors or electron acceptors for the matrix, which would otherwise be an insulator. 
     The systems of dopants and matrix molecules belong to one of two classes. In the first class, the dopants are acceptors of photo-excited electrons from the organic matrix or donors of photo-excited holes to the matrix. In the second class, the dopants are photo-excitable donors of electrons to the organic matrix or acceptors of photo-excited holes from the matrix. Photo-excitations can result from the absorption of light by either the matrix molecules or dopants. Each class involves a particular alignment between highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) of the dopants and matrix molecules. 
     In the first class, the HOMO of the matrix molecules has a higher energy than the HOMO of the dopants, and the LUMO of the matrix molecules also has a higher energy than the LUMO of the dopants. For this alignment of energy levels, dopants have higher electron affinities and higher ionization potentials than matrix molecules. Exemplary of this class are systems in which the matrix includes poly(dialkoxyphenylenevinylene)s and the dopants are selected from C 60 , metal-phthalocyanines, thia-pyrylium, squarylium, azo-compounds, perylene, anthanthrone, and nanocrystalline CdSe. 
     In the second class, the HOMO of the matrix molecules has a lower energy than the HOMO of the dopants, and the LUMO of the matrix molecules also has a lower energy than the LUMO of the dopants. For this orbital alignment, the dopants have lower electron affinities and lower ionization potentials than the matrix molecules. Exemplary of the class are systems where the matrix includes poly(α,α′-dicyanophenylenevinylene)s and the dopants are poly(dialkoxyphenylenevinylene)s. 
     In photosensitive layer  22 , dopant concentrations are fixed to produce desired conductivities when suitably illuminated by light source  14 . Preferred conductivities result from between about 10 19  and about 10 21  mobile charge carriers per centimeter cubed when suitably illuminated. To achieve such charge carrier concentrations, organic materials include significant volume fractions of dopants. The volume fraction occupied by dopants is typically greater than 0.1 percent, preferably at least 1.0 percent, and often 10 percent or more. 
     Light source  14  excites electrons either from dopant sites to the matrix or from the matrix to dopant sites to convert photosensitive layer  22  from an insulating state to a conducting state. Thus, the conductivity of photosensitive layer  22  depends on both the dopant density and the illumination intensity from the light source  14 . The dependencies of the conductivity on the dopant density and the illumination intensity are often approximately linear. 
     The conductivity of channel region  26  varies linearly with both the channel width and the inverse of the channel length. A preselected dark-state resistance fixes the ratio of the width to length of the channel region  26 . The dark-state resistance determines the leakage current through the photosensitive switch  12 . A desired minimum breakdown voltage determines the minimum length for the channel region  26  of the photosensitive switch  12 . 
     A person of skill in the art could determine suitable channel dimensions and dopant fractions based on preselected values of the dark-state and light-state channel resistances, the intensity of light source  14 , and the channel breakdown voltage. 
     FIG. 2 shows a control circuit  34  based on optically controlled switch  10  of FIG.  1 . The control circuit  34  includes a direct current (DC) voltage source  36  and a voltage divider  38 . In the voltage divider  38 , the optically controlled switch  10  and a fixed resistor  40  connect in series. The fixed resistor  40  is a voltage source for a load element  42 , e.g., a capacitor or inductor. The resistance of the optically controlled switch  10  controls the current through the fixed resistor  40  and thus, the voltage drop applied across the load element  42 . 
     The optically controlled switch  10  includes light source  14  and photosensitive switch  12  of FIG.  1 . Exemplary light sources  14  include light emitting diodes (LED) and diode lasers. The light source  14  may include an optical waveguide, e.g., an optical fiber, that delivers light from a remote source to the photosensitive switch  12 . A voltage, V, used to modulate the light source  14  controls the resistance of photosensitive switch  12 . 
     FIG. 3 is a flow chart for a method  44  of controlling a circuit via an optically controlled variable switch, e.g., switch  12  of FIG.  3 . The method  44  includes applying an external voltage across a photosensitive switch located in the circuit (step  46 ). The method  44  also includes modulating the intensity of a light source, e.g. light source  14  of FIG. 2, that illuminates the photosensitive organic resistor while the external voltage is applied across the photosensitive organic switch (step  48 ). The modulated light intensity changes the resistance of the photosensitive switch and thus, the current that the external voltage produces in the circuit. The changed current changes the voltage drop across a load element, e.g., load element  42  in FIG.  2 . 
     The induced change in the voltage drop across the photosensitive switch is greater than any photovoltaic voltage induced across the photosensitive switch. Preferably, the change in the voltage drop is at least ten times any produced photovoltaic voltage. 
     Referring again to FIG. 2, exemplary control circuit  34  functions as a digitally modulated (DM) voltage source for load element  42 . In the DM voltage source, light source  14  functions as an optical modulator that produces a repeating sequence of bright and dark periods, e.g., ON and OFF periods of a diode laser or LED. The relative lengths of the bright and dark periods are varied to apply different average voltages across fixed resistor  40  and load element  42 . 
     FIG. 4 shows a micro-electromechanical (MEM) device  50  controlled by control circuit  34  of FIG.  3 . The MEM device  50  includes a flexible stalk  52  and a top piece  54 . The stalk  52  connects the top piece  54  to substrate  16 . The top piece  54  includes a first plate  56  of a capacitor and a reflector  58 . A second plate  60  of the capacitor is located on the substrate  16 . The capacitor is load element  42  of the control circuit  34  shown in FIG.  3 . The control circuit  34  determines the charge state of the capacitor thereby controlling the orientation of the reflector  58  on the MEM device  50 . 
     The control circuit  34  functions as a DM voltage source for charging the capacitor that controls the orientation of MEM device  50 . In the DM voltage source, light source  14  shines a light beam with a modulated intensity on photosensitive resistor  12 . The light intensity is modulated at a frequency that is higher than the time constant for mechanical resonance in the MEM device  50 , e.g., at least 5-10 times the mechanical resonance frequency. At such high frequencies, the average charge on plates  56 ,  60  determines the mechanical reaction of MEM device  50  to the driving voltage. The average charge on the plates  56 ,  60  depends on the relative lengths of the bright and dark portions of the illumination cycle. 
     Digital modulation of light source  14  requires a high frequency voltage source, V. The voltage source, V, can be a digital source, but the voltage source, V, typically has a maximum amplitude that is much smaller than that of the voltage modulating the charging and discharging of the capacitor of MEM device  50 . The voltage applied to capacitor is typically in the range of 0 volts-1000 volts and is preferably in the range of about 100 volts-300 volts. For such high voltages, electrically controlled DM voltage sources are often more expensive than the optically controlled DM voltage source formed from control circuit  34  and DC voltage source  36  of FIG.  3 . 
     An exemplary DC source  36  has a voltage of about 100-300 volts. For such a source a dark-state resistance of about 10 10  ohms is preferable to avoid substantial power dissipation in the dark-state. For such a resistance, channel region  26  typically has a length of at least 0.5 microns and preferably a length of 1-100 microns and a width of about 1,000 microns. The channel region  26  is highly inter-digitated to reduce to overall transverse extend of the region  26  (FIG.  4 ). Such channel dimensions also provide breakdown voltages of in excess of 150 volts. 
     In other embodiments of system  50 , photosensitive switch  12  is replaced by a photosensitive resistor (not shown). The photosensitive resistor has a photosensitive channel region  26  that includes either organic or inorganic materials. Exemplary inorganic materials include amorphous selenium (Se), silicon (Si), cadmium sulfide (CdS), and cadmium selenide (CdSe). These inorganic materials may be doped with well-known electron acceptors or donors. 
     Other embodiments of the invention will be apparent to those skilled in the art in light of the specification, drawings, and claims of this application.