Photovoltaic Generation and Device

1. Field of the Invention

The invention is concerned with photovoltaic generation.

2. Description of the Prior Art

Photovoltaic devices have been available in a variety of commercial forms for some years. Commercially available devices have invariably depended upon a junction effect--that is, upon the junction field separation of photo-generated electron-hole pairs; and as a consequence, maximum generated voltages are, in magnitude, equal to the bandgap of the material. One such class of devices is exemplified by the selenium cell which has, for many years, been used as a photographic light meter. Voltages in the selenium cell are generated at the selenium-metal interface defined at one electrode.

A development which grew out of the emphasis on semiconductor technology stimulated by the transistor takes the form of the silicon solar cell. Here, a planar junction between p and n regions serves to divide the carrier pairs again generated within carrier lifetime distance of the junction by light absorbed in a thin outer region, and the voltage generated is equal to the approximate 1.1 volt silicon bandgap. Junction devices of this nature are described in 25, Journal of Applied Physics, page 676 (1954).

A second type of photovoltage, also limited to bandgap value may occur in the absence of a junction. The responsible effect, the "Dember effect" depends on an absorption gradient in a uniform medium. See 109, Journal of the Electrochemical Society, page 688 (1962). An analogous effect depends on a composition gradient--see 29, Reviews of Modern Physics, page 308 (1957).

Anamolously high photovoltages have been observed. The best known example concerns zinc sulfide, and a number of technical papers reporting voltages well above the bandgap have appeared. See, for example:

102, Physical Review, No. 3, page 705 et seq. May (1956);

109, Physical Review, page 1860 (1958 ),

109, Physical Review, page 601 (1958),

113, Physical Review, page 1204 (1959), and

40, Journal of Applied Physics, page 66 (January 1969). Workers have considered these anamolous effects mystifying and have attributed them to a variety of mechanisms. For example, in zinc sulfide, the effect has been attributed to junctions resulting from the stacking faults. These crystalline faults, generally lamalar in form, define interfaces between successive Wurtzite and cubic forms which coexist at room temperature.

A mechanism which may give rise to very high photovoltages is described. This mechanism, typically responsive to radiation within the wavelength range of from 5 microns to 0.1 microns, upon which the devices of the invention are based, relies on a bulk effect in accordance with which electronic transfer results from excited dipoles associated with light absorbing centers. The original impetus and the origin of the only net directionality of such electrons is the result of dipole orientation. Such dipoles associated with dopants--preferably absorbing in the visible--are dispersed as solute within poled spontaneously polarized material. This material, which may be single crystal or polycrystalline, is necessarily within the class which permits the well-known pyroelectric response. It is well known that all such materials also permit ferroelectricity. The inventive requirement, however, does not include the switching and hysteresis properties of ferroelectric--it being required only that there be a net polarization sufficient to result in alignment of the dipolar dopant.

A recently developed class of devices is dependent upon the emission resulting from excitation of absorbing dipolar species in a polarized environment. See 5, Optics Communication, page 45 (1972). The only condition that need be superimposed upon materials suitable for such use is that of charge transfer. This requirement is conveniently described in terms of the necessary photoconductivity characterized by the dopant species in the pure form.

The additional inventive requirement is for appropriate electrodes defining extreme surfaces in the net polar direction.

The FIGURE is a schematic representation of apparatus suitable for the practice of the invention.

The FIGURE depicts a body 2 consisting of a polarized host exhibiting a net moment having a component in the direction defined by electrodes 3 and 4. The host of body 2 is transparent to radiation depicted as arrow 5 but is doped with color centers which absorb radiation within the visible spectrum. Arrow 5A shown in phantom represents an alternative species in which light is introduced through a transparent electrode 4. Devices using this arrangement are common and electrode materials exhibiting appropriate transparency are exemplified, for example, by tin oxide, thin deposited gold, indium oxide, etc. These color centers have a ground state dipolar moment, as described, and, upon absorption, result in charge transfer to produce the photocurrent which is the essence of the invention. Photocurrent flows via conductors 6 and 7 through load 8 to complete the electrical circuit. Load 8 may be any form of electrical device, desirably impedance-matched to photovoltaic cell 1.

The essential requirement of the invention is a color center or "absorbing species". This center absorbs radiation, preferably within the visible spectrum, to liberate electrons by a charge transfer mechanism. The center is characterized by a ground state dipole moment of at least 0.01 Debye. The required charge transfer mechanism is satisfied in any such center which in the pure form evidences photoconductivity. Photoconductivity, a measure of electron liberation upon absorption of light, that is, of the transfer of electrons to the conduction band, or, in the instance of holes, to the valence band, is sufficient if the light conductivity in a uniform body of material consisting of the color centers is at least 10.sup..sup.-14 (ohm-cm).sup..sup.-1 (Wcm.sup..sup.-2).sup..sup.-1.

The concentration of this dopant material generally lies within the range of from 0.005 to 5 atom percent. A number of factors are considered in determining this range; and, depending upon these factors, a preference exists for one or a different portion. A first consideration has to do with absorption of the concerned radiation. As in transducers of this general nature, it is desired that conversion essentially be high. For any given system, concentration should be sufficiently high that concerned radiation is absorbed at a level such that no more than about 1/eth is transmitted, where e is the natural logarithin base. This distance is conventionally known as the "absorption length". It may be stated in usual terminology that the concentration must be such that the thickness of the body in a relevant radiation direction be at least one absorption length. As an example, Fe.sup.2.sup.+ at a concentration of 0.1 atom percent gives rise to an absorption length of 0.025 cm in a transparent host. The same requirement may give rise to a concentration as high as 5 atom percent for the same dopant or absorbing species for a body which is 5 microns thick. In general, a minimum dimension of the body corresponding to greater than about 3 absorption lengths gives little further advantage.

A completely engineered device, in accordance with the invention, must take account of the mean-free path of the electron which is propelled from the excited dipole. The requirement that this dopant be photoconductive inherently requires an average mean-free path sufficient to avoid recapture by the very center of electron origin. To usefully contribute to the photovoltaic effect, the liberated electron should be captured by a second color center. Once propelled, the electron moves in the field direction prescribed by the polarization of the host. This useful directional traversal ordinarily continues until a first collision. The effect of the first collision, generally with a lattic phonon (a vibrational quantum), is to randomize direction of travel. The entire effective energy conversion upon which the invention is based is, therefore, a consequence of this first travel period before collision. In an optimum case, it would be desirable to have a sufficiently long mean-free path, such that essentially all of the initially propelled electrons travel without collision to a trapping center. This trapping center should be a second color center which can, through the mechanism of dipolar excitation and charge transfer, in turn, liberate an electron upon absorption of a quantum of radiation energy. In a preferred embodiment, a concentration of color centers should be sufficient to result in an average spacing equal to the electronic mean-free path. For many systems, however, other considerations noted result in far greater spacing. As an illustration, Example 1, utilizing a material consisting of 0.1 atom percent Fe.sup.2.sup.+ in LiNbO.sub.3 is found to owe its effectiveness to a net mean-free path of only about 1 Angstrom unit. The conclusion is reached that the spacing of centers in this example was perhaps 10 times this total mean-free path. Under many systems, that is, in applications in which high current flow is not required, lesser concentrations may be adequate. A concentration sufficiently small as to be equivalent to a spacing of 100 times the total mean-free path may, in a material of sufficiently low conductivity, result in attainment of open circuit voltages of a thousand volts/cm or greater in periods of one second for a light intensity of 1 w/cm.sup.2 for typical structures.

The host material is, under most circumstances, desirably transparent to the radiation of concern. For preferred devices in accordance with the invention in which dependence is had upon a wavelength within the visible spectrum, that is, from 7,000 to 4,000 Angstrom units, the host should exhibit an absorption in the relevant spectral range of no greater than ten percent for the radiation transmission direction.

Since the inventive phenomenon depends upon charge accumulation or, at the very least, upon net directional flow, there is the expected requirement that resistivity be sufficiently high under operating conditions to permit this fundamental mechanism. In broad terms, resistivity must be at just such a minimum level--i.e., sufficient to permit charge accumulation due to charge transfer from excited dipole liberation. In terms of the design parameters set forth above, a dark resistivity of at least 10.sup.6 ohm-cm is generally adequate. A preferable minimum is 10.sup.12 ohm-cm. While this parameter is described in terms of the host material, the dopant may also contribute so that the resistivity minimum states is that for the entire body.

In general, the device function is dependent only upon the dopant-host combination described. The phenomenon of electronic or upper band edge absorption is, however, well known; and the possibility of modifying composition to better take advantage of particular available radiation may exist. So inclusion of other dopant material to effect energy transfer--e.g., a dopant to provide a first photon absorption for a multiple photon event--may be provided where available radiation peaks at a wavelength below that of the upper absorption edge of the dopant species.

In the preferred embodiment, polarization of the host is perfect, both as to direction (defining the shortest distance between electrodes) and quantity (all host dipoles contributing). While this condition is generally attainable in a perfect single crystal and nearly attainable in relatively dislocation-free polycrystals, it may not be attainable in materials otherwise satisfactorily applied. The minimum requirement is for a net polarization in the electrode direction. Device efficiency may be considered primarily in terms of the potential energy distortion at the dopant site and other considerations which together affect the net electronic mean-free path. The primary effect of the host remanent polarization is to impart directionality and thereby increase this most significant parameter. The most significant device requirement with regard to the host polarization is simply that there be sufficient remanence to maintain the body in the saturated condition.

While discussion has been in terms of crystalline material--the class traditionally associated with electrical polarization effect and, undoubtedly, the preferred class for the purposes of the invention--another class of material may be utilized. A class of organic material, exemplified by polyvinylidene fluoride and polyvinyl fluoride, has been reported as exhibiting the remanent polarization required for the purposes of the invention. Such a material may be utilized as the host for a color species having the required solubility.

Other material considerations are not directly relevant to the operative mechanism. These are the usual requirements of chemical and physical stability under the operating conditions. So, for example, the soluble pyroelectric, such as, poled ADP, may serve a long-term device use only if encapsulated or otherwise protected from any moisture-containing ambient.

General requirements have been set forth in the preceding section. Such requirements--e.g., device thickness--are dependent upon such factors as color center concentration, absorption efficiency, etc. Other considerations are concerned with desired photovoltage, as well as photocurrent, expected or available radiation intensity, etc. Still other design considerations are conventional (e.g., impedance matching). Electrode design should be such as to minimize contact resistance. Where design is such that electrode transparency is desired, electrode material, such as, indium oxide and tin oxide, may be utilized for radiation wavelengths within the visible spectrum. In general, electrode area should at least approach the total area of the relevant body surface for charge accumulation. This desideratum arises from the expectation of high current.

As in other primary generators, highest utilization efficiency coincides with a condition of impedance matching. In general, devices of the invention are characterized by high impedance values--e.g., 10.sup.6 ohm or greater. Effective utilization, therefore, generally requires similarly high impedance loads. It is comtemplated that these devices will be most valuably used for biasing high voltage, low current drain apparatus.

A single crystal of LiNbO.sub.3 containing 0.1 atom percent Fe.sup.2.sup.+ was prepared by Czochralski pulling from a melt containing 0.1 weight percent Fe.sub.2 O.sub.3 and 99.9 weight percent LiNbO.sub.3 in accordance with the procedure described in 27, Journal of Physics and Chemistry of Solids, page 983 (1966). The grown crystal evidenced an absorption coefficient of 38 cm.sup..sup.-1 (an absorption length of .noteq.0.025) and a dark resistivity of 10.sup.14 ohm-cm. A crystalline body of dimensions 1 cm by 1 cm by 0.0225 cm was cut and polished and electrodes of thin gold were affixed to the broad faces by evaporation. Wire leads were run to an electrooptic modulator of a characteristic impedance of approximately 10.sup.15 ohms which was greater than that of the body. Electrodes, therefore, defined a thickness of crystalline material of approximately 0.025 cm which had been poled in that direction. The body was then irradiated with 5145 Angstrom units wavelength unmodulated radiation from an argon-ion laser with a peak intensity approximately 1 w/cm.sup.2. Photovoltages as determined from the modulator transmission attained a value of well in excess of 1,000 volts (>10.sup.5 volts/cm) in a period of one second. Continuous radiation of the value noted resulted in a substantially unvarying dc voltage of the stated value for an instant radiation period of 1/2 hour.

Many experiments showing similar results were conducted using a variety of materials, operational parameters, and apparatus arrangements. Such varying parameters and other conditions resulted in essentially dc voltages, i.e., from about 1 Hz to 0 Hz. Depending on conditions, photovoltages were over a broad range up to a maximum of about 10,000 volts which for the conditions of the latter experiment was the maximum available due to dielectric breakdown of the surrounding air. That the effect was steady state (as distinguished from the transitory effect associated with pyroelectricity) was established by a continuance of essentially constant photovoltages at such levels for periods of up to about a day. Such experiments established the concept of a saturating voltage which occurs, since at high radiation intensities the photocurrent divided by the photoresistance is independent of intensity.

The conclusion is reached that the inventive matter, in its most important aspects, is concerned with steady state conditions--i.e., dc or near dc. Photovoltages of photocurrents to be detected generally represent saturation values which for incident radiation intensities even as high as 1 w/cm.sup.2 imply time constants of the order of a second. In general, therefore, devices operated in accordance with the described phenomenon contemplate incident radiation also of an essentially dc character as described above.