Abstract:
A high-efficiency single heterojunction solar cell wherein a thin emitter layer (preferably Ga 0 .52 In 0 .48 P) forms a heterojunction with a GaAs absorber layer. The conversion effiency of the solar cell is at least 25.7%. The solar cell preferably includes a passivating layer between the substrate and the absorber layer. An anti-reflection coating is preferably disposed over the emitter layer.

Description:
CONTRACTUAL ORIGIN OF THE INVENTION 
     The U.S. Government has rights in this invention under Contract No. DE-ACO2-83H10093 between the U.S. Department of Energy and the National Renewable Energy Laboratory, a Division of the Midwest Research Institute. 
    
    
     BACKGROUND OF THE INVENTION 
     1 . Field of the Invention 
     This invention relates generally to solar cells. More particularly, this invention relates to solar cells having a heterojunction. Even more particularly, this invention relates to solar cells having a GaAs absorber layer. 
     2. Description of the Prior Art 
     The current state-of-the-art GaAs solar cells having a GaAs absorber layer and a GaAs emitter layer are limited in their efficiency (less than about 24.8%) by the quality of the GaAs emitter layer. The high recombination velocity at the GaAs surface (even when passivated, e.g., with AlGaAs) reduces the V oc , J sc , and fill factor of these devices 
     One obstacle to increasing the conversion efficiency of GaAs solar cells has been the quality of the GaAs emitter. The blue portion of the incident light spectrum is absorbed mainly in the emitter. Hence, the effective lifetime of photogenerated minority carriers in the emitter must be long enough to assure their complete collection. However, there are several loss mechanisms in the GaAs that limit this collection efficiency. First, most of the current in the device must flow laterally in the emitter. Therefore, its sheet resistance must be small. For a given emitter thickness, one is forced to use a high doping density, which in turn reduces the minority-carrier lifetime. Second, defects at the front surface also reduce the minority-carrier lifetime. 
     U.S. Pat. No. 4,017,332 (James) discloses a cell for converting received light energy to electrical energy, comprising: four layers of differing types of semiconductive materials stacked to form three opposite conductivity junctions, wherein the outer two &#34;active&#34; junctions are formed of confronting layers with matched lattice constants to provide a plurality of energy converters, and the center connective junction is formed by two confronting intermediate layers which have purposely mismatched lattice constants to provide a lattice defect site surrounding the center junction. 
     U.S. Pat. No. 4,591,654 (Yamaguchi et al.) discloses an InP solar cell comprising a p-type InP single crystal substrate having a defined carrier concentration and an n-type InP layer containing a dopant of at least one element selected from groups VIA including S and Se disposed on the substrate. 
     The Conference Record, 13th IEEE Photovoltaic Specialists Conference, June, 1978, pages 886-891, discloses a (AlGa)As-GaAs-Ge dual junction cell made monolithically depositing GaAs and Ge layers epitaxially on the back side of a state-of-the-art AlGaAs/GaAs solar cell to provide two photovoltaicallyactive junctions, in the GaAs and in the Ge, respectively. 
     IEEE Transactions on Electron Devices, Vol. ED-27, No. 4, April 1980, pages 822-831, discloses materials for fabrication of cells including the use of Ga x  In 1-x  P (x being less than 0.35 ) layers to increase the efficiency of multi junction solar cells beyond that of single-junction cells. 
     However, these references only disclose multi- or single-junction devices that utilize GaInP, either as a buffer layer (as in the Yamaguchi et al. reference) or as a homojunction material in a multijunction device. 
     Even though the current state-of-the-art has to a significant extent resolved many of the mechanical problems encountered by providing workable contacts to the various layers in the stack or stacking cells in a relatively efficient manner (i.e., through the use of n-on-p GaAs solar cells) these cells have been found to be limited in conversion efficiency due to the quality of the n-GaAs emitter layer. Further, in the n-on-p GaAs solar cells (and also in the p-on-n type), the high recombination velocity at the GaAs surface, even when passivated with AlGaAs, reduces the V oc ,J sc , and the fill factor of these devices. 
     SUMMARY OF THE INVENTION 
     One object of the present invention is to provide an improved high-efficiency solar cell that surpasses the efficiency of the current state-of-the-art GaAs solar cells that are limited in efficiency to less than 24.8% due to the quality of the GaAs emitter layer. 
     Another object of the invention is to provide an improved high-efficiency solar cell that surpasses the efficiency of the current state-of-the-art GaAs solar cell by not using the GaAs emitter layer, but instead using a thin emitter layer of Al y  Ga x  In 1-y-x  P, to form a heterojunction with a GaAs absorber layer, where x+y is in the range of 0.47 to 0.57. 
     Another object of the invention is to provide an improved heterojunction solar cell which includes a more desirable emitter layer to increase efficiency. 
     A further object of the invention is to use a thin emitter layer of defined composition over a GaAs absorber layer in a heterojunction solar cell having a 1-sun energy conversion efficiency of greater than 25%. 
     Additional objects, advantages, and novel features of the invention shall be set forth in part in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned by the practice of the invention. The objects and the advantages of the invention may be realized and attained by means of the instrumentalities and in combinations particularly pointed out in the appended claims. 
     To achieve the foregoing and other objects and in accordance with the purpose of the present invention, as embodied and broadly described herein, the heterojunction solar cell may comprise: 
     (a) a substrate (e.g., a semiconductor); 
     (b) an absorber layer comprising GaAs; and 
     (c) an emitter layer in contact with the absorber layer; wherein the emitter layer comprises Al y  Ga x  In 1-y-x  P, where x+y is in the range of 0.47 to 0.57. 
     Either x or y could be zero, if desired. The absorber layer may be n or p conductivity type, and the emitter layer is, correspondingly, p or n conductivity type. In other words, the absorber layer and emitter layer are of complementary (i.e., opposite) conductivity types (i.e., one layer is p-type and the other layer is n-type. The emitter layer has a band gap energy of at least 1.8 eV. More preferably, the band gap energy of the emitter layer is at least 2.3 eV. 
     The emitter layer used in the solar cell of this invention is a semitransparent semiconductor that has a low sheet resistance, a high minority-carrier lifetime, and a lattice constant equal or very similar to that of GaAs. AlGaAs is not an ideal material for the emitter layer because its quality (or minority-carrier lifetime) is degraded by trace levels of oxygen in the material, whereas the quality of the AlGaInP emitter layer is not affected. 
     The improved solar cells of this invention should exhibit improved efficiencies (greater than 25%). 
     Other advantages of the apparatus and techniques of the invention will be apparent from the following detailed description and the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 shows a schematic cross section of a high-efficiency single heterojunction GaInP/GaAs solar cell of the invention having an (anti-reflective) AR coating and a GaAs substrate. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In general, the invention pertains to the fabrication and characterization of a single heterojunction AlGaInP/GaAs solar cell with an efficiency greater than 25% (one sun, AM 1.5). The present invention addresses the problem of the limited efficiency of conventional GaAs solar cells attributed to the quality of the GaAs emitter layer and the fact that the high recombination velocity at the GaAs surface (even when passivated with AlGaAs) reduces the V oc , J sc , and fill factor of these solar cell devices. 
     A schematic of a single heterojunction GaInP/GaAs solar cell having an AR coating is shown in FIG. 1. A passivating layer of p-GaInP 2  (p-Ga 0 .5 In 0 .5 P) is coated on the GaAs substrate. The structure was grown in a vertical, air-cooled reactor at one atmosphere using organometallic chemical vapor deposition (OMCVD), the detailed aspects of which are described in J. M. Olson and A. Kibbler, J. Crystal Growth, 77, 182 (1986), incorporated herein by reference. The Group III source gases were trimethylindium, trimethylgallium and trimethylaluminum; the Group V source gases were arsine and phosphine. The dopant sources were diethylzinc and hydrogen selenide. 
     Generally, the device described here was grown at a temperature T g  =700° C. The Ga 0 .5 In 0 .5 P layer was grown at a rate of R g  =80-100 nm/minute and a ratio of phosphine partial pressure to trimethylgallium plus trimethylindium partial pressure (called the V/III ratio) of 30. For the GaAs layers, R g  =120-150 nm/minute and V/III=35 Also the absorber was doped with Zn to a nominal level of 0.5-4×10 17  cm -3 , and the emitter layer was doped with Se at about 10 18  cm -3 . However, the optoelectronic properties and photovoltaic quality of the materials set forth above are complex and coupled functions of T g , V/III, dopant type and concentration, substrate quality, and the precise composition of the GaInP alloy. See S. R. Kurtz, J. M. Olson and A. Kibbler, Applied Physics Letters, 57, 1922 (1990), incorporated herein by reference. Therefore, it is possible that one skilled in the art could find a set of conditions, composition, and dopant type and concentration that will yield even higher efficiencies. 
     The front and back contacts of these devices were electroplated with gold. A high dopant concentration is used in both the GaAs substrate and the top contacting layer (not shown), and therefore, no thermal annealing of either contact is required. The front contact is defined by photolithography and obscures approximately 5% of the total cell area. The cell perimeter is also defined by photolithography and a mesa etch that uses a sequential combination of concentrated hydrochloric acid and an ammonia:peroxide:water solution. The ammonia/peroxide solution is also used to remove the contacting layer between the gold grid fingers. The anti-reflection (AR) coating preferably is a double layer of evaporated ZnS and MgF 2 , with thicknesses of 0.12 and 0.065 microns, respectively. 
     The thickness of the emitter layer may generally vary from about 500 to 2000 angstroms. Of course, if an electrically conductive anti-reflection coating is present over the emitter layer, then the emitter layer can be very thin (e.g., 20 angstroms). 
     The emitter layer must have sufficient conductivity so as not to limit the fill factor. Also, the emitter layer must not be so thick as to reduce the V oc  and J sc . 
     The emitter layer may be represented by the formula Al y  Ga x  In 1-y-x  P where x+y is in the range of 0.47 to 0.57. Both x and y may vary from zero to 0.57. When x is zero, the emitter layer is composed of Al y  In 1-y  P, and when y zero the emitter layer is composed of Ga x  In 1-x  P. The Al y  In 1-y  P emitter layer is more transparent and less conductive than Ga x  In 1-x  P. 
     The emitter layer is approximately lattice-matched with the absorber layer (i.e., coherent epitaxy is required). The band gap energy of the emitter layer is at least 1.8 eV and may be higher. 
     The thickness of the absorber layer may vary in thickness. A preferred thickness is in the range of about 1 to 5 microns. The band gap energy of the absorber layer is about 1.42 eV. 
     Generally, the absorber layer and the substrate are of the same conductivity type (either n- or p-type). The emitter layer is of the opposite conductivity type of the absorber layer. 
     Preferably a passivating layer is disposed between the substrate and the absorber layer. The purpose of the passivating layer is to prevent recombination of electrons and holes at the back surface of the absorber layer. The passivating layer is of the same conductivity type as the absorber layer. 
     Various types of passivating materials are known, e.g., GaInP 2  AlGaAs, ZnSe, etc. and may be used herein. 
     The substrate is preferably GaAs. Other useful substrates include germanium, silicon, GaInP, AlGaAs, alone or on silicon. 
     If desired, a mirror could be included at the back surface of the absorber layer, or at the top surface of the substrate. The use of a mirror to reflect light back through the absorber layer is well known in the art. 
     The foregoing i s considered as illustrative only of the principles of the invention. Further, because numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly all suitable modifications and equivalents may be resorted to falling within the scope of the invention as defined by the claims which follow.