IL46896A

IL46896A - Semiconductive device

Info

Publication number: IL46896A
Application number: IL46896A
Authority: IL
Original assignee: Innotech Corp
Priority date: 1974-03-27
Filing date: 1975-03-21
Publication date: 1977-07-31
Also published as: GB1501015A; DE2512898A1; FR2266312B3; IL46896A0; FR2266312A1; NL7503616A; JPS50134392A

Description

Semicoriductive Device -! E DE I E EMPLOYING Λ , WIDi- P.AMDGAP MATERIAL AS AN ACTIVE LAYER ABSTRACT OF THE .DISCLOSURE A semiconductive heterojunction device particularly uscf\l no Γ, photovoltaic device such as a solar cell ccm" rd ir s a hetero unction formed between a first layer of semiconductor material exhibiting one type of electronic conductivity (N or P) and a second layer of a compositionally different material ex- hibiting the other Ntype of electronic conductivity (P or N) , 8 Which second layer has an energy bandgap relatively wider than 9 that of the semiconductor material and an electron affinity less 10 than or equal to the electron affinity of the semiconductor. 11 Preferably, the wider bandgap material is a glassy amorphous 12 material which possess or is doped to possess a low resistivity 7 13 below about 10 ohm-cm. In devices employing N-type wider band-14 gap layers, the conduction band energy level of the. wider band-15 gap material is preferably at substantially the same energy l. i! |,pye,L Q9 t fi conduction band enercv level af th^ na oje-p ban5· 17 gap material at electrical neutrality. In devices employing 18 P-type wider bandgap layers, the valence band energy level of the 19 wider bandgap material is preferably at substantially the same 20 energy level as the valance band energy level of the narrower ¾1 bandgap material. 22 BACKGROUND OF THE INVENTION 23 The present invention relates to a semiconductive 24 hetero unction device which is particularly useful as a photo25 voltaic device such as a solar cell. 26 Conventional solar cells typically comprise a P-N 27 junction formed in a monocrystalline silicon substrate. Typical28 ly, an N-type surface region is diffused into a P-type silicon 29 substrate and ohmic contacts are applied. In operation, the 30 device is exposed to solar radiation; and photons, incident P~type substrate where they are absorbed in the production of electron-hole pairs. Holes created in the junction region (or which diffuse to the junction region)- are. swept by the built-in voltage to the N-type surface region where they either leave the device as photocurrent or accumulate to produce a photo-induced open circuit voltacje: The conversion efficiency of conventional solar cells, * · 8 however, is seriously limited by a number of factors. One such 9 factor is that the built-in voltage is limited by the relatively 10 narrow bandgap of the N-type silicon and the limited extent to 11 which both layers of silicon can be doped. While the built-in 12 voltage of the device can be increased by increased doping of 13 both layers forming the junction , such excess doping tends to 14 reduce conversion efficiency of the device by reducing its _LC» coT lection efficiency. As a consequence, the open circuit 16 voltages of typical silicon -solar cells are only about 50 per cen 17 of the silicon bandgap. 18 A second factor limiting the conversion efficiency of 19 conventional silicon solar cells is the fact that silicon tends 20 to absorb high energy photons (photons of blue and violet light) 21 near the surface, typically within a micron thereof. As a 22 consequence, many of the high energy photons are absorbed near 23 the surface of the N-type region and the carriers generated by 24 this absorption recombine at the surface. Such recombined 25 carriers are thereby lost as sources of photocurrent. 26 Yet a third limiting factor is the fact that lower 27 energy photons (photons of red and near infra-red light) tend 28 to penetrate deeply into silicon before they are absorbed. Whi] 29 minority carriers created by deep absorption can contribute to ' to permit them to drift into the junction region, the high temperature diffusion step required to form the N-type region significantly reduces minority carrier lifetime in the -type substrate. As a consequence, many carriers created by deep 5 absorption are lost to the photocurren . 6 SUMMARY OF THE INVENTION ■ ' ■ \—: 7 It has been discovered by the present applicants that 8 semiconductive heterojunction devices can be made into photo9 voltaic devices having improved conversion efficiencies by. 10 properly selecting the bandgaps and electron affinities of 11 the constituent materials. 12 Specifically, an improved photovoltaic junction device, 13 in accordance with the invention, /comprises heterojunction 14 formed between a first layer of semiconductor material exhibit15 ing one type of e ectronic conductivity (N or P) and a second 16 layer of a compos tionally different material exhibiting the 17 other type of electronic conductivity (P or N) , which second 18 layer has an energy bandgap relatively larger than that of the 19 semiconductor material and an electron affinity less than or 20 equal to the electron affinity of the semiconductor. Preferably 21 the wider bandgap material is a glassy amorphous material which 22 possess or is doped to possess a low resistivity below about 23 10^ ohm-cm. 24 In preferred embodiments, energy band levels of the •25 materials are substantially aligned. Specifically, in devices 26 employing N-type wider bandgap layers, the conduction band energ .27 level of the wider bandgap material is preferably at substan28 tially the same energy level as the conduction band energy level ' 29 of the narrower bandgap material at electrical neutrality. In energy level of the wider bandgap material is preferably at substantially the same energy level as the valence band energy leve of the narrower bandgap material. _. · ■ ■ ·■· Photovoltaic devices in accordance with the invention achieve improved energy conversion by virtue of a higher built- in voltage and increased collection efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages, nature, and various features of the present invention will appear more fully upon consideration of the illustrative embodiments now to be described in detail in connection with the accompanying drawings.

In the drawings: ' ....

FIG. 1 is a cross-section of a semiconductive junction device in accordance with the invention especially useful as a photovoltaic device; I i FIGS. 2Ά ana 2Q are energy level diagrams showing preferred energy relationships for the device of FIG. 1; FIG. 3 is a graphical illustration showing the relationship among current, voltage and incident radiation for a typical device in accordance with the invention; and FIG. 4 is a graphical illustration showing the quantum efficiency of a typical device in accordance with the invention as a function of wavelength.

DETAILED DESCRIPTION OF THE DRAWINGS Referring to the drawing, FIG. 1 is a schematic cross- section of a photovoltaic semiconductive junction device comprising a first active layer 10 of a semiconductor material, such as 1 crystalline silicon, exhibiting one type of electronic conductivity (N or P),and a second active layer 11 of a compositionally other type of electronic conductivity (P or N, respectively) , wider bandgap layer 11 is disposed in contact with the narrov/er bandgap semiconductor layer .10, thereby. forming a .junction 12 therewith. Preferably, the wider bandgap material is a glassy amorphous material, such as indium trioxide, which has been dop to exhibit conductivity in the semiconducting range, (i.e., it v exhibits or is doped to exhibit a low resistivity below about 1 ohm-cm. ) .

Electrodes 13 and 14 are applied to layers 10 and 11, respectively, for making ohmic electrical contact therewith.

Electrode 13, for example, can be aluminum for P-type silicon and tin for N-type silicon. Electrode 14 can also be alumnium for N-type. wider bandgap layers, and tin for P-type. Preferabl the electrode contacting the layer to be directly exposed to incident radiation is a transparent conductive material such as or tin cxide, /a educed area electrode, having A gkLd ox "finge " configuration in order to permit exposure of the layer surface. Typically, the. wider bandgap layer 11 is the directly exposed layer, and electrode 14 is the transparent or reduced area electrode .

Electrodes 13 and 14 are, in turn, electrically couple to an energy receiving element 15, such as a battery or detector (e.g., current detector or voltage detector), in a manner suitable for transmitting energy in the form of electrical current from the junction device to the receiving element 15.

The narrower bandgap semiconductor material of layer 10 can be a conventional monocrystalline semiconductor, such as silicon or gallium arsenide; a polycrystalline semiconductor suc as a polycrystalline film of silicon or Si Ge ; or a semicon- . 1—X ducting glassy amorphous material having a relatively narrow \ . In a device in accordance with the invention, the bandgap of the wider bandgap material is a least twice the bandgap of the narrower bandgap material. Thus, when the -narrower bandgap material is silicon, the bandgap of the wider bandgap layer should be larger than 2.22 electron volts; and when the narrower^bandgap material is gallium arsenide, it should be larger than about 2.80 electron volts. This larger bandgap permits utilization of a larger portion of the narrower bandgap in the built-in voltage, thereby increasing the voltage factor of the device.

Preferred materials for use as wider bandgap layers in devices according to the invention are glassy amorphous · materials, and particularly transparent glassy amorphous materi The term "glassy amorphous material" (or "glassy material"), as used herein, defines those materials which exhibit only shori-- . . cerm ordering. The term is intended to. include not only glasses but also those "amorphous" materials which have any appreciable short-range ordering. However, it is intended to exclude both crystalline substances (such as silicon and silicon dioxide) and true amorphous materials having no appreciable ordering. Glasse which comprise a specific class of glassy amorphous materials, are typically quenched liquids having a viscosity in excess of 18 about 10 poise at ambient temperature. They are generally characterized by: (1) the existence of a single phase; (2) gradual softening and subsequent melting with increasing tempera ture rather than sharp melting characteristics; (3) conchoidal fracture; and (4) the absence of crystalline X-ray diffraction peaks. The principal advantages of glassy amorphous materials for use in solar cells are their relatively wide bandgaps and th The term "transparent glassy amox-phous material" as useu herein refers to those glassy materials which are substantially transparent to visible light. More specifically, a glassy layer in order -to be considered transparent for purposes of making a solar cell, in accordance with the invention, should transmit mor than about -80 per cent of incident light having wavelengths in th region between abouNt-4000 angstroms and about.7000 angstroms.

In heterojunction devices in accordance with the. inventi the disparity in bandgap widths of the constituent layers results in potential spikes or notches in the junction as shown in FIGS. 2A and 2B. These potential spikes or notches can be undesirable barriers to carrier flow. Accordingly, when the device is to be used as a photovoltaic device, the electron affinities of the larger bandgap material and the narrower bandgap material are preferably chosen to minimize the notch impedance while retaining a high open Circuit: voltage. This r sul c n be. effected in < device having an N-type wider bandgap material by choosing as the wider bandgap material, a material having an electron affinity approximately equal to that of the narrow bandgap material or les than that of the narrow bandgap material by an amount not greater than the narrower bandgap. This requirement places the conductio bands of the. two layers at substantially the same energy level, and thereby reduces the notch to a very thin dimension through which carriers can readily tunnel. While the electron affinity of the wider bandgap material can be slightly higher than that of the narrower bandgap material, the open circuit voltage is reduced proportionately by the excess. Correspondingly, in a devic having a P-type larger bandgap material , the wider bandgap material is chosen to have an electron affinity which is smaller than that of the narrower bandgap material by an amount in excess of the narrower bandgap. This requirement places the valence band of the larger bandgap material at substantially the same energy level as that of the narrower bandgap material and minimizes notch impedance consistent with maintaining a high open circuit voltage.

The bandgaps and electron affinities of component materials are well-defined and generally tabulated quantities. · Bandgaps are generally tabulated as such, and electron affinities can be obtained by subtracting tabulated barrier heights from tab ulated work functions. (For tabulations of typical materials, se American Institute of Physics Handbook, 3rd Ed., 1972 (McGraw-Hil In instances where work functions of the wider bandgap and narrower bandgap materials are known or where the difference between the two work functions are known (as determined, for example, by measurement of the built-in voltage) , a more precise ; selection of e preferired electron affinities is possible. In such cases wherein the wider bandgap material is an N-type material, the electron affinity of the wider bandgap material is ideally substantially equal to the electron affinity of the narrower bandgap material less the absolute value of the difference between the work functions of the two materials. If the wider bandgap material is a P-type material, its electron affinity is ideally substantially equal to the electron affinity of the narrower bandgap material increased by the absolute value of the difference between the work functions.

Materials which have been found particularly advantageou for use" as wider bandgap materials in conjunction with convention semiconductors (such as silicon, germanium, gallium arsenide, gallium phosphide, and silicon carbide) , include indium trioxide (InO-j) , tin oxide (SnC^) f cadmium oxide (CdO) , antimon trioxide 1 (Sb 0 ) , lead oxide (iJbO) , vanadium oxide (Vo0_) , germanium oxide 2 (GeO^) , vanogermanate glasses, vanophosphate glasses, lead silica 3 glasses, and glassy mixtures of the above. , Materials preferred for use with substrates of P-type 5 silicon, P-type germanium, P-type gallium arsenide, and P-type 6 gallium phosphide include indium trioxide, tin oxide, cadmium 7 oxide, antimony trioxide, and mixtures thereof. 8 For N-type silicon substrates, a glass of the kind 9 having the following oxide components, doped with indium trioxide 0 is preferred. 1 Oxide Component Preferred Mole per Cent Permissible Range 2 PbO 0.2 0.2% - 2.6% 3 B203 34.6 34.6% - 37.6% 4 A1203 0.8 · 0.8% - 0.9% 5 ZnO 52.5 52.5% - 52.6 % e !! Ce02 2.2 01 - 2.Z¾ 7 Si02 9.7 6.3% - 9.7% 8 TABLE 1 9 Preferred materials for use with P-type gallium arsenide 0 substrates include lead oxide, germanium oxide, vanadium oxide, 1 vanogermanate glass, vanophosphate glass, and mixtures thereof. 2 For N-type gallium arsendie substrates, a glassy amor3 phous material having the composition set forth in Table 1 and 4 doped with indium trioxide is preferred. 5 The thickness of the layer of wider bandgap material is 6 preferably not more than about several microns and not less than 7 about a few hundred angstroms. In general, the layer should be 8 as thin as is possible consistent with maintaining a sufficient 9 thickness to avoid significant tunneling of carriers therethrough 0 By making the layer sufficiently thin, even materials which are ) not normally' considered transparent can be formed into layers which transmit sufficient quantities of light to be useful.

The device of FIG. 1 can be fabricated by any one of several techniques. Conveniently, a thin layer of glassy amorphous material is deposited on a semiconductor substrate by a conventional technique such as vacuum evaporation. The glassy layer is then doped to a low resistivity in the semiconducting range. The doping can be effected either by conventional diffu- sion doping or by the doping techniques disclosed in expending application Se ial Mo. 227,03-2 f"i"l'ed" by Seymour .orrin on Febi rar 3 rj—3r9 2, and assigned to applicant's assignee. The electrodes are then deposited, e.g., by vacuum evaporation or sputtering, and the resulting structures are packaged. The electrodes are then connected to an energy receiving element, such as a battery in charging polarity. ! . jn 'operation as ¾ solar Call, the device of l ±£■ exposed .to solar radiation and electrical energy flows from the device to the energy receiving element. While applicants do not wish to be bound by theory, it is believed that the wider bandgap material acts as a window through which incident solar radiation passes to the semiconductor substrate. Photons of the radiation are absorbed in the substrate by the creation of electron-hole pairs. In devices having N-type substrates, holes which are created in the transition region (or which diffuse thereto) are swept by the built-in voltage to layer 11 where they either contribute to the photocurrent or accumulate to develop an open circuit equilibrium voltage. In devices having P-type substrates electrons are swept into the layer 11 with the same result.

The structure and fabrication of such devices may be furtlier understood by reference to the following specific example attributable to energy band notching. r EXAMPLE 2 A photovoltaic device was fabricated by depositing on a one square centimeter square of gallium phosphide, a P-type glass having the following composition: Oxide Component Mole Per Cent PbO ' 49.5 Si02 49.5 p'2o3 1.0 " TABLE 2 The gallium phosphide substrate contained N-type zinc . . 17 impurities at concentration Of about 2 x 10 atoms per cubic centimeter. The above-composition glass was deposited by the well-known sedimentation technique1 to form a layer about 5000A thick. The glass was then doped by applying to it a layer of IIboron oxide estimated -to be less than about IQO angstrom^ thicK and heating the structure to 350°C for 10 minutes. A chrome electrode about 500 angstroms thick was deposited on the glass and a 1000 angstrom gold electrode applied to the' gallium phosphide. . ' The resulting structure acted as a photovoltaic device and exhibited no appreciable impedance attributable to energy band notching.

EXAMPLE 3 A photovoltaic device was fabricated in substantially th same manner as set forth in Example 2, except that N-type gallium arsenide was substituted for the N-type gallium phosphide. The resulting device acted as a photovoltaic device and exhibited no appreciable impedance attributable to energy, band notching.

EXAMPLE 4 A photovoltaic device was fabricated by depositing a 5000 angstrom layer of the preferred glass composition set forth in Table 1 on a one centimeter square of N-type silicon carbide 18 doped with nitrogen to a concentration of 2 x 10 . A chrome electrode was applied to the glass and a gold electrode was applied to the silicon carbide. The resulting structure acted as a photovoltaic device and exhibited no appreciable impedance 8 attributable to energy band notching. · . 9 The characteristics of devices in accordance with the 0 invention are relatively superior over those of conventional 1 silicon solar cells. While applicants do not wish to be bound by 2 theory, it is believed that these favorable characteristics resul 3 from improved collection efficiency and a higher built-in voltage 4 Collection efficiency is improved for at least two reasons. Firs 5 the wider bandgap material is substantially transparent to high ^ " energy photo s , thereby redixcirig losses du£ -to Surface reeombin || ations . Second, due to the fact that the wider bandgap material 8 can generally be doped at lower temperatures than those used in 9 fabricating conventional diffused junctions, the device can be 0 made with an enhanced minority carrier lifetime in the semicon-1J]ductor substrate. This enhanced lifetime enables more minority 2 carriers produced by the deep penetration of low energy photons 3 to diffuse to the junction and thereby contribute to the photo-4 current. 5 The higher built-in voltage results from the disparity 6 of bandgaps of the two layers resulting in a more nearly complete 7 utilization of the semiconductor bandgap in the built-up voltage. 8 In addition, the device offers considerable promise of 9 II increased yields and lower fabrication costs due to the relativel 0IIsimple process steps required for its fabrication.

■ I is understood that the above-described embodiments are illustrative of only a few of the many possible specific embodiments which can represent applications of the principles of the invention. Numerous and varied other arrangements can be readily devised in accordance with these principles by those skilled in the art without departing from the spirit and scope of the invention, v

Claims

CLAIMS TOAT IS C I 18 TS l

1. A semiconductive heterojunction device of the type comprisi -a pair of layers of compositionally different semiconductor materials disposed in contact with one another, each of said layers exhibiting a different kind of electronic conductivity and the material of each of said layers characterized by a band separating a conduction energy level band from a valence energy level band and by an electron affinity; and means for electrically contacting each of said layers; said device characterized in that: the bandgap of the material of one of said semiconduc layers is more than about twice the bandgap of the material of other of said semiconductor layers; and the electron affinity of the material of the layer , i g th l rger fcaiiuyap js ≤ubst¾»tx¾ll eq o to or* less than the electron affinity of the layer having the smaller bandgap material . ~

2. A device according to claim 1 further characterized in that the electron affinity of the larger bandgap material is less tha the electron affinity of the smaller bandgap material by at leas the amount of the smaller bandgap.

3. A device according to claim 1 further characterized in that the conduction energy bands of the materials of said two layers are at substantially the same energy level.

4. A device according to claim 1 further characterized in that: . the layer of material having the larger bandgap exhibi N-typc electronic conductivity: the layer of material having the smaller bandgap exhibits P-type electronic conductivity; and the conduction energy bands of the materials of said two layers are at substantially the same energy level.

5. *. Λ device accordin to claim 1 further characterized in that the valence energy bands of the materials of said two layers are at substantially the same energy levels.

6. A device according to claim 1 further characterized in that: the layer of material having the larger bandgap exhibits P-type electronic conductivity; the smaller bandgap material exhibits N-type electroni conductivity; and the valence energy bands of the materials of said two lsye S ¾re at..substantially -the Same energy ievei.

7. A device according to claim 1 further characterized in that the material of at least one of said layers is glassy amorphous material.

8. A device according to claim 1 further characterized in that the material of at least one of said layers is a material selected from the group consisting of indium trioxide, tin oxide cadmium oxide, antimony trioxide , lead oxide, vanadium oxide , germanium oxide, vanogermanate glasses, vanophosphate glasses, lead silicate glasses and glassy mixtures of the above.

9. A device according to claim 1 further characterized in that the material of the layer having the narrower bandgap is P-type silicon and in that the material of the layer having the larger bandgap is selected from the group consisting of indium trioxide tin oxilc, cadmium oxide, antimony trioxide and glassy .mixtures thereof. /

10. A device according to claim 1 further characterized in tha the material of the layer having the narrower bandgap is an N-type semiconductor and in that the material of the layer having the larger bandgap ^is a glass of the type which is formed by the following oxide components in the following mole percentage ranges: . ', Component Mole Per Cent PbO 0.2 - 2.6 B203 . 34.6 - 37.6 A1203 0.8 - 0.9 ZnO 52.5 - 52.6 CeO 0 - 2.2 46896/ 2 ,

11. 1 1. A photovoltaic device for generating electrical energy in response to incident electromagnetic radiation in a predetermined spectral range comprising: a semicoriductive substrate exhibiting one kind of electronic conductivity; disposed upon said substrate and fom.ing a junction therewith, a continuous layer . of glassy amorphous material exhibiting the other kind of electronic conductivity, said layer of glassy amorphous material being substantially transparent to electromagnetic radiation in said predetermined spectral range, having a resistivity of le ss than about 7 ' ■ ■ ' · ' . 10 ohm cm and having an electron affinity which is substantially equal to or less . than the electron affinity of the material forming said semiconductive substrate for providing reduced junction impedance to carrier flow . means for electrically contacting said substrate; and means for electrically contacting said ί layer of glassy amorphous material while permitting electromagnetic radiation in said predetermined spectral range to impinge upon said layer of glassy amorphous material.

12. A device according to claim 11, wherein said glassy amorphous material is glass.

13. A device according to claim 11, wherein said means for electrically 46896/2 comprising a transparent conductive material.

14. A device for utilising the energy of electromagnetic radiation in a predetermined spectra range comprising: a semiconductive substrate exhibiting one kind of electronic conductivity; disposed upon said substrate and forming a junction therewith, a continuous layer of glassy amorphous ' material exhibiting the other kind of electronic conductivity and having an electron affinity which is substantially equal to or less than the electron affinity of the material forming said semiconductiv substrate for providing reduced junction impedance to carrier flow means for electrically contacting said substrate; means for electrically contacting said layer of glassy amorphous material; means for receiving and utilising electrical energy; and circuit means for transmitting electrical energy generated between said means for contacting said substrate and said means for contacting said layer of glassy amorphous r aterial to said means for receiving and utilising electrical energy. 46896/2

15. A device according to claim 14, wherein said glassy amorphous material is substantially transparent to electromagnetic radiation in said predetermined spectral range and has a specific resistivity of less than about 10 7. ohm cm.

16. A device according to claim 14, wherein said glassy amorphous material is glass. COHEN ZEDEK & SPISBACH P. O. BOX 33116, TEL AVIV. ATTORNEYS FOR APPLICANTS