GB2116366A

GB2116366A - Liquid junction photovoltaic device with tunnelable interfacial layer

Info

Publication number: GB2116366A
Application number: GB08305601A
Authority: GB
Inventors: Dr Rajendra Singh
Original assignee: Energy Conversion Devices Inc
Current assignee: Energy Conversion Devices Inc
Priority date: 1982-03-01
Filing date: 1983-03-01
Publication date: 1983-09-21
Also published as: IT8319838A0; AU1196083A; IT8319838A1; JPS58194264A; GB8305601D0; FR2522445A1; DE3307165A1; IT1161588B; NL8300756A

Abstract

A liquid junction photovoltaic device includes a semiconductor base (32) with a top junction surface, a tunnelable interfacial layer (34) deposited on the junction surface and an electrolyte (14) in electrically conducting relationship with the semiconductor base (32). A conducting semiconductor (40) may be deposited on the interfacial layer (34) between the interfacial layer (34) and the electrolyte (14). The semiconductor base may comprise multiple semiconductor layers (44, 45, 46) alternating in type between n and p type semiconductors to provide a series connection of two or more photovoltaic junctions (45, 47). <IMAGE>

Description

SPECIFICATION Liquid junction photovoltaic device with tunnelable interfacial layer This invention relates to photovoltaic devices and in particular to liquid junction photovoltaic devices which include a tunnelable interfacial layer deposited on a semiconductor layer.

Photovoltaic semiconductor devices which convert solar or other radiant energy into electrical energy are well known and have been used for a number of years to provide a power source for satellites and remote ground facilities.

However, broad application of photovoltaic devices has not been feasible because of the high cost of growing and cutting the required silicon crystals and the high sensitivity of such devices to crystalline defects and impurities along the current collecting junction surface.

The present invention provides a photo electrochemical device to be coupled between a first and a second electrode to convert solar energy impinging on the device into electrical energy comprising: a semiconductor base having a contact surface electrically coupled to the first electrode and comprising at least one semiconductor layer having a junction surface with an electron affinity Xs and a band gap Eggs; a tunnelable interfacial layer with an electron affinity Xj and a band gap Egi disposed on the junction surface of the at least one semiconductor layer; and an electrolyte disposed in electrical conducting relationship between the second electrode and the interfacial layer.

The preferred embodiment of this invention will now be described by way of example, with reference to the drawings accompanying this specification in which:~ Fig. 1 is a simplified illustration of the present invention in electrical conducting relationship between two electrodes.

Fig. 2 is a prior art simplified illustration of the physical relationship between the electrolyte, interfacial layer and semiconductor device.

Fig. 3 is an equilibrium energy band diagram for the illustration in Fig. 2.

Fig. 4 is a simplified illustration of an embodiment of the invention incorporating a conducting-semiconductor layer between the electrolyte and the interfacial layer.

Fig. 5 is an equilibrium energy band diagram for the embodiment of the invention illustrated in Fig. 4.

Fig. 6 is a simplified illustration of a third embodiment in accordance with the invention where the semiconductor base incorporates a plurality of thin film semiconductors in stacked arrangement where the thin film semiconductors alternate in type between n-type and p-type semiconductors.

Referring initially to Fig. 1, an electrolyte container 12 contains a suitable electrolyte 14 with a utilization circuit or load 16 electrically interconnected between a first electrode 18 and a second electrode 20 immersed in the electrolyte 14. The first electrode 18 is electrically interconnected to a semiconductor-interfacial layer combination 22. The semiconductor-interfacial layer combination 22 is positioned so that electrical energy will be generated when solar energy 24 impinges on the surface of the semiconductor2interfacial layer combination 22.

The present invention, in its essential components, comprises the electrolyte 14 and the semiconductor-interfacial layer structure 22.

It will be appreciated, of course, that the means by which the electrolyte is contained is of no consequence so long as the electrolyte is in an electrical conducting relationship to the semiconductor-interfacial layer structure. In addition, it will be appreciated that the second electrode 20 may consist of a suitable light transmissive structure or may be positioned out of the path of solar energy so that the solar energy will impinge on the junction between the interfacial layer and the semiconductor.

Referring to Fig. 2, a prior art embodiment is illustrated comprising a first electrode 30 coupled in electrically conducting relationship to a semiconductor base 32. A thin tunnelable interfacial layer 34 is either externally deposited or grown on the junction surface 35 of the semiconductor base 32. A conductive electrolyte 36 is then placed in a containment means (not shown) in electrically conducting relationship to the interfacial layer 34 with a second electrode 38 positioned in the electrolyte 36 so that radiant energy 39 passing through the electrolyte and interfacial layer into the surface of the semiconductor base 32 will cause electrical energy to be generated and flow between the first electrode 30 and the second electrode 38.

The semiconductor base may be either crystalline, such as a single crystal of silicon, polycrystalline, or amorphous and may be either an n-type or a p-type semiconductor. For example, in one prior embodiment, an electrolyteinterfacial layer-semiconductor (EIS) device was fabricated utilizing an n-type tellurium-doped GaAs semiconductor with a carrier concentration of about 5x 1022 m#3.

The first electrode 30 on the back of the semiconductor base 32 may be formed utilizing conventional deposition techniques well known in the art and may be any suitable conducting metal or alloy such as aluminum.

The interfacial layer 34 has a tunnelable thickness so that current will pass through the interfacial layer 34 even though in bulk form, the interfacial layer is generally an insulator. In order to assure the existence of the tunnel effect, the interfacial layer is preferably between about 10 angstroms and 40 angstroms in thickness depending upon the semiconductor base. The interfacial layer may be either a native dielectric which, for example, is grown generally by oxidizing the surface of the semiconductor base 32, or may be an externally deposited dielectric.

In the latter case, the interfacial layer may be any of a number of different oxides including Nub205, Sb203, SiO2, TiO2, Ta2O5 or other suitable materials which meet the criteria to be described hereafter.

The electrolyte 36 may likewise be any of a number of different compounds or mixtures. For example, (1 MK2Se, 1 MSe, 1 MKOH) or (1 MNa2S, 1 MS, MNaOH).

In order for the electrolyte, interfacial layer material and the semiconductor base to interact together in such a way to generate electrical energy in response to impingement by solar energy, certain criteria should be met. First, the electrolyte and the semiconductor base should be selected to have an oxidation-reduction (redox) potential and a work function respectively so that the junction surface of the semiconductor base 32 will be inverted. Such a condition will be assured if the band gap, Egl of the interfacial layer 34 is greater than or equal to the electron affinity of the semiconductor base, Xs, minus the electron affinity of the interfacial layer, Xi, plus the band gap energy, Egs, of the semiconductor base 32.In addition, the redox potential, Vredox, for an EIS solar cell with an n-type semiconductor base, should be greater than the sum of the electron affinity and the band gap of the semiconductor base. Similarly, for EIS solar cells with a p-type semiconductor base, the electrolyte should be selected with a redox potential which is less than the electron affinity of the semiconductor base.

Referring to Fig. 3, a simple equilibrium energy band diagram for an n-type EIS solar cell is illustrated where Egl and Egs denote the band gaps of the interfacial layer 34 and the semiconductor base 32 respectively; fel denotes the electrolyteto-insulator barrier height and is related to the redox potential of the redox couple in the electrolyte with respect to vacuum level; glr, is the surface potential of the semiconductor base; An denotes the distance between the Fermi level and the conduction band in the bulk of the semiconductor base; d is the thickness of the interfacial layer; 05j is the energy difference between the semiconductor base (conductor band edge) and the interfacial layer conduction band edge, and Avis the potential drop across the interfacial layer junction.

Referring next to Fig. 4, an alternative embodiment of the present invention is illustrated which incorporates a conducting semiconductor layer 40 between the interfacial layer 34 and the electrolyte 36. As previously indicated, one of the purposes of interposing the interfacial layer 34 is to protect the semiconductor base from the corrosive effects of the electrolyte 36. However, because the interfacial layer is extremely thin, nonidealities can occur which will affect the efficiency and longevity of the solar cell device.

One nonideality which can be present in the ultrathin interfacial layer is a pinhole. Pin-holes allow the electrolyte to come into intimate contact with a small region of the semiconductor base 32 allowing the corrosive effects of the electrolyte 36 to attack the semiconductor base 32 and eliminating the beneficial effects of the interfacial layer on the photovoltaic properties in the small region. In addition, the pinhole region will act as a Schottky or heterojunction diode where saturation current will depend both on the pinhole area and on the properties of the Schottky or heterojunction diode when an interfacial layer is not deliberately introduced. If the surface of the semiconductor 32 is not inverted, the pinholes can also be responsible for lowering the open circuit voltage by about 100 millivolts.Of course, as previously indicated, one advantage of the present device with the material selected so as to create an inverted surface at the junction surface of the semiconductor base 32 is to substantially decrease the effects of such pinholes. Indeed, it has been found that if the surface is strongly inverted, over 107 pinholes per square meter of diameter of 1 micron or less can be tolerated without degrading device properties.

Another nonideality associated with the interfacial layer is the oxide trap arising either from its amorphous nature or from the presence of foreign atoms or ions. Even though direct tunneling is an effective transport mechanism for the thin layers involved, comparable currents could traverse the oxide by a hopping process with large trap densities. Since hopping is most probable when the energy change per hop is small, a concentration of traps at a given energy in the oxide will improve communication between the electrolyte and the semiconductor at that energy. A large density of oxide traps near the edge of the minority carrier band in the semiconductor will enhance the communication between the electrolyte and this band.

Another nonideality possible with an ultrathin interfacial layer may be non-stoichiometry.

However, if the surface of the semiconductor base is strongly inverted, the non-stoichiometry has no effect on the device performance.

While inversion of the surface of the semiconductor base 32 minimizes the effect of most nonidealities, it is still desirable to protect the semiconductor base from the electrolyte particularly at the pinhole regions across the interfacial layer. In order to achieve this goal, and in accordance with another embodiment of the invention, after the ultrathin interfacial dielectric layer is deposited, a wide band gap (greater than 3.2 ev, so it does not absorb light in the visible spectrum) conducting semiconductor having a thickness of about 750 angstroms (A/4 or its odd multiple to get an antireflective coating effect where A is the average solar energy wavelength at which current power is maximum) is deposited to cover the top of the interfacial layer 34. The resultant three layer structure is then immersed in the electrolyte 36. The work function of the wide band gap conducting semiconductor 40 is selected in such a way that it is equivalent to the redox potential of the electrolyte 36. Again, as previously described, the surface of the semiconductor base should be inverted in order to obtain optimum operation. This can be insured by selecting the work function of the conducting semiconductor and the redox potential of the electrolyte.

Specifically, the optimum value for an n-type semiconductor base may be given by: VNHE=(W~4.7) > VFBn+#nV redox where; VFBn is the flat band potential of the n-type semiconductor 2kT (ND) inV= 1 n q nl k is Baltzman's constant t is the temperature ND is the carrier concentration in the semiconductor or base q is the electronic charge is isthe intrinsic carrier concentration in the semiconductor base W is the work function of the conducting semiconductor, and VNHE is the redox potential of the electrolyte redox as measured on the normal hydrogen electrode scale.

Similarly, for p-type semiconductors; VNHE=(W#4.7) < V#Bp where: VFBP is the flat band potential of the p type semiconductor; 2KT (NA) #ins= 1n q nl Egs is the band gap of the semiconductor base; and W is the work function of the con ducting semiconductor NA is the carrier concentration in the interfacial layer q is the electronic charge nj is the intrinsic carrier concentration in the semiconductor base The equilibrium energy band diagram of the electrolyte-conducting semiconductor-interfacial layer-semiconductor base (ESIS) device is shown in Fig. 5 for the n-type semiconductor base material where:: V =Vredox xi redox w' =W-x1 x,=electron affinity of insulator xj=electron affinity of semiconductor x5 VFBn=-=#n q W is the work function of conducting semiconductor is isthe semiconductor insulator barrier height Vredox is the oxidation-reduction potential of electrolyte with respect to vacuum level VFBn is the flat band potential of semi conductor is isthe difference between Fermi level and the conduction band in the bulk of the semiconductor base InV is the surface potential of the semi conductor base when the surface is inverted The conducting semiconductor may be ZnO, SnO2, indium tin oxide or any other suitable oxide material which is conducting, which has an antireflective effect so that maximum light penetrates through the conducting semiconductor, and which has a sufficiently wide band gap so that it does not absorb radiant energy in the spectrum used to activate the semiconductor to generate electrical energy.

Referring next to Fig. 6, another embodiment of the invention is illustrated which provides two or more photovoltaic semiconductor layers which operate as if connected in series whereby a higher voltage can be achieved. Such a solar cell device is particularly important in order to increase the efficiency of hydrogen production in the photoelectrolysis of aqueous electrolytes.

The semiconductor base in Fig. 6 therefore comprises a conductive substrate 42 on which a thin film n-type semiconductor layer 44 is deposited according to conventional deposition techniques. A p-type semiconductor 46 is then deposited on top of the n-type semiconductor and a second n-type semiconductor 48 deposited on top of the p-type semiconductor 46. Each of these semiconductor layers 44, 46 and 48 are thin film semiconductor layers and are partially light transmissive and partially light absorptive. Tunnel junctions 45 and 47 100~200 angstroms thick having the configuration n±p+ are provided between layers 44 and 46 and 46 and 48 respectively by suitable doping so that each junction is highly conductive.The doping concentration of the junction regions 45 and 47 are substantially similar to that of the interfacial layer 34 and are formed in a similar way. The interfacial layer 34 is then deposited on top of the last n-type semiconductor 48 by either oxidizing the top layer of the n-type semiconductor or by suitable external deposition techniques by which another composition is deposited on the top-most n-type semiconductor. The semiconductor may be polycrystalline, amorphous or mixed phases. The work function of the wide band gap conducting semiconductor 40 and the redox potential of the electrolyte 36 are preferably matched as closely as possible. The substrate may be made of glass, stainless steel, or any other such material.

It will be appreciated that any number of semiconductor layers may be stacked in accordance with the invention. However, it will also be appreciated that as more layers are stacked, the amount of solar energy penetrating to the bottom layer to be converted to electricity is decreased, thus decreasing the benefit of such additional semiconductor layers. Also, the arrangement of the type of semiconductor may be inverted so that the semiconductor layer 44 is a p-type, the semiconductor layer 46 is an n-type and the semiconductor layer 48 is a p-type.

It has been found that the open circuit voltage of the specific npn device illustrated in Fig. 6 is almost doubled while the current density is about one-half or slightly more, when compared to the current density of a photovoltaic device with a single semiconductor layer such as that illustrated in Fig. 4. It will also be appreciated that impedance matching requirements between semiconductor layers are relaxed if the photovoltaic device is used in hydrogen production. The system illustrated in Fig. 6 is therefore an effective system for producing hydrogen more efficiently than a single semiconductor layer device.

Example 1 An n-type tellerium-doped GaAs wafer with a carrier concentration of about 5 times 1022 m-3 was obtained and degreased with xylene and then chemomechanically polished with a 1% solution of bromine in methanol using a polishing pad. To remove the mechanical surface damage caused by the chemomechanical polishing, the crystal was chemically etched in NHH {NH4OH/H2O#H2O in a ratio of 10/1/1) for 15 seconds and was subsequently etched in SHH (K2SO4/H20#H2O in a ratio of 10/1/1) for one minute. After etching, the wafer was thoroughly washed with deionized water and then dried in a nitrogen atmosphere.

The interfacial layer oxide was grown by placing the GaAs wafer in a quartz tube and passing oxygen saturated with water vapor over it for a period of 50 hours. The back contact was provided by thermal evaporation of Ge-Au alloy on the back surface and subsequent annealing in forming gas at 4000C.

The oxidized GaAs wafer was exposed to the electrolyte comprising a 1/1 mixture of AlCI3/butyl pyridinium chloride (BFC). Photovoltaic characteristics were measured under AM 1 illumination. The open circuit voltage was increased from a typical value of 590 millivolts without the interfacial layer to 640 millivolts.

Other parameters and the short circuit current density and fill factor were relatively unchanged.

Preferred embodiments of the present invention addresses the problems encountered with known photovoltaic semiconductor devices by utilizing an amorphous or polycrystalline silicon semiconductor base, a tunnelable interfacial layer, and an electrolyte where each is selected so that inversion along the semiconductor surface will occur. This inversion minimizes the effect of crystalline defects and impurities so that the structure does not depend strongly on surface states and other defects of the interfacial layer. By eliminating the requirement for a crystalline semiconductor and reducing the sensitivity to surface stage abnormalities, inherently simple and inexpensive devices, easily manufactured utilizing mass production techniques, can be obtained.The resultant photovoltaic cells have been found to compare very favorably to ideal p-n junction photovoltaic devices in operating characteristics and efficiency.

The resultant photovoltaic devices are useful both in the direct conversion of solar energy into electricity and the production of hydrogen gas through electrolysis in an electrolyte. In addition, the ultrathin interfacial layer provides a protective covering which prevents the electrolyte from contacting and corroding the semiconductor surface. Thus, the life and stability of the device is greatly increased.

Preferred embodiments of the present invention comprise a photoelectrochemical device for converting solar energy directly into electrical energy. The photoelectrochemical device is coupled to a utilization circuit or other load via a first and a second electrode. Preferably the photoelectrochemical device itself comprises a semiconductor base structure having a contact surface on one of its sides which is electrically coupled to the first electrode and includes at least one semiconductor layer having a junction surface where the semiconductor layer has an electron affinity, Xs, and a band gap energy, Erg5.

A tunnelable interfacial layer with an electron affinity, Xj, and a band gap energy, Egj, is deposited on the junction surface of the uppermost semiconductor layer. Finally, an electrolyte is retained in electrically conducting relationship between the second electrode and the interfacial layer.

In one preferred embodiment, the semiconductor base structure is a single layer ntype semiconductor and the electrolyte is selected to have a redox potential which is greater than the sum of the electron affinity and the band gap of the single layer semiconductor base.

Alternatively, the semiconductor base may be a single layer p-type semiconductor in which event the electrolyte is selected to have a redox potential which is less than the electron affinity of the semiconductor base.

In accordance with the preferred embodiments of the invention, the interfacial layer and the single layer semiconductor base are selected so that the band gap of the tunnelable interfacial layer, Egj, is greater than or equal to the electron affinity of the single semiconductor layer minus the electron affinity of the interfacial layer plus the band gap energy of the single semiconductor layer.

To be tunnelable, the interfacial layer should have a thickness in the range of about 10 angstroms to about 40 angstroms. In addition, the electrolyte and the semiconductor layer are selected so that the redox potential of the electrolyte and the work function 6f the semiconductor layer cause the junction surface of the semiconductor layer to be inverted.

Because the thickness of the interfacial layer is so small, defects may exist which allow the electrolyte to penetrate the interfacial layer at isolated locations and chemically attack the semiconductor layer. Consequently, in another preferred embodiment of the invention, a conducting semiconductor layer is disposed over the interfacial layer between the interfacial layer and the electrolyte. The conducting semiconductor is selected so that solar energy is not absorbed by the conducting semiconductor.

The thickness of the conducting semiconductor is selected to have an anti-reflective (AR) effect. In accordance with a preferred embodiment of the invention, the conducting semiconductor further has a work function which is substantially equal to the redox potential of the electrolyte.

In yet another preferred embodiment of the invention, the semiconductor base comprises a conductive substrate with the contact surface on one of its sides, a first surface on its other side and a plurality of semiconductor layers deposited on the first surface in electrical conducting relationship thereto where the layers are provided to alternate in type between an n-type semiconductor and a p-type semiconductor. Each of the semiconductor layers is semitransparent so that a portion of the solar energy passing through is absorbed by the semiconductor layer and converted into electrical energy.

The utilization of plurality of semiconductor layers of alternating types causes the photovoltaic effect at each semiconductor layer junction to be additive thereby increasing the open circuit voltage of the system in much the same way that two batteries can be connected in series to increase the resultant output voltage.

While various embodiments and examples of the present invention have been described above, it will be appreciated that many variations in materials and configurations are possible without departing from the spirit of the present invention.

It is therefore the object of the claims to encompass all such modifications and variations that fall within the true spirit and scope of the invention.

Claims

1. A photoelectrochemical device to be coupled between a first and a second electrode to convert solar energy impinging on the device into electrical energy comprising: a semiconductor base having a contact surface electrically coupled to the first electrode and comprising at least one semiconductor layer having a junction surface with an electron affinity X5 and a band gap Erg5; a tunnelable interfacial layer with an electron affinity Xi and a band gap Eg disposed on the junction surface of the at least one semiconductor layer; and an electrolyte disposed in electrical conducting relationship between the second electrode and the interfacial layer.

2. The photoelectrochemical device according to claim 1 wherein the semiconductor base is a single layer, n-type semiconductor and the electrolyte is selected from the group of electrolytes having a redox potential greater than the sum of the electron affinity at zero bias and the band gap of the semiconductor layer.

3. The photoelectrochemical device according to claim 1 wherein the semiconductor base is a single layer, p-type semiconductor and the electrolyte is selected from the group of electrolytes having a redox potential which is less than the sum of the electron affinity at zero bias and the band gap of the semiconductor layer.

4. The photoelectrochemical device according to claim 1 wherein the base semiconductor is a single layer and the interfacial layer and the semiconductor layer are selected so that: Egi2Xs~Xl+Egs

5. The photoelectrochemical device according to any one of claims 1 to 4 wherein the interfacial layer has a thickness in the range of 10 angstroms to 40 angstroms.

6. The photoelectrochemical device according to any one of claims 1 to 5 wherein the semiconductor base is a single layer and the electrolyte and the semiconductor layer are selected to have a redox potential and a work function respectively so that the junction surface of the semiconductor layer is inverted.

7. The photoelectrochemical device according to any one of claims 1 to 6 further comprising a conducting semiconductor layer deposited over the interfacial layer between the interfacial layer and the electrolyte for protecting the interfacial layer and the semiconductor base from the electrolyte, the conducting-semiconductor selected so that light energy in a selected spectrum passes through the conducting semiconductor, the thickness of the conducting semiconductor being selected so that the conducting semiconductor is anti-reflective.

8. The photoelectrochemical device according to any one of claims 2, 3, 4, 5 or 6 further comprising a conducting semiconductor layer deposited over the interfacial layer between the interfacial layer and the electrolyte or protecting the interfacial layer and the semiconductor base from the electrolyte, the conducting semiconductor selected so that light energy in a selected spectrum passes through the conducting semiconductor, the thickness of the conducting semiconductor being selected so that the conducting semiconductor is anti-reflective.

9. The photoelectrochemical device according to claim 7 wherein the conducting semiconductor has a band gap greater than 3.2 ev. and a thickness which is substantially equal to nA/4 where n is an odd integer and A is the average wavelength of the energizing light impinging on the junction surface.

10. The photoelectrochemical device according to claim 7 wherein the conducting semiconductor has a work function which is a linear function of the redox potential of the electrolyte.

11. The photoelectrochemical device according to claims 1 or 7 wherein the semiconductor base comprises: a conductive substrate having a first surface on one of its sides with the contact surface being on its other side; and a plurality of semiconductive layers, the layers alternating in type between an n-type semiconductor and a p-type semiconductor, each of the semiconductive layers absorbing impinging solar energy for generating an electrical current, whereby the open circuit voltage of the system is increased as a consequence of the inclusion of the plurality of semiconductor layers.

12. The photoelectrochemical device according to claim 10 wherein each of the plurality of semiconductive layers is a film sufficiently thin to enable each layer to absorb only a portion of impinging solar energy.

13. The photoelectrochemical device according to claim 10 wherein the plurality of semiconductor layers comprises a first n-type semiconductor layer deposited on the substrate, a second p-type semiconductor layer deposited on the first n-type semiconductor layer and a third ntype semiconductor layer deposited on the second p-type semiconductor layer between the second p-type semiconductor layer and the interfacial layer for defining a first current collection junction between the first and second semiconductor layers and a second current collection junction between the third semiconductor layer and the electrolyte.

14. A photoelectrochemical device substantially as hereinbefore described with reference to and as illustrated in Figure 1, Figures 4 and 5 or Figure 6 of the accompanying drawings.

15. A photoelectrochemical device substantially as hereinbefore described in Example 1.