DE3307165A1 - ELECTROCHEMICAL PHOTOELEMENT - Google Patents

Description

Patent ^attorneys:

DipL-Ing. Hane-Jürgen Müller "*" * * ° 3307165

DipL-Chem. Dr. Gerhard Sciiupfner £

DipL-Ing. Hans-Peter Gauger

Luclto-erahn-Str. 38 - D 8000 monks 80

Energy Conversion Devices, Inc 1675 Went Maple Road

Troy, Michigan υ .. ·.;, Λ.

E LEKTRO C HRM Ί Γ> Γ HE Γ, F 0 T OELEME NT

Electrochemical photo element

The invention relates to photovoltaic barrier elements, in particular with a liquid transition, which has a tunnelable boundary surface deposited on a semiconducting layer. * ci. "do not have.

Semiconductor junction photo elements that convert solar or other radiant energy into electrical energy, are well known and have been used for several years as power supplies for satellites and remote ground facilities used. A widespread application of barrier layer photo elements is not possible because of the high cost associated with pulling and cutting the required silicon crystals, as well as the high sensitivity of such components Crystal defects and impurities along the upstream Transition surface.

The present invention addresses these problems and employs an amorphous or polycrystalline silicon semiconductor base a tunnelable interface layer and an electrolyte, each selected so that an inversion occurs along the semiconductor surface. This inversion minimizes the effect of crystal defects and impurities, so that the build-up is not largely due to surface conditions and other defects of the interface layer is dependent. Because there is no need for a crystalline semiconductor and there is no sensitivity against surface condition abnormalities are inherently simple and inexpensive

Photo elements that can be mass-produced in a simple manner are obtained. The resulting barrier photo elements are cheaper than ideal barrier photo elements in terms of operating characteristics and efficiency with pn junction.

The resulting barrier photo elements are both useful for converting solar energy directly into electricity Can also be used for the generation of hydrogen gas by electrolysis in an electrolyte. In addition, it forms extremely thin interface layer provides a protective coating that causes contact and corrosion of the semiconductor surface prevents the electrolyte. This considerably increases the service life and stability of the photo elements.

The invention is directed to an electrochemical photo element for the direct conversion of solar energy into electrical energy Energy. The electrochemical photo element is connected to a consumer circuit via a first and a second electrode or another load. The photo element itself comprises a semiconductor base structure with a contact area on one side that is electrically coupled to the first electrode, and has at least one semiconductor layer with a transition area, the semiconductor layer having an electron affinity X and a bandgap energy E.

s gs

A tunnelable interface layer with an electron affinity X. and a ^ .Bandabsfeartds - Erreirg'ire E '"". ~' Is on deposited on the transition surface of the top semiconductor layer. After all, an electrolyte is in electrical

* ft

maintained conductive relationship between the second electrode and the interface layer.

In one embodiment, the semiconductor base structure is an η-conductive single-layer semiconductor, and the electrolyte ¹ Su is selected such that its redox potential 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 p-type single layer semiconductor, in which case the electrolyte is selected so that its redox potential is less than the electron affinity of the semiconductor base.

According to the invention, the interface layer and the single layer semiconductor base are selected so that the band gap E. of the tunnelable interface layer is greater than or equal to the electron affinity of the single semiconductor layer minus the electron affinity of the interface layer plus the bandgap energy of the semiconductor single layer is.

In order to be tunnelable, the interface layer should have a thickness of approx. 10-40 Å *. Also are the electrolyte and the semiconductor layer selected so that the redox potential of the electrolyte and the work function of the semiconductor layer cause an inversion of the transition area of the semiconductor layer.

Because the thickness of the interface layer is so small, there may be defects that prevent the electrolyte from penetrating through the interface layer at individual points and

enable chemical attack of the semiconductor layer. As a result, in a further embodiment of the Invention an electrically conductive semiconductor layer over the interface layer between this and the electrolyte intended. The conductive semiconductor is chosen so that it does not absorb solar energy. The thickness of the conductive Semiconductor is chosen so that a reflection-reducing effect is achieved. According to the preferred embodiment According to the invention, the current-conducting semiconductor also has a work function which is essentially equal to the redox potential of the electrolyte is.

According to another embodiment of the invention, the semiconductor base comprises an electrically conductive substrate with the Contact area on one side, a first surface on its other side and a plurality of semiconductor layers, deposited on the first surface in conductive relationship thereto, these layers being so arranged are that they are alternately η-type and p-type semiconductors. Every semiconductor layer is semi-permeable, so that part of the solar energy penetrating it is absorbed by the semiconductor layer and in electrical energy is converted.

The use of a plurality of semiconductor layers with alternating conductivity has the consequence that the junction photo effect at every semiconductor junction is additive, which increases the open circuit voltage of the system in much the same way that it does two batteries can be connected in series to increase the resulting output voltage.

The electrochemical photo element according to the invention, the can be coupled between a first and a second electrode for converting solar energy incident on the photo element in electrical energy, is characterized by a semiconductor base with a contact surface that communicates with the '!.' ■ en electrode is electrically coupled and at least comprises a semiconductor layer with a junction area having an electron affinity X and a bandgap energy E has, through a tunnelable interface layer with an electron affinity X. and a band gap energy E., Which is provided on the transition surface of the at least one semiconductor layer, and through an electrolyte that is in electrically conductive relationship between the second electrode and the interface layer is arranged.

The invention is explained in more detail, for example, with the aid of the drawing. Show it:

Fig. 1 is a simplified representation of the component according to the invention in electrically conductive Relationship between two electrodes;

Fig. 2 is a simplified representation of the physical

Relationship between the electrolyte, the interface layer and the semiconductor device according to the state of the art;

Fig. 3 is an equilibrium energy band diagram illustrating Fig. 2;

Fig. 4 is a simplified representation of an embodiment of the invention, wherein a conductive Semiconductor layer is provided between the electrolyte and the interface layer;

Figure 5 is an equilibrium energy band diagram for the Embodiment according to FIG. 4; and

6 shows a simplified representation of a third

Embodiment of the invention, wherein the semiconductor base is a plurality of thin-film semiconductors has in a stacked arrangement and the thin film semiconductors alternately one after the other η-conducting and p-conducting semiconductors are.

According to Fig. 1, an electrolyte container 12 contains a suitable electrolyte 14, and a consumer circuit or load 16 is electrically between a first electrode 18 and a second electrode 20, which is immersed in the electrolyte 14, connected. The first electrode 18 is electrically connected to a semiconductor interface layer combination 22. This is positioned so that electrical energy is generated when applied to the surface of the semiconductor-interface layer combination 22 solar energy 24 hits. The present invention comprises as essential components the electrolyte 14 and the semiconductor interface layer combination or structure 22.

Of course, the means in which the electrolyte is contained are irrelevant as long as the electrolyte is in electrically conductive relationship to the semiconductor interface layer structure is located. It should also be noted that the second electrode 20 can either consist of a suitable transparent structure or outside of the Path of solar energy can be arranged so that the solar energy on the transition between the interface layer and the semiconductor hits.

Fig. 2 shows an embodiment according to the prior Technology, with a first electrode 30 which is electrically conductively connected to a semiconductor base 32. One thin tunnelable interface layer 34 is on the transition surface 35 of the semiconductor base 32 either from the outside IU ^ Brought or raised on this. A senior Electrolyte 36 is then in a container (not shown) in electrically conductive relationship to the interface layer 34 arranged, wherein a second electrode 38 is arranged in the electrolyte 36 so that the electrolyte and radiant energy penetrating the interface layer and entering the surface of the semiconductor base 32 39 causes the generation of electrical energy which flows between the first electrode 30 and the second electrode 38.

The semiconductor base is either crystalline, e.g. B. a silicon single crystal, polycrystalline or amorphous and can be an n- or a p-conducting semiconductor. For example, in an earlier embodiment, an electrolyte interface layer semiconductor device was used (EIS component) manufactured using an η-conductive tellurium-doped GaAs halide

22 -3 conductor with a carrier concentration of approx. 5x10 m

The first electrode 30 on the rear side of the semiconductor base 32 can be carried out using conventional deposition methods be made and of any suitable electrically conductive metal or alloy, e.g. B. made of aluminum, exist.

The interface layer 34 has a tunnelable thickness so that current can pass through the interface layer 34, though

the interface layer is normally an insulator in terms of material. In order to ensure the presence of the tunnel effect, the interface layer preferably has a thickness between approximately 10 and 40 8, depending on the semiconductor base. The interface layer can either be a natural dielectric, e.g. B. usually grows by oxidation of the surface of the semiconductor base 32, or it can be an externally deposited dielectric. In the latter case, the interface layer can be one of a number of different oxides, e.g. B. Nb ₃ O ₅ , Sb ₃ O ₃ , SiO ₃ , TiO ₃ , Ta ₃ O ₅ , or another suitable material that meets the criteria mentioned below.

The electrolyte 36 can also be one of several different compounds or mixtures, e.g. B. (IMK ₉ Se, IMSe, IMKOH) or (IMNa ₃ S, IMS, IMNaOH).

So that the electrolyte, the material of the interface layer and the semiconductor base interact in such a way occur that electrical energy is generated due to the impact of solar energy, are different criteria to meet. First, the electrolyte and semiconductor base should be selected to be one Have a redox potential or a work function such that the transition surface of the semiconductor base 32 is inverted. This condition is met if the band gap E. of the interface layer 34 is greater than or equal to the electron affinity X of the semiconductor base minus the electrons

affinity X. of the interface layer plus the band gap

stand energy E of the semiconductor base 32 is. Furthermore ³ gs

should be the redox potential V, for an EIS solar cell

with an η-conducting semiconductor base be greater than the sum of the electron affinity and the band gap of the semiconductor base. Similarly conductive p-EIS for solar cells with semiconductor-based, the electrolyte should have a redox potential that is less than the electron affinity of ¹ p '^ conductor base is.

Figure 3 shows a simple equilibrium energy band diagram for an η-conductive EIS solar cell, where E. and E are the band gaps of the interface layer 34 and the

Denote semiconductor base 32; φ. is the electrolyte-insulator interface height and is related to the redox potential of the redox couple in the electrolyte the vacuum level; "ψ" is the surface potential of the

Semiconductor base; £ denotes the distance between the Fermi level and the conduction band inside the semiconductor base; d is the thickness of the interface layer; φ. is

5 X

the energy difference between the semiconductor base (conduction band edge) and the conduction band edge of the interface layer; and V. is the voltage drop at the interface layer transition.

Fig. 4 shows another embodiment of the invention, in which a conductive semiconductor layer 40 between the Interface layer 34 and the electrolyte 36 is provided. As already mentioned, the interposition of the Interface layer 34 has the purpose of protecting the semiconductor base against the corrosive effects of the electrolyte 36. There However, if the interface layer is extremely thin, conditions deviating from the ideal can occur, which affect the efficiency and adversely affect the life of the solar cell component.

One flaw that may be present in the extremely thin interface layer is e.g. B. a pinhole. Feinlunker allow the electrolyte to come into close contact with a small area of the semiconductor base 32 so that the corrosion processes of the electrolyte 36 attack the semiconductor base 32 and the beneficial effects of the interface layer to eliminate the barrier properties in the small area. The pinhole area is also effective as a Schottky or heterojunction diode, if an interface layer is not intentionally introduced. If the Surface of the semiconductor 32 is not inverted, the pinholes can also be responsible for the Open circuit voltage is reduced by approx. 100 mV, as already mentioned, there is one of the advantages of the specified component, the material of which is selected so that on the Transition surface of the semiconductor base 32 creates an inverted surface, in the effects of such Significantly reduce pinholes. Indeed, it has been found that when the surface is strongly inversed, more than

7 2

10 pinholes per m with a diameter of 1 µm or less can be tolerated without the properties of the component being impaired.

Another flaw in connection with the interface layer which affects the ideal state is the oxide trap, which result either from the amorphous nature of the interface layer or from the presence of foreign atoms or ions. Tunneling is an effective transport mechanism for the thin layers affected here, however, comparable currents could traverse the oxide with high trap densities due to a jump process. There

Such jump processes are most likely when the change in energy per jump is small, caused by a trap concentration at a certain energy in the oxide the communication between the electrolyte and the semiconductor ., η 1 of this energy improved. A high density of Oxidfal-. * near the edge of the minority carrier tape in the semiconductor enhances communication between the electrolyte and this tape.

Another state deviating from the ideal state in the case of an extremely thin interface layer can be the non-stoichiometry be. However, when the surface of the semiconductor base is highly inverted, the non-stoichiometry is affected does not affect the functionality of the component.

Because of the inversion of the surface of the semiconductor base 32, the effect of most of them becomes from the ideal state deviating states minimized, but it is nevertheless desirable to the semiconductor base against an electrolyte protect, especially in the pinhole areas at the interface layer. To reach this goal, is according to a further embodiment of the invention after the application of the extremely thin dielectric Interface layer is a conducting semiconductor with a wide band gap (greater than 3.2 eV, so it is light in the visible Spectrum not absorbed) with a thickness of approx. 750 α (λ / 4 or its odd multiple, um to achieve a reflection-reducing coating effect, where X is the average solar energy wavelength at which the power output is maximum) applied so that it covers the surface of the interface layer 34. the

resulting three-layer structure is then incorporated into the electrolyte 36 submerged. The work function of the wide band gap conductive semiconductor 40 is selected so that it is equal to the redox potential of the electrolyte 36. Again, the surface of the semiconductor base should be inverted in order to achieve optimal functionality. This can be ensured that the work function of the conductive semiconductor and the redox potential of the Electrolytes are selected accordingly.

In particular, the optimal value results for an n-type Semiconductor base as follows:

V ^NHE - (W- 4,7)> V "_ + f inV redox ^FBn

V " _o = the flat band potential of the n-type

Semiconductor

k = Boltzmann's constant

T = temperature

N_ = charge carrier concentration in the semiconductor or the base

q = the electronic charge

n. = intrinsic conduction density of the carriers in the semiconductor base

W = the work function of the conductive semiconductor and

NHE = the redox potential of the electrolyte oxidation reduction, measured on the normal Hydrogen electrode scale.

The following also applies to p-conducting semiconductors:

_V NHE = (W- 4,7) <V _FBp + Egs redox

* ⁷ Bd ⁼ ^ ^as fl ^acne band potential of the p-conducting semiconductor,

Egs = band gap of the semiconductor base, W = work function of the conductive one

Semiconductor N. = the carrier concentration in the Interface layer

q is the electronic charge and n. = the intrinsic conductivity of the carriers in the Semiconductor base.

The device's equilibrium energy band diagram Electrolyte - conductive semiconductor - interface layer semiconductor base (= ESIS component) is in Fig. 5 for a η-conducting semiconductor base material shown, where:

w ¹ = W - X ₁ of Isolator ^x i = Electron affinity of Semiconductor = Electron affinity ^V FBn "Τ" - Φη

W = the work function of the conductive semiconductor

ίο

χ ¹ = the barrier layer height of the semiconductor insulator V- = the redox potential of the electrolyte in relation to the negative pressure level

V__ * the flat band potential of the semiconductor φ = the difference between the Fermi level and the

Conduction band in the bulk of the semiconductor base ijr.y. = the surface potential of the semiconductor base

with inverted surface.

The conductive semiconductor can be ZnO, SnO ₂ , indium tin oxide or any other suitable oxide material which is conductive, has an anti-reflective effect so that the maximum amount of light penetrates the conductive semiconductor, and which has a sufficiently wide band gap that there is no radiant energy absorbed in the spectrum that is used to activate the semiconductor for generating electrical energy.

Fig. 6 shows a further embodiment of the invention wherein two or more junction semiconductor layers are provided that function as if they were connected in series, whereby a higher voltage is obtained. Such a solar cell component is of particular importance in increasing the efficiency of hydrogen generation in the photoelectrolysis of aqueous electrolytes.

Therefore, the semiconductor base of Fig. 6 comprises a conductive substrate 42 on which an η-type thin film semiconductor is applied with conventional deposition processes. According to this, the η-conductive semiconductor is a p-conductive semiconductor 46 is deposited, and on top of it is another one

η-conductive semiconductor 48 deposited. Each of these semiconductor layers 44, 46 and 48 is a thin film semiconductor layer, and they are partly translucent and partly light-absorbing. Tunnel junctions 45 and 47 with a thickness \ "^ n 100-200 8 and η -ρ configuration are between ^ n layers 44 and 46 as well as 46 and 48 by suitable Doping in such a way that each junction is highly conductive, educated. The dopant concentration of the transition zones 45 and 47 is essentially the same as that of the interface layer 34 and formed in a similar manner. Then the interface layer 34 is on the surface of the last Applied η-conductive semiconductor 48, either by oxidation of the top layer of the η-conductive semiconductor or by suitable external deposition processes, with which a different composition on the uppermost n-type Semiconductor is deposited. The semiconductor can be polycrystalline, amorphous or a mixed phase semiconductor. The work function of the wide band gap conductive semiconductor 40 and the redox potential of the electrolyte 36 are preferably matched to one another as largely as possible. The substrate can be glass, stainless steel, or another be such material.

It should be noted that according to the invention, any desired Number of semiconductor layers can be stacked on top of one another. However, it should also be noted that when stacking always more layers the solar energy penetrating to the lowest layer, which is to be converted into electricity, in terms of quantity continues to decrease, as a result of which the advantages of such further semiconductor layers are reduced. Also can the sequence of conductivity types of semiconductors is reversed

are, so that the semiconductor layer 44 p-type, the Semiconductor layer 46 η-conductive and the semiconductor layer is p-conductive.

It has been found that the open circuit voltage of the specific npn device of FIG. 6 is almost doubled during an increase in current density of about half or slightly more is obtained compared to the current density of a barrier layer photo element with only one semiconductor layer as shown in FIG. 4. Furthermore, it should be noted that the Resistance matching requirements are less stringent when the junction photo element is used Hydrogen generation is used. The component of FIG. 6 is thus an effective system for generating Hydrogen with a higher efficiency than is possible with a component with only one semiconductor layer.

The following example serves to explain the invention.

example

An η-conductive tellurium-doped GaAs plate with a

22 -3 charge carrier concentration of approx. 5 χ 10 m was prepared, degreased with xylene and then chemomechanically polished with a 1% bromine solution in methanol using a buffing disk. To remove the mechanical surface damage caused by chemomechanical polishing, the crystal was _{chemically etched in NHH (NH 4} OH / H ₂ O ₂ / H ₂ O in a ratio of 10/1/1) for 15 s and then for 1 min in SHH (H ₂ SO./H ₂ O ₂ /

-aar- / 3

H ₂ O in the ratio 10/1/1). After the etching, the wafer was washed thoroughly with deionized water and then dried in a nitrogen atmosphere.

Oc ■ The interface oxide layer was grown by Q. »s GaAs platelets placed in a quartz tube and during a period of 50 h of water vapor-saturated oxygen was passed over the platelet. The normally closed contact was by thermal vapor deposition of a Ge-Au alloy on the Back surface and subsequent annealing in forming gas at 400 ° C.

The oxidized GaAs platelets were added to the electrolyte, consisting of a 1/1 mixture of AlClg / butylpyridinium chloride (BPC). The barrier properties were measured under AMI lighting. The open circuit voltage increased from a typical value of 590 mV without the interface layer to 640 mV. The remaining parameters as well the short-circuit current density and the fill factor were relatively unchanged.

Claims (13)

Pate claims ntan Electrochemical photo element that can be coupled between a first and a second electrode to convert solar energy incident onto the photo element into electrical energy,
characterized by a semiconductor base (32) having a contact area electrically coupled to the first electrode (18, 30) and having at least one semiconductor layer (22) with a junction area having an electron affinity X and a bandgap energy E; a tunnelable interface layer (34) with an electron affinity X. ^ and a bandgap energy E., which is provided on the surface of the at least one semiconductor layer; and an electrolyte (14) disposed in electrically conductive relationship between the second electrode and the interface layer. 2. Electrochemical photo element according to claim 1, characterized in that that the semiconductor base (32) is an η-conductive single-layer semiconductor and that the electrolyte (14) is selected from the group of electrolytes whose redox potential is greater than the sum of the electron affinity with no bias and the band gap of the semiconductor layer. 3. Electrochemical photo element according to claim 1, characterized, that the semiconductor base (32) is a p-conductive single-layer Semiconductor (46) and the electrolyte is selected from the group of electrolytes whose redox potential is lower than the sum of the electron affinity with no bias and the band gap of the semiconductor layer. 4. Electrochemical photo element according to claim 1, characterized in that that the semiconductor base (32) is a single layer and that the interface layer (34) and the semiconductor layer (22) are selected as follows: ^E gi> ^X s - ^X i 5. Electrochemical photo element according to one of claims 1-4, characterized, that the interface layer has a thickness in the range of 10-40 8. 6. Electrochemical photo element according to one of claims 1-5, wherein the semiconductor base is a single layer, characterized, that the electrolyte (14) and the semiconductor layer (22) are selected so that they have such a redox potential or have such a work function that the transition surface of the semiconductor layer is inverted. 7. Electrochemical photo element according to one of claims 1-6, marked by one over the interface layer (34) between this and the electrolyte (14) applied conductive semiconductor layer (40) to protect the interface layer (34) and the Semiconductor base (32) with respect to the electrolyte,. -> b ^ 'the conductive semiconductor is selected so that light energy permeates it within a certain spectrum, and the thickness of the conductive semiconductor is selected in such a way that that it has a reflection-reducing effect. 8. Electrochemical photo element according to one of claims 2, 3, 4, 5 or 6, marked by a conductive semiconductor layer applied over the interface layer (34) between this and the electrolyte (14) (40) to protect the interface layer (34) and the semiconductor base (32) against the electrolyte, wherein the conductive semiconductor is selected so that light energy within a selected spectrum permeates it, and the thickness of the conductive semiconductor is selected to be anti-reflective. 9. Electrochemical photo element according to claim 7, characterized in that that the band gap of the conductive semiconductor is greater than 3.2 eV and that its thickness is substantially equal to nA / 4 is, with η = an odd integer and Λ the mean wavelength of the incident on the transition surface activating light. 10. Electrochemical photo element according to claim 7, characterized in that that the work function of the conductive semiconductor is a linear function of the redox potential of the electrolyte is. 11. Electrochemical photo element according to one of the claims or 7, characterized, that the semiconductor base is formed as follows: with a conductive substrate (42) with a first Surface on one side to which the contact surface is opposite; and - With a plurality of semiconductor layers (44, 46, 48) with alternating n- and p-conductivity, with each semiconductor layer incident solar energy absorbs and generates an electric current, reducing the open circuit voltage of the system as a result of the Incorporation of the plurality of semiconductor layers is increased. 12. Electrochemical photo element according to claim 10, characterized in that that each of the plurality of semiconductor layers (44, 46, 48) is a sufficiently thin film that each layer is only one Part of the incident solar energy is absorbed. 13. Electrochemical photo element according to claim 10, characterized in that that the plurality of semiconductor layers comprises: a first, η-conductive semiconductor layer (44) which is applied to the substrate (42), a second, p-conducting semiconductor layer (46) which is applied to the first, η-conducting semiconductor layer (44) is and a third, η-conductive semiconductor layer (48), which is on the second, p-conductive semiconductor layer (46) between
this and the interface layer (34) is applied to form a first current consumption transition (4 5) between the first and the second semiconductor layer and x.ines second current consumption transition (47) between the third semiconductor layer and the electrolyte (14).