DE102014224004A1 - Electronic component with intermediate layer between n- and p-doped semiconductor layer - Google Patents

Abstract

The invention relates to an electronic component, in particular a solar cell, having an absorber layer of intrinsic or doped semiconductor material (2), which may additionally contain a binder material (3) of a contact layer of doped semiconductor material (5), wherein the layers (2, 3, 5) ² are electrically arranged between an electrical contact (1) and an electrical contact (6), which may be located on opposite sides of the arrangement, but also in the closed layer (3) also side by side on only one side of the arrangement, characterized an intermediate layer (4) is provided in the transition region between the layers (2) and (5) of semiconductor material, which forms a barrier for the majority charge carriers of the absorber layer (2) or for the minority charge carriers of the contact layer 5.

Description

The invention relates to an electronic component, in particular a solar cell.

In a solar cell, the absorption of light causes freely moving electrons and holes, also called holes. The freely moving electrons and holes are connected to selective contacts, as z. B. formed by rectifying contacts band bending (an electric field), spatially separated from each other. The separate charge carriers are supplied via electrical contacts to an electrical load.

In order for a band bending, that is to say an internal electric field, to be present, a solar cell usually comprises a pn contact between a p-type and an n-doped layer of semiconductor material. In a p-doped semiconductor, the freely movable positive charge carriers, ie the holes or defect electrons, which are then called majority charge carriers, predominate. In the case of n-doping, the freely mobile electrons predominate, which then form the majority charge carriers here.

In solar cells, a distinction is made between solar cells of the p-type and n-type. When p-type, the relatively thick, consisting of semiconductor material absorption layer of the solar cell is p-doped. The then thin n-doped layer is referred to as emitter. The emitter collects the minority charge carriers accelerated by the electric field, which have become free in the absorption layer by light absorption. N-type solar cells have a relatively thick layer of n-doped material. The essentially significantly thinner emitter layer is then p-doped.

The publication WO 2010/000581 A2 discloses the production of a solar cell with a monograin membrane. The grains of the monograin membrane are made of semiconductor material with pn junction. The document WO 2010/000581 A2 describes a CZTS solar cell, that is a solar cell with copper-zinc-tin sulfide as a semiconductive compound.

According to document US 5,899,704 can in a solar cell with back and front electrode or electrical back and front contact minority carrier by diffusion reach the return electrode and reduce by recombination with the majority charge carriers the current transported through this stream. There are proposed in the document measures that counteract such a diffusion of minority carriers.

It is an object of the invention to provide a powerful electronic component.

The object is achieved by electronic component, in particular a solar cell, with the features of claims 1 or 2. Advantageous embodiments emerge from the subclaims.

To achieve the object, an electronic component according to claim 1 comprises an electrical contact for deriving the majority charge carriers, a layer with p- or n-doped semiconductor material, a layer with opposite (ie n- or p-) doped semiconductor material and an electrical contact for the majority charge carriers of this oppositely doped layer. An intermediate layer is provided in the contact region of p-doped semiconductor material and the layer of n-doped semiconductor material. The intermediate layer acts as an additional barrier for the majority charge carriers of the absorber layer.

To achieve the object, an electronic component according to claim 2 comprises an electrical back contact, a layer with p-doped semiconductor material, a layer with n-doped semiconductor material and an electrical front contact. An intermediate layer is provided between the p-type semiconductor material layer and the n-type semiconductor material layer. The interlayer acts as an additional barrier for majority carriers. The intermediate layer does not have to be installed exactly at the interface, it only has to be inside the band bend, ie in the electric field. In addition, a layer of intrinsic semiconductor material may be provided between the two doped layers.

The electronic component is in particular a solar cell, preferably of the p-type. If, for example, freely movable electrons are generated in the relatively thick, p-doped absorption layer of the solar cell by light absorption, these will flow into the n-doped selective contact due to the band bending (the electric field) produced at the emitter. The holes, so the majority charge carriers are retained by the band bending and accelerated due to the electric field in the opposite direction.

Although the electric field exerts a force on the majority charge carriers, which is directed in the desired direction, it has surprisingly been found to be performance enhancing to arrange an additional barrier, ie an obstacle, for the majority charge carriers in the transition region between the p- and n-doped layers , This additional barrier for the majority carriers significantly reduces the number of charge carriers traveling through the barrier by thermal excitation (traveling against the electric field). Namely, such majority excited carriers penetrating into the transition region thermally excited from both sides (the n- and the p-doped side) recombine in the contact region and reduce the efficiency of the solar cell.

Such carriers recombining in the contact region also reduce the blocking behavior of diodes and transistors, the photon yield of light-emitting diodes and semiconductor lasers and the sensitivity of photodiodes. In all these cases, by incorporating energetic barriers for the majority charge carriers on the respective side of a pn junction, a significant improvement in behavior can be achieved. Thus, the invention improves the performance of a variety of electronic components.

In principle, an intermediate layer acting as a barrier for majority charge carriers has proved to be advantageous within a transition. This applies to all electronic components with pn junction, a semiconductor / metal junction (Schottky contact) or a transition between an intrinsic semiconductor and a p- or n-doped semiconductor, in which charge carriers at a transition through to an electrical contact can recombine. In the case of a transition between an intrinsic semiconductor and an n-doped semiconductor, the intermediate layer is thus in the transition region between the intrinsic semiconductor and the n-doped semiconductor and acts as a barrier for holes. In the case of a transition between an intrinsic semiconductor and a p-doped semiconductor, the intermediate layer acts as a barrier for electrons in the conduction band.

The electronic component may be p-type solar cells. The interlayer is then a barrier to holes.

Such barriers are caused by band bending or energetic jumps in the respective band edges. Metal oxides are suitable as a barrier for holes. Tin oxide and / or zinc oxide have been found to be a material which is generally well suited for this purpose and is capable of enhancing performance as a barrier to holes or holes. Moreover, they are also suitable, after high doping, as transparent conductive oxides (transparent conductive oxides, TCO) for electron-conducting contact. Here, however, an additional thin intermediate layer is introduced in the pn junction, which in the case of today's (eg CdTe, CuS, SnS, Cu (InGa) Se ₂ - (CIGS) or kesterite) solar cells at the transition between the said p-type semiconductor, an n-CdS buffer layer and the highly doped n-TCO. Here, an additional thin oxidic intermediate layer near the pn-junction, z. B. between p-type semiconductor and n-buffer by forming a hole barrier in the valence band advantageous.

The intermediate layer is preferably very thin to provide a particularly efficient electronic component. In one embodiment, the intermediate layer is thinner than 100 nm, preferably thinner than 50 nm, particularly preferably thinner than 10 nm, in order to provide a particularly high-performance component.

At least one of the layers with doped semiconductor material is regularly thicker by a multiple than the intermediate layer. This layer may be at least five times, preferably at least fifty times, more preferably at least one hundred times thicker than the intermediate layer, at least five times. In particular, the layer thicker by a multiple is the absorption layer of a solar cell. In principle, this layer is also thicker by a multiple than the other layer with doped semiconductor material. In the case of the solar cell, the other layer of doped semiconductor material is the emitter layer.

Since, in the case of solar cells, it is usually a p-type solar cell, the layer thicker by a multiple generally comprises p-doped semiconductor material.

In one embodiment, at least one of the layers with doped semiconductor material is more than 500 nm thick, preferably more than 1 μm thick, specifically for the creation of a solar cell with a suitably thick absorption layer. Advantageously, the absorption layer is not thicker than 200 microns, more preferably not thicker than 100 microns, to keep the cost of materials in relation to the benefit low. Preferably, a monocrystal membrane is used as the absorption layer in order to produce a technically simple manner.

At least one of the layers with doped semiconductor material is advantageously thinner than 100 nm, preferably thinner than 50 nm. In the case of a solar cell, this layer generally forms the emitter layer. The emitter layer is preferably at least 1 nm thick.

One of the layers of doped semiconductor material preferably comprises Cu ₂ Zn _x Sn ₁ -x (S _y Se _1-y ) ₄ with x, y = 0-1 such as Cu ₂ ZnSnS ₄ (x = 0.5, y = 1) ), Cu ₂ ZnSnSe ₄ (x = 0.5, y = 0) as a semiconductor material to provide a particularly environmentally friendly electronic device. CZTSSe (x = 0.5, y = 0.8) is another example of a particularly suitable material for the absorption layer.

In one embodiment, one of the layers comprising doped semiconductor material comprises a sulfide or oxy-sulfide, frequently cadmium sulfide and / or zinc sulfide, in particular in the case of a solar cell.

As a material for the electrical contacts transparent aluminum-doped zinc oxide or indium-doped tin oxide (ITO) and graphite are suitable and in particular in the case of a solar cell, in which an electrical contact should be transparent in principle. Preferably, the zinc or tin oxide is highly doped (> 10 ¹⁷ cm ^-3 ).

In particular, the invention is suitable for creating a CZTS solar cell or a CIGS or CdTe solar cell. These are preferably 1.5 μm to 2.5 μm thick.

It can be used organic or inorganic semiconductor materials.

The invention will be explained in more detail by way of examples.

Show it:

1 : Schematic structure of solar cells produced according to the invention;

2 : High-resolution XPS spectra of Sn 3d, S 2p and O 1s orbitals on the surface of CZTS powders without (a-c) and after bromine etching (d-f) to demonstrate the formation of oxidized tin by etching;

The 1a schematically shows a possible structure of solar cells produced according to the invention. A layer 1 made of graphite serves as electrical back contact. On the shift 1 From graphite is a comparatively thick monograin membrane with grains 2 and binder 3 which serves as an absorption layer. The grains have a diameter of several 10 microns. The grains 2 consist of a p-doped semiconductor material. The binder 3 consists of an electrically non-conductive material. At the top are the grains 2 opposite the binder 3 out. At the opposite bottom are areas of grains 2 together with binder 3 has been removed to make an ohmic electrical contact to the layer 1 manufacture. On the monocorn membrane 2 . 3 is a much thinner tin oxide layer 4 applied as an intermediate layer. This layer is a few nanometers thin. Above is a thin buffer layer 5 of n-doped semiconductor material, e.g. From CdS. The buffer layer 5 is also a few nanometers thin, but usually thicker than the intermediate layer. On the buffer layer 5 is in the form of a layer 6 a transparent electrical front contact, which may consist of doped or intrinsic plus doped zinc oxide. There are two different arrangements in the two experiments. The first is described correctly, here are the SnO ₂ and then the CdS applied to the monograin membrane. Another electrical contact can be arranged on a substrate which serves as a carrier. In the case of a solar cell, this is usually not transparent back contact.

The following experiments were carried out.

A monocrystal layer or monocrystal membrane was made of Cu ₂ ZnSnS ₄ crystals with a mean diameter of 60 microns as in WO 2010/000581 A2 produced described. This was coated with a tin oxide layer in a commercial ALD apparatus of Ultratech, Waltham, MA, USA (Cambridge Nano Tech Fiji F200 ALD). Here, tetrakis (dimethylamino) tin (IV), (99.99% -Sn) (TDMASn) is used as the tin precursor. The TDMASn was maintained at 60 ° C to adjust the necessary vapor pressure. Water (H ₂ O) was used as the oxygen precursor, and argon at a flow rate of 40 standard cm ^{3 was} used as the carrier gas. The precursor gases were fed to the argon stream by transferring fast switching valves. The final tin oxide layer thicknesses were adjusted by the number of TDMASn and H ₂ O cycles. The sample table was heated to 120 ° C. With 10 tin oxide cycles at at a pressure of 0.45 torr, a layer thickness of 2 nm measured by electron microscopy was deposited.

The ALD conditions for depositing the ultra-thin (2 nm) tin oxide layer are shown in the table below. parameter SnO cycles Temperature (° C) Pressure (Torr) 10 120 0.45

Thermal post-treatment ("annealing") of the interlayer after ALD was not performed in the examples described herein, but has been found to be useful in other experiments.

After application of the tin oxide intermediate layer, a standard buffer layer or emitter layer, by chemical bath deposition (CBD, see " David B. Mitzi: Solution processing of inorganic materials. Wiley-Interscience, 2009, ISBN 978-0470406656, p. 200 ") Applied. For this purpose, the surface was for 10 min with a freshly prepared and tempered at 70 ° C to 75 ° C aqueous solution of the precursor compounds cadmium acetate Cd (Ac) ₂ or zinc acetate Zn (Ac) ₂ , ammonium acetate NH ₄ Ac, thiourea and 30 % Ammonium hydroxide NH ₄ OH brought into contact. In this case, the buffer material (here cadmium sulfide or zinc sulfide) has been deposited as a thin film.

The buffer materials used were the standard material cadmium sulfide and, for comparison, the hitherto less widely used zinc sulfide. This comparison has demonstrated that the improvement achieved by tin oxide is independent of the type of buffer material used. The CBD conditions are shown in the table below.

After applying the buffer layer, aluminum-doped zinc oxide was sputtered on as a window electrode or transparent front contact in a standard system from AJA International, Inc., N. Scituate, MA, USA (AJA-ATC1800).

The back of the monograin membrane became appropriate WO 2010/000581 A2 polished on the back and contacted with graphite paste. The solar cells thus obtained were measured with a calibrated CZTS solar cell in a calibration laboratory of the FHG ISE in Freiburg, Germany, in a calibrated solar simulator test stand.

The following table summarizes the results. Parameters of the solar cells determined from the current-voltage curves of the solar cells: interlayer buffer layer V _oc [mV] FF [%] Jsc [mA / cm ² ] Eff. [%] SnO _x ALD 10 cycles CdS 30 nm 724 55.8 12.9 5.21 none CdS 30 nm 666 53,30 13.75 4.88 SnO _x ALD 10 cycles ZnS 5 nm 719 53.91 13.43 5.21 SnO _x ALD 10 cycles ZnS 10 nm 707 50.68 13.33 4.78 none ZnS 10 nm 617 53.8 12.5 4.14

The open-circuit voltage V _{oc of} the reference cell of 666 mV corresponds to the normally-obtained no-load voltage of CZTS solar cells. This is clearly exceeded in all solar cells produced with a tin oxide intermediate layer. It has been repeatedly observed that a 5 nm thick ZnS layer gives higher cell voltage and higher efficiency than a 10 nm thick buffer layer. The use of CdS gave even higher cell voltage. However, the efficiency has not increased further because of the slightly lower short-circuit current at CdS.

According to a second example, standard monocrystal membrane solar cells were prepared as in the WO / 2010/000581 A2 described. CZTS powder was compared as starting material with and without tin oxide interlayers. In this case, however, the powder material was subjected to different etching processes before it was embedded in the epoxy layer of the membrane. A control powder was not etched, a powder was etched only in a 1 volume percent solution of bromine in methanol (Br ₂ / MeOH), a sample was also etched, but then additionally with a 1 mole percent alkaline KCN solution, a fourth sample was etched only in this alkaline KCN solution.

A detailed examination of the surface was carried out by scanning electron microscopy (Zeiss HR SEM ULTRA 55 with angle selective backscattered electron detector (SEM-AsB) and energy dispersive spectroscopy (EDX)), by X-ray photoelectron spectroscopy (XPS) Kratos Analytical AXIS ULTRA DLD spectrometer with monochromatic Al Ka X-ray source and achromatic Mg Ka / Al Ka dual-anode X-ray source from Shimadzu, Tokyo, Japan).

The XPS studies (see 2 ) show that the surface of the CZTS powders after etching with the bromine / methanol solution was coated with a thin tin oxide layer. This was also retained in a subsequent KCN etching process. It did not exist on the non-etched surface and also did not form during the subsequent deposition of the CdS buffer layer, as determined by sputtering followed by CdS deposition by ion bombardment in the XPS spectrometer followed by surface analysis. This also means that a component is formed in which a few nanometers thick SnO interlayer is located between the CZTS absorber layer and the CdS buffer layer. The SnO interlayer and CdS buffer layer surround the particles as in FIG 1b to see. These intermediate layers are then interrupted by the polishing on the back. Although they form a kind of "short circuit" through the membrane in principle, but because of the low layer thicknesses of the intermediate layers is so high impedance that it does not affect the function of the device.

However, the influence of the intermediate layers on the achieved photovoltage was more important for the efficiency of the different solar cells.

From the current-voltage characteristics, the solar cell parameters can be determined by fitting the curves and taking an equivalent circuit diagram based on the classic diode model with parallel and series resistance. The following table contains the values for a solar cell with unetched (reference) and etched absorber material. The etched and therefore a tin oxide intermediate layer containing solar cells show higher efficiencies due to increased photovoltage (V _oc ). Parameters of solar cells that have been improved without (reference cell) and with different etching methods: V _oc mV J _sc , mA / cm ² FF% η% reference 706 14.2 58 5.85 Bromo-etched 743 14.6 63 6.88

In this case, since the forming tin oxide intermediate layer was formed only as a side reaction of etching the surface to remove foreign compounds deposited on the surface of the CZTS powder, the film thickness could not be controlled. Nevertheless, there was a clear improvement of the solar cells compared to the non-etched reference cell.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• WO 2010/000581 A2 [0005, 0033, 0039, 0042]
• US 5899704 [0006]

Cited non-patent literature

• David B. Mitzi: Solution processing of inorganic materials. Wiley-Interscience, 2009, ISBN 978-0470406656, p. 200 [0036]

Claims (13)

Electronic component, in particular solar cell, with an absorber layer of intrinsic or doped semiconductor material ( 2 ), which additionally contain a binder material ( 3 ) can contain a contact layer of doped semiconductor material ( 5 ), the layers ( 2 . 3 . 5 ) ¹ electrically between an electrical contact ( 1 ) and an electrical contact ( 6 ) are arranged on opposite sides of the assembly, but also in the closed layer ( 3 ) can also be located side by side on only one side of the arrangement, characterized in that an intermediate layer ( 4 ) in the transition region between the layers ( 2 ) and ( 5 ) is provided of semiconductor material which forms a barrier for the majority charge carriers of the absorber layer ( 2 ) or for the minority carriers of the contact layer 5 forms. Electronic component, in particular a solar cell, with a layer of p-doped semiconductor material ( 2 . 3 ), a layer of n-doped semiconductor material ( 5 ), the layers ( 2 . 3 . 5 ) with semiconductor materials between an electrical back contact ( 1 ) and an electrical front contact ( 6 ), characterized in that an intermediate layer ( 4 ) between the layers ( 2 . 3 . 5 ) is provided with semiconductor material which forms a barrier for freely movable majority charge carriers. Component according to claim 1 or 2, characterized in that the intermediate layer ( 4 ) is formed from a metal oxide, preferably from tin oxide and / or zinc oxide. Component according to one of the preceding claims, characterized in that one of the layers ( 2 . 3 ) of semiconductor material is many times thicker than the intermediate layer ( 4 ) and / or is many times thicker than the other layer ( 5 ) with doped semiconductor material. Component according to the preceding claim, characterized in that the layer thicker by a multiple ( 2 . 3 ) p-doped semiconductor material ( 2 ). Component according to one of the preceding claims, characterized in that the intermediate layer ( 4 ) is thinner than 20 nm, preferably thinner than 10 nm. Component according to one of the preceding claims, characterized in that at least one of the layers ( 2 . 3 ) is more than 100 nm thick with semiconductor material, preferably more than 1 micron thick. Component according to one of the preceding claims, characterized in that the layer ( 5 ) with doped semiconductor material is thinner than 150 nm, preferably thinner than 20 nm. Component according to one of the preceding claims, characterized in that one of the layers ( 2 . 3 ) comprising doped semiconductor material Cu ₂ Zn _x Sn _1-x (S _y Se _1-y ) ₄ with x, y = 0-1 as semiconductor material. Component according to one of the preceding claims, characterized in that one of the layers ( 5 ) comprising cadmium sulfide and / or zinc sulfide with doped semiconductor material or the corresponding oxysulfides as pure components or components of solid solutions. Component according to one of the preceding claims, characterized in that an electrical contact ( 1 . 6 ) Comprises aluminum-doped zinc oxide or graphite. Component according to one of the preceding claims, characterized in that a layer ( 2 . 3 ) is a monocrystal membrane with semiconductor material. Component according to one of the preceding claims, characterized in that the component is a CZTS solar cell, a CdTe or a CIGS solar cell.

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