DE102008043206A1 - Solar cell, particularly semiconductor solar cell such as wafer solar cell, has structure with change-over face that is stretched between two areas, where structure is made up of amorphous, multi-crystalline or mono-crystalline silicon - Google Patents

Abstract

The solar cell has a structure (1) with a change-over face (5) that is stretched between two areas (2,4), where the structure is made up of amorphous, multi-crystalline or mono-crystalline silicon. A part of the change-over face is enclosed by an intermediate semiconductor that has two intermediate layers.

Description

The The invention relates to a semiconductor solar cell with reduced shunt formation.

One significant problem in the operation of semiconductor solar cells, for example as wafer solar cells is the so-called shunt formation. This phenomenon occurs when individual solar cells in a solar cell module or Solar cell areas are shaded in a solar cell while the surface of the other solar cells (areas) continues to be illuminated. Due to their interconnection, it can happen that the shaded solar cells (areas) thereby in the reverse direction be poled. When the applied reverse voltage exceeds a breakdown voltage, it comes to a breakthrough. As the breakdown voltage usually by defects or impurities in the semiconductor material of Solar cell is locally reduced, such breakthroughs occur also preferably locally at these sites, which is locally very high Current densities can cause the solar cell to be local Heavily heated, damaged or even completely gets destroyed.

Around To avoid such breakthroughs is common Tried the solar cell with the highest possible breakdown voltage produce in the hope that this voltage in normal operation is not exceeded. The problem of shunt formation however, it can be done with it due to defects and impurities do not prevent. This is usually tried at the production of the solar cell as homogeneous as possible and vacancies to reach free active zone. This sets however very high demands on the manufacturing processes and the used semiconductor materials and therefore more expensive solar cell production.

Another known measure for avoiding shunt formation is the provision of tens diodes, which are arranged parallel to the pn junction of the solar cell. In EP 0 537 781 B1 such a solar cell is disclosed. The decimal transitions incorporated in the optically active layer of the solar cell act there as bypass diodes, which in the case of a reverse polarity of the solar cell bring about a controlled breakdown at a desired breakdown voltage. The decimal transitions in the known solar cell are formed island-shaped and therefore cause here also a punctual breakthrough at the desired breakdown voltage. Similar to a shunt formation, the resulting breakdown currents thus occur in a locally restricted region, which can lead to high current densities. In addition, the structure must be off EP 0 537 781 B1 have a relatively complicated three-dimensional structure, which makes the preparation consuming and expensive.

It The object of the invention is to propose a solar cell in which shunt formation is reduced or avoided. This should also reliable, reproducible and as inexpensive as possible respectively.

The The object is achieved by a solar cell having the features of the claim 1 solved. Advantageous developments of the invention are listed in the subclaims.

Of the Invention is based on the consideration, with the help of Between semiconductors the breakdown voltage along a surface reduce, which is a significant proportion of the transition area between occupies the two areas of the structure. This can be the breakthrough behavior the solar cell is controlled in the reverse direction become. Due to the reduced breakdown voltage is also the total applied maximum power limited in a breakthrough arises, so that damage to the solar cell less probably is. In addition, the breakthrough over a larger Distributed area, so that the breakthrough currents do not focus on local flaws and heat up these sites. HotSpot formation is thereby effectively prevented.

This consideration Can work on both metal-semiconductor junctions, therefore Schottky junctions, as well as on semiconductor junctions or p-n transitions apply. Both transitions have a directional conduction behavior. Becomes a such transition in the reverse direction or reverse direction operated, so there is a very low current flow at low voltage. However, the voltage increases a transition-dependent breakdown voltage, This is a breakthrough in which the current flow increases relatively abruptly.

something similar applies to a metal-insulator-semiconductor junction (MIS transitions). In this case, one of the areas is an insulator or insulating area which a tunnel junction between a metal layer and the semiconductor region forms.

The transition area describes a two-dimensional structure, which is the transition defined between the first area and the second area. at For example, it can be useful for a p-n transition be defined as the area at which a change of Doping of a p-type doping to n-doping takes place. Depending on the design, it is the transition area thus either a real, physical entity or one fictitious area.

The fact that the intermediate semiconductor layer encloses a part of the transition surface means that, starting from a basic structure with immediate Touch of the two areas, is inserted along a partial area between the two areas of the intermediate semiconductor. The transition between the two areas thus takes place indirectly through the intermediate semiconductor. It is thus provided that the two regions can partially meet directly one another, but they are connected to one another along a larger part of the transition via the intermediate semiconductor.

present is the structure described here in relation to a Basic structure set in which an intermediate semiconductor layer is not is present and the first and the second area directly adjacent to each other limits. Although the structure described here has the advantage that as an essentially one-dimensional layered structure may be formed, wherein the doping density along only one Coordinate perpendicular to a layer plane varies, it should be clarified here be that with the concept of structure not a coherent structure must be meant, in which the two areas and the intermediate semiconductors each are connected entities. Rather, as are explained below in the description of the figures, also Islanding possible by making the structures island-shaped are distributed on a substrate.

It However, it is essential that throughout Solar cell is an essential part of the transition area is enclosed by means of the intermediate semiconductor. In contrast to the stand The technology is the energy conversion of the solar cell mainly in or in an environment of the intermediate semiconductor and thus in the same place, in the case of blocking also the controlled breakthrough takes place. Cargo carrier separation and breakthrough are thus spatially united. The main advantage here is a spatial broad performance entry in the breakthrough case. Preferably, they are at least 60%, 70%, 80%, 90% or 95% of the transitional area enclosed by the intermediate semiconductor.

In A preferred embodiment provides that essentially the entire transition area by means of the intermediate semiconductor is enclosed. This means that transitions, where the two areas meet directly, if at all, only available for process reasons. This is to ensure be that the controlled voltage breakdown in reverse polarized solar cell over the entire active area of the Solar cell takes place. This leads to a breakthrough case spatially essentially homogeneous power input.

Either in the above explained as well as in the following listed embodiments, it must not be in the intermediate semiconductor mandatory to act on a microscopic layer. For example when the two areas and the intermediate semiconductor means Doping generated from a one-piece semiconductor substrate are, it is the intermediate semiconductors only to an intermediate semiconductor region, with continuous transitions to adjacent areas of the solar cell.

According to one appropriate training are the first area and / or the second region formed from the same material, like the intermediate semiconductor. In cases where one of the areas is formed of a metal, the intermediate semiconductor must made of the same material as the other of the two areas be. In all cases, the intermediate semiconductor with a be formed integrally with or with both areas, wherein then the doping later, so after an application or depositing a base material on a substrate is added to the different areas and the intermediate semiconductor form.

at an advantageous embodiment include the set Between semiconductor parameters an intermediate semiconductor doping and / or an intermediate semiconductor width. Here, the intermediate semiconductor width conveniently as a shortest distance from a first interface between the first region and the intermediate semiconductor and a second interface defined between the intermediate semiconductor and the second region be. The interfaces can turn as surfaces be defined, along each of which a gradient of the doping profile reaches a maximum value in the structure. The intermediate semiconductor width is smaller than 5 μm in a preferred embodiment, smaller than 3 μm, smaller than 2 μm or smaller than 1 μm.

Of the Between semiconductors points in a convenient Execution of a same doping on, as one of two areas, with a higher contrast Between semiconductor doping. So if the area in question p-doped is, the intermediate semiconductor doping may be referred to as p + doping become; corresponding to an n-doped region as n + doping.

advantageously, is the intermediate semiconductor doping along the intermediate semiconductor width essentially plateau-shaped. Such a plateau-shaped Doping is at least approximately, for example by means of epitaxial growth of the intermediate semiconductor achievable. she has the advantage that the electrical properties of the so formed Structure are easily calculable and thus controllable.

In a preferred embodiment provided that a transition zone between the first region and the intermediate semiconductor and / or between the second region and the intermediate semiconductor has a substantially stepped doping profile. This means that there is a relatively abrupt doping transition.

According to one advantageous development, the intermediate semiconductor comprises at least two intermediate layers. In this case, there are more selectable ones Parameters for an optimal setting of the electrical Properties of the structure.

at an expedient design has one at the first Semiconductor region adjacent first interlayer a same Doping on, as the first area, with a contrast higher first interlayer doping, and / or a The second intermediate layer adjoining the second region has a same doping on how the second area, with a contrast higher second interlayer doping. On the basis to the name of the two areas in a solar cell can the intermediate layers as base-side intermediate layer and emitter-side Intermediate layer can be called.

In a preferred embodiment, it is provided that the intermediate semiconductor doping, the first interlayer doping and / or the second interlayer doping are in a range between 0.3 and 10 × 10 ¹⁷ cm ^-3 . The intermediate semiconductor doping, the first interlayer doping and / or the second interlayer doping are in an advantageous embodiment in a range between 0.5 and 2 × 10 ¹⁷ cm ^-3 .

advantageously, the structure is based on, amorphous, multicrystalline or monocrystalline silicon. This is again preferred Purified metallurgical silicon (UMG silicon) or as polycrystalline silicon. For example, a Silicon wafer can be used as a substrate, wherein the wafer material can simultaneously serve as an area of the structure. On the substrate can then the other parts of the structure by means of deposition methods and / or be applied by doping.

The Invention will be described below with reference to exemplary embodiments explained with reference to the figures. Hereby show:

1 a basic structure of a solar cell in cross section;

2 a structure of a solar cell with an intermediate semiconductor arranged between two regions in cross section;

3 a cross section of another solar cell with several island-shaped structures;

4 a diagram with a schematic profile of a doping profile of the basic structure according to 1 ;

5 a schematic profile of a doping profile of the structure according to 2 ;

6 a cross-section of another embodiment of the structure of the solar cell with a two-layer intermediate semiconductor; and

7 a schematic profile of a doping profile of the structure according to 6 ,

The embodiments described below are pure semiconductor structures. For this reason, the terms first and second semiconductor region are used for the first and second regions, the term semiconductor structure for structure, and the term semiconductor structure for basic structure. However, the features discussed herein are essentially equivalent to metal-semiconductor structures such as those found in Schottky junctions. In the case includes one of the areas 2 . 4 a metal or a metal alloy.

The 1 is a schematic representation of a cross-sectional view of a structure, which in the present case as a semiconductor structure 1' referred to as. The semiconductor basic structure 1' includes a first semiconductor region 2 and a second semiconductor region 4 , The two semiconductor areas 2 . 4 meet one another directly and form a transition surface along their contact plane 5 , The present basic semiconductor structure 1' is constructed as a layered structure, so that the two semiconductor regions 2 . 4 are formed as planar semiconductor layers. The semiconductor areas 2 . 4 are doped, wherein the second semiconductor region 4 one to the first semiconductor region 2 has opposite doping such that on the semiconductor base structure 1' incident light is converted into electrical energy.

Deviating from this shows the 2 a semiconductor structure 1 with a first semiconductor region 2 a second semiconductor region 4 and an intermediate semiconductor disposed therebetween 6 , Although the two semiconductor regions 2 . 4 not directly meet, can also here a transition area 5 between the two semiconductor regions 2 . 4 To be defined. Regardless of the exact definition of the transition surface 5 , it must span a plane that is neither unique in the first semiconductor region 2 still clearly in the second semiconductor region 4 lies. In the in 2 illustrated semiconductor structure 1 is the transition area 5 essentially completely from the intermediate semiconductor 6 enclosed. This means that the two semiconductor areas 2 . 4 only via the intermediate semiconductor 6 connected to each other.

Completely enclosing the transition surface 5 through the intermediate semiconductor 6 However, it is not absolutely necessary as long as the intermediate semiconductor 6 a substantial part of the transition area 5 encloses. That means it can be outside the intermediate semiconductors 6 Areas are present in which the two semiconductor regions 2 . 4 like in the 1 Touch each other directly.

Also in the 2 illustrated semiconductor structure 1 has a one-dimensional layer structure. In contrast, the shows 3 a schematic cross-sectional view of a solar cell with a substrate, the same time as a second semiconductor region 4 for semiconductor structures formed in insular form on the substrate 1 acts. The semiconductor structures 1 are island-shaped and may for example have circular or rectangular or square bases. Again, the semiconductor structures 1 each a first semiconductor region 2 and a second semiconductor region 4 on, between which a transitional area 5 is definable, wherein the second semiconductor region 4 Part of the substrate is. The transition area 5 is from the intermediate semiconductor 6 enclosed, which is designed cup-shaped for this purpose.

The embodiment according to the 3 can be used for back-contacted solar cells, with the bottom side of the semiconductor structure 1 serves as a light incident side, while the back side contact is mounted on the upper side where both semiconductor regions 2 . 4 are accessible from the outside.

The 4 shows a schematic representation of a possible doping profile of the semiconductor base structure 1' from the 1 along a layer depth perpendicular to the transition surface 5 , The layer depth is plotted along the abscissa, while the ordinate shows a donor doping density or electron density (n _e ) at the top and an acceptor doping density or hole density (n _h ) plotted logarithmically downwards. The position of the transition surface 5 is shown as a dashed line representing the first semiconductor region 2 with a first doping density 12 from the second semiconductor region 4 with a second doping density 14 separates. In the present case, therefore, the first semiconductor region 2 n-doped, while the second semiconductor region 4 p-doped.

A possible doping profile of in 2 illustrated semiconductor structure 1 is in the 5 drawn schematically, whereby here too the layer depth perpendicular to the transition surface 5 plotted along the abscissa. The doping profile shown here could also be in one of the semiconductor structures 1 present, which in 3 are shown, in which case along the abscissa a layer depth perpendicular to the transition surface 5 is plotted from the center of one of the cylindrical semiconductor structures 1 out starts and out through the transition area 5 runs.

Again, the transition area 5 shown as a dashed line between the first semiconductor region 2 with the first doping density 12 and the second semiconductor region 4 with the second doping density 14 is arranged. Between the two semiconductor areas 2 . 4 is an intermediate semiconductor 6 arranged, which has a same doping type as the second semiconductor region 4 has, namely p-doped. The intermediate semiconductor 6 may also be like the first semiconductor region in other embodiments 2 be doped, in the present case, n-doped. However, this does not mean that the second semiconductor region 4 and the intermediate semiconductor 6 necessarily include the same dopant.

So that the semiconductor structure 1 has a lower breakdown voltage in the reverse direction than the corresponding semiconductor base structure 1' , In the present embodiment, is the intermediate semiconductor 6 with an intermediate semiconductor doping 16 which is higher than the second doping density 14 , Since it is at the 5 is a schematic representation show the second doping density 14 as well as the intermediate semiconductor doping 16 within the intermediate semiconductors 6 essentially plateau-shaped profiles. In practice, an approximation to such an ideal course will at best be achieved by epitaxial deposition techniques. Also, the abrupt or stepped doping transition shown here in a transition zone 7 between the second semiconductor region 4 and the intermediate semiconductor 6 will probably be more gradual in practice.

Another embodiment for a semiconductor structure 1 with an intermediate semiconductor 6 is in the 6 shown. Here is the intermediate semiconductor 6 from a first intermediate layer 6a and a second intermediate layer 6b , For better illustration, the transition surface 5 in the second intermediate layer 6b located. However, it can instead be along a boundary plane between the two intermediate layers 6a . 6b or in the first interlayer 6a be defined. Even if the two parts of the intermediate semiconductors 6 as intermediate layers 6a . 6b are referred to here is a plate-shaped or planar structure with egg a substantially one-dimensional doping profile is not absolutely necessary. Instead, a three-dimensional structure can also be selected here, for example as in the US Pat 3 shown.

A possible course of the doping profile of the semiconductor structure 1 from the 6 show the 7 , The two intermediate layers 6a . 6b , whose associated areas are marked with underlined reference characters in the illustration, have a first interlayer doping 16a and a second interlayer doping 16b on. Conveniently, the transition surface 5 in the form of a dashed line, which runs through the intersection of the doping profile with the abscissa.

In the foregoing description, it is mainly the doping densities 12 . 14 . 16 . 16a . 16b the different areas of the semiconductor structure 1 been received. For the controlled choice of the breakdown voltage, however, the dimensions of these areas are also important, in particular the layer thickness of the intermediate semiconductor 6 , also called intermediate semiconductor width 17 is designated and in the 2 . 3 . 5 and 7 is shown as a bar. For setting the breakdown voltage and other properties that are important for the operation of the solar cell, thus are due to the introduction of an intermediate semiconductor 6 at least two intermediate semiconductor parameters, namely the intermediate semiconductor doping 16 and the intermediate semiconductor width 17 , When the intermediate semiconductor 6 beyond that the two intermediate layers 6a . 6b include, with independent selection of the two interlayer dopings 16a . 16b as well as layer thicknesses (in the 7 not shown) of the two intermediate layers 6a . 6b a total of at least four parameters to choose from.

In addition to setting the breakdown voltage, the intermediate semiconductor 6 as a bonding layer for better connection of the two semiconductor regions 2 . 4 or for the control of further electrical, optical and / or mechanical properties of the semiconductor layer 1 serve. Further, for this purpose, further layers between the two intermediate layers 6a . 6b be provided.

1

structure (The semiconductor structure)

1'

basic structure (Semiconductor base structure)

2

first Area (first semiconductor area)

4

second Area (second semiconductor area)

5

Transition surface

6

Between semiconductor

6a, 6b

first and second intermediate layer

7

Transition zones (between intermediate semiconductors / intermediate layers and regions)

12

first doping density

14

second doping density

16

Between semiconductor doping

16a, 16b

first and second interlayer doping

17

Between semiconductor Width

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• - EP 0537781 B1 [0004, 0004]

Claims (14)

Solar cell comprising a structure ( 1 ) with a transition surface ( 5 ), which between a first area ( 2 ) and a second area ( 4 ) and one at the first area ( 2 ) and the second area ( 4 ) adjacent intermediate semiconductors ( 6 ), whereby at least one of the areas ( 1 ; 2 ) is a doped semiconductor region and the other region ( 2 ; 1 ) is a metal region, an insulating region or a further semiconductor region doped in the opposite direction to the semiconductor region, and wherein intermediate semiconductor parameters of the intermediate semiconductor ( 6 ) are set such that the structure ( 1 ) has a lower breakdown voltage than a basic structure ( 1' ), where the first area ( 2 ) directly to the second area ( 4 ), characterized in that a substantial part of the transition surface ( 5 ) by means of the intermediate semiconductor ( 6 ) is enclosed. Solar cell according to claim 1, characterized in that at least 60%, 70%, 80%, 90% or 95% of the transition surface ( 5 ) by means of the intermediate semiconductor ( 6 ) are enclosed. Solar cell according to claim 2, characterized in that substantially the entire transition surface ( 5 ) by means of the intermediate semiconductor ( 6 ) is enclosed. Solar cell according to one of the preceding claims, characterized in that the first region ( 2 ) and / or the second area ( 4 ) are formed from the same material as the intermediate semiconductor ( 6 ). Solar cell according to one of the preceding claims, characterized in that the adjusted intermediate semiconductor parameters provide an intermediate semiconductor doping ( 16 ) and / or an intermediate semiconductor width ( 17 ). Solar cell according to claim 5, characterized in that the intermediate semiconductor width ( 17 ) is smaller than 5 μm, smaller than 3 μm, smaller than 2 μm or smaller than 1 μm. Solar cell according to claim 5 or 6, characterized in that the intermediate semiconductor ( 6 ) has a same doping type as one of the two regions ( 2 ; 4 ), with a higher intermediate semiconductor doping ( 16 ). Solar cell according to one of claims 5 to 7, characterized in that the intermediate semiconductor doping ( 16 ) along the intermediate semiconductor width ( 17 ) is substantially plateau-shaped. Solar cell according to one of claims 5 to 8, characterized in that a transition zone ( 7 ) between the first area ( 2 ) and the intermediate semiconductor ( 6 ) and / or between the second area ( 4 ) and the intermediate semiconductor ( 6 ) has a substantially stepped doping profile. Solar cell according to one of the preceding claims, characterized in that the intermediate semiconductor ( 6 ) at least two intermediate layers ( 6a . 6b ). Solar cell according to claim 10, characterized in that one at the first semiconductor region ( 2 ) adjacent first intermediate layer ( 6a ) has the same doping type as the first region ( 2 ), with a higher intermediate layer doping ( 16a ), and / or one on the second area ( 4 ) adjacent second intermediate layer ( 6b ) has a same doping type as the second region ( 4 ), with a comparatively higher second interlayer doping ( 16b ). Solar cell according to one of claims 5 to 9 or 11, characterized in that the intermediate semiconductor doping ( 16 ), the first interlayer doping ( 16a ) and / or the second interlayer doping ( 16b ) are in a range between 0.3 and 10 × 10 ¹⁷ cm ^-3 . Solar cell according to claim 12, characterized in that the intermediate semiconductor doping ( 16 ), the first interlayer doping ( 16a ) and / or the second interlayer doping ( 16b ) are in a range between 0.5 and 2 × 10 ¹⁷ cm ^-3 . Solar cell according to one of the preceding claims, characterized in that the structure ( 1 ) is made of amorphous, multicrystalline or monocrystalline silicon, which is preferably present in processed metallurgical purity or as polycrystalline silicon.