DE102013217653A1 - Photovoltaic solar cell - Google Patents

Abstract

The invention relates to a photovoltaic solar cell having at least one front-side electrode and at least one rear-side electrode and a photoelectrically active absorber layer arranged between front and rear electrode, which absorber layer contains at least 30% by mass of germanium. The invention is characterized in that the absorber layer has a thickness of less than 40 nm, that the solar cell has a front side reflection layer and a back side reflection layer, wherein the absorber layer is disposed between the front and rear side reflection layers and the optical path length between the front and back side reflection layers is smaller than ½ λ Tauc with a wavelength λTauc corresponding to the Tauc band gap of the absorber layer.

Description

The invention relates to a photovoltaic solar cell according to the preamble of claim 1.

Such photovoltaic solar cells, which are typically referred to as "thin-film solar cell", have at least one front-side electrode and at least one rear-side electrode and a photoelectrically active absorber layer arranged between the front and rear-side electrodes.

Electromagnetic radiation penetrating into the photovoltaic solar cell via the front side, typically sunlight, is at least partially absorbed in the photoelectrically active absorber layer, and a charge carrier generation occurs, ie. H. Generation of electron-hole pairs. The electron-hole pairs are separated at a pn junction and supplied to the front and the backside electrode, so that electrical energy can be tapped off via external contacts.

In the context of this description, the term "pn junction" also encompasses those pn junctions in which an intrinsic layer is arranged between the doped layers, so-called pin junctions as a subgroup of the pn junctions. In the context of this description, this term likewise includes structures in which one or both of the doped layers is or are replaced by a material which causes charge separation even without doping due to the position of the Fermi level or its work function, in particular in the form of one known Schottky contact (metal-semiconductor junction). In this respect, the term "pn junction" in the context of this description also denotes any layer sequence consisting of an intrinsic semiconductor layer or a sequence of multiple intrinsic semiconductor layers, on one side of which a layer of a material having a Fermi level is located near the conduction band edge of at least one of the intrinsic materials, and at the other side of which a layer of a material having a Fermi level near the valence band edge of at least one of the intrinsic materials is arranged.

The development of thin-film solar cells is currently aimed primarily at reducing energy consumption and material and energy costs in the production process while maximizing the efficiency of the solar cell. Regardless of the photoelectrically active absorber material used, in particular the layer thickness of the absorber material plays an essential role: With increasing layer thickness of the absorber layer, the absorption and thus the short-circuit current density is typically increased, while the open-circuit voltage and the fill factor of the solar cell decrease. An essential challenge for the further development of thin-film solar cells is therefore to lower the layer thickness of the absorber layer, with little or no reduction in the absorption of the solar cell.

The present invention relates to such photovoltaic thin-film solar cells, in which the absorber layer contains at least 30 percent by mass (corresponding to a mass fraction of 0.3), preferably more than 50 percent by mass of germanium. Solar cells with absorber layers of amorphous or microcrystalline germanium or a germanium alloy are already known; such a solar cell is in US 2012/0319111 A1 described.

A known production method provides for the use of the PECVD technology (plasma enhanced chemical vapor deposition) to deposit the absorber layer. In addition, various sputtering processes for the production of germanium-containing layers are known. However, these deposition methods which are preferred per se have the disadvantage that absorber layers produced therewith, in particular those with a high germanium content, have a rather low electronic quality, in particular with regard to the charge carrier lifetime, so that a negative effect on the efficiency of the solar cell is the result.

A known approach for reducing the influence of the low carrier lifetime on the efficiency of solar cells is the reduction of the absorber layer thickness, which is usually associated with a reduction of the absorption of the solar cell and thus also with a limitation of the efficiency.

To improve the coupling of light into thinner absorber layers, in particular in the long-wave range, in which there is typically a lower absorption in the absorber layer, therefore, different approaches are known:
A roughening of the interfaces, in particular on the light incidence side, pursues the goal of scattering the incident light into different angles, so that the effective optical path length of the light in the absorber and thus also the probability of absorption increase.

Furthermore, attempts are being made to achieve the same effect by means of selectively produced optical scattering structures (for example nanoparticles), such scattering structures typically being arranged in or on the back of the absorber layer.

Although these methods can improve the absorption and thus the efficiency of the solar cell; However, this in turn requires in addition sometimes complex and difficult to control großskalig usable and thus costly production steps, such as additional wet-chemical etching of substrates, the production and positioning of nanoparticles or the use of lithographic processes.

In addition, rough surfaces can make controlled, homogeneous layer growth more difficult.

The invention is therefore based on the object of providing a photovoltaic solar cell which makes it possible to reduce the thickness of the absorber layer in comparison to previously known, in particular germanium-containing solar cells, with or without a comparatively slight reduction in absorption. The solar cell according to the invention should also be less expensive to manufacture compared to the aforementioned processes in order to reduce costs.

This object is achieved by a photovoltaic solar cell according to claim 1. Advantageous embodiments of the photovoltaic solar cell according to the invention can be found in claims 2 to 15. Hereby, the wording of all claims is explicitly incorporated by reference in the description.

The photovoltaic solar cell according to the invention has at least one front-side electrode and at least one rear-side electrode and a photoelectrically active absorber layer arranged between the front and rear-side electrodes.

As with prior art photovoltaic solar cells thus takes place at least partial absorption in the solar cell penetrating electromagnetic radiation in the photoelectrically active absorber layer. In this case, the absorber layer contains at least 30 percent by mass, preferably at least 50 percent by mass of germanium.

It is essential that the absorber layer has a thickness of less than 40 nm, that the solar cell has a front side reflection layer and a back side reflection layer, wherein the absorber layer is disposed between front and rear side reflection layers, and the solar cell is formed such that the optical path length between the front and back side reflection layers for incident light perpendicular to the cell is smaller ½ λ _Tauc , with a wavelength corresponding to the Tauc band gap E _{τ of} the absorber _layer λ _Tauc (λ _Tauc = 1240 / E _τ , where E _τ in eV and λ _{Tauc are given} in nm).

The optical path length between the front-side reflection layer and the back-reflection layer for perpendicularly incident light must be determined in a manner known per se. In particular, the optical path length between the front-side _reflection layer and the back- _reflection layer for perpendicularly incident light can be determined by multiplying the respective refractive index at λ _Tauc by the respective layer thickness for the intermediate layers and summing the individual values thus obtained. In the present specification, the term "refractive index" is understood to mean the real part of the complex refractive index.

The definition of the tauc band gap is well known in the literature, such as in Tsao, C.Y., Weber, J.W., Campbell, P., Conibeer, G., Song, D., & Green, M.A. (2010). In-situ low temperature growth of poly-crystalline germanium thin film on glass by RF magnetron sputtering. Solar Energy Materials and Solar Cells, 94 (9), 1501-1505. doi: 10.1016 / j.solmat.2010.02.035 summarized. It also discusses the different procedures for determining direct and indirect band gaps. The term "band gap" in the context of this invention refers to the commonly used physical definition of the energy band gap of a semiconductor.

The invention is based on the knowledge that a thinner absorber layer, which is thinner compared to previously known solar cells, can be selected provided that the optical path length is as described above is trained. For this is a condition for forming an optical resonance between the front and back reflection layer, which substantially increases the absorption in the absorber layer. An optimization in particular of the short-circuit current density of the solar cell has hitherto been carried out by increasing the absorber thickness. Elaborate examinations and simulations have shown, however, that an action contrary to this doctrine by thinning the layers arranged between the front side and rear side reflection layer, in particular also the absorber layer, then leads to an increase in the short circuit current, in particular, when an optical resonance occurs.

The above-described combination in the solar cell according to the invention comprising the front and rear reflection layer, an absorber layer having a thickness of less than 40 nm and a configuration corresponding to the aforementioned optical path length condition typically already leads to the formation of an advantageous optical resonance, at least an optical resonance is thereby favored.

The solar cell according to the invention thus offers an alternative to the techniques described above for light scattering, in order to increase the absorption in the absorber layer with reduced absorber layer thickness. In the solar cell according to the invention, an optical resonance is formed so that in particular with a germanium-containing photoelectrically active absorber layer having a thickness less than 40 nm, however, sufficient absorption can be achieved for a comparatively high efficiency. With the solar cell according to the invention high efficiencies can thus be achieved even with absorber layers with comparatively poor electronic material quality, in particular comparatively high open-circuit voltages and filling factors.

Since no roughened surfaces or other optical scattering centers are necessary for the above-described effect of the optical resonance, the production of the photovoltaic solar cell according to the invention is less complicated in terms of process and thus more cost-effective. Nevertheless, it is also within the scope of the invention to combine the solar cell according to the invention with one or more of the light scattering techniques described above, for example in order to achieve a folded solar cell structure, similar to FIG Vanecek, M., Babchenko, O., Purkrt, A., Holovsky, J., Neykova, N., Poruba, A., ... Kroll, U. (2011). Nanostructured three-dimensional thin film silicon solar cells with very high efficiency potential. Applied Physics Letters, 98 (16), 163503. doi: 10.1063 / 1.3583377 shown.

The front-side electrode of the solar cell can be formed in a manner known per se as a contacting structure, in particular a metallic contacting structure, for example in the form of a contacting grid. Advantageously, the front-side electrode is designed as a surface electrode, which covers the front side of the solar cell with an area fraction of at least 80%, preferably at least 90%, particularly preferably over the whole area. The surface electrode is preferably designed to be substantially transparent to incident electromagnetic radiation, at least in the absorption region of the photoelectrically active absorber layer. It is within the scope of the invention, the surface electrode as per se known transparent electrode, for example by means of TCO (Transparent Conductive Oxide), in particular from one of the materials ZnO: Al (AZO), SnO ₂ : F (FTO), In ₂ O ₃ : F (ITO).

The front-side reflection layer is also preferably formed over the entire surface. Likewise, it is within the scope of the invention to eliminate surface areas, for example, to form the front-side reflection layer only with an area fraction (with respect to a front side of the absorber layer) of at least 80%, preferably at least 90%.

It is within the scope of the invention to form the front side reflection layer in the form of a plurality of completely or partially superimposed layers or structures of different materials. For example, the reflection layer may be formed as a metal-lattice structure, in particular of silver, in which the metal covers the surface of the solar cell only to a small extent, for example 10%, while on the front side of this metal lattice a further layer is formed from a TCO such that both layers together provide at least 80% coverage of the cell. In this case, since both materials have high reflectivity at the interface with amorphous or microcrystalline silicon, for example, they together form a front-side electrode 1 and at the same time a front side reflection layer 6 ,

In particular, it is also within the scope of the invention to form the front side reflection layer as a known Bragg reflector, in particular with the aim of achieving wavelength-dependent transmission and reflection properties thereof.

The front-side reflection layer is preferably designed to transmit electromagnetic radiation, which penetrates from the front into the photovoltaic solar cell, in the direction of the absorber layer. In particular, the front-side _{reflection layer is} preferably designed to be substantially transparent in a wavelength range ½ λ _Tauc to λ _Tauc , in particular preferably in a wavelength range 300 nm to λ _Tauc for electromagnetic radiation penetrating into the solar cell from the front side. Preferably, electromagnetic radiation of a corresponding wavelength which has penetrated into the reflective layer is to be absorbed when passing through the front-side reflection layer to a fraction of less than 20%, preferably less than 10%. This can be ensured according to the well-known Lambert-Beer law either by choosing a material with a low absorption coefficient or by a correspondingly thin design of the front side reflection layer.

The task of the front-side reflection layer is preferably to form a partially reflecting interface, in particular together with the absorber layer or the layer adjoining the front-side reflection layer in the direction of the absorber layer, in order to enable the utilization of interferences. As is well known, the reflectivity of an interface can be determined using the complex refractive indices of the adjacent materials using the generally known Fresnel equations as the sum of the squares of the Fresnel coefficients derived therefrom. For the purposes of this description, the term "reflectivity" is understood to mean exclusively the reflection at a single interface determined in accordance with the above provision, without consideration of interference effects.

According to this definition, the front-side reflection layer is preferably formed such that the interface between the front-side reflection layer and the layer adjacent to the front-side reflection layer in the direction of the absorber layer has a reflectivity of at least 10%, preferably more than 15%, in particular preferably more than 20%, in particular Wavelength range between ½ λ _Tauc and λ _Tauc . This makes it possible for electromagnetic waves reflected on the rear side reflection layer to interfere destructively with the electromagnetic waves reflected directly on the front side reflection layer upon selection of the layers located between the front and back reflection layers according to claim 1.

The backside reflection layer is also advantageously flat, in particular over the entire surface. It is also within the scope of the invention to save surface areas. Preferably, the backside reflection layer is formed with an area ratio {with respect to a backside of the absorber layer} of at least 80%, preferably at least 90%. It is within the scope of the invention to form the back reflection layer in the form of several completely or partially superimposed layers of different materials. In particular, it is within the scope of the invention to form the rear-side reflection layer as a known Bragg reflector, in particular with the aim of achieving wavelength-selective transmission and reflection properties of the same.

An advantageous embodiment consists, for example, in the shaping of the rear-side reflection layer as a known Fabry-Pérot resonator, preferably consisting of a TCO layer arranged between two thin metal layers, since this provides a simple possibility for achieving a wave-selective partial transparency of the solar cell.

The rear-side reflection _{layer is} preferably designed to have a reflectivity of at least 80%, preferably 90%, for electromagnetic radiation at least on the side facing the absorber layer at least in a wavelength range ½ λ _Tauc to λ _Tauc , preferably in a wavelength range 250 nm to λ _Tauc .

However, it is also within the scope of the invention to form the rear-side reflection layer such that it has only minor, but preferably at least 10% reflectivity, more preferably 20% reflectivity, for electromagnetic radiation impinging on the absorber side in a subregion of the optical wavelengths.

A particularly structurally simple and thus inexpensive advantageous embodiment of the photovoltaic solar cell results in an advantageous embodiment in which the front-side electrode is simultaneously formed as a front-side reflection layer. Likewise, it is alternatively or additionally advantageous to design the backside electrode simultaneously as a backside reflection layer in order to achieve a cost-effective production method.

The front-side reflection layer is preferably composed of one or more of the group consisting of oxides (in particular one or more of ZnO: Al, SnO ₂ : F (also called FTO), In ₂ O ₃ : F (also called ITO), SiO ( : B or: P), SiC (: B or P), SiN (: B or: P), or more complex alloys of Si, O, C and N.), thin metal layers, preferably silver, aluminum, gold, preferably with Layer thickness <20 nm formed.

The backside reflective layer is preferably formed from one or more of the materials as already listed for the front side reflective layer.

When forming the front side reflection layer simultaneously as the front side electrode, the formation of the front side reflection layer can be performed as described above, wherein the front side reflection layer must additionally have a lateral electric conductivity, preferably has a sheet resistance smaller than 20 Ω / □, preferably smaller than 10 Ω / □.

When forming the backside electrode at the same time as a backside reflective layer, the formation is preferably made of one or more of the materials as already listed for the front side reflective layer.

The absorber layer is preferably formed as a germanium layer, particularly preferably as an amorphous or microcrystalline germanium layer. This makes it possible to resort to production methods known per se for forming the layer, wherein due to the small thickness (less than 40 nm) of the absorber layer correspondingly short production times and a reduced recombination rate of charge carriers in the absorber layer are achieved.

As described above, the photovoltaic solar cell according to the invention at least partially absorbs the incident electromagnetic radiation in the photoelectrically active absorber layer, followed by charge carrier separation of electron hole pairs in the absorber layer. These are separated at a pn junction and separately fed to a respective associated electrode for charge carrier removal. It is within the scope of the invention to provide separate doped layers in a conventional manner for forming a pn junction.

Likewise, in known manner, one or both of the electrode layers and / or one or both of the reflection layers themselves may be formed as a doped or metal electrode layer for forming a pn junction or Schottky contact, for example when using a TCO or metal Front reflection layer whose Fermi level is close to the conduction band edge of the absorber. In this case, it is not absolutely necessary to form an additional doped layer between the absorber and the reflection layer.

In a further preferred embodiment of the solar cell according to the invention, at least one further doped intermediate layer for charge carrier separation is arranged between absorber layer and front side reflection layer and / or between absorber layer and backside reflection layer.

Preferably, this further doped intermediate layer is formed as a layer which absorbs less in comparison to the absorber layer. This is due to the fact that such layers typically have low charge carrier lifetimes and correspondingly small diffusion lengths of the minority charge carriers, such that electron hole pairs generated in these doped intermediate layers do not contribute or only slightly contribute to the electrical energy that can be tapped off at electrodes.

The further doped intermediate layer is therefore preferably formed with a lower optical absorption, in particular in the range ½ λ _Tauc to λ _Tauc , preferably in the wavelength range 300 nm to λ _Tauc .

In a preferred embodiment, the doped intermediate layer has a similar refractive index as the adjacent layer in the direction of the absorber, such that the reflectivity at the interface between these two layers is less than 5%. This results in the advantage that light is reflected almost exclusively at the intended in the direction of the absorber facing boundary surfaces of the reflective layers, instead of additionally strong reflection on the facing towards the absorber boundary layer of the doped intermediate layer.

This is particularly important if the corresponding reflection layer is a metal layer and if a phase jump associated with the reflection is to be utilized. Even if the optical path length is to be increased without increasing the absorber thickness, this can be advantageous.

In a further preferred embodiment, the aforementioned doped intermediate layer is formed of a different material than the photoelectrically active absorber layer, in particular preferably of a material having a higher Tauc band gap than the absorber layer, in order to achieve a lower absorption in the doping layers than the absorber layer ,

Here, for example, the combination of an absorber layer of amorphous germanium or a silicon-germanium alloy with a dating layer of amorphous or microcrystalline silicon is particularly advantageous. In the case of amorphous silicon, the interface between doped intermediate layer and absorber layer exhibits a reflectivity of less than 5% in the wavelength range between 350 nm and 1200 nm, while for microcrystalline silicon for 250 nm to 600 nm below 6% and between 600 nm and 1200 nm also less than 5%. Microcrystalline silicon is a particularly advantageous material for use in doped interlayers in combination with an amorphous germanium absorber since both materials have similar bandgaps and electron affinities.

Typically, the use of different absorber and doping layer materials leads to a band discontinuity in the area between the absorber layer and doped intermediate layer, which forms a barrier for charge carriers and thus increases their probability of residence in the region between the doped and undated regions. Due to the increased density of defects due to doping, there is therefore a greater recombination of charge carrier pairs there. In this case, it is advantageous that at least one undated buffer intermediate layer is arranged between the absorber layer and the doped intermediate layer. The undated buffer intermediate layer preferably has a band gap greater than the absorber band gap, preferably equal to or greater than the band gap of the doped intermediate layer. In particular, it is within the scope of the invention to form the undated buffer intermediate layer such that a gradual development of the band gap between the absorber layer and the doped intermediate layer is produced.

Due to the buffer layer, no barrier or a reduced barrier for the charge carriers occurs at the interface between doped and undated material, so that their local probability of residence and thus the recombination rate are lowered. This increases the voltage and fill factor of the cell.

Furthermore, the buffer interlayer has the advantage that the optical resonance frequency between the front-side reflection layer and the back-reflection layer can be influenced by the material and the layer thickness of the buffer interlayer. It is therefore possible to optimally optimize the remaining layers for optimum efficiency (apart from the effect of the optical resonance) and specify the desired resonance frequency of the optical resonance by means of the choice of the parameters of the intermediate buffer layer, in particular the thickness of the intermediate buffer layer be without the parameters, in particular the thicknesses of the other layers have to be changed again for this purpose. In particular, when providing a buffer interlayer, a thinner absorber layer is typically possible with comparable solar cell absorption as compared to a bufferless interlayer design.

The intermediate buffer layer is preferably made of silicon or a silicon alloy, in particular a silicon alloy with one or more of the elements O, C, N, Ge.

In particular, the buffer intermediate layer may consist of the same material as the further doped intermediate layer or of a material with a comparatively larger band gap. Alternatively or additionally, the buffer interlayer can be formed as a thin oxide layer, for example silicon dioxide layer, in this case without the advantage of easy adaptability of the optical path length.

In order to achieve a similar effect as in the case of a gradual bandgap in the buffer layer, it is therefore also within the scope of the invention to carry out a buffer interlayer as a consequence of a plurality of nominally intrinsic layers having different band gaps.

It is also within the scope of the invention to provide a buffer intermediate layer without further doped intermediate layer. In this case, it is advantageous that the intermediate buffer layer consists of a material which has the same or a larger band gap than the absorber layer.

Preferably, the total thickness of all buffer interlayers and doped interlayers is less than 100 nm, preferably less than 50 nm, more preferably less than 20 nm.

In a further preferred embodiment, the optical path length between the front and rear _{reflection layer is} less than 1/3 λ _Tauc , further preferably ¼ λ _Tauc . This is particularly advantageous when the backside reflective layer is formed as a metallic layer. This is due to the fact that due to the high absorption of the photoelectrically active absorber layer, especially when using a back metal reflector due to the phase jumps in the reflection at said reflector typically a red shift of the optical resonances occurs, so that the optical path length between the front side reflection layer and back reflection layer and thus the Thickness of the intermediate layers must be chosen even lower, in order to achieve the desired optical resonance. Therefore, the thickness of the absorber can often be further reduced without reducing the absorption of the solar cell, again with the effect of increased open-circuit voltage and increased solar cell filling factor.

The thickness of the absorber layer is preferably less than 40 nm, more preferably less than 30 nm, for cost saving, efficiency increase and optical resonance. In the case of a plurality of germanium-containing absorber layers, it is preferable that the thickness of each such absorber layer is less than 40 nm, more preferably less than 30 nm. Typically, in a solar cell satisfying the conditions of claim 1, an optical resonance is achieved or can be achieved by further thinning the layers located between the front and back reflection layers. The optical resonance is then typically exhibited by an increase in short circuit current density as these layers thin.

The solar cell according to the invention can be designed as a so-called "single solar cell" which has a pn junction.

In an advantageous embodiment, the solar cell according to the invention is designed as a per se known multiple solar cell with a plurality of pn junctions. Such "multijunction" solar cells are known per se and allow a higher overall efficiency, since the provision of multiple pn junctions and absorbers with different band gaps a broader spectrum of incident electromagnetic radiation can be absorbed more efficiently. The use of two pn junctions with the same absorber material can in this context increase the efficiency compared to the single solar cell cause, as a rule, the thickness of the individual absorbers are reduced compared to the thickness used in the single cell and thus the recombination can be reduced. It is essential that an optical resonance as described above is formed at least with regard to one of the germanium-containing absorber layers.

In an advantageous embodiment of the solar cell according to the invention, at least one second photoelectrically active absorber layer is arranged between the front and rear reflection layer in the manner of a construction of a multi-junction solar cell, wherein the second photoelectric absorber layer preferably has a different bandgap to the first photoelectrically active absorber layer.

If a plurality of absorber layers are arranged between two reflection layers, which have different band gaps, the definition of λ _Tauc refers to the germanium-containing absorber with the lowest band gap, ie the highest value of λ _Tauc , since, as already mentioned, typically long-wave light is absorbed the poorest and the optical resonance in the cell with the corresponding absorber can achieve the strongest layer thickness-reducing effect. Nevertheless, it is also within the scope of the invention to make the optical path length smaller and to optimize the optical resonance to a subcell having a higher bandgap.

In a further preferred embodiment of the solar cell, this contains, in addition to the pn junctions arranged between the front and rear reflection layer, further photoelectrically active absorber layers or pn junctions which are not arranged between a front side reflection layer and a rear side reflection layer, in particular as an upper partial solar cell cell.

In a further advantageous embodiment, a plurality of partial solar cells of the multi-junction solar cell each have a front and a rear-side reflection layer, between which one or more absorber layers are respectively arranged. In this preferred embodiment, the reflection layers can thus be optimized for the respective absorber or absorbers. In particular, it is advantageous to the Forming a back reflection layer of an upper part solar cell of the multiple solar cell simultaneously as a front side reflection layer of an underlying lower part solar cell of the multiple solar cell, such a layer being hereinafter referred to as an intermediate reflection layer and an intermediate electrode, respectively.

At least the absorber layer of the solar cell according to the invention is preferably deposited by means of PECVD. Likewise, the absorber layer and / or further layers can be sputtered by means of rf magnetron, sputtered by magnetron or hot wire CVD (HWCVD).

All of the Germanium and / or silicon-containing layers mentioned in the context of this description are preferably deposited as hydrogenated layers, since this typically results in improved material quality. These are material layers which have a hydrogen content of up to 20 atomic percent to saturate defect states.

Furthermore, the names of alloys refer exclusively to the chemical elements contained, without including their respective mass fractions. Accordingly, for example, the term "SiO" stands for an alloy of silicon and oxygen with arbitrary proportions of both elements.

Furthermore, the deposited semiconductor layers may be slightly n-type or p-type due to process-related contamination or structurally related. The term "intrinsic" therefore includes in the context of this description nominal intrinsic semiconductors, ie those semiconductors whose Fermi energy were not changed by heavy doping (about 10 ¹⁵ cm ^-3 or more) with elements of another main group such that they are n- or p Conductively behave, as well as those semiconductors, which naturally behave easily without doping p- or n-type and whose dark conductivity was reduced by counter-doping.

The solar cell according to the invention can either be embodied as a "substrate solar cell" known per se, that is to say on a transparent or non-transparent substrate, preferably aluminum, arranged on the back side. Alternatively, an embodiment of the solar cell according to the invention takes place as a likewise known "superstrate solar cell". In this embodiment, the solar cell is arranged on the back on a light-transmitting substrate, preferably glass.

It is within the scope of the invention, front side of the solar cell according to the invention in a conventional manner to arrange one or more layers to achieve antireflective properties or encapsulation of the solar cell.

Since the conductivity of the electrode layers in thin-film solar cells is often insufficient to ensure current transport over long distances, it is customary to divide larger thin-film solar cells by different structuring methods into several smaller cells and connect them in series to form a solar module (monolithic series connection). This happens, for example, by the production of laser trenches after different production steps of the solar cell. It is therefore within the scope of the invention to form the solar cell according to the invention in a manner known per se as a monolithic series-connected solar module.

The solar cell according to the invention is particularly suitable for forming a large-area, monolithically interconnected thin-film solar cell. Likewise, the training as a thin-film concentrator solar cell is within the scope of the invention.

Further preferred features and embodiments of the invention will be described below with reference to figures and embodiments. Showing:

1 a first embodiment of a solar cell according to the invention, which is designed as a single resonator and as a single-junction solar cell;

2 A second embodiment of a solar cell according to the invention, which is designed as a single resonator and as a tandem solar cell,

3 A third embodiment of a solar cell according to the invention, which is designed as a single resonator and as a triple solar cell,

4 A fourth embodiment of a solar cell according to the invention, which is designed as a multiple resonator and triple solar cell, and

5 A fifth embodiment of a solar cell according to the invention, which differs from the in 4 is distinguished by the formation of the front side reflection layer as a-SiO (p) layer.

The figures show schematic, not to scale representations of the respective layer structure. When using the solar cells shown in the figures, the incidence of light from above, d. H. the front of the solar cell is overhead and the bottom or back of the solar cell is shown underneath. The solar cells are designed as large-area, monolithically interconnected thin-film solar cells and can accordingly have other known, not shown, elements and structures, in particular for interconnection.

All shown solar cells or partial solar cells are designed such that the respective p-doped layers are arranged on the light incident side of the corresponding absorber layer. The solar cells resulting in obvious manner by interchanging the positions of the p- and n-doped layers of the respective individual cells are likewise within the scope of the invention and represent further exemplary embodiments.

Like reference numerals in the figures indicate the same or equivalent elements.

1 shows a first embodiment of a solar cell according to the invention with a front side electrode 1 a backside electrode 2 and an absorber layer 3 , Between front side electrode 1 and absorber layer 3 is a first doped intermediate layer 4 and further a first undoped buffer interlayer 5 arranged.

Between the absorber layer 3 and the backside electrode 2 is a second undoped buffer interlayer 5a and a second doped intermediate layer 4a arranged. The absorber layer 3 has a thickness less than 40 nm and is formed as an intrinsic amorphous germanium layer.

It is essential that the front side electrode 1 at the same time as a front-side reflection layer 6 and the backside electrode 2 at the same time as a backside reflection layer 7 is trained:
The front side reflection layer 6 is designed to absorb light in the wavelength range 300 nm to 1200 nm to the lowest possible content. Light which is inside the solar cell on the front side reflection layer 6 that is, from the first doped interlayer 4 starting at the interface between the first doped intermediate layer 4 and front side reflection layer 6 , is reflected at least in the above-mentioned wavelength range to a proportion of 15% or more. This is due to the very different complex refractive indices of the front surface reflection layer 6 and first doped intermediate layer 4 founded. The refractive indices are approximately 2 for ZnO: Al and 4 for a-Si and μc-Si, respectively, but are wavelength-dependent and are not sufficient for a precise calculation of the reflection coefficients, since the complete complex refractive index is needed for this purpose.

The backside reflective layer 7 is formed as a silver layer and the boundary surface formed with the layer adjacent to the absorber thus reflecting at least in the wavelength range 600-1200 nm to a proportion of at least 90%.

The following is an example calculation for the determination of the optical path length between front and rear reflection layer for the verification of the resonance condition according to claim 1:
The Tauc band gap of amorphous germanium is about 1.1 eV, the corresponding wavelength λ _Tauc of light is about 1127 nm (1240 nm divided by 1.1 for the bandgap energy), so that ½ λ _Tauc corresponds to a wavelength of about 564 nm. At this wavelength, the real part of the complex refractive index (refractive index) of amorphous germanium is about 4.4, μc-Si is about 3.5, and a-Si is about 3.7. With the underlined layer thicknesses given in Table 1 for the embodiment described here, an optical path length between front side reflection layer accordingly results 6 and backside reflective layer 7 of 138nm (5nm * 3.5 + 5nm * 3.5 + 15nm * 4.4 + 5nm * 3.7 + 5nm * 3.7), which is less than ½ λ _tauc , especially even less than ¼ λ _tauc .

This results in an optical resonance in a relevant for the absorption function of the absorber layer wavelength range, in the present case approximately between 600 and 800 nm, so that due to the greatly increased absorption in spite of the small thickness of the absorber layer 3 a high efficiency of the solar cell can be achieved.

In this embodiment, therefore, an optical resonator is formed, which is why this solar cell is also referred to as a single resonator.

The following table shows further advantageous embodiments of a solar cell structure according to FIG 1 listed, with several preferably specified parameter or parameter ranges and underlined the parameter value of the embodiment are given.

In this description the usual abbreviations for materials are used in semiconductor solar cells, for example "μc" (eg μc-Si) for microcrystalline material, "a" (eg a-Si) for amorphous material, "(p) "Or" (n) "(eg" a-Si (p) ") for a p- or n-type doping and" (i) "(eg" a-Si (i) " ) for intrinsic material. A colon ":" (eg "ZnO: Al") denotes a corresponding alloy. The addition (optional) in this and the following tables means that preferably the respective layer can also be omitted. (Reference numeral) layer name material thickness endowment ( 1 . 6 ) Front side electrode, front side reflection layer ZnO: Al (50 nm to 1000 nm); 600 nm ( 4 ) (optional) doped intermediate layer μc-Si (p) ; a-Si (p) (5 nm to 10 nm); 5 nm Boron (10 ¹⁷ -10 ²⁰ cm ^-3 ) 10 ¹⁹ cm ^-3 ( 5 ) (optional) buffer interlayer μc-Si (i) ; a-Si (i) (5 nm to 20 nm); 5 nm - ( 3 ) Absorber layer a-Ge (i) ; .mu.C-Ge (i) (1 nm to 25 nm); 15 nm - ( 5a ) (optional) 2. buffer interlayer a-Si (i) ; c-Si (i) (5 nm to 20 nm); 5 nm - ( 4a ) (optional) 2. doped intermediate layer a-Si (n) ; c-Si (n) (5 nm to 10 nm); 5 nm Phosphorus (10 ¹⁷ -10 ²⁰ cm ^-3 ) 10 ¹⁹ cm ^-3 ( 2 . 7 ) Backside electrode backside reflection layer Ag (5 nm to 500 nm); 250 nm -

In 2 a second embodiment of a solar cell according to the invention is shown. In order to avoid repetition, only the essential differences to those in 1 the solar cell described:
This in 2 illustrated second embodiment of a solar cell according to the invention is designed as a tandem solar cell, ie there are two absorber layers and two pn junctions, both absorber layers are designed as intrinsic amorphous germanium layers.

In addition to the already too 1 layers described absorber layer 3 , doped interlayers 4 and 4a and undoped buffer interlayers 5 and 5a has the in 2 illustrated solar cell, a second absorber layer 3a on which front side a third undoped buffer interlayer 5b and a doped intermediate layer 4b and at the back a fourth undoped buffer interlayer 5c and a fourth doped intermediate layer 4c is arranged. This second pn junction consisting of third doped intermediate layer 4b third buffer interlayer 5b , second absorber layer 3a fourth buffer interlayer 5c and fourth doped intermediate layer 4c is in addition to in 1 illustrated first pn junction between front side reflection layer 1 and first doped intermediate layer 4 arranged so that there are two pn junctions in the resonator formed by the two reflection layers here. In addition, this cell is different from the one in 1 depict a cell for increasing the reflection with a separate front-side reflection layer 1 executed, which between third doped intermediate layer 4b and front side reflection layer 1 is arranged.

The formation of a tandem solar cell to increase the efficiency by utilizing two different absorber band gaps or reduction of recombination compared to a single cell is on known. Essential in the second embodiment according to 2 is that here is the front side electrode 1 not simultaneously as a front-side reflection layer 6 is performed. Instead, it's a separate front side reflection layer 6 between front side electrode 1 and third doped intermediate layer 4b arranged. The backside electrode 2 is also here as a backside reflective layer 7 educated.

The front side reflection layer 6 is in this case in a conventional manner as a thin silver layer, such as in DE 10 2012 201 284 A1 described, trained and is due to their thin training for incident electromagnetic radiation in the wavelength range 300 nm to 1200 nm only weakly absorbing and simultaneously has as a result of the highly different complex refractive indices of silver and microcrystalline silicon within the solar cell on the interface between the third doped intermediate layer 4b and front side reflection layer 6 impinging electromagnetic radiation, at least in the above-mentioned wavelength range, a high reflectivity. In the wavelength range between 600 nm and 1200 nm, the reflectivity of the interface between the two materials according to the aforementioned definition is over 90%. Due to the thin design of this layer in combination with a suitable choice of the optical path length of the front side electrode as well as between the front side reflection layer and the back reflection layer, a high transmission of the silver and a strong absorption in both absorber layers nevertheless results.

Further preferably embodiments of a solar cell according to the invention with the in 2 shown structure are listed in the following table, with several preferably specified parameter or parameter ranges and underlined the parameter value of the embodiment are given.

The optical path length between front side reflection layer 6 and backside reflective layer 7 is at execution of the absorber layer ( 3 ) as an amorphous germanium layer here with the layer thicknesses given in the table with a value of about 250 nm smaller ½ λ _Tauc (560 nm), as can be calculated assuming the above values for the refractive indices for amorphous germanium and amorphous and microcrystalline silicon. When the absorber layer ( 3 ) as the microcrystalline germanium layer, the wavelength λ _{Tauc is} to be chosen according to the band gap of this material, since the band gap of microcrystalline germanium is smaller than that of amorphous germanium. Since the refractive indices of all participating materials at higher values of λ _{Tauc are} at the same time somewhat lower than when the absorber layer is carried out ( 3 ) as an amorphous germanium layer, in this case also the optical path length is smaller than ½ λ _Tauc . (Reference numeral) layer name material thickness endowment ( 1 ) Front side electrode ZnO: Al (50 nm to 1000 nm); 80 nm - ( 6 ) Front reflection layer Ag (1 nm to 15 nm); 10 nm - ( 4b ) (optional) 3. doped intermediate layer μc-Si (p) ; a-Si (p) (5 nm to 10 nm); 5 nm Boron (10 ¹⁷ -10 ²⁰ cm ^-3 ) 10 ¹⁹ cm ^-3 ( 5b ) (optional) 3. buffer interlayer μc-Si (i) ; a-Si (i) (5 nm to 10 nm); 5 nm - ( 3a ) 2. absorber layer a-Ge (i) (1 nm to 20 nm); 8 nm - ( 5c ) (optional) 4. buffer interlayer a-Si (i) ; c-Si (i) (0 nm to 10 nm); 5 nm - ( 4c ) (optional) 4. doped intermediate layer a-Si (n) ; c-Si (n) (5 nm to 10 nm); 10 nm Phosphorus (10 ¹⁷ -10 ²⁰ cm ^-3 ) 10 ¹⁹ cm ^-3 ( 4 ) (optional) doped intermediate layer μc-Si (p) ; a-Si (p) (5 nm to 10 nm); 10 nm Boron (10 ¹⁷ -10 ²⁰ cm ^-3 ) 10 ¹⁹ cm ^-3 ( 5 ) (optional) buffer interlayer μc-Si (i) ; a-Si (i) (5 nm to 10 nm); 5 nm - ( 3 ) Absorber layer a-Ge (i) ; .mu.C-Ge (i) (1 nm to 20 nm); 8 nm - ( 5a ) (optional) 2. buffer interlayer a-Si (i) ; c-Si (i) (0 nm to 10 nm); 5 nm - ( 4a ) (optional) 2. doped intermediate layer a-Si (n) (5 nm to 10 nm); 5 nm Phosphorus (10 ¹⁷ -10 ²⁰ cm ^-3 ) 10 ¹⁹ cm ^-3 ( 2 . 7 ) Backside electrode backside reflection layer Ag (5 nm to 500 nm); 250 nm -

In 3 a third embodiment of a solar cell according to the invention is shown, which is designed as a triple solar cell. This solar cell thus has three absorbers and three pn junctions. The formation of a triple solar cell to increase the efficiency is known per se. In the following, to avoid repetition, only the essential differences from those in FIG 2 the solar cell described:
The solar cell according to 3 has a third absorber layer 3b on which at the back a fifth doped intermediate layer 4d and on the front side a sixth doped intermediate layer 4e is arranged. The layer stack sixth doped intermediate layer 4e , third absorber layer 3b and fifth doped interlayer 4d is between the third doped interlayer 4b and the front side reflection layer 6 arranged and forms a third pn junction. As already mentioned, the formation of a triple solar cell with three absorber layers 3 . 3a and 3b with partially different band gaps to increase the efficiency known per se.

It is essential that in contrast to the embodiment of 2 in this embodiment, the front-side electrode 1 again simultaneously as a front-side reflection layer 6 and the backside electrode 2 at the same time as a backside reflection layer 7 is trained. Furthermore, the first absorber layer is compared to 2 formed as a microcrystalline germanium layer instead of an amorphous germanium layer.

The front side reflection layer 6 is designed to be transparent for penetrating into the solar cell from the front electromagnetic radiation at least in the wavelength range 300 nm to 1200 nm. For electromagnetic radiation, which within the solar cell to the interface between the third doped intermediate layer 4b and front side reflection layer 6 is the front-side reflection layer 6 formed in the above wavelength range with a reflectivity of at least 10%. This is due to the high differences in the complex refractive indices of the layers mentioned (rough guide values: refractive index of about 2 for ZnO: Al and about 4 for a-Si, for a precise calculation of the reflectivity the complete complex refractive indices are to be used).

Further preferably embodiments of a solar cell according to the invention with a solar cell structure according to 3 are listed in the following table, with several preferably specified parameters or parameter ranges and given in thick print the parameter value of the embodiment.

The exact bandgap of μc-Ge may vary depending on the deposition temperature and manufacturing process. For the determination of the optical path length here a value of 0.85 to 0.9 eV is assumed, corresponding to a wavelength of about 1340 nm to 1460 nm. Since this is the lowest of the band gaps involved, a resonance to support the absorption in the μc-Ge containing Subcell be formed so that the definition of λ _Tauc refers to this material, ie λ _Tauc corresponds in this example depending on the band gap between 1380 nm and 1460 nm. Since the refractive indices of the materials involved to higher wavelengths hardly change Continue to use the same values as in the previous examples. The optical path length between front side reflection layer 6 and backside reflective layer 7 Accordingly, for the underlined in the following table parameter is about 655 nm and is thus below ½ λ _Tauc (690 nm to 730 nm). (Reference numeral) layer name material thickness endowment ( 1 . 6 ) Front side electrode, front side reflection layer ZnO: Al (50 nm to 1000 nm); 600 nm - ( 4d ) 5. doped intermediate layer a-Si (p) (5 nm to 10 nm); 10 nm boron ( 3b ) 3. absorber layer a-Si (i) (50 nm to 80 nm); 80 nm - ( 4e ) 6. doped intermediate layer a-Si (n) ; c-Si (n) (5 nm to 10 nm); 10 nm Phosphorus (10 ¹⁷ -10 ²⁰ cm ^-3 ) 10 ¹⁹ cm ^-3 ( 4b ) 3. doped intermediate layer μc-Si (p) ; a-Si (p) (5 nm to 10 nm); 10 nm Boron (10 ¹⁷ -10 ²⁰ cm ^-3 ) 10 ¹⁹ cm ^-3 ( 5b ) (optional) 3. buffer interlayer μc-Si (i) ; a-Si (i) (0 nm to 5 nm); 5 nm - ( 3a ) 2. absorber layer a-Ge (i) ; (1 nm to 10 nm); 10 nm - ( 5c ) (optional) 4. buffer interlayer a-Si (i) ; c-Si (i) (0 to 5 nm); 5 nm - ( 4c ) 4. doped intermediate layer a-Si (n) ; c-Si (n) (5 nm to 10 nm); 10 nm Phosphorus (10 ¹⁷ -10 ²⁰ cm ^-3 ) 10 ¹⁹ cm ^-3 ( 4 ) Doped intermediate layer μc-Si (p) ; a-Si (p) (5 nm to 10 nm); 10 nm Boron (10 ¹⁷ -10 ²⁰ cm ^-3 ) 10 ¹⁹ cm ^-3 ( 5 ) (optional) buffer interlayer μc-Si (n) ; a-Si (i) (5 nm to 10 nm); 5 nm - ( 3 ) Absorber layer .mu.C-Ge (i) (1 nm to 10 nm); 10 nm - ( 5a ) (optional) 2. buffer interlayer a-Si (i) ; c-Si (i) (0 nm to 5 nm); 5 nm - ( 4a ) (optional) 2. doped intermediate layer a-Si (n) ; c-Si (n) (0 nm to 10 nm); 5 nm Phosphorus (10 ¹⁷ -10 ²⁰ cm ^-3 ) 10 ¹⁹ cm ^-3 ( 2 . 7 ) Backside electrode backside reflection layer Ag (5 nm to 500 nm); 250 nm -

In 4 Finally, a fourth embodiment of a solar cell according to the invention is shown, which analogous to the in 3 illustrated solar cell is also designed as a triple solar cell.

In order to avoid repetition, only the essential differences from those in 3 the solar cell described:
This in 4 illustrated fourth embodiment of a solar cell according to the invention differs significantly from the third embodiment according to 3 , in that an intermediate electrode 8th between fourth doping layer 4c and first doping layer 4 is arranged. The intermediate electrode 8th is also an intermediate reflection layer 9 educated. This results in two optical resonators:
A first resonator is interposed between the front reflection layer 6 trained front side electrode 1 and the intermediate reflection layer 9 formed and a second optical resonator is between the intermediate reflection layer 9 and as the second backside reflective layer 7a trained back electrode 2 formed so that the intermediate reflection layer 9 at the same time as a backside reflection layer 7 as well as front side reflection layer 6a is executed. The solar cell according to this embodiment is therefore also referred to as a multiple resonator.

The second, rear-side arranged optical resonator fulfills with the parameters given in the following table the condition that the optical path length between intermediate reflection layer 9 and second backside reflective layer 7a is smaller than ½ λ _tauc , here the bandgap of the first absorber layer 3 is crucial, since it represents the layer with the lowest band gap lying between the above-mentioned reflection layers. Already hereby a solar cell according to the invention is realized.

In addition, the first front side resonator having the parameters given in the following table satisfies the condition that the optical path length between the front surface reflection layer 6 and intermediate reflection layer 9 is smaller than ½ λ _Tauc , with the bandgap of the third absorber _layer corresponding here 3b is decisive.

However, it is also within the scope of the invention to form only a germanium-containing partial solar cell according to the above descriptions for the formation of an optical resonance, since the invention relates mainly to the improved coupling of light into such germanium-containing partial solar cells. Accordingly, for the thickness of the third absorber layer 3b a second parameter range specified (provided with a "*"), in which this absorber layer 3b containing upper part cell does not necessarily satisfy the resonance condition. The resulting solar cell is, as described above, also within the scope of the invention.

Further preferably embodiments of a solar cell according to the invention with a structure according to 4 are shown in the following table, wherein in some cases several preferably specified parameter or parameter ranges and underlined the parameter value of the embodiment are given. (Reference numeral) layer name material thickness endowment ( 1 . 6 ) Front side electrode, front side reflection layer ZnO: Al (50 nm to 1000 nm); 100 nm - ( 4d ) 5. doped intermediate layer a-Si (p) ; a-SiO (p) (5 nm to 10 nm); 10 nm Boron (10 ¹⁷ -10 ²⁰ cm ^-3 ) 10 ¹⁹ cm ^-3 ( 3b ) 3. absorber layer a-Si (i) (50 nm to 75 nm); * (50 nm to 200 nm) 70 nm - ( 4e ) 4. doped intermediate layer a-Si (n) ; c-Si (n) (5 nm to 10 nm); 10 nm Phosphorus (10 ¹⁷ -10 ²⁰ cm ^-3 ) 10 ¹⁹ cm ^-3 ( 8th . 7 . 6a . 9 ) Inter-electrode Backside reflection layer Second front-reflection layer Inter-reflection layer ZnO: Al ; SiO (n), SiC (n), SiN (n), Ag, Al, Au (1 nm to 100 nm) 40 nm Phosphorus (only for the Si alloys mentioned here) (10 ¹⁷ -10 ²⁰ cm ^-3 ) 10 ¹⁹ cm ^-3 ( 4b ) 3. doped intermediate layer μc-Si (p) ; a-Si (p) (5 nm to 10 nm); 5 nm Boron (10 ¹⁷ -10 ²⁰ cm ^-3 ) 10 ¹⁹ cm ^-3 ( 5b ) (optional) 3. buffer interlayer μc-Si (i) ; a-Si (i) (5 nm to 20 nm); 5 nm - ( 3a ) 2. absorber layer a-Ge (i) ; .mu.C-Ge (i) (1 nm to 25 nm); 10 nm - ( 5c ) (optional) 4. buffer interlayer a-Si (i) ; c-Si (i) (5 nm to 20 nm); 5 nm - ( 4c ) 4. doped intermediate layer a-Si (n) (5 nm to 10 nm); 5 nm Phosphorus (10 ¹⁷ -10 ²⁰ cm ^-3 ) 10 ¹⁹ cm ^-3 ( 4 ) Doped intermediate layer μc-Si (i) ; a-Si (i) 5 nm Boron (10 ¹⁷ -10 ²⁰ cm ^-3 ) 10 ¹⁹ cm ^-3 ( 5 ) (optional) buffer interlayer μc-Si (i) ; a-Si (i) 5 nm - ( 3 ) Absorber layer a-Ge (i) ; .mu.C-Ge (i) (1 nm to 15 nm); 5 nm - ( 5a ) (optional) 2. buffer interlayer a-Si (i) ; c-Si (i) 5 nm - ( 4a ) (optional) 2. doped intermediate layer a-Si (n) ; c-Si (n) 5 nm Phosphorus (10 ¹⁷ -10 ²⁰ cm ^-3 ) 10 ¹⁹ cm ^-3 ( 2 . 7a ) Backside electrode second backside reflection layer Ag (5 nm to 500 nm); 250 nm -

Another, preferably embodiment differs from the one in FIG 4 represented solar cell by the 5th doped intermediate layer 4d instead of of amorphous silicon of an amorphous silicon alloy with a widened band gap to silicon, for example, a-SiO (p) or a-SiC (p) is formed. This expansion can be either weak or pronounced. If the expansion is weak, only a small refractive index change occurs, so that the reflectivity at the interface between the fifth doped intermediate layer 4d and third absorber layer 3b low and that between front side reflection layer 6 and 5. doped interlayer remains high.

With medium expansion, however, there is an unfavorable further increase in the reflectivity of the boundary layer between the aforementioned doping layer and absorber layer and an unfavorable reduction in the reflectivity of the boundary layer between the aforementioned front-side reflection layer and doping layer, with the result of a weakening of the resonance.

In this case it is advantageous, as in 5 5, to make the fifth doped intermediate layer expanded in such a way that the refractive index difference with respect to the previous front-reflection layer embodied as ZnO: Al layer 6 or front side electrode 1 small and thus the reflectivity between the two layers is low. At the same time, the reflectivity between the previous fifth doped intermediate layer increases 4d and the third absorber layer 3b , leaving the 5th doped interlayer 4d (here: the front side reflection layer 6 ) performs the task of a front-side reflection layer. The ZnO: Al layer then only takes on the function of the front side electrode 1 ,

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

• US 2012/0319111 A1 [0006]
• DE 102012201284 A1 [0095]

Cited non-patent literature

• Tsao, C.Y., Weber, J.W., Campbell, P., Conibeer, G., Song, D., & Green, M.A. (2010). In-situ low temperature growth of poly-crystalline germanium thin film on glass by RF magnetron sputtering. Solar Energy Materials and Solar Cells, 94 (9), 1501-1505. doi: 10.1016 / j.solmat.2010.02.035 [0019]
• Vanecek, M., Babchenko, O., Purkrt, A., Holovsky, J., Neykova, N., Poruba, A., ... Kroll, U. (2011). Nanostructured three-dimensional thin film silicon solar cells with very high efficiency potential. Applied Physics Letters, 98 (16), 163503. doi: 10.1063 / 1.3583377 [0023]

Claims (15)

Photovoltaic solar cell, with at least one front-side electrode ( 1 ) and at least one backside electrode ( 2 ) and one between the front and rear side electrodes ( 2 ) arranged photoelectrically active absorber layer ( 3 . 3a ), which absorber layer ( 3 . 3a ) contains at least 30% by mass of germanium, characterized in that the absorber layer ( 3 . 3a ) has a thickness of less than 40 nm, the solar cell has at least one front-side reflection layer ( 6 . 6a ) and at least one backside reflective layer ( 7 . 7a ), wherein the absorber layer ( 3 . 3a ) between front ( 6 . 6a ) and backside reflective layer ( 7 . 7a ) and that the optical path length between front ( 6 . 6a ) and backside reflective layer ( 7 . 7a ) is at least perpendicular to the cell incident light smaller ½ λ _Tauc , with one of the Tauc band gap of the absorber layer ( 3 . 3a ) corresponding wavelength λ _Tauc . Solar cell according to claim 1, characterized in that the front side electrode ( 1 ) covers the solar cell with at least 80% surface area, preferably that the front side electrode ( 1 ) is formed over the entire surface. Solar cell according to one of the preceding claims, characterized in that the front-side reflection layer ( 6 . 6a ) for electromagnetic radiation on the absorber layer ( 3 . 3a . 3b ) facing side, at least in the range ½ λ _Tauc to λ _{Tauc has} a reflectivity of at least 10%, preferably more than 15%, in particular preferably more than 20%. Solar cell according to one of the preceding claims, characterized in that the backside side reflection layer ( 7 . 7a ) is formed, for electromagnetic radiation at least on the absorber layer ( 3 . 3a . 3b ) facing side, at least in a wavelength range ½ λ _Tauc to λ _Tauc , preferably in a wavelength range 250 nm to λ _Tauc to have a reflectivity of at least 80%, preferably 90%. Solar cell according to one of the preceding claims, characterized in that the front-side electrode ( 1 ) simultaneously as a front-side reflection layer ( 6 . 6a ) is formed and / or that the rear side electrode ( 2 ) simultaneously as a backside reflection layer ( 7 . 7a ) is trained. Solar cell according to one of the preceding claims, characterized in that the absorber layer ( 3 . 3a ) is formed as germanium layer, in particular amorphous or microcrystalline, in particular hydrogenated germanium layer. Solar cell according to one of the preceding claims, characterized in that between absorber layer ( 3 . 3a . 3b ) and front side reflection layer ( 6 . 6a ) and / or between absorber layer ( 3 . 3a . 3b ) and backside reflective layer ( 7 . 7a ) at least one further doped intermediate layer ( 4 . 4a . 4b . 4c . 4d . 4e ) is arranged for charge carrier separation, in particular a layer having a lower absorption coefficient than it has the absorber layer, in particular in the wavelength range ½ λ _Tauc to λ _Tauc . Solar cell according to claim 7, characterized in that the doped intermediate layer ( 4 . 4a . 4b . 4c . 4d . 4e ) in about the optical refractive index in the direction of the absorber layer ( 3 . 3a ) adjacent layer, in particular that refractive index of doped intermediate layer and in the direction of the absorber layer adjacent layer are selected such that the reflectivity at the interface between these two layers is less than 5%. Solar cell according to one of claims 7 to 8, characterized in that absorber layer ( 3 . 3a . 3b ) and doped interlayer ( 4 . 4a . 4b . 4c . 4d . 4e ) have different materials and that between absorber layer ( 3 . 3a . 3b ) and doped intermediate layer an undoped buffer intermediate layer ( 5 . 5a . 5b . 5c ), which undoped buffer interlayer ( 5 . 5a . 5b . 5c ) preferably has a band gap equal to or greater than the bandgap of the doped intermediate layer. Solar cell according to one of the preceding claims, characterized in that the optical path length between front ( 6 . 6a ) and backside reflective layer ( 7 . 7a ) is less than 1/3 λ _Tauc , in particular smaller ¼ λ _Tauc . Solar cell according to one of the preceding claims, characterized in that the front ( 6 . 6a ) and / or the backside reflective layer ( 7 . 7a ) is formed as a metallic layer, in particular as a silver layer. Solar cell according to one of the preceding claims, characterized in that the solar cell is designed as a multiple solar cell, with at least one second absorber layer ( 3 . 3a . 3b ), which further absorber layer ( 3 . 3a . 3b ) preferably a different bandgap to the first absorber layer ( 3 . 3a . 3b ), in particular, which second absorber layer ( 3 . 3a . 3b ) Comprises silicon. Solar cell according to claim 12, characterized in that first and second absorber layer ( 3 . 3a . 3b ) between the same front ( 6 . 6a ) and backside reflective layer ( 7 . 7a ) are arranged. Salarzelle according to claim 12, characterized in that the solar cell, a second front side reflective layer ( 6a ) and a second backside reflective layer ( 7a ) and the second absorber layer ( 3 . 3a . 3b ) between the second front-side reflection layer ( 6a ) and second backside reflective layer ( 7a ), in particular that the solar cell has an intermediate reflection layer ( 9 ), both as a second front-side reflection layer ( 6a ) and first backside reflective layer ( 7 ) is trained. Multiple solar cell, consisting of a plurality of partial solar cells, characterized in that at least one of the partial solar cells is designed according to one of the preceding claims.

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