EP2429962A1

EP2429962A1 - Thin-film solar cell and method for producing a thin-film solar cell

Info

Publication number: EP2429962A1
Application number: EP10720348A
Authority: EP
Inventors: Burkhard Speit; Eveline Rudigier-Voigt; Silke Wolff; Wolfgang Mannstadt
Original assignee: Schott AG
Current assignee: Schott AG
Priority date: 2009-05-12
Filing date: 2010-05-05
Publication date: 2012-03-21
Also published as: KR101029385B1; US20100288361A1; WO2010130359A1; CN101887922A; KR20100122467A; DE102009050987B3; TW201103879A; CN101887922B; JP2010267967A

Abstract

The claimed thin-film solar cell comprises as least one multi-component substrate glass containing Na2O, the substrate glass is not phase-separated and has a ß-OH content of 25 - 80 mMol/l. The claimed method for producing a thin-film solar cell comprises the following steps: a) a multi-component substrate glass containing Na2O is provided, said substrate glass has a ß-OH content of 25 - 80 mMol/l and the substrate glass is not phase-separated, b) a metal layer is applied to the substrate glass, said metal layer forming an electric rear contact of the thin-film solar cells, c) an intrinsic p-conductive polycrystalline layer is applied, said layer being made of a composite semi-conductor material, in particular a CIGS-composite semi-conductor material, having at least one high temperature step at a temperature of > 550 °C d) a p/n-junction is applied. The glass is preferably composed of the following, in mole %: SiO2 61 - 70.5; AI2O3 8.0 - 15.0; B2O3 0 - 4.0; Na2O 0,5 - 18.0; K2O 0.05 - 10.0; Li2O + Na2O + K2O 10.0 - 22.0; MgO 0 - 7.0; CaO 0 - 5.0; SrO 0 - 9.0; BaO 0 - 5.0; CaO + SrO + BaO + ZnO 0.5 - 11.0; TiO2 + ZrO2 0 - 4.0; SnO2 + CeO2 0 - 0.5; As2O3 + Sb2O3 + P2O5 + La2O3 0 - 2.0; F2 + Cl2 0 - 2.

Description

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Thin-film solar cell and process for producing a thin-film solar cell

The invention relates to a thin-film solar cell and a method for producing a thin-film solar cell.

The so-called thin-film technology is today a major competitor to the established c-Si wafer technology in photovoltaics. Large-scale deposition processes at mostly lower efficiencies make these technologists attractive in terms of manufacturing costs and thus the so-called € / Wp. An advantage of thin-film technology is a comparably short value-added chain, since semiconductor, cell and module manufacturing are in one hand. Nevertheless, cost-cutting measures are also playing an increasingly important role for thin-film technology in photovoltaics.

The cost reduction potentials are above all a reduction in material consumption, a reduction in process times and, associated with this, a higher throughput as well as an increase in the yield. Solar cell concepts based on thin films mainly live on coating technologies on a large area. A major challenge is the homogeneous coating of large areas (> 1 m ² ), in particular edge effects or non-homogeneous ion exchange effects from, for example, the glass substrate locally affect the quality of the layers produced, which is macroscopically in a reduction of the yield but also the energy conversion efficiency of the module reflected.

Compound semiconductor-based thin film solar cells such as CdTe or CIGS (having the general formula Cu (In x _-x , Gaχ) (Si- _y , Sey) ₂ ) show excellent stability as well as very high energy conversion efficiencies

BESTATIGUNGSKOPIE Solar cell structure is known for example from US 5,141,564. These materials are characterized in particular by the fact that they are direct semiconductors and absorb the sunlight effectively even in a relatively thin layer (about 2 μm). The deposition technologies for such thin photoactive layers require high process temperatures to achieve high efficiencies. Typical temperature ranges lie between 450 and 600 ^° C., the maximum temperature being limited in particular by the substrate. For large area applications, glass is generally used as the substrate. Because of economic considerations, ie the low cost, as well as a coefficient of thermal expansion (CTE) which fits approximately to the semiconductor layers, floated soda lime glass (window glass) is used as the substrate, as can be seen from the publications DE 43 33 407, WO 94/07269 , 555 ° C and limits all subsequent processes to about 525 ° C, since otherwise it leads to so-called "sagging", ie warping, and begins to bend The substrate to be coated is and the nearer the process temperature approaches the transformation temperature (Tg) of the glass, warping or bending leads to problems, in particular in so-called inline processes / plants, for example at the locks and as a result the throughput and / or the yield become worse Higher temperatures, say> 550 ^° C., can be achieved, for example, on metal foils, Ti foils, which withstand these temperatures, as described in WO 2005/006393 However, such systems have the disadvantage that they do not react because of their inherent conductivity are suitable for a monolithically integrated series connection of the modules and a Beschichtungsu It is extremely difficult to achieve this level of subsidiarity on a large scale. Solar cells on metal foil are generally connected in series. Due to their low weight, such modules are particularly suitable for extraterrestrial applications. In principle, glass substrates are preferable for terrestrial applications, besides the static properties and the lighter ones Processing, above all because of the significantly higher achievable efficiencies.

It is generally known that an improvement in the electrical properties of such compound semiconductor-based thin film solar cells can be achieved when they are deposited at high temperatures, ie,> 550 ^° C. In detail, this means that a deposition process of such compound semiconductor thin films at high temperatures could be optimized in terms of process control, ie higher deposition and cooling rates, and their performance as a photovoltaic device, ie, superior crystalline quality. As already mentioned, soda-lime glass is not suitable for this purpose.

DE 100 05 088 and JP 11-135819 A describe glass substrates for thin-film photovoltaic modules based on compound semiconductors. In DE 100 05 088, the CTE has been adapted, which corresponds to the CTE of the first layer, the back contact (for example made of molybdenum). On such substrates, CTE mismatch of the glass substrate and the CIGS semiconductor layer does not ensure the layer adhesion of the CIGS layer on the Mo-coated substrate glass. In addition, these substrates contain boron, which especially at high temperatures, ie> 550 ⁰ C, can outgas from the substrate and acts as a semiconductor poison in the CIGS. It would be desirable to have a substrate that can contain boron, but this can not outgas and thus is harmless to the deposition process and thus the semiconductor layer. JP 11-135819 A describes substrates which have no CTE mismatch. However, these glasses contain a high proportion of alkaline earth ions. wasjdazu results in that the mobility of the alkali ions in the substrate is drastically reduced or prevented. It is generally known that alkali ions play an important role during the layer deposition of the compound semiconductor thin films, and therefore it is desirable to have a substrate for the deposition process that allows for spatially and temporally homogeneous alkali ion delivery. In addition, this alkali Lonenbeweglichkeit further limited by the unfavorable molar ratio of S1O2 / AI2O3> 8. Such glass structures are dominated by the structural element of the Si ⁴⁺ oxygen tetrahedron without sufficient diffusion paths which builds up the structural element Al ³⁺ / Na ⁺ in the oxygen anions sublattice. DE 196 16 679 C1 and DE 196 16 633 C1 describe a material with a similar glass composition. However, this material may contain arsenic, which is a semiconductor poison for these layer systems and in particular can outgas at high temperatures and thus contaminate the semiconductor layer. Therefore, this material is not suitable as a glass substrate for CIGS-based solar cells. At this point it is either to use arsenic-free substrates by alternative refining agents to prevent the outgassing of arsenic over applied barrier layers or to make it difficult to degas via a targeted modification of the glass substrate.

Furthermore, it is known that sodium has a positive influence on crystal structure and crystal density but also on crystallite size and orientation. Various approaches will be discussed among experts, but key aspects are the improved chalcogen incorporation into the crystal lattice and the passivation of grain boundaries. These phenomena inevitably lead to significantly better semiconductor properties, in particular to a reduction of the recombination in the bulk material and thus a higher open circuit voltage. This then results in a higher efficiency result. However, the spatially and in particular temporal release of alkali ions from the substrate into the semiconductor layer during the deposition process is very inhomogeneous when using soda-lime glasses. In WQ-94 / Q7-269 this solution is solved so that a barrier layer is applied to the glass surface before the back contact coating (usually Si _x Ny, SiO _x N _y or Al ₂ O ₃ ) and so the sodium diffusion from the glass blocked in the semiconductor layer. Sodium is then added in a further process step as a layer on the barrier layer or on the back contact layer separately. often in the form of NaF ₂ , which significantly increases process times and costs.

Thin polycrystalline layers / layer packages based on Cu (In _I-X1 Ga _x ) (Si _y , Se _y ) ₂ can be prepared in principle by a number of processes, including co-evaporation and the so-called sequential process. In addition, methods such as liquid coatings or electrodeposition associated with a temperature step in chalcogen atmosphere are also suitable. A deposition method that is particularly suitable for large areas and has a relatively stable process window compared to others is the sequential process. This process allows relatively short process times, in the range of a few minutes, in which case the limiting factor is the cooling of the substrate, and thus promises a high degree of economy. In addition, the process is based on furnace processes, which are known in particular from the thick-film doping of silicon for photovoltaics, and enables a comparatively simple process control (US 2004115938). In this process, a molybdenum layer is first applied to the substrate, which has the function of the back contact. Furthermore, a so-called metallic precursor layer consisting of Cu, In and / or Ga, for example, deposited by sputtering and then thermally reacted in chalcogen atmosphere at temperatures of at least 500 ⁰ C. In this last process step, the back of the glass substrates can be attacked , In this case, for example, the SO ₂ or SeO ₂ fraction contained in the sulfur or selenium vapor can react with the sodium ions in the soda lime glass surface to form water-soluble Na ₂ SO ₄ or Na ₂ SeO ₄ , as a result of which the glass surface can be damaged sigmfikant-geschädigtwer_d.en_kann. In addition, cracking of the IITL layer may occur, for example due to heat inhomogeneities of the layer package during the coating process, spatially non-homogeneous diffusion of the alkali ions from the glass into the layer or generation of mechanical stresses of the glass in the event of rapid cooling. In particular with regard to the temperature profiles, the scale up from the laboratory scale (10 x 10 cm ² ) on industrial scale (currently 125 x 65 cm ² ) is not completely mastered. Another disadvantage of this deposition method is that frequently a detachment of the absorber layer from the back contact layer is observed, which can already lead to a poor yield during solar cell production, but in particular occurs in outdoor applications, that is as a result of thermal cycling in the day / night or season change. From US 4,915,745 or DE 43 33 407 it is known that an adhesion promoter can be created by means of intermediate layers. However, it would be desirable to dispense with such an additional intermediate step.

Corrosion resistance is a key issue for thin film solar cells in general and for CIGS semiconductor based solar cells in particular. Corrosion-inducing processes can be: the handling of the glass samples, the external weathering, in particular with regard to long-term stability requirements (up to 20 years) and the CIGS deposition process itself, since such corrosion effects occur particularly when the substrate is exposed to high temperatures in an S / Reinforce se atmosphere.

The object of the invention is therefore to find a comparison with the prior art improved thin-film solar cell. Furthermore, it is an object of the invention to find a comparison with the prior art improved method for producing a thin-film solar cell. The solar cell according to the invention should also be economical to produce by means of known methods or by means of the method according to the invention and have a higher efficiency.

It is another object of the present invention to provide a method for producing a high-efficiency thin-film solar cell on a highly corrosion-stable, temperature-resistant and functional substrate glass, wherein the HaI- bleiterdepositionprozess at least one high-temperature step, ie a temperature of> 55O ⁰ C, should include. Requirements that are placed on the invention are, moreover, the overcoming:

the temperature limitation by the glass substrate with simultaneous CTE adaptation to the layer system,

thermally induced substrate glass warping, in particular in the case of flat modules, as occurs at high temperatures in the case of soda lime substrate glass,

of semiconducting poisons which can be incorporated into the semiconductor layer at high temperatures during the deposition process, as described in the prior art DE 100 05 088, DE 196 16 679,

DE 196 16 633 corresponding glass substrates is the case,

the spatially and temporally inhomogeneous alkali ion entry into the semiconductor layer during the deposition process without additional process steps, in contrast to WO 94/07269,

the thickness limitation of the glass substrate by both the insufficient rigidity of the glass substrate itself and the process conditions in the deposition,

- of corrosion problems,

- of liability problems,

- of inhomogeneities during crystal growth itself,

the process time limitation, in particular during the cooling process, but also faster separation process (throughput),

- not sufficiently high efficiencies,

- of low yields.

The object is achieved according to claim 1 by means of a thin-film solar cell comprising at least one Na ₂ O-containing multi-component substrate glass, wherein the substrate glass is not phase-separated and has a content of ß-OH of 25 to 80 mmol / l. Furthermore, it has been shown that it is advantageous if the substrate glass of the solar cell according to the invention

a transformation temperature Tg of greater than 550 ^° C., in particular of greater than 600 ^° C., and / or

- a coefficient of thermal expansion 0 _20/300 of greater than 7.5 x 10 ^"6 / K, in particular from 8.0 x 10 ^{" 6} ZK to 9.5 x 10 ^"6 / K, in the temperature range from 20 ⁰ C to 300 ⁰ C. and / or

less than 1% by weight of B ₂ O ₃ , less than 1% by weight of BaO and less than 3% in total of CaO + SrO + ZnO (sum of CaO + SrO + ZnO <3% by weight) contains and / or

- Has a molar ratio of the substrate glass components (Na ₂ O + K ₂ O) / (lvlgO + CaO + SrO + BaO) greater than 0.95 and / or

- Has a molar ratio of the substrate glass components Siθ ₂ / Al ₂ θ3 of less than 8.8, in particular of less than 7.

It is particularly advantageous if all the features mentioned above are fulfilled.

Furthermore, it has been found that the solar cell can be a planar, curved, spherical or cylindrical thin-film solar cell. Preferably, the solar cell according to the invention is a substantially planar (flat) solar cell or a substantially tubular solar cell, wherein preferably flat substrate glasses or tubular substrate glasses are used. In principle, the solar cell according to the invention is subject to no restriction with regard to its shape or to the shape of the substrate glass. In the case of a tubular solar cell, the outer diameter of a tubular substrate glass of the solar cell is preferably 5 to 5 mm, and the wall thickness of the tubular substrate glass is preferably 0.5 to 10 mm.

With regard to the method, the object is achieved according to claim 4 as follows. The inventive method for producing a thin-film solar - -

Cell, in particular a solar cell according to claim 1 or 2, comprises at least the following steps: a) providing a Na ₂ O-containing multicomponent substrate glass, wherein the substrate glass has a content of β-OH of 25 to 80 mmol / l and wherein the substrate glass is not phase-demixed b) applying a metal layer to the substrate glass, wherein the metal layer forms an electrical back contact of the thin-film solar cells, c) applying an intrinsically p-type polycrystalline layer of a compound semiconductor material, in particular a CIGS

Composite semiconductor material comprising at least one high-temperature step at a temperature> 550 ⁰ C, d) applying a p / n transition, in particular via a combination of buffer layer and subsequent window layer.

In the case of non-monolithically integrated series connection, preferably a metallic front side contact is applied.

The term metal layer here includes all suitable, electrically conductive layers.

The solar cells according to the invention and the solar cells produced by the process according to the invention have an over 2% absolute higher efficiency compared to the prior art.

Step b) preferably comprises applying a metal layer to the substrate, wherein the metal layer forms an electrical back contact of the thin-film solar cells, and is a single-layer or multi-layer system of suitable materials, more preferably a monolayer system of molybdenum ,

Step c) preferably comprises the application of an intrinsically p-type polycrystalline layer of a compound semiconductor material, particularly preferably based on CIGS, with at least one high-temperature step in the temperature range 550 ⁰ C <T <700 ⁰ C, particularly preferably 600 ° C <T <700 ° C.

Step d) preferably comprises the application of an intrinsically n-type buffer layer of a semiconducting material, more preferably of CdS, In (OH), InS o.a. and a window layer of a transparent conductive material, more preferably ZnO: Al, ZnO: Ga or SnO: F, said window layer consisting of an intrinsic layer and a heavily doped layer.

A substrate glass is not phase-separated in the sense of this invention if it has less than 10, preferably less than 5, surface defects in a surface area of 100 × 100 nm ² after a conditioning experiment. The conditioning experiment was carried out as follows: At 500 to 600 ⁰ C, a flow of compressed air in the range between 15 to 50 ml / min and a flow of sulfur dioxide gas (SO ₂ ) in the range 5 to 25 ml / min, for a period of 5 to 20 minutes, the substrate glass surface to be examined is gassed. Irrespective of the glass type, a crystalline coating forms on the substrate glass. After washing off the crystalline coating (eg by means of water or an acidic or basic aqueous solution so that the surface is not further attacked), the surface defects per substrate glass surface area are determined microscopically, as shown for example in FIG. If less than 10, in particular less than 5, surface defects are present in a surface area of 100 × 100 nm ² , the substrate glass is considered not to be phase-separated. All surface defects which have a diameter of> 5 nm are counted.

The β-OH content of the substrate glass was determined as follows. The equipment used for the quantitative determination of the water over the OH stretching vibration around 2700 nm is the commercially available Nicolet FTIR spectrometer with attached computer evaluation. It was first measured the absorption in the wavelength range of 2500-6500 nm and determined the absorption maximum around 2700 nm. The absorption coefficient α was then calculated with the sample thickness d, the net transmission Tj and the reflection factor P: α = 1 / d * Ig (1 / Ti) [cm ^-1 ], where T ₁ = T / P with the transmission T. The water content is further calculated from c = α / e, where e is the practical extinction coefficient [l * mol ^"1 * cm ^{" 1} ] and is used for the abovementioned evaluation range as a constant value of e = 110 l * Mor ¹ * cm ^{" 1} based on moles of H ₂ O used. The e value is taken from the work of H. Frank and H. Scholze from the "Glastechnische Berichte", Volume 36, Volume 9, page 350.

Thin-film solar cell is briefly referred to in the text for simplicity as solar cell, also in the dependent claims. Substrate glass in the sense of this application may also comprise a superstrate glass.

With Na ₂ θ-containing multicomponent substrate glass in the sense of this invention is meant that the substrate is glass in addition to Na ₂ θ further composition components such as B ₂ O _3, BaO, CaO, SrO, ZnO, K ₂ O, MgO, SiO ₂ and Al ₂ O ₃ , but also non-oxide components such as F, P, N may contain.

This invention enables the development of a low-cost, high-efficiency monolithic integrated photovoltaic module based on compound semiconductors, such as CdTe or CIGS. Cost-effective refers in the context of the invention to a possible small € / watt costs due mainly by higher efficiencies, faster process times and thus higher throughput, and higher yields.

The invention includes a substrate glass, in addition to its carrier function plays an active role in the semiconductor manufacturing process and in particular by an optimal CTE adaptation at high temperatures to the photoactive compound semiconductor thin film, and a high thermal (ie a high stiffness ness) and chemical (ie high corrosion resistance) stability.

The invention also includes tandem, multijunction or hybrid thin film solar modules from a high temperature process deposited on a substrate glass, and a process for making such modules. Furthermore, it is included in the invention that the solar module can have both flat, spherical, cylindrical or other geometric shapes. In particular embodiments, the glass may be colored.

Preferred technical features of the substrate glass included in the invention are: (i) highly corrosion resistant, (ii) material without spatial phase separation, (iii) As, B free, (iv) high temperature stable, (v) coefficient of thermal expansion (CTE) adjusted, (vi ) Na content, (vii) Mobility Na in glass, (viii) Stiffness (Ew-Tg)> 200 ⁰ C.

Preferred technical features of the process: (i) large-area process, (ii) high temperatures (> 55O ⁰ C ₁ especially> 600 ⁰ C), (iii) more homogeneous process, ie faster process times and thus higher throughput, (iv) higher yield.

Preferably, the method according to the invention for producing a thin-film solar cell comprises at least one or all of the following steps:

a) providing a substrate which meets the required conditions. b) cleaning and preconditioning of the substrate glass by acid leaching of surface impurities and near-surface sodium ions in hydrochloric acid wash lotion, c) forming a metal layer on the substrate, the metal layer forming an electrical back contact in the thin-film solar cell structure and preferably a monolayer system is, without structural steps or fractures, d) forming an intrinsically p-type polycrystalline layer of a compound semiconductor material, particularly preferably based on CIGS, with at least one high-temperature step, e) forming a p / n junction by introducing a thin buffer layer, for example a few nm thick CdS layer, and following an n-type, transparent TCO, such as ZnO or ZnO: Al or their combination. f) forming a monolithic series connection between the various deposition steps or applying a front contact grid consisting of metal fingers and bus bars, g) encapsulation of the thin-film module.

Surprisingly, high-alkali aluminosilicate glass systems met the requirements for a substrate glass for the thin-film solar cell produced in a high-temperature process. In a particular embodiment, the high-temperature CIGS manufacturing technology could be brought to the application with SubstratgJastempera- temperatures of up to 700 ⁰ C, at the same time was adjusted to the CIGS semiconductor layer of the CTE of the substrate. Accordingly, 2% higher efficiencies of CIGS cells could be achieved compared to the standard process at temperatures - 525 ° C.

The requirements for the substrate glass for a production process by means of a high-temperature step are particularly well met by glass compositions (mol .-%) in the following range:

SiQ ₂ 61 - 70.5

AI ₂ O ₃ 8.0 - 15.0

B ₂ O ₃ 0 - 4.0

Na ₂ O 0.5 - 18.0

K ₂ O 0.05 - 10.0

Li ₂ O + Na ₂ O + K ₂ O 10.0 - 22.0 MgO 0-7.0

CaO 0-5.0

SrO 0-9.0

BaO 0-5.0

MgO + CaO + SrO + BaO 0, in particular> 0.5, preferably> 5

CaO + SrO + BaO + ZnO 0.5-11.0

TiO ₂ + ZrO ₂ 0-4.0

SnO ₂ + CeO ₂ 0-0.5, in particular 0.01-0.5, preferably 0.1-0.5

As ₂ O ₃ + Sb ₂ O ₃ + P ₂ O ₅ + La ₂ O ₃ 0-2.0

F ₂ + Cl ₂ 0-2, in particular 0-1.0 β-OH content (mmol / liter) 25-80

SiO ₂ / Al ₂ O ₃ 4,2-8,8

Alkali oxides / Al ₂ O ₃ 0.6-3.0

Alkaline earth oxides / Al ₂ O ₃ 0.1-1.3

Number of surface defects <10

The glasses were melted in 4-liter platinum crucibles from conventional raw materials. To ensure a certain constituent of water in the glass, the AI raw material AI (OH) _{3 was} used and also an oxygen burner in the furnace chamber of the gas-heated melting furnace (Oxyfueltechnik) was used to achieve the high melting temperatures at oxidizing melt guide. The raw materials were placed over a period of 8 hours at melting temperatures of 1580 ⁰ C and then held for 14 hours at this temperature. The molten glass was then poured within 8h to i4GQ G the mixture is cooled! Tund-anschließendJn a 500 ⁰ C preheated graphite mold stirring. This mold was placed immediately after casting in a pre-heated to 650 ° C cooling oven, which cooled to 5 ° C / h to room temperature. The glass samples necessary for the measurements were then removed from this block. Surprisingly, it has been found that these glasses have a high homogeneity with regard to blowing when melting under oxidizing conditions when nitrates of the alkali and / or alkaline earth metal components are used.

Table 1: Exemplary embodiments of substrate glasses, as used according to the invention, composition components in mol%, molar ratios.

Example 1 2 3 4 5 6 7

SiO ₂ 64.88 68.65 66.32 63.77 66.26 66.83 70.04

AI ₂ O ₃ 11, 07 11, 2 7,96 11,01 10,91 10,91 13,22

B ₂ O ₃ 0.45 3.65 0 0 0 0 0

Li ₂ O 2.49 0.49 0 0 0 0 1, 06

Na ₂ O 11, 61 8.02 3.57 12.59 11, 3 11, 3 3.52

K ₂ O 6.07 1, 34 8.5 3.58 3.82 3.82 5.14

MgO O O 6.56 3.25 3.25 0 0.3

CaO 0.56 4.53 0 0 0.12 0.12 1, 63

SrO O 0.31 7.98 0 0 2.0 0

BaO O O 2.22 0 2.0 0 1, 38

ZnO 4.0 0.4 0 0 0 0 0

TiO ₂ + ZrO ₂ OO 3.41 0.66 1, 23 0.66 2.68

SnO ₂ + CeO ₂ 0.14 0.16 0.02 0.02 0.19 0.19 0.14

^■ F2 + GI2- 0.1 0 0.2 0.5 0.59 0.59 0

As ₂ O ₃ + Sb ₂ O ₃ + P ₂ O ₅ + La ₂ O ₃ O 1.0 0.05 0.35 0.33 0.33 0

CaO + SrO + ZnO 4.56 5.24 7.98 0 0.12 2.12 1, 63

(Na ₂ O + K ₂ O) /

(MgO + CaO + SrO + BaO) 31, 55 1, 88 0.72 4.98 2.82 7.13 2.61

SiO ₂ / Al ₂ O ₃ 5.9 6.1 8.3 5.8 6.1 6.1 5.3 - -

α 20/300 (10 ^"6 / K) 8.9 7.55 8.5 8.6 9.1 8.7 7.55

Tg ( ⁰ 595 573 655 610 593 579 661

Ew ( ⁰ C) 812 763 898 852 821 822 884

Ew-Tg ( ⁰ C) 217 190 243 242 228 243 223

Number of surface defects << 1100 <10 <10 <10 <10 <10 <10 β-OH content in mmol / l 52 51 47 31 26 29 63

The molar ratios of the two glass formers SiO ₂ to Al ₂ O ₃ are responsible for the achievement of high operating temperatures of the substrate glasses, since they determine the slope of the viscosity in the transformation point (Tg) to the softening point. Such so-called long glasses can be thermally stressed not only up to the transformation point without deformation but also up to about 100 ⁰ C below the softening point (Ew) of the glasses. Thus, even when using high temperatures, ie> 550 ° and <700 ° C, it can be ensured that no thermally induced substrate swellings occur. However, the essential requirement of the CTE adaptation to the subsequent layer system must be solved simultaneously. Decisive, especially for the high expansion coefficient of Boroalumi- silikatgläser, is the molar ratio of the sum of alkali metal ions to Al ₂ O ₃ . Here meets only surprisingly found here very close ratio alkali oxides / alumina of 0.6 to 3.0 the two requirements of high Tg between 580 and 680 ⁰ C and at the same time high thermal expansion coefficient greater than 7.5 x 10 ^"6 ZK and thus fulfills the required CTE.

In the manufacture of semiconductors in general, it is extremely critical when ^■ Malbleitergifte-in-the-P Fozess-go, -. Diejdie_se reduce dje performance of the film dramatically. In the production of CIGS-based solar cells in the high-temperature process, it is important to prevent semiconductor poisons, such as iron, arsenic or boron, from outgassing or diffusing out of the glass, since these elements can become active recombination centers and lead to a drop in open circuit voltage and short-circuit current , surprisingly - -

For example, it was found that the glasses of the above glass compositions meet exactly the requirements, as they are iron-free, but a water content of> 25 mmol / liter, preferably> 40 mmol / liter and more preferably> 50 mmol / liter , Thus, the semiconductors are chemically bound and can not get into the process even at temperatures> 550 ⁰ C.

The water content can be determined with commercially available spectral measuring devices in the wavelength range from 2500 to 6000 nm using appropriate calibration standards.

FIG. 1 shows by way of example the water content (β-OH) of a glass substrate according to the invention in comparison to the prior art. Infrared measurement in the wavelength range 2500 - 6000 nm with the maximum of the β-OH absorption of the water at 2800 nm from a soda-lime glass, a glass according to JP 11-135819A and example glass 4.

The selective delivery of alkali ions, particularly sodium, both in time and space (over the coating surface) homogeneously throughout the semiconductor deposition step, is critically important for the fabrication of high efficiency compound solar cell based solar cells, especially under the condition that additional process steps, such as the addition of sodium, should be eliminated to realize a cost-effective process.

Surprisingly, it has been found that this is achieved only with substrate lenses that no spatial phase separation with alkali-rich or alkaline iiarmen ^~ areas-comprise invGegensatz for example, boron-containing AIu- minosilikatgläsern or low water aluminosilicates as described in DE 100 05 088 DE 196 16 679, DE 196 16 633. The substrate glass should release Na ions / Na atoms at temperatures around the Tg, which requires an increased mobility of the alkali ion. Surprisingly, it was found that the mobility of the alkali ions in water-containing glasses, such as those of the above composition, despite the increased proportion of alkaline earth ions, which meet the requirement of high Tg with high thermal expansion, however, the diffusion of smaller sodium Ions in the glass structure by the alkaline earth ions hinder, continues to exist. The ion mobility of the sodium ions and their easier exchangeability in the glasses according to the invention is positively influenced especially by the residual water content in the glass structure, which with the selection of water-rich raw materials in the crystal lattice such as by Al (OH) 3 instead of Al ₂ O ₃ and by Oxygen-rich gas atmosphere in the melting process, also known as oxyfuel, can be realized. Surprisingly, it was found that the found ratio of SiO ₂ / Al ₂ O _{3 is also} necessary for a high alkali ion mobility.

For substrates which show no phase separation but high alkali-ion mobility, the alkali ions can be spatially homogeneously distributed over the entire substrate surface to the overlying layers or diffused therethrough. The release of the alkali ions does not break even at higher temperatures,> 600 ⁰ C, not from. In addition, such a substrate exhibits improved adhesion properties for the functional layers of molybdenum and compound semiconductors deposited thereon. In a high-temperature process, compound semiconductor layers can grow up ideally, ie homogeneous crystal growth over the surface and thus a higher yield can be realized, as well as ensuring a sufficiently large alkali ion reservoir during the deposition process. In a further conditioning step alkali ions can be exchanged _L K example, specifically in the upper region-of the glass substrate ^, Li by Na or vice versa. Thus glasses of different compositions, s. Table 1, are conditioned so that they allow a spatially and temporally homogeneous alkali ion delivery of exactly one species. Composite semiconductor thin film solar cells, especially those produced in a high temperature step under a corrosive atmosphere, must have a high corrosion resistance. Unexpectedly, it was found that a hydrolytic resistance of the above-described glasses of <0.5 μg / g Na ₂ O significantly reduces the risk of corrosion. The hydrolytic resistance is determined according to DIN ISO 719. Here, the substrate glass is ground into Glasgries with 300 to 500 microns grain size and then placed for one hour in 98 ⁰ C hot, demineralized water. The aqueous solution is then analyzed for the alkali content.

These glasses, like soda-lime glasses, show a reaction with SO2 / SeO2, but in contrast to the soda-lime glasses, without any visible corrosion of the surface, as when cleaning with water. In Figure 2, a corroded glass surface (pictured on the left, soda lime substrate glass) is shown as compared to the uncorroded surface (shown on the right) of a substrate glass as appropriate for a solar cell according to the present invention. This is due not only to the high mobility of the sodium ions in the glass lattice, which are replenished in the reaction with the chalcogenide oxides from deeper layers below the surface, the phase stability of the glass. This allows a homogeneous diffusion of the sodium ions to the surface and thus prevents a visibly corroded surface.

The so-called stiffness (dimensional stability at high temperatures> 600 ° C.) can be estimated, inter alia, by the difference between Ew-Tg (in ⁰ ° C.) Requirement of at least 200 ^° C. is necessary in order to obtain thinner substrates than the currently used 3 bis-3 ₇ 5-mm _r dτh. Thinner substrate glasses <2.5 mm, _zu allow. as a result, the cooling section can be, for example, after the coating process of> 600 ⁰ C significantly reduce to room temperature, which reduces processing time and investment costs. also mean lower Material and manufacturing costs for the substrate glass itself, which is the price difference compared to Kalknat- reduces glass lens and thus contributes to a better competitiveness of these substrate glasses.

The substrate glass of the above composition was prepared and formulated so that it has a high dimensional stability at temperatures> 600 ⁰ C. This dimensional stability can be expressed by the so-called stiffness, which is indicated, inter alia, by the modulus of elasticity of the glasses of> 70 kN / mm ² and by the large difference between softening point (EW) and transformation point (Tg). Surprisingly, it was found that with a temperature difference of Ew-Tg of> 200 ⁰ C allows a reduction of the substrate glass thickness of the prior art with 3 to 3.5 mm to less than 2.5 mm without loss of rigidity of the substrate glass.

By this substrate glass thickness reduction, a faster heat transfer through the substrate glass can be realized, which allows accelerated process control in the semiconductor deposition process and thus savings in process time. In particular, this allows, for example, the cooling section to be significantly reduced, which, in addition to the process times, also significantly reduces the investment costs. Thinner substrate glasses also mean lower material and manufacturing costs for the substrate glass itself, and can lead to a more positive cost balance in the production of solar cells through lossless transport of the substrate glasses including layers in in-line systems. Bent substrate glass is problematic, for example, in process chamber locks and can lead to a significant yield loss. In addition, it is of enormous advantage for the lamination process, if the solar cells are not bent, again, not quite planar substrate glass can lead to a yield loss.

In figure 34stder_Einfluss _^ derJ3 | shown as_kojτipjDnenten and especially the influence of the component Al2O3 a Alumosilikatsubstratglases on the modulus of elasticity (kN / cm ²⁾ (after http://glassproperties.com). In addition to the basic mobility of the alkali ions, the diffusion paths of the overlying layers are of crucial importance, for example through the back contact layer into the semiconductor layer. Surprisingly, it has been found that the avoidance of structural steps and / or fractures in the back contact layer, as solved in the invention for example by a single-stage back contact layers, is of central importance for this purpose. This is particularly important for ensuring a spatially and temporally homogeneous alkali-ion distribution.

It is known that substrate glass surfaces age over time and lose their originally active surface. Surprisingly, it has been found that coating the glass surface with a metal film preserves this activity. This is especially true for a coating with tungsten, silver, vanadium, tantalum, chromium, nickel, more preferably molybdenum. The metal film has a thickness of 0.2 to 5 microns, more preferably 0.5 to 1 micron and a conductivity of 0.6 x 10 ⁵ to 2 x 10 ⁵ Ohm.cm, more preferably 0.9 x 10 ⁵ bis 1, 4 x 10 ⁵ ohm.cm.

Furthermore, it was surprisingly found that due to the unobservable phase separation (as described above) of the high temperature stable substrate glasses of the above composition with a corresponding stability against crystallization a particularly good adhesion of the metal back contact on the substrate glasses. Frequently observed adhesion problems with soda lime glasses, such as the detachment of the layer in some places, also called chocolate paper, did not result for substrates coated with the metal back contact according to the invention, particularly preferred when the metal back contact is a single layer system with few or no structural steps. Surprisingly, it has been found that the unobserved phase separation of the above-mentioned substrate glasses also leads to an excellent adhesion of the CIGS layer on the metal back contact compared to conventional substrates. In the so-called sequential process cavities were often found on soda-lime glass at the interface CIGS layer / back contact, so-called underground garages, only small islands serve the adhesion. In contrast, for the invention solar cells on the basis of the above-mentioned substrate glasses, in particular in conjunction with a high temperature step, a full-surface adhesion were due to the spatially and temporally homogeneous sodium ions delivery of the substrate glasses and spatially temporally homogeneous by avoiding structural steps spatially temporal diffusion of these through the metal back contact layer.

A substrate glass with a higher Tg than standard soda-lime glass allows for higher process temperatures during semiconductor deposition. It is known that higher deposition temperatures during chalcopyrite formation can lead to a significant minimization of crystal imperfections, to below the detection limit, such as the so-called CuAu order. This is especially true for the sequential process described above. Surprisingly, the semiconductor layers of the solar cells according to the invention with a substrate glass of the above composition and wherein a semiconductor layer was deposited at temperatures> 600 ⁰ C meet the demand for a higher crystallinity and thus fewer defects. This can be seen in FIG. 4 on the basis of the Ramaπspektreπ. In FIG. 4, the A1 mode of a CIGS absorber layer is deposited according to the invention at high temperatures, and the A1 mode of a CIGS layer deposited on soda-lime glass is shown. The lower half-width of the solar cell according to the invention is a direct measure of better crystal quality and thus fewer defects. The mode shows in the CIGS layer according to the invention in the high-temperature step (T> 550 ⁰ C) deposited on the basis of a substrate glass of the composition described lower half-width than a CIGS layer in the conventional process prepared on a Kalknatronsubstratglas (larger half-width).

Surprisingly, it has been shown that higher process temperatures also allow for faster processing. In particular, processes at the crystal formation front run faster and, for example, the incorporation of the elements on the corresponding crystal sites is accelerated. In the case of sequential processing, one essential mechanism is the diffusion of the individual - -

Atoms to the surface where the reactions with the chalcogen atoms take place. A higher temperature causes a higher diffusion rate of the elements to the reaction surface, and thus a faster transport of the elements required for crystal formation to the crystallization front. Typical ramp ramps are in the range 5 to 10 K / s, with hold times to maximum temperature of about 5 minutes and typical cooling ramps in the range of 3 to 4 K / s. Surprisingly, it was found that based on the substrate glasses with the above composition / heating ramps> 10 K / s could be realized and especially cooling profiles> 4 K / s, more preferably> 5 K / s. Furthermore, it was found that despite accelerated heating and cooling ramps and a maximum temperature significantly greater than 550 ⁰ C no outgassing from the substrate glasses were found with the above composition in contrast to conventional substrate glasses such as soda lime glasses.

FIG. 5 shows a solar cell produced according to the prior art, in particular an SEM micrograph of a cross section through the zonal structure of a multilayer molybdenum layer (three-layer process sequence) on a substrate glass (left in the image). Visible here are three steps in the molybdenum layer (center).

FIG. 6 shows a solar cell according to the invention, in particular a SEM micrograph of a cross section through the columnar, stepless structure of a molybdenum layer of a solar cell according to the invention, the molybdenum layer being applied by means of a single-layering process.

Claims

claims

1. thin-film solar cell comprising at least one Na ₂ O-containing multi-component substrate glass, characterized in that the substrate glass is not phase-separated and has a content of β-OH of 25 to 80 mmol / l.

2. Thin-film solar cell according to claim 1, characterized in that the substrate glass

- a coefficient of thermal expansion α ₂ o _{/ 3} oo of greater than 7.5 x 10 ^"6 ZK, in particular from 8.0 x 10 ^{" 6} / K to 9.5 x 10 ^"6 / K, in the temperature range from 20 ⁰ C to 300 ⁰ C and / or,

contains less than 1% by weight of B ₂ O ₃ , less than 1% by weight of BaO and less than 3% in total of CaO + SrO + ZnO and / or

has a molar ratio of the substrate glass components (Na ₂ O + K ₂ O) / (MgO + CaO + SrO + BaO) greater than 0.95 and / or

- Has a molar ratio of the substrate glass components SiO ₂ / Al ₂ O ₃ of less than 8.8, in particular of less than 7.

3. Thin-film solar cell according to claim 1 or 2, characterized

^"That the solar cell-a-planar _^ _^ gew.ölbt spJiärisch or cylindrically shaped thin-film solar cell.

4. A process for producing a thin-film solar cell, characterized in that the process comprises the following steps: a) providing a Na ₂ O-containing multicomponent substrate glass, wherein the substrate glass has a content of β-OH of 25 to 80 mmol / l and wherein the substrate glass is not phase-separated, b) applying a metal layer to the substrate glass, wherein the metal layer is an electrical C) applying an intrinsically p-type polycrystalline layer of a compound semiconductor material, in particular of a CIGS compound semiconductor material, comprising at least one high-temperature step at a temperature> 550 ⁰ C, d) applying a p / n transition.