Patent Application
20100288351
The subject matter described and claimed herein below is also described in German Patent Application No. 10 2009 020 955.7, filed on May 12, 2009 in Germany, and German Patent Application No. 10 2009 050 988.7, filed on Oct. 28, 2009 in Germany. These German Patent Applications provide the basis for respective claims of priority of invention for the thin-film solar cell and process claimed herein below under 35 U.S.C. 119 (a)-(d).
1. The Field of the Invention
The invention relates to a thin-film solar cell.
2. The Description of the Related Art
The future market development of photovoltaics, in particular for photovoltaic plants connected to the grid, is critically dependent on the cost reduction potential in the production of solar cells. A great potential is seen in the production of thin-film solar cells, since significantly less photoactive material is required for efficient conversion of sunlight into electricity than in the case of conventional crystalline, silicon-based solar cells. In thin-film solar cells, photoactive semiconductor materials, especially indirect semiconductors such as silicon-based materials (a distinction is made here between amorphous or microcrystalline and crystalline silicon or layers thereof) and direct semi-conductors such as highly absorbing compound semiconductors of groups II to VI of the Periodic Table of the Elements (for example CdTe) or of groups Ito III to VI2, e.g. Cu(IN1-xGax)(Se1-ySy)2 (CIGS) are deposited on inexpensive, sufficiently heat-resistant substrates, e.g. molybdenum-coated substrate glasses, in layers of a few μm in thickness. The cost reduction potential is especially based on the lower semiconductor material consumption and the great ability to automate production. However, the efficiencies of commercial thin-film solar cells which have hitherto been achieved remain significantly behind those of crystalline, silicon-based solar cells (thin-film solar cells: about 10-15% efficiency; crystalline silicon-based solar cells comprising silicon wafers: about 15-18% efficiency).
Apart from solar cells comprising soda-lime float glasses as substrate glass for thin-film photovoltaic applications, solar cells having other substrate glass types or further substrate glass types which are said to be suitable for photovoltaics are also known.
DE 699 16 683 T2 discloses substrate glasses for VDUs having a coefficient of thermal expansion of from 6.0×10−6/K to 7.4×10−6/K in the temperature range from 50° C. to 350° C. which are also said to be suitable for solar cells.
Solarization-stable aluminosilicate glasses having a total content of CaO, SrO and BaO of from 8 to <17% by weight as substrate for solar collectors are disclosed in EP 0 879 800 A1.
Thin-film solar cells, in particular on the basis of compound semiconductors, comprising a glass substrate having a coefficient of thermal expansion of from 6×10−6/K to 10×10−6/K are disclosed in JP 11-135819 A. The glass substrate here has the following composition in percent by weight: SiO2 from 50 to 80, Al2O3 from 5 to 15, Na2O from 1 to 15, K2O from 1 to 15, MgO from 1 to 10, CaO from 1 to 10, SrO from 1 to 10, BaO from 1 to 10, ZrO2 from 1 to 10, and is characterized by an “Annealing Point” (temperature at a viscosity of the glass of 1013 dPas) of greater than 550° C.
Substrate glasses for use in thin-film photovoltaics, in particular on the basis of compound semiconductors, are disclosed in DE 100 05 088 C1. The glasses have a B2O3 content of from 1 to 8% by weight and a total content of alkaline earth metal oxides (MgO, CaO, SrO and BaO) of from 10 to 25% by weight.
It is an object of the invention to provide a thin-film solar cell which is improved over the prior art. The solar cell of the invention should also be able to be produced economically by known processes and it should have a higher efficiency.
This object is achieved by a thin-film solar cell comprising at least one Na2O-containing multicomponent substrate glass, The Na2O-containing multicomponent substrate glass (substrate glass) must have at least all of the following features:
A thin-film solar cell will hereinafter be referred to as a solar cell in the interests of simplicity, including in the dependent claims. For the purposes of the present patent application, the term substrate glass can also encompass a superstrate glass.
For the purposes of the present invention, the expression Na2O-containing multicomponent substrate glass means that the substrate glass can contain not only Na2O, but also additional composition components, such as B2O3, BaO, CaO, SrO, ZnO, K2O, MgO, SiO2 and Al2O3, and also nonoxidic components, e.g. anionically bound components such as F, P, N.
Such solar cells according to the invention can be produced by known processes, with the process parameters possibly having to be adapted. Known processes for producing the semiconductor layers on the substrate glass or on a previously coated substrate glass are, for example, the sequential process (reaction of metallic layers in a chalcogen atmosphere), co-vaporization (virtually simultaneous vaporization of the individual elements or element compounds) and liquid coating processes with a subsequent heating step in a chalcogen atmosphere. It has surprisingly been found that, particularly in the deposition of the semiconductor layers, it is possible to use far higher process temperatures than in the case of conventional soda-lime substrate glasses without the substrate glass becoming disadvantageously deformed for a later lamination process, and the solar cells of the invention have an efficiency which is over 2% absolute higher than that of known solar cells having soda-lime substrate glasses.
It has been found that a B2O3 content of the substrate glass of above 1% by weight has an adverse effect on the efficiency Of the solar cell. Boron atoms can presumably migrate from the substrate glass into the semiconductor by vaporization or diffusion. This presumably leads to defects within the semi-conductor layer which are electrically active and cause increased recombination, as a result of which the performance of the solar cell is reduced.
On the other hand, a content of BaO of less than 1% by weight and a content of one or all of the following substrate glass components CaO, SrO and/or ZnO of less than 3% by weight (sum of CaO+SrO+ZnO<3% by weight, preferably <0.5% by weight) have a positive effect on the mobility of the sodium ions in the substrate glass during production of the solar cell, which leads to an increase in the efficiency of the solar cell. It is important that the molar ratio of the substrate glass components, (Na2O+K2O)/(MgO+CaO+SrO+BaO), must be greater than 0.95, preferably from >0.95 to 6.5, in order to increase the efficiency of the solar cell of the invention compared to a known solar cell.
The solar cell of the invention preferably comprises a substrate glass, which contains less than 0.5% by weight of B2O3, in particular no B2O3 apart from unavoidable traces. Furthermore, the solar cell of the invention preferably comprises a substrate glass which contains less than 0.5% by weight of BaO, in particular no BaO apart from unavoidable traces. For particular solar cells, it is advantageous for the substrate glasses to be free of B2O3 and/or BaO apart from unavoidable traces, in particular for less than 1000 ppm of B2O3 and/or less than 1000 ppm of BaO to be present.
In a further preferred embodiment of the invention, the solar cell comprises a substrate glass which contains a total of less than 2% by weight of CaO+SrO+ZnO in the substrate glass components, which leads to a higher mobility of the alkaline metal ions in the substrate glass during production of the solar cell and thus to a more effective solar cell.
The solar cell preferably comprises a substrate glass containing at least 5% by weight of Na2O, in particular at least 8% by weight of Na2O.
In a further preferred embodiment, the solar cell comprises a substrate glass containing not more than 18% by weight of Na2O and preferably not more than 16% by weight of Na2O.
The molar ratio of the substrate glass components SiO2/Al2O3 is preferably less than 6 and greater than 5.
According to the invention, the solar cell preferably has an aluminosilicate substrate glass, in particular an aluminosilicate substrate glass having a glass transition temperature Tg of >550° C., which comprises the following composition components (in mol %):
wherein the components are present in the substrate glass in the following molar ratios:
Furthermore the solar cell of the invention preferably has an aluminosilicate substrate glass which comprises the following composition components (in mol %):
wherein the components are present in the substrate glass in the following molar ratios:
Apart from these preferred compositions, the substrate glass can contain additional components customary in glass production, e.g. refining agents, in the customary amounts, in particular up to 1.5% by weight of sulphate and/or up to 1% by weight of chloride.
Furthermore, it is necessary for the solar cell to have a substrate glass having a coefficient of thermal expansion α20/300 of greater than 7.5×10−6/K, in particular from 8.0×10−6/K to 9.5×10−6/K, in the temperature range from 20° C. to 300° C. Thus, it has been found to be advantageous to match the coefficient of thermal expansion of the substrate glass to that of the photoactive semiconductor layer, for example a CIGS layer.
In a particular embodiment of the invention, the solar cell has a substrate glass which has an electrical conductivity of greater than 17×10−12 S/cm at 25° C., with the electrical conductivity of the substrate glass at 250° C. being greater by a factor of 104, preferably greater by a factor of 105 and particularly preferably greater by a factor of 106, than the electrical conductivity of the substrate glass at 25° C.
If Si-based or CdTe-based thin-film solar cells are produced according to the invention, the substrate glasses described are particularly well suited, since in the case of these substrate glasses ions can be exchanged, preferably by a chemical route. The sodium ions which are undesirable in these cases can thus easily be replaced by other ions, e.g. lithium or potassium ions. These substrate glasses are therefore also suitable for special CIGS solar cells in which Na is added as dopant (e.g. as NaF2), since they have an intrinsic Na barrier due to the ion-exchanged surface; an additional layer acting as a barrier layer is not necessary. For this purpose, the substrate glasses are, for example, dipped into a potassium salt melt, e.g. a KNO3 melt at from 400° C. to 520° C., for a particular time interval, which is determined essentially by the thickness of the exchange layer in the substrate. If dipping is carried out, for example, at 450° C. for 10 hours, a virtually sodium ion-free surface layer having a surface depth of at least 20 μm and having potassium ions on the sodium ion sites is formed on the surface of the substrate glass.
These ion exchange properties can also be utilized in fracture-resistant covering glasses for these solar cells according to the invention, with a compressive stress being generated in the surface by replacement of the smaller sodium ion by the larger potassium ion. This significantly improves the mechanical strength of the covering glass at an unaltered transparency.
In the solar cells of the invention, the sodium ions of the substrate glass are therefore preferably replaced at least partly by other cations, in particular by potassium ions, to a surface depth of 20 μm, so that the sodium ion content in the surface layer is reduced compared to the total sodium ion content of the substrate glass.
The substrate glass of a solar cell according to the invention is preferably coated with at least one molybdenum layer, with the molybdenum layer preferably having a thickness of from 0.25 to 3.0 μm, particularly preferably from 0.5 to 1.5 μm.
The solar cell is preferably a thin-film solar cell based on silicon or a thin-film solar cell based on compound semiconductor material, for example CdTe, CIS or GIGS.
Furthermore, it has been found that the solar cell can be a planar, curved, spherical or cylindrical thin-film solar cell.
The solar cell of the invention is preferably an essentially planar (flat) solar cell or an essentially tubular solar cell, with flat substrate glasses or tubular substrate glasses preferably being used. The solar cell of the invention is in principle not subject to any restrictions with respect to its shape or the shape of the substrate glass.
In the case of a tubular solar cell, the external diameter of a tubular substrate glass of the solar cell is preferably from 5 to 100 mm and the wall thickness of the tubular substrate glass is preferably from 0.5 to 10 mm.
In a further preferred embodiment of the invention, the solar cell has functional layers. The functional layers of the solar cell preferably comprise conductive and transparent conductive materials, photosensitive compound semiconductor materials, buffer materials and/or metallic back contact materials. If at least two solar cells are connected in series, a thin-film photovoltaic module is formed and is protected from environmental influences by encapsulation, in particular by encapsulation with SiO2, plastics and films, e.g. EVA (ethylene-vinyl acetate), surface coating layers or/and a further substrate glass. The further substrate glass can be the same substrate glass as is already present in the solar cell or else can be another substrate glass, e.g. a substrate glass which has been pre-stressed by ion exchange.
The solar cell preferably has at least one photoactive semiconductor which has been applied to the substrate glass or a previously coated substrate glass at a temperature of >550° C. This temperature is preferably less than the glass transition temperature Tg of the substrate glass.
The solar cell is preferably a thin-film solar cell based on compound semiconductors, as will be illustrated by way of example below.
The thin-film solar cells according to the invention based on II-VI or I-III-VI compound semiconductors, such as CdTe or CIGS of the general formula
Cu(In1-xGax)(S1-ySey)2
have a better crystallinity compared to the prior art and thus an increased open circuit voltage and a higher efficiency.
These compound semiconductors applied in the form of thin layers or packets of layers to the substrate glasses meet important prerequisites such as in the case of CIGS a band gap (1.0<Eg<2.0 eV) which is very well matched to the spectrum of sunlight by mixing of the ternary compounds and a high absorption of incident light (absorption coefficient >2×104 cm−1) for use thereof in solar cells.
Thin, polycrystalline layers or packets of layers of easily variable Cu(In1-xGax)(S1-ySey)2 compositions can in principle be produced in a number of stages by a series of processes (e.g. simultaneous vapor deposition of the elements, sputtering with a subsequent reactive gas step, CVD, MOCVD, co-vaporization, electro-deposition or liquid deposition with a subsequent heating step in a chalcogen atmosphere, etc.). As such, CIGS layers or packets of layers have intrinsic p conduction. The p/n junction in such material systems is then formed by introducing a thin buffer layer (e.g. a CdS layer or the like having a thickness of a few nanometers) and subsequently deposited n-conducting, transparent oxides (TCO=Transparent Conductive Oxides, e.g. ZnO or ZnO(AI)). To avoid parasitic absorption the buffer layer is made very thin, while the TCO layer additionally must have a high electrical conductivity in order to ensure virtually loss-free output of the current.
The efficiencies of Cu(In1-xGax)(S1-ySey)2 cells produced on a pilot or production scale are at present in the range from 10 to 15%. Customary module formats made up of individual solar cells connected in series in a monolithically integrated fashion have a size on the order of 60×120 cm2 while ensuring the homogeneity of the layers (thickness, composition) over the entire module area.
In one embodiment as shown in
The substrate glass preferably comprises an aluminosilicate glass as is known, for example, from the documents DE 196 16 633 C1 and DE 196 16 679 C1, but it must have the composition and properties recited in the appended claims. Also its coefficient of thermal expansion α20/300 must be matched to that of the semiconductor. A contact layer, here of metallic molybdenum, is applied to the substrate glass. The actual photoactive semiconductor is located thereon. On top of this, a buffer layer of, for example, CdS and on top of that a window (here a transparent, conductive layer (TCO)) through which sunlight can penetrate through to the semiconductor are applied.
An important requirement which a suitable substrate glass must meet results from the temperatures prevailing in the coating process. To achieve high deposition rates or a very good crystalline quality of the layers, the phase diagram of Cu(In1-xGax)(S1-ySey)2 indicates that temperatures above at least 550° C. are necessary. Higher temperatures, in particular temperatures above 600° C., lead to even better results with respect to the deposition rate and crystallinity of the layers. Since the substrate glass to be coated is generally positioned very close to a radiation source, in particular embodiments suspended over the vaporization sources used in the coating process, the substrate glass should have a very high heat resistance. As a rough guide, the glass transition temperature (Tg) in accordance with DIN 52 324 of the glass should accordingly be above at least 550° C. The higher the Tg, the lower the risk of deformation of the substrate glass during coating at temperatures close to Tg. A process temperature below Tg also prevents introduction of stresses into the substrate glass and thus into the layer system as a result of rapid cooling, which is usually the case in CIGS coating processes.
Not only the glass transition temperature (Tg), but also the viscosity behavior up to the softening temperature (ST), defined as the temperature of the glass at a glass viscosity of 107.6 dPas in accordance with DIN 52 312, has to be taken into account, with a very large difference between Tg and ST (“long glass”) reducing the risk of thermal deformation of the substrate at coating temperatures above 600° C.
To prevent splitting-off of the layer systems on cooling after the coating process, the substrate glass also has to be matched to the thermal expansion of the back contact (e.g. molybdenum, about 5×10−6/K) and even better to the semiconductor layer deposited thereon (e.g. about 8.5×10−6/K for CIGS).
Furthermore, it is known that sodium can be incorporated into the semiconductor so as to increase the efficiency of the solar cell as a result of improved chalcogen incorporation into the crystal structure of the semiconductor. The substrate glass therefore has not only to serve as support material but also has an additional function: namely the targeted release, both in terms of time and physical location (homogeneously over the area of the coating), of sodium. The glass should release sodium ions/atoms at temperatures around Tg, which requires increased mobility of the sodium ions in the glass. As an alternative, a barrier layer (e.g. an Al2O3 layer) which completely prevents diffusion of sodium ions can be applied to the glass surface before coating with molybdenum. Sodium ions then have to be added separately (e.g. in the form of NaF2) in a further process step, which increases process times and costs.
In addition, attention has to be paid to sufficient chemical resistance against environmental influences, in particular water (moisture, wetness, rain), because of the usual placement of the solar cells (outdoors) and also against other aggressive reagents which may be used in the production process. The layers themselves are protected from the environment by encapsulation with SiO2, plastic, surface coatings and/or a covering glass.
Table I below shows properties of substrate glasses for CIGS thin-film solar cells compared to the prior art, which are suitable for the solar cells of the invention.
Surprisingly, boron- and barium-free aluminosilicate glasses in particular meet the requirements for use as substrate glass for thin-film photovoltaics, since, for example in high-temperature CIGS production technology, substrate glass temperatures of up to 700° C. are reached during coating. In particular, efficiencies of CIGS thin-film solar cells which were more than 2% absolute above those of the prior art were achieved by means of the properties according to the invention of the substrate glasses, i.e. an efficiency of 14% was achieved instead of, for example, 12% using a conventional substrate glass.
It has surprisingly also been found that these glasses have a high homogeneity with respect to bubble content on melting under oxidizing conditions when nitrates of the alkali metal and/or alkaline earth metal components, e.g. KNO3, Ca(NO3)2, are used.
Large bubbles, i.e. bubbles which are visible to the naked eye (diameter >80 μm), are counted by the naked eye in a polished glass cube having an edge length of 10 cm. Size and number of smaller bubbles are measured/counted in 10 cm×10 cm×0.1 cm glass plates having a good surface polish by means of a microscope at a magnification of 400-500×.
Examples of the composition and properties of the substrate glass used in the solar cells of the invention may be found in Table II below (composition of the glasses in mol %).
The glasses were melted from conventional raw materials, i.e. carbonates, nitrates, fluorides and oxides of the components, in 4 litre platinum crucibles. The raw materials were introduced at melting temperatures of 1580° C. over a period of 8 hours and subsequently maintained at this temperature for 14 hours. The glass melt was subsequently cooled while stirring to 1400° C. over a period of 8 hours and subsequently cast into a graphite mold, which was preheated to 500° C. This casting mold was introduced immediately after casting into a cooling oven which has been preheated to 650° C. and cooled down at 5° C./min to room temperature. The glass specimens necessary for the measurements were subsequently cut from this block.
Apart from the known methods of determining the typical glass properties, the determination of the conductivity is of particular importance here. The dielectric measurements were carried out using the impedance spectrometer alpha-Analyser from Firma Novocontrol, Limburg, and the associated temperature control unit. In the measurement, a usually round plate of the glass specimen having a diameter of typically 40 mm and a thickness of from about 0.5 to 2 mm is provided on both sides with conductive silver contacts. The specimen is clamped from the upper side and underside by means of gilded brass contacts in a specimen holder and placed in a cryostat. The electrical resistance and the capacitance of the arrangement can then be measured as a function of frequency and temperature by balancing of a bridge. In the case of known geometries, the conductivity and the dielectric constant of the material can then be determined.
The relatively high electrical conductivity at room temperature (typical values of glasses are in the range from 10−14 to 10−17S/cm; 25° C.), the high temperature dependence of the conductivity and the low activation energy of <1 eV measured on all exemplary glasses are a measure of the high sodium ion mobility of these substrate materials. In addition, it can be seen from the linear behavior of the temperature dependence of the electrical conductivity in the Arrhenius plot (
The glasses not only can be used without deformation at temperatures of about 100° C.-150° C. above those of the prior art, but are also found to be reliable dopant sources for the crystallization process of, for example, I-III-VI2 compound semiconductors such as CIGS due to the increased sodium ion mobility; these compound semiconductors can therefore grow to a higher degree of perfection in a temperature range which is about 100° C.-150° C. higher.
This high mobility is a prerequisite for the crystalline growth of the compound semiconductor layers, in particular the CIGS layers, and the photovoltaic properties which can then be achieved, if it is taken into account that the sodium ions must diffuse through a 0.5-1 μm thick molybdenum layer on the substrate glass before they reach the crystallization zone and/or must travel from the vapor phase as sodium atoms into the growing semiconductor layer.
The positive effect of the sodium ions on the chalcogen incorporation in the semiconductor crystal not only produces an improved crystalline structure and crystal density but also influences the crystalline size and orientation. The sodium ion is, inter alia, incorporated into the grain boundaries of the system and can contribute, inter alia, to a reduction in charge carrier recombination at the grain boundaries. These phenomena lead automatically to considerably improved semiconductor properties, in particular to a reduction in the recombination in the bulk material and thus to an increased open-circuit potential. This naturally shows up, in particular, in the efficiency with which the solar spectrum can be converted into electric power.
This ion mobility in the substrate glasses can be influenced further in a positive fashion by, preferably, a surface treatment in acidic or alkaline solutions, for example in such a way that ion mobility occurs earlier at relatively high temperatures or uniform diffusion of the sodium ions or more uniform evaporation of sodium from the surface is present.
Furthermore, it has surprisingly been found that a significant increase in the efficiency of a thin-film solar cell can be achieved in a simple manner when the solar cell has at least one Na2O-containing multicomponent substrate glass which has the composition and properties as recited in the appended claims and is not phase demixed and has a content of β—OH of from 25 to 80 mmol/l. These substrate glass features include that the Na2O-containing multicomponent substrate glass contains less than 1% by weight of B2O3, less than 1% by weight of BaO and a total of less than 3% by weight of CaO+SrO+ZnO, that the molar ratio of the substrate glass components, Na2O+K2O)/(MgO+CaO+SrO+BaO, is greater than 0.95, that the molar ratio of the substrate glass components SiO2/Al2O3 is less than 7 and that the substrate glass has a glass transition temperature Tg of greater than 550° C., in particular greater than 600° C.
A substrate glass is not phase demixed for the purposes of the present invention when it has fewer than 10, preferably fewer than 5, surface defects in a surface region of 100×100 nm2 after a conditioning experiment. The conditioning experiment was carried out as follows:
The substrate glass surface to be examined is subjected at 500-600° C. to a flow of compressed air in the range from 15 to 50 ml/min and a flow of sulphur dioxide gas (SO2) in the range from 5 to 25 ml/min for a time of from 5 to 20 minutes. Regardless of the type of glass, this results in formation of a crystalline coating on the substrate glass. After washing off the crystalline coating (e. g, by means of water or an acidic or basic aqueous solution so that the surface is not attacked further), the surface defects per unit area of the substrate glass surface are determined by microscopy. If fewer than 10, in particular fewer than 5, surface defects are present in a surface region of 100×100 nm2, the substrate glass is considered not to be phase demixed. All surface defects having a diameter of >5 nm are counted.
The β-OH content of the substrate glass was determined as follows. The apparatus used for the quantitative determination of water via the OH stretching vibration at 2700 nm is a commercial Nicolet FTIR spectrometer with attached computer evaluation. The absorption in the wavelength range 2500-6500 nm was firstly measured and the absorption maximum at 2700 nm was determined. The absorption coefficient a was then calculated from the specimen thickness d, the pure transmission T1 and the reflection factor P:
α=1/d*Ig(1/Ti)[cm−1],
wherein Ti=TIP with the transmission T. Then, the water content is calculated from c=α/e, wherein e is the practical extinction coefficient [l*mol−1*cm−1] and for the above-mentioned evaluation range is used as a constant value of e=110*mol*cm−1 based on mol of H2O. The e value is taken from the work by H. Frank and H. Scholze in “Glastechnischen Berichten”, Volume 36, No. 9, page 350.
The objects, features and advantages of the invention will now be illustrated in more detail, with reference to the accompanying figures in which:
While the invention has been illustrated and described as embodied in thin-film solar cells, it is not intended to be limited to the details shown, since various modifications and changes may be made without departing in any way from the spirit of the present invention.
Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention.
What is claimed is new and is set forth in the following appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2009 020 955.7 | May 2009 | DE | national |
| 10 2009 050 988.7 | Oct 2009 | DE | national |
