Classifications

• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
  • H10F77/00—Constructional details of devices covered by this subclass
  • H10F77/10—Semiconductor bodies
  • H10F77/12—Active materials
  • H10F77/126—Active materials comprising only Group I-III-VI chalcopyrite materials, e.g. CuInSe2, CuGaSe2 or CuInGaSe2 [CIGS]
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
  • H10F10/00—Individual photovoltaic cells, e.g. solar cells
  • H10F10/10—Individual photovoltaic cells, e.g. solar cells having potential barriers
  • H10F10/16—Photovoltaic cells having only PN heterojunction potential barriers
  • H10F10/167—Photovoltaic cells having only PN heterojunction potential barriers comprising Group I-III-VI materials, e.g. CdS/CuInSe2 [CIS] heterojunction photovoltaic cells
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E10/00—Energy generation through renewable energy sources
  • Y02E10/50—Photovoltaic [PV] energy
  • Y02E10/541—CuInSe2 material PV cells

Definitions

• the present invention relates to a layer system, in particular for thin-film solar cells, solar modules or the like. According to the preamble of claim 1 and solar cells or solar cell modules with this layer system.
• Thin-film systems for solar cells and solar modules are well known and available in various designs, depending on the substrate and deposited materials on the market. The materials are selected so that the incident solar spectrum is maximally utilized.
• thin-film cells made of semiconductor materials are commercially available, for example, from cadium telluride (CdTe), or copper indium (gallium) sulfur selenium compounds (Cu (In / Ga) (Se / S)), known as CIS or CIGS cells, where depending on the cell type S may be sulfur and / or selenium.
• Amorphous silicon (a: Si: H) is most commonly used for thin-film cells.
• CIS thin-film cells achieve approximately the same efficiencies as multicrystalline silicon modules.
• CdS has so far been wet-chemically separated in a CBD process (chemical bath process).
• CBD process chemical bath process
• CdS buffer layer contains the toxic heavy metal cadmium. This results in higher production costs since increased safety precautions in the production process, e.g. when disposing of the wastewater, must be taken. Disposal of the product may also result in higher costs to the customer as the manufacturer may be forced to take back, dispose of or recycle the product, depending on local legislation, and pass the resulting costs on to customers.
• these materials are not yet suitable as a buffer for the solar cells based on Cu (In, Ga) (S, Se) 2 for commercial use, since they do not have the same efficiencies (ratio of irradiated power to the generated electrical power of a solar cell) such as those with a CdS buffer layer, which are up to nearly 20% for laboratory cells in small areas and between 10% and 12% for large-area modules. Furthermore, they show too much instability, hysteresis or efficiency degradation when exposed to light, heat and / or moisture.
• CdS is a direct semiconductor with a direct electronic band gap of about 2.4 eV and therefore in a Cu (In 5 Ga) (S, Se) 2 / CdS / ZnO solar cell even at CdS Layer thicknesses of a few 10 nm, the incident light is absorbed. Since there are many crystal defects, ie recombination centers, in this region of the heterojunction and in the buffer material, the charge carriers generated recombine in this layer immediately. Thus, the light absorbed in the buffer layer is lost for the electric yield, that is, the efficiency of the solar cell becomes smaller, which is disadvantageous for a thin film cell.
• the object of the present invention is therefore to provide a layer system based on Cu (In, Ga) (S, Se) 2 , in particular for solar cells, solar modules or the like, with a buffer layer which has high efficiency and high stability, wherein the production should be cost-effective and environmentally friendly.
• the invention is characterized in that the layer system comprises an absorber layer of a compound semiconductor material with chalcopyrite structure (Cu (In, Ga) (S i -y , Se y ) 2 , where 0 ⁇ y ⁇ 1) and a first buffer layer, wherein the first buffer layer In 2 (S i -X , Se x ) 3 + 5 (indium sulfide selenide) comprises and 0 ⁇ x ⁇ 1 and -1 ⁇ ⁇ 1 (x and ⁇ can therefore also be 0).
• the In 2 (S u x , Se x ) 3 + g buffer layer is further formed amorphous.
• the solar cells produced with this layer system show high efficiencies combined with high long-term stability. Since toxic substances are no longer used, the manufacturing process is gentler and cheaper and there are no follow-up costs, as with CdS buffer layers.
• Amorphous in the context of the present invention means that in X-ray diffraction studies the signals reflecting a crystalline structure are below the detection limit.
• the Raman spectrum (at 488nm excitation wavelength) of the strata in the range of 220cm “1 and 380cm " 1 wavenumbers shows a broad band with maximum at 290cm '1 in which no discrete lines are resolvable
• the absorber layers were either amorphous or crystalline, and surprisingly, only the layer systems yielded particularly good efficiencies of greater than 12% for the indium sulfide layer
• the absorber was grown amorphously, with layers of crystalline indium sulfide yielding efficiencies between 6 and 12%.
• the first buffer layer does not contain impurities, i. it is not deliberately provided with other elements, such as oxygen, carbon or chlorine, and contains these at most within the framework of production-technically unavoidable concentrations of less than or equal to 1 mol%. This ensures a high degree of efficiency.
• ⁇ is advantageously between -0.5 and +0.5, preferably between -0.2 and 0.
• a value is thereby determined.
• the high efficiency is largely determined by the amorphous design of the first buffer layer.
• studies show a reciprocal relationship between efficiency and crystallinity for the layers and deposition methods used here.
• amorphous layers can be produced at lower temperatures, thereby avoiding massive interdiffusion of the elements between buffer and absorber, and generally facilitating production.
• Amorphous semiconductor layers have hitherto been used in solar cells only in amorphous silicon germanium layer systems as well as in heterotransitions of crystalline and amorphous silicon. For chalcopyrite semiconductor combinations with amorphous buffer or amorphous front electrodes are not yet known.
• the layer system according to the invention Another advantage of the layer system according to the invention is that the cells have a higher stability to temperature, light, moisture, as well as mechanical and electrical stress. Hysteresis effects are often observed in the previous chalcopyrite-based solar cells: the efficiency can be degraded by the action of heat and / or moisture. Although the efficiency can be partially restored by lighting. In the worst case, however, irreversible losses remain. In some cases, the efficiency after production of the cells sets only by long lighting (light soaking). In the case of the solar cells produced with the layer system according to the invention, measurements of the aging effect under moist heat (85% relative humidity, 85 ° C.
• the selenium content x is between 0 and 0.5, preferably between 0 and 0, 1.
• the band gap of the first buffer layer and its band matching to the absorber layer can be made of Cu (In 5 Ga) (S 5 Se) 2 , the efficiency being highest at these selenium contents.
• the selenium content is thus a fine tuning of the band gap and the band adaptation to In 2 (S i -x , Se x ) 3 + ⁇ possible, whereby the efficiency can be further increased.
• the absorber layer comprises Cu (In 5 Ga) (S 5 Se) 2 with a ratio of the molar concentrations [S] / ([Se] + [S]) at the surface of the absorber layer between 10% and 90 %, in particular 20% to 65% and preferably 35%, whereby the sulfur is incorporated into the anion lattice of the chaclopyrite structure.
• This also makes it allows fine tuning of the band gap and band alignment with respect to In 2 (S i. X, Se x) 3 + ⁇ achieve.
• the sulfur concentration in the absorber has a decreasing gradient from the surface, that is to say the boundary surface facing the first buffer layer, to the interior of the absorber, whereby the efficiency is also positively influenced.
• the sulfur gradient can be adjusted during the manufacturing process of the CIS absorbers by the seleniumation and sulfurization process of metallic layers by means of suitable temperature and gas time profiles.
• the molar ratio S / (Se + S) decreases from the surface of 20% to 60% to values around 5-10% in the interior of the absorber layer, so that the ratio S / (Se + S) is integrally significantly lower, as on the surface. Toward the back contact, the ratio S / (Se + S) can also increase again.
• the gradient of sulfur leads to a gradient in the bandgap. That a decreasing gradient of the band gap in the base of a solar cell can lead to improved efficiencies is known from various solar cell technologies.
• the gradient additionally allows an adaptation of the band structures at the heterojunction of the indium sulfide / Cu (In, Ga) (S, Se) 2 layer system.
• the In 2 (S i -x , Se x ) 3 + ⁇ layer is between 10 nm and 200 nm, in particular between 40 nm and 140 nm, preferably 60 nm thick, since the light absorption by the In 2 (S i -X , Se x ) 3 + ⁇ buffer layer is low.
• the layer system according to the invention comprises a second buffer layer, which is preferably arranged between the first buffer layer and a front electrode.
• the force applied to the first buffer layer second buffer layer comprises undoped Zni -z Mg z O, where 0 ⁇ z ⁇ 1.
• Your Layer thickness is suitably up to 200 nm, in particular 20 nm to 140 nm, preferably 60 nm.
• a front electrode which comprises a transparent conductive oxide (TCO).
• TCO transparent conductive oxide
• ITO indium tin oxide
• ZnO doped ZnO, in particular Al- or Ga-doped ZnO being preferred.
• the support in the layer system according to the invention is preferably a metal, glass, plastic or ceramic substrate, glass being preferred.
• transparent carrier materials in particular plastics.
• a back electrode e.g. Molybdenum (Mo) or other metals, provided.
• Mo molybdenum
• this has a molybdenum sublayer adjacent to the absorber and a silicon nitride sublayer (SiN) adjacent to the Mo sublayer.
• the method for producing such a layer system is such that at least the deposition of the first buffer layer is carried out in vacuo, wherein preferably the production of the entire layer system takes place in a vacuum.
• Another advantage is that the vacuum process prevents the incorporation of oxygen or hydroxide. Hydroxide components in the buffer layer are suspected to be responsible for transients of the effect of heat and light.
• the deposition of the buffer layer at temperatures less than or equal to 150 ° C takes place, especially less equal to 130 0 C, preferably between 5O 0 C and 100 0 C. Therefore, the cost of the vacuum system can be reduced.
• indium sulfide buffer on Cu (In 1 Ga) Se 2 which does not include sulfur, either at temperatures higher than 150 0 C must be deposited or the entire cell assembly must be tempered after deposition of the ZnO front electrode at higher temperatures.
• temperatures for the production of the layer are significantly lower than 150 0 C possible and the cell structure must not be post-annealed. This could be due to the fact that the sulfur-containing surface of Cu (In, Ga) (S, Se) 2 already possesses the correct band matching to the buffer in the absorber / buffer layer structure according to the invention.
• the absorber layer is expediently applied in an Avancis RTP ("rapid thermal processing") process by first depositing a precursor layer onto the substrate with back electrode: the elements Cu, In and Ga are deposited by sputtering and amorphous selenium by thermal evaporation
• the substrate temperature is below 100 ° C., so that the layers are essentially left unreacted as a metal alloy plus elemental selenium rapid annealing process (RTP rapid thermal processing) in a sulfur-containing atmosphere to Cu (In, Ga9 (S, Se) 2 chalcopyrite reacts.
• the first buffer layer is thermally evaporated in a high vacuum, wherein the process conditions are preferably selected such that this buffer layer is deposited amorphously on the absorber layer.
• Essential for the layer system according to the invention is thus the growth of the amorphous layer on the polycrystalline absorber. Since thin layers grow differently on different substrates, detection is only possible on the actual layer system absorber / buffer or in the complete solar cell, but not on witness glasses (control samples), by X-ray diffraction in grazing incidence and by Raman spectroscopy. It is also conceivable that both the first and the second buffer layer, if used, are applied by means of radio-frequency sputtering (RF sputtering).
• the front electrode is again preferably applied in a DC magnetron sputtering process.
• FIG. 1 shows a schematic cross-sectional view of the layer system according to the invention
• 2 is a graph showing the efficiency distribution of 32 cells with indium sulfide selenide buffer.
• FIG. 3 results of Raman spectroscopy (vertical axis) on a thin-film cell with an indium sulfide-selenide amorphous buffer layer (horizontal axis)
• FIG. 4 shows results of Raman spectroscopy (vertical axis) on a thin-layer cell with a crystalline indium sulfide-selenide buffer layer (horizontal axis), wherein additionally calculated lines are plotted at the positions of the CuInSe 2 , CuInS 2 , ZnO and In 2 S 3 phases are,
• Fig. 5 shows the correlation between Raman line width of the peak at 326 cm “ (vertical axis) and line width of the (1 1 1) reflection of cubic indium sulfide selenide (horizontal axis) and
• Fig. 6 shows the relationship between Raman line width of the peak at 326 cm -1 (horizontal axis) and efficiency (vertical axis).
• FIG. 1 shows purely schematically a preferred exemplary embodiment of the layer system 1 according to the invention in a cross-sectional view.
• the layer system 1 comprises a substrate 2, a back electrode 3, an absorber layer 4, a first buffer layer 5, a second buffer layer 6 and a front electrode 7.
• the production process begins in the usual way with the thermal deposition of the back contact layer 3 of Mo on a glass substrate 2.
• the back electrode 3 of Mo can also be a layered electrode of a first, applied to the glass substrate SiN sublayer and a second, applied thereto Mo Partial layer can be used, which improves the properties of this back contact.
• the absorber layer 4 made of Cu (In 5 Ga) (S 5 Se) 2 is applied to the back electrode 3.
• the first buffer layer 5 of In 2 (S u x , Se x ) 3 + ⁇ , the second buffer layer 6 of undoped ZnO and then a front electrode 7 made of ZnO doped with aluminum are then produced. The following parameters are used.
• the back electrode 3 made of molybdenum has a layer thickness of 400 nm.
• the absorber layer 4 is produced with a thickness of 1.5 ⁇ m using the AVANCIS RTP process.
• the surface of the absorber layer 4 has an anion composition [S] / ([Se] + [S]) of about 35%.
• the gallium concentration is below 1% at the surface.
• the In 2 (S i x , Se x ) 3 + ⁇ buffer layer 5 was thermally evaporated in a high vacuum.
• the selenium content of the buffer layer x is 0 to 3%.
• the stoichiometry deviation ⁇ is about -0.1 (which corresponds to an In 2 S 2 9).
• the layer thickness of the first buffer layer is 80 nm.
• the front electrode is 1200 nm ZnO: Al deposited with DC magnetron sputtering.
• the process temperatures during deposition of the absorber layer 4 and the subsequent layers 5, 6, 7 are significantly below 150 0 C, namely 7O 0 C.
• FIGS. 3 and 4 The results of Raman spectroscopy studies on finished solar cell devices are shown in FIGS. 3 and 4. These Raman spectra were performed at room temperature with an argon ion excitation laser at a wavelength of 488 nm for each of a solar cell with an amorphous ( Figure 3) and a crystalline In 2 (S i -x , Se x ) 3 + ⁇ layer (Fig. 4) measured.
• the crystalline In 2 (S i -x , Se x ) 3 + ⁇ layer is characterized by a structured spectrum in which the single lines are clearly visible at the positions of the spectrum of ⁇ -In 2 S 3 .
• the observed Raman modes can be assigned to the contributing phases CuInSe 2 , CuInS 2 , In 2 S 3, and ZnO as follows: the line at 179 cm -1 and the neighboring side modems at 210-220 cm -1 of the phase CuInSe 2 , the line at 190 cm -1 of the phase CuInS 2 , the lines at 189, 244, 266, 306, 326 and 367 cm -1 of the phase In 2 S 3 and the lines at 430 and 570 cm -1 including the Flank towards smaller wavenumbers towards the phase ZnO.
• This layer shows the diffractogram of cubic In 2 S 3 by X-ray diffraction (XRD), which also proves the crystalline nature of the sample.
• XRD X-ray diffraction
• the Raman spectrum shows an unstructured mountain in the wavenumber range, which also contains the signals of the crystalline indium sulfide, in particular in the region of 220 to 380 cm "1.
• the X-ray diffraction there are no reflections in the X-ray diffraction that could be assigned to an indium sulfide phase.
• Fig. 5 shows the relationship between the Raman line width of the peak at 326 cm -1 and the line width of the XRD (1 1 1) reflection of In 2 (S i -X , Se x ) 3 + ⁇ layers Indium sulfide reflections in the diffractogram, the width of the Raman lines correlates well with the width of the (1 1 1) reflection from the X-ray diffraction pattern, the wider the line of the (1 1 1) reflection, the wider the Raman line and the more amorphous
• the structure of the sample is shown in Fig. 5.
• the determined relationship between the two line widths is also given by formula
• the width of the Raman line was determined by fitting Gauss-Lorentz lines to the line positions of In 2 S 3 .
• Fig. 6 again shows the relationship between the ramen line width of the peak at 326 cm -1 and the efficiency for the In 2 (S i -x , Se x ) 3 + ⁇ layers It can be seen that the broader the Raman line, ie the more amorphous the structure is higher, the efficiency of the layers is higher, which means that very good solar cells, ie cells with high efficiency, in the process parameters studied here and the absorbers used are characterized by an unstructured Raman spectrum, greatly broadened Raman line and the absence of crystalline The determined relationship between the Raman line width of the peak at 326 cm -1 and efficiency is also given by formula in Fig. 6. For strongly amorphous layers, the determination of the line width becomes inaccurate completely unstructured spectrum lead to the best solar cell efficiencies.

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• 2008
  • 2008-05-19 DE DE102008024230A patent/DE102008024230A1/de not_active Withdrawn
• 2009
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  • 2009-05-19 EP EP09749622.8A patent/EP2281310B1/de active Active
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