US008062922B2 (12) United States Patent (10) Patent No.: US 8,062.922 B2 Britt et al. (45) Date of Patent: Nov. 22, 2011 (54) BUFFER LAYER DEPOSITION FOR (56) References Cited THIN-FILMI SOLAR CELLS U.S. PATENT DOCUMENTS (75) Inventors: Jeffrey S. Britt, Tucson, AZ (US); Scot 3,148,084 A 9, 1964 Hill et al. Albright, Tucson, AZ (US); Urs 4,143,235 A 3, 1979 Duisman Schoop, Tucson, AZ (US) 4,204,933 A 5/1980 Barlow et al. s s 4,366,337 A 12/1982 Alessandrini et al. 4,642,140 A 2f1987 Noufi et al. (73) Assignee: Global Solar Energy, Inc., Tucson, AZ 4,778.478 A 10/1988 Barnett (US) 5,112,410 A 5, 1992 Chen 5,578,502 A 1 1/1996 Albright et al. (*) Notice: Subject to any disclaimer, the term of this 6,268,014 B1 7/2001 Eberspacher et al. patent is extended or adjusted under 35 (Continued) U.S.C. 154(b) by 203 days. OTHER PUBLICATIONS (21) Appl. 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Intr. vol. 76, Issue 6, Article 062206, May 18, 2005, 5pgs. 2007/O169809 A1 7/2007 Van Duren et al. 2007/024.3657 A1 10, 2007 Basol et al. * cited by examiner U.S. Patent Nov. 22, 2011 Sheet 1 of 6 US 8,062.922 B2 U.S. Patent Nov. 22, 2011 Sheet 3 of 6 US 8,062.922 B2 Fig. 5 62ag. 62c- 62bC) 36a- A ; , , V42 A ; , -36C W -36b 32 O O 38aA. 52 38c/ Nso 33bA 34 is il SR 36C 36a- , W *- , 42 , A. O 38a, 3. N50 3é. 52 34 O O U.S. Patent Nov. 22, 2011 Sheet 4 of 6 US 8,062.922 B2 3 2 22. 2 2 2 2 22. 2.2 2.2 2 2 32 76 76 76 76 76 U.S. Patent Nov. 22, 2011 Sheet 5 of 6 US 8,062.922 B2 Fig. 10 - O O 102 TRANSPORT SUBSTRATE WEB THROUGH DEPOSITION REGION 104 PRE-RINSE SUBSTRATE WEB 106 HEAT-METAL CONTAINING SOLUTION 108 DISPENSE METAL-CONTAINING SOLUTION 11 O DISTRIBUTE METAL-CONTAINING SOLUTION 11 2 DISPENSE CHALCOGEN-CONTAINING SOLUTION 11 4. REPLENISH METAL-CONTAINING SOLUTION 11 6 REPLENISH CHALCOGEN-CONTAINING SOLUTION 11 8 ADJUST LONGITUDINAL SLOPE OF SUBSTRATE WEB US 8,062,922 B2 1. 2 BUFFER LAYER DEPOSITION FOR other types of PV cells, such as crystalline silicon PV cells, THIN-FILMI SOLAR CELLS thin film PV cells require less light-absorbing material to create a working cell, and thus can reduce processing costs. CROSS-REFERENCE TO RELATED Thin film based PV cells also offer improved cost by employ APPLICATION ing previously developed deposition techniques widely used in the thin film industries for protective, decorative, and func This application claims priority under 35 U.S.C. S 119 and tional coatings. Common examples of low cost commercial applicable foreign and international law of U.S. Provisional thin film products include water permeable coatings on poly Patent Applications Ser. No. 61/068.459, filed on Mar. 5, mer-based food packaging, decorative coatings on architec 2008, which is hereby incorporated by reference in its 10 tural glass, low emissivity thermal control coatings on resi entirety. dential and commercial glass, and scratch and anti-reflective BACKGROUND coatings on eyewear. Adopting or modifying techniques that have been developed in these other fields has allowed a reduc The field of photovoltaics generally relates to multi-layer 15 tion in development costs for PV cell thin film deposition materials that convert sunlight directly into DC electrical techniques. power. The basic mechanism for this conversion is the pho Furthermore, thin film cells, particularly those employing a tovoltaic (or photoelectric) effect, first correctly described by Sunlight absorber layer of copper indium diselenide, copper Einstein in a seminal 1905 scientific paper for which he was indium disulfide, copper indium aluminum diselenide, or awarded a Nobel Prize for physics. In the United States, copper indium gallium diselenide, have exhibited efficiencies photovoltaic (PV) devices are popularly known as solar cells. approaching 20%, which rivals or exceeds the efficiencies of Solarcells are typically configured as a cooperating sandwich the most efficient crystalline cells. In particular, copper of p-type and n-type semiconductors, in which the n-type indium gallium diselenide (CIGS) is stable, has low toxicity, semiconductor material (on one “side' of the sandwich) and is truly thin film, requiring a thickness of less than two exhibits an excess of electrons, and the p-type semiconductor 25 microns in a working PV cell. As a result, to date CIGS material (on the other “side' of the sandwich) exhibits an appears to have demonstrated the greatest potential for high excess of holes, each of which signifies the absence of an performance, low cost thin film PV products, and thus for electron. Near the p-n junction between the two materials, penetrating bulk power generation markets. Valence electrons from the n-type layer move into neighbor Thin film PV materials may be deposited either on rigid ing holes in the p-type layer, creating a small electrical imbal 30 glass Substrates, or on flexible Substrates. Glass Substrates are ance inside the solar cell. This results in an electric field in the relatively inexpensive, generally have a coefficient of thermal vicinity of the junction. expansion that is a relatively close match with the CIGS or When an incident photon excites an electron in the cell into other absorber layers, and allow for the use of vacuum depo the conduction band, the excited electron becomes unbound sition systems. However, such rigid substrates suffer from from the atoms of the semiconductor, creating a free electron/ 35 various shortcomings, such as a need for Substantial floor hole pair. Because, as described above, the p-n junction cre space for processing equipment and material storage, special ates an electric field in the vicinity of the junction, electron/ ized heavy duty handling equipment, a high potential for hole pairs created in this manner near the junction tend to Substrate fracture, increased shipping costs due to the weight separate and move away from junction, with the electron and delicacy of the glass, and difficulties in installation. As a moving toward the n-type side, and the hole moving toward 40 result, the use of glass Substrates is not optimal for large the p-type side of the junction. This creates an overall charge Volume, high-yield, commercial manufacturing of multi imbalance in the cell, so that if an external conductive path is layer functional thin film materials such as photovoltaics. provided between the two sides of the cell, electrons will In contrast, roll-to-roll processing of thin flexible sub move from the n-type side back to the p-type side along the strates allows for the use of compact, less expensive vacuum external path, creating an electric current. In practice, elec 45 systems, and of non-specialized equipment that already has trons may be collected from at or near the surface of the n-type been developed for other thin film industries.