Introduction to Thin Film CIGS Solar Cells By Junce Zhang, EE, UC Santa Cruz jzhang37@ucsc.edu

Four Generation of Solar Cells

Introduction of CIGS CIGS = CuInxGa1-xSe2 (copper indium gallium selenide) Compound semiconductor ( I-III-VI) Direct gap semiconductor, band gap can be adjusted. Heterojunction solar cells High efficiency (≈20.3% in small area, ≈13% in large area modules) Very good stability in outdoor tests Applications: solar power plants power supply in aerospace decentralized power supply power supply for portable purposes The abbreviation CIS stands for copper indium diselenide and CIGS for copper indium gallium diselenide. The x describes the ratio between indium and gallium. CIS and CIGS are compound semiconductors of the I-III-VI group. The band-gap structure is a complex heterojunction system. That means that thejunction is formed by different semiconducting materials with unequal band gaps. These thin film solar cells show efficiencies of 19% in the case of small area modules in laboratory and of about 13 % in large area modules. They show a very good stability in outdoor tests concerning mechanical load and radiation hardness against electrons and protons. These solar cells are used in solar power plants, as power supply in aerospace, decentralized power supply or in portable purposes. http://www.copper.org/innovations/2007/05/images/civilian_flex_panel.jpg http://www.rgp.ufl.edu/publications/explore/v12n2/images/thin-film.jpg http://www.baulinks.de/webplugin/2007/i/0732-wuerthsolar1.jpg http://www.esa.int/images/ISS_2004_web400.jpg

CIGS Device Structure CIGS 1-2.5 µm ZnO, ITO 2500 Å CdS 700 Å Mo 0.5-1 µm Glass, Metal Foil, Plastics 7

CIGS Device Structure Back contact Layer Mo (0.5-1 µm) Deposited by RF Sputtering Function: to reflect most unabsorbed light back into the absorber. Absorber Layer CIGS (1-2.5 µm) Band gap: 1.02eV (CuInSe2) to 1.65 eV (CuGaSe2) p-type Buffer Layer CdS (700 Å) Deposited by chemical bath deposition (CBD) Band gap: 2.5eV Function: protection of CIGS layer. Window Layer: ZnO (2500 Å) Deposited by RF sputtering / Atomic Layer Deposition(ALD) Band gap: 3.3eV, n-type(Doping with Al 1020 cm-3) Function: transparent conducting oxide to collect and move electrons out of the cell. Meyer, Thorsten: Relaxationsphänomene im elektrischen Transport von Cu(In,Ga)Se2, 1999.

Growth Methods for Thin Films I coevaporation process: evaporation of Cu, In, Ga and Se from elemental sources precise control of evaporation rate by EIES & AAS or mass spectrometer required substrate temperature between 300-550°C inverted three stage process: evaporation of In, Ga, Se deposition of (In,Ga)2Se3 on substrate @ 300°C evaporation of Cu and Se deposition at elevated T Þ smoother film morphology Þ highest efficiency There are three most used methods for producing thin films of CIGS. The first one is the coevaporation process. This method is named coevaporation because in the beginning all needed elements were evaporated in vacuum at the same time. The thin film is produced by evaporating Cu, In, Ga, Se from elemental sources. In order to achieve the favored film composition, a precise control of the particular evaporation rates is necessary. There an electron impact emission spectrometer and an atom absorption spectrometer or a mass spectrometer is used. But the process also requires a substrate temperature between 300 and 550°C for a certain time during film growth. There are several processes of coevaporation, but one of the most favored ones is the inverted three-stage process, which you can see on the right. At first In, Ga, and selenium are evaporated with different rates and deposited as (In,Ga)2Se3 at 300°C on the substrate. Afterwards Cu and selenium are evaporated and deposited on the substrate at elevated temperatures. At last In, Ga, and selenium are evaporated again. The inverted three-stage process leads to smoother film morphology and to high efficiency solar cells. Hamakawa, Yoshihiro: Thin Film Solar Cells, Springer, 2004.

Growth Methods for Thin Films II sequential process: selenization from vapor: substrate: soda lime glass coated with Mo deposition of Cu and In, Ga films by sputtering selenization under H2Se atmosphere thermal process for conversion into CIGS advantage: large-area deposition disadvantage: use of toxic gases (H2Se) annealing of stacked elemental layers substrate: soda lime glass coated with Mo deposition of Cu and In, Ga layers by sputtering deposition of Se layer by evaporation rapid thermal process advantage: large-area deposition avoidance of toxic H2Se The second method to produce thin films of CIGS is the sequential process. There are two different techniques according to the selenization of the film. The first possibility is the selenization from vapor. The substrate is a soda lime glass coated with a thin film of molybdenum as the back contact of the solar cell. Cu and In,Ga layers are sequentially deposited on the substrate by sputtering. The different layers are selenized in a hydrogen selenide atmosphere and converted into a CIGS thin film by a thermal process. The advantage of this process compared with the coevaporation process is that large-area depositions of CIGS films are commercially producible. But there is also a big disadvantage: hydrogen selenide, which is a highly toxic gas, is used for selenization. The other technique is the annealing of stacked elemental layers. The substrate and deposition technique of Cu and In, Ga are equal to the selenization from vapor. The only differences are that a layer of selenium is deposited by evaporation and that there is a rapid thermal process in an inert atmosphere. With this technique large-area deposition of CIGS films are also commercially producible, but the main advantage is that the hydrogen selenide atmosphere can be avoided. Hamakawa, Yoshihiro: Thin Film Solar Cells, Springer, 2004.

Growth Methods for Thin Films III roll to roll deposition: substrate: polyimide/ stainless steel foil coated with Mo ion beam supported low temperature deposition of Cu, In, Ga & Se advantages: low cost production method flexible modules and high power per weight ratio disadvantages: lower efficiency Mo Cu,Ga,In,Se CdS ZnO The third method to produce thin film CIGS is the roll to roll deposition method. In contrast to the other methods with glass substrates, the substrate is flexible. Polyimides or stainless steel foils are used as substrates coated with a thin film of molybdenum. In the case of a stainless steel foil as the used substrate an insulator between the steel foil and the molybdenum layer is necessary for interconnection. Polyimides are used because of the high electric strength and its high dimensional stability under heat. Cu, In, Ga and selenium are deposited by the ion beam supported low temperature deposition method. The huge advantages of this method are the low cost production of CIGS solar cells that flexible modules are producible with a high power per weight ratio. But these advantages involve lower efficiency. http://www.solarion.net/images/uebersicht_technologie.jpg

Fabrication Technology cell processing: substrate wash #1 deposition of metal base electrode patterning #1 formation of p-type CIGS absorber monolithical integration: during cell processing fabrication of complete modules deposition Ni/Al collector grid deposition of antireflection coating deposition of buffer layer patterning #2 deposition of n-type window layer patterning#3 You heard about the needed materials and the used deposition techniques to form a CIGS based circuit, so that we the cell processing now. This description of the cell processing is just schematically and does not include all steps in detail. It is started with washing the substrate. The deposition of the metal base electrode is followed by the first patterning, which is done by a laser. Afterwards the structure is cleaned and the p-type CIGS-based absorber is superimposed. The CIGS layer is covered by a buffer layer consisting of cadmium sulfide. Subsequently the existing structure is patterned by a CNC machine (Computerized Numerical Control). After that the n-type window layer is deposited on top of the whole composition and is also patterned by a CNC machine. This process step is followed by the metallization and the deposition of the antireflection coating consisting of magnesium difluorite. The monolithical integration is the connection in series of single cell. In contrast to crystalline Si solar cells it takes place during the cell processing, so that complete modules are produced, not only single cells that must be connected to modules. substrate Hamakawa, Yoshihiro: Thin Film Solar Cells, Springer, 2004.

sources: 2.Hamakawa, Yoshihiro: Thin Film Solar Cells, Springer, 2004. 1.NREL http://www.nrel.gov/ 2.Hamakawa, Yoshihiro: Thin Film Solar Cells, Springer, 2004. 3.Meyer, Thorsten: Transport on Cu(In,Ga)Se2, 1999. 4.Dimmler, Bernhard: CIS-Solar Cells, 2006. 5.Nanosolar, Ultra-Low-Cost Solar Electricity Cells, 2009.

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