User:Rfassbind/sandbox/CIGS structure

My notes

 * Buffer Layer for CIGS
 * Buffer, p. 9,, Applications of atomic layer deposition in solar cells

Properties
CIGS is a I-III-VI2 compound semiconductor material composed of copper, indium, gallium, and selenium. The material is a solid solution of copper indium selenide (often abbreviated "CIS") and copper gallium selenide, with a chemical formula of CuInxGa(1-x)Se2, where the value of x can vary from 1 (pure copper indium selenide) to 0 (pure copper gallium selenide). It is a tetrahedrally bonded semiconductor, with the chalcopyrite crystal structure. The bandgap varies continuously with x from about 1.0 eV (for copper indium selenide) to about 1.7 eV (for copper gallium selenide).



CIGS has an exceptionally high absorption coefficient of more than 105/cm for 1.5 eV and higher energy photons. CIGS solar cells with efficiencies around 20% have been claimed by the National Renewable Energy Laboratory (NREL), the Swiss Federal Laboratories for Materials Science and Technology (Empa), and the German Zentrum für Sonnenenergie und Wasserstoff Forschung (ZSW), which is the record to date for any thin film solar cell.

Structure
The most common device structure for CIGS solar cells is shown in the diagram (see Figure 1: Structure of a CIGS device).

Substrate
Soda-lime glass of about of 1–3 milimetres thickness is commonly used as a substrate, because the glass sheets contains sodium, which has been shown to yield a substantial open-circuit voltage increase, notably through surface and grain boundary defects passivation. However, many companies are also looking at lighter and more flexible substrates such as polyimide or metal foils.

Back contact
A molybdenum (Mo) metal layer is deposited (commonly by sputtering) which serves as the back contact and reflects most unabsorbed light back into the CIGS absorber.

CIGS absorber
Following molybdenum deposition a p-type CIGS absorber layer is grown by one of several unique methods (see section Production).

Buffer
A thin n-type buffer layer is added on top of the CIGS absorber. The buffer is typically cadmium sulfide (CdS) deposited via chemical bath deposition.

ZnO
The buffer is overlaid with a thin, intrinsic zinc oxide layer (i-ZnO) which is capped by a thicker, aluminum (Al) doped ZnO layer.

The i-ZnO layer is used to protect the CdS and the absorber layer from sputtering damage while depositing the ZnO:Al window layer, since the latter is usually deposited by DC sputtering, known as a damaging process. The Al doped ZnO serves as a transparent conducting oxide to collect and move electrons out of the cell while absorbing as little light as possible.

The CuInSe2-based materials that are of interest for photovoltaic applications include several elements from groups I, III and VI in the periodic table. These semiconductors are especially attractive for solar applications because of their high optical absorption coefficients and versatile optical and electrical characteristics, which can in principle be manipulated and tuned for a specific need in a given device.

Conversion efficiency
CIGS is mainly used in the form of polycrystalline thin films. The best efficiency achieved as of Septemberer 2014 was 21.7%. A team at the National Renewable Energy Laboratory achieved 19.9%, a record at the time, by modifying the CIGS surface and making it look like CIS. These examples were deposited on glass, which meant the products were not mechanically flexible. In 2013, scientists at the Swiss Federal Laboratories for Materials Science and Technology developed CIGS cells on flexible polymer foils with a new record efficiency of 20.4%. These display both the highest efficiency and greatest flexibility.

The U.S. National Renewable Energy Laboratory confirmed 13.8% module efficiency of a large-area (meter-square) production panel, and 13% total-area (and 14.2% aperture-area) efficiency with some production modules. In September 2012 the German Manz AG presented a CIGS solar module with an efficiency of 14.6% on total module surface and 15.9% on aperture, which was produced on a mass production facility. MiaSolé obtained a certified 15.7% aperture-area efficiency on a 1m2 production module, and Solar Frontier claimed a 17.8% efficiency on a 900 cm2 module.

Higher efficiencies (around 30%) can be obtained by using optics to concentrate the incident light. The use of gallium increases the optical band gap of the CIGS layer as compared to pure CIS, thus increasing the open-circuit voltage. Gallium's relative abundance, compared to indium, lowers costs.

Conventional crystalline silicon
Unlike conventional crystalline silicon cells based on a homojunction, the structure of CIGS cells is a more complex heterojunction system. A direct bandgap material, CIGS has very strong light absorption and a layer of only 1–2 micrometers (µm) is enough to absorb most of the sunlight. By comparison, a much greater thickness of about 160–190 µm is required for crystalline silicon.

The active CIGS-layer can be deposited in a polycrystalline form directly onto molybdenum (Mo) coated on a variety of several different substrates such as glass sheets, steel bands and plastic foils made of polyimide. This uses less energy than smelting large amounts of quartz sand in electric furnaces and growing large crystals, necessary for conventional silicon cells, and thus reduces its energy payback time significantly. Also unlike crystalline silicon, these substrates can be flexible.

In the highly competitive PV industry, pressure increased on CIGS manufacturers, leading to the bankruptcy of several companies, as prices for conventional silicon cells declined rapidly in recent years. However, CIGS solar cells have become as efficient as multicrystalline silicon cells—the most common type of solar cells. CISG and CdTe-PV remain the only two commercially successful thin-film technologies in a globally fast-growing PV market.

Other thin films
In photovoltaics "thinness" generally is in reference to so-called "first generation" high-efficiency silicon cells, which are manufactured from bulk wafers hundreds of micrometers thick. Thin films sacrifice some light gathering efficiency but use less material. In CIGS the efficiency tradeoff is less severe than in silicon. The record efficiencies for thin film CIGS cells are slightly lower than that of CIGS for lab-scale top performance cells. In 2008, CIGS efficiency was by far the highest compared with those achieved by other thin film technologies such as cadmium telluride photovoltaics (CdTe) or amorphous silicon (a-Si). CIS and CGS solar cells offer total area efficiencies of 15.0% and 9.5%, respectively. In 2015, the gap with the other thin film technologies has been closed, with record cell efficiencies in laboratories of 21.5% for CdTe (FirstSolar) and 21.7% for CIGS (ZSW). (See also NREL best research cell efficiency chart. )