Tech Hardware Supply Chain - Gazhoo

Bhavin Shah
(852) 2800-8538
bhavin.a.shah@jpmorgan.com
Asia Pacific Equity Research
20 April 2009
film layer. The morphology, or physical structure, of the resulting layer affects
efficiency and other characteristics of the final cell.
Figure 148: Sputtering diagram
Source: Wikipedia.
CIGS absorber layer
In CIGS thin film, the critical absorber layer consists of a complex alloy of copper,
indium, gallium, and selenium. Copper, indium, and selenium (CIS) exhibits a high
degree of light absorption, requiring a very thin layer to capture maximum energy
from incident light, and is thus a good material for use in PV cells. However, the
band-gap of CIS is 1eV, below an optimal band-gap of 1.4-1.5eV for the solar
spectrum. The addition of gallium is a means to raise the cell band-gap from 1eV up
to 1.7eV, closer to that of the solar spectrum and therefore increase cell conversion
efficiency.
Various processes have been developed in the lab to form CIGS layers. The most
common approach is to use either thermal co-evaporation or sputtering, whereby
constituent elements are deposited in precise concentrations forming a layer 2-3
microns thick on the molybdenum coated substrate. Nanosolar, a private startup, is
developing a novel approach to manufacture CIGS cells utilizing nanoparticles in a
proprietary process that resembles ink-jet printing.
Deposition of CIGS can occur simultaneously with all four components applied
during a single step or in multiple steps, which typically involves the initial
deposition of a copper or selenium rich precursor layer, followed by the addition of
any deficient elements to bring the thin film to desired final composition. Irrespective
of the process used, the critical ratio of copper to indium and gallium should be in the
range of 0.86-0.96 to produce a high efficiency cell. Varying the deposition rate of
gallium during CIGS processing allows for grading the band-gap along the depth of
the layer. Higher band-gap at the front and at the back of the layer increases overall
efficiency by creating a band-gap profile that can absorb a larger light spectrum and
minimize back recombination.
Window layer
As the name suggests, the window layer is an optically transparent material that
allows light to pass though to the absorber layer and acts as an “N-type” electron
source in the cell junction. CdS, is commonly used as the window layer in CIGS cells
as it results in high cell efficiency and is typically applied using a relatively low-cost
chemical bath deposition process. Cadmium is a toxic substance; however, only a
247