Page 363 - A Comprehensive Guide to Solar Energy Systems

368 A ComPRehensiVe Guide To soLAR eneRGy sysTems

18.4 Flexible Cell Formations

Probably the ultimate advantage of thin-film technology is the application of roll-to-roll
manufacturing for production of monolithically interconnected solar modules for low
capex, lightweight, flexible modules leading to low energy payback time because of high
throughput processing and low cost of overall system. Out of all the thin film technologies,
CiGs cells have shown the most promise and progress for flexible products, owing to its
high efficiency achieved under substrate configuration. A large number of activities on
highly efficient, stable and flexible thin film modules based on CiGs has recently drawn
much interest for flexible solar cells on metal and plastic foils. Apart from the expected
high efficiency and long term stability for terrestrial applications, flexible CiGs has excel-
lent potential for space application because of their space-radiation-tolerant properties
which are 2–4 times superior to the conventional si and GaAs cells.
high record efficiencies of flexible CiGs solar cells are 17.7% on stainless steel, 18.6%
on enamelled steel and 20.4% on polymer foil [16–18].

18.5 Challenges

The main challenge for most thin film technologies is their stability. Amorphous silicon
cell efficiency falls instantly at module level with current commercial PV module efficien-
cies ranging around 4%–8% [25]. The SWE [26] causes a-Si cells to suffer light induced deg-
radation and the short orders in the amorphous material and dangling bonds also reduce
efficiency [10]. An important issue in CdTe solar cell technology is the formation of effi-
cient and stable ohmic contact on p-CdTe layer and avoiding Schottky contacts forming.
Another key issue for both CdTe and CiGs, is the toxicity of the materials involved. CdTe
technology relies on the toxic cadmium Cd, but there is some manoeuvrability in CiGs
technology in the elimination of very thin (typically ∼50 nm) CdS buffer layer.
Some key points in favor of CdTe in terms of environmental impact however are:

• Cd is produced as a by-product of Zn and can either be put to beneficial uses or
discharged to the environment posing another risk.
• CdTe in PV is much safer than other current Cd uses.
• CdTe PV uses Cd 2500 times more efficiently than ni-Cd batteries.
• occupational health risks are well managed.
• Absolutely no emission during PV operation.
• A risk from fire emission is minimal.
• CdTe technology and modules are safe and don’t pose significant risks.

similarly for perovskites, critical issues involve the stability of the organic spiro omeTAd
layer; and the need for Pb-free compounds in the cells, to cover the environmental con-
cerns [27]. In addition to improving material stability, the prevention of water, air and high
energy photons getting into the device and destroying the perovskite are current technol-
ogy aims. Currently, relatively low temperatures of around 95°C can cause degradation.