Page 357 - A Comprehensive Guide to Solar Energy Systems

362 A ComPRehensiVe Guide To soLAR eneRGy sysTems

• Flexible and lightweight PV facilitates several attractive applications.

A summary of the thin film properties is given in Table 18.1.
However, the efficiency and the robustness of thin film PV technologies have to match
or exceed the current c-Si technology to be competitive in the market share. Once opti-
mized, the manufacturing methods can be scaled up and provide factors of magnitude
cheaper processing cost and low energy payback time in comparison to c-Si. Currently,
amorphous silicon (a-Si), cadmium telluride (CdTe), and copper-indium gallium di-selenide
(CiGs) are considered the mainstream thin-film technologies. The world record efficien-
cy of 23.3% has been obtained by nReL for CiGs cells under 14.7× concentration and of
22.6% without concentration by ZSW [2]. CdTe technology is not far behind with a record
research-cell efficiency of 22.1% obtained by First solar [2] and for CiGs and CdTe this
substantial progress has occurred over a short span of time [2]. For amorphous silicon the
stabilized record cell efficiency is behind at 14% by AisT [2]. So, although a-Si had prom-
ising manufacturing targets a decade ago, it lost out to competition due to its intrinsic
issues of light dependent degradation referred to as Staebler Wronsky Effect (SWE). With
the cost of c-si modules available well below $0.4 per Wp and with life time guarantees of
25 years, a-Si technology, although a robust option, cannot compete without efficiencies
levels matching around 20%.
Recent developments on solid state perovskite based solar cells (a kind of solid state
analogue of dye-sensitised solar cells) are equally promising. In year 2012–13 alone the
efficiency figures acquired an unprecedented leap from near 4% [3] to 17% and are now
currently 22.7% [2,4]. These technologies with their low-temperature and cheaper pro-
cessing cost on flexible substrates [5,6] make them potentially very attractive as a cost
effective option for high volume manufacturing. However, there are issues of instability
and intrinsic issues that need to be addressed. The material, methyl ammonium lead
iodide (MAPI), or CH 3 NH 4 PbI 3 , is labile and can be attacked by moisture in the atmo-
sphere as well as undergo a phase change at 93°C. These are considered to be the major
scientific and engineering challenges. Worldwide attempts are underway to overcome
these challenges along with the replacement of Pb in the materials by Sn (Tin) or other
benign elements to overcome the environmental concerns.

Table 18.1 Summary of Thin Film Properties
c-Si a-Si (stabilised) CdTe CIGS Perovskite
Absorption ∼1 × 10 4 1.7 × 10 6 1.1 × 10 6 >1 × 10 5 1.5 × 10 4
coefficient/cm −1
Direct band gap/eV 3.4 1.75 1.44 1–1.6 1.55
Sufficient 170–200 1 3–5 1–2 0.6–2
thickness/µm
Record cell 26.6 14 22.1 22.6 22.7
efficiency/%