The Bridge - Issue 3, 2020 - 8

Feature

THE FUTURE OF RENEWABLE ENERGY GENERATION: Photovoltaic Materials

Low production rates are improved through
development of low-cost precursors and high-speed
processes. If successful, these approaches could
significantly reduce costs, but they involve multiple
innovations and development of novel material
processing technologies [2,3].

limitation on PV production [8]. However, scarcity
concerns, along with the toxicity of Cd, have led to
efficient recycling of CdTe PV materials, thus improving
sustainability. All of these established thin films are
subject to one common constraint: minimal ability to
manipulate the bandgap.

Thin Film Materials: CIGS, CZTS, and CdTe
Existing thin film technologies are based on a number
of inorganic thin film absorbers. These materials
all interact strongly with light, allowing them to be
thin. copper indium gallium sulfide (CIGS) is a longresearched semiconductor. While boasting impressive
champion efficiencies, the semiconductor properties
rely on a balance between the four elements.
This has represented a clear technical challenge in
translating laboratory-scale efficiencies to large-scale,
high yield production [4, 5]. A number of these
hurdles were overcome, but the decreased prices of
Si-based competitors undercut the cost advantage
for terrestrial solar - an application where other
aspects of the CIGS value proposition, e.g., light
weight, flexibility, are less important. The inability to
compete in terrestrial solar represents a significant
barrier to entry, as losing the volume represented
by this sector leaves companies difficult to sustain.
A related technology, copper zinc tin sulfide (CZTS),
targeted a set of materials that were abundant and
lower in cost, but CTZS appears to have more issues
with fundamental electronic defect structure. This
precludes it from competitive efficiency [6,7]. The
final inorganic absorber is cadmium telluride (CdTe)arguably the most successful PV technology - with
extremely high-speed production and very low cost.
Relative to CIGS, the reduced materials complexity
enables excellent manufacturability with reasonably
high efficiency, culminating in excellent dollar per Watt
value. However, CdTe has notable limitations. First,
Cd and Te are both relatively scarce in the earth's
crust. However, the real issue with the availability as
it relates to scaling a PV material to terawatt levels is
a combination of not only how much is present in
comparison to what is needed for PV and competing
uses, but also the relative accessibility along with the
cost of access, e.g., refining it. In the case of Cd and
Te, the elements' availability appears to place some

The Future of PV Materials

THE BRIDGE

When considering the future of PV absorber materials
in light of previous technologies, we can establish
some critical characteristics:
1) Ability to be manufactured rapidly at scale
(to reduce cost)
2) Strong absorption (to minimize material use
and facilitate carrier extraction)
3) Tunable bandgap (to enable tandems)
The latter two criteria come from well-known
fundamental materials considerations. The former
criterion is a bit more elusive. Considerable efforts
are being made in using computational approaches
to identify the next PV material. These efforts
seek abundant, readily forming materials with
semiconductor properties that are defect tolerant,
i.e., robust against disorder, either structurally and/or
electronically [9,10]. In the context of a PV absorber,
defect tolerance enables rapid manufacturing. Among
next generation PV materials, that is, materials that
have been explored seriously as PV absorbers only
in the last 20 years, two have a degree of defect
tolerance: organic photovoltaics (OPVs) and the even
more nascent metal halide perovskites (MHPs).
Organic PV
The OPV approach is to tailor organic molecules to
absorb light and produce long-lived carriers. In such
molecular solids, care must be given to the details
of the intermolecular packing and how it impacts the
resulting film [11]. Specifically, the use of a molecular
motif can negatively impact charge transport, often
limiting the harvesting ability of photogenerated
carriers. OPV materials have great tunability in their
optoelectronic properties and their interaction with
light can be tailored to be very strong in a specific
range of wavelengths. However, absorption is typically
more selective than inorganic semiconductors.

https://hkn.ieee.org/ https://hkn.ieee.org/

The Bridge - Issue 3, 2020

Table of Contents for the Digital Edition of The Bridge - Issue 3, 2020