The Bridge - Issue 3, 2020 - 7

THE FUTURE OF RENEWABLE ENERGY GENERATION: Photovoltaic Materials

appropriate bandgap to most effectively harvest solar
irradiance. Given BOS expenses, there is a significant
premium on solar cells having high efficiency as
increases in efficiency provide an economic lever
to reduce overall costs. We focus on 1-sun, i.e.,
unconcentrated, operation most typical to residential
and utility-scale PV. (For latitudes of the United States,
1-sun is equivalent to 100 W/m2.)

PV Challenges
From a semiconductor perspective, III-V materials,
e.g. gallium arsenide (GaAs), gallium indium arsenide
(GaInAs), are well matched for PV applications.
These semiconductors interact strongly with light,
have highly mobile carriers, and have direct, tunable
electronic gaps. This last feature is critical to reaching
the highest efficiencies, as it permits minimal thermal
loss [1]. Additionally, III-Vs are lightweight, permitting
a range of potential deployment options; however,
III-V materials are not deployed terrestrially in most
applications. Rather, the elemental semiconductor
silicon (Si), which has relatively poor absorption and
no real tunability of its electronic gap, dominates
the existing PV material landscape. This discrepancy
highlights the importance of high-volume and lowcost manufacturing as an additional requirement
for designing PV materials. Si, which is an excellent
semiconductor, is used in multiple technologies; and
benefits from research relating to those technologies
as well as their associated supply chains. This situation
enables manufacturers to produce Si at scale, leading
to reduced production cost.
Life cycle analysis (LCA) and fundamental aspects of
energy input are also important aspects that should
be considered in evaluation of sustainability and
feasibility. Energy payback time (EPT) for a solar cell is
simply an assessment of the energy input to produce
a solar cell, versus the operational time required for
that solar cell to harvest enough energy to enable
its production. This concept of EPT incorporates the
complexity of mining and refining the materials (which
is often energy-intensive) as well as energy inputs to
production of all components of a solar cell. When
considering deployment of solar energy at terawatt
scale, EPT can provide insight about manufacturing
as well as recycling and reuse of materials. EPT

Feature

combined with LCA can provide important insight
regarding the sustainability of a technology. Given that
PVs are semiconductors, the resulting modules and
systems at large scale could, if sustainability is not
considered, represent a massive volume of electronic
waste materials.

Conventional PV
SINGLE JUNCTIONS
The need for efficiency argues for tandems that
are stacked semiconductor junctions with different
bandgaps. The different bandgaps allow more
photons across the spectrum to be absorbed.
All tandem PVs are predicated on having efficient
single junctions to pair. This reason, coupled with
a marked increase in complexity, has prevented
existing terrestrially deployed technology from
leveraging tandems.
Current PV Materials and Their Limits
We can assess key material characteristics for future
PV materials based on established PV technologies.
While Si dominates PV material based on EPT as
well as LCA considerations, the next generation of
PV materials will likely be thin films. While innovation
in Si is possible to continue to reduce costs, there is
little headroom in single-junction efficiency due to
fundamental Si material properties. The remainder of
this discussion will focus on thin film absorbers when
looking to the future.
III-V Materials
As indicated, III-V materials are impressive PV
materials; however, to enjoy their remarkable
efficiencies, they require single crystalline, highly
pure, and highly perfect materials. More critically,
they also require high embodied energy precursor
materials, which demand extreme efficiency from
resulting devices. To enable single crystal fabrication,
bulk crystal substrates are required, which are
energy intensive to produce. To achieve high-quality
semiconductors, growth of the active layer material
on these costly substrates occurs at relatively
low deposition and production rates relative to
polycrystalline thin films. There are active efforts to
develop new processing approaches to address these
limitations. Low deposition rates are addressed by
enabling reuse of the bulk crystalline substrate.

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The Bridge - Issue 3, 2020

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