The Bridge - Issue 3, 2020 - 9

THE FUTURE OF RENEWABLE ENERGY GENERATION: Photovoltaic Materials

This selectivity provides unique opportunities for
semitransparent applications, e.g., tinted windows or
building integrated PV, but limits the overall efficiency.
The selectivity of OPV can also enable tandems and
multijunction configurations, but neither has been
realized to date [4]. OPV has recently made significant
gains in single junction efficiencies [4]. This indicates
that the molecular motif can inhibit traditional
electronic defects, but the challenge of controlling the
microstructure to enable efficiency remains [12-15].
The vast space of molecular design, abundant atomic
components, and amenability to a variety of process
approaches makes efficient OPV an interesting
path forward.
Pb Metal Halide Perovskites
MHPs are perhaps the most promising and
potentially transformational PV material in a
generation. These halide-based compounds
can be organic/inorganic hybrids or made with
completely inorganic substituents [16]. In the last
decade, Pb-based compounds have increased
in champion cell efficiencies more rapidly than
any other thin film technology and are now the
most efficient polycrystalline device systems at the
laboratory scale. Pb-based MHP materials have
strong broad absorption and have a defect tolerant
electronic structure that enables them to be readily
manufactured [17,18]. MHPs are unique in their
ability to address the manufacturability challenge,
coupled with materials that have demonstrated the
latter criteria of strong absorption and tunability. These
attributes of MHPs also offer avenues to improve
all older technologies. Specifically, MHPs have been
demonstrated to improve Si and CIGS absorbers
to exceed the efficiency they can achieve on their
own [19-21]. Similar to III-V materials, all-perovskite
tandems have also been demonstrated but have yet
to exceed the high performance of single-junction
perovskite devices. Given the rapid advances, we
expect that this shortcoming will be overcome in the
near future [22-26].

Feature

Future Outlook
PV design is a demanding task, as it requires a
complex multilayer semiconductor to operate
outdoors for 30 years. The question of stability is tied
to EPT, in that EPT sets the threshold for operational
stability. A successful PV technology will operate at
timeframes of one or more orders of magnitude
greater than its EPT. While MHPs have very low EPTs
relative to Si, no owner of a PV system, individual
or utility, wants to deal with frequent maintenance.
This makes stability and durability critical questions to
address the technical challenge of stability is especially
monumental [27]. Field test data is available for Si
technologies, from real solar cells in real modules
that have been in operation for extended times, e.g.,
30 years. However, the evolution of Si PVs hampers
the ability to predict stability based on established
field data. This necessitates re-evaluation using
understanding and insight, as well as knowledge
of operational stresses. The prevalence of Si across
technologies has been leveraged for determining the
stability of Si PV. CdTe is a small fraction of deployed
PV, but the approach to evaluating its stability also
uses basic material insight. In the case of MHP, these
materials were not comprehensively studied until
approximately a decade ago, when they emerged for
PV applications. To enable the use of these materials
in deployed PV, the question of stability must be
addressed. That in turn requires basic material
understanding to make multidecade predictions for
MHP devices [28]. This frames the challenge for next
generation PV materials beyond the three criteria
enumerated earlier.
Stability and durability challenges for PV materials
are also critical to their sustainability. While MHPbased PV has yet to demonstrate multidecade
stability, projections indicate that the dramatically
reduced EPT of MHP-based PV could make even
more modest lifetimes of 1.5 decades competitive
with current PV technologies [29]. Lifetimes on
these timescales are less than those of established
Si and CdTe technologies, but they are of interest
particularly if recycling or refurbishment can be carried
out efficiently and at low cost. Despite the nascent
state of MHP PV, investigations of how to recycle and

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The Bridge - Issue 3, 2020

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