3 research outputs found
Impacts of Ion Segregation on Local Optical Properties in Mixed Halide Perovskite Films
Despite
the recent astronomical success of organic–inorganic perovskite
solar cells (PSCs), the impact of microscale film inhomogeneities
on device performance remains poorly understood. In this work, we
study CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> perovskite films
using cathodoluminescence in scanning transmission electron microscopy
and show that localized regions with increased cathodoluminescence
intensity correspond to iodide-enriched regions. These observations
constitute direct evidence that nanoscale stoichiometric variations
produce corresponding inhomogeneities in film cathodoluminescence
intensity. Moreover, we observe the emergence of high-energy transitions
attributed to beam induced iodide segregation, which may mirror the
effects of ion migration during PSC operation. Our results demonstrate
that such ion segregation can fundamentally change the local optical
and microstructural properties of organic–inorganic perovskite
films in the course of normal device operation and therefore address
the observed complex and unpredictable behavior in PSC devices
Dimension- and Surface-Tailored ZnO Nanowires Enhance Charge Collection in Quantum Dot Photovoltaic Devices
The use of zinc oxide
(ZnO) nanowires improves charge collection,
and consequently power conversion efficiency, in quantum dot (QD)
based photovoltaic devices. However, the role of the nanowire geometry
(e.g., density, length, and morphology, etc.) relative to the QD properties
remains unexplored, in part due to challenges with controlled nanowire
synthesis. Here, we independently tailor nanowire length and the active
device layer thickness to study charge collection in lead sulfide
(PbS) QD photovoltaic devices. We then demonstrate consistently high
internal quantum efficiency in these devices by applying quantum efficiency
and total reflectance measurements. Our results show that significant
losses originate from ZnO nanowire–QD interfacial recombination,
which we then successfully overcome by using nanowire surface passivation.
This geometry-tailored approach is generally applicable to other nanowire–QD
systems, and the surface passivation schemes will play a significant
role in future development of nanostructured photovoltaics
Transition Metal-Oxide Free Perovskite Solar Cells Enabled by a New Organic Charge Transport Layer
Various
electron and hole transport layers have been used to develop
high-efficiency perovskite solar cells. To achieve low-temperature
solution processing of perovskite solar cells, organic n-type materials
are employed to replace the metal oxide electron transport layer (ETL).
Although PCBM (phenyl-C<sub>61</sub>-butyric acid methyl ester) has
been widely used for this application, its morphological instability
in films (i.e., aggregation) is detrimental. Herein, we demonstrate
the synthesis of a new fullerene derivative (isobenzofulvene–C<sub>60</sub>–epoxide, IBF–Ep) that serves as an electron
transporting material for methylammonium mixed lead halide-based perovskite
(CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3–<i>x</i></sub>Cl<sub><i>x</i></sub>) solar cells, both in the normal
and inverted device configurations. We demonstrate that IBF–Ep
has superior morphological stability compared to the conventional
acceptor, PCBM. IBF–Ep
provides higher photovoltaic device performance as compared to PCBM
(6.9% vs 2.5% in the normal and 9.0% vs 5.3% in the inverted device
configuration). Moreover, IBF–Ep devices show superior tolerance
to high humidity (90%) in air. By reaching power conversion efficiencies
up to 9.0% for the inverted devices with IBF–Ep as the ETL,
we demonstrate the potential of this new material as an alternative
to metal oxides for perovskite solar cells processed in air