37 research outputs found
Novel Plasma-Assisted Low-Temperature-Processed SnO<sub>2</sub> Thin Films for Efficient Flexible Perovskite Photovoltaics
The
recent evolution of solution-processed hybrid organic–inorganic
perovskite-based photovoltaic devices opens up the commercial avenue
for high-throughput roll-to-roll manufacturing technology. To circumvent
the thermal limitations that hinder the use of metal oxide charge
transport layers on plastic flexible substrates in such technologies,
we employed a relatively low-power nitrogen plasma treatment to achieve
compact SnO<sub>2</sub> thin-film electrodes at near room temperature.
The perovskite photovoltaic devices thus fabricated using N<sub>2</sub> plasma-treated SnO<sub>2</sub> performed on par with thermally annealed
SnO<sub>2</sub> electrodes and resulted in a power conversion efficiency
(PCE) of ca. 20.3% with stabilized power output (SPO) of ca. 19.1%
on rigid substrates. Furthermore, the process is extended to realize
flexible perovskite solar cells on indium tin oxide (ITO)-coated polyethylene
terephthalate (PET) substrates with champion PCE of 18.1% (SPO ca.
17.1%), which retained ca. 90% of its initial performance after 1000
bending cycles. Our investigations reveal that deep ultraviolet irradiation
associated with N<sub>2</sub> and N<sub>2</sub>O plasma emission plays
a major role in obtaining good quality metal oxide thin films at lower
temperatures and offers promise toward facile integration of a wide
variety of metal oxides on flexible substrates
Facile Photochemical Synthesis of Graphene-Pt Nanoparticle Composite for Counter Electrode in Dye Sensitized Solar Cell
A low temperature route to synthesize graphene oxide–Pt
nanoparticle hybrid composite by light assisted spontaneous coreduction
of graphene oxide and chloroplatinic acid without reducing agent is
demonstrated. Analysis indicates the importance of light as energy
provider and ethanol as hole scavenger in the formation of small Pt
nanoparticles (∼3 nm) on graphene oxide as well as graphene
oxide reduction. Spray coating was used to deposit the hybrid material
as a counter electrode in dye sensitized solar cells (DSCs). An efficiency
of 6.77% for the hybrid graphene counter electrode has been obtained,
higher than the control device made by low temperature sputtered Pt
as counter electrode. Compatibility of the hybrid material with flexible
plastic substrates was demonstrated yielding DSCs of an efficiency
of 4.05%
Modulating Cationic Ratios for High-Performance Transparent Solution-Processed Electronics
Amorphous oxide semiconductors such
as indium zinc tin oxide (IZTO) are considered favorites to serve
as channel materials for thin film transistors (TFTs) because they
combine high charge carrier mobility with high optical transmittance,
allowing for the development of transparent electronics. Although
the influence of relative cationic concentrations in determining the
electronic properties have been studied in sputtered and PLD films,
the development of printed transparent electronics hinges on such
dependencies being explored for solution-processed systems. Here,
we study solution-processed indium zinc tin oxide thin film transistors
(TFTs) to investigate variation in their electrical properties with
change in cationic composition. Charge transport mobility ranging
from 0.3 to 20.3 cm<sup>2</sup>/(V s), subthreshold swing ranging
from 1.2 to 8.4 V/dec, threshold voltage ranging from −50 to
5 V, and drain current on–off ratio ranging from 3 to 6 orders
of magnitude were obtained by examining different compositions of
the semiconductor films. Mobility was found to increase with the incorporation
of large cations such as In<sup>3+</sup> and Sn<sup>4+</sup> due to
the vast s-orbital overlap they can achieve when compared to the intercationic
distance. Subthreshold swing decreased with an increase in Zn<sup>2+</sup> concentration due to reduced interfacial state formation
between the semiconductor and dielectric. The optimized transistor
obtained at a compositional ratio of In/Zn/Sn = 1:1:1, exhibited a
high field-effect mobility of 8.62 cm<sup>2</sup>/(V s), subthreshold
swing of 1.75 V/dec, and current on–off ratio of 10<sup>6</sup>. Such impressive performances reaffirm the promise of amorphous
metal oxide semiconductors for printed electronics
High Stability Bilayered Perovskites through Crystallization Driven Self-Assembly
In this manuscript
we reveal the formation of bilayered hybrid perovskites of a new lower
dimensional perovskite family, (CHMA)<sub>2</sub>(MA)<sub><i>n</i>−1</sub>Pb<sub><i>n</i></sub>I<sub>3</sub> with <i>n</i> = 1–5, with high ambient stability
via its crystallization driven self-assembly process. The spun-coated
perovskite solution tends to crystallize and undergo phase separation
during annealing, resulting in the formation of 2D/3D bilayered hybrid
perovskites. Remarkably, this 2D/3D hybrid perovskites possess striking
moisture resistance and displays high ambient stability up to 65 days.
The bilayered approach in combining 3D and 2D perovskites could lead
to a new era of perovskite research for high-efficiency photovoltaics
with outstanding stability, with the 3D perovskite providing excellent
electronic properties while the 2D perovskite endows it moisture stability
Poor Photovoltaic Performance of Cs<sub>3</sub>Bi<sub>2</sub>I<sub>9</sub>: An Insight through First-Principles Calculations
Bismuth-based
halide perovskite derivatives have now attracted
huge attention for photovoltaic (PV) applications after the unparalleled
success of lead-based halide perovskites. However, the performances
of PV devices based on these compounds are poor, despite theoretical
predictions. In this Article, we have investigated the electronic
structure and defect formation energies of Cs<sub>3</sub>Bi<sub>2</sub>I<sub>9</sub> using density functional theory (DFT) calculations.
The calculated electronic bandstructure indicates an indirect bandgap
and high carrier effective masses. Our calculations reveal a large
stability region for this compound; however, deep level defects are
quite prominent. Even the varying chemical potentials from the stoichiometric
region do not eliminate the presence of deep defects, ultimately limiting
photovoltaic efficiencies
Perovskite–Hematite Tandem Cells for Efficient Overall Solar Driven Water Splitting
Photoelectrochemical water splitting half reactions on semiconducting photoelectrodes have received much attention but efficient overall water splitting driven by a single photoelectrode has remained elusive due to stringent electronic and thermodynamic property requirements. Utilizing a tandem configuration wherein the total photovoltage is generated by complementary optical absorption across different semiconducting electrodes is a possible pathway to unassisted overall light-induced water splitting. Because of the low photovoltages generated by conventional photovoltaic materials (e.g., Si, CIGS), such systems typically consist of triple junction design that increases the complexity due to optoelectrical trade-offs and are also not cost-effective. Here, we show that a single solution processed organic–inorganic halide perovskite (CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>) solar cell in tandem with a Fe<sub>2</sub>O<sub>3</sub> photoanode can achieve overall unassisted water splitting with a solar-to-hydrogen conversion efficiency of 2.4%. Systematic electro-optical studies were performed to investigate the performance of tandem device. It was found that the overall efficiency was limited by the hematite’s photocurrent and onset potential. To understand these limitations, we have estimated the intrinsic solar to chemical conversion efficiency of the doped and undoped Fe<sub>2</sub>O<sub>3</sub> photoanodes. The total photopotential generated by our tandem system (1.87 V) exceeds both the thermodynamic and kinetic requirements (1.6 V), resulting in overall water splitting without the assistance of an electrical bias
Interfacial Charge Transfer Anisotropy in Polycrystalline Lead Iodide Perovskite Films
Solar cells based on organic–inorganic
lead iodide perovskite
(CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>) exhibit remarkably high
power conversion efficiency (PCE). One of the key issues in solution-processed
films is that often the polycrystalline domain orientation is not
well-defined, which makes it difficult to predict energy alignment
and charge transfer efficiency. Here we combine ab initio calculations
and photoelectron spectroscopy to unravel the electronic structure
and charge redistribution at the interface between different surfaces
of CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> and typical organic
hole acceptor Spiro-OMeTAD and electron acceptor PCBM. We find that
both hole and electron interfacial transfer depend strongly on the
CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> surface orientation: while
the (001) and (110) surfaces tend to favor hole injection to Spiro-OMeTAD,
the (100) surface facilitates electron transfer to PCBM due to surface
delocalized charges and hole/electron accumulation at the CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>/organic interfaces. Molecular dynamic
simulations indicate that this is due to strong orbital interactions
under thermal fluctuations at room temperature, suggesting the possibility
to further improve charge separation and extraction in perovskite-based
solar cells by controlling perovskite film crystallization and surface
orientation
Low-Temperature Chemical Transformations for High-Performance Solution-Processed Oxide Transistors
The
challenges associated with low-temperature solution-processed
metal oxide network formation have hindered the realization of high-performance
solution-based electronic circuitry at temperatures lower than 200
°C. Here, UV irradiation is embarked upon as a route to effectively
transform the chemical precursors to semiconducting metal oxides with
high electrical quality. High-performance UV-irradiated indium oxide
(In<sub>2</sub>O<sub>3</sub>) and indium zinc oxide (IZO) thin film
transistors with mobility greater than 30 cm<sup>2</sup>/(V s) have
been obtained from nitrate-based precursors. The chemical transformation
has been monitored by detailed spectroscopic studies, physical characterization,
and temperature-dependent electrical transport measurements. In comparison
to thermal annealing, UV annealing seems to result in higher M–O–M
network formation (depicted by M–O bonds in XPS), better removal
of chemical impurities (depicted by FTIR and XPS), and structural
relaxation driven electron doping, transforming the oxygen vacancies
to act as shallow donors (depicted by TFT characteristics, XPS, XRD,
and Urbach studies). Our results provide new insight into how UV irradiation
drives metal oxide network formation and passivates the subgap density
of states (DOS)
Unravelling the Effects of Cl Addition in Single Step CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> Perovskite Solar Cells
The
effect of film morphology on the photovoltaic performances
is illustrated. Pin holes free and uniform films could be formed by
slowing down the crystallization rate through additional of excess
CH<sub>3</sub>NH<sub>3</sub><sup>+</sup>. Though better film coverage
is obtained, the device series resistance increases as excess CH<sub>3</sub>NH<sub>3</sub>I increases. Thermogravimetric analysis confirms
the presence of residual excess CH<sub>3</sub>NH<sub>3</sub>I in the
final film. When chlorine is added to the CH<sub>3</sub>NH<sub>3</sub><sup>+</sup> rich system, CH<sub>3</sub>NH<sub>3</sub>Cl is formed
and can easily sublime during annealing at 100 °C. A little trace
of excess CH<sub>3</sub>NH<sub>3</sub>Cl can be detected from thermogravic
analysis, and the residual chlorine atoms match with XPS reading
Highly Spin-Polarized Carrier Dynamics and Ultralarge Photoinduced Magnetization in CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> Perovskite Thin Films
Low-temperature
solution-processed organic–inorganic halide perovskite CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> has demonstrated great potential
for photovoltaics and light-emitting devices. Recent discoveries of
long ambipolar carrier diffusion lengths and the prediction of the
Rashba effect in CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>, that
possesses large spin–orbit coupling, also point to a novel
semiconductor system with highly promising properties for spin-based
applications. Through circular pump–probe measurements, we
demonstrate that highly polarized electrons of total angular momentum
(<i>J</i>) with an initial degree of polarization <i>P</i><sub>ini</sub> ∼ 90% (i.e., −30% degree of
electron spin polarization) can be photogenerated in perovskites.
Time-resolved Faraday rotation measurements reveal photoinduced Faraday
rotation as large as 10°/μm at 200 K (at wavelength λ
= 750 nm) from an ultrathin 70 nm film. These spin polarized carrier
populations generated within the polycrystalline perovskite films,
relax via intraband carrier spin-flip through the Elliot-Yafet mechanism.
Through a simple two-level model, we elucidate the electron spin relaxation
lifetime to be ∼7 ps and that of the hole is ∼1 ps.
Our work highlights the potential of CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> as a new candidate for ultrafast spin switches in spintronics
applications