6 research outputs found
Highly Controlled Codeposition Rate of Organolead Halide Perovskite by Laser Evaporation Method
Organolead-halide
perovskites can be promising materials for next-generation
solar cells because of its high power conversion efficiency. The method
of precise fabrication is required because both solution-process and
vacuum-process fabrication of the perovskite have problems of controllability
and reproducibility. Vacuum deposition process was expected to achieve
precise control; however, vaporization of amine compound significantly
degrades the controllability of deposition rate. Here we achieved
the reduction of the vaporization by implementing the laser evaporation
system for the codeposition of perovskite. Locally irradiated continuous-wave
lasers on the source materials realized the reduced vaporization of
CH<sub>3</sub>NH<sub>3</sub>I. The deposition rate was stabilized
for several hours by adjusting the duty ratio of modulated laser based
on proportional-integral control. Organic-photovoltaic-type perovskite
solar cells were fabricated by codeposition of PbI<sub>2</sub> and
CH<sub>3</sub>NH<sub>3</sub>I. A power-conversion efficiency of 16.0%
with reduced hysteresis was achieved
Glancing Angle Deposition of Copper Iodide Nanocrystals for Efficient Organic Photovoltaics
We report a simple method to achieve efficient nanostructured
organic
photovoltaics via patterning copper iodide (CuI) nanocrystals on indium
tin oxide by glancing angle deposition. The strong interfacial interaction
between zinc phthalocyanine (ZnPc) and CuI leads to the formation
of nanopillar arrays with lying-down molecular order, which greatly
improve light absorption and surface roughness for exciton dissociation.
Optimized ZnPc/C<sub>60</sub> bilayer cell has a power conversion
efficiency of 4.0 ± 0.1%, which is about 3-fold larger than that
of conventional planar cell
Understanding Device-Structure-Induced Variations in Open-Circuit Voltage for Organic Photovoltaics
We
investigate the structural influences on the device performance, especially
on open-circuit voltage (<i>V</i><sub>OC</sub>) in squaraine
(SQ)/fullerene (C<sub>60</sub>) bilayer cells. Simply changing the
SQ thickness could lead to 40% variation in <i>V</i><sub>OC</sub> from 0.62 to 0.86 V. The ionization potential (IP) of SQ
films and recombination at the anode surface as well as donor/acceptor
(D/A) interface sensitively vary with film thicknesses, which account
for the shifts in <i>V</i><sub>OC</sub>. The anode recombination
can be effectively suppressed by preventing direct contact between
C<sub>60</sub> and the anode with a buffer layer, delivering an elevated <i>V</i><sub>OC</sub>. Through polarized infrared–multiple-angle
incidence resolution spectroscopy measurement, the molecular structure
of SQ films is found to gradually evolve from lying-down on indium–tin
oxide substrates with noncentrosymmetric orientation at low thicknesses
to random structure at high thicknesses. The different molecular orientation
may yield different strengths of electronic coupling, which affects
the charge-carrier recombination and thus <i>V</i><sub>OC</sub>. Moreover, the oriented SQ films would spontaneously compose aligned
dipole moments at the D/A interface because of the strong dipolar
effects in SQ molecules identified by density functional theory calculations,
whereas no aligned interfacial dipole moment exists in the random
structure. The resulting interfacial dipole moments would form an
electric field at the D/A interface, leading to variations in the
IP and thus impacting <i>V</i><sub>OC</sub>. Our findings
demonstrate that <i>V</i><sub>OC</sub> in organic photovoltaic
cells is critically associated with the molecular orientation that
affects the charge-carrier recombination and interfacial dipole alignment,
which should be seriously taken into consideration for the design
of organic molecules and optimization of the cell efficiency
Crystallization Dynamics of Organolead Halide Perovskite by Real-Time X‑ray Diffraction
We analyzed the crystallization process
of the CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> perovskite by observing
real-time X-ray diffraction immediately after combining a PbI<sub>2</sub> thin film with a CH<sub>3</sub>NH<sub>3</sub>I solution.
A detailed analysis of the transformation kinetics demonstrated the
fractal diffusion of the CH<sub>3</sub>NH<sub>3</sub>I solution into
the PbI<sub>2</sub> film. Moreover, the perovskite crystal was found
to be initially oriented based on the PbI<sub>2</sub> crystal orientation
but to gradually transition to a random orientation. The fluctuating
characteristics of the crystallization process of perovskites, such
as fractal penetration and orientational transformation, should be
controlled to allow the fabrication of high-quality perovskite crystals.
The characteristic reaction dynamics observed in this study should
assist in establishing reproducible fabrication processes for perovskite
solar cells
Adjustment of Conduction Band Edge of Compact TiO<sub>2</sub> Layer in Perovskite Solar Cells Through TiCl<sub>4</sub> Treatment
Perovskite
solar cells (PSCs) without a mesoporous TiO<sub>2</sub> layer, that
is, planar-type PSCs exhibit poorer cell performance as compared to
PSCs with a porous TiO<sub>2</sub> layer, owing to inefficient electron
transfer from the perovskite layer to the compact TiO<sub>2</sub> layer
in the former case. The matching of the conduction band levels of
perovskite and the compact TiO<sub>2</sub> layer is thus essential
for enhancing PSC performance. In this study, we demonstrate the shifting
of the conduction band edge (CBE) of the compact TiO<sub>2</sub> layer
through a TiCl<sub>4</sub> treatment, with the aim of improving PSC
performance. The CBE of the compact TiO<sub>2</sub> layer was shifted
to a higher level through the TiCl<sub>4</sub> treatment and then
shifted in the opposite direction, that is, to a lower level, through
a subsequent heat treatment. These shifts in the CBE were reflected
in the PSC performance. The TiCl<sub>4</sub>-treated PSC showed an
increase in the open-circuit voltage of more than 150 mV, as well
as a decrease of 100 mV after being heated at 450 °C. On the
other hand, the short-circuit current decreased after the treatment
but increased after heating at temperatures higher than 300 °C.
The treated PSC subjected to subsequent heating at 300 °C exhibited
the best performance, with the power conversion efficiency of the
PSC being 17% under optimized conditions
Epitaxial Growth of C<sub>60</sub> on Rubrene Single Crystals for a Highly Ordered Organic Donor/Acceptor Interface
The
highly ordered epitaxial growth of C<sub>60</sub> films on
rubrene single crystals was demonstrated. The C<sub>60</sub> crystals
growth commensurate with rubrene (001) surface lattice was confirmed
by reflection high energy electron diffraction and X-ray diffraction
and grazing incident wide-angle X-ray scattering. Depending on growth
conditions, several surface morphologies (rounded, hexagonal, and
triangular grains) of C<sub>60</sub> grains were observed at the initial
growth process. Large grains with layer-by-layer step and terrace
structure of C<sub>60</sub> were observed at the terrace region of
rubrene (001) surface. The growth of high-crystallinity C<sub>60</sub> films was achieved at high substrate temperature and slow deposition
rate. Long migration length on the substrate which was given by enough
substrate temperature enabled the formation of large single crystalline
domains