5 research outputs found
Improved Quantum Efficiency of Highly Efficient Perovskite BaSnO<sub>3</sub>‑Based Dye-Sensitized Solar Cells
Ternary oxides are potential candidates as an electron-transporting material that can replace TiO<sub>2</sub> in dye-sensitized solar cells (DSSCs), as their electronic/optical properties can be easily controlled by manipulating the composition and/or by doping. Here, we report a new highly efficient DSSC using perovskite BaSnO<sub>3</sub> (BSO) nanoparticles. In addition, the effects of a TiCl<sub>4</sub> treatment on the physical, chemical, and photovoltaic properties of the BSO-based DSSCs are investigated. The TiCl<sub>4</sub> treatment was found to form an ultrathin TiO<sub>2</sub> layer on the BSO surface, the thickness of which increases with the treatment time. The formation of the TiO<sub>2</sub> shell layer improved the charge-collection efficiency by enhancing the charge transport and suppressing the charge recombination. It was also found that the TiCl<sub>4</sub> treatment significantly reduces the amount of surface OH species, resulting in reduced dye adsorption and reduced light-harvesting efficiency. The trade-off effect between the charge-collection and light-harvesting efficiencies resulted in the highest quantum efficiency (<i>i</i>.<i>e</i>., short-circuit photocurrent density), leading to the highest conversion efficiency of 5.5% after a TiCl<sub>4</sub> treatment of 3 min (<i>cf</i>. 4.5% for bare BSO). The conversion efficiency could be increased further to 6.2% by increasing the thickness of the BSO film, which is one of the highest efficiencies from non-TiO<sub>2</sub>-based DSSCs
Solvent-Engineering Method to Deposit Compact Bismuth-Based Thin Films: Mechanism and Application to Photovoltaics
Bismuth-based materials
have been studied as alternatives to lead-based
perovskite materials for photovoltaic applications. However, poor
film quality has limited device performance. In this work, we developed
a solvent-engineering method and show that it is applicable to several
bismuth-based compounds. Through this method, we obtained compact
films of methylammonium bismuth iodide (MBI), cesium bismuth iodide
(CBI), and formamidinium bismuth iodide (FBI). On the basis of film
growth theory and experimental analyses, we propose a possible mechanism
of film formation. Additionally, we demonstrate that the resultant
compact MBI film is more suitable to fabricate efficient and stable
photovoltaic devices compared to baseline MBI films with pinholes.
We further employed a new hole-transporting material to reduce the
valence-band offset with the MBI. The best-performing photovoltaic
device exhibits an open-circuit voltage of 0.85 V, fill factor of
73%, and a power conversion efficiency of 0.71%, the highest reported
values for MBI-based photovoltaic devices
Zn<sub>2</sub>SnO<sub>4</sub>‑Based Photoelectrodes for Organolead Halide Perovskite Solar Cells
We
report a new ternary Zn<sub>2</sub>SnO<sub>4</sub> (ZSO) electron-transporting
electrode of a CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> perovskite
solar cell as an alternative to the conventional TiO<sub>2</sub> electrode.
The ZSO-based perovskite solar cells have been prepared following
a conventional procedure known as a sequential (or two-step) process
with ZSO compact/mesoscopic layers instead of the conventional TiO<sub>2</sub> counterparts, and their solar cell properties have been investigated
as a function of the thickness of either the ZSO compact layer or
the ZSO mesoscopic layer. The presence of the ZSO compact layer has
a negligible influence on the transmittance of the incident light
regardless of its thickness, whereas the thickest compact layer blocks
the back-electron transfer most efficiently. The open-circuit voltage
and fill factor increase with the increasing thickness of the mesoscopic
ZSO layer, whereas the short-circuit current density decreases with
the increasing thickness except for the thinnest one (∼100
nm). As a result, the device with a 300 nm-thick mesoscopic ZSO layer
shows the highest conversion efficiency of 7%. In addition, time-resolved
and frequency-resolved measurements reveal that the ZSO-based perovskite
solar cell exhibits faster electron transport (∼10 times) and
superior charge-collection capability compared to the TiO<sub>2</sub>-based counterpart with similar thickness and conversion efficiency
<i>A</i>‑Site Cation in Inorganic <i>A</i><sub>3</sub>Sb<sub>2</sub>I<sub>9</sub> Perovskite Influences Structural Dimensionality, Exciton Binding Energy, and Solar Cell Performance
Inspired
by the rapid rise in efficiencies of lead halide perovskite
(LHP) solar cells, lead-free alternatives are attracting increasing
attention. In this work, we study the photovoltaic potential of solution-processed
antimony (Sb)-based compounds with the formula <i>A</i><sub>3</sub>Sb<sub>2</sub>I<sub>9</sub> (<i>A</i> = Cs, Rb,
and K). We experimentally determine bandgap magnitude and type, structure,
carrier lifetime, exciton binding energy, film morphology, and photovoltaic
device performance. We use density functional theory to compute the
equilibrium structures, band structures, carrier effective masses,
and phase stability diagrams. We find the <i>A</i>-site
cation governs the structural and optoelectronic properties of these
compounds. Cs<sub>3</sub>Sb<sub>2</sub>I<sub>9</sub> has a 0D structure,
the largest exciton binding energy (175 ± 9 meV), an indirect
bandgap, and, in a solar cell, low photocurrent (0.13 mA cm<sup>–2</sup>). Rb<sub>3</sub>Sb<sub>2</sub>I<sub>9</sub> has a 2D structure,
a direct bandgap, and, among the materials investigated, the lowest
exciton binding energy (101 ± 6 meV) and highest photocurrent
(1.67 mA cm<sup>–2</sup>). K<sub>3</sub>Sb<sub>2</sub>I<sub>9</sub> has a 2D structure, intermediate exciton binding energies
(129 ± 9 meV), and intermediate photocurrents (0.41 mA cm<sup>–2</sup>). Despite remarkably long lifetimes in all compounds
(54, 9, and 30 ns for Cs-, Rb-, and K-based materials, respectively),
low photocurrents limit performance of all devices. We conclude that
carrier collection is limited by large exciton binding energies (experimentally
observed) and large carrier effective masses (calculated from density
functional theory). The highest photocurrent and efficiency (0.76%)
were observed in the Rb-based compound with a direct bandgap, relatively
lower exciton binding energy, and lower calculated electron effective
mass. To reliably screen for candidate lead-free photovoltaic absorbers,
we advise that faster and more accurate computational tools are needed
to calculate exciton binding energies and effective masses
Improving the Carrier Lifetime of Tin Sulfide via Prediction and Mitigation of Harmful Point Defects
Tin monosulfide (SnS) is an emerging
thin-film absorber material
for photovoltaics. An outstanding challenge is to improve carrier
lifetimes to >1 ns, which should enable >10% device efficiencies.
However, reported results to date have only demonstrated lifetimes
at or below 100 ps. In this study, we employ defect modeling to identify
the sulfur vacancy and defects from Fe, Co, and Mo as most recombination-active.
We attempt to minimize these defects in crystalline samples through
high-purity, sulfur-rich growth and experimentally improve lifetimes
to >3 ns, thus achieving our 1 ns goal. This framework may prove
effective
for unlocking the lifetime potential in other emerging thin-film materials
by rapidly identifying and mitigating lifetime-limiting point defects