9 research outputs found
Trap and Transfer. Two-Step Hole Injection Across the Sb<sub>2</sub>S<sub>3</sub>/CuSCN Interface in Solid-State Solar Cells
In solid-state semiconductor-sensitized solar cells, commonly known as extremely thin absorber (ETA) or solid-state quantum-dot-sensitized solar cells (QDSCs), transfer of photogenerated holes from the absorber species to the p-type hole conductor plays a critical role in the charge separation process. Using Sb<sub>2</sub>S<sub>3</sub> (absorber) and CuSCN (hole conductor), we have constructed ETA solar cells exhibiting a power conversion efficiency of 3.3%. The hole transfer from excited Sb<sub>2</sub>S<sub>3</sub> into CuSCN, which limits the overall power conversion efficiency of these solar cells, is now independently studied using transient absorption spectroscopy. In the Sb<sub>2</sub>S<sub>3</sub> absorber layer, photogenerated holes are rapidly localized on the sulfur atoms of the crystal lattice, forming a sulfide radical (S<sup>–•</sup>) species. This trapped hole is transferred from the Sb<sub>2</sub>S<sub>3</sub> absorber to the CuSCN hole conductor with an exponential time constant of 1680 ps. This process was monitored through the spectroscopic signal seen for the S<sup>–•</sup> species in Sb<sub>2</sub>S<sub>3</sub>, providing direct evidence for the hole transfer dynamics in ETA solar cells. Elucidation of the hole transfer mechanism from Sb<sub>2</sub>S<sub>3</sub> to CuSCN represents a significant step toward understanding charge separation in Sb<sub>2</sub>S<sub>3</sub> solar cells and provides insight into the design of new architectures for higher efficiency devices
CdSeS Nanowires: Compositionally Controlled Band Gap and Exciton Dynamics
CdS, CdSe, and ternary CdSe<sub><i>x</i></sub>S<sub>(1–<i>x</i>)</sub> are some of the most widely studied II–VI semiconductors due to their broad range of applications and promising performance in numerous systems. One-dimensional semiconductor nanowires offer the ability to conduct charges efficiently along the length of the wire, which has potential charge transport benefits compared to nanoparticles. Herein, we report a simple, inexpensive synthetic procedure for high quality CdSeS nanowires where the composition can be easily modulated from pure CdSe to pure CdS by simply adjusting the Se:S precursor ratio. This allows for tuning of the absorption and emission properties of the nanowires across the visible spectrum. The CdSeS nanowires have a wurtzite crystal structure and grow along the [001] direction. As measured by femtosecond transient absorption spectroscopy, the short component of the excited state lifetime remains relatively constant at ∼10 ps with increasing Se; however, the contribution of this short lifetime component increased dramatically from 8.4% to 57.7% with increasing Se content. These CdSeS nanowires offer facile synthesis and widely adjustable optical properties, characteristics that give them broad potential applications in the fields of optoelectronics, and photovoltaics
CdSeS Nanowires: Compositionally Controlled Band Gap and Exciton Dynamics
CdS, CdSe, and ternary CdSe<sub><i>x</i></sub>S<sub>(1–<i>x</i>)</sub> are some of the most widely studied II–VI semiconductors due to their broad range of applications and promising performance in numerous systems. One-dimensional semiconductor nanowires offer the ability to conduct charges efficiently along the length of the wire, which has potential charge transport benefits compared to nanoparticles. Herein, we report a simple, inexpensive synthetic procedure for high quality CdSeS nanowires where the composition can be easily modulated from pure CdSe to pure CdS by simply adjusting the Se:S precursor ratio. This allows for tuning of the absorption and emission properties of the nanowires across the visible spectrum. The CdSeS nanowires have a wurtzite crystal structure and grow along the [001] direction. As measured by femtosecond transient absorption spectroscopy, the short component of the excited state lifetime remains relatively constant at ∼10 ps with increasing Se; however, the contribution of this short lifetime component increased dramatically from 8.4% to 57.7% with increasing Se content. These CdSeS nanowires offer facile synthesis and widely adjustable optical properties, characteristics that give them broad potential applications in the fields of optoelectronics, and photovoltaics
Wide Dynamic Range Sensing with Single Quantum Dot Biosensors
Single-particle analysis of biosensors that use charge transfer as the means for analyte-dependent signaling with semiconductor nanoparticles, or quantum dots, was examined. Single-particle analysis of biosensors that use energy transfer show analyte-dependent switching of nanoparticle emission from off to on. The charge-transfer-based biosensors reported here show constant emission, where the analyte (maltose) increases the emission intensity. By monitoring the same nanoparticles under various conditions, a single charge-transfer-based biosensor construct (one maltose binding protein, one protein attachment position for the reductant, one type of nanoparticle) showed a dynamic range for analyte (maltose) detection spanning from 100 pM to 10 μM while the emission intensities increase from 25 to 175% at the single-particle level. Since these biosensors were immobilized, the correlation between the detected maltose concentration and the maltose-dependent emission intensity increase could be examined. Minimal correlation between maltose detection limits and emission increases was observed, suggesting a variety of reductant-nanoparticle surface interactions that control maltose-dependent emission intensity responses. Despite the heterogeneous responses, monitoring biosensor emission intensity over 5 min provided a quantifiable method to monitor maltose concentration. Immobilizing and tracking these biosensors with heterogeneous responses, however, expanded the analyte-dependent emission intensity and the analyte dynamic range obtained from a single construct. Given the wide dynamic range and constant emission of charge-transfer-based biosensors, applying these single molecule techniques could provide ultrasensitive, real-time detection of small molecules in living cells
Monitoring a Silent Phase Transition in CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> Solar Cells via <i>Operando</i> X‑ray Diffraction
The
relatively modest temperature of the tetragonal-to-cubic phase
transition in CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> perovskite
is likely to occur during real world operation of CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> solar cells. In this work, we simultaneously
monitor the structural phase transition of the active layer along
with solar cell performance as a function of the device operating
temperature. The tetragonal to cubic phase transition is observed
in the working device to occur reversibly at temperatures between
60.5 and 65.4 °C. In these <i>operando</i> measurements,
no discontinuity in the device performance is observed, indicating
electronic behavior that is insensitive to the structural phase transition.
This decoupling of device performance from the change in long-range
order across the phase transition suggests that the optoelectronic
properties are primarily determined by the local structure in CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>. That is, while the average
crystal structure as probed by X-ray diffraction shows a transition
from tetragonal to cubic, the local structure generally remains well
characterized by uncorrelated, dynamic octahedral rotations that order
at elevated temperatures but are unchanged locally
High-Work-Function Molybdenum Oxide Hole Extraction Contacts in Hybrid Organic–Inorganic Perovskite Solar Cells
We
investigate the effect of high work function contacts in halide
perovskite absorber-based photovoltaic devices. Photoemission spectroscopy
measurements reveal that band bending is induced in the absorber by
the deposition of the high work function molybdenum trioxide (MoO<sub>3</sub>). We find that direct contact between MoO<sub>3</sub> and
the perovskite leads to a chemical reaction, which diminishes device
functionality. Introducing an ultrathin spiro-MeOTAD buffer layer
prevents the reaction, yet the altered evolution of the energy levels
in the methylammonium lead iodide (MAPbI<sub>3</sub>) layer at the
interface still negatively impacts device performance
Degradation of Highly Alloyed Metal Halide Perovskite Precursor Inks: Mechanism and Storage Solutions
Whereas the promise
of metal halide perovskite (MHP) photovoltaics
(PV) is that they can combine high efficiency with solution-processability,
the chemistry occurring in precursor inks is largely unexplored. Herein,
we investigate the degradation of MHP solutions based on the most
widely used solvents, dimethylformamide (DMF) and dimethyl sulfoxide
(DMSO). For the MHP inks studied, which contain formamidinium (FA<sup>+</sup>), methylammonium (MA<sup>+</sup>), cesium (Cs<sup>+</sup>), lead (Pb<sup>2+</sup>), bromide (Br<sup>–</sup>), and iodide
(I<sup>–</sup>), dramatic compositional changes are observed
following storage of the inks in nitrogen in the dark. We show that
hydrolysis of DMF in the precursor solution forms dimethylammonium
formate, which subsequently incorporates into the MHP film to compromise
the ability of Cs<sup>+</sup> and MA<sup>+</sup> to stabilize FA<sup>+</sup>-based MHP. The changes in solution chemistry lead to a modification
of the perovskite film stoichiometry, band gap, and structure. The
solid precursor salts are stable when ball-milled into a powder, allowing
for the storage of large quantities of stoichiometric precursor materials
Curtailing Perovskite Processing Limitations via Lamination at the Perovskite/Perovskite Interface
Standard layer-by-layer
solution processing methods constrain lead–halide
perovskite device architectures. The layer below the perovskite must
be robust to the strong organic solvents used to form the perovskite
while the layer above has a limited thermal budget and must be processed
in nonpolar solvents to prevent perovskite degradation. To circumvent
these limitations, we developed a procedure where two transparent
conductive oxide/transport material/perovskite half stacks are independently
fabricated and then laminated together at the perovskite/perovskite
interface. Using ultraviolet–visible absorption spectroscopy,
external quantum efficiency, X-ray diffraction, and time-resolved
photoluminesence spectroscopy, we show that this procedure improves
photovoltaic
properties of the perovskite layer. Applying this procedure, semitransparent
devices employing two high-temperature oxide transport layers were
fabricated, which realized an average efficiency of 9.6% (maximum:
10.6%) despite series resistance limitations from the substrate design.
Overall, the developed lamination procedure curtails processing constraints,
enables new device designs, and affords new opportunities for optimization
Targeted Ligand-Exchange Chemistry on Cesium Lead Halide Perovskite Quantum Dots for High-Efficiency Photovoltaics
The
ability to manipulate quantum dot (QD) surfaces is foundational
to their technological deployment. Surface manipulation of metal halide
perovskite (MHP) QDs has proven particularly challenging in comparison
to that of more established inorganic materials due to dynamic surface
species and low material formation energy; most conventional methods
of chemical manipulation targeted at the MHP QD surface will result
in transformation or dissolution of the MHP crystal. In previous work,
we have demonstrated record-efficiency QD solar cells (QDSCs) based
on ligand-exchange procedures that electronically couple MHP QDs yet
maintain their nanocrystalline size, which stabilizes the corner-sharing
structure of the constituent PbI<sub>6</sub><sup>4–</sup> octahedra
with optoelectronic properties optimal for solar energy conversion.
In this work, we employ a variety of spectroscopic techniques to develop
a molecular-level understanding of the MHP QD surface chemistry in
this system. We individually target both the anionic (oleate) and
cationic (oleylÂammonium) ligands. We find that atmospheric moisture
aids the process by hydrolysis of methyl acetate to generate acetic
acid and methanol. Acetic acid then replaces native oleate ligands
to yield QD surface-bound acetate and free oleic acid. The native
oleylÂammonium ligands remain throughout this film deposition
process and are exchanged during a final treatment step employing
smaller cationsî—¸namely, formamidinium. This final treatment
has a narrow processing window; initial treatment at this stage leads
to a more strongly coupled QD regime followed by transformation into
a bulk MHP film after longer treatment. These insights provide chemical
understanding to the deposition of high-quality, electronically coupled
MHP QD films that maintain both quantum confinement and their crystalline
phase and attain high photovoltaic performance