Xiaoyun Yu

Self-assembled 2D WSe2 thin films for photoelectrochemical hydrogen production

WSe2—a layered semiconductor that can be exfoliated into atomically thin two-dimensional sheets—offers promising characteristics for application in solar energy conversion. However, the lack of controllable, cost-effective methods to scalably fabricate homogeneous thin films currently limits practical application. Here we present a technique to prepare controlled thin films of 2D WSe2 from dispersions of solvent-exfoliated few-layer flakes. Flake self-assembly at a liquid/liquid interface (formed exceptionally from two non-solvents for WSe2) followed by substrate transfer affords large-area thin films with superior 2D flake alignment compared with traditional (liquid/air) self-assembly techniques. We further demonstrate, for the first time, solar-to-hydrogen conversion from solution-processed WSe2 thin films. Bare photoelectrodes with a thickness of ca. 25 nm exhibit sustained p-type photocurrent under simulated solar illumination, and up to 1.0 mA cm–2 at 0 V versus reversible hydrogen electrode with an added water reduction catalyst (Pt). The importance of the self-assembled morphology is established by photoelectrochemical and conductivity measurements.

Direct Light-Driven Water Oxidation by a Ladder-Type Conjugated Polymer Photoanode

A conjugated polymer known for high stability (poly[benzimidazobenzophenanthroline], coded as BBL) is examined as a photoanode for direct solar water oxidation. In aqueous electrolyte with a sacrificial hole acceptor (SO32–), photoelectrodes show a morphology-dependent performance. Films prepared by a dispersion-spray method with a nanostructured surface (feature size of ∼20 nm) gave photocurrents up to 0.23 ± 0.02 mA cm–2 at 1.23 VRHE under standard simulated solar illumination. Electrochemical impedance spectroscopy reveals a constant flat-band potential over a wide pH range at +0.31 VNHE. The solar water oxidation photocurrent with bare BBL electrodes is found to increase with increasing pH, and no evidence of semiconductor oxidation was observed over a 30 min testing time. Characterization of the photo-oxidation reaction suggests H2O2 or •OH production with the bare film, while functionalization of the interface with 1 nm of TiO2 followed by a nickel–cobalt catalyst gave solar photocurrents of 20–30 μA cm–2, corresponding with O2 evolution. Limitations to photocurrent production are discussed.

Enhancing the Thermal Stability of Solution‐Processed Small‐Molecule Semiconductor Thin Films Using a Flexible Linker Approach

Using flexible aliphatic chains to link conjugated molecular semiconductors affords a polymeric material that possesses defined conjugated segments but extended covalent connectivity, which enhances crystallinity and thermal stability in field-effect transistors and bulk heterojunction solar-cell devices when used as an additive.

Evaluating spinel ferrites MFe2O4 (M = Cu, Mg, Zn) as photoanodes for solar water oxidation: prospects and limitations

The search for ideal semiconductors for photoelectrochemical solar fuel conversion has recently recognized the spinel ferrites as promising candidates due to their optoelectronic tunability together with superb chemical stability. However, a systematic understanding of the main material factors limiting their performance is currently lacking. Herein, nanostructured thin-film electrodes of three representative spinels, namely CuFe2O4 (CFO), MgFe2O4 (MFO) and ZnFe2O4 (ZFO), are prepared by a solution-based approach and their photoelectrochemical (PEC) properties are comprehensively characterized. Annealing post-treatments together with the deposition of NiFeOx overlayers are found to improve the native n-type response, although a dominant bulk recombination (especially in MFO) limits the saturation photocurrents (below 0.4 mA cm−2 at 1.23 V vs. RHE). Likewise, prominent Fermi level pinning due to surface states at around 0.9 V vs. RHE in all cases appears to limit the photovoltage (to ca. 300 mV). Rapid-scan voltammetry is used to gain insight into the surface states and the operation of the overlayer. Interestingly, the NiFeOx is ineffective at mitigating Fermi level pinning, but clearly participates as an electrocatalyst to improve the overall performance. Generally, these results evidence the potential and current intrinsic limitations of the spinel ferrites—establishing a roadmap for the optimization of these materials as photoanodes for solar water oxidation.

Defect Mitigation of Solution-Processed 2D WSe2 Nanoflakes for Solar-to-Hydrogen Conversion

Few-atomic-layer nanoflakes of liquid-phase exfoliated semiconducting transition metal dichalcogenides (TMDs) hold promise for large-area, high-performance, low-cost solar energy conversion, but their performance is limited by recombination at defect sites. Herein, we examine the role of defects on the performance of WSe2 thin film photocathodes for solar H2 production by applying two separate treatments, a pre-exfoliation annealing and a post-deposition surfactant attachment, designed to target intraflake and edge defects, respectively. Analysis by TEM, XRD, XPS, photoluminescence, and impedance spectroscopy are used to characterize the effects of the treatments and photoelectrochemical (PEC) measurements using an optimized Pt-Cu cocatalyst (found to offer improved robustness compared to Pt) are used to quantify the performance of photocathodes (ca. 11 nm thick) consisting of 100-1000 nm nanoflakes. Surfactant treatment results in an increased photocurrent attributed to edge site passivation. The pre-annealing treatment alone, while clearly altering the crystallinity of pre-exfoliated powders, does not significantly affect the photocurrent. However, applying both defect treatments affords a considerable improvement that represents a new benchmark for the performance of solution-processed WSe2: solar photocurrents for H2 evolution up to 4.0 mA cm-2 and internal quantum efficiency over 60% (740 nm illumination). These results also show that charge recombination at flake edges dominates performance in bare TMD nanoflakes, but when the edge defects are passivated, internal defects become important and can be reduced by pre-annealing.

Amorphous Ternary Charge-Cascade Molecules for Bulk Heterojunction Photovoltaics

Ternary bulk heterojunctions with cascade-type energy-level configurations are of significant interest for further improving the power conversion efficiency (PCE) of organic solar cells. However, controlling the self-assembly in solution-processed ternary blends remains a key challenge. Herein, we leverage the ability to control the crystallinity of molecular semiconductors via a spiro linker to demonstrate a simple strategy suggested to drive the self-assembly of an ideal charge-cascade morphology. Spirobifluorene (SF) derivatives with optimized energy levels from diketopyrrolopyrrole (DPP) or perylenediimide (PDI) components, coded as SF-(DPP)4 and SF-(PDI)4, are synthesized and investigated for application as ternary components in the host blend of poly(3-hexylthiophene-2,5-diyl):[6,6]phenyl-C61-butyric acid methyl ester (P3HT:PCBM). Differential scanning calorimetry and X-ray/electron diffraction studies suggest that at low loadings (up to 5 wt %) the ternary component does not perturb crystallization of the donor:acceptor host blend. In photovoltaic devices, up to 36% improvement in the PCE (from 2.5% to 3.5%) is found when 1 wt % of either SF-(DPP)4 or SF-(PDI)4 is added, and this is attributed to an increase in the fill factor and open-circuit voltage, while at higher loadings, the PCE decreased because of a lower short-circuit current density. A comparison of the quantum efficiency measurements [where light absorption of SF-(DPP)4 was found to give up to 95% internal conversion] suggests that improvement due to enhanced light absorption or to better exciton harvesting via resonance energy transfer is unlikely. These data, together with the crystallinity results, support the inference that the SF compounds are excluded to the donor:acceptor interface by crystallization of the host blend. This conclusion is further supported by impedance spectroscopy and a longer measured charge-carrier lifetime in the ternary blend.

Carrier dynamics and transport properties in few-layer MoS2 nanoflakes(Conference Presentation)

In the last few years, tremendous research interest has been focused on two-dimensional transition metal dichalcogenides, following progress in processing of layers with atomic-size thickness. Among them, molybdenum disulfide (MoS2) nanoflakes have shown unique optical and electrical properties. They are excellent absorbers, despite being ultrathin, with high exciton-binding energies that make excitonic transitions evident even at room temperature. Bulk and few-layer MoS2 are indirect band gap semiconductors, while its monolayer is a direct band gap semiconductor. Also, bound trions have been reported in monolayer MoS2, due to strong interactions between excitons and charges and spatial confinement of the photoexcited species. Ultrafast spectroscopy has brought important clarification to the aforementioned properties in MoS2 nanoflakes and has unraveled their carrier dynamics and transport properties. However, the strong dependence of the fundamental properties on the nanoflake size and preparation process clearly shows that additional research is needed to understand the rich and unusual photophysics in this system. Therefore, we have used time-resolved absorption and THz transmission spectroscopy to shed light on the photophysical properties of solution-processed, few-layer MoS2 nanoflakes with subpicosecond temporal resolution. Using different excitation photon energies and fluences, we have resolved the carrier and exciton relaxation, the recombination processes and the corresponding time scales. Also, we have used the spectrum of the complex photoconductivity in the THz region to study the carrier transport properties in the nanoflakes as a function of number of layers.