Photogenerated Charge Harvesting and Recombination in Photocathodes of Solvent-Exfoliated WSe₂

Understanding and optimizing the effects of edge states and nanoflake dimensions on the photon harvesting efficiency in ultrathin transition-metal dichalcogenide (TMD) semiconductor photoelectrodes is critical to assessing their practical viability for solar energy conversion. We present herein a novel filtration-based separation approach to systematically vary the TMD nanoflake dimensions and edge density of solution-processed large-area multiflake WSe₂ photocathodes. Photoelectrochemical measurements in both aqueous electrolyte (for water reduction) and a sacrificial redox system, together with a continuum-based charge transport model, reveal the role of the edge sites and the effects of the flake size on the light harvesting, charge transport, and recombination. A selective passivation technique using atomic layer deposition is developed to address detrimental recombination at flake edges. Edge-passivated WSe₂ films prepared with the smallest flakes (∼150 nm width, 9 nm thickness) demonstrate an internal quantum yield of 60% (similar to bulk single-crystal results). An optimized (1 sun) photocurrent density of 2.64 mA cm^–2 is achieved with 18-nm-thick flakes (700 nm width) despite transmitting ∼80% of the accessible photons. Overall, these results represent a new benchmark in the performance of solution-processed TMDs and suggest routes for their development into large-area low-cost solar energy conversion devices

Effects of Molecular Weight on Microstructure and Carrier Transport in a Semicrystalline Poly(thieno)thiophene

The ultimate control over chain self-assembly is key to unravel and optimize the relationship between film microstructure and charge carrier mobility in solution processable conjugated polymer semiconductors. Here we employ preparatory size exclusion chromatography to produce fractions of a poly­(thieno)­thiophene polymer, coded PBTTT-C₁₂, with varying number-average molecular weight, <i>M</i>_n, from 5.8 to 151 kDa and low polydispersity index of 1.1–1.4. Solution processing of these samples into bottom-contact, bottom gate, field effect transistors reveals a strong dependence of transistor performance on the molecular weight. Further analysis of the films’ microstructure and crystallinity show three distinct regions: fiber formation (ca. 5–20 kDa), terrace formation (20–50 kDa), and a rough morphology (50–150 kDa). The performance of low-<i>M</i>_n films was found to increase rapidly with increasing chain length, and while the best transistor performance was found with the terrace morphology, films not exhibiting the terraced morphology (using 80 kDa polymer) were capable of similar performance. In addition, by blending only 5 wt % of a high molecular weight fraction into a low-<i>M</i>_n film, we demonstrate the ability to drastically increase the measured charge carrier mobility of the low-<i>M</i>_n material without attaining a terraced morphology. This illustration suggests a viable route to easily increase the processability and transistor performance of low molecular weight conjugated polymeric or oligomeric semiconductors. In addition, GIXRD and thermal analysis of select fractions further indicate that the films of higher molecular weight exhibit a reduced side-chain crystallinity due to chain entanglement; the degree of backbone crystallinity remains more constant

Enhancing the Charge Separation in Nanocrystalline Cu₂ZnSnS₄ Photocathodes for Photoelectrochemical Application: The Role of Surface Modifications

Cu₂ZnSnS₄ (CZTS) colloidal inks were employed to prepare thin-film photocathodes that served as a model system to interrogate the effect of different surface treatments, viz. CdS, CdSe, and ZnSe buffer layers along with methylviologen (MV) adsorption, on the photoelectrochemical (PEC) performance using aqueous Eu³⁺ redox electrolyte. PEC experiments revealed that ZnSe and CdSe overlayers outperform traditional CdS, and the additional surface modification with MV was found to further boost the charge extraction. By analyzing the photocurrent onset behavior and measuring the open circuit photopotentials, insights are gained into the nature of the observed improvements. While a more favorable conduction band offset rationalizes the improvement offered by CdSe, charge transfer through midgap states is invoked for ZnSe. Improvement offered by MV treatment is clearly caused by both the shifting of the flat-band potential and a charge-transfer mediation effect. Overall, this work suggests promising alternative surface treatments for CZTS photocathodes for PEC energy conversion

Optimization and Stabilization of Electrodeposited Cu₂ZnSnS₄ Photocathodes for Solar Water Reduction

Cu₂ZnSnS₄ (CZTS) is a promising p-type semiconductor that has not yet been extensively investigated for solar fuel production via water splitting. Here, we optimize and compare two different electrodeposition routes (simultaneous and sequential) for preparing CZTS electrodes. More consistent results are observed with the simultaneous route. In addition, the effect of etching and the presence of a CdS buffer layer on the photocurrent are investigated. Finally, we demonstrate for the first time the stabilization of these electrodes using protecting overlayers deposited by atomic layer deposition (ALD). Our best performing protected electrodes (Mo/CZTS/CdS/AZO/TiO₂/Pt) exhibited a photocurrent of over 1 mA cm^–2 under standard one sun illumination conditions and a significant improvement in stability over unprotected electrodes

Multiflake Thin Film Electronic Devices of Solution Processed 2D MoS₂ Enabled by Sonopolymer Assisted Exfoliation and Surface Modification

The solvent-assisted exfoliation of transition metal dichalchogenides (TMDs) is a promising method for preparing scalable quantities of two-dimensional nanomaterials dispersed in a liquid phase. However, low concentrations and the restacking/aggregation of TMD layers remain challenges to the solution based preparation of large-area electronic devices. Here we present advances in the exfoliation and solution processing of 2D MoS₂ that are subsequently leveraged to prepare and electronically probe homogeneous multiflake thin-film devices. We report that sonopolymer, formed when using 1,2-dichlorobenzene (DCB) as a solvent, plays a critical role in affording stable dispersions (up to 0.5 mg mL^–1) of few-layer MoS₂ flakes in the 2H phase after only 6 h of low-power sonication. After removing the sonopolymer using a washing procedure, alkyl-trichlorosilane surfactants were used to prevent the restacking of 2D MoS₂ layers and create stable dispersions with concentrations as high as 85 mg mL^–1. In spin-coated multiflake thin films as thin as 20 nm, electron transport parallel to the substrate was quantifiable over channel lengths of 50 μm owing to the homogeneous film formation. By further varying the alkyl-trichlorosilane chain length we show that a trade-off between dispersibility (film homogeneity) and electronic insulation from the surfactant leads to a maximum (multiflake) electron mobility of 0.02 cm² V^–1 s^–1 using hexyl-trichlorosilane modified MoS₂ in the direction perpendicular to the substrate as measured by space-charge limited current devices

The Transient Photocurrent and Photovoltage Behavior of a Hematite Photoanode under Working Conditions and the Influence of Surface Treatments

Hematite (α-Fe₂O₃) is widely recognized as a promising candidate for the production of solar fuels via water splitting, but its intrinsic optoelectronic properties have limited its performance to date. In particular, the large electrochemical overpotential required to drive the water oxidation is known as a major drawback. This overpotential (0.4 – 0.6 V anodic of the flat band potential) has been attributed to poor oxygen evolution reaction (OER) catalysis and to charge trapping in surface states but is still not fully understood. In the present study, we quantitatively investigate the photocurrent and photovoltage transient behavior of α-Fe₂O₃ photoanodes prepared by atmospheric pressure chemical vapor deposition, under light bias, in a standard electrolyte, and one containing a sacrificial agent. The accumulation of positive charges occurring in water at low bias potential is found to be maximum when the photocurrent onsets. The transient photocurrent behavior of a standard photoanode is compared to photoanodes modified by either a catalytic or surface passivating overlayer. Surface modification shows a reduction and a cathodic shift of the charge accumulation, following the observed change in photocurrent onset. By applying an electrochemical model, the values of the space charge width (5–10 nm) and of the hole diffusion length (0.5–1.5 nm) are extracted from photocurrent transients’ amplitudes with the sacrificial agent. Characterization of the photovoltage transients also suggests the presence of surface states causing Fermi level pinning at small applied potential. The transient photovoltage and the use of both overlayers on the same electrode enable differentiation of the two overlayers’ effects and a simplified model is proposed to explain the roles of each overlayer and their synergetic effects. This investigation demonstrates a new method to characterize water splitting photoelectrodesespecially the charge accumulation occurring at the semiconductor/electrolyte interface during operation. It finally confirms the requirements of nanostructuring and surface control with catalytic and trap passivation layers to improve iron oxide’s performance for water photolysis

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

The Role of Excitons and Free Charges in the Excited-State Dynamics of Solution-Processed Few-Layer MoS₂ Nanoflakes

Solution-processed semiconducting transition metal dichalcogenides are emerging as promising two-dimensional materials for photovoltaic and optoelectronic applications. Here, we have used transient absorption spectroscopy to provide unambiguous evidence and distinct signatures of photogenerated excitons and charges in solution-processed few-layer MoS₂ nanoflakes (10–20 layers). We find that photoexcitation above the direct energy gap results in the ultrafast generation of a mixture of free charges in direct band states and of excitons. While the excitons are rapidly trapped, the free charges are long-lived with nanosecond recombination times. The different signatures observed for these species enable the experimental extraction of the exciton binding energy, which we find to be ∼80 meV in the nanoflakes, in agreement with reported values in the bulk material. Carrier-density-dependent measurements bring new insights about the many-body interactions between free charges resulting in band gap renormalization effects in the few-layer MoS₂ nanoflakes