Tuning p-Si(111) Photovoltage via Molecule|Semiconductor Electronic Coupling.

Photoelectrochemical (PEC) device efficiency depends heavily on the energetics and band alignment of the semiconductor|overlayer junction. Exerting energetic control over these junctions via molecular functionalization is an extremely attractive strategy. Herein we report a study of the structure-function relationship between chemically functionalized pSi(111) and the resulting solar fuels performance. Specifically, we highlight the interplay of chemical structure and electronic coupling between the attached molecule and the underlying semiconductor. Covalent attachment of aryl surface modifiers (phenyl, Ph; nitrophenyl, PhNO2; anthracene, Anth; and nitroanthracene, AnthNO2) resulted in high-fidelity surfaces with low defect densities (S < 50 cm/s). Electrochemical characterization of these surfaces in contact with methyl viologen resulted in systematically shifted band edges (up to 0.99 V barrier height) and correspondingly high photoelectrochemical performance (Voc up to 0.43 V vs MV2+) consistent with the introduction of a positive interfacial dipole. We extend this functionalization to HER conditions and demonstrate systematic tuning of the HER Voc using pSi(111)-R|TiO2|Pt architecture. Correlation of the shifts in barrier height with the photovoltage provides evidence for nonideality despite low surface recombination. Critically, DFT calculations of the electronic structure of the organic-functionalized interfaces show that the molecule-based electronic states effectively hybridized with the silicon band edges. A comparison of these interfacial states with their isolated molecular analogues further confirms electronic coupling between the attached molecule and the underlying semiconductor, providing an induced density of interfacial states (IDIS) which decreases the potential drop across the semiconductor. These results demonstrate the delicate interplay between interfacial chemical structure, interfacial dipole, and electronic structure.

Multiscale Computational Design of Functionalized Photocathodes for H2 Generation.

We present an integrated computational approach combining first-principles density functional theory (DFT) calculations with wxAMPS, a solid-state drift/diffusion device modeling software, to design functionalized photocathodes for high-efficiency H2 generation. As a case study, we have analyzed the performance of p-type Si(111) photocathodes functionalized with a set of 20 mixed aryl/methyl monolayers, which have a known synthetic route for attachment to Si(111). DFT is used to screen for high-performing monolayers by calculating the surface dipole induced by the functionalization. The trend in the calculated surface dipoles was validated using previously published experimental measurements. We find that the molecular dipole moment is a descriptor of the surface dipole. wxAMPS is used to predict the open-circuit voltage (efficiency) of the photocathode by calculating the photocurrent versus voltage behavior using the DFT surface dipole calculations as inputs to the simulation. We find that Voc saturates beyond a surface dipole of ∼0.3 eV, suggesting an upper limit for achievable device performance. This computational approach provides a possibility for the rational design of functionalized photocathodes for enhanced H2 generation by combining the angstrom-scale results obtained using DFT with the micron-to-nanometer scale capabilities of wxAMPS.

Computational Analysis of the Interplay between Deep Level Traps and Perovskite Solar Cell Efficiency.

New deposition methods of halide perovskites are being developed with the aim of improving solar cell power conversion efficiency by controlling the physiochemical properties of the perovskite film. In the case of methylammonium lead iodide (MAPbI3), deep level traps limit efficiency by participating in charge carrier recombination. Prior work has shown that the solar cell efficiency of MAPbI3 solar cells varied significantly with deposition method; specifically, efficiencies of 13.5 and 17.7% were observed for MAPbI3 processed with a one- and two-step method, respectively. However, the origin of the difference in efficiency remains unclear. In this study, we analyze the interplay between deep level traps and efficiency by simulating the photoexcited charge carrier pathway across solar cells processed via the one- and two-step method using finite-element drift-diffusion modeling. We determined that in the case of one-step processing, the traps propagate throughout the bulk, while for two-step, the traps congregate at the interface where the MAPbI3 was grown (mesoporous TiO2). Composition and structural analysis are used to propose a plausible explanation as to why the difference in processing changes the spatial distribution of the traps.

Identifying Charge Transfer Mechanisms across Semiconductor Heterostructures via Surface Dipole Modulation and Multiscale Modeling.

The design and fabrication of stable and efficient photoelectrochemical devices requires the use of multifunctional structures with complex heterojunctions composed of semiconducting, protecting, and catalytic layers. Understanding charge transport across such devices is challenging due to the interplay of bulk and interfacial properties. In this work, we analyze hole transfer across n-Si(111)- R|TiO2 photoanodes where - R is a series of mixed aryl/methyl monolayers containing an increasing number of methoxy units (mono, di, and tri). In the dimethoxy case, triethylene glycol units were also appended to substantially enhance the dipolar character of the surface. We find that hole transport is limited at the n-Si(111)- R|TiO2 interface and occurs by two processes- thermionic emission and/or intraband tunneling-where the interplay between them is regulated by the interfacial molecular dipole. This was determined by characterizing the photoanode experimentally (X-ray photoelectron spectroscopy, voltammetry, impedance) with increasingly thick TiO2 films and complementing the characterization with a multiscale computational approach (first-principles density functional theory (DFT) and finite-element device modeling). The tested theoretical model that successfully distinguished thermionic emission and intraband tunneling was then used to predict the effect of solution potential on charge transport. This prediction was then experimentally validated using a series of nonaqueous redox couples (ferrocence derivatives spanning 800 mV). As a result, this work provides a fundamental understanding of charge transport across TiO2-protected electrodes, a widely used semiconductor passivation scheme, and demonstrates the predictive capability of the combined DFT/device-modeling approach.

Design Strategy for the Molecular Functionalization of Semiconductor Photoelectrodes: A Case Study of p-Si(111) Photocathodes for H2 Generation.

Functionalization of semiconductor photoelectrodes is actively pursued as an approach to improve the efficiency of photoelectrochemical reactions by modulating the semiconductor's barrier height, but the selection of molecules for functionalization remains largely empirical. We propose a simple but effective design strategy for the organic functionalization of photocathodes for high-efficiency hydrogen generation based on first-principles density functional theory (DFT) calculations. The surface dipole of the functionalized photocathode determines its barrier height, which can be optimized to enhance charge separation at the semiconductor-electrolyte interface. Focusing on p-Si(111) photocathodes functionalized with different mixed aryl/methyl monolayers, we use DFT to systematically investigate the effect of - the presence and distribution of pi bonds, binding atom (the atom in the functional group that bonds with the Si surface), functional group length, and electrophilic substituent groups - on the surface dipole and charge rearrangement at the functionalized surface. We find that the most important factors affecting the surface dipole are the intrinsic molecular dipole moment of the organic moiety, the presence of electrophilic substituent groups, and the binding atom. Using these findings, a three-step design strategy is proposed for the experimental realization of high-performing functionalized p-Si(111) photocathodes by maximizing the surface dipole.

Computational insights into charge transfer across functionalized semiconductor surfaces.

Photoelectrochemical water-splitting is a promising carbon-free fuel production method for producing H2 and O2 gas from liquid water. These cells are typically composed of at least one semiconductor photoelectrode which is prone to degradation and/or oxidation. Various surface modifications are known for stabilizing semiconductor photoelectrodes, yet stabilization techniques are often accompanied by a decrease in photoelectrode performance. However, the impact of surface modification on charge transport and its consequence on performance is still lacking, creating a roadblock for further improvements. In this review, we discuss how density functional theory and finite-element device simulations are reliable tools for providing insight into charge transport across modified photoelectrodes.

Computational Approaches to Photoelectrode Design through Molecular Functionalization for Enhanced Photoelectrochemical Water Splitting.

Photoelectrochemical water splitting is a promising carbon-free approach to produce hydrogen from water. A photoelectrochemical cell consists of a semiconductor photoelectrode in contact with an aqueous electrolyte. Its performance is sensitive to properties of the photoelectrode/electrolyte interface, which may be tuned through functionalization of the photoelectrode surface with organic molecules. This can lead to improvements in the photoelectrode's properties. This Minireview summarizes key computational investigations on using molecular functionalization to modify photoelectrode stability, barrier height, and catalytic activity. It is discussed how first-principles density functional theory, first-principles molecular dynamics, and device modeling simulations can provide predictive insights and complement experimental investigations of functionalized photoelectrodes. Challenges and future directions in the computational modeling of functionalized photoelectrode/electrolyte interfaces within the context of experimental studies are also highlighted.

Platinum-Enhanced Electron Transfer and Surface Passivation through Ultrathin Film Aluminum Oxide (Al2O3) on Si(111)-CH3 Photoelectrodes.

We report the preparation, stability, and utility of Si(111)-CH3 photoelectrodes protected with thin films of aluminum oxide (Al2O3) prepared by atomic layer deposition (ALD). The photoelectrodes have been characterized by X-ray photoelectron spectroscopy (XPS), photoelectrochemistry (Fc in MeCN, Fc-OH in H2O), electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV) simulation. XPS analysis of the growing Al2O3 layer affords both the thickness, and information regarding two-dimensional versus three-dimensional mode of growth. Impedance measurements on Si(111)|CH3|Al2O3 devices reveal that the nascent films (5-30 Å) exhibit significant capacitance, which is attenuated upon surpassing the bulk threshold (∼30 Å). The Al2O3 layer provides enhanced photoelectrochemical (PEC) stability evidenced by an increase in the anodic window of operation in MeCN (up to +0.5 V vs Ag) and enhanced stability in aqueous electrolyte (up to +0.2 V vs Ag). XPS analysis before and after PEC confirms the Al2O3 layer is persistent and prevents surface corrosion (SiOx). Sweep-rate dependent CVs in MeCN at varying thicknesses exhibit a trend of increasingly broad features, characteristic of slow electron transport kinetics. Simulations were modeled as slow electron transfer through a partially resistive and electroactive Al2O3 layer. Lastly, we find that the Al2O3 ultrathin film serves as a support for the ALD deposition of Pt nanoparticles (d ≈ 8 nm) that enhance electron transfer through the Al2O3 layer. Surface recombination velocity (SRV) measurements on the assembled Si(111)|CH3|Al2O3-15 device affords an S value of 4170 cm s(-1) (τ = 4.2 µs) comparable to the bare Si(111)-CH3 surface (3950 cm s(-1); τ = 4.4 µs). Overall, the results indicate that high electronic quality and low surface defect densities can be retained throughout a multistep assembly of an integrated and passivated semiconductor|thin-film|metal device.