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 H 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 V 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 H generation by combining the angstrom-scale results obtained using DFT with the micron-to-nanometer scale capabilities of wxAMPS.

A method for predicting the stability and interfacial dipole of mixed functionalized surfaces using first-principles density functional theory calculations is described, and calculated trends are consistent with previously published experimental data. Predicting the interfacial dipole is critical for photovoltaic and photoelectrochemical applications because the dipole can be tailored to enhance device performance by improving charge separation at the interface. To demonstrate the approach, the enthalpy of reaction and interfacial dipole as a function of coverage of 3,4,5-trifluorophenylacetylenyl (TFPA) moieties on Si(111) was analyzed for both mixed methyl/TFPA and mixed chlorine/TFPA-terminated surfaces. The enthalpy of reaction calculations show that the affinity for functionalization improves as a function of TFPA coverage for the mixed chlorine surface but instead remains constant for the mixed methyl surface across all coverages. The results indicate that the trend in enthalpy of reaction is a good predictor of the affinity for functionalization and the stability of the resulting surface. The interfacial dipole calculations show that the shift in dipole relative to the H-terminated Si(111) surface increases as a function of TFPA coverage with the mixed chlorine surface having a more positive shift than the mixed methyl across all coverages. We find that there are significant interactions between TFPA and neighboring −Cl or −CH3 moieties that increase the magnitude of the interfacial dipole. This suggests that the magnitude of an interfacial dipole can be tuned by adjusting the chemical makeup of a mixed monolayer. All trends presented in this work were validated against experimental observations found in literature for both mixed methyl/TFPA and chlorine/TFPA surfaces.

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.