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Revealing the Role of Confined Molecular H $_2$ in the Passivation of Defective Silicon Using First-Principles Simulations

Using first-principles simulations, this study demonstrates that confined molecular H $_2$ within porous regions near amorphous/crystalline silicon interfaces enables a competitive double-hydrogen pathway for Si-H depassivation and rapid repassivation, providing an atomistic explanation for passivation recovery during light soaking.

Original authors: Hania Azzam (Theory and Computation of Energy Materials, Chair of Theory and Computation of Energy Materials, Faculty of Georesources and Materials Engineering, RWTH Aachen University, 52062 Aachen, G

Published 2026-08-24

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Original authors: Hania Azzam (Theory and Computation of Energy Materials, Chair of Theory and Computation of Energy Materials, Faculty of Georesources and Materials Engineering, RWTH Aachen University, 52062 Aachen, Germany), Tobias Binninger (Theory and Computation of Energy Materials), Benedikt Fischer (Energy Materials and Devices - Photovoltaics), Uwe Rau (Energy Materials and Devices - Photovoltaics), Michael Eikerling (Theory and Computation of Energy Materials, Chair of Theory and Computation of Energy Materials, Faculty of Georesources and Materials Engineering, RWTH Aachen University, 52062 Aachen, Germany)

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Silicon is the bedrock of modern solar power, the material that captures sunlight and turns it into electricity. For this material to work efficiently, its surface must be perfectly smooth and free of "dead spots" where electrons get stuck and lost. To fix these dead spots, scientists coat the silicon with hydrogen atoms, which act like tiny plugs, filling in the gaps and keeping the electricity flowing. However, this protection is not always permanent. Under the intense heat and bright light of the sun, these hydrogen plugs can sometimes pop off, leaving the silicon vulnerable again. This cycle of losing and regaining protection is a major puzzle for engineers trying to make solar panels that last longer and perform better. The key to solving this mystery lies in understanding exactly how hydrogen moves and behaves when it is trapped inside the tiny, porous spaces of the silicon material.

A team of researchers has used powerful computer simulations to watch this invisible dance of atoms, revealing a surprising new way that hydrogen protects and then repairs itself. They focused on a specific question: when hydrogen atoms detach from the silicon surface, do they simply float away as individual atoms, or do they pair up to form a tiny molecule before doing something else? By building detailed digital models of silicon with missing atoms and empty pockets, the scientists discovered that hydrogen often prefers to stick together in pairs, forming a confined molecule inside these empty spaces. This finding challenges the old idea that hydrogen only acts as a single, solitary traveler. Instead, the study suggests that these paired hydrogen molecules are active participants in the repair process, capable of breaking apart and reattaching to the silicon surface to fix the damage, a process that happens much faster than previously thought.

The researchers began by constructing virtual models of silicon crystals, introducing specific defects where atoms were missing, creating small voids or cavities. In these empty spaces, they placed hydrogen atoms in various arrangements to see which ones were the most stable. They found that while a single hydrogen atom can sit comfortably in the silicon, two hydrogen atoms are even happier when they join forces to form a molecule inside a cavity. This pairing releases energy, making the molecule a very stable resident in these tiny pockets. The team then simulated the process of "depassivation," where the protective hydrogen plugs are knocked off the silicon surface. They compared two scenarios: one where a single hydrogen atom leaves, and another where two hydrogen atoms leave at the same time to form a molecule inside the nearby cavity.

The results were counterintuitive. Breaking two bonds to create a molecule seemed like it should require much more energy than breaking just one. However, the simulations showed that the energy released by the two hydrogen atoms snapping together to form a molecule almost completely paid for the cost of breaking the second bond. In fact, in many cases, the path involving the formation of the molecule was just as easy, or even easier, than the path where a single atom wandered off alone. This meant that the formation of these confined molecules is not just a storage method for hydrogen, but a direct mechanism for the silicon to lose and regain its protection.

The speed of this repair process depends heavily on the electrical nature of the silicon. The researchers tested how the process behaved in different types of silicon, specifically those that are positively charged, negatively charged, or neutral. They found that in positively charged silicon, the barrier for the hydrogen molecule to break apart and reattach to the surface was incredibly low. Once the molecule was sitting in the cavity near the damaged spot, it could jump back onto the silicon surface in a fraction of a second, effectively healing the defect almost instantly. This rapid recovery suggests that the presence of these molecular hydrogen pockets could explain why solar cells sometimes get better at blocking defects after being exposed to light and heat for a while. The light and heat likely help the hydrogen move around and find these empty pockets, and once they are there, the repair happens almost automatically.

This work provides a clear, atom-by-atom explanation for a phenomenon that has been observed in solar cells for years but not fully understood. It shows that the hydrogen in silicon is not a static shield but a dynamic system where molecules form, break, and reform to maintain the material's health. The study confirms that the creation of these confined hydrogen molecules is a viable and efficient pathway for both damaging and healing the silicon surface. While the simulations focused on specific types of defects, the principles likely apply to the complex, porous layers found in real-world solar cells. By understanding that hydrogen can act as a paired molecule to facilitate rapid repair, scientists can now look for ways to design solar materials that encourage this beneficial behavior, potentially leading to more durable and efficient solar energy technology.

Technical Summary: Revealing the Role of Confined Molecular H2 in the Passivation of Defective Silicon

Problem Statement
Hydrogen passivation of silicon dangling bonds is fundamental to the performance of silicon-based optoelectronic devices, particularly solar cells. While the formation of Si–H bonds mitigates recombination centers, the stability of this passivation is dynamic. Recent experiments on silicon heterojunction (SHJ) solar cells under intense light soaking have revealed complex degradation and recovery mechanisms involving Si–H bond breaking and repassivation. These processes are hypothesized to occur in porous regions near amorphous/crystalline interfaces where hydrogen can exist in molecular form. However, the atomistic reaction pathways remain unclear. Specifically, it is unknown whether molecular hydrogen ( $H_2$ ) acts merely as a stored species or if it directly participates in the local depassivation (breaking Si–H bonds) and repassivation (reforming Si–H bonds) processes. Previous computational work has characterized hydrogen configurations in bulk silicon, point defects, and surfaces, but has not fully addressed the energetics and kinetics of a "double-H" process where two neighboring Si–H bonds break to form a confined $H_2$ molecule within a defect cavity.

Methodology
The authors employed first-principles density functional theory (DFT) simulations using the Quantum ESPRESSO package to investigate these mechanisms.

Electronic Structure: Calculations utilized the PBE generalized-gradient approximation functional with projector augmented wave (PAW) pseudopotentials. Spin-polarized calculations were performed to accurately model open-shell configurations with dangling bonds.
Systems Modeled: The study analyzed three distinct environments:
1. Bulk Silicon: Interstitial hydrogen configurations (bond-centered, antibonding, tetrahedral) and molecular $H_2$ at tetrahedral sites.
2. Point Defects: A monovacancy ( $V_1$ ) with sequential hydrogenation ( $V_1H_n$ ).
3. Multivacancy Cavities: Connected vacancy clusters ( $V_4, V_6, V_{10}$ ) representing nano-cavities, with fully passivated walls and depassivated states containing confined $H_2$ .
4. Surface: A symmetric Si(100) slab to model the extended surface limit of Si–H bond breaking.
Thermodynamics: Formation energies were evaluated within a grand-canonical framework, referencing silicon to bulk crystalline silicon and hydrogen to an isolated gas-phase $H_2$ molecule. This allowed for the ranking of stability across varying hydrogen chemical potentials ( $\Delta\mu_H$ ).
Kinetics: Minimum-energy pathways for depassivation and repassivation were computed using the Nudged Elastic Band (NEB) method. Both forward (depassivation) and reverse (repassivation) activation barriers were determined.
Doping Effects: The influence of carrier type was modeled by adding (n-type) or removing (p-type) electrons from the supercell, compensated by a homogeneous background charge.
Validation: Key barrier estimates were validated using hybrid functional (HSE06) single-point calculations to ensure robustness against PBE self-interaction errors.

Key Contributions and Results
The study compares two competing pathways for Si–H depassivation:

Single-H Process: Breaking one Si–H bond to release an atomic hydrogen into an interstitial site.
Double-H Process: Breaking two neighboring Si–H bonds to release two hydrogen atoms that recombine into a single confined $H_2$ molecule within a vacancy or cavity.

Thermodynamic Findings:

In bulk silicon, molecular $H_2$ at a tetrahedral interstitial site is energetically favored over two separated bond-centered H atoms.
For the monovacancy ( $V_1$ ), the single-H depassivation pathway is energetically preferred over the double-H pathway.
However, for multivacancy cavities ( $V_4, V_6, V_{10}$ ), the formation of confined $H_2$ significantly lowers the energy cost. The reaction energy for the double-H process in the $V_6$ cavity is 1.574 eV, which is lower than the 1.829 eV required for the single-H process in the monovacancy. This indicates that the stabilization of the confined $H_2$ molecule overcompensates for the energy required to break the second Si–H bond.

Kinetic Findings:

Forward Barriers (Depassivation): The activation barriers for the double-H process in the $V_6$ cavity are comparable to, or lower than, those for the single-H process in $V_1$ , particularly under n-type and p-type doping conditions.
Reverse Barriers (Repassivation): A critical finding is the extremely low reverse activation barrier for the double-H pathway under p-type conditions. The barrier for the reformation of two Si–H bonds from a confined $H_2$ molecule is calculated to be 0.154 eV (PBE) and 0.142 eV (HSE06).
Doping Dependence: The repassivation barrier is highly sensitive to doping, decreasing significantly in p-type silicon compared to undoped or n-type conditions.

Significance and Claims
The paper claims to provide a plausible atomistic explanation for passivation recovery observed during light soaking and illuminated annealing in porous regions near amorphous/crystalline silicon interfaces.

Direct Participation of $H_2$ : The work challenges the view of molecular hydrogen solely as a storage reservoir, demonstrating that confined $H_2$ can be a direct reactant in local repassivation.
Mechanism for Recovery: The low reverse barrier (0.154 eV) in p-type conditions suggests that once confined $H_2$ is positioned near two dangling bonds, the local repassivation step is thermally accessible at room temperature.
Scope of Application: The authors explicitly state that their model describes a local reaction pathway within a crystalline vacancy framework. They do not claim to model the full device-level dynamics, amorphous disorder, or the specific role of illumination in the transport of hydrogen to the defect. Instead, they propose that illumination and annealing likely facilitate the redistribution and transport of hydrogen to the defect site, while the calculated low barrier explains the rapid local recovery step once the hydrogen arrives.
Broader Context: The findings suggest that similar double-H mechanisms may be relevant in other photovoltaic materials, such as perovskite/silicon tandem devices, where hydrogen passivation plays a critical role.

In summary, the study establishes that in environments with sufficient free volume (such as multivacancy cavities), the formation of confined molecular hydrogen creates a kinetically competitive and energetically favorable pathway for both the degradation and, crucially, the rapid recovery of silicon passivation.

Technical Summary: Revealing the Role of Confined Molecular H2 in the Passivation of Defective Silicon

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