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Ba- and Mg-Doped Perovskites on Engineered TiO2 Interfaces for Efficient HTL-Free Solar Cells

This study demonstrates that combining a low-temperature amorphous TiO₂ electron transport layer with Mg²⁺ and Ba²⁺ cationic doping in perovskite absorbers significantly enhances HTL-free solar cell performance by reducing bulk defect density, engineering the bandgap, and optimizing key photovoltaic parameters like short-circuit current and open-circuit voltage.

Original authors: Adil Alshoaibi, Stephen C. Nnochin

Published 2026-08-13

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: Adil Alshoaibi, Stephen C. Nnochin

Original paper licensed under CC BY 4.0 (https://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

Imagine the world is trying to solve a massive energy puzzle, and one of the most promising pieces is a type of solar cell made from a special crystal called "perovskite." Think of these crystals as tiny, super-efficient factories that turn sunlight into electricity. For a long time, scientists have been trying to build these factories using a simple, cheap blueprint that skips a few expensive parts, hoping to make solar power accessible to everyone. However, these simplified blueprints often have a flaw: the walls of the factory are a bit leaky, and the floor is a bit bumpy, causing the electricity to get lost before it can be used.

To fix this, researchers are trying two main tricks. First, they are smoothing out the "floor" (a layer called Titanium Dioxide, or TiO₂) so the electricity can flow without tripping. Second, they are trying to "season" the crystal walls with special ingredients (like Magnesium and Barium) to make them stronger and less likely to let energy escape. The big question is: if you mix these two tricks together, do you get a super-charged solar cell, or does the recipe just get messy? This is the story of a new study that tries to answer that question by building a virtual solar cell on a computer to see what happens before they even mix the chemicals in a lab.

The Virtual Solar Cell Experiment

In this study, two researchers, Adil Alshoaibi and Stephen C. Nnochin, decided to play the role of solar cell architects, but instead of building a physical device right away, they built a highly detailed digital twin using a computer program called SCAPS-1D. Their goal was to see if they could fix the "leaky" solar cells by combining a smooth, glass-like floor with a crystal wall that had been seasoned with two different types of "salt": Magnesium (Mg) and Barium (Ba).

The Smooth Floor: The Amorphous TiO₂
First, they looked at the floor of the solar cell, which is made of Titanium Dioxide (TiO₂). In many solar cells, this floor is made of tiny, jagged crystals that can create holes or "pinholes" where electricity leaks out. The researchers used a special low-temperature method to create a floor that is completely smooth and glass-like (amorphous). Imagine pouring a thick, smooth batter onto a pan instead of scattering jagged rocks; this smooth layer acts like a perfect, hole-free carpet that prevents the electricity from short-circuiting and leaking away.

The Seasoned Walls: Mg and Ba Doping
Next, they tackled the main wall of the factory, the perovskite crystal. They decided to "dope" it, which is a fancy word for adding a tiny bit of a different ingredient to change how the crystal behaves. They chose two ingredients from the "alkaline earth metal" family: Magnesium and Barium.

Magnesium (Mg): This is a tiny atom. When they added it, it acted like a tightener, squeezing the crystal lattice together. This created a "compressive strain," which the researchers found helped make the crystal grains (the individual blocks of the wall) grow very large and distinct.
Barium (Ba): This is a much larger atom. When added, it acted like a spacer, stretching the crystal lattice. This created a "tensile strain," which helped the grains pack together tightly into a dense, interconnected network.

The Reality Check: What Actually Got In?
Here is where the story gets interesting. The researchers planned to add 5% of these ingredients to their mix. However, when they checked the actual chemical composition of their samples (using a tool called EDX), they found that the crystals were picky eaters. Only about 1.21% of the Barium and 0.74% of the Magnesium actually made it into the crystal structure. The rest didn't fit and stayed out. This is a crucial detail: the computer model had to be built based on these actual low numbers, not the high numbers they originally planned to use.

The Computer Simulation Results
With their smooth floor and their "seasoned" walls, the researchers ran their computer simulations to see how much electricity the virtual solar cells could produce. They compared three versions: a plain one (no seasoning), a Magnesium-doped one, and a Barium-doped one.

The Magnesium Winner (Voltage): The Magnesium-doped solar cell turned out to be the king of voltage. In the simulation, it achieved an open-circuit voltage (VOC) of 1.16 V. This is like having a very strong water pressure pushing the electricity through the system. The simulation predicted this version could reach a power efficiency of 19.4%. The large, distinct grains created by the Magnesium helped reduce the number of defects (flaws) in the crystal, allowing the voltage to soar.
The Barium Winner (Current): The Barium-doped solar cell was the king of current. It produced the highest short-circuit current density (JSC) of 23.6 mA/cm². This is like having a massive volume of water flowing through a wide pipe. The dense, interconnected grains created by the Barium made it easier for electrons to move quickly, resulting in a predicted efficiency of 18.2%.
The "Leak" Fix: Both seasoned versions were much better at stopping the "hysteresis" (a wobble in the performance where the solar cell acts differently depending on which way you measure it). The plain cell had a hysteresis index of 17.4%, meaning it was quite unstable. The Magnesium version dropped this to 7.2%, and the Barium version to 13.9%. This suggests that adding these ingredients helps the crystal hold its shape and behave consistently.

The Secret Sauce: Fewer Defects
The most important discovery from the computer model was why these cells worked so well. The simulation showed that the main reason for the improvement was a massive drop in "bulk defect density" (the number of flaws inside the crystal). The plain crystal had a defect density of 5 × 10¹⁶ cm⁻³, but the doped versions dropped this to 8 × 10¹⁴ cm⁻³. That is a 100-fold reduction in flaws! The researchers suggest that the smooth, hole-free floor combined with the "seasoned" walls creates a perfect environment where electricity can flow without getting stuck or lost.

What This Means (and What It Doesn't)
It is important to remember that this study is a simulation based on real material measurements. The researchers did not build a working solar cell that generated this much power yet; they built a mathematical model that predicts what would happen if these materials were combined perfectly. They explicitly state that their results are "predicted" and that the actual dopant levels were lower than intended, which means there is still work to be done to get more of the Magnesium and Barium into the crystals.

However, the study provides a very strong roadmap. It suggests that if scientists can figure out how to get more of these ingredients into the crystal and build the physical device to match their computer model, they could create solar cells that are not only cheaper (because they don't need extra layers) but also more efficient and stable. The paper concludes that this "dual-front" approach—smoothing the floor and seasoning the walls—is a promising path forward for the future of solar energy.

Technical Summary: Ba- and Mg-Doped Perovskites on Engineered TiO2 Interfaces for Efficient HTL-Free Solar Cells

Problem Statement
Perovskite solar cells (PSCs), particularly those utilizing methylammonium lead iodide (MAPbI₃), face significant hurdles in commercialization, including lead toxicity, moisture instability, and performance degradation in simplified architectures. Specifically, Hole Transport Layer (HTL)-free devices suffer from critical challenges related to interfacial charge recombination and electrical shunting. In the absence of an HTL, the TiO₂ electron transport layer (ETL)/perovskite junction becomes the sole charge-selective interface. Surface defects on TiO₂ (such as oxygen vacancies) and bulk defects within the perovskite act as recombination centers, while direct contact with the back electrode creates additional leakage pathways. Current literature lacks a systematic computational exploration of the synergistic effects between engineered TiO₂ interfaces and specific cationic doping (Mg²⁺ and Ba²⁺) to simultaneously address bulk and interfacial quality in HTL-free architectures.

Methodology
This study employs a dual approach combining low-temperature material synthesis with numerical device modeling:

Material Synthesis: The researchers synthesized amorphous TiO₂ ETLs via a sol-gel process using titanium isopropoxide (TTIP) at varying concentrations (5.5%, 6.0%, 6.5%). Perovskite precursors were prepared by doping MAPbI₃ with Barium (Ba) and Magnesium (Mg) chlorides. Films were deposited on FTO/glass substrates using spin-coating.
Characterization: Structural and optical properties were analyzed using X-ray Diffraction (XRD), Energy Dispersive X-ray Spectroscopy (EDX), Field-Emission Scanning Electron Microscopy (FE-SEM), and UV-Vis-NIR spectrophotometry.
Numerical Modeling: The SCAPS-1D simulator was utilized to model the FTO/TiO₂/perovskite/Au heterojunction. Experimental data (bandgaps, mobilities, defect densities derived from XRD/SEM/EDX) served as input parameters. The simulation evaluated J-V characteristics, power conversion efficiency (PCE), and hysteresis under AM 1.5G illumination. Notably, the study did not fabricate or measure experimental J-V curves; all performance metrics are derived from the simulation.

Key Contributions and Results

Engineered TiO₂ Interface: XRD analysis confirmed that the sol-gel synthesized TiO₂ is amorphous, regardless of TTIP concentration. This amorphous phase provides a pinhole-free, conformal interface that minimizes leakage currents and suppresses shunting, a critical advantage for HTL-free devices.
Cationic Doping Effects (Structural & Optical):
- Ba-Doping: Incorporation of Ba²⁺ (ionic radius ~1.35 Å) induced tensile lattice strain, leading to larger, densely packed, and interconnected grain structures (average diameter ~0.56 µm). This resulted in improved crystallinity and a bandgap widening to 1.97 eV.
- Mg-Doping: Incorporation of Mg²⁺ (ionic radius ~0.72 Å) induced compressive strain. While low concentrations initially reduced crystallinity, optimized doping led to significantly larger grain clusters (up to ~2.3 µm) and a bandgap of 1.95 eV.
- Dopant Incorporation Discrepancy: EDX analysis revealed a significant gap between nominal precursor stoichiometry (5%) and actual lattice incorporation (1.21% for Ba, 0.74% for Mg), indicating thermodynamic solubility limits under the employed low-temperature conditions.
Device Performance (SCAPS-1D Simulation):
- Defect Reduction: The primary driver for performance enhancement was a modeled 100-fold reduction in bulk defect density ( $N_t$ ), decreasing from $5 \times 10^{16}$ cm⁻³ in pristine devices to $8 \times 10^{14}$ cm⁻³ in doped variants.
- Mg-Doped Performance: Mg-doping optimized the open-circuit voltage ( $V_{OC}$ ) to ~1.16 V and Fill Factor (FF) to 0.72, yielding a predicted peak PCE of 19.4%. This is attributed to lattice compression enhancing the built-in potential and minimizing series resistance.
- Ba-Doped Performance: Ba-doping maximized the short-circuit current density ( $J_{SC}$ ) to ~23.6 mA/cm², resulting in a predicted PCE of 18.2%. The dense, interconnected morphology facilitated superior charge carrier transport.
- Hysteresis Suppression: Both doped variants showed significantly reduced hysteresis indices (7.2% for Mg, 13.9% for Ba) compared to the pristine device (17.4%), suggesting effective suppression of ion migration and interfacial charge accumulation.

Significance and Claims
The paper claims to provide a computational framework demonstrating that a "dual-front" strategy—combining an amorphous, pinhole-free TiO₂ ETL with cationic doping of the perovskite absorber—can effectively mitigate the recombination losses inherent to HTL-free architectures. The authors assert that while Ba-doping excels in defect passivation and current extraction, Mg-doping offers superior voltage alignment and fill factors.

The study modestly acknowledges its limitations: the results are based on numerical modeling rather than experimental device fabrication, and the actual dopant incorporation levels were lower than intended. Consequently, the authors conclude that while the simulation predicts a two-order-of-magnitude improvement in defect mitigation and efficiencies approaching 20%, the translation of these findings to physical prototypes requires further optimization of synthesis kinetics to increase dopant solubility and the implementation of alternative back contacts (e.g., carbon or graphite) to further mitigate rear-interface recombination. The work serves as a guide for optimizing HTL-free heterojunction PSCs through combined structural engineering and electronic defect reduction.

The Virtual Solar Cell Experiment

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