Beyond lead halide perovskites: visible light photovoltaics with phase engineered bismuth-based oxide double-perovskites, Bi2MCrO6 (M = Fe, Mn)

This study reports the optoelectronic characterization of solution-deposited Bi2FeCrO6 and Bi2MnCrO6 thin films as stable, lead-free alternatives for solar cells, demonstrating a 3.56% efficiency for the BMCO-based device and predicting significantly higher performance through future defect control.

The Gist

Imagine the world of solar panels as a bustling city trying to catch sunlight and turn it into electricity. For a while, the city's favorite building material was a special type of "lead brick" (lead halide perovskites). These bricks were amazing at catching light, but they had two big problems: they were toxic (like lead poisoning) and they crumbled easily when exposed to normal air and humidity.

The researchers in this paper decided to stop using the toxic, fragile bricks and start building with something new: Bismuth-based oxide double-perovskites. Think of these as sturdy, non-toxic "bi-bricks" made from elements found naturally in the earth, like Bismuth, Iron, Manganese, and Chromium.

Here is a breakdown of their journey, using simple analogies:

1. The New Building Blocks (The Materials)

The team created two specific types of these new bricks:

• BFCO: Made with Iron.
• BMCO: Made with Manganese.

They grew these materials as very thin films (about the thickness of a human hair, or roughly 400 nanometers) on glass. When they looked at them under a microscope, they saw that the atoms were arranged in a specific, orderly pattern called a "monoclinic double-perovskite." It's like arranging Lego bricks in a specific, complex shape that allows them to hold together well.

2. The Hidden Flaws (Defects)

However, the bricks weren't perfect. Inside the material, there were "glitches" or defects.

• The Mix-up: In a perfect brick, every Iron or Manganese atom should have a specific electrical charge. But in these films, some atoms had the wrong charge (like having a mix of +2, +3, and +4 charges).
• The Missing Pieces: There were also missing oxygen atoms, creating tiny holes (vacancies) in the structure.
• The Analogy: Imagine a factory assembly line where some workers are wearing the wrong uniforms or are missing entirely. This causes traffic jams. In solar cells, these "traffic jams" are called deep-level defects. They trap the electricity (electrons and holes) before it can get out, which lowers the efficiency of the solar panel.

3. Catching the Light (Optical Properties)

Despite the flaws, the materials were excellent at catching sunlight.

• The Sponge Effect: The paper found that these materials are like super-sponges for visible light. They absorb light very strongly (high absorption coefficient), meaning even a thin layer can catch a lot of the sun's energy.
• The Energy Gap: They calculated the "bandgap" (the energy threshold needed to start the electricity flow). BMCO had a slightly smaller gap (1.71 eV) than BFCO (1.97 eV), making it slightly better at capturing a broader range of sunlight.

4. Building the Solar Cell (The Device)

The team built a sandwich-like solar cell to test these materials:

1. Bottom Bun (FTO/SnO2): A glass base with a conductive layer and an electron-transport layer (a slide for electrons).
2. The Filling (BFCO or BMCO): The new bismuth material acting as the light catcher.
3. Top Bun (Spiro-OMeTAD/Ag): A layer to help holes (positive charges) move out, topped with a silver electrode.

5. The Results: How Well Did They Work?

When they tested the solar cells under sunlight:

• The Iron Brick (BFCO): It worked, but not great. It converted about 1.07% of the sunlight into electricity.
• The Manganese Brick (BMCO): It performed better, converting about 3.56% of the sunlight.

Why wasn't it perfect?
The researchers noticed the electricity output curve was "wobbly" (showing a "red kink" and "crossover"). This is like a car engine that sputters instead of running smoothly. The paper attributes this to the defects mentioned earlier. The "traffic jams" inside the material prevented the electricity from flowing freely, limiting the voltage and current.

6. The Crystal Ball (Simulation)

Since they couldn't easily fix the defects in the lab immediately, the team used a computer simulation (SCAPS-1D) to ask: "What if we could make these bricks perfect?"

• The Prediction: They simulated a scenario where they reduced the defects (the "traffic jams") to a very low level.
• The Outcome: The computer predicted that if they could clean up the material and control the defects, the BMCO solar cell could jump from 3.56% efficiency all the way up to nearly 20%.

Summary

This paper is a proof-of-concept. It says: "We found a new, non-toxic, stable material (BMCO) that is great at absorbing light. Right now, it's a bit messy inside, which limits its performance to about 3.5%. But, if we can learn to make the inside of the material cleaner and more organized, our computer models say it has the potential to become a highly efficient solar cell (around 20%), offering a safe and stable alternative to the toxic lead-based ones we use today."

Technical Summary

Technical Summary: Beyond Lead Halide Perovskites: Visible Light Photovoltaics with Phase Engineered Bismuth-Based Oxide Double-Perovskites

Problem Statement
The commercial viability of lead-based halide perovskite solar cells is hindered by lead toxicity and notorious ambient instability. Consequently, there is an urgent need to explore alternative, environmentally benign materials that offer high efficiency and stability. Transition metal oxide (TMO) double-perovskites present a promising class of materials due to their chemical stability, abundance, low toxicity, and large absorption coefficients. However, a critical prerequisite for their adoption as absorber layers is a comprehensive optoelectronic evaluation. While bismuth-based oxide double-perovskites, specifically Bi2FeCrO6 (BFCO), have shown potential, their detailed optoelectronic characteristics vis-à-vis photovoltaic (PV) response require further investigation. Furthermore, the potential of the Mn-based analog, Bi2MnCrO6 (BMCO), for PV applications remains largely unexplored in thin-film form.

Methodology
The study employed a solution-processing route to deposit thin films of BFCO and BMCO (350–450 nm) on Fluorine-doped Tin Oxide (FTO) coated glass substrates. The films were synthesized using stoichiometric metal nitrates dissolved in a solvent mixture of acetic acid, acetylacetone, and 2-methoxyethanol, followed by spin coating and multi-step annealing to achieve the intended monoclinic P21/c phase.

Solar cell devices were fabricated with an architecture of FTO/SnO2/Absorber (BFCO or BMCO)/Spiro-OMeTAD/Ag, where SnO2 and Spiro-OMeTAD served as the electron and hole transport layers, respectively.

Characterization techniques included:

• Structural/Morphological: Powder X-ray diffraction (pXRD), Field Emission Scanning Electron Microscopy (FESEM), and Atomic Force Microscopy (AFM).
• Chemical/Defect Analysis: Energy Dispersive X-ray Spectroscopy (EDS) and X-ray Photoelectron Spectroscopy (XPS) to determine elemental composition and oxidation states.
• Optoelectronic: UV-Visible spectroscopy for absorption and bandgap determination, Ultraviolet Photoelectron Spectroscopy (UPS) for band-edge alignment, and Mott-Schottky analysis for carrier density estimation.
• Device Performance: Current density-voltage (J-V) measurements under AM 1.5G illumination.
• Simulation: Numerical modeling using SCAPS-1D to simulate device performance under varying defect densities, series resistance, and shunt resistance.

Key Contributions and Results

1. Structural and Morphological Characterization:
   Both BFCO and BMCO thin films were confirmed to possess a double-perovskite structure with monoclinic P21/c symmetry. FESEM analysis revealed that BFCO films exhibited a compact structure with a distribution of grain sizes, whereas BMCO films consisted of ultrafine grains forming clusters with some porosity. AFM indicated comparable surface roughness (RMS ~20 nm) for both. EDS confirmed cationic stoichiometry close to the ideal 2:1:1:6 ratio, though slight excess bismuth and oxygen were noted.

2. Defect Structure and Oxidation States:
   XPS analysis revealed mixed valence states for the transition metals. BFCO contained Fe2+/Fe3+ and Cr2+/Cr3+/Cr4+, while BMCO contained Mn2+/Mn3+/Mn4+ and Cr2+/Cr3+/Cr4+. The presence of multiple oxidation states suggests the formation of charged point defects, such as oxygen vacancies and antisite defects. The study proposes that achieving long-range B-site ordering (critical for bandgap tuning) is challenging due to similar ionic radii and valences of the B-cations, but specific oxidation state combinations (e.g., Mn4+/Mn2+ and Cr2+/Cr4+ in BMCO) could facilitate better ordering compared to BFCO.

3. Optoelectronic Properties:

   • Absorption: Both materials demonstrated strong optical absorption in the visible range with coefficients ($\alpha$) of $10^4$–$10^5$ cm$^{-1}$. BMCO exhibited a higher absorption coefficient than BFCO.
   • Bandgap: Tauc plot analysis indicated indirect bandgaps of 1.97 eV for BFCO and 1.71 eV for BMCO.
   • Defect Levels: Urbach energy calculations yielded values of 0.7 eV (BFCO) and 0.66 eV (BMCO), corresponding to deep defect levels ($E_t > 0.3$ eV). These deep states are identified as a primary factor limiting open-circuit voltage ($V_{OC}$) via Shockley–Read–Hall (SRH) recombination.
   • Band Alignment: UPS measurements determined the Fermi levels, valence band maxima (VBM), and conduction band minima (CBM), confirming the suitability of SnO2 and Spiro-OMeTAD as transport layers for these absorbers.
   • Carrier Density: Mott-Schottky analysis revealed n-type behavior with carrier densities of $\sim 1.59 \times 10^{17}$ cm$^{-3}$ for BFCO and a significantly higher $\sim 1.07 \times 10^{20}$ cm$^{-3}$ for BMCO.

4. Photovoltaic Performance:

   • Experimental: The fabricated solar cells achieved power conversion efficiencies (PCE) of 1.07% for BFCO and 3.56% for BMCO. The BMCO device showed superior performance, attributed to its higher carrier density, larger absorption coefficient, and better band alignment. However, both devices exhibited non-ideal J-V characteristics, including "red kinks" and crossover behaviors, which the authors attribute to deep-level defects and non-Ohmic contacts.
   • Simulation: SCAPS-1D simulations were used to reproduce experimental J-V curves by optimizing defect density ($N_t$), series resistance ($R_S$), and shunt resistance ($R_{SH}$). The simulations indicated that the current low efficiencies are primarily limited by high defect densities ($N_t \sim 10^{16}$–$10^{18}$ cm$^{-3}$).

Significance and Claims
The paper establishes that bismuth-based oxide double-perovskites (BFCO and BMCO) are viable candidates for lead-free photovoltaics, demonstrating significant visible light absorption and suitable band alignments for device integration. The study highlights that while current efficiencies are modest, the material system possesses intrinsic potential.

The authors claim that the primary bottleneck for performance is the high density of deep-level defects and mixed cation valences. Through numerical simulation, they predict that if defect concentrations can be reduced to moderate levels ($\sim 10^{13}$ cm$^{-3}$) and interface resistances optimized, the conversion efficiency for BMCO-based devices could theoretically approach 20%, and BFCO-based devices could reach 17%. The work underscores the necessity of process optimization—specifically controlling oxygen partial pressure and cation ordering—to minimize antisite defects and oxygen vacancies, thereby unlocking the high-efficiency potential of these non-toxic, stable materials.