Carrier Localization in Pnictogen-Based Chalcohalides from Defect-Bound Hot Polarons

This study reveals that in the pnictogen-based chalcohalide BiSBr, vacancies introduced during synthesis or post-treatment induce extrinsic carrier localization through the formation of defect-bound hot polarons, which divert excited carriers from cooling to the band edge and thereby limit solar absorber efficiency.

The Gist

The Big Picture: Why Some Solar Materials Get "Stuck"

Imagine you are trying to run a marathon (representing electricity flowing through a solar panel). In a perfect world, runners (electrons) would sprint freely from the start line to the finish line.

For a long time, scientists thought that a specific family of solar materials (called pnictogen-based chalcohalides, like the one studied here, BiSBr) was naturally bad at this. They believed the material's internal structure was like a maze with narrow, twisting corridors that forced runners to slow down and get stuck immediately. This "getting stuck" is called carrier localization, and it stops solar cells from working efficiently.

However, this new study says: "Wait a minute. The material isn't naturally a maze. It's actually a wide-open highway. The problem is the construction zones."

The Discovery: It's Not the Road, It's the Potholes

The researchers compared two versions of the same material:

1. The "Bulk" Film: Large, smooth crystals.
2. The "Nanocrystal" (NC) Film: Tiny, fragmented crystals with lots of edges and surfaces.

The Result:

• The Bulk Film acted like a highway. The runners (electrons) could sprint freely for a long time.
• The Nanocrystal Film acted like a traffic jam. The runners got stuck almost instantly.

Since the chemical makeup was the same, the difference had to be the defects (imperfections) created during the making of the tiny crystals. The smaller the crystal, the more "potholes" (vacancies where atoms are missing) it had on its surface.

The Culprit: "Defect-Bound Hot Polarons"

This is the most complex part, so let's use a metaphor.

When sunlight hits the material, it creates "hot" electrons. Think of these as high-speed race cars zooming down the track.

• In a perfect material: These cars slow down gradually as they lose energy, eventually reaching a cruising speed (the "band edge") where they can travel efficiently to do work.
• In the defective material: The missing atoms (vacancies) create a special kind of trap. When a hot race car hits one of these potholes, it doesn't just stop; it gets stuck in a deep hole and starts vibrating violently against the sides of the hole.

The scientists call this a "Defect-Bound Hot Polaron."

• Hot: The electron still has a lot of energy (it's not cold yet).
• Polaron: The electron has dragged the surrounding atoms with it, creating a little "bubble" of distortion that traps it.
• Defect-Bound: This bubble only forms because there is a hole (defect) in the material.

Because the electron is stuck in this vibrating hole, it cannot move to the finish line. It gets diverted from the main road, effectively disappearing from the pool of usable electricity.

How They Proved It

The team used several clever tricks to see this happening:

1. Positron Annihilation Spectroscopy: They shot tiny particles (positrons) into the material. These particles love to hang out in empty spaces (holes). They found that the tiny crystals had way more empty spaces (defects) than the big crystals.
2. Laser "Push" Experiments: They used a laser to kick the electrons. In the defective samples, the electrons were so trapped in their "holes" that the laser couldn't easily kick them back out to move around. In the clean samples, the electrons were free to move.
3. Vibrational Analysis: They listened to the "music" of the atoms. The defective samples had a unique, noisy vibration pattern that only happens when an electron is trapped and shaking the atoms around it.

The Takeaway

The paper concludes that these materials are not naturally bad at conducting electricity. In fact, if you make them perfectly, they are excellent.

The reason they usually perform poorly is that the manufacturing process often leaves behind tiny defects (missing atoms). These defects act like traps that catch the high-energy electrons before they can settle down and do their job.

In short: The material is a great highway, but we need to fix the potholes (defects) to stop the race cars (electrons) from getting stuck in the mud.

Technical Summary

Technical Summary: Carrier Localization in Pnictogen-Based Chalcohalides from Defect-Bound Hot Polarons

Problem Statement
Pnictogen-based solar absorbers, such as those containing Bi³⁺ and Sb³⁺, are promising non-toxic alternatives to lead-halide perovskites (LHPs). However, their photovoltaic performance is severely limited by carrier localization, a phenomenon where charge carriers become trapped within a unit cell, drastically reducing mobility and diffusion lengths. While recent studies have identified intrinsic factors (e.g., low electronic dimensionality, high deformation potentials) that cause localization in some materials, these explanations do not account for all observations. Furthermore, existing research has largely focused on "cold" carriers, neglecting the role of defects in the dynamics of "hot" carriers immediately following photoexcitation. The specific mechanism by which defects influence carrier localization in structurally one-dimensional pnictogen chalcohalides, particularly regarding hot carrier cooling, remains poorly understood.

Methodology
The authors investigated the pnictogen-based chalcohalide BiSBr as a model system to disentangle intrinsic transport properties from extrinsic defect effects. The study employed a multi-faceted approach:

1. Sample Synthesis: Two distinct sample types were prepared to vary defect density: bulk thin films (solution-processed) and nanocrystals (NCs, hot-injection synthesized). NCs possess a high surface-to-volume ratio, theoretically leading to higher defect concentrations.
2. Structural and Compositional Analysis: X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and positron annihilation spectroscopy (PAS) were used to confirm phase purity, identify chemical stoichiometry (specifically sulfur deficiency), and quantify open-volume defects (vacancies and clusters).
3. Spectroscopic Characterization:
   • Optical Pump Terahertz Probe (OPTP): Measured photoconductivity decay kinetics to assess carrier mobility and localization timescales.
   • Photoluminescence (PL) and Transient Absorption Spectroscopy (TAS): Used to probe carrier relaxation pathways, identify above-gap emission, and track hot carrier dynamics.
   • Pump-Push-Probe TAS: Employed a dual-pulse scheme to re-excite trapped carriers and quantify the population of cold carriers versus trapped states.
   • Raman Spectroscopy: Correlated vibrational modes with observed carrier-phonon coupling.
4. First-Principles Calculations: Density Functional Theory (DFT) was utilized to calculate defect formation energies, transition levels, acoustic deformation potentials, Fermi isosurfaces, and polaron binding energies. Simulations specifically modeled the impact of sulfur vacancies ($V_S$) and defect clusters on lattice distortion and wavefunction localization.

Key Results

• Intrinsic Transport Properties: Contrary to the general trend in pnictogen-based materials, pristine (bulk) BiSBr exhibits band-like transport. First-principles calculations revealed low acoustic deformation potentials and high electronic dimensionality (2D in the conduction band, 3D in the valence band) due to quasi-bonding between ribbons and regular free volume. OPTP measurements confirmed this, showing slow photoconductivity decay ($t_{1/2} > 8.5$ ps) in bulk films.
• Defect-Induced Localization: In contrast, NC samples with higher defect densities exhibited rapid photoconductivity decay ($t_{1/2} < 0.35$ ps), indicative of strong carrier localization. PAS and XPS confirmed that NCs contain higher concentrations of sulfur vacancies ($V_S$) and larger defect clusters compared to bulk films.
• Defect-Bound Hot Polarons: The study identified a mechanism where vacancies introduce localized states above the conduction band minimum (CBM). Photoexcited hot carriers are captured by these defect states before cooling to the band edge. Strong coupling between these carriers and local lattice vibrations (specifically LO phonon modes like $B_{2g}$, $B_{1g}$, and $A_g$) leads to the formation of defect-bound hot polarons.
• Experimental Signatures:
  • Above-Gap Emission: Both bulk and NC samples showed broad PL emission above the bandgap (2.22 eV and 2.45 eV), which was significantly more pronounced in NCs and persisted at low temperatures (7 K), indicating phonon-assisted emission from localized hot states.
  • TA Spectral Shifts: TAS measurements revealed a distinct blueshift in photoinduced absorption (PIA) within the first 9 ps for NCs, consistent with the formation of small polarons retaining excess energy.
  • Carrier Depletion: Pump-push-probe experiments demonstrated that NCs have a significantly reduced population of cold carriers available for re-excitation compared to bulk films, confirming that hot carriers are intercepted by defect states.

Key Contributions

1. Mechanism Identification: The paper establishes that carrier localization in BiSBr is not an intrinsic property of the lattice but is extrinsically driven by defect-bound hot polarons. This distinguishes the material from other pnictogen-based absorbers where localization is often attributed solely to intrinsic lattice coupling.
2. Hot Carrier Dynamics: The work extends the understanding of carrier localization to the "hot" regime, demonstrating that defects can intercept hot carriers during the cooling process, preventing them from reaching the band edge and reducing the mobile carrier population.
3. Defect Chemistry Correlation: By combining PAS, XPS, and DFT, the authors quantitatively link specific defect types (sulfur vacancies and clusters) to the formation of localized vibrational modes that facilitate polaron trapping.
4. Design Principles: The study provides a framework for understanding how defect engineering can inadvertently degrade performance in materials that are intrinsically capable of high mobility.

Significance
The authors claim that this work challenges the prevailing view that self-trapping is an inherent, unavoidable feature of all perovskite-inspired materials (PIMs). Instead, they posit that in materials like BiSBr, which possess favorable intrinsic transport properties, performance limitations are primarily dictated by defect engineering. The findings suggest that to realize efficient pnictogen-based solar absorbers, it is critical to control defect formation—specifically sulfur vacancies—to prevent the formation of defect-bound hot polarons. This shifts the design paradigm from solely optimizing intrinsic electronic structure to actively managing defect chemistry to preserve hot carrier mobility and extraction efficiency.