Reversible Photo-Induced Trap Formation in Mixed-Halide Perovskites for Photovoltaics

Table of Contents

1. Introduction & Overview

Hybrid organic-inorganic perovskites, particularly mixed-halide variants like (CH₃NH₃)Pb(Br_xI_1-x)₃ (MAPb(Br,I)₃), have emerged as promising materials for high-efficiency, low-cost photovoltaics. A key advantage is the ability to tune the optical bandgap ($E_g$) continuously from approximately 1.6 eV (iodide-rich) to 2.3 eV (bromide-rich) by varying the halide ratio (x). This tunability makes them suitable for single-junction and tandem solar cell applications. However, a persistent challenge has been the failure of mixed-halide perovskite solar cells to achieve the high open-circuit voltages ($V_{OC}$) expected from their larger bandgaps when bromide content is high (x > 0.25). This work investigates the origin of this voltage deficit, uncovering a reversible, light-induced phenomenon that fundamentally limits performance.

2. Core Findings & Experimental Results

The study reveals a dynamic and reversible transformation in MAPb(Br,I)₃ thin films under illumination, with direct consequences for their optoelectronic properties.

2.1 Optical Property Changes Under Illumination

Under constant illumination equivalent to 1 sun (100 mW/cm²), the photoluminescence (PL) spectrum of mixed-halide perovskites undergoes a dramatic change in less than a minute. A new, red-shifted PL peak emerges at approximately 1.68 eV, regardless of the initial alloy composition's bandgap (for x > ~0.2). Concurrently, sub-bandgap absorption increases around 1.7 eV. These observations are hallmark signatures of the formation of new electronic trap states within the material's bandgap. These states act as non-radiative recombination centers, which typically lower photoluminescence quantum yield and, critically for solar cells, reduce $V_{OC}$.

2.2 Structural Evidence from X-ray Diffraction

X-ray Diffraction (XRD) measurements provided structural insight. Upon illumination, the single, sharp XRD peaks characteristic of a homogeneous mixed-halide phase were observed to split. This peak splitting is direct evidence of phase segregation, indicating the material separates into distinct crystalline domains with different lattice constants.

2.3 Reversibility of the Phenomenon

A crucial and surprising finding is the complete reversibility of this process. When the illuminated sample is placed in the dark for several minutes, the red-shifted PL peak disappears, the sub-bandgap absorption decreases, and the XRD peaks revert to their original, single-phase line shape. This cyclability distinguishes it from permanent photodegradation pathways.

3. Proposed Mechanism: Halide Segregation

The authors hypothesize that the observed effects are caused by photo-induced halide segregation. Under photoexcitation, electron-hole pairs are generated, creating a local driving force for ion migration. Iodide ions (I⁻), being more mobile and polarizable than bromide ions (Br⁻), are believed to migrate and cluster together, forming iodide-rich minority domains. Conversely, the surrounding matrix becomes enriched in bromide.

This creates a heterogeneous structure: the iodide-rich domains have a narrower bandgap (~1.68 eV) than the surrounding bromide-rich matrix. These low-bandgap domains act as efficient "sinks" or traps for photogenerated charge carriers. They become the dominant recombination centers, pinning the PL emission energy and, by extension, the quasi-Fermi level splitting that determines $V_{OC}$ in a solar cell, to the lower bandgap of the iodide-rich phase.

4. Implications for Photovoltaic Performance

This mechanism directly explains the poor $V_{OC}$ performance of mixed-halide perovskite solar cells, especially those with high bromide content intended for wider bandgaps. Despite an initial, homogeneous film with a large bandgap (e.g., 1.9 eV), under operating conditions (sunlight), the material spontaneously forms low-bandgap (1.68 eV) trap regions. The device's $V_{OC}$ becomes limited by these regions rather than the intended bulk bandgap. This represents a fundamental efficiency loss pathway and a critical challenge for the stability of mixed-halide perovskites in optoelectronic devices.

5. Technical Details & Analysis

5.1 Mathematical Description of Bandgap Tuning

The bandgap ($E_g$) of the mixed-halide perovskite MAPb(Br_xI_1-x)₃ does not follow a simple linear Vegard's law but can be empirically described. For a first approximation, the bandgap tuning with composition $x$ can be modeled as: $$E_g(x) \approx E_g(\text{MAPbI}_3) + [E_g(\text{MAPbBr}_3) - E_g(\text{MAPbI}_3)] \cdot x - b \cdot x(1-x)$$ Where $b$ is a bowing parameter accounting for the non-linear behavior. The formation of iodide-rich domains under light effectively reduces the local $x$ to near 0, reverting $E_g$ to ~1.6 eV.

5.2 Experimental Setup & Data Analysis Framework

Analysis Framework Example (Non-Code): To diagnose photo-induced segregation in a lab setting, a standard protocol can be established:

1. Baseline Characterization: Measure initial PL spectrum, absorption spectrum, and XRD pattern of the pristine film in the dark.
2. Light-Soaking Stress Test: Illuminate the sample with a calibrated solar simulator (1 Sun, AM1.5G spectrum) while monitoring the PL spectrum in real-time using a fiber-coupled spectrometer.
3. Kinetic Analysis: Plot the intensity of the emergent ~1.68 eV PL peak versus illumination time. Fit the data to a first-order kinetic model: $I(t) = I_{max}(1 - e^{-t/\tau})$, where $\tau$ is the characteristic time constant for segregation.
4. Reversibility Check: Cease illumination and monitor the decay of the 1.68 eV peak in the dark. Fit the recovery to a similar exponential decay model.
5. Structural Correlation: Perform XRD on the light-soaked state (quickly transferring the sample) and again after full recovery in the dark to confirm reversible peak splitting.

This systematic framework allows for quantifying the severity and kinetics of the segregation effect in different material formulations.

6. Critical Analysis & Expert Perspective

Core Insight: Hoke et al. didn't just find a new degradation mode; they identified a fundamental operational instability intrinsic to mixed-halide perovskites under bias. The voltage of your cell isn't defined by the film you fabricate, but by the film that evolves under light. This is a game-changer for the perceived versatility of halide tuning.

Logical Flow: The logic is elegant and damning. 1) Mixed-halide cells underperform on $V_{OC}$. 2) Light causes a red-shift in PL to a fixed, low energy. 3) Light also causes XRD peak splitting. 4) Conclusion: Light drives reversible phase separation into I-rich (low-$E_g$, high-recombination) and Br-rich domains. The $V_{OC}$ is pinned by the I-rich traps. It's a direct, mechanistic explanation for a major performance roadblock.

Strengths & Flaws: The paper's strength is its multidisciplinary correlation of optical and structural data to propose a compelling physical model. The reversibility finding is critical—it's not irreversible damage, but a dynamic equilibrium. However, the 2015 work is a phenomenological report. It speculates on ion migration but doesn't prove it with direct techniques like ¹²⁷I NMR or in-situ TEM, nor does it model the exact driving force (e.g., strain, polaron formation). Later work by Slotcavage, Snaith, and Stranks would build on this, showing it's a universal issue in mixed-halide and even mixed-cation systems, exacerbated by higher light intensity and lower temperatures—a counter-intuitive point this early paper misses.

Actionable Insights: For researchers and commercial developers, this paper sounds a loud alarm: simply tuning halides for bandgap is a trap (pun intended). The community's response, evident in subsequent literature, bifurcated: 1) Avoid the problem: Focus on pure iodide (FAPbI₃) for mainstream cells, using cation engineering (e.g., Cs, FA, MA mixtures) for stability, not halide mixing for bandgap. 2) Mitigate the problem: Explore strategies to suppress ion migration via grain boundary passivation, strain engineering, or using larger, less mobile A-site cations. For tandem cells requiring a wide-bandgap (~1.8 eV) top cell, the search shifted to low-bromide or bromine-free alternatives (e.g., tin-lead alloys). This paper forced a strategic pivot in material design philosophy.

7. Future Applications & Research Directions

While a challenge for photovoltaics, understanding and controlling photo-induced phase segregation opens doors in other areas:

• Programmable Photonics: The reversible, light-written structural change could be harnessed for optical memory or switching elements where specific light patterns define low-bandgap conductive pathways.
• Light-Emitting Diodes (LEDs): Controlled segregation could be used to create embedded low-energy emission centers for broad-spectrum or white-light emission from a single material.
• Fundamental Research: The system serves as a model for studying photo-induced ion transport and phase transitions in soft, ionic semiconductors.
• Future PV Research Directions: Current efforts focus on:
  1. Developing kinetic stabilization strategies using surface ligands or 2D/3D heterostructures to suppress ion migration at operational timescales.
  2. Exploring alternative wide-bandgap perovskites with reduced halide mobility, such as those with mixed cations (Cs/FA) or low-dimensional perovskites.
  3. Utilizing external fields (electric, strain) to counteract the photo-driving force for segregation.

8. References

1. Hoke, E. T. et al. Reversible photo-induced trap formation in mixed-halide hybrid perovskites for photovoltaics. Chem. Sci. 6, 613–617 (2015). DOI: 10.1039/c4sc03141e
2. Slotcavage, D. J., Karunadasa, H. I. & McGehee, M. D. Light-Induced Phase Segregation in Halide-Perovskite Absorbers. ACS Energy Lett. 1, 1199–1205 (2016).
3. National Renewable Energy Laboratory (NREL). Best Research-Cell Efficiency Chart. https://www.nrel.gov/pv/cell-efficiency.html (Accessed continuously, illustrating the efficiency evolution post-2015).
4. Stranks, S. D. & Snaith, H. J. Metal-halide perovskites for photovoltaic and light-emitting devices. Nat. Nanotechnol. 10, 391–402 (2015).
5. Bischak, C. G. et al. Origin of Reversible Photoinduced Phase Separation in Hybrid Perovskites. Nano Lett. 17, 1028–1033 (2017).