Impact of Decomposed PbI₂ on Scattering Mechanisms in CH₃NH₃PbI₃ Perovskite Films

Background

HOIPs have rapidly advanced as high‑efficiency optoelectronic materials for photovoltaics, photodetectors, LEDs, and lasers. Perovskite solar cells now routinely exceed 20% efficiency, driven by their favorable band gaps, high absorption, and scalable processing. However, a comprehensive understanding of charge‑transport mechanisms—especially scattering at GBs—remains incomplete. Previous studies report carrier mobilities in the 1–10 cm²/V·s range, often limited by GB barriers and acoustic‑phonon scattering. Conflicting evidence exists regarding GB impact: some work indicates potential barriers reduced under illumination, while others point to deep traps at GBs. This study addresses these gaps by examining how decomposed PbI₂ influences scattering in MAPbI₃ films.

Methods

MAPbI₃ films were prepared by vapor‑solution reaction, following established protocols. PbI₂ (99.99%) was dissolved in DMF (99.9%) at 1 mol L⁻¹ and stirred at 70 °C for 3 h. Films were spin‑coated at 4000 rpm for 30 s and annealed at 70 °C for 10 min. MAI powder and PbI₂ films were placed in separate zones of a tubular furnace under vacuum. After 10 min pumping, the zones were heated to 180 °C (MAI) and 140 °C (PbI₂) for >100 min. Resulting MAPbI₃ films, darkened in color, were subsequently annealed at 100 °C, 120 °C, or 145 °C for 1 h after isopropanol rinsing. All steps were conducted in ambient air (≈45 % RH).

Microstructure was characterized by XRD (Bruker MXP‑III) and SEM (Hitachi S‑3400 N‑II). Time‑resolved photoluminescence (TRPL) employed a fluorescence spectrophotometer with time‑correlated single‑photon counting (FLS920, Edinburgh). Temperature‑dependent Hall measurements (LakeShore 8400 series) used Al electrodes in a van der Pauw geometry under 0.6 T. Measurements spanned 300–350 K in 10 K steps with an Ar atmosphere.

Results and Discussion

Figure 1 shows XRD patterns before and after annealing. Samples annealed at 120 °C retained pure MAPbI₃ peaks at 14.04°, 28.42°, and 43.08° (110, 220, 330). Annealing at 145 °C introduced a new peak at 12.56°, corresponding to the (001) plane of PbI₂, indicating perovskite decomposition above 145 °C. This aligns with prior reports that thermal treatment above 145 °C releases CH₃NH₃⁺, leaving PbI₂.

The XRD pattern for MAPbI₃ films before and after annealing at 120 °C and 145 °C

SEM images (Figure 2) reveal compact, conformal films across all treatments. At 145 °C, bright contrast at GBs suggests PbI₂ formation, corroborating the XRD data. Thus, annealing above 145 °C drives PbI₂ segregation to GBs.

The SEM images for MAPbI₃ films before (a) and after annealing at 120 °C (b) and 145 °C (c)

Hall mobilities (Figure 3) were ~0.6–1 cm²/V·s for unannealed, 100 °C, and 120 °C films, matching literature values. Remarkably, the 145 °C film achieved ~5 cm²/V·s—an order of magnitude increase. This suggests reduced scattering, likely due to PbI₂ passivation of GBs, lowering potential barriers.

Hall mobilities of all MAPbI₃ films at room temperature

Temperature‑dependent Hall data (Figure 4a) show that mobility rises with temperature for unannealed and 120 °C films—characteristic of GB‑limited transport, where carriers overcome potential barriers. Applying Seto’s model, ln μ versus 1000/T yields barrier heights of 208 meV (unannealed) and 147 meV (120 °C). In contrast, the 145 °C film displays a T⁻².⁰ dependence, indicative of acoustic‑phonon scattering and negligible GB influence. Thus, decomposed PbI₂ at GBs effectively suppresses barrier heights, shifting the dominant scattering mechanism.

a, b Temperature-dependent Hall mobilities of the MAPbI₃ films before and after annealing at 120 °C and 145 °C

TRPL analysis (Figure 5, Table 1) reveals a stable long‑lived component (~20 ns) across all samples, while the fast component increases from 1.39 ns (unannealed) to 6.05 ns (145 °C). The extended fast decay reflects reduced surface/interface recombination, further confirming PbI₂ passivation at GBs.

The TRPL decay spectra of the MAPbI₃ films before and after annealing at 120 °C and 145 °C

Conclusions

MAPbI₃ films produced via vapor‑solution reaction exhibited pure perovskite phases and typical optoelectronic properties. Thermal annealing at 145 °C induced PbI₂ formation at GBs, which effectively passivated GB barriers. This passivation suppressed GB scattering, shifted the dominant mechanism to acoustic‑phonon scattering, and elevated Hall mobility to 5 cm²/V·s—substantially higher than the 0.6 cm²/V·s of unannealed films.

Availability of Data and Materials

The datasets supporting this study are available from the corresponding author upon request.

Abbreviations

GBs:

Grain boundaries

HOIPs:

Hybrid organic‑inorganic perovskites

MAPbI₃:

CH₃NH₃PbI₃

SEM:

Scanning electron microscopy

SKPM:

Scanning Kelvin probe microscopy

TRPL:

Time‑resolved photoluminescence

XRD:

X‑ray diffraction