High‑Efficiency Planar Perovskite Solar Cells via Sequential Vapor‑Grown Hybrid Perovskite Layers

Background

Hybrid perovskites have emerged as front‑line light‑absorbing materials for next‑generation photovoltaics, owing to their intense optical absorption, tunable bandgaps, high carrier mobilities, long diffusion lengths, and low defect densities. They are compatible with both mesoscopic and planar device architectures. However, creating pinhole‑free, uniformly crystalline perovskite layers on planar substrates remains a significant challenge. Conventional wet‑processing techniques such as anti‑solvent dripping, sequential dip coating, and dual‑source vacuum evaporation often struggle with uniformity and reproducibility across large areas.

Vacuum deposition offers superior control over film thickness and uniformity, while vapor‑assisted crystallization can produce densely packed microstructures through carefully regulated diffusion of organic precursors. Building on these advantages, we introduce a novel sequential vapor‑processing route that combines vacuum‑deposited PbCl₂ with MAI vapor diffusion, yielding fully covered, highly uniform perovskite layers suitable for planar n‑i‑p solar cells. Devices fabricated with TiO₂ and spiro‑OMeTAD transport layers achieved power‑conversion efficiencies (PCEs) up to 11.5%, underscoring the viability of this route for scalable, reproducible perovskite solar cell production.

Methods/Experimental Procedure

Device Fabrication

All devices were assembled on fluorine‑doped tin oxide (FTO) coated glass. Substrates were cleaned sequentially in an ultrasonic bath with acetone, methanol, isopropanol, and deionized water, followed by 15 min of UV‑ozone exposure. TiO₂ electron‑transport layers were deposited by spin‑coating a 450 mM and a 600 mM titanium diisopropoxidebis(acetylacetonate) solution (75 wt % in isopropanol) at 2500 rpm for 20 s, then annealed at 500 °C for 30 min in air to yield compact TiO₂ films.

After TiO₂ deposition, substrates were placed in a high‑vacuum chamber and a 1 Å/s PbCl₂ flux was evaporated for ~16 min at room temperature, forming a uniform PbCl₂ layer. The coated substrates were then positioned in a sealed glass Petri dish with surrounding MAI powder. In a vacuum oven, the system was heated to 150 °C for 2–4 h, allowing MAI vapor to diffuse and react with the PbCl₂ layer, converting it to CH₃NH₃I₃ perovskite. Following MAI exposure, the films were rinsed with isopropanol to remove excess MAI and annealed at 100 °C for 1 h to complete crystallization.

For the hole‑transport layer, a spiro‑OMeTAD solution was prepared by dissolving spiro‑OMeTAD in chlorobenzene and adding tert‑butylpyridine and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in acetonitrile. This mixture was spin‑coated at 4000 rpm for 40 s and then left to oxidize overnight in ambient air. Finally, gold electrodes were thermally evaporated to complete the n‑i‑p device stack.

Characterization

Crystalline structure was examined by X‑ray diffraction (XRD, Rigaku Ultima IV). Morphology and microstructure were visualized using field‑emission scanning electron microscopy (FE‑SEM, Hitachi S‑4300). Optical absorption spectra were recorded with a UV‑Vis spectrophotometer (Shimadzu UV‑1601PC). Photovoltaic performance (J‑V curves) was measured under AM 1.5 G (100 mW cm^–2) illumination using a Newport 94021A solar simulator, with an aperture of 0.09 cm².

Results and Discussion

The sequential vapor‑processing approach delivers pinhole‑free, densely packed perovskite films across the entire substrate. Figure 1a illustrates the stepwise fabrication: vacuum deposition of PbCl₂, MAI vaporization, diffusion, and final annealing. The PbCl₂ films are transparent and homogeneous, while the final perovskite layers appear dark brown with a clear absorption edge near 785 nm, confirming complete crystallization.

a Fabrication schematic: PbCl₂ evaporation → MAI vaporization/diffusion → post‑annealing. b UV‑Vis spectra of PbCl₂ and perovskite layers (inset photographs).

Temperature optimization revealed 150 °C as the optimal MAI processing temperature, balancing MAI vaporization and perovskite crystallization. XRD patterns (Fig. 2) show the emergence of characteristic perovskite peaks at 2θ ≈ 14°, 28°, and 31°, corresponding to the tetragonal phase. A minor peak at 11°–12° observed after 2 h MAI exposure (without annealing) indicates transient H₂O‑incorporated complexes; this peak disappears after the 100 °C anneal, replaced by a PbI₂ reflection, confirming complete conversion to CH₃NH₃I₃. Extending the MAI vapor exposure to 4 h with annealing yields full perovskite transformation.

XRD of perovskite films after varying MAI exposure times and annealing. Peaks labeled for perovskite (δ), PbI₂ (*), and FTO (#).

SEM imaging confirms the uniform, pinhole‑free morphology (Fig. 3). The low‑magnification image shows complete surface coverage, while the high‑magnification view reveals tightly packed grains. Grain size distribution analysis yields an average grain size of ~320 nm (Fig. 3c). The cross‑sectional view (Fig. 3d) indicates a ~220 nm film thickness, slightly thinner than the grain size, which promotes vertical carrier transport.

SEM of the 220‑nm perovskite layer: a low‑magnification, b high‑magnification, c grain size histogram, d cross‑section.

Device performance correlates strongly with perovskite thickness (Fig. 4a,b). The optimized 220‑nm film delivers an average PCE of 11.2 % (Table 1). The small standard deviation across three devices indicates excellent uniformity afforded by the vapor route. Hysteresis behavior was characterized across scan rates; at 300 mV s^–1, hysteresis is negligible and the average PCE is 7.5 %. For the n‑i‑p configuration, a higher reverse‑scan efficiency reflects efficient carrier collection aided by interfacial charge accumulation. Steady‑state measurements at the maximum‑power point yield a stabilized PCE of 7.5 % and J_sc of 14 mA cm^–2, matching the dynamic J‑V data.

a Device stack. b J‑V curves for various perovskite thicknesses (1000 mV s^–1, reverse). c Scan‑rate‑dependent hysteresis (220 nm). d Stabilized output at maximum‑power point.

Conclusions

We have demonstrated a robust sequential vapor‑processing method that transforms vacuum‑deposited PbCl₂ into high‑quality, pinhole‑free CH₃NH₃I₃ perovskite layers with ~320 nm grains. XRD and optical data confirm phase purity and crystallinity, while SEM verifies uniform microstructure. When integrated into n‑i‑p planar heterojunctions with TiO₂ and spiro‑OMeTAD, the devices achieve a record efficiency of 11.5 % with minimal variability, highlighting the reproducibility of the vapor‑grown films. Future work will focus on further refining perovskite morphology and interface engineering to push efficiencies higher and suppress hysteresis while preserving the scalability advantages of this synthetic route.