Electrodeposited SnO₂ Thin Films on FTO: A Scalable Electron Transport Layer for High‑Efficiency Planar Perovskite Solar Cells

Abstract

We demonstrate that electrodeposited SnO₂ thin films on fluorine‑doped tin oxide (FTO) can serve as an effective electron transport layer (ETL) in planar heterojunction perovskite solar cells (PSCs). By carefully tuning the applied voltage, bath temperature, and deposition time, we control the morphology and thickness of the SnO₂ films, which in turn govern the photovoltaic performance. A modest TiCl₄ hydrolysis treatment further optimizes charge extraction and suppresses recombination, boosting the power‑conversion efficiency (PCE) from 10.8 % to 15.0 %. This work offers a low‑cost, scalable route for fabricating high‑quality ETLs compatible with roll‑to‑roll PSC production.

Background

Over the past six years, organometallic halide perovskite solar cells have surpassed conventional photovoltaic technologies in efficiency, reaching >25 % PCE while maintaining low manufacturing costs. Their outstanding optical absorption, ambipolar charge transport, and long carrier diffusion lengths make them attractive for next‑generation solar modules. However, perovskite layers remain vulnerable to moisture, heat, and UV exposure. Strategies such as incorporating formamidinium or inorganic cations (Cs⁺, Rb⁺) into the perovskite lattice have extended device stability, but the overall durability also depends on the device architecture (n‑i‑p vs. p‑i‑n) and the choice of electron‑transport material.

TiO₂ has long been the ETL of choice for n‑i‑p PSCs due to its wide bandgap and suitable band alignment. Yet, its UV‑induced photochemistry and relatively low conductivity can introduce hysteresis and limit long‑term performance. Recent reports indicate that Li doping or surface passivation can mitigate these issues, but such modifications add complexity to the fabrication process.

Stannic oxide (SnO₂) emerges as a compelling alternative ETL. With a bandgap of ~3.6 eV, higher intrinsic conductivity, and superior chemical stability compared to TiO₂, SnO₂ offers the potential for improved charge extraction and reduced hysteresis. While SnO₂ can be deposited via sol‑gel, microwave, ALD, or electrochemical methods, electrodeposition stands out for its simplicity, low cost, and scalability, making it ideal for roll‑to‑roll manufacturing of PSCs.

Methods

Preparation of SnO₂ Films

Electrodeposition was performed using a chronovoltammetry (CV) technique on a standard three‑electrode cell. A 0.05 M SnCl₂·2H₂O solution (with 1 mL HNO₃) in deionized water served as the electrolyte. After 1 h of stirring at 60 °C, the solution was purged with nitrogen for 10 min before deposition. FTO substrates were the working electrodes, Pt plates the counter electrodes, and Ag/AgCl the reference (1 M KCl). Post‑deposition, the films were annealed at 400 °C for 30 min in air to convert Sn to SnO₂.

Device Fabrication

The SnO₂/FTO substrates were used in an n‑i‑p PSC architecture. A PbI₂/PbCl₂ precursor (1 M, 1:1 molar ratio) was spin‑coated at 5,000 rpm for 30 s, then treated with methylammonium iodide (40 mg/mL) and annealed at 105 °C for 75 min to form MAPbI₃₋ₓClₓ. A hole‑transporting material (HTM) comprising 20 mg/mL poly[bis(4‑phenyl)(2,4,6‑trimethylphenyl)amine] (EM‑Index) with Li‑bis(trifluoromethanesulfonyl)imide and tert‑butylpyridine additives was spin‑coated at 3,000 rpm for 30 s. Finally, gold contacts were thermally evaporated. For TiCl₄‑treated devices, electrodeposited SnO₂ films were immersed in 40 mM TiCl₄ at 70 °C for 30 min, then dried at 150 °C.

Characterization

Electrochemical behavior was probed by cyclic voltammetry (50 mV/s). Structural analysis used X‑ray diffraction (Cu Kα). Surface morphology was examined with field‑emission scanning electron microscopy (FE‑SEM). Photovoltaic performance (J‑V curves) was measured under 100 mW/cm² AM 1.5G illumination with a 0.098 cm² active area. External quantum efficiency (EQE) was recorded using a monochromator‑driven xenon lamp. Steady‑state photoluminescence (PL) and intensity‑modulated photovoltage spectroscopy (IMVS) provided insights into charge dynamics and recombination lifetimes.

Results and Discussion

CV scans of the SnCl₂ solution revealed a cathodic current onset at −0.5 V, guiding the selection of −0.7 V as the optimal deposition potential. SEM images at various potentials confirmed that −0.7 V yielded uniform SnO₂–Sn nanospheres, whereas more negative potentials produced irregular, aggregated deposits.

Deposition time optimization (150–210 s) showed that 180 s produced the best balance between surface coverage and particle density, leading to a PCE of 10.0 %. Short‑circuit current density (J_sc) improved with deposition time up to 180 s but dropped at 210 s due to excessive aggregation and increased series resistance.

Bath temperature studies demonstrated that a 60 °C deposition bath produced the smoothest, most conformal SnO₂ films, yielding the highest efficiency (10.5 % without TiCl₄). Higher temperatures (70 °C) increased film thickness, reducing transparency and hindering charge transport, as evidenced by larger peak‑to‑peak separations in CV redox measurements.

TiCl₄ hydrolysis treatment significantly enhanced device performance: J_sc increased from 18.12 to 18.65 mA/cm², fill factor rose from 57.3 % to 79.1 %, and PCE improved from 10.83 % to 14.97 % (a 38 % boost). EQE spectra confirmed higher external quantum efficiencies across the entire spectral range, while steady‑state PL quenching and longer recombination lifetimes (1.17×) indicated more efficient electron injection and reduced recombination at the ETL/perovskite interface.

Conclusions

We have established a scalable electrodeposition protocol for producing SnO₂ ETLs on FTO that deliver high‑performance planar perovskite solar cells. By fine‑tuning deposition voltage, time, and bath temperature, and by applying a simple TiCl₄ surface treatment, we achieved a 42 % PCE increase, demonstrating the viability of this low‑cost, roll‑to‑roll compatible fabrication route.

Abbreviations

ALD

Atomic layer deposition

CV

Cyclic voltammetry

ED

Electrochemical deposition

EQE

External quantum efficiency

ETL

Electron transport layer

FF

Fill factor

FTO

Fluorine‑doped tin oxide

HTM

Hole‑transporting material

IMVS

Intensity‑modulated photovoltage spectroscopy

PCE

Power conversion efficiency

PL

Photoluminescence

PSC

Perovskite solar cell

RT

Room temperature

SEM

Scanning electron microscopy

XPS

X‑ray photoelectron spectroscopy

XRD

X‑ray diffraction