Optimizing MAPbI3 Perovskite Solar Cells with Oblique-Angle Sputtered ITO Electrodes

Background

Indium tin oxide (ITO) remains the most widely used transparent conductive oxide (TCO) in optoelectronic devices, offering ~96 % visible transmittance and ~10 Ω/sq sheet resistance ^[1–5]. Tailoring ITO’s optical and electrical characteristics—through annealing, gas‑ratio control, or glancing‑angle deposition (GLAD)—has yielded anisotropic films with columnar microstructures and direction‑dependent properties ^[9–10]. Perovskite solar cells (PSCs) based on CH₃NH₃PbI₃ (MAPbI₃) have achieved remarkable efficiencies ^[11–18], yet most devices employ isotropic TCO substrates. This study explores the use of oblique‑sputtered ITO as a substrate for planar MAPbI₃ PSCs, examining how surface anisotropy impacts film growth and device performance.

Methods

ITO glass (1.5 × 1.5 cm²) was cleaned sequentially with acetone, ethanol, and deionized water (5 min ultrasonication) and dried with nitrogen. A 100 nm ITO layer was sputtered at 5 mTorr Ar, varying the deposition angle (0°, 15°, 30°, 45°, 60°, 80°) and time (5–30 min). Post‑deposition, films were annealed at 300 °C for 30 min. PEDOT:PSS (Clevios™ PH 1000) was spin‑coated on the oblique ITO (5000 rpm, 30 s) and annealed at 110 °C for 10 min. MAPbI₃ was deposited via a two‑step spin‑coating process (1000 rpm, 10 s; 5000 rpm, 20 s) with a 1 mL DMSO/GBL (1:1) precursor solution (1.25 mmol PbI₂, 1.25 mmol MABr). During the 5000 rpm step, 100 µL anhydrous toluene was dripped to induce rapid crystallization. Films were annealed at 100 °C for 10 min. A 50 nm PCBM layer (20 mg mL⁻¹ chlorobenzene) was spin‑coated (3000 rpm, 30 s), followed by 20 nm Ag evaporation under vacuum. Devices had a 0.5 × 0.2 cm² active area defined by a shadow mask. Structural analysis used XRD (Cu Kα) and FESEM; wettability was assessed via water contact angle. J‑V characteristics were measured with a Keithley 2420 under 1000 W m⁻² AM 1.5G illumination.

Results and Discussion

Crystallinity of MAPbI₃ films remained unchanged across sputtering angles; XRD peaks at 14.28°, 28.5°, 30.61°, and 31.93° matched (110) MAPbI₃, (220) MAPbI₃, (110) SnO₂, and (222) In₂O₃ respectively, with domain sizes ≈71.8 nm (Scherrer). FESEM revealed grain growth with increasing sputtering angle, confirming that ITO surface roughness and wettability modulate PEDOT:PSS wetting and perovskite nucleation. Water contact angle measurements showed a linear correlation between the angle and grain size, indicating that a more hydrophobic PEDOT:PSS surface (achieved via higher oblique angles) promotes larger perovskite grains.

Photoluminescence spectra displayed a dominant emission at 768 nm, unaffected by the underlying ITO. PL quenching intensified with higher sputtering angles, evidencing improved exciton separation at the PEDOT:PSS/perovskite interface—most pronounced at 80° due to optimal wettability.

Device performance peaked at a 30° sputtering angle. The 15‑min deposition yielded the best conductivity while maintaining sufficient light scattering. Table 1 lists the key performance metrics: J_SC = 20.46 mA cm⁻², V_OC = 0.92 V, FF = 60 %, and Eff = 11.30 %. Increasing the angle beyond 30° raised sheet resistance and diminished efficiency, illustrating the trade‑off between optical and electrical properties.

Varying sputtering time further optimized performance (Table 2). A 15‑min deposition provided the optimal film thickness and conductivity, achieving the same 11.3 % efficiency as the 30° angle.

Conclusions

We demonstrated that oblique‑sputtered ITO substrates modulate MAPbI₃ film morphology and PSC performance. The optimal configuration—a 30° sputtering angle and 15‑min deposition—delivers 11.3 % efficiency, balancing conductivity and light‑scattering. Beyond 30°, increased sheet resistance outweighs optical benefits, reducing device yield. These findings highlight the importance of tailoring TCO microstructure to enhance perovskite photovoltaic performance.

Abbreviations

FESEM

Field‑emission scanning electron microscope

GLAD

Glancing angle deposition

ITO

Indium tin oxide

J‑V

Current density–voltage

MAPbI₃

CH₃NH₃PbI₃

PEDOT:PSS

Poly(3,4‑ethylenedioxythiophene) polystyrene sulfonate

TCO

Transparent conductive oxide

XRD

X‑ray diffractometer