Thin-Film Solar Cell

SURFACE

Overview

Fabricated a thin-film solar cell using Copper Oxide (CuO) as the p-type absorber and Zinc Oxide (ZnO) as the n-type emitter layer on a Fluorine-doped Tin Oxide (FTO) glass substrate. The goal was a complete CuO/ZnO heterojunction device, characterized at each fabrication step using AFM and SEM, with final IV measurements taken on a Keithley 2400 under both dark and illuminated conditions.

CuO's narrow bandgap makes it an effective light absorber; ZnO provides high electron mobility and optical transparency. FTO serves as the transparent bottom contact.

Layer Stack

ITO Glass (bottom contact) → Copper Oxide Cu₂O (p-type, 2.17 eV bandgap, light absorber) → Zinc Oxide ZnO (n-type emitter, high electron mobility) → Aluminum (top contact)

TOPSOIL

Process

Electrodeposition of Copper Oxide

Cleaned substrate in ultrasonic bath with IPA and DI water
Prepared 0.25M CuSO₄ solution; adjusted pH to 10 with NaOH and 3M lactic acid at ~50°C
Masked substrate edges with Kapton tape to preserve FTO contact
Deposited at -1.0V for 40 minutes with graphite counter electrode
Rinsed with DI water and dried

Thermal Oxidation

Removed Kapton; placed samples in box furnace at 425°C for 1 hour
Cooled to below 20°C before opening

ZnO Sputtering

Re-masked edges; used Zn target with pure O₂ as process gas
Sputtered ~5 minutes

Aluminum Contact Deposition

Kept masking; used Al target with Ar sputter gas; ~8 minutes
Final thermal oxidation at 425°C for 1 hour

Fabricated CuO/ZnO solar cell on FTO glass substrate

Fabricated solar cell on FTO glass substrate

Microscopy Results

AFM of CuO After Deposition

AFM after CuO deposition showed pronounced surface texturing. Some peaks appeared incomplete, likely due to inspecting the sample mid-deposition.

AFM measurement of CuO surface after deposition

AFM scan of CuO surface after electrodeposition

SEM Results

Copper Oxide pre-annealing showing cubic/prismatic crystal morphology

Copper Oxide pre-annealing: cubic/prismatic crystal morphology

CuO post-anneal showing porous surface with nanowire formation

CuO post-anneal: porous surface with nanowire formation

ZnO after sputtering

Aluminum contact layer

Failed deposition attempt with non-uniform copper coverage

Failed attempt: loose contact during deposition caused non-uniform coverage and spherical particle formation

IV Characterization

I-V curve under dark conditions

I-V curve under illumination

I-V curve comparison: dark vs. illuminated

Dark IV confirmed diode behavior. Under illumination, the curves suggested significant recombination losses consistent with the surface defects and non-uniform coverage visible in SEM. Shunt resistance was low, indicating leakage paths likely from the incomplete aluminum coverage and porous CuO layer.

Challenges & Lessons

Handling samples between furnace steps introduced contamination; cleaning between each step (ultrasonic IPA + DI water) would reduce surface defects
Loose contact during copper deposition on one sample caused non-uniform current density, visible as discontinuous regions and spherical particles in SEM
Without introducing O₂ gas flow during oxidation, the growth rate diverged from simulation predictions

Future Work

Tighter process control at each step, particularly cleaning between depositions
Introduce proper dry O₂ flow during thermal oxidation steps
Investigate conformal ZnO deposition to reduce interface defects

Thanks to Prof. Iulian Gherasoiu and Tyler Allen for guidance and lab support.

BEDROCK

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