High‑Performance Dye‑Sensitized Solar Cells Using Screen‑Printed Multi‑Walled Carbon Nanotube Counter Electrodes

Background

Dye‑sensitized solar cells (DSSCs) are a leading third‑generation photovoltaic technology, prized for their low manufacturing cost, facile fabrication, and competitive efficiency. A typical DSSC comprises a TiO₂ photoanode on FTO glass, a photosensitizing dye, an iodide/tri‑iodide liquid electrolyte, and a counter‑electrode (CE) that catalyzes the reduction of I₃⁻ to I⁻. Platinum has long been the CE material of choice because of its excellent catalytic activity and conductivity. However, Pt’s high cost and susceptibility to corrosion in iodide‑based electrolytes limit the commercial scalability of DSSCs. Consequently, research has focused on alternative low‑cost catalysts such as carbon black, metal alloys, metal sulfides, and conducting polymers. Among these, nanostructured carbons—CNPs, MWCNTs, and GFs—offer high conductivity, large specific surface area, and photochemical stability, making them promising Pt replacements.

Traditional fabrication methods for carbon‑based CEs, such as chemical vapor deposition, drop‑coating, spin‑coating, and spray‑coating, often involve complex procedures and yield non‑uniform films. Screen‑printing provides a simple, scalable, and controllable alternative, allowing precise tuning of film thickness and uniformity through multiple printing passes. In this work we employ screen‑printing to fabricate thin films of CNPs, MWCNTs, GFs, and their composites on FTO glass and systematically evaluate their impact on DSSC performance.

Methods/Experimental

Fabrication of TiO₂‑Based Photoelectrodes of DSSCs

TiO₂ nanoparticle (NP) paste was prepared by mixing 6 g TiO₂ NPs (Degussa P25) with 20 g terpineol, 1 ml acetic acid, and 15 g ethanol (solution‑I). Separately, 3 g ethyl cellulose and 27 g ethanol were mixed (solution‑II). The two solutions were blended in a planetary mixer for 3 min and then heated to evaporate ethanol. Using a screen‑printer, the paste was deposited onto cleaned FTO glass (0.6 cm × 0.6 cm) to a thickness of ~23 µm. The films were sintered at ~500 °C for 30 min to remove organics, then immersed in a 0.3 mM N719 dye solution for 24 h.

Fabrication of Nanostructured Carbon Materials‑Based CEs

Carbon composites were prepared by dispersing 0.2 g each of CNPs, MWCNTs, and GFs in 1 g terpineol plus 0.1 g ethyl cellulose, followed by sonication in ethanol for 2 h. After evaporation on a hot plate, the resulting viscous paste was screen‑printed onto FTO glass (0.6 cm × 0.6 cm). Seven paste formulations—CNP, MWCNT, GF, CNP/MWCNT, CNP/GF, MWCNT/GF, and CNP/MWCNT/GF—were printed. A 400 °C heat treatment for 15 min removed residual organics. CE thickness was tuned by varying the number of printing passes. For comparison, a Pt CE was deposited by ion sputtering (1.2 kV, 7 mA).

a Schematic of fabricating carbon nanoparticle (CNP)/multi‑walled carbon nanotube (MWCNT)/graphene flake (GF) composites for counter electrodes (CEs) of dye‑sensitized solar cells (DSSCs) and b photograph and components of DSSC assembled in the present study

Manufacturing and Characterization of DSSCs

The photoanode and CE were sealed in a sandwich configuration with a 60 µm hot‑melt polymer film, then heated at 120 °C for 4 min. An iodide‑based liquid electrolyte (AN‑50, Solaronix) was injected into the interspace, and the holes were sealed with a cover glass. DSSCs were illuminated under AM 1.5 (100 mW cm⁻²) using a PEC‑L11 solar simulator calibrated with a Si photodiode. Current‑density voltage (J‑V) curves and electrochemical impedance spectra (EIS) were recorded with a Keithley SMU 2400 under illumination. Structural characterization employed SEM (Hitachi S‑4200, 15 kV) for morphology and thickness, BET (ASAP 2020) for surface area, Raman spectroscopy (Ramboss 500i, 532 nm) for defect analysis, and cyclic voltammetry (CV) in a three‑electrode cell (Pt counter, calomel reference) in 10 mM LiI, 1 mM I₂, 0.1 M LiClO₄ acetonitrile.

Results and Discussions

Raman spectra of CNPs, MWCNTs, and GFs exhibited the characteristic D (≈1355 cm⁻¹) and G (≈1579 cm⁻¹) peaks. The D/G intensity ratios were 0.95 (CNPs), 1.01 (MWCNTs), and 0.97 (GFs), indicating that MWCNTs possess the highest defect density, which enhances catalytic activity at the CE‑electrolyte interface.

Raman spectra of CNPs, MWCNTs, and GFs

BET analysis revealed surface areas of 24.7 m² g⁻¹ (CNPs), 311.8 m² g⁻¹ (MWCNTs), and 269.5 m² g⁻¹ (GFs). Composite formulations showed increasing specific surface area with the inclusion of MWCNTs, confirming that MWCNTs are key to enhancing interfacial charge transfer.

a Nitrogen adsorption/desorption curves. b Pore volume distributions of CNP, MWCNT, GF, MWCNT/GF, CNP/GF, CNP/MWCNT, and CNP/MWCNT/GF powders

SEM imaging (Fig. 4) shows that CNPs form large aggregates that poorly adhere to FTO, whereas MWCNTs create a porous, interconnected network that promotes efficient ion transport. GFs tend to stack into planar layers with limited interlayer contact. Composite coatings (CNP/MWCNT, MWCNT/GF, CNP/MWCNT/GF) combine the high surface area of MWCNTs with the mechanical stability of GFs and CNPs, yielding a uniform, well‑bonded CE surface.

Top‑ and cross‑sectional views of various carbon materials, including CNP, MWCNT, GF, MWCNT/GF, CNP/GF, CNP/MWCNT, and CNP/MWCNT/GF stacked on the surface of FTO glass using screen‑printing process (scale bars: 0.5 µm top, 5 µm cross‑section)

CV measurements (Fig. 5) demonstrate that Pt and MWCNT‑based CEs exhibit clear oxidation/reduction peaks for the I₃⁻/I⁻ redox couple, whereas pure GF and CNP CEs lack such features, underscoring their poor catalytic activity. Peak‑to‑peak separations (ΔEₚ) are 0.62 V for Pt, 0.77 V for MWCNT, and up to 1.03 V for the composite CEs, reflecting enhanced charge‑transfer kinetics in the latter.

a Cyclic voltammetry of Pt‑, CNP‑, MWCNT‑, and GF‑coated CEs. b Cyclic voltammetry of Pt‑ and carbon composite‑coated CEs measured at 50 mV s⁻¹ in 10 mM LiI, 1 mM I₂, 0.1 M LiClO₄ acetonitrile

Photovoltaic testing (Fig. 6) shows that CNP‑based CEs yield high short‑circuit current density (J_sc ≈ 17 mA cm⁻²) but low open‑circuit voltage (V_oc ≈ 0.5 V) and fill factor (FF ≈ 0.25), resulting in a PCE of only 0.22 %. GFs and CNP/GF CEs exhibit similar shortcomings due to limited surface area. In contrast, MWCNT‑based CEs and their composites deliver J_sc > 12 mA cm⁻², V_oc ≈ 0.62 V, FF ≈ 0.75, and PCEs exceeding 5 %. The CNP/MWCNT composite achieves 5.67 % PCE, nearly identical to the Pt benchmark (5.70 %). These data confirm that a high‑surface‑area MWCNT network is critical for efficient electron transfer at the CE‑electrolyte interface.

The comparison of photovoltaic performances of DSSCs composed of various carbon materials‑ and Pt‑based CEs in terms of a J_sc, b V_oc, c FF, and d PCE

J‑V and EIS data (Fig. 7a‑c, Table 1) further illustrate that MWCNT and composite CEs exhibit lower charge‑transfer resistance (R_ce) than Pt, while their recombination resistance (R_rec) is higher, indicating reduced electron recombination and longer electron lifetimes (τ_e). Bode plots reveal τ_e values above 50 ms for composite CEs, surpassing the 30 ms observed for Pt‑based devices, which translates into higher FF and overall efficiency.

Comparison of a current density‑voltage curves, b Nyquist plots, and c Bode plots for the DSSCs composed of various carbon materials‑ and Pt‑based CEs

Conclusions

Our systematic study confirms that CNPs alone are unsuitable as Pt replacements in DSSC counter electrodes because of aggregation and poor interfacial contact. In contrast, MWCNT‑rich composites—especially CNP/MWCNT, MWCNT/GF, and CNP/MWCNT/GF—form a highly networked, high‑surface‑area CE layer that promotes rapid triiodide reduction and suppresses recombination. Consequently, DSSCs equipped with these screen‑printed carbon composites achieve PCEs on par with Pt‑based devices, offering a scalable, low‑cost alternative for commercial DSSC production.

Abbreviations

BET:

Brunauer‑Emmett‑Teller

CEs:

Counter electrodes

CNPs:

Carbon nanoparticles

DSSCs:

Dye‑sensitized solar cells

EIS:

Electrochemical impedance spectroscopy

FF:

Fill factor

FTO:

Fluorine‑doped tin oxide

GFs:

Graphene flakes

MWCNTs:

Multi‑walled carbon nanotubes

PCE:

Power conversion efficiency

SEM:

Scanning electron microscopy