Comparative Study of Nanohole‑ and Nanopillar‑Patterned Metal Electrodes for Enhanced Organic Solar Cells

Abstract

Patterned metallic electrodes (PMEs) are a proven strategy to increase light harvesting in thin‑film organic solar cells (OSCs). Two dominant PME geometries—nanohole and nanopillar arrays—have been tested experimentally, yet their relative merits remain unclear. In this work, we perform a comprehensive theoretical comparison of these two architectures in a PSBTBT:PC₇₁BM active layer. By optimizing the height, period, and filling ratio of each PME, we find that both designs yield an identical integrated absorption efficiency of 82.4 % in the 350–850 nm range—an improvement of 9.9 % over a planar reference. Despite the similar spectral performance, the underlying light‑trapping mechanisms differ: nanohole PMEs rely on dipole‑like localized plasmon resonances (LPRs) and surface plasmon polaritons (SPPs) localized at the ridge tops, whereas nanopillar PMEs activate analogous modes on the pillar tops while also exciting additional SPPs at the bottom. The nanopillar configuration offers a moderate filling ratio (≈30 %) and a more forgiving fabrication window, making it a practical choice for high‑efficiency OSCs.

Background

Sub‑wavelength metallic nanostructures can drastically alter the optical response of OSCs, enabling plasmonic–photonic hybrid modes that enhance absorption in ultrathin active layers. Two main PME families exist:

• Nanohole PMEs – isolated voids in a continuous metal film; the surrounding metal can be deposited after imprinting the active layer, yielding nanopillar contacts on the organic surface.
• Nanopillar PMEs – isolated metal pillars atop a continuous film, achieved by imprinting the active layer with pillars and then evaporating metal to cover them.

Both approaches improve optical, electrical, and morphological properties, but their comparative performance and optimal design parameters have not been systematically explored. This study fills that gap.

Methods

The OSC stack considered is ITO / PEDOT:PSS / PSBTBT:PC₇₁BM / Ag. The active layer (85 nm) uses PSBTBT:PC₇₁BM for its broad absorption (350–900 nm). Two device models were built:

• Device A – nanohole PME (period = p_A, width = D_A, height = h_A).
• Device B – nanopillar PME (period = p_B, width = D_B, height = h_B).

Finite‑Difference Time‑Domain (FDTD) simulations were performed with periodic boundaries along x and y, perfectly matched layers at the top and bottom, and TM/TE illumination from the ITO side. Material optical constants were taken from the literature. The absorption efficiency (η) and integrated absorption efficiency (η_I) over 350–850 nm (AM1.5G weighted) were extracted for each geometry.

Results and Discussion

Optimal geometries

• Device A (nanohole) – h_A = 45 nm, f_A = 0.10 (D_A = 35 nm, p = 350 nm).
• Device B (nanopillar) – h_B = 65 nm, f_B = 0.30 (D_B = 105 nm, p = 350 nm).

Both configurations achieve η_I = 82.4 % (≈ 9.9 % higher than the planar control, 75.0 %). Device B’s moderate filling ratio offers a larger design tolerance and easier nanoimprinting fabrication compared to Device A’s shallow, tightly packed structure.

Spectral analysis

• Device B consistently outperforms the planar reference across the full spectrum.
• Device A shows a dip around 650 nm but compensates with stronger absorption below 550 nm, yielding identical η_I.
• Both devices exhibit five major enhancement peaks (λ_1–5) where the relative absorption change Δη reaches 219–222 % at the band edge (~830 nm).

Field‑distribution insights

• At λ₁ and λ₂, dipole‑like LPRs coexist with SPPs trapped at the top of the metallic protrusions, hybridizing to concentrate light within the active layer.
• Short‑wavelength peaks (λ < 600 nm) arise from multiple SPP modes excited at the bottom of the PMEs, producing a broad enhancement bump with several subsidiary peaks.
• Variation of the period shows that 350 nm optimally couples localized resonances with bent surface modes, maximizing η_I while maintaining angle‑insensitive performance for both TM and TE polarizations.

Conclusions

This systematic comparison demonstrates that both nanohole‑ and nanopillar‑patterned metal electrodes can boost OSC absorption by nearly 10 % while preserving carrier transport. The integrated absorption efficiency of 82.4 % is achieved with the same active‑layer thickness as the planar control, indicating minimal material penalty. Device B (nanopillar) is recommended for practical implementation due to its easier fabrication and robust design tolerance. The study highlights the critical role of hybrid plasmonic–photonic resonances in light trapping and provides design guidelines for next‑generation high‑efficiency OSCs.