[Paper Review] Light trapping in a 30-nm organic photovoltaic cell for efficient carrier collection and light absorption

Cheng‐Chia Tsai, Richard R. Grote+3 more|arXiv (Cornell University)|Jan 20, 2012

Organic Electronics and Photovoltaics3 citations

TL;DR

This paper proposes a 30-nm P3HT-based organic photovoltaic cell with a patterned ITO top electrode that enhances light absorption and carrier collection through engineered gap-plasmon modes and photonic crystal resonances. By optimizing 2D cone-shaped ITO nanostructures, the design achieves up to 50% photon collection efficiency—representing a 3.7-fold improvement over unpatterned cells—while maintaining high angular tolerance and polarization-insensitive performance.

ABSTRACT

We describe surface patterning strategies that permit high photon-collection efficiency together with high carrier-collection efficiency in an ultra-thin planar heterojunction organic photovoltaic cell. Optimized designs reach up to 50% photon collection efficiency in a P3HT layer of only 30 nm, representing a 3- to 5-fold improvement over an unpatterned cell of the same thickness. We compare the enhancement of light confinement in the active layer with an ITO top layer for TE and TM polarized light, and demonstrate that the light absorption can increase by a factor of 2 due to a gap-plasmon mode in the active layer.

Motivation & Objective

• Address the fundamental trade-off in organic photovoltaics between exciton diffusion length (~10–15 nm) and optical absorption length.
• Overcome limited photon absorption in ultra-thin active layers (e.g., 30 nm P3HT) by enhancing light trapping without increasing thickness.
• Simultaneously improve both photon collection efficiency and carrier collection efficiency in a planar heterojunction architecture.
• Design a top-surface ITO pattern that supports multiple light confinement mechanisms: gap modes, grating resonances, and photonic crystal modes.
• Achieve polarization-insensitive and angle-tolerant absorption enhancement for practical solar energy harvesting.

Proposed method

• Employed 2D photonic crystal structures with ITO cones, cylinders, and blocks in a square lattice to engineer subwavelength optical modes.
• Utilized finite-difference time-domain (FDTD) simulations to model electromagnetic field distributions and absorption in the P3HT layer.
• Engineered a low-index active layer (P3HT, n≈1.7) sandwiched between high-index ITO (n≈2.0) and a metal back contact to excite gap-plasmon modes.
• Optimized geometric parameters (period, height, diameter, etching depth) to maximize coupling to both photonic crystal modes and gap modes.
• Compared TE and TM polarized light responses to evaluate polarization dependence and identify optimal geometries.
• Evaluated angular dependence of absorption across incident angles up to 70° to assess real-world performance.

Experimental results

Research questions

• RQ1 Can a 30-nm P3HT layer achieve high photon collection efficiency through surface patterning of the ITO top electrode?
• RQ2 To what extent can gap-plasmon modes in a low-index active layer between high-index ITO and metal contacts enhance light absorption?
• RQ3 How does the geometry of the ITO pattern (cylinders, blocks, cones) affect the coupling efficiency to gap modes and photonic resonances?
• RQ4 Can 2D periodic structures eliminate polarization dependence and improve angular tolerance in ultra-thin OPVs?
• RQ5 What is the maximum achievable photon collection efficiency in a 30-nm active layer using combined photonic and plasmonic engineering?

Key findings

• The optimal 2D cone array structure achieves a photon collection efficiency of 49.46%, representing a 3.7-fold enhancement over the unpatterned reference (13.33%).
• The 2D cone array achieves up to 55.80% absorbance at 12° incident angle along the diagonal, exceeding 50% for angles up to 70°.
• The gap-plasmon mode in the P3HT layer contributes to a near-doubling of absorption, increasing by a factor of 2 compared to unpatterned cells.
• Block and cone geometries outperform cylindrical arrays in coupling to gap modes due to in-plane asymmetry, leading to 25% higher absorbance than cylinder arrays.
• The structure maintains high performance across a broad angular range, with absorbance >50% higher than unpatterned reference until incident angles exceed 70°.
• The design enables simultaneous enhancement of both photon collection and carrier collection by confining light within the active layer while maintaining efficient charge extraction.