Rand, Peumans and Forrest (J. Appl. Phys. 96, 7519 (2004)) showed that an ultrathin layer of ~5-nm silver nanoclusters, used as the recombination contact of a tandem organic solar cell, enhances optical absorption in the surrounding organic dielectric over distances of order 10 nm and far from the cluster surface-plasmon resonance, and they proposed encapsulating the clusters in thin insulating shells to keep this enhancement while suppressing exciton quenching at the metal. They left open the experimental and theoretical program needed to generalize, validate, and optimize this plasmonic enhancement: testing realistic insulating shells, controlling cluster morphology, extending the idealized two-dimensional modeling to fully three-dimensional random island films, coupling the optics to exciton generation, diffusion, dissociation, and nonunity charge collection, and treating the quantum-size and scattering-dominated size regimes that lie outside the quasistatic, size-independent-dielectric-function treatment. The 2008-2020 plasmonic-photovoltaics literature has substantially addressed the individual items, while the combined core — a quantitative, experimentally validated, coupled optical-electronic framework determining whether realistic (shell-encapsulated, statistically distributed) nanoclusters yield a net efficiency gain in real devices — remains open.

Background

Organic photovoltaic (OPV) cells are thin-film solar cells whose light-absorbing layers are organic semiconductors. In a donor–acceptor (D–A) bilayer, absorbed photons create excitons (bound electron–hole pairs) rather than free charges; an exciton must reach a D–A interface, within its diffusion length LDL_D (typically 5–10 nm in small-molecule organics), to dissociate into free carriers. Because the optical absorption length is much longer (~100 nm), films thick enough to absorb the sun efficiently are too thick for most excitons to be collected. One remedy is the tandem cell: several ultrathin D–A subcells stacked in series, separated by ~1-nm-thick evaporated metal layers that serve both as recombination contacts (joining the subcells electrically) and as optical elements. Rand, Peumans, and Forrest (2004) studied such tandem cells with silver nanocluster recombination layers (isolated ~5-nm-diameter Ag islands with ~10-nm mean center-to-center spacing). They measured enhanced absorption of copper phthalocyanine (CuPc) films deposited on the cluster layers, persisting up to ~10 nm from the clusters and extending to wavelengths well beyond the cluster localized surface plasmon resonance (the collective oscillation of the metal's conduction electrons; its wavelength was ~470 nm when the clusters were embedded in CuPc). To explain this they solved Maxwell's equations by the finite-element method (FEM) in a two-dimensional cross-section, modeling the experimental random island film as an idealized one-dimensional chain of circular or elliptical (axial ratio b/a = 0.5) particles with a single interparticle spacing, and found strong interparticle dipolar coupling that amplifies, broadens, and redshifts the near-field enhancement (the region of elevated optical intensity I/I0I/I_0 around and between the particles). A transfer-matrix optical model (a standard method computing interference in plane-parallel multilayer stacks) that folded this enhancement into the tandem stack ITO/CuPc/PTCBI/Ag-cluster/CuPc/PTCBI/Ag (ITO, indium tin oxide, is the transparent conducting anode, and PTCBI, a perylene-bisbenzimidazole, the acceptor) reproduced the experimentally observed ~30% increase in short-circuit current density JSCJ_{SC} and the power conversion efficiency of about 2.4% versus 1.9% without the clusters. The modeling, however, assumed a classical, size-independent dielectric function for the metal, idealized cluster geometries, unity exciton dissociation and charge-collection efficiency, and complete quenching of any exciton reaching the metal. Because bare metal clusters quench excitons at their surface, the authors proposed encapsulating Ag clusters in a thin insulating shell and dispersing them in the organic films, so that near-field enhancement survives while quenching sites are removed — a proposal they did not test. The question was subsequently taken up by the broader field of plasmonic photovoltaics, in which metal nanostructures are used to trap light in thin solar cells through near-field enhancement and far-field scattering, and where a key design tension is that small (<~10 nm) particles respond mostly by (often parasitic) absorption while larger particles (~tens of nm and up) scatter, and that any metal in or near the active layer can also recombine or trap carriers.

Problem Statement

Determine whether, and by how much, long-range plasmonic absorption enhancement from metal nanoclusters embedded in organic photovoltaic cells yields a net improvement in device efficiency for realistic shell-encapsulated, statistically distributed nanoclusters, by evaluating the 2004 proposal of Rand, Peumans, and Forrest across the regimes its original analysis excluded: (1) shell-encapsulated Ag clusters in thin insulating shells, where the effect of realistic shell materials and thicknesses on the near-field amplitude, spectral shape, and spatial extent of the enhancement and on exciton quenching dynamics is unmeasured; (2) cluster morphologies (size, axial ratio, interparticle spacing) realizing the modeled parameter regimes and the tradeoffs among enhancement amplitude, spectral overlap with the organic absorbers, and parasitic loss in actual devices; (3) fully three-dimensional, randomized island films with the full statistical distributions of particle shapes and orientations that evaporation actually produces, in place of the two-dimensional cross-section and idealized chain/array geometries; (4) the competition between optical enhancement and carrier-loss pathways in different material systems, under nontrivial exciton generation, diffusion, and dissociation and nonunity charge-collection efficiencies rather than the unity-collection and complete-metal-quenching assumptions; and (5) the quantum-size and nonlocal regime for particles with R<1 R<1\,nm, where the size-independent local dielectric function fails, and the crossover to scattering-dominated response for larger particles (2R≳30 2R\gtrsim30\,nm), where what fails is the quasistatic approximation rather than the dielectric function. The single scientific objective uniting these is the determination of whether, and by how much, realistic (shell-encapsulated, statistically distributed) nanocluster enhancement changes device efficiency in real organic photovoltaic devices and which gain and loss channels dominate; either sign of the net result resolves the question, and items (1)-(5) delimit the regimes a defensible determination must address rather than being independent deliverables each requiring separate resolution.

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