Commercial white LEDs (WLEDs) use yellow-emitting cerium-doped yttrium aluminum garnet phosphor, along with an InGaN-based blue LED. However, such phosphors suffer from the following disadvantages: (1) limited phosphor performance due to thermal degradation, (2) significant backscattering losses, and (3) poor absorption. Current commercial WLEDs have a luminous efficacy varying from 75~100 lm/W, with some showing higher values, but with a trade-off in the color rendering indices (CRIs). To work towards the US Department of Energy target of a luminaire of 200 lm/W, it is necessary to develop new designs for phosphors for WLEDs with high efficiency and better color rendering.
Here, we propose and study theoretically core-shell (CS) and core-shell-shell (CSS) metal-semiconductor nanowires (NWs) as phosphor components in white LEDs, using a Mie formalism for absorption and a Green’s function approach for emission. Coupling of the plasmon resonance oscillations at the metal surface with the electric fields of the incident light enables an enhanced absorbance of CS NWs of 0.6-0.9 for blue light compared to the absorbance of 0.2-0.4 observed in the CS quantum dots. We have predicted that the External Quantum Efficiency (EQE) can be enhanced by almost 11 times for red phosphors, by 36 times for yellow phosphors and as high as four orders of magnitude for the green phosphors relative to the bare semiconductor nanowires, when carefully choosing the semiconductor and metal materials and dimensions. CSS NWs further improve values of the EQE by as much as 60% relative to the CS nanowires for red phosphors and 3 times for yellow phosphors, due to the addition of another enhanced electric field from the semiconductor core to the Purcell factor.
One of the critical challenges for achieving solar-to-hydrogen efficiency greater than 10% (100 W/m2), especially in metal oxide photoelectrodes, is the poor internal quantum efficiency arising from high, bulk and surface, recombination and insufficient light absorption. Plasmonic light harvesting has emerged as a promising strategy to address this challenge. However, most designs are photocatalyst specific and employ precious metals, making large scale applications infeasible. We present metal-photocatalyst core-shell and semiconductor-metal-photocatalyst coremultishell nanowires as a novel class of multi-functional plasmonic photoelectrodes. By combining the optical resonances with the localized surface plasmon resonance within the proposed structures, we achieve extreme light absorption in the visible range within ultrathin photocatalyst layers. Such enhanced absorption ensures that the photocharges are preferentially generated very close to the photocatalyst-electrolyte interface and can effectively drive the reaction forward, thereby improving the internal quantum efficiency. Specifically, for nanowires in an aqueous electrolyte, we demonstrate the effectiveness of aluminum and copper to confine light and establish them as plasmonic alternatives to precious metal counterparts such as silver and gold therefore enabling cheap and scalable plasmonics. Further, we probe the absorption as a function of the permittivity of the electrolyte and show that the absorption in such nanowires is large even for high permittivity electrolytes. Hematite and copper(I) oxide have been chosen as the test materials to validate the generality of this approach. Notably, for hematite, we show that aluminum is more effective than copper, while for a broadband absorber such as copper(I) oxide, we show that both aluminum and copper are equally effective for plasmonic light harvesting.