Se-Woong Baek

Plasmonic Forward Scattering Effect in Organic Solar Cells: A Powerful Optical Engineering Method

In this report, plasmonic effects in organic photovoltaic cells (OPVs) are systematically analyzed using size-controlled silver nanoparticles (AgNPs, diameter: 10 ~ 100 nm), which were incorporated into the anodic buffer layer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). The optical properties of AgNPs tuned by size considerably influence the performance levels of devices. The power conversion efficiency (PCE) was increased from 6.4% to 7.6% in poly[N-9-hepta-decanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2,1,3-benzothiadiazole)] (PCDTBT):[6,6]-phenyl C71-butyric acid methyl ester (PC70BM) based-OPVs and from 7.9% to 8.6% in polythieno[3,4-b]thiophene/benzodithiophene (PTB7):PC70BM based-OPVs upon embedding the AgNPs. The external quantum efficiency (EQE) was significantly enhanced by the absorption enhancement due to the plasmonic scattering effect. Finally, we verified the origin of the size-dependent plasmonic forwarding scattering effect of the AgNPs by visualizing the scattering field with near-field optical microscopy (NSOM) and through analytic optical simulations.

Au@Ag Core–Shell Nanocubes for Efficient Plasmonic Light Scattering Effect in Low Bandgap Organic Solar Cells

In this report, we propose a metal-metal core-shell nanocube (NC) as an advanced plasmonic material for highly efficient organic solar cells (OSCs). We covered an Au core with a thin Ag shell as a scattering enhancer to build Au@Ag NCs, which showed stronger scattering efficiency than Au nanoparticles (AuNPs) throughout the visible range. Highly efficient plasmonic organic solar cells were fabricated by embedding Au@Ag NCs into an anodic buffer layer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), and the power conversion efficiency was enhanced to 6.3% from 5.3% in poly[N-9-hepta-decanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2,1,3-benzothiadiazole)] (PCDTBT):[6,6]-phenyl C71-butyric acid methyl ester (PC70BM) based OSCs and 9.2% from 7.9% in polythieno[3,4-b]thiophene/benzodithiophene (PTB7):PC70BM based OSCs. The Au@Ag NC plasmonic PCDTBT:PC70BM-based organic solar cells showed 2.2-fold higher external quantum efficiency enhancement compared to AuNPs devices at a wavelength of 450-700 nm due to the amplified plasmonic scattering effect. Finally, we proved the strongly enhanced plasmonic scattering efficiency of Au@Ag NCs embedded in organic solar cells via theoretical calculations and detailed optical measurements.

High-performance hybrid plastic films: a robust electrode platform for thin-film optoelectronics

We report a novel flexible hybrid plastic film that can be used as a robust electrode platform for typical thin-film optoelectronic devices. Silver nanowires (AgNWs) were embedded on the surface of a glass-fabric reinforced transparent composite (GFRHybrimer) film to form a flexible transparent conducting substrate with excellent opto-electrical properties, superior thermal stability, and impressive mechanical flexibility. A highly efficient and flexible inverted organic solar cell with a power conversion efficiency (PCE) of 5.9% under 100 mW cm−2 AM 1.5G illumination was fabricated on the AgNW–GFRHybrimer film. The AgNW–GFRHybrimer film exhibits potential as an excellent transparent electrode for low cost flexible optoelectronic devices.

Broadband energy transfer to sensitizing dyes by mobile quantum dot mediators in solar cells

The efficiency of solar cells depends on absorption intensity of the photon collectors. Herein, mobile quantum dots (QDs) functionalized with thiol ligands in electrolyte are utilized into dye–sensitized solar cells. The QDs serve as mediators to receive and re–transmit energy to sensitized dyes, thus amplifying photon collection of sensitizing dyes in the visible range and enabling up–conversion of low-energy photons to higher-energy photons for dye absorption. The cell efficiency is boosted by dispersing QDs in electrolyte, thereby obviating the need for light scattering1 or plasmonic2 structures. Furthermore, optical spectroscopy and external quantum efficiency data reveal that resonance energy transfer due to the overlap between QD emission and dye absorption spectra becomes dominant when the QD bandgap is higher than the first excitonic peak of the dye, while co–sensitization resulting in a fast reduction of oxidized dyes is pronounced in the case of lower QD band gaps.

Coupled Near‐ and Far‐Field Scattering in Silver Nanoparticles for High‐Efficiency, Stable, and Thin Plasmonic Dye‐Sensitized Solar Cells

Here, we report plasmonically enhanced thin dye-sensitized solar cells (DSSCs) in an imidazolium-dicyanamide based ionic liquid, in which size-controlled metal (silver) nanoparticles (AgNPs) with passivation layers of a few nanometers are arranged into the electrolyte and photo-electrodes. It was revealed that the AgNPs in the electrolyte and the photo-electrode have distinct effects on device performance via different coupling mechanisms. Strong far-field scattering is critical in the electrolyte while near-field scattering is efficient in the photo-electrode. Indeed, we find that the power conversion efficiency of the DSSC can be substantially improved by a synergistic arrangement of the AgNPs in the electrolyte and the photo-electrode. Furthermore, an imidazolium-dicyanamide based nonvolatile ionic liquid electrolyte for MNPs is demonstrated to provide thin plasmonic DSSCs with good stability.

A Resonance‐Shifting Hybrid n‐Type Layer for Boosting Near‐Infrared Response in Highly Efficient Colloidal Quantum Dots Solar Cells

A new configuration of a plasmonic quantum dots solar structure is proposed. Gold-silver core-shell metal nanoparticles (Au@Ag NCs) are incorporated into the TiO2 layer (Au@Ag NCs-HL) of PbS-based solar cells. The TiO2 layer enables the Au@Ag NCs to have broad plasmonic responses and the external quantum efficiency and absorption of the plasmonic devices are significantly enhanced. The electrical performance of the solar cells is also improved.

Improved Internal Quantum Efficiency and Light-Extraction Efficiency of Organic Light-Emitting Diodes via Synergistic Doping with Au and Ag Nanoparticles

This paper reports the distinct roles of Au and Ag nanoparticles (NPs) in organic light-emitting diodes (OLEDs) depending on their sizes. Au and Ag NPs that are 40 and 50 nm in size, respectively, are the most effective for enhancing the performance of green OLEDs. The external quantum efficiencies (EQEs) of green OLEDs doped with Au and Ag NPs (40 and 50 nm, respectively) are improved by 29.5% and 36.1%, respectively, while the power efficiencies (PEs) are enhanced by 47.9% and 37.5%, respectively. Furthermore, combining the Au and Ag NPs produces greater enhancements. The EQE and PE of the codoped OLEDs are improved by 63.9% and 68.8%, respectively, through the synergistic behavior of the different NPs. Finite-difference time-domain simulations confirm that the localized surface-plasmon resonance of the Au NPs near 580 nm improves the radiative recombination rate (krad) of green-light emitters locally (<50 nm), while the Ag NPs cause relatively long-range and broadband enhancements in krad. The simulations of various domain sizes verify that the light-extraction efficiency (LEE) can be enhanced by more than 4.2% by applying Ag NPs. Thus, size-controlled Au and Ag NPs can synergistically enhance OLEDs by improving both the internal quantum efficiency and LEE.

Efficient Green Organic Light-Emitting Diodes by Plasmonic Silver Nanoparticles

The resonance effect between the localized surface plasmon mode and the excited excitons in organic materials can enhance the optical and electrical performances of organic light-emitting diodes (OLEDs). Here, we show an enhancement in the luminescence efficiency of a green OLED by employing silver nanoparticles without an optical microcavity effect. The enhanced photoluminescence and exciton decay rate support the idea that the improved luminescence efficiency is due to the increased spontaneous emission rate of the emitter. Our results indicate that the plasmonic near-field interaction of the light-matter can be a strong engineering tool for high-efficiency organic optoelectronic devices in the display industry.

A Colloidal‐Quantum‐Dot‐Based Self‐Charging System via the Near‐Infrared Band

A novel self-charging platform is proposed using colloidal-quantum-dot (CQD) photovoltaics (PVs) via the near-infrared (NIR) band for low-power electronics. Low-bandgap CQDs can convert invisible NIR light sources to electrical energy more efficiently than wider spectra because of reduced thermalization loss. This energy-conversion strategy via NIR photons ensures an enhanced photostability of the CQD devices. Furthermore, the NIR wireless charging system can be concealed using various colored and NIR-transparent fabric or films, providing aesthetic freedom. Finally, an NIR-driven wireless charging system is demonstrated for a wearable healthcare bracelet by integrating a CQD PVs receiver with a flexible lithium-ion battery and entirely embedding them into a flexible strap, enabling permanent self-charging without detachment.

A hydro/oxo-phobic top hole-selective layer for efficient and stable colloidal quantum dot solar cells

In this report, we explore the underlying mechanisms by which doped organic thin films as a top hole-selective layer (HSL) improve the performance and stability of colloidal quantum dot (CQD)-based solar cells. Molecular dynamics-based theoretical studies prove that the hydro/oxo-phobic properties of the HSL serve to efficiently passivate the CQD solid. Furthermore, the robust and outstanding electrical properties of the HSL, simultaneously ensure a high power conversion efficiency (PCE) and increase the stability performance of CQD-based solar cells. As a result, a best PCE of 11.7% in a lead sulfide (PbS)-based CQD solar cell is achieved and over 90% of the initial performance is retained after 1 year storage under ambient conditions.