Yuying Hao

Improved performance of organic solar cells by incorporating silica-coated silver nanoparticles in the buffer layer

It is demonstrated that the use of silica-coated silver nanoparticles (AgNPs) in the buffer layer improves the performance of organic solar cells (OSCs). It is found that only large sized AgNPs are advantageous for increasing the electric field distribution in the active layer, and therefore, increasing light absorption, caused by the localized surface plasmonic resonance and far-field scattering. Furthermore, the scattering of silica-coated AgNPs is more important to the light harvesting because of the existence of the silica coating. It is also demonstrated that the silica coating is favorable for enhancing the exciton dissociation because of the reduction of the exciton quenching that occurred at the interface between the bare AgNPs and the active layer. Furthermore, silica-coated AgNPs also promote hole transport and extraction, which is presumably explained by the introduction of “dopant” levels within the band gap of the poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) and reduction of hole trapping of a bare silver surface. The combination of all these benefits results in a 25.4% improvement in photocurrent density and an increase of 19.2% in power conversion efficiency. This work indicates that using silica-coated AgNPs as light trapping elements is more efficient than using bare AgNPs in plasmonic organic solar cells. The systematic exploration of the optical and electrical effects of silica-coated AgNPs contributes to a more comprehensive understanding of the mechanism of performance improvement of the plasmonic OSCs.

Efficient multiband absorber based on one-dimensional periodic metal–dielectric photonic crystal with a reflective substrate

We propose an efficient multiband absorber comprised of a truncated, one-dimensional periodic metal-dielectric photonic crystal and a reflective substrate. The reflective substrate is essentially an optically thick metallic film. Such a planar device is easier to fabricate compared to absorbers with complicated shapes. For a four-unit cell device, all four of the absorption peaks can be optimized with efficiencies higher than 95 percent. Moreover, those absorption peaks are insensitive to the polarization and incident angle. The influences of the geometrical parameters and the refractive index of the dielectric on the device performance also are discussed. Furthermore, we found that the number of absorption peaks within each photonic band precisely corresponds to the number of unit cells because the truncated photonic crystal lattices select resonant modes. We also show that the total absorption efficiency gradually increases when there are more periods of the metal-dielectric composite layer placed on top of the metallic substrate. We expect this work to have potential applications in solar energy harvesting and thermal emission tailoring.

High-efficiency, broad-band and wide-angle optical absorption in ultra-thin organic photovoltaic devices

Metal nanogratings as one of the promising architectures for effective light trapping in organic photovoltaics (OPVs) have been actively studied over the past decade. Here we designed a novel metal nanowall grating with ultra-small period and ultra-high aspect-ratio as the back electrode of the OPV device. Such grating results in the strong hot spot effect in-between the neighboring nanowalls and the localized surface plasmon effect at the corners of nanowalls. These combined effects make the integrated absorption efficiency of light over the wavelength range from 400 to 650 nm in the active layer for the proposed structure, with respect to the equivalent planar structure, increases by 102% at TM polarization and by 36.5% at the TM/TE hybrid polarization, respectively. Moreover, it is noted that the hot spot effect in the proposed structure is more effective for ultra-thin active layers, which is very favorable for the exciton dissociation and charge collection. Therefore such a nanowall grating is expected to improve the overall performance of OPV devices.

Efficiency enhancement in organic solar cells by incorporating silica-coated gold nanorods at the buffer/active interface

The performance of organic solar cells (OSCs) can be greatly improved by incorporating silica-coated gold nanorods (Au@SiO2 NRs) at the interface between the hole transporting layer and the active layer due to the plasmonic effect. The silica shell impedes the aggregation effect of the Au NRs in ethanol solution as well as the server charge recombination on the surface of the Au NRs otherwise they would bring forward serious reduction in open circuit voltage when incorporating the Au NRs at the positions in contact with the active materials. As a result, while the high open circuit voltage being maintained, the optimized plasmonic OSCs possess an increased short circuit current, and correspondingly an elevated power conversion efficiency with the enhancement factor of ~11%. The origin of performance improvement in OSCs with the Au@SiO2 NRs was analyzed systematically using morphological, electrical, optical characterizations along with theoretical simulation. It is found that the broadband enhancement in absorption, which yields the broadband enhancement in exciton generation in the active layer, is the major factor contributing to the increase in the short circuit current density. Simulation results suggest that the excitation of the transverse and longitudinal surface plasmon resonances of individual NRs as well as their mutual coupling can generate strong electric field near the vicinity of the NRs, thereby an improved exciton generation profile in the active layer. The incorporation of Au@SiO2 NRs at the interface between the hole transporting layer and the active layer also improves hole extraction in the OSCs.

Dual-Layer Nanostructured Flexible Thin-Film Amorphous Silicon Solar Cells with Enhanced Light Harvesting and Photoelectric Conversion Efficiency

Three-dimensional (3-D) structures have triggered tremendous interest for thin-film solar cells since they can dramatically reduce the material usage and incident light reflection. However, the high aspect ratio feature of some 3-D structures leads to deterioration of internal electric field and carrier collection capability, which reduces device power conversion efficiency (PCE). Here, we report high performance flexible thin-film amorphous silicon solar cells with a unique and effective light trapping scheme. In this device structure, a polymer nanopillar membrane is attached on top of a device, which benefits broadband and omnidirectional performances, and a 3-D nanostructure with shallow dent arrays underneath serves as a back reflector on flexible titanium (Ti) foil resulting in an increased optical path length by exciting hybrid optical modes. The efficient light management results in 42.7% and 41.7% remarkable improvements of short-circuit current density and overall efficiency, respectively. Meanwhile, an excellent flexibility has been achieved as PCE remains 97.6% of the initial efficiency even after 10 000 bending cycles. This unique device structure can also be duplicated for other flexible photovoltaic devices based on different active materials such as CdTe, Cu(In,Ga)Se2 (CIGS), organohalide lead perovskites, and so forth.

Plasmonic broadband absorber by stacking multiple metallic nanoparticle layers

High efficiency, broadband plasmonic absorbers are constructed based on a stack of alternating metallic nanoparticle layers (MNLs) and SiO2 slabs on top of a reflective Ag substrate. Experimental results show that the stacks with thick MNLs absorb light better than those with thin MNLs when the number of MNL/SiO2 cells (N) is small (e.g., 1 or 2), but the situation gets reversed when N is greater than 3. When the nominal thickness of MNL is as thin as 5 nm, the acquired Ag nanoparticles are so small that light penetration through all of the stacked MNLs in the proposed design is possible. Thus, an increase in N leads to a growing number of light trapping elements. Our simulation reveals that the Ag nanoparticles at different layers are hybridized to excite rich localized plasmonic resonances, resulting in multiple absorption peaks at optical frequencies and thus a broader absorption band. The broadband absorbers with an integrated absorption efficiency of 96% over the 300–1100 nm wavelength range were ach...

Efficient tandem organic light-emitting device based on photovoltaic-type connector with positive cycle

We designed a tandem organic light-emitting device based on an organic photovoltaic-type charge generation connector (CGC) of fullerene carbon 60/copper(II) phthalocyanine. The CGC can absorb a portion of photons radiated from emission zone and form excitons which disassociated into free charges at PN junction interface without energy barrier, leading to low driving voltage and better charge balance. The efficiency increases remarkably with increasing current density, even beyond two folds compared with single unit device under higher current density, meaning slower roll-off. The whole process is a positive cycle, and actually enhances the utilization of internal radiation and the overall performance of tandem device.

Optical and electrical properties of [N,N′-bis(salicylidene)-ethylenediamine]zinc as an electroluminescent material

The variability of photoluminescence (PL) and electroluminescence (EL) properties of [N,N′-bis(salicylidene)-ethylenediamine]zinc [Zn(salen)] was investigated. It was found that the PL spectra of Zn(salen) powder are dependent on synthesis temperature, and recrystallization solvent and those of Zn(salen) film are dependent on deposition vacuum degree. The EL properties of Zn(salen) are not only dependent on deposition vacuum degree but also dependent on driving voltage. The variability of PL and EL properties of Zn(salen) can be related to the various molecular geometries of Zn(salen).

Tungsten based anisotropic metamaterial as an ultra-broadband absorber

The trapped rainbow effect has been mostly found on tapered anisotropic metamaterials (MMs) made of low loss noble metals, such as gold, silver, etc. In this work, we demonstrate that an anisotropic MM waveguide made of high loss metal tungsten can also support the trapped rainbow effect similar to the noble metal based structure. We show theoretically that an array of tungsten/germanium anisotropic nano-cones placed on top of a reflective substrate can absorb light at the wavelength range from 0.3 micrometer to 9 micrometer with an average absorption efficiency approaching 98%. It is found that the excitation of multiple orders of slow-light resonant modes is responsible for the efficient absorption at wavelengths longer than 2 micrometer, and the anti-reflection effect of tapered lossy material gives rise to the near perfect absorption at shorter wavelengths. The absorption spectrum suffers a small dip at around 4.2 micrometer where the first order and second order slow-light modes get overlapped, but we can get rid of this dip if the absorption band edge at long wavelength range is reduced down to 5 micrometer. The parametrical study reflects that the absorption bandwidth is mainly determined by the filling ratio of tungsten as well as the bottom diameter of the nano-cones and the interaction between neighboring nano-cones is quite weak. Our proposal has some potential applications in the areas of solar energy harvesting and thermal emitters.

High efficiency planar Sn–Pb binary perovskite solar cells: controlled growth of large grains via a one-step solution fabrication process

One-step solution fabrication of high-performance Sn-including perovskite solar cells (PSCs) is very challenging due to the rapid crystallization of the Sn-based perovskite layer, leading to a poor film morphology and low surface coverage. In this work, a well-controlled one-step method, assisted by a multi-step solvent treatment, is developed for the growth of a high-quality CH3NH3Pb(1−x)SnxI3 (0 ≤ x ≤ 1) perovskite film on a planar PEDOT:PSS substrate. The CH3NH3Sn0.25Pb0.75I3 perovskite films consisting of densely packed and uniformly distributed large crystal grains were obtained using sec-butyl alcohol solvent engineering and N,N-dimethylformamide solvent annealing under an N2 atmosphere. The CH3NH3Sn0.25Pb0.75I3-based PSCs with a maximum power conversion efficiency (PCE) of 12.08% and an average PCE of 11.01% were obtained. The PSCs also exhibit excellent performance reproducibility, good air stability and weak hysteresis behavior. The enhancement in the performance of the PSCs is attributed to the well-crystallized CH3NH3Sn0.25Pb0.75I3 film, resulting in simultaneous improvement in charge–carrier transport properties and reduction in charge–carrier recombination, a very promising approach to obtain high performance Sn-including perovskite solar cells.