Kong Liu

Improved photovoltaic performance of silicon nanowire/organic hybrid solar cells by incorporating silver nanoparticles.

Silicon nanowire (SiNW) arrays show an excellent light-trapping characteristic and high mobility for carriers. Surface plasmon resonance of silver nanoparticles (AgNPs) can be used to increase light scattering and absorption in solar cells. We fabricated a new kind of SiNW/organic hybrid solar cell by introducing AgNPs. Reflection spectra confirm the improved light scattering of AgNP-decorated SiNW arrays. A double-junction tandem structure was designed to manufacture our hybrid cells. Both short-circuit current and external quantum efficiency measurements show an enhancement in optical absorption of organic layer, especially at lower wavelengths.

Efficient hybrid plasmonic polymer solar cells with Ag nanoparticle decorated TiO2 nanorods embedded in the active layer

A hybrid plasmonic polymer solar cell, in which plasmonic metallic nanostructures (such as Ag, Au, and Pt nanoparticles) are embedded in the active layer, has been under intense scrutiny recently because it provides a promising new approach to enhance the efficiency of the device. We propose a brand new hybrid plasmonic nanostructure, which combines a plasmonic metallic nanostructure and one-dimensional semiconductor nanocrystals, to enhance the photocurrent of the device through a strong localized electric field and an enhanced charge transport channel. We demonstrate that when Ag nanoparticle decorated TiO2 nanorods were introduced into the active layer of polymer-fullerene based bulk heterojunction solar cells, the photocurrent significantly increased to 14.15 mA cm(-2) from 6.51 mA cm(-2) without a decrease in the open voltage; thus, the energy conversion efficiency was dramatically enhanced to 4.87% from 2.57%.

Fully understanding the positive roles of plasmonic nanoparticles in ameliorating the efficiency of organic solar cells

Herein, we constructed inverted PBDTTT-CF:PC70BM bulk-heterojunction organic solar cells by introducing Au nanoparticles to a ZnO buffer layer and a great improvement in energy conversion efficiency has been realized. To discover the positive roles of such plasmonic nanoparticles in the process of solar energy conversion, photovoltaic devices with the same architecture but different sized Au nanoparticles were purposely fabricated and it has been observed that the overall efficiency can be remarkably improved from 6.67% to 7.86% by embedding 41 nm Au nanoparticles in the buffer layer. The devices with other sizes of Au nanoparticles show a relatively low performance. Subsequent investigations including finite difference time domain simulation and transient photoluminescence studies reveal that the existence of the plasmonic particles could not only improve the optical absorption and facilitate the exciton separation, but can also benefit the collection of charge carriers. Thus, this paper provides a comprehensive perspective on the roles of plasmonic particles in organic solar cells and insights into the photo energy conversion process in the plasmonic surroundings.

Efficacious engineering on charge extraction for realizing highly efficient perovskite solar cells

Efficient extraction of photogenerated charge carriers is of significance for acquiring a high efficiency for perovskite solar cells. In this paper, a systematic strategy for effectively engineering the charge extraction in inverted structured perovskite solar cells based on methylammonium lead halide perovskite (CH3NH3PbI3−xClx) is presented. Intentionally doping the chlorine element into the perovskite structure is helpful for obtaining a high open circuit voltage. The engineering is carried out by modifying the aluminium cathode with zirconium acetylacetonate, doping the hole transport layer of nickel oxide (NiOx) with copper and using an advanced fluorine doped tin oxide (FTO) substrate. This improves the bandgap alignment of the whole device, and thus, is of great benefit for extracting the charge carriers by promoting the transport rate and reducing the trap states. Consequently, an optimized power conversion efficiency of 20.5% is realized. Insights into how to extract charge carriers efficiently with a minimum energy loss are discussed.

Insights into the Influence of Work Functions of Cathodes on Efficiencies of Perovskite Solar Cells

Though various efforts on modification of electrodes are still undertaken to improve the efficiency of perovskite solar cells, attributing to the large scope of these methods, it is of significance to unveil the working principle systematically. Herein, inverted perovskite solar cells based on indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)/CH3 NH3 PbI3 /phenyl-C61-butyric acid methyl ester (PC61 BM)/buffer metal/Al are constructed. Through the choice of different buffer metals to tune work function of the cathode, the contact nature of the active layer with the cathode could be manipulated well. In comparison with the device using Au/Al as the electrode that shows an unfavorable band bending for conducting the excited electrons to the cathode, the one with Ca/Al presents a dramatically improved efficiency over 17.1%, ascribed to the favorable band bending at the interface of the cathode with the active layer. Details for tuning the band bending and the corresponding charge transfer mechanism are given in a systematic manner. Thus, a general guideline for constructing perovskite photovoltaic devices efficiently is provided.

Moiré fringes characterization of surface plasmon transmission and filtering in multi metal-dielectric films

We use moire fringes to experimentally characterize the transmission and filtering effect of surface plasmons (SPs) in the alternatively stacked metal-dielectric layers (ASMDLs). Two gratings with unequal periods are introduced at the two sides of films structure to excite SPs and generate moire fringes observable in the far-field. The observed moire fringes with specific period agree well with simulations and clearly demonstrate SP with corresponding wave vector transmission and filtering in ASMDLs. By applying variant incidence angle, the fringes contrast fluctuation further illustrate the transmission of SP in a specific wide wave vector band and with an uneven amplitude transmission profile.

Anisotropic characteristics and morphological control of silicon nanowires fabricated by metal-assisted chemical etching

Low-cost fabrication methods enabling the morphological control of silicon nanowires are of great importance in many device application fields. A top-down fabrication method, metal-assisted chemical etching, is proved to be a feasible solution. In this paper, some novel approaches based on metal-assisted chemical etching, alkaline solution etching, and electrochemical anodic etching are presented for fabricating micro- and nano-structures, which reveal the anisotropic characteristics of metal-assisted chemical etching in silicon. A new model is proposed to explain the motility behavior of Ag particles in metal-assisted chemical etching of silicon. It is shown that Ag particle forms a self-electrophoresis unit and migrates into Si substrate along [100] direction independently. Diameter and length control of silicon nanowires are achieved by varying Ag deposition and etching durations of metal-assisted chemical etching, respectively, which provide a facilitation to achieve high-aspect-ratio silicon nanowires at room temperature in a short period. These results show a potential simple method to microstructure silicon for devices application, such as solar cells and sensors.

Turning a disadvantage into an advantage: synthesizing high-quality organometallic halide perovskite nanosheet arrays for humidity sensors

Though power conversion efficiencies of perovskite solar cells of over 21% have been reported, the degradation of the performance of these cells in the presence of moisture still inhibits the commercialization of these otherwise promising devices. Herein, we decided to utilize the moisture sensitivity of perovskite materials and fabricate a moisture detector based on high-quality CH3NH3PbI3−xClx nanosheet arrays. The resulting devices present a high sensitivity towards humidity with device resistance significantly dropping from 1.28 × 108 Ω at 30% relative humidity (RH) to 7.39 × 104 Ω at 90% RH and a response even faster than that of the commercial psychrometer. This response was even steeper than those of commercial psychrometers. Moreover, our device showed good reversibility and impressive specificity. Thus, the humidity sensitivity of perovskites, while disadvantageous for solar cell applications, was found to be quite applicable in a new field.

Hybrid silicon nanocone–polymer solar cells based on a transparent top electrode

Instead of using a metal grid line as the top electrode for Si nanocone/PEDOT:PSS hybrid solar cells, herein we utilize a fully covered thin layer of transparent MoO3/Ag/ZnS to efficiently collect photo-generated charge carriers without sacrificing photon absorption. Vertical silicon nanocone (SiNC) arrays with short cone lengths are prepared by a metal-assisted chemical etching technique. Such a unique structure favors the subsequent infiltration of PEDOT:PSS into the nanocone array. In addition to decent antireflection and light trapping properties, which can be attributed to the nanostructure array, the power conversion efficiency (PCE) of the hybrid solar cells reaches 5.12% upon optimizing the length of the nanocone and the thickness of the PEDOT:PSS layer. This paper not only provides an elaborate investigation on Si-based hybrid solar cells, but also gives a universal methodology for the preparation of transparent electrodes for other categories of photoelectric devices.

Recent Development in ITO-free Flexible Polymer Solar Cells

Polymer solar cells have shown good prospect for development due to their advantages of low-cost, light-weight, solution processable fabrication, and mechanical flexibility. Their compatibility with the industrial roll-to-roll manufacturing process makes it superior to other kind of solar cells. Normally, indium tin oxide (ITO) is adopted as the transparent electrode in polymer solar cells, which combines good conductivity and transparency. However, some intrinsic weaknesses of ITO restrict its large scale applications in the future, including a high fabrication price using high temperature vacuum deposition method, scarcity of indium, brittleness and scaling up of resistance with the increase of area. Some substitutes to ITO have emerged in recent years, which can be used in flexible polymer solar cells. This article provides the review on recent progress using other transparent electrodes, including carbon nanotubes, graphene, metal nanowires and nanogrids, conductive polymer, and some other electrodes. Device stability is also discussed briefly.