Xiaojuan Shen

Hybrid heterojunction solar cell based on organic-inorganic silicon nanowire array architecture.

Silicon nanowire arrays (SiNWs) on a planar silicon wafer can be fabricated by a simple metal-assisted wet chemical etching method. They can offer an excellent light harvesting capability through light scattering and trapping. In this work, we demonstrated that the organic-inorganic solar cell based on hybrid composites of conjugated molecules and SiNWs on a planar substrate yielded an excellent power conversion efficiency (PCE) of 9.70%. The high efficiency was ascribed to two aspects: one was the improvement of the light absorption by SiNWs structure on the planar components; the other was the enhancement of charge extraction efficiency, resulting from the novel top contact by forming a thin organic layer shell around the individual silicon nanowire. On the contrary, the sole planar junction solar cell only exhibited a PCE of 6.01%, due to the lower light trapping capability and the less hole extraction efficiency. It indicated that both the SiNWs structure and the thin organic layer top contact were critical to achieve a high performance organic/silicon solar cell.

Solvent-free ionic liquid/poly(ionic liquid) electrolytes for quasi-solid-state dye-sensitized solar cells

Photochemically stable poly(ionic liquids) (poly(ILs)) including poly(1-butyl-3-vinylimidazolium bromide) ([PBVIm][Br]) and poly(1-butyl-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide) ([PBVIm][TFSI]) were synthesized and dissolved in the room temperature ionic liquids (ILs) to form quasi-solid-state electrolytes for dye-sensitized solar cells (DSSCs), without using any volatile organic solvent. DSSCs based on IL/[PBVIm][TFSI] gel electrolyte yielded a power conversion efficiency of 4.4% under a simulated air mass 1.5 solar spectrum illumination at 100 mW cm−2. The superior long-term stability of fabricated DSSCs indicated that the DSSCs based on solvent-free IL/poly(ILs) gel electrolytes could overcome the drawbacks of cells containing volatile solvents.

High-performance photoelectrochemical cells from ionic liquid electrolyte in methyl-terminated silicon nanowire arrays.

Photoelectrochemical (PEC) cells based on silicon nanowire arrays (SiNWs) have, to date, exhibited modest power conversion efficiency (PCE) and suffered serious degradation, though they exhibit advantageous properties of charge-transfer/transport properties at the radial-junction and strong light-trap capabilities. The main challenge for this low-cost PEC cell is the surface photooxidation and photocorrosion of the silicon surface when contacting with the electrolyte. In this report, SiNWs derivatized with covalently attached methyl groups, prepared via a two-step chlorination/methylation procedure, demonstrate excellent stability even in the presence of water. Furthermore, SiNWs PEC cells utilizing a room temperature ion liquid (IL) acting as an electrolyte solvent display neglectable surface oxidation. A PEC cell based on a platinum (Pt) nanodots decorated and methylated (-CH(3)) SiNWs electrode in combination with an IL electrolyte yields a PCE of 6.0% and shows excellent stability under simulated air mass (AM) 1.5 solar spectrum irradiation, while the PCE of a PEC cell based on planar silicon only exhibits 0.003%. The inherent performance of these structures indicates that a -CH(3) (Pt) SiNWs electrode in combination with an IL is a new approach to develop a high-performance and low-cost solar cell.

Hole electrical transporting properties in organic-Si Schottky solar cell

In this work we investigated the hole electrical transporting properties effect on the organic-Si hybrid Schottky solar cells. By changing the post-annealing atmosphere of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) film, the power conversion efficiencies of the Schottky Si/PEDOT:PSS cell boosted from 6.40% in air to 9.33% in nitrogen. Current-voltage, capacitance-voltage, external quantum efficiency, and transient photovoltage measurements illustrated that the enhanced power conversion efficiency of the cell was ascribed to the increase in both conductivity and work function (WP) of PEDOT:PSS film. The increased conductivity reduced the series resistance (RS) within the cell, and the higher WP generated the larger built-in potential (Vbi) which resulted in the improvement of the open-circuit voltage. In addition, the decreased RS and enlarged Vbi were beneficial for the efficient charge transport/collection, contributing to the enhancement of the fill factor. Our results indicated...

Robust and aligned carbon nanotube/titania core/shell films for flexible TCO-free photoelectrodes.

Carbon nanotube (CNT)/semiconducting oxide hybrids are an ideal architecture for light-harvesting devices, in which the CNTs are expected to not only act as a scaffold but also provide fast transport paths for photogenerated charges in the oxide. However, the current potential of CNTs for charge transport is largely suppressed due to the nanotubes not being interconnected but isolated by the low conductive oxide coatings. Herein, a flexible and conductive CNT/TiO(2) core/shell heterostructure film is reported, with aligned and interconnected CNTs wrapped in a continuous TiO(2) coating. Without using additional transparent conducting oxide (TCO) substrates, this unique feature of the film boosts the incident photon-to-electron conversion efficiency to 32%, outperforming TiO(2) nanoparticle electrodes fabricated on TCO substrates. Moreover, the film shows high structural stability and can generate a stable photocurrent even after being bent hundreds of times.

Improved Work Function of Poly(3,4-ethylenedioxythiophene): Poly(styrenesulfonic acid) and its Effect on Hybrid Silicon/Organic Heterojunction Solar Cells

Hybrid silicon/organic solar cells have been recently extensively investigated due to their simple structure and low-cost fabrication process. However, the efficiency of the solar cells is greatly limited by the barrier height as well as the carrier recombination at the silicon/organic interface. In this work, hydrochloroplatinic acid (H2PtCl6) is employed into the poly(3,4-ethlenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) solution, and the work function (WF) of the PEDOT:PSS layer has been successfully improved. Based on the Pt-modified PEDOT:PSS layer, the efficiency of the silicon/PEDOT:PSS cell can be increased to 11.46%, corresponding to ~20% enhancement to the one without platinum (Pt) modification. Theoretical and experimental results show that, when increasing the WF of the PEDO:PSS layer, the barrier height between the silicon/PEDOT:PSS interface can be effectively enhanced. Meanwhile, the carrier recombination at the interface is significantly reduced. These results can contribute to better understanding of the interfacial mechanism of silicon/PEDOT:PSS interface, and further improving the device performance of silicon/organic solar cells.

An enzyme-like imprinted-polymer reactor with segregated quantum confinements for a tandem catalyst

This study was aimed at addressing the present challenge in tandem catalysts, as to how to furnish catalysts with tandem catalytic-ability without involving the precise control and man-made isolation of different types of catalytic sites. This objective was realized by constructing an enzyme-like imprinted-polymer reactor made of a unique polymer composite inspired from the compartmentalization of cells, a composite of a reactive imprinted polymer (containing acidic catalytic sites), and encapsulated metal nanoparticles (acting as catalytic reduction sites). The compilation of two types of catalytic sites with admissible access allowed this reactor to behave like compartments of cells for enzymatic reactions and hence catalytically constituted two quantum interaction-segregated domains, which led to the occurrence of catalytic tandem processes. Unlike the reported functional reactors that run tandem catalysis by largely depending on the precise control and man-made isolation of different types of catalytic sites, tandem catalysis in this reactor run naturally with segregated quantum confinements, which does not involve the precise control and isolation of different types of catalytic sites. This protocol presents new opportunities for the development of functional catalysts for complicated chemical processes.

High efficiency conjugated polymer/Si hybrid solar cells with tetramethylammonium hydroxide treatment

Conjugated polymer/Si hybrid solar cells are fabricated based on a Si nanowire array (SiNW) substrate prepared by metal-assisted electroless etching. With a facile method involving immersing the SiNW substrate in anisotropic tetramethylammonium hydroxide (TMAH) solution just for few minutes, the power conversation efficiency (PCE) of the hybrid solar cell can be 12.36%, which is 69.5% higher than that of pristine one. The efficiency improvement mechanism is discussed and analyzed in detail, and all the results demonstrate that with the TMAH treatment, the density of the SiNWs is reduced, and the electrochemical contact between the PEDOT:PSS and SiNWs as well as the rear contact is obviously improved, all of which significantly suppress the charge recombination in the hybrid solar cells. Our major investigation emphasizes the importance of the TMAH treatment to nanostructured Si and renders a promising approach to obtain low-cost and high performance conjugated polymer/Si hybrid solar cells.

Enhancement of light-emitting properties by simultaneously patterning and controlling molecular alignment in polyfluorene thin films

Soft nano-imprinting lithography is used to fabricate PFO nanogratings. On the one hand, soft nano-imprinting lithography has been well explored to create nanostructures and induce molecular orientation in conjugated polymer thin films because of its limited cost, high throughput, and small feature size. On the other hand, the pure α-conformation PFO nanogratings with highly ordered molecular chains can be achieved with graphoepitaxial alignment without any molecular chain degradation. Hence, herein, we demonstrated that large-area PFO (poly(9,9-dioctylfluorene)) nanogratings could be fabricated by this effective way with high accuracy of patterning and chain alignment controllability. The molecular chain alignment degree can be tuned due to entropic and geometrical reasons; this has been verified by polarized absorption and GIWAXD (grazing incidence wide angle X-ray diffraction). Moreover, both the optical and electrical pumping properties have been characterized. In addition, not only optimized polarizability was observed at the most oriented situation, but also 20% more light was extracted from this PLED device.