Charge Transfer in Nanowire-Embedded PEDOT:PSS and Planar Heterojunction Solar Cells.

Hybrid metallic nanowire-embedded, highly conductive poly(3,4-ethylenedioxy thiophene):polystyrenesulfonate (PEDOT:PSS) with synergetic properties is indispensable for enhancing the performances of conductive polymer-based electronic devices. Here, we report embedment of silver nanowires (AgNWs), with diameter ∼100 nm and a high concentration (500 mg/mL) of nanowires dispersed in either ethanol or isopropanol, in PEDOT:PSS and compare the effects of the nanowire-dispersing solvents as well as its thicker diameter and high concentration on the overall properties and particularly its charge transfer characteristics and planar heterojunction solar cell (HSC) properties. Furthermore, electrostatic force microscopy is applied to elucidate the direct charge transfer from AgNWs to the PEDOT:PSS matrix. The AgNW-embedded PEDOT:PSS-based planar HSCs show a very high open-circuit voltage of over 638 mV and a high power conversion efficiency greater than 15.3% and without any significant influence from the AgNW dispersing solvents. While charge transfer in PEDOT:PSS without AgNWs occurs through the conducting PEDOT grains, enhanced charge transfer is realized in AgNW-embedded PEDOT:PSS with charge transport from PEDOT grains to AgNWs and then to PEDOT grains before reaching the top electrode in the HSC. The AgNW-embedded PEDOT:PSS hybrid materials pave a simple way to enhance the charge transfer performance in not only HSCs but also other hybrid or heterojunction electronics.

Highly Conducting Hybrid Silver-Nanowire-Embedded Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate) for High-Efficiency Planar Silicon/Organic Heterojunction Solar Cells.

Embedding nanowires, such as silver nanowires (AgNWs), in a transparent conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) to enhance its conductivity is technologically important for improving the performances of devices comprising transparent conductive layers. Addition of nanowires in the highly conducting form of cosolvent (ethylene glycol) or mixed-cosolvent (ethylene glycol and methanol) modified PEDOT:PSS could change the nanowire structure and significantly alter the conductivity. Here, we report a simple method to embed AgNWs in PEDOT:PSS efficiently to improve its conductivity. By incorporating nanowires in the mixed cosolvent matrix prior to addition into PEDOT:PSS, this method preserves the structure of the nanowires while enabling conductivity enhancement. In contrast, the addition of AgNWs into cosolvent-premodified PEDOT:PSS leads to breaking of nanowires and conductivity impediment. The hybrid films with efficiently embedded AgNWs and mixed-cosolvent-modified PEDOT:PSS show a sheet resistance of 104 Ω/□, which is among the lowest ever reported for the as-deposited films, with conductivity enhancement of 33% relative to that of mixed-cosolvent-modified PEDOT:PSS. The resulting planar heterojunction solar cell (HSC) based on AgNW-embedded PEDOT:PSS exhibits a power conversion efficiency of greater than 15%. This demonstrates the importance of reducing sheet resistance by integrating nanowires into the PEDOT:PSS matrix as effective charge-transfer conduits interconnecting the highly conducting quinoid chains. The present approach to efficiently embed AgNWs in PEDOT:PSS could be readily extended to other nanowires or nanoparticles for improving the performance of PEDOT:PSS for applications in not just HSCs but indeed other electronic devices that require both transparent and highly conductive layers.

Efficient photoelectrochemical water splitting on ultrasmall defect-rich TaOx nanoclusters enhanced by size-selected Pt nanocluster promoters.

Formation of nanoclusters has attracted a lot of attention in recent years because of their distinct properties from isolated atoms and bulk solids. Here, we focus on the catalytic properties of supported transition metal oxide nanoclusters, such as TaO2, with a well-defined size distribution below 10 nm. We show that their catalytic performance can be greatly enhanced by introducing a reaction promoter such as Pt. Different combinations of precisely size-selected, defect-rich TaOx and Pt nanoclusters are produced by a gas-phase aggregation technique in a special DC magnetron sputtering system. Argon flow rate and aggregation length are carefully optimized to control the sizes of these ultrasmall TaOx and Pt nanoclusters by using a quadrupole mass filter, and TEM studies reveal the different crystalline nature of TaOx (amorphous) and Pt (crystalline) nanoclusters. We have further demonstrated the size-dependent photoanode activity of (TaOx, Pt) nanocluster systems in a photoelectrochemical water splitting reaction, where the Pt nanocluster promoters are found to provide a significant enhancement in the photocurrent density, approximately tripled that was observed from just TaOx nanocluster catalysts alone. The photocurrent density and photoconversion efficiency tend to reduce when Pt nanoclusters become overpopulated due to blocking of the photosensitive TaOx surface. Reducing the Pt nanocluster size resolves this problem by incorporating a greater number of smaller nanocluster promoters without blocking TaOx, which leads to further enhancement in the photocurrent density. The enhanced photocatalytic activity is attributed to synergetic effects introduced by the Pt nanoclusters that act as temporary charge storage sites to facilitate effective separation of a large number of electron-hole pairs, generated from a large number of active sites on the defect-rich amorphous TaOx nanoclusters upon illumination.

Performance Enhancement by Secondary Doping in PEDOT:PSS/Planar-Si Hybrid Solar Cells.

Solar cells depend on effectively absorbing light and converting it into electrical current. It is therefore essential to increase conductivity and to limit both reflection and parasitic absorbance to achieve higher photoconversion efficiency. Here, we examine the effect of post-treatment on the absorbance and conductivity of hybrid solar cells comprised of p-type poly(3,4-ethylene-dioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) on an n-type silicon substrate. Three sets of cells based on pristine PEDOT:PSS film, cosolvent enhanced PEDOT:PSS film using ethylene glycol as a cosolvent, and post-treated PEDOT:PSS film using a novel 1:1 binary mixture of ethylene glycol and methanol have been studied. Markedly different film conductivities have been found for the pristine (∼0.8 S/cm), cosolvent added (637 S/cm), and post-treated films (1334 S/cm). The photoconversion efficiency obtained over a large set of samples (72 cells) was used to evaluate the cosolvent addition and post-treatment. Post-treatment is found to reproducibly provide films with not only the highest conductivities but also the highest efficiencies along with higher open-circuit voltage and fill factor but lower short-circuit current density when compared to those of the cosolvent added films. The decrease in the latter is attributed to the increase in absorbance in the PEDOT:PSS film. The present work illustrates the delicate challenge in improving the conductivity and carrier collection efficiency of the cells not at the expense of other properties such as absorption.

Plasmonic gold nanoparticles for ZnO-nanotube photoanodes in dye-sensitized solar cell application.

Surface modification of nanostructured metal oxides with metal nanoparticles has been extensively used to enhance their nanoscale properties. The unique properties of metal nanoparticles associated with their controllable dimensions allow these metal nanoparticles to be precisely engineered for many applications, particularly for renewable energy. Here, a simple electrodeposition method to synthesize gold nanoparticles (GNPs) on electrochemically grown ZnO nanotubes (NTs) is reported. The size distribution and areal density of the GNPs can be easily controlled by manipulating the concentration of AuCl3 electrolyte solution, and the deposition time, respectively. An excellent enhancement in the optical properties of ZnO NTs surface-decorated with GNPs (GNP/ZnO-NT), especially in the visible region, is attributed to their surface plasmon resonance. The plasmonic effects of GNPs, together with the large specific surface area of ZnO NTs, can be used to significantly enhance the dye-sensitized solar cell (DSSC) properties. Furthermore, the Schottky barrier at the Au/ZnO interface could prevent electron back transfer from the conduction band of ZnO to the redox electrolyte and thus could substantially increase electron injection in the ZnO conduction band, which would further improve the overall performance of the constructed DSSCs. The GNP/ZnO-NT photoanode has been found to increase the efficiency of the DSSC significantly to 6.0% from 4.7% of the pristine ZnO-NT photoanode, together with corresponding enhancements in short-circuit current density from 10.4 to 13.1 mA cm(-2) and in fill factor from 0.60 to 0.75, while the open-circuit voltage remain effectively unchanged (from 0.60 to 0.61 V). Surface decoration with GNPs therefore provides an effective approach to creating not only a high specific surface area for superior loading of dye molecules, but also higher absorbance capability due to their plasmonic effect, all of which lead to excellent performance enhancement for DSSC application.

Reversible structural transformation and enhanced performance of PEDOT:PSS-based hybrid solar cells driven by light intensity.

Hybrid solar cells made of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) ( PEDOT: PSS) and appropriate amounts of a cosolvent and a fluorosurfactant on planar n-type silicon substrates showed a photoconversion efficiency (PCE) of above 13%. These cells also exhibited stable, reproducible, and high external quantum efficiency (EQE) that was not sensitive to light-bias intensity (LBI). In contrast, solar cells made of pristine PEDOT: PSS showed low PCE and high EQE only under certain measurement conditions. The EQE was found to degrade with increasing LBI. Here we report that the LBI-sensitive variation of EQE of the low-PCE cells is related to a reversible structural transformation from a quinoid to a benzoid structure of PEDOT.

Size-selected TiO2 nanocluster catalysts for efficient photoelectrochemical water splitting.

Nanoclusters (NCs) are of great interest because they provide the link between the distinct behavior of atoms and nanoparticles and that of bulk materials. Here, we report precisely controlled deposition of size-selected TiO2 NCs produced by gas-phase aggregation in a special magnetron sputtering system. Carefully optimized aggregation length and Ar gas flow are used to control the size distribution, while a quadrupole mass filter provides precise in situ size selection (from 2 to 15 nm). Transmission electron microscopy studies reveal that NCs larger than a critical size (∼8 nm) have a crystalline core with an amorphous shell, while those smaller than the critical size are all amorphous. The TiO2 NCs so produced exhibit remarkable photoelectrochemical water splitting performance in spite of a small amount of material loading. NCs of three different sizes (4, 6, and 8 nm) deposited on H-terminated Si(100) substrates are tested for the photoelectrochemical catalytic performance, and significant enhancement in photocurrent density (0.8 mA/cm(2)) with decreasing NC size is observed with a low saturation voltage of -0.22 V vs Ag/AgCl (0.78 V vs RHE). The enhanced photoconductivity could be attributed to the increase in the specific surface area and increase in the number of active (defect) sites in the amorphous NCs. The unique advantages of the present technique will be further exploited to develop applications based on tunable, size-selected NCs.

Interfacial micropore defect formation in PEDOT:PSS-Si hybrid solar cells probed by TOF-SIMS 3D chemical imaging.

Conducting p-type polymer layers on n-type Si have been widely studied for the fabrication of cost-effective hybrid solar cells. In this work, time-of-flight secondary ion mass spectrometry (TOF-SIMS) is used to provide three-dimensional chemical imaging of the interface between poly(3,4-ethylene-dioxythiophene):polystyrenesulfonate (PEDOT:PSS) and SiOx/Si in a hybrid solar cell. To minimize structural damage to the polymer layer, an Ar cluster sputtering source is used for depth profiling. The present result shows the formation of micropore defects in the interface region of the PEDOT:PSS layer on the SiOx/Si substrate. This interfacial micropore defect formation becomes more prominent with increasing thickness of the native oxide layer, which is a key device parameter that greatly affects the hybrid solar cell performance. Three-dimensional chemical imaging coupled with Ar cluster ion sputtering has therefore been demonstrated as an emerging technique for probing the interface of this and other polymer-inorganic systems.