Strategic Design of Three-Dimensional (3D) Urchin-Like Pt–Ni Nanoalloys: How This Unique Nanostructure Boosts the Bulk Heterojunction Polymer Solar Cells Efficiency to 8.48%

In this study, a simple and systematic shape-controlled synthetic protocol for tailoring nanoscale structures to generate large and monodispersed of three-dimensional (3D) urchin-like Pt–Ni multipods (MPs) and spherical nanoparticles (NPs) is reported, for which the mechanism of production is elaborated in detail. We then demonstrate, for the first time, that the 3D urchin-like Pt–Ni MPs possess good solution processability and substantially enhance both short-circuit current density (<i>J</i>_sc) and fill factor (FF) and consequently increase the overall power conversion efficiencies (PCEs), because of the combination of multiple scattering processes of incident light, improved conductivity, and facilitating the charge transport in the active layer. PSC fabricated using 5% Pt–Ni MPs embedded in a blend of poly­{[4,8-bis­(2-ethyl-hexyl-thiophene-5-yl)-benzo­[1,2-b:4,5-b′]­dithiophene-2,6-diyl]-<i>alt</i>-[2-(2′-ethyl-hexanoyl)-thieno­[3,4-<i>b</i>]­thiophen-4,6-diyl]} (PBDTTT-C-T) and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) leads to compelling device PCEs of 8.48%, in comparison to 7.38% of the reference device (PBDTTT-C-T:PC₇₁BM, fabricated and tested under the same conditions). This study thus demonstrates a novel approach to enhance the photovoltaic performance, in combination with 3D urchin-like nanoalloys

Graphene Anodes and Cathodes: Tuning the Work Function of Graphene by Nearly 2 eV with an Aqueous Intercalation Process

To expand the applications of graphene in optoelectronics and microelectronics, simple and effective doping processes need to be developed. In this paper, we demonstrate an aqueous process that can simultaneously transfer chemical vapor deposition grown graphene from Cu to other substrates and produce stacked graphene/dopant intercalation films with tunable work functions, which differs significantly from conventional doping methods using vacuum evaporation or spin-coating processes. The work function of graphene layers can be tuned from 3.25 to 5.10 eV, which practically covers the wide range of the anode and cathode applications. Doped graphene films in intercalation structures also exhibit excellent transparency and low resistance. The polymer-based solar cells with either low work function graphene as cathodes or high work function graphene as anodes are demonstrated

Highly Sensitive Graphene–Semiconducting Polymer Hybrid Photodetectors with Millisecond Response Time

Graphene–semiconducting light absorber hybrid photodetectors have attracted increasing attention because of their ultrahigh photoconductive gain and superior sensitivity. However, most graphene-based hybrid photodetectors reported previously have shown a relatively long response time (on the order of seconds) caused by numerous long-lived traps in these hybrid systems, which greatly restricts device speed. In this work, graphene–thieno­[3,4-<i>b</i>]­thiophene/benzodithiophene polymer hybrid photodetectors fabricated on self-assembled-monolayer (SAM)-functionalized SiO₂ substrates are demonstrated with a maximum responsivity of ∼1.8 × 10⁵ A W^–1 and a relatively short photocurrent response time of ∼7.8 ms. The fast and highly sensitive device characteristics provide great potential in low-light imaging applications. The hybrid photodetector on the SAM-coated SiO₂ substrate shows better performance in responsivities and response times as compared with those of the device on the bare SiO₂ substrate. The improved responsivities are attributed to a significant increase in carrier mobility in graphene channels by introducing SAM-modified substrates. In addition, SAM functionalization is capable of effectively removing multiple surface traps and charged impurities between graphene sheets and SiO₂ substrates, which prevents the long-lived trapping of photocarriers at graphene/SiO₂ interfaces and remarkably decreases device response time

High-Efficiency Small-Molecule-Based Organic Light Emitting Devices with Solution Processes and Oxadiazole-Based Electron Transport Materials

We demonstrate high-efficiency small-molecule-based white phosphorescent organic light emitting diodes (PHOLEDs) by single-active-layer solution-based processes with the current efficiency of 17.3 cdA^‑1 and maximum luminous efficiency of 8.86 lmW^‑1 at a current density of 1 mA cm^‑2. The small-molecule based emitting layers are codoped with blue and orange phosphorescent dyes. We show that the presence of CsF/Al at cathodes not only improves electron transport in oxadiazole-containing electron transport layers (ETLs), but also facilitates electron injection through the reacted oxadiazole moiety to reduce interface resistance, which results in the enhancement of current efficiency. By selecting oxadiazole-based materials as ETLs with proper electron injection layer (EIL)/cathode structures, the brightness and efficiency of white PHOLEDs are significantly improved

Revealing Ordered Polymer Packing during Freeze-Drying Fabrication of a Bulk Heterojunction Poly(3-hexylthiophene-2,5-diyl):[6,6]-Phenyl-C61-butyric Acid Methyl Ester Layer: In Situ Optical Spectroscopy, Molecular Dynamics Simulation, and X‑ray Diffraction

Formation of ordered poly­(3-hexylthiophene-2,5-diyl) (P3HT) molecular stacking during the freeze-drying process is tracked with in situ spectroscopy of Raman scattering, absorption, and photoluminescence. Raman spectra of pristine P3HT dissolved in 1,2-dichlorobenzene show that P3HT polymers undergo drastic ordered aggregation upon being lower than 0 °C, at which the solubility of P3HT is reached, as evidenced by the emergence of pronounced red-shifted, narrow Raman peaks (1422 and 1435 cm^–1) caused by intermolecular coupling. The absorption and photoluminescence spectra bear similar temperature dependence as the results of Raman. Aggregation of P3HT is further confirmed by coarse-grained molecular dynamics simulation showing the enhanced order parameters of distance and orientation between P3HT chains upon cooling. The incorporation of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) does not significantly alter the P3HT packing configuration, as verified by nearly identical Raman features observed in P3HT:PCBM mixing solution upon cooling. While optical spectroscopy and MD simulation portrayed the short-range order of P3HT aggregates, grazing-incident X-ray diffraction exposed the long-range order by the pronounced diffraction spots corresponding to the lamellar stacking of P3HT. This study demonstrates the ability of Raman spectroscopy to reveal the short-range order of polymer packing, while the in situ monitoring illustrates that the ability of freeze-drying to separate molecular aggregation from solvent removal thus is advantageous for photovoltaic device fabrication without resorting to trial and error

Monolayer MoS₂ Heterojunction Solar Cells

We realized photovoltaic operation in large-scale MoS₂ monolayers by the formation of a type-II heterojunction with p-Si. The MoS₂ monolayer introduces a built-in electric field near the interface between MoS₂ and p-Si to help photogenerated carrier separation. Such a heterojunction photovoltaic device achieves a power conversion efficiency of 5.23%, which is the highest efficiency among all monolayer transition-metal dichalcogenide-based solar cells. The demonstrated results of monolayer MoS₂/Si-based solar cells hold the promise for integration of 2D materials with commercially available Si-based electronics in highly efficient devices

Shape-Dependent Light Harvesting of 3D Gold Nanocrystals on Bulk Heterojunction Solar Cells: Plasmonic or Optical Scattering Effect?

In the work, mechanisms behind various 3D nanocrystals enhanced performance of bulk heterojunction solar cells were studied comprehensively. Four types of gold nanoparticles (NPs) with distinctly different shapes and great uniformity were designed and synthesized, including cubes, rhombic dodecahedra (RD), edge- and corner-truncated octahedra (ECTO), and triangular plates, to systematically probe their influences on photovoltaics. RD and triangular plates show a higher growth rate, while slower growth favors cubes and ECTO formation by controlling the reduction agent and capping ion amount. NPs with increasing corners and proper size of cross-section induce stronger near-field coupling and far-field scattering in P3HT:PC₆₁BM-based active layers. Both finite-difference time-domain simulation and UV–visible absorption spectra firmly support that RD exhibit the strongest localized surface plasmon resonance and optical scattering. With optimized conditions, a high power conversion efficiency exceeding 4% was reproducibly achieved

Highly Efficient Visible Light Photocatalytic Reduction of CO₂ to Hydrocarbon Fuels by Cu-Nanoparticle Decorated Graphene Oxide

The production of renewable solar fuel through CO₂ photoreduction, namely artificial photosynthesis, has gained tremendous attention in recent times due to the limited availability of fossil-fuel resources and global climate change caused by rising anthropogenic CO₂ in the atmosphere. In this study, graphene oxide (GO) decorated with copper nanoparticles (Cu-NPs), hereafter referred to as Cu/GO, has been used to enhance photocatalytic CO₂ reduction under visible-light. A rapid one-pot microwave process was used to prepare the Cu/GO hybrids with various Cu contents. The attributes of metallic copper nanoparticles (∼4–5 nm in size) in the GO hybrid are shown to significantly enhance the photocatalytic activity of GO, primarily through the suppression of electron–hole pair recombination, further reduction of GO’s bandgap, and modification of its work function. X-ray photoemission spectroscopy studies indicate a charge transfer from GO to Cu. A strong interaction is observed between the metal content of the Cu/GO hybrids and the rates of formation and selectivity of the products. A factor of greater than 60 times enhancement in CO₂ to fuel catalytic efficiency has been demonstrated using Cu/GO-2 (10 wt % Cu) compared with that using pristine GO

13% Efficiency Hybrid Organic/Silicon-Nanowire Heterojunction Solar Cell <i>via</i> Interface Engineering

Interface carrier recombination currently hinders the performance of hybrid organic–silicon heterojunction solar cells for high-efficiency low-cost photovoltaics. Here, we introduce an intermediate 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) layer into hybrid heterojunction solar cells based on silicon nanowires (SiNWs) and conjugate polymer poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS). The highest power conversion efficiency reaches a record 13.01%, which is largely ascribed to the modified organic surface morphology and suppressed saturation current that boost the open-circuit voltage and fill factor. We show that the insertion of TAPC increases the minority carrier lifetime because of an energy offset at the heterojunction interface. Furthermore, X-ray photoemission spectroscopy reveals that TAPC can effectively block the strong oxidation reaction occurring between PEDOT:PSS and silicon, which improves the device characteristics and assurances for reliability. These learnings point toward future directions for versatile interface engineering techniques for the attainment of highly efficient hybrid photovoltaics

The Effects of Fluorine-Contained Molecules on Improving the Polymer Solar Cell by Curing the Anomalous S‑Shaped <i>I</i>–<i>V</i> Curve

In this study, we investigate the effects of fluorinated poly­(3,4-ethylene dioxythiophene):poly­(styrenesulfonate) buffer layer on the performance of polymer photovoltaic cells. We demonstrate for the first time, the deterioration of the device performance can be effectively mended by modifying the interface between the active layer and buffer layer with heptadecafluoro-1,1,2,2-tetra-hydro-decyl trimethoxysilane (PFDS) and perfluorononane. Device performance shows a substantial enhancement of short-circuit current from 7.90 to 9.39 mA/cm² and fill factor from 27% to 53%. The overall device efficiency was improved from 0.98% to 3.12% for PFDS modified device. The mechanism of S-shape curing is also discussed. In addition, the stability of modified devices shows significant improvement than those without modification. The efficiency of the modified devices retains about half (1.88%) of its initial efficiency (4.1%) after 30 d compared to the unmodified ones (0.61%), under air atmosphere