Additional file 1 of MiRKAT-S: a community-level test of association between the microbiota and survival times

PDF file includes supplemental tables (Tables S1–S3) and figures (Figures S1–S2). (PDF 933 kb

Composition-Dependent Light-Induced Dipole Moment Change in Organometal Halide Perovskites

In this work we investigate the compositional dependence of electric dipole moment in AMX₃ (A: organic; M: metal; X: halogen) perovskite structures using modulation electroabsorption (EA) spectroscopy. By sampling various device structures, we show that the second harmonic EA spectra reflect the intrinsic dipolar property of perovskite films in a layered configuration. A quantitative analysis of the EA spectra of CH₃NH₃PbI₃, NH₂CHNH₂PbI₃, and CH₃NH₃Sn_0.4Pb_0.6I₃ is provided to compare the impact of the organic and metal cations on the photoinduced response of dipole moment. Based on the EA results, we propose that the A and M cations could both largely affect the dielectric and dipolar properties of the perovskite materials, but through different mechanisms, such as ionic polarization, rotation of molecular dipoles and charge migration. These processes occur at different time scales and thus result in a frequency-dependent dipole response

Carrier-Activated Polarization in Organometal Halide Perovskites

Organometal halide perovskite solar cells exhibit a strong polarization effect under light illumination. This unique property, although widely observed, has not been well understood. In this work, we carried out a systematic investigation on this phenomenon by varying the perovskite composition and device configurations. We find that the light-enhanced strong polarization is a general phenomenon that occurs in all tested perovskite materials. The organic molecular dipoles affect the polarization at high frequency range, while the photoexcited free carriers and the interface between perovskite and its neighboring layers dominate the low-frequency response. In particular, our study suggests that the giant low-frequency capacitance enhancement originates from native defects and their accompanying defect dipoles, which need to be activated by photogenerated charge carriers. The high flexibility of the perovskite lattice facilitates the formation of these defects, and the dipole effect is enhanced at the interface regions where an electric double-layer capacitor is formed. This work sheds light on the understanding of the light-induced polarization mechanisms of perovskite materials

In Situ Probing of the Charge Transport Process at the Polymer/Fullerene Heterojunction Interface

The polymer/fullerene interface (PFI) in polymer solar cells (PSCs) provides an energetic offset for exciton dissociation while at the same time influencing local transport of photocarriers adjacent to the interface. In this paper, we introduce a heterojunction field-effect transistor (FET) structure in charge modulation spectroscopy (CMS) to enable in situ probing of the charge transport process at PFIs. The PFIs formed by fullerene/crystalline polymer and fullerene/amorphous polymer systems are studied and compared, respectively. By correlating the steady-state and frequency-dependent CMS responses of pure polymer, polymer/fullerene bilayer, and polymer/fullerene blend FETs, we demonstrate that through different charge localization effects the interface fullerene molecules can influence the hole transport in both crystalline and amorphous polymer phases. We propose a trade-off between charge transfer and charge transport at PFIs with an aim to enhance the engineering of molecular orientation and packing at the donor–acceptor interface for high-performance PSCs

Photocurrent Enhancement of HgTe Quantum Dot Photodiodes by Plasmonic Gold Nanorod Structures

The near-field effects of noble metal nanoparticles can be utilized to enhance the performance of inorganic/organic photosensing devices, such as solar cells and photodetectors. In this work, we developed a well-controlled fabrication strategy to incorporate Au nanostructures into HgTe quantum dot (QD)/ZnO heterojunction photodiode photodetectors. Through an electrostatic immobilization and dry transfer protocol, a layer of Au nanorods with uniform distribution and controllable density is embedded at different depths in the ZnO layer for systematic comparison. More than 80 and 240% increments of average short-circuit current density (<i>J</i>_sc) are observed in the devices with Au nanorods covered by ∼7.5 and ∼4.5 nm ZnO layers, respectively. A periodic finite-difference time-domain (FDTD) simulation model is developed to analyze the depth-dependent property and confirm the mechanism of plasmon-enhanced light absorption in the QD layer. The wavelength-dependent external quantum efficiency spectra suggest that the exciton dissociation and charge extraction efficiencies are also enhanced by the Au nanorods, likely due to local electric field effects. The photodetection performance of the photodiodes is characterized, and the results show that the plasmonic structure improves the overall infrared detectivity of the HgTe QD photodetectors without affecting their temporal response. Our fabrication strategy and theoretical and experimental findings provide useful insight into the applications of metal nanostructures to enhance the performance of organic/inorganic hybrid optoelectronic devices

Spectroscopic Study of Charge Transport at Organic Solid–Water Interface

Charge transport in an organic solid and its coupling with the neighboring aqueous biological environment dictates the performance of many organic bioelectronic devices. Understanding how the transport property at the solid–water interface is influenced by the surface structure characteristics of the organic solid is essential for rational design of organic bioelectronics and chemical sensors. However, <i>in situ</i> probing such structure–property relationships has been difficult due to lack of experimental techniques with sufficient sensitivity to the water-buried interface. Here, we demonstrate a charge accumulation spectroscopy (CAS)-based protocol, exploiting water-gated organic field-effect transistor as the testing platform, to investigate the structure-dependent localization of polaronic charge carriers at the organic semiconductor–liquid interface. Our results reveal that the degree of charge delocalization is reduced drastically when the charge carriers are moved from the bulk semiconductor to the semiconductor–water interface, suggesting the existence of a highly disordered surface layer in contact with water. It is also found that the charge delocalization could be further reduced by intercalation of chloride ions (from salt water) in the semiconductor surface layer. This study suggests that the spectroscopic signatures of polaronic charge carriers could be a sensitive probe to detect the structure-dependent charge localization at organic solid–liquid interfaces

Flexible Piezoelectric-Induced Pressure Sensors for Static Measurements Based on Nanowires/Graphene Heterostructures

The piezoelectric effect is widely applied in pressure sensors for the detection of dynamic signals. However, these piezoelectric-induced pressure sensors have challenges in measuring static signals that are based on the transient flow of electrons in an external load as driven by the piezopotential arisen from dynamic stress. Here, we present a pressure sensor with nanowires/graphene heterostructures for static measurements based on the synergistic mechanisms between strain-induced polarization charges in piezoelectric nanowires and the caused change of carrier scattering in graphene. Compared to the conventional piezoelectric nanowire or graphene pressure sensors, this sensor is capable of measuring static pressures with a sensitivity of up to 9.4 × 10^–3 kPa^–1 and a fast response time down to 5–7 ms. This demonstration of pressure sensors shows great potential in the applications of electronic skin and wearable devices

Energy Level Modification in Lead Sulfide Quantum Dot Thin Films through Ligand Exchange

The electronic properties of colloidal quantum dots (QDs) are critically dependent on both QD size and surface chemistry. Modification of quantum confinement provides control of the QD bandgap, while ligand-induced surface dipoles present a hitherto underutilized means of control over the absolute energy levels of QDs within electronic devices. Here, we show that the energy levels of lead sulfide QDs, measured by ultraviolet photoelectron spectroscopy, shift by up to 0.9 eV between different chemical ligand treatments. The directions of these energy shifts match the results of atomistic density functional theory simulations and scale with the ligand dipole moment. Trends in the performance of photovoltaic devices employing ligand-modified QD films are consistent with the measured energy level shifts. These results identify surface-chemistry-mediated energy level shifts as a means of predictably controlling the electronic properties of colloidal QD films and as a versatile adjustable parameter in the performance optimization of QD optoelectronic devices

Solution-Processed Ambipolar Organic Thin-Film Transistors by Blending p- and n‑Type Semiconductors: Solid Solution versus Microphase Separation

Here, we report solid solution of p- and n-type organic semiconductors as a new type of p–n blend for solution-processed ambipolar organic thin film transistors (OTFTs). This study compares the solid-solution films of silylethynylated tetraazapentacene <b>1</b> (acceptor) and silylethynylated pentacene <b>2</b> (donor) with the microphase-separated films of <b>1</b> and <b>3</b>, a heptagon-embedded analogue of <b>2</b>. It is found that the solid solutions of (<b>1</b>)_<i>x</i>(<b>2</b>)_1–<i>x</i> function as ambipolar semiconductors, whose hole and electron mobilities are tunable by varying the ratio of <b>1</b> and <b>2</b> in the solid solution. The OTFTs of (<b>1</b>)_0.5(<b>2</b>)_0.5 exhibit relatively balanced hole and electron mobilities comparable to the highest values as reported for ambipolar OTFTs of stoichiometric donor–acceptor cocrystals and microphase-separated p-n bulk heterojunctions. The solid solution of (<b>1</b>)_0.5(<b>2</b>)_0.5 and the microphase-separated blend of <b>1:3</b> (0.5:0.5) in OTFTs exhibit different responses to light in terms of absorption and photoeffect of OTFTs because the donor and acceptor are mixed at molecular level with π–π stacking in the solid solution

Ternary Bulk Heterojunction Photovoltaic Cells Composed of Small Molecule Donor Additive as Cascade Material

To explore the potential of ternary blend bulk heterojunction (BHJ) solar cells as a general platform for improving the performance of organic photovoltaics, we studied a ternary BHJ system based on poly­(3-hexylthiophene) (P3HT), [6,6]-phenyl C61 butyric acid methyl ester (PC₆₁BM), and DTDCTB. The optimized ternary structure containing a weight ratio of 20% DTDCTB as the cascade material demonstrates a ∼25% improvement of the power conversion efficiency (PCE) as compared to the binary P3HT/PC₆₁BM solar cells. A systematic spectroscopic study is carried out to elucidate the underlying mechanism of charge transfer in the ternary system. Wavelength-dependent external quantum efficiency measurement confirms the contribution of DTDCTB to the enhanced photocurrent. Photoinduced absorption spectroscopy and transient photovoltage measurement reveal unambiguously that charges generated in DTDCTB are efficiently transferred to and subsequently transported in P3HT and PC₆₁BM. The results also suggest that despite the realization of cascade charge transfer, the bimolecular charge recombination process in the ternary system is still dominated by the P3HT/PC₆₁BM interface