International audienceSoil water erosion and shallow landslides depend on aggregate stability and soil shear strength. We investigated the effect of vegetation on both soil aggregate stability and shear strength (through direct shear tests) in former croplands converted to vegetated erosion protection areas within the context of China's sloping land conversion programme. Four treatments were analysed in plots comprised of (i) 4 year old crop trees, Vernicia fordii, where understory vegetation was removed; (ii) V. fordii and the dominant understory species Artemisia codonocephala; (iii) only A. codonocephala and (iv) no vegetation. Soil samples were taken at depths of 0-5 cm and 45-50 cm. Root length density (RLD) in five diameter classes was measured, soil organic carbon (SOC), hot water extractable carbon (HWEC), texture and Fe and Al oxides were also measured. We found that mean weight diameter after slow wetting (MWDSW) in the A-horizon, was significantly greater (0.94-1.01 mm) when A. codonocephala was present compared to plots without A. codonocephala (0.57-0.59 mm). SOC and RLD in the smallest diameter class (< 0.5 mm), were the variables which best explained variability in MWDSW. A significant positive linear relationship existed between MWDSW and soil cohesion but not with internal angle of friction. As herbaceous vegetation was more efficient than trees in improving aggregate stability, this result suggests that the mechanisms involved include modifications of the cohesive forces between soil particles adjacent to plant roots and located in the enriched in SOC rhizosphere, thus also affecting shear strength of the corresponding soil volume. Thus, vegetation stabilised soil under different hierarchical levels of aggregate organisation, i.e. intra- and inter-aggregate. Our results have implications for the efficacy of techniques used in land conversion programs dedicated to control of soil erosion and shallow landslides. We suggest that mixtures of different plant functional types would improve soil conservation on slopes, by reducing both surface water erosion and shallow substrate mass movement. Planting trees for cropping or logging, and removing understory vegetation is most likely detrimental to soil conservation

Ion migration has been regarded as the major cause of photocurrent hysteresis. Here we use photoluminescence (PL) and optical images, combined with Galvanostatic measurement, to detect the ionic motion. We observe an irreversible PL and optical transmittance change after electric poling. By comparing a neat perovskite film with the sample coated by poly(methyl methacrylate) (PMMA), polyethylene glycol (PEG), and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), we found that PCBM effectively inhibits ionic motion near the surface of the perovskite.We further evidenced the donor−acceptor complex formed between PCBM and perovskite, implying the mechanism of inhibited ion migration by PCBM. We close by demonstrating that PCBM can also be introduced on the top of perovskite fim in an n−i−p TiO 2 planar structure, to achieve an average 14% steady-state output over 2.3 × 10 5 s (∼64 h). This work highlights the importance of inhibiting ionic motion in perovskite solar cells.

We design a series of metal-free donor-π-bridge molecules (denoted VB0–VB4) based on a new donor groupullazine donoras sensitizers for dye sensitized solar cell (DSSC) applications. Density functional theory (DFT) and time-dependent DFT calculations reveal that the physical properties of dyes, including spectral response, light harvesting efficiency, and electron injection rate, are systematically improved by combining ullazine donor to a series of length changing π bridges. Dye VB2 is the best candidate thanks to its outstanding performance on key parameters and achieving a balance between competing factors. Compared to two other series of moleculesL and M dyes, which differ from VB dyes by only the donor groupVB dyes have the largest light harvesting efficiency and the largest number of electrons injected to the conduction band of TiO2. These results suggest that the ullazine group can serve as an excellent donor for future DSSC applications.

Light‐induced interlayer ultrafast charge transfer in 2D heterostructures provides a new platform for optoelectronic and photovoltaic applications. The charge separation process is generally hypothesized to be dependent on the interlayer stackings and interactions, however, the quantitative characteristic and detailed mechanism remain elusive. Here, a systematical study on the interlayer charge transfer in model MoS2/WS2 bilayer system with variable stacking configurations by time‐dependent density functional theory methods is demonstrated. The results show that the slight change of interlayer geometry can significantly modulate the charge transfer time from 100 fs to 1 ps scale. Detailed analysis further reveals that the transfer rate in MoS2/WS2 bilayers is governed by the electronic coupling between specific interlayer states, rather than the interlayer distances, and follows a universal dependence on the state‐coupling strength. The results establish the interlayer stacking as an effective freedom to control ultrafast charge transfer dynamics in 2D heterostructures and facilitate their future applications in optoelectronics and light harvesting.

In this work we target on accurately predicting energy conversion efficiency of dye-sensitized solar cells (DSC) using parameter-free first principles simulations. We present a set of algorithms, mostly based on solo first principles calculations within the framework of density functional theory, to accurately calculate key properties in energy conversion including sunlight absorption, electron injection, electron−hole recombination, open circuit voltages, and so on. We choose two series of donor-πacceptor dyes with detailed experimental photovoltaic data as prototype examples to show how these algorithms work. Key parameters experimentally measured for DSC devices can be nicely reproduced by first-principles with as less empirical inputs as possible. For instance, short circuit current of model dyes can be well reproduced by precisely calculating their absorption spectra and charge separation/recombination rates. Open circuit voltages are evaluated through interface band offsets, namely, the difference between the Fermi level of electrons in TiO 2 and the redox potential of the electrolyte, after modification with empirical formulas. In these procedures the critical photoelectron injection and recombination dynamics are calculated by realtime excited state electronic dynamics simulations. Estimated solar cell efficiency reproduces corresponding experimental values, with errors usually below 1−2%. Device characteristics such as light harvesting efficiency, incident photon-to-electron conversion efficiency, and the current−voltage characteristics can also be well reproduced and compared with experiment. Thus, we develop a systematic ab initio approach to predict solar cell efficiency and photovoltaic performance of DSC, which enables large-scale efficient dye screening and optimization through high-throughput first principles calculations with only a few parameters taken from experimental settings for electrode and electrolyte toward a renewable energy based society.

Organometal halide perovskite solar cells (PSCs) have emerged as one of the most promising photovoltaic technologies with efficiencies exceeding 20.3%. However, device stability problems including hysteresis in current−voltage scans must be resolved before the commercialization of PSCs. Transient absorption measurements and first-principles calculations indicate that the migration of oxygen vacancies in the TiO 2 electrode under electric field during voltage scans contributes to the anomalous hysteresis in PSCs. The accumulation of oxygen vacancies at the electrode/perovskite interface slows down charge extraction while significantly speeding up charge recombination at the interface. Moreover, nonadiabatic molecular dynamics simulations reveal that the charge recombination rates at the interface depend sensitively (with 1 order of magnitude difference) on the locations of oxygen vacancies. By intentionally reducing oxygen vacancies in the TiO 2 electrode, we substantially suppress unfavorable hysteresis in the PSC devices. This work establishes a firm link between microscopic interfacial structure and macroscopic device performance of PSCs, providing important clues for future device design and optimization.

Organic field-effect transistors (OFETs) featuring a photoactive hybrid bilayer dielectric (PHBD) that comprises a self-assembled monolayer (SAM) of photochromic diarylethenes (DAEs) and an ultrathin solution-processed hafnium oxide layer are described here. We photoengineer the energy levels of DAE SAMs to facilitate the charging and discharging of the interface of the two dielectrics, thus yielding an OFET that functions as a nonvolatile memory device. The transistors use light signals for programming and electrical signals for erasing (≤3 V) to produce a large, reversible threshold-voltage shift with long retention times and good nondestructive signal processing ability. The memory effect can be exercised by more than 10(4) memory cycles. Furthermore, these memory cells have demonstrated the capacity to be arrayed into a photosensor matrix on flexible plastic substrates to detect the spatial distribution of a confined light and then store the analog sensor input as a two-dimensional image with high precision over a long period of time.

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