This article evaluates the efficiency of density functional theory calculations when used in conjunction with Scharber's model to predict the power conversion efficiency of organic solar cells. Thirty polymers were investigated, and their calculated electronic properties were assessed against their reported experimental values. The energy level calculations have a relatively small standard deviation of about 0.2 eV after a correction for a systematic overestimation. The optical band gap and the open-circuit voltage are obtained within an accuracy of 0.09 eV and 0.10 V, respectively. Also, the model provides an indication of the maximum value for the short-circuit current and an interesting guiding tool to identify promising suitable polymers to reach high power conversion efficiencies. After validating the present numerical approach against known devices, new polymers that could reach a power conversion efficiency ranging from 8 to 11% are presented.

Second-order Raman scattering has been extensively studied in carbon-based nanomaterials, for example, nanotube and graphene, because it activates normally forbidden Raman modes that are sensitive to crystal disorder, such as defects, dopants, strain, and so forth. The sp-hybridized carbon systems are, however, the exception among nanomaterials, where first-order Raman processes usually dominate. Here we report the identification of four second-order Raman modes, named D, D, D and D, in exfoliated black phosphorus (P(black)), an elemental direct-gap semiconductor exhibiting strong mechanical and electronic anisotropies. Located in close proximity to the A and A modes, these new modes dominate at an excitation wavelength of 633 nm. Their evolutions as a function of sample thickness, excitation wavelength, and defect density indicate that they are defect-activated and involve high-momentum phonons in a doubly resonant Raman process. Ab initio simulations of a monolayer reveal that the D' and D modes occur through intravalley scatterings with split contributions in the armchair and zigzag directions, respectively. The high sensitivity of these D modes to disorder helps explaining several discrepancies found in the literature.

Black phosphorus (BP), a 2D semiconducting material of interest in electronics and photonics, exhibits physical properties characterized by strong anisotropy and band gap energy that scales with reducing layer number. However, the investigation of its intrinsic properties is challenging because thin layer BP are photo oxidized in ambient conditions and the energy of their electronic states shift in different dielectric environment. We prepared free-standing samples of few layer BP in glovebox conditions and probed the dielectric response in vacuum using Scanning Transmission Electron Microscopy and Electron Energy Loss Spectroscopy (STEM-EELS). Thresholds of the excitation energy are measured at 1.9 eV, 1.4 eV and 1.1 eV for the mono-bi-and tri-layer BP, respectively and these values are used to estimate the corresponding optical band gaps.A comparison of our results with electronic structure calculations indicates that the variation of the quasi-particle gap is larger than that of the exciton binding energy. The dispersion of the plasmons versus momentum for 1-3 layer BP and bulk BP highlights a deviation from parabolic to linear dispersion and strong anisotropic fingerprints. Summary 1-Exfoliation and TEM grid preparation. 2-Layer number determination based on HAADF Z-contrast. 3-Computational details (gap and binding energy)1-Exfoliation and TEM grid preparation.

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