In this work, the effect of [6,6]-phenyl-C 61butyric acid methyl ester (PCBM) orientation on the electronic and optical properties of the regioregular poly(3hexylthiophene) (P3HT):PCBM blend interface was studied by means of first-principles density functional theory calculations with G 0 W 0 approximation plus the Bethe− Salpeter equation (BSE). The band structures and photoresponse are shown to depend on the PCBM orientation. The origin of the two main optical absorption peaks is determined, and the effect of PCBM rotation on optical properties is revealed. The calculated lowest charge transfer complex state energy, exciton binding energy, and the absorption spectrum for the flat-lying model are in good agreement with the experimental values, which indicates the flat-lying structure is the predominant interface structure in the experiments. The lowestenergy configuration is also determined as the flat-lying orientation in our calculations. Our results further suggest that the dissociation of excitons and charge transfer at the interface is more efficient for the PCBM flat-lying orientation than that for the upright-standing one, which provides a possible explanation for the increased performance of the P3HT:PCBM devices after a thermal annealing treatment.

The two major classes of unconventional superconductors, cuprates and Fe-based superconductors, have magnetic parent compounds, are layered, and generally feature square-lattice symmetry. We report the discovery of pressure-induced superconductivity in a nonmagnetic and wide band gap 1.95 eV semiconductor, Cu2I2Se6, with a unique anisotropic structure composed of two types of distinct molecules: Se6 rings and Cu2I2 dimers, which are linked in a three-dimensional framework. Cu2I2Se6 exhibits a concurrent pressure-induced metallization and superconductivity at ∼21.0 GPa with critical temperature (T c) of ∼2.8 K. The T c monotonically increases within the range of our study reaching ∼9.0 K around 41.0 GPa. These observations coincide with unprecedented chair-to-planar conformational changes of Se6 rings, an abrupt decrease along the c-axis, and negative compression within the ab plane during the phase transition. DFT calculations demonstrate that the flattened Se6 rings within the CuSe layer create a high density of states at the Fermi level. The unique structural features of Cu2I2Se6 imply that superconductivity may emerge in anisotropic Cu-containing materials without square-lattice geometry and magnetic order in the parent compound.

Poly(2,5-bis(3-tetradecylthiophen-2yl)thieno(3,2-b)thiophene) (PBTTT-C14) is an important electro-optical polymer, whose three-dimensional crystal structure is somewhat ambiguous and the fundamental electronic and linear optical properties are not well known. We carried out first-principles calculations to model the crystal structure and to study the effect of side-chains on the physical structure and electronic properties. Our calculations suggest that the patterns of side-chain has little direct effect on the valence band maximum and conduction band minimum but they do have impact on the bandgap through changing the π-π stacking distance. By examining the band structure and wave functions, we conclude that the fundamental bandgap of the PBTTT-C14 crystal is determined by the conduction band energy at the Q point. The calculations indicate that the bandgap of PBTTT-C14 crystal may be tunable by introducing different side-chains. The significant peak in the imaginary part of the dielectric function arises from transitions along the polymer backbone axis, as determined by the critical-point analysis and the large optical transition matrix elements in the direction of the backbone.

Although NiO exhibits great potential as a hole transport layer in polymer organic electronic devices, our understanding of the atomic structure of organics/NiO interfaces and the effect of NiO surface modification on device performance is still limited. Here, we report the study of the structure and electronic properties of the monomer of the regioregular poly(3-hexylthiophene) (rr-P3HT)/NiO (100) interface by means of first-principles calculations. Different adsorption sites and orientations were studied, and a global minimum configuration was determined. The backbone of P3HT monomer prefers to orient along the OO direction, while the side chains prefer to align in the NiO direction of the NiO (100) surface. Although a significant contribution to the adsorption energy comes from the side chains, strong electronic coupling is found between the backbone of P3HT and NiO. Our calculations indicate that the interfacial electronic structure of organics/ NiO is key to device performance. Further, the calculated interfacial electronic structures demonstrate the critical role of oxygen in enhancing hole transport and electron blocking between P3HT and NiO and provide a possible explanation for the increased performance in O 2 -plasma-treated NiO devices observed in experiments.

Given the recent success in achieving efficient organic photovoltaic solar cells based on thieno [3,4-b]thiophene/benzodithiophene polymers (PTB7) and growing efforts to further improve the power conversion efficiency of the PTB7-based devices, a detailed atomic-scale picture of the electronic structure and the excitonic properties of PTB7 crystal is highly desirable.We report electronic and optical properties of PTB7 on the basis of first-principles density functional theory and GW many-body plus Bethe-Salpeter equation (GW-BSE) calculations. It is established that the first two highest valence bands (HVB) and the first two lowest conduction bands (LCB) originate from the benzodithiophene (BDT) and thieno [3,4-b]thiophene (TT) functional units, respectively, thus confirming the donor-acceptor nature of PTB7. A significant difference of band-splitting between HVBs and LVBs is found and its origins are explained. Our results strongly suggest that the strength of the inter-chain π-π interaction is not only a function of inter-chain distance, but is also highly dependent on the nature of the fused rings. The experimental optical absorption spectrum of PTB7 is well reproduced and explained by our 2 GW-BSE calculations. Further analysis shows that the nature of the lowest singlet (triplet) excitons in polymeric crystals such as PTB7 differs from that of organic molecular crystals. A possible reason is explored by combining BSE calculations with a simple Hamiltonian model.