Improving the performance of arylamine-based hole transporting materials in perovskite solar cells： Extending π-conjugation length or increasing the number of side groups？

In this work, we prepared three simple arylamine-based hole transporting materials from commercially available starting materials. The effect of extending z-conjugation length or increasing the number of side groups compared with reference compound on the photophysical, electrochemical, hole mobility properties and performance in perovskite solar cells were further studied. It is noted that these two kinds of molecular modifications can significantly lower the HOMO level and improve the hole mobility, thus improving the hole injection from valence band of perovskite. On the other hand, the compound with more side groups showed higher hole injection efficiency due to lower HOMO level and higher hole mo- bility compared with the compound with extending π-conjugation length. The perovskite solar cells with the modified molecules as hole transporting materials showed a higher efficiency of 15.40% and 16.95%, respectively, which is better than that of the reference compound （13.18%）. Moreover, the compound with increasing number of side groups based devices showed comparable photovoltaic performance with that of conventional spiro-OMeTAD （16.87%）.

DIMENSIONS OF ATACTIC POLY(α-METHYLSTYRENE)CHAINS WITH SIDE GROUPS

An improved configurational-confomational statistical method is developed and the mean-square radius of gyration for atactic poly（α-methylstyrene）（PαMS） chains is studied, in which the effect of large side groups is considered. The deduced formulas, based on the rotational isomer state theory, are used to investigate the configuration-dependent properties of the atactic polymer chain, and the statistical correlation of the unperturbed polymer chain dimension and structure parameters are calculated. For the fraction of meso dyads Wm= 0.4, the dependence of the radius of gyration Rg and the intrinsic viscosity [η] on the molecule mass M are Rg = 2.63×10^-2 M^0.50 nm and [η] = 7.36 × 10^-2 M^0.497, respectively, which are in agreement with the previous experimental data for the PαMS samples. A small hump is detected in the curve of the characteristic ratio of the unperturbed mean-square radius of gyration versus the chain length for short PαMS chains. The Rg increases linearly with the temperature T, and the effects of the chain length and the tacticity on the temperature coefficient are remarkable. These are quite different from the results for PαMS chains not considering side groups or for the monosubstituted polystyrene chain.

Negative differential resistance behaviors in OPE derivatives combined C_(60) molecular junctions modulated with side groups

By applying non-equilibrium Green's functions (NEGF) in combination with the density functional theory (DFT), we investigate the electronic transport properties of molecular junctions constructed by OPE derivatives with different side groups combined C60 molecules. The results show that the side groups play an important role in the properties of electron transport. Negative differential resistance (NDR) is observed in such devices. Especially for the molecule with electron-donating group ( OCH3), two NDR appear at different bias voltage regions. And the mechanism is proposed for the NDR behavior, owing to the shift of the molecular orbitals caused by the change in molecule charge.

Tuning the type of charge carriers in N-heterocyclic carbene-based molecular junctions through electrodes

Designing tunable molecular devices with different charge carriers in single-molecule junctions is crucial to the nextgeneration electronic technology.Recently,it has been demonstrated that the type of charge carriers depends on and can be tuned by controlling the molecular length and the number of interfacial covalent bonds.In this study,we show that the type of charge carriers can also be tuned by controlling the material and shape of electrodes.N-heterocyclic carbenes(NHCs)have attracted attention because of their ability to form strong,substitutional inert bonds in a variety of metals.Also,NHCs are more stable than the widely used thiol group.Therefore,we use electrodes to tune the type of charge carriers in a series of NHCs with different side groups.The ab initio calculations based on non-equilibrium Green’s formalism combined with density functional theory show that the dominant charge carrier switches from electrons to holes when gold electrodes are changed into platinum ones.The nature of the charge carriers can be identified by variations in the transport spectra at the Fermi level(EF),which are caused by the side groups.The projections of transport spectra onto the central molecules further validate our inferences.In addition,the transmission coefficient at EF is found to be dependent on the atomic interface structure.In particular,for the NHC without methyl or ethyl side groups,connecting a protruding atom on the electrode surface significantly enhances the transportability of both electrode materials.Overall,this study presents an effective approach to modifying transport properties,which has potential applications in designing functional molecular devices based on NHCs.