Self-assembled monolayers (SAMs) based on Br-2PACz ([2-(3,6dibromo-9H-carbazol-9-yl)ethyl]phosphonic acid) 2PACz ethyl]phosphonic acid) and ethyl]phosphonic acid) molecules were investigated as hole-extracting interlayers in organic photovoltaics (OPVs). The highest occupied molecular orbital (HOMO) energies of these SAMs were measured at À 6.01 and À 5.30 eV for Br-2PACz and MeO-2PACz, respectively, and found to induce significant changes in the work function (WF) of indium-tin-oxide (ITO) electrodes upon chemical functionalization. OPV cells based on PM6 (poly [(2,6-(4,8-bis(5-(2-ethylhexyl-3- ([6,6]-phenyl-C71-bu-tyric acid methyl ester) using ITO/Br-2PACz anodes exhibited a maximum power conversion efficiency (PCE) of 18.4 %, outperforming devices with ITO/MeO-2PACz (14.5 %) and ITO/poly (3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PE-DOT : PSS) (17.5 %). The higher PCE was found to originate from the much higher WF of ITO/Br-2PACz (À 5.81 eV) compared to ITO/MeO-2PACz (4.58 eV) and ITO/PEDOT : PSS (4.9 eV), resulting in lower interface resistance, improved hole transport/extraction, lower trap-assisted recombination, and longer carrier lifetimes. Importantly, the ITO/Br-2PACz electrode was chemically stable, and after removal of the SAM it could be recycled and reused to construct fresh OPVs with equally impressive performance.

The influence of halogen substitutions (F, Cl, Br, and I) on the energy levels of the self‐assembled hole‐extracting molecule [2‐(9H‐Carbazol‐9‐yl)ethyl]phosphonic acid (2PACz), is investigated. It is found that the formation of self‐assembled monolayers (SAMs) of [2‐(3,6‐Difluoro‐9H‐carbazol‐9‐yl)ethyl]phosphonic acid (F‐2PACz), [2‐(3,6‐Dichloro‐9H‐carbazol‐9‐yl)ethyl]phosphonic acid (Cl‐2PACz), [2‐(3,6‐Dibromo‐9H‐carbazol‐9‐yl)ethyl]phosphonic acid (Br‐2PACz), and [2‐(3,6‐Diiodo‐9H‐carbazol‐9‐yl)ethyl]phosphonic acid (I‐2PACz) directly on indium tin oxide (ITO) increases its work function from 4.73 eV to 5.68, 5.77, 5.82, and 5.73 eV, respectively. Combining these ITO/SAM electrodes with the ternary bulk‐heterojunction (BHJ) system PM6:PM7‐Si:BTP‐eC9 yields organic photovoltaic (OPV) cells with power conversion efficiency (PCE) in the range of 17.7%–18.5%. OPVs featuring Cl‐2PACz SAMs yield the highest PCE of 18.5%, compared to cells with F‐2PACz (17.7%), Br‐2PACz (18.0%), or I‐2PACz (18.2%). Data analysis reveals that the enhanced performance of Cl‐2PACz‐based OPVs relates to the increased hole mobility, decreased interface resistance, reduced carrier recombination, and longer carrier lifetime. Furthermore, OPVs featuring Cl‐2PACz show enhanced stability under continuous illumination compared to ITO/PEDOT:PSS‐based cells. Remarkably, the introduction of the n‐dopant benzyl viologen into the BHJ further boosted the PCE of the ITO/Cl‐2PACz cells to a maximum value of 18.9%, a record‐breaking value for SAM‐based OPVs and on par with the best‐performing OPVs reported to date.

Recent studies have examined the possibility of growing honeycomb silicene and germanene, the silicon and germanium analogues of graphene. Here we use first-principles calculations to examine the relative stability of a number of other single-layer structures that are derived from prominent surface reconstructions of group-IV semiconductors. We find that Si single-layers with the geometry of the √3 × √3 reconstruction are more stable than honeycomb silicene. Likewise, honeycomb germanene is less stable than germanium ultrathin films with a √3 × √3 or 7 × 7 arrangement. √3 × √3 Si and Ge single layers are metals with strong peaks at the Fermi level, and, in the case of Ge, they bear a magnetic moment. Overall, the results demonstrate that free-standing Si and Ge nano-sheets differ in key structural and electronic properties from graphene, and may thus provide systems with alternative functionalities.

Germanane (GeH) and silicane (SiH) are the fully hydrogenated forms of germanene and silicene, the Ge- and Si-analogues of graphene. Here we use density-functional theory calculations to probe the properties of GeH and SiH sheets and their dependence on applied uni-axial compression. We find that GeH polymorphs with distinct hydrogen arrangements have markedly different energy band gaps. We also show that, when compressed, GeH forms superstructures with parts in low- and wide-gap geometries, enabling the creation of alternating polymorph nano-ribbons. An alternative to superstructure formation is the adoption by GeH of a corrugated form with extreme bending. Silicane shows similar behavior under compression, with either high corrugation, or successive parts with different geometries. Finally, we demonstrate that interaction with a substrate can influence the relative stability of GeH overlayer polymorphs. Overall, the results reveal ways to enhance the functionalities of these two-dimensional materials through the formation of superstructures with sizeable quantum well effects or outstanding mechanical response.

In the quest for the construction of silicene, the silicon analogue of graphene, recent experimental studies have identified a number of distinct ultrathin Si over-layer structures on a Ag(111) surface. Here we use first-principles calculations to probe associated atomic-scale mechanisms that can give rise to this rich behavior of Si wetting layers. We find that the interaction between the Si film and the Ag substrate, neither too strong nor too weak, combined with the possibility of buckling, allows for the incorporation of a number of excess Si adatoms in continuous overlayers with a honeycomb network topology. Depending on the Si coverage, we thus obtain a hierarchy of Si mono-atomic films, in agreement with experiments.