Molecular monolayer doping (MLD) presents an alternative to achieve doping of silicon in a nondestructive way and holds potential for realizing ultrashallow junctions and doping of nonplanar surfaces. Here, we report the mixing of dopant-containing alkenes with alkenes that lack this functionality at various ratios to control the dopant concentration in the resulting monolayer and concomitantly the dopant dose in the silicon substrate. The mixed monolayers were grafted onto hydrogen-terminated silicon using well-established hydrosilylation chemistry. Contact angle measurements, X-ray photon spectroscopy (XPS) on the boron-containing monolayers, and Auger electron spectroscopy on the phosphorus-containing monolayers show clear trends as a function of the dopant-containing alkene concentration. Dynamic secondary-ion mass spectroscopy (D-SIMS) and Van der Pauw resistance measurements on the in-diffused samples show an effective tuning of the doping concentration in silicon.
We have studied the electronic and magnetic properties of the interface between C60 molecules and a Fe(001) surface. X-ray absorption spectroscopy and x-ray magnetic circular dichroism studies of C60 monolayers on Fe(001) surfaces show that hybridization between the frontier orbitals of C60 and continuum states of Fe leads to a significant magnetic polarization of C60 π∗-derived orbitals. The magnitude and also the sign of this polarization were found to depend markedly on the excitation energy. These observations underline the importance of tailoring the interfacial spin polarization at the Fermi level of ferromagnet/organic semiconductor interfaces for applications in organic spintronics.
The bulk-heterojunction concept was originally developed to allow for a large active interfacial area between the donor and acceptor materials in organic solar cells. [ 1 ] The energy offsets between the lowest unoccupied molecular orbital (LUMO) levels of the donor and acceptor are tuned to effi ciently dissociate photogenerated excitons, i.e. an offset on the order of > 0.5 eV is effi ciently promoting electron transfer to the acceptor and prohibiting back-transfer to the donor. However, after dissociation the charge carriers are still Coulombically bound at zero external fi eld within the Coulomb radius r c = e 2 /4 π ε ε 0 kT , where e is the electron charge, ε ( ε 0 ) is the relative (absolute) dielectric permittivity, k the Boltzmann constant and T is the temperature. The formation of a ground-state charge transfer (CT) state opens up (non-)radiative recombination channels at the interface, where the Coulombically bound carriers can eventually recombine both geminately and non-geminately, causing recombination losses in solar cells.Recent reports on charge transfer state formation in the ground state of the polymer/fullerene mixtures suggests that the CT state arises from a wave function overlap of the polymer and fullerene molecules, [2][3][4][5][6][7] whereby a new inter-bandgap charge transfer complex state is formed at the donor-acceptor interface. Experimental evidence show that this CT state has an energy lower than the bandgap of both the donor and the acceptor materials. [ 8 , 2 , 3 ] Two routes for populating the CT states have been reported. The fi rst is by relaxation from singlet states formed via either above-bandgap excitation or injection from contacts. [ 9 , 10 ] Here the charge pair migrates to a donor-acceptor interface and minimizes its energy by populating the CT state at the interface. The second route for populating a CT complex is by direct optical excitation using subbandgap light. [ 11 , 12 ] Besides providing a recombination channel for Coulombically bound electron-hole pairs at donor-acceptor interfaces, there is also evidence of charge generation via the CT state. In regioregular-(rr) poly (3-hexylthiophene):[6,6]-phenyl-C 61 -butyric acid methyl ester (P3HT:PCBM) a substantial number of sub-bandgap generated charges escape from the interface and can contribute to the solar cell photocurrent. [ 11 , 13 ] The CT state is also shown to be closely linked to the open circuit voltage of bulk heterojunction solar cells. [ 13 ] The main obstacle for materials with low dielectric constants (and consequently a large r c ) to be operational in effi cient solar cells is avoiding geminate recombination of the Coulombically bound charge pairs. The carrier generation in homogeneous polymeric semiconductors is of Onsager-type, which means that the generation is governed by the Brownian motion of the geminate pair within their mutual Coulomb potential. The criterion for this process is that the hopping distance is much shorter than the Coulomb radius. If a geminate pair can separate this di...
The two-dimensional (2D) semiconductor molybdenum disulfide (MoS) has attracted widespread attention for its extraordinary electrical-, optical-, spin-, and valley-related properties. Here, we report on spin-polarized tunneling through chemical vapor deposited multilayer MoS (∼7 nm) at room temperature in a vertically fabricated spin-valve device. A tunnel magnetoresistance (TMR) of 0.5-2% has been observed, corresponding to spin polarization of 5-10% in the measured temperature range of 300-75 K. First-principles calculations for ideal junctions result in a TMR up to 8% and a spin polarization of 26%. The detailed measurements at different temperature, bias voltages, and density functional theory calculations provide information about spin transport mechanisms in vertical multilayer MoS spin-valve devices. These findings form a platform for exploring spin functionalities in 2D semiconductors and understanding the basic phenomena that control their performance.
In this short review, we will give examples on how photoelectron spectroscopy (PES) assisted by models on interface energetics can be used to study properties important to bulk heterojunction type organic photovoltaic devices focusing on the well-known bulk heterojunction blend of poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) and its model system P3HT:C 60 . We also will discuss some of the limitations of PES as applied to organic semiconductors (OS) and photovoltaic devices and finish with reviewing recent theoretical advances that now enable calculation of relevant parameters at (hybrid) interfaces measured by PES.
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