Laurie J. Phillips

Growth, disorder, and physical properties of ZnSnN2

We examine ZnSnN2, a member of the class of materials contemporarily termed “earth-abundant element semiconductors,” with an emphasis on evaluating its suitability for photovoltaic applications. It is predicted to crystallize in an orthorhombic lattice with an energy gap of 2 eV. Instead, using molecular beam epitaxy to deposit high-purity, single crystal as well as highly textured polycrystalline thin films, only a monoclinic structure is observed experimentally. Far from being detrimental, we demonstrate that the cation sublattice disorder which inhibits the orthorhombic lattice has a profound effect on the energy gap, obviating the need for alloying to match the solar spectrum.

Improved electrical mobility in highly epitaxial La:BaSnO3 films on SmScO3(110) substrates

Heteroepitaxial growth of BaSnO3 and Ba1−xLaxSnO3 (x = 7%) lanthanum doped barium stannate thin films on different perovskite single crystal (SrTiO3 (001) and SmScO3 (110)) substrates has been achieved by pulsed laser deposition under optimized deposition conditions. X-ray diffraction measurements indicate that the films on either of these substrates are relaxed due to the large mismatch and present a high degree of crystallinity with narrow rocking curves and smooth surface morphology while analytical quantification by proton induced X-ray emission confirms the stoichiometric La transfer from a polyphasic target, producing films with measured La contents above the bulk solubility limit. The films show degenerate semiconducting behavior on both substrates, with the observed room temperature resistivities, Hall mobilities, and carrier concentrations of 4.4 mΩ cm, 10.11 cm2 V−1 s−1, and 1.38 × 1020 cm−3 on SmScO3 and 7.8 mΩ cm, 5.8 cm2 V−1 s−1, and 1.36 × 1020 cm−3 on SrTiO3 ruling out any extrinsic contribution from the substrate. The superior electrical properties observed on the SmScO3 substrate are attributed to reduction in dislocation density from the lower lattice mismatch.

In situ stepwise synthesis of functional multijunction molecular wires on gold electrodes and gold nanoparticles.

Interest in molecular electronics has focused on materials that act as diodes 2] or wires, and on the development of innovative techniques for contacting to single molecules and ultrathin films. They include, for example, the in situ synthesis of molecular wires on solid supports, in which recent advances have resulted in the bridging of nanometersized gaps between electrodes by the coupling of aminoterminated molecules between opposing surface-based aldehyde groups on the electrode coatings. For photovoltaic applications, molecules may be elongated in situ on nanoparticles with differently colored linked units to provide charge-transport pathways along the conjugated wirelike backbones. This elongation method is highly versatile: it combines a facile synthesis, user-defined functionality during the initial self-assembly of the precursors to provide reactive surface-based groups, and the sequencing of chemical building blocks (donors, acceptors, and bridges) to match the energy levels with those of the electrodes. The current– voltage (I–V) characteristics, which are usually symmetrical for conventional wires, may be tuned by controlling the molecular sequence. Donors and acceptors give rise to rectifying characteristics when isolated at opposite ends and the direction of electron flow at forward bias is from the cathode to acceptor on one side and from the donor to anode on the other. Improved rectification has been achieved by incorporating a cyclohexane s bridge that inhibits intramolecular charge transfer and maintains the integrity of the electron-donating and electron-accepting moieties. This additional bridge has resulted in the highest rectification ratio to date from a molecular diode. Molecular wires were synthesized in situ on gold electrodes from a self-assembly reaction of a meta-substituted thiophenol with a terminal aldehyde group to act as a template for growth that is perpendicular to the substrate (Figure 1). These embryonic wires were elongated by reacting first with H2N-wire-NH2 and then OHC-wire-CHO to couple the wire-like units through imino groups. The sequence is user-defined and the wires may be elongated to any appropriate length by repeating the process before terminating with a mono-substituted component. In this study, plasma-cleaned substrates were immersed in a solution of 4-[(3-mercaptophenylimino)methyl]benzaldehyde in acetone (0.1 mg cm , step 1) for 4 h and then rinsed with acetone to remove physisorbed material. When assembled on the electrodes of 10 MHz quartz crystals, a Sauerbray analysis of the limiting frequency change gave an area of 0.3 nm molecule , which approximates to the molecular cross-section. Chemisorption was verified by X-ray photoelectron spectroscopy (XPS): a doublet at 162.1 eV (S 2p3/2) and 163.3 eV (S 2p1/2) is characteristic of the Au S link, and a peak at 399.1 eV (N 1s) corresponds to the imino nitrogen atom. Quantitative analysis of the areas under the S 2p and N 1s peaks, which was fitted using a Gaussian-Lorentzian function and corrected for atomic sensitivity factors, confirmed an N:S ratio of about 1:1, consistent with the atomic ratio within the self-assembled molecule. Mercaptoaniline (HS C6H4 NH2) binds to the gold surface through both the nitrogen and sulfur substituents, as shown by the characteristic XPS data of each bond: 162.0 and 163.2 eV (S 2p, Au S); 163.6 and 164.8 eV (S 2p, SH); 398.3 eV (N 1s, Au N); 400.4 eV (N 1s, NH2). As such, 4-[(3mercaptophenylimino)methyl]benzaldehyde 1 was chosen as the starting building block for the molecular wires (Figure 1). However, mercaptoaniline was used in a preliminary study to demonstrate the in situ growth of molecular wires on gold nanoparticles. When functionalized by 3-mercaptoaniline, these nanoparticles exhibited NH2 IR absorption stretching modes at 3200 and 3300 cm , but subsequent reaction with terephthalaldehyde resulted in the loss of them and the Figure 1. Structure of the self-assembled monolayer (SAM), which was obtained from 4-[(3-mercaptophenylimino)methyl]benzaldehyde precursor 1, and aldehyde and amine substrates that were used for in situ elongation of the wire.

Molecular Bridging of Silicon Nanogaps

The highly doped electrodes of a vertical silicon nanogap device have been bridged by a 5.85 nm long molecular wire, which was synthesized in situ by grafting 4-ethynylbenzaldehyde via C-Si links to the top and bottom electrodes and thereafter by coupling an amino-terminated fluorene unit to the aldehyde groups of the activated electrode surfaces. The number of bridging molecules is constrained by relying on surface roughness to match the 5.85 nm length with an electrode gap that is nominally 1 nm wider and may be controlled by varying the reaction time: the device current increases from ≤1 pA at 1 V following the initial grafting step to 10-100 nA at 1 V when reacted for 5-15 min with the amino-terminated linker and 10 μA when reacted for 16-53 h. It is the first time that both ends of a molecular wire have been directly grafted to silicon electrodes, and these molecule-induced changes are reversible. The bridges detach when the device is rinsed with dilute acid solution, which breaks the imine links of the in situ formed wire and causes the current to revert to the subpicoampere leakage value of the 4-ethynylbenzaldehyde-grafted nanogap structure.

Core-shell ITO/ZnO/CdS/CdTe nanowire solar cells

Radial p-n junction nanowire (NW) solar cells with high densities of CdTe NWs coated with indium tin oxide (ITO)/ZnO/CdS triple shells were grown with excellent heterointerfaces. The optical reflectance of the devices was lower than for equivalent planar films by a factor of 100. The best efficiency for the NW solar cells was η = 2.49%, with current transport being dominated by recombination, and the conversion efficiencies being limited by a back contact barrier (ϕB = 0.52 eV) and low shunt resistances (RSH < 500 Ω·cm2).

Functional molecular wires

The properties of self-assembled molecules may be tuned by sequentially coupling components on a gold surface, the molecular electronics toolbox of chemically reactive building blocks yielding molecular wires with diode-like current-voltage (I-V) characteristics. The bias for rectification in each case is dependent upon the sequence of electron-donating and electron-accepting moieties and similar behaviour has been achieved for four different contacting techniques.

In-depth analysis of chloride treatments for thin-film CdTe solar cells

CdTe thin-film solar cells are now the main industrially established alternative to silicon-based photovoltaics. These cells remain reliant on the so-called chloride activation step in order to achieve high conversion efficiencies. Here, by comparison of effective and ineffective chloride treatments, we show the main role of the chloride process to be the modification of grain boundaries through chlorine accumulation, which leads an increase in the carrier lifetime. It is also demonstrated that while improvements in fill factor and short circuit current may be achieved through use of the ineffective chlorides, or indeed simple air annealing, voltage improvement is linked directly to chlorine incorporation at the grain boundaries. This suggests that focus on improved or more controlled grain boundary treatments may provide a route to achieving higher cell voltages and thus efficiencies.

Synthesis of Covalently Linked Molecular Bridges between Silicon Electrodes in CMOS-Based Arrays of Vertical Si/SiO2/Si Nanogaps

Silicon nanogaps were bridged in situ by grafting 4-ethynylbenzaldehyde to activate the electrodes and coupling 2,6-diaminoanthra-9,10-quinone to link the coatings. The bridged structures exhibit currents of 11-14 nA at 1 V. The process is reversed by soaking in acidified solution, which causes the current to diminish. Copyright

Stability and Performance of CsPbI2Br Thin Films and Solar Cell Devices

In this manuscript, the inorganic perovskite CsPbI2Br is investigated as a photovoltaic material that offers higher stability than the organic-inorganic hybrid perovskite materials. It is demonstrated that CsPbI2Br does not irreversibly degrade to its component salts as in the case of methylammonium lead iodide but instead is induced (by water vapor) to transform from its metastable brown cubic (1.92 eV band gap) phase to a yellow phase having a higher band gap (2.85 eV). This is easily reversed by heating to 350 °C in a dry environment. Similarly, exposure of unencapsulated photovoltaic devices to water vapor causes current (JSC) loss as the absorber transforms to its more transparent (yellow) form, but this is also reversible by moderate heating, with over 100% recovery of the original device performance. NMR and thermal analysis show that the high band gap yellow phase does not contain detectable levels of water, implying that water induces the transformation but is not incorporated as a major component. Performances of devices with best efficiencies of 9.08% (VOC = 1.05 V, JSC = 12.7 mA cm-2 and FF = 68.4%) using a device structure comprising glass/ITO/c-TiO2/CsPbI2Br/Spiro-OMeTAD/Au are presented, and further results demonstrating the dependence of the performance on the preparation temperature of the solution processed CsPbI2Br films are shown. We conclude that encapsulation of CsPbI2Br to exclude water vapor should be sufficient to stabilize the cubic brown phase, making the material of interest for use in practical PV devices.

NH

The CdCl2 treatment is a key step in CdTe solar cell fabrication. However, despite its near ubiquitous use, the process is nonideal as CdCl2 is both expensive and potentially hazardous to utilize in processing. In this paper, we report on the development of a NH4Cl replacement to the CdCl2 process, which is a low-cost noncarcinogenic alternative. Comparative cells were fabricated and compared via C -V, J-V, scanning electron microscopy, and external quantum efficiency analysis. Further process optimization led to device efficiencies of up to 11.5%, achieved using this new process, with VOC values of up to 832 mV, which is relatively high.