Magnesium Fluoride Electron-Selective Contacts for Crystalline Silicon Solar Cells.

In this study, we present a novel application of thin magnesium fluoride films to form electron-selective contacts to n-type crystalline silicon (c-Si). This allows the demonstration of a 20.1%-efficient c-Si solar cell. The electron-selective contact is composed of deposited layers of amorphous silicon (∼6.5 nm), magnesium fluoride (∼1 nm), and aluminum (∼300 nm). X-ray photoelectron spectroscopy reveals a work function of 3.5 eV at the MgF2/Al interface, significantly lower than that of aluminum itself (∼4.2 eV), enabling an Ohmic contact between the aluminum electrode and n-type c-Si. The optimized contact structure exhibits a contact resistivity of ∼76 mΩ·cm(2), sufficiently low for a full-area contact to solar cells, together with a very low contact recombination current density of ∼10 fA/cm(2). We demonstrate that electrodes functionalized with thin magnesium fluoride films significantly improve the performance of silicon solar cells. The novel contacts can potentially be implemented also in organic optoelectronic devices, including photovoltaics, thin film transistors, or light emitting diodes.

Three-dimensional nanotub submicrometer diffraction gratings for solar cells.

Diffraction gratings are a promising approach for reducing reflection and achieving light-trapping in solar cells. Using square lattices as a base structure, we investigate a novel bi-periodic nanotub three-dimensional grating structure and compare it with established textured structures for thin-film and wafer applications. For wafer application, simulations show that optimal AR coated nanotubs demonstrated solar weighted reflectance (SWR) of 2% compared to AR coated square pyramids with values 1.9%. Nanotubs also show SWR below 8% for polar angles to 60°. Simulated short-circuit current thin-film cells with nanotubs using smaller dimensions show higher yields (3-6 mA/cm2 average) compared to square pyramids. For periods greater than 700 nm at aspect ratios of 0.7 and greater, nanotubs have reduced current attributed to the increased planar surface area of the nanotub base, and evident in increased SWR. A simple nanoimprint lithography process was employed in experiments to define a square array of circular holes, utilizing a polydimethylsiloxane (PDMS) stamp applied onto a sol-gel imprint resist. The underlying silicon was then wet etched to produce the nanotub textures of 200 nm height and 513 nm period. AR coated nanotub wafers were produced via plasma enhanced chemical vapor deposition (PECVD), with an experimental and theoretical SWR of 6.4% and 5.4%, respectively.

The reversible and stereoselective N- to C-bonded rearrangement of tris(2-pyridylmethyl)-1,4,7-triazacyclononanecobalt(III).

The hexaaminecobalt(III) complex [Co(tmptacn)]3+ (tmptacn = 1,4,7-tris(2'-pyridylmethyl)-1,4,7-triazacyclononane) undergoes a novel base-catalyzed N- to C-bonded rearrangement in which a tacn nitrogen is displaced by the alpha-carbon which deprotonates and binds to the metal ion as a carbanion. The X-ray structure establishes the configuration for the regio- and stereoselectively (100%) formed product. The reaction involves both ring expansions and ring contraction. The carbanion is part of a strained four-membered ring. The kinetics are reported for the N- to C-rearrangement, shown to be retentive for the optically resolved (+)-[Co(tmptacn)]3+ reactant, and also the kinetics for a competitive and somewhat faster base-catalyzed racemization reaction of this complex. The reaction is completely but very slowly reversed in acid, also with retention, and in D2O/D+ there is 1:1 D-incorporation into the two sets of inequivalent tacn carbons. Extensive 1D and 2D NMR studies establish mechanistic details, and alternative mechanisms are proposed for the forward and reverse reactions. In neutral solution, there is a competitive oxidation reaction for the reverse C- to N-bonded process, involving the regio- and stereoselective (100%) incorporation of an alpha-OH substituent into the tacn ring.

Synthesis, structure, and kinetics and stereochemistry of base-catalyzed hydrolysis of meso- and rac-[Co2(tmpdtne)Cl2]4+, bis(pentaamine) complexes devoid of deprotonatable NH centers.

The bis(pentadentate) ligand tmpdtne binds two Co(II) centers, and the entity is readily oxidized to the dicobalt(III) derivative [Co(2)(tmpdtne)Cl(2)](4+) which has been separated into two isomeric forms. NMR studies establish these as meso and rac isomers arising from the different or same absolute configurations for the asym configuration about each Co(III) center. Each dinuclear ion base hydrolyses to the dihydroxo derivative [Co(2)(tmpdtne)(OH)(2)](4+) with retained asym configurations about each metal ion and also retained rac or meso configurations. The kinetics for the stepwise loss of the two Cl(-) ligands is uniphasic, and data are presented to show that the loss of the first chloride is rate determining and is followed by very rapid intramolecular and loss of the second Cl(-) via a hydroxo-bridged species to yield the observed dihydroxo derivative. Meso and rac forms of the latter have been crystallized. The X-ray crystal structure of the rac-dihydroxo complex is reported, and it establishes the configurations of all the complexes reported. The (1)H NMR spectra for the hydroxo ions show very high field Co-OH resonances (ca. delta-0.5 ppm) not observed previously for such ions, and this result is discussed in the context of published (1)H NMR data for bridged Co-OH-Co species. The base hydrolysis kinetics for the dichloro ions are first order in [OH(-)], and deprotonation at an alpha-CH(2) center (alpha to a pyridyl) is identified as the source of the catalysis, since there is no NH center available for deprotonation on the ligand. These data further support the new pseudoaminate base hydrolysis mechanism first reported in 2003. The values of k(OH) for the second-order base-catalyzed reaction are ca. 4.0 M(-1) s(-1) for both the rac and meso isomers, and these results are discussed in terms of the increased acidities of these 4+ cations compared to their 2+ ion counterparts.

Pyridyl-based pentadentate ligands: base-catalyzed hydrolysis of asym-[Co(dmptacn)Cl]2+.

The [Co(dmptacn)Cl](2+) (dmptacn = 1,4-bis(pyridylmethyl)-1,4,7-triazacyclononane) complex has been shown to be the asym isomer through 1D and 2D NMR studies, its optical resolution, and the single-crystal X-ray structure of its perchlorate salt. The kinetics of base-catalyzed hydrolysis establishes the usual [OH(-)] dependence (k(OH) = 0.040 M(-1) s(-1), 25 degrees C, I = 1.0 M, NaCl), but D-exchange experiments reveal that substantial if not complete reaction proceeds via the new pseudoaminate mechanism, i.e., via deprotonation at an alpha-CH(2) center rather than the NH. The significant kinetic isotope effect (k(H)/k(D) = 2.1) is interpreted in terms of rate-limiting deprotonation followed by reprotonation of the conjugate base at a rate competitive with loss of Cl(-). NMR and polarimetric studies establish geometric and optical retention for the hydrolysis reaction and exclude even the transient formation of a sym isomer intermediate.

The rediscovery of Alfred Werner's second hexol.

A compound originally prepared by Alfred Werner but structurally misassigned has been shown by three independent X-ray structural analyses to be a hexanuclear species which can now be regarded as Werner's second hexol, his first being that famous for being the first non-carbon containing species to be optically resolved.

Alfred Werner's inorganic counterparts of racemic and mesomeric tartaric acid: a milestone revisited.

Both isomers of [(en)(2)Co(micro-NO(2))(micro-NH(2))Co(en)(2)](4+) synthesized by Alfred Werner in 1913 have been optically resolved and are therefore the classic inorganic analogues of the sugars threose and erythrose, rather than the rac- and meso-tartaric acids, as Werner believed. The nitro bridge is unsymmetrically N-O bonded, and each dinuclear ion is therefore asymmetric, a fact also clearly apparent in the (1)H and (13)C NMR spectra.

N-methylmonothiocarbamatopentamminecobalt(III): restricted C-N bond rotation and the acid-catalyzed O- to S-bonded rearrangement.

High-resolution (1)H and (13)C NMR studies on the linkage isomers [(NH(3))(5)CoOC(S)NHCH(3)](2+) and [(NH(3))(5)CoSC(O)NHCH(3)](2+) reveal that the O-bonded form exists as a 5:1 mixture of Z and E isomers arising from restricted rotation about the C-N bond. Similarly, restricted rotation is observed (at 20 degrees C) for the S-bonded isomer (Z/E ca. 18:1), but not for the isoelectronic carbamate ion [(NH(3))(5)CoOC(O)NHCH(3)](2+), nor for the unsubstituted carbamato complex [(NH(3))(5)CoOC(O)NH(2)](2+). An analysis of the variable-temperature NMR data for the O-bonded carbamato and urea complexes has provided quantitative data on the rotational barriers, and these ions involve much faster C-N bond rotations than the thiocarbamato complexes. The acid-catalyzed reaction of [(NH(3))(5)CoOC(S)NHCH(3)](2+) is confirmed, but there is much less parallel hydrolysis (ca. 2%) than previously reported (40 +/- 10%) for 0.1 M HClO(4). In 1 M HClO(4), [(NH(3))(5)CoSC(O)NHCH(3)](2+) and [(NH(3))(5)CoOH(2)](3+) are formed in parallel as an 83:17 mixture. The kinetic data suggest that the protonated form is at least 20-fold more reactive than the free ion and that the linkage isomerization and hydrolysis pathways are both acid-catalyzed, the latter clearly more so than the rearrangement.

A unique mechanism for base catalyzed hydrolysis of pentaaminecobalt(III) complexes containing picolyl residues.

A novel [Co(pentaamine)Cl](2+) complex having all tertiary amine or pyridine donors has been synthesized (pentaamine = 1,4-bis(2'-pyridyl)-7-methyl-1,4,7-triazacyclononane). This asym-[Co(dmpmetacn)Cl](2+) species has been completely characterized through 1D and 2D NMR studies, and through the X-ray structure for the ZnCl(4)(2)(-) salt. Despite the lack of an activating NH center, remarkably its hydrolysis to [Co(pentaamine)OH](2+) is base catalyzed (k(OH) 0.70 M(-)(1) s(-)(1), 25 degrees C, I = 1.0 M, NaCl). Detailed NMR studies reveal that the base catalyzed substitution leads to the exchange of just one deuterium in one of the two -CH(2)- pyridyl arms, that is approximately trans to the leaving group, and this occurs during and not after base hydrolysis. Quenching experiments for the reaction of asym-[Co(dmpmetacn)Cl](2+) and control experiments on H/D exchange for the product asym-[Co(dmpmetacn)OD](2+) in OD(-) show that each act of deprotonation at the acidic methylene leads to loss of Cl(-). This is the first established case of base catalyzed substitution for a complex where the effective site of deprotonation is at a pyridyl group. A pronounced kinetic isotope effect is observed for the species perdeuterated at the pyridyl methylenes (k(H)/k(D) = 5.0), consistent with rate limiting deprotonation which is a rare event in Co(III) substitution chemistry. The activation afforded by the carbanion is discussed in terms of a new process coined the pseudo-aminate mechanism.