We have developed a Kramers-Kronig consistent analytical expression to fit the measured optical functions of hydrogenated amorphous silicon (a-Si:H) based alloys, i.e., the real and imaginary parts of the dielectric function (⑀ 1 ,⑀ 2) ͑or the index of refraction n and absorption coefficient ␣͒ versus photon energy E for the alloys. The alloys of interest include amorphous silicon-germanium (a-Si 1Ϫx Ge x :H) and silicon-carbon (a-Si 1Ϫx C x :H), with band gaps ranging continuously from ϳ1.30 to 1.95 eV. The analytical expression incorporates the minimum number of physically meaningful, E independent parameters required to fit (⑀ 1 ,⑀ 2) versus E. The fit is performed simultaneously throughout the following three regions: ͑i͒ the below-band gap ͑or Urbach tail͒ region where ␣ increases exponentially with E, ͑ii͒ the near-band gap region where transitions are assumed to occur between parabolic bands with constant dipole matrix element, and ͑iii͒ the above-band gap region where (⑀ 1 ,⑀ 2) can be simulated assuming a single Lorentz oscillator. The expression developed here provides an improved description of ⑀ 2 ͑or ␣͒ in the below-band gap and near-band gap regions compared with previous approaches. Although the expression is more complicated analytically, it has numerous applications in the analysis and simulation of thin film a-Si:H based p-in and n-i-p multilayer photovoltaic devices. First, we describe an approach whereby, from a single accessible measure of the optical band gap, the optical functions can be generated over the full solar spectrum for a sample set consisting of the highest quality intrinsic a-Si:H based alloys prepared by plasma-enhanced chemical vapor deposition using the principle of maximal H 2 dilution. Second, we describe quantitatively how such an approach can be modified for sample sets consisting of lower quality alloy materials. Finally, we demonstrate how the generated optical functions can be used in simulations of the absorption, reflection, and quantum efficiency spectra of a-Si:H based single-junction and multijunction solar cells.

Ammonia–borane (AB) has been in the spotlight for its much touted potential as an onboard vehicular hydrogen delivery material. Over the past decade, catalyzed dehydrogenation/dehydrocoupling reactions for releasing H2 from the maximum available 3 equiv in AB have gained significant momentum. In this Perspective, we focus on the homogeneous AB dehydrogenation catalysis, by both transition metal (TM)-based and metal-free systems. Several questions pertaining to underlying mechanisms, nature of intermediates, and catalyst efficacy have surfaced as the multitude of discoveries in the field has built up at a fast pace. The varied fate of the dehydrogenation reactions of AB with different catalysts yielding different end products ranging from polyaminoborane (PAB) to polyborazylene (PBZ) and the ability/inability of catalysts to release more than 1 equiv of H2 from AB have fuelled the genesis of several mechanistic hypotheses. However, the copious investigations on the experimental and theoretical fronts have led to some convergent views. We try to highlight the general consensus on mechanistic underpinnings and the crucial role of important intermediates in determining the fate of catalysis for this family of dehydrocoupling reactions. We also point out the unresolved issues along with a short note on the regeneration of AB.

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We report a hydrogen-related defect that establishes the direct role of hydrogen in stabilizing the silicon dangling bonds created in the Staebler-Wronski effect in hydrogenated amorphous silicon. A specific NMR signal due to paired hydrogen atoms occurs only after optical excitation, exists at an intensity that is consistent with the density of optically induced silicon dangling bonds, and anneals at temperatures that are consistent with the annealing of the optically induced silicon dangling bonds. At this defect the hydrogen atoms are 2.3+/-0.2 A apart.

We have conducted mechanistic investigations using dispersion-corrected hybrid density functional theory on three different homogeneous processes: (a) hydrogenation of styrene using H2, (b) dehydrogenation of amine–borane, and (c) transfer hydrogenation of styrene using amine–borane catalyzed by a boryl-ligated Co-based catalytic system, LCo­(N2) (where L = meridional bis-phosphinoboryl (PBP) ligand), recently developed by Peters and co-workers (Lin, T.-P; Peters, J. C. J. Am. Chem. Soc. 2013, 135, 15310–15313). Our studies reveal that all three catalytic processes are facilitated by the same active species, which is of the form LCo­(H)2. The formation of the active catalytic species in turn determines the rate-determining barrier (RDB) for the hydrogenation reactions of the olefin and also for the dehydrogenation reaction of amine–borane. We predict that the RDB for hydrogenation of styrene under H2 atmosphere is 17.3 kcal/mol, which occurs through a channel that involves switching of a singlet electronic ground state (S0) of the organometallic catalytic species to its low-lying triplet electronic state (T1) and returning back to the singlet surface through minimum energy crossing points along the reaction coordinate. Alternatively, we estimate the RDB to be 19.4 kcal/mol, slightly higher than that of the previous channel, if only the singlet spin state surface is considered. We find that the associated RDB for both the dehydrogenation of amine–borane (NMe2H-BH3) and transfer hydrogenation of styrene by amine–borane are higher than the hydrogenation of olefin using H2(g) and is predicted to be 24.7 kcal/mol. In addition, we show that in the reaction involving amine–borane, the active catalytic species (LCo­(H)2) can get deactivated by forming a hydridoborane cobalt tetrahydridoborate complex, which happens through an SN2 type nucleophilic attack by the LCo­(H)2 on amine–borane.