P. Jackson et al.: Cu(In,Ga)Se 2 solar cells with new record efficiencies up to 21.7%

We report the resonant Raman spectra of individualized ͑=debundled͒ single-walled carbon nanotubes ͑SWNTs͒ having diameters d ϳ 0.8-1.3 nm subject to compression up to 10.5 GPa. Both SWNTs in watersurfactant dispersions as well as SWNTs deposited onto glass microfibers and compressed with methanolethanol were studied. In the low pressure regime ͑Ͻ1 GPa͒, linear and reversible up-shifts of the Raman bands associated with the radial breathing vibrational mode ͑RBM͒ and tangential G mode were observed. The pressure derivative of the RBM frequency increases with increasing d and, unexpectedly, is larger for metallic than for semiconducting tubes. Above 1-2 GPa, RBM bands of additional SWNT species appear. This is due to pressure-induced shifts into resonance and broadening of the corresponding optical transitions. The latter could be quantified by photoluminescence ͑PL͒ measurements of the corresponding semiconducting tubes that are also reported here. Disappearance of the RBM and G bands contributed by nanotubes with d ϳ 0.8-0.9 and ϳ1.2-1.3 nm at ϳ10 and ϳ4 GPa, respectively, is tentatively assigned to extensive radial deformation of the nanotubes, in accordance with theoretical predictions. After the application of several GPa pressure, a significant loss of Raman signals and a relative increase of the defect-induced D band are found, in particular for metallic SWNTs. We attribute these and other irreversible effects in the Raman spectra to defects generated in nanotubes under compression ͑most likely via chemical processes͒. Comparable structural deterioration of semiconducting nanotubes is evidenced by strong irreversible changes in their PL spectra.

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New processing developments in the Cu(In,Ga)Se 2 (CIGS)-based solar cell technology have enabled best cell efficiencies to exceed 21%. The key innovation involves the alkali postdeposition treatment (PDT) of the CIGS film. Furthermore, the range of optimal CIGS growth parameters and the minimal thickness of the CdS buffer layer is affected by the process modifications. In 2013, we reported a 20.8% record device with PDT. Later optimizations, e.g., in the composition profile and CdS buffer layer thickness as discussed in this study, enabled us to increase the photocurrent density with only a slight loss in open-circuit voltage and unchanged fill factor, resulting in the current world record of 21.7% efficiency. Furthermore, a record efficiency of 21.0% could be achieved with a Cd-free Zn(O,S) buffer layer. This contribution presents measurements, simulations, and a discussion of the photocurrent increase.

Low-energy, dark excitonic states have recently been predicted to lie below the first bright (E11) exciton in semiconducting single-walled carbon nanotubes [Phys. Rev. Lett. 93, 157402 (2004)10.1103/PhysRevLett.93.157402]. Decay into such deep excitonic states is implicated as a mechanism which reduces photoluminescence quantum yields. In this study we report the first direct observation of deep excitons in SWNTs. Photoluminescence (PL) microscopy of suspended semiconducting single-walled carbon nanotubes (SWNTs) reveals weak emission satellites redshifted by approximately 38-45 and approximately 100-130 meV relative to the main E11 PL emission peaks. Similar satellites, redshifted by 95-145 meV depending on nanotube species, were also found in PL measurements of ensembles of SWNTs in water-surfactant dispersions. The relative intensities of these deep exciton emission features depend on the nanotube surroundings.

We have studied the convergence properties of embedded and constrained cluster models of proton transfer in zeolites. We applied density functional theory to describe clusters and ONIOM to perform the embedding. We focused on converging the reaction energy and barrier of the O(1) to O(4) jump in H-Y zeolite as well as vibrational and structural aspects of this jump. We found that using successively larger clusters in vacuo gives convergence of this reaction energy to 14 ± 2 kJ mol(-)(1) and the barrier to 135 ± 5 kJ mol(-)(1) at a cluster size of 5 Å, which contains 11 tetrahedral (Si or Al) atoms. We embedded quantum clusters of various sizes in larger clusters with total radii in the range 7-20 Å, using the universal force field as the lower level of theory in ONIOM. We found convergence to the same values as the constrained clusters, without the use of reactive force fields or periodic boundary conditions in the embedding procedure. For the reaction energy, embedded cluster calculations required smaller clusters than in vacuo calculations, reaching converged reaction energies for quantum systems containing at least 8 tetrahedral atoms. In addition, optimizations on embedded clusters required many fewer cycles, and hence much less CPU time, than did optimizations on comparable constrained clusters.

Photoluminescence spectra of water−surfactant dispersions of semiconducting single-walled carbon nanotubes (SWNTs) show large shifts of interband transition energies upon freezing and cooling the dispersions to 16 K. This is accompanied by an increase of the emission intensities up to ∼10 times in the presence of poly(vinylpyrrolidone). The shifts develop mainly in the temperature interval of ∼100−240 K and are reversible by cycling the temperature. Two groups of nanotubes classified by the value of (n-m) mod 3, where n,m are structure indices, demonstrate opposite shifts, the largest ones from nanotubes with small chiral angles. The experimental data agree well with calculations of Yang et al. [Phys. Rev. B 1999, 60, 13874] for SWNTs under axial compression and indicate that large stresses of up to ∼5 GPa are generated in individual nanotubes by thermal contraction of the ice matrix.

Photoluminescence ͑PL͒ laser microscopy was applied to determine optical transition energies E 11 and E 22 of individual semiconducting single-walled carbon nanotubes ͑SWNTs͒ suspended on top of carbon nanotube "forests," grown by chemical vapor deposition ͑CVD͒ on silicon substrates. A uniform increase of E 11 and E 22 energies by 40-55 and 24-48 meV, respectively, was found for 19 different ͑n , m͒ nanotube species suspended in air or a vacuum-relative to SWNTs in a reference water-surfactant dispersion. CVD-grown SWNTs embedded in paraffin oil and 1-methylnaphthalene show nearly the same PL peak positions as SWNTs in aqueous dispersion, indicating similar dielectric screening of excitons in SWNTs in these media.