J. O. Ossó

Anisotropic optical properties of single crystalline PTCDA studied by spectroscopic ellipsometry

We report anisotropic spectra of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) in the optical energy range 1.5–3.7 eV, measured at room temperature by spectroscopic ellipsometry. The results were obtained on single crystals of a-PTCDA grown by temperature gradient sublimation. Spectra were measured at highly symmetric positions of the plane of incidence relative to the sample crystallographic axes and are presented as pseudo-values of refractive index hni and extinction coefficient hki, which reflect a very large anisotropy of these optical functions. The spectroscopic properties of the excited states in the molecular plane (1 0 2) are analyzed; in particular, we identify clear Davydov splittings proving the formation of coherent excitons in the crystal. Out-of-plane, we observe a relatively narrow absorption peak

Controlled molecular alignment in phthalocyanine thin films on stepped sapphire surfaces

We report a detailed study of the growth and structure of thin films of copper hexadecafluorophthalocyanine (F16CuPc) on sapphire. These films show very good out-of-plane order and have X-ray rocking widths of around 0.02°. If prepared under suitable conditions on A-plane sapphire substrates, the molecules align without significant azimuthal dispersion. Growth on MgO (001) and oxidized silicon wafers resulted in a comparable out-of-plane structure, but showed no azimuthal order. We find that the azimuthal alignment on sapphire is induced by the step edges along the c-axis of the sapphire, which serve as templates for the growth. For growth at different substrate temperatures, we find a monotonic change of the molecular out-of-plane tilt angle, as obtained from Raman scattering, which is accompanied by a change of the out-of-plane lattice parameter.

Photoluminescence spectroscopy of pure pentacene, perfluoropentacene, and mixed thin films

We report detailed temperature dependent photoluminescence (PL) spectra of pentacene (PEN), perfluoropentacene (PFP), and PEN:PFP mixed thin films grown on SiO(2). PEN and PFP are particularly suitable for this study, since they are structurally compatible for good intermixing and form a model donor/acceptor system. The PL spectra of PEN are discussed in the context of existing literature and compared to the new findings for PFP. We analyze the optical transitions observed in the spectra of PEN and PFP using time-dependent density functional theory calculations. Importantly, for the mixed PEN:PFP film we observe an optical transition in PL at 1.4 eV providing evidence for coupling effects in the blend. We discuss a possible charge-transfer (CT) and provide a tentative scheme of the optical transitions in the blended films.

Strong optical anisotropies of F16CuPc thin films studied by spectroscopic ellipsometry

We have used spectroscopic ellipsometry to measure the optical functions of F16CuPc (copper-hexadecafluoro-phthalocyanine, C32F16N8Cu) films grown by organic molecular beam deposition simultaneously onto MgO(001), SiO2/Si, and A-plane (1120) sapphire substrates. The latter allows one to produce, under suitable conditions, highly ordered films with interesting anisotropic optical properties. These films are shown to be oriented with two principal axes of the dielectric tensor on the film surface, with one of them almost aligned with the c axis of the substrate. The main spectral features, corresponding to the strong Q- and B-band absorptions of phthalocyanines, are polarized along the other in-plane axis which is perpendicular to c. The Q band contains a particularly sharp excitonic peak near 1.55 eV whose intensity variations point to different molecular stackings in the films.

Self-organization of phthalocyanines on Al2O3 (1120) in aligned and ordered films

We studied the self-organization process of F 16 CuPc films (20–30 ML) on stepped Al 2 O 3 (11 2 0) substrates. X-ray diffraction measurements revealed a highly ordered layered structure with the molecules in a nearly upright configuration. The morphology, investigated by atomic force microscopy, consisted of long (several microns) and narrow (20–100 nm) needlelike terraces unidirectionally aligned along one of the main crystallographic directions of the Al 2 O 3 (11 2 0) surface. High resolution atomic force microscopy images revealed in-plane molecular order with the molecular stacking direction parallel to the needlelike terraces. Such anisotropic morphology is the result of a self-organization process of F 16 CuPc in elongated crystallites driven to a preferential orientation by the interaction with the substrate. Spectroscopic ellipsometry showed that these films exhibit anisotropic optical properties correlated with the molecular arrangement.

Evolution of strain and composition during growth and capping of Ge quantum dots with different morphologies

We follow the growth of islands with different shapes by monitoring the strain relaxation by reflection high energy electron diffraction (RHEED). Comparing a bimodal ensemble of pyramids and domes with a monomodal distribution of C-induced domes, we observe different relaxation pathways and a growth mode change from Stranski‐Krastanow to Volmer‐Weber. We also study the changes induced by the capping process with Si. Small strains in thin cap layers are revealed by spectroscopic ellipsometry. Raman spectroscopy is employed to probe the built-in strain and silicon intermixing in different types of islands, evidencing that smaller islands are enriched in Si and effectively recompressed, whereas bigger relaxed dots remain substantially unaffected. (Some figures in this article are in colour only in the electronic version)

Microscale Characterization of Surface Recombination at the Vicinity of Laser-Processed Regions in c-Si Solar Cells

Laser firing processes have emerged as a technologically feasible approach for the fabrication of local point contacts or local doped regions in advanced high-efficiency crystalline-Si (c-Si) solar cells. In this work, we analyze the local impact induced by the laser pulse on the passivation layers, which are commonly present in advanced c-Si solar cell architectures to reduce surface recombination. We use microphotoluminescence (PL) measurements with a spatial resolution of 7 μm to evaluate the passivation performance at the surroundings of laser-processed regions (LPRs). In particular, we have studied LPRs performed on SiCx/Al2O3- and Al2O3- passivated c-Si wafers by an infrared (1064 nm) laser. Micro-PL results show that passivation quality of c-Si surface is affected up to about 100 μm away from the LPR border and that the extension of this damaged zone is correlated with the laser power and to the presence of capping layers. In the final part of the work, the observed decrease in passivation quality is included in an improved 3-D simulation model that gives accurate information about the recombination velocities associated with the studied LPRs.

SNOM characterization of self-assembled organic nanocrystals

We have investigated the optical properties of self‐assembled diindenoperylene nanocrystals grown epitaxially on a copper hexadecafluorophthalocyanine film by SNOM imaging and near‐field photoluminescence. The coupled SNOM‐spectrometer system allows us to study the optical response of a single nanocrystal beyond the diffraction limit. Our results demonstrate the existence of a diindenoperylene wetting layer connecting the nanocrystals.