Marc A. Gluba

Perovskite Solar Cells with Large-Area CVD-Graphene for Tandem Solar Cells

Perovskite solar cells with transparent contacts may be used to compensate for thermalization losses of silicon solar cells in tandem devices. This offers a way to outreach stagnating efficiencies. However, perovskite top cells in tandem structures require contact layers with high electrical conductivity and optimal transparency. We address this challenge by implementing large-area graphene grown by chemical vapor deposition as a highly transparent electrode in perovskite solar cells, leading to identical charge collection efficiencies. Electrical performance of solar cells with a graphene-based contact reached those of solar cells with standard gold contacts. The optical transmission by far exceeds that of reference devices and amounts to 64.3% below the perovskite band gap. Finally, we demonstrate a four-terminal tandem device combining a high band gap graphene-contacted perovskite top solar cell (Eg = 1.6 eV) with an amorphous/crystalline silicon bottom solar cell (Eg = 1.12 eV).

Identification of nitrogen and zinc related vibrational modes in ZnO

Zinc oxide films with natural zinc and isotopically pure Z68n were grown by pulsed laser deposition on sapphire substrates. Prior to and after ion implantation with N2+ the samples were characterized with Raman spectroscopy. After implantation the well-known N-related vibrational modes at 273.9 and 509.5 cm−1 are observed. In the isotopically pure Z68nO samples the vibrational modes exhibit a shift of 5.4 and 1.6 cm−1 to smaller wave numbers. As a result of the experimental data the vibrational modes at 273.9 and 509.5 cm−1 are attributed to a ZnI–NO and ZnI–OI complex, respectively. This is consistent with ab initio calculations based on density functional theory.

Kinetic properties of TiN thin films prepared by reactive magnetron sputtering

Results of investigations of kinetic properties of TiN thin films prepared by dc reactive magnetron sputtering are presented. It is established that the TiN thin films are polycrystalline and possess semiconductor n-type conduction. The carrier concentration is ∼1022 cm−3, while electron scattering occurs at ionized titanium atoms.

Strain relaxation in graphene grown by chemical vapor deposition

The growth of single layer graphene by chemical vapor deposition on polycrystalline Cu substrates induces large internal biaxial compressive strain due to thermal expansion mismatch. Raman backscattering spectroscopy and atomic force microscopy were used to study the strain relaxation during and after the transfer process from Cu foil to SiO2. Interestingly, the growth of graphene results in a pronounced ripple structure on the Cu substrate that is indicative of strain relaxation of about 0.76% during the cooling from the growth temperature. Removing graphene from the Cu substrates and transferring it to SiO2 results in a shift of the 2D phonon line by 27 cm−1 to lower frequencies. This translates into additional strain relaxation. The influence of the processing steps, used etching solution and solvents on strain, is investigated.

Embedded graphene for large-area silicon-based devices

Macroscopic graphene films buried below amorphous and crystalline silicon capping layers are studied by Raman backscattering spectroscopy and Hall-effect measurements. The graphene films are grown by chemical vapor deposition on copper foil and transferred to glass substrates. Uncapped films possess charge-carrier mobilities of 2030 cm2/Vs at hole concentrations of 3.6 × 1012 cm−2. Graphene withstands the deposition and subsequent crystallization of silicon capping layers. However, the crystallinity of the silicon cap has large influence on the field-induced doping of graphene. Temperature dependent Hall-effect measurements reveal that the mobility of embedded graphene is limited by charged-impurity and phonon-assisted scattering.

Improved passivation of the ZnO/Si interface by pulsed laser deposition

Zinc oxide thin-films were grown on crystalline silicon employing magnetron sputtering and pulsed laser deposition. Bulk and interface properties were investigated using scanning electron microscopy, Raman backscattering, photoluminescence, and infrared spectroscopic ellipsometry. Sputter deposited ZnO samples reveal a large degree of disorder and an interface defect density of ≈1012 cm−2. A significant improvement of the structural quality is observed in samples grown by pulsed laser deposition. The bulk defect density is further reduced, when introducing monatomic oxygen during deposition. Simultaneously, the defect density at the ZnO/Si interface decreases by about a factor of five. Implications for devices containing ZnO/Si interfaces are discussed.

Polarity driven morphology of zinc oxide nanostructures

Catalyst-free ZnO nanostructures were grown by pulsed-laser deposition on c-oriented sapphire. The nanostructure morphology can be controlled by introducing a nucleation layer. Depending on the doping concentration in the nucleation layer two distinct types of nanostructures are observed. On intrinsic nucleation layers nanowires form, while Al-doping results in a honeycomb network of nanoscale walls. From scanning-electron microscopy and X-ray photoelectron spectroscopy a correlation between nanostructure morphology and the polarity of the nucleation layers is derived.

Polarity of pulsed laser deposited ZnO nanostructures

ZnO nanostructures were grown by pulsed laser deposition on planar ZnO with different surface polarities. While for planar layers of pulsed laser deposited ZnO polarity control is feasible, the polarity relation of ZnO nanostructures to their substrate layers is not yet investigated. Depending on the polarity of the nucleation layer, two distinct morphologies were found, namely, nanorods on O-polar and nanowalls on Zn-polar ZnO. Convergent beam electron diffraction was performed to reveal the polarity of the ZnO nanostructures. The evolution of ZnO nanostructures is described in terms of a growth rate and a surface diffusion model.

Fine Art of Thermoelectricity

A detailed study of hitherto unknown electrical and thermoelectric properties of graphite pencil traces on paper was carried out by measuring the Hall and Seebeck effects. We show that the combination of pencil-drawn graphite and brush-painted poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) films on regular office paper results in extremely simple, low-cost, and environmentally friendly thermoelectric power generators with promising output characteristics at low-temperature gradients. The working characteristics can be improved even further by incorporating n-type InSe flakes. The combination of pencil-drawn n-InSe:graphite nanocomposites and brush-painted PEDOT:PSS increases the power output by 1 order of magnitude.