N. H. Nickel

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.

Stress in undoped and doped laser crystallized poly-Si

Raman measurements were performed on laser crystallized poly-Si on different substrates. Observed shifts of the Si LO–TO phonon peak are caused by stress originating from the film-substrate interface. The principal cause of the stress is the difference in the thermal expansion coefficients of substrate and film. Consequently, the amount of thermal stress critically depends on the choice of substrate. In the case of undoped samples on quartz, profiler and x-ray measurements confirmed the occurrence of tensile stress in the films. In the case of heavily doped samples, the change of the lattice parameter determined by x-rays is probably to a significant extend responsible for additional Raman shifts.

Radiation Hardness and Self‐Healing of Perovskite Solar Cells

The radiation hardness of CH3 NH3 PbI3 -based solar cells is evaluated from in situ measurements during high-energy proton irradiation. These organic-inorganic perovskites exhibit radiation hardness and withstand proton doses that exceed the damage threshold of crystalline silicon by almost 3 orders of magnitude. Moreover, after termination of the proton irradiation, a self-healing process of the solar cells commences.

Excimer laser crystallization of amorphous silicon on molybdenum coated glass substrates

Hydrogenated amorphous silicon (a-Si:H) films on molybdenum coated glass substrates were crystallized using a XeCl excimer laser. Structural information on the resulting polycrystalline silicon (poly-Si) films was obtained from scanning electron microscopy and electron backscattering diffraction measurements. The average grain size varies with laser fluence. The maximum average grain size in the super lateral growth energy–density range is considerably smaller for poly-Si on Mo coated substrates than for poly-Si on quartz. In addition, the metal layer affects the laser fluence necessary to achieve super lateral growth. Samples crystallized under super lateral growth conditions show a preferential surface orientation along the {111} direction. Intermixing of Mo and silicon is not observed.

Laser-induced self-organization in silicon-germanium thin films

We report on the formation of self-organized structures in thin films of silicon-germanium (Si1−xGex) with 0.3<x<0.7 after exposing the films to laser irradiation. Amorphous SiGe samples that are exposed to a single laser pulse exhibit a ripple structure that changes to a hillock structure when the samples are irradiated with additional laser pulses. The topographic structure is coupled to a periodic compositional variation of the SiGe alloy. The periodicity length of the structure after a single laser pulse is in the range of 0.3–1.1 μm, depending on Ge content, layer thickness, and laser fluence, and rapidly grows with increasing number of laser pulses. In situ conductivity measurements during solidification support the theoretical instability analysis that we have done, based on the Mullins–Sekerka theory, to elucidate the nature of this phenomenon. Moreover, as theoretically predicted, the self-organization phenomenon can be turned off by increasing the solidification velocity.

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.

Electrical transport in laser-crystallized polycrystalline silicon-germanium thin-films

We report on the electrical transport properties of intentionally undoped, laser-crystallized polycrystalline silicon-germanium thin-films. The electrical transport in this material strongly depends on the alloy composition and the crystallization procedure. At low temperatures the undoped germanium-rich samples show an unexpected high p-type conductivity with a weak temperature dependence. Posthydrogenation results in a pronounced decrease in the conductivity and a change in the dominating low temperature transport behavior. The results are discussed in terms of a grain-boundary dominated transport model.

Fabrication and characterization of anisotype heterojunctions n-TiN/p-CdTe

Photosensitive heterojunctions n-TiN/p-CdTe were fabricated for the first time by means of titanium nitride thin film deposition (n-type conductivity) by the reactive magnetron sputtering onto freshly etched single crystal substrates CdTe (1 1 0) of p-type conductivity. The temperature dependences of the height of the potential barrier and series resistance of the n-TiN/p-CdTe heterojunction were investigated. The dominating current transport mechanisms through the heterojunctions under investigation were determined at forward and reverse bias. The heterojunctions under investigation generate open-circuit voltage Voc = 0.35 V, short-circuit current Isc = 1.88 mA см−2 and fill factor FF = 0.51 under illumination 80 mW сm−2.