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 e

Akzeptorkomplexe und Oberflächen des ZnO Atomare und elektronische Struktur

Zinkoxid ist ein transparenter Halbleiter mit einer ausgeprägten Dotierasymmetrie. Während reines Zinkoxid bereits Elektronenleitung zeigt, ist eine Akzeptordotierung zur Erzeugung von Defektelektronen ungleich schwerer zu realisieren. Das vorliegende Buch behandelt die grundlegenden Ursachen für dieses Ungleichgewicht die Passivierung potentieller Akzeptoren durch Bildung von Komplexen und die Erzeugung kompensierender Donatoren an den polaren Oberflächen des Zinkoxid

Transition metal acceptor complexes in zinc oxide

The incorporation of hydrogen shallow donors gives rise to Mn2 fine and hyperfine lines in the electron paramagnetic resonance of ZnO. This cannot be explained by recharging of isolated Mn atoms. First principles density functional calculations reveal that transition metals readily form complexes with acceptors by a charge transfer process or by bond formation. This mechanism has implications on both complex partners. Acceptors are neutralized and uncommon charge states of the transition metals are stabilized. Implications for dilute magnetic ZnO are discusse

Phase diagrams of polar surface reconstructions of zinc oxide

Due to the formation of surface dipoles the truncated bulk like 0001 surfaces of zinc oxide ZnO are electrostatically unstable. These dipoles can neither be compensated by the formation of surface dimers nor by a relaxation of near surface layers. Hence, the driving force for the reconstruction of ZnO must be a deviation of the surface stoichiometry. To study this stabilization effect, we have performed density functional theory calculations on 2 2 and 3 3 periodic non stoichiometric reconstructions of the ZnO polar surfaces. Among the 24 possible 2 2 periodic structures that preserve bulk symmetry, a class of ad atom reconstructions shows the lowest energy of formation. To facilitate comparability with experimental conditions we calculate the Gibbs free energy of each reconstruction as a function of temperature and chemical conditions. However, the established calculation method is ambiguous in case of non stoichiometric surfaces and must be extended by taking into account the temperature dependence of the ZnO enthalpy of formation. From the resulting data phase diagrams of the stable reconstructions of ZnO polar surfaces are constructed which are discussed in terms of epitaxial growt

Defects in Compound Semiconductors Caused by Molecular Nitrogen

The interaction of nitrogen molecules N2 with the host lattice of compound semiconductors is investigated using first principles density functional calculations. In ZnO it is found that N2 causes localized states in the band gap either by forming an N2O molecule or by breaking a Zn O bond. This mechanism contributes to the observed low nitrogen doping efficiency in ZnO. The appearance of localized states caused by N2 was also found in other semiconductors such as MgO and NaC

Properties of nitrogen molecules in ZnO

For ZnO the use of nitrogen has been suggested to achieve p type doping. Unfortunately, the doping efficiency is rather low. One cause can be the formation of N2 molecules that are widely believed to be inert. Here, we show that molecular nitrogen can cause defects in the ZnO lattice with localized states in the band gap either by forming an N2O molecule or by breaking a Zn O bond without forming a chemical bond with the host lattice. Interestingly, bond breaking is energetically more favorable. This hitherto unexpected behavior of nitrogen molecules is also observed in other compound semiconductor

Interstitial zinc clusters in zinc oxide

Doped zinc oxide ZnO exhibits anomalous Raman modes in the range of 270 to 870 cm 1. Commonly, the resonance at 275 cm 1 is attributed to the local vibration of Zn atoms in the vicinity of extrinsic dopants. We revisit this assignment by investigating the influence of isotopically purified zinc oxide thin films on the frequency of the vibrational mode around 275 cm 1. For this purpose undoped and nitrogen doped ZnO thin films with Zn isotope compositions of natural Zn, 64Zn, 68Zn, and a 1 1 mixture of 64Zn and 68Zn were grown by pulsed laser deposition. The isotopic shift and the line shape of the Raman resonance around 275 cm 1 are analyzed in terms of three different microscopic models, which involve the vibration of i interstitial zinc atoms bound to extrinsic defects, ii interstitial diatomic Zn molecules, and iii interstitial zinc clusters. The energy diagram of interstitial Zn Zn bonds in a ZnO matrix is derived from density functional theory calculations. The interstitial Zn Zn bond is stabilized by transferring electrons from the anti bonding orbital into the ZnO conduction band. This mechanism facilitates the formation of interstitial Zn clusters and fosters the common n type doping asymmetry of Zn

ESR investigations on hydrogen induced hyperfine splitting features in ZnO

Low temperature X band electron spin resonance measurements ESR were performed on as grown and post hydrogenated zinc oxide ZnO single crystals. As grown ZnO exhibits a single strong line at g 1.957 that was identified as hydrogen. In contrast, post hydrogenated ZnO shows a distinctly different ESR spectrum. Besides the intensification of the hydrogen donor line, additional hyperfine lines become observable. They are attributed to manganese Mn impurities probably introduced from the growth process. A Mn concentration of 6x10 13cm3 was estimate

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 amp; 8776;1012cm 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 discusse