Adnan Hammud

Bio: Adnan Hammud is an academic researcher from University of Konstanz. The author has contributed to research in topics: Thin film & Amorphous silicon. The author has an hindex of 4, co-authored 5 publications receiving 38 citations.

Nanocrystal Formation in Silicon Oxy-Nitride Films for Photovoltaic Applications : Optical and Electrical Properties

Abstract: Thin films of nanocrystalline SiOxNy are studied in view of their application in silicon heterojunction (SHJ) solar cells. In particular, the formation of the nanocrystals and their effects on the electrical and optical properties of the films are investigated. The role of the oxygen content on the properties of the layers is clarified as well. The obtained layers show very high conductivity (44 S/cm), low activation energy (1.85 meV) and high Tauc gap (2.5 eV), promising features for their application in photovoltaics.

Crystalline Nature of Metal Spikes and Silicon Inclusions in Ag/Al Screen-Printing Metallization

Abstract: For contacting boron emitters by screen-printing metal pastes, up to now, it has been necessary to add a small amount of Al to the Ag paste to facilitate a reasonable contact resistivity. With the addition of Al to the Ag paste, deep Ag/Al spikes appear, which can be deep enough to penetrate the emitter and, therefore, affect the emitter and space charge region, and, finally, affect the performance of the solar cell. In this paper, a transmission electron microscopy (TEM) analysis of these Ag/Al spikes is presented. The crystalline nature of the Ag/Al spikes is revealed for different surface structures of the crystalline Si wafer and different Al contents in the screen-printing paste. This result is confirmed by X-ray diffraction measurements of etched-back contacts. Additionally, TEM energy-dispersive X-ray spectroscopy facilitates the examination of the Si-rich inclusions found in the Ag/Al spikes. They prove to be multicrystalline Si precipitates with at least 99 at% Si. The observations help to understand the contact formation process of Al containing Ag screen-printing pastes and support the previously presented model.

Underdense a-Si:H film capped by a dense film as the passivation layer of a silicon heterojunction solar cell

Abstract: Underdense hydrogenated amorphous silicon (a-Si:H) prepared by plasma-enhanced chemical vapor deposition was used as a passivation layer in silicon heterojunction (SHJ) solar cells. By reducing the thickness of the underdense a-Si:H passivation layer from 15 nm to 5 nm, the open circuit voltage (Voc) of the corresponding SHJ solar cell increased significantly from 724.3 mV to 738.6 mV. For comparison, a widely used transition-zone a-Si:H passivation layer was also examined, but reducing its thickness from 15 nm to 5 nm resulted in a continuous Voc reduction, from 724.1 mV to 704.3 mV. The highest efficiency was achieved using a 5-nm-thick underdense a-Si:H passivation layer. We propose that this advantageous property of underdense a-Si:H reflects its microstructural characteristics. While the porosity of a-Si:H layer enables H penetration into the amorphous network and the a-Si:H/c-Si interface, a high degree of disorder inhibits the formation of the epitaxial layer at the a-Si:H/c-Si interface during pos...

Contacting boron emitters on n-type silicon solar cells with aluminium-free silver screen-printing pastes

Abstract: In the production of n-type Si solar cells, B diffusion is commonly applied to form the p+ emitter. Up to now, Ag screen-printing pastes, generally used to contact P emitters, had been incapable of reliably contact B emitters. Therefore, a small amount of Al is generally added to Ag pastes to allow for reasonable contact resistances. The addition of Al, however, results in deep metal spikes growing into the Si surface that can penetrate the emitter. Losses in open-circuit voltage are attributed to these deep metal spikes. In this investigation we demonstrate, that state-of-the-art Al-free Ag screen-printing pastes are capable to contact BBr3-based B emitters covered with different dielectric layers and reach specific contact resistances <1 mΩ cm2. Bifacial n-type solar cells with Al-free Ag pastes on both sides show efficiencies of up to 18.3% and series resistances <0.5 Ω cm2. (© 2016 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

Improving the oxygen barrier properties of PET polymer by radio frequency plasma-polymerized SiOxNy thin film

Abstract: Plasma-polymerized silicon oxynitride (SiOxNy) oxygen barrier thin film was deposited on polyethylene terephthalate substrate by plasma enhanced chemical vapor deposition at low temperature. The deposition reactor utilizes capacitively coupled plasma operating at radio frequency (13.56 MHz). Nitrogen incorporation during the polymerization leads to a SiOxNy dense film with low defects which assists to improve the oxygen barrier properties. The gas mixture of tetraethyl orthosilicate as organosilicon precursor with oxygen and nitrogen gases was used to deposit the transparent polymerized SiOxNy thin films. The effects of nitrogen flow rate on deposition rate, refractive index, surface morphology, surface wettability, chemical structure and binding composition and oxygen permeability of the barrier films were investigated. Moreover, the plasma parameters were monitored by optical emission spectroscopy. SiOxNy oxygen barrier films showed 89–91% transparency. Under the optimal deposition conditions the minimum oxygen permeability of 0.08cm3/cm2day bar was obtained.

Structural Investigation of Printed Ag/Al Contacts on Silicon and Numerical Modeling of Their Contact Recombination

Abstract: Ag/Al pastes allow for a sufficiently low contact resistivity of less than 5 mΩ cm2 with boron-doped p+ emitters. A drawback of those pastes is an enlarged recombination at the silicon/metal interface below those contacts, compared with Ag pastes. For previous Ag/Al pastes from 2013, the observed recombination is even higher than theoretically expected for a fully metal-covered surface. Newly developed Ag/Al pastes allow for a significant reduction of the recombination below the contact, compared with a 2013 Ag/Al paste; for example, the $J_{\rm{0e,met}}$ of an $\mathrm{92 \Omega / \text{sq}}$ . p+ emitter has decreased from 3420 down to 1014 fA/cm2 due to the newly developed paste. For an $R_{\rm{sheet}}$ of 137 Ω/sq, the $J_{\rm{0e,met}}$ is 1399 fA/cm2 . Structural investigations of those contacts reveal the microscopic appearance of the contacted region. There are contact spikes of metal grown into the silicon. Those spikes cover 1–1.2% of the entire printed finger area. With values for area fraction and depth of the spikes, we conduct simulations of $J_{\rm{0e,met}}$ . With these simulations, we are able to explain the enlarged recombination at the contact interface and describe the experimentally measured $J_{\rm{0e,met}}$ for both Ag/Al pastes described in this paper.