Amorphe Siliziumoxidschichten zur Oberflächenpassivierung und Kontaktierung von Heterostruktur-Solarzellen aus amorphem und kristallinem Silizium

Atomic hydrogen plays a dominant role in the passivation of crystalline silicon surfaces by layers of amorphous silicon. In order to research into this role, this thesis presents the method of hydrogen effusion from thin amorphous films of silicon (a-Si:H) and silicon oxide (a-SiOx:H). The oxygen concentration of the sub-stoichiometric a-SiOx:H films ranges up to 10 at.-%. The effusion experiment yields information about the content and thermal stability of hydrogen and about the microstructure of the films. A mathematical description of the diffusion process of atomic hydrogen yields an analytical expression of the effusion rate depending on the linearly increasing temperature in the experiment. Fitting of the calculated effusion rates to measured effusion spectra yields the diffusion coefficient of atomic hydrogen in a-SiOx:H. With increasing oxygen concentration, the diffusion coefficient of hydrogen in the a-SiOx:H films decreases. This is attributed to an increasing Si-H bond energy due to back bonded oxygen, resulting in a higher stability of hydrogen in the films. This result is confirmed by an increasing thermal stability of the p-type c-Si passivation with a-SiOx:H of increasing oxygen concentrations up to 5 at.-%. The passivation reaches very low recombination velocities of below 10 cm/s at the interface. However, for higher oxygen concentrations up to 10 at.-%, the passivation quality decreases significantly. Here, infrared spectroscopy of Si-H vibrational modes and hydrogen effusion show an increase of hydrogen-rich interconnected voids in the films. This microstructure results in a high amount of molecular hydrogen (H2) in the layers, which is not suitable for the saturation of c-Si interface defects. Annealing of the films at temperatures around 400°C leads to a release of H2 from the voids, as a result of which Si-Si bonds in the material reconstruct. Subsequently, hydrogen migration in the films is dominated by the diffusion of atomic hydrogen being suitable for the saturation of interface defects. This shows in a distinct increase of the passivation quality of annealed films. The heterostructure back contact for solar cells on p-type c-Si consists of an undoped, full area passivation film, followed by a boron doped p-type contact layer. If highly doped, this layer generates a back surface field effect which decreases the recombination by reflecting minority charge carriers from the defect rich interface. An undoped passivation layer of a-Si:H improves the passivation by saturating interface defects. Effusion measurements on stacks of undoped and doped layers show an increase of the hydrogen diffusion in the undoped layer due to the presence of the doped layer. The doped layer shifts the Fermi level of the undoped layer towards the valence band edge. This shift increases the diffusion coefficient of hydrogen due to an effective reduction of the Si-H bond energy. As a consequence, the presence of the doped contact layer increases the mobility of atomic hydrogen in the passivation layer, with atomic hydrogen being necessary for the passivation of interface defects. The thickness of the passivation layer critically affects both the passivation and the electrical conductance of the back contact. Whereas the passivation quality steadily improves with increasing thickness, the conductance drops when exceeding a critical thickness in a range of 4 nm to 8 nm. Temperature dependent measurements yield a thermal activation of the conductance with an activation energy of approximately 0.5 eV, which is attributed to the valence band offset between a-Si:H and c-Si. This high barrier requires a tunneling process for majority charge carriers and explains the abrupt decrease of conductance with increasing passivation layer thickness. Passivation layers of a-SiOx:H form a blocking junction on p-type c-Si, which is explained by an oxygen induced n-type conduction of a-SiOx:H. Therefore, full area contacts require local through-connections of the a-SiOx:H layers. However, these films offer themselves as passivating interlayers at the solar cell front side between p-type base and n-type emitter. Symmetrical test structures on p-type c-Si allow for studying two-stage emitter structures consisting of undoped and phosphorous doped a-SiOx:H layers. Low and thermally stable emitter saturation current densities result, allowing for high open circuit voltages of above 700 mV in solar cells. However, the efficiency of solar cells with these emitter structures is limited by a high contact resistance between n-type a-SiOx:H and ZnO:Al at the front side as well as a non-optimized back contact.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2010

Author dc:creator

• Einsele, Florian

Contributors dc:contributor

• Rau, Uwe

• info:eu-repo/classification/ddc/620
• Heterosolarzelle
• Photovoltaik
• Passivierung
• Effusion <Physik>
• Silicium
• Amorpher Halbleiter
• Siliciumoxide
• Ingenieurwissenschaften
• heterostructure solar cell
• passivation
• effusion
• amorphous silicon
• silicon oxide

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

ger

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-113916%22

OAI record oai:identifier

https://publications.rwth-aachen.de/oai2d?verb=GetRecord&metadataPrefix=oai_dc&identifier=oai%3Apublications.rwth-aachen.de%3A51644