Lichtstreuende Oberflächen, Schichten und Schichtsysteme zur Verbesserung der Lichteinkopplung in Silizium-Dünnschichtsolarzellen

This doctoral thesis addresses the light yield and the optical loss mechanisms in thin-film solar cells based on amorphous (a-Si:H) and microcrystalline (µc-Si:H) silicon. The study focuses on the optimization of the aluminum-doped zinc oxide (ZnO:Al) front contact, which has to combine low series resistance and high transparency. The two essential premises (high transmission and conductivity) do not completely fulfill the requirements for this front contact. Due to the low absorption of thin (approx. 1 µm) silicon layers especially in the infrared spectral region, light trapping inside the silicon solar cell structure is essential as well. This study employs rf-magnetron sputtered ZnO:Al films from ceramic targets. These films can be surface-textured by wet-chemical etching in diluted hydrochloric acid leading to a particularly efficient light scattering and subsequent light trapping. In a comprehensive material study the ZnO:Al films are characterized electrically, optically and structurally. Three different regimes of film properties are identified in interdependence of two deposition parameters substrate temperature and target doping concentration. Moreover, the influence of vacuum-annealing on ZnO:Al film properties is studied. An investigation of the vacuum-annealing behavior of the reference front contact shows a carrier concentration decrease by a factor of three without any significant change in carrier mobility. Accordingly, the transmission in the long wavelength part of the spectrum is increased. Thus, vacuum-annealed ZnO:Al films function as an experimental model system: a variation of the annealing intensity leads to a gradually increased transmission at the expense of higher conductivity. Vacuum annealing maintains the surface topography and therefore separates the light-scattering ability from the electro-optical properties which are usually both determined by the deposition parameters. By applying the experimental model system in thin-film silicon solar cells, the relationship between carrier concentration of the front contact and cell current density is studied. For application in solar modules an optimized balance between optical and electrical needs is provided at a front-contact carrier concentration of 2.0e-20 cm-3. First a-Si:H/µc-Si:H/µc-Si:H-multi-junction modules with an initial efficiency of 11.1% (aperture area of 64 cm2) confirm the potential for efficiency increase in the order of 5% relative. The material study identifies deposition-parameter pairs that enable optimally balanced electro-optical properties combined with efficient light scattering without additional post-deposition treatment. Using front contacts accordingly, microcrystalline silicon solar cells with cell current densities of up to 25.0 mA/cm2 (intrinsic silicon layer thickness: 1.1 µm) and 26.9 mA/cm2 (1.9 µm) are presented in this theses. These short-circuit cell current densities are among the highest publicized values for silicon thin-film solar cells deposited onto ZnO:Al substrates and demonstrate an increase of more than 1 mA/cm2 compared to the currently used, high-quality reference front contact. Finally, the limitation of light trapping is investigated by experimental results combined with quantum-efficiency calculations based on theoretical models. A detailed analysis shows that a considerable amount of photons is reflected primarily or secondarily and cannot be used due to the low absorption of the thin intrinsic silicon layer assuming Lambertian light scattering within the solar-cell and light-trapping scheme investigated in this study. An optical improvement of the back reflector (introducing a silicon dioxide interlayer) and the application of refractive-index matching (titanium dioxide between front contact and silicon absorber) are examined not only in experiments but also through calculations in respect to the potential for current gain. The results of individually implemented optical improvements help to estimate the effect of combining the approaches and shows that a cell current density of about 30 mA/cm2 is in reach.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2007

Author dc:creator

• Berginski, Michael

Contributors dc:contributor

• Wuttig, Matthias

• info:eu-repo/classification/ddc/530
• Dünnschichtsolarzelle
• Zinkoxid
• Magnetronsputtern
• Photovoltaik
• Silicium
• Ätzen
• Hochfrequenzsputtern
• Sputtern
• Transparent-leitendes Oxid
• Physik
• Lichteinkopplung
• Lighttrapping
• optische Verluste
• Quanteneffizienz
• Zellstromdichte
• Optical-losses
• Quantum-Efficiency
• Cell-Current-Density

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

ger

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-112495%22

OAI record oai:identifier

https://publications.rwth-aachen.de/oai2d?verb=GetRecord&metadataPrefix=oai_dc&identifier=oai%3Apublications.rwth-aachen.de%3A49927