Compared to wafer based solar cells, thin film solar cells greatly reduce material cost and thermal budget due to low temperature process. Monolithically manufacturing allows large area fabrication and continuous processing. In this work, several photovoltaic thin films have been developed by rf-PECVD including a-Si:H and μc-Si, both intrinsic and doped on Corning 4 inch glass substrate at low temperature. The conductivity of n type and p type μc-Si at 180ºC was 17S/cm and 7.1E-2S/cm, respectively. B dopants either in a-Si:H or μc-Si films require higher plasma power to get active doping. The B2H6-to-SiH4 flow ratio for p type μc-Si lies from 0.01 to 0.025. Chamber conditions have critical effect on film quality. Repeatable and superior results require a well-established cleaning passivation procedure. Moreover, μc-Si films have been deposited from pure silane on glass substrate by modified rf-ICP-CVD. The deposition rate has been dramatically increased to 5Å/s due to little H2 dilution with crystalline fraction was around 69%, and 6.2Å/s with crystalline fraction 45%. Microstructure started to form at 150ºC with a thin incubation layer on the glass substrate, and became fully dense conical conglomerates around 300nm where conductivity and crystallinity saturated. Additionally, a-SiGe:H films have been developed by modified rf-ICP-CVD. The optical band gaps have been varied from 1.25 to 1.63eV by changing SiH4-to-GeH4 ratio. Also high temperature resulted in low bandgap. Cross-section TEM showed some microcrystllites appeared near interface region. Heterojunction solar cells on p type c-Si wafer have been fabricated using films developed in this thesis. Interference fringes in EQE disappeared on either textured substrate or cells with lift-off contacts. Maximum EQE was 87% around 700nm. I-V curves have also been studied where the interesting kink suggests a counter-diode has formed between emitter region and contacts.

Compared to wafer based solar cells, thin film solar cells greatly reduce material cost and thermal budget due to low temperature process. Monolithically manufacturing allows large area fabrication and continuous processing. In this work, several photovoltaic thin films have been developed by rf-PECVD including a-Si:H and μc-Si, both intrinsic and doped on Corning 4 inch glass substrate at low temperature. The conductivity of n type and p type μc-Si at 180ºC was 17S/cm and 7.1E-2S/cm, respectively. B dopants either in a-Si:H or μc-Si films require higher plasma power to get active doping. The B2H6-to-SiH4 flow ratio for p type μc-Si lies from 0.01 to 0.025. Chamber conditions have critical effect on film quality. Repeatable and superior results require a well-established cleaning passivation procedure. Moreover, μc-Si films have been deposited from pure silane on glass substrate by modified rf-ICP-CVD. The deposition rate has been dramatically increased to 5Å/s due to little H2 dilution with crystalline fraction was around 69%, and 6.2Å/s with crystalline fraction 45%. Microstructure started to form at 150ºC with a thin incubation layer on the glass substrate, and became fully dense conical conglomerates around 300nm where conductivity and crystallinity saturated. Additionally, a-SiGe:H films have been developed by modified rf-ICP-CVD. The optical band gaps have been varied from 1.25 to 1.63eV by changing SiH4-to-GeH4 ratio. Also high temperature resulted in low bandgap. Cross-section TEM showed some microcrystllites appeared near interface region. Heterojunction solar cells on p type c-Si wafer have been fabricated using films developed in this thesis. Interference fringes in EQE disappeared on either textured substrate or cells with lift-off contacts. Maximum EQE was 87% around 700nm. I-V curves have also been studied where the interesting kink suggests a counter-diode has formed between emitter region and contacts.

Mit der vorliegenden Arbeit wird das Wachstum von polykristallinem Silicium auf Glas bei niedrigen Temperaturen aus metallischen Lösungen in einem Zweistufenprozess untersucht. Im ersten Prozessschritt werden nanokristalline Siliziumschichten (nc-Si) hergestellt, entweder durch die direkte Abscheidung auf geheizten Substraten oder durch als ''Amorphous-Liquid-Crystalline''(ALC)-Umwandlung bezeichnete metall-induzierte Kristallisation. Im zweiten Prozessschritt dienen die Saatschichten als Vorlage für das Wachstum von deutlich größeren Kristalliten durch stationäre Lösungszüchtung. Die ALC-Prozessdauer konnte durch umfassende Parameterstudien signifikant reduziert werden. Die Charakterisierung der durch direkte Abscheidung auf geheizten Substraten entstehenden nc-Si Saatschichten offenbarte, dass es sich dabei um individuelle Saatkörner handelt, die in eine quasi-amorphe Matrix eingebettet sind. Die Oxidation der Saatschichten vor dem zweiten Prozessschritt wurde als ein wesentliches Hindernis für das Wachstum identifiziert. Als erfolgreichste Lösung zur Überwindung dieses Problems hat sich ein anfänglicher Rücklöseschritt erwiesen. Da diese Methode jedoch schwierig zu kontrollieren ist, wurde ein UV-Laser-System entwickelt und installiert. Erste Resultate zeigen epitaktisches Wachstum an den Stellen, an denen das Oxid entfernt wurde. Bei der Lösungszüchtung auf ALC-Schichten beginnt das Wachstum an einigen größeren Saatkristallen, von wo aus umliegende Gebiete lateral überwachsen werden. Obwohl Kristallitgrößen bis zu 50 Mikrometern erreicht wurden, war es noch nicht möglich, geschlossene Schichten zu erzielen. Durch Lösungszüchtung auf nc-Si Saatschichten hingegen konnte dieses Ziel erreicht werden. Geschlossene, polykristalline Si-Schichten wurden erzeugt, auf denen alle Si-Kristallite miteinander verbunden sind. Neben den Wachstumsexperimenten wurden 3D-Simulationen durchgeführt, in denen u.a. unterschiedliche Heizerkonfigurationen simuliert wurden. / The subject of this thesis is the investigation of the growth of polycrystalline silicon on glass at low temperatures from metallic solutions in a two-step growth process. In the first process step, nanocrystalline Si (nc-Si) films are formed either by direct deposition on heated substrates, or by a metal-induced crystallization process, referred to as amorphous-liquid-crystalline (ALC) transition. In the second process step, these seed layers serve as templates for the growth of significantly larger Si crystallites by means of steady-state solution growth. Extensive parameter studies for the ALC process helped to bring down the process duration significantly. Characterization of the nc-Si seed layers, formed by direct deposition on heated substrates, showed that the layer is composed of individual seeds, embedded in a quasi-amorphous matrix. The oxidation of the seed layers prior to the second process step was found to be a major obstacle. The most successful solution has been an initial melt-back step. As the process is hard to control, though, a UV laser system has been developed and installed. First promising results show unobstructed epitaxial growth where the oxide has been removed. Steady-state solution growth on ALC seed layers was found to start from a few larger seed crystals, and then cover the surrounding areas by lateral overgrowth. Although crystallites with sizes of up to 50 micrometers were obtained, it was not yet possible to achieve full surface coverage with a continuous layer. By solution growth on nc-Si seed layers, however, it was eventually possible to achieve this goal. Continuous, polycrystalline Si layers were grown, on which all Si crystallites are interlocked. The growth experiments were accompanied by 3D simulations, in which e.g. different heater configurations have been simulated.