Manufacturing and characterization of amorphous silicon alloys passivation layers for silicon hetero-junction solar cells

Nel presente lavoro di tesi magistrale sono stati depositati e caratterizzati film sottili (circa 10 nm) di silicio amorfo idrogenato (a-Si:H), studiando in particolare leghe a basso contenuto di ossigeno e carbonio. Tali layer andranno ad essere implementati come strati di passivazione per wafer di Si monocristallino in celle solari ad eterogiunzione HIT (heterojunctions with intrinsic thin layer), con le quali recentemente è stato raggiunto il record di efficienza pari a 24.7% . La deposizione è avvenuta mediante PECVD (plasma enhanced chemical vapour deposition). Tecniche di spettroscopia ottica, come FT-IR (Fourier transform infrared spectroscopy) e SE (spettroscopic ellipsometry) sono state utilizzate per analizzare le configurazioni di legami eteronucleari (Si-H, Si-O, Si-C) e le proprietà strutturali dei film sottili: un nuovo metodo è stato implementato per calcolare i contenuti atomici di H, O e C da misure ottiche. In tal modo è stato possibile osservare come una bassa incorporazione (< 10%) di ossigeno e carbonio sia sufficiente ad aumentare la porosità ed il grado di disordine a lungo raggio del materiale: relativamente a quest’ultimo aspetto, è stata sviluppata una nuova tecnica per determinare dagli spettri ellisometrici l’energia di Urbach, che esprime la coda esponenziale interna al gap in semiconduttori amorfi e fornisce una stima degli stati elettronici in presenza di disordine reticolare. Nella seconda parte della tesi sono stati sviluppati esperimenti di annealing isocrono, in modo da studiare i processi di cristallizzazione e di effusione dell’idrogeno, correlandoli con la degradazione delle proprietà optoelettroniche. L’analisi dei differenti risultati ottenuti studiando queste particolari leghe (a-SiOx e a-SiCy) ha permesso di concludere che solo con una bassa percentuale di ossigeno o carbonio, i.e. < 3.5 %, è possibile migliorare la risposta termica dello specifico layer, ritardando i fenomeni di degradazione di circa 50°C.

Study of the absorption of organic compounds on biopolymers

This work has been conducted in order to determine the solubility and diffusion coefficients of different aromatic substances in two different grades of polylactic acid (PLA), Amorphous (PDLLA) and Crystalline (PLLA); in particular the focus is on the following terpenes: Linalool, α-Pinene, β-Citronellol and L-Linalool. Moreover, further analyses have been carried out with the aim to verify if the use of neat crystalline PLA, (PLLA), a chiral substrate, may lead to an enantioenrichment of absorbed species in order to use it as membrane in enantioselective processes. The other possible applications of PLA, which has aroused interest in carry out the above-mentioned work, concerns its use in food packaging. Therefore, it is interesting and also very important, to evaluate the barrier properties of PLA, focusing in particular on the transport and absorption of terpenes, by the packaging and, hence, by the PLA. PLA films/slabs of one-millimeter thickness and with square shape, were prepared through the Injection Molding process. On the resulting PLA films heat pretreatment processes of normalizing were then performed to enhance the properties of the material. In order to evaluate solubility and diffusion coefficient of the different penetrating species, the absorption kinetics of various terpenes, in the two different types of PLA, were determined by gravimetric methods. Subsequently, the absorbed liquid was extracted with methanol (MeOH), non- solvent for PLA, and the extract analyzed by the use of High Performance Liquid Chromatography (HPLC), in order to evaluate its possible enantiomeric excess. Moreover, PLA films used were subjected to differential scanning calorimetry (DSC) which allowed to measure the glass transition temperature (Tg) and to determine the degree of crystallinity of the polymer (Xc).

Determination of local density of states in amorphous oxide semiconductors with Kelvin Probe Force Microscopy

Amorphous semiconductors are important materials as they can be deposited by physical deposition techniques on large areas and even on plastic substrates. Therefore, they are crucial for transistors in large active matrices for imaging and transparent wearable electronics. The most widely applied candidate for amorphous thin film transistors production is Indium Gallium Zinc Oxide (IGZO). It is attracting much interest because of its optical transparency, facile processing by sputtering deposition and notable improved charge carrier mobility with respect to hydrogenated amorphous silicon a-Si:H. Degradation of the device and long-term performance issues have been observed if IGZO thin film transistors are subjected to electrical stress, leading to a modification of IGZO channel properties and subthreshold slope. Therefore, it is of great interest to have a reliable and precise method to study the conduction band tail, and the density of states in amorphous semiconductors. The aim of this thesis is to develop a local technique using Kelvin Probe Force Microscopy to study the evolution of IGZO DOS properties. The work is divided into three main parts. First, solutions to the non-linear Poisson-Boltzmann equation of a metal-insulator-semiconductor junction describing the charge accumulation and its relation to DOS properties are elaborated. Second macroscopic techniques such as capacitance voltage (CV) measurements and photocurrent spectroscopy are applied to obtain a non-local estimate of band-tail DOS properties in thin film transistor samples. The third part of my my thesis is dedicated to the KPFM measurements. By fitting the data to the developed numerical model, important parameters describing the amorphous conduction band tail are obtained. The results are in excellent agreement with the macroscopic characterizations. KPFM result is comparable also with non-local optoelectronic characterizations, such as photocurrent spectroscopy.