Towards poly-crystalline silicon thin film solar cells : on the microstructure and the crystallization kinetics of hydrogenated amorphous silicon

Abstract

Amongst the thin-film based approaches for photovoltaics, which aim to combine high conversion efficiencies (> 10%) and low cost manufacturing (<1$/Wp), poly-crystalline silicon (poly-Si) based solar cells (<10 µm thick) are nowadays considered a promising candidate. Poly-Si couples the high quality crystalline Si technology with the potential for large area and low-cost thin film manufacturing. One of the most followed approaches is the development of poly- Si on inexpensive (e.g. glass) substrates upon solid-phase crystallization (SPC), i.e. an annealing procedure under a controlled temperature ramp up to 600-650 °C, of plasma- deposited amorphous silicon (a-Si:H) films. Here the challenge is the development of large (in the range of few µm) grain poly-Si and large area, high growth rate a-Si:H films have generated in the recent years several (fundamental) research questions. Among these, the impact of the a-Si:H microstructure on the crystallization kinetics (incubation-nucleation-grain development) represents the key towards large grain poly-Si and process up-scaling. The approach chosen in this Ph.D. project initiates from the development of state-of-the art poly-Si layers characterized by grain sizes developing up to a 1 µm in diameter, as obtained upon SPC of expanding thermal plasma deposited a-Si:H layers [1]. Furthermore, a selected experimental approach based on an extensive use of in situ and ex situ surface and bulk film diagnostics is applied with the purpose of disclosing the impact of the a-Si:H film properties on the crystallization kinetics. In particular, a-Si:H bulk parameters such as the hydrogen content and microstructure parameters (hydrogen distribution in several SiHx configurations, medium range order) are studied in depth [2-3]. In detail, larger grains are found to be promoted by an increase in the a-Si:H microstructure parameter R* [2]. R* represents the order (low R*)/disorder (high R*) in the matrix according to the Si-H bond distribution in mono-/di-vacancies (–low stretching mode-LSM) and nano-sized voids (–high stretching mode-HSM), and it is quantified by the integrated IR absorption band ratio IHSM/ (ILSM+IHSM). Furthermore, the incubation time towards nuclei formation appears to be not only controlled by the medium range order in the a-Si:H films, as previously reported in literature, but also by the density of nano-sized voids which undergo a faster hydrogen out-diffusion and chemical bond rearrangement towards a higher medium range order and more ordered microstructure. Next to the insight into the impact of the a-Si:H properties on the crystallization kinetics, an in-depth study is also devoted to the control of the a-Si:H properties by means of a proper tuning of film growth- related parameters such as the ion flux and energy involved during the plasma deposition of thin a-Si:H layers [4]. In view of the manufacturing challenges of high throughput, the expanding thermal plasma approach is further explored [5] for the deposition of high growth rate a-Si:H films, in the range of 11- 58 nm/s. Poly-Si layers characterized by large grains (~1.5 µm) were obtained from disordered a-Si:H films deposited at 11- 25 nm/s. The study confirmed the role of the medium range order and the R* parameter in affecting the crystallization kinetics of a-Si:H, i.e. the incubation time and the grain size development, respectively. The present thesis addresses the results of the research efforts towards high conversion efficiency, high throughput thin-?lm poly-Si solar cells. It addresses a significant contribution to the insight on the SPC kinetics, the impact of a-Si:H microstructure on the incubation step and grain development and demonstrates the potential for large grain (~1.5 µm), high growth rate (~25 nm/s) poly-Si from SPC of ETP deposited a-Si:H films.