Single and multi-junction thin film silicon solar cells for flexible photovoltaics

This thesis investigates amorphous (a-Si:H) and microcrystalline (μc-Si:H) solar cells deposited by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) in the n-i-p or substrate configuration. It focuses on processes that allow the use of non transparent and flexible substrates such as plastic foil with Tg < 180°C like poly-ethylene-naphtalate (PEN). In the first part of the work, we concentrate on the light trapping properties of a variety of device configurations. One original test structure consists of n-i-p solar cells deposited directly on glass covered with low pressure chemical vapour deposition (LP-CVD) ZnO. For this device, silver is deposited below the LP-CVD ZnO or white paint is applied at the back of the substrate as back reflector. This avoids the parasitic plasmonic absorptions in the back reflectors, which are observed for conventional rough metallic back contacts. Furthermore, the size and morphology of the LP-CVD ZnO is varied and the relation between the substrate morphology and the short circuit current density (Jsc) is experimentally explored. As a result, the Jsc can be increased by 23% for a-Si:H and 28% for μc-Si:H solar cells compared to the case of flat substrate and the role of the size and shape can be clearly separated. We also explore the optical behavior of single and multi-junction devices prepared with different back and front contacts. The back contact consists either of a 2D periodic grid with moderate slope, or of LP-CVD ZnO with random pyramids of various sizes. The front contacts are either a 70 nm thick, nominally flat ITO or a rough 2 microns thick LP-CVD ZnO. We observe that, for a-Si:H, the cell performance is critically dependent on the combination of thin flat or thick rough front TCOs and the back contact. Indeed, for a-Si:H, a thick LP-CVD ZnO front contact provides more light trapping on the 2D periodic substrate. The Jsc relatively increases by 7 % with LP-CVD ZnO compared to ITO. Then, we study the influence of the thick and thin TCOs in conjunction with thick absorbers like triple junction or mc-Si:H solar cells. Because of the different nature of the optical systems, thick (> 1 micron) against thin (