Low Temperature Thin-Film Silicon Solar Cells on Flexible Plastic Substrates

Providing energy for a steadily increasing world population is one of the major tasks of our time. Solar energy has the ability to satisfy this demand in a clean, sustainable and environmentally friendly manner. Though solar module prices have fallen considerably within the last years, costs need to be further reduced to achieve comprehensive gridparity. A possible approach for reduced module costs is provided by thin-film technologies and the use of low cost polymer substrates. In addition, this research eld offers the opportunity to provide exible and lightweight solar modules, gaining increasing attention for applications in building integrated photovoltaics, exible electronics and mobile power applications. Roll-to-roll manufacturing in turn provides an additional tool for possible cost reductions by high-throughput production. The use of low cost transparent plastic films enables to produce solar cells in the socalled superstrate concept, where sunlight enters through the substrate. However, these types of substrates limit the applicable process temperature range. The aim of this thesis is to investigate the in uence of low deposition temperatures (120 $^{\circ}$C) on the properties of functional layers and to establish a link between material properties and the performance of thin-film silicon solar cells on plastic substrates. This work demonstrates that the deterioration in electrical properties of amorphous silicon (a-Si:H) layers, due to a lower deposition temperature, can be compensated by careful adjustment of deposition gas ow mixtures, resulting in an efficiency of 9.1% for an a-Si:H solar cell on glass substrate. Microcrystalline silicon ($\mu$c-Si:H) layers are less sensitive to a reduction in deposition temperature and by implementation in an a-Si:H/$\mu$c-Si:H tandem solar cell, an efficiency of 9.8% was achieved with great potential for future improvement. The low temperature a-Si:H solar cells exhibit a strong improvement in all photovoltaic parameters, particularly in the fill factor, after post-deposition annealing at 120 $^{\circ}$C. Extensive studies were carried out to understand the underlying physical processes and to link the changes in individual layers upon annealing to changes in solar cell performance. Changes in layer properties were investigated as a function of annealing time and consequent in uence on the solar cell performance was analyzed. Measurements of external quantum efficiencies of p- as well as n-side illuminated solar cells in addition to variable intensity measurements revealed a strong positive effect of the post-deposition annealing on the charge carrier collection efficiency. Possible contributions from the $\mu \tau$-products of electrons and holes in the intrinsic absorber layer, as well as the built-in field in the solar cell were analyzed and discussed, and supported by computer simulations. Annealing effects present in a-Si:H solar cells on PET substrates and $\mu$c-Si:H solar cells on glass substrates were treated as well