This title appears in the Scientific Report : 2014 
Coupling Incident Light to Guided Modes in Thin-Film Tandem Solar Cells with Intermediate Reflector
Hoffmann, Andre (Corresponding Author)
Bittkau, Karsten / Zhang, Chao / Merdzhanova, Tsvetelina / Rau, Uwe
Photovoltaik; IEK-5
2014
2014
40th IEEE Photovoltaic Specialist Conference, Denver, CO (USA), 2014-06-08 - 2014-06-13
Conference Presentation
Helmholtz Interdisciplinary Doctoral Training in Energy and Climate Research (HITEC)
Thin Film Photovoltaics


In thin-film silicon tandem solar cells, hydrogenated amorphous (a-Si:H) and microcrystalline (µc-Si:H) silicon solar cells are connected in series. Due to inherent material properties, the thickness of the a-Si:H top cell has to be small. Different light management concepts are applied to increase the top cell photocurrent generation. Typically, textured interfaces as well as intermediate reflectors (IRs) between top and bottom cell are incorporated into the device structure. The design and optimization of the device structure with IR on textured substrates is often done by rigorous optical simulations assuming large calculation domains.We will show in our contribution that such optical simulations provide much more information about the physical mechanism of light trapping inside the tandem solar cells. In particular, we investigate the light coupling to guided optical modes in the a-Si:H top cell by the interplay between the textured interfaces and the chosen type of IR. This study is done by Fourier analysis of the local light intensity distribution inside the absorber layers. This analysis provides information about light intensity as a function of planar wave vector. Guided modes are characterized by their large planar wave vector which forbids light propagation in air.The amount of light intensity in guided modes inside the a-Si:H absorber layer is correlated to the experimentally determined external quantum efficiency in the top cell for different types of IR and different front side textures. Those results show the importance of an efficient light coupling to guided modes in order to achieve high efficiency thin-film solar cells.