Light management by intermediate reflectors in silicon-based tandem solar cells

Multijunction solar cells provide a route to further increase the efficiency of solar cells. By stacking different band-gap materials, thermalization losses can be decreased. Maximizing the efficiency of series-connected multijunction solar cells gets complex as a variety of different layers is involved. The application of nano-scale textures which scatter and diffract the light in order to increase the light path and absorption. An advanced light management combining light trapping and spectral distribution is necessary to gain maximal output. In this work, light management in silicon-based multijunction solar cells by intermediate reflectors (IR) is studied. As soon as the thickness of absorber layers in multijunction devices is physically limited, IRs increase the light path in the sub cells and contribute to the matching of currents and power of the sub cells in series-connected multijunction solar cells. As each element added to a working device increases its complexity, the understanding of their interplay and underlying loss mechanisms is crucial to obtain an improvement of the device. Thin-film silicon tandem solar cells made of hydrogenated amorphous and microcrystalline silicon (also called 'micromorph') are chosen as model system for the analysis of the optics in silicon-based multijunction solar cells, as they are a well established industrially up-scalable technology and exhibit the important characteristics of other multijunction cell material systems: Thin-film layers, nano-structured surfaces, as well as physical limitations of thicknesses and a broad utilization of the solar spectrum.The combination of thin-film layer stack and nano-structured surfaces demands for a treatment of the solar cell as a nano-optical device. Motivated by the results of experimental studies, tandem solar cells are modeled by thin-film optics and rigorous optical simulations. An agreement between simulation to experimental results allows for an investigation and optimization of these devices.First, the optical losses induced by intermediate reflectors in tandem solar cells were studied. Therefore, tandem solar cells without and with micro-crystalline silicon oxide intermediate reflectors were fabricated on different aluminum -doped zinc oxide front contacts which had partially been treated to decrease absorption within the zinc oxide. Increased cell reflectance and parasitic absorption due to the IR were observed. The comparison to tandem solar cells on non-treated front contacts showed that in spite of the increased reflectance, parasitic absorption in the front contacts due to the IR is negligible. The results are supported by optical simulations of tandem solar cells with intermediate reflector which, additionally, allowed to quantify parasitic losses. Furthermore, it is shown that the decrease of the refractive index of the silicon oxide intermediate reflector results in a decrease of parasitic absorption and an increase of total tandem cell current if layer thicknesses are adapted. In this case, parasitic reflection of light by the IR out of the solar cell is determined to be the dominating optical loss mechanism.Second, the reflection losses were assigned to the low spectral selectivity of reflection of single layer IRs. Basing on this, a 1D photonic crystal produced of conductive and low absorptive thin-film materials which are available in-situ in thin-film silicon solar cell production is designed for a flat layer stack. The influence of surface texture on the effectiveness of the multilayer IR is studied and it is shown that the IR functionality remains for a textured solar cell. A multilayer IR consisting of aluminum-doped zinc oxide (ZnO:Al) and microcrystalline silicon oxide (μc-SiOx:H) layers was successfully integrated into a nano-textured thin-film silicon solar cell and decreases the bottom cell losses in comparison to other IRs while still increasing the top cell current density by ΔJsc,top = 1.2mA/cm2. A further optimization of material choice and layer quantity shows that the potential of further increasing top cell enhancement and spectral selectivity by increasing the number of layers exists but that possible gains are low.Third, the top cell current increase was investigated in detail. Therefore, a new method to extract the angular distribution of light within a thin amorphous silicon film from rigorous optical simulations was developed and applied to a-Si:H cells on both, periodic gratings and randomly textured front structures. For photonic structures, absorption in the silicon film is assigned to the coupling of incident light to leaky waveguide modes by the grating. It is shown that in case of silicon films on randomly textured surfaces, these waveguide modes are present as well but broadened due to the superposition of a variety of distinct periods and heights. As a quantity for light trapping in thin films, the integrated scattering intensity $ISI$ could be established which describes the coupling to waveguide modes and correlates well to the absorption enhancement in a-Si:H film with different reflectors.Fourth, absorption and transmission enhancements are investigated on a variety of periodic and randomly textured substrates. In a broad experimental and simulation study, tandem solar cells on various substrates have been deposited and reproduced by rigorous optical simulations. An artificial modification of the substrate texture, revealed the font texture scattering distribution and the surface angles at the IR to be the decisive surface parameters to determine the effectiveness of the IR. A large dependence of the functionality of the IR on the angular intensity distribution in the top cell is observed. In particular, two different reflection modes of the IR are observed: In reflection mode (A) below the critical angle of total internal reflection, light propagates into the IR and the reflectance was dependent on interference of the incident light and light reflected at the back IR interface. The effectiveness of the IR on surface textures comprising low surface angles and low parallel wave vector transfer as APCVD SnO2 or sputter-etched ZnO:Al textures can be well reproduced by a thin-film model applying the angular scattering distribution at the front side. If the critical angle is surpassed, light was totally internally reflected and tunnels through the IR as an evanescent wave. This reflection mode (B) results in large absorption enhancements for thick intermediate layers but is always connected to reflection losses. A spectral reflectance parameter SRP was introduced. It was used as a tool to analyze the influence of IR refractive index and thickness on the absorption enhancement and bottom cell transmission losses. The theory was successfully applied to understand the interplay of front side texture and IR reflectance on the relevant randomly textured surfaces. The study of the IR effectiveness on photonic structures emphasized the importance to examine the waveguide modes, which do not just increase top cell enhancement, but further induce parasitic reflection losses in the NIR range which are disadvantageous to the current of further bottom cells. The analysis reveals that these losses can be decreased by broadening the resonances as it is the case in randomly textured solar cells. Simulations on tandem solar cells on LPCVD ZnO:B reproduce the results of previous experimental studies