Doped nanocrystalline silicon oxide for use as (intermediate) reflecting layers in thin-film silicon solar cells

In summary, this thesis shows the development and nanostructure analysis of doped silicon oxide layers. These layers are applied in thin-film silicon single and double junction solar cells. Concepts of intermediate reflectors (IR), consisting of silicon and/or zinc oxide, are applied in tandem cells. Multi-stack Bragg reflector IRs are tested in tandem cells, increasing the top cell current output. Finally, mechanical polishing is applied on intermediate reflectors, creating asymmetrically textured IRs. Doped silicon oxide layers have proven their versatility as multipurpose layers in thin-film silicon solar cells. In chapter 3, the search for device grade n- and p-doped silicon oxide material is described. The nanostructure of silicon oxide films with a wide array of optical and electrical properties is studied in detail by TEM, Raman, FTIR and XPS. Silicon oxide is found to be a very heterogeneous material with complex nanostructure. Both the amorphous and crystalline phases of silicon oxide are studied in detail. Differences are found between the p- and n-doped materials. It is found that the n-doped material has a nanostructure of silicon crystal grains embedded in an amorphous silicon oxide matrix. The p-doped material, however, contains silicon filaments in an amorphous silicon oxide matrix. These filaments are of intrinsic amorphous silicon with crystalline silicon grains. Intrinsic amorphous silicon is mainly responsible for good conductivity in both n-doped and p-doped silicon oxide, however, minimum crystalline content is also required. Finally, the relations between each phase and element content is related to optical and electrical properties. N-doped silicon oxide used as a back reflector in single junction solar cells reflects unabsorbed light back into the absorber layer, increasing its current output. The blue part of the spectrum is absorbed in one pass, therefore the response in the red part of the spectrum is expected to increase. However, an increase in the blue part of the spectrum is observed as well and is the topic of chapter 4. This increase is attributed to a combination of factors, but mostly to the prevention of a native oxide formation on the standard a-Si:H n-layer. The standard n-layer is covered with the n-doped silicon oxide layer which prevents the standard layer from oxidizing in ambient air. The silicon oxide also provides a better contact interface with silver. Other factors increasing the blue response include: 1. The lower activation energy of n-doped silicon oxide in comparison with the standard a-Si:H n-layer. 2. The changing of the band states due to the larger bandgap of n-doped silicon oxide in reference to n-doped a-Si:H. 3. The thinner a-Si:H n-layer as the one in the reference cell is twice as thick. 4. The lower parasitic plasmonic absorption in the silver back contact due to the common interface with silicon oxide. P-doped silicon oxide exhibits anti-reflective properties, increasing cell current output in the blue part of the spectrum as well. An initial efficiency of 11.4% is achieved with the application of both p- and n-doped silicon oxide layers in a single junction a-Si:H solar cell. Intermediate reflector concepts are explored in chapter 5. Distributed Bragg Reflectors (DBR) have tunable reflective properties and are an interesting candidate for intermediate reflectors in tandem cells. They exhibit nearly the same reflectance range under various angles of incidence. DBRs can be easily designed with the help of optical simulation software such as ASA. The design sequence is as follows: 1. The desired reflectance range inside a solar cell is simulated by varying the thickness of each material. 2. This stack is then simulated in a glass – air environment. 3. The stack is deposited on a glass substrate. 4. The measured reflectance is compared with the air – glass simulation. If a good fit is achieved, the DBR will give the desired simulated reflectance inside the cell. DBRs greatly enhance the top cell current in a tandem cell, reaching up to 13,5 mA/cm2 in a 175 nm-thick a-Si:H layer. Current matching and lowering of Voc remain issues. Texture control in the IR is important in order to provide good light scattering for both the top and bottom cells of a tandem and to provide a good substrate for the growth of a defect-free nanocrystalline absorber layer. An approach to modify the texture of ZnO serving as an asymmetric IR in a tandem cell is developed. Because of the excellent performance of the top amorphous silicon cell deposited on an Asahi VU substrate, it is beneficial to keep this substrate texture for the top cell and integrate different textures (with larger surface features) in the layers processed after the top cell. Two approaches to create an asymmetrically-textured IR are chosen: wet etching and mechanical polishing. The wet etching approach is done with two dilution levels of HCl. Then the IR interface facing the top cell has a typical Asahi VU texture while the IR interface facing the bottom cell has larger surface features beneficial for long-wavelength scattering. The second approach is about applying mechanical polishing to silicon oxide and ZnO IRs. This approach successfully flattened the Asahi-induced texture, leaving it in the IR interface facing the top cell and on the other flat side allowing higher-quality nc-Si:H growth.