Nanostructured back reflectors for improved light absorption in silicon thin film solar cells
Date of Issue2018-11-19
School of Electrical and Electronic Engineering
Research Techno Plaza
The study on photovoltaic is an important topic in the sustainable and clean energy research field, driven by the deteriorating energy crisis and environmental pollutions due to fossil fuel shortage and combustion. Currently, crystalline silicon (c-Si) based solar cell dominates the photovoltaic market. However, its high material cost has weakened its competitiveness against fossil fuels. To reduce the material cost, silicon thin film based solar cell that uses less material is favourable. However, this will result in reduced optical absorption and consequently lower cell efficiency. To address the issue, nanostructures can be incorporated into the thin film solar cell for light trapping and to improve the optical absorption. In this work, we study nanostructured back reflector (BR) for improved light absorption in hydrogenated amorphous silicon thin film solar cells. The nanostructured BRs are patterned and fabricated based on polystyrene sphere (PS) assisted lithography. We adopt “substrate” (n-i-p) structure and highly reflective Ag layer and optical spacer ZnO layer are deposited on glass substrate as the BR materials. The hydrogenated amorphous silicon thin films are grown on top of the BRs by the plasma enhanced chemical vapor deposition (PECVD) technique. To improve the patterning capability and develop a well-controlled fabrication process for the BRs, we study the etching behavior of PS spheres in electron cyclotron resonance (ECR) oxygen plasma. By tuning the oxygen plasma condition including plasma input power, etching mode, shielding of the ion flux with Faraday cage, we successfully isolated the effects of oxygen radical and of energetic ions on the etching behaviors of PS spheres. The reasons behind PS spheres melting and shape deformations are identified, and negative impacts during BR fabrication are avoided. As a result, we developed a well-controlled oxygen plasma etching process for PS spheres using the ECR plasma source. After etching, smaller PS sphere sizes are achieved, which enables us to control the dimensions of the nanostructures formed using the PS assisted lithography. After that, we study light trapping in hydrogenated amorphous silicon thin film solar cells fabricated by the PECVD technique on various nanostructured BRs. The BRs are patterned using different PS sphere sizes without oxygen plasma etching step. We have investigated the correlation between the optical properties of the BRs and the performance of the corresponding fabricated solar cells. We have also introduced a mixture of two different sizes of PS spheres patterned BRs, and have obtained solid experimental evidence of improved light trapping performance of such BRs, as compared with those patterned using single size polystyrene spheres. Overall, we have achieved high performing nanostructured amorphous silicon solar cells with an initial power conversion efficiency of 8.79 %, and over 20 % enhancement of the short-circuit current compared with the reference flat BR solar cell without the nanostructures. Due to the usage of Ag and ZnO material and Ag nanostructures in the BRs for light scattering and absorption enhancement, parasitic losses are also inevitably introduced in the device. In this thesis, we also study in detail through optical modelling, the parasitic losses and light trapping in hydrogenated amorphous silicon thin film solar cells fabricated by the PECVD technique on nanostructured BRs. The BRs are patterned using single PS sphere size with oxygen plasma etching step to reduce and control the dimension of the PS spheres. By using O2 plasma etching of the PS spheres, we fabricated hexagonal nanostructured BRs. With the help of rigorous modeling, we study the parasitic losses in different BRs, in the non-active layers, and the light enhancement effect in the silicon absorber layer. Moreover, the simulation results have been compared and verified with experimental data. We have demonstrated hexagonal nanostructured amorphous silicon thin film solar cells with a power conversion efficiency of 7.7 % and around 34.7 % enhancement of the short-circuit current density, compared with flat amorphous silicon thin film solar cell. In summary, nanostructured silicon thin film solar cells are fabricated, characterized and simulated. The etching behavior of polystyrene (PS) spheres under electron cyclotron resonance (ECR) generated oxygen plasmas are studied. Various nanostructured Ag/ZnO back reflectors (BRs) patterned by single or double PS sphere sizes are fabricated and their optical performance are characterized and correlated with the corresponding solar cell performance. Hexagonal nanostructured Ag/ZnO back reflectors and their corresponding nanostructured silicon thin film solar cells are simulated and verified against experimental results. The parasitic losses in the device are investigated.
DRNTU::Engineering::Electrical and electronic engineering