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Title: Optics in perovskites and low-mobility materials based solar cells and color vision in perovskite sensors
Authors: Qarony, Md Wayesh
Degree: Ph.D.
Issue Date: 2019
Abstract: The perovskite material system allows for the realization of perovskite/silicon tandem solar cells with high energy conversion efficiencies (PCE) >30%. No other solar cell technology exhibits such high potential in reaching high PCE while maintaining low cost manufacturing. The short circuit current of perovskite/silicon tandem solar cells has to be increased to realize solar cells with PCE exceeding the records of single junction solar cells. Photon management allows for increasing the short circuit current and PCE of perovskite single junction and perovskite/silicon tandem solar cells due to improved light incoupling and/or light trapping. In the first part of this study, the light incoupling is studied for perovskite and amorphous silicon solar cells using moth eye surface texture, since moth eye surface texture represents a refractive index grating that allows for an efficient incoupling of light in the solar cell while minimizing reflection losses. Perovskite has a rather low refractive index of approx. 2.5, while amorphous silicon exhibits a refractive index of approx. 4.5 comparable to that of crystalline silicon. Due to largely different refractive indices, different device designs must be selected to allow for an efficient light incoupling in the solar cell. Three different solar cell structures are compared: a solar cell where only the front surface is covered with a moth eye structure, a solar cell where all interfaces are moth eye textured; and thirdly, a structure with all interfaces being textured by the moth eye texture except for the metal back reflector. The differences in the refractive indices between perovskite (approx. 2.5) and amorphous silicon (approx. 4.5) distinctly affect the coupling of light in the solar cell. The influence of the moth eye texture on the solar cell properties and the difference between perovskite and silicon solar cells is discussed. Then, the study is moved to an electrically flat but optically rough perovskite solar cell design, which combines the benefit of reaching high short circuit currents due to improved photon management, while maintaining the integrity of perovskite films processed on planar substrates or surfaces. Consequently, the perovskite films exhibit electrical properties comparable to planar perovskite solar cells, which allows for achieving solar cells with high open circuit voltage and fill factors. Electrically flat but optically rough perovskite solar cell structures can be successfully implemented because the perovskite material system and the contact materials of the solar cells exhibit comparable refractive indices. This sets the perovskite material system with its low refractive index apart from other high refractive index materials like silicon or germanium. Hence the proposed design of an electrically flat but optically rough solar cell can not be applied to silicon or germanium solar cells. The design of perovskite single junction and perovskite/siliocn tandem solar cells is discussed, and electrically flat but optically rough solar cells are compared to electrically and optically rough solar cells. At the end of this part, the formation of perovskite films on the textured substrates and the influence of the perovskite realistic interface morphology on the short circuit current and quantum efficiency of perovskite/silicon tandem solar cells are investigated utilizing a 3D morphological algorithm.
The next part of the thesis discusses on an efficient 3D architecture for low charge carrier mobility like organics and amorphous silicon materials based solar cells that exhibit PCE distinctly below the Shockley Queisser upper theoretical limit. However, thin film silicon materials and organic solar cells can be fabricated at low temperatures on large areas and/or flexible substrates allowing for low cost manufacturing. In this study, a 3D solar cell architecture is proposed for enhancing the short circuit current and PCE of such solar cells by increasing the optical thickness while the electrical thickness of the solar cell remains unchanged. A simple procedure is proposed to reach the maximal energy conversion of 3D solar cells without using complex numerical calculations. The design of single-junction perovskite and 3D organic solar cell is performed by 3D Finite Difference Time Domain (FDTD) optical simulations, while the design of perovskite/silicon tandem solar cells is described by using a hybrid approach, which combines FDTD optical simulations of perovskite top and experimentally measured crystalline bottom solar cells. The last part of the thesis is about optical color sensors based on multi-bandgap perovskite materials that overcome the limit of conventional image sensors used in smartphones and digital cameras. The investigated sensor structure is color aliasing or color Moire free error, while conventional sensors using optical filters are limited by this error. The spectral sensitivity of vertically stacked sensor is up to 3 times higher than the spectral sensitivity of filter-based sensors. For the optical-constant calculation of perovskite-based alloys with suitable band gaps, the energy shift model has been adopted. The spectral sensitivities of the sensors were colorimetrically characterized and compared to sensors in literature including conventional sensors using optical filters. Up to our knowledge for the first time it could be shown that a vertically stacked three color sensor exhibits color error equal to, or smaller than errors of conventional sensors using optical filters. Detail on the used materials, the device design, and the colorimetric analysis are provided.
Subjects: Hong Kong Polytechnic University -- Dissertations
Perovskite solar cells
Solar cells
Solar energy
Energy conversion
Pages: xxix, 206 pages : color illustrations
Appears in Collections:Thesis

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