Partially Asymmetric Grating In High Absorption Thin Film Solar Cells And Broadband Reflectors

Subwavelength gratings can be used to design high-performance anti-reflection or reflective surfaces,which have many applications in the fields of spectroscopy,integrated optoelectronics,solar cells and communications.The conventional ordered symmetrical subwavelength gratings could excite a finite number of mode resonances and achieve high transmission or reflection efficiency in a narrow spectrum range.However,in some practical applications,such as optical communication,optical sensing,and thin film solar cells,besides transmission or reflection efficiency,there are some other performance requirements for optoelectronic devices in polarization,incidence angle,and bandwidth.However,optical devices based on conventional simple symmetrical gratings are difficult to meet the requirements of these comprehensive performances at the same time.Recent studies have shown that when local asymmetry is introduced,the number and wavelength of mode resonance may change,which may improve the comprehensive performance of optical devices.Therefore,in this thesis,two design methods of locally asymmetric structure sub-wavelength gratings are proposed.They are applied to the design of thin-film solar cells and polarization-independent reflectors,respectively.The optical properties of grating devices are numerically calculated by rigorous coupled-wave analysis method,and the physical mechanism of light manipulation by local asymmetric grating in optical devices is analyzed.The results show that when the local asymmetric sub-wavelength grating is applied to the above optical devices,the performance of polarization,incidence angle or bandwidth is improved.It is proved that the optical devices based on asymmetric sub-wavelength gratings can help to meet the complex and diverse application scenarios.The main works of this thesis are embodied in the following three aspects.(1)Two new design methods of locally asymmetric sub-wavelength gratings are proposed.To form local asymmetric structures in a long period,we propose for the first time to obtain a local asymmetric grating by changing the rotation angle of sub-structure or combining different length periodic structures.The asymmetric long-period grating is composed of rotated substructure elements or different length periodic structure elements.Compared with the conventional method of position change or shape parameter change,the two new methods not only reduce the number of variables,making the design simple,but also meet the purpose of designing local asymmetric grating,which can further enhance the polarization,incidence angle or bandwidth performance of the device.According to the requirements of practical application,the two methods are used to design thin-film solar cells and polarization-independent broadband reflectors,respectively.(2)Rotated Square pillar Array Locally Asymmetric Grating Thin Film Solar Cells.In order to obtain efficient and stable optical absorption for thin film solar cells with wide spectral range and large change of incident angle,a rotated square pillar array grating is designed by the first method,changing the rotation angle of substructure to form a local asymmetric structure grating.Compared with the un-patterned stack slab and the optimized uniform-SPAG cells,absorption enhancements of 78.54 % and 3.21 % are obtained in optimized rotated-SPAG,respectively.Moreover,at any oblique incidence angles,the absorption of the optimized rotated-SPAG cell are always higher than that of the optimized uniform-SPAG cell.These phenomena are mainly due to the introduction of rotation disorders,which not only diffracts light into more orders,but also concentrates more diffraction energy into those orders,exciting more quasi-guided modes in silicon absorption layer,and thus resulting an increase of absorption.Furthermore,both the low incidence angular sensitivity and high structural tolerance indicate stable absorption performance for the rotated-SPAG cell.The maximum optical absorption only decreases by 10.08 % within ±60° oblique incidence range;and less than 2 % absorption decrease is also found when the height or width of the sub-squarepillar slightly deviates within ±20 nm from the optimum.Additionally,another rotation case on spiral grating indicates that rotation disorders may be also beneficial to other single and composite structures.Generally,our results indicate not only the proposed rotated square pillar array asymmetric grating would be promising to improve light trapping efficiently and stably,but also the way of introducing disorders through rotation,which is a new kind of disorder differentiated from shape and relative position disorders to cause symmetry damage,would be useful for designing various high absorption pseudo-unordered surface structures.Generally speaking,the rotating square column array grating is not only beneficial to improve the efficiency and stability of light capture,but also effective for the design of various high absorption pseudo-disordered surface structures by introducing disorder to cause symmetry damage through rotation.(3)Series Asymmetric 1-D Strip Grating Non-polarized Broadband Reflector.To meet the demands of both polarization and bandwidth performance of the reflectors in optical communication and sensing systems,polarization-independent reflection with ultra-wide bandwidth is obtained by combining local asymmetric grating with orthogonal series arrangement.Firstly,based on the local asymmetric structure,the four leakage modes of the grating could be distributed over a wide spectrum in TM mode,which makes the local asymmetric grating have an ultra-wideband reflection of 469 nm with reflectivity greater than 98%.Secondly,making use of the 90-degree rotational symmetry in orthogonal series arrangement,the polarization-independent broadband reflection of 492 nm with reflectivity greater than 97 % is obtained with an optimized bi-layer grating system.Finally,the spacing distance between the two series connection gratings are studied.It shows that asymmetric series gratings not only can achieve wide-spectrum polarization-independent reflection with both large spacing and specific small spacing distances,they also can be applied to optical filters and optical sensors when changing the spacing distances.In addition,to detect the performance of optical devices in the follow-up study,a highprecision coupling testing platform for passive optical devices is built.The platform uses stepper motor to precisely control the three-dimensional displacement operation.At the same time,the cross-ball guide with small friction and large contact area is used to ensure the stability and accuracy of displacement operation.The control accuracy can reach 50 nanometers.The test platform also has semi-automatic optical alignment through the combination of computer and stepper motor,which greatly reduces the test time and operation difficulty.The high precision coupling testing platform can not only provide incident light at different angles for the performance test of reflectors conveniently and accurately,but also meet the performance test of optical devices in silicon-based optoelectronic integrated chips in the future.