Large-scale Fabrication And Analysis Of Flexible ?-? Multijunction Solar Cells

Flexible ?-? multijunction solar cells,because of their high-efficiency,lightweight and stable flexibility,are expected to be widely used in space applications such as high-altitude airships,near-space drones and commercial aerospace,as well as ground mobile energy systems such as new energy vehicles and smart wearable equipment.This dissertation focuses on the technical problems existing in lift-off and transfer of large-size epilayers,and the preparation of flexible substrates in the manufacturing process of flexible high-efficiency ?-? multijunction solar cells.Meanwhile,the optoelectronic reciprocity relation and failure mechanism of multijunction solar cells have been systematically analyzed.The main results are listed below:1.The effect of the buffers thickness on the mismatched InGaAs heteroepitaxial strain relaxation has been calculated.The equilibrium dislocation density,which contributes to the strain release,is close to the maximum by optimizing the thickness of a single buffer,and the stress release effect is the best.High-quality epitaxial materials are the key to realizing the high efficiency of multijunction solar cells.GaInP/GaAs/InGaAs three-junction solar cells grown on the GaAs substrates have been analyzed.The surface atomic force microscopy and cross-sectional transmission electron microscopy images show perfect heterojunction interface and crystal quality.In addition,1.96/1.55/1.17/0.83 eV AlGaInP/AlGaAs/InGaAs/InGaAs four-junction solar cells have been realized,based on the epitaxy of the three-junction solar cells.The AlInGaAs compositionally graded buffers limit almost all misfit dislocations to the internal interfaces without threading into the active region,and the mismatch strain of the two InGaAs subcells has been basically relaxed.2.Breaking through the conventional flexible solar cell preparation technology,a thin film epilayers transfer scheme combining electroplating process and a low-temperature bonding process has been innovatively proposed,which halves the preparation process of flexible solar cells and greatly reduces the residual stress between the flexible substrate and the epilayers.The solar cells are softer,have high yield and are more suitable for mass production.Flexible four-inch three-junction solar cells based on this scheme are successfully fabricated with a weight-area density of 169 g/m2 and a photoelectric conversion efficiency of 34.68%.In addition,the technical route will not be limited by the wafer size;i.e.,the wafers of six-inch,eight-inch,or even larger can be fabricated due to the simplicity of the process.This scheme is universal and suitable for ?-? multijunction solar cells and can be expected to achieve mass production of large-size flexible multijunction solar cells with low cost.The optical model of light trapping and photon recycling based on the native textured back-surface reflector of the three-junction solar cells has been constructed by the transfer matrix method code.Based on this optical model,the bi-layer TiOx/SiO2(44/83 nm)reducing the 300?1350 nm spectrum reflectance ratio and the four-layer ZnS/MgF2/ZnS/MgF2(45/17/7/71 nm)reducing the 300-1650 nm spectrum reflectance ratio antireflection coatings(ARC)have been designed.3.In view of the key problem that the short-circuit current density of the four-junction inverted metamorphic solar cells is low under the normal open-circuit voltage,the failure analysis of four-junction solar cells has been carried out.The sharp decrease in short-circuit current density is not due to the leakage channels caused by the mismatched buffers dislocations penetrating into the active region of the InGaAs subcells,but resulted from Al-O deep-level defects in the first grown AlGaInP subcell which increase non-radiative recombination and reduce the optical properties of the materials.The four-junction solar cell with an efficiency of 34.9%and an open-circuit voltage of 3.53 V has been obtained by optimizing the growth of AlGaInP subcell using a low-oxygen trimethylaluminum source.4.Since the subcells current density versus voltage curves of multijunction tandem solar cells cannot be directly measured,the optoelectronic reciprocity relation between the EQE and the different injection current densities EL is used to calculate the subcells current density versus voltage characteristic curves of multijunction solar cells.The analysis of individual subcells is the key to evaluating the performance of multijunction solar cells.The photoelectric characteristics of subcells are analyzed based on the optoelectronic reciprocity relation of multijunction solar cells,and the targeted optimization guidance of subcells is realized.The key to improving the overall efficiency of four-junction solar cells is to reduce the deep-level recombination of the AlGaInP top cell and the bulk recombination of the 0.83 eV InGaAs bottom cell.For the bottom cell,a micro-nanostructure texture back-surface reflector can be designed to realize photon recycling and light trapping to increase the open-circuit voltage and short-circuit current density.Compared with low-bandgap bottom cells,manufacturing high-bandgap top cells is more challenging.It is critical to reduce the Shockley-Read-Hall recombination,especially for the high Al composition AlGaInP top cell.The Al content and the material growth quality must be balanced.It is a friendly solution to appropriately increase the Al content of the emitter in the absorber layers,which is able to increase the open-circuit voltage while ensuring a sufficient diffusion length.