Zinc Oxide/Silicon Heterojunction Solar Cell Simulation and Experimental Evaluation
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A new type of solar cell—ZnO/Si hetero-junction solar cells were explored for their potentially low cost application. The main focus of this work is to study the properties and characteristics of n-ZnO/p-Si heterojunction solar cells. In order to find the optimal design structure of ZnO/p-Si solar cells, numerical modeling using AMPS-1D was performed on various thicknesses, doping densities and junction structures. All the input parameters were chosen based on experimental values, literature values, theory and reasonable estimates. In addition, graded doping layer and back surface field (BSF) layer were also introduced to achieve improvement on the performance. Due to the nature of heterojunctions, defects are very easily formed at the junction interface. The ZnO/p-Si solar cell simulations with a Si interface layer of 50nm, defect density of 1 x 10 14 cm −3 , near mid-gap defect location, and capture cross section of 1 x 10 −10 – 1 x 10 −8 cm 2 were also performed. Result showed a "bending" I-V curve and low fill factor as we expected. The influences of defect layer thickness and defect location were discussed. Results also showed that the main reason for the bad performance is the defects in the Si surface rather than the ZnO surface. Those defects could be affected by a combination of interface layer thickness, defect density, defect capture cross section and defect location. After the simulation work, ZnO/p-Si heterojunction solar cell devices have been fabricated. Specific studies like SEM, XRD, I-V measurement were performed. The "bending" I-V shape and low fill factor were predicted by our AMPS-1D simulation. The transport mechanism was discussed by using space charge limited current (SCLC) theory. Even the best cell (300nm AZO/ 50nm ZnO/ 550um p-Si) only showed a short circuit current of 4.6 mA/cm 2 , an open circuit voltage of 0.15V and an efficiency less than 1%. Because ZnO/n-Si solar cells give normally better efficiency of ∼8% if compared with ZnO/p-Si solar cells, a comparison of these two types of solar cells was performed in the last chapter of this thesis. For ZnO/n-Si solar cells, dark current follows the traditional exponential equation, indicating a traditional p-n junction transport mechanism. However, ZnO/p-Si solar cells can only be explained by the SCLC mechanism. Then, a possible method to improve ZnO/p-Si solar cells performance was proposed. By adding a thin layer (5nm or 1um) of highly doped n-Si between ZnO and p-Si layer can greatly improve the performance. This structure was also simulated by using AMPS-1D, increasing the efficiency from 0.4% to 11.7%.