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Multijunction Solar Cells from Monolithic Integration of Dilute Nitrides on Gallium Arsenide (GaAs) and Silicon (Si) Wafers : defect studies

Abstract : Multi-junction solar cells based on III-V compounds have reached very high power conversion efficiencies (46%). However, the fabrication methods that are generally used are complex and expensive for non-monolithic bonded and inverted solar cells. This thesis is devoted to the study of prospective methods to increase the efficiency of monolithic solar cells. The work is focused on the study of electronically active defects in the materials constituting the solar cells by means of photoelectric and capacitance techniques (admittance spectroscopy, DLTS,…) and it can be divided into three parts. The first part deals with single-junction solar cells wherein the absorber is made of i-layers of 1 eV bandgap InGaAsN compounds with various thicknesses grown as sub-monolayer digital alloys (SDA) of InAs/GaAsN by molecular-beam epitaxy (MBE) on GaAs wafers. The cell with 900 nm thick InGaAsN exhibits the best photovoltaic performance and no defects could be evidenced from capacitance techniques. When the thickness is increased to 1200 nm, defects were detected, but their concentration is low so it did not strongly affect the photoelectric properties. Further increase to 1600 nm of the layer thickness was shown to lead to a higher defect concentration causing a change in the band diagram of the structure and lowering the lifetime of photogenerated carriers. This could explain the drastic drop of the external quantum efficiency, and the overall poor performance of the solar cell. The second part is devoted to the study of single- and multi-junction solar cells with active layers of (In)GaP(As)N grown by molecular beam epitaxy (MBE) on GaP and Si wafers, respectively. More precisely, the active layers were either quaternary alloys of GaPAsN or SDAs of InP/GaPN. We found that p-i-n type solar cells with active layers of i-GaPAsN showed better performance than p-n type solar cells with active layers of n-GaPAsN due to higher EQE values. Moreover, solar cells with an i-GaPAsN absorber layer show better photoelectric properties and lower defect concentrations, than those with an SDA InP/GaPN absorber layer. Different defect levels were detected by capacitance methods in these materials and their parameters were described in detail. We showed that a suitable post-growth treatment could improve the electronic quality of the GaPAsN layer and the solar cell properties. Also, a triple-junction solar cell was fabricated with active layers of i-GaPAsN and i-GaPN. All subcells were found to be operating, leading to a large open circuit voltage (>2.2 V), but the overall performance is limited by the low value of the quantum efficiency due to low thicknesses of i-layers that should be increased for better absorption. Finally, the third part is devoted to the study of GaP layers grown on Si wafers at temperatures below 400 °C using an original method called plasma-enhanced atomic-layer deposition (PE-ALD). Indeed, it uses a plasma-enhanced chemical vapor deposition equipment and it is based on the alternate interaction of the wafer surface with Ga and P atoms coming from injected trimethylgallium and phosphine, respectively. We also grew layers using a continuous process (providing simultaneously the P and Ga atoms) and observed that their electric and structural properties were poorer than that grown by the proposed PE-ALD method. The influence of growth conditions on the GaP/Si heterostructures was explored. We found that low RF-plasma power leads to better photoelectric, structural and defect-related properties, due to a better passivation of the silicon wafer. In addition, we demonstrated that, contrary to results reported in the literature using MBE processes, our growth process does not affect the electronic properties of phosphorous doped n-Si wafers, while slight changes were observed in boron-doped p-Si wafers containing Fe-related defects, however without deactivation of the doping nor strong degradation of the electronic properties.
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Submitted on : Wednesday, September 26, 2018 - 2:29:06 PM
Last modification on : Wednesday, October 14, 2020 - 4:10:37 AM
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  • HAL Id : tel-01881959, version 1


Artem Baranov. Multijunction Solar Cells from Monolithic Integration of Dilute Nitrides on Gallium Arsenide (GaAs) and Silicon (Si) Wafers : defect studies. Micro and nanotechnologies/Microelectronics. Université Paris Saclay (COmUE); Saint Petersburg Academic University (Saint Petersburg), 2018. English. ⟨NNT : 2018SACLS137⟩. ⟨tel-01881959⟩



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