| This work was supported by the National Basic Research Program of China (2010CB327600), the National Natural Science Foundation of China (61020106007and61376019), the Natural Science Foundation of Beijing (4142038), the Specialized Research Fund for the Doctoral Program of Higher Education (20120005110011), and the111Program of China (B07005). The authors also acknowledge support from the EDA center of Chinese Academy of Sciences.III-V nanowire (NW) solar cells have been a topic in the research of photovoltaic devices recently. III-V compound semiconductor materials are mainly direct-bandgap ones, which have an appropriate range of light absorption and high absorptance. Nanowires’unique1D structure makes the synthesis of axial/radial p-n junction and heterostructures in a single nanowire possible. The pure "axial" or "radial" tandem solar cell also has its limit, which blocks the efficiency more optimum. To solve the problems mentioned above, the photovoltaic performance of composite III-V nanowire solar cells is thoroughly investigated in this work, which focus on the electrical and optical properties of the III-V nanowire solar cells. The main achievements are listed as follows:1. A novel NW composite nanostructure for high-efficiency solar cells is proposed in this work, which is based on the in-depth analysis of the "axial" and "radial" structures. The structure consists of several core-shell p-n junctions with different III-V materials which are axially connected by the tunnel diode in a NW. The stacked subcells with different band gaps can exploit the solar spectrum very effectively, while the core-shell p-n junctions in each subcell provide an efficient separation and collection of photocarriers.2. The3D optical analysis of the proposed structure is carried out using the finite-difference time-domain (FDTD) method. The optical properties such as optical field distribution, the absorptance, reflectance and transmittance of each subcell are analyzed in detail. The results show that the proposed structure has a good anti-reflection and light-trapping ability. And the absorptance of the cell maintains a high level in the absorption range. Then, the3D optical generation profiles are incorporated into the finite-element mesh of the NWs in the electrical tool. The current matching can be achieved by tuning the diameter of the NWs. After the optimization, a high conversion efficiency of19.9%is obtained, which is30%higher than that of thesingle-junction nanowire solar cells with the same material.3. The effect of the morphology of NW arrays such as the surface, D/P ratio, taper and oblique on the photovoltaic performance of NWs is deeply investigated using the FDTD method and finite-element method. It is demonstrated that:(1) the surface recombination can be effectively suppressed by the surface passivation, which leads to a high short-circuit current.(2) An optimum D/P ratio is required to achieve the best conversion efficiency.(3) The tapered NWs provide a better anti-reflection and light absorption than the normal ones, while an optimum taper angle is also required to achieve the best conversion efficiency.(4) For the case of axial p-i-n solar cells, the incident light can be coupled into the i region of oblique NWs more efficiently than the vertical NWs, which leads to a higher efficiency.The research mentioned above are expected to play an important and directive role in the fabrication process improvement of the III-V nanowire solar cells. |
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