Numerical Analysis and Device Optimization of Radial p-n Junction GaAs/Al x Ga 1-x As Core- Shell Nanowire Solar Cells Cheng G. Lim and Helge Weman Department of Electronics and Telecommunications, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway. Abstract - Based on the transfer-matrix method and the complex wave impedance approach, this unified electrical and optical numerical simulation thoroughly analyzes the impacts of the design parameters on the transport mechanisms and device characteristics of radial p-n junction GaAs/Al x Ga 1-x As core-shell nanowire solar cells. By optimizing the doping density of the core and shell, core radius, shell thickness, nanowire length as well as the Al mole fraction of the n-type Al x Ga 1-x As shell, the optimized device exhibits an open-circuit voltage of ~0.94V, a short-circuit current of ~55.5 pA (effective short- circuit current density is ~40.9 mA/cm 2 ), and a fill-factor of ~0.76. Hence, this clearly shows that radial p-n junction GaAs/Al x Ga 1-x As core-shell nanowire solar cell on Si substrate is capable of achieving an unprecedented solar cell efficiency of ~30% for single-junction GaAs solar cells in a cost-effective way. I. INTRODUCTION Recently, radial p-n junction core-shell nanowire solar cells [1]-[6] have attracted a lot of attention because this device architecture enables effective light absorption and efficient carrier collection to be achieved simultaneously. This feature is unique to the radial p-n junction core-shell nanowire structure because absorption of sunlight takes place axially along the core of the nanowire whereas the collection of photo-generated carriers happens radially out of the core of the nanowire. Apart from that, nanowire array inherently exhibits enhanced light absorption due to their intrinsic light trapping effects and reduced light reflection without using anti-reflection coatings. Therefore, it is expected that the radial p-n junction core-shell nanowire structure could achieve higher solar cell efficiencies that cannot be obtained using the planar device geometry. The nanowire device concept also offers an additional cost advantage because III-V compound semiconductor nanowires can be grown directly on cheaper substrates. Hence, radial p-n junction III-V compound semiconductor core-shell nanowires would make it possible to develop low-cost and high-efficiency solar cells. However, despite the above advantages of the radial p-n junction core-shell nanowire structure, the reported solar energy conversion efficiencies for radial p-n junction core- shell nanowire solar cells are less than 10% [3]-[6] when illuminated with the one-sun AM1.5G solar spectrum regardless of the material system used. Clearly, something must be amiss and it is the aim of this work to gain insights into improving the solar energy conversion efficiency of radial p-n junction GaAs/Al x Ga 1-x As core-shell nanowire solar cells. The approach taken in this numerical study is to vary the doping density of the core and shell, core radius, shell thickness, nanowire length, and Al mole fraction of the Al x Ga 1- x As shell to study the influences of these design parameters on the device characteristics of these unique devices. This finite- element analysis was based on the transfer-matrix method and complex impedance approach, and the simulations were performed using a commercial technology-computer-aided- design package called Sentaurus. II. RESULTS AND DISCUSSIONS An example of the radial p-n junction GaAs/Al x Ga 1-x As core-shell nanowire solar cells analyzed in this study is shown in Fig. 1. In order to keep the computation loads manageable, only a single radial p-n junction GaAs/Al x Ga 1-x As core-shell nanowire was considered and light-trapping effects were excluded in the simulations. The focus of the initial simulations was on studying the effects of the doping densities of the core and shell on the device characteristics. For this purpose, a value of 100 nm was considered for the core diameter and shell thickness, and the solar cell efficiency for a radial p-n junction GaAs/Al 0.2 Ga 0.8 As core-shell nanowire solar cell as a function of the core and shell doping densities are shown in Fig. 2. Clearly, the result showed that the combination of a high core doping density and low shell doping density would lead to high solar cell efficiencies. That is because higher core doping with lower shell doping leads to larger quasi-Fermi-level splitting and stronger electric field. Consequently, open-circuit voltage and short-circuit current improved considerably and resulted in higher solar cell efficiencies. In this result, an optimum solar Fig. 1. Schematic diagram showing the device structure of a radial p-n junction GaAs/AlxGa1-xAs core-shell nanowire solar cell. 978-1-4673-6310-5/13/$31.00 ©2013 IEEE NUSOD 2013 11