Design High-Efficiency III–V Nanowire/Si Two-Junction Solar Cell

Wang et al. Nanoscale Research Letters (2015) 10:269 DOI 10.1186/s11671-015-0968-2 NANO EXPRESS Open Access Design High-Efficiency III–V Nanowire/Si Two-Junction Solar Cell Y Wang1, Y Zhang1, D Zhang1*, S He1 and X Li2* Abstract In this paper, we report the electrical simulation results of a proposed GaInP nanowire (NW)/Si two-junction solar cell. The NW physical dimensions are determined for optimized solar energy absorption and current matching between each subcell. Two key factors (minority carrier lifetime, surface recombination velocity) affecting power conversion efficiency (PCE) of the solar cell are highlighted, and a practical guideline to design high-efficiency two-junction solar cell is thus provided. Considering the practical surface and bulk defects in GaInP semiconductor, a promising PCE of 27.5 % can be obtained. The results depict the usefulness of integrating NWs to construct high-efficiency multi-junction III–V solar cells. Keywords: Two-junction solar cells; Nanowire; Current matching; Photovoltaics Background Multi-junction solar cells are constituted by series-connected multiple semiconductor layers (subcells) to convert the energy of light directly into electricity by the photovoltaic effect. By dividing the broad solar spectrum into smaller sections, the multi-junction solar cell can convert solar energy more efficiently. Till now, efficiencies exceeding 44 % have been achieved at high concentration ratios for conventional triple-junction cell cells [1]. However, the high efficiency of thin-film multi-junction solar cell is gained at the cost of increased complexity and manufacturing price. The development of high-efficiency, low-cost photovoltaic device is urgently needed. Due to their potential to realize low-cost and high energy conversion efficiency solar cells, semiconductor nanowire array (NWA) is a topic of intense research for photovoltaic applications. Recently, many theoretical and experimental works, which are focused on the design of single III–V NWA p-n junction, are widely reported [2, 3]. Meanwhile, some axially connected nanowire two-junction cells are also theoretically designed to obtain high-efficiency solar cells [4]. Considering the fact that many kinds of III–V NWAs have been successfully prepared on heterosubstrates (such as Si) [5], the concept of multi-junction * Correspondence: ; 1 College of Electrical Engineering and Automation, Anhui University, Hefei, China 2 Key Laboratory of Material Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China solar cell can also be used in the design of photovoltaic devices. As compared with conventional multi-junction solar cell, the integration of NWAs in photovoltaic applications has the following merits. Firstly, NWA with well-defined geometrical structures exhibits higher light absorption than their thin-film counterparts of the same thickness, due to the integration of NWA in solar cells significantly changes the mechanism of light absorption [6]. Secondly, when the top NWA cell is designed in radial p-n structure, the short collection lengths facilitate the efficient collection of photogenerated carriers, and herein, the Shockley-Quisser efficiency limit for nanowire arrays is higher than for the conventional thin film [7]. Finally, the integration of semiconductor NWA on low-cost substrates can help substantially reduce the final cost of PV fabrication. In view of this, we have recently presented a study on two-junction III–V NWA/Si solar cell, which is consisted by III–V NWA with p-n junction on the active Si substrate. We show that outstanding light harvesting rooted from the strong light trapping and the formation of Fabry-Pérot optical cavity in the NWA enables the cell to produce high photocurrent [8]. In this paper, we further report the electrical performance via simulation, where the physical dimensions are taken by considering the condition of current matching and maximized solar energy absorption. The impact of III–V NWA quality in terms of minority carrier lifetime, surface recombination velocity (SRV) on the solar cell performance is presented, © 2015 Wang et al. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. Wang et al. Nanoscale Research Letters (2015) 10:269 and a practical guideline to design high-efficiency III–V NWA/Si two-junction solar cell is provided. Methods Experiments Figure 1a shows the schematics of the proposed twojunction solar cell, which is comprised of vertically aligned core-shell structure Ga0.35In0.65P (1.7 eV) NWs [9, 10] on active thickness (T) 400 μm Si (1.12 eV) substrates. By balancing the absorption and consumption of source materials, in this study, we fixed the length (L) of GaInP NW at 2 μm. The NWA cell was connected with Si thin-film cell by the tunnel junction which can be assumed to be perfect. The SiO2 layer is added between the shell of NWs and the Si substrate. Transparent conductive oxide (TCO) and metal contacts are placed at the top and bottom of the structure. The optical properties of GaInP NWA/Si solar cells are investigated using Sentaurus Electromagnetic Wave Solver module package, a finite-difference time-domain (FDTD) full field electromagnetic simulation tool [11]. In order to simplify the calculation, the Si thin film is assumed infinite thick with no transmission loss by using a perfect match layer (PML) adjacent underneath the Si film. To investigate the potential photovoltaic efficiency of top NWA solar cells, a coupled optoelectronic simulation is presented. Based on the position-dependent light absorption obtained by FDTD simulation, the threedimensional (3D) carrier generation rate is obtained from the divergence of the Poynting vector. The currentvoltage behaviors of the GaInP NWA subcells are calculated at three-dimensional by using the weighted optical generation profiles [12]. The electrical behaviors of the bottom Si cell are calculated at two-dimensional with Beer-Lambert absorption model while a reflection loss Page 2 of 5 is adopted from the FDTD simulations. Ohmic contacts are assumed to be ideal at the front and rear surfaces of the device. The contact resistance has a minimum effect on the cell performance for resistivity of less than 0.01 Ω cm2. The tunnel diode connecting the subcells is assumed to be perfect. Electrical simulation takes into account the doping-dependent mobility, Auger, radiative, and Shockley-Read-Hall (SRH) recombinations. Results and Discussion Current Matching Process Figure 1b shows the ultimate photocurrent as a function of NW diameters from 60 to 200 nm. One can find the photocurrents of the two subcells match each other closely at the NW diameters of 100 and 180 nm. For hetero-epitaxial growth (...truncated)