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Engineering

Developing High Performance GaP/Si Heterojunction Solar Cells

Published: November 16th, 2018

DOI:

10.3791/58292

1School of Electrical, Computer, and Energy Engineering, Arizona State University

Here, we present a protocol to develop high-performance GaP/Si heterojunction solar cells with a high Si minority-carrier lifetime.

To improve the efficiency of Si-based solar cells beyond their Shockley-Queisser limit, the optimal path is to integrate them with III-V-based solar cells. In this work, we present high performance GaP/Si heterojunction solar cells with a high Si minority-carrier lifetime and high crystal quality of epitaxial GaP layers. It is shown that by applying phosphorus (P)-diffusion layers into the Si substrate and a SiNx layer, the Si minority-carrier lifetime can be well-maintained during the GaP growth in the molecular beam epitaxy (MBE). By controlling the growth conditions, the high crystal quality of GaP was grown on the P-rich Si surface. The film quality is characterized by atomic force microscopy and high-resolution x-ray diffraction. In addition, MoOx was implemented as a hole-selective contact that led to a significant increase in the short-circuit current density. The achieved high device performance of the GaP/Si heterojunction solar cells establishes a path for further enhancement of the performance of Si-based photovoltaic devices.

There has been a continuing effort on the integration of different materials with lattice mismatches in order to enhance overall solar cell efficiency1,2. The III-V/Si integration has the potential to further increase the current Si solar cell efficiency and replace the expensive III-V substrates (such as GaAs and Ge) with a Si substrate for multijunction solar cell applications. Among all III-V binary material systems, gallium phosphide (GaP) is a good candidate for this purpose, as it has the smallest lattice-mismatch (~0.4%) with Si and a high indirect bandgap. These features can enable high-quality integra....

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CAUTION: Please consult all relevant material safety data sheets (MSDS) before dealing with chemicals. Please use all appropriate safety practices when performing a solar cell fabrication including the fume hood and personal protective equipment (safety glasses, gloves, lab coat, full-length pants, closed-toe shoes).

1. Si Wafer Cleaning

  1. Clean Si wafers in Piranha solution (H2O2/H2SO4) at 110 °C.
    1. To produce Piranha soluti.......

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Atomic force microscopy (AFM) images and high-resolution x-ray diffraction (XRD) scans, including the rocking curve in the vicinity of the (004) reflection and the reciprocal space map (RSM) in the vicinity of (224) reflection, were collected for the GaP/Si structure (Figure 1). The AFM was used to characterize the surface morphology of the MBE-grown GaP and XRD was used to examine the crystal quality of GaP layer. The effective minority-carrier lifetime of t.......

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A nominal 25 nm-thick GaP layer was epitaxially grown on a P-rich Si surface via MBE. To grow a better quality of GaP layer on Si substrates, a relatively low V/III (P/Ga) ratio is preferable. A good crystal quality of GaP layer is necessary to achieve high conductivity and low density of recombination centers. The AFM root-mean-square (RMS) of the GaP surface is ~0.52 nm showing a smooth surface with no pits, indicative of high crystal quality with a low threading dislocation density (Figure 1a

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The authors would like to thank L. Ding and M. Boccard for their contributions in processing and testing of the solar cells in this study. The authors acknowledge funding from the U.S. Department of Energy under contract DE-EE0006335 and the Engineering Research Center Program of the National Science Foundation and the Office of Energy Efficiency and Renewable Energy of the Department of Energy under NSF Cooperative Agreement No. EEC-1041895. Som Dahal at Solar Power Lab was supported, in part, by NSF contract ECCS-1542160.

....

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Name Company Catalog Number Comments
Hydrogen peroxide, 30% Honeywell 10181019
Sulfuric acid, 96% KMG electronic chemicals, Inc. 64103
Hydrochloric acid, 37% KMG electronic chemicals, Inc. 64009
Buffered Oxide Etch 10:1 KMG electronic chemicals, Inc. 62060
Hydrofluoric acid, 49% Honeywell 10181736
Acetic acid Honeywell 10180830
Nitride acid, 69.5% KMG electronic chemicals, Inc. 200288

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