Electrodeposition of Metal Matrix Composites and Materials Characterization for Thin-Film Solar Cells

Metal matrix composites, which consist of silver-multiwalled carbon nanotube-silver, layer-by-layer stacks, can electrically bridge the cracks (>40 μm) that appear in semiconductor substrates and the composite grid lines.

December 1, 2019

Air Force Research Laboratory, Kirtland Air Force Base, New Mexico

The current trend in both space and terrestrial photovoltaics is to implement high-efficiency, thin-film-based solar cells to reduce weight and materials cost while improving performance. For space photovoltaics, multi-junction (MJ) solar cells have been used almost exclusively due to their high efficiency and high radiation hardness. The efficiency of state-of-practice triple-junction (TJ) cells used in space today is approximately 30% under 1-sun Air Mass 0 (AM0) spectrum.

Figure 1. Schematic architecture of IMM cell

Multijunction technology involves stacking different bandgap subcells electrically and optically in series, connected by tunnel junctions, to effectively capture and utilize the solar spectrum. State-of-practice TJ cells consist of GaInP₂ and (In)GaAs subcells grown lattice-matched via metal organic vapor phase epitaxy on an active Ge substrate.

In recent years, for improved performance over the state-of-practice TJ cells, other cell architectures are being explored, such as inverted metamorphic multijunction (IMM) solar cells. Figure 1 shows a schematic view of IMM cell architecture, where InGaP and InGaAs subcells are sequentially grown on a lattice-matched substrate. After completing the growth, only the stack of active layers is exfoliated and transferred to a lightweight handling substrate, where the stack is inverted, such that the InGaP layer faces the sun. This new architecture eliminates the use of Ge substrates, as well as the Ge bottom cell found in traditional TJ solar cells, providing better current matching, higher efficiency, and lighter weight.

As the cells become thinner, however, it is expected that cell fracture will be a greater concern when these thin-film cells are subjected to thermomechanical stress, such as prolonged temperature fluctuations encountered in low earth orbit operation. The mismatch in thermal expansion coefficients of semiconductor and metal is an inherent engineering problem.

Figure 2. SEM micrographs of CNTs bridging gaps

To minimize the detrimental effects of severed metal busbars and gridlines on solar cell performance, this research explored incorporating multiwalled carbon nanotubes (MW-CNTs) into silver (Ag), forming a metal matrix composite (MMC). It was discovered that with proper CNT surface functionalization and judiciously designed composite microstructure, the CNTs in MMC gridlines can electrically bridge gaps greater than 40 μm. The scanning electron micrograph (SEM) images in Figure 2 conceptually demonstrate this composite engineering strategy, where the CNTs mechanically and electrically bridge the gaps in severed MMC gridlines, providing redundant electrical conduction pathways.

Only 9-μm-wide gaps are shown here because of the practical difficulty of transferring fractured samples with large gaps into the SEM vacuum chamber. In addition to gap-bridging, this research demonstrates that the MMC gridlines, which are strained to failure by greater than 40 μm displacement, can “self-heal” to re-establish electrical conduction, when the gap is closed. This self-healing is proven to be repeatable over many strain-to-failure/closed-gap cycles. Most importantly, preliminary device characterization on MMC-enhanced commercial TJ cells has shown substantially improved crack-tolerance compared to the cells with conventional evaporation-based metallization.

This work was done by Sang M. Han of the University of New Mexico for the Air Force Research Laboratory. AFRL-0279