Next-generation multijunction solar cells: The promise of II-VI materials

Abstract

High concentration photovoltaic (HCPV) systems offer the highest photovoltaic (PV) conversion efficiencies. Also, as production is beginning to ramp up, HCPV is becoming cost competitive with thin-film poly-CdTe and crystalline Si systems in high solar insolation regions. High solar concentrations, X ∼ 500, are used to increase cell efficiencies and greatly reduce the cell area per unit of incident solar radiation, thereby greatly reducing the cell cost per watt. The monolithic three-junction (3J) solar cells presently used in HCPV systems typically consist of two epitaxial III-V homojunctions, such as GaInP and GaInAs, grown on an active Ge substrate by metal-organic chemical vapor deposition (MOCVD). The III-V bandgaps are chosen to match the currents generated in each junction and minimize the energy lost to thermalization of the electron-hole pairs generated, subject to the constraint of approximate lattice matching. We propose using cells consisting of one or more CdTe-based II-VI homojunctions grown on large-area active Si substrates by high-throughput MBE or a less expensive high-vacuum deposition technique as an alternative to III-V based multijunction cells grown by MOCVD. The bandgap of Si is more optimal than that of Ge for two-junction (2J) or 3J cells, and lattice mismatches affect the efficiencies of such cells only slightly, which allows greater freedom in the choice of bandgaps, and thus the potential for higher efficiencies. Also, such cells could be manufactured at a much lower cost due to the larger area, much lower cost and superior mechanical properties of Si substrates as compared to Ge substrates. The much lower cell cost also would enable medium concentration PV systems that would require more cell area, but with simplified, less expensive tracking and optics, resulting in lower overall system costs. Promising initial results from material-property measurements and single-junction and 2J CdZnTe/Si cell characterization results are given. Both the promise of the proposed technology and the challenges it faces are discussed.

Determining the nonparabolicity factor of the CdO conduction band using indium doping and the Drude theory

“…Nevertheless, research into Cd-based TCOs has recently picked up, mainly in response to the development of high efficiency thin-film solar cells which already contain a significant amount of Cd. [3] Much progress has already been made and doped CdO thin-films with resistivities below 10 −4 Ωcm and visible transparency above 80% can be readily deposited by several techniques. [4,5,6,7,8,9] Practical applications aside, CdO is an excellent material in which to study fundamental physics of degenerate semiconductors.…”

Growing high-quality ternary CdMnTe epilayers by molecular beam epitaxy on Si substrates and its mechanism

“…High-quality low-cost Si substrates with large areas are commercially available and II-VI compound semiconductor materials with wide band-gaps have potential for use in visible-wavelength optoelectronics. Based on successful developments in the infrared sensing and imaging industry, some researchers have proposed the growth of the II-VI compound semiconductor material system, CdZnTe/CdTe, on Si substrates for use in multijunction solar cells [2][3][4][5]. High-quality and uniform CdTe epilayers on Si substrates have been grown by molecular beam epitaxy [4], and their characterizations have shown that the minority carrier lifetime of CdTe epilayers may be insensitive to the structural defects that are induced by the lattice mismatch [4,6,7].…”

Growth and characterization of CdTe absorbers on GaAs by MBE for high concentration PV solar cells

“…This problem is thought to be related with structural defects in the polycrystalline CdTe absorbers and interfaces [3]. In the early 1990s, there was a brief interest in CdTe single crystal solar cell devices [3]. However, solar cell community lost interest in CdTe single crystal solar cells due to high production costs and small area of the suitable substrates.…”

“…Low solar cell efficiencies of the polycrystalline CdTe based solar cells are correlated with the low open circuit voltages (V oc ). This problem is thought to be related with structural defects in the polycrystalline CdTe absorbers and interfaces [3]. In the early 1990s, there was a brief interest in CdTe single crystal solar cell devices [3].…”

“…However, solar cell community lost interest in CdTe single crystal solar cells due to high production costs and small area of the suitable substrates. In the late 2000s, a new approach to form single and multi-junction single crystal CdTe solar cell devices is reported [3].…”