Basic Concepts and Interfacial Aspects of High-Efficiency III-V Multijunction Solar Cells

Abstract

Among various types of solar cells, MOVPE-grown triple-junction III-V compound semiconductors are today's most efficient photovoltaic devices with conversion efficiencies exceeding 40%. A next-generation multijunction cell with four or more junctions and optimized band gaps is expected to break the present record efficiency surpassing the 50% mark. High band gap material combinations that are lattice matched to GaAs are already well established, but the required low band gap combinations containing a band gap around 1eV are still to be improved. For this purpose, we have developed a low band gap tandem (two-junction) solar cell lattice matched to InP. For the top and bottom subcells InGaAsP (Eg = 1.03 eV) and InGaAs (Eg = 0.73 eV) were utilized, respectively. A new interband tunnel junction was used to connect the subcells, including thin and highly doped layers of n-type InGaAs and p-type GaAsSb. The delicate MOVPE preparation of critical interfaces was monitored with in-situ reflectance anisotropy spectroscopy (RAS). After a contamination-free transfer, the RAS signals were then benchmarked in ultrahigh vacuum (UHV) with surface science techniques like low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS). XPS measurements revealed that the sharpest InGaAs/GaAsSb interface was achieved when the GaAsSb layer in the tunnel junction of the solar cell was grown on III-rich (2×4)- or (4×2)-reconstructed InGaAs(100) surfaces. The improved interface preparation had a positive impact on the overall performance of the tandem cell, where slightly higher efficiencies were observed for the cells with the III-rich-prepared tunnel junction interfaces.

Efficient direct solar-to-hydrogen conversion by in situ interface transformation of a tandem structure

“…For III–V semiconductors, their adjustable optoelectronic properties, the high control of doping levels, the formation of tunnel junctions and abrupt interfaces enable such an adaptation181920. Application in photoelectrochemical devices necessitates in addition a careful conditioning of the interfaces of the absorber with the electrolyte and with the electrocatalyst, also considering molecular details of the surface chemistry2122.…”

Increased cell efficiency in InGaAs thin film solar cells with dielectric and metal back reflectors

“…InGaAsP is a promising quartenary III-V semiconductor material that can be grown lattice matched to InP. Ge bottom cell of InGaP/GaAs/Ge three-junction solar cells could be replaced with a tandem InGaAsP/InGaAs cell to increase the photovoltaic efficiency [8,9]. We modeled an InGaAsP topcell for such a two junction tandem cell.…”

Minority carrier lifetime analysis of bulk and interfaces on MOVPE grown III-V Low bandgap solar cell materials

“…Such a four junction cell was just recently realized by wafer bonding reaching an efficiency of 43,6% at 319 suns [15]. Since theoretically conversion efficiencies lying above 60% practical conversion efficiencies in the range of 50% appear to be feasible [6]. As the diffusion length and thus the minority carrier lifetime is a key material parameter that can a priori determine the potential solar cell performance we report on suitable nondestructive time resolved photoluminescence (TRPL) methods using a time correlated single photon counting (TCSPC) setup to get information about the electronic quality of the grown GaInAsabsorber layer and GaInAs to InP interface transition layer.…”