G.J. Bauhuis

High rate epitaxial lift-off of InGaP films from GaAs substrates

Centimeter sized, crack-free single crystal InGaP films of 1 μm thickness were released from GaAs substrates by a weight-induced epitaxial lift-off process. At room temperature, the lateral etch rate of the process as a function of the applied Al0.85Ga0.15As release layer thickness was found to have a maximum of 3 mm/h at 3 nm. Using 5-nm-thick AlAs release layers, the etch rate increased exponentially with temperature up to 11.2 mm/h at 80 °C. Correlation of the experimental data with the established theoretical description of the process indicate that the model is qualitatively correct but fails to predict the etch rates quantitatively by orders of magnitude.

Broadband and Omnidirectional Anti-reflection Coating for III/V Multi-junction Solar Cells

Graded refractive index layers reduce the reflection and increase the coupling of light into a substrate by optical impedance matching at the interfaces. Due to the optical impedance matching, reflections at the interfaces are not possible for a broad wavelength range, rendering this type of anti-reflection coating a promising candidate for III/V multi-junction solar cells. Graded refractive index layers can be modeled using a transfer-matrix method for isotropic layered media. We derive the transfer-matrix method and we show calculations of the reflection from and the transmission into an AlInP layer coated with different anti-reflection coatings. We describe a new type of anti-reflection coating based on tapered semiconductor nanowires and we show reflection and transmission measurements of those kind of anti-reflection coatings on top of different substrates.

Maximal power output by solar cells with angular confinement

Angularly selective filters can increase the efficiency of radiatively limited solar cells. A restriction of the acceptance angle is linked to the kind of utilizable solar spectrum (global or direct radiation). This has to be considered when calculating the potential enhancement of both the efficiency and the power output. In this paper, different concepts to realize angularly selective filters are compared regarding their limits for efficiency and power output per unit area. First experimental results of a promising system based on a thin-film filter as the angularly selective element are given to demonstrate the practical relevance of such systems.

Deep junction III-V solar cells with enhanced performance

The influence of junction depth in III–V solar cell structures was investigated for GaAs and InGaP cells. Typical III–V solar cells employ a shallow junction design. We have shown that for both investigated cell types, a deep junction close to the back of the cell structure performs better than shallow junction cells. At the maximum power point the deep junction cells operate mainly in the radiative recombination regime, while in the shallow junction cells non-radiative recombination is dominant. The steeper slope of the IV curve boosts the fill-factor by 3–4%, which is thereby the most improved cell parameter. In order to minimize collection losses in the upper part of the solar cell, the optimal thickness of the GaAs deep junction cell is only two-thirds of a shallow junction cell. The associated lower cell current is more than compensated by the higher fill-factor and open circuit voltage. The best deep junction GaAs cell shows a record efficiency of 26.5% for a GaAs cell on substrate. In the thinner InGaP deep junction cell the absence of current loss, leads to 1.6% higher efficiency than for the shallow junction cell.

Inverted thin film InGaP/GaAs tandem solar cells for CPV applications using epitaxial lift off

The epitaxial lift-off technique has been applied to dual-junction III–V solar cells grown in inverted order (subcell with highest band gap is grown first). It is shown that growing in inverse order is not trivial since both the tunnel junction and the InGaP subcell perform differently.

Steps to minimize the characterization errors in steady state IV curve measurements taken with C.O.T.S. equipment

The continuing search for clean energy solutions is driving a huge increase in the number of laboratories, non-profit and commercial, working on solar cell development. These laboratories need to characterize their solar cells in order to steer their development process. The most popular method is the measurement of the current voltage curve, better known as the IV curve. IV curves can be measured by Commercial Off The Shelf (COTS) equipment which typically consists of a steady state solar simulator, a probe station, an electronic load, a reference cell, and software driven data-acquisition and analysis means.

Study of wet chemical etching of AlxGa1-xInP2 films using hydrochloric acid

The etching behavior of Al x Ga 1-x InP 2 (0 ≤ x ≤ 1) in aqueous HCl was investigated for layers on their native GaAs substrates as well as for layers after releasing from their substrate and transferring to a foreign plastic carrier utilizing the epitaxial lift-off (ELO) technique. For InGaP 2 layers on their native substrates the activation energy of the etching rate was determined to be 22 kcal/mol for HCl concentrations of both 6 and 12 M. The surface roughness of the partially etched Al x Ga 1-x InP 2 layers as determined with atomic force microscopy (AFM) was found to decrease with increasing aluminum fraction and to be smaller for 6 M than for 12 M HCl. Al x Ga 1-x InP 2 layers on foreign plastic carriers were often found to be not etched in HCl, in contrast to layers on substrates. This could not be attributed to a single cause and it is suggested that the nonetching behavior is related to a combination of factors, like exposure of the layers to the ELO process and strain induced by the foreign carrier. AFM studies showed an increased density of irregularities at the surfaces of the Al x Ga 1-x InP 2 samples that later showed nonetching behavior.

Thin-Film III–V Solar Cells Using Epitaxial Lift-Off

Epitaxial lift-off is used to create thin-film III–V solar cells without sacrificing the GaAs wafer. It is based on selective etching of an AlAs release layer between the wafer and the cell structure using an HF solution. The wafer can be reused for subsequent deposition runs thereby reducing the cost of the cells. The thin-film cell can be transferred to any new carrier, e.g. glass, plastic, silicon, or metal foil. Although epitaxial lift-off was first demonstrated in 1978, it took until the 1990s to make significant progress in understanding the process and devising new ways to increase the etch rate. The first single-junction epitaxial lift-off cells were made in 1996. Thin-film cells offer new cell applications based on their flexibility, low weight, and possibility to deposit the cell structure in reverse order. Today the world record for single-junction cells is held by a thin-film GaAs cell, who’s performance is partly based on the increased photonrecycling factor in cells with a back contact acting as a mirror. Also state of the art tandem and inverted metamorphic thin-film cells have been demonstrated.

InGaP/GaAs Inverted Dual Junction Solar Cells For CPV Applications Using Metal-Backed Epitaxial Lift-Off

The epitaxial lift‐off (ELO) technique has been combined with inverted III–V PV cell epitaxial growth with the aim of employing thin film PV cells in HCPV systems. In a stepwise approach to the realization of an inverted triple junction on a MELO platform we have first grown a GaAs single junction PV cell to establish the basic layer release process and cell processing steps followed by the growth, fabrication and test of an inverted InGaP/GaAs dual junction structure.

Measurement of an InGaAsP/InGaAs tandem solar cell under GaAs

We have developed a low band gap tandem (two-junction) solar cell lattice-matched to InP, which is designed to work under a InGaP/GaAs tandem in a four-junction configuration. For the top and bottom subcells InGaAsP (Eg = 1.03 eV) and InGaAs (Eg = 0.73 eV) were utilized, respectively. A new tunnel junction was used to connect the subcells, including thin layers of n-type InGaAs and p-type GaAsSb. The delicate critical interfaces were prepared employing metal organic vapor phase epitaxy (MOVPE) and were monitored with optical in-situ spectroscopy (reflectance anisotropy spectroscopy, RAS). After a contamination-free transfer, the in-situ signals were then benchmarked in ultrahigh vacuum (UHV) with surface science techniques. X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED) 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.