E.J. Haverkamp

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.

Results of the Fifth International Spectroradiometer Comparison for Improved Solar Spectral Irradiance Measurements and Related Impact on Reference Solar Cell Calibration

This paper reports on the results of the fifth spectral irradiance measurement intercomparison and the impact these results have on the spread of spectral mismatch calculations in the outdoor characterization of reference solar cell and photovoltaic (PV) devices. Ten laboratories and commercial partners with their own instruments were involved in the comparison. Solar spectral irradiance in clear sky condition was measured with both fast fixed and slow rotating grating spectroradiometers. This paper describes the intercomparison campaign, describes different statistical analysis used on acquired data, reports on the results, and analyzes the impact these results would have on the primary calibration of a c-Si PV reference cell under natural sunlight.

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.

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.

Ultra thin tunnel junction for use in III–V multijunction solar cells

In the ideal case, a III-V tunneljunction (TJ) used in a concentrator setup can handle high current, has a low resistance and does not absorb photons with an energy higher than the subcell(s) below it. In practice, TJs for concentrator systems are made of GaAs or AlGaAs with a low Al fraction, so absorption is present and causes an efficiency loss. Simulations show that in a standard 40 nm thick GaAs TJ the current density loss due to absorption is 1.7 mA/cm2. This value can be reduced by replacing one of the GaAs layers by AlGaAs and/or using a thinner TJ. We have tested Al0.1GaAs/GaAs TJs with a total thickness of 17.5 nm. Two n-type dopants were investigated: Te and Si. For the p-type layer C was used. Excellent TJs with peak current densities above 600 A/cm2 and a series resistance of 0.5 mΩ cm2 were obtained. In this ultra thin TJ current density losses were reduced to 0.6 mA/cm2.