Anastasia Soeriyadi

Material and Device Improvement of GaAsP Top Solar Cells for GaAsP/SiGe Tandem Solar Cells Grown on Si Substrates

With its wide bandgap and good diode performance, GaAsP is an excellent candidate for the top cell in a silicon-based multijunction tandem device. Even though the material is not lattice matched to silicon, inclusion of a graded SiGe buffer between the GaAsP layer and the Si substrate has previously been demonstrated to enable lattice matching. The SiGe layer may then serve as a high-quality current-matched bottom cell to form a tandem dual-junction structure. This paper describes the design, fabrication, analysis, and improvement of the GaAsP top solar cell in a three-terminal GaAsP/SiGe tandem solar cell on a silicon substrate. Uncertified GaAsP top cell efficiencies have been improved from 8.4% to 18.4% with bandgap voltage offsets (Woc) of 0.48 and 0.31 V under concentration factors of 1 and 20 ×, respectively. This progress is made by improved III-V material quality, reduced series resistance, and an addition of antireflection coating. Improving the optics, material quality, and fill factor (FF) should further improve the efficiency of the GaAsP top cell in this tandem structure grown on an Si substrate.

Current matched GaAsP/SiGe tandem device on Si over 20% efficiency under indoor measurement

Lattice matched and current matched GaAsP/SiGe tandem solar cell on Si has the potential of 40% efficiency. This paper describes our design, fabrication and improvement of this tandem solar cell. This tandem device has achieved efficiencies of 20.6% and 20.2% under 1X and 2.2X, respectively. Current matching between top cell and bottom cell is realized by manipulating the bottom cell active area and lamp spectrum during JV measurements. Improving the optics and cell structure should lead to current matching in this tandem device and produce an efficiency over 25% and over 30% under 1X and 20X standard AM1.5G, respectively.

Large area efficient interface layer free monolithic perovskite/homo-junction-silicon tandem solar cell with over 20% efficiency

Monolithic perovskite/silicon tandem solar cells show great promise for further efficiency enhancement for current silicon photovoltaic technology. In general, an interface (tunnelling or recombination) layer is usually required for electrical contact between the top and the bottom cells, which incurs higher fabrication costs and parasitic absorption. Most of the monolithic perovskite/Si tandem cells demonstrated use a hetero-junction silicon (Si) solar cell as the bottom cell, on small areas only. This work is the first to successfully integrate a low temperature processed (≤150 °C) planar CH3NH3PbI3 perovskite solar cell on a homo-junction silicon solar cell to achieve a monolithic tandem without the use of an additional interface layer on large areas (4 and 16 cm2). Solution processed SnO2 has been effective in providing dual functions in the monolithic tandem, serving as an ETL for the perovskite cell and as a recombination contact with the n-type silicon homo-junction solar cell that has a boron doped p-type (p++) front emitter. The SnO2/p++ Si interface is characterised in this work and the dominant transport mechanism is simulated using Sentaurus technology computer-aided design (TCAD) modelling. The champion device on 4 cm2 achieves a power conversion efficiency (PCE) of 21.0% under reverse-scanning with a VOC of 1.68 V, a JSC of 16.1 mA cm−2 and a high FF of 78% yielding a steady-state efficiency of 20.5%. As our monolithic tandem device does not rely on the SnO2 for lateral conduction, which is managed by the p++ emitter, up scaling to large areas becomes relatively straightforward. On a large area of 16 cm2, a reverse scan PCE of 17.6% and a steady-state PCE of 17.1% are achieved. To our knowledge, these are the most efficient perovskite/homo-junction-silicon tandem solar cells that are larger than 1 cm2. Most importantly, our results demonstrate for the first time that monolithic perovskite/silicon tandem solar cells can be achieved with excellent performance without the need for an additional interface layer. This work is relevant to the commercialisation of efficient large-area perovskite/homo-junction silicon tandem solar cells.

Spectral response of steady-state photoluminescence from GaAs1-xPx layers grown on a SiGe/Si system

Measuring the spectral response of photoluminescence (SRPL) in solar cells has recently attracted attention as it can be used as a contactless relative measure of external quantum efficiency (EQE) prior to full device fabrication. However, this technique requires that the monitored PL spectrum originates mainly from a region in the solar cell with uniformly distributed majority carriers. For a stack of thin films with a similar material composition, the slightly different emission spectrum from each layer may lead to the superposition of several luminescence peaks. This letter presents the measurement of the SRPL from GaAsP tandem solar cells and outlines a method for separating the individual layer contributions. Good agreement between measured SRPL and EQE at short wavelengths has been achieved, and the deviations at longer wavelengths have been analyzed. This study also reveals unexpected bandgap narrowing resulting from a variable material composition within the active region.

Improved GaAsP/SiGe tandem on silicon outdoors and under concentration

Tandem solar cells made from III-V layers grown on silicon have the potential for high efficiencies at relatively low cost. Recent work has produced a GaAsP/SiGe tandem with over 20% efficiency based on indoor measurements. In this work, improvements are made to this device and outdoor performance is reported for the first time. Light trapping and substrate thinning techniques developed on a SiGe single junction are adapted to the demands of tandem cell processing. Devices are fabricated with front metalization optimized for low and high concentration applications, with potential efficiencies over 25%.

Spatially resolved EL and PL coupling of a dual junction solar cell

Tandem solar cell design has been leading the solar cell efficiency table. The high radiative recombination of the III-V materials employed in tandem structure becomes an important aspect to consider on the design stage. It is well known that the reabsorption of such process affects the efficiency of each cell and thus the overall system. A measurement technique of analyzing such effect is presented in this paper, limited to the luminescent coupling in a dual-junction solar cell system. The material presented here provides a unique opportunity to get a deeper understanding on the amount as well as the relation between the two luminescent coupling phenomena.

Performance improvement for epitaxially grown SiGe on Si solar cell using a compositionally graded SiGe base

Silicon germanium (SiGe) is a material with high mobility and relatively low bandgap making it an attractive candidate for the bottom subcell in a III-V tandem solar cell grown on silicon (Si) substrate. This paper reports on the performance improvement of an epitaxially grown SiGe on Si solar cell by growing a higher Ge composition SiGe layer in the base. The purpose of growing a higher Ge composition SiGe layer in the base is to improve the light absorption. The first iteration of this structure was an Si0.18Ge0.82 solar cell fabricated with a 1 μm thick Si0.12Ge0.88 layer in the base. This solar cell had a lower efficiency compared with the reference solar cell without the Si0.12Ge0.88 layer. One of the main reasons for the lower efficiency is believed to be the high threading dislocation density (TDD) caused by the abrupt change of lattice constant between Si0.18Ge0.82 and Si0.12Ge0.88 in the base. In order to reduce the TDD, the second iteration of the structure was fabricated with a compositionally ...

Short ciruit current improvement of SiGe solar cell in a gallium arsenide phosphide - silicon germanium dual junction solar cell on Si substrate

Light trapping capability is analyzed for the SiGe solar cell in the GaAsP-SiGe tandem device. One-dimensional model predicts an achievable Jsc of 17.46 mA/cm2 below the top cell, with the electrical parameter from quantum efficiency fitting. Experimentally, a Jsc of 15.59 mA/cm2 was achieved from the Si.lsGe.s 2 cell, with textured Si02 back surface reflector and a single layer SiN, anti-reflection coating. A step-by-step path to reach the 21 mA/cm2 Jsc is provided.

Optical and electrical analysis of graded buffer layers in III–V/SiGe on silicon tandem solar cells

A graded buffer layer is needed in order to grow III-V/SiGe on silicon substrate because of the lattice mismatch. In this work, optical absorption in the graded buffer layer is quantified by measuring the transmittances. We demonstrate a 7 μm graded buffer layer with Ge composition from 0% to 82% is equivalent to a 2 μm SiGe film with 82% Ge composition in terms of optical absorption. A SiGe solar cell was fabricated and characterized. Quantum efficiency measurements show that around 1 mA/cm2 short circuit current is generated from the graded buffer layer in the fabricated solar cell.

GaAsP Hall mobility characterization for GaAsP/SiGe tandem solar cell on Si substrate

This work discusses Hall effect measurements by the van der Pauw (vdP) method of thin gallium arsenide phosphide (GaAs.84P.16) grown on silicon (Si) substrates with graded silicon germanium (SiyGe1-y) working as a buffer layer. The material characterized is used as the top cell of a recently demonstrated III-V/SiGe on Si tandem cell. We report for the first time data of the Hall mobility of the material obtained through measurement of samples with different doping levels and thicknesses and at various temperatures. This data is then used for predicting device performance using a 1-D diode model. The technique can be applied as a material screening test to ensure optimal device performance.