PoTaSi – Demonstration of the Potential of Monolithic Tandem Solar Cells Made of III-V Semiconductors and Silicon

Duration: 10/2017 - 09/2020
Contracting Authority/ Sponsors: Federal Ministry of Economics and Energy (BMWi)
Project Focus:
Triple solar cell with thin layer stack of epitaxially grown III-V compound semiconductors
© Fraunhofer ISE
Image of a manufactured triple solar cell using scanning electron microscopy. The thin layer stack of epitaxially grown III-V compound semiconductors was bonded onto the silicon sub-cell, which is about 280 μm thick. The colouring was done according to an EDX element analysis.
IV characteristics of first and second generation III-V//Si tandem solar cells
IV-characteristics of the first and second generation of III-V//Si tandem solar cells, which were produced within the framework of PoTaSi. The triple solar cells made of III-V semiconductors and silicon were measured in the Fraunhofer ISE CalLab PV Cells under AM1.5g standard conditions (cell area 4 cm2).

Increasing the efficiency of solar cells is one of the key factors in further reducing electricity generation costs. Multi-junction solar cells make this possible by converting the solar spectrum more efficiently in the different subcells. In the project "PoTaSi”, tandem solar cells made of III-V semiconductors are produced and analyzed on a silicon solar cell, whereby the monolithic integration of the partial cells is carried out by means of wafer bonding. The aim is to determine the technological potential of this tandem technology using optimized cell structures, calibrated measurements and optoelectronic simulation. To this end, the best available technologies are used and further developed in all areas.

Higher efficiencies of solar cells at the same or even lower production costs are a central development goal in the field of photovoltaics in order to reduce electricity generation costs. In addition, higher efficiencies allow significant savings in cell and module components such as semiconductor material, metals, glass, frames, connectors or EVA, thus contributing to higher sustainability and lower resource consumption. Silicon solar cells currently dominate the PV market with a share of more than 92%. Building on this high market penetration, a tandem cell with silicon as the bottom cell and an upper cell made of thin III-V semiconductor layers is a particularly promising approach for achieving a substantial increase in efficiency beyond the maximum theoretical efficiency for silicon of 29.4%.

In fact, a triple-junction solar cell made of AlGaInP/AlGaAs//Si with an area of 4cm² was recently demonstrated for the first time at Fraunhofer ISE as part of the project, with an efficiency of 34.1% under the AM1.5g spectrum. The III-V semiconductor layers have a thickness of only about 5μm and were transferred from a GaAs substrate to silicon by wafer bonding (monolithic integration). This is the proof of a monolithic tandem solar cell with silicon, which has only 2 contacts and nevertheless an efficiency above the theoretical limit (Shockley-Queisser) of single-junction solar cells. At the same time, it is known that efficiencies of up to 37% are possible through a further adjustment of the structure. For this purpose, the quality of the III-V absorbers must be further improved and losses due to different current generation in the subcells must be avoided. Surface passivation by means of passivating contacts has already been realized for the silicon bottom cell. In addition, the absorption was improved by optical structures for light guidance on the rear side.

The technological optimization is supported by optical and electrical simulations, which hand in hand lead to a maximization of the efficiency. In this project monolithic III-V//Si triple-junction solar cells with efficiencies in the range of 35% are aimed at. In addition to high efficiency, the cell concept promises very high stability (negligible degradation), as already known from III-V space solar cells.