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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - SiTaSol (Application relevant validation of c-Si based tandem solar cell processes with 30 % efficiency target)

Teaser

Summary

Solar cells from crystalline silicon wafers have been dominating the photovoltaic market so far due to the availability and stability of c-Si and the decades of Si technology development which were leading to two orders of magnitude in cost reduction. However, conversion efficiencies are limited for single-junction c-Si devices and efficiencies exceeding 25% in production are hardly achievable. Therefore, it becomes increasingly important to find solutions how to increase efficiency by moving towards more advanced technologies. But at the same time such technologies may significantly benefit from building on the success of c-Si solar cell devices. The EU-funded project SiTaSol develops a next generation photovoltaic technology with approximately 30 % higher performance compared to state-of-the-art c-Si solar cells. It answers important questions leading to low manufacturing costs and low environmental impact. The III-V/Si solar cells will be attractive not only for the PV power market but also for the growing market of mobile applications where solar cells are integrated into cars, wearable electronics, unmanned vehicles or consumer devices to extend grid independent operation. Furthermore, the project will lead the way to lower cost manufacturing of conventional GaAs based devices like LEDs, sensors, transistors, switches and photodetectors which are the basis of growing markets for â€œSmartâ€ technologies.

Work performed

During the first 2.5 years of the SiTaSol project, we investigated processes leading to economic manufacturing of GaInAsP/Si tandem solar cells with an efficiency potential exceeding 30%. This included the design and manufacturing of a new high throughput MOVPE reactor, currently used to develop GaAs with growth rates exceeding 100 Âµm/h as well as Ga(In)AsP compounds with approximately 1.7 eV bandgap, suitable as top absorber above a Silicon bottom solar cell. The bottom solar cell consists of a silicon wafer with a diffused or implanted pn-junction. For the direct growth of the III-V compounds onto this Silicon bottom cell, it was important to find economic processes for preparing the surface with very low roughness and defined surface step structure. The typical polishing steps used in the microelectronics industry were not compatible with the solar cell cost targets but we could identify a process which allows significant cost reduction and it has been already shown that low roughness values in the nanometer range are achieved with this process as well as Galliumphosphide growth with low defect density. This is a very encouraging result for our Concept A approach in which the III-V layers are grown directly on Silicon. The development of the Silicon bottom solar cell targeted higher current densities by using light trapping structures on the rear side. Here self-organized nanostructures as well as nano-patterned resist structures have been investigated and both were leading to a current increase on the order of 1.5 mA/cm2. These photonic structures act as diffusor for the sunlight reaching the back side of the silicon solar cell and therefore enable higher absorption. Cells have been optimized both for concept A and B, reaching open circuit voltages of >680 mV and 651 mV respectively. For Concept B, the III-V layers are grown on GaAs using a sacrificial layer which allows lifting the III-V solar cell structure after the growth and transferring it onto a Silicon bottom solar cell. We have been working on wet-chemical as well as laser lift-off and have found that the removal of the III-V layer stack is possible with both methods. But currently one of the main challenges is the fragility of the lifted layers which tend to break into little pieces. We have worked on methods which allow the attaching of the III-V layer stack to the Silicon bottom cell before the lift-off but this has so far not very successful. The attachment of the thin III-V layer to the silicon solar cell was investigated using spray pyrolysis of a transparent conductive oxide layer which has sufficient mechanical stability and forms the electrical contact between the III-V and Silicon cells. Here first results have proven that ohmic contacts can be formed to p-GaAs and n-Si and that the interface is sufficiently stable for further processing. But the combination of p-GaAs and n-Si has been leading to technological challenges in both adhesion and ohmic resistance. So far, the resistance is remaining high at 1-2 Ohm cm2. Both solar cell Concepts A and B require additional manufacturing steps for the formation of metal contacts and anti-reflection coatings. Here we investigated ink-jet printing of low-cost metals like Cu. Laser sintering of the metals is developed to achieve low resistance contacts and contact resistances of 10 mOhm*cm2 were achieved for a printed Cu contact on n-GaAs. After the first two years of the project we have concluded that Concept A has a higher economic and technological potential and further developments will target the implementation of all SiTaSol technologies into this GaAsP/Si tandem solar cell. Besides technological advancement, a detailed life cycle assessment has been performed for the different process routes leading to III-V/Si tandem solar cell devices. We have found that the III-V/Si tandem solar cells can be beneficial in terms of environmental impact due to the higher efficiency and that the electricity of the III-V epi

Final results

The results which are expected from SiTaSol are the demonstration of III-V/Si tandem solar cells which are produced using economic manufacturing methods. Many groups work on the implementation of III-V solar cells with c-Si but they neglect that costs are two orders of magnitude above what c-Si photovoltaics delivers today. First we investigate wet chemical processes to prepare a c-Si wafer, suitable for the direct growth of a III-V cell on silicon. This is a completely new field which has not been investigated yet but which is mandatory to achieve the costs necessary for large scale photovoltaics. Second we build new epitaxy equipment to reduce cost of the III-V layer growth. In SiTaSol we develop a scalable high-throughput MOVPE reactor which will allow significantly higher growth rates at low V/III ratios and high precursor efficiency. We develop also low cost processes for the front contact metallization and back-end processing of these solar cells. The target is to bring all of these new technologies together and demonstrate the value of the III-V/Si tandem technology.

Potential environmental, health and safety issues as well as socio-economic and environmental aspects from a life-cycle perspective are addressed by performing a detailed life cycle analysis and connecting this to other EU projects in the field of III-V compound semiconductors. Potential health risks will be investigated as well as legal issues. Recycling of precious metals in the epitaxy growth of the III-V compound semiconductor layers is studied in detail as this is an important economic- as well as sustainability topic.