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Application relevant validation of c-Si based tandem solar cell processes with 30 % efficiency target

Periodic Reporting for period 3 - SiTaSol (Application relevant validation of c-Si based tandem solar cell processes with 30 % efficiency target)

Reporting period: 2019-11-01 to 2021-01-31

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 performance 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 developed a next generation III-V/Si tandem solar cell with a target conversion efficiency of 30 %. It answered important questions for reaching low manufacturing costs and low environmental impact. III-V/Si solar cells are 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 leads 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 “Smart” technologies.
In SiTaSol we investigated two process routes with high potential to reach GaInAsP/Si tandem solar cells with efficiencies exceeding 30% and low manufacturing costs. One process used the transfer of the III-V thin film from a GaAs substrate onto Silicon, with promising results reaching up to 29% efficiency under AM1.5g for a wafer bonded AlGaAs//Si device and up to 8.6% conversion efficiency for a GaInP/GaAs/Si triple-junction cell realized by low cost ZnO gluing – the latter is the first of its kind. This process of low-cost gluing and substrate reuse for the GaAs has been shown to be challenging and therefore the direct growth of GaAsP on silicon was prioritized after the first half of the project. Here we achieved very high growth rates up to 100 µm/h on a new large area MOVPE reactor which was developed during the project. Also, a new solar cell structure was developed on this machine, leading to first GaAsP/Si tandem devices with 20.8 % efficiency. A world record performance was reached for a new GaInP/GaAs/Si triple-junction cell grown directly onto the low-cost silicon substrate which were developed in SiTaSol. Manufacturing processes for low cost metal contacts were developed using different printing methods. Unfortunately, many unforeseen technological difficulties prevented us from implementing the full process flow into the final solar cell devices. On the other hand, many new components have been successfully demonstrated, including a very positive life-cycle analysis for the III-V/Si tandem solar cells showing the high value of the technology, an energy yield analysis and a cost reduction roadmap towards industrial exploitation was also developed. We have published our findings in numerous scientific publications and have presented them at conferences. Some parts of the development may lead into direct industrial exploitation while realizing a III-V/Si tandem solar cell which competes on the market will still require further R&D in the future.
The results expected from SiTaSol were the demonstration of III-V/Si tandem solar cells with economic manufacturing methods. Many groups work on the implementation of III-V solar cells with c-Si but they neglect that costs have two orders of magnitude above c-Si photovoltaic devices today. Firstly, we investigated wet chemical processes to prepare a c-Si wafer, suitable for the direct growth of the III-V layers on silicon. This is a completely new field which has not been investigated before and very important progress was made in SiTaSol by developing a low-cost preparation process for the silicon wafers. This has already led to at least a30% cost reduction and the wafers have been shown to be suitable for high quality III-V crystal growth. Also, lifetime values up to 17.000µs show the outstanding quality of these silicon substrates, well-suited for high efficiency photovoltaics. Secondly, we built new epitaxy equipment to reduce the cost of the III-V layer growth. This reactor holds up to thirty-one 4-inch wafers and has shown growth rates up to 100 µm/h for GaAsP, MO efficiency of 42% and good levels of homogeneity across the susceptor and wafers. These are encouraging results which can immediately reduce the cost of the MOVPE processes, and they inspire further developments in the future. We developed low cost front metal contacts and back-end processes for these solar cells. For the first time inkjet and Aerosol-jet printing were used to form the seed metallisation for the front contact and metal fingers of III-V/Ge reference solar cells with promising results. Transferring the process to the GaAsP/Si tandem solar cells with higher surface roughness turned out to be challenging due to issues with the wetting of the ink. Further R&D will be required in the future to solve this issue. The best dual-junction solar cells on silicon in the project reach 29% conversion efficiency showing the potential of these tandem devices. Directly grown triple-junction cells on the low-cost Si wafers result in 25.9% efficiency and directly grown GaAsP/Si cells on the high-throughput MOVPE reactor reach 20.8% efficiency. This is below the overall target of 30% efficiency but still clearly at the top level of the international photovoltaic community. Therefore, the SiTaSol project has very clearly extended the state-of-the-art and shown a path towards cost efficient III-V/Si tandem solar cells. This is further supported by a detailed life-cycle assessment which was performed within SiTaSol, an energy yield analysis and cost model predicting clear pathways for further developments.