The realization of sustainable and circular regional value chains for annual photovoltaic (PV) markets at a scale of multiple terawatts, sets new unprecedented societal and technical challenges expressed in new value-chain-metrics. The new challenging value-chain-metrics are related to efficiency, costs, reliability, circularity, life cycle analyses, sustainable business models, regional independence and human capital. The unique physical characteristics of lightweight and flexible thin-film silicon PV (LF-TFSiPV) technology makes this technology an excellent candidate for future circular markets that are complimentary to the conventional glass encapsulated crystalline silicon photovoltaic technology. The application benefits of solar foils are i) customized panels to facilitate new application opportunities in the urban environment, industrial regions and transport sector, ii) new innovative designs for BoS that allow large scale utility applications on land or water at extreme low LCoEs. iii) and the design rules for a fully circular value chain for LF-TFSiPV. 

In this work, advances in multi-junction PV devices based on both thin-film silicon alloys (amorphous and nanocrystalline) and germanium alloys (amorphous and nanocrystalline) will be discussed. We will elaborate on the roadmaps of the various device architectures for solar foils with higher efficiencies and its relation to supporting and photovoltaic materials. 

Amorphous silicon and nanocrystalline silicon are fairly known materials in the photovoltaic field. The record efficiency of a thin film silicon multijunction device is 14.8%. To try to push the efficiency further, this work looks at the fine tuning of these materials, including the TCOs used. In particular, the bandgap of amorphous silicon can be tuned by controlling the deposition parameters, which can be used to increase the voltage of the solar cell. Using a combination of low bandgap and high bandgap materials can allow for maximum current and voltage output of the solar cell with state-of-the-art long term stability. 

Amorphous germanium can be a good candidate for a bottom cell absorber in multijunction devices. However, it has not been studied thoroughly for photovoltaic applications. This work explores the deposition parameter space (temperature, pressure, RF power, GeH₄ flow, etc.) and how it affects the optoelectrical parameters and stability of the films. A thorough characterization was conducted, including FTIR, spectroscopic ellipsometry, XPS, dark and photoconductivity measurements. This work reports on thin-film germanium alloys with bandgaps of 0.8-0.9 eV and intrinsic electrical behaviour, which will be a suitable low band gap absorber for implementing in a solar cell.