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In this paper the opto-electrical nature of hydrogenated group IV alloys with optical bandgap energies ranging from 1.0 eV up to 2.3 eV are studied. The fundamental physical principles that determine the relation between the bandgap and the structural characteristics such as material density, elemental composition, void fraction and crystalline phase fraction are revealed. Next, the fundamental physical principles that determine the relation between the bandgap and electrical properties such as the dark conductivity, activation energy, and photoresponse are discussed. The unique wide range of IV valence alloys helps to understand the nature of amorphous (a-) and nanocrystalline (nc-) hydrogenated (:H) germanium films with respect to the intrinsicity, chemical stability, and photoresponse. These insights resulted in the discovery of i) a processing window that results in chemically stable Ge:H films with the lowest reported dark conductivity values down to 4.6•10 -4 (𝛀 •cm) -1 for chemical vapor deposited Ge:H films, and ii) O, C and Sn alloying approaches to improve the photoresponse and chemical stability of the a/nc-Ge:H alloys.

A logical next step for achieving a cost price reduction per Watt peak of photovoltaics (PV) is multijunction PV devices. In two-terminal multijunction PV devices, the photo-current generated in each subcell should be matched. Intermediate reflective layers (IRLs) are widely employed in multijunction devices to increase reflection at the interface between subcells to enhance current generation in the subcell(s) positioned before the IRL, in reference to the incident light. In this work, the results of over 65 multijunction devices are presented, in order to explore the effect of different current matching approaches. The influence of variations in absorber thickness as well as thickness variations of different IRLs based on silicon-oxide, various transparent conductive oxides (TCO), and metallic layers on all-silicon multijunction PV devices is studied. Specifically, hybrid, 2-terminal, monolithically integrated silicon heterojunction (SHJ) and thin film nanocrystalline silicon (nc-Si:H) and amorphous silicon (a-Si:H) tandem and triple junction devices are processed. Based on these experiments, certain design rules for optimal current matching operation in multijunction devices are formulated. Finally, taking these design rules into account, record allsilicon multijunction devices are processed. Conversion efficiencies close 15% and V oc ≈ 2 V are demonstrated for triple junction SHJ/nc-Si:H/a-Si:H devices. Such conversion efficiencies for a wireless, high-voltage wafer-based all-silicon 2-terminal multijunction PV device opens the way for efficient autonomous solar-to-fuel synthesis systems as well as other wireless innovative approaches in which the multijunction solar cell is used not only as a photovoltaic current-voltage generator, but also as an ion-exchange membrane, electrochemical catalysts, and/or optical transmittance filter.