Abstract
Two terminal multi-junction (MJ) photovoltaic (PV) devices are well established concepts to increase the solar-to-electrical power conversion in reference to single PV junctions. In multi-junction PV devices two consecutive sub-cells are interconnected using a tunnel recombination junction (TRJ) in which the light excited holes of one sub-cell recombine with the light excited electrons of the other sub cell. An ideal TRJ is an ohmic contact with non-rectifying behaviour. TRJ’s based on p- and n-doped silicon-oxides have been successfully applied in a variety of hybrid multi-junction PV devices in which tunnelling and trap-assisted tunnelling over width of 5–20 nm rules the TRJ’s recombination kinetics. In this contribution the qualitative fundamental working principles of tunnel recombination junctions based on p- and n-doped silicon and silicon-oxide alloys are revealed using both electrical modelling and experiments based on a unique set of tandem lab cells (four types based on four different PV materials) combined with structural variations in TRJ architectures. The study results in design rules for the integration of silicon-oxide based TRJ’s and provides fundamental insights into the sensitivity of the electrical performance of the TRJ’s to doping concentrations, to alignment of the conduction and valence bands of consecutive sub-cells, to the nature of interface defects, to the growth of amorphous and crystalline phases and its dependence on substrate or seed layers and to the nanoscale thicknesses of the TRJ layers. • Experimental results of over 100+ different tandem device architectures are presented. • Four types of tandem PV devices are presented based on four different silicon alloys. • p-layer design is structurally varied across different multijunction PV architectures. • Energy band diagrams provide fundamental insight into the working principles of TRJ’s. • The study results in design rules for the integration of silicon-oxide based TRJ’s.
Highlights
Multijunction photovoltaic (PV) devices are a logical step for further reducing the cost price per Watt peak of PV, by increasing the yield per area
Considering the sensitivity of device performance to the p-layer properties in particular, in this work, the effect of changes to the p-layer are investigated across four different multijunction PV device architectures. These are tandem devices consisting of a wafer-based silicon heterojunction (SHJ) subcell and subcells with hydrogenated nanocrystalline silicon absorbers, hydrogenated amorphous silicon germanium (a-SiGe:H) absorbers and hydrogenated amorphous silicon (a-Si:H) absorbers, in various configurations
There are several ways in which the properties of the p-doped layer in a tunnel recombination junction (TRJ) can result in losses to the Voc and fill factor (F F) in a tandem device
Summary
Multijunction photovoltaic (PV) devices are a logical step for further reducing the cost price per Watt peak of PV, by increasing the yield per area. This allows for the isolation of the impact of the TRJ, from the opto-electrical nature of the subcells, on the device performance To achieve this isolation, and considering the sensitivity of device performance to the p-layer properties in particular, in this work, the effect of changes to the p-layer are investigated across four different multijunction PV device architectures. Considering the sensitivity of device performance to the p-layer properties in particular, in this work, the effect of changes to the p-layer are investigated across four different multijunction PV device architectures These are tandem devices consisting of a wafer-based silicon heterojunction (SHJ) subcell and subcells with hydrogenated nanocrystalline silicon absorbers, hydrogenated amorphous silicon germanium (a-SiGe:H) absorbers and hydrogenated amorphous silicon (a-Si:H) absorbers, in various configurations. Electrical simulations are performed to qualitatively support findings from the experimental results, in order to reveal the fundamental operation mechanisms that determine the performance of tunnel recombination junctions
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