Integration of a-Ge:H nanocavity solar cells in tandem devices

•Amorphous germanium (a-Ge:H) replaces µc-Si:H absorber in micromorph devices.•A resonant nanocavity is used to enhance the bottom cell absorption.•Additional undesired resonances are induced by the top cell.•Rough interfaces suppress top cell induced resonances, but keep nanocavity effect.•Current density similar to micromorph devices reachable with 20 nm a-Ge:H absorber.

By taking advantage of spectrally broad resonances, nanocavity-enhanced a-Ge:H solar cells with an absorber layer thickness below 20 nm can reach current densities similar to micron-thick µc-Si:H devices. However, as nanocavity-enhanced devices are highly reliant on interference effects, further spectrally narrow resonance patterns are generated if an additional top cell is added to form a multijunction solar cell. This may complicate the integration of a-Ge:H nanocavity solar cells in tandem devices. We show that conventionally textured TCO substrates can be employed to suppress the top cell induced interferences, while the required broadband resonance of the a-Ge:H bottom cell nanocavity is maintained. This approach is realized in an a-Si:H/a-Ge:H tandem solar cell with an only 20 nm thick a-Ge:H bottom cell absorber. The spectrally broad quantum efficiency curve of the bottom cell corresponds to a photocurrent density of 12.3 mA cm−2, which is comparable to values reached in micromorph devices.

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