Resonant-cavity-enhanced a-Ge:H nanoabsorber solar cells for application in multijunction devices

Highlights: • A resonant-cavity-enhanced a-Ge:H nanoabsorber solar cell with improved fill factor is presented. • Specific design for bottom cell application reduces negative impact of resonance minima at lower wavelengths. • A nanorod-textured intermediate reflector avoids additional interference with top cell resonances. A resonant-cavity-enhanced solar cell with a 25 nm thick amorphous germanium nanoabsorber is demonstrated. The device is specifically designed for use as bottom cell in multijunction devices. In this case, instead of relying on broadband absorption, only red light needs to be absorbed in the bottom cell. This allows exploiting the narrow resonance patterns of long Fabry-Pérot cavities to achieve efficient absorption inside the nanoabsorber. It is shown that this approach can only be integrated into multijunction devices when suppressing inter-subcell coherence. To achieve this, the intermediate reflector of the tandem device is textured using electrochemically grown zinc oxide nanorods.