Multijunction (MJ) solar cells present the best option for significantly increasing the absolute module efficiency beyond the single-junction Shockley-Queisser limit of 34%. However, MJ solar-cell designs typically use current-matched configurations that combine expensive III-V semiconductor pn junctions and require complicated and costly fabrication methods. Voltage-matched (VM) solar cells address the series-resistance limitation imposed by current-matching tunnel junctions and minimize variations in power output due to diurnal spectral variations and intensity. (Diurnal variations can have a large impact on spectrally optimized pn-junction current from individual junctions and thus on the total current from the MJ cell.) VMMJ solar cell architecture developed by NREL scientists overcomes existing limitations of combining CdTe and Si pn junctions into tandem solar cells while using common manufacturing techniques.
The ideal combination of bandgap energies for a tandem solar cell under one-Sun illumination is approximately 1.1 eV and 1.7 eV. The bandgap energy of Si or Ge is near the ideal 1.1 eV value for a bottom sub-cell. A number of polycrystalline or amorphous thin film materials may be used for the top sub-cells. CdTe (1.45 eV) has a bandgap energy that is slightly lower than the ideal value but would still be viable. CdTe may also be alloyed with Zn, Se, or S to increase the bandgap energy. Other possible thin film materials may include polycrystalline CIGS (0.9 – 2.5 eV), CZTS (1.4 – 1.5 eV), a-Si (1.7 eV), or microcrystalline Si.
VMMJ solar cells use a modified architecture to integrate PV technologies into a tandem solar cell. Junction layers are processed separately and then combined via a transparent, electrically insulating barrier. This approach facilitates the serial interconnection of sub-cells into strings. The simplicity of the architecture enables the different components to be fabricated using existing methods and eliminates problems due to processing incompatibilities. One possible configuration is a wafer-based c-Si solar cell platform. This platform leverages a mature Si-based PV industry and abundant Si solar cells. Interdigitated back contacts simplify interconnections and enable high efficiency due to reduced optical shading losses. Multiple PV technologies can be used as the front sub-cells. Another possible platform uses a substrate/superstrate combination, which enhances module efficiency while maintaining low processing costs. This platform will be able to retain the current fabrication technologies with small modifications and is ideal for a CIGS substrate and CdTe superstrate combination.
To learn more, please contact Bill Hadley at:
ROI 14-31 & 12-62
Applications and Industries
- Commercial power generation
- Residential power generation
- Space-constrained locations
- Small remote power stations
- Consumer electronics
- Military, mobile, weight-constrained applications
- Substantial increase in module efficiency over single junction technologies
- Draws on well-developed PV technology base for shortened development times
- MJ devices benefit from any cost savings/efficiency improvements realized for single junction counterparts
- Preserves additional functionality, such as flexibility
- Connection of pn junctions in parallel instead of series eliminates the need for heavily doped tunnel junctions
Possible Multijunction Solar-Cell Combinations
- Other polycrystalline/crystalline semiconductor combinations