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Voltage-matched multijunction solar cell architectures for integrating PV technologies

National Renewable Energy Laboratory

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Technology Marketing Summary

The U.S. Department of Energy SunShot Initiative aims to reduce the total installed cost of solar energy systems to $.06 per kilowatt-hour (kWh) by the year 2020. Reducing the cost of solar electricity requires that solar cell modules become more efficient and much less expensive. Multijunction (MJ) solar cells present the best option for significantly increasing the absolute module efficiency beyond the single junction Schockley Queisser limit of 33%. To date, MJ solar cell designs have been based solely on “current-matched” configurations that typically only combine expensive III-V semiconductor pn junctions and require complicated and costly fabrication methods. Combining different, low cost PV technologies (i.e. c-Si, CdTe, CIGS, etc.) within an MJ solar cell could provide an alternative way to improve PV module efficiencies as well as keep manufacturing costs and complexity down.

Voltage-matched (VM) solar cells address these issues by eliminating efficiency losses in current-matched MJ solar cells. VM addresses series resistance imposed by tunnel junctions and minimizes variations in power output due to diurnal spectral variations in the solar spectrum and intensity. Variations in the solar spectrum throughout the day can have a large impact on the current that is output by individual pn junctions, but spectral variations will have a much smaller impact on the voltage output. Therefore, the performance of voltage-matched multijunction (VMMJ) solar cells suffers less from diurnal variations than current-matched multijunction solar cells. However, until now VM architectures have been difficult to realize. The process was originally envisioned for single crystalline, heteroepitaxial pn junction stacks. Many etch/deposition steps are required to isolate and interconnect sub-cells. Finally, geometry of the bottom sub-cells is constrained by the top sub-cells. VMMJ solar cell architecture developed by NREL scientists overcomes existing limitations of combining CdTe and Si pn junctions into tandem solar cells while utilizing common manufacturing techniques already in practice.

The bandgap energies of the materials comprising the pn junctions in the monolithic tandem voltage-matched solar cells can span the range of 0.25 eV to 2.5 eV. However, 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 is near the ideal 1.1 eV value for a bottom sub-cell. A wafer of Ge or any other crystalline semiconductor may also be used in place of a Si wafer for the bottom sub-cells, but this may increase the cost of the top device. 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, but are not limited to, polycrystalline CIGS (0.9 – 2.5 eV), CZTS (1.4 – 15 eV), a-Si (1.7 eV), or microcrystalline Si.

Combining crystalline Si and polycrystalline CdTe cells is advantageous because they are the most widely used PV technologies and have the greatest knowledge and manufacturing bases. This combination therefore presents one of the best possibilities for achieving a levelized cost of electricity that is comparable to conventional energy sources, and for being manufactured on a large scale. Incorporating these materials into a current-matched MJ solar cell configuration, however, is impractical because it is difficult to form a high performance tunnel junction between tandem CdTe and Si solar cells. The current-matching constraint also limits the design of the top and/or bottom sub-cells, potentially lowering the overall performance of the device.

Description

VMMJ solar cells use a modified architecture to integrate PV technologies into a tandem solar cell. Junction layers are processed separately and are then combined via a compliant 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. Any two or more semiconductor materials may be combined.

One possible platform 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. Any PV technology can be used for the front sub-cells (a-Si, polycrystalline, single crystalline III-V). 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.

Possible Combinations

·        c-Si/CdTe

·        c-Si/III-V

·        c-Si/a-Si

·        CIGS/CdTe

·        CIGS/III-V

·        Other polycrystalline/crystalline semiconductor combinations

Benefits

·        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

Applications and Industries

·        Commercial power generation

·        Residential power generation

·        Space-constrained locations

·        Small remote power stations

·        Consumer electronics

·        Military, mobile, weight-constrained applications

Patents and Patent Applications
ID Number
Title and Abstract
Primary Lab
Date
Patent 9,287,431
Patent
9,287,431
Superstrate sub-cell voltage-matched multijunction solar cells
Voltage-matched thin film multijunction solar cell and methods of producing cells having upper CdTe pn junction layers formed on a transparent substrate which in the completed device is operatively positioned in a superstate configuration. The solar cell also includes a lower pn junction formed independently of the CdTe pn junction and an insulating layer between CdTe and lower pn junctions. The voltage-matched thin film multijunction solar cells further include a parallel connection between the CdTe pn junction and lower pn junctions to form a two-terminal photonic device. Methods of fabricating devices from independently produced upper CdTe junction layers and lower junction layers are also disclosed.
National Renewable Energy Laboratory 03/15/2016
Issued
Technology Status
Technology IDDevelopment StageAvailabilityPublishedLast Updated
NREL ROI 12-62, 12-69, 14-12, 14-31ProposedAvailable02/27/201502/27/2015

Contact NREL About This Technology

To: Bill Hadley<Bill.Hadley@nrel.gov>