NREL's III-V Team Demonstrates Record Efficiency Dual-Junction Solar Cell
June 1, 2017
Scientists at the U.S. Department of Energy's National Renewable Energy Laboratory (NREL) have set a record efficiency for a dual-junction solar cell, surpassing the previous mark by a full percentage.
Under one sun of illumination, the efficiency was certified at 32.6%.
The series-connected device comprises a 1.7-eV GaInAsP top cell and a lattice-mismatched, 1.1-eV GaInAs bottom cell, grown monolithically by atmospheric pressure metal-organic vapor-phase epitaxy (MOVPE). The inverted metamorphic multijunction (IMM) device strategy is compatible with substrate removal and reuse technologies, which are important for cost reduction and attractive to realize lightweight and flexible solar cells. The new result will be presented in June at the 44th IEEE Photovoltaic Specialists Conference in Washington, D.C.
Tandem solar cells enable increased power-conversion efficiencies compared to single-junction cells. The tandem design splits the incident spectrum for more optimal absorption, which reduces the thermalization losses and leads to a higher voltage. This new result builds on the team’s earlier demonstration of 1.7-eV tunable-bandgap GaInAsP solar cells and lattice-mismatched (or metamorphic) 1.1-eV GaInAs solar cells. The material quality is excellent in both alloys, despite the propensity for phase separation in the GaInAsP alloy and the lattice mismatch in the GaInAs alloy. The bandgaps are much closer to the ideal combination than the higher bandgaps of previous GaInP/GaAs record dual-junction cells that are more commonly employed due to their lattice-matched nature. The team believes that evolving both alloy bandgaps toward the global optimum of 1.6 eV/0.9 eV may yet lead to even higher efficiency.
Higher cell and module efficiencies translate directly into an increase in energy production while reducing the land capital expenditure and installation cost of a system, thus influencing the balance-of-systems economics. Tandem cells could be particularly valuable for area-constrained or high-performance applications and are being developed at NREL for concentrator photovoltaic applications. The bandgap combination for this record cell also represents a natural guide for the practical efficiency attainable for silicon-based tandems, which would optimally be designed with a ~1.7-eV top junction.
NREL scientists John Geisz, Myles Steiner, Nikhil Jain, Kevin Schulte, Dan Friedman, Ryan France, Bill McMahon, and Emmett Perl developed this tandem solar cell as part of a larger ongoing project to design a six-junction concentrator solar cell with a target efficiency of >50% at high illumination intensity. These and similar tandem cells have also led to record solar-to-hydrogen generation efficiencies via photoelectrochemical water splitting, in a project led by Todd Deutsch, James Young, and John Turner.
This research is supported by the DOE's SunShot Initiative, a national effort to drive down the cost of solar electricity and support solar adoption. Learn more at energy.gov/sunshot. Partial funding also comes from the DOE's Fuel Cell Technologies Office.