Image courtesy of NREL
[Left] Next generation multijunction solar cell with a dilute nitride middle junction (red) [Right] A transition from delocalized to localized electron trapping is directly observed in the photoluminescence (PL) spectra as the magnetic field is increased up to 57 Tesla. Schematics (bottom right) illustrate the electron wavefunction overlap in delocalized nitrogen (N) superclusters at low magnetic fields and (top left) electron wavefunction localization at isolated N clusters at high magnetic fields.
High magnetic fields are used for the first time to unravel the unusual behavior of electron trapping and its effect on the electronic properties for emerging gallium arsenide nitride (GaAs1-xNx) photovoltaic materials.
The new insight into electron trapping in dilute nitride semiconductors will help mitigate their undesirable transport properties in order to utilize them for ultra-high efficiency solar cells.
The highest efficiency multijunction photovoltaic (PV) devices require materials for each junction that have carefully selected bandgaps (the energy gap between the conducting and insulating states for a material) and have the same atomic spacing as the germanium substrates. The dilute nitride semiconductor alloy gallium arsenide nitride [GaAs1-xNx (x < 0.3)] is one of the only materials that meets both of these requirements and has recently been used to achieve world record solar cell efficiencies. However, electron trapping by unintentionally formed nitrogen cluster states degrades the electrical properties of this alloy, and the behavior of these clusters must be better understood in order to control their undesirable effects. Using photoluminescence (PL) spectroscopy under a magnetic field, researchers at the National Renewable Energy Laboratory (NREL) and the National High Magnetic Field Laboratory (NHMFL) at the Los Alamos National Laboratory (LANL) have identified the existence of nitrogen superclusters that funnel electrons to even deeper electronic trap states. At very high magnetic fields, the individual nitrogen clusters comprising the superclusters are decoupled, restoring their localized nature. These measurements indicate that controlling formation of clustering during growth will reduce their deleterious effects, leading to improved electron transport and better photovoltaic performance.
DOE Office of Science, Basic Energy Sciences Program. Work performed at the National High Magnetic Field Laboratory Los Alamos was supported by the NSF, DOE and State of Florida.
K. Alberi, S.A. Crooker, B. Fluegel, D.A. Beaton, A.J. Ptak and A. Mascarenhas, Magnetic Field-Induced Delocalized to Localized Transformation in GaAs:N, Phys. Rev. Lett., 110, 156405 (2013)
Collaborations, Non-DOE Interagency Collaboration