FY2010 - Oak Ridge National Laboratory

Director’s R&D Fund—
Science for Extreme Environment: Advanced Materials and Interfacial Processes for Energy
05451
Designing High-Efficiency Photovoltaic Heterostructures
by Interfacing Polar and Nonpolar Oxides at the Atomic Scale
Ho Nyung Lee, Satoshi Okamoto, Gerald E. Jellison, Jr., Matthew F. Chisholm, and David J. Singh
Project Description
This project is aimed at obtaining fundamental understanding of oxide interfaces in a photovoltaic (PV)
context. If successful, these insights will be enabling for new approaches to high-efficiency generation III
PVs. Of necessity, these high-efficiency cells require new materials with appropriate band gaps and
electronic properties, and entirely new approaches may be required. Although complex oxides are
normally quite stable, there has been very little interest in their applications to PVs because most oxides
have a large optical band gap. Furthermore, doping both n-type and p-type to obtain high-mobility
materials is not yet possible in typical oxides, so charge separation is more difficult. However, recent
advances in complex oxide synthesis have demonstrated that the interfaces in polar-nonpolar
heterostructures can be electronically reconstructed, leading to conducting layers with high carrier
mobility. Here, we will strive to gain fundamental insight into the role of the interface on PV properties
by investigating transition metal oxide heterostructures. The ultimate goal of this project is to show that
polar interfacial oxide heterostructures can yield efficient PVs. We will investigate perovskite-based
interfacial oxide heterojunctions seeking systems that offer (1) high mobility, (2) efficient solar light
absorption, and (3) charge separation. The focus of this program is entirely new, will build on strengths,
and will fundamentally differentiate ORNL from other PV programs, thereby giving ORNL a competitive
advantage in pursuing new funding opportunities.
Mission Relevance
The quest for new energy materials is central to a broad range of research programs. Thus, the methods
and prototype materials developed in this project will undoubtedly impact on the mission of the DOE
Office of Basic Energy Sciences (BES) in both directly and indirectly related fields. Moreover, the work
undertaken here is a DOE BES opportunity for world leadership in a new class of materials for highefficiency
PV applications. Since 50% efficient PV is the stated DOE goal for third-generation PVs, this
will give us a comparative advantage for understanding and developing new energy materials.
Results and Accomplishments
Based on atomic scale p-n junctions, we have found that the electronic ground states are strongly
influenced by the potential difference between the p- and n-type interfaces imposed by the layer
thickness. This implies that when the interfaces are properly constructed, the charge carriers generated
from the solar light illuminating can be separated. While superlattices with very thin individual layers
(n = 1 and 2) are highly resistive (>10 6 ohm), superlattices with greater individual layers (n = 5 and 10)
have shown semiconducting behaviors with a carrier density ~10 13 /cm 2 . We also have found that when
atomic layers are substituted by perovskites with a high absorption coefficient, the overall band gap can
be lowered by about 50%. This has been confirmed by substituting the Mott insulator LaCoO 3 for Bi 2 O 2
layers in the layered perovskite Bi 4 Ti 3 O 12 . Our spectroscopic ellipsometry confirmed the band gap
reduction from 3.3 to 1.6 eV. Moreover, by using recently developed density functional theory for
accurately determining an oxide’s band gap, we have determined for the first time the band gap of
Bi 4 Ti 3 O 12 theoretically, which has been also confirmed to match well with the experimentally
determined E g .
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