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Bucket brigade photovoltaic effect in ferroelectrics

The periodic structure of ferroelectric materials allows them to behave as a group of current sources connected in series, producing photovoltages larger than each material's band gap would seem to permit.
12 December 2011, SPIE Newsroom. DOI: 10.1117/2.1201112.003983

Ferroelectric materials have a built-in, spontaneous electrical polarization. The polarization's direction can be switched by an applied electric field. The electric fields caused by the polarization influence charge flow. As a result, ferroelectrics have found numerous applications as sensors and in solid-state memory. It is an attractive idea to use the built-in electric fields to drive a photovoltaic (PV) current. Indeed, it has been known for quite some time that some ferroelectric materials can develop very large photovoltages under illumination. Although the effect can be spectacular, its detailed understanding has been elusive.

The study of above-band-gap photovoltages that occur in some ferroelectrics and also in polycrystalline thin-film semiconductors dates from the late 1950s. Pankove summarized the state of the field in 1980 as follows: “Although this phenomenon is spectacular and most intriguing, its understanding is at the stage of tentative models."1 However, recent progress in the growth of single-crystal ferroelectric thin films with control over the polarization structure has set the stage for the development of a fundamental understanding of these high-voltage PV effects.2, 3

A major limitation is incomplete knowledge of the exact nature of the electric fields that produce the effect, e.g., whether they surround impurity atoms or occur between different grains in polycrystalline materials.4, 5 This uncertainty is eliminated in thin films of ferroelectric bismuth iron oxide (BiFeO3 or BFO). Such layered, single-crystal BFO can be made with exquisite control over the areal patterning of domains of differing electrical polarization. Figure 1(a) shows a piezo force microscopy image of a thin film of BFO. The narrow yellow lines are due to the large electric field that develops at the ‘walls’ (only 2nm wide in the film shown) between the domains (150nm across). The ferroelectric domain structure is highly periodic and extends over large distances up to hundreds of microns. Figure 1(b) shows a schematic band diagram for BFO along a direction perpendicular to the stripes. The polarization step between the domains leads to a potential step at the domain walls, and the resulting band structure has a sawtooth shape.

Figure 1. (a) Piezoresponse force microscopy image showing well-aligned arrays of domain walls in an epitaxial BiFeO3 film (bismuth iron oxide, or BFO). The domains of opposite electrical polarization are 150nm wide, and the walls between them (light contrast) are 2nm in width. (b) Schematic band structure and photovoltaic (PV) current flow. The polarization step at the domain walls creates a sawtooth electrical potential that is reflected in the position of the conduction (red) and valence (blue) band edges. The wall width and the height of the potential step are exaggerated for clarity. (Adapted from Seidel et al.)6

Illumination of BFO thin films with simulated sunlight leads to a dramatic PV effect.7 Figure 2(c) shows that for top contacts positioned such that the current flow in the film is perpendicular to the domain walls, photovoltages far in excess of the 2.7eV band gap of BFO are generated. The photovoltage is linear in the number of domain walls between the contacts, suggesting an additive effect. The direction of the current flow is reversed if the sample's polarization direction is switched by applying a strong electric field, an effect that has no analogy in normal photovoltaics. For contacts positioned such that current flows along the domain walls, photoconductivity but not a net PV effect (i.e., non-zero open circuit voltage) is observed.

Under illumination, both excess electrons and holes are generated (in the dark, BFO is highly insulating).8 The sawtooth potential shown in Figure 1(b) causes electrons to accumulate on one side of the wall, while oppositely charged holes are repelled as shown in Figure 2(c). On the other side of the wall, holes accumulate and electrons are driven away. As a result, the carrier recombination rate (the key PV loss mechanism) is reduced next to the walls as both an electron and hole are required for this process to occur: see Figure 2(d). In the middle of the domains, the carrier populations are more equal, and the higher recombination rate allows for the domains to connect electrically in series. The domain walls act as current sources, pumping electrons from one domain to another as in a bucket brigade. While the photovoltage per wall is low, about 10mV in this case, the contribution of thousands of walls in series leads to the large observed values. Our analysis of the internal quantum efficiency for individual domain walls shows it can be surprisingly high and approaches 10% for photons of energy above the band gap.

Figure 2. (a) Schematic of ferroelectric BFO film with top electrical contacts such that current flow traverses the domain walls. The film is illuminated from above. (b) Measured current-voltage characteristics under white-light illumination (200mW/cm2) showing an open circuit voltage of 16V, which far exceeds the 2.7eV band gap of BFO. (c) Carrier concentrations for electrons (red) and holes (blue) under illumination as a function of position: two domain walls (shaded) are depicted. Electrons and holes accumulate on opposite sides of the walls. (d) Carrier recombination rate R as a function of position across the domain walls. Near the walls, R exceeds the photogeneration rate G as indicated by the shading. These regions act as nanoscale sources of PV current. (Adapted from Seidel et al.)6

Although we found the effect experimentally in BiFeO3 thin films, the bucket brigade mechanism should operate in any system with a similar periodic potential. Our current research is focused on boosting the PV efficiency by inducing the effect in materials that more effectively absorb the solar spectrum. This could potentially lead to both high-current and high-voltage performance. This work is a significant advance in the fundamental understanding of PV effects in ferroelectrics.

Thin-film deposition was supported by the Helios Solar Energy Research Center, which is supported by the Director, Office of Science, Office of Basic Energy Sciences of the US Department of Energy. Photovoltaic measurements and modeling were supported by the Laboratory Directed Research and Development Program of Lawrence Berkeley National Laboratory. LBNL operates under US Department of Energy contract DE-AC02-05CH11231.

Joel W. Ager
Lawrence Berkeley National Laboratory
Berkeley, CA

Joel Ager is a staff scientist in the Materials Sciences Division. He is a project leader in the Joint Center for Artificial Photosynthesis and a principal investigator in the Electronic Materials Program. His research interests include fundamental study of carrier transport and optical spectroscopy in compound semiconductors, development of new PV materials and mechanisms, and investigation of the unique properties of isotopically enriched silicon. He has published over 200 papers, review articles, and book chapters.

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