Traditional imaging technologies have been used to investigate overall solar efficiency, but many of the methods only offer surface views. A new – and “exciting” – ultra-fast laser technique developed at the Department of Energy's Lawrence Berkeley National Laboratory provides a deeper look and maps a solar cell in three dimensions.
Photonics & Imaging Technology spoke with Edward Barnard, a principal scientific engineering associate at the Molecular Foundry, a user facility located within the lab.
Photonics & Imaging Technology: What is the benefit of mapping thin-film solar cells in 3D?
Edward Barnard: A lot of imaging techniques look at the surface of a solar cell. Below the surface, however, are possible defects, such as the boundaries between crystalline grains or the critical junctions between layers.
The real goal for this technique was to look at the micron and submicron scale, where the efficiency in a solar cell could be lost. We're looking in three dimensions, so we can see below the surface, see laterally, and, instead of getting just an aggregate efficiency number, we can actually hone in on where we're losing carrier lifetime in the system.
P&IT: What does the carrier lifetime demonstrate?
Barnard: In a solar cell, you try to extract the electrons that are excited by sunlight so you can drive a circuit. If they don't stay excited for long enough, you will never extract those electrons out of the solar cell, and this is an example of a loss mechanism that lowers efficiency. We wanted to map this carrier lifetime throughout the material to pinpoint where these losses occur. We can find locations in the solar cell where the carrier lifetime is reduced, which allows researchers to start improving solar cell efficiency by targeting those areas.
P&IT: How does the imaging technique work?
Barnard: We're exciting the solar cell with a laser. We're looking at how long the excited electrons stay active before they send light back out. This carrier lifetime is what we're measuring, and we're mapping that in three dimensions.
P&IT: What is special about the laser that you use?
Barnard: We have an ultra-fast infrared laser that can excite the electron carriers using nonlinear optical absorption. Typically, infrared light goes right through the solar cell. If you shine infrared light on a solar cell, nothing happens.
However, if you put enough photons and enough light in one place and at one time, you can get the energy of two or more of those photons to excite an electron. And you only get this absorption in the highest-intensity spots of the laser focus. This two-photon excitation allows us to excite below the surface of the material, unlike normal photoexcitation of a solar cell, which is predominantly absorbed near the surface.
P&IT: From a technology perspective, how is the solar cell mapped in three dimensions?
Barnard: Since we can excite the solar cell both at the surface and below the surface, our technique allows us to measure the lifetime at any position within the solar cell. We do this by moving the sample through the laser focus spot using a three-axis piezoelectric stage. At each position of the sample, we record the time when light is reemitted from the solar cell, to create a map of carrier lifetime.
P&IT: What are the drawbacks of previous imaging methods?
Barnard: Traditional lifetime mapping, which is done with visible light instead of infrared light, gives you information about what's happening at the surface. A lot of the interesting phenomenon and problems that you may have with a solar cell, however, are not just at the surface; they are below that top 100-200 nm.
P&IT: Are there techniques that don't use light?
Barnard: An imaging technique that does not use light is high-resolution transmission electron microscopy (TEM). TEM can tell you what atoms are where, with single atom precision, but the technique doesn't tell you how well it operates as a solar cell. The process also requires destructive cross-sectioning of the sample.
By combining the ability to see in 3D, we see our method as giving the ability to scientists to explore the insides of a solar cell in action.
P&IT: When was this imaging technology used?
Barnard: To develop this technique, we collaborated with a small startup company which was a user at the Molecular Foundry, a DOE Office of Science user facility located at Berkeley Lab. A company called PLANT PV Inc. [based in Alameda, California] was interested in creating and characterizing new materials for solar cells. That's where I became involved and came up with a way to look below the surface of the material.
We extended the technique to three dimensions, working with the National Renewable Energy Lab, where the collaborators there were interested in developing and providing cadmium telluride (CdTe) solar cells. We mapped them in 3D and compared them with other techniques that they were looking at, to give us a picture of where the efficiency was being lost in each cell.
P&IT: What did you learn about solar cells from that trial?
Barnard: We looked at, specifically with NREL, comparing these cadmium telluride solar cells in two states: an initial as-deposited state, where the material is initially grown, and then its post-processed state with cadmium chloride, which is a common processing step in this type of solar cell. We were trying to understand what the cadmium chloride did to the solar cell at the micro scale.
What we found was that when you initially grow these materials, there are regions between crystals that have very poor optical properties and short lifetimes. They clearly show up in these three-dimensional maps, where the boundaries between these crystals are basically as short of a lifetime as we could measure with our system.
When you treat this material with the cadmium chloride, however, we found that the lifetime at these grain boundaries was improved significantly such that we wouldn't even see a contrast between the crystals and the boundary between the crystals. People knew that cadmium chloride improved efficiency, but they weren't able to localize where the effects were happening.
P&IT: What's next with this technology, and how will you be using it?
Barnard: The next project we're working on with other collaborators is to try to extend this to other solar cell materials. We're looking at CIGS (Copper indium gallium selenide) and Perovskite solar cells, and how defects in materials can also affect the lifetime and, therefore, the efficiency of these cells.
[The imaging technology] can be used in solar cells, many other optical materials, any active electronic material, or anything you can excite with light and see light coming out of it.