Characterization of Semiconductor Bulk and Interfacial Properties
LaboratoryNational Renewable Energy Laboratory (NREL)
Capability ExpertTodd Deutsch, James Young
Node Readiness Category1: Photoelectrochemical (PEC)
This capability involves characterizing semiconductors using photoelectrochemical (PEC) methods to measure their bulk and interfacial properties to determine their suitability for photoelectrolysis. We have practical experience contacting a variety of semiconductor configurations and fabricating samples into photoelectrodes suitable for PEC testing. Once we fabricate electrodes and measure their surface areas, we use a suite of characterization methods to determine unknown semiconductor properties by the following procedure: We determine the conductivity type of an unknown material by monitoring the open-circuit potential response upon illumination, which is important to establish the reverse-bias conditions used for all subsequent testing. We then measure band gap energy to within ±0.01 eV as well as determine whether the electronic transition is direct or indirect with our custom-built photocurrent spectroscopy system. Once the bandgap is known, we use the appropriate reference cells and calibrated light sources to measure (photo)current-potential performance under simulated reference illumination (AM1.5 G). We then determine the conduction/valence band edge alignment by measuring the flatband potential across a range of electrolytes with varying pHs using three different techniques; photocurrent onset, VOC under intense illumination, and Mott-Schottky analysis. The doping density of the semiconductor is calculated from the slope of the Mott-Schottky response. We measure incident photon-to-current efficiency (IPCE) to get a wavelength-dependent conversion efficiency that can be integrated over a reference spectrum (AM1.5G) to corroborate the photocurrents obtained under broadband illumination. The reflectance may also be monitored during the IPCE measurement to calculate wavelength-dependent internal quantum efficiency.
Also available is a scanning electrochemical microscope (SECM), an electroanalytical scanning probe tool that can be used to image substrate topography and local reactivity with high resolution. The SECM can be operated in several modes that make it a useful tool in a variety of applications. Some examples include:
- Determining values for kinetic rate constants
- Mapping and quantifying differences in H2 (or O2) production at surfaces
- Corrosion detection/monitoring
- Studying adsorbed surface species on an electrode.
Inline capillary mass spectrometry (ThermoStar GSD320, Pfeiffer Vacuum, 1-300 amu) is also available through this capability node for the purpose of sampling gas streams near atmospheric pressure continuously (up to one data point per second) and in real-time. Analog mass/charge scans can be performed to identify unknown species by their mass/charge ratio, or the response of a known species can be monitored continuously over time and quantified when suitable calibration standards are used. For hydrogen and oxygen yield measurements, a NIST-traceable primary calibration gas standard and calibrated mass flow controllers are available. Other relevant calibration gas standards may be obtained as needed. As examples, hydrogen/oxygen cross-over could be quantified under various operating conditions or degradation reactions that produce vapor-phase species could be detected and monitored.
Electrode sizes from 0.01 cm2 up to several cm2 can be tested. For safety reasons, we do not test materials that contain Cd (e.g., CdSe). Custom cells may be required to accommodate various electrode geometries.
These capabilities have been funded by the FCTO for >15 years. We have trained dozens of undergraduate, graduate, and postdoctoral researchers and tested thousands of c-Si, a-Si, oxide, nitride, carbide, phosphide, arsenide, selenide, sulfide, bismide, and antimonide semiconductors and co-wrote a book based on our approach.
We have 5-6 characterization stations that include potentiostats, frequency response analyzers, and light sources permitting a relatively high PEC characterization throughput. A scientist unskilled in this area could gain moderate proficiency on these characterization techniques within a few days of training. Its high availability would allow this capability to serve as a user facility if a large number of sample characterizations is needed.
This capability can screen unknown photoelectrode candidate materials and, by evaluating their intrinsic bulk and interfacial properties, determine their potential to direct sunlight toward water splitting. The co-location of all measurement equipment and capability experts offers convenience, support, and quick turnaround for users.
PEC characterization flow chart for a single-absorber material.
1. "Accelerating materials development for photoelectrochemical (PEC) hydrogen production: Standards for methods, definitions, and reporting protocols," Zhebo Chen, Thomas F. Jaramillo, Todd G. Deutsch, Alan Kleiman-Shwarsctein, Arnold J. Forman, Nicolas Gaillard, Roxanne Garland, Kazuhiro Takanabe, Clemens Heske, Mahendra Sunkara, Eric W. McFarland, Kazunari Domen, Eric L. Miller, John A. Turner, Huyen N. Dinh, J. Mater. Res. 25(1), 3-16 (2010).
2. "Direct solar-to-hydrogen conversion via inverted metamorphic multijunction semiconductor architectures," JL Young, MA Steiner, H Döscher, RM France, JA Turner, TG Deutsch. Nat. Energy 2, 17028 (2017).
3. "Photoelectrochemical Water Splitting: Standards, Experimental Methods, and Protocols," Zhebo Chen, Huyen N. Dinh, Eric Miller, Todd G. Deutsch, Kazunari Domen, Keith Emery, Arnold J. Forman, Nicolas Gaillard, Roxanne Garland, Clemens Heske, Thomas F. Jaramillo, Alan Kleiman-Shwarsctein, Kazuhiro Takanabe, John Turner, eds: Zhebo Chen, Huyen N. Dinh, Eric Miller. New York: Springer, 2013.
4. "Photoelectrochemical Characterization and Durability Analysis of GaInPN Epilayers" Todd G. Deutsch, Jeff L. Head, John A. Turner. J. Electrochem. Soc. 155(9) B903, (2008).