Characterization of Semiconductor Bulk and Interfacial Properties

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:

1. 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.
2. 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.
3. 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).
4. 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.
5. The doping density of the semiconductor is calculated from the slope of the Mott-Schottky response.
6. 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.
7. The reflectance may also be monitored during the IPCE measurement to calculate wavelength-dependent internal quantum efficiency.

Efficiency Benchmarking

This capability involves testing water-spitting semiconductors and semiconductor-based devices using simulated and actual solar (on-sun) illumination to validate solar-to-hydrogen (STH) conversion efficiency. The first step is to take incident photon-to-current efficiency measurements to get the wavelength dependent conversion efficiency for each subcell absorber junction. This is necessary to calculate a spectral correction factor to adjust measured photocurrent densities to a reference spectrum for objective comparison of performance to other devices. Outdoor measurements are taken using collimating tubes to isolate the direct component of the solar spectrum to minimize errors due to coupling of the diffuse component of solar radiation to the semiconductor by the photoreactor cell. Continuous research-quality measurements of the characteristics of local solar irradiance are recorded at the Solar Radiation Research Laboratory (SRRL) at NREL every minute from over 80 instruments including pyranometers, pyroheliometers, pyrgeometers, anemometers and other meterological sensors. Details on the measurements and data sets are online. This data is used to calculate real-time STH efficiencies from short-circuit current density measurements of the device. These broadband STH efficiency measurements are corroborated by integrating the IPCE over the reference spectrum and making Faradaic efficiency measurements with a capillary mass spectrometer. For measurements of STH efficiency over time, we have an EKO STR-22G Sun Tracker that has a 20 kg payload capacity and a pointing accuracy better than 0.01°.

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 crossover could be quantified under various operating conditions or degradation reactions that produce vapor-phase species could be detected and monitored.