In order to reach a carbon-neutral economy, the discovery of new, stable and high-quality photoactive materials for energy conversion applications is needed. One of the most essential properties of high-quality photoactive materials is the lack of deep defects inside the material`s bandgap, yet it poses a very challenging experimental task.
To-date, the ability to reliably assess the real potential of novel semiconductor candidates at early stages of their discovery using “conventional” methods is rather limited, often via computational theory. Experimental characterization only happens later, often involving constructing full device structure, introducing additional variables. Thus, for assessing the real potential for applications of novel semiconductors, early-stage experimental characterization to unravel structure-property-performance relations (and bridge the theory-experiment gap) is crucial, but is mostly missing.
In our lab, we bridge this gap by utilizing two unique non-conventional Highly-Sensitive Opto-Electronic Spectroscopic methods: Surface Photovoltage (SPV) and Constant Final State Yield Spectroscopy (CFSYS), in order to investigate how the crystalline structure and chemical composition of novel semiconductors affect the distribution of intrinsic electronic defect states in their bandgap, and, in turn, the properties that determine device performance. Moving a step forward, we study the formation of extrinsic defects upon contacting the semiconductor with other materials and their effect on device-relevant properties.
A. Surface Photovoltage (SPV)
SPV spectroscopy is based on probing the charge separation of photoexcited carriers under light excitation with varying wavelengths, and allows to probe changes in the surface potential upon preferential separation of different photoexcited charge carriers.1,2 In our lab, instead of using the “conventional” vibrating Kelvin Probe (KP) technique to measure the surface potential, we use the Fixed-Capacitor probe configuration (FC-SPV, see Figure below) and use either a modulated or pulsed light excitation source. The main advantages of FC-SPV compared to “conventional” SPV based on a Kelvin Probe, relies on the fact that the probe is mechanically fixed, allowing us to use modulated light excitation combined with lock-in detection, resulting in significantly higher sensitivities (3-4 orders of magnitudes gain). Thus, by carefully performing sub-band gap SPV spectroscopy, defect-related transitions can be directly detected.
1T. Dittrich and S. Fengler, Surface Photovoltage Analysis of Photoactive Materials (WORLD SCIENTIFIC EUROPE, 2020).
2L. Kronik and Y. Shapira, Surf. Interface Anal. 31, 954 (2001).
B. Constant Final State Yield Spectroscopy (CFSYS)
Ultraviolet photoelectron spectroscopy (UPS) is a specialized variant of photoelectron spectroscopy used to study the electronic structure of materials. In UPS, ultraviolet (UV) photons with specific energy levels are directed at a sample, causing the emission of electrons from its surface. These emitted electrons, known as photoelectrons, carry information about the sample's electronic states and energy levels.
CFSYS has a similar working principle as the “conventional” common UPS method.3,4 The main difference between UPS and CFSYS lies in the detection mode and excitation source. In CFSYS, rather than using a single excitation wavelength and detecting the different kinetic energies of the emitted electrons, the analyzer is set to detect one specific kinetic energy (usually slightly above the Vacuum Level) and instead of the He-1 photon source (photon energy of 21.2 eV) used for UPS, for CFSYS, a near-UV source is coupled with a monochromator to excite the sample with variable photon energies between 3.7 to 7.3 eV. These differences result in a significantly higher S/N with much larger dynamic range compared to UPS, gaining several orders of magnitudes (as shown in the Figure below), thus allowing for the detection of low deep defect densities down to 1015 cm-3.
3L. Korte and M. Schmidt, J. Non. Cryst. Solids 354, 2138 (2008).
4T. Sato, H. Kinjo, J. Yamazaki, and H. Ishii, Appl. Phys. Express 10, 011602 (2017).