Microwave/THz Instrumentation Assesses, Resolves PV-Material Performance Data

This article appeared in Electronic Design and has been published here with permission.

What you’ll learn:

The relationship between RF energy and assessing potential photovoltaic-material performance.
The two overlapping ways to excite and measure PV materials such as perovskites.
How the conflicting test results were resolved.

It may seem from a first glance that there’s little in common between the dc performance of solar cells and their constituent photovoltaic (PV) materials and the RF world of microwaves and terahertz (THz) energy. Of course, a close connection exists at the atomic-physics level, as the mobility and lifetime of electrons and holes in the PV material translate into light-induced current flow in PV systems when energized by electromagnetic radiation.

Those two parameters—mobility and lifetime—are among the most important properties of a semiconductor to be used as a solar cell, as large charge-carrier mobilities are desirable to enable efficient charge injection and extraction.

Both parameters can be measured without contacts via spectroscopic methods using terahertz or microwave radiation. However, the resultant measurement data found in literature often differs by orders of magnitude. This has made it difficult to use the results for reliable assessments of material quality.

Higher-Accuracy PV-Material Performance Determination

In recent years, perovskite semiconductors in particular have attracted attention, as they’re relatively inexpensive, easy to process, and enable high efficiencies. Now, researchers at Helmholtz-Zentrum Berlin (HZB) took a major step in more accurately determining the potential performance of PV materials such as perovskites without the need to fabricate solar cells or make physical contact. They combined the expertise of 15 laboratories to quantitatively model and analyze the current-voltage characteristics of a solar cell from such measurements.

Note: If you’re not familiar with HZB, it’s the result of the merger of two research institutions—the Hahn-Meitner-Institut (established in 1959) and BESSY GmbH (1979). With approximately 1,100 employees, it’s one of the largest non-university research centers in Berlin.

TRMC and OPTP

Both charge-carrier mobilities and lifetimes can be probed without physical contact by Time-Resolved Microwave Conductivity (TRMC) and Optical-Pump Terahertz-Probe (OPTP) spectroscopy, making them excellent tools for the characterization of photovoltaic materials (Fig. 1). OPTP can monitor fast processes like trapping into defect states and exciton formation, while TRMC adds the slower processes such as long-living trapped carriers or weak recombination in advanced materials, e.g., halide perovskites.

1. OPTP and TRMC in brief: Illustration of (a) optical-pump terahertz-probe measurements (OPTP) and (b) time-resolved microwave conductivity measurements (TRMC). (c) Photoconductivity transients measured on an InP wafer and on a perovskite thin film. (d) Time-resolved terahertz spectroscopy (TRTS) on InP—a frequency-resolved version of OPTP—yields the mobility spectra of the real and imaginary parts of the mobility.

The contactless nature of the measurement is a major advantage of OPTP and TRMC over more conventional techniques such as dc-conductivity and Hall-effect measurements. That’s because it overcomes interface issues such as contact resistance, unintentional doping, and bandwidth limitation, and it allows for the study of bare films and even powders. However, these measurements of an electrical potential corresponding to solar-cell operating conditions can’t be applied directly due to the lack of those same contacts.

New Measurement Process

Therefore, the OPTP and TRMC measurement conditions must be translated to the specifics of the thin-film device, which results in different interpretations due to subtleties and difference in the models used by the different institutions. To overcome this obstacle, the HZB team worked with the other institutions to develop a consensus on the interpretation of TRMC and OPTP measurements, using a halide perovskite (Cs,FA,MA)Pb(I,Br)₃ thin film as a case study.

This measurement process is a complex multistep sequence:

The first step is to characterize carrier trapping and exciton formation (an exciton is a bound state of an electron and an electron hole, which are attracted to each other by the electrostatic coulomb force) as the corresponding quantum yields are crucial for assigning relevant lifetimes and electron and hole mobilities (Fig. 2).
Second, material is exposed to continuous “1 sun” illumination while OPTP and TRMC spectroscopy are measured after pulsed excitation.
Third, OPTP spectroscopy (Fig. 3) or TRMC spectroscopy (Fig. 4) is used to measure the mobilities of charge carriers in electric fields that alternate at THz or GHz frequencies, respectively, which must be translated to the dc-transport through the thin-film solar cells.
Finally, the precision of the results was determined by an inter-laboratory comparison. Resolving the differences and providing final data with confidence was a primary goal of the project.

2. Probing exciton formation and charge-carrier trapping: Illustration of competing charge-carrier dynamics, including cooling to the band-edge, trapping, exciton formation, and recombination.

3. The OPTP setup is based on a femtosecond laser system. It generates single-cycle pulses of terahertz radiation by optical rectification of an optical femtosecond laser pulse or by photoconductive switches. These generated terahertz pulses propagate through free space to the sample, and the transmitted or reflected part is detected by electro-optical sampling with another fs-laser pulse in a second crystal or by a photoconductive switch. An additional optical pump pulse photo-excites the sample and causes a pump-induced change in the terahertz transmission (or reflection).

4. In a TRMC system, the sample is probed using microwave frequency (typically 5-30 GHz, here 8 GHz) radiation. Microwaves are generated using a source such as a voltage-controlled oscillator (VCO) or Gunn diode and directed into a microwave cavity via a small aperture (iris). The distribution and intensity of the microwaves in the cavity depend strongly on the cavity and iris properties, but conventionally a resonance condition is established where one full wavelength fills the cavity.

Both techniques probe the formation and kinetics of photoexcited electrons, holes, and excitons by measuring photoconductivity transients, as well as mobilities. Pulses from an optical pump induce photogeneration of charge carriers in the (perovskite) sample. Depending on their mobilities, these carriers are accelerated laterally in an electric field of a terahertz probe pulse or by a standing microwave in a cavity, which alternate at terahertz or gigahertz frequencies, respectively.

The photogenerated carriers then acquire an oscillating drift velocity, and thereby a part of the terahertz or microwave radiation is absorbed. The corresponding pump-induced change in the transmitted terahertz electric field (ΔE/E) or in the reflected microwave power (ΔP/P) is measured and analyzed in a first step for the sheet photoconductivity or photoconductance. TRMC and OPTP measurements yield mobilities for charge carriers oscillating at GHz and THz frequencies.

To characterize dc-device performance, the effective mobility relevant for direct transport through the thin-film device must be estimated, which is accomplished by additional data analysis and conversion to determine the desired parameters.

By combining the expertise of 15 laboratories, the HZB work identified error sources. Thus, it provides a “best practice” model for the interpretation measurements on photovoltaic materials to better predict their potential performance in solar cells (Fig. 5).

5. Variations in terahertz and microwave-derived mobility data: (a) with respect to the participating laboratories and (b) categorized by error type.

The work is detailed in their lengthy and intense, yet informative and readable, 16-page paper “Predicting Solar Cell Performance from Terahertz and Microwave Spectroscopy” published in Advanced Energy Materials (with 29 authors from the 15 institutions!). It’s bolstered by a 23-page Supporting Information document.