Recombination Dynamics in Thin-film Photovoltaic Materials via Time-resolved Microwave Conductivity

The overall goal of this experiment is to use reflected microwave power to measure the recombination dynamics of photo-induced charge carriers and thin-film photovoltaic materials. This method is useful in the field of photovoltaics because it measures photo-induced charge carrier decay via trap states and band-to-band recombination processes. The main advantage of this technique is that it doesn't require electrical contacts to the sample.

This means that the measured lifetimes and conductivities are not distorted by the presence of cathodes. First, gather some of the necessary elements to perform the measurements including a prepared measurement sample. This thin-film perovskite sample has been characterized and its wavelengths of interest chosen.

In addition to the sample, have ready a microwave cavity, network analyzer, and laser. The sample will be mounted in this custom microwave cavity, which is made of four quarter-wave plates screwed together. An iris couples microwaves into the cavity, and a fiber port on the microwave short allows optical access.

A Teflon sample holder slots into the cavity. A defuser ensures that the incident light is spatially uniform while a tuning screw modifies the cavity quality factor. The laser is tunable over visible to infrared wavelengths.

Required smaller optical elements include an off-axis parabolic mirror coupler and neutral-density filters. With the laser wavelength at the desired value, direct its beam to an off-axis coupler. Use a beam card to check the alignment with the optical axis of the mirror.

The spot should enter the coupler approximately at its center. Next, obtain an optical fiber that will eventually couple to the cavity. Connect one end of the fiber to the fiber coupler.

Connect the other end to a power sensor and meter. Use low laser power and start monitoring the output of the fiber while adjusting the fiber coupler. Adjust the coupler's tilted angle in an effort to maximize the fiber coupling.

Continue to monitor the power meter and stop when it registers maximum power. The tilt angle adjustments on the coupler should be done slowly making sure not to bump anything. Refine the coupling by gradually increasing the laser power and further adjusting the coupler orientation.

To determine the cavity loss factor, compare the laser power out of the fiber to the power incident on the sample. First, turn on the laser and measure the power output through the fiber. Next, work with the cavity to measure the power at the sample position.

Since the cavity is made up of quarter-wave plates, disassemble it to access the sample position. When disassembled, place a mask the size of the holder at the sample position where the electric field is maximum. Position a power sensor behind the mask to measure the power at the sample.

Now connect the coupled fiber the the cavity. Measure the power at the sample position for the chosen wavelength. Next, disconnect the fiber, then remove the power sensor and mask from the sample position, and reassemble the cavity to prepare it for later steps.

The sample holder is designed to center to the sample on the cavity. Place the sample onto the sample holder. Be sure to orient the sample holder to face the laser input port.

Insert the sample holder into the cavity. Calibration of the cavity sensitivity requires a network analyzer and a circulator. Connect port one of the network analyzer to the circulator input port one.

Connect port two of the circulator to the microwave coupler of the cavity. Any radiation reflected from the sample will exit the circulator at port three. Connect this port to the network analyzer port too.

Next, use the network analyzer to perform a two-port S21 measurement of the reflected power. Recenter the display at the empty cavity resonance frequency, which is about 6.5 gigahertz for this cavity. Then set the frequency span of the display to one gigahertz.

Locate and mark the resonance frequency of the loaded cavity and center it. Finally, zoom in before recording the resonance curve. Now begin tuning the cavity's quality factor.

To do this, adjust the tuning screw on the cavity. Observe the resonance curve to optimize the quality factor. An optimum quality factor is often a compromise between time resolution and measurement accuracy.

The cavity sensitivity factor may be calculated from the resonance curve of the loaded cavity. The next step is to set up the time-resolved microwave conductivity or TRMC measurement. Tune the laser to one of the wavelengths of interest and set the output power to maximum.

Next, use a power sensor to measure the laser power transmitted through the fiber. Remove the fiber from the power sensor, then connect the fiber to the optical input of the cavity. At the other end of the cavity, the network analyzer and circulator remain connected as for the cavity sensitivity calibration.

The analyzer is now in a transient mode and this schematic makes clear the new functions of the network analyzer ports. Port one is the microwave source. Port two is the detector.

The laser input is not in the diagram, and the amplifier in the diagram is embedded in the network analyzer. A popular alternative setup uses a microwave diode in oscilloscope as the detector. Now work with the network analyzer.

Enable transient detection by setting the frequency span of the network analyzer to zero. Set the measurement frequency to the resonance frequency of the loaded cavity measured previously. Next, configure the microwave source.

Set the microwave source frequency to measurement frequency, and the output power to zero dBm. Next, arrange for the laser to trigger the network analyzer. Connect the laser trigger output to the network analyzer.

Enable the external trigger and find the trigger offset so as to capture the rise of the signal with a few microseconds of dark signal before the laser pulse. Finally, adjust the time base of the analyzer such that the transient tail is much longer than the initial decay. Record the resulting time-resolved microwave conductivity trace.

Once the optimal parameters are known, use them to automate the time-resolved microwave conductivity or TRMC measurements. To gather data on the complex conductivity, repeat this measurement at various microwave frequencies. Select the frequency sweep by reviewing the cavity resonance curve and choosing several frequencies that span the curve.

Use the network analyzer to set the measurement frequency to the first frequency in the sweep. Adjust the source frequency to the measurement frequency. Use the previously found optimal measurement parameters to obtain a TRMC trace.

This transient has the background subtracted. Repeat this measurement for each chosen frequency. To obtain intensity-dependent TRMC data, begin by tuning the laser to a wavelength of interest at maximum power.

Remove the fiber from the cavity and measure the power transmitted through the fiber. Reconnect the fiber to the cavity before continuing. Start automated data gathering by entering the experimental parameters.

Turn on the laser. Obtain a transient average over at least 100 traces to account for a laser shot-to-shot power variations. Once this is found, turn the laser off.

Place a neutral-density filter between two irises in the beam path before turning the laser on again. Repeat the measurement steps for this setup with different neutral-density filters. Ideally, measurements will be taken as power spanning over two orders of magnitude.

This log-log plot is of a fit of the time-resolved microwave conductivity data using a kinetic model convolved with a Gaussian instrument response function. Using a fit, it is possible to determine carrier lifetimes and trap densities, and to characterize direct and trap-mediated recombination processes. Each trace in this plot corresponds to a transient conductivity at an excitation intensity between 10 to the 12th and 10 to the 14th observed protons per centimeter square.

The intensities were varied using neutral-density filters. The transient conductivity data allows for the extraction of the intensity dependence of the total carrier mobility. The excitation wavelength for this data is 530 nanometers.

These traces represent a deconstruction of a TRMC trace and to contributions from the real and imaginary components of the conductivity. The red traces data taken at the resonance frequency of the dark-loaded cavity. The green trace gives the decay the real part of the conductivity.

The decay of the polarization is in blue. The polarization decay tail in blue is significantly smaller than that of the conductivity in green. This is consistent with decay via localized trap states.

Once mastered, each TRMC trace may be taken in a few minutes. On performing these experiments, it's important to correctly record all the laser power settings including the power out of the fiber, the wavelength, and any neutral-density filters used. Time-resolved photoluminescence measurements can be used in conjunction with TRMC measurements to decouple the direct recombination pathways from the combined recombination processes measured using TRMC.

This technique paved the way to explore charge carrier recombination processes in a huge range of semiconducting materials including thin-film photovoltaic sensitizers, carbon nanotubes, and titanium dioxide nanoparticles. Don't forget that working with high power lasers can be extremely dangerous and precautions such as enclosing the laser beam and wearing appropriate glasses should always be taken.