A new instrument at Brazil’s Sirius accelerator enables researchers to observe solar cells in operation

Scientists at the Brazilian Center for Research in Energy and Materials (CNPEM) in Campinas, São Paulo state, developed an instrument for the Sirius particle accelerator that can observe the interior of solar cells on a nanoscale while they are operating. Rather than analyzing isolated components in a laboratory setting, this new platform enables the evaluation of complete devices generating electricity under real operating conditions. It simulates daily exposure to light, heat, and humidity.

The team created a system with high mechanical stability to prevent the samples from shifting out of the X-ray beam’s focus, even under extreme temperature fluctuations. Understanding how the atomic structure and chemistry of these materials react to the environment is essential to investigating the causes of their degradation. This technology will be a crucial tool for optimizing emerging solar technologies, such as perovskite cells, paving the way for cheaper, more efficient, and more durable solar energy sources.

“We sought to reproduce the normal operating conditions of the solar cell and use the available techniques at the Sirius accelerator to observe what happens within the material,” says Rodrigo Szostak, a researcher at the Brazilian National Synchrotron Light Laboratory (LNLS-CNPEM) and the first author of the study published in August in the journal Photon Science.

The work was supported by FAPESP through a postdoctoral fellowship awarded to Szostak.

As the researcher explains, X-rays offer a key advantage in this type of research. Due to their penetrating power, X-rays allow for examination of the different layers of the device with minimal sample preparation. This reduces the risk that the experimental procedure itself might alter what is intended to be observed.

The platform was designed primarily for studying perovskite solar cells, a technology that has advanced rapidly in recent years due to the high efficiency of these materials and the ability to manufacture them using simple processes at low temperatures without a vacuum.

Szostak has been following this trajectory since the beginning of his career. He conducted one of the first studies on perovskite solar cells in Brazil. “These cells have only been in development for about 15 or 20 years, but companies are already beginning to launch commercial products based on them. At the same time, several questions remain unanswered, and the academic community continues to conduct research to understand what’s happening,” he says.

One of the most important questions concerns stability. While a solar cell is operating, its material is not static. Light, heat, humidity, and the electrical voltage itself can cause changes to its structure and composition. Some of these changes are reversible, but others can contribute to the degradation of the device.

“A solar cell undergoes temperature fluctuations every day. As they say in the field, it goes through ‘cycling.’ We want to understand the effect of these cycles, as well as those caused by light and humidity, on its performance,” the researcher points out.

What sets the platform apart is its ability to correlate these macroscopic variables with what occurs on much smaller scales. “We can measure the device’s efficiency by varying temperature and humidity while simultaneously using X-ray techniques to examine its chemical composition, physical form [morphology], atomic structure, and even its electronic structure,” Szostak notes.

Achieving this required developing a stable sample holder and a small experimental environment in which the cell can be heated, illuminated, exposed to different atmospheres, and electrically connected.

One of the most significant challenges was temperature. To microscopically track the changes caused by heating, the same region of the cell must be observed repeatedly. However, repeated exposure to X-rays can heat, expand, and displace the device. Given the scale at which the experiment is conducted, even an almost imperceptible displacement can move the selected region out of the field of view.

“Since we’re working with a beam about 200 nanometers wide – hundreds of times thinner than a human hair – we need the sample to remain stationary,” Szostak explains. “Our engineering team worked with us to develop a way to secure the sample and accommodate this expansion.”

Heating without losing the target

The solution began with the development of a small chip on which the solar cell itself is built. The substrate is a piece of glass coated on one side with indium tin oxide (ITO). This material serves as a transparent, electrically conductive base upon which the solar cell material (perovskite) is built. On the other side of the glass are two extremely thin platinum circuits: one functions as a heater, and the other as a temperature sensor. Passing current through the first circuit produces heat, and the electrical resistance of the second circuit allows the temperature of the sample to be determined precisely.

“The system concentrates the heating in an area of just 3 by 3 millimeters. The smaller the heated region, the less thermal expansion the experimental setup tends to experience. Tests with an infrared camera showed that the temperature in this small area is quite uniform: at 150 °C, the relative variation was only 0.85 °C,” Szostak reports.

The mount itself also required precision engineering solutions. It uses a titanium alloy with low thermal expansion and flexible elements that allow the glass to expand without accumulating stresses that could cause it to break while keeping the cell firmly in place. The system was thermally decoupled from the structures responsible for moving the sample to reduce heat transfer and unwanted displacements.

“That’s one of the innovations of this work. We have excellent positional stability. I reviewed the literature, and as far as we know, our holder likely offers the best positional stability ever achieved for this type of experiment,” says the researcher.

In tests conducted at Sirius, the sample was repeatedly subjected to temperatures of 30 °C and 150 °C for approximately 20 hours. Over this 120 °C range, the vertical displacement was just 5 micrometers, and the horizontal displacement was about 1 micrometer. More importantly, these displacements were reproducible and reversible. When the temperature returned to its initial value, the sample practically returned to its previous position.

After thermal stabilization, the residual movement was small enough to not cause significant distortions in the images produced during the scans. Temperature control also proved to be fast and precise. The programmed temperature was reached in less than two minutes, and at 150 °C, it remained stable with general fluctuations of less than 0.1 °C for 50 minutes. The ITO-based chip can be used at temperatures up to 150 °C. In experiments that do not require a conductive substrate, the system can reach 300 °C.

Light, heat, and humidity

Temperature is just one of the controlled variables. The device features a small, sealed chamber in which gases can circulate, and humidity levels at the inlet and outlet can be monitored. This allows for the simulation of different atmospheric conditions and investigation of the role of humidity in the degradation of perovskites, for example.

“Another essential component is the lighting. To study a solar cell in operation, it must be illuminated while X-rays take measurements. A xenon lamp would better reproduce all the colors and characteristics of sunlight [the solar spectrum], but it would be too large for the space available in the experimental station. We therefore opted for a broadband white LED, chosen because, among the LEDs evaluated, it had the spectral distribution closest to sunlight and covered the wavelength range absorbed by the perovskite cell,” says Szostak.

However, the issue of lighting has not yet been fully resolved. When placed directly in front of the cell, the LED can provide an irradiance equivalent to that of the sun, about 100 milliwatts per square centimeter. However, this position blocks certain X-ray transmission geometries. In the lateral configuration required to allow the beam to pass freely, the intensity reaching the cell corresponds to approximately 30% of the reference solar radiation. Improving this system is among the next steps of the project.

The cell under X-rays

The researchers conducted the experimental demonstration presented in the article with this setup. They fabricated a complete perovskite cell on the chip and subjected it to conventional electrical characterization and various X-ray analyses. The researchers obtained its current-voltage curves and monitored its response when the illumination was turned on and off.

This was made possible thanks to the characteristics of the Carnaúba beamline, one of 12 beamlines installed at the Sirius accelerator. It is designed to perform analyses using an extremely focused X-ray beam. “The apparatus functions like a multi-technique microscope. We have a very narrow X-ray beam, which allows us to examine the electronic structure, atomic structure, and chemical composition of materials in high resolution, in two or three dimensions,” Szostak explains.

At the Tarumã station, where the device was installed and which is part of the Carnaúba beamline, different techniques provide complementary information. X-ray diffraction investigates the crystalline structure, X-ray fluorescence maps the distribution of chemical elements, and X-ray absorption spectroscopy provides information on the chemical state of the elements, bond distances, and coordination environments. The station also supports X-ray-excited optical luminescence and other techniques, some of which are still in the implementation phase.

“Basically, the Carnaúba beamline is a super-powerful microscope with which we can examine matter in many different ways,” the researcher summarizes. “And its field of application is much broader than that of solar cells: it includes batteries, plants, seeds, biological materials, and even paleontological samples.”

In the experiment with the perovskite cell, X-ray fluorescence produced maps of the iodine and lead distribution in the device. On the other hand, diffraction made it possible to visualize individual perovskite grains about two micrometers in diameter and to obtain information about their spatial orientation and the exact distance between the atoms in their structure, or the crystal lattice parameters. These parameters are important because they allow one to estimate material deformations related to properties such as electronic structure, ion migration, defect formation, and stability.

To demonstrate this capability, the researchers mapped the crystal structure of the perovskite at 40 °C, then repeated the measurement after raising the temperature to 65 °C. The expected increase in the lattice parameter, or the characteristic dimension of the crystal structure, could be observed directly. Thus, the sequence of maps revealed not only the microscopic structure of the cell, but also how it transformed in response to a controlled external variable.

“It’s precisely this combination that gives the platform its main potential. A single region of the cell can undergo cycles of heating, illumination, humidity, or electrical voltage, while different techniques simultaneously or sequentially monitor what happens to its chemical composition, crystal structure, and performance. In this way, it’ll be possible to investigate the relationships between microscopic changes and the loss – or improvement – of the cell’s ability to convert light into electricity,” the researcher explains.

The equipment is no longer just an experimental prototype. Developed during Szostak’s project, it is now a multi-user facility available to the research community. Several experiments are currently underway. Although it has been optimized for perovskite solar cells, it can be used for other applications as well. It can also be used to study organic solar cells, solid-state dye-sensitized cells, quantum dot cells, light-emitting diodes (LEDs), and other devices built on compatible substrates. “Any device or material that can be deposited on glass and that one wishes to study under heating conditions can, in principle, be investigated,” the researcher states.

About São Paulo Research Foundation (FAPESP)

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