XJTU Molecular Press Annealing Strengthens Perovskite Solar Cells

Solar cells produced from low-cost, printable materials are approaching leading performance levels, but they commonly break down under heat.

A new protective process is particularly relevant because large-scale clean power depends on devices that can continue operating in demanding environments.

Heat remains the weak link

For many highly efficient prototypes, fabrication is the most challenging stage, as heat can destroy an otherwise promising solar layer.

Researchers at Xi’an Jiaotong University (XJTU) in China set out to prevent heat-related damage before it could become widespread.

Dr. Chao Liang led the project, with his work concentrating on vulnerable thin-film interfaces.

Protecting the surface while it was heated made small imperfections less likely to develop into failures over the longer term.

Why perovskites are challenging

Many laboratories make solar cells with perovskites, crystalline materials that absorb light exceptionally effectively. Their appeal is that they can deliver high efficiencies.

In contrast with silicon, their structure relies on electrical attraction, meaning heat can weaken important connections.

When these connections deteriorate, ion migration – the movement of charged atoms through a material – becomes more likely, while electrical losses increase over time.

This mobility also explains why perovskite devices may deteriorate rapidly unless their surfaces remain orderly and chemically stable.

How iodine escapes

Creating an even perovskite film generally requires annealing, a controlled heating stage that encourages crystals to develop smoothly after the wet coating has dried.

Iodine may leave the upper surface during this heating, making the crystal left behind more susceptible to damage.

Where an iodine vacancy occurs – a missing iodine atom within the crystal – surrounding lead ions lose bonding partners and the local structure becomes distorted.

Defects originating at the surface may subsequently move deeper into the perovskite, reducing electrical transport and paving the way for more rapid later degradation.

A solvent-free pressing process

To prevent the initial damage, the researchers applied molecular press annealing, a hot-pressing process that attaches molecules to the perovskite surface.

They pressed a thin template film onto the perovskite without using solvent, forming a compact capping layer as the material was heated.

The film contained 2-pyridylethylamine, a small organic molecule rich in nitrogen, whose two binding points supported the surface.

The team presented the procedure as a means of avoiding surface damage during heating rather than repairing defects once they have formed.

Molecules secure the surface

The central molecule attached itself to exposed lead atoms at the surface, providing the crystal with stronger bonds at high temperatures.

Its two-point binding stabilised the lead-iodine framework, which reduced both the formation of vacancies and the spread of existing vacancies.

As defects declined, charges travelled more smoothly through the film, allowing more captured sunlight to arrive at the electrodes as electricity.

This benefit relies on maintaining a uniform contact, as breaks in the pressed layer could still provide iodine with routes to escape.

Perovskite efficiency at larger scale

For years, the National Renewable Energy Laboratory chart has shown perovskites advancing quickly, and this outcome moved close to the leading results.

Under standard certification, the group recorded 26.5 percent efficiency for a small device and 23.0 percent for a 2.5-square-inch (16 square centimetre) module.

The improvements largely resulted from more even charge collection, as fewer defects meant less energy was lost as heat within the cell.

Nevertheless, strong performance from a press-ready film is only one stage on the route to mass production across full solar cell panel lines.

Performance in damp heat

Efficiency figures have limited value when a solar cell loses output after a few months, so the team exposed devices to demanding heat and humidity conditions.

While operating at 185 degrees Fahrenheit (85 degrees Celsius) and 60 percent relative humidity, the cells retained 98.6 percent of their initial output after 1,617 hours.

When stored in the dark under room conditions, they retained 97.2 percent after 5,280 hours, indicating gradual chemical change without light exposure.

Although these trials cannot account for every outdoor condition, they indicate that addressing early defects can provide benefits later.

Reusing one template

A method that succeeds only once is insufficient, so the researchers assessed whether the imprinting film could be repeatedly reused.

A single 2-pyridylethylamine template produced 30 consecutive devices, while the process lowered surface-treatment material costs by more than 47 times.

By eliminating additional solvent washes, the method also lowered the chance of dissolving or making the fragile upper layer rougher.

Reuse can reduce solar cell production costs, though factories would still require dependable pressure equipment and quality controls to ensure each press is consistent.

Towards more durable solar cell hardware

Perovskite devices continue to face obstacles beyond the laboratory, particularly where they must withstand rain, dust and daily changes in temperature.

Protective packaging needs to exclude water and oxygen for years, while seals must remain secure as materials expand.

Most high-efficiency perovskites contain lead as well, requiring manufacturers to prevent leakage during operation and manage waste responsibly.

Molecular press annealing addresses one route to failure, but practical solar cell panels will require several protective measures to work together in sunlight.

By pressing a protective molecule into position during heating, XJTU transformed a hazardous fabrication stage into a controlled surface treatment.

Should other teams scale up the pressing process and demonstrate long outdoor operating lifetimes, this method could help perovskites compete in commercial markets.