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The Netherlands' energy transition is progressing, with solar panels playing a pivotal role in the shift toward renewable energy. While many homes already feature classic silicon-based solar panels on their roofs, the next steps forward can be taken with perovskite solar cells.

These perovskite solar cells are not just a scientific curiosity; they represent a leap forward in our quest for sustainable energy. Over the past decade, their efficiency has rapidly increased, and their versatile nature makes them applicable in various applications. Perovskite solar cells are strong contenders for the future of solar power.

Core of a perovskite cell

The core of a perovskite solar cell is the perovskite absorption layer, a semiconductor that converts sunlight into electrical charges: electrons (negative) and holes (positive).

The electrons and holes are separately conducted through an electron transport layer (ETL) and a hole transport layer (HTL), which surround the perovskite layer on the bottom and top.

These layers are covered with conductive electrodes to extract the current. The entire structure is extremely thin, allowing perovskite solar cells to be applied as a thin layer on various surfaces.

Recombination problems

To further enhance and sustain their performance, it is essential to focus on a critical parameter of these solar cells: the open circuit voltage (VOC).

The VOC represents the maximum voltage a solar cell can produce. One significant factor limiting the VOC is non-radiative recombination.

In an ideal scenario, all recombination would be radiative (emitting light), and non-radiative recombination occurs when energy is lost. This non-radiative recombination can happen within the perovskite material or at the interfaces and it reduces the overall efficiency of the solar cell. Thus, it should be minimized and better understood.

Photoluminescence

To address this challenge, turned to a technique called absolute photoluminescence (PL). PL refers to the phenomenon where there is emission of light by a semiconducting material after it has absorbed light.

The term "absolute" implies that every emitted photon (light particle) is accounted for. It is a very powerful technique, since Remmerswaal and her colleague were able to determine the quasi-Fermi level splitting (QFLS) with it. This QFLS correlated directly with the maximum achievable VOC. Essentially, she measured voltage with light!

Deeper understanding

During the 4 years of her PhD research, Willemijn Remmerswaal demonstrated that this technique affords a deeper understanding of non-radiative losses and provided strategies for reducing them.

Her analysis showed that defects within the perovskite material and at its interfaces cause significant voltage losses. She also studied how different perovskite materials with varying bandgaps – the energy difference between the valence band and conduction band – impacted non-radiative recombination.

By assessing the QFLS, Remmerswaal and her colleagues compared the contributions to these losses. The results revealed that non-radiative recombination increases in pristine perovskite layers with higher bandgaps.

However, the most significant losses were identified at the perovskite – ETL interface. These losses were mitigated through interface passivation, which, rather than improving energy level alignment at the perovskite-ETL interface, worked by eliminating direct contact between the perovskite semiconductor and the ETL.

Voltage-bias-dependent

Furthermore, by combining optical and electrical measurements, Remmerswaal explored voltage-bias-dependent absolute PL. She developed an automated setup to analyze these cells under various conditions, revealing how factors such as ionic motion influence their performance.

She also explored the potential of tandem solar cells. This is a solar cell that combines multiple light absorbing layers to maximize efficiency. By selectively targeting each layer with the correct color of light, she was able to identify and address specific limitations, further pushing the boundaries of what these solar cells can achieve.

Advancements in perovskite solar cells highlight efforts to improve solar energy efficiency. As the Netherlands advances its energy transition, such innovations are crucial for a sustainable future.

Title of PhD thesis: . Supervisors: René Janssen and Martijn Wienk.