Solar cells are among the most promising renewable energy technologies today. Therefore, a detailed understanding of the dynamics and energetics of photon-to-electron conversion within the solar cell is essential for developing higher-efficiency devices. Since these processes occur on ultrafast timescales, time-resolved measurements are required to directly study the underlying mechanisms within the solar cell to identify factors that might limit device efficiency.

In this presentation, I will show how we employ time-resolved X-ray photoelectron spectroscopy (PES) to probe the ultrafast dynamics of charge-carrier generation and recombination. The method is based on a pump–probe scheme in which a visible laser pulse, operating at a lower repetition rate, excites the sample, while synchronized X-ray pulses at a higher repetition rate probe the resulting excited states. By systematically varying the time delay between the laser and X-ray pulses, the rapid generation of charge carriers (photovoltage) can be resolved with picosecond resolution. In addition, the combination of a high X-ray and low laser repetition rate enables sampling over longer timescales, allowing both the rise and decay of the photovoltage to be measured within a single experiment.[1,2]

This approach allows investigation of dynamics spanning from picoseconds to microseconds. By modifying the sample architecture, we can directly study how individual layers influence charge transport within the device.

We have applied this technique to AgBiS₂ quantum dot solar cells[3] to investigate how different ligand choices and combinations affect charge transport. Four samples with different active-layer structures were studied: (I) three quantum-dot layers using EDT as the ligand, (II) three layers using TBAI, (III) one EDT-treated layer followed by two TBAI-treated layers, and (IV) the reverse sequence, with two TBAI-treated layers followed by one EDT-treated layer. The results show pronounced differences in charge-carrier separation and recombination dynamics, leading to varying photovoltage dynamics depending on the architecture. These findings provide insight into how (and why) ligand selection affects device efficiency.

More generally, this measurement technique is applicable to a wide range of device types, provided the samples are stable under illumination, offering a powerful tool for gaining a deeper understanding of ultrafast processes in different solar-cell technologies.