Hidden surface photovoltages revealed by pump probe KPFM

Area of Science:

• Surface science
• Photovoltaics
• Materials science

Background:

• Understanding light-induced charge dynamics is crucial for optimizing photovoltaic devices.
• Conventional surface potential microscopy can miss complex charge behaviors.

Purpose of the Study:

• To investigate light-induced surface potential dynamics in alumina-passivated silicon and PTB7-PC71BM organic films.
• To differentiate and analyze multiple photo-induced charge distributions with opposing polarities and distinct dynamics.

Main Methods:

• Utilizing pump-probe Kelvin probe force microscopy (pp-KPFM) and non-contact atomic force microscopy (nc-AFM) under ultrahigh vacuum.
• Acquiring data in a data cube mode and employing batch processing for 2D dynamical maps.

Main Results:

• Successfully identified and separated two types of photo-induced charge distributions with opposite potential shifts and different dynamics.
• Observed sequential events in silicon, including carrier separation, trapping, recombination, and trap release, consistent with band-bending and passivation layer effects.
• Revealed unique positive surface photovoltage (SPV) components in organic films attributed to specific interfaces, and confirmed acceptor clusters as electron-trapping centers.

Conclusions:

• The study demonstrates that pp-KPFM can resolve complex, time-resolved surface potential dynamics previously hidden in conventional SPV imaging.
• Findings highlight the importance of time-resolved techniques for accurate quantitative measurements of photo-response in photovoltaic materials.
• Results provide a deeper understanding of charge carrier behavior at interfaces, essential for advancing solar cell technology.