Probing carrier lifetimes in photovoltaic materials using subsurface two-photon microscopy

Area of Science:

• Materials Science
• Semiconductor Physics
• Photovoltaics

Background:

• Accurate measurement of bulk minority carrier lifetime is crucial for evaluating photoactive materials in photovoltaic cells.
• Traditional one-photon time-resolved photoluminescence decay measurements often yield inaccurate results due to surface carrier generation and defect interference.

Purpose of the Study:

• To develop and demonstrate a method for accurately measuring the bulk minority carrier lifetime in photovoltaic semiconductors.
• To decouple surface and bulk recombination processes for more reliable material evaluation.
• To generate multi-dimensional spatial maps of optoelectronic properties within the bulk of these materials.

Main Methods:

• Utilizing two-photon absorption microscopy for sub-surface optical excitation.
• Comparing results with traditional one-photon time-resolved photoluminescence decay measurements.
• Analyzing carrier generation and recombination dynamics in photovoltaic semiconductor samples, including Cadmium Telluride (CdTe).

Main Results:

• Two-photon absorption enables excitation below the material surface, circumventing surface defect interference.
• This technique accurately measures bulk minority carrier lifetime, unlike the one-photon method which can underestimate it significantly (e.g., by 10× in CdTe).
• Multi-dimensional spatial maps of bulk optoelectronic properties were successfully generated.

Conclusions:

• Two-photon microscopy provides a superior method for accurately determining the bulk minority carrier lifetime in photovoltaic materials.
• This advancement allows for more reliable characterization of semiconductors, essential for improving solar cell efficiency.
• The ability to map bulk properties spatially opens new avenues for understanding and optimizing photovoltaic device performance.