Subsurface photoluminescence lifetime imaging of photovoltaic materials using multiphoton tomography

Multiphoton tomography based on tunable femtosecond near infrared 80 MHz laser radiation has been used to map twophoton- excited time-resolved photoluminescence with picosecond temporal resolution from in-bulk thin photovoltaic layers. The time-resolved photoluminescence reflects carrier lifetimes and is therefore an important measure for the efficiency of a solar cell. Conventional one-photon solar cell characterization methods are dominated by surface effects and cannot provide information on subsurface carrier dynamics. In contrast, by using two-photon excitation, subsurface carrier dynamics can be monitored in 3D, providing morphological and spatial information on local defects and crystalline grain boundaries We present results on time-resolved photoluminescence and second harmonic generation imaging in forward and backward directions of CdTe/CdS films by time-resolved single photon counting and false-color photoluminescence mapping. High-resolution two-photon optical sectioning was carried out with a modified multiphoton FLIM tomograph MPTflex employing near-infrared sensitive photodetectors.