(775d) Characterizing Defects in Photovoltaic Semiconductors with Optical Spectroscopy

Authors: 
Hages, C. J., Helmholtz-Zentrum Berlin
Unold, T., Helmholtz-Zentrum Berlin
Understanding the defect properties of photovoltaic absorbers is critically important for high performance device fabrication. Such properties play a crucial role in the charge carrier dynamics which determine device performance. In this work we demonstrate how time-resolved photoluminescence (TRPL) is a useful technique to investigate charge carrier dynamics and defect properties in photovoltaic absorbers and solar cells. Useful material parameters, such as minority carrier lifetime and mobility, are often reported from TRPL measurements. Additionally, photoluminescence (PL) decay times are often used as a metric for absorber quality during process optimization. However, accurately interpreting the PL decay time can be quite complicated as the charge carrier dynamics are influenced by various recombination, transport, and charge trapping effects. Such metrics are particularly relevant for chalcopyrite materials, which often employ complicated band structures for high performance, as well as for kesterite materials, where short PL decay times are commonly reported for this highly defective material. Therefore, careful consideration is needed when analyzing TRPL data of such non-ideal absorbers and devices to correctly interpret the limiting absorber defect properties.

In this work, high-resolution voltage-, temperature-, and intensity-dependent TRPL data for chalcopyrite, kesterite, and perovskite devices and absorbers are analyzed to determine the origin of the PL decay time. Additionally, a supercontinuum excitation source is used to investigate the excitation energy-dependence of the PL decay time. Such measurements allow us to distinguish the various PL decay rate limiting mechanisms for these materials. We find chalcopyrite absorbers – including CuInSe2 (CISe) and Cu(In,Ga)Se2 (CIGSe) – from a variety of processing conditions and compositional profiles follow the expected behavior for Shockley–Read–Hall (SRH) recombination limited decay times. The impact of carrier diffusion and surface recombination are demonstrated. In contrast, TRPL analysis of kesterite devices – including Cu2ZnSnSe4 (CZTSe) and Cu2ZnSn(S,Se)4 (CZTSSe) from various processing conditions – suggests that PL decay times are not directly related to the recombination lifetime for CZTSSe. A model for minority carrier trapping in shallow donor states is proposed to explain the measured TRPL behavior for kesterites. These results illustrate the importance of measurement conditions and careful analysis when interpreting TRPL data, and further elucidate the defect related device limitations in chalcogenide solar cells.