Photocarrier dynamics in CIGS, CZTS, and related materials revealed by ultrafast optical spectroscopy

We discuss the photocarrier dynamics in emerging thin-film solar-cell materials, Cu(In,Ga)Se-2 (CIGS), Cu2ZnSnS4 (CZTS), and CdTe-based nanocrystals (NCs), using time-resolved photoluminescence and femtosecond pump-probe spectroscopy. In CIGS and CZTS samples, we found long-lived free photocarriers and slow intraband relaxation at room temperature. These distinctive behaviors of the free carriers are considered common features in multinary semiconductors and are related to the complicated near-band edge structure due to potential fluctuations, which might affect solar-cell efficiencies. We revealed the relationship in CdTe-based type-II NCs between hot carrier transfer and intraband relaxation, providing a better understanding of the charge separation mechanism in multinary semiconductors. We also demonstrated the usefulness of ultra-fast optical spectroscopy techniques for clarifying the photocarrier dynamics in various semiconducting bulk and nano-structure materials for solar cells. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim