Na-Dependent Ultrafast Carrier Dynamics of CdS/Cu(In,Ga)Se₂ Measured by Optical Pump-Terahertz Probe Spectroscopy

To investigate the origin of the Na effect on photovoltaic (PV) devices, Cu(In,Ga)Se₂ (CIGS) and CdS/CIGS layers were grown on borosilicate (BS) and soda-lime glass (SLG), respectively. The defect states and nonequilibrium carrier dynamics of the samples were measured using photoluminescence (PL) and optical pump-THz probe (OPTP) spectroscopy. From the PL results, we discovered that different shallow donor-acceptor levels were formed in the CIGS layer grown on BS and SLG, respectively. In the OPTP results, relaxation times of photocarriers excited from the CdS/CIGS layer were clearly distinguishable, and are explained by the formation of different defect states depending on substrates. In BS, deep defect level 'DX states' were formed in the E_g near the p-n junction, which induce trapping photocarriers, resulting in shortening relaxation time. In SLG, there was no "DX state", which clearly demonstrates the positive effect of Na atoms at the p-n junction on performance of PV devices.