Ultrafast Photocarrier Dynamics at the p–n Junction in Cu(In,Ga)Se₂ Solar Cell with Various Zn(O,S) Buffer Layers Measured by Optical Pump–Terahertz Probe Spectroscopy

We fabricated Cu(In,Ga)Se 2 (CIGS) solar cells with three types of Zn(O,S) buffer layer produced by different techniques: S thermal cracking, chemical bath deposition (CBD), and reactive sputtering. The highest cell efficiency was obtained from the CIGS solar cell with the CBD-Zn(O,S) buffer layer. The solar cell performance is predominantly determined by the population of defect states distributed in the p–n junction and CIGS bulk layer. We utilized the optical pump–terahertz probe (OPTP) spectroscopy to measure the carrier lifetimes governed by the trapping time at defect states and found out short (τ s ) and long (τ l ) lifetimes related to the surface defects and Cu vacancy (V Cu ) defects, respectively. Both τ s and τ l increased after deposition of a Zn(O,S) layer, which is attributed to the surface curing and the decrease of V Cu defect states with the substitution of Zn atoms for V Cu, resulting in “Zn Cu ”. To investigate band alignment at the p–n junction depending on the different buffer types, we measured the valence band offset (Δ E V ) along the depth direction using X-ray photoemission spectroscopy with sputtering process. Based on the Δ E V values, we suggested a band alignment schematically at the interface of CIGS/Zn(O,S). Taking the carrier lifetimes and barrier height (conduction band offset, Δ E C ) in band alignment into account, we established an important correlation between cell efficiency and interfacial properties that the performance of the CIGS solar cell can be improved with a sufficiently long carrier lifetime of τ l (approximately a nanosecond) and appropriate Δ E C (0.3–0.4 eV) at the p–n junction.