Control of Defect States of Kesterite Solar Cells to Achieve More Than 11% Power Conversion Efficiency
Md Hamim Sharif, Temujin Enkhbat, Enkhjargal Enkhbayar, JunHo Kim
Abstract
Due to their inherent nature, kesterite solar cells exhibit complex secondary phases and intrinsic lattice defects. As a result, the power conversion efficiencies (PCEs) of kesterites are considerably lower than the theoretical limit. One of the main culprits of limitation is the open-circuit voltage deficiency (Vocdef) due to CuZn antisite-related deep defects that originate from the similar ionic radii of constituent elements of Cu and Zn. Partial replacement of Zn with Cd is known to mitigate the defect states from CuZn antisite-related defects. In this regard, we studied the defect physics of Cd-alloying effects on CZTSSe solar cells by varying the Cd/(Cd + Zn) ratio from 0.04 to 0.3. We further investigated Cd-alloyed CZTSSe (CZCTSSe) devices using temperature-dependent admittance spectroscopy, current–voltage characteristics, and time-resolved photoluminescence measurements. Spectroscopic characterization and analysis revealed that the trap energy level, Urbach energy, and band tailing decreased with low Cd alloying (Cd/(Cd + Zn) < 0.08) and increased again with further Cd alloying (Cd/(Cd + Zn) > 0.08) in CZCTSSe absorbers. Experimental results indicate that the alloying amount of Cd has a huge impact on the defect states of absorber films, which dominates the PCE of CZCTSSe solar cells. With fine tuning of the Cd/(Cd + Zn) ratio = 0.06, a CZCTSSe device efficiency of 11.73% was achieved without an antireflection coating using aqueous spray pyrolysis.