Se-Assisted Performance Enhancement of Cu₂ZnSn(S,Se)₄ Quantum-Dot Sensitized Solar Cells via a Simple Yet Versatile...

The earth-abundant Cu₂ZnSnS₄ (CZTS) quantum dots (QDs) have emerged as one potential substitute to toxic cadmium or rare indium QDs, but their application in quantum dot-sensitized solar cells (QDSSCs) is still limited by the improper particle size and the rigorous synthesis and ligand exchange conditions. Herein, we developed a one-pot hot injection method by using Tri-n-octylphosphine oxide (TOPO) as the solvent and oleylamine as the capping agent to synthesize Cu₂ZnSn(S,Se)₄ (CZTSSe) QDs with adjustable size and narrow size distribution. The key feature of this approach is that we can take advantage of the high-temperature nucleation, low-temperature growth, and strong reducibility of NaHB₄ to prepare small-sized CZTSSe QDs without using 1-dodecanethiol (DDT) and to extend the light harvesting range through Se incorporation. After Se incorporation, it turns out that the conduction band (CB) level of CZTSSe QDs decreases, implying that the injection driving force of the electron to the CB of TiO₂ films becomes weaker and a larger recombination would be induced at the TiO₂/QDs/electrolyte interface. Benefiting from the broadened optoelectronic response range, the induced higher J_sc (16.80 vs 14.13 mA/cm²) finally leads to the increase of the conversion efficiency of CZTSSe QDSSC from 3.17% to 3.54% without further modification. Despite the fact that the efficiency is still far behind those of literature reported values through use of other chalcogenide sensitizers, this DDT-free approach solves the main hindrance for the application of CZTSSe QDs in QDSSCs and holds a more convenient way for ligand exchange, light absorption improvement, and particle size control.