Tailoring Energy Structure of Low-Toxic Ternary Ag−Bi−S Quantum Dots through Solution-Phase Synthesis for Quantum-Dot-Sensitized Solar Cells

Low-toxic multinary semiconductor quantum dots (QDs) showing a photoresponse in a wide wavelength range from visible to near-IR wavelength regions have been intensively investigated for fabricating efficient solar light conversion systems. Recently, AgBiS₂ QDs have attracted much attention for their application to improve the performance of QD solar cells because they have a large absorption coefficient in the visible and near-IR regions. In this report, we describe solution-phase preparation of ternary Ag−Bi−S QDs with different sizes and with different Ag/Bi ratios. The average size of resulting spherical Ag−Bi−S QDs was controllable from 2.7 to 8.1 nm by adjusting the reaction temperature from 373 to 473 K, while the Ag fraction of obtained QDs also increased with an increase in particle size. The absorption onset wavelength shifted from 850 to 1200 nm in the near-IR region as the particle size increased. AgBiS₂ QDs with an almost stoichiometric composition, obtained at reaction temperatures of 423 K and 473 K, contained no deep intragap states and exhibited a p-type semiconductor behavior, while QDs prepared at a reaction temperature of 393 K or lower had non-stoichiometric Ag-deficient compositions producing intragap defect states. Sensitized solar cells fabricated with stoichiometric AgBiS₂ QDs exhibited a photoresponse in visible and near-IR wavelength regions, the optimal PCE being 0.74% with AgBiS₂ QDs of 6.2 nm in diameter that were prepared at 423 K.