ИСТИНА

An influence of surface ligands of lead sulfide quantum dots (QDs) on the optical properties and photoconductivity of the QD condensate layers was studied. The ligands of various organic molecules with different length were used. On QDs, long length molecules were replaced successfully by short length ones and the result of the procedure was confirmed by IR spectroscopy. Based on the studied QD layers, photovoltaic diodes of the ITO\PEDOT:PSS\QDs PbS\ZnO\Al structure were fabricated. The diodes showed the short-circuit current of 2-4 mA/cm2 for QDs layers with either 1,6-hexanedithiol (HDT) or hydrazine ligands. On the contrary, the diodes with QDs layers with an oleic acid (OA) ligand showed neglegibly low photocurrents. The conductivity measured by the four probe technique increases exponentially with decreasing ligand molecule length on QDs (Figure 1). Measured by CELIV method in samples of ITO\SiO2\QD PbS\Al structure, the charge carrier mobility values for QDs with HDT and hydrazine ligands are equal to 7,1·10-6 and 13,6·10-6 cm2/(V·s), respectively. The mobility agrees well the conductivity, which confirms the key role of ligand molecules in the transport of excitons and charge carriers in the QD layers. The obtained data for PbS QDs correlate with those for PbSe QDs in Ref.1 Thus, the replacement of the original long OA molecules by relatively short HDT and hydrazine molecules, which can chemically bind QDs to each other, leads to the improved hopping transport of charge carriers between QDs. The study also reveals that it is the length of the ligand molecules rather than their chemical structure defines the conductivity and hopping transport of charge carriers between QDs in the QD layers. This work was supported by RFBR (project № 16-29-09623).