Study of hole mobility in poly(N-vinylcarbazole) films doped with CdSe/ZnS quantum dots encapsulated by 11-(N-carbazolyl) undecanoic acid (C11)

We report the experimental study of hole transport in poly(vinylcarbazole) (PVK) films doped with colloidal CdSe/ZnS core-shell quantum dots (QDs) determined using the Time-of-Flight (TOF) method. The miscibility between PVK and the QDs is improved by capping the QDs with a novel 11-(N-carbazolyl) undecanoic acid (C11) ligand instead of commonly used organic ligands, such as oleic acid. The study of the hole mobility of the pristine and doped PVK films with a doping concentration of the C11 capped QDs ranging from 1.61 x 10(17) to 7.10 x 10(18) dots/cm(3) was performed as a function of electric field and temperature in the range of 10(5)-10(6) V/cm and 298-338K, respectively. Upon increasing the QD concentration, a decrease of hole mobility was observed by up to nearly 2 orders in magnitude at a doping concentration of 3.87 x 10(18) dots/cm(3) at T = 298 K. This suggests that the QDs induce shallow hole traps. The field and temperature dependence of the hole mobility was characterized using the Bassler disorder model and showed an increase of the energetic disorder (sigma) from 124 to 204meV as well as of the spatial disorder (Sigma) from 0.95 to 5 when the concentration of the QDs was increased to 3.87 x 10(18) dots/cm(3). At higher concentration of the QDs (7.10 x 10(18) dots/cm(3)), an increase of the hole mobility was observed suggesting hopping of the holes through the QD clusters. In addition, we also found that for this high doping concentration, the field dependence of the hole mobility was no longer in agreement with the Bassler disorder model. One should consider that at this doping concentration, the volume occupied by the inorganic (CdSe + ZnS) and organic (C11) components of the QDs in the doped film was estimated to be 14.6 and 15.8 volume %, respectively. This implies that the volume fraction of the inorganic material is very close to the percolation threshold, which amounts to 17 volume % for small spherical particles embedded in a three dimensional matrix. Furthermore, the conductivity data suggest a qualitative change in film properties between the samples with 3.87 x 10(18) and 7.10 x 10(18) dots/cm(3). The study of film morphology by atomic force microscopy (AFM) experiment shows that while for the film with 3.87 x 10(18) dots/cm(3) the surface of the film has still the same features as that of a pristine PVK film, this is no longer the case for the film with 7.10 x 10(18) dots/cm(3), where shallow holes with a diameter of 100 to 200 nm become visible. These holes with the size much larger than the diameter of an individual QD likely correspond to clusters of the QDs. Upon further increasing the QD concentration to 9.68 x 10(18) dots/cm(3), the density of these holes is also increased. A correlation between the conductivity data and the film morphologies indicates that the presence of these QD clusters in the sample with 7.10 x 10(18) dots/cm(3) does not only change the homogeneity and roughness of the film but also leads to a significant change in the shape of the density of states of the energy sites for hopping holes resulting in a field and temperature dependence of the hole mobility that is no longer compatible with the Gaussian disorder model for this sample. Furthermore, the presence of these hole structures observed with AFM might imply a formation of large QD clusters in the polymer film, which form continuous pathways for charge carrier hopping between the opposite electrodes. (c) 2013 AIP Publishing LLC.