Figure 3: Impact of doping on electronic structure of QDs.
From: Heterovalent cation substitutional doping for quantum dot homojunction solar cells
(a) UPS (UV excitation by HeI=21.21 eV) valence band-onset spectral region and measured EF−EVB differences for PbS and two Bi:PbS unbiased QD thin film samples, and (b) cutoff region of the spectra with the samples biased at −5 V, including measured EF shift by doping. (c) Part of cyclic voltammetry graphs at dark for PbS and Bi:PbS (precursor Bi/Pb=3.2%) QDs. Marked by arrows are the positions of the first oxidation and reduction peaks attributed to the valence band (HOMO) at −5.13 eV and conduction band (LUMO) at −4.13 eV of PbS, respectively. The x axis is the energy product of the elementary charge and applied voltage, referenced against Evac=0 using the ferrocene/ferrocenium couple. For Bi:PbS (Bi/Pb=3.2%) an additional conduction tail EcT (0.3 eV below LUMO) reduction peak at 4.43 eV and a mirror oxidation one during the reverse scan, are observed. (d) Comparison of the EV, EC levels as measured by cyclic voltammetry for pure and doped QD films with 1 eV bandgap (with broadening due to size distribution of single dots). EV is constant with respect to vacuum and the Fermi levels are positioned with respect to EV according to UPS. The crystal models of PbS and Bi:PbS QDs of 3.6 nm diameter, consisting of x6 rock-salt unit cells in each direction and truncated at their {110} and {111} edge and corner planes, respectively, indicate that a doped QD with Bi/Pb=4.5% contains ~728 and 33 atoms of Pb and Bi, respectively, assuming that Bi substitutes Pb. (e) Dependence of Voc and Jsc of an inverted homojunction on temperature with extrapolation of linear Voc-T regions towards T→0 K.
