Artem Pimachev

Electronic structure calculations of PbS quantum rods and tubes

We study absorption spectra, optical and HOMO-LUMO gaps, and the density of states for PbS quantum rods (QRs) and tubes (QTs). We find some similarities and also differences in QR and QT properties. For both QRs and QTs, the optical and HOMO-LUMO gaps reach the plateaus for small lengths. We find that tubes are as stable as rods. The optical spectra exhibit a peak that can be due to the electron-hole interaction or be a prototype of an Se–Sh transition in the effective mass approximation. We also calculate the density of states by the density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. The TDDFT density of states function is shifted towards the red side by 0.5 eV indicating the strong e-h interaction.

Large enhancement in photocurrent by Mn doping in CdSe/ZTO quantum dot sensitized solar cells

We find a large enhancement in the efficiency of CdSe quantum dot sensitized solar cells by doping with manganese. In the presence of Mn impurities in relatively small concentrations (2.3%) the photoelectric current increases by up to 190%. The average photocurrent enhancement is about 160%. This effect cannot be explained by a light absorption mechanism because the experimental and theoretical absorption spectra demonstrate that there is no change in the absorption coefficient in the presence of the Mn impurities. To explain such a large increase in the injection current we propose a tunneling mechanism of electron injection from the quantum dot LUMO state to the Zn2SnO4 (ZTO) semiconductor photoanode. The calculated enhancement is approximately equal to 150% which is very close to the experimental average value of 160%. The relative discrepancy between the calculated and experimentally measured ratios of the IPCE currents is only 6.25%. For other mechanisms (such as electron trapping, etc.) the remaining 6.25% cannot explain the large change in the experimental IPCE. Thus we have indirectly proved that electron tunneling is the major mechanism of photocurrent enhancement. This work proposes a new approach for a significant improvement in the efficiency of quantum dot sensitized solar cells.

Internal relaxation in dye sensitized solar cells based on Zn2SnO4 nanostructures

In this work we study the effect of internal relaxation in a (Bu(4)N)(2)Ru(dcbpyH)(2)(NCS)(2) (N719) dye molecule in a dye sensitized solar cell. Experimentally measured light intensity dependencies of short circuit current and open circuit voltage for two different types of photoanodes, ZTO (Zn(2)SnO(4)) nanorods and nanoparticles, are explained in the framework of the proposed microscopic theory. This theory is based on a density matrix equation with a Markovian relaxation term. The computational results are in favor of the fast relaxation inside the unoccupied and occupied bands rather than slow interband electron-hole recombination. The difference in experimental dependencies for ZTO nanorods and nanoparticles is explained by the difference in the electron transfer matrix elements, and therefore, the electron transfer injection constants for the different morphologies of the photoanodes.

Giant photocurrent enhancement by transition metal doping in quantum dot sensitized solar cells

A huge enhancement in the incident photon-to-current efficiency of PbS quantum dot (QD) sensitized solar cells by manganese doping is observed. In the presence of Mn dopants with relatively small concentration (4 at. %), the photoelectric current increases by an average of 300% (up to 700%). This effect cannot be explained by the light absorption mechanism because both the experimental and theoretical absorption spectra demonstrate several times decreases in the absorption coefficient. To explain such dramatic increase in the photocurrent we propose the electron tunneling mechanism from the LUMO of the QD excited state to the Zn2SnO4 (ZTO) semiconductor photoanode. This change is due to the presence of the Mn instead of Pb atom at the QD/ZTO interface. The ab initio calculations confirm this mechanism. This work proposes an alternative route for a significant improvement of the efficiency for quantum dot sensitized solar cells.

Coexistence of Two Electronic Nano-Phases on a CH3NH3PbI3–xClx Surface Observed in STM Measurements

Scanning tunneling microscopy is utilized to investigate the local density of states of a CH3NH3PbI3-xClx perovskite in cross-sectional geometry. Two electronic phases, 10-20 nm in size, with different electronic properties inside the CH3NH3PbI3-xClx perovskite layer are observed by the dI/dV mapping and point spectra. A power law dependence of the dI/dV point spectra is revealed. In addition, the distinct electronic phases are found to have preferential orientations close to the normal direction of the film surface. Density functional theory calculations indicate that the observed electronic phases are associated with local deviation of I/Cl ratio, rather than different orientations of the electric dipole moments in the ferroelectric phases. By comparing the calculated results with experimental data we conclude that phase A (lower contrast in dI/dV mapping at -2.0 V bias) contains a lower I/Cl ratio than that in phase B (higher contrast in dI/dV).

Tunable bandgap in halogen doped 2D nitrogenated microporous materials

The quest for new materials with extraordinary electronic, magnetic, and optical properties leads to the synthesis of 2D nitrogenated microporous materials with the hole diameter of 1.16 nm. We computationally study the evolution of the energy bandgaps, optical, and transport properties with the following substituents: hydrogen, fluorine, chlorine, and iodine. We find that such a small perturbation by these atoms has a tremendous impact on the electronic properties of these materials. Indeed, the direct energy bandgaps can be tuned from 1.64 to 0.96 eV by the substituents from hydrogen to iodine. The optical gaps demonstrate similar dependence. From the transport properties, we calculate the effective masses of π-conjugated microporous polymers and find that the conduction electron effective masses are insensitive to halogen substituents while for some low-lying energy valence bands the effective masses can be drastically increased from 0.71 to 2.98 me and 0.28 to 0.58 me for the heavy and light holes, re...

Effects of Mn dopant locations on the electronic bandgap of PbS quantum dots

Dilute magnetic semiconductors (DMSs) are typically made by doping semiconductors with magnetic transition metal elements. Compared to the well-understood bulk and thin film DMS, the understanding of the magnetic element doping effects in semiconducting quantum dots (QDs) is relatively poor. In particular, the influence of the dopant locations is rarely explored. Here, we present a comprehensive study of the effects of Mn doping on the electronic density of states of PbS QDs. Based on the results observed by scanning tunneling microscopy, X-ray diffraction, electron paramagnetic resonance, and density functional theory calculations, it is found that the Mn doping causes a broadening of the electronic bandgap in the PbS QDs. The sp-d hybridization between the PbS host material and Mn dopants is argued to be responsible for the bandgap broadening. Moreover, the locations of the Mn dopants, i.e., on the surface or inside the QDs, have been found to play an important role in the strength of the sp-d hybridization, which manifests as different degrees of the bandgap change.Dilute magnetic semiconductors (DMSs) are typically made by doping semiconductors with magnetic transition metal elements. Compared to the well-understood bulk and thin film DMS, the understanding of the magnetic element doping effects in semiconducting quantum dots (QDs) is relatively poor. In particular, the influence of the dopant locations is rarely explored. Here, we present a comprehensive study of the effects of Mn doping on the electronic density of states of PbS QDs. Based on the results observed by scanning tunneling microscopy, X-ray diffraction, electron paramagnetic resonance, and density functional theory calculations, it is found that the Mn doping causes a broadening of the electronic bandgap in the PbS QDs. The sp-d hybridization between the PbS host material and Mn dopants is argued to be responsible for the bandgap broadening. Moreover, the locations of the Mn dopants, i.e., on the surface or inside the QDs, have been found to play an important role in the strength of the sp-d hybridiza...