Mott-Schottky Analysis and Impedance Spectroscopy of TiO2/6T and ZnO/6T devices.

Schottky junctions have been realized by evaporating gold spots on top of sexithiophen (6T), which is deposited on TiO 2 or ZnO with e-beam and spray pyrolysis. Using Mott-Schottky analysis of 6T/TiO2 and 6T/ZnO devices acceptor densities of 4.5x10(16) and 3.7x10(16) cm(-3) are obtained, respectively. For 6T/TiO2 deposited with the e-beam evaporation a conductivity of 9x10(-8) S cm(-1) and a charge carrier mobility of 1.2x10(-5) cm2/V s is found. Impedance spectroscopy is used to model the sample response in detail in terms of resistances and capacitances. An equivalent circuit is derived from the impedance measurements. The high-frequency data are analyzed in terms of the space-charge capacitance. In these frequencies shallow acceptor states dominate the heterojunction time constant. The high-frequency RC time constant is 8 micros. Deep acceptor states are represented by a resistance and a CPE connected in series. The equivalent circuit is validated in the potential range (from -1.2 to 0.8 V) for 6T/ZnO obtained with spray pyrolysis.

Photoluminescence study of sexithiophene thin films.

To determine the exciton diffusion length of sexithiophene (6T) thin films, quenching of the photoluminescence (PL) of vacuum-deposited 6T films on TiO2 and on quartz has been investigated. For films with a thickness of more than 22 nm and at temperatures below 100 K, additional PL lines appear in luminescence spectra. This feature is related to the structural properties of 6T films. The PL intensity is thermally activated with an activation energy of 18 meV on TiO2 and 6 meV on quartz. When 6T is applied on TiO2, exciton quenching occurs for films up to 120 nm. For 6T on quartz this value is reduced to 60 nm. By comparing the relative luminescence intensities of 6T on quartz and on TiO2 substrates, an exciton diffusion length of 60 +/- 5 nm is derived.

Supercrystals of CdSe quantum dots with high charge mobility and efficient electron transfer to TiO2.

Thermal annealing of thin films of CdSe/CdS core/shell quantum dots induces superordering of the nanocrystals and a significant reduction of the interparticle spacing. This results in a drastic enhancement of the quantum yield for charge carrier photogeneration and the charge carrier mobility. The mobile electrons have a mobility as high as 0.1 cm(2)/(V x s), which represents an increase of 4 orders of magnitude over non-annealed QD films and exceeds existing literature data on the electron mobility in CdSe quantum dot films. The lifetime of mobile electrons is longer than that of the exciton. A fraction of the mobile electrons gets trapped at levels below the conduction band of the CdSe nanocrystals. These electrons slowly diffuse over 50-300 nm on longer times up to 20 micros and undergo transfer to a TiO2 substrate. The yield for electron injection in TiO2 from both mobile and trapped electrons is found to be >16%.

Study of electronic defects in CdSe quantum dots and their involvement in quantum dot solar cells.

To enhance efficiencies of quantum dot CdSe/TiO(2) based solar cells, understanding of the space charge at the CdSe/TiO(2) interface is crucial. In this paper, the presence of a shallow acceptor in the CdSe quantum dots is found by means of a detailed impedance and Mott-Schottky (C(-2)-phi) study. Furthermore, it is clearly shown that this acceptor density decreases strongly with increasing quantum dot size. The presence of these defect states may give rise to Auger recombination in small quantum dots and therewith decrease the efficiency of quantum-dot-sensitized solar cells.

Spray-deposited CuInS(2) solar cells.

Spray deposition of CuInS(2) offers an attractive route towards industrial production of thin-film solar cells. With spray deposition it is possible to make nanocomposites of n-type TiO(2) and p-type CuInS(2). Upon application of an In(2)S(3) buffer layer, solar cells can be made with efficiencies of ∼7%, being comparable to that of amorphous silicon. Rapid thermal annealing is not involved in the production of these solar cells. In order to further improve the performance, the concentration of electronic defect states in the bandgap must be reduced. Towards this end a detailed study has been undertaken to elucidate the role of associated point defects in the recombination of electron-hole pairs. Especially with transient absorption spectroscopy it is possible to make an accurate assessment of the fundamental electronic processes that are involved. We find electronic states in the bandgap related to the presence of anti-site defects. In addition, indium vacancies are also involved. State-to-state recombination occurs, indicating that the involved defects are associated. An electronic state located at 1.1 eV above the valence band, which is related to indium on a copper position, has a lifetime of about 20 µs at room temperature. The lower lying states related to copper on indium positions, and indium vacancies, are populated from this 1.1 eV state.

In spite of recent doubts carrier multiplication does occur in PbSe nanocrystals.

Efficient carrier multiplication has been reported for several semiconductor nanocrystals: PbSe, PbS, PbTe, CdSe, InAs, and Si. Some of these reports have been challenged by studies claiming that carrier multiplication does not occur in CdSe, CdTe, and InAs nanocrystals, thus raising legitimate doubts concerning the occurrence of carrier multiplication in the remaining materials. Here, conclusive evidence is given for its occurrence in PbSe nanocrystals using femtosecond transient photobleaching. In addition, it is shown that a correct determination of carrier-multiplication efficiency requires spectral integration over the photobleach feature. The carrier multiplication efficiency we obtain is significantly lower than what has been reported previously, and it remains an open question whether it is higher in nanocrystals than it is in bulk semiconductors.

Nanocomposite three-dimensional solar cells obtained by chemical spray deposition.

The present study is focused on low-cost preparation of thin film TiO2/CuInS2 nanocomposite three-dimensional (3D) solar cells. With the aid of a simple spray deposition method, we have been able to obtain 3D solar cells, with a remarkable energy conversion efficiency of 5%. The new 3D solar cell design has the potential to breakdown the price barrier and to open up new production technologies for low-cost photovoltaic solar cells.