Andriy Nadtochiy

Photovoltage transients at fullerene-metal interfaces

Photovoltage (PV) transients are studied in C60–Pb and C60–Au thin films. The morphology of the C60 layers is characterized by x-ray diffraction and atomic force microscopy, which evidence the formation of a nanocrystalline C60 layer on polycrystalline Pb and Au underlayers. In contrast to Au substrate, Pb crystallites with a (111) texture are predominantly formed. The signs of the PV signals developed at the C60–Pb and C60–Au interfaces are found to be opposite due to very different workfunction values of the two metals. The evolution of the PV rise and decay curves with increasing light illumination intensity is completely different at the C60–Pb and C60–Au interfaces. The rise for the C60–Pb interface speeds up considerably with the increase in intensity, which is markedly different from the behavior at C60–Au, which exhibits nearly unchanged curve shapes. The PV decay time for C60–Au is also only weakly affected by varying light intensity. In contrast, increasing the illumination intensity causes the ...

Photovoltage Performance of Ge/Si Nanostructures Grown on Intermediate Ultrathin SiOX Layers

An enhanced photovoltage is reported to occur in Ge/Si structures with a SiOx layer having a thickness of 0.5-2 nm and placed between a Si substrate and Ge nanoislands. The effect is interpreted in terms of an increased separation distance for photoexcited electrons and holes occurring in the stress fields generated in the oxidized Ge/SiOx/Si structure. The electron-hole separation is modeled utilizing finite-element method techniques, and a good agreement between the experimentally observed enhancement and the computationally increased inter-charge distance is obtained. It is also found that insertion of the oxide layer accelerates the photovoltage decay. This result is interpreted in terms of competing processes, involving the direct recombination of the separated electrons and holes and multi-trapping behavior typical of disordered systems caused by Ge islands.

Charge carrier lifetime recovery in γ-irradiated silicon under the action of ultrasound

Ultrasonic treatment of γ-irradiated silicon can lead to a significant (up to about 70%) recovery of the minority carrier lifetime reduced by the irradiation. A mechanism of the ultrasound-induced recovery is proposed, which is based on the release of vacancies from E-type centers followed by their trapping on defect sinks. It is suggested that the role of defect sinks can be played by A-type microdefects.

Charge-carrier relaxation in sonochemically fabricated dendronized CaSiO3–SiO2–Si nanoheterostructures

We present systematic studies of charge-carrier relaxation processes in sonochemically nanostructured silicon wafers. Impedance spectroscopy and transient photovoltage techniques are employed. It is found that interface potential in Si wafers remarkably increases upon their exposure to sonochemical treatments in Ca-rich environments. In contrast, the density of fast interface electron states remains almost unchanged. It is found that the initial photovoltage decay, taken before ultrasonic treatments, exhibits the involvement of shorter- and longer time recombination and trapping centers. The decay speeds up remarkably due to cavitation treatments, which is accompanied by a substantial quenching of the photovoltage magnitude. It is also found that, before the treatments, the photovoltage magnitude is markedly non-uniform over the wafer surface, implying the existence of distributed sites affecting distribution of photoexcited carriers. The treatments cause an overall broadening of the photovoltage distribution. Furthermore, impedance measurements monitor the progress in surface structuring relevant to several relaxation processes. We believe that sonochemical nanostructuring of silicon wafers with dendronized CaSiO3 may enable new promising avenue towards low-cost solar energy efficiency multilayered solar cell device structures.

Effects of Ultrasonic Cleaning on Carrier Lifetimes and Photovoltage in Monocrystalline Silicon

Effects of a kHz-frequency ultrasonic cleaning of silicon wafers on free carrier lifetimes and the photovoltage magnitude are addressed. It is found that the initial photovoltage decay, taken before ultrasonic treatments, can be fitted to a double-exponent form, exhibiting the involvement of shorter- and longer time recombination and trapping centers. The decay speeds up remarkably due to the treatment, and the rapid component of the decay grows at the expense of the slow component. It is also found that, before the treatment, the decay time is markedly non-uniform over the wafer surface, implying the existence of distributed sites affecting carrier lifetimes. The cleaning causes an overall smoothening of the lifetime distribution, which is accompanied by the above shortening. A likely explanation of the effects is based on two facts: (i) the cavitating bubbles are capable of locally removing the surface oxide layer affecting the dangling bonds on the bare Si surface, and (ii) the oxygen and hydrogen, decomposed in water at elevated pressures and temperatures occurring inside a cavitating bubble, can micro-precipitate the Si wafer thus affecting the recombination rate.