Nicola Wrachien

Thermal stress effects on Dye-Sensitized Solar Cells (DSSCs)

Abstract Since the DSSCs gain heat during exposure to sunlight increasing its own temperature, we have studied the role of temperature on the degradation of DSSCs. We have performed pure thermal stresses keeping the devices at a constant temperature inside a climatic chamber and monitoring the electrical parameters during stress. We found that temperature alone strongly impacts on the DSSC performances, enhancing the degradation of the sensitizer and then reducing the photo-generated current.

Effects of Positive and Negative Stresses on III–V MOSFETs With

We subjected III-V InGaAs MOSFETs to positive and negative gate stresses. The stress polarity strongly affects the degradation kinetics of the gate current. Positive stress features a remarkable increase of the gate current, a net negative trapped charge, a large telegraphic noise, and soft breakdown, before the occurrence of the final catastrophic breakdown. Negative stress features only positive trapped charge until the hard breakdown.

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We investigate ferroelectric random access memories subjected to X-ray and proton irradiations. We address the radiation damage dependence on irradiation temperature, its stability during annealing and cycling, and the effects of supply voltage and packaging. The radiation damage strongly depends on the irradiation temperature. Immediately after proton or X-ray irradiation, we detect only stuck bits without data corruption, at least at doses up to 9 Mrad(Si) at room temperature. The radiation damage anneals in time as long as several weeks, and the recovery rate is accelerated by either electrical cycling or high temperature annealing. The radiation tolerance is much higher if the device is irradiated unpowered. Finally, we present a degradation model that accounts for the irradiation temperature dependence.

Gate Dielectric

In this work we present the instabilities observed in organic Thin-Film Transistors when subjected to stress test in different bias, temperature and illumination conditions. C-V measurements, indicate the presence of two distinct trapping phenomena. Appreciable charge trapping can be achieved using relatively high biases for long times (1000s). Illumination strongly enhances charge trapping only under positive gate biases, while it has no effect on the charge trapping/detrapping under negative gate bias. Charge detrapping is thermally activated coherently with trapping/detrapping at the SiO2/pentacene interface from hydrogenoid species. A first order model explaining the observed relaxation kinetics is also presented.

Ionizing Radiation Effect on Ferroelectric Nonvolatile Memories and Its Dependence on the Irradiation Temperature

In this paper, we study the reliability of sensitized solar cells. We carried out accelerated illumination stresses with the purpose to extract a degradation law as a function of time, illumination intensity, and temperature in order to understand which are the most important mechanisms involved in the degradation. The main responsible for degradation during illumination stresses is the formation of defects and chemical species at TiO 2/sensitizer/electrolyte interface, which reduces the charge transfer at the interface and the ion migration across the electrolyte. Since the sunlight exposure increases the temperature of the dye-sensitized solar cell (DSC), we have studied the role of temperature on the degradation of DSCs by pure thermal stresses. We found that temperature alone strongly impacts the DSC performance, enhancing the degradation of the sensitizer and then reducing the photogenerated current.

Light, bias, and temperature effects on organic TFTs

We present new results on heavy-ion irradiation of nanocrystal non-volatile addressable memory arrays. We show that the effects of a single ion hit are negligible on these devices due to the discrete nature of the storage sites. We estimate that, in order to observe an appreciable threshold voltage shift, at least three to four ion hits are needed. Despite several cells experiencing multiple hits they are still functional after the irradiation, showing no changes on the retention characteristics. These results highlight an outstanding improvement of the nanocrystal technology over the conventional floating gate memories in terms of radiation tolerance, which are encouraging for a potential application in radiation harsh environments.

Reliability Study of Ruthenium-Based Dye-Sensitized Solar Cells (DSCs)

We irradiated Organic Thin-Film Transistors using light sources with peak wavelength in the near-UV spectrum. Organic Thin-Film Transistors have been found very sensitive to near UV rays. Irradiation generates negative trapped charge due to photogeneration, as well as neutral defects, which degrade the mobility and transconductance. We observed noticeable degradation even with 400-nm wavelength, which lies between visible and UV-light. Stronger variations were observed with the shortest wavelength, with a transconductance drop of -85% after 33000-s irradiation with 310 nm UV. We found that the mobility degradation is permanent after storing the devices in vacuum at least for 50 days.

Radiation Tolerance of Nanocrystal-Based Flash Memory Arrays Against Heavy Ion Irradiation

We investigate Total Ionizing Dose effects on 4 Mbit Phase Change Memories (PCM) arrays. We demonstrate a high robustness of PCM against ionizing radiation. We irradiated PCM with 8-MeV electrons. Only small variations are measured in the cell distributions after irradiation. The primary cause of these variations is the degradation of the Bit-Line and the Word-Line selector MOSFETs. Finally, radiation does not compromise the functionality of the SET and the RESET operations.

Near-UV Irradiation Effects on Pentacene-Based Organic Thin Film Transistors

Proton irradiation of nanocrystals memories produces peculiar radiation effects on the electrical characteristics of these devices, owing to their thin tunnel oxide and to the presence of nanocrystals replacing the conventional flash memory floating gate. In this work, we show that the data retention capability is compromised only after high fluences and that irradiated devices do not show accelerated degradation during subsequent electrical stresses. The presence of nanocrystals instead of a floating gate reduces also the quantity of charge lost during irradiation, indicating these devices as possible candidates for space and avionic environments

Total Ionizing Dose Effects on 4 Mbit Phase Change Memory Arrays

We performed gate ramp voltage stress on III-V InGaAs based MOSFETs. Stress induces trapped charge and it also leads to interface trap generation, which has detrimental effects on the subthreshold slope and on the transconductance. At high electric fields, before the hard breakdown, a very low-frequency high-current random telegraph noise appears at the gate, which seems to be not correlated with the soft breakdowns commonly observed in other devices.