D. Bari

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.

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

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.

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

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.

Enhanced permanent degradation of organic TFT under electrical stress and visible light exposure

Abstract We subjected OTFTs to electrical stress under various illumination conditions, in the visible-light range. Negligible intrinsic degradation is observed without stress and light permanent effects of light without stress on OTFT properties are very modest. When light and stress are applied simultaneously, the degradation is much stronger, than stress alone. We found that the acceleration factor is over 2 if blue light is shed on the devices.

Reliability study of dye-sensitized solar cells by means of solar simulator and white LED

Abstract In this work, we take into account a LED-based light source as an alternative to AM1.5 solar simulator to perform the reliability study on dye-sensitized solar cells (DSCs). We performed accelerated optical stress by means of high power white LED and during stress we performed DC and EIS measurements with both white LED and AM1.5 solar simulator in order to find, if any, differences in kinetics degradation. During stress we also performed characterization measurements using monochromatic LED sources in order to understand if it adds more information about the DSCs degradation mechanism. We found that DC parameters feature different degradation rates depending on characterization source and differences also appear on degradation kinetics shape. The DSC characterization performed with monochromatic light sources show strong differences in degradation rate and in degradation kinetics shape as well depending on wavelength sources.

Low-energy UV effects on Organic Thin-Film-Transistors

We subjected Organic Thin-Film-Transistors with different gate dielectrics interface treatment to visible light and low energy UV irradiation. Devices with silicon nanoparticles only feature a remarkable temporary charge trapping, regardless the irradiation wavelength. On the contrary, devices without silicon nanoparticles feature temporary trapped charge under visible light, and permanent mobility degradation if they are irradiated with a wavelength shorter than 420nm.

Effects of constant voltage stress on p- and n-type organic thin film transistors with poly(methyl methacrylate) gate dielectric

Abstract We performed constant voltage stress on oligothiophene-based p- and n-type organic thin-film-transistors, in the accumulation regime. The stress induced charge trapping, mobility degradation and defect generation. The specific kinetics depends not only on the applied bias value and polarity, but also on the semiconductor type. Stress on PMMA capacitors (without the semiconductor film) revealed that most of the degradation is largely originated by the interaction with the semiconductor layer, which enhances charge injection.

Improved Tolerance Against UV and Alpha Irradiation of Encapsulated Organic TFTs

In this work, we analyzed the effects of alpha and ultraviolet irradiation on encapsulated and non-encapsulated thin film-transistors. Up to -35% mobility variation occurs on unencapsulated devices after UV irradiation. The same UV irradiation induced less than -3% on encapsulated devices. By investigating the charge trapping kinetics, we show that the increased robustness comes from the reduced air absorption, rather than solely attenuating UV the components. Beside the degradation mechanisms seen on conventional MOS structures, alpha irradiation also induces another form of degradation in air, on devices without encapsulation, due to the reaction of the exposed pentacene with ozone or ionized oxygen generated by alpha, through air ionization. The encapsulation partially mitigates this mechanism, but it is ineffective in reducing the ionizing radiation effects intrinsic to the MOS structure.

Effects of positive and negative constant voltage stress on organic TFTs

We subjected Organic TFTs to positive and negative constant gate voltage stresses. Stress not only induces temporary charge trapping, but also permanent transconductance degradation. The permanent degradation is accelerated if the devices are stressed under illumination. Negative stress degrades the TFT much faster than the positive stress, at the same voltage. Furthermore, hard breakdowns are observed on devices stressed under light, with VGS= -65V. The degradation mainly comes from stress-induced interface trap generation, rather than photochemical reactions as observed with UV irradiations.

Organic Thin Film Transistor degradation under sunlight exposure

We irradiated Organic Thin-Film Transistors with wavelengths ranging in the visible and near ultraviolet parts of the solar spectrum, reporting strong degradation in few hours. We modeled the sunlight-induced permanent degradation, taking into account the mobility reduction. The model closely fits the experimental data, allowing the prediction of the device permanent degradation under sunlight exposure.