| dc.description.abstract | The main focus of the present work is addressed to the field of organic
electronics, which has attracted increasing interest for the
development of flexible, large area and low cost electronic
applications, from light emitting diodes to thin film transistors and
solar cells. The present work describes initially, the application of
low-frequency electronic noise spectroscopy for the characterization
of organic electronic devices as an innovative and non-destructive
technique. In particular the role of the modification induced by
thermal stress on the electronic transport parameters under dark
conditions of a bulk-heterojunction polymer solar cell have been
investigated in detail.
The investigated organic solar cell is based on a blend between
poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C6l-butyric acid
methyl ester (PCBM), representing the classical reference structure
regarding the polymer:fullerene type devices. Before the irreversible
modification of the active layer, the solar cell has been modeled at low
frequencies as a parallel connection between a fluctuating resistance
RX(t) and a capacitance CX. Under dc biasing, the carriers injected into
the active layer modify the equivalent electrical impedance thus
changing the noise spectra. The experimental spectral trace can be
interpreted by means of a theoretical model based on the capacitance
Cμ, which takes into account the excess of minority carriers in the
blend, and the device resistance Rrec. The measured electric noise is of
1/f-type up to a cut-off frequency fX, after which a 1/f3 dependence
has been observed. The analysis of fX gives information regarding the
recombination lifetime of the electrons in the active layer, while the
voltage dependence of the Cμ provides information about the density
of states for the lowest unoccupied molecular orbital (LUMO) level in
the PCBM material. Furthermore, the voltage fluctuations
spectroscopy has been used to detect modifications of the active layer
due to thermal stress. The temperature has been identified as one of
the external parameters that can accelerate the parameter degradation.
The analysis of the flicker and the thermal noise at low frequency
reveals a decrease of the charge carrier zero-field mobility after a
thermal cycle. This effect has been related to morphological changes
of the solar cell active layer and the interface between the metal
contact and the blend. Moreover, the influence of the solvent
additives during the film preparation stage on the electronic transport
in the solar cells has been studied by means of noise spectroscopy, and
a detailed comparison of the optoelectronic properties of solar cells
prepared with different blends has been made.
On one side, a P3HT/PCBM based bulk heterojunction solar cell is
one of the most prominent candidates for a polymer solar cell, but on
the other side, its conversion efficiency is limited by poor longwavelength
absorption. One way to increase the conversion efficiency
is to modify the active layer absorption by the addition of materials,
that increase the absorption of light in the red and infrared spectral
region. One of the most promising materials for this task are inorganic
quantum dots (QDs). In the present study we choose InP/ZnS quantum
dots with an emission peak wavelength of about 660 nm. ... [edited by Author] | it_IT |
