Organic semiconductor material and device characterization by low-frequency noise and admittance spectroscopy of polymer: fullerene solar cells and silicon/organic thin film heterodiodes

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]