Francesco Santoni

Modeling and simulation of energetically disordered organic solar cells

The aim of this work is to present a consistent model for simulation of organic solar cells (OPV) with a correct description of mobility, density of state, organic-metal contacts, and exciton. We simulate the photoconversion by means of an integration of the optical and electrical part: light absorption is calculated with a Transfer Matrix Model and the charge transport is computed using Drift Diffusion approach including the effect of energetically disorder materials. Most model parameters are directly taken from experiment. The model is used to study the effect of energetic disordered materials and cell thickness on the performance of the cell in terms of short circuit current, open circuit voltage, and fill factor. Based on the results of this model, it will be possible to design and predict the optimal thickness of OPV toward higher efficiencies.

Geometric conductive filament confinement by nanotips for resistive switching of HfO2-RRAM devices with high performance

Filament-type HfO2-based RRAM has been considered as one of the most promising candidates for future non-volatile memories. Further improvement of the stability, particularly at the “OFF” state, of such devices is mainly hindered by resistance variation induced by the uncontrolled oxygen vacancies distribution and filament growth in HfO2 films. We report highly stable endurance of TiN/Ti/HfO2/Si-tip RRAM devices using a CMOS compatible nanotip method. Simulations indicate that the nanotip bottom electrode provides a local confinement for the electrical field and ionic current density; thus a nano-confinement for the oxygen vacancy distribution and nano-filament location is created by this approach. Conductive atomic force microscopy measurements confirm that the filaments form only on the nanotip region. Resistance switching by using pulses shows highly stable endurance for both ON and OFF modes, thanks to the geometric confinement of the conductive path and filament only above the nanotip. This nano-engineering approach opens a new pathway to realize forming-free RRAM devices with improved stability and reliability.

Modeling of Filamentary Conduction in Organic Thin Film Memories and Comparison With Experimental Data

Following the experimental evidences of filament forming in organic thin film memories, we developed a semiclassical drift-diffusion model of electrical conductivity in the filament. We show that the global behavior of a memory device and the total current can be accounted for by fully-formed and well-connected filaments. We investigated and ruled out the eventual influence of coherent quantum tunneling in disconnected filaments. It is also shown how a heating model of the filament can be used to check if assumptions on the number of filaments and their radii are physically plausible.

Charge transport modelling in organic semiconductors: From diodes to transistors, memories and energy harvesters

Organic semiconductors are playing an increasingly important role for the fabrication of many electronic and optoelectronic devices such as organic light emitting diodes (OLEDs) [1], organic photovoltaics (OPVs) [2], organic thin-film transistors (OTFTs) [3] and organic memories [4]. Amorphous and/or regular assembly of polymers/small molecules can form such materials which typically exhibit complex electronic properties. Thus, a unified, comprehensive and transferable model of charge transport and injection in organic semiconductors is highly desired to help the understanding, development and optimization of organic devices. In this work, we will present the efforts made to identify a consistent scheme for charge transport in organic semiconductors and applications of this model to the description of diodes, resistive memories, OTFTs and OPVs.

A universal drift-diffusion simulator and its application to OLED simulations

A universal simulation tool for electronic devices based on a semi-classical drift-diffusion (DD) model is presented. The core of the model is a fully-coupled system of Poisson equation for the electrostatic potential and drift-diffusion transport equations. Both charged and neutral (e.g. excitons) carriers are supported. One transport equation is associated to each carrier. The number of carriers can be set at user level. The equation system can be defined in 1, 2 and 3 dimensions, and it is solved using finite element methods (FEM). The simulator has many potential application, from simple semiconductors with electrons and holes transport, to far more complex device structures, such as the host-guest system of an OLED emitter layer including singlet and triplet excitons. The simulation of an OLED emitter layer is presented, including the thermally activated delayed fluorescence (TADF) effect.