Simulation and modelling of charge transport in dye-sensitized solar cells based on carbon nano-tube electrodes

Abstract

For a better understanding of the mechanisms of dye-sensitized solar cells (DSSCs), based on carbon nano-tube (CNT) electrodes, a phenomenological model is proposed. For modelling purposes, the meso-scopic porous CNT electrode is considered as a homogeneous nano-crystalline structure with thickness L. The CNT electrode is covered with light-absorbing dye molecules, and interpenetrated by the tri-iodide (I−/I3−) redox couple. A simulation platform, designed to study coupled charge transport in such cells, is presented here. The work aims at formulating a mathematical model that describes charge transfer and charge transport within the porous CNT window electrode. The model is based on a pseudo-homogeneous active layer using drift–diffusion transport equations for free electron and ion transport. Based on solving the continuity equation for electrons, the model uses the numerical finite difference method. The numerical solution of the continuity equation produces current–voltage curves that fit the diode equation with an ideality factor of unity. The calculated current–voltage (J–V) characteristics of the illuminated idealized DSSCs (100 mW cm−2, AM1.5), and the different series resistances of the transparent conductor oxide (TCO) layer were introduced into the idealized simulated photo J–V characteristics. The results obtained are presented and discussed in this paper. Thus, for a series resistance of 4 Ω of the TCO layer, the conversion efficiency (η) was 7.49% for the CNT-based cell, compared with 6.11% for the TiO2-based cell. Two recombination kinetic models are used, the electron transport kinetics within the nano-structured CNT film, or the electron transfer rate across the CNT–electrolyte interface. The simulations indicate that both electron and ion transport properties should be considered when modelling CNT-based DSSCs and other similar systems. Unlike conventional polycrystalline solar cells which exhibit carrier recombination, which limits their efficiency, the CNT matrix (in CNT-based cells) serves as the conductor for majority carriers and prevents recombination. This is because of special conductivity and visible–near-infrared transparency of the CNT. Charge transfer mechanisms within the porous CNT matrix and at the semiconductor–dye–electrolyte interfaces are described in this paper.