Prediction of an order of magnitude for electron and hole mobilities using 1D simulations
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Prediction of an order of magnitude for electron and hole mobilities using 1D simulations Damir Aidarkhanov 1, 2, Adam Raba 2, Yann Leroy 2 and Anne-Sophie Cordan 2 1 Renewable Energy Department, Nazarbayev University Research and Innovation System, Nazarbayev University, 53 Kabanbay Batyr Ave., Astana 010000, Republic of Kazakhstan. 2 Institut d'Électronique du Solide et des Systèmes, Strasbourg University, Télécom Physique Strasbourg, Illkirch, France. ABSTRACT Organic photovoltaics has attracted much effort and many research groups during the past decade, because of low-cost and easy fabrication techniques. Despite the great progress that has been achieved in increasing the conversion efficiencies of the devices, there are still several problems to be solved to make the solar cells commercially viable, especially for cells based on bulk heterojunctions. The purpose of this work is to supply techniques for predicting the order of magnitude of the charge carrier mobilities of bulk heterojunction devices, on the basis of easy-to-perform measurements for experimentalists. A one dimensional model of a bulk heterojunction cell was used, and then simulations were performed in order to obtain the photocurrent as a function of an effective applied voltage. Plotted in a double logarithmic scale, the resulting curves exhibit different signatures depending on the mobilities of the charge carriers. These signatures could be helpful for experimentalists in order to predict an order of magnitude for both the electron mobility and the hole mobility. INTRODUCTION One of the main characteristics of the organic solar cells, affecting their performance, is the charge carrier mobility in the active media. Thus, it is necessary to keep track of the changes in electron and hole mobilities which can be related to changes of the device performance. There are some experimental techniques available to extract mobilities of the electrons and holes. But they usually require specific devices other than solar cells. An alternative way for the parameter extraction is the simulation of the device performance. Bulk heterojunction (BHJ) solar cells have been simulated for steady state and transient cases [1-6]. In the literature, the general route for modeling is to solve numerically onedimensional Poisson and continuity (drift-diffusion) equations, including different boundary conditions, recombination and generation rates, carrier mobility and space-charge effects. The nanoscale morphology of the BHJ region is neglected and the heterojunction media is approximated by one active layer with an effective dielectric constant, and electron and hole mobilities. Therefore, the device is described as an active layer of thickness L sandwiched between two metal electrodes of different work functions (Figure 1).
Figure 1. Schematic diagram of a BHJ solar cell and its energy diagram of metalinsulator-metal picture for an applied voltage Va. THEORETICAL ASPECT The starting point for the device simulation is the one-dimensional (1D) model of Koster et al. [
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