Professor, Åbo Akademi University
Determination of Charge-Carrier Mobility and Built-In Potential in Thin-Film Organic MIM Diodes from Extraction Current Transients
Staffan Dahlström(1), Oskar J. Sandberg(1,2), Mathias Nyman(1), and Ronald
- Physics and Center for Functional Materials, Faculty of Science and Engineering, Åbo Akademi University, FINLAND
(2) Department of Physics, Swansea University, United Kingdom
The charge-carrier mobility is a key parameter for optimal device performance in the field of organic electronics. In low and moderate mobility materials, the Charge Extraction by a Linearly Increasing Voltage (CELIV) is a commonly used technique for mobility measurements. However, the original CELIV method is only applicable to devices with blocking contacts, not taking into account diffusion of carriers from the contacts, which is a severe limitation since most operating devices have Ohmic contacts.
In this work, we have extended the CELIV method for measuring the charge-carrier mobility in any thin-film diode with a metal-insulator-metal (MIM) structure. In contrast to the original CELIV theory, which assumes a uniform charge-carrier distribution in the active layer of the device, we derive an analytical expression for determining built-in potential and mobility in the case of a non-uniform charge-carrier distribution where charges have diffused into the active layer from the contacts. The theory is verified by drift-diffusion simulations and demonstrated experimentally on organic solar cells. Not accounting for the diffusion of carriers from the contacts overestimates the mobility up to more than an order of magnitude (see Fig. 1). We believe that this work can be very useful for future characterization of thin-film MIM as well as p-i-n diodes, e.g. thin-film solar cells.
Professor Ronald Österbacka received his PhD in Physics in 1999 från Åbo Akademi. During his PhD he spent one year at the University of Utah under the guidance of Prof. Z. Valy Vardeny. Österbacka was an Academy of Finland Post Doctoral Research fellow (2000-2003) and Academy Research fellow (2014) before taking up the position as Full Professor of physics in 2005. His research interests are electro-optical properties of disordered organic semiconductors and novel devices for paper electronics. Österbacka has submitted three patent applications on ion-modulated transistors. He is the editor-in-chief for the IoP journal Flexible and Printed Electronics. Ronald Österbacka is the director of the Center for Functional Materials since 2011. He is also member of the Swedish Academy of Engineering Sciences and Finnish Society for the Science and Letters. Österbacka has been active in arrangeing numerous conferences, most notably he was the chairman for the main conference on Synthetic Metals, ICSM2014 (www.icsm2014.fi) held in Turku June 30th to July 5th, 2014 gathering 600 scientists from all over the world.
The Organic Electronics group headed by Österbacka has made fundamental discoveries in the development of ion-modulated organic transistors and paper electronics, as well as charge extraction techniques for measurements of charge transport and recombination in low-mobility materials. Österbacka has published more than 180 papers, and have been cited more than 5500 times (without self-citations) earning him an h-factor of 37 (WoS as of July 2018).
The research goals within the Center for Functional Materials (www.funmat.fi) at Åbo Akademi University (www.abo.fi) has been to create and demonstrate new functionalities for future interactive products by combining advanced chemistry and complex materials with printing technologies. Our approach has been to design and make materials, substrates, devices and processes for large-area (printed) functional solutions. We have successfully demonstrated electronic devices such as indicators; resistors; transistors, ring oscillators; displays, as well as a number of gas-phase and liquid phase sensors on a specially designed, fully recyclable paper substrate in a roll-to-roll process using our unique printing laboratory.