BM22016/10.26125/0fz8-ar47

Molecular doping for organic eletronic devices: doping concept and influence of trap Levels

Author(s): Christian Koerner, Max L. Tietze, Torben Menke, and Karl Leo*

Publication: Bunsenmagazin, Issue 2 2016, Aspekte, Seiten: 41 - 48

Publisher: Deutsche Bunsen-Gesellschaft für physikalische Chemie e.V., Frankfurt

Language: English

DOI: 10.26125/0fz8-ar47

 

Introduction

Organic electronics is an emerging technology that has attracted great interest in the last decades. The primary goal is not to replace existing technologies, but to create a new form of electronics which cannot be realized by standard inorganic semiconductor technologies. Important factors are the feasibility of, e.g., flexible devices, broad tunability of material properties, the possibility of low-temperature roll-to-roll processing, and the low weight of final products. The range of applications starts with organic light emitting diodes (OLED), which are already established on the display market, moving on to organic solar cells (OSC) and organic transistors (OFET), currently entering the market. The basis of all those technologies are -conjugated organic molecular or polymeric materials exhibiting semiconducting properties. In contrast to inorganic semiconductors, their transport properties are fundamentally different, particularly, charge carrier mobilities are orders of magnitude lower than in inorganic crystalline solids. The reasons are the weak intermolecular coupling due to van-der-Waals forces, a strong charge localization due to reduced screening of the charge, a broad density of states (DOS) due to the disordered nature of thin films, and further (shallow or deep) trap states due to impurities or strong morphological disorder. Additionally, the intrinsic charge carrier concentration is rather low due to the large band gap. Unwanted impurities create trap states of comparable density, such that the overall conductivity is low.

To drive high currents through OFETs or OLEDs or efficiently extract charge carriers in OSCs, doping was introduced equivalent to inorganic semiconductors.[1, 2, 3] The resulting increase in conductivity is due to an increased free charge carrier concentration and often also a higher mobility, leading to less transport losses and an Ohmic behavior at device contacts[4]. Doping was also used to realize organic tunnel diodes for a precise tuning of the breakdown voltage[5, 6] as well as for defining the threshold voltage in organic inversion and depletion transistors[ 7]. Recently, doping was used for in-depth investigations of the trap distributions by controlled trap filling such that trapped charges can respond to an electrical signal.[8, 9]

Despite the success of this concept in the last years, the doping process itself is still not fully understood. Previous works have suggested different physical mechanisms to explain, e.g., the usually observed rather low doping efficiencies.[10, 11, 12] However, as we discuss below, in many cases the concepts from inorganic semiconductors can reasonably describe the experimental observations.

Technically, the challenge remains to find suitable dopants with either deep energy levels for p-doping, or high levels for n-doping. Especially the latter leads to instability of most n-dopants against oxidation, such that synthesis and handling of those compounds must be performed under exclusion of any atmospheric contact. Therefore, new material concepts were developed in order to find air stable dopant compounds that develop their doping capability just upon processing, like molecular dimers that dissociate upon thermal evaporation.[13, 14, 15, 16]

In this paper, we review the common understanding of the doping mechanism in organic small molecule semiconductors. In particular, we summarize our recent experimental progress and advances in the description of the doping process and the doping efficiencies. Furthermore, we will highlight the importance of trap states for the doping process, how those trap states can be analyzed, and how doping can contribute to their quantitative characterization. For more in-depth information about molecular doping and its history, we refer to previous works.[2, 12, 17, 18, 19, 20]

 

Cite this: Christian Koerner, Max L. Tietze, Torben Menke, Karl Leo* (2016): Molecular doping for organic eletronic devices: doping concept and influence of trap Levels. Bunsenmagazin 2016, 2: 41-48. Frankfurt am Main: Deutsche Bunsen-Gesellschaft für physikalische Chemie e.V. DOI: 10.26125/0fz8-ar47

 

 

 

 

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