Abstract
In this work, we measure the hole mobility in the model polymer system poly(3-hexylthiophene-2,5-diyl) by using different measurement techniques. Our main purpose is to determine how the recently developed charge extraction by a linearly increasing voltage technique for metal–insulator–metal devices (MIM-CELIV) compares to other commonly used methods, such as space charge limited currents and time-of-flight. Our findings suggest that the MIM-CELIV technique gives a slightly lower mobility than the other techniques, which is understandable given that the method measures the mobility of relaxed charge carriers in the dark unlike, for example, time-of-flight where charge carriers are photogenerated. In addition, we scrutinize the accuracy and reliability of the techniques used, showing that differences in mobility smaller than a factor of two are not detectable unless statistics from many devices are available.
Highlights
Organic semiconductors such as polymers and small molecules hold great potential for future applications in electronic components and devices
Both the space-charge limited currents (SCLCs) and the dark injection SCLC (DI-SCLC) techniques require a hole-only or an electron-only diode, where at least one of the contacts must be Ohmic for charge injection, i.e., the current is not limited by injection from the contact
The applied field in the CELIV measurements can be approximated from the voltage applied at tmax divided by the active layer thickness
Summary
Organic semiconductors such as polymers and small molecules hold great potential for future applications in electronic components and devices. These materials can typically be processed from the solution, which allows for low-cost and large-scale production technologies such as roll-to-roll printing. One of the main drawbacks of organic semiconductors is their low charge carrier mobility; the performance of many devices, such as transistors and solar cells, is often limited by the poor charge transport in the semiconductor layers. In the field of organic electronics, reliable characterization of charge transport is important—preferably using in-device techniques. The arguably most common technique for determining the charge carrier mobility is based on space-charge limited currents (SCLCs).. In the space-charge limited conduction regime, the current should be proportional to the applied voltage squared; in organic materials, a stronger voltage dependence is often seen due to trapping or other space-charge effects, which can result in large errors.
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