Probing charge motion in next-generation semiconductors with scanned probe microscopy

Dwyer, Ryan Patrick

Probing charge motion in next-generation semiconductors with scanned probe microscopy

Abstract

Scanned probe microscopy has allowed researchers to explore spatial variations in charge generation and transport in solar-cell films prepared on a conductive substrate with a best-case resolution of 2 nanometers. In this thesis, we introduce new scanned probe measurements to measure light- and voltage-induced changes to capacitance, surface potential, and electric fields with better time resolution. We demonstrate the measurements on organic and perovskite semiconductors. First, we present a new method for measuring photocapacitance transients. We demonstrate the ability of this indirect, ``phase kick'' technique to record multi-exponential photocapacitance transients on timescales ranging from 40 microseconds to 10 milliseconds in the organic donor:acceptor blend PFB:F8BT. The technique's ability to measure subcycle, nanosecond charge dynamics is demonstrated by measuring the 34 nanosecond sample electrical charging time. Along with the measurement, we present an accurate approximate model for the cantilever dynamics during the photocapacitance measurement. We use the model to explain the origin of the signal in our new phasekick electric force microscopy measurement and the alternative feedback-free time-resolved electric force microscopy. We show that for sample time constants faster than the inverse cantilever angular frequency, feedback-free time-resolved electric force microscopy is sensitive mainly to the size of the abrupt phase shift induced by the abrupt step change in the tip-sample capacitive force. Second, we present a new method for measuring the vector electric field using frequency-modulated Kelvin probe force microscopy. During a Kelvin probe force microscopy linescan, we sinusoidally modulate the cantilever position along the direction perpendicular to the linescan. We determine the electric field along both the linescan direction and the modulation direction simultaneously by numerical differentiation and lock-in detection respectively. We demonstrate the technique by recording linescans of the in-plane electric field vector in the vicinity of a patch of trapped charge in a DPh-BTBT organic field-effect transistor. The measured electric field depends strongly on experimental parameters: the Kelvin probe force microscopy feedback loop bandwidth, the linescan speed, the position modulation amplitude, and the position modulation frequency. We demonstrate how to optimally choose these experimental parameters for our new vector electric field measurement.

Date Issued

2017-08-30

Keywords

Physical chemistry; Materials Science; organic semiconductors; scanned probe microscopy

Committee Chair

Marohn, John A.

Committee Member

Lee, Stephen
Loring, Roger F.

Degree Discipline

Chemistry and Chemical Biology

Degree Name

Ph. D., Chemistry and Chemical Biology

Degree Level

Doctor of Philosophy

Rights

Attribution 4.0 International

Rights URI

https://creativecommons.org/licenses/by/4.0/

Types

dissertation or thesis