ORGANICPV
Time-Resolved Electrostatic Force Microscopy (trEFM)
Before discussing the specifics of time-resolved EFM, we will first briefly review steady-state EFM. More detailed descriptions of conventional steady- state EFM can be found elsewhere.25, 26
EFM is a
form of AC mode imaging. While most AFM users are familiar with more common forms of AC mode imaging that exploit van der Waals interactions between the tip and the sample to image sample topography (e.g. “intermittent contact mode AFM”), EFM makes use of the fact that the cantilever’s oscillatory motion is also sensitive to longer-range electrostatic interactions between the tip and sample. In one common EFM imaging mode, these electrostatic interactions are monitored by measuring their effect on cantilever resonance frequency while the cantilever is made to vibrate some distance (~10 nm) above the sample surface. The interactions with the surface at this distance are dominated by local electrostatic force gradients, and the shift in cantilever resonance frequency is proportional to both the local capacitive gradient and the potential difference between the tip and the sample. The frequency shift (∆f) can be written as:27
present implementation, we monitor the time- dependent frequency shift in Equation 1 that may result, for instance, from the rapid accumulation of photogenerated charge in a solar cell following illumination, or the fast trapping and detrapping kinetics of charge carriers on sub-ms time scales.
Figure 2a depicts the operation of a trEFM experiment to measure photogenerated charge. In the dark, the semiconductor slab is mostly depleted of charge carriers. The sample is then illuminated with a light pulse and the photoexcitation of the OPV material generates charge carriers. Due to the applied voltage on the tip (in our experiments typically 5-10 V), these
51
Here, C is the capacitance, z is the relative tip height, Vtip
Vsample
is the voltage applied to the tip, and is the local potential in the sample. In a
typical steady-state EFM experiment, a line is scanned in AC mode to record the sample topography; the tip is then raised a preset distance (for example 20 nm) to beyond the range of the short-range van der Waals forces, and the shift in the resonant frequency of the cantilever is measured while a voltage is applied between the tip and sample.
While conventional EFM has been useful in the characterization of a variety of static or quasi-static processes in organic electronic devices,11, 28 parameters such as surface potential and capacitive gradient fail to provide direct information about the local efficiency of a thin-film solar cell. To address this limitation, we have extended the capability of EFM to enable the study of time- dependent phenomena at sub-ms time scales using time-resolved EFM (trEFM). With trEFM, we can measure the transient behavior in the electrostatic force gradient. Specifically, in our
Figure 2. (A) Schematic depiction of how photogenerated charge carriers cause an increase in the capacitive gradient and a change in the surface potential and thus a shift in the resonance frequency. The time rate of change in this shift is what is measured by trEFM. (B) Representative plot of the resonance frequency shift versus time following photoexcitation. At time t=0 ms, the LED is turned on, causing an exponential decay in the frequency shift. By finding the time constant of this decay we can extract a relative charging rate. (C) Topography and (D) charging rate image for the same area of a PFB:F8BT sample, dissolved in xylene with 1:1 composition. (E) Spatially-averaged charging rates in films with different PFB:F8BT ratios are quantitatively consistent with the trend exhibited by EQE measurements
www.solar-pv-management.com Issue II 2010
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