Implementation of data-cube pump–probe KPFM on organic solar cells

Abstract

An implementation of pump–probe Kelvin probe force microscopy (pp-KPFM) is reported that enables recording the time-resolved surface potential in single-point mode or over a 2D grid. The spectroscopic data are acquired in open z-loop configuration, which simplifies the pp-KPFM operation. The validity of the implementation is probed by measurements using electrical pumping. The dynamical photoresponse of a bulk heterojunction solar cell based on PTB7 and PC71BM is subsequently investigated by recording point-spectroscopy curves as a function of the optical power at the cathode and by mapping 2D time-resolved images of the surface photovoltage of the bare organic active layer.

Nanoscale Charge Accumulation and Its Effect on Carrier Dynamics in Tri-cation Perovskite Structures

“…The so-called pump–probe-based approaches have been used in optical spectroscopy for decades and recently have been applied to electric SPM modes in order to resolve the local decay of the surface photovoltage in organic donor–acceptor blends, V cpd changes in an organic transistor structure with microsecond temporal resolution, and picosecond relaxations of photovoltage in low-temperature grown GaAs. 24 , 27 , 28 Limitations and possible artifacts of this approach have also been investigated by a number of groups. 27 , 29 Murawski et al reported on a dual-closed loop system that removes crosstalk artifacts in the topography channel originating from an incorrect voltage applied by the KPFM feedback loop.…”

“…These approaches enable non-invasive imaging down to picosecond temporal resolution 21,22 . Improvement of the time resolution has been achieved by various means, such as recording the cantilever deflection read-out signal with ultra-high sampling rate as well as by overcoming the averaging limitation by applying high frequency pulses in excitation and detection [23][24][25][26] . The latter so-called pump-probe based approaches have been used in optical spectroscopy for decades and recently have been applied to SPM in order to resolve the local decay of the surface photovoltage in organic donor-acceptor blends, contact potential difference (CPD) changes in an organic transistor structure with microsecond temporal resolution, and picosecond relaxations of photovoltage in low temperature grown GaAs 27,28 .…”

“…As recent developments of advanced scanning probe microscopy (SPM) techniques now allow for the characterization of electrically and optically active materials with high temporal resolution, they offer insight into the spatial variation of charge carrier dynamics on multiple timescales. − These approaches enable noninvasive imaging down to picosecond temporal resolution. , Improvement of the time resolution has been achieved by various means, such as recording the cantilever deflection read-out signal with ultrahigh sampling rate as well as by overcoming the averaging limitation by applying high-frequency pulses in excitation and detection. − These approaches have been of particular interest for electrostatic force microscopy (EFM) and Kelvin probe force microscopy (KPFM) applications. The EFM and KPFM techniques rely on the electrostatic force arising between the AFM probe and the sample, separated by a distance z, upon application of an AC voltage.…”

Nanoscale Mapping of Photo‐Induced Charge Carriers Generated at Interfaces of a Donor/Acceptor 2D‐Assembly by Light‐Assisted‐Scanning Tunneling Microscopy

“…This is in agreement with the observations made from KPFM measurements on D/A bulk heterojunctions, where local variations of SPV have been attributed to local phase composition and/or morphology changes. [ 46 ] The latter observation could come from the fact that the shape and spatial extent of the molecular orbitals probed at +0.5 V (empty states) are not identical to those measured at −0.5 V (occupied states). Nevertheless, we observe no obvious correlation between the profiles and (from Figure 6b), which tends to prove that the dissymmetry of the photocurrents measured at the D/A junction level does not come from a variation between the exponential decays of the molecular states′ wave functions probed in the tunnel junction at + V t and − V t , respectively.…”

Probing Charge Carrier Properties and Ion Migration Dynamics of Indoor Halide Perovskite PV Devices Using Top‐ and Bottom‐Illumination SPM Studies

“…This understanding can be achieved by performing macroscopic optical measurements, e.g., current‐voltage [ 5 ] and photoluminance, [ 6 ] in conjunction with scanning probe microscopy (SPM) with high spatial resolution, [ 7 ] such as conductive atomic force microscopy (c‐AFM), [ 8 ] which maps electronic transport capability, [ 9 ] ion migration, [ 10 ] surface hysteresis, [ 11 ] ferroelectric polarization, [ 12 ] and face orientation of PSCs. [ 11,13 ] In addition to c‐AFM, Kelvin probe force microscopy (KPFM) has been implemented to evaluate the device performance [ 14 ] and to gain an improved understanding of the local charge carrier movement mechanism, which includes charge carrier generation, [ 15 ] accumulation, [ 16 ] recombination, [ 17 ] transport, [ 18 ] relaxation, [ 19 ] and extraction at the electrode. [ 7a,20 ]…”