High-Resolution Kelvin Probe Force Microscopy Imaging of Interface Dipoles and Photogenerated Charges in Organic Donor–Acceptor Photovoltaic Blends

Abstract

We present noncontact atomic force microscopy and Kelvin probe force microscopy studies of nanophase segregated photovoltaic blends based on an oligothiophene-fluorenone oligomer and [6,6]-phenyl C70 butyric acid methyl ester. We carried out a complete analysis of the influence of the tip-surface interaction regime on the topographic, in-dark contact potential and surface photovoltage contrasts. It is demonstrated that an optimal lateral resolution is achieved for all channels below the onset of a contrast in the damping images. With the support of electrostatic simulations, it is shown that in-dark contact potential difference contrasts above subsurface acceptor clusters are consistent with an uneven distribution of permanent charges at the donor-acceptor interfaces. A remarkable dependence of the surface photovoltage magnitude with respect to the tip-surface distance is evidenced and attributed to a local enhancement of the electromagnetic field at the tip apex.

Analysis and modification of defective surface aggregates on PCDTBT:PCBM solar cell blends using combined Kelvin probe, conductive and bimodal atomic force microscopy

“…However, if the aggregates are made of acceptor molecules, hole conduction would be deficient (see Fig. 5) and localization of electrons would cause negative potential changes [40]. Furthermore, the measured thickness of the cluster layer is similar to the diameter of fullerenes if we consider partially buried PCBM molecules or molecules deformed by AFM indentation.…”

Direct Imaging of Space‐Charge Accumulation and Work Function Characteristics of Functional Organic Interfaces

“…Ultimately, this approach makes possible to obtain reliable information of the local work function neglecting current‐dependent and nonequilibrium effects such as phase variations on applied voltage . In particular, SKPM is ideal for investigating charge heterogeneity, where the surface electronic properties appear to be dependent on packing order and dominant coupling types in material assemblies, as well as on the degree of strain or composition of organic systems. In the case of organic semiconductor heterostructures, the device efficiency strongly relies on aspects such as the accurate quantification and local spatial overview for the energy‐level tuning of the charge electronic states in the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO).…”

“…This is of great relevance for organic solar cells (OSCs), once this drives the open circuit voltage . Similarly, the tuning of the energy‐level alignment at the metal/organic interface is paramount in order to optimize charge extraction in OSCs as well as the charge injection in organic light‐emitting diodes and organic field‐effect transistors . Due to the charge characteristics at the organic interface, diverse electrical processes, such as charge transfer and charge recombination, can take place.…”

“…[18] Similarly, the tuning of the energy-level alignment at the metal/organic interface is paramount in order to optimize charge extraction in OSCs as well as the charge injection in organic light-emitting diodes and organic field-effect transistors. [17,[19][20][21] Due to the charge characteristics at the organic interface, diverse electrical processes, such as charge transfer and charge recombination, can take place. Therefore, the quantification of the spatial dimensions of the interface dipole or charge accumulation layer becomes highly relevant to determine the electrical performance of functional organic interfaces.…”

Two competing interpretations of Kelvin probe force microscopy on semiconductors put to test

“…In combination with sample illumination techniques, properties such as the band gap, carrier diffusion length, and recombination rate can be obtained [9][10][11]. The importance of KPFM is reflected in its application in a broad range of popular material science topics, such as new photovoltaic materials [12][13][14][15], two-dimensional materials [16][17][18][19], nanowires [20][21][22], topological insulators [23], plasmonic structures [24,25], and photocatalytic systems [26,27].Reviews of KPFM and its applications can, e.g., be found in Refs. [28,29].Like the classic vibrating Kelvin probe, the quantity measured with KPFM is generally interpreted as the contact potential difference (CPD) [28][29][30].…”