Local Probing of Photocurrent and Photoluminescence in a Phase‐Separated Conjugated‐Polymer Blend by Means of Near‐Field Excitation

Abstract

In this paper scanning near‐field microscopy is used to characterize polymer blends for photovoltaic applications, and fluorescence imaging and photoconductivity are combined to elucidate the spatial distribution and relative efficiency of current generation and photoluminescence in different domains of compositionally heterogeneous films. Focus is placed on a binary system consisting of poly[(9,9‐dioctylfluorene)‐alt‐benzothiadiazole] (F8BT) and poly[(9,9‐dioctylfluorene)‐alt‐(bis(N,N′‐(4‐butylphenyl))‐bis(N,N′‐phenyl‐1,4‐phenylenediamine))] (PFB), spun from xylene solutions, so as to obtain phase separation on micrometer and nanometer length scales. Protruding regions with diameters of about 5 μm in the topography image coincide with regions of high photocurrent (PC) and luminescence; these regions are identified as being F8BT‐rich. A general method to estimate the photoluminescence efficiency in the different domains of phase‐separated blends is proposed. As expected, lack of enhancement of the PC signal at the boundaries between protruding and lower‐lying phases indicate that these microscale boundaries play a small role in the charge generation by exciton splitting. This is consistent with the domains compositional inhomogeneity, and thus with finer phase separation within the domains. We also provide an analysis of the extent to which the metallized probe perturbs the near‐field photocurrent signal by integrating Poisson's equation. Finally, by using a Bethe–Bouwkamp model, the energy absorbed by the polymer film in the different regions is estimated.

Photoconductive NSOM for mapping optoelectronic phases in nanostructures

“…It was ascertained that they were mixed domains due to photocurrent from both the phases. Riehn et al studied the PFB:F8BT system by p-NSOM and concluded that regions rich in PFB had a certain F8BT concentration and vice versa [88]. The current contribution from the PFB rich domains was about four times that of an F8BT region confirming that the donor absorption was responsible for the majority of charge carrier generation.…”

“…However, effects of light interaction with a metal layer or tip are generally not considered in the image analysis. Although, Riehn et al had solved the Poisson equation with a commercial finite element package in order to arrive at perturbation effects of a metalized nearfield at the vicinity of a photoconductive material [88].…”

High‐resolution morphological and electrical characterisation of organic bulk heterojunction solar cells by scanning probe microscopy

“…Near-field scanning optical microscopy (NSOM) has also been successively applied to characterise conjugated polymers [21][22][23] and organic BH solar cells. [24][25][26][27] This specific scanning probe method is based on a tip probe in which an optical fibre is embedded to locally illuminate the sample. This allows local characterisation of photoconductive properties and raises therefore significant interest in the field of organic photovoltaics.…”

“…Solid-state NMR spectroscopy and relaxometry are powerful non-invasive and non-destructive techniques to investigate the segmental chain dynamics and the molecular miscibility of polymer blends and copolymers at the nanoscale. [24][25][26][27] This specific scanning probe method is based on a tip probe in which an optical fibre is embedded to locally illuminate the sample. This allows local characterisation of photoconductive properties and raises therefore significant interest in the field of organic photovoltaics.…”

“…53 NSOM can also be considered as a powerful method to locally map optical and photoelectrical properties of organic semiconducting structures. [21][22][23][24][25][26][27][28][29] Yet, as previously mentioned, the large diameter of the NSOM probe still hampers high-resolution measurements. Furthermore NSOM based characterisation requires excitation intensities $10 3 higher than those to be considered in an operating solar cell.…”