Unambiguous Measurement of Local Hole Current in Organic Semiconductors Using Conductive Atomic Force Microscopy

Abstract

Conductive atomic force microscopy (C-AFM) is a widely used tool for studying the charge transport properties of organic semiconductor films with nanoscale resolution. Local hole current is commonly measured by electrically contacting the film with a high work function C-AFM probe on top and an underlying electrode coated with a hole transport layer. The two voltage polarities, corresponding to the probe injection and substrate injection of holes, are both found in the C-AFM literature; nevertheless, there has been a lack of consideration about the possible influence of voltage polarity on image contrast and charge transport mechanisms. By analyzing local hole current maps and current−voltage curves for three organic semiconductors (a small molecule and two polymers), we find that probe and substrate injection leads to drastically different hole current maps and charge transport mechanisms. Specifically, the substrate injection of holes exhibits ohmic characteristics at low voltages and space-charge-limited current behavior at elevated voltages. Conversely, the probe injection of holes leads to injection-limited current that is sensitive to the state of the probe− sample interface. These measurements provide a blueprint for ensuring that C-AFM measurements are unambiguously probing the bulk properties of organic semiconductor films.

Remarkable Electrical Connectivity in the Acceptor Phase of Y6- and Fullerene-Based Bulk Heterojunction Solar Cells

“…A positive voltage polarity, corresponding to hole injection from the PEDOT:PSS/ ITO substrate, was used for mapping hole current because the associated hole injection barrier is small, leading to bulk limited current; conversely, the large hole injection barrier during probe injection leads to injection limited current that is sensitive to the state of the surface. 57 Before considering the electrical connectivity of the fullerene phase of p-DTS(FBTTh 2 ) 2 :PC 71 BM BHJs on PEI/ITO, we first compared the BHJ's film structure on PEI/ITO with that on the standard PEDOT:PSS/ITO substrate. The atomic force microscope (AFM) topography of the BHJ on PEI/ITO and PEDOT:PSS/ITO, shown in Figures 3a and 3d, exhibits a similar morphology and surface roughness of 1.7 and 1.9 nm, respectively.…”

“…Similar measurements and analysis were performed with p -DTS­(FBTTh 2 ) 2 and PC 71 BM films on PEDOT:PSS-coated ITO to determine a voltage window of 2–4 V to measure hole transport through the donor phase of a p -DTS­(FBTTh 2 ) 2 :PC 71 BM BHJ (see Figure S3). A positive voltage polarity, corresponding to hole injection from the PEDOT:PSS/ITO substrate, was used for mapping hole current because the associated hole injection barrier is small, leading to bulk limited current; conversely, the large hole injection barrier during probe injection leads to injection limited current that is sensitive to the state of the surface …”

A multi-resistance wide-range calibration sample for conductive probe atomic force microscopy measurements

“…Alternatively, current variation maps are acquired at a given applied voltage by scanning the AFM tip in contact mode across a defined sample surface area [5]. Owing to its versatility and high resolution in probing the local conductivity of materials, C-AFM has been extensively used in studying semiconductors [6,7], two-dimensional materials [8][9][10], memristive devices [11][12][13][14][15], photoelectric systems [16][17][18], dielectric films [19][20][21][22][23], molecular electronics [24][25][26][27][28][29], organic and biological systems [30][31][32][33][34], and quantum devices [35][36][37]. Various technical methods have been developed in C-AFM to cope with the diversity of its applications, including advanced sensors and lownoise preamplifiers [2,[38][39][40].…”