Nanoscale investigation of bulk heterojunction organic solar cell by scanning capacitance force microscopy

Abstract

We developed a frequency-modulation atomic force microscopy (FM-AFM) system combined with Kelvin probe force microscopy (KFM) and scanning capacitance force microscopy (SCFM). Using the developed system, we investigated the surface structure and electrical characteristics of a bulk heterojunction (BHJ) sample consisting of a mixture of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and poly(3-hexylthiophene) (P3HT). The BHJ structure consisted of PCBM and P3HT molecules. These materials are often used as the active and charge transport layers in an organic solar cell, respectively. The results suggest that we succeeded in visualizing the phase separation in the BHJ sample using SCFM measurements. We illustrated the energy band diagrams from the KFM results and showed that band bending occurs from the organic material side at the interface with the indium–tin oxide substrate. Furthermore, the KFM and SCFM results suggested that charge separation occurs in the BHJ structure.

Cross-sectional observation in nanoscale for Si power MOSFET by atomic force microscopy/Kelvin probe force microscopy/scanning capacitance force microscopy

“…KFM is a measurement method of surface potential at the nanoscale. [4][5][6][7][8][9][10][11][12][18][19][20][21][22] The surface potential was detected by the contact potential difference (CPD) between the cantilever probe tip and sample surface. The modulation signal V mod , (=V V t cos dc ac w + ( )) consisted of a DC bias voltage (V dc ) and AC bias voltage (V ac ) oscillating at an angular frequency ω.…”

“…Subsequently, the SCFM was used as a carrier density distribution mapping method for the semiconductor material. 5,[7][8][9][10][11][23][24][25] A differential capacitance (∂C/∂V ) was achieved in the metal-oxide-semiconductor structure by the arrangement of the metal-coated probe tip, oxide layer on the sample surface, and semiconductor substrate. (∂C/∂V ) was used to calculate the F 3w component.…”

“…The amplitude and phase component of F 3ω depend on the majority carrier density and type of semiconductor, respectively. 5,[7][8][9][10][11][23][24][25] The primary resonance frequency of the cantilever in the experiment was 71.140 kHz. In the KFM, the modulation frequency was set at 23.713 kHz corresponding to one-third of the primary resonance frequency.…”

“…In high-performance device development, an evaluation analysis technology is required. Scanning probe microscopy (SPM) [3][4][5][6][7][8][9][10][11][12] is used as it achieves a high resolution. The SPM as an evaluation method aims to provide a nanoscale observation of power semiconductor devices regardless of the material and structure types.…”

“…The device having these characteristics is referred to as Si-SJ MOSFET, which was cut and polished to expose its crosssectional structure. Moreover, we constructed an atomic force microscopy (AFM) 3,[5][6][7][8][9][10][11][12][13][14][15][16] system based on a frequency modulation (FM) method 17) combined with Kelvin probe force microscopy (KFM) [4][5][6][7][8][9][10][11][12][18][19][20][21][22] and scanning capacitance force microscopy (SCFM). 5,[7][8][9][10][23][24][25] We simultaneously observed the surface topography, surface potential, and differential capacitance of the Si-SJ MOSFET with an exposed internal structure using the AFM / KFM / SCFM system to analyze the same location, where the device was operated by applying a bias voltage.…”