Scanning Photocurrent Microscopy

We have developed photoelectronic transport imaging [69] or scanning photocurrent microscopy (SPCM) [70] as an easy and fast method for determining 

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McNeill CR, Frohne H, Holdsworth JL, Dastoor PC (2004) Direct influence of morphology on current generation in conjugated polymer methanofullerene solar cells measured by near-field scanning photocurrent microscopy. Synth Met 147 101... 

The ehapter is organised as follows. After the introduction, a brief overview of the fundamentals of CNTs and CNT-based FETs will be given. The next section describes a range of chemical functionalisation schemes that have been devised for the performance enhancement of CNT-FETs. The subsequent section is devoted to the characterisation of as-prepared and functionalised CNT-FETs through electrical transport measurements and scanning photocurrent microscopy. In this context, the relevant device parameters of the FETs such as saturation behaviour, field-effeet mobility, transconductance and sub-threshold slope will be analysed and compared. The chapter concludes with future perspectives for the fabrication of CNT-based FETs. 

Figure 26.16 Scanning photocurrent microscopy (SPCM) of an ambipolar s-SWCNT-FET with data recorded in the p-type (a, c, e) and n-type (b, d, f) regimes. (a, b) SPCM images showing the spatial variation of the photocurrent at zero drain-source bias. An optical image of the device is overlaid in order to identify the position of the contacts (black...

C.R. McNeill and P.C. Dastoor, Photocurrent pattern formation in polymer/methanofuUerene blends imaged by near-field scanning photocurrent microscopy. J. Appl. Phys., 99, 033502... 

Since the cross section of planar surface cell LECs can be imaged by a microscope, a number of interesting experiments can be done to investigate the operation mechanisms. In a p-n diode, there is a built-in electric field at the p-n junction. This field can be measured by optical beam induced current (OBIC) microscopy, a technique in which a focused laser beam is scanned across the device while the photocurrent is monitored. When the beam excites a region that... 

Deeper insight can be gained from a scanning near-field optical microscopy (SNOM) study by McNeill et al., who resolved the local photocurrent obtained on MDMO-PPV PCBM toluene cast blends [138]. The authors revealed that the photocurrent was considerably reduced on top of the small hUls caused by the PCBM clusters (Fig. 25), whereas it stayed nearly constant over the surface of chlorobenzene cast blends [ 138]. 

Another impedance-based imaging technique for laterally resolved characterization of thin films or electrochemical systems is Scanning Photo-induced Impedance Microscopy (SPIM) [44]. It is based on photocurrent measurements at field-effect structures. In their simplest arrangement, field-effect structures consist of a semiconductor substrate with a thin insulator, and a gate electrode. This gate electrode can be a metal film resulting in the structure Metal Insulator Semiconductor (MIS) or, alternatively. Electrolyte Insulator Semiconductor structures are used, in which the electrolyte is in direct contact with the insulator, and a reference electrode is required to fulfill the function of the gate electrode. 

A probe consisting of an optical fiber coated with gold was used as the SECM tip to image simultaneously the electrochemical and photoelectrochemical activity of pitting precursor sites on an oxide film (11b, 115) by combining SECM with scanning photo-electrochemical microscopy (116). It was found that the area of high electrochemical activity is correlated with lower substrate photocurrent in the area. 

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