Studies of length scales in semiconducting polymers using scanning photocurrent contrast microscopy

Abstract

Semiconducting polymer films. when photoexcited with defined beam profiles generate excess charge carriers from the electronically excited state, which eventually spread over a volume exceeding the beam-sample cross-section. The thesis focuses on the studies of these length scales in model polymer systems which are of fundamental importance in understanding the transport mechanisms. and are crucial parameters for device development. A modified scanning photocurrent contrast microscopy technique was designed and implemented for these studies. Patterned device substrates with dimensions over a large range (1 μm to 2 mm) were used in these experiments. This approach essentially utilized a photocurrent-contrast imaging of the polymer film sandwiched between the patterned substrate and a top Schottky-type electrode, where the incident light beam scans regions beyond the uniform electric field region of the overlapping electrodes. The electron/hole transport processes can be independently evaluated by positioning the narrow-light beam selectively over the anode/cathode regions, enabling the estimation of the efficacy of hole transport vis-à-vis electron transport. Sizable photocurrent-signals between the electrodes are observed even at lateral distances several microns away from the counter electrode. While this current in general decays monotonically as a function of distance from the counter electrode, the functional forM.S. depends on the specific microscopic properties of the polymer semiconductor, hence the method is particularly suited to directly estimate the degree of disorder and spatial anisotropy in the electrical transport parameters. These length scales in the first approximation corresponds to the mobility-lifetime product (μτ). The values of these intrinsic parameters are extracted for variety of semiconducting polymers (poly(3-hexylthtiophene)(P3HT), Poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene) (MEHPPV) and Poly(9,9-dioctylfluorene-alt-bithiophene) (PFO-TT)). The implications of these results are discussed in detail in the thesis. An outcome of the studies led to the explanation of lateral photovoltaic (LPV) effect observed in these systems.. LPV is observed using a local optical probe on device structures consisting of the semiconducting polymer film with a Schottky type back contact and a front pair of ohmic contacts. The measurements carried out for a large range of interelectrode length ranging from 25 μm to 2 cm is studied in detail as a function of temperature, wavelength, and modulation frequency of the photoexcitation. A spreading impedance approach in the context of a discrete circuit element model is used to obtain a quantitative understanding of the spatial dependence and the frequency response of the LPV.