Direct Estimate of Transport Length Scales in Semiconducting Polymers

Abstract

Conjugated semiconducting polymers form a class of materials that utilize facile polymer-processing methods and exhibit practically all the functional phenomena present in traditional semiconductors. [1,2] The successful use of organic semiconductors in optoelectronic applications such as lightemitting diodes [3] (LEDs), organic solar cells, [4] photodetectors, [5] photo-organic field-effect transistors [6] (FETs), optical sensors, [7] and light-emitting FETs [8,9] depend on the development of our understanding of electronic processes to a level comparable to that of their inorganic counterparts. The prevalence of hole and electron transport coexisting has been stressed recently in the context of interesting phenomena observed in these materials. [8,10] The estimation of transport length scales of both carriers-electrons and holes-are important to understand the transport mechanisms as well as to arrive at an optimum design for ambipolar optoelectronic devices such as light-emitting FETs, photo-FETs, and solar cells. The process of charge-carrier generation upon photoexcitation of the polymer semiconductor films is reasonably well documented. The results have been largely interpreted in terms of a primary process such as excitonic mechanisms and subsequent processes involving charge generation and transport.[11] Photocarrier transport studies of the polymer semiconductors are normally carried out using steady-state photoconductivity and transient photoconductivity in sandwich or surface cell configuration. The analysis and interpretation of these results is complicated by the simultaneous presence of both electrons and holes in the same region resulting in recombination processes, trap kinetics, and interfacial processes. The coexistence of drift and diffusion processes, especially in ambipolar systems, and the non-validity of Einstein's relation in the large-field regime makes the extraction of the diffusivity from mobility measurements, especially in anisotropic systems, difficult from standard experimental techniques. [12,13] Even though the standard well-developed methods have yielded accurate quantitative estimates of parameters such as mobility and carrier-lifetime distribution, the method introduced in this paper will complement these measurements with additional insight into properties such as the anisotropy, and correlation between lateral and longitudinal transport processes.Recent observations from our laboratory on polymer-based position-sensitive photocurrent device structures that function over large length scales, also indicate the need for such studies.[7] These position-sensitive planar device structures based on poly(3-hexylthiophene) (P3HT) and poly(2-methoxy-5(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEHPPV) essentially have a pair of front ohmic electrodes and a common back Schottky electrode. The photogenerated holes drift towards the ohmic counter electrode driven by a lateral photovoltage caused by nonuniform illumination of the polymer/Al Schottky interface [7] . In this report, we des...

Nonuniform Nanowire Doping Profiles Revealed by Quantitative Scanning Photocurrent Microscopy

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Enhanced Stability and Optimized Morphology Induced by Electric‐Field‐Assisted Annealing of Bulk Heterojunction Solar Cells

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