Organic Polymer Semiconductor Superlattices

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ORGANIC POLYMER SEMICONDUCTOR SUPERLATTICES Samson A. Jenekhel and Wen-Chang Chen Department of Chemical Engineering, University of Rochester Rochester, New York 14627 Abstract We describe the synthesis and study of the first organic polymer semiconductor superlattices designed as periodic block conjugated copolymers, (-AxBy-)m. The observed variation of electronic spectra and wavelength of the lowest energy absorption maxima with block length y are interpreted in terms of quantum confinement size effects predicted The periodic block conjugated copolymers were for semiconductor superlattices. synthesized by a two-step strategy that ensure strict control of sequence, block length and periodicity. It is suggested that organic semiconductor superlattices provide a rational and systematic approach to the molecular engineering of electronic, optical, nonlinear optical, and electro--optical properties in polymeric materials and hold promise for molecular electronics and molecular photonics. Introduction

Our interest in the "molecular engineering" of polymeric materials with specified or novel electronic, linear optical, nonlinear optical, and electro-optical properties [2-9] recently led to the concept of organic semiconductor superlattices 2-4,6]. A semiconductor superlattice is a periodic structure[10-17], (-AxBy-)m, consisting of alternating layers (x and y) of two semiconductors with unit cells A and B and different bandgaps A and EB (E > E , say). In such a periodic structure, the small gap semiconductor (B) forms a series of square potential wells and the large gap semiconductor (A) forms a series of potential bariers in the direction of the periodicity. The height of the potential wells formed depend primarily on the magnitude of the gap difference, AEg = EA B

E9. In a superlattice (SL) structure the barrier thickness (dA) is small enough for charge carriers in adjacent wells of width (dB) to interact; otherwise, the periodic structure is a multiple quantum well (QW). Superlattices and quantum well structures have been widely studied in inorganic semiconductors where the barrier thickness, well width, and number of layers (m) of the repeating unit cell of the periodic structure can be precisely controlled by thin film deposition processes, such as molecular beam epitaxy (MBE) or metal-organic chemical vapor deposition (MOCVD)[11-17]. The possible experimental approaches to organic semiconductor superlattices might include Langmuir-Blodgett (LB) and vapor phase thin film deposition processes. However, although these two techniques are conceptually analogous to the MBE and MOCVD processes, they do not appear adequate for preparing true semiconductor superlattices without the prior design of the SL and QW structures into the macromolecular chains. Since polymers are periodic structures by nature, the required SL or QW structure, (-AxBy-)m, is thus a multiblock conjugated copolymer containing sufficiently long blocks (x>l and y>l) of repeating unit cells A and B of the conjugated homopolymers (-A-)m -

A

B

and (-B