J. Phys. Chem. B 2001, 105, 2479-2482
2479
Nanolayered Heterojunctions of Donor and Acceptor Conjugated Polymers of Interest in Light Emitting and Photovoltaic Devices: Photoinduced Electron Transfer at Polythiophene/Polyquinoline Interfaces Maksudul M. Alam and Samson A. Jenekhe* Department of Chemical Engineering, Benson Hall, Box 351750, UniVersity of Washington, Seattle, Washington 98195-1750 ReceiVed: December 13, 2000; In Final Form: February 14, 2001
Four new nanolayered heterojunctions of electron-donating poly(3-hexylthiophene-2,5-diyl) and poly(3octylthiophene-2,5-diyl) with two electron-accepting polyquinolines have been prepared, and the photophysical processes occurring at the polymer/polymer interfaces were investigated by photoluminescence (PL) spectroscopy and picosecond time-resolved PL decay dynamics. Observed nearly complete quenching of the steady-state photoluminescence and a factor of 160 shortening of the fluorescence lifetime of the polyquinolines suggested photoinduced electron transfer from polythiophene to polyquinoline at the polyquinoline/ polythiophene interfaces. A new, weak and red shifted, emission band centered at 630 nm and reduced lifetimes show the formation of intermolecular exciplexes at the polyquinoline/polythiophene interfaces. This is the first example of intermolecular exciplex formation between two π-conjugated polymers. Exciton diffusion lengths in the polyquinolines were estimated to be 17-22 nm. These results suggest that nanolayered polythiophene/polyquinoline heterojunctions are promising new material systems for the development of photovoltaic devices and for model studies of the photophysics of multicomponent conjugated polymers.
Nanoscale thin films of conjugated polymers are widely used for the fabrication of optoelectronic and electronic devices such as light emitting diodes (LEDs),1,2 photovoltaic cells,3 electrophotographic imaging receptors,4 lasers,5 and field effect transistors.6 Multicomponent conjugated polymers such as multilayered thin films,1-4 phase-separated blends,7 and block copolymers8 are of increasing interest as a means of both improving device performance and engineering new capabilities such as selfassembly8,9 into this class of materials. Enhancement of polymer LED performance has been observed in many multilayered thin films owing to improved injection of both electrons and holes, whereas multicolor electroluminescence (EL) has been reported in layered heterojunctions1,2 and phase-separated blends.7 Similarly, enhancement of photoconductive and photovoltaic properties has been reported in layered heterojunctions and phase-separated blends due to efficient charge photogeneration and separation.3,10,11 Although interfaces in multicomponent conjugated polymers can be expected to play critical roles in their properties and the performance of devices made from them, few studies have so far focused on creating and studying controlled conjugated polymer/polymer interfaces.2a,12 Here, we focus on novel nanoscale polythiophene/polyquinoline heterojunctions, which are excellent model systems for investigating the electronic and optical properties of polymer/polymer interfaces and how such interfaces mediate photophysical and chargetransfer processes in nanostructured multicomponent conjugated polymer systems. The solid-state photophysics of π-conjugated homopolymers are well known to depend on the degree of intermolecular interactions and the resulting morphology.13,14 We expect the photophysics of multicomponent conjugated polymers to be even richer in scope and strongly dependent on the spatial scale of phase separation and the relative magnitudes of the ionization potentials and electron affinities of the components.8,9,11 Indeed,
photoinduced electron transfer,10-12 exciplex formation,13 and energy transfer12a have been observed in extensive prior studies of conjugated polymer/organic molecule heterojuctions or composites such as poly(p-phenylenevinylene)(PPV)/C60,10a MEH-PPV/C60,10a,b polythiophene/C60,10c,d and polybenzobisoxazole/tris(p-tolyl)amine13 systems. Various photoinduced processes arising from intermolecular interactions across interfaces have been implicated in light emitting and photovoltaic devices based on polymer/polymer heterojunctions or blends, including charge transfer in PPV/poly(benzobisimidazolebenzophenanthroline ladder),3a poly(p-pyridyl vinylene)/polythiophene,12b polythiophene/CN-PPV,12b MEH-PPV/CN-PPV12a and PPV/polythiophene,12b and energy transfer in PPV/CNPPV.12a Thus, a better understanding of the photoinduced processes occurring at polymer/polymer interfaces can help in improving electronic and optoelectronic devices based on conjugated polymers. However, few suitable electron accepting (ntype) conjugated polymers have been available to facilitate the creation and investigation of polymer/polymer heterojunctions with controllable electronic structures.2a We report studies of the photophysical and charge-transfer processes occurring at the interfaces of nanoscale heterojunctions of electron donating (p-type) poly(3-hexylthiophene-2,5-diyl) (PHT) and poly(3-octylthiophene-2,5-diyl) (POT) and electron accepting (n-type) poly(2,6-(4-phenyl)quinoline) (PPQ) and poly(2,2′-(p,p′-biphenylene)-6,6′-bis(4-phenylquinoline)) (PBPQ) (Figure 1). Several nanolayered heterojunctions (PPQ/PHT, PPQ/POT, PBPQ/PHT, and PBPQ/POT) were prepared and investigated by optical absorption and steady-state photoluminescence (PL) spectroscopies and picosecond time-resolved PL decay dynamics. The photoluminescence of the normally highly fluorescent polyquinolines2 was almost completely quenched in the heterojunctions as a result of photoinduced electron transfer from the polythiophene layer. Studies of the PL
10.1021/jp004474w CCC: $20.00 © 2001 American Chemical Society Published on Web 03/10/2001
2480 J. Phys. Chem. B, Vol. 105, No. 13, 2001
Figure 1. (A) Chemical structures of π-conjugated polymers used to fabricate nanolayered heterojunctions. (B) Schematic cross-section of p-type/n-type heterojunctions.
quenching as a function of the layer thicknesses allowed us to estimate the exciton diffusion lengths in the polyquinolines for the first time. The synthesis and characterization15 and the electrochemical15b and electroluminescent2b properties of π-conjugated polyquinolines, including PPQ and PBPQ, have previously been reported. Regioregular polythiophenes PHT and POT were purchased from Aldrich Chemical Co.16 PPQ and PBPQ thin films were deposited onto glass substrates by spin-coating from their formic acid solutions followed by vacuum drying at 60 °C for 12 h to remove the solvent. Thin films of PHT and POT were spincoated from chlorobenzene solutions onto the vacuum-dried PPQ (or PBPQ) layer and then dried in a vacuum at 60 °C for 6 h to remove the solvent. We note that polyquinoline is neither swellable nor soluble in chlorobenzene, and hence bilayer heterojunctions could be prepared in this way.2a,15 The film thicknesses were measured by an Alpha-step profilometer (Model 200, Tencor Northern, San Jose, CA) with an accuracy of (1 nm and confirmed by an optical absorption coefficient technique. In the case of very thin films (
