New Photoluminescent Display Devices - American Chemical Society

Chapter 16

New Photoluminescent Display Devices Christoph Weder, Andrea Montali, Christian Sarwa, Cees Bastiaansen, and Paul Smith

Downloaded by TUFTS UNIV on October 14, 2016 | http://pubs.acs.org Publication Date: September 16, 1999 | doi: 10.1021/bk-1999-0735.ch016

Department of Materials, Institute of Polymers, Ε Τ Η Zurich, U N O C14, CH-8092 Zürich, Switzerland

L i q u i d crystal displays represent the dominant flat panel display technology, despite their limitations in brightness and efficiency originating from the use of absorbing polarizers and color filters. This paper reviews novel concepts of photoluminescent l i q u i d crystal displays that employ photoluminescent polarizers. These polarizers exhibit highly anisotropic absorption and / or emission and efficiently combine two separate features: the polarization o f light and the generation of bright color. A new photophysical effect - polarizing energy transfer - can be used to drive the efficiency of these elements to the theoretical limit: it enables to produce photoluminescent polarizers w h i c h optimally harvest incident light by isotropic absorption but emit the absorbed energy in highly linearly polarized fashion. Experiments suggest that the novel concepts can simplify device design and substantially increase device brightness, contrast, and efficiency.

Despite the growing research interests in inorganic (7) and, more recently, organic (2) electroluminescent ( E L ) light-emitting devices, and other techniques, such as plasma display panels (3) or vacuum fluorescent displays (4), liquid crystal displays ( L C D s ) have maintained their dominant position in the field of flat-panel displays (5,6). The m a i n advantages o f L C D s are their versatility, low-voltage operation and semiconductor compatibility; but their limited brightness and energy efficiency, as well as the often unsatisfying viewing angle of L C D s , leave ample room for further improvement (6). The severe limitations in brightness and efficiency of L C D s arise chiefly from the use o f dichroic sheet polarizers and - i n case of color devices absorbing color filters since these elements convert a large fraction (> 85 %) of the incident light into thermal energy. A s an alternative to dichroic polarizers which typically transmit less than 40 % of unpolarized incident light, polarizers have recently been proposed that are based on selective reflection (7) or scattering (8) of one polarization and allow recycling of the reflected or scattered light. The ultimate efficiency of these polarizers is, in principle, unity, thus, doubled compared to

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Hsieh and Wei; Semiconducting Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1999.


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absorbing polarizers. However, color applications based on these elements still rely on color filters which typically absorb at least 70 % of white light (6). The use of photoluminescent ( P L ) materials, which act as "active" color filters and, therefore, might enhance the visual performance of L C D s , was previously suggested (9-11). Several principal possibilities exist to incorporate luminescent materials into L C D s , including the use of fluorescent L C s or the dissolution or dispersion of luminescent molecules in a conventional L C layer (9), or the application of P L plates (10) or front-face screens (77). However, the proposed devices suffer from a number of drawbacks, related to the limited stability of the fluorescent dyes or L C s or both, the difficulty to produce pixilated devices, depolarization effects, or the required thickness (> 1 mm) and (large) area of the luminescent layer (70). This paper reviews novel concepts o f photoluminescent l i q u i d crystal displays that comprise P L polarizers w h i c h efficiently combine two separate features: the polarization of light and the generation of bright color. Experiments suggest that this approach can simplify device design and substantially increase device brightness, contrast, and efficiency, and - in specific configurations - viewing angle. Photoluminescent Polarizers Uniaxially oriented, P L materials usually exhibit anisotropic, i.e., linearly polarized, absorption and emission. This phenomenon has been known for inorganic crystals for more than a century (72) and was reported for oriented blends of ductile polymers and low-molecular weight P L materials as early as the 1930's (13). Since, the effect has been shown i n a variety of materials and using a diversity of orientation methods (14) including, for example, P L liquid crystal systems (75) or l o w molecular P L materials u n i a x i a l l y g r o w n onto orienting substrates, such as oriented poly(tetrafluoroethylene) (16). Direct deposition through friction or rubbing (77), or the Langmuir-Blodgett technique (18), as well as mechanical deformation of pure conjugated polymer films (79) have also been demonstrated to yield P L layers or films which exhibit anisotropic optical properties. However, the degree of orientation and, hence, the dichroic ratios obtained with these methods are usually only modest, typically well below 10. B y contrast, the tensile deformation of guest-host systems, in which the guest molecules adopt the orientation of the host, was found to be a most promising technique for the production of P L polarizers with significant anisotropic optical properties. This concept has been used i n the past by different research groups for the preparation of polarized P L films based on blends o f various formanisotropic low-molecular and oligomeric compounds and a variety o f matrix polymers (e.g. polystyrene or polyethylene) (14,20,21). Rather surprisingly, this method has only recently been adapted for blend films of conjugated polymers; a blend of poly(2-methoxy-5-(2'ethyl-hexyloxy)-p-phenylenevinylene ( M E H - P P V ) and ultra-high molecular weight polyethylene ( U H M W - P E ) was used i n the initial experiment (22). W i t h the above described relevance of polarized photoluminescence for L C D applications i n mind, we recently have systematically investigated the structureproperty relations o f oriented films based on blends o f poly(2,5-dialkoxy-pphenyleneethynylene)s (PPEs) and U H M W - P E (23). P P E s exhibit an ideal matrix of properties with respect to the preparation of such P L polarizers (23-30), including large P L quantum efficiencies i n solution and solid state (24), and an extremely stiff, linear polymer backbone that enables maximum orientation. Reported here are yellow-green lieht-emitting P L polarizers based on E H O - O P P E (M ~ 10,000 and ~ 84,000 g m o n (24,27), a highly soluble P P E derivative, substituted with linear and sterically hindered alkyloxy groups i n an alternating pattern, as well as blue lightemitting P L polarizers based on PPE-copolymers (co-PPE blue) (29) o f controlled conjugation length (Figure 1). Uniaxially oriented films with contents of typically 1 n



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Figure 1. C h e m i c a l structures of the poly(2,5-dialkoxy-/?-phenyleneethynylene) derivatives E H O - O P P E and co-PPE blue.


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to 2 % w / w of P P E i n U H M W - P E were prepared by solution casting, drying and subsequent tensile drawing i n the solid state (23). The pristine, solution-cast films were drawn at temperatures of 90 - 130 °C (i.e., in the narrow temperature window above the glass transition of the P P E guest, and below the melting point of the polyethylene) to a series of different draw-ratios ( λ = final length / initial length) that ranges from 10 to 80. The thickness of drawn films of the maximum draw ratio of 80 was i n the order of 2 μπι. The anisotropic photophysical behaviour of these drawn films was studied employing polarized U V / V i s absorption and steady-state P L spectroscopy, using unpolarized light for excitation (23). The anisotropic characteristics are expressed i n terms of dichroic ratios, which are defined for absorption ( D R ) and emission ( D R ) as the ratio between the respective spectra measured with polarization parallel (p) and perpendicular (s) to the drawing direction. Note, that different phonon bands are observed for p- and j-polarized light, making the dichroic behavior wavelengthdependent, with maximum distortion at absorption and emission maxima; thus, to better reflect the 'average' dichroic behavior, all evaluations are based on integration of the respective spectra. Highly linearly polarized absorption and P L emission was observed for the oriented films, as visualized in Figure 2 for a film containing 2 % of E H O - P P E (M ~ 84,000 gmol" ) of a draw ratio of 80. The polarized emission spectra o f this f i l m reflect state-of-the-art optical anisotropy, characterized by dichroic ratios, D R , of 27 (23). Matching dichroic ratios were determined for absorption and emission experiments, suggesting that no molecular reorientation, and no energy transfer processes, which could eventually limit the emission dichroic ratio, are present. The significant influence of the draw ratio on the dichroic ratio i n all systems under investigation is characterized by an initial linear increase of dichroic ratio, with a tendency to level off at λ > 50 (23). Comparing the properties of blends based on E H O - O P P E with M of 10,000 gmol and 84,000 gmol it was found, that the molecular weight of the conjugated polymer has a notable influence on the orientation; the high-molecular weight material exhibits m a x i m u m orientability, which is understood i n terms of its more favourable molecular aspect ratio. It was also observed that the orientation of the P P E was not influenced by the blend composition i n the concentration regime of 1 - 2 % (w/w) of the P P E . X - r a y and electron diffraction experiments as well P L spectra indicate that the orientation process appears to induce a transformation of an initially phase-separated system into an apparent molecular dispersion of the conjugated polymer guest in the P E host (23). It is also noteworthy that the oriented films were found to be extremely stable: the materials could be stored under ambient conditions (exposure to air and light) for months without any noticeable change of their properties. The latter, most relevant, phenomenon is attributed to the encapsulation of the conjugated polymer in the highly crystalline P E matrix (22,23) and the outstanding intrinsic stability of the P P E backbone.


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Photoluminescent Display Devices P L polarizers can combine, and directly replace, the standard polarizer and color filter in conventional L C D s and, using an appropriate, for example ultraviolet ( U V ) backlight, result in efficient, colored P L L C D s (31,32). W e have designed and fabricated devices that use the P L polarizers described above, based on blends of U H M W - P E and yellow-green light-emitting E H O - O P P E (2 % w / w , M of 84,000 gmol" , λ = 80), blue light-emitting c o - P P E blue (10 % w / w , λ = 80) or orange-red light-emitting M E H - P P V (1 % w/w, λ = 80). In backlit P L display devices either the light used to photoexcite the P L polarizer, or light emitted from the P L polarizer may be switched by a twisted nematic (TN) electrooptical (EO) light valve (Figure 3). Depending on the selected n

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Figure 2. Polarized absorption (a) and photoluminescence (b) spectra of a P L polarizer ( λ = 80) based on a 2 % w/w E H O - O P P E / U H M W - P E blend, recorded for absorption and emission parallel (solid line) and perpendicular (dashed line) to the drawing direction. (Adapted with permission from ref. 31.) Copyright 1998 American Association for the Advancement o f Science.

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Viewer Sheet polarizer TN L C cell E H O - O P P E P L layer

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New Photoluminescent Display Devices - American Chemical Society

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