Exciton emission from printed EL devices using solution‐processed ZnO nanocrystals

Abstract

ZnO is an excellent material candidate for ultraviolet (UV)‐light emitting devices based on its exciton emission. We have applied solution‐processed ZnO NCs to printed electroluminescence (EL) devices as their emission layers (EMLs) to develop a novel flexible lightweight, and flat‐panel UV‐light source. In this article, first, the structural properties of ZnO NCs fabricated from a solution route are presented together with the results of photoluminescence (PL). We address to suppression of the defect‐related green‐band PL from ZnO NCs by controlling the crystal size. Next, we describe device properties of the printed EL devices. Specifically, we present the impact on the use of pentaceneon the device properties; pentacene acts as the hole‐transport layer, resulting in the exciton emissions in EL. In addition, it passivates the rear surface of ZnO NC EML, which is effective in suppressing the green‐band EL. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Table of Contents

“…Sample fabrication Figure 1 illustrates the structure of a thin-film LED composed of an aluminosilicate glass substrate ͑Corning, 1737͒, tin-doped indium oxide ͑ITO͒ transparent electrodes, a pentacene HTL, a ZnO NC EML, and Al dot electrodes. 12,13 The 100 nm thick ITO coated glass substrates were purchased from Sanyo Vacuum Industries, Osaka, Japan. Pentacene has been widely used in organic LEDs as a HTL ͑Ref.…”

“…[7][8][9][10][11][12][13] First, the ZnO NC powder was dispersed in a mixture of resin and organic solvent, yielding a printing ink. A layer of this ink was printed on the pentacene layer.…”

“…Until now, research has been focused on organic semiconductors as the material most applicable for PE, 2 much less effort has been devoted to using inorganic semiconductors in PE. Despite this, inorganic semiconductor nanostructures, such as quantum dots, 3-6 nanocrystals ͑NCs͒, [7][8][9][10][11][12][13] nanoparticles, [14][15][16] nanorods, 17,18 and nanowires 19,20 have an excellent potential for PE because various inorganic semiconductor nanostructures, particularly II-VI semiconductors nanostructures have been chemically synthesized via a solution route, [3][4][5][6][7][8][9][10][11][12][13][14][15][17][18][19][20] which lends itself to solution-based deposition techniques. We demonstrated thinfilm electroluminescent ͑EL͒ devices using printed emission layers ͑EMLs͒ containing Mn-doped [7][8][9] or Cu-doped 10 ZnS NCs synthesized by coprecipitation methods.…”

“…[11][12][13] ZnO is a prom-ising material for the light-emitting applications, because ZnO has a direct band gap of 3.37 eV at room temperature and a relatively large exciton binding energy of 60 meV, 21 which results in its highly efficient exciton emission in the ultraviolet ͑UV͒ region of a wavelength of around 380 nm. UV LEDs can be coupled with visible phosphors to yield high color tunability.…”

“…As a result, extensive effort has been devoted to structural, optical, and photoluminescence ͑PL͒ studies of solution-processed ZnO nanostructures. [11][12][13][17][18][19][20][22][23][24][25][26][27][28][29][30] The PL studies revealed the nanostructure size is a key factor in achieving efficient UV-PL. 26,27 Moreover, EL studies investigating UV emission caused by the radiative recombination of excitons have been reported for solution-processed ZnO nanostructures, 9,10,[17][18][19][20] and they revealed that a hole transport layers ͑HTL͒ plays a crucial role in achieving UV emissions.…”