2、紫外有机电致发光存在的问题和解决途径
<1)紫外发光材料的稳定性有待提高因为紫外发光材料的带隙较宽,决定了其电子共轭长度较短,分子尺寸较小,所以许多小分子发光材料的结晶温度较低薄膜稳定性差,影响器件的光电性能例如PBD的结晶温度只有60℃,TAZ的结晶温度也低于70℃目前常用的解决途径是制备螺旋结构和星形结构材料,例如spiro-PBD和螺旋双芴等;
<2)色纯度不高紫外发光器件往往伴随着可见光区域的发光,究其原因种类众多,主要包括:
紫外发光材料的薄膜存在结构缺陷致使附近激子发可见光;发光材料与邻近材料因为相互作用而产生激基复合物;空穴或电子无法有效注入到发光层,导致激子形成区域不在紫外发光层中目前已报道的解决途径是改进材料结构,增大分子尺寸或者是合成非对称的分子结构例如,螺旋双芴相对双芴类材料具有较大的分子结构,除可以提高成膜性外,还能减小与周围分子的相互作用,避免形成激基复合物或缔合物;
<3)紫外发光器件的激子形成区域不容易控制有机材料带隙普遍较窄,决定了紫外发光器件的激子限制材料的选择性非常小,这就要求必须实现载流子向紫外发光的平衡注入与传输,使激子形成区域在紫外发光层中然而,紫外发光材料的价带能级普遍较低,空穴往往很难注入到紫外发光层中目前的解决办法,一方面是合成限制激子和阻挡载流子的新型材料例如,日本大坂大学研究表明,紫外发光器件的发光效率与空穴阻挡层紧密有关,为此他们合成一系列的硼烷类衍生物,促进向宽带隙发光材料电子注入,同时,也对空穴载流子起到阻挡作用[21]另一种解决办法是提高空穴向发光层的注入能力其中,可以提高阳极界面的空穴注入能力,例如采用PEDOT:
PSS[22]F16CuPc[3]帮助空穴注入,实现器件的纯紫外发光,也可以选择合适价带能级的空穴传输材料,实现空穴向发光层的有效注入[4,22]
结论
随着社会进步,市场需求对紫外光源提出了更高的要求紫外有机发光相对于其它发光技术,具有面发光低驱动电压高发光效率低制备成本全视角等诸多优势,其应用前景更加广阔尽管紫外有机发光研究较少,存在发光材料种类较少发光效率不高器件寿命较低等问题,但随着有机半导体理论的持续完善以及有机光电子器件制备技术的不断成熟,相信紫外发光将逐一解决各种问题,达到应用要求,迅速发展起来
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