Armstrong Neal R, Wang Weining, Alloway Dana M, Placencia Diogenes, Ratcliff Erin, Brumbach Michael
Department of Chemistry, University of Arizona, Tucson, Arizona 85721, USA.
Macromol Rapid Commun. 2009 May 19;30(9-10):717-31. doi: 10.1002/marc.200900075. Epub 2009 May 11.
Heterojunctions created from thin films of two dissimilar organic semiconductor materials [organic/organic' (O/O') heterojunctions] are an essential component of organic light emitting diode displays and lighting systems (OLEDs, PLEDs) and small molecule or polymer-based organic photovoltaic (solar cell) technologies (OPVs). O/O' heterojunctions are the site for exciton formation in OLEDs, and the site for exciton dissociation and photocurrent production in OPVs. Frontier orbital energy offsets in O/O' heterojunctions establish the excess free energy controlling rates of charge recombination and formation of emissive states in OLEDs and PLEDs. These energy offsets also establish the excess free energy which controls charge separation and the short-circuit photocurrent (J(SC) ) in OPVs, and set the upper limit for the open-circuit photopotential (V(OC) ). We review here how these frontier orbital energy offsets are determined using photoemission spectroscopies, how these energies change as a function of molecular environment, and the influence of interface dipoles on these frontier orbital energies. Recent examples of heterojunctions based on small molecule materials are shown, emphasizing those heterojunctions which are of interest for photovoltaic applications. These include heterojunctions of perylenebisimide dyes with trivalent metal phthalocyanines, and heterojunctions of titanyl phthalocyanine with C(60) , and with pentacene. Organic solar cells comprised of donor/acceptor pairs of each of these last three materials confirm that the V(OC) scales with the energy offsets between the HOMO of the donor and LUMO of the acceptor ($E_{{\rm HOMO}^{\rm D} } - E_{{\rm LUMO}^{\rm A} }$).
由两种不同有机半导体材料的薄膜制成的异质结[有机/有机'（O/O'）异质结]是有机发光二极管显示器和照明系统（OLED、PLED）以及基于小分子或聚合物的有机光伏（太阳能电池）技术（OPV）的重要组成部分。O/O'异质结是OLED中激子形成的位点，也是OPV中激子解离和光电流产生的位点。O/O'异质结中的前沿轨道能量偏移建立了控制OLED和PLED中电荷复合速率和发光态形成的过剩自由能。这些能量偏移还建立了控制OPV中电荷分离和短路光电流（J(SC)）的过剩自由能，并设定了开路光电势（V(OC)）的上限。我们在此回顾如何使用光发射光谱法确定这些前沿轨道能量偏移，这些能量如何随分子环境变化，以及界面偶极子对这些前沿轨道能量的影响。展示了基于小分子材料的异质结的近期实例，重点强调了那些对光伏应用有意义的异质结。这些包括苝二酰亚胺染料与三价金属酞菁的异质结，以及钛氧基酞菁与C(60)以及与并五苯的异质结。由这最后三种材料中的每一种的供体/受体对组成的有机太阳能电池证实，V(OC)与供体的HOMO和受体的LUMO之间的能量偏移（$E_{{\rm HOMO}^{\rm D} } - E_{{\rm LUMO}^{\rm A} }$）成比例。
