Cao Yong, Parker Ian D, Yu Gang, Zhang Chi, Heeger Alan J
UNIAX Corporation, 6780 Cortona Drive, Santa Barbara, California 93117-3022, USA.
Nature. 1999 Feb 4;397(6718):414-417. doi: 10.1038/17087.
Some conjugated polymers have luminescence properties that are potentially useful for applications such as light-emitting diodes, whose performance is ultimately limited by the maximum quantum efficiency theoretically attainable for electroluminescence, ,. If the lowest-energy excited states are strongly bound excitons (electron-hole pairs in singlet or triplet spin states), this theoretical upper limit is only 25% of the corresponding quantum efficiency for photoluminescence: an electron in the π-band and a hole (or missing electron) in the π-band can form a triplet with spin multiplicity of three, or a singlet with spin multiplicity of one, but only the singlet will decay radiatively. But if the electron-hole binding energy is sufficiently weak, the ratio of the maximum quantum efficiencies for electroluminescence and photoluminescence can theoretically approach unity. Here we report a value of ∼50% for the ratio of these efficiencies (electroluminescence:photoluminescence) in polymer light-emitting diodes, attained by blending electron transport materials with the conjugated polymer to improve the injection of electrons. This value significantly exceeds the theoretical limit for strongly bound singlet and triplet excitons, assuming they comprise the lowest-energy excited states. Our results imply that the exciton binding energy is weak, or that singlet bound states are formed with higher probability than triplets.
一些共轭聚合物具有发光特性，这对于诸如发光二极管等应用可能是有用的，发光二极管的性能最终受到电致发光理论上可达到的最大量子效率的限制。如果能量最低的激发态是强束缚激子（单重态或三重态自旋态的电子 - 空穴对），那么这个理论上限仅为相应光致发光量子效率的25%：π能带中的一个电子和π能带中的一个空穴（或缺失的电子）可以形成自旋多重性为3的三重态，或自旋多重性为1的单重态，但只有单重态会通过辐射衰变。但是，如果电子空穴结合能足够弱，电致发光和光致发光的最大量子效率之比理论上可以接近1。在这里，我们报告了聚合物发光二极管中这些效率（电致发光：光致发光）之比的值约为50%，这是通过将电子传输材料与共轭聚合物混合以改善电子注入而实现的。假设能量最低的激发态由强束缚单重态和三重态激子组成，这个值显著超过了它们的理论极限。我们的结果表明，激子结合能较弱，或者单重态束缚态比三重态更有可能形成。
