Lei Dang Yuan, Zhang Lei, Ong Hock Chun
Department of Applied Physics, The Hong Kong Polytechnic University , Hong Kong, China.
Department of Electrical and Computer Engineering, National University of Singapore , 4 Engineering Drive 3, Singapore 117583.
ACS Appl Mater Interfaces. 2015 Oct 28;7(42):23496-500. doi: 10.1021/acsami.5b05871. Epub 2015 Oct 9.
Increasing light extraction efficiency in the forward direction is being extensively pursued due to its crucial role in realizing top-emitting organic and inorganic light emitting devices. Various surface plasmon polariton (SPP)-based strategies for emission enhancement and light extraction have been developed to improve the top-emitting efficiency of these devices. However, the role of surface roughness of both semiconductor film and metal electrode in improving the emission efficiency of a practical device has not been thoroughly studied yet. In this work, the influence of surface roughness of a top metal electrode on the photoluminescence enhancement of a ZnO thin film is investigated experimentally and numerically based on an insulator-metal-semiconductor system. It is found that the generic surface roughness of the metal electrode plays an encouraging role in increasing the forward-emission intensity by facilitating cross-coupling of SPPs on the two opposite sides of the metal layer. More importantly, the forward emission can be further enhanced by capping a high-index polymer layer on the metal electrode to bridge the momentum mismatch between the two SPPs modes. The experimental observations are well explained by the SPPs cross-coupling mechanism that models the radiation power of a dipolar emitter underneath the metal electrode as a function of the metal surface roughness. Our work opens up the possibility of using cross-coupling of SPPs as an effective means to fabricate high-brightness top-emitting devices without the need of complicated nanoscale patterning.
由于提高正向光提取效率在实现顶部发射有机和无机发光器件中起着关键作用，因此人们正在广泛地进行相关研究。为了提高这些器件的顶部发射效率，已经开发了各种基于表面等离激元极化激元（SPP）的发射增强和光提取策略。然而，半导体薄膜和金属电极的表面粗糙度在提高实际器件发射效率方面的作用尚未得到充分研究。在这项工作中，基于绝缘体-金属-半导体系统，通过实验和数值方法研究了顶部金属电极的表面粗糙度对ZnO薄膜光致发光增强的影响。研究发现，金属电极的一般表面粗糙度通过促进金属层两侧SPP的交叉耦合，在增加正向发射强度方面发挥了积极作用。更重要的是，通过在金属电极上覆盖一层高折射率聚合物层来弥合两种SPP模式之间的动量失配，可以进一步增强正向发射。实验观察结果通过SPP交叉耦合机制得到了很好的解释，该机制将金属电极下方偶极发射器的辐射功率建模为金属表面粗糙度的函数。我们的工作开辟了利用SPP交叉耦合作为一种有效手段来制造高亮度顶部发射器件的可能性，而无需复杂的纳米级图案化。
