Planar Black Silicon: disentangling optical and electrical properties of textured interfaces using transformation optics

对太阳能电池的吸收材料进行纳米纹理化导致入射太阳光的强的和光谱宽带吸收增强。然而，纳米织构化界面同时降低了光电器件的电子特性。这否认了吸收改善的完全收获，并且不允许将其转化为器械效率的可比改善。相比之下，使吸收体材料未受损的外部光管理结构在光学上远非最佳，并且它们提供的吸收增强显著减弱。为了解决这个问题，我们提出利用变换光学的概念来设计光子结构，该光子结构保持吸收体材料的几何非线性，使其在电学上完整，同时在光学上像具有优化的散射和散射特性的纹理界面一样起作用。在变换光学程序之后，可以利用麦克斯韦方程的不变性来推导出一种材料分布，该材料分布以与模板表面纹理（例如传统黑硅）完全相同的方式调节光流动。通过这样做，我们有效地推导出一个不均匀的平面层，当放置在吸收层的顶部，可以提供反射和光捕获，从而吸收增强，相当于模板表面纹理没有其电子退化。我们将追求介电渐变折射率光管理结构的实验实现，并展示其集成到太阳能电池中。作为构建模块，我们将利用高折射率介电纳米结构和通过先进的原子层沉积技术共形沉积的不同材料的薄膜层。据我们所知，我们利用转换光学推导光管理结构的方法是开创性的。我们的设计方法是从目前常见的转换光学应用程序的出发，并将打开转换光学概念的应用程序在设计各种真实的设备的结构。太阳能转换是直接贝内于该项目成果的明显且最重要的应用。 然而，这里开发的方法也可能有助于其他光学领域新概念的发展。例如，人们还可以推导出发光二极管（LED）中的替代光输出耦合结构，这与我们在这里主要要做的相反。

Nanotexturing a solar cell’s absorber material leads to a strong and spectrally broadband absorption enhancement of incident sun light. However, nanotexturing the interfaces degrades simultaneously the electronic properties of the optoelectronic device. This denies the complete harvest of the absorption improvement and does not allow to translate it to a comparable improvement in device efficiency. In contrast, external light management structures that leave the absorber material undamaged are optically far from optimum and they provide substantially weaker absorption enhancement.To solve this problem, we propose to capitalize on the notion of transformation optics to design photonic structures that preserve the geometrical flatness of the absorber material, to leave it electrically intact, while acting optically like a textured interface with optimized antireflection and scattering properties. Following transformations optics procedures, the invariance of Maxwell’s equations can be exploited to deduce a material distribution that regulates the light flow in exactly the same way as a template surface texture, e.g. conventional black silicon, would do. By doing so, we are effectively deducing an inhomogeneous planar layer that, when placed on top of the absorber layer, can provide antireflection and light trapping, thus absorption enhancement, equivalent to the template surface texture without its electronic degradation. We will pursue the experimental realization of the dielectric graded refractive index light management structures and demonstrate its integration into a solar cell. As building blocks, we will utilize dielectric high refractive index nanostructures and thin film layers of different materials conformally deposited by an advanced atomic layer deposition technique.Our approach of deducing light management structures with transformation optics, to the best of our knowledge, is pioneering. Our design approach is a departure from currently common transformation optics applications and will open up applications of transformation optics concepts in designing structures for various real devices. Solar energy conversion is the obvious and most important application that would directly benefit from the findings of this project. However, the methods to be developed here may also contribute to the development of novel concepts in other fields of optics. For example, one can also deduce alternative light outcoupling structures in light emitting diodes (LEDs), which is the inverse of what we mainly aim to do here.