Perovskite solar cells based on self-assembled monolayer materials for charge transport achieve high efficiency. However, the control of the electrical properties and coverage of these few nanometre-thick monolayers is challenging. In particular, the morphology of the underlying substrate — a transparent conductive oxide electrode — has been reported to affect the quality of the layer. Yet, a deeper understanding of the factors at play is needed. Now, Suzana Kralj, Monica Morales-Masis, and team at the University of Twente and King Abdullah University of Science and Technology show a correlation between the microstructure of the electrode and the work function — the minimum energy needed to extract an electron from the surface of a material — of the self-assembled monolayers.

The researchers compare conductive oxide electrodes with different microstructures, that is, amorphous and polycrystalline with nanometre- or micrometre-scale grains. The large grain size and different crystal orientations of the polycrystalline sample with micrometre grains lead to an inhomogeneous spatial distribution of the work function. Such inhomogeneity is retained when the self-assembled monolayer is deposited on top of the electrode, making charge transport less effective. On the other hand, Kralj et al. show that the use of amorphous buffer layers, such as nickel oxides, could ensure a uniform work function when using polycrystalline electrodes. They conclude that electrodes or buffer layers with an amorphous structure or no preferential crystal orientations are needed to afford high-efficiency solar cells.

基于自组装单层材料进行电荷传输的钙钛矿太阳能电池实现了高效率。然而，控制这些几纳米厚的单分子层的电性能和覆盖范围具有挑战性。特别是，据报道，底层基板（透明导电氧化物电极）的形态会影响层的质量。然而，需要更深入地了解起作用的因素。现在，Suzana Kralj、Monica Morales-Masis 以及特温特大学和阿卜杜拉国王科技大学的团队展示了电极的微观结构与功函数（从表面提取电子所需的最小能量）之间的相关性材料的自组装单层。

研究人员比较了具有不同微观结构的导电氧化物电极，即非晶态和具有纳米或微米级晶粒的多晶。微米晶粒的多晶样品的大晶粒尺寸和不同的晶体取向导致功函数的空间分布不均匀。当自组装单层沉积在电极顶部时，这种不均匀性仍然存在，从而导致电荷传输效率降低。另一方面，Kralj 等人。研究表明，使用非晶缓冲层（例如氧化镍）可以确保使用多晶电极时具有均匀的功函数。他们的结论是，需要具有非晶结构或无优先晶体取向的电极或缓冲层来提供高效太阳能电池。