2D organic-inorganic perovskites pave the way to novel low-cost solar cells

Detlef Smilgies, CHESS, Cornell University
Kui Zhao, Shaanxi Key Laboratory for Advanced Energy Devices, China

Organic-inorganic lead iodide perovskites are a much studied materials class that have reached solar cell efficiencies above 22% – on par with amorphous silicon – in only a few years after discovery.

cesium-doped four-layer 2D perovskite

Figure 1. Structure of a cesium-doped four-layer 2D perovskite.

Such organic perovskites can be prepared from a solution of readily available lead salts and organic cations (e.g., CH3NH3I) and yield the desired high-performance material under a gentle thermal anneal at 100°C. The process seems to be also relatively insensitive to impurities, as opposed to the ultra-high purity demands and high process temperatures of amorphous silicon, the most commonly used material for solar cells currently. However, the major challenge to large-scale use of organic-inorganic perovskite solar cells is that these materials are prone to degradation under the influence of moisture.

This problem has been much reduced in a 2D perovksite solar cell, reported by Kui Zhao and his coworkers from China and King Abdullah University of Science and Technology (KAUST). They have fabricated high quality plate-like 2D crystallites, only a few perovskite layers thick, whose surfaces are passivated with hydrophobic chains – in a way a self-organized encapsulation on the nanoscale. Zhao et al. have shown that such 2D perovskites can reach similar efficiencies as the original 3D crystalline counterparts but display superior moisture and thermal stability when doped with cesium. Their results were recently published in the renowned journal Energy & Environmental Science [1].

2D perovskite
Figure 2. GIWAXS images from a 2D perovskite, revealing the vertical orientation of the platelets. Cesium doping (Cs0 – none, Cs5 – 5 wt%, Cs15 – 15 wt%) gives rise to a more compact crystal lattice.

Grazing-incidence wide-angle scattering (GIWAXS) data were obtained at CHESS D-line by Rahim Munir of the Amassian group at KAUST, with assistance by CHESS staff scientists Ruipeng Li and Detlef Smilgies. The analysis of the scattering images showed that the 2D perovskite platelets are oriented perpendicular to the substrate which is favorable for the charge transport in a solar cell.

perovskite during solution-casting
Figure 3. Structure formation in pure and 5% doped perovskite during solution-casting.

In-situ analysis during spin coating solution at D-line revealed that the lead perovskite is formed at ca. 6 sec. Cesium doping prolongs the crystal formation, and may thus contribute to forming larger crystallites. In a device structure the Cs-doped 2D perovskite material yielded 13.7% efficiency, the highest reported thus far for this class of materials. As an added benefit, the unencapsulated Cs-doped solar cells also exhibited higher stablility in 30%, 65%, and 85% relative humidity at room temperature and thermal stability at 80 ºC than their undoped counterparts. These encouraging results should get hybrid perovskite materials one step closer to the commercialization.

Reference:

[1] Xu Zhang, Xiaodong Ren, Bin Liu, Rahim Munir, Xuejie Zhu, Dong Yang, Jianbo Li, Yucheng Liu, Detlef-M. Smilgies, Ruipeng Li, Zhou Yang, Tianqi Niu, Xiuli Wang, Aram Amassian, Kui Zhao and Shengzhong (Frank) Liu: "Stable High Efficiency Two-Dimensional Perovskite Solar Cells via Cesium Doping", Energy Environ. Sci., 2017, 10, 2095-2102  (DOI: 10.1039/C7EE01145H).