2D perovskite materials could hold key to develop higher efficiency solar cell.

Research led by Prof. Honhzheng Chen from Zhejiang University fabricated 2D perovskite films with high efficiency. The fabrication of high quality film with large grains oriented grown along the direction of film thickness is of great importance for the two‐dimensional (2D) Ruddlesden‐Popper perovskite‐based solar cells (PVSCs) with high power conversion efficiency (PCE). In this work, for the first time, the team realized the fabrication of high quality 2D BA2MAn‐1PbnI3n+1 (BA+ = butylammonium, MA+ = methylammonium, n = 5) perovskite film, with enlarged grain size over 1 µm and preferential orientation growth by introducing a second spacer cation (SSC+) into the precursor solution. As evidenced by dynamic light scattering (DLS) measurement, SSC+ addition can induce aggregation in the precursor solution. The precursor aggregates are favorable for the formation of large crystal grains via inducing nucleation and decreasing the nucleation sites.

Ruddlesden-Popper perovskite presents better moisture resistance and higher forming energy.
Three-dimensional (3D) hybrid perovskite solar cells (PVSCs) have experienced unprecedented rise in power conversion efficiency (PCE). Along with PCE exceeding 24%, increasing attention has been paid to improve the environmental stability of the 3D perovskites for further practical application. Two-Dimensional (2D) Ruddlesden-Popper perovskite with chemical formula of A2Bn-1MnX3n+1 presents better moisture resistance than the 3D counterpart because of the advantages of enhanced hydrophobicity and higher forming energy.

Optimized orientation and grain size
The perovskite film with optimized orientation and grain size is desired in fabricating high performance PVSCs. Enlarging the grain size of perovskite polycrystalline film can reduce the grain boundaries, which are regarded as non-radiative recombination centers for photogenerated charge carriers and harmful to charge transport. For the 2D perovskite film, the impact of crystal orientation and grain size will be more significant on the device performance, because of the anisotropy in crystal structure and the resulting anisotropic movement of the charge carriers. The in-grain movement of the charge carriers between the adjacent [Bn-1MnX3n+1] 2- slabs will be blocked because of the insulating nature of the spacer cation layer, on one hand. On the other hand, the spacer cation will constrain the growth of crystal grains during film forming, and tend to concentrate on the grain boundary, which increases the number of the grain boundaries and hinders the between-grain charge transport. That is to say, both of the in-grain and between-grain blocking of the charge transport in the 2D perovskite film lead to poor PCE of the 2D PVSCs.

To ensure the fast in-grain charge transport, one of the effective approach is to realize preferred orientation growth of the [Bn-1MnX3n+1] 2- slabs in the 2D perovskite crystals perpendicular to the substrate. For the between-grain charge transport, enlarging the grain size and reducing the grain boundaries along the direction of film thickness will be helpful. Recently, various approaches, including hot-casting, additive assistant, solvent engineering, have been developed to promote the vertical growth of the perovskite film to improve the charge transport. Owing to the improvement of the film quality, together with the application of new spacer cations (such as allylammonium, 2-thiophenemethylammonium) and the increase of n value, the PCE of the 2D PVSCs achieved obvious progress in the past three years. Kanatzidis et al. improved the orientation of BA2MAn-1PbnI3n+1 (BA+ = butylammonium, MA+ = methylammonium, n=5) film with increased grain size from nm to μm scale by using mixture of DMF/DMSO as solvent and achieved a PCE of 10%, the highest efficiency for BA2MA4Pb5I16 based PVSCs so far. It is still important to explore the new method to fabrication high quality film with large grains oriented grown along the direction of film thickness and find out the effective mechanisms positively impact the film quality.

2D perovskite with high efficiency of 14.09%
Herein, for the first time, a second spacer cation (SSC+ ) such as phenylethylammonium (PEA+ ) was introduced into the precursor solution of the BA2MAn-1PbnI3n+1 (n = 5) perovskite. By adjusting the precursor aggregation induced by SSC+ , high quality 2D perovskite film with preferential orientation growth and enlarged grain size over 1 μm was prepared. With PEAI addition in precursor solution, the optimized planar-structured PVSCs presented a maximum PCE of 14.09%, which is the highest value of the BA2MA4Pb5I16 based 2D PVSCs. The unsealed device showed good moisture stability by maintaining around 90% of its initially efficiency after 1000 hours’ exposure to the air (Hr = 25±5%).

PL spectra of BA2MA4Pb5I16 perovskite films
The steady-state photoluminescence (PL) spectra excited from quartz side of the corresponding BA2MA4Pb5I16 perovskite films. The dominated peak strengthens and red-shifts from 743 nm to 749 nm when the amounts of PEAI increase from 0 to 0.1, indicating the improvement of film quality with enlarged grain size. Additional emission peaks at 576 nm, 613 nm and 647 nm can be detected for all the cases, ascribing to the 2D components with lower n values of 2, 3, 4. The 0.1 PEAI based perovskite film exhibits the weakest intensity of these three peaks, indicating the low content of low-n phase. Considering the wider bandgap and the larger exciton binding energy of the low-n phase, the decrease of the low-n phase will be helpful to light harvesting and exciton separation as well.

Dynamic light scattering (DLS) measurements
To understand the effect of SSC+ on film morphology, dynamic light scattering (DLS) measurements are conducted for the precursor solutions with and without PEAI addition. Colloidal particles with sizes less than 10 nm exist in these precursor solutions. Precursor aggregates with size of 3~6 µm can be detected in the case of 0.1 PEAI addition. But for the system without PEAI addition, no precursor aggregates can be detected. Besides PEA+ , this kind of aggregation is also observed when adding other SSC+ , like benzylammonium (PMA+ ) and butylammonium (isoBA+ ), into the precursor solution, despite their difference in shape and size. Based on these observations, we argue that the aggregation in the precursor solution induced by SSC+ plays an important role in forming high quality film with large grains.

As illustrated in the figure bellow, without PEAI addition, a large number of nuclei will precipitate from the supersaturated precursor solution without aggregates inside, which leads to the formation of perovskite film with small grains. In the presence of PEAI, the existence of precursor aggregates might induce nucleation and decrease the nucleation sites, which is favorable for the formation of large crystal grains.

In conclusion, we demonstrated a new route to fabricate high quality 2D BA2MA4Pb5I16 Ruddlesden-Popper perovskite film on ITO/PEDOT:PSS substrate by adding the second spacer cation, such as PEA+ , into the precursor solution. The second spacer cation induced aggregation in precursor solution plays an important role in the formation of high quality 2D perovskite film with large grains oriented grown along the direction of film thickness. With optimized PEAI addition, 0.1 PEAI, the obtained 2D perovskite film exhibits preferable orientation growth and large grain size over 1 µm. Planar-structured PVSCs with inverted structure presented a maximum PCE of 14.09%. The unsealed device remained over 90% of its original PCE after 1000 hours’ storage in air atmosphere with humidity of 25±5%. This work provides a new approach toward high quality 2D perovskite films, and is meaningful for the fabrication of highly efficient and stable PVSCs.

Reference: 
The Second Spacer Cation Assisted Growth of 2D Perovskite Film with Oriented Large Grain for Highly Efficient and Stable Solar Cells.
Xiaomei Lian, Jiehuan Chen, Minchao Qin, Yingzhu Zhang, Tian Shuoxun, Xinhui Lu, Gang Wu, Hongzheng Chen.
Angewandte Chemie, First published: 07 May 2019;https://doi.org/10.1002/anie.201902959