2D-Quasi-2D-3D Hierarchy Structure for Tin Perovskite Solar Cells with Enhanced Efficiency (9.41%) and Stability

A team led by Prof. Zhijun Ning, the School of Physical Science and Technology at ShanghaiTech University, fabricated a Sn-based hierarchy structure perovskite in a one-step process, comprising highly parallel-orientation 2D PEA2Snl4 on the surface of 3D FASnl3. The hierarchy structure delivers significant enhanced stability and oxidation resistance in air atmosphere. They have developed a Sn-based perovskite films in planar structure solar cells and achieved a PCE up to 9.41 %.
The final paper: “2D-Quasi-2D-3D Hierarchy Structure for Tin Perovskite Solar Cells with Enhanced Efficiency and Stability.” was published in Joule.

Details of the research are as follows: 
Over the past few year, organic-inorganic hybrid halide perovskite solar cells (PSCs) have attracted tremendous attention and rapidly achieved a significant high-power conversion efficiency (PCE) of 23.3%. 
However, toxicity and instability of Pb-based PSCs remains to be issues that limit their large-scale application. A bunch of Pb-free PSCs based on elements such as tin (Sn), germanium (Ge), bismuth (Bi), stibium (Sb), and copper (Cu) have been investigated. Among these alternatives, Sn-based PSCs display promising device performance owing to their suitable band gap (1.2-1.4 eV), small exciton binding energy (18 meV), and high carrier mobility. However, Sn-based perovskites suffer from self-doping due to the oxidation of Sn2+ to Sn4+ and Sn vacancies, leading to high intrinsic carrier density, instability, and poor reproducibility.

Low-dimensional perovskite, namely perovskite sandwiched by layers of large organic molecules can effectively boost stability of PSCs as a result of “nanoscale encapsulation.” Organic molecules such as phenylethyl amine (PEA) butylamine (BA) were introduced to form low-dimensional Sn perovskite, achieving a PCE of 5.9 % with improves stability. However, for low-dimensional perovskite, especially Sn-based materials, a key issue to obtain long-term stability and high efficiency simultaneously demands prompt solution.

Here the team introduced removable pseudohalogen ammonium thiocyanate (NH4SCN, hereafter described as SCN) to manipulate the crystal growth process of perovskite film. They demonstrate that the addition of SCN separates the nucleation and crystallization growth processes, leading to the formation of 2D-quasi-2D-3D hierarchy structure perovskite (HSP). The final HSP shows great resistance to oxidation, giving rise to reduce carrier density and enhanced carrier mobility. As a result, the PSCs based on this HSP achieved a record PCE up to 9.41% for lead-free perovskite.

Hierarchy Structure Characterization
Typically, the perovskite film was prepared based on a spin coating process using a precursor of PEA0.15FA0.85Snl3 and SnF2 in a mixture of N, N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) with different amount of SCN. An annealing process was subsequently performed to remove the solvent residue and turn it into perovskite phase. Scanning electron microscope (SEM) images indicate that both the control film without SCN and the film with 5% SCN show smooth morphology with small pinholes (Fig. 1A and 1B). When the amount of SCN increase, the diameter of the pinholes are remarkably enlarged.

To investigate the structure of the perovskite films, X-ray diffraction (XRD) was conducted for films with and without the addition of SCN (Fig. 1C and 1D). Three dominant diffraction peals at angles of 14.0゜, 28.3゜, and 42.9゜ are ascribed to the (100), (200), and (300)lattice planes for 3D orthorhombic (Amm2) FASnl3. HSP with the inclusion of 5% SCN shows two additional diffraction peaks at angles of 5.5゜ and 27.4゜, assigned to the crystallographic planes (200) and (1000) of 2D PEA2Snl4.
Figure 1. Perovskite Film Morphology and Structure Characterization.(Cited from the research)
(A and B) SEM images of the control film (A) and HSP (B). 
(C and D) XRD spectra of the control film (C) and HSP (D). The subscript 2D of (hkl) represents crystal planes of PEA2SnI4 perovskite. The small diffraction angle indicates the formation of a 2D structure as the addition of SCN in film fabrication.

We carried out energy-dispersive X-ray spectroscopy (EDS) analysis and X-ray photoelectron spectroscopy (XPS) to analyze the composition of the perovskite film. EDS results indicate that the atomic ratio of sulfur in the sample is zero, implying the complete removal of the SCN addictive. Such a conclusion is further supported by the absence of an S element single in the XPS measurements. This indicates that the diffraction peaks related to 2D perovskite derive from PEA2Snl4 rather than structure containing SCN-.

They exploited grazing-incidence wide-angle X-ray scattering (GIWAXS) to characterize the structure of HSP and the control (Fig. 2). Scattering spectra at different incident angles of 0.2゜, 1.0゜, 1.5゜, and 2゜ were measured to track the perovskite structure evolution from the shallow to the bottom side. Bragg spots and Debye-Scherrer rings belonging to low-dimensional and 3D perovskite polycrystal-line films are indexed. The Bragg spots indicate strongly preferential orientation for low-dimensional polycrystalline films in the shallow surface where the c-planes of perovskite extend parallel to the substrate (Fig. 2A and 2D). As the incident angle increase to 2゜, three Debye-Scherrer rings emerge (Fig. 2B and 2E), indicating the existence of 3D perovskite grains with random orientation in the depths of the films. The (001) and (004) Bragg spots (Fig. 2A) located on the qz axis can be ascribed to quai-2D perovskite PEA2FASnl7 containing double-layer Snl6 octahedra. The (002)2D spot above (001) in Fig. 2D can be ascribed to a single-layer 2D perovskite of PEA2Snl4. As the incident angle increases to 0.5゜, 1.0゜, and 1.5゜, the Debye-Scherrer rings are enlarged, indicating a low-dimensional perovskite with fewer layer of Snl6 octahedra, which tends to be distributed to the surface of the films, and 3D perovskite is concentrated at the bottom (Fig. 2C and 2F). The team of Prof. Zhijun Ning infers that the films a 2D-quasi-2D-3D structure with parallel orientation and layered distribution, and hence name it “hierarchy structure perovskite” (HSP) to show the structure evolution trend with the increase of film depth.
Figure 2. Hierarchy Structure Characterization and Schematic Structure of Perovskite Films. (Cited from the research)
(A–C) Structure characterization of the control film. 
(D–F) Structure characterization of HSP. 

Device Performance Characterization 
The team fabricated solar cells based on an inverted structure, with NiOx as a hole-transporting layer and PCBM as an electron-transporting layer. The CBM of perovskite is higher than PCBM, and the VBM matches with NiOx which enables effective carrier transport into transporting layers. The PSCs based on HSP induced by 5% SCN show the highest solar conversion efficiency of 9.41% under AM 1.5 G solar illumination at 100 mW/cm2 (1 sun). The light intensity was calibrated by mean of a KG-5 Si diode with a solar simulator (Enli Tech SS-F5-3A solar simulator). The current density reaches 22.0mA/cm2, remarkably higher than the device with a 20% PEA molecule. The integrated Jsc value (20.8 mA/cm2) from the external quantum efficiency (EQE) curve agrees well with Jsc. In comparison with the device on the control film, the device based on HSP shows improved FF and Voc. This can be attributed to the increase of carrier mobility and the reduction of defects arising from the reduction of Sn oxidation. When more than 5 % SCN additive is incorporated, Jsc decreases from 22.0 to 17.3 mA/cm2. This can be ascribed to reduction of carrier mobility with the increase of the 2D structure. To evaluate the real performance of the HSP device, steady-state PCE measurement was carried out, and the HSP device showed a quite stable power output. Moreover, a large-area device of ~0.09 cm2 achieved a high PCE of 8.82%.
Figure 3. HSP Device Performance. (Cited from the research)
(A)J–V curves of an HSP champion device measured using both forward and reverse scan mode; the insert image is the energy level diagram of the device. (B) EQE of an HSP champion device. (C) PCE histogram of HSP devices from several fabrication batches. (D) Normalized PCE of an HSP device stored in a N2 atmosphere glovebox.

Prof. Zhijun Ning earns the Highly Cited Researchers 2018
Clarivate Analytics announced the Highly Cited Researchers 2018, and Prof. Zhijun Ning is selected in the cross-field category. This list recognizes world-class researchers selected for their exceptional research performance, demonstrated by production of multiple highly cited papers that rank in the top 1% by citations for field and year in Web of Science.

Zhijun Ning, professor of School of Physical Science and Technology, ShanghaiTech University, aims to apply synthetic strategy to create novel optoelectronic materials including nanocrystals, perovskite, and organic molecules for applications like solar cells, photocatalysis, luminescence, and photodetectors. The team of Prof. Zhijun Ning is particularly interested in leveraging chemistry method to address interface and surface problems that generally exist for nanomaterials and realizing high performance optoelectronic devices.

Referemce:2D-Quasi-2D-3D Hierarchy Structure for Tin Perovskite Solar Cells with Enhanced Efficiency and Stability.
Fei Wang, Xianyuan Jiang, Hao Chen, Yuequn Shang, Hefei Liu, Jingle Wei, Wenjia Zhou, Hailong He, Weimin Liu, Zhijun Ning.
Joule, Volume 2, Issue 12, 19 December 2018, Pages 2732-2743