Now, the highest certified efficiency among lead-free perovskite solar cells (PSCs) belongs to Sn-based devices because of its superior optoelectronic properties, such as ideal bandgap and excellent carrier mobility, which is considered as the most promising lead-free perovskite material.
In recent years, tin perovskite has been become the frontier of perovskite devices due to its superior optoelectronic properties. Researchers have put much effort into tin perovskite materials and devices, and significant improvement of PCE of Sn PSCs has been obtained by methods such as dimensional engineering, crystal control, anti-oxidation additives and surface passivation.
As a widely-used method to improve the devices performance, low-dimensional (quasi-2D) structure perovskite with higher crystal quality has effectively promote the PCE of Sn PSC. However, the number of [SnI6]4- layer member (n) of low-dimensional tin perovskite is general even distributed with a low n value. The structures with low n value have high exciton binding energies, which lead to fast bimolecular recombination, decreasing carrier diffusion and currents of devices.
The short-circuit current density (JSC) of Sn PSCs based on quasi-2D structure is usually lower than 20 mA/cm2. As a result, it is necessary to fabricate high n value structure with high orientation, which is expected to boom the JSC.
Compared with the developed lead perovskite, the crystal kinetic process of tin perovskite is quite different, such as fast and uncontrollable crystal rate, which makes it a challenge to regulate the tin perovskite growth kinetic process.
Furthermore, there is a few characterizations to understand the crystallization process of tin perovskite, which makes the lack of theoretical guidance to the methods of tin perovskite film growth.
Recently, the Zhijun Ning group of Shanghaitech University and collaborators reported the method of regulating the crystallization kinetics of tin perovskite thin films and improving crystal quality through phenethylammonium thiocyanate (PEASCN).
In this work, 17.5% phenylethylamonium bromide (PEABr) is added into 3D perovskite to fabricate low-dimension structure, which is called PEABr group; based on PEABr group, 2.5% PEABr is replaced by PEASCN in order to control the crystallization kinetic process of tin perovskite, which is called PEABr-PEASCN group. Both of the two kinds of films are fabricated by anti-solvent spin-coating method.
The n values of two films were compared by absorption spectra (Abs) and time-resolved photoluminescence (TRPL) measurement. As shown in Fig.1(a), contrast with the 1.46 eV bandgap of PEABr film, the bandgap of PEABr-PEASCN film is red shifted to 1.44 eV, which is closer to the bandgap of pure 3D FASnI3 film (1.40 eV), meaning the increase of n value in PEABr-PEASCN films. The photoluminescence (PL) spectra indicate that the wavelength of PEABr-PEASCN peak is longer consistent with the conclusion above. Furthermore, carrier recombination rate constants of these films were then calculated by fitting the TRPL curves (Fig.1 (b)).
The bimolecular recombination rate constant of PEABr-PEASCN film and PEABr film is 1.3+ 10-8 cm3 s-1 and 2.0+ 10-8 cm3 s-1, respectively. The lower bimolecular recombination rate can be ascribed to the increased ratio of high member of quasi-2D structures in PEABr-PEASCN films, which reduces exciton binding energy. The characterization results above prove the n value is improved after PEASCN added, which is conducive to the fabrication of high member low-dimensional structure.
To analyze the composition of crystal structure, grazing incidence wide-angle X-ray scattering (GIWAXS) was employed. As shown in Fig.2 (a), in comparison with PEABr film, the intensity of 1L structure (n=1) diffraction pattern of PEABr-PEASCN film is significantly decreased, while the 2L structure (n=2) diffraction intensity is chiefly improved. The change of diffraction pattern intensity can be attributed to reduction of 1L structure and the gain of 2L structure when PEASCN exists. Fig. 2(b) describes the relationship between diffraction intensity and azimuth.
The diffraction intensity of (100) plane of PEABr-PEASCN film is concentrated at the azimuth angle around 90 degrees , indicating that the film is highly oriented and grows perpendicularly to the substrate, which is profit to the carrier transport in perovskite film. The film structure is further characterized by X-ray diffraction (XRD). As shown in Fig.2 (c), the full width at half maximum (FWHM) of (100) diffraction peaks are 0.074 degrees for PEABr-PEASCN film, smaller than that of PEABr film (0.082 degrees ), which can be ascribed to the enhanced crystallinity and improved crystal quality derived from the increase of high member quasi-2D structure in PEABr-PEASCN film.
The crystallization kinetics experiment revealed the mechanism of PEASCN regulating n value and improving crystal quality. The processes of film growth were tracked by quasi-in situ GIWAXS measurement, as shown in Fig. 3(a). Before annealing, only (100) peak can be observed in the PEABr films, while low-dimensional structure signals appear after annealing process; in contrast, PEABr-PEASCN film shows both (100) and (002)2L peaks before annealing. It indicates the 2D structure can grow without annealing on the condition that PEASCN exists.
The evolution of free energy of different structures during crystallization is shown in Figure 3 (b). In comparison with low-dimensional structure consisting PEABr, the reaction barrier of PEASCN low-dimensional structures decreases significantly, indicating that low-dimensional structures can form at the beginning of fabrication. The pre-formed low-dimensional structures play the role as templates that leading bottom crystal ordered grow. The orientation of perovskite films that guided by enough template during crystallization improved, as well as higher crystallinity.
However, the formation time of low-dimensional structure consisting PEABr is later than that of bulk structures because of the larger reaction activation energy, which means less template takes part in crystallization, leading a disordered PEABr film with low crystallinity. Meanwhile, 2L structure is a more stable thermodynamic product than 1L structure when SCN- anions exist, hence PEASCN precisely induce the formation of 2L low-dimensional structure early. The diffraction signal intensity of 2L structure is in coherence during annealing, indicating 2L structure is mainly distributed on the surface of perovskite film, which is beneficial to improve the stability of the film and reduce the defects on the film surface
Benefitted from the improvement of high member structure ratio, enhanced orientation and crystallinity, the photoelectric performances of PEABr-PEASCN films boom. The electron diffusion length and PL lifetime of PEABr-PEASCN are 480 nm and 126 ns, respectively, larger than 410 nm and 105 ns of PEABr. As shown in Fig. 4(a), the solar cell based on PEABr-PEASCN film achieved a 14.6% PCE, as high as the record Sn PSC certified efficiency, significantly higher than PEABr device efficiency (13.4%).
Higher PCEs can be ascribed to the enhanced JSC. Fig. 4(b) shows statistical PCE and JSC from PEABr and PEABr-PEASCN Sn PSCs. The champion JSC of PEABr-PEASCN is 20.6 mA cm-2, while the average JSC is 20.2 mA cm-2, obviously higher than that of PEABr devices (19.0 mA cm-2). The stability of PEABr-PEASCN is also enhanced. The encapsulated PEABr-PEASCN solar cell maintains 99.7% of the initial PCE in a N2 glovebox for 1,000 h (Fig.4 (c)).
In summary, PEASCN can increase the proportion of high member quasi-2D films, improve the crystal orientation and crystallinity, and further enhance the photoelectric performance of the prepared devices by regulating the crystallization kinetics of tin perovskite. Furthermore, the quasi-in situ characterization method adopted in this work deepens the understanding of the crystallization kinetics of tin perovskite. This crystal growth control method provides an effective approach for the film structure regulation.
The co-first authors are Hansheng Li and Zihao Zang, Ph.D. students of School of Physical Science and Technology, Shanghaitech University. The corresponding authors are the Prof. Zhijun Ning of Shanghaitech University and Prof. Yuanyuan Zhou of the Hong Kong Baptist University. Prof. Philip C. Y. Chow at University of Hongkong, and Prof. Kam Sing Wong from Hong Kong University of Science and Technology assisted the transient spectroscopy measurement.
Research Report:High-member low-dimensional Sn-based perovskite solar cells
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