Hybrid perovskites have been explored quite extensively since their first use as a light absorbing layer in solar cells. Within a decade the solar cell efficiencies of perovskite based solar cells reached the solar cell power conversion efficiencies (PCE) of commercially available Si based solar cells, resulting in a significant academic and commercial interest. Though the solar cell efficiencies of perovskite solar cell are high and comparable to that of Si based solar cells the stability of the perovskite layer under ambient conditions is a major concern from their commercialization point of view. Protecting perovskites from moisture is a challenge that has been difficult to address.
Several attempts have been made to prolong the lifetime of the perovskites solar cell either by encapsulating the entire device or by coating the perovskite layer with lower dimensional hybrid perovskite materials. The latter approach has shown a significant improvement in the device stability under ambient conditions. A large number 2D perovskites have been explored and used as a coating/passivating materials on the top of the 3D perovskite layer. Though there are several reports on 2D perovskite coated 3D perovskite solar cells, the use of long chain alkylammonium cation based 2D perovskites has rarely been studied, most likely due to the possibility that the bulky cations might hamper charge transport between the perovskite and hole transporting material.
Our research group at KTH Royal Institute of Technology, Sweden has investigated the effect of water-resistant long chain alkylammonium cations based 2D perovskites ((CnH2n+1NH3)2PbI4 where n = 14, 16, and 18 and hereon called C14, C16, and C18) on the solar cell performance and stability of the solar cell devices. In our previous work we demonstrated that the C14, C16, and C18 perovskites have unparalleled water stability when compared to 3D perovskites. In the case of C14, photoluminescence was observable from powders even after immersion in water for 96 hours. Conventional 3D perovskites completely decompose within seconds of immersion in water. The 2D coated 3D perovskites retain much of the moisture stability of the original 2D perovskites as will be expanded upon below.
Figure 1.(a) X-ray diffraction (XRD) patterns of 3D-only and 2D perovskite-coated thin films, (b) Ultraviolet–visible (UV–Vis) absorption spectra, the inset shows a small red shift in the absorption band edge of 3D perovskite after 2D perovskite coating, (c) current-voltage characteristic of 3D-only and C18 perovskite-coated PSCs, (d) Incident photon-to-electron-conversion efficiency (IPCE) spectra and the corresponding integrated Jsc of 3D-only and C18 perovskite-coated PSCs, (e) Variation in PCE of nonencapsulated 3D-only PSCs and C14, C16, and C18 perovskite-coated PSCs kept under ambient atmosphere. The figures are taken from our published work (https://www.nature.com/articles/s43246-021-00200-8).
In this study, we have shown a notable suppression of a PbI2 impurity in the 3D perovskite after coating with a 2D perovskite layer (see Fig. 1a). Furthermore, the interaction of 2D perovskite with 3D perovskite leads to the formation of an entirely new crystallographic phase. This has not been previously observed and is entirely novel. Previousstudies where an encapsulating layer of 2D perovskite is used on top of 3D perovskite films have not shown theformation of new phases. Our detailed study using X-ray diffraction demonstrates that cations in the 3D perovskite interact with the 2D perovskite and generates the new crystal phase; the exact crystal structure of this new phase is yet unexplored but will be the focus of future work. There is strong possibility that this new phase can have improvedphysical, optical and photovoltaic properties over the present 2D perovskites reported in the literature.
UV–Vis absorption spectra (see Fig. 1b) shows no significant change in the light absorption of the 3D perovskite after 2D perovskite coating, moreover we observed a significant enhancement in the photoluminescence intensity aftercoating with the 2D perovskite, which is attributed to a decrease in the concentration of electronic defects in the perovskite. In short, the 2D perovskite coating improves the perovskite film quality and improves the physical properties of the 3D perovskite. We further investigated the effect of 2D perovskite coating on the photovoltaic performance.
For champion solar cells, we obtained an efficiency of 18.19% for 3D only and 16.79% for 3D@2D based perovskite solar cells (see Fig. 1c). The 2D perovskite coating enhances the open circuit voltage of the solar cells compared to the 3D only solar cells. We further noticed that the slight drop in the photovoltaic efficiencies in 2D perovskite-coated cells is due to the lower photon-conversion efficiency compared with the 3D device (see Fig. 1d). Amongst the three different 2D perovskites we obtained highest PCE vales for C14 coated solar cells, and the PCE values gradually decreased with increasing the length of the alkyl chain. The concentration of the 2D perovskite solution also plays an important role in achieving the high PCEs as the higher concentration of 2D perovskites leads to a significant drop in the PCE values due to the formation of a thick 2D perovskite layer. The steady-state photocurrent-density measurements show stable Jsc and PCE for both 3D only and 3D@C18 devices, when monitored at maximum power point for about an hour.
When we looked at the cross-sectional SEM images the 2D perovskite-coated solar cell devices we could not observe any additional layer on the top of the 3D perovskite layer suggesting formation of very thin layer of 2D perovskites on the top of the 3D perovskite, and this could be reason why we do not observe any diffraction peaks from the 2D phase. For the stability test the 3D only and 2D perovskite-coated solar cell devices were kept under ambient condition and the photovoltaic parameters were monitored for over 200 days (see Fig. 1e), the 2D perovskite coated solar cells showed quite good resistance towards moisture and only about 15% drop in the solar cell efficiency observed within this period.
This work proves that even long chain organic cation based 2D perovskites can be successfully used in encapsulating the 3D perovskites without much compromising the solar cell efficiencies.
More details can be found in our article published in Communications Materials (https://www.nature.com/articles/s43246-021-00200-8).
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