Conjugated Organic Cations to Improve the Optoelectronic Properties

Aug 2, 2017 - Use of C6H5NH3I as an organic cation instead of CH3(CH2)3NH3I in the synthesis of 2D/3D perovskites (C6H5NH3I)2(CH3NH3I)n−1(PbI2)n lea...
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Conjugated Organic Cations to Improve the Optoelectronic Properties of 2D/3D Perovskites Jesús Rodríguez-Romero, Bruno Clasen Hames, Ivan Mora-Sero, and Eva M. Barea ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00654 • Publication Date (Web): 02 Aug 2017 Downloaded from http://pubs.acs.org on August 2, 2017

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ACS Energy Letters

Conjugated Organic Cations to Improve the Optoelectronic Properties of 2D/3D Perovskites Jesús Rodríguez-Romero, Bruno Clasen Hames, Iván Mora-Seró* and Eva M. Barea* Institute of Advanced Materials (INAM), Universitat Jaume I, 12006 Castelló, Spain Email: [email protected], [email protected] 1 August 2017 Abstract The use of anilinium iodide (C6H5NH3I) as an organic cation instead of butylammonium iodide (CH3(CH2)3NH3I), in the synthesis of a 2D/3D perovskite (C6H5NH3I)2(CH3NH3I)n-1(PbI2)n (with n=1 to 5, where n is the number of inorganic layers) lead to a material with significantly improved photovoltaic and optoelectronic properties with respect to the common butylammonium iodide based perovskite, which is one of the most studied material so far. TOC:

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Two-dimensional (2D) hybrid perovskites (HPVK) are structures alternating organic and inorganic layers, arising of the inclusion of a large organic cation providing Goldschmidt´s tolerance factor higher than 1.1 This fact generates a separation of a determined number of inorganic layers (n), which can range from one to infinity, which corresponds to a three-dimensional (3D) arrangement. A variation of this pure organicinorganic structure can be obtained by the addition of a small organic cation, MA (CH3NH3+) in most of the cases, providing a Goldschmidt´s tolerance factor adequate for perovskite formation, making that the inorganic part becomes a hybrid structure. This organic-inorganic hybrid structures have been named as 2D/3D HPVK. Actually, 2D/3D perovskite-based solar cells has been emerged as an alternative to pure 3D perovskites with the aim to improve their long term stability, which is a key factor in a future device commercialization.2, 3 In the last years, several studies have been reported on 2D/3D perovskites prepared using mainly two ammonium salts: i) phenylethylammonium iodide (PEA), with 4.73% efficiency for a layered material (n=3)4 and 15.3% for a quasi 3D material (n=60)5 and ii) butylammonium iodide (BAI) (from 4.02% to 12.52%, observed only in layered materials i.e. n≤5).6-8 Recently, has been shown one year stable 2D/3D perovskite based devices fabricated using 5ammonium valeric acid iodide with an efficiency up to 11%.2 It is clear that development in the photovoltaic performance is impressive, however, considering the number of organic molecules that can be introduced, as ammonium salts, within the perovskite structure, the results obtained so far can be considered very limited since, in general, the studies has been focused only to a aliphatic non-conjugated ammonium salts. In this study we report on the synthesis of the 2D/3D HPVK (C6H5NH3I)2(CH3NH3I)n-1(PbI2)n (n=1 to 5) call hereafter AnyIPbn, using anilinium iodide (C6H5NH3I) and its performance in a complete perovskite solar cells, see Suppporting Information for material preparation. At the same time it has been carried out a comparative studies using the well known 2D/3D perovskite (BuNH3I)2(MAI)n5 1(PbI2)n, call hereafter BAIPbn, that has been considered as reference. The films were prepared through of a modification of the successful hot casting method.7 The X-ray diffraction (XRD) pattern, for the first perovskite films, which were fabricated without posterior annealing, shows a peak at 12.72°, (PbI2 005 reflection) which is indicative of an incomplete formation of the perovskite lattice (Figure S1). To overcome this drawback, it was studied different annealing times for perovskite films, obtaining 10 min as the most adequate time (superior times produce degradation of the film and the peak of 12.72° starts to increase again). The two characteristic peaks for layered structures, at 14.2° and 28.52°, associated to (111) and (202) crystallographic planes, can also be observed, see Figure S1.5 The current-potential (J-V) curves for the champion devices (for each one of the n evaluated i.e n=1 to n=5) are plotted in Figure 1a and 1b for cell prepared with AnyIPbn and BAIPbn perovskites, respectively. J-V curves show a systematic increase in the

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power conversion efficiency (PCE) from n=1 to n=5 for both HPVK systems and, in all the cases, the performance of the device with anilinium exhibits a significantly higher performance than its respective reference prepared with butylammonium. The conversion efficiency values obtained for the AnyIPbn family devices are among 0.66% (n=1) to 5.94% (n=5), see Table S3, while the BAIPbn family exhibit photoconversion efficiencies ranging from 0.01% (n=1) and 3.02% (n=5) (Table S4). This evolution of the photovoltaic behavior with n value is a clear indicator of the 2D/3D structure of the material formed.9 The great improvement comparing both kind of devices it is observed at the photocurrent, Jsc. In the case of AnyIPbn devices Jsc ranges from 1.2 mA/cm2 (n=1) up to 10.46 mA/cm2 (n=5). BAIPbn cells present a Jsc higher than 6 mA/cm2 just for n=5, while for AnyIPbn this threshold is overcome for n≥2. The use anilinium instead of butylammonium also improves fill factor, FF, and photovoltage, Voc, see Tables S3 and S4. Whereas the increase of FF is significant for all the n range, the augmentation of Voc is especially significant for low n.

Figure 1: J-V curves of champion devices prepared with a) AnyIPbn and b) BAIPbn 2D/3D HPVK layers, the photovoltaic parameters are depicted in Table S3 and Table S4. c) Electroluminescence obtained from devices prepared with both AnyIPbn and BAIPbn at different applied bias. d) Electroluminescence obtained AnyIPbn samples with different n at 3V applied bias.

In addition to the improvement of the performance of solar cell by the use of AnyIPbn

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2D/3D HPVK, the goodness of this new 2D/3D material for the development of another kind of optoelectronic devices has also been analyzed. Perovskites have been increasingly applied in a wide range of areas, including photodetectors,10 optical amplifiers,11 lasers12 and light emitting diodes (LEDs)13 due to their incredible optoelectronic properties based on the low non-radiative recombination. Considering that the TiO2 and Spiro-OMeTAD layer used respectively as electron and hole selecting contact can be also used as efficient injecting layers,14 electroluminescence (EL) measurements were performed comparing the two photoactive materials AnyIPbn and BAIPbn. In Figure 1c, it is possible to observe the response of the devices under different applied bias. It can be clearly appreciated that the EL AnyIPbn devices is practically two orders of magnitude higher than the obtained for BAIPbn based devices at the same applied voltage. The same effect can be observed for devices prepared with n=3, see Figure S2. However, in this case no EL signal was detected for BAIPbn. The analysis of samples with different n also provides interesting conclusions. The optical absorption spectra of each of the families studied show significant differences (Figure S3a, and S3b). While for BAIPbn family the spectra exhibit the corresponding progression of the excitonic band as we move from n = 1 to n=3,5 the AnyIPbn family show an impressive red shift for the band edge from n≥2, suggesting that the excitons are practically ionized into free carriers, even for films with n=2. In fact no shift of the EL at 772 nm is observed for AnyIPbn samples for n≥2, see Figure 1d, suggesting a common electronic state from which the radiative recombination is carried out.9, 15 That effect is really interesting, demonstrating that this systems combine both of the imperative requirements to be used as photovoltaic material, the good charge transport and high stability.8, 15 Recently this behavior was observed in BAIPbn family, but it was observed from n=3 to n=5.9 2D/3D perovskites evolves to a 3D behavior as n increases the use of conjugated molecules as anilinium produces a faster evolution allow to obtain significant efficiencies with relative low n, see Fig. 1a. In summary, we report by the first time the synthesis of AnyIPbn 2D/3D HPVK and its implementation on optoelectronic devices as solar cells and LEDs. The use of conjugated anilinium cation enhances significantly the performance of both kind of devices. The higher photoconversion and photoemission efficiencies observed, compared to the BAIPbn HPVK, can be attributed to an increase of the electronic transport properties of the material, as suggest the higher Jsc, without a deleterious effect on the non-radiative recombination, as point out the increase of EL and Voc. Concerning optoelectronic properties, the absence of emission or absorption shifts, obtained through UV-Vis spectroscopy and EL, suggest that the excitons, for n≥2 films are practically ionized into the free carriers. Consequently AnyIPbn is a good candidate in the fabrication of optoelectronic devices. Although 2D/3D HPVKs show higher stability than 3D counterparts, they present the important drawback of a lower performance due to the transport properties limitations. This work reflects the undoubted interest of the use of conjugated cations to improve the transport properties of 2D/3D HPVK and could

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have important implications in the field.

Acknowledgment. This study was supported by the University Jaume I (project UJI-B2016-35), by MINECO of Spain (project MAT2016-76892-C3-1-R) by Generalitat Valenciana (Project PROMETEOII/2014/020). J.R.R thanks to CONACyT-México for a postdoctoral fellowship. B.C.H is grateful to the support of the National Council of Technological and Scientific Development (CNPq), Brazil, through the Science without Borders program. Supporting Information Available: Experimental methods. Photovoltaic performance. XRD measurements. Electroluminescence of n=3 devices. Light Absorption measurements.

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