Letter Cite This: Nano Lett. 2019, 19, 150−157
pubs.acs.org/NanoLett
Bifunctional Organic Spacers for Formamidinium-Based Hybrid Dion−Jacobson Two-Dimensional Perovskite Solar Cells Yang Li,†,‡,§ Jovana V. Milic,́ *,† Amita Ummadisingu,† Ji-Youn Seo,† Jeong-Hyeok Im,† Hui-Seo Kim,∥ Yuhang Liu,† M. Ibrahim Dar,† Shaik M. Zakeeruddin,† Peng Wang,‡ Anders Hagfeldt,∥ and Michael Grätzel*,† Laboratory of Photonics and Interfaces. É cole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland Center for Chemistry of Novel & High-Performance Materials, Department of Chemistry, Zhejiang University, Hangzhou 310028, China § Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China ∥ Laboratory of Photomolecular Science. É cole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland †
Nano Lett. 2019.19:150-157. Downloaded from pubs.acs.org by UNIV OF NEW ENGLAND on 01/10/19. For personal use only.
‡
S Supporting Information *
ABSTRACT: Three-dimensional (3D) perovskite materials display remarkable potential in photovoltaics owing to their superior solar-to-electric power conversion efficiency, with current efforts focused on improving stability. Two-dimensional (2D) perovskite analogues feature greater stability toward environmental factors, such as moisture, owing to a hydrophobic organic cation that acts as a spacer between the inorganic layers, which offers a significant advantage over their comparatively less stable 3D analogues. Here, we demonstrate the first example of a formamidinium (FA) containing Dion−Jacobson 2D perovskite material characterized by the BFAn−1PbnI3n+1 formulation through employing a novel bifunctional organic spacer (B), namely 1,4-phenylenedimethanammonium (PDMA). We achieve remarkable efficiencies exceeding 7% for (PDMA)FA2Pb3I10 based 2D perovskite solar cells resisting degradation when exposed to humid ambient air, which opens up new avenues in the design of stable perovskite materials. KEYWORDS: Two-dimensional perovskites, layered perovskites, bifunctional spacers, molecular design
H
2D perovskites.11,16 The RP perovskites are the dominant class of 2D perovskites, featuring a monovalent (+1) organic spacer (S′), most commonly ammonium cation (RNH3+), in the S′2An−1MnX3n+1 composition that forms a double layer between the inorganic layers with an offset per unit cell (Figure 1a).10−12 On the other hand, the DJ perovskites comprise divalent (+2) organic spacers within the interlayer that require a single cation per unit of the SAn−1MnX3n+1 composition, which stack with either a perfect alignment or a minor displacement depending on the steric demands of the spacer cation (Figure 1b).16−18 This nature of the spacer cation is reflected in structural differences between the RP and DJ perovskites, the former featuring higher flexibility compared to the latter, rendering the inorganic stacks of the DJ perovskites more uniform and closer to each other.16 These characteristics in turn affect the optoelectronic properties and consequently, the photovoltaic performance of these categories of 2D perovskites. There has until now only been a single report on the DJ hybrid perovskite archetype, which was based on MA as the central organic (A) cation and (aminomethyl)-
ybrid organic−inorganic perovskite solar cells (PSCs) have reached remarkable photovoltaic (PV) performances with solar-to-electric power conversion efficiencies (PCE) exceeding 23%.1−4 The three-dimensional (3D) perovskite materials feature structures with a general formula AMX3 (A = monovalent central organic cation, such as Cs+, FA+ = formamidinium, MA+ = methylammonium, or GUA+ = guanidinium; M = divalent metal ion, commonly Pb2+, Sn2+; X = halide ion, I−, Br−, Cl−).3 Despite their outstanding performance, 3D perovskite solar cells still suffer from limited stability in certain environmental conditions, under oxygen and moisture in particular.5−7 This renders twodimensional (2D) hybrid perovskites an emerging class of thinfilm semiconductors that are particularly attractive due to their superior stability when compared to their 3D analogues.8−16 2D hybrid perovskites feature a general formula S′2An−1MnX3n+1 or SAn−1MnX3n+1 (S′ and S are mono and divalent organic spacer cations, respectively), which represents a layered structure of 3D perovskite slabs separated by the organic spacer layers (Figure 1).10,12,16 They can be classified based on the number of inorganic 3D perovskite layers (n = 1, 2, 3, 4, etc.). In addition, depending on the organic spacer group, two main archetypes are known, namely, the Ruddlesden−Popper (RP) and Dion−Jacobson (DJ) layered © 2018 American Chemical Society
Received: September 2, 2018 Revised: November 4, 2018 Published: December 12, 2018 150
DOI: 10.1021/acs.nanolett.8b03552 Nano Lett. 2019, 19, 150−157
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Structural Characteristics and Optoelectronic Properties of 2D-B DJ Perovskites. The unique structural features of DJ hybrid perovskites are predominantly determined by the organic interlayer spacer cation.16−18 Unlike the RP system, where the organic spacer consists of two layers of interacting monovalent cations (Figure 1a), the DJ phases comprise a single divalent interlayer cation between the 3D perovskite inorganic slabs (Figure 1b). This structural characteristic is expressed by the general formula SAn−1MnX3n+1 for the DJ phases. It results in fewer degrees of freedom for the DJ system with a stronger orbital overlap within the spacer layer, which is further altered depending on the steric demand of the spacer, its interaction with the inorganic slabs via hydrogen bonding, as well as interaction between the adjacent spacer moieties.16 We introduce here a novel bifunctional (B) spacer group, namely, 1,4-phenylenedimethanammonium (PDMA), which features an aromatic core equipped with two flexible methylammonium groups (Figure 2a). This molecular design Figure 1. Structural characteristics of two-dimensional layered perovskites. Schematic representation of the structure of (a) Ruddlesden−Popper (S′2An−1PbnI3n+1 composition with monovalent spacers S′) and (b) Dion−Jacobson phases (SA n−1 Pb n I 3n+1 composition with divalent spacers S) with different number of inorganic layers (n). Blue octahedra represent the {PbI64−} units, light blue spheres represent the central A cations, whereas the cyan rods represent the organic spacer groups (either S′ or S).
piperidinium organic spacer, which has already demonstrated promising performances comparable to the RP analogues.16 Although the PCE of the MA-containing n = 3 RP-based PSCs exceeds 11% (12.5% for n = 4 composition) by employing hotcasting techniques,10−12 the efficiencies for preparation at ambient temperature in absence of additives and antisolvents are only ∼4% for RP and ∼1% for DJ analogues (7.3% for n = 4 DJ composition).8−16 On the contrary, the efficiencies for FA-based 2D PSCs in the absence of any additional treatments remain only ∼1% even using the antisolvent-based method, whereas the highest reported efficiency is 6.88% upon treatment with thiourea.19,20 However, the stability of these systems remains underexplored,16,19,20 while the potentially more stable FA-based DJ analogues have not been reported. In order to overcome this “glass ceiling” of the performance of 2D perovskites and achieve long-term device stability, design of spacer groups is required at the molecular level, in conjunction with exploiting alternative 2D perovskite formulations. Herein, we report the first homologous series of Dion− Jacobson 2D perovskites that comprise FA as a central (A) cation within the 3D perovskite structure with the number (n) ranging from 1 to 4, which are linked via bifunctional (B) 1,4phenylenedimethanammonium (PDMA) in a BFAn−1PbnI3n+1 formulation. We demonstrate the effect of the spacer on the structural and optoelectronic properties of the perovskite films. We also explore the PV performance of different compositions in mesoscopic solar cell devices prepared by spin-coating of the precursor solutions in the absence of additives and antisolvents. As a result, we achieve PCEs exceeding 7% for (PDMA)FA2Pb3I10 formulations, which is to the best of our knowledge the highest value reported for single A cation FAbased 2D PSCs. This record performance is accompanied by long-term stability over the period of two months in ambient air with a relative humidity of 30−50%. This prototype highlights the potential of 2D layered DJ PSCs.
Figure 2. Structure and morphology of 2D-B perovskites. (a) Top: Schematic representation of the envisaged Dion−Jacobson structure of 2D-B perovskite materials with different BFA n−1 Pb n I 3n+1 compositions (n = 1−4). Bottom: Structural features of the compositional elements, specifically the FAPbI3 crystal structure representation (FA = formamidinium) and the bifunctional (B) 1,4phenylenedimethanammonium (PDMA) spacer group (DFT optimization of the spacer geometry and the corresponding NMR spectra are shown in Figures S1 and S2). (b) Plane-view scanning electron microscopy (SEM) images of perovskite films based on different BFAn−1PbnI3n+1 compositions (n = 1−4). Scale bar represents 1 μm. (c) Cross-sectional SEM image of the BFA2Pb3I10-based photovoltaic device of FTO/cp-TiO2/mp-TiO2/perovskite/spiro-OMeTAD/Au architecture. FTO = fluorine-doped tin oxide; cp = compact layer; mp = mesoporous layer; spiro-OMeTAD = 2,2′,7,7′-tetrakis(N,N-di4-methoxyphenylamino)-9,9′-spirobifluorene. 151
DOI: 10.1021/acs.nanolett.8b03552 Nano Lett. 2019, 19, 150−157
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Nano Letters was assumed to provide a rigid 2D backbone through π−π interactions within the hydrophobic spacer layer that would contribute to a higher crystallinity of the resulting perovskite material. In addition, the ammonium moieties are introduced to provide the sites for interaction with the inorganic slabs via hydrogen bonding. The ammonium groups are connected through flexible methylene linkers that feature higher level of adaptability to the geometrical mismatches in the overall structure to enable a more robust structural framework. The effect of the PDMA spacer group on the DJ perovskite structure was investigated in the (PDMA)FAn−1PbnI3n+1 formulation of various compositions defined by the number of layers (n, Figure 2a) that we further refer to as “2D-B perovskites”. The 3D perovskite slabs were based on the FA cations, which have been previously shown to exhibit superior stabilities compared to the MA analogues, presumably as a result of the resonance stabilization and lower basicity.19,20 Moreover, since 2D systems with n > 4 are conventionally referred to as quasi-2D perovskites, owing to their predominant 3D character,12 we focused on the lower compositional representatives (n = 1−4) throughout the study. The synthesis of the spacer and the preparation of the perovskite films are detailed within the Supporting Information. The perovskite films are prepared by spin-coating solutions comprising stoichiometric amounts of the corresponding precursors, namely, PDMAI2, PbI2, and FAI, onto the designated substrate (either microscopic glass/mp-Al2O3 or FTO/compact (cp)TiO2/mesoporous (mp)-TiO2, as specified in the corresponding figure captions and the Supporting Information). This was performed by using the antisolvent-free deposition and subsequent annealing before characterization. The structural differences of the films based on the (PDMA)FAn−1PbnI3n+1 formulation as a function of the number of inorganic layers (n) were directly reflected in their morphology, which was investigated by means of scanning electron microscopy (SEM). The top-view SEM images of films on FTO/cp-TiO2/mp-TiO2 substrates reveal their morphological differences (Figure 2b). Accordingly, the (PDMA)PbI4 (n = 1) system features large grain sizes up to >5 μm, whereas the (PDMA)FAPb2I7 (n = 2) analogue displays needle-like structures separated by numerous voids in the film. On the contrary, both (PDMA)FA2Pb3I10 (n = 3) and (PDMA)FA3Pb4I13 (n = 4) perovskite films are characterized by comparable island-like motifs covering the film surface (Figure 2b). Apart from the effect on the film morphology, the structural properties of 2D perovskites uniquely affect their optoelectronic properties, which were investigated in perovskite films, as well as the complete devices of conventional architecture based on FTO/cp-TiO 2 /mp-TiO 2 /perovskite/2,2′,7,7′tetrakis(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene (spiro-OMeTAD)/Au. The corresponding layers and their dimensions have been analyzed by cross-sectional SEM, showing that the thickness of the perovskite films is ∼200 nm (Figure 2c). In general, 2D layered perovskites are known to act as natural quantum wells, featuring an optical band gap trend with the energy (Eg) decreasing with the increasing number (n) of inorganic slabs from n = 1 to n = ∞ (FAPbI3).9,11,14 Accordingly, in the (PDMA)FAn−1PbnI3n+1 series, the band gap changes from 2.42 eV (n = 1) to 1.51 eV (n = ∞). This is directly manifested in the changes of the color of the corresponding films (Figure 3a), from yellow (n = 1), though red (n = 2) and dark brown (n = 3−4), to black (n
Figure 3. Optoelectronic properties of 2D-B perovskites. (a) Images of perovskite films and (b) the corresponding UV−vis absorption and steady-state photoluminescence (PL) spectra of BFAn−1PbnI3n+1 perovskite compositions (n = 1−4) on FTO/cp-TiO2/mp-TiO2 substrates. B = PDMA.
= ∞). In addition, the UV−vis absorptions spectra display exitonic features typical of 2D perovskites, which gradually disappear with an increase in the number of inorganic layers (Figure 3b).12,14,21 An analogous trend was observed in the steady-state photoluminescence (PL) spectroscopy measurements (Figure 3b), as the position of the emission signals features a gradual bathochromic shift with the increasing number of layers (n). Furthermore, the PL intensity was also dependent on the composition, as the n = 1 system displays the most intense PL signals that gradually decrease with an increase in n, which follows the trends observed with the RP phases.12,14,21 The optical properties of DJ perovskites were shown to differ from those of the RP phases in such a way that the PL emission energy is red-shifted by approximately 0.1 eV with respect to the RP analogues.16 The most common 2D RP perovskites, comprising of n-buthylammonium (BA) spacers, display a band gap of 1.68 eV estimated from PL measurements for (BA)2FA2Pb3I10 composition,19,20 which in case of the (PDMA)FA2Pb3I10 analogue decreases to 1.59 eV. In addition, the PL spectra of the n ≥ 2 systems exhibit large apparent Stokes shifts (>100 nm; Figure 3b), which can be rationalized from the perspective of the presence of the so-called “edge states” that are characteristic of the layered 2D perovskite materials, as reported by Blancon et al.14 Specifically, it has been shown that n > 2 compositions feature an inherent conversion of the photogenerated exciton to long-lived carriers at the edges of the layered perovskites, which are “protected” in these states from nonradiative recombination. This is of interest to photovoltaic or light-emissive applications, and the presence of these states can be revealed by the PL signals additional to those of the short-lived main exciton emission. They are more red-shifted and longer lived, displaying higher PL intensity, and are commonly associated with the layered “edge states” (LES).14 The steady-state PL spectra of both the n = 2 and n = 3 perovskite film compositions feature additional PL signals corresponding to the main exciton emissions and indicative of the presence of the LES (Figure S3). Moreover, time-resolved PL measurements suggest that the LES signals correspond to longer lifetimes as compared to the main emission (Figures S3, S4), similarly to those previously reported for phenylethylammonium 2D systems.22,23 In addition to the effect of LES on the properties of 2D layered perovskites, such as the large Stokes shift, it has been shown that presence of multiple phases in the films of n > 1 152
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tensity with the increasing number of inorganic slabs.10−12 This is an indication of the decrease in crystallinity in the film. In this regard, the low angle lattice reflections at ∼8° are most apparent for the n = 1 and n = 2 compositions, which can be associated with (002) and (020) lattice reflections, respectively, based on the literature reports.12,16 Presence of other phases in n ≥ 2 compositions, as indicated by UV-vis and PL spectra, as well as the use of mesoscopic substrate, are also likely to affect the resulting XRD patterns.24−30 Moreover, all the 2D perovskite films show a dominant (111) lattice reflection at 2θ ∼ 14° (Figure 4b). Using the full-width-at-half-maximum (fwhm) of the (020) and (202) low angle reflections between 6.5° and 8.5°, the crystallite size was estimated by means of the Scherrer equation.31 The crystallite size was found to decrease from 74 nm for the n = 1 systems to 71, 61, and 46 nm in the case of the n = 2, 3 and 4, respectively. These results indicate that the increasing number of inorganic layers decreases the crystallite size in this class of 2D perovskites. This complements our observations on the morphology of the films revealed by SEM, where larger grains were seen for n = 1. Moreover, the decrease in luminescence for n > 1 is in part related to the crystallinity of the corresponding films.12 Photovoltaic Performance of 2D-B DJ Perovskite Materials. The structural and optoelectronic properties of 2D perovskite materials particularly affect their prospective applications. While n = 1 and n = 2 compositions are known to adopt a preferentially parallel orientation (Figure 4c,d), the n > 2 compositions display a preference for perpendicular orientation with respect to the substrate (Figure 4e).12 This preference is in accordance with the manifestation of the edge states for the orientation of the perovskite layers normal to the substrate,14 and is directly related to the charge-extraction efficiency of interest to photovoltaic applications, typically more effective for n ≥ 3 compositions.12 The effects of the structure on the morphology and optoelectronic properties was probed in the mesoscopic devices of the FTO/cp-TiO2/mpTiO2/perovskite/spiro-OMeTAD/Au architecture with an active aperture area of 0.16 cm2 (0.4 cm × 0.4 cm). The J− V metrics of perovskite solar cells were recorded under standard AM1.5 illumination under a light intensity of 100 mW cm−2 (Figure 5a,b). The incident photon-to-electron conversion efficiency (IPCE) spectra and the integrated current density derived from the IPCE (JSCIPCE) of the devices comprising of various representatives of the (PDMA)FAn−1PbnI3n+1 series (n = 1−4) reveal the highest JSCIPCE throughout the spectrum for n = 3, 4 compositions, which is in accordance with the UV−vis absorption and the literature on the RP perovskite solar cells (Figure 5a).12 The IPCE spectra for n ≥ 2 feature an onset above 720 nm that is indicative of the formation of additional 3D phases in the films typical of quasi-2D perovskites. However, it is further evident that the JSCIPCE saturation occurs around 700 nm, which suggests that the JSCIPCE predominantly originates from the absorption in the spectral range below 700 nm, which exemplifies the contribution of the 2D perovskite to the current density. This is further reflected in the PV performance of the corresponding devices (Figure 5b), which display short-circuit current densities (JSC) of >11 mA cm−2 for n = 3−4 systems, whereas they are only ∼5 mA cm−2 for n = 2 and ∼2 mA cm−2 for n = 1 representatives. However, the open circuit voltage (VOC) follows the opposite trend, being larger for n = 1 (0.9 V) and n = 2 (>0.8 V) systems as compared to the n = 3−4 (∼0.8 V), which is in accordance
compositions plays a role in altering their performance through internal photoinduced charge separation and transfer, funneling excitons toward the lowest band gap state.24−30 Lower dimensional phases are apparent in the UV−vis absorption spectra of n = 2−4 compositions, as well as in the steady state PL spectra (Figure S3), in addition to the predominant ones defined by the (PDMA)FAn−1MnX3n+1 stoichiometry, indicating possible contribution of such interphase charge transfer cascade processes as well. Moreover, the contribution of other lower bandgap phases cannot be ruled out. The origin of these effects of the structure on the properties of the (PDMA)FAn−1MnX3n+1 films was further investigated by means of X-ray diffraction (XRD). The reflections of these films were indexed assuming an analogy to the MA-based DJ perovskites, whose XRD spectra have been reported and indexed by Mao et al.16 The diffractograms of all the perovskite films on FTO/cp-TiO2/mp-TiO2 substrates reveal low angle reflections below 10° characteristic of the 2D perovskite materials (Figure 4a), which gradually decrease in relative in-
Figure 4. Structural properties of 2D-B perovskites. (a) X-ray diffraction (XRD) patterns on FTO/cp-TiO2/mp-TiO2 substrate for films with different (PDMA)FAn−1PbnI3n+1 perovskite composition (n = 1, 2, 3, and 4) with indexes of the corresponding planes based on the literature reports.12,16 Dashed lines indicate the signals from the substrate. (b) XRD pattern in the area of 6.5−8.5° angle range corresponding to the (002) and (020) reflections employed to estimate the crystallite size based on the Scherrer equation (a.u., arbitrary units). Schematic representation of the crystal structure packing of the films with different BFAn−1PbnI3n+1 perovskite compositions based on the analogy with other reported 2D layered perovskites12,16 (n): (c) n = 1, (d) n = 2, and (e) n = 3 (the n = 4 system is envisaged to feature comparable properties to n = 3). B = PDMA. 153
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Figure 5. Photovoltaic characteristics of the 2D-B layered perovskites. (a) Incident photon to current efficiency (IPCE) spectra with short circuit photocurrent density (dashed lines) derived by integrating the IPCE over the standard AM1.5 solar emission. The vertical dashed line indicates the approximate current density saturation point for n > 2 compositions. (b) Current density−voltage (J−V) curves for FTO/cp-TiO2/mp-TiO2/ perovskite/spiro-OMeTAD/Au devices featuring perovskite films based on (PDMA)FAn−1PbnI3n+1 perovskite composition (n = 1−4). (c) J−V curves of the champion cell in forward (from JSC to VOC) and reverse (from VOC to JSC) scanning direction under standard AM1.5 solar radiation. The photovoltaic metrics derived from the two J−V curves are shown in the inset. (d) Power conversion efficiency (PCE) statistics of solar cells based on (PDMA)FA2Pb3I10 perovskite composition. J−V curves are recorded at a scanning rate of 50 mV s−1 in reverse and forward direction under standard AM1.5 solar radiation, unless stated otherwise. PV metrics are shown in the Supporting Information.
Figure 6. Resilience of unsealed 2D-B layered perovskites to ambient air. (a) Evolution of photovoltaic metrics over time of unsealed PSCs of aperture area of 0.16 cm2 based on (PDMA)FA2Pb3I10 (n = 3) perovskite films under full solar intensity upon storage under dark condition at ambient air (∼30−50% humidity) and ambient temperature (for operational stability refer to the Supporting information, Figure S10). Contact angle measurements of the (b) FAPbI3 and (c) (PDMA)FA2Pb3I10 perovskite films with a water droplet.
the film thickness, concentration (Figure S5, and Tables S1, S2), solvent engineering (Figure S6, Table S3), and the annealing temperature (Figure S7, Table S4; for more details, refer to the Supporting Information). The champion device displays a JSC of 11.49 mA cm−2, VOC of 0.844 V, and FF of 0.72, with an average PCE of 6.85% and 7.11% in the reverse scan direction (Figure 5c; the maximum power point tracking
with the gradual decrease of the band gap with the increasing number of layers. We have therefore focused on the photovoltaic performance of (PDMA)FA2Pb3I10 perovskite films in a conventional mesoscopic FTO/cp-TiO2/mp-TiO2/ perovskite/spiro-OMeTAD/Au device architecture with an active aperture area of 0.16 cm2 (0.4 cm × 0.4 cm). The film preparation conditions were optimized from the perspective of 154
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dinium (FA) central cation formulations and 1,4-phenylenedimethanammonium (PDMA) as a bifunctional spacer group. This design gave remarkable efficiencies for FA-based 2D perovskite solar cells exceeding 7%, accompanied by excellent stability in humid ambient air, which holds promise for perovskite solar cell research in the future. Further optimization of the materials and devices is therefore a subject of an ongoing investigation.
data is shown in Figure S9). The ideality factor was found to be 1.45, which is comparable to conventional PSC devices (Figure S8). The incident photon-to-electron conversion efficiency (IPCE) spectrum of the champion PSC (shown in Figures 5a and S9) reveals the integrated current density derived from the IPCE spectrum of 11.24 mA cm−2, in good agreement with the JSC value obtained from the J−V curves, which excludes any notable spectral mismatch between our simulator and the AM1.5 solar source. The PV metrics for over 40 devices (Figure 5d) shows that the average PCE is around 6% with reproducibility of the PSC preparation approach. In order to identify the underlying reasons for such performance, we measured charge carrier mobilities for different perovskite compositions (for details, see the Supporting Information, Figure S11−S14). The electron mobilities were found to be 4.02 × 10−3 cm2 V−1 s−1 for n = 3 and as high as 8.76 × 10−3 cm2 V−1 s−1 for n = 1 representatives (Table S5), presumably as a result of their better orientation and crystallinity as suggested by the XRD analysis. Remarkably, these values were comparable to those reported for the hot-casting deposition method (11 × 10−3 cm2 V−1 s−1),10 despite using room temperature deposition. Moreover, the mobilities were superior to those reported in the most recent recordperforming MA-based 2D perovskites (1.76 × 10−3 cm2 V−1 s−1).32 We thereby ascribe the photovoltaic performance to the enhanced charge carrier mobility of the layered perovskites as a result of the spacer layer in conjunction with the trends in the band gap of the material. The unique role of the spacer in the applied device architecture should be further investigated. Resilience of 2D-B DJ Perovskite Materials to Ambient Air. The resilience of photovoltaic performance of this new class of 2D PSCs to ambient air was scrutinized by measuring the stability under humid ambient air conditions, as shown by the evolution of photovoltaic metrics over time (Figure 6). Environmental degradation of the perovskite solar cells is commonly associated with the decomposition of the perovskite layer by exposure to moisture.5−7 In this regard, hydrophobicity of the films is an important criterion in longterm stability toward environmental factors. In order to probe the hydrophobicity of the best PV performing (PDMA)FA2Pb3I10 (n = 3) perovskite, we measured the contact angles of the corresponding films using a water droplet. The angle of the droplet on the perovskite film indicates decreased wetting compared to the pristine FAPbI3 films (Figure 6b), which suggests increased hydrophobicity of the 2D perovskite surface (Figure 6c). The shelf life and resilience to ambient air was evaluated by monitoring the photovoltaic performance metrics upon storage at ambient air with relative humidity of 30−50% (Figure 6a). Remarkably, the unencapsulated 2D perovskites maintained >85% of the initial PCE after 60 days, suffering only a minor decrease in JSC and VOC after 10 and 40 days of exposure to humid ambient air, respectively, whereas the PV performance remained nearly unaltered in dry air (Figure S10). Moreover, even under accelerated aging conditions, the perovskite solar cells show >60% retention of the efficiency after 500 h, as demonstrated via maximum power point tracking at 60 °C under inert nitrogen atmosphere (Figure S10). This highlights the potential of the bifunctional aromatic spacers within novel 2D Dion−Jacobson formulations to improve both the performance and the stability of perovskite solar cells. In summary, we present the first example of 2D hybrid Dion−Jacobson perovskite solar cells based on the formami-
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.8b03552. Materials and methods, solar cell preparation, device characterization, DFT calculations, NMR spectra, timeresolved photoluminescence, photovoltaic performance optimization and metrics, UV−vis and PV metrics upon long-term aging, and mobility measurements (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: jovana.milic@epfl.ch. *E-mail: michael.graetzel@epfl.ch. ORCID
Jovana V. Milić: 0000-0002-9965-3460 Hui-Seo Kim: 0000-0002-9928-3033 M. Ibrahim Dar: 0000-0001-9489-8365 Peng Wang: 0000-0002-6018-1515 Michael Grätzel: 0000-0002-0068-0195 Author Contributions
The manuscript was written by J.V.M. and Y. Li through contributions of all authors. The study was conceptualized by J.V.M., who designed and characterized the organic spacer, identified perovskite compositions, performed the preliminary device fabrication and characterization, and coordinated the study. Y. Li optimized the device fabrication and performed the device characterization, stability studies, as well as the UV−vis absorption and steady-state PL measurements. Y. Liu performed the protonation to obtain the spacer precursor. J.V.M. and J.-H.I. performed the plane-view SEM, A.U. the XRD and TRPL, and J.-Y.S. the cross-sectional SEM and contact angle measurements. H.-S.K. and A.H. conducted the charge mobility measurements. S.M.Z., M.I.D., and P.W. were involved in the discussions throughout the study. S.M.Z. was involved in the project coordination, and M.G. supervised the project. All authors discussed the results, contributed to the study, and revised the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS J.V.M, M.I.D., S.M.Z., and M.G. are grateful to the European Union’s Horizon 2020 research and innovation program under Grant Agreement No. 687008 (GOTSolar). J.-H.I., M.G., and S.M.Z thank the King Abdulaziz City for Science and Technology (KACST), the SNSF NRP70 “Energy Turnaround”, and the SNSF for a joint research project (IZLRZ2_164061) under Scientific & Technological Cooperation Programme Switzerland-Russia. H.-S.K acknowledges the financial support from the GRAPHENE Flagship Core 2 155
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Letter
Nano Letters
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project supported by the European Commission H2020 Programme under contract 785219. P.W acknowledges the financial support from the National 973 Program (2015CB932204) and the National Science Foundation of China (No. 91733302). Authors are grateful to Lichen Zhao (EPFL) for discussions in a preliminary investigation, Linfeng Pan (EPFL) for help on the XRD measurement, and Algirdas Dučinskas (EPFL) for helpful discussions on structural representations.
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DOI: 10.1021/acs.nanolett.8b03552 Nano Lett. 2019, 19, 150−157
