Composition Engineering in Two-Dimensional Pb-Sn Alloyed

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Composition Engineering in Two-Dimensional Pb-Sn Alloyed Perovskites for Efficient and Stable Solar Cells Yani Chen, Yong Sun, Jiajun Peng, Pavel Chabera, Aliriza Honarfar, Kaibo Zheng, and Ziqi Liang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06256 • Publication Date (Web): 05 Jun 2018 Downloaded from http://pubs.acs.org on June 5, 2018

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ACS Applied Materials & Interfaces

Composition Engineering in Two-Dimensional Pb-Sn Alloyed Perovskites for Efficient and Stable Solar Cells Yani Chen,† Yong Sun,† Jiajun Peng,† Pavel Chábera,‡ Aliriza Honarfar,‡ Kaibo Zheng,‡,§,* and Ziqi Liang†,* †

Department of Materials Science, Fudan University, Shanghai 200433, China Department of Chemical Physics and NanoLund, Lund University, Box 124, 22100, Lund, Sweden § Department of Chemistry, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark ‡

ABSTRACT: Environmentally friendly tin (Sn)-based metallic halide perovskites suffer from oxidation and morphological issues. Here we demonstrate the composition engineering of Pb-Sn alloyed two-dimensional (2D) Ruddlesden–Popper perovskites, (BA)2(MA)3Pb4−xSnxI13, for efficient and stable solar cell applications. Smooth thin films with high surface coverage are readily formed without using any additive owing to the self-assembly characteristic of 2D perovskites. It is found that Sn plays a significant role in improving the crystallization and crystal orientation while narrowing the bandgap of Pb–Sn 2D perovskites. Photophysical studies further reveal that the optimal Sn ratio (25 mol%) based sample exhibits both minimized trap density and weakened quantum confinement for efficient charge separation. Consequently, the optimized (BA)2(MA)3Pb3SnI13 based solar cells yield the best power conversion efficiency close to 6% with suppressed hysteresis. KEYWORDS organic–inorganic hybrid perovskites, two-dimensional, tin-based perovskites, composition engineering, planar solar cells

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Owing to their superior ambient stability, organic‒inorganic lead halide perovskite solar cells are excellent candidates for solution-processable thin-film solar energy conversion owing to the impressively boosted power conversion efficiencies (PCEs) from 3.8% to 22.1% in the past several years.1‒6 However, the environmental toxicity of lead severely limits the further commercialization. Thus, the development of eco-friendly lead-free or -less alternatives is highly imperative. For instance, germanium(II) (Ge2+),7 tin(II) (Sn2+),8,9 antimony(III) (Sb3+),10 and bismuth(III) (Bi3+)11 have been employed to replace Pb2+ completely or partially. Among them, Sn-based perovskites gave rise to narrower optical bandgaps (Egs), which have thus far achieved reasonably the best device performance. Noticeably, in 2014, CH3NH3SnI3 perovskite solar cells demonstrated the PCEs of 5‒6%.9,12 Following those work, a variety of strategies such as the addition of SnF213 and pyridine14 have been attempted to further improve the cell performance, which was however not successful. The critical challenge of retarding the device improvement lies mainly in the facile oxidization of Sn2+ to Sn4+, which causes detrimental p-type self-doping and decreases the diffusion lengths of photoexcited carriers.12 Another limiting factor is the uncontrollable film morphology of Sn-based perovskites owing to the fast crystallization between SnI2 and MAI.15,16 The emerging two-dimensional (2D) Ruddlesden–Popper perovskites have been demonstrate to exhibit appealing environmental stability owing to the hydrophobic nature of their insulating organic spacers,17−19 which prevents the penetration of water and oxygen molecules and might help resolve the oxidization issue of Sn-perovskites. 2 ACS Paragon Plus Environment

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Moreover, they possess good capability of self-assembly, which ensures the formation of high-quality film without the need of any additive.20 However, the insulating organic spacers also lead to the unfavorable large Eg for Pb-based 2D perovskites. The inclusion of Sn can resolve this issue as demonstrated in the case of 3D perovskites.9,12−14 Yet, to the best of our knowledge, there only exists a few reports on Sn-based 2D perovskites. For example, Adachi et al. successfully applied (C6H5C2H4NH3)2SnI4 to field-effect transistors, which exhibited a record hole mobility of 15 cm2 V−1 s−1 at room temperature.21 Very recently, Kanatzidis et al. reported two series of new Sn-based 2D perovskites, (HA)Pb1−xSnxI4 and (BZA)2Pb1−xSnxI4,

where

HA

and

BZA

stand

for

histammonium

and

benzylammonium cations, respectively.22 These two systems displayed a varying trend of Egs with an increasing Sn fraction. Lately, the same group for the first time employed (BA)2(MA)n−1SnnI3n+1 as the photoactive layer in solar cells. By using triethylphosphine additive, the oxidation of Sn2+ was suppressed and a PCE of 2.5% was obtained for those of n = 4.23 However, this performance is still poor as compared to Pb based counterparts. In

this

work,

we

develop

Pb-

and

Sn-alloyed

2D

perovskites,

(BA)2(MA)3Pb4−xSnxI13, for planar solar cell applications. On the one hand, the self-assembly characteristic of 2D perovskites give rise to the formation of high-quality (BA)2(MA)3Pb4−xSnxI13 films without the aid of any additive. On the other hand, the addition of Sn diminishes the use of Pb to some extent, which is more environment-friendly. Furthermore, it greatly reduces Eg, enhances light absorption, 3 ACS Paragon Plus Environment


and improves charge transport when comparing to analogous Pb-based 2D perovskites. Therefore, such alloyed Pb-Sn devices afford the best PCE of 5.96% when the molar ratio of Pb/Sn = 3:1. Five sets of (BA)2(MA)3Pb4−xSnxI13 2D perovskites were obtained by one-step spin-coating method. The precursor solutions were prepared by mixing PbI2, SnI2, C4H9NH2, HI and CH3NH3I at a stoichiometric ratio of 4−x : x : 2 : 2 : 3 (x = 0, 1, 2, 3, 4) in dimethyl formamide (DMF), which corresponds to Pb/Sn = 4:0, 3:1, 2:2, 1:3, 0:4, respectively. We first conducted the XRD measurement to study the effect of Sn substitution on crystal phases of these 2D perovskites. The initial (BA)2(MA)3Pb4I13 perovskite displays exclusively two weak diffraction peaks at 14.35° and 28.51°, which are assigned to characteristic (111) and (202) lattice planes of 2D perovskites, respectively (see Figure S1 in Supporting Information, SI).18 As shown in Figure 1a, when Pb is partially replaced by Sn, the peak intensity is remarkably increased, implying that the insertion of Sn can siginificacntly enhance the crystallization of 2D perovskites. The corresponding spectral expansions in the diffraction regions of 14.5– 15.0° and 27.2–29.2° are shown in Figure 1b.


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Figure 1. (a) XRD patterns of (BA)2(MA)3Pb4−xSnxI13 2D perovskites and (b) their magnified profiles in the diffraction regions of 14.5–15.0º and 27.2–29.2º, respectively. As the Sn ratio increases, both (111) and (202) planes are gradually up-shifted towards high angles. This indicates the conversion of Pb-based perovskites to Sn-containing counterparts where the parameters of unit cells become smaller, which is in good accordance with previous report.22 In particular, two additional peaks around 13.8 and 27.7º appear when the Sn fraction is increased up to 75 and 100%, suggesting the formation of second phases, which may be ascribed to the oxidation of Sn2+ to Sn4+. Figure 2 compares the optical absorption spectra of these alloyed Sn−Pb 2D perovskites(BA)2(MA)3Pb4−xSnxI13 and their corresponding Egs are displayed in Figure S2. With increasing Sn proportions, the absorption spectra continuously 5 ACS Paragon Plus Environment


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red-shift. As a result, the decreasing Eg values are estimated for (BA)2(MA)3Pb4I13, (BA)2(MA)3Pb3SnI13,

(BA)2(MA)3Pb2Sn2I13,

(BA)2(MA)3PbSn3I13,

and

(BA)2(MA)3Sn4I13 to be 2.09, 2.01, 1.79, 1.77 and 1.67 eV, respectively.

Figure 2. UV-vis absorption spectra of (BA)2(MA)3Pb4−xSnxI13 2D perovskites. We then examined the film morphology by using field-emission scanning electron microscopy (FE-SEM). It can be seen that the (BA)2(MA)3Pb4I13 film is very smooth (Figure 3a), while an incorporation of Sn makes the film become textured due to the increasing crystallization. At the Sn fraction of 25 mol%, the film consists of crystal grains with an average size of 150 nm (Figure 3b). When the Pb/Sn = 2:2 mol%, the film gets coarse and rectangular grains show up (Figure 3c). As the Sn content continues to increase, the crystals seem to merge with each other, accompanied by pin-holes, which are detrimental to device performance (Figure 3d,e). Subsequently, we performed grazing-incidence wide-angle X-ray scattering (GIWAXS) measurement. As shown in Figure 2f, the neat Pb based perovskites display continuous rings while the Sn−Pb analogues exhibit discrete spots (Figure 2g‒


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j). This indicates a significant improvement of both crystallization and crystal orientation upon the inclusion of Sn, which is consistent with XRD results.

Figure 3. (a‒e) Top-view SEM images of (BA)2(MA)3Pb4−xSnxI13 perovskites, and (f‒ j) their corresponding GIWAXS images. Given these favorable optical and crystalline characteristics, we fabricated planar solar

cells

with

a

normal

device

structure

of

ITO/PEDOT:PSS/(BA)2(MA)3Pb4−xSnxI13/PCBM/Bphen/Al as schematically shown in Figure 4a. Figure 4b presents the current density versus voltage (J‒V) curves of (BA)2(MA)3Pb4−xSnxI13based planar photovoltaic cells measured under illumination by a calibrated AM 1.5G solar simulator at reverse scan. The detailed photovoltaic parameters of these four group devices are summarized in Table 1. The device based on neat Pb-based 2D perovskite counterpart exhibits a PCE of 3.61%. As the Sn content is increased, the open-circuit voltage (VOC) of the cells is gradually decreased, which can be attributed to the reduced Eg of Sn-based 2D perovskites as indicated from their absorption spectra. Meanwhile, the short-circuit current (JSC) first increases when Pb/Sn = 3:1 and then decreases when the Sn content is larger than 25%. The increased Jsc in (BA)2(MA)3Pb3SnI13 based devices is attributable to the enhanced light absorption, while the decreased JSC for other Sn-based samples originate 7 ACS Paragon Plus Environment


presumably from those traps generated by second phases as shown in XRD patterns. As a result, the optimal (BA)2(MA)3Pb3SnI13 based device yields the best PCE of 5.96% with large VOC = 0.80 V, JSC = 12.05 mA cm‒2 and FF = 61.81%. Figure 4c displays the external quantum efficiency (EQE) profiles of (BA)2(MA)3Pb4−xSnxI13 based devices. Their integrated current densities are 4.50, 8.13, 4.68, 1.81 mA cm−2 for x = 0, 1, 2, 3, respectively, which are in accordance with the Jsc varying trend despite that the values are comparatively lower. For the neat Sn-based device, the EQE is yet not detectable owing to its low phonon-to-electron conversion efficiency. Finally, device stabilities of these devices are evaluated with encapsula-tion in N2 atmosphere as shown in Figure S3. It turns out that the (BA)2(MA)3Pb3SnI13 based device displays the best stability, which preserves 90% of its original PCE after 450 h storage.


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Figure 4. (a) Schematic of ITO/PEDOT:PSS/(BA)2(MA)3Pb4−xSnxI13/PCBM/Bphen/Al cells. (b) Representative J‒V characteristics and (c) EQE profiles of (BA)2(MA)3Pb4−xSnxI13 based regular solar cells under light irradiation of 100 mW/cm2 at reverse scan. (d) Light intensity dependences of JSC of the devices with Pb:Sn = 4:0 (dark) and 3:1 (red). The lines are linearly fitted to the data. Interestingly, the hysteresis phenomenon becomes alleviated as the Sn fraction is larger (Figure S4). Therefore, we investigated how the Sn fraction influences charge transport dynamics of these above perovskite solar cells. First, the dependence of JSC on light intensity was measured to probe charge recombination behavior in devices. In principle, JSC follows a power-law dependence on light intensity, JSC ∝ Pinα where α is the power-law exponent.24 As shown in Figure 4d, α of (BA)2(MA)3Pb4I13 and (BA)2(MA)3Pb3SnI13 based solar cells are 0.67 and 0.75, respectively, suggesting that an introduction of Sn can slightly suppress bimolecular recombination, which is similar to the role of FA in the (BA)2(MA1−xFAx)3Pb4I13 system.25

Table 1. Summary of Photovoltaic Parameters of (BA)2(MA)3Pb4−xSnxI13 Perovskites Based Planar Solar Cells (BA)2(MA)3Pb4−xSnxI13 Pb:Sn = 4:0 Pb:Sn = 3:1 Pb:Sn = 2:2 Pb:Sn = 1:3 Pb:Sn = 0:4

VOC (V)

FF (%)

PCE (%)

Reverse Forward Reverse

JSC (mA/ cm2) 6.24 5.77 12.05

1.06 0.90 0.80

54.66 45.28 61.81

3.61 2.35 5.96

Forward Reverse Forward Reverse Forward Reverse Forward

10.32 6.07 5.18 1.97 1.64 0.47 0.45

0.64 0.54 0.50 0.36 0.36 0.24 0.26

53.97 59.35 48.92 48.39 48.19 46.46 47.54

3.56 1.95 1.27 0.34 0.28 0.05 0.06

Scanning direction



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Second, we performed charge extraction by a linearly increasing voltage (CELIV) measurement. As shown in Figure S5 and Table S1, the calculated overall carrier mobilities of these perovskites-based devices all fall within the range of ~10−4 cm2 V−1 s−1. The charge mobility in perovskites devices is principally dominated by the interaction between charge carriers and interfacial structure imperfections, for instance, defects at the interface between the perovskite layer and the electrodes or HTMs, etc.26. Thus, we can conclude that such interaction in this work is insensitive to the varied Sn ratio. The mobility value is also on the same order as that in 3D perovskites devices27, implying that the oxidized Sn4+ species and insulating organic spacers of 2D Pb-Sn alloyed perovskites would not noticeably influence the charge carrier transport in devices. In order to acquire in-depth insight into intrinsic charge carrier dynamics, both transient

absorption

(TA)

and

time-resolved

photoluminescence

(TRPL)

measurements on neat films were conducted. Figure 5a−e show the time evolution of TA spectra for 2D perovskites with various Pb:Sn ratios. They are constituted of multi-bleach bands, which corresponds to the state filling of 2D quantum wells with different thickness of inorganic layer.28 When the Sn faction is more than 50%, the majority of charge carriers tend to reside at the thinner quantum well phase (smaller n) as indicated by the negative bleach bands in Figure 5c−e. Consequently, the larger exciton binding energy in thin quantum wells would hinder charge dissociation. This could be one of the origins for the low photocurrent in the devices with larger Sn content. We then performed the singular value decomposition (SVD) fitting of the TA 10 ACS Paragon Plus Environment

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spectra to obtain the excited state lifetime as summarized in Figure 5f−j. In general, the TA spectra can all be fitted by one fast (3−31 ps) component and the other slow (0.1−12 ns). The fast component can be attributed to the bi-exciton recombination (i.e., bimolecular recombination, see Figure S6 for details). The relative amplitude of the fast component over the slow one first decreases and then increases with increasing Sn ratio, where the lowest value occurs at the Pb:Sn ratio of 3:1. This confirms the above argumentation from Figure 4d that the bimolecular recombination can be surpassed by the 25% Sn addition.

Figure 5 (a−e) TA spectra at selected delay time after light excitation for (BA)2(MA)3Pb4−xSnxI13 perovskites with (f−j) corresponding SVD global fitting results.



To assign the slow component in TA dynamics, we also performed the TRPL measurement of identical samples and the results are shown in Figure 6. The PL decays can be well fitted by bi-exponential functions as summarized in Table S2. We found that the lifetimes of dominate slow component in TRPL is consistent with the slow components extracted from TA in Figure 5f−j. This indicates the slow component in TA can be assigned as first order recombination of singlet excitons. Such single exciton state lifetimes exhibit the consistent dependence on Sn ratio with that of device performance as shown in Figure 5f−j, where the sample with Pb:Sn = 3:1 shows the longest lifetime of 12 ns. This indicates the trap densities within the lattice is minimized.

Figure 6. TRPL decays of (BA)2(MA)3Pb4−xSnxI13 perovskites. In Pb-Sn mixed 3D perovskites based solar cells, the device performance is concurrently affected by the Sn addition through three ways: 1) the lattice distortion that induces trap states, 2) Eg that is tunable by competition of spin– orbit coupling between Pb2+ and Sn2+ ions, and 3) charge carrier mobilities that are modulated by carrier-lattice interaction.29 In 2D perovskites, as the quantum 12 ACS Paragon Plus Environment

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confinement of 2D perovskites restrict the up-limit of carrier mobilities in the range of 1−2 cm2/Vs,30 there is little room for the Sn substitution to tune the intrinsic carrier transport capability by changing the carrier-lattice interaction. On the other hand, the Sn addition greatly dominates the assembly of 2D quantum wells and affects the n value. Although the underlying synthetic mechanism remains to be discovered, the Sn addition below 50 mol% would formulate the majority 2D phases with the maximum quantum well thickness (n = 3 or 4), which facilitates exciton dissociation as well as charge transport. Moreover, the trap densities within 2D perovskites are dominated by the Sn addition. As the structural distortion in Sn-based perovskites is generally less than that in Pb-based analogues,31,32 the initial Sn addition is expected to diminish the lattice deficiency. However, the rearrangement of 2D quantum wells induced by further Sn addition would increase trap densities. As a result, the trap density of the optimal Sn ratio (25 mol%) based sample with the best solar cell performance is much less than that in 3D perovskites counterparts (60~80 mol%).29 This can also account for the improved stability for the (BA)2(MA)3Pb3Sn1I13 based device. To conclude, we have demonstrated the solar cell applications of Pb-Sn alloyed

2D

perovskites.

As

compared

to

neat

Pb

counterpart,

(BA)2(MA)3Pb4−xSnxI13 2D perovskites exhibit significantly enhanced optical absorption, crystallization and crystal orientation. Moreover, the film morphology is notably improved in comparison to traditional Sn-based 3D 13 ACS Paragon Plus Environment


perovskites, which can be ascribed to the self-assembly characteristic of 2D perovskites. The composition-optimal (BA)2(MA)3Pb3SnI13 based regular planar solar cell yields the best PCE up to ~6%. Importantly, time-resolved spectroscopical studies reveal that the lattice mismatch in optimized devices has been minimized with less trap centers and longer charge carrier lifetime, while the thickness of the quantum wells is maximized with more efficient charge seperation and transportation. This work exemplifies an effective avenue towards efficient, stable and eco-friendly 2D Pb-less perovskite solar cells.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.xxxxxxx. Experimental details, XRD patterns, stability data, hysteresis measurement, CELIV profiles, TRPL spectra and detailed TA kinetics (PDF). AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Z.L.). *E-mail: [email protected] (K.Z.). Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS This work is supported by Inter-Governmental International Cooperation Project of Science and Technology Commission of Shanghai Municipality (STCSM) under grant No. 17520710100 (Z.L.). K.Z. acknowledges the support from Danish Council for Independent Research No. 7026-0037B and Swedish Research Council No. 2017-05337. REFERENCES 14 ACS Paragon Plus Environment

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Composition Engineering in Two-Dimensional Pb-Sn Alloyed

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