Journal ofMaterials Chemistry A

PAPER

Boosting the per

aSchool of Chemical and Biological Engineer

National University, 1, Gwanak-ro, Gwan

E-mail: joonhoh@snu.ac.krbDepartment of Chemical Engineering, Poha

(POSTECH), Pohang, Gyeongbuk 37673, RepcDepartment of Chemistry, Pohang Universit

Pohang, Gyeongbuk 37673, Republic of Kore

† Electronic supplementary informationS1–S3. See DOI: 10.1039/c8ta05916k

Cite this: J. Mater. Chem. A, 2018, 6,18173

Received 20th June 2018Accepted 31st August 2018

DOI: 10.1039/c8ta05916k

rsc.li/materials-a

This journal is © The Royal Society of C

formance and stability of quasi-two-dimensional tin-based perovskite solar cellsusing the formamidinium thiocyanate additive†

Hongki Kim,ab Yoon Ho Lee,ab Taecheon Lyu,c Jong Heun Yoo,ab Taiho Park b

and Joon Hak Oh *a

The poor oxidative stability of tin-based perovskites has been an obstacle to their widespread

implementation in high-performance solar cells. Herein, quasi-two-dimensional (quasi-2D) tin-based

perovskite solar cells are fabricated with significantly improved performance and stability, by introducing

an additional formamidinium thiocyanate (FASCN) additive into quasi-2D tin-based perovskites. The

incorporation of the FASCN additive greatly prevents quasi-2D tin-based perovskites from oxidation

during film formation, through strong chemical interactions with the tin component (Sn2+). Moreover, it

results in a coarser perovskite grain and a higher degree of crystallinity in the out-of-plane direction,

leading to enhanced optoelectronic performance of quasi-2D tin-based perovskites. The best-performing

tin-based perovskite solar cell shows an efficiency of 8.17% under reverse scan with a steady-state

efficiency of 7.84% at a maximum power point (MPP), while retaining over 90% of its initial efficiency after

1000 hours in a glovebox filled with nitrogen. These results demonstrate a versatile, yet simple

methodology that can be applied to other lead-free perovskites suffering from poor oxidative stability.

Introduction

Organic–inorganic hybrid lead-based perovskite solar cells haveshown tremendous growth in power conversion efficiency (PCE)over recent years,1–4 due to their outstanding optoelectronicproperties such as high absorption coefficient, long diffusionlength, and low defect density.5–10 Despite remarkable strides inperformance, one of the biggest barriers to their commerciali-zation is the toxicity of lead. In this regard, efforts to developlead-free perovskite solar cells have been made by utilizing IVAgroup ions such as Sn2+ and Ge2+ or VA group ions such as Bi3+

and Sb3+, or by implementing halide double perovskites.11–14

Among these candidates, tin-based perovskites are beingregarded as promising alternatives due to their lower toxicityand excellent optoelectronic properties.11,15 Moreover, theirnarrower optical band gap (1.2–1.4 eV) is closer to the optimumvalue represented by the Shockley–Queisser limit (1.34 eV),making tin-based perovskites more ideal light absorbers forsolar cells and other optoelectronic devices, such as

ing, Institute of Chemical Processes, Seoul

ak-gu, Seoul 08826, Republic of Korea.

ng University of Science and Technology

ublic of Korea

y of Science and Technology (POSTECH),

a

(ESI) available: Fig. S1–S19 and Tables

hemistry 2018

photodetectors and light-emitting diodes operating in the nearinfrared region.16–18 Nevertheless, tin-based solar cells have notyet attained performance levels comparable to those of lead-based perovskite solar cells because of the strong oxidativetendency of Sn2+ to form Sn4+, which induces an unintentionalself-p-doping in tin-based perovskites, leading to the shorting ofsolar cell devices even in a nitrogen-lled glovebox. Therefore,many efforts have been devoted to reducing the extent of tinoxidation.19–36

Recently, several efficient approaches have been explored toimprove the device performance of tin-based perovskite solarcells, i.e., (1) implementation of mixed halide or cation tin-based perovskites,22,23 (2) introduction of a sequential deposi-tion route,24 and (3) fabrication of quasi-two-dimensional(quasi-2D) or hollow-structured tin-based perovskites.27–32 Inparticular, remarkable improvements in performance andstability have been achieved through the structural modica-tion of three-dimensional (3D) into quasi-2D tin-based perov-skite structures by introducing hydrophobic long alkyl chaincations.29–31,33 G. Kanatzidis and co-workers rstly reportedquasi-2D tin-based perovskite solar cells by fabricating 2DRuddlesden–Popper (CH3(CH2)3NH3)2(CH3NH3)n�1SnnI3n+1perovskites, where n is the number of the tin iodide octahedralstacks conned by long organic spacers. The fabricated quasi-2D Sn3I10 (n ¼ 3) and Sn4I13 (n ¼ 4) perovskite solar cellsexhibited much improved stability compared to 3D methyl-ammonium tin iodide (MASnI3) perovskite solar cells despiteslightly low efficiency below 3% of PCE.29 Later, Z. Ning and co-

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workers reported quasi-2D tin-based perovskites by introducingphenethylammonium iodide (PEAI) into 3D formamidinium tiniodide (FASnI3) perovskites, achieving a PCE of 5.94% with 20%stoichiometric ratio of PEAI to formamidinium iodide (FAI).30

Further, M. A. Loi and co-workers extended their studies toquasi-2D tin-based perovskites with a relatively lower stoichio-metric ratio of PEAI to FAI and achieved a PCE as high as 9%.31

The implementation of quasi-2D tin-based perovskitesprovides an efficient methodology to minimize the extent ofoxidation by passivating easily oxidizable tin-based perovskitesfrom moisture, leading to signicantly improved performanceand stability of tin-based perovskite solar cells. However, quasi-2D tin-based perovskites still suffer from poor oxidativestability and rapid increases in carrier density, which eventuallyresult in the loss of their intrinsic semiconducting properties. Inaddition, recent efforts employing quasi-2D tin-based perovskiteshave been focused on merely tuning the stoichiometric ratio ofPEAI to FAI from a structural point of view. Therefore, essentialstrategies that take into account tin chemistry are required tomitigate the intrinsic instability and improve the performance oftin-based perovskite solar cells along with the methodology froma structural point of view. Based on the chemical nature of tin, N-,S-, and O-based electron donors can coordinate with tin halides(SnX2, X ¼ halide anions) by donating a lone pair electron to theempty orbital in the valence shell of divalent tin.37–41 Accordingly,it is expected that coordination between Sn2+ ions and appro-priate ligands could block the reaction of Sn2+ ions with oxygen,resulting in effective suppression of the oxidation of Sn2+ to Sn4+.

Taking this into account, herein we fabricated quasi-2D tin-based perovskite solar cells by introducing the formamidiniumthiocyanate (FASCN) additive into quasi-2D tin-based perov-skites and investigated the effects of the FASCN additive on theperformance and stability of quasi-2D tin-based perovskite solarcells. We found that the thiocyanate anion (SCN�) could coor-dinate with the vacant orbital of Sn2+, thereby possibly shieldingthe tin ion and suppressing oxidation of Sn2+ to Sn4+ oxidationduring lm formation. This effect resulted in both improvedstability and better optoelectronic performance. Moreover,excellent morphological improvement with addition of theFASCN additive was observed simultaneously. Thus, the quasi-2D tin-based perovskite solar cells exhibited greatly enhancedphotovoltaic performance and stability due to both chemicaland morphological effects, attaining a PCE as high as 8.17%under reverse scan with a steady-state efficiency of 7.84% ata maximum power point (MPP) in comparison with that of 3DFASnI3 (PCE �1.84%), while retaining over 90% of their initialPCE aer 1000 hours in a nitrogen-lled glovebox (H2O <2.2 ppm, O2 < 5 ppm). We propose the method described hereinas an effective means of improving the efficiency and stability oflead-free, tin-based perovskite solar cells through the simpleintroduction of the FASCN additive.

ExperimentalMaterials

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, PVP Al 4083) was purchased from Heraeus Clevios. Tin

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iodide (SnI2) was purchased from Alfa Aesar. FAI, PEAI, andFASCN were purchased from GreatCell Solar. Tin uoride(SnF2), anhydrous N,N-dimethyl formamide (DMF), dimethylsulfoxide (DMSO), chlorobenzene (CB), and toluene werepurchased from Sigma-Aldrich. 6,6-Phenyl C61 butyric acidmethyl ester (PC60BM) was obtained from EM-index. All chem-icals were used without further purication as received.

Device fabrication

Indium tin oxide (ITO)-coated glasses were sonicated sequen-tially in soapy water, de-ionized water, acetone and isopropylalcohol. Cleaned substrates were treated with UV-generatedozone for 20 min. A PEDOT:PSS dispersion was ltered andthen spin-coated onto the ITO substrates at 5000 rpm for 40 sand dried at 150 �C for 30 min. The coated ITO/PEDOT:PSSsubstrates were transferred to a nitrogen-lled glovebox (H2O< 2.2 ppm, O2 < 5 ppm). Quasi-2D tin-based perovskiteprecursor solutions with different stoichiometric ratios of PEAIto FAI were prepared by dissolving SnI2, FAI, PEAI, and SnF2with the molar ratio of 1 : (1 � x) : x : 0.1 in a DMF : DMSOsolvent (8 : 2 volume ratio). The concentration of the precursorsolution was 0.6 M. Quasi-2D tin-based perovskite precursorsolutions containing FASCN were prepared by adding theFASCN additive to the perovskite precursor with differentproportions of FASCN to SnI2. The perovskite precursor solutionwas spin-coated under two-step coating conditions of 1000 rpmfor 10 s and 5000 rpm for 30 s, and then annealed at 100 �C for30min. Toluene was used as the antisolvent for the second spin-coating process. The PC60BM solution (20 mg ml�1) in CB wasltered and spin-coated onto the perovskite lm at 2000 rpm for60 s. Then, a 100 nm thick Al layer was thermally evaporatedunder vacuum (<10�6 mbar) through a shadowmask. The activearea of the device was 0.12 cm2.

Film and device characterization

Fourier transform infrared (FTIR) spectroscopy (VERTEX 70;Bruker) was performed on prepared solutions with a spectralrange of 4000 to 500 cm�1. X-ray photoelectron spectroscopy(XPS) (ESCALAB 250) was performed on prepared perovskitelms aer etching for 20 s. X-ray diffraction (XRD) diffracto-grams were obtained using an X-ray diffractometer (D8Advance; Bruker) with Cu Ka radiation (l ¼ 1.5406 A) at 40 kV/40 mA. Grazing-incidence wide-angle X-ray scattering (GIWAXS)measurements were performed at the PLS-II 3C SAXS I beamlineof Pohang Accelerator Laboratory in Korea. X-rays from the in-vacuum undulator (IVU) were selected (Ek ¼ 9.81 keV and l ¼1.26 A) using a Si (111) double crystal monochromator. TheGIWAXS patterns were recorded using a 2D charge-coupleddevice (CCD) detector (SX165; Rayonix), and the X-ray irradia-tion time was 1–6 s depending on the saturation level of thedetector. Scanning electron microscopy (SEM) images wereobtained using a Hitachi cold SEM microscope. Absorptionspectra were measured using a UV-Vis-NIR spectrophotometer(Cary 5000; Agilent). Steady-state and time-resolved photo-luminescence (PL) spectra were measured using a cavity-dumped Ti:sapphire laser. The excitation pulse was 4.5 nJ at

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Paper Journal of Materials Chemistry A

a wavelength of 415 nm, which was doubled to 830 nm (380kHz, 20 fs). The PL signal was collected by a 25.4 mm parabolicmirror and dispersed by a grating monochromator (SP-300;Acton Research) with a pitch of 150 g mm�1. Steady-state PLspectra were recorded using a CCD (DV-420; Andor). Time-resolved PL spectra were measured by time-correlated single-photon counting using a fast silicon avalanche photodiode(Id100; ID Quantique SA) as the detector. Photocurrent density–voltage (J–V) curves and solar cell performance were measuredusing a Keithley 2400 Source Meter with an AM 1.5G solarsimulator (100 mW cm�2). Current–voltage (I–V) curves ofelectron-only devices for extracting trap densities weremeasured at forward bias ranging from 0 V to 1.5 V usinga Keithley 2400 Source Meter under dark conditions. Capaci-tance–voltage (C–V) measurements were carried out in the darkat voltages ranging between �0.5 V and 0.5 V at a constantfrequency of 10 kHz and an AC amplitude of 20 mA. Electro-chemical impedance spectra (EIS) were measured using anIviumStat at a forward bias under AM 1.5G illumination(100 mW cm�2) and frequencies ranging between 100 Hz and 1MHz with an AC amplitude of 10 mA. Z-view soware was usedfor Nyquist tting.

Results and discussionPreparation of quasi-2D tin-based perovskite solar cells

We rst prepared quasi-2D tin-based perovskite precursorsolutions with different PEAI molar percentages of 0% (FASnI3),3%, 5%, 10%, and 20% by mixing SnI2, FAI, PEAI, and SnF2 ata molar ratio of 1 : (1 � x) : x : 0.1 in a DMF : DMSO solvent(8 : 2 volume ratio). Perovskite lms were fabricated witha solvent engineering approach using anti-solvent dropping.Structural effects of the PEAI spacer on the FASnI3 lm weredetermined by XRD analyses (Fig. S1†). The lm yieldeddiffraction peaks at 14.1�, 24.2�, 28.2�, 31.6�, 40.5�, and 43.2�,which were ascribed to the (100), (120)/(102), (200), (122), (222),and (300) planes, respectively, representing an orthorhombicphase structure having a random crystallographic phase withvarious orientation planes. Notably, with the incorporation ofPEAI, the diffraction peaks corresponding to the (100) and (200)lattice planes were largely enhanced, while those correspondingto the (120), (122), and (222) planes were drastically suppressed.The suppression of (120), (122), and (222) planes by preferential(h00) planes indicates that the vertical growth of the 3D FASnI3crystal was conned by the PEAI spacer to a parallel growth,resulting in a quasi-2D crystal structure with a preferentialorientation with respect to the substrate.42,43 The GIWAXSanalyses were conducted to further investigate the crystalorientation of these quasi-2D tin-based perovskites (Fig. S2†).The FASnI3 lm exhibited a completely isotropic intensitydistribution, represented by Debye–Scherrer rings, implyinga random crystal orientation. In contrast, distinct Bragg spotswere observed with increasing proportions of PEAI, where the(100) peak appeared strongly along the qz direction and the(011) and (022) peaks appeared along the qy direction. Thesedata imply that the crystal orientation was such that the (100)plane was parallel to the substrate.29–31 This oriented structure

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could facilitate charge transport in the vertical directionbecause charge transfer would not be disturbed by the insu-lating PEAI spacer. Compared to the FASnI3 lm, a smoothersurface morphology was observed with the controlled orienta-tion facilitated by the introduction of PEAI (Fig. S3†). An opti-mized quasi-2D tin-based perovskite solar cell with an invertedplanar structure on ITO-coated glass with PEDOT:PSS as thehole transporting layer, FASnI3 with 20% molar ratio of PEAI toFAI as the absorber, PC60BM as the electron transporting layer,and Al as the metal electrode yielded a PCE of 5.07%, showinga comparable performance level with the previously reportedquasi-2D tin-based perovskite solar cell.30 The highest PCE of5.74% in quasi-2D tin-based perovskite solar cells was obtainedunder reverse scan with 10% PEAI (Fig. S4 and Table S1†), whichis the reference cell in this study.

Study on the effects of the FASCN additive on quasi-2D tin-based perovskites

Further improvements in the performance and stability ofquasi-2D tin-based perovskite solar cells were attained using theFASCN additive. To investigate the effect of the FASCN additiveon tin components, SnI2 solutions were prepared with differentadditive compositions in a DMF : DMSO (8 : 2 volume ratio)solvent and stored in air (Fig. 1a). Before exposure to air (0 min),the color of the solutions containing the SnF2 compensator(10 mol%) was slightly brighter than that of those lacking SnF2.This indicates that SnF2 can act as an antioxidant.19,26 Onceexposed to air, all of the solutions turned dark red, beginning atthe surface, due to the oxidation of Sn2+ ions. Interestingly, inthe rst 8 minutes of aging, the rate of discoloration of a solu-tion containing only the FASCN additive (5 mol%) without SnF2was slightly lower than that of a solution containing neitherSnF2 nor the FASCN additive. This indicates the suppression oftin oxidation by the FASCN additive. With assistance of theFASCN additive, the solution containing both SnF2 and theFASCN additive showed the slowest degradation rate as shownin 8–20 min. The effect of the FASCN additive on the rate ofoxidation was more clearly observed in solutions containingrelatively higher concentrations of FASCN, even without SnF2(Fig. S5†). These results suggest that the FASCN additive inter-acts strongly with the tin component of the perovskite precursorsolution to effectively suppress the oxidation of Sn2+ ions. Forspectroscopic conrmation of this interaction, FTIR spectros-copy was conducted on prepared DMF : DMSO solutions con-taining FASCN, FASCN + SnI2, FASCN + SnI2 + SnF2, and FASCN+ SnI2 + SnF2 + FAI + PEAI (Fig. 1b). The vibrational peak cor-responding to the C^N stretching vibration of SCN� was clearlyobserved at 2055 cm�1 with pure FASCN in a DMF : DMSOsolvent.44–46 This peak red-shied to 2043 cm�1 with the incor-poration of SnI2, and to 2040 cm�1 with the incorporation ofSnF2 (Fig. 1c). It has been shown that the empty orbital in thevalence shell of SnX2 functions as an electron acceptor forelectron-donating (O-, S-, or N-based) ligands.37–41 Accordingly,the gradual red-shi of the SCN vibrational peak is due to thepartial donation of a lone pair electron from the sulfur atom ofthe SCN group to a vacant orbital of tin, forming an SnX2–donor

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Fig. 1 (a) Photographs of SnI2 solutions in a DMF : DMSO (8 : 2 volume ratio) solvent with different additive compositions depending onexposure time to air (w/o SnF2 and FASCN contains neither of them, with only FASCN or SnF2 contains each content, and with SnF2 and FASCNcontains both of them), (b) FTIR spectra of DMF : DMSO solutions containing different precursor compositions in the presence of the FASCNadditive in the overall scanning range, (c) FTIR spectra of a region specified for the SCN� vibrational peak. XPS Sn (3d) spectra of tin-basedperovskite films (d) without and (e) with the FASCN additive (5 mol%) where the peaks were deconvoluted at 486.6 eV (Sn2+, green) and 487.4 eV(Sn4+, red) (etching time: 20 s).

Journal of Materials Chemistry A Paper

complex. The same peak was shied to a slightly higher wave-number of 2044 cm�1 with the incorporation of FAI and PEAI.This is due to the donation of a partial electron cloud from theN-donor groups of FAI and PEAI to SCN, which had becomerelatively electronic-decient as a result of its interaction withtin. The formation of a coordination complex between SnX2 andthe FASCN additive was also conrmed by XRD analyses(Fig. S6†). The SnI2 lm lost its intrinsically preferred crystallinephase, a high-temperature yellow b-SnI2 phase, and adopteda randomly oriented a-SnI2 phase with hardly traceable inten-sity aer the incorporation of the FASCN additive.47,48 The XPSanalysis (Fig. 1d–e) was used to investigate the effects of the

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strong interaction between the FASCN additive and tin on theextent of oxidation in the resulting perovskite lms. The Sn (3d)band can be deconvoluted into two peaks at 486.6 and 487.4 eV,associated with Sn2+ and Sn4+, respectively.49 The XPS spectraindicate that the 10% PEAI lm with the FASCN additive con-tained much less Sn4+ than those without the FASCN additive.These results explicitly imply that the FASCN additive exten-sively suppresses the oxidation of Sn2+ to Sn4+ by forminga complex with the tin component in the precursor solution,which could act as a shield to prevent Sn2+ from reacting withoxygen, thereby preventing tin precursors from oxidationduring lm fabrication.

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Paper Journal of Materials Chemistry A

Fig. 2a shows a GIWAXS image of the 10% PEAI lm with theFASCN additive. Compared to the 10% PEAI lm without theFASCN additive (Fig. S2†), more distinct and clear Bragg spotswere observed, implying enhanced crystal orientation withrespect to the direction parallel to the substrate, and its sche-matic crystal structure is given in Fig. 2b. The improved crystal-linity in the out-of-plane direction can alleviate the interferencein charge transport by the insulating organic spacers, which leadsto an increase in the short circuit current (Jsc) of the nal device.To investigate the improved crystallinity of the (h00) diffractionplanes representing out-of-plane orientation, the XRD analysis ofthe 10% PEAI lms with different amounts of FASCN was per-formed. Notably, sharper and stronger diffraction peaks corre-sponding to (h00) planes were observed with the addition ofFASCN (Fig. 2c). Crystallographic enhancement due to the FASCNadditive is evidenced by a decrease in the full width at halfmaximum (FWHM) of the peaks corresponding to the (100) and(200) planes (Fig. 2d). This decrease indicates improved crystal-linity with a more ordered crystal orientation.

The morphological effects of the FASCN additive on the 10%PEAI lms were investigated using SEM (Fig. 3a–e). In theabsence of the FASCN additive, the lms exhibited somepinholes, which could potentially lead to direct contact betweenthe electron transporting layer and hole transport layer, therebyimpeding efficient charge transport and resulting ina decreased ll factor (FF) and poor device performance.50 Asthe FASCN additive was added, grain boundaries became clearand slightly enlarged grain sizes were observed, which are inagreement with the results of XRD analyses. The optimized lmwith 5 mol% FASCN additive showed a compact and uniformlm coverage without pinholes. The enlargement of grain sizeswith the addition of FASCN may be due to the coarsening of

Fig. 2 (a) GIWAXS image and (b) schematic crystal structure of the quasi-additive. (c) XRD diffractograms of the 10% PEAI films with different amouof FASCN concentration.

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perovskite grains by FA gas, as in the case of methylamine(CH3NH2, MA) gas used in the lead-based perovskites where MAgas is known to enlarge the grain size.51–59 In our case, we inferthat SCN� reacts with FA+ to generate FA gas and release HSCNgas, rather than being intercalated into the perovskite lattice.This is conrmed by the XPS spectrum for S (2p) core electronsin the tin-based perovskite lm, which indicates no evidence forthe presence of SCN� in the nal lm under the detection limitof XPS (Fig. S7†). To further investigate whether the SCN�

worked as an additive or was doped into the quasi-2D tin-basedperovskite in our case, we conducted time-of-ight secondaryion mass spectrometry (ToF-SIMS), which has a more delicatedetection limit (parts per billion) compared with XPS, in orderto detect the remaining amount of SCN� in the nal lm asshown in Fig. S8.† A very small amount of SCN� is expected toremain aer annealing considering the detection limit of ToF-SIMS. Notably, the intensity of sulfur in the surface regionwas lower than that in the bulk region, implying that SCN�

reacts with FA+ more easily in the surface region to generate FAgas, which is consistent with the result of the lead-basedperovskite.57 In addition, because there is a loss of SCN� aerannealing, it is not reasonable to conclude that SCN� pene-trated into the lattice and formed a thermodynamically stableperovskite crystal. Furthermore, we could not observe anycrystallographic or optical shi by lattice expansion whichcould occur if SCN� penetrated into the perovskite lattice,60,61

verifying the absence of doping effect in our case. Accordingly,the generation of FA gas may result in larger grains through thereconguration and healing of the initial perovskite grains ina similar way MA gas worked in lead-based perovskites.54

However, pinholes were gradually observed with excess FASCN,possibly due to large amounts of FA vapor being released at

2D tin-based perovskite film (10% PEAI) with the addition of the FASCNnts of FASCN additive. (d) FWHM for (100) and (200) peaks as a function

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Fig. 3 SEM images of quasi-2D tin-based perovskite films (10% PEAI) with FASCN additive amounts of (a) 0 mol%, (b) 2.5 mol%, (c) 5 mol%, (d)7.5 mol%, and (e) 10mol%. Scale bar in all images indicates 2.5 mm. The inset in each image shows its correspondingmagnified SEM image, wherethe scale bar in the inset indicates 1 mm.

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once. Overall, the incorporation of the FASCN additive not onlyproduces a tin-based perovskite lm which is less damaged byoxidation through chemical interactions, but also enhancesmorphological quality, leading to improved crystallinity.

Fig. 4a and b show absorption, steady-state and time-resolved PL spectra of the 10% PEAI lms with differentamounts of the FASCN additive. The absorption spectra revealthat the optical band gap was not affected by the addition ofPEAI or FASCN. The absorption onset occurs near 900 nm,similar to that of the 3D FASnI3 perovskite.19,26 This means thatquantum connement effects by the incorporation of PEAI arenegligible for the 10% PEAI sample, as reported for quasi-2Dlead-based perovskites with a similar molar percentage ofPEAI.43,62,63 The PL intensity of tin-based perovskite lms withthe FASCN additive was strongest around 890 nm, while thereference cell yielded the lowest emission. Carrier lifetimes ofthe 10% PEAI lms prepared with 0, 2.5, 5, 7.5, and 10 mol%FASCN additive were 1.91, 2.71, 6.40, 6.10, and 5.24 ns,respectively (Fig. 4b). These results imply that the incorporationof the FASCN additive facilitated the radiative recombination ofphoto-generated charges, while simultaneously inhibiting non-radiative recombination by reducing the defect density relatedto tin vacancies and improving lm quality.

Photovoltaic properties of quasi-2D tin-based perovskite solarcells with the FASCN additive

The effects of the FASCN additive on the performance of quasi-2D tin-based perovskite solar cells are represented by averagephotovoltaic parameters andmaximum PCEs obtained from the

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optimized 20 devices, including Jsc, open circuit voltage (Voc),FF, and PCE for each FASCN incorporation amount as shown inTable 1 and Fig. S9.† The reference cell yielded the lowestphotovoltaic parameters, with an average PCE of 5.09% underreverse scan (Jsc ¼ 18.5 mA cm�2, Voc¼ 0.48 V, and FF¼ 57.2%).The average Jsc, Voc, and FF of the device improved progressivelywith the addition of FASCN, reaching an optimized average PCEof 7.66% under reverse scan with 5 mol% FASCN additive (Jsc ¼21.8 mA cm�2, Voc ¼ 0.53 V, and FF ¼ 66.5%). The improve-ments in Jsc, Voc, and FF in devices containing FASCN are due tothe formation of chemically stable tin-based perovskiteabsorbers and morphological enhancements, which result ina reduced number of tin vacancy defects and improved chargetransport properties, respectively. With the addition of excessFASCN, average Jsc, Voc, and FF gradually decreased, yielding anaverage PCE of 6.00%with 10mol% FASCN. This is attributed toincreasing pinhole size and correspondingly poor lm coverage,implying that the decrease in FF is the primary reason for thedegraded device performance with excess FASCN additive.Fig. 5a shows a cross-sectional SEM image of a fabricateddevice, including a 200 nm thick perovskite lm with 40 nmPC60BM and 50 nm PEDOT:PSS, which act as electron- and hole-transporting layers, respectively. Fig. 5b displays the J–V curvesof the best-performing tin-based device, which contained5 mol% FASCN, and shows a Jsc of 22.5 mA cm�2, Voc of 0.53 V,FF of 68.3%, and PCE of 8.17%, while the best-performingreference cell shows a Jsc of 19.9 mA cm�2, Voc of 0.50 V, FF of57.5%, and PCE of 5.74% under reverse scan. The obtained bestefficiency presents the highest enhancement from the 3DFASnI3 device compared to other studies reporting similar

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Fig. 4 (a) Absorption and steady-state PL spectra of the 10% PEAI filmswith different amounts of FASCN additive and (b) their correspondingtime-resolved PL spectra.

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performance levels for the 3D FASnI3 device (Table S3†). Thereliability of the best-performing devices without or with theFASCN additive was conrmed by steady-state photocurrentmeasurements at their MPP for 150 s, displaying a stabilizedPCE of 5.28% and 7.84%, respectively (Fig. S10†). Smallerhysteresis was observed with the FASCN additive for the opti-mized device compared to the reference cell, further demon-strating the reliability of the device (Fig. S11†). The J–V curvesand photovoltaic parameters of the best-performing devicesmade with other proportions of FASCN are shown in Fig. S12and Table S2,† respectively. Fig. 5c shows the external quantumefficiency (EQE) of the optimized cell with or without the FASCN

Table 1 Photovoltaic performances of quasi-2D tin-based perovskite so

FASCN [mol%] Jsc [mA cm�2] Voc [V]

0 18.5 (�0.79) 0.48 (�0.01)2.5 19.6 (�1.06) 0.51 (�0.01)5 21.8 (�0.80) 0.53 (�0.01)7.5 21.7 (�0.68) 0.52 (�0.01)10 20.0 (�0.86) 0.51 (�0.01)

a Average value. b Standard deviation. c Maximum value obtained from th

This journal is © The Royal Society of Chemistry 2018

additive. The cell with FASCN showed a much higher EQE overthe whole wavelength range and the Jsc, calculated from theEQE, matched well with the corresponding value obtained fromthe J–V curve. Fig. 5d shows the PCE distribution of 50 devices,indicating better reproducibility afforded by the FASCN addi-tive. Stability tests were performed in a nitrogen-lled glove box(H2O < 2.2 ppm, O2 < 5 ppm) for 1000 hours as shown in Fig. 5eand S13.† The device containing the FASCN additive retainedover 90% of its initial efficiency aer 1000 hours. In contrast,the reference cell retained just 60% of its initial efficiency. Theimproved stability is attributed to the enhanced chemicalstability of the tin-based perovskite against oxidation duringlm formation, and the suppressed penetration of oxygen andmoisture into the perovskite lm by morphological improve-ment. As shown in Fig. S14,† under ambient conditions with40% relative humidity at room temperature, the FASCN-applieddevice showed greatly improved stability, retaining 40% of itsinitial efficiency aer 30 min, while the reference one retainedaround 20%. To also examine the stability under continuousillumination, we monitored the PCEs of optimized devices for15 hours under continuous 1 sun illumination at their MPP. Asshown in Fig. S15,† the FASCN-applied device displayedenhanced stability under working conditions compared to thereference, showing a much slower decay in the PCE at the MPP.At the initial time, both devices exhibited steep decays.However, within a short time, the device with the FASCN addi-tive showed gradual stabilization and a slow decay, while thereference device showed a fast decay trend at a similar rate tothe initial. The slow decay rate implies that the FASCN-applieddevice is less sensitive to defect formation, which facilitates iondefect migration in bulk and accumulation at junction inter-faces under continuous working conditions, deteriorating thesteady-state efficiency.64,65 Consequently, these results clearlydemonstrate improved defect tolerance in the FASCN-applieddevice. C–V measurements were performed to determine thecharge density prole of our perovskite lms (Fig. 5f). The slopeof the C–V plot was used to extract charge carrier density usingthe Mott–Schottky equation as dened by eqn (1),66

1

C2¼ 2

330A2eNd

�V � Vfb � kBT

e

�(1)

where 3 is the dielectric constant of the perovskite lm, 30 is thepermittivity of vacuum, A is the area of the electrode, e is theelementary electronic charge, Nd is the charge carrier density,V is the at band potential, kB is the Boltzmann constant, andT is the temperature. A remarkable reduction in carrier density

lar cells prepared with different amounts of FASCN additive

FF [%] PCEAVG [%] PCEMAX [%]

57.2 (�1.51) 5.09a (�0.27)b 5.74c

59.3 (�2.06) 5.89 (�0.28) 6.5466.5 (�1.32) 7.66 (�0.29) 8.1759.5 (�2.74) 6.66 (�0.36) 7.3358.9 (�2.83) 6.00 (�0.35) 6.59

e optimized 20 devices.

J. Mater. Chem. A, 2018, 6, 18173–18182 | 18179

Fig. 5 (a) Cross-sectional SEM image of a fabricated device. (b) J–V curves under AM 1.5G illumination for the best-performing quasi-2D tin-based perovskite solar cell with the FASCN additive compared to the reference cell (inset shows the schematic illumination of the deviceconfiguration) and corresponding (c) EQE spectra and integrated Jsc calculated from EQE. (d) Distribution in PCEs for 50 devices in each case. (e)Stability test represented by the normalized PCEs of unencapsulated devices in a nitrogen-filled glove box over 1000 hours (H2O < 2.2 ppm, O2 <5 ppm). (f) C–V measurement results (down and left axes) and calculated depletion width (top and right axes).

Journal of Materials Chemistry A Paper

(6.67 � 1015 cm�3) was observed with the addition of 5 mol%FASCN compared to that of the reference cell (2.17 � 1016

cm�3). The carrier density decreased further with increasingproportions of FASCN additive (Fig. S16†), which indicates theeffectiveness of FASCN in suppressing the oxidation of tin. Thisreduced carrier density with the FASCN additive was furtherconrmed by dark I–V curves, where the extracted shunt resis-tance (Rsh) showed a higher value (145.2 kU cm2) with theFASCN additive compared to that (16.32 kU cm2) without theFASCN additive (Fig. S17†). The higher Rsh implies a higherresistance to increasing carrier density due to the self-p-doping,which leads to low leakage current. The width of the depletionregion, calculated from C–V measurements, is shown in Fig. 5f(top and right axes). The depletion region gradually broadenedwith increasing reverse voltage until fully depleted. In lmscontaining FASCN, the fully depleted region at a reverse voltageof �5 V was 190 nm. Without FASCN, the depleted region wasonly 113 nmwide, indicating that a stronger eld is formed withthe FASCN additive. This, in turn, leads to better junctionproperties and facilitates the extraction or separation of photo-generated charges under light illumination. Dark I–V curves ofelectron-only devices were measured to determine the trapdensity of our perovskite lms. The I–V plot in Fig. S18a†consists of a linear ohmic region at low voltages, a trap-llingregion, where current increases sharply as empty trap statesare lled, and a space charge-limited region that obeys theMott–Gurney law. The voltage at the point where the trap beginsto ll is represented by the trap-lled limit voltage (VTFL) andtrap densities can be obtained by eqn (2),67

18180 | J. Mater. Chem. A, 2018, 6, 18173–18182

VTFL ¼ entL2

2330(2)

where nt is the trap density and L is the lm thickness. Trapdensity decreased from 1.87 � 1016 cm�3 in the reference cell to9.36 � 1015 cm�3 in lms containing 5 mol% FASCN(Fig. S18b†). This is attributed to a reduction in the number oftin vacancy defects and improved lm quality. In contrast,excess FASCN led to an increase in trap density due to poor lmcoverage. Recombination resistance (Rrec) was extracted fromEIS (Fig. S19a–e†) to verify the extent of charge recombination inour perovskite lms. The resulting Nyquist plots featureda major arc in agreement with previous reports for tin-basedperovskites22,35,68 and corresponding Rrec values were extractedas a function of applied forward voltage (Fig. S19f†). The opti-mized cell yielded the highest Rrec over the entire voltage range,thereby conrming the suppression of charge recombination.

Conclusions

In summary, we fabricated efficient, tin-based perovskite solarcells with over 8% efficiency using the FASCN additive in quasi-2D tin-based perovskites. The incorporation of the FASCNadditive not only signicantly suppressed the oxidation of tin-based perovskites via strong interaction with tin but alsoimproved the crystallographic and morphological quality bycreating coarser perovskite grains. These two effects resulted insignicant enhancements in both device performance andstability. Our work demonstrates, for the rst time, that

This journal is © The Royal Society of Chemistry 2018

Paper Journal of Materials Chemistry A

thiocyanate-based additives can be effectively applied to tin-based perovskite solar cells, exerting a remarkable impact onthe device performance and stability. We anticipate that thisapproach can be applied to other lead-free perovskite solar cellscontaining metal components prone to oxidation to realizesignicant improvements in device performance and stability.

Conflicts of interest

There are no conicts to declare.

Acknowledgements

This work was supported by the National Research Foundationof Korea (NRF) grant (2017R1E1A1A01074090), the Nano Mate-rial Technology Development Program (2017M3A7B8063825),and the global PhD fellowship (GPF) program of the NRF fun-ded by the Ministry of Science and ICT (MSIT), Korea.

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