Air-Exposure Induced Dopant Redistribution and Energy Level

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Air-Exposure Induced Dopant Re-distribution and Energy Level Shifts in Spin Coated Spiro-MeOTAD Films Yabing Qi Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm504022q • Publication Date (Web): 29 Dec 2014 Downloaded from http://pubs.acs.org on December 30, 2014

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Air-Exposure Induced Dopant Re-distribution and Energy Level Shifts in Spin Coated Spiro-MeOTAD Films

Journal: Manuscript ID: Manuscript Type: Date Submitted by the Author: Complete List of Authors:

Chemistry of Materials cm-2014-04022q.R2 Article 26-Dec-2014 Hawash, Zafer; Okinawa Institute of Science and Technology Graduate University (OIST), Energy Materials and Surface Sciences Unit (EMSS) Ono, Luis; Okinawa Instute of Science and Technology Graduate University, Energy Materials and Surface Sciences Unit Raga, Sonia; Okinawa Institute of Science and Technology (OIST), Energy Materials and Surface Sciences Lee, Michael; Okinawa Institute of Science and Technology, Energy Materials and Surface Sciences Unit Qi, Yabing; Okinawa Institute of Science and Technology Graduate University, Energy Materials and Surface Sciences Unit

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Air-Exposure Induced Dopant Re-distribution and Energy Level Shifts in Spin Coated Spiro-MeOTAD Films.

3

Zafer Hawash, Luis K. Ono, Sonia R. Raga, Michael V. Lee, Yabing Qi*

4 5

Energy Materials and Surface Sciences Unit (EMSS), Okinawa Institute of Science and Technology Graduate Uni‐ versity (OIST), 1919‐1 Tancha, Onna‐son, Okinawa 904‐0495 Japan.

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7 ABSTRACT: Doping properties of 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene (spiro‐MeOTAD) 8 Hole Transport Layer (HTL) are investigated by X‐ray Photoelectron Spectroscopy (XPS), Ultraviolet Photoelectron Spec‐ 9 troscopy (UPS), and Atomic Force Microscopy (AFM) under air exposure. XPS results reveal that 3 hours exposure of Li‐ 10 bis(trifluoromethanesulfonyl)‐imide (LiTFSI) doped spiro‐MeOTAD to air results in the migration of LiTFSI from the bot‐ 11 tom to the top across the spiro‐MeOTAD film. AFM images reveal the presence of pinholes with an average diameter of 2 12 ~135 nm and a density of ~3.72 holes/µm . In addition, cross sectional Scanning Electron Microscope (SEM) images reveal 13 that these pinholes form channels across the doped spiro‐MeOTAD film. Optical microscopy and Fourier Transform In‐ 14 frared (FTIR) microscopy images confirm the presence of large pinholes with diameters in the range of 1‐20 m and a 2 15 density of ~289 holes/mm as well. The presence of pinholes may play a major role in the migration processes of the 16 LiTFSI within the spiro‐MeOTAD film as well as on the degradation processes of solar cells. This is further confirmed by 17 the rapid decreasing efficiency of perovskite solar cells with solution prepared doped spiro‐MeOTAD layers when exposed 18 to air.

19

49 circuit voltage (Voc) and LiTFSI improved the short‐ 50 circuit current (Jsc) and hence resulted in overall en‐ 10 51 hancement of ~2.56% in PCE. Such optimized doped 52 spiro‐MeOTAD was used as a standard HTL for the solid‐ 11‐13 53 state DSSCs together with different organic dyes. The 54 highest certified efficiency of 6.08% has been reported 14 55 using the C220 dye molecules. More recently, when 56 combined with organometallic perovskite absorber, the 3,15 57 same optimized HTL resulted in ~9% PCE. Currently, 58 the same recipe that includes the t‐BP and LiTFSI doped 59 spiro‐MeOTAD as HTL prevails in perovskite based solar 16‐18 60 cells and reported to achieve high efficiencies >15%.

1. INTRODUCTION

20 Solar energy‐to‐electricity power conversion efficiencies 21 (PCEs) of solid‐state dye‐sensitized solar cells (DSSCs) 22 have rapidly progressed in the last few years. One of the 23 leading improvements of the solid‐state DSSC compared 24 to the previous DSSC was the replacement of the liquid 25 electrolyte with solid organic hole transport layers 1,2 26 (HTLs). The replacement of the electrolyte with 2,2′,7,7′‐ 27 tetrakis(N,N‐di‐p‐methoxyphenylamine)‐9,9′‐ 28 spirobifluorene (spiro‐MeOTAD) HTL not only solved the 29 problem of leakage and sealing issues, but also culminat‐ 30 ed as one of the most important HTLs for solid‐state 3‐5 31 DSSC as well as for perovskite based solar cells. Spiro‐ 32 MeOTAD is widely used HTL in currently high‐ 33 performance solid‐state cells, mostly due to its high sta‐ 34 bility (glass‐transition temperature Tg = 121 °C), high sol‐ 6‐9 35 ubility, and amorphous nature. Material infiltration 36 into the mesosporous TiO2 layers of up to few microme‐ 8 37 ters has been reported. The first report employing spiro‐ 38 MeOTAD mixed with Li(CF3SO2)2N (Li‐ 39 bis(trifluoromethanesulfonyl)‐imide, LiTFSI) and 40 N(PhBr)3‐SbCl6 (tris (4‐bromophenyl)ammoniumyl hexa‐ 2 41 chloroantimonate) as additives had a low PCE of ~0.74%. 42 Interfacial charge recombination processes taking place 43 between dye/TiO2, HTL/TiO2, and HTL/dye were pin‐ 2,10 44 pointed as the origin for the low efficiencies. Thus, the 45 composition of the HTL was further modified blending 4‐ 46 tert‐butylpyridine (t‐BP) and LiTFSI, which reduced 47 charge recombination under optimized conditions. The 48 addition of t‐BP showed 100% improvement in the open‐

61 Spiro‐MeOTAD in its pristine form suffers from the low 19‐21 62 hole mobility and conductivity. Thus, inclusion of do‐ 63 pant (e.g., LiTFSI) helps not only generate additional 64 charge carriers, but also tune the electronic properties 22‐24 65 such as the energy level alignment at the interfaces. 66 The doping of spiro‐MeOTAD is generally associated with 67 the oxidation reaction and efforts have been made to find 22‐27 68 suitable dopants that effectively dope spiro‐MeOTAD. 69 In the presence of LiTFSI dopant, spiro‐MeOTAD is not 70 readily oxidized in the presence of light or in dark and the 71 oxidation reaction is only promoted upon air 19,28,29 72 exposure. In contrast to organic solar cells, where air 73 exposure leads to degradation of devices, air exposure is 74 necessary during device fabrication in order to obtain a 28,30 75 working perovskite cell. On the other hand, funda‐ 76 mental understanding remains elusive regarding the pro‐ 77 cesses taking place when the LiTFSI doped spiro‐ 78 MeOTAD film is exposed to air. In this work, we studied

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1 the electronic structure and the chemical composition of 2 the doped spiro‐MeOTAD and its interactions with air 3 exposure. Comparisons are made to undoped spiro‐ 4 MeOTAD and spiro‐MeOTAD films with only t‐BP. Seg‐ 5 regation of LiTFSI to the top surface evolves as a function 6 of air exposure time. Microscopy techniques revealed the 7 presence of pinholes in those films, which is a possible 8 factor to facilitate the diffusion of the dopant upon air 9 exposure as well as on the degradation processes of the 10 active materials (e.g., perovskite). Our combined tech‐ 11 niques of spectroscopy and microscopy presented in this 12 work is expected to help the screening process of finding 13 alternative suitable HTL materials with pinhole free films, 14 high stability upon air exposure, and good energy level 15 alignments.

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59 Spectroscopy (UPS, Kratos AXIS ULTRA HAS, He‐Iα = 60 21.22 eV) and the data analysis were performed in Origin 61 Pro 9. The analysis of UPS was complemented by X‐ray 62 Photoemission Spectroscopy (XPS, Kratos AXIS ULTRA 63 HAS, monochromated Al Kα = 1486.6 eV). XPS was per‐ 64 formed to monitor the chemical states of the as‐prepared 65 spiro‐MeOTAD films, as well as air and nitrogen exposed 66 films. The binding energy (BE) for UPS and XPS was cali‐ 67 brated by measuring the Fermi edge (EF = 0 eV) and Au‐ 68 4f7/2 (84.0 eV) on a clean Au surface. The estimated ener‐ 69 gy resolutions of UPS and XPS were 0.14 eV and 0.7 eV, 70 respectively. Analysis of the XPS data was performed in 71 CasaXPS 2.3.16 software. UV and X‐ray induced sample 72 damage was monitored by taking consecutive five spectra 73 and comparing those spectra. The time acquisition for 74 each scan varied from 10 to 20 seconds depending on the 75 core level regions. The five scans were averaged to a single 76 spectrum if no changes were observed among these five 77 spectra. Individual spin‐coated films of spiro‐MeOTAD, 78 LiTFSI, and t‐BP were examined under UV and X‐ray ra‐ 79 diation. A small peak corresponding to the LiF compound 80 was observed at higher BE in F‐1s region (~685 eV) as well 81 as TFSI at lower BE (~402 eV) in the N‐1s region, both of 82 which were a result of the decomposition of LiTFSI (Fig. 31 83 S3) upon UV and X‐ray exposures. Thus, special care was 84 taken to minimize the UV and X‐ray exposure time when 85 acquiring UPS/XPS on doped spiro‐MeOTAD films. No X‐ 86 ray and UV induced damages were observed on spiro‐ 87 MeOTAD molecules. The XPS detection of spin‐coated or 88 drop‐casted t‐BP compound on Au film was below the 89 sensitivity of the system when the samples were loaded 90 into the vacuum chamber. Because of the high vapor 91 pressure of t‐BP, it was not possible to obtain reference 92 UPS and XPS spectra for t‐BP spin‐coated on Au. For the 93 investigation of the influence of t‐BP on spiro‐MeOTAD, 94 t‐BP solution with ten times higher concentration was 95 mixed with spiro‐MeOTAD and films were prepared by 96 spin coating.

16 17 2. EXPERIMENTAL SECTION: 18 Doped spiro‐MeOTAD solution (Fig. 1) was prepared 19 according to the standard literature procedure reported 10 20 elsewhere. Briefly, spiro‐MeOTAD (SHT‐263, Merck 21 KGaA) was dissolved in chlorobenzene and mixed with 4‐ 22 tert‐butylpyridine (t‐BP, Sigma) and acetonitrile (52 23 mg/100 L) dissolved Li‐bis(trifluoromethanesulfonyl)‐ 24 imide (LiTFSI, Sigma). The final solution had concentra‐ 25 tions of 56.4 mM of spiro‐MeOTAD, 187.9 mM of LiTFSI, 26 and 30.46 mM of t‐BP. Thus, the corresponding molar 27 ratios of spiro‐MeOTAD : LiTFSI : t‐BP were 1 : 3.33 : 0.54 28 based on the nominal concentrations. For photoemission 29 spectroscopy studies, thin films of doped spiro‐MeOTAD 30 were prepared by spin coating in the nitrogen glove box 31 for 60 s at a speed of 2000 rpm. Spin coating was per‐ 32 formed on thermally evaporated 120 nm of Au (Denton 33 Vacuum Evaporator, Model DV‐502V) on heavily doped Si 34 substrates with a thin native SiO2 layer (SAMCO Inc., 35 0.013 cm). The final thickness of the films was approxi‐ 36 mately 240 nm determined by a profilometer (Dektak 37 stylus profiler, Bruker). 38 Perovskite solar cells with doped spiro‐MeOTAD were 39 prepared on glass substrates coated with F‐doped SnO2 ‐1 40 (FTO, Pilkington 7 Ω sq ). FTO was etched with Zn pow‐ 41 der and HCl, and subsequently cleaned by brushing with 42 detergent, rinsing with milliQ water and sonicating with 43 2‐propanol. An 80 nm‐thick TiO2 compact layer was de‐ 44 posited by spray pyrolysis using a mixture of Ti (IV) iso‐ 45 propoxyde, acetylacetone and anhydrous ethanol with the 46 weight ratio of 3:3:2. Perovskite film precursor was depos‐ 47 ited by spin coating at 2000 rpm for 45 s from a mixture 48 of methylammonium iodide (MAI) and PbCl2 with the 49 weight ratio of 2.5:1 in dimethylformamide. Perovskite 50 crystal growth was performed by annealing the films on a 51 hot plate at 110°C for 45 min in a N2 glovebox with O2 and 52 H2O levels below 0.1 ppm. HTL consisting of LiTFSI and t‐ 53 BP doped spiro‐MeOTAD solution described above was 54 deposited by spin coating at 2000 rpm for 60 seconds. 55 Finally, top gold contacts (70 nm) were deposited by 56 thermal evaporation in a vacuum chamber.

97 The morphological characterization of the films was 98 performed ex situ by AFM in tapping‐mode (MFP‐3D se‐ 99 ries, Asylum Research) and the quantitative analysis was 100 conducted using WSxM 5.0 software. The cross sectional 101 Scanning Electron Microscope (SEM) images were taken 102 at 1.5 kV with Through the Lens Detector (TLD) for de‐ 103 tecting secondary electrons (FEI, Helios NanoLab 650). 104 Multiple samples were prepared for the cross sectional 105 SEM by cleaving manually the doped spiro‐MeOTAD de‐ 106 posited on Au(120 nm)/SiO2(native oxide)/Si substrate by 107 using a diamond scribe from the backside of the sample. 108 Air exposure experiments were performed on doped spi‐ 109 ro‐MeOTAD films exposed to the laboratory air in dark 110 (samples stored in a stainless steel container). The meas‐ 111 ured laboratory humidity and temperature were approxi‐ 112 mately 37 % and 24 C, respectively. Controlled nitrogen 113 exposure experiments were conducted in the loadlock of 114 the XPS system (nitrogen gas pressure = 1 atm). Time evo‐ 115 lution of films’ chemical states were also monitored by 9

57 The electronic properties of the doped spiro‐MeOTAD 116 storing the samples in vacuum (210 Torr) for the same 58 films were characterized by Ultraviolet Photoelectron 117 period of times used for the different gas exposures to

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UPS Intensity (arb. units)

(a) 39 h (v) 18 h (iv) 6 h (iii) 3 h (ii) fressh (i) 3

2

1

0

Energyy w.r.t Ef (eV)

(b) 5 4

Energy w.r.t. EF

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Figure 1. Mollecular structtures of LiTFS SI, t‐BP, and s spiro‐ MeOTAD co omposing the e doped spirro‐MeOTAD hole transport ma aterial.

ated with Fo ourier 8 Molecular information was investiga nfrared (FTIR)) microscopy,, using an Ag gilent 9 Transform In 10 Cary 620 FTIR microscope e coupled to a a Cary 670 mid‐IR m 11 spectrometer. Spectral im mages were ob btained using nor‐ 12 mal and high h magnificatio on optics usin ng high NA reeflec‐ 13 tive lenses. Th he incident IR R beam passed through thee spi‐ gold and passsed a 14 ro‐MeOTAD films, reflectted off the g 15 second time through the ffilms using trransflection m mode. 16 Peaks from sspiro‐MeOTA AD, t‐BP, and d LiTFSI weree ob‐ esolution of th he images waas on the order of a 17 served. The re 18 few microns, so only the la arger 1‐20 micron holes cou uld be 19 observed by F FTIR microsco opy. 20 Solar cell efficiencies w were extracteed from currrent‐ 21 voltage (I‐V V) curves measured by a source m meter 22 (Keithley, 242 20) under 1‐ssun illuminatiion (AM 1.5G G, 100 ‐2 23 mW cm ) usiing a solar sim mulator (New wport, Oriel So ol1A). 24 For the stab bility tests, th he “air‐exposeed” samples were 25 stored in a pe etri dish in laaboratory air w wrapped in allumi‐ 26 num foil and d the “vacuum m exposed” saamples stored d in a 27 vacuum cham mber with the pressure keptt at 10‐7 Torr. 28 29 3. RESULTS A AND DISCUS SSION:



3 2 1

EF

0

-1

1 rule out any measurementt‐induced artiifacts (such a as UV diation). No indication i of radiation ind duced 2 and X‐ray rad o‐MeOTAD o oxida‐ 3 changes (e.g., dopant segrregation, spiro 4 tion, and frragmentation of differentt chemical com‐ UPS and XPS on the doped d spi‐ 5 pounds) was observed by U constant halo ogen light (sttrong 6 ro‐MeOTAD films with c 7 intensity in th he visible rang ge) illuminatio on.

4 14 4.13 4.21 4.16 4.13 4.1

-0.54 -0.56 -0.76

-1.00 -1.0 04

HO OMO

-2 0

3

6

18

39 9

Air expo osure time (h) Fig gure 2. (a) UPS specttra (He−Iα = 21.22 eV V) corrresponding t to the HOMO O region of th he as‐prepare ed dop ped spiro‐Me eOTAD film (i) and the same samp ple wh hen exposed to air for 3 hours (ii), 6 hours (iii), 18 hou urs (iv), an nd 39 hourss (v). (b) Correspondin C ng sch hematic enerrgy diagram of doped spiro‐MeOTA s AD film m. 156 0.54 eV (Fig. 2a a(i)). After the initial threee hours air exp po‐ 157 surre, the HOMO O level of thee doped spiro o‐MeOTAD fiilm 158 sho owed a slight shift towardss high BEs setttling at 0.56 eV 159 (Fiig. 2a(ii)). An additional 3 hours of exposure (F Fig. 160 2a((iii)) induced a large shift bringing the HOMO leadiing 161 edg ge to 0.76 eeV followed b by an overall decrease in tthe 162 inttensity of the HOMO featu ures. With a total t of 18 or 39 163 ho ours of air exposure (Figs. 2a(iv),(v)), thee HOMO furth her 164 shiifted with the e leading edg ges at 1.00 eV V and 1.04 eeV, 165 resspectively. An n almost com mplete disapp pearance of tthe 166 HO OMO featuress was observed after the 18 hours air exp po‐ 167 surre. Neverthele ess, the high s sensitivity of our UPS systeem 168 ma ade it still possible to extract unambigu uously the leaad‐ 169 ing g edge position that wass distinguishaable above tthe 170 background sig gnal. As seen n from Fig. 2a 2 a substanttial 171 HO OMO level sh hift occurred during the first 6 hours aand 172 satturation was rreached after r 18 hours. Fig gure 2b displaays 173 the e schematic en nergy diagram m showing thee time evolutiion

30 The electro onic propertiees of the as‐prrepared doped d spi‐ nction () and d HOMO lead ding edge valu ues 31 ro‐MeOTAD film changes in the elec ctronic propeerties 174 of the work fun 175 w.r r.t. E on the d doped spiro‐M MeOTAD film ms with air exp po‐ F 32 after air expo osure were pro obed by UPS with He‐I so ource

ues were deteermined from the cutoff ed dge 33 (Fig. 2). The l leading edge o of the highest occupied mo olecu‐ 176 surre. The  valu pectra (Fig. S11a). 34 lar orbital (HOMO) with r respect to the Fermi level ( w.r.t. 177 observed in the high BE side of the UPS sp he extracted   values weree constant oveer the entire air 35 EF) for the as‐‐prepared dop ped spiro‐MeO OTAD films w was at 178 Th ഥ = 4.155  0.03 eV). T 179 exp posure time in n this experim ment ( The

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(a) C-1s (v) 39 h XPS Intensity (arb. units)

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(b) Li-1s x4

(c) F-1s

(v) (v) (iv)

(iv) 18 h

(iv) (iii)

(iii) 6 h

(iii) (ii)

(ii) 3 h

(ii) (i)

(i) fresh 295

290 285 Binding Energy (eV)

(i) 280


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Figure 3. XPS spectra (Al−Kα = 1486.6 eV) corresponding to (a) C‐1s, (b) Li‐1s, and (f) F‐1s core levels of the as‐prepared doped spiro‐MeOTAD films (i) and subsequent after 3 hours (ii), 6 hours (iii), 18 hours (iv), and 39 hours air exposure (v). 1 ionization energy (IE) was calculated to be 4.67 eV for the 2 as‐prepared doped spiro‐MeOTAD film, which is ~0.3 eV 3 lower than the IE measured on the pristine spiro‐ 21 4 MeOTAD film prepared by vacuum evaporation and ~0.1 5 eV lower than the pristine spiro‐MeOTAD film prepared 6 by solution processing (Fig. S1b). The observed differences 7 are possibly due to the different preparation methods 8 used in these studies. Interestingly, the IE was observed 9 to increase as a function of the air exposure time, from 10 the initial value of 4.67 eV for the as‐prepared film to a 11 saturation value of ~5.1 eV after 18 hours air exposure. At 12 the same time, as will be discussed later in the XPS sec‐ 13 tion, the as‐prepared LiTFSI doped spiro‐MeOTAD shows 14 a small amount of LiTFSI at the top surface. This indicates 15 that a strong electronic interaction between spiro‐ 16 MeOTAD molecules and LiTFSI prevails within the film 17 upon air exposure. Two factors may contribute to the ob‐ 18 served IE changes: (1) chemical changes of the spiro‐ 19 MeOTAD induced by dopants or air exposure contamina‐ 32 20 tion (interaction induced by atmospheric aerosols), (2) 21 the diffusion of dopants populating at the top‐most sur‐ 22 face. 23 After each UPS measurement, XPS was recorded to 24 monitor the chemical states of the doped spiro‐MeOTAD 25 films. Figure 3 shows the XPS C‐1s (a), Li‐1s (b), and F‐1s 26 (c) core levels of the doped spiro‐MeOTAD film for the 27 as‐prepared sample (i) and the same sample after the dif‐ 28 ferent air exposure periods: after 3 hours (ii), 6 hours (iii), 29 18 hours (iv), and 39 hours (v). The XPS O‐1s, S‐2p, and N‐ 30 1s core levels were also measured and displayed in Fig. S2. 31 The as‐prepared doped spiro‐MeOTAD film shows the 32 overall C‐1s spectrum shape that is similar to the pristine 21,22,33,34 33 spiro‐MeOTAD. A small F‐1s signal (~2.2%) corre‐ 34 sponding to LiTFSI (Figs. 3b, c, and 4) were observed 35 meaning that a diluted LiTFSI concentration is present at 36 the top surface on the as‐prepared film. Because doped 37 spiro‐MeOTAD films are composed of three chemical

38 compounds (Fig. 1), the overlapping of CH, CC, and 39 CN signals from spiro‐MeOTAD and t‐BP molecules are 40 unavoidable. Thus, curve fitting analysis in the C‐1s region 41 is challenging. XPS measurements on spiro‐MeOTAD 42 mixed with t‐BP film compared with pristine spiro‐ 43 MeOTAD film did not show any difference (Fig. S4). 44 Although spectra for the t‐BP could not be measured by 45 XPS, the presence of t‐BP was confirmed by FTIR. Un‐ 46 doped and doped spiro‐MeOTAD films were measured by 47 FTIR. Additional peaks were observed in the doped films 35 48 that correlate to peaks from t‐BP in reference spectra 19,36 49 and LiTFSI spectra in the literature. Further detailed 50 description is given in the Supporting Information. 51 In contrast, the LiTFSI compound was observed to 31 52 show the characteristic signatures in XPS such as Li and 53 high oxidative‐shifted carbon (CF3) (Figs. 3b,c), which 54 enables unambiguous detection. The C‐1s peak maximum 55 of the as‐prepared LiTFSI doped spiro‐MeOTAD was lo‐ 56 cated at a BE of ~284.5 eV. For comparison, the typical C‐ 57 1s peak maximum BE value reported for the undoped spi‐ 21,22,33,34

58 ro‐MeOTAD is ~285.1285.4 eV. Thus, XPS also 59 confirmed that the dopants induced a Fermi level shift 60 towards the HOMO by ~0.60.9 eV, consistent with the 61 p‐type doping previously discussed for the UPS analysis 62 (Fig. 2). After 3 hours air exposure a new peak appeared at 63 ~688.6 eV that was attributed to F‐1s of the CF3 groups 31 64 in TFSI. Such an observation confirms the re‐ 65 distribution of the LiTFSI within the doped spiro‐ 66 MeOTAD film to reach the top surface driven by the air 67 exposure. Thus, the as‐prepared films seem to have a 68 phase segregated concentrations of spiro‐MeOTAD and 69 LiTFSI within the film where high concentrations of 70 LiTFSI were initially located at the bottom region of the 71 film (i.e. the region close to the Au substrate). Upon air 72 exposure, the LiTFSI diffusion starts to take place segre‐ 73 gating to the top surface of the film. Because of the high

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XPS atomic ratio (%)

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0.6

O-1s

0.4

C-1s

17 nm

(a)

F-1s 0.12 0.10 0.08 0.06

S-2p

N-1s

0.04

1.0µm

0.02

Li-1s

0.00 0

10

20

30

0 24 nm

40

Air exposure time (hours)

(b)

Figure 4. XPS atomic ratio variations for C‐1s, O‐1s, S‐2p, N‐1s, Li‐1s and F‐1s when doped spiro‐MeOTAD is exposed to air. 1 atomic sensitivity factor (ASF) of the F‐1s signal 2 (ASF=4.43) compared to the Li‐1s (ASF=0.06) and C‐1s 37 3 (ASF=1), the changes in the C‐1s and Li‐1s regions are 4 minimal (Figs. 3a,b(ii)). Clear signatures of the LiTFSI 5 were only observed after 18 hours (Fig. 3) with the Li‐1s 6 peak centered at ~56 eV, C‐1s at 293 eV, and S‐2p at ~170 31,34,38 7 eV (Fig. S2b). The O‐1s and N‐1s regions (Fig. S2) 8 corroborate as well with the picture of the segregation of 9 LiTFSI to the top‐surface. In the S‐2p region; the appear‐ 10 ance of doublet peaks (2p3/2 = 169.3 eV and 2p1/2 = 170.5 11 eV) due to spin‐orbit splitting was observed after the ini‐ 38 12 tial 3 hours air exposure. The S‐2p photoemission was

1.0µm 0

Figure 5. Tapping‐mode AFM topography images of as prepared doped spiro‐MeOTAD, and after 24 hours air exposure.

13 assigned to the SO2 group of the LiTFSI. In O‐1s re‐ 14 gion, the as‐prepared film showed two components, one 15 centered at ~532 eV and the other at ~533 eV. The O‐1s 16 peak at ~532 eV, belongs to LiTFSI and was detected in 17 the as‐prepared film due to the high ASF value of O‐1s 18 (ASF=2.93). After three hours air exposure, the lower BE 19 shoulder (~532 eV) was observed to increase in intensity, 38 20 which can be assigned to the LiTFSI. The other high BE

38 after 6 hours air exposure (Fig. 3a(iii)), which could be 39 associated with air exposure contamination such as aero‐ 32 40 sols. At the moment, it is difficult to assign the observed 34,40‐42 41 peak. A similar peak was reported by Schölin et al. 42 and was assigned as the oxidized form of spiro‐MeOTAD. 43 In addition, the C‐1s peak maximum at 284.4 eV corre‐

21 component was assigned to the CaOC group (where Ca 21 22 corresponds to the aromatic C) in spiro‐MeOTAD. In the 23 N‐1s core level, Fig. S2c, a gradual broadening of the peak 24 with increasing air exposure time was observed. Two 25 components were deconvoluted with one at ~399.5 eV

46 ~285287 eV (corresponding to carbon from CC, CN,

38

26 and the other at ~400 eV, which can be assigned to CN 27 group in spiro‐MeOTAD and imide group in LiTFSI, re‐ 21,38 28 spectively. XPS Li‐1s and F‐1s spectra measured on an 29 as‐prepared pure LiTFSI film (Fig. S3) show the peak posi‐ 30 tion and overall shape similar to the doped spiro‐ 31 MeOTAD film case. Thus, it is expected that the majority 32 of the LiTFSI molecules were found in its intact form; i.e, 39 33 dissociation of Li from TFSI was negligible (if any) in 34 our experiments. 35 Additional secondary processes were observed from our 36 XPS analysis induced by air exposure. For example, an 37 additional peak in C‐1s centered at ~289 eV was observed

21,22

44 sponding to CH carbon in spiro‐MeOTAD decreases 45 substantially in intensity while in the BE range between 47 and CO in spiro‐MeOTAD) is less affected by the air 48 exposure. As described in the experimental section, cau‐ 49 tion was taken to minimize the X‐ray time exposure when 50 acquiring UPS and XPS on doped spiro‐MeOTAD films. 51 Thus, the aforementioned secondary processes observed 52 in doped‐spiro‐MeOTAD (appearance of ~289 eV peak 53 and significant decay of the 284.4 eV peak in C‐1s) are 54 expected to be induced mainly from the air exposure step. 55 In addition, we did not observe similar behavior in the 56 XPS core levels or HOMO leading edges of the doped 57 spiro‐MeOTAD films when samples were kept in vacuum 58 or in dry N2 environment (Fig. S5, S6, S7 and S8). 59 The effect of air exposure over time on the doped spiro‐ 60 MeOTAD film surface induced by the presence of gas 61 molecules were monitored by the quantitative analysis of 62 the XPS data (Fig. 4). XPS relative atomic ratios of O‐1s,



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Figure 6. Cro oss sectional SEM of doped spiro‐MeOT TAD film on A Au. (a) pinholles form chan nnels across t the doped spiiro‐ MeOTAD fillm indicated w with arrows i in high magnification imag ge, and (b) th he pinholes ob bserved from m the top surfa ace of the film a and from the cross section n. 1 N‐1s, C‐1s, F‐‐1s, S‐2p and Li‐1s were ca alculated by sum‐ 2 ming the inte egrated peak a area values of f all species in each 3 region follow wed by normaalization on th he basis of th he re‐ 4 spective ASF F values. Each spiro‐MeO OTAD molecu ule is 5 composed of 81 C atoms, 8 8 O atoms, an nd 4 N atoms (Fig. 6 1). The t‐BP molecules, ass discussed in n the XPS secction, 7 facilitates the e solubility of o the LiTFSII, and will m mostly the surface wh hen transferreed to vacuum m.28 In 8 desorb from t 9 the case of LiT TFSI dopant, each moleculle is composed d of 2 10 C atoms, 6 F atoms, 1 Li attom, 1 N atom m, 4 O atoms a and 2 ncrease in O‐‐1s, F‐1s, S‐2p, and 11 S atoms (Fig.. 1). A clear in 12 Li‐1s atomic rratios was observed as air exposure tim me in‐ 13 creases mean ning that a dyn namic processs of re‐distribu ution 14 of the LiTFSI to the top‐su urface takes place. On the o other 15 hand, the ato omic ratios of C‐1s and N‐1s decrease sub bstan‐ 16 tially. If LiTF FSI molecules segregate to the top surfaace in 17 their intact fo orm, then thee atomic ratio os of C‐1s and d N‐1s ant would be expected to i ncrease, but i if one 18 from the dopa 19 LiTFSI molec cule displacees a spiro‐M MeOTAD molecule 20 then the overrall atomic rattios of C‐1s sh hould decreasse, on 21 the other han nd it is difficu ult to expect t the overall rattio of 22 N‐1s without knowing each h componentt concentratio on on 23 the surface. T Therefore, thee secondary prrocesses cann not be 24 neglected here. As descriibed above, the t photoemiission 25 spectroscopy analysis upon n air exposuree was complexx due 26 to the different gas molecu ules present i in ambient airr (N2, 27 O2, H2O, CO2, etc.). Furthe ermore, speciial care in thee XPS 28 analysis has t to be taken, co onsidering th he effects of ad dven‐ 29 titious carbon n compoundss typically asssigned as conttami‐ 30 nation in th he literature.40‐42 To pin‐p point the gass ele‐ 31 ment(s) respo onsible for the segregation n of the LiTFSI and 32 unravel the observed secondary proce esses, furtherr sys‐ 33 tematic studies to expose tthe doped spiiro‐MeOTAD films 34 under controlled environm ments of O2, H H2O + N2, and d syn‐ 35 thetic air are currently und der further invvestigation.

205 as‐‐prepared film ms as well as in n the air expo osed films. Bassed 206 on n the quantitattive analysis o of those images, the integrat‐ 207 ed area of all p pinholes correesponds to ab bout 6% of tthe 208 tottal surface are ea with averaage diameters of 135 nm (F Fig. 209 S9). Similar h holes were aalso detected d when spirro‐ 210 Me eOTAD films were spin co oated on diffeerent substrattes 211 inc cluding the on nes pre‐coateed with perovsskite films, su ug‐ 212 gessting the univ versal presencce of these pin n‐holes in spirro‐ 213 Me eOTAD filmss prepared byy spin coatin ng. In additio on, 214 optical microscopy images o of doped spirro‐MeOTAD on 215 gla ass substrate showed largee pinholes with diameters in 216 the e range of 1‐220 m and a density of ~2289 holes/mm m , 217 wh hich was extrracted from 22×2 mm2 areaa measuremen nts 218 (Fiig. S10). FTIR under high m magnification n can also imaage 219 the ese micron‐sccale holes and d provides ad dditional conffir‐ 220 ma ation (Fig. S111). The existen nce of pinholees is reported to 221 lea ad to shorts b between the d different layers in the organ nic 222 dev vices 43 and solutions havve been soug ght in achieviing 223 rob bust devices.443‐45 These pin nholes in spirro‐MeOTAD aare 224 alsso likely the cause for the v very short lifetime common nly 225 observed for pe erovskite solaar cells that u use spin coatted 226 spiiro‐MeOTAD films as HTM M. The effects can be possib bly 227 tw wo‐fold: (1) pinholes faccilitate moistture migratiion 228 thrrough spiro‐M MeOTAD to reach perovskite layer aand 229 hence causing the degrada ation; (2) pin nholes facilitaate 230 com mponent elem ments from perovskite (e.g g. iodine) to m mi‐ 231 gra ate to top surrface and deg grade perovsk kite (decompo osi‐ 232 tio on). On the b basis of such o observations, to increase tthe 233 life etime of pero ovskite solar ccells, it is necessary to op pti‐ 234 miize the prepa aration proceedure of solu ution processsed 235 doped spiro‐Me eOTAD to avo oid pinhole fo ormation, e.g. by 236 usiing different s solvents, mixiing with an ad dditive, adding g a 237 top p capping layeer, etc. 2

103 To further pr rovide insightss on the morp phology of tho ose 104 pin nholes, cross sectional SEM M images weree taken on mu ul‐ 36 Figure 5 shows mages of thee as‐ 105 tip s AFM ttopography im ple doped spirro‐MeOTAD film samples (Fig. 6). Bassed 37 prepared dop ped spiro‐MeeOTAD film (a) and afteer 24 106 on n the high maagnification S SEM image prresented in F Fig. 38 hours air exp posure (b). As we can see fr from Fig. 5, a large 107 6a,, pinholes aree observed to f form channelss across the fiilm 39 density of pin nholes of 3.722 holes/µm2 w was detected in n the 108 (~2240 nm depth h). The overaall top surface image on tthe

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F Figure 7. Evollution of the photovoltaicc parameters with time; op pen circuit vo oltage (a), sh hort circuit cu urrent densitty ((b), fill factorr and (c), and d power conve ersion efficie ency (d). The points repressent the averrage values frrom 6 sample es w with the corre esponding sta andard deviattion. 1 spiro‐MeOTA AD film revealls the high deensity of holess cor‐ 2 roborating th he AFM images (Fig. 5) prevviously shown n. As

85 tio on is remarkaably faster witthin the first two days of air 86 sto orage. On the other hand, t the solar cellss stored in vaccu‐ 87 um m for 8 days,, the photocu urrent remain ned almost tthe 88 sam me, while photovoltage waas slightly en nhanced and fill 89 fac ctor decreased d, which overrall resulted in an efficien ncy 90 decrease of app proximately 14 4%. The largee decrease of JJsc 91 wa as observed fo or the air‐expo osed solar cellls, which is po os‐ 92 sib bly related to o the perovsk kite degradatiion induced by 93 mo oisture.46,47 Th his implies that water molecules diffu use 94 thrrough the sp pin‐coated spiiro‐MeOTAD layer to reaach 95 the e perovskite fi film and most likely this process is acceleer‐ 96 ate ed by the pressence of pinho oles.

n the Experim mental Sectio on, samples were 3 described in he doped sp piro‐MeOTAD D(240 4 prepared by cleaving th 5 nm)/Au(120 n nm)/SiO2(natiive oxide)/Si substrate by u using cribe from th he backside off the sample. This 6 a diamond sc edure was observed to tear apart 7 mechanical clleavage proce film together with the dopeed spiro‐MeO OTAD 8 the soft gold f 9 film leaving p portion of thee underneath ssubstrate exposed. aminated from m the 10 In addition, tthe gold film appears dela 11 underneath su ubstrate and a at the edges o of the image in n Fig. 12 6(a) and (b),, respectively y, because thee adhesion o of the 13 evaporated go old on silicon dioxide is verry poor.

42

14 To further confirm the effect of the e pinholes in con‐ h air exposuree effects, we exxamined the ttime‐ 15 junction with 16 dependent pe erformance off perovskite ssolar cells em mploy‐ 17 ing spin‐coatted doped spiiro‐MeOTAD as HTL by ex xpos‐ 18 ing them to a ambient cond ditions as welll as to vacuum m (as 19 reference). F Figure 7 show ws the time‐‐evolution of f two 20 batches of do oped spiro‐M MeOTAD baseed solar cells that 21 were exposed d in air and vaacuum for 8 d days. A progreessive 22 decrease of alll the photovo oltaic parametters were obseerved 23 over time un nder air expossure, which w was translated d to a 24 decrease of e efficiency of 6 68% after 8 daays. This degrada‐

4 4. CONCLUSI ION:

93 In summary, o I our XPS studyy reveals thatt (1) freshly prre‐ 94 parred spiro‐MeO OTAD showss a very low concentration c of 95 LiT TFSI dopants at the top surface; (2) air e exposure causses 96 som me characteriistic elementss (e.g. F, S, an nd Li) belongiing 97 to LiTFSI to migrate m to thee top surface. This migratiion 98 acrross the bulk k film seems to be facilitaated by the tw wo 99 typ pes of pinholles with diffeerent sizes, tthe presence of 100 wh hich is confirm med by AFM,, SEM, and op ptical microscco‐ 101 py. The small‐sized pinholess are partially y filled up up pon 102 airr exposure. Th he presence off pinholes mayy strongly affeect 103 the e stability of t the perovskitee based solar cells using sp pin

7



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1 coated spiro‐MeOTAD as HTL. Therefore, it is necessary 2 to optimize the preparation method for spin coated 3 doped spiro‐MeOTAD. 4

5 ASSOCIATED CONTENT 6 Supporting Information 7 UPS and XPS spectra on undoped spiro‐MeOTAD films, spi‐ 8 ro‐MeOTAD + t‐BP films, LiTFSI films, doped spiro‐ 9 MeOTAD films stored in vacuum, and doped spiro‐MeOTAD 10 films stored in N2, AFM images analysis, and optical micros‐ 11 copy image of doped spiro‐MeOTAD. FTIR microscopy on 12 undoped and doped spiro‐MeOTAD films. This material is 13 available free of charge via the Internet at 14 http://pubs.acs.org. 15 AUTHOR INFORMATION 16 Corresponding Author 17 * E‐mail: [email protected]. 18 ACKNOWLEDGMENT 19 This work was supported by funding from the Okinawa Insti‐ 20 tute of Science and Technology Graduate University. 21 ABBREVIATIONS 22 spiro‐MeOTAD, 2,2′,7,7′‐tetrakis(N,N‐di‐p‐ 23 methoxyphenylamine)‐9,9′‐spirobifluorene; HTL, Hole 24 Transport Layer; XPS, X‐ray Photoelectron Spectroscopy; 25 UPS, Ultraviolet Photoelectron Spectroscopy; FTIR, Fourier 26 Transform Infrared; SEM, Scanning Electron Microscope; 27 AFM, Atomic Force Microscopy; LiTFSI, Li‐ 28 bis(trifluoromethanesulfonyl)‐imide; PCE, power conversion 29 efficiency; DSSCs, solid‐state dye‐sensitized solar cells; t‐BP, 30 4‐tert‐butylpyridine; Jsc, short‐circuit current; Voc, open‐ 31 circuit voltage; MAI, methylammonium iodide; BE, binding 32 energy; I‐V, current‐voltage; HOMO, highest occupied mo‐ 33 lecular orbital; , work function; IE, ionization energy; ASF, 34 atomic sensitivity factor. 35 REFERENCES 36 (1) Hagen, J.; Schaffrath, W.; Otschik, P.; Fink, R.; Bacher, A.; 37 Schmidt, H. W.; Haarer, D. Synthetic Met. 1997, 89, 215. 38 (2) Bach, U.; Lupo, D.; Comte, P.; Moser, J. E.; Weissortel, F.; 39 Salbeck, J.; Spreitzer, H.; Gratzel, M. Nature 1998, 395, 583. 40 (3) Kim, H. S.; Lee, C. R.; Im, J. H.; Lee, K. B.; Moehl, T.; 41 Marchioro, A.; Moon, S. J.; Humphry-Baker, R.; Yum, J. H.; Moser, 42 J. E.; Gratzel, M.; Park, N. G. Sci. Rep. 2012, 2, 591. 43 (4) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; 44 Snaith, H. J. Science 2012, 338, 643. 45 (5) Liu, M.; Johnston, M. B.; Snaith, H. J. Nature 2013, 501, 46 395. 47 (6) Burschka, J.; Dualeh, A.; Kessler, F.; Baranoff, E.; Cevey48 Ha, N. L.; Yi, C.; Nazeeruddin, M. K.; Gratzel, M. J. Am. Chem. Soc. 49 2011, 133, 18042. 50 (7) Snaith, H. J.; Moule, A. J.; Klein, C.; Meerholz, K.; Friend, 51 R. H.; Grätzel, M. Nano Lett. 2007, 7, 3372. 52 (8) Dewalque, J.; Colson, P.; Thalluri, G. K. V. V.; Mathis, F.; 53 Chêne, G.; Cloots, R.; Henrist, C. Org. Electron. 2014, 15, 9. 54 (9) Ariga, K.; Yamauchi, Y.; Rydzek, G.; Ji, Q.; Yonamine, 55 Y.; Wu, K. C. W.; Hill, J. P. Chem. Lett. 2014, 43, 36. 56 (10) Krüger, J.; Plass, R.; Cevey, L.; Piccirelli, M.; Grätzel, M.; 57 Bach, U. Appl. Phys. Lett. 2001, 79, 2085.

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In nsert Table of Contentts artwork h here

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Air-Exposure Induced Dopant Redistribution and Energy Level

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