Enhanced Device Efficiency and Long-Term Stability via Boronic Acid

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Surfaces, Interfaces, and Applications

Enhanced device efficiency and long-term stability via boronic acid-based self-assembled monolayer modification of ITO in planar perovskite solar cell. Duygu Ak#n Kara, Koray Kara, Gorkem Oylumluoglu, Mesude Zeliha Yigit, Mustafa Can, Jae Joon Kim, Edmund K. Burnett, D. Leonardo Gonzalez Arellano, Sumeyra Buyukcelebi, Faruk Ozel, Özlem Usluer, Alejandro L. Briseno, and Mahmut Kus ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10445 • Publication Date (Web): 08 Aug 2018 Downloaded from http://pubs.acs.org on August 8, 2018

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ACS Applied Materials & Interfaces

Enhanced device efficiency and long-term stability via boronic acidbased self-assembled monolayer modification of ITO in planar perovskite solar cell. Duygu Akın Kara*†‡=, Koray Kara§‡, Gorkem Oylumluoglu*†, Mesude Zeliha Yigit∆, Mustafa Can&, Jae Joon Kim=, Edmund K. Burnett=, D. Leonardo Gonzalez Arellano=, Sümeyra Buyukcelebi‡, Faruk Ozel#, Ozlem Usluer=, Alejandro L. Briseno*=$, Mahmut Kus*ǁ‡. †

Muğla Sıtkı Koçman University, Department of Physics, 48000, Mugla, Turkey

‡

Selçuk University, Advanced Technology and Application Center, 42250, Konya, Turkey

=

University of Massachusetts Amherst, Polymer Science and Engineering, Amherst, MA, 01003, United States

§

Selçuk University, Department of Physics, 42250, Konya, Turkey

∆

Izmir Katip Çelebi University, Department of Material Science and Engineering, 35000, Izmir, Turkey

&

Izmir Katip Çelebi University, Department of Engineering Sciences, 35000, Izmir, Turkey

#

Karamanoğlu Mehmetbey University, Department of Material Science and Engineering, 70200, Karaman, Turkey

$

The Pennsylvania State University, Department of Chemistry, University Park, PA 16802, United States

ǁ

Selçuk University, Department of Chemical Engineering, 42250, Konya, Turkey

KEYWORDS: Perovskite, Solar Cell, Interface Engineering, Surface modification, SAM Treatment, Long Term Stability Supporting Information Placeholder

ABSTRACT: Interfacial engineering is essential for the development of highly efficient and stable solar cells through minimizing energetic losses at interfaces. Self-assembled monolayers (SAMs) have been shown as a handle to tune the work function of ITO, improving photovoltaic cell performance and device stability. In this study, we utilize a new class of boronic acid-based fluorineterminated SAMs to modify ITO surfaces in planar perovskite solar cells. The SAM treatment demonstrates an increase of the work function of ITO, an enhancement of the short circuit current and a passivation of trap states at the ITO/PEDOT:PSS interface. Device stability improves upon SAM modification, with efficiency decreasing only 20 % after one month. Our work highlights a simple treatment route to achieve hysteresis free, reproducible, stable and highly efficient (16 %) planar perovskite solar cells.

INTRODUCTION Converting cheap, efficient and sustainable energy from the sun is one of the big challenges for this era in renewable energy concept. In the last decade, organic-inorganic hybrid solar cells have shown to be a promising candidate due to their low fabrication costs and high power conversion efficiencies (PCE)1–4. This stems from the advancement of perovskite materials, which have high carrier mobilities, long exciton diffusion lengths, strong optical absorption spectrum, and suitable band gaps for solar cell applications. Due to these features, perovskite based devices have reached PCE values of over 20%5. In lead halide perovskite solar cells, there are various device architectures including mesoscopic (utilizing a TiO2 or Al2O3 oxide layer)6,7 or planar heterojunction

(p-i-n or n-i-p)8–10. Planar devices offer the ability of low temperature processing, enabling roll to roll fabrication of flexible cells8,11. The general chemical formula of organometalic halide perovskite absorber material is ABX3, A is an organic cation (such as CH3NH3, NH2CHNH2), B is a metallic cation (such as Pb, Sn) and X is halide anion (such as Cl, Br, I). Methylammonium lead iodide (CH3NH3PbI3) is a widely used member of the perovskite family having excellent optical and electronic properties: direct band gap of 1.55 eV, large absorption coefficient for the entirely visible range, and ambipolar charge characteristics.9,10 Understanding the interface between electrodes and organic layers is one of the most important issues in profiling device parameters such as open circuit voltage (VOC), short circuit current density

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(JSC) and fill factor (FF) besides of absorber layer morphology problems in solar cells.11,12 Indium tin oxide (ITO) is the preferred anode contact in planar heterojunction perovskite solar cells as well as other organic electronic devices13,14, due to its optical transparency and high electrical conductivity15,16. Yet, for enhanced solar cell performance, work function modification is often required. This can be achieved through UV ozone and oxygen plasma treatments or chemical approaches to modify the work function and surface energy.17 Work function adjustment by plasma induced treatments are temporary and unstable in atmospheric conditions18. PEDOT:PSS (Poly(3,4ethylenedioxylenethiophene):poly(styrenesulfonic acid)) is the most commonly used hole transport material (or electron blocking layer) due to its solution-processability and high conductivity. However, the acidic nature of PEDOT:PSS and electrical inhomogeneity give rise to a decrease in chemical stability at the ITO/PEDOT:PSS interface and a decay in electron blocking properties over time, respectively19–21. Self-assembled monolayer (SAM) treatment can chemically modify the ITO creating a permanent dipole moment that adjusts the work function of ITO and changes the surface wettability of the substrate16. SAM treatment is a convenient way to tailor the work function and improve stability in optoelectronic devices. There are many reports utilizing SAM modified ITO surfaces in organic lightemitting diodes (OLEDs) and solar cells but they have not been

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investigated in planar heterojunction perovskite solar cells to date22–24. In our previous work, boronic acid with different functional group (1-OMe) has been used for modification of c-TiO2 surface in perovskite solar cells25. In this work, we investigate the effects of SAM modified ITO with a series of boronic acid-based fluorineterminated derivatives (functional group with 1F, 2F, 3F) on the stability and efficiency of perovskite solar cells. The Boronic acid is anchoring the ITO surface by replacing the terminating –OH groups of ITO and tuning the substitute organic groups attached to the boron atoms allow to change surface dipole (it is affecting the work function of ITO). X-Ray Photoelectron spectroscopy show that the boronic acid based SAMs were bonded chemically to ITO surface uniformly. Ultraviolet photoelectron spectroscopy (UPS) and Kelvin-Probe Force Microscopy (KPFM) demonstrate that SAM treatment permanently changes the work function of ITO, allowing increased hole collection and improving short circuit photocurrent density. Device performance is also measured over 30 days, revealing an increase in stability upon SAM modification. This treatment allows for a permanent work function increase in ambient conditions and acts as a protective layer between the ITO and PEDOT:PSS to prevent degradation of the ITO electrode, improving long term device stability. The present study is, to our knowledge, the first report demonstrating ITO modification with boronic-based SAM in solution processed planar heterojunction perovskite solar cells.

Figure 1. (a) Chemical structures of self-assembled monolayers and net dipole moment direction of SAMs used in this work. (b) Device architecture of planar heterojunction perovskite solar cell with SAM-modified ITO electrode. (c) Energy level diagram of a device with a SAM between ITO and PEDOT:PSS layers.


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RESULTS AND DISCUSSION Work Function and Wettability The functional groups of the three self-assembled molecules with Fluorine-functionalized boronic acid derivatives were employed in this study. The chemical structures of the SAMs and planar device architecture are shown in Figure 1a-b. These SAMs modify the ITO to closely match the energy level alignment of the ITO/PEDOT:PSS interface (Figure 1c). Decreasing the energy barrier between the work function (WF) of ITO and HOMO level of PEDOT:PSS is critical for enhancing effective hole collection. Various fluorine terminated SAMs also exhibit different electron withdrawing characteristics since various para- substituents affect the dipole moment magnitude. The degree of fluorination has a large effect on the strength of the dipole moment and work function shift. The net dipole moment magnitudes and directions were calculated using density functional theory (DFT) with a Becke’s 3-parameter exchange and Lee-Yang-Parr correlation (B3LYP) hybrid functional and a 6-31G(d,p) basis set implemented in the Gaussian 09 revision D.0126 and shown in Table 1. From DFT calculations, it is worth to note that the geometries of boronic acid SAMs on the ITO surface lead to perpendicular dipole moment to the surface normal (Figure S1) and as the dipole on the surface gets larger so does the work function change bigger27. We have presented the changes in work function induced by the boronic acid with various degree of fluorinations. Boronic acid has not the variety of bonding modes on ITO as distinct from Phosphonic acids. It brings that boronic acids provides the controllable and easy application for modification of metal oxide surfaces28. To confirm the work function modification of the ITO electrode ultraviolet photoelectron spectroscopy (UPS) and Kelvin-Probe Force Microscopy (KPFM) measurements were carried out. UPS measurements were carried out on an Omicron SPHERA and PHI QUANTUM 2000 ESCA hemispherical spectrometer with He I (21.22 eV) excitation source. The UPS displays the secondary

electron cut off region (a sample bias of -3 V applied), from which the work function of the surfaces is extracted for modified and non-modified ITO (Figure 2a). An unmodified ITO exhibits a work function value of 4.68 eV and modified ITO surfaces have work function values of 4.87 eV, 5.11 eV and 5.38 eV for 1FSAM, 2F-SAM and 3F-SAM, respectively (Table 1). Based on the work function increase, it can be concluded that there exists an interfacial dipole (eD) at the surface of ITO. In principle, a positive eD formation arise from permanent dipole moment of fluorinated SAMs29. According to energy level alignment of electronic states, a favorable photo-carrier collection at the anode side is expected with a closer work function of ITO but lower than the HOMO level of PEDOT:PSS30. The UPS results show that the 2F-SAM modified surface is more suitable in terms of hole collection in comparison with 1F and 2F SAM-modified surfaces. The WF values of all surfaces obtained from UPS and KPFM measurements before and after treatment are shown in Table 1. KPFM results also show similar tendency in terms of work function variation with UPS. ITO surface work function has a value of 4.72 eV and values of surface WF with SAM modification are 4.92, 5.20 and 5.42 eV for 1F, 2F and 3F, respectively. UPS and KPFM results are concomitant with each other in terms of energy levels matching which are shown in Figure 2b. 1F and 2F SAMs show more efficient energy level matching for effective hole collection rather than 3F-SAM which dominates hole accumulation at interface and has difficulty to transition of holes from PEDOT:PSS to the anode side. To determine the surface coverage of the boronic acid SAMs on ITO, X-ray Photoelectron spectroscopy (XPS) measurement has been analyzed. Figure S2a-g shows the combined survey scan and high-resolution spectra of the 1F,2F,3F-SAM modified samples: F (1s) and B (1s) peaks emitted by boronic acid based SAMs as well as In (3d), Sn (3d), O (1s) peaks emitted by the respective underlaying metal oxide surface.

Figure 2. a) UPS spectra of the secondary electron cutoff region of UVO-treated ITO and fluorine-SAMs modified ITO surfaces. b) Graph shows the relative work function of ITO and SAM modified surfaces as a function of the µz, dipole moments, as measured by UPS in vacuum and KPFM in air.



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Table 1. DFT calculated perpendicular dipole moment results of SAMs and comparison of modified and bare ITO surface properties with different techniques. Dipole moment µz (Debye)

Work function (UPS) (eV)

Work function (KPFM) (eV)

Contact angle ( o)

ITO

---

4.68

4.72

43.4

1F-SAM

4.03

4.87

4.92

73.9

2F- SAM

4.32

5.11

5.20

86.9

3F- SAM

5.85

5.38

5.42

83.1

The detected elemental compositions for 2F-SAM modified ITO surface were recorded as (at. %) 12.84% of C (1s), 58.24% of In (3d5), 7.10% of Sn (3d5), 17.19% of O (1s), 3% F (1s) and 1.63 of B (1s). It is important that similar coverages on ITO achieved by each modifier. By comparing the F(1s) and In (3p3/2) peak areas rate for each SAM (1F, 2F and 3F) spectrum and considering the number of fluorine atoms for each modifier on the ITO surface can be determined27. Adjusting F (1s) and In (3p3/2) peak ratios for the number of fluorine atoms, excepting the fluorine atoms in the ortho position, all SAMs show the similar surface coverage (Table S1). Contact angle measurements (Table 1) were investigated to determine the wettability of an ITO surface after SAM modification. The surface becomes more hydrophobic upon the introduction of the SAM (from 43.4 °- to 86.9 °) in comparison with the untreated form. Surface topography and roughness which is related to the wettability of the samples with and without treatment have been investigated by Atomic Force Microscope (AFM) and Scanning Electron Microscopy (SEM). AFM images of ITO and PEDOT:PSS coated surfaces show no large difference between ITO and SAM treated surfaces (see Figure S3-S4). As can be seen in Figure S5, SEM results of the perovskite surfaces demonstrate that the perovskite morphology is not affected by SAM treatment. Take into consideration that the size of the SAMs is very small as compared with the measured surface roughness. Therefore, we can conclude that the SAM treatment of ITO does not have a significant effect on the morphology of the upper layers (see Figure S3-S4 and Table S2).

planar heterojunction perovskite solar cell with and without SAMs were fabricated (Figure 1). In this study, reference and modified perovskite solar cells were produced using the solvent washing method highlighted in our previous work31. Figure 3a displays the current-voltage characteristics of the solar cells fabricated on non-modified ITO and modified by the three different boronic-based SAMs. The SAM-modified cells exhibit an increase in short circuit current density (JSC), open circuit voltage (VOC), fill factor (FF), and efficiency in comparison to the reference cells (Table 2). The effect of modification between ITO and PEDOT:PSS bilayer on JSC and FF is crucial to obtain more efficient power conversion efficiency in planar heterojunction perovskite solar cells. Maximum increase in JSC was observed using 2F-SAM modified cell due to ideal energy level alignment in comparison with the other two SAMs (Figure 3a). Decreasing the energy barrier between ITO and PEDOT:PSS allows for better contact, resulting in improved hole collection. Thus, short circuit current (JSC) increases from 19.48 mA/cm2 to 22.20 mA/cm2 by SAM modification. All photovoltaic figures-ofmerit are summarized in Table 2. As a result, a 20% efficiency improvement is observed for 2F-SAM modification. To investigate the improvement of hole extraction between ITO and PEDOT:PSS layer, the current densities of the hole only devices were compared, as shown in Figure S6, current density of hole only device with 2F-SAM is higher than the hole only device with non-modified. Figure 3b shows the increase in performance was found to be statistically relevant and SAM-modified cells show better average efficiency (approximately 16%) than non-modified counterparts (12%) with 90% reproducibility.

Current-Voltage Characteristics To understand the effect of the self-assembled monolayer on the performance of perovskite solar cells, one-step solution processed

Table 2: Device figure of merit parameters of the perovskite solar cells with and without SAM treatment under illumination of AM 1.5 G, 100 mW/cm2. Voc (mV)

JSC (mA/cm2)

FF

PCE (%)

Rs (Ω.cm2)

Rsh (Ω.cm2)

940±7

19.48±0.7

0.68±0.03

12.49±0.2

14.8

1324

1F-SAM

990±8

20.47±0.5

0.70±0.03

14.04±0.3

6.7

2830

2f- SAM

980±5

22.20±0.4

0.72±0.02

15.66±0.2

4.0

2945

3f- SAM

950±9

18.69±0.8

0.75±0.04

13.23±0.4

4.4

3030

Non-modified


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Figure 3. a) J-V characteristic of the perovskite solar cell with and without Fluorine SAM (1F, 2F, 3F) modification under A.M 1.5 G. b) Statistical efficiency graphs based on the number of solar cell with and without 2F-SAM treatment c) and d) show the hysteresis behavior of non-modified and 2F-SAM modified perovskite solar cell forward (0 V_1.1 V) and reverse (1.1 V_0 V) scan directions under illumination at 300 K, respectively. e) incident photon-to-carrier efficiency (IPCE) with integrated current density behavior and f) steady state photocurrent of perovskite solar cell with and without 2F-SAM modification. To improve the understanding of the influences of the SAM treatment on cell performance: hysteresis effect, steady state photocurrent, and reproducibility characteristics were investigated32. The changes between forward and reverse scan direction of 2F-SAM modified and non-modified devices are small, yet the 2F-SAM modified cell shows less hysteresis than the non-modified counterparts as shown in Figure 3c-d. The decreased hysteresis could be associated with improved interfacial energy level alignment and efficient hole collection33Figure 3e compares the incident photon-to-current conversion efficiency (IPCE) for perovskite solar cells with and without 2F-SAM modification. 2F-SAM modified solar cell has a stronger IPCE response than the non-modified solar cell in the wavelength range 450-750 nm. That can be associated with better charge extraction and recombination reduction34. The integrated current density values of 21.25 mA/cm2 and 17.9 mA/cm2 for 2F-SAM modified and non-modified solar cells, respectively, from the IPCE measurements are close to JSC obtained from J-V characterization. In Figure 3f, the variation in photocurrent density of the optimized solar cells was recorded under A.M. 1.5 G 1 sun intensity by applying a bias voltage near the maximum output point. The output curves of the perovskite solar cell without any surface modification exhibit maximum output current density of ~18.6 mA/cm2 while for the perovskite solar cell with 2F-SAM, the steady state output current density was ~21.3 mA/cm2. These results clearly demonstrate device performance enhancement with 2F-SAM modification. The dark current voltage response was examined using Shockley Read Hall equation to further

understand the fundamental device behavior. In this equation, Jo, the reverse saturation current density, n, the ideality factor, q is the electron charge, V is the applied bias, KB is the Boltzmann constant and T is the temperature.

exp

1 (1)

Dark current density voltage characteristics of the 2F-modified and non-modified perovskite solar cell are shown in Figure 4a. The obtained device parameters such as Jo, and n, using diode equation 1 are summarized in Table S3. The decreased dark current density and lower values of Jo and n values in the 2FSAM modified cells result from less charge recombination at the ITO/PEDOT:PSS interface35The electronic charge transport properties at the interface of the inserted 2F-SAM layer in the perovskite solar cells was investigated using impedance spectroscopy. We measured the impedance spectra at applied D.C bias of VOC and a frequency range from 1 kHz to 1 MHz with AC amplitude of 20 mV under illumination of 1 sun. Nyquist plots (imaginary and real part of the impedance) are shown in Figure 4b. It is clearly shown that 2F-SAM modified perovskite solar cell exhibit higher recombination resistance due to lower recombination and more efficient charge transport at the interfaces than the non-modified counterpart. This also explains the enhanced VOC upon SAM modification36,37. Light intensity measurements were carried out to understand the recombination mechanism affecting device performance 38 by using following equations:



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Figure 4. (a) Current -voltage curves of the 2F-SAM modified and non-modified perovskite solar cell under dark condition. (b) Impedance spectroscopy of perovskite solar cell with and without 2F-SAM modification in the form of Nyquist plot. (c) VOC dependence, and (d) JSC dependence on the light intensities for perovskite solar cell for 2F-SAM modified and non-modified perovskite solar cell.

∝

∝

(2) (3)

where q is the electron charge, KB is the Boltzmann constant and T is the temperature in Kelvin and α is fitting parameter of the data yield. From equation 2, the slope of the VOC versus natural logarithm of light intensity graph is KBT /q which can be related to the recombination mechanism39,40. As shown in Figure 4c, the slope for optimized devices with and without 2F-SAM are 1.13 KBT/q and 1.48 KBT /q, respectively. 2F-SAM modified device displays a weaker light intensity dependence thus bimolecular recombination is more dominant for 2F-SAM modified cells while additional mechanisms such as trap assisted interfacial recombination mechanisms are valid for non-modified solar cell devices, which show a stronger VOC dependence35. This result indicates that the 2F-SAM modification reduced the recombination loss at interface. Figure 4d shows the JSC dependence on light intensity for studied solar cell devices. From equation 3, where, α is the slope of natural logarithm of JSC vs natural logarithm of light intensity graph, α=1 indicates bimolecular recombination and independency of the light intensity in short circuit conditions.

Device Stability Long-term stability of perovskite solar cells has been investigated by current density -voltage characteristics for 30 days without any

encapsulation process under glove-box conditions. Current density voltage curves can be seen in Figure S7 for all devices within 30 days drastic variation of the J-V shape. Device figuresof-merit are shown in Figure 5, with SAM treatment increasing long term stability of perovskite solar cells. Non-modified devices lost ∽75% of their original efficiency within 30 days, compared to a 20% reduction in 2F-SAM modified solar cell PCE in the same period and conditions. As shown in Figure 5c, while fill factor values of unmodified cells fell rapidly about 60% in 30 days, the cell with 2F-SAM modification preserved its fill factor about 70%. Open circuit voltage of 2F-SAM modified cells shows minimal reduction of 40 mV over 30 days whereas unmodified cells are result in a significant decrease of about 150 mV. This demonstrating that SAM modification of the ITO surface useful route to restrict ITO degradation. In addition, we investigated the stability of ITO/PEDOT:PSS interface in SAM modified and nonmodified solar cell devices for 30 days in terms of conductivity variation with the following ITO/PEDOT:PSS/Al and ITO/SAM/PEDOT:PSS/Al device architecture. We discovered that non-modified devices lost 15% of its initial conductivity whereas SAM modified devices did not see a reduction in conductivity in fact an increase roughly 5% after 30 days (see Figure S8) was observed. It confirms that the SAM layer stabilizes the interface conductivity of the device. Ion mobility has also been shown to be a major factor with device stability, and a SAM layer appears to reduce the effect of this mobility41.


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Figure 5. Long term stability results of 2F-SAM modified and non-modified perovskite solar cell. a) short circuit current density b) open circuit voltage c) fill factor, and d) cell efficiency in perovskite solar cells over 30 days. Metal ions have been shown to be released from ITO to the inner layer of the device and consequently degrade it and harm the device stability as well36. In the presence of the SAM layer, gap states induced by metal ions performing as trapping sites for photo-induced carriers could be prevented and resulting to improve cell stability43.

CONCLUSION In summary, we have reported a simple solution-processed interfacial-modification method using self-assembled monolayers to enhance the cell efficiency and long-term stability of planar perovskite solar cells. SAM treatment is demonstrated as an effective way to adjust the work function of ITO to reduce the energetic barrier between ITO and PEDOT:PSS. Interfacial modifications were carried out using a new series of fluorinefunctionalized boronic acid SAMs. The 2F-SAM modification exhibited the greatest increase in performance, reaching power conversion efficiencies of 16%. Furthermore, interface modification via SAM treatment significantly improves long term cell stability. 2F-SAM provides a permanent ITO work function modification and prevents the degradation of ITO due to the acidic effect of PEDOT:PSS and maintaining the conductivity of the PEDOT:PSS layer. This simple interface modification approach is critical to improve efficiency and stability of perovskite solar cells as well as other devices that are acid sensitive.

EXPERIMENTAL Solar cell fabrication Commercially, available ITO- coated glasses used as a substrate. After usual cleaning process (15 minutes water, acetone and isopropanol respectively) substrates exposed to UV-ozone treatment to activate the surfaces. All molecules used as SAM have been purchased from Sigma-Aldrich. 1 mM of selfassembled molecules dissolved in DMSO (dimethyl sulfoxide). A monolayer of the molecules was formed through chemical bonding between boronic acid and ITO for 24 hours in ambient air and room temperature. The substrates were then removed and rinsed with copious fresh DMSO and were dried. PEDOT:PSS deposited on-to SAM-modified ITO glasses as hole injection layer by spin coating at 5000 rpm for 40 s and annealed at 120 ℃ for 20 minutes. Methylamonioumiodide and Lead iodide dissolved in γButyrolactone (GBL) equimolar ratio 1.23 M and stirring at least 3 h at 80 ℃. Perovskite layers were deposited by spin coating using a prepared solution. The anti-solvent which is employed to precipitate perovskite precursors to surface drop during spin coating at 3500 rpm for 40 s. Afterward obtained perovskite layer annealed at 80 ℃ for 5 minutes and PCBM solution which is dissolved in Chloroform:Chlorobenzene (1:3) 3% wt, spin-coated as electron transport layer at 2500 rpm for 60 s and annealed 80 ℃ for 20 minutes to evaporate residual solvents. All of the fabrication stages were carried out in ambient conditions. Finally,



100 nm Ag (99.99% purity) thermally evaporated as cathode electrodes by glove-box integrated PVD system. Reference solar cells fabricated by this process are described below except for SAM treatment.

Device characterization All measurements completed in MBraun M200 glove–box system under nitrogen atmosphere. Glove-box integrated ATLAS solar simulator used as AM1.5 light source. Photocurrent and voltage characterization done by Keithley 2400 source meter. Kelvin probe characterizations were completed by NTMDT NTEGRA Solaris for AFM. UPS measurements have been completed by ESCA+S, Electron Spectroscopy for Chemical Analysis Instrument (Scienta Omicron Nanotechnology) which provides spectral analysis of kinetic/binding energies of electrons emitted from the material surface under the UV (He I line, 21.2 eV) photo-excitation. XPS data collected by Thermo scientific equipped with X-Ray source Al K Alpha micro-focused monochromator and ion gun energy ranges (100-4000 eV). IPCE measurements done by NEWPORT/ ORIEL instrument. Impedance data were taken by Agilent 4294A. Contact angle measurements were taken by Data Physics OCA 50 instrument with water droplets. All calculations were performed with Gaussian 09 software1 which is licensed-program in UMASSAmherst university. The ground state geometries of materials were optimized and frequency analysis (no imaginary frequency) with density functional theory (DFT), B3LYP 23 functional and 6311G++(d,p) basis sets. All the data on the device which have been realized over at least 10 devices, presented in this article.

ACKNOWLEDGMENT We acknowledge to Haydar Akdag and Dr. Cisem Kirbiyik for their valuable discussion during the study. D.A.K. thank to s (Tubitak 2211-C special areas -1649B031502037) and to Tubitak research fellowship (Tubitak 2214/A-1059B141501315). K.K thank to Turkey Scholarship Council (2214/A 1059B141501316). J.J.K, D.L.G.A, E.K.B, and A.L.B acknowledge the Office of Naval Research (N00014-16-1-2612 and N000147-14-1-0053).

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ASSOCIATED CONTENT Supporting Information Supporting information contents detailed perovskite solar cell preparation, roughness variations, Contact Angle images of SAM modified and non modified ITO surfaces. XPS survey scan and detailed high-resolution data for SAM modified ITO surfaces can be found in supporting information. In addition AFM and SEM images of ITO, ITO/ PEDOT:PSS and ITO/PEDOT:PSS/PEROVSKITE surfaces of cells with and without treatment are summarized. Stability and conductivity tests of SAM modified and non-modified solar cells and PEDOT:PSS surfaces, respectively can be seen in supporting information. The Supporting Information is available free of charge on the ACS Publications website.

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AUTHOR INFORMATION Corresponding Author

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* E-mail: [email protected] (DAK) * E-mail: [email protected] (MK)

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* E-mail: [email protected] (GO) (13) * E-mail: [email protected] (ALB)

Notes The authors declare no competing financial interest.

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Ball, J. M.; Lee, M. M.; Hey, A.; Snaith, H. J. LowTemperature Processed Meso-Superstructured to ThinFilm Perovskite Solar Cells. Energy Environ. Sci. 2013, 6 (6), 1739–1743. Park, N. G. Organometal Perovskite Light Absorbers toward a 20% Efficiency Low-Cost Solid-State Mesoscopic Solar Cell. J. Phys. Chem. Lett. 2013, 4 (15), 2423–2429. Kamat, P. V. Quantum Dot Solar Cells. The next Big Thing in Photovoltaics. J. Phys. Chem. Lett. 2013, 4 (6), 908–918. Green, M. A.; Ho-Baillie, A.; Snaith, H. J. The Emergence of Perovskite Solar Cells. Nat. Photonics 2014, 8, 506. Yang, W. S.; Noh, J. H.; Jeon, N. J.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. Il. High-Performance Photovoltaic Perovskite Layers Fabricated through Intramolecular Exchange. Science (80-. ). 2015, 348 (6240), 1234–1237. Kim, H.-S.; Lee, C.-R.; Im, J.-H.; Lee, K.-B.; Moehl, T.; Marchioro, A.; Moon, S.-J.; Humphry-Baker, R.; Yum, J.-H.; Moser, J. E.; Grätzel, M.; Park, N.-G. Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591. Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science (80-. ). 2012, 338 (6107), 643–647. Malinkiewicz, O.; Yella, A.; Lee, Y. H.; Espallargas, G. M.; Graetzel, M.; Nazeeruddin, M. K.; Bolink, H. J. Perovskite Solar Cells Employing Organic ChargeTransport Layers. Nat. Photonics 2013, 8, 128. Jeng, J. Y.; Chiang, Y. F.; Lee, M. H.; Peng, S. R.; Guo, T. F.; Chen, P.; Wen, T. C. CH3NH3PbI3 Perovskite/fullerene Planar-Heterojunction Hybrid Solar Cells. Adv. Mater. 2013, 25 (27), 3727–3732. Eperon, G. E.; Burlakov, V. M.; Docampo, P.; Goriely, A.; Snaith, H. J. Morphological Control for High Performance, Solution-Processed Planar Heterojunction Perovskite Solar Cells. Adv. Funct. Mater. 2014, 24 (1), 151–157. Docampo, P.; Ball, J. M.; Darwich, M.; Eperon, G. E.; Snaith, H. J. Efficient Organometal Trihalide Perovskite Planar-Heterojunction Solar Cells on Flexible Polymer Substrates. Nat. Commun. 2013, 4, 1–6. Ma, H.; Yip, H.-L.; Huang, F.; Jen, A. K.-Y. Interface Engineering for Organic Electronics. Adv. Funct. Mater. 2010, 20 (9), 1371–1388. Feng, W.; Rangan, S.; Cao, Y.; Galoppini, E.; Bartynski, R. A.; Garfunkel, E. Energy Level Alignment of polythiophene/ZnO Hybrid Solar Cells. J. Mater. Chem. A 2014, 2 (19), 7034–7044. Scharber, M. C.; Mühlbacher, D.; Koppe, M.; Denk, P.;


Page 9 of 11 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(15)

(16)

(17)

(18)

(19)

(20)

(21)

(22)

(23)

(24)

(25)

(26)

ACS Applied Materials & Interfaces Waldauf, C.; Heeger, A. J.; Brabec, C. J. Design Rules for Donors in Bulk-Heterojunction Solar Cells - Towards 10 % Energy-Conversion Efficiency. Adv. Mater. 2006, 18 (6), 789–794. Yip, H. L.; Hau, S. K.; Baek, N. S.; Ma, H.; Jen, A. K. Y. Polymer Solar Cells That Use Self-AssembledMonolayer-Modified ZnO/metals as Cathodes. Adv. Mater. 2008, 20 (12), 2376–2382. Khodabakhsh, S.; Sanderson, B. M.; Nelson, J.; Jones, T. S. Using Self-Assembling Dipole Molecules to Improve Charge Collection in Molecular Solar Cells. Adv. Funct. Mater. 2006, 16 (1), 95–100. Zhou, Y.; Shim, J. W.; Fuentes-Hernandez, C.; Sharma, A.; Knauer, K. A.; Giordano, A. J.; Marder, S. R.; Kippelen, B. Direct Correlation between Work Function of Indium-Tin-Oxide Electrodes and Solar Cell Performance Influenced by Ultraviolet Irradiation and Air Exposure. Phys. Chem. Chem. Phys. 2012, 14 (34), 12014–12021. Lee, K. H.; Jang, H. W.; Kim, K.-B.; Tak, Y.-H.; Lee, J.L. Mechanism for the Increase of Indium-Tin-Oxide Work Function by O2 Inductively Coupled Plasma Treatment. J. Appl. Phys. 2004, 95 (2), 586–590. Sapochak, L. S.; Padmaperuma, A. B.; Cai, X.; Male, J. L.; Burrows, P. E. Inductive Effects of Diphenylphosphoryl Moieties on Carbazole Host Materials: Design Rules for Blue Electrophosphorescent Organic Light-Emitting Devices. J. Phys. Chem. C 2008, 112 (21), 7989–7996. Yan, H.; Lee, P.; Armstrong, N. R.; Graham, A.; Evmenenko, G. A.; Dutta, P.; Marks, T. J. HighPerformance Hole-Transport Layers for Polymer LightEmitting Diodes. Implementation of Organosiloxane Cross-Linking Chemistry in Polymeric Electroluminescent Devices. J. Am. Chem. Soc. 2005, 127 (9), 3172–3183. Wong, K. W.; Yip, H. L.; Luo, Y.; Wong, K. Y.; Lau, W. M.; Low, K. H.; Chow, H. F.; Gao, Z. Q.; Yeung, W. L.; Chang, C. C. Blocking Reactions between Indium-Tin Oxide and Poly (3,4-Ethylene Dioxythiophene):poly(styrene Sulphonate) with a SelfAssembly Monolayer. Appl. Phys. Lett. 2002, 80 (15), 2788–2790. Can, M.; Havare, A. K.; Aydin, H.; Yagmurcukardes, N.; Demic, S.; Icli, S.; Okur, S. Electrical Properties of SAM-Modified ITO Surface Using Aromatic Small Molecules with Double Bond Carboxylic Acid Groups for OLED Applications. Appl. Surf. Sci. 2014, 314, 1082–1086. Cao, J.; Yin, J.; Yuan, S.; Zhao, Y.; Li, J.; Zheng, N. Thiols as Interfacial Modifiers to Enhance the Performance and Stability of Perovskite Solar Cells. Nanoscale 2015, 7 (21), 9443–9447. Hou, F.; Su, Z.; Jin, F.; Yan, X.; Wang, L.; Zhao, H.; Zhu, J.; Chu, B.; Li, W. Efficient and Stable Planar Heterojunction Perovskite Solar Cells with an MoO3/PEDOT:PSS Hole Transporting Layer. Nanoscale 2015, 7 (21), 9427–9432. Kırbıyık, Ç.; Kara, K.; Kara, D. A.; Yiğit, M. Z.; Đstanbullu, B.; Can, M.; Sariciftci, N. S.; Scharber, M.; Kuş, M. Enhancing the c-TiO2based Perovskite Solar Cell Performance via Modification by a Serial of Boronic Acid Derivative Self-Assembled Monolayers. Appl. Surf. Sci. 2017, 423, 521–527. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji,

(27)

(28)

(29)

(30)

(31)

(32)

(33)

(34)

(35)

(36)

H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery Jr., J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö.; Foresman, J. B.; Ortiz, J. V; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision D.01. Gaussian Inc. 2009, Wallingford CT. Hotchkiss, P. J.; Li, H.; Paramonov, P. B.; Paniagua, S. A.; Jones, S. C.; Armstrong, N. R.; Brédas, J. L.; Marder, S. R. Modification of the Surface Properties of Indium Tin Oxide with Benzylphosphonic Acids: A Joint Experimental and Theoretical Study. Adv. Mater. 2009, 21 (44), 4496–4501. Hotchkiss, P. J.; Jones, S. C.; Paniagua, S. A.; Sharma, A.; Kippelen, B.; Armstrong, N. R.; Marder, S. R. The Modification of Indium Tin Oxide with Phosphonic Acids: Mechanism of Binding, Tuning of Surface Properties, and Potential for Use in Organic Electronic Applications. Acc. Chem. Res. 2012, 45 (3), 337–346. Lee, H.; Puodziukynaite, E.; Zhang, Y.; Stephenson, J. C.; Richter, L. J.; Fischer, D. A.; Delongchamp, D. M.; Emrick, T.; Briseno, A. L. Poly(sulfobetaine Methacrylate)s as Electrode Modifiers for Inverted Organic Electronics. J. Am. Chem. Soc. 2015, 137 (1), 540–549. Zhou, H.; Chen, Q.; Li, G.; Luo, S.; Song, T.; Duan, H.S.; Hong, Z.; You, J.; Liu, Y.; Yang, Y. Interface Engineering of Highly Efficient Perovskite Solar Cells. Science (80-. ). 2014, 345 (6196), 542–546. Kara, K.; Kara, D. A., Kirbiyik, C.; Ersoz, M.; Usluer, O.; Briseno, A. L.; Kus, M. Solvent Washing with Toluene Enhances Efficiency and Increases Reproducibility in Perovskite Solar Cells. RSC Adv. 2016, 6 (32), 26606–26611. Snaith, H. J.; Abate, A.; Ball, J. M.; Eperon, G. E.; Leijtens, T.; Noel, N. K.; Stranks, S. D.; Wang, J. T. W.; Wojciechowski, K.; Zhang, W. Anomalous Hysteresis in Perovskite Solar Cells. J. Phys. Chem. Lett. 2014, 5 (9), 1511–1515. Azpiroz, J. M.; Mosconi, E.; Bisquert, J.; De Angelis, F. Defect Migration in Methylammonium Lead Iodide and Its Role in Perovskite Solar Cell Operation. Energy Environ. Sci. 2015, 8 (7), 2118–2127. Zhang, H.; Xue, L.; Han, J.; Fu, Y. Q.; Shen, Y.; Zhang, Z.; Li, Y.; Wang, M. New Generation Perovskite Solar Cells with Solution-Processed Amino-Substituted Perylene Diimide Derivative as Electron-Transport Layer. J. Mater. Chem. A 2016, 4 (22), 8724–8733. Zang, Y.; Li, C. Z.; Chueh, C. C.; Williams, S. T.; Jiang, W.; Wang, Z. H.; Yu, J. S.; Jen, A. K. Y. Integrated Molecular, Interfacial, and Device Engineering towards High-Performance Non-Fullerene Based Organic Solar Cells. Adv. Mater. 2014, 26 (32), 5708–5714. Dualeh, A.; Moehl, T.; Tétreault, N.; Teuscher, J.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M. Impedance Spectroscopic Analysis of Lead Iodide Perovskite-



(37)

(38)

(39)

(40)

(41)

(42)

(43)

Sensitized Solid-State Solar Cells. ACS Nano 2014, 8 (1), 362–373. Xu, X.; Liu, Z.; Zuo, Z.; Zhang, M.; Zhao, Z.; Shen, Y.; Zhou, H.; Chen, Q.; Yang, Y.; Wang, M. Hole Selective NiO Contact for Efficient Perovskite Solar Cells with Carbon Electrode. Nano Lett. 2015, 15 (4), 2402–2408. Kyaw, A. K. K.; Wang, D. H.; Gupta, V.; Leong, W. L.; Ke, L.; Bazan, G. C.; Heeger, A. J. Intensity Dependence of Current-Voltage Characteristics and Recombination in High-Efficiency Solution-Processed Small-Molecule Solar Cells. ACS Nano 2013, 7 (5), 4569–4577. Cowan, S. R.; Roy, A.; Heeger, A. J. Recombination in Polymer-Fullerene Bulk Heterojunction Solar Cells. Phys. Rev. B 2010, 82 (24), 245207. Wetzelaer, G. A. H.; Kuik, M.; Lenes, M.; Blom, P. W. M. Origin of the Dark-Current Ideality Factor in Polymer:fullerene Bulk Heterojunction Solar Cells. Appl. Phys. Lett. 2011, 99 (15), 153506. de Jong, M. P.; van IJzendoorn, L. J.; de Voigt, M. J. A. Stability of the Interface between Indium-Tin-Oxide and poly(3,4-Ethylenedioxythiophene)/poly(styrenesulfonate) in Polymer Light-Emitting Diodes. Appl. Phys. Lett. 2000, 77 (14), 2255–2257. Liu, J.; Shao, S.; Meng, B.; Fang, G.; Xie, Z.; Wang, L.; Li, X. Enhancement of Inverted Polymer Solar Cells with Solution-Processed ZnO-TiOX Composite as Cathode Buffer Layer. Appl. Phys. Lett. 2012, 100 (21), 213906. Zhang, Y.; Chen, L.; Hu, X.; Zhang, L.; Chen, Y. Low Work-Function Poly(3,4-Ethylenedioxylenethiophene): Poly(styrene Sulfonate) as Electron-Transport Layer for High-Efficient and Stable Polymer Solar Cells. Sci. Rep. 2015, 5, 12839.


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Enhanced Device Efficiency and Long-Term Stability via Boronic Acid

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