Effect of TiO2 modification with amino-based self-assembled monolayer on inverted organic solar cell

Accepted Manuscript Title: Effect of TiO2 Modification with Amino-Based Self-Assembled Monolayer on Inverted Organic Solar Cell Authors: Cem Tozlu, Adem Mutlu, Mustafa Can, Ali Kemal Havare, Serafettin Demic, Sıddık Icli PII: DOI: Reference:

S0169-4332(17)31784-1 http://dx.doi.org/doi:10.1016/j.apsusc.2017.06.128 APSUSC 36328

To appear in:

APSUSC

Received date: Revised date: Accepted date:

14-3-2017 29-5-2017 12-6-2017

Please cite this article as: Cem Tozlu, Adem Mutlu, Mustafa Can, Ali Kemal Havare, Serafettin Demic, Sıddık Icli, Effect of TiO2 Modification with AminoBased Self-Assembled Monolayer on Inverted Organic Solar Cell, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.06.128 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Effect of TiO2 Modification with Amino-Based Self-Assembled Monolayer on Inverted Organic Solar Cell

Cem Tozlu*a, Adem Mutlua, Mustafa Can**b, Ali Kemal Havarec, Serafettin Demicd, Sıddık Icli a

Department of Energy System Engineering, Faculty of Engineering, Karamanoglu Mehmetbey University, 70100 Karaman, Turkey

b

Department of Engineering Sciences, Faculty of Engineering, Izmir Katip Celebi University, Cigli, 35620 Izmir, Turkey

c

Department of Electrical and Electronics Engineering, Toros University, Mersin, Turkey d

Department of Material Science and Engineering, Faculty of Engineering, Izmir Katip Celebi University, Cigli, 35620 Izmir, Turkey

* Corresponding author. Fax: +90 338 2262214 ** Corresponding author. Fax: +90 232 3293999 E-mail addresses: [email protected] (C. Tozlu), [email protected] (M. Can)

HIGHLIGHTS

   

Modified TiO2 was used as electron selective electrode in organic solar cell Two functional self-assembled monolayer (SAM) molecules are proposed The effective work function of TiO2 is tuned significantly by SAM molecules Type of the functional units on SAM affects OSC performance significantly

Abstract The effects of surface modification of titanium dioxide (TiO2) on the performance of inverted type organic solar cells (i-OSCs) was investigated in this study. A series of benzoic acid derivatized self-assembled monolayer (SAM) molecules of 4'-[(hexyloxy)phenyl]amino-3,5-biphenyl dicarboxylic acid (CT17) and 4'-[1-naphthyl (phenyl)amino]biphenyl-4-carboxylic acid (CT19) were utilized to modify the interface between TiO2 buffer layer and poly-3 hexylthiophene (P3HT) : [6,6]-phenyl C61 butyric acid methyl ester (PC61BM) active layer having the device structure of ITO/ TiO2/SAM/P3HT: PC61BM/MoO3/Ag. The work function and surface wetting properties of TiO2 buffer layer served as

1

electron transporting layer between ITO and PC61BM active layer were tuned by SAM method. The solar cell of the SAM modified devices exhibited better performance . The power conversion efficiency (PCE) of i-OSCs devices with bare TiO2 electrodes enhanced from 2.00% to 2.21% and 2.43% with CT17 and CT19 treated TiO2 electrodes, respectively. The open circuit voltage (Voc) of the SAM treated TiO2 devices reached to 0.60 V and 0.61 V, respectively, while the Voc of untreated TiO2 was 0.57 V. The water contact angle of i-OSCs with CT17 and CT19 SAMs was also higher than the value of the unmodified TiO2 electrode. These results show that inserting a monolayer at the interface between organic and inorganic layers is an useful alternative method to improve the performance of iOSCs.

Keywords: Surface modification, SAM, interface, TiO2 ,organic solar cell

Introduction Organic solar cells (OSCs) have emerged as a potential candidate for photovoltaic applications compared to the silicon solar cells due to their simple production techniques, low manufacturing costs, flexibility, and light-weight sources of renewable energy [1-7]. Most of the OSCs are bulk heterojunction (BHJ) type devices which contain a mixture of π-conjugated polymers (donor) and fullerene derivatives (acceptor) from nanoscale interpenetrating networks. The device performance strongly depends on

photoinduced charge carrier

concentrations flowing through the OSC device under operation. The enhancement of the photoinduced charge carriers in the solar cell is achieved by providing low series resistance (Rs) and high parallel resistance (Rp) as well as a photo-absorption of an active layer. Therefore, an ohmic contact has to be established between metal contacts and the energy level of the highest occupied molecular orbital (HOMO) of donor material and the lowest unoccupied molecular orbital (LUMO) of acceptor material. Many tremendous efforts have been put to increase the PCE of BHJ solar cells for commercial structures [8-13]. A conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonete) (PEDOT:PSS) has been widely used as an anode buffer layer on ITO in common OSCs structure due to its advantages such as its high work function value, high transparency in the visible range and high hole mobility [8, 9, 12]. In contrast to its advantages, acidic nature of the PEDOT:PSS can lead to device degradation in long term performance [14]. Metal oxides are promising materials to be used as buffer layers in OSCs because of their better environmental stability, higher optical transparency, easier routes for their 2

syntheses and ohmic contact behaviors [15]. In order to obtain higher values of short circuit current (Jsc), open circuit voltage (Voc) and fill factor (FF) from OSC devices, a sufficient ohmic contact at the interface of inorganic and organic layers has to be established [16]. Metal oxides with low work function value such as TiO2 and ZnO have been used as cathode buffer material as a hole blocking layer in an i-OSC device due to matching the Fermi level of metal oxides with the LUMO of the electron-accepting fullerene derivatives as well as an optical window for interference and atmospheric oxygen barrier [17-21]. Unbounded oxygen or metal atoms on the metal oxide surface create high density of surface states which lead to recombination of photoinduced charge carriers at the interface between the metal oxide/organic active layer. To overcome the effects of surface states on the charge carriers, an interfacial modification technique has to be applied to the interface. The atomic modification of surface with molecules having capable of SAM formation is a cost effective technique to decrease these surface states [22, 23]. The interfacial modification with SAMs has many advantages on the development of the electronic device technology due to their in facile, fast and simple processing ways to alter the interfacial energy level offset. The performance of the devices of OSC type, dye sensitized solar cell (DSSC) and organic field effect transistor (OFET) can be further improved by the modification of the semiconductor/organic or metal/organic active with SAMs [24, 25]. By modifying the ITO surface with poly(dimethyl diallylammonium chloride) (PDDA) ), a useful SAM forming material, the rough surface of the ITO becomes smoother leading to effectively collect more charge carriers. Modification of the ITO surface with PDDA results in the decrease of the recombination charge carriers and increase of Jsc [26]. Furthermore, the surface properties of ITO can be easily converted from hydrophilic character to hydrophobic in pentacene/C60 bilayer heterojunctions by the application of SAM process. When the ITO/pentacene interface treated with phosphonic acid based surface modifiers, the work function of ITO layer is increased due to its improved hydrophobicity and decreased surface energy [27]. According to our knowledge up to now, TiO2 layer treated with SAM molecules having amino groups is not reported in the literature as an example for OSC application. In this study, we demonstrated the surface modification of TiO2 with a series of SAM modifiers using the molecules of CT17 and CT19 compounds as the cathode interlayer for electron injection. The effects of substituting functional groups such as naphthyl and hexloxy on the SAM were investigated on the performance of BHJ solar cell. We refer to CT17 treated TiO2 and CT19 treated TiO2 as TiO2/CT17 and TiO2/CT19, respectively. The effective work 3

function of TiO2 layer depends on the molecular groups of amino. Here, the physical and photovoltaic properties of BHJ solar cell TiO2 cathode buffer layer treated with SAM are reported. Material and Methods Materials All solvents and reagents were of puriss quality and used as received. Copper (I) iodide, 1bromohexane were purchased from Fluka. Acetone, dichloromethane, toluene, 18-crown-6, phenantroline, n-buthyllithium, 1,2-dimethoxyethane (DME), tetrahydrofuran (THF), trimethyl

borate,

[1,10-bi(diphenylphospino)ferrocene]dichloropalladium(II)

(DPPF),

titanium(IV) isopropoxide (Ti(OC3H7)4), ethanol (C2H5OH), acetic acid (CH3COOH) and trimethylamine (C3H9N) were purchased from Sigma-Aldrich. 4-Iodophenol and 4bromoaniline were purchased from Alfa Aesar. Potassium carbonate and potassium hydroxide were purchased from Riedel de Haen. PCBM and P3HT were purchased from Lumtec and used as received. Synthesis Synthesis of 4'-[1-naphthyl(phenyl)amino]biphenyl-4-carboxylic acid Pd(OAc)2 (0.045 g; 0.2 mmol) and P(t-Bu)3 (0.06 g; 0.3 mmol) were placed in a two necked ball and round-bottomed flask and allowed to stand at room temperature for 15 min. Nphenyl-1-naphthylamine (1 g; 4.6 mmol), dibromobenzene (1.44 g; 6 mmol) and dry toluene (13 mL) were then added to the flask in the given order, and the whole mixture was heated on oil bath until the temperature reached 80 °C. Upon reaching this temperature, t-BuOK (0.576 g, 6 mmol) in powder form was added and then the mixture was boiled under reflux for 20 hours. Then the flask was cooled to room temperature and glacial acetic acid (30 mL) was added to the reaction flask. The resulting mixture was extracted with CH2Cl2 (3 x 3 mL), the organic phases were combined. The organic phase was washed first with saturated NaHCO3 and (50 mL) followed by brine (40 mL). The removal of organic solvent produced brown liquid which was then purified by passing the crude product through a short silica gel column with a solvent mixture of CH2Cl2:P.E (2:1). The pre-purified crude product was again subjected to column purification to obtain a white solid matter was formed and it proceeded directly to the next step.

4

To a solution of N-(4-bromophenyl)-N-phenylnaphthalen-1-amine (374 mg, 1 mmol), [4(methoxycarbonyl) phenyl]boronic acid (252 mg, 1.4 mmol) in anhydrous THF (16 mL) , Pd(dppf)Cl2 (17 mg,0.048 mmol) were added. The reaction temperature was gradually increased and when it reached to 50 °C, 2 mL of K2CO3 (1M) was added and stirred at 90 °C for 22 h. At the end, the reaction mixture was let cool to room temperature and the reaction mixture was quenched by the addition of water (40 mL) and CH2Cl2 (40 mL). The organic phases were combined and evaporated. The crude product was purified by column chromatography (silica gel, CH2Cl2:P.E 2:1) and then dried in vacuo at (50 oC) to produce an off-white solid. Methyl 4'-[1-naphthyl(phenyl)amino]biphenyl-4-carboxylate:

1

H NMR

(CHCl3): 8.13 (d, 2H), 8.06 (d, 1H), 7.95 (d, 1H), 7.85 (d, 1H), 7.64 (d, 2H), 7.49 (d, 4H), 7.42 (dd, 2H), 7.29 (t, 2H), 7.20 (d, 2H), 7.12 (d, 2H), 3.95 (s, 3H). In a two necked flask, methyl 4'-[1-naphthyl (phenyl) amino]biphenyl-4-carboxylate (0.27 g, 1 mmol) was boiled in 1 mL of 2 N KOH in THF/MeOH. After cooling to room temperature and then acidification (2N HCl) resulted in the formation of whitish particles. The solid was collected on sintered glass funnel and dried under vacuum (whitish solid). 4'-[1Naphthyl(phenyl)amino]biphenyl-4-carboxylic acid, (CT19); 1H NMR (CHCl3): 8.13 (d, 2H), 7.96 (d, 1H), 7.91 (d, 1H), 7.81 (d, 1H), 7.64 (d, 2H), 7.49 (m, 4H), 7.38 (t, 2H), 7.25 (t, 2H), 7.14 (d, 2H), 7.06 (d, 2H), 6.99 (t, 1H); FT-IR (KBr, cm-1): –OH, 3450; C=O, 1686; C=C–H, 3040; (C=C–Ph) 1534 ve 1492. Synthesis of 4' -bis[4-(hexyloxy)phenyl] amino biphenyl-3,5-dicarboxylic acid To a solution of (4-{bis[4-(hexyloxy)phenyl]amino}phenyl)boronic acid (730 mg, 1.4 mmol), which was synthesized in our previously article [28], dimethyl 5-bromoisophthalate (273 mg, 1 mmol) in anhydrous THF (16 mL), Pd(dppf)Cl2 (17 mg, 0.048 mmol) were added. The reaction temperature was gradually increased and when it reached to 50 °C, 2 mL of K 2CO3 (1M) was added and stirred at 90 °C for 22 h. At the end, the reaction mixture was let cool to room temperature and the reaction mixture was quenched by the addition of water (40 mL) and CH2Cl2 (40 mL). The organic phases were combined and evaporated. The crude product was purified by column chromatography (silica gel, CH2Cl2:PE, 3:1) and then dried in vacuo at

(50

o

C)

to

produce

oily

green

molecules.

Dimethyl

4'-{bis[4-

(hexyloxy)phenyl]amino}biphenyl-3,5-dicarboxylate: 1H NMR (CHCl3): 8.58 (s, 1H), 8.39 (s, 2H), 7.46 (d, 2H), 7.08 (d, 4H), 7.00 (d, 2H), 6.85 (d, 4H), 3.94 (s, 10H), 1.77 (s, 4H), 1.46 (s, 4H), 1.27 (s, 8H), 0.91 (s, 6H)

5

In

a

two

necked

flask,

dimethyl

4'-{bis[4-(hexyloxy)phenyl]amino}biphenyl-3,5-

dicarboxylate (0.318 g, 0.5 mmol) was boiled in 2 mL of 2 N KOH in THF/MeOH. After cooling to room temperature and then acidification (2N HCl) resulted in the formation of greenish particles. The solid was collected on sintered glass funnel and dried under vacuum (greenish solid). 1H NMR (CDCl3): 13.61 (s, 2H), 8.33 (s, 1H), 7.90 (s, 1H), 7.47 (s, 1H), 6.90 (m, 2H), 6.74 (m, 10H), 3.91 (s, 4H), 1.81 (s, 4H), 1.41 (s, 12H), 0.97 (s, 6H); FT-IR (KBr, cm-1): –OH, 3466; C=O, 1702; (C–H, aliphatic), 2933 ve 2863; C=C–H, 3040; –C=C–, 1602; (C=C, Ph) 1506 ve 1468; C–O, 1239. Synthesis of TiO2 TiO2 film was prepared by sol-gel route as described in the previous report [29]. Ti(OC3H7)4 was added to ethanol and stirred one hour at room temperature. CH3COOH and C3H9N were added to TiO2 solution in ethanol in the given order. Then finally the solution was stirred vigorously for 3 h in order to obtain homogeneous solution and aged for over night. Instrumentation The cyclic voltammetry (CV) measurements were conducted by CHI 660 B potentiostat. The contact potential differences (CPD) were characterized by a Kelvin probe microscopy (KPM) (NTD-MDT, Ntegra Solaris). All the contact potential differences were measured with respect to the conduct Pt (NSG03/Pt) with a force constant of 0.35-6.06 N/m and resonance frequency of 47-150 kHz at room temperature. Contact angle measurements were obtained by Data Physics OCA 50 instrument with water droplets. The dark and light current density-voltage curves of OSCs measurements (J-V) were performed with a Keithley 2400 source meter in the glove box, OAI Trisol (class AAA, 1000W) coupled with an AM1.5 filter was used as the light source and a light intensity of a 1000 W/m2 was used in all measurements. A certified Si reference solar cell was used to calibrate the light intensity before each measurement. OSC fabrication The fabricated OSC devices with TiO2/SAMs and TiO2 cathode electrode configurations and the molecular structure of CT17 and CT19 are shown in Fig. 1. The ITO substrates (with a sheet resistance of 15 Ω/sq) were placed in an ultrasonic bath and cleaned in highly pure deionized water, acetone and isopropyl alcohol, respectively. After drying the ITO substrates over N2 stream gas, UV ozone treatment was applied to them for 30 minutes. Before coating the TiO2 layer, already prepared TiO2 solution was filtered through a 0.45 𝜇m filter. It was 6

spin-coated on the ITO surface as a cathode buffer layer at 1500 rpm for 1 min. Then each substrate spin-coated by TiO2 solutions was transferred to high temperature furnace and annealed at 450℃ for 1h to obtain a thin film of TiO2 in anatase crystalline form. To deposit SAMs, TiO2 coated substrates were dipped into 1mM SAM solutions of CT17 and CT19 (dissolved in acetonitrile) and kept in solutions for 24 hours at room temperature. After formation of SAMs, spin-coating technique was applied to deposit the photoactive layers on bare and SAM treated TiO2 layers with a solution of P3HT and PCBM (1:1 w/w) in chlorobenzene. MoO3 buffer layer with a thickness of 10 nm thickness and Ag anode electrodes of 80 nm were deposited by thermal evaporation under high vacuum at 7x10-7 torr with the use of a shadow mask in this given order to complete the device fabrication.

Fig.1 Results and Discussion

Cyclic voltammetry measurements of SAM molecules CV is an important technique to determine the electron transfer characteristic of formed monolayer film. In order to investigate the effect of SAM modification using different acceptor and donor groups on TiO2 surface and checking the oxidation potential shift of ITO/TiO2/SAM electrodes with respect to the untreated ITO/TiO2 surface, CV technique is a very simple and powerful tool. For this purpose, a solution of 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) dissolved in acetonitrile solution was used as a redox active compound with a scan rate of 50 mV/s. While ITO/TiO2 and ITO/TiO2/SAM cathode electrodes were used independently as working electrode, a saturated Ag/AgCl and platinum wire were used as reference electrode and platinum counter electrode, respectively. The ITO/TiO2 working electrode was the reference electrodes to observe the monolayer effect on the oxidation potential that happens on TiO2 surface. As seen in Fig. 2, the observed peaks clearly indicate the oxidation process occurring on CT17 and CT19 treated TiO2 electrodes in the given potential interval when this behavior is compared to the bare TiO2 electrode which has no electrochemical activity. The oxidation potential was shifted to higher values by substituting naphthyl group (CT19) compared with hexyloxy group (CT17) on the phenyl carboxylic acid. The oxidation process brought about by CT17 SAM molecule treated TiO2 has occurred with a high current rate at lower potential than that of CT19. Better performance

7

of ITO/TiO2/CT17 working electrode is probably due to better electron donating property of hexyloxy group substituted on phenyl. Oxidation and calculated HOMO values of SAM modified TiO2 working electrodes (ITO/TiO2/CT17 and ITO/TiO2/CT19) are tabulated in Table 1. Also, these CV results of cathode electrodes indicate that the SAM molecules have been coated satisfactorily on TiO2 surface.

Table 1

Fig. 2

Contact angle measurements The contact angle measurements based on the principle of wetting surface with water are another technique that proves the surface modification. The change on wetting properties of SAM treated and bare TiO2 surface were determined by the use of contact angle measurements at room conditions. A deionized water-drop with a volume of 2 µL was used to observe how water-droplet spreads out on each surface. The coverage of water droplet on the SAM coated TiO2 as well as the bare TiO2 surface and the contact angle values are given in Fig. 3 and Table 2, respectively. As seen in Fig.3-a, the lowest contact angle value was obtained from bare TiO2 surface because of the hexyloxy terminated surface. The treatment of TiO2 surface with CT17 and CT19 SAM molecules induced changes in the polarity of surface and the surface energy as well. The contact angle was obtained at highest value on the CT19 coated TiO2 surface. CT19 molecule contains apolar naphthyl and phenyl groups while CT17 has polar hexyloxy substituted phenyl groups as shown in Fig. 1. The presence of hexyloxy group on phenyl which conforms the outer periphery of the molecule after achoring on TiO2 bring in polar feature to the surface compared to bare naphthyl and phenyl groups and therefore the surface coated with CT17 exhibits more hydrophilic character because of etheric oxygen atom in the molecule than CT19.

Table 2

8

Fig. 3

X-Ray photoelectron spectroscopy X-Ray photoelectron spectroscopy (XPS) was used to determine chemical compositions of TiO2 surface treated with CT17 and CT19 molecules. Fig. 4 shows a survey and high resolution spectrum of C(1s) peak obtained from TiO2/CT17 and TiO2/CT19 surface. The peaks, Ti(2p), O(1s), C(1s) and N(1s) are seen in survey spectra with binding energies of 458.4, 529.6, 284.6 and 399.7 eV, respectively. The corresponding binding energy values of functional group for CT19 and CT17 on TiO2 surface are summarized in Table 3. Ti peaks from survey spectra in Fig. 4 (a and d) confirm the success of immobilization of SAM molecules on TiO2 surface as a thin layer. In order to analyze carbon atomic bond on the surface, the high resolution spectrum of C(1s) was fitted with a Shirley background. The signals coming from TiO2 surface with CT17 molecule with binding energies of 284.6, 285.9 and 288.3 eV are attributed to C—C and C—H, C—N and O—C=O bonds [30], respectively. The peak of O—C=O at 288.6 eV clearly indicates successfully the formation of the covalent bonding between the carboxylic acid (—COOH) head group of SAM molecule and hydroxyl (—OH) group of TiO2 surface. The peaks with binding energies of 284.6, 285.8 and 288.2 eV as seen in Fig.4 (e) are obtained from TiO2 surface with CT19 molecule. Since these peak values do corresponds to the same covalent bonds as in CT17 modified TiO2 surface, they must be identical or vice versa. The indication of immobilized SAM molecules on TiO2 surface is strongly supported by O—C=O covalent bonding observed sapecifically 288.2 eV. O(1s) high resolution spectra of both SAM molecules are given in Fig.4(c and f) for deeper analysis of surface atomic bondings between surface atoms and head functional group (— COOH) SAM molecules. The peaks at 529.6 and 531.5 eV for TiO2 surface with CT17 corresponds to lattice oxide, i.e the O2‒ in TiO2 [31] and C=O [32], respectively. The high resolution spectra of O(1s) for TiO2 surface with CT19 is almost the same as CT17 with the value of 529.62 and 531.28 eV.

Fig.4

Atomic force microscopy (AFM) 9

Atomic force microscopy (AFM) images were applied to investigate the effects of SAM molecules on TiO2 surface from their topographic information. Fig. 5 shows the experimental surface topography images of bare TiO2 and SAM treated TiO2 surfaces. As they imply, island like structures are formed on TiO2 surface after SAM modification compared to the bare TiO2 surface. Moreover, the contact potential difference results (VCPD) were measured by the KPM to confirm work function changes in SAM treated and bare TiO2 surfaces and observe the dipole effects on the surfaces. The KPM method, a compatible version of AFM, is a wellknown technique for measuring work functions of electrodes and CPD values are determined by the potential differences between the tip and the sample. While the VCPD between bare TiO2 electrode and conduct Pt tip was measured as 0.333 V, the contact potential differences between CT17&CT19 coated TiO2 electrode and conduct Pt tip were obtained as 0.686 V and 0.783 V, respectively. As given in Table 4, the effective work function of untreated TiO2 (4.32 eV) is larger than that of TiO2/CT17 (3.96 eV) and TiO2/CT19 (3.87 eV). Fig. 5

The permanent molecular dipole direction of SAM molecules directly affects the effective work function of TiO2. The dipole orientation of CT17 and CT19 molecules is in the same direction which is obviously away from TiO2. The carboxylic acid terminal group on CT17 and CT19 behaves like electron withdrawing moiety with respect to the other end part of these molecular structures (naphthyl&phenyl and hexyloxy substituted phenyl groups). For this reason, after the esterification reaction occurred between carboxylic acid part of CT17&CT19 molecules and TiO2 surface, while partial negative charges are mostly accumulated on the carboxylate part, the counter partial positive charges are formed on TiO2 surface. Therefore, the permanent dipole is directed outwards from TiO2 surface as shown in Fig. 6. The effective work functions of TiO2/CT17 and TiO2/CT19 are smaller than that of untreated TiO2 (Fig.7).

Fig.6

Fig. 7

Current–Voltage Characteristics of the Devices

10

Fig.8 shows the dark and light current density-voltage (J-V) characteristics of the i-OSCs having SAM treated and untreated TiO2. The measurements of solar cells are carried out under 100 mW/cm2, A.M. 1.5G, at room temperature conditions and its parameters are listed in Table 5. The highest Voc value of the solar cell is 610 mV obtained from OSC with TiO2/CT19 electrode while the other Voc values are 600 mV and 580 mV for TiO2/CT17 and TiO2, respectively. The increase in Voc value is attributed to the effective work function value and the highest voltage is obtained from TiO2/CT19 with smallest effective work function. The fermi level pinning at the interface between cathode contact and PCBM leads to increase in Voc value when there is a material with lower work function value than the LUMO level of PCBM and this material has been interfaced with PCBM. The theoretical Voc is achieved if the ohmic contact is established between LUMO of PCBM and cathode buffer layer. The mismatching ohmic contact between LUMO of PCBM and cathode buffer layer leads to a decrease in Voc value due to the differences in LUMO level of PCBM and HOMO level of P3HT [33, 34]. For this reason, the maximum value of the Voc of the i-OSCs structure is directly related to the position of the fermi level of the electron and hole buffer layers, as well as the difference in LUMO level of PCBM and HOMO level of P3HT. The power conversion efficiency (PCE) of i-OSCs with TiO2/CT19 and TiO2/CT17 reaches the value of 2.43% and 2.21%, respectively while this value is 2% for untreated TiO2 buffer layer. Other solar cell parameters are summarized in Table 5 and J-V curves for each i-OSCs under dark and light conditions are given in Fig. 8. The values of short circuit (Jsc) with TiO2, TiO2/CT17 and TiO2/CT19 electrode configurations were obtained as 5.30, 5.35 and 5.82 mA/cm2, respectively. These results clearly indicates that the devices with SAM treated TiO2 buffer electrodes exhibit better Jsc values when compared to untreated TiO2 buffer electrodes.

Fig. 8

Table 5

The fill factor (FF) of the fabricated devices is also improved from 65.7% to 68.9% by using SAM treated TiO2 buffer electrodes. The series resistance (Rs) and the parallel resistance (Rp) of solar cell are one of the important parameters that affect directly the FF and Jsc values. The Rs values were measured as 16.6, 12.2 and 11.2 Ω. cm2 from the devices of TiO2, TiO2/CT17 11

and TiO2/CT19, respectively. The Rs values of i-OSCs are decreased in the device having SAMs treatment when compared to the one without SAM curing. Moreover, the Rp values of i-OSCs with SAMs treated TiO2 buffer layer are higher than the untreated SAM devices. The calculated Rp values are 1.20, 1.40 and 1.29 kΩ. cm2 for the devices ofTiO2, TiO2/CT17 and TiO2/CT19, respectively. Our results show that the interface modification between active layer and charge transport layer improves the solar cell parameter effectively by the change of fermi level in inorganic part of i-OSCs. To show the solar cell parameters effective on the performance of i-OSCs, the measurement of the incident photon to current conversion efficiency (IPCE) are carried out for fabricated devices as shown in Fig. 9. The highest IPCE value is obtained as 45.4% by TiO2/CT19 device, which is higher than that of TiO2/CT17 (43.6%) and bare TiO2 (38.0%). Our results clearly indicate that one of the strategy to increase the performance of solar cell is to improve the FF, Rs and Rp values by the modification interface property between active and transport layer.

Fig. 9

Conclusion This study is focused on the effect of interface between inorganic TiO2 buffer layer and organic active layer

as well as the device physics on the solar cell applications. The

monolayer formation by carboxylic acid terminated molecules were used systematically to modify the surface of TiO2 cathode buffer layer. The modification of the interface between contacting materials results in the improvement of solar cell parameters such as Voc, Isc, FF and PCE. The main factor in the improvement of Voc is the tuning work function of cathode buffer layer at the interface between organic and inorganic parts of solar cell. The molecular dipole orientation of SAM at the interface is also one of the reasons for the improvement of the solar cell performance. This strategy emerges as an alternative method of developing organic active layers to improve the solar cell efficiency. The well establishment of the energy levels of the materials corresponding to formation of interfaces leads to improved solar cell efficiency by changing the effective parameters in a positive aspect.

Acknowledgements

12

This article is dedicated to Prof. Dr. Sıddık Icli on the occasion of his 70th birthday. The authors are grateful to Karamanoglu Mehmetbey University (Project 11-M-13) for financial support.

References [1] C.W. Tang, Two‐layer organic photovoltaic cell, Applied Physics Letters, 48 (1986) 183185. [2] F.C. Krebs, S.A. Gevorgyan, J. Alstrup, A roll-to-roll process to flexible polymer solar cells: model studies, manufacture and operational stability studies, Journal of Materials Chemistry, 19 (2009) 5442-5451. [3] H. Hoppe, N.S. Sariciftci, Organic solar cells: An overview, J. Mater. Res, 19 (2004) 1924-1945. [4] C.J. Brabec, N.S. Sariciftci, J.C. Hummelen, Plastic solar cells, Advanced functional materials, 11 (2001) 15-26. [5] C. Lungenschmied, G. Dennler, H. Neugebauer, S.N. Sariciftci, M. Glatthaar, T. Meyer, A. Meyer, Flexible, long-lived, large-area, organic solar cells, Solar Energy Materials and Solar Cells, 91 (2007) 379-384. [6] S.E. Shaheen, C.J. Brabec, N.S. Sariciftci, F. Padinger, T. Fromherz, J.C. Hummelen, 2.5% efficient organic plastic solar cells, Applied Physics Letters, 78 (2001) 841-843. [7] S. Günes, H. Neugebauer, N.S. Sariciftci, Conjugated polymer-based organic solar cells, Chemical reviews, 107 (2007) 1324-1338. [8] W.-J. Yoon, P.R. Berger, 4.8% efficient poly (3-hexylthiophene)-fullerene derivative (1: 0.8) bulk heterojunction photovoltaic devices with plasma treated Ag O x/indium tin oxide anode modification, Applied Physics Letters, 92 (2008) 6.

13

[9] Z. Hu, J. Zhang, Z. Hao, Y. Zhao, Influence of doped PEDOT: PSS on the performance of polymer solar cells, Solar Energy Materials and Solar Cells, 95 (2011) 2763-2767. [10] F. Liu, S. Shao, X. Guo, Y. Zhao, Z. Xie, Efficient polymer photovoltaic cells using solution-processed MoO 3 as anode buffer layer, Solar Energy Materials and Solar Cells, 94 (2010) 842-845. [11] V. Shrotriya, G. Li, Y. Yao, C.-W. Chu, Y. Yang, Transition metal oxides as the buffer layer for polymer photovoltaic cells, Applied Physics Letters, 88 (2006) 073508. [12] X. Fan, C. Cui, G. Fang, J. Wang, S. Li, F. Cheng, H. Long, Y. Li, Efficient Polymer Solar Cells Based on Poly (3‐hexylthiophene): Indene‐C70 Bisadduct with a MoO3 Buffer Layer, Advanced functional materials, 22 (2012) 585-590. [13] Y.-C. Tseng, A.U. Mane, J.W. Elam, S.B. Darling, Ultrathin molybdenum oxide anode buffer layer for organic photovoltaic cells formed using atomic layer deposition, Solar Energy Materials and Solar Cells, 99 (2012) 235-239. [14] M. De Jong, L. Van Ijzendoorn, M. De Voigt, Stability of the interface between indiumtin-oxide and poly (3, 4-ethylenedioxythiophene)/poly (styrenesulfonate) in polymer lightemitting diodes, Applied Physics Letters, 77 (2000) 2255-2257. [15] S. Chen, J.R. Manders, S.-W. Tsang, F. So, Metal oxides for interface engineering in polymer solar cells, Journal of Materials Chemistry, 22 (2012) 24202-24212. [16] S. Lattante, Electron and hole transport layers: their use in inverted bulk heterojunction polymer solar cells, Electronics, 3 (2014) 132-164. [17] R. Lin, M. Miwa, M. Wright, A. Uddin, Optimisation of the sol–gel derived ZnO buffer layer for inverted structure bulk heterojunction organic solar cells using a low band gap polymer, Thin Solid Films, 566 (2014) 99-107. [18] T. Yang, W. Cai, D. Qin, E. Wang, L. Lan, X. Gong, J. Peng, Y. Cao, Solution-processed zinc oxide thin film as a buffer layer for polymer solar cells with an inverted device structure, The Journal of Physical Chemistry C, 114 (2010) 6849-6853. [19] C. Waldauf, M. Morana, P. Denk, P. Schilinsky, K. Coakley, S. Choulis, C. Brabec, Highly efficient inverted organic photovoltaics using solution based titanium oxide as electron selective contact, Applied Physics Letters, 89 (2006) 233517. [20] J.Y. Kim, S.H. Kim, H.H. Lee, K. Lee, W. Ma, X. Gong, A.J. Heeger, New Architecture for high‐efficiency polymer photovoltaic cells using solution‐based titanium oxide as an optical spacer, Advanced materials, 18 (2006) 572-576. [21] V. Vohra, K. Kawashima, T. Kakara, T. Koganezawa, I. Osaka, K. Takimiya, H. Murata, Efficient inverted polymer solar cells employing favourable molecular orientation, Nature Photonics, 9 (2015) 403-408. [22] M. Salinas, Interface Engineering with Self-assembled Monolayers for Organic Electronics, (2014). [23] I. Campbell, S. Rubin, T. Zawodzinski, J. Kress, R. Martin, D. Smith, N. Barashkov, J. Ferraris, Controlling Schottky energy barriers in organic electronic devices using selfassembled monolayers, Physical Review B, 54 (1996) R14321. [24] S. Kobayashi, T. Nishikawa, T. Takenobu, S. Mori, T. Shimoda, T. Mitani, H. Shimotani, N. Yoshimoto, S. Ogawa, Y. Iwasa, Control of carrier density by self-assembled monolayers in organic field-effect transistors, Nature materials, 3 (2004) 317-322. 14

[25] S. Wooh, T.-Y. Kim, D. Song, Y.-G. Lee, T.K. Lee, V.W. Bergmann, S.A. Weber, J. Bisquert, Y.S. Kang, K. Char, Surface modification of TiO2 photoanodes with fluorinated self-assembled monolayers for highly efficient dye-sensitized solar cells, ACS applied materials & interfaces, 7 (2015) 25741-25747. [26] M. Deng, W. Shi, C. Zhao, B. Chen, Y. Shen, ITO surface modification for inverted organic photovoltaics, Frontiers of Optoelectronics, 8 (2015) 269-273. [27] A. Sharma, A. Haldi, W.J. Potscavage Jr, P.J. Hotchkiss, S.R. Marder, B. Kippelen, Effects of surface modification of indium tin oxide electrodes on the performance of molecular multilayer organic photovoltaic devices, Journal of Materials Chemistry, 19 (2009) 5298-5302. [28] M. Can, H. Bilgili, K. Demirak, B. Gultekin, S. Koyuncu, M. Kus, C. Zafer, S. Demic, S. Icli, Push-pull type material having spirobifluorene as p-spacer for dye sensitized solar cells, , International Journal of Hydrogen Energy, 41 (2016) 21293–21299 [29] O. Pakma, N. Serin, T. Serin, Ş. Altındal, On the energy distribution profile of interface states obtained by taking into account of series resistance in Al/TiO 2/p–Si (MIS) structures, Physica B: Condensed Matter, 406 (2011) 771-776. [30] B. Yameen, M. Ali, M. Álvarez, R. Neumann, W. Ensinger, W. Knoll, O. Azzaroni, A facile route for the preparation of azide-terminated polymers.“Clicking” polyelectrolyte brushes on planar surfaces and nanochannels, Polymer Chemistry, 1 (2010) 183-192. [31] E. Binkauskiene, A. Lugauskas, M. Krunks, I.O. Acik, V. Jasulaitiene, G. Saduikis, Interaction of Chrysosporium merdarium with titanium oxide surface, Synthetic metals, 160 (2010) 906-910. [32] F. Montagne, J. Polesel-Maris, R. Pugin, H. Heinzelmann, Poly (N-isopropylacrylamide) thin films densely grafted onto gold surface: preparation, characterization, and dynamic AFM study of temperature-induced chain conformational changes, Langmuir, 25 (2008) 983-991. [33] V. Mihailetchi, P. Blom, J. Hummelen, M. Rispens, Cathode dependence of the opencircuit voltage of polymer: fullerene bulk heterojunction solar cells, Journal of Applied Physics, 94 (2003) 6849-6854. [34] C.J. Brabec, A. Cravino, D. Meissner, N.S. Sariciftci, T. Fromherz, M.T. Rispens, L. Sanchez, J.C. Hummelen, Origin of the open circuit voltage of plastic solar cells, Advanced functional materials, 11 (2001) 374-380.

15

Figure Captions:

Figure 1. The schematic illustration of organic solar cells with SAMs treated TiO2 and the chemical structure of (a) CT17 and (b) CT19 molecules Figure 2. The cyclic voltammetry results of CT17 and CT19 on TiO2 surface with respect to TiO2 electrode Figure 3. Water contact angle images of (a) bare TiO2, (b) CT17 treated TiO2 and (c) CT19 treated TiO2 surface Figure 4. (a) XPS survey spectra of CT17 treated TiO2, (b) the high resolution spectra of C(1s) and (c) O(1s) peak of CT17 treated TiO2, (d) XPS survey spectra of CT19 treated TiO2, (e) the high resolution spectra of C(1s) and (f) O(1s) peak of CT19 treated TiO2 Figure 5. AFM topography images of (a) bare TiO2, (b) CT17 and (c) CT19 treated TiO2 surface Figure 6. The energy band diagram of i-OSCs with SAMs treated and the interface dipole directions of CT17 and CT19 molecules. Figure 7. The work functions of untreated TiO2, CT17 and CT19 treated TiO2 electrodes. Figure 8. Current density-voltage characteristics of i-OSCs without and with CT17 and CT19 treated TiO2 electrodes under AM.1.5 simulated illumination with an intensity of 100 mW/cm2 and the dark condition. Figure 9. The IPCE spectra of i-OSCs with TiO2/SAM electrodes and only bare TiO2 electrode.

Fig 1.

16

Fig. 2

Fig 3.

17

Fig 4.

18

19

20

Fig.5 21

(a)

(b)

22

(c) Fig. 6

23

Fig. 7

Fig.8

24

Fig. 9

25

Table Captions:

Table 1. The HOMO energy levels of SAMs Table 2. The contact angle values of the bare and SAM modified TiO2 surface Table 3. Corresponding binding energy values of C, O and N atoms for CT17 and CT19 treated TiO2 electrodes Table 4. The average value of contact potential difference between surfaces and conducting tip and work function of bare and SAMs treated TiO2 electrodes. Table 5. Solar cell parameters of i-OSCs with SAM treated TiO2 and untreated TiO2 cathode electrodes Table 1. CT17

CT19

Eox (V)

1.16

1.33

EHOMO(eV)

5.56

5.73

Table 2. Cathode Electrode Configuration

Contact Angle Values (°)

ITO/TiO2

9.9

ITO/TiO2/CT17

26.4

ITO/TiO2/CT19

70.1

Table 3. C 1s (eV)

O 1s (eV)

N 1s (eV)

Samples O2-

C=O

TiO2/CT17

529.68

531.50

284.64

285.92

288.33

399.70

TiO2/CT19

529.62

531.28

284.60

285.80

288.16

399.70

C—C and C—H

C—N

O=C—O

26

Table 4. Cathode Electrode Configuration

Work Function (eV)

CPD (V)

ITO/TiO2

4.32

0.333

ITO/TiO2/CT17

3.96

0.686

ITO/TiO2/CT19

3.87

0.783

Table 5. Cathode Electrode

VOC

JSC

FF

RS

RP

(mV)

(mA/cm2)

(%)

(ohm.cm2)

(ohm.cm2)

Configuration

Average

Best

PCE

PCE

(%)

(%)

ITO/TiO2

570

5.30

65.7

16.6

1206

1.94

2.00

ITO/TiO2/CT17

600

5.35

68.9

12.2

1403

2.19

2.21

ITO/TiO2/CT19

610

5.82

68.4

11.2

1295

2.40

2.43

27