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Syntheses, Charge Separation and Inverted Bulk Heterojunction Solar Cell Application of Phenothiazine-Fullerene Dyads Gwendolyn D. Blanco, Arto J. Hiltunen, Gary N. Lim, Chandra Bikram KC, Kimmo Kaunisto, Tommi Vuorinen, Vladimir N. Nesterov, Helge J. Lemmetyinen, and Francis D'Souza ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00561 • Publication Date (Web): 18 Mar 2016 Downloaded from http://pubs.acs.org on March 21, 2016

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ACS Applied Materials & Interfaces

Syntheses, Charge Separation and Inverted Bulk Heterojunction Solar Cell Application of Phenothiazine-Fullerene Dyads Gwendolyn D. Blanco,a Arto J. Hiltunen,b Gary N. Lim,a Chandra B. KC,a Kimmo M. Kaunisto,b* Tommi K. Vuorinen,c Vladimir N. Nesterov,a Helge J. Lemmetyinen,b Francis D’Souza*a a

Department of Chemistry, University of North Texas, 1155 Union Circle, #305070, Denton,

TX 76203-5017, USA. b

Department of Chemistry and Bioengineering, Tampere University of Technology, Finland.

c

VTT Technical Research Centre of Finland Ltd.

ABSTRACT: A series of phenothiazine-fulleropyrrolidine (PTZ-C60) dyads having fullerene either at the C-3 aromatic ring position or at the N-position of phenothiazine macrocycle were newly synthesized and characterized. Photoinduced electron transfer leading to PTZ•+-C60•charge separated species was established from studies involving femtosecond transient absorption spectroscopy. Due to the close proximity of the donor and acceptor entities, the C-3 ring substituted PTZ-C60 dyads revealed faster charge separation and charge recombination processes than that observed in the dyad functionalized through the N-position. Next, inverted organic bulk heterojunction (BHJ) solar cells were constructed using the dyads in place of traditionally used PCMB and an additional electron donor material poly(3-hexylthiophene), P3HT. The performance of the C-3 ring substituted PTZ-C60 dyad having polyethylene glycol substituent produced power conversion efficiency of 3.5 % under inverted bulk heterojunction (BHJ) configuration. This was attributed to optimal BHJ morphology between the polymer and dyad that was further promoted by the efficient intramolecular charge separation and slow charge recombination promoted by the dyad within the BHJ structure. The present finding demonstrate PTZ-C60 dyads as materials of high prospective to build organic photovoltaic devices. KEYWORDS: fullerene, phenothiazine, photoinduced electron transfer, femtosecond transient spectroscopy, inverted bulk heterojunction, organic photovoltaics

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1. INTRODUCTION Among the light-to-electricity conversion devices, organic photovoltaic (OPV) devices are becoming increasingly popular and have been investigated extensively during the last two decades.1-5 Lower manufacturing cost by utilizing various printing techniques and solution processable materials, light weight solar modules, and flexible large-area applications are some of the salient features that OPVs offer.6-9 In a typical OPV assembly, electron donor and acceptor materials are blended together to form a film with interpenetrating donor-acceptor network structure during evaporation of the solvent.10-13 Often, such a bulk heterojunction (BHJ) is composed of a conjugated polymer, such as poly(3-hexylthiophene), P3HT, as a light absorbing/electron donating material, and soluble fullerene derivatives, such as [6,6]-phenyl-C61butyric acid methyl ester, PCBM, as an electron acceptor. Through molecular engineering, the miscibility and energy levels of the donor and the acceptor are often tuned to improve the overall efficiency of the cell. Recently, power conversion efficiencies (PCEs) in the range of 10% have been reached for organic solar cells.14-17 Fulleropyrrolidines, a class of vicinally disubstitued-organo-fullerenes, are common type of acceptors used in building photosynthetic supramolecular model compounds and OPVs.18-28 These are synthesized through the 1,3-dipolar cycloaddition of azomethine ylides,29 with several attributes including high electron affinity, small reorganization energy, and good charge transport ability. Importantly, their structures can be modified by organic addends or donor entities. Incidentally, phenothiazine (PTZ) an electroactive heterocyclic compound with electronrich sulfur and nitrogen atoms, have recently attracted considerable interest as electron donors and sensitizers in dye sensitized solar cells (DSSCs).30 Consequently, donor-acceptor dyads comprised of phenothiazine and fulleropyrrolidine (PTZ-C60) have been investigated for their ability to undergo photochemical charge separation31-33 and applications in organic solar cells.34,35 It has been suggested that the donor-acceptor dyads provide sufficient HOMO–HOMO and LUMO–LUMO offsets of the donor and the acceptor entities to help exceed the Coulombic attraction of the hole-electron pair, necessary to overcome the exciton binding energy to avoid the back charge transfer.34,35 In the present investigation, we have synthesized a new series of PTZ-C60 dyads wherein the phenothiazine C-3 position (dyads 1 and 2) or heteroatom N position (dyad 3) have been functionalized to possess fullerene (see Figure 1 for structures). Additionally, dyads 1 and 2 are


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substituted with N-hexyl or tri(ethylene glycol) chains, respectively, for better solubility and miscibility needed in device fabrication. The newly synthesized compounds have been characterized by spectral (1H, 13C and MALDI-mass) and electrochemical studies. Photoinduced electron transfer has been established from studies involving femtosecond transient absorption studies in polar and nonpolar solvents, and substitution effect of phenothiazine macrocycle on charge stabilization has been arrived. Finally, inverted BHJ solar cells have been constructed and their film morphology and photovoltaic performance have been evaluated with respect to phenothiazine ring substitution.

Figure 1. Chemical structures of the tailored phenothiazine-fullerene dyads synthesized and studied in inverted BHJ-OPVs. 2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Buckminsterfullerene, C60 (99.95% purity), was obtained from SES Research (Houston, TX). Tetra-n-butyl ammonium perchlorate, (n-C4H9)4NClO4, was obtained from Fluka Chemicals. All the chromatographic materials and solvents were procured from Fisher Scientific and were used as received. P3HT (poly(3-hexylthiophene-2,5-diyl)) was purchased from Rieke Metals. PCBM (phenyl-C60-butyric-acid-methyl-ester) was purchased from Nano-C. Alq3 (tris-(8-hydroxyquinoline)aluminum) and zinc acetate dihydrate were purchased from Sigma-Aldrich. ITO coated glass substrates (25-35 Ω/Sq) were purchased from Solems.


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2.2 Synthesis of PHZ-C60 dyads Synthesis of 10-hexyl-phenothiazine : Phenothiazine (1 gm, 5.02 mmol), 1-bromohexane (0.7 ml, 5.02 mmol), and sodium hydride (360 mg, 15 mmol) were added into the ice cold DMF (30 ml) solution under nitrogen and stirred for 5 hours. After that the reaction mixture was poured into ice cold water and extracted with dichloromethane. The organic layer was collected and dried over anhydrous sodium sulfate. After evaporation of the solvent the crude compound was purified over silica column and the desired compound was eluted by CH2Cl2:Hexanes (40:60 v/v). Yield-85%. 1H NMR- (CDCl3:400 MHz), 0.90 (m, 3H, hexyl-H), 1.30 (m, 4H, hexyl-H), 1.42 (m, 2H, hexyl-H), 1.80 (m, 2H, hexyl-H), 3.85 (t, 2H, hexyl-H), 6.90 (m, 4H, Ar-H), 7.18 (m, 4H, Ar-H). Synthesis of 10-hexyl-3-formyl phenothiazine : The mixture of 10-hexyl-phenothiazine (500 mg, 1.76 mmol), phosphorus oxychloride ( 5.28 mmol, 0.49 ml) and DMF (7 ml) was stirred for 15 minutes at room temperature and then refluxed at 90 oC for overnight. After cooling to room temperature, the solution was neutralized by saturated solution of NaOH checked by litmus paper. The mixture was extracted by CH2Cl2. The organic layer was collected and dried over anhydrous sodium sulfate. After evaporation of the solvent, the crude compound was purified over silica column. The desired compound was eluted by CH2Cl2:Hexanes (60:40 v/v). Yield40%. 1H NMR; (CDCl3:400 MHz), 0.95 (m, 3H, hexyl-H), 1.10 (m, 4H, hexyl-H), 1.22 (m, 2H, hexyl-H), 1.80 (m, 2H, hexyl-H), 3.85 (t, 2H, hexyl-H), 6.85 (m, 2H, Ar-H), 6.90 (m, 1H, Ar-H), 7.10 (m, 1H, Ar-H), 7.16 (m, 1H, Ar-H), 7.56 (d, 1H, Ar-H), 7.63 (dd, 1H, Ar-H), 9.78 (s, 1H, CHO). Synthesis of 10-hexyl-3-(1-methyl-3,4-[60]fulleropyrrolidine-2-yl) phenothiazine, 1: 10hexyl-3-formyl phenothiazine (90 mg, 0.28 mmol), sarcosine (80 mg, 0.90 mmol) and C60 (432 mg, 0.6 mmol) were refluxed in 50 ml of dry toluene for 12 hours. After evaporation of the solvent the crude compound was purified over silica column. The desired compound was eluted by toluene:hexane (60:40 v/v). Yield – 40%. 1H NMR (CDCl3:400 MHz), 0.90 (m, 4H, hexylH), 1.18 (m, 3H, hexyl-H), 1.32 (m, 2H, hexyl-H), 1.70 (m, 2H, hexyl-H), 2.70 (s, 3H, Me-H), 3.75 (t, 2H, hexyl-H), 4.12 (d, 1H, fulleropyrrolidine-H), 4.72 (d, 1H, fulleropyrrolidine-H), 4.90 (d, 1H, fulleropyrrolidine-H), 6.71 (d, 1H, Ar-H), 6.80 (m, 2H, Ar-H), 7.05 (m, 2H, Ar-H), 7.10


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(m, 2H, Ar-H).

13

CNMR (400 MHz; CDCl3), δ (ppm). 17.1, 26.2, 29.8, 32.3, 34.5, 43.2, 50.5,

71.2, 72.1, 116.1, 122.3, 127.3, 131.5, 135.2, 136.3, 139.5, 141.5 (m), 142.4 (m), 143.1, 144.1, 144.2, 144.3, 144.8, 145.1, 145.5, 145.8, 146.1, 146.3, 146.9, 147.2, 148.3, 152.1, 153.1, 155.1. FT-IR peaks (cm-1): 2921.75, 2944.27, 2850.29, 2775.93, 2159.43, 2024.33, 1730.78, 1602.96, 1495.59, 1464.16, 1441.01, 1428.97, 1370.61, 1330.69, 1282.97, 1244.05, 1179.51, 1121.68, 1105.64, 1036.99. Mass, [MALDI-TOF], calculated – 1059.15, obtained – 1059.1. Synthesis phenothiazine,

of 2:

10-triethylenemonomethylether-3-(1-methyl-3,4-[60]fulleropyrrolidine-2-yl) 10-Triethylene

monomethylether-3-formyl

phenothiazine30

(90

mg,

0.24mmol), sarcosine (80 mg, 0.90 mmol) and C60 (432 mg, 0.6 mmol) were refluxed in 50 ml of dry toluene for 12 hours. After evaporation of the solvent the crude compound was purified over silica column. The desired compound was eluted by toluene:hexane (100% toluene). Yield – 40%. 1H NMR (CDCl3:400 MHz), 2.68 (s, 3H, fulleropyrrolidine-H), 3.30 (s, 3H, PEG-H), 3.45 (m, 2H, PEG-H), 3.58 (m, 6H, PEG-H), 3.78 (t, 2H, PEG-H), 4.01 (t, 2H, PEG-H), 4.15 (d, 1H, fulleropyrrolidine-H), 4.72 (d, 1H, fulleropyrrolidine-H), 4.88 (d, 1H, fulleropyrrolidine-H), 6.82 (m, 3H, Ar-H), 7.05(m, 2H, Ar-H), 7.15 (m, 1H, Ar-H), 7.35 (m, 1H, Ar-H). 13CNMR (400MHz; CDCl3), δ (ppm). 43.1, 49.5, 61.2, 69.2, 70.5, 71.2, 71.8, 73.1, 74.1, 84.1, 116.1, 123.2, 124.5, 127.3, 131.2, 136.1, 139.2, 139.9, 141.5, 141.9, 142.1, 142.5, 142.8, 143.1, 143.3, 143.8 (m), 144.2 (m), 144.3, 144.5, 144.8, 145.2 (m), 145.7, 146.1, 146.3, 146.7, 147.1, 151.2, 151.4, 155.2. FT-IR peaks (cm-1): 2862.88, 2776.94, 2531.61, 2159.26, 2032.53, 1977.04, 1734.58, 1600.77, 1495.21, 1460.86, 1330.13, 1264.77, 1177.49, 1103.92, 1028.89. Mass, [MALDI-TOF], calculated – 1121.18, obtained, 1126.1 and 1143.1 (Na+ adduct). Synthesis of 10-(4-phenyl(1-methyl-3,4-[60]fulleropyrrolidine-2-yl))phenothiazine, 3: 10-(4formylphenyl) phenothiazine30 (90 mg, 0.29 mmol), sarcosine (80 mg, 0.9 mmol), and C60 (432 mg, 0.6 mmol) were refluxed in 50 ml dry toluene for 12 hours. After evaporation of toluene the crude compound was purified over silica column. The desired compound was eluted by toluene:hexane (70:30 v/v). Yield-45%. 1H NMR (CDCl3; 400 MHz), 2.95 (s, 3H, Me-H), 4.42 (d, 2H, fulleropyrrolidine-H), 5.02 (d, 1H, fulleropyrrolidine-H), 5.10 (s, 1H, fulleropyrrolidineH), 6.48 (m, 2H, Ar-H), 7.10 (m, 2H, Ar-H), 7.21 (m, 2H, Ar-H), 7.31 (m, 2H, Ar-H), 7.43 (m, 2H, Ar-H), 7.88 (m, 2H, Ar-H). FT-IR peaks (cm-1): 2920.35, 2851.08, 2777.67, 2327.87,


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1722.81, 1584.19, 1506.06, 1481.16, 1456.58, 1443.30, 1338.35, 1290.07, 1261.08, 1177.90, 1105.47, 1075.85, 1005.53. Mass, [MALDI-TOF], calculated – 1051.09, obtained, - 1051.0.

2.3. Devices fabrication In order to be consistent with our previous work,36 a similar device fabrication method was implemented. Briefly, zinc acetate was spin coated on N2 plasma cleaned ITO substrates from a 50 mg/ml zinc acetate solution in 96 vol% 2-methoxyethanol and 4 vol% ethanolamine and then converted to ZnO by annealing at 300 °C. The active layer components, C60-derivatives and P3HT, were dissolved in 1,2-dichlorobenzene with roughly 1:1 mass ratio and 0.8:1 for PCBM (C60-derivative:P3HT). In the optimization experiment with dyad 2, a 0.8:1 mass ratio was used. In both cases the concentration of the active layer solution was 25 mg/ml. The BHJ type active layer was spin coated from the described solutions and then annealed at 110 °C for 10 min in vacuum. Finally, a 3 nm Alq3 layer and a 50 nm Ag layer were thermally evaporated in high vacuum on top (Edwards Auto 306). After evaporation the solar cells were post annealed at 120 °C for 10 min in vacuum. The measurements were done the next day following the preparation in order to allow the oxidation of the silver electrode. All preparation steps were done in air at room temperature unless otherwise mentioned. 2.4. Methods The UV-visible spectral measurements were carried out with a Shimadzu 2550 double grating UV-Vis spectrophotometer. The steady-state fluorescence emission was monitored by using a Varian (Cary Eclipse) Fluorescence Spectrophotometer or a Horiba Jobin Yvon Nanolog spectrofluorimeter equipped with photomultiplier (for UV-visible) and InGaAs (for near-IR) detectors. A right angle detection method was used for fluorescence measurements at room temperature. All the solutions were purged prior to spectral measurements using argon gas. The 1

H and

13

C NMR spectra were recorded on a Varian 400 MHz spectrometer. Tetramethylsilane

(TMS) was used as an internal standard. Cyclic voltammetry was recorded on a Princeton Applied Research potentiostat/galvanostat Model 263A using a three electrode system. A platinum button electrode was used as the working electrode, while a platinum wire served as the counter electrode and an Ag/AgCl


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electrode was used as the reference electrode. Ferrocene/ferrocenium redox couple was used as an internal standard. All the solutions were purged prior to electrochemical measurements with nitrogen gas. The oxidation and the first reduction potentials were related to HOMO and LUMO energy levels by setting oxidation energy of ferrocene to -4.8 eV against the vacuum level. Spectroelectrochemical study was performed by using cell assembly (SEC-C) supplied by ALS Co., Ltd. (Tokyo, Japan). This assembly comprised of a Pt counter electrode, a 6 mm Pt Gauze working electrode, and an Ag/AgCl reference electrode in a 1.0 mm path length quartz cell. The optical transmission was limited to 6 mm covering the Pt Gauze working electrode. Influence of the different acceptors on BHJ film properties were studied by using field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM) imagining. FE-SEM instrument (Carl Zeiss Ultra 55) was operated at 1 kV and 5 kV acceleration voltages in inlens mode. Topography AFM images were recorded in noncontact mode (XE-100, Park Systems Inc., USA). Silicon probes (ACTA-SS, Applied NanoStructures Inc., USA) with a nominal resonance frequency of 200–400 kHz, spring constant of 25–75 N m–1, a pyramidalshaped tip (radius < 5 nm) were used. Images were acquired with a scan speed of 0.50–1.0 Hz depending on scan size. Femtosecond Laser Flash Photolysis: Femtosecond transient absorption spectroscopy experiments were performed using an Ultrafast Femtosecond Laser Source (Libra) by Coherent incorporating diode-pumped, mode locked Ti:Sapphire laser (Vitesse) and diode-pumped intra cavity doubled Nd:YLF laser (Evolution) to generate a compressed laser output of 1.45 W. For optical detection, a Helios transient absorption spectrometer coupled with a femtosecond harmonics generator both provided by Ultrafast Systems was used. The source for the pump and probe pulses were derived from the fundamental output of Libra (Compressed output 1.45 W, pulse width 100 fs) at a repetition rate of 1 kHz. About 95% of the fundamental output of the laser was introduced into harmonic generator which produces second and third harmonics of 400 and 267 nm besides the fundamental 800 nm for excitation, while the rest of the output was used for generation of white light continuum. In the present study, the second harmonic 400 nm excitation pump was used in all the experiments. Kinetic traces at appropriate wavelengths were assembled from the time-resolved spectral data. Data analysis was performed using Surface Xplorer software supplied by Helios manufacturer. All measurements were conducted in Ar degassed solutions at 298 K.


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Solar cell characterization: The current-voltage (IV) curves of the model solar cells were measured using an Agilent E5272A source measurement unit. The solar cells were illuminated using a 450 W Xe-lamp through a 315 nm -710 nm band pass filter (KG3 from Thorlabs). The intensity of light was set to match 100 mW/cm2 AM1.5G spectrum with a certified reference solar cell from Oriel Instruments. Surface area of a metal electrode was used to determine the photovoltaic parameters of each device (the whole electrode area was illuminated and no mask was used to limit the illumination area). The IV sweeps of each cell were performed repeatedly and no considerable variation between the measurements was observed. 3. RESULTS AND DISCUSSION 3.1

Synthesis, characterization, and electrochemistry. The synthesis of dyads 1 and 2

involved first appending the desired N-alkyl substituents followed by formylation by the reaction of POCl3 in DMF. Next, the phenothiazine-aldehyde derivatives were reacted with C60 and sarcosine in toluene.29 The X-ray structure and crystal packing of 10-hexyl-3-formyl phenothiazine are shown in Figure S1.37 In the structure, due to the presence of central sulphur and nitrogen atoms, phenothiazine ring was found to be slightly bowed. Some intermolecular type interactions between two phenothiazine rings were also observed. That is, the formyl oxygen was at 2.456 – 2.583Å closer to the hydrogen of the second phenothiazine ring (see Figure S1a) establishing a dimeric type structure. The atomic coordinates, bond lengths and angles, anisotropic displacement parameters, hydrogen coordinates are given in the supporting information (Tables S1-S5). Synthesis of dyad 3 involved first synthesis of N-substituted benzaldehyde following by standard Prato’s fulleropyrrolidine synthesis.29 The newly synthesized compounds were characterized by 1H and

13

C NMR, MALDI-TOF-mass, spectral

and electrochemical methods (see supporting information for 1H and

13

C NMR, FT-IR, and

MALDI-TOF-mass spectra of the dyads). Figure 2a shows the optical absorption spectra of the dyads and of the control compounds, phenothiazine, 10-hexyl-3-formyl phenothiazine, and 2-phenylfulleropyrrolidine, in odichlorobenzene (DCB). Pristine phenothiazine revealed a broad peak at 318 nm while for 10hexyl-3-formyl phenothiazine, this peak was red shifted to 384 nm. Fulleropyrrolidine had peaks at 324 nm and a sharp peak at 432 nm. The phenothiazine and fulleropyrrolidine peaks in the dyads overlapped in the 300-450 nm range. In the 400-600 nm range, weak absorbance due to


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the presence of fullerene and a sharp peak at 432 nm, characteristic of fulleropyrrolidine, were also observed (shown by asterisk) for all dyads. Steady-state emission spectra recorded for (10hexyl-3-formyl)phenothiazine and (10-triethylene monomethylether-3-formyl)phenothiazine revealed a broad emission at 520 nm while this emission for 10-(4-formylphenyl)phenothiazine was at 450 nm in DCB (see Figure S19 in the supporting information). However, for the dyads no emission from either phenothiazine (400-650 nm range)32 or fulleropyrrolidine (675-800 nm range)30 entities was observed suggesting occurrence of excited state events in these dyads.

Figure 2. (a) Absorption spectra of dyads 1-3 (blue, black and red lines) and the control compounds (concentration = 5 x 10-5 M each) in DCB, and (b) spectral changes observed during oxidation of 10-triethylene monomethylether-3-formyl phenothiazine in DCB containing 0.2 M (TBA)ClO4. Electrochemical and spectroelectrochemical studies of the dyads were also performed. Figure 3 shows cyclic voltammograms of dyads 1 and 3 in DCB containing 0.1 M (TBA)ClO4.


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Within the potential window of the solvent, three reversible reductions were observed (oxidation process of internal ferrocene redox couple is shown in asterisk in Figure 3a). Table 1 lists the potential of the dyads. Control experiments on 2-phenyl fulleropyrrolidine confirmed that all of the three reductions were due to the reduction of fullerene entity. On the anodic side, a reversible oxidation at 0.31 V vs. Fc/Fc+ for oxidation of the phenothiazine entity in case of 1 and 2 were observed while for 3, a slightly anodically shifted reversible oxidation at 0.33 V vs. Fc/Fc+ was observed. Free-energy calculations performed according to Rehm-Weller approach38,39 (DCB as solvent) suggested electron transfer from phenothiazine to fulleropyrrolidine after the singlet state excitation of either of the chromophores was possible. Free-energy change, ∆G, values of 1.05+0.3 eV and -0.35+0.3 eV from 1PHZ* and 1C60*, respectively, were obtained suggesting thermodynamic feasibility of the intramolecular charge separation.

Figure 3. Cyclic voltammograms of (a) dyad 1 and (b) dyad 3 in DCB containing 0.1 M (TBA)ClO4. Scan rate = 100 mV/s. Table 1. Redox potential (V. vs. Fc/Fc+) and HOMO-LUMO gap of investigated dyads in dichlorobenzene containing 0.1 M (TBA)ClO4.


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Compound

Ox Potential, V

Red. Potential, V

HOMO-LUMO gap, V

1

0.31

-1.10, -1.49, -2.03

1.41

2

0.31

-1.10, -1.48, -2.02

1.41

3

0.33

-1.11, -1.48, -2.04

1.44

PCBM

--

-1.09, -1.46

In order to assist interpretation of transient spectral results of electron transfer products, spectroelectrochemical study of the donor, phenothiazine was carried out. Figure 2b shows the spectral changes observed during the first electrochemical oxidation of 10-triethylene monomethylether-3-formyl phenothiazine (precursor compound of dyad 1) at an applied potential of 0.70 V vs. Ag/AgCl. During the course of oxidation, new peaks at 460, 534, 702 and 784 nm were observed. The highest peak intensity was observed for the 534 nm band. Therefore, possible appearance of transient peaks at these wavelengths and those of fulleropyrroline radical anion at 1020 nm31 serves as a proof for occurrence of charge separation in these dyads. 3.2

Femtosecond transient absorption spectroscopy. Figure 4 presents femtosecond transient

spectra of pristine phenothiazine and 10-hexyl-3-formyl phenothiazine control compounds in benzonitrile. Immediately after excitation, pristine phenothiazine singlet excited state revealed bands at 559 and 610 nm (see Figure 4a). With time, these peaks diminished in intensity with the appearance of peaks at 525 nm and a broad peak in the 600-650 nm range likely to the triplet state formation. Similarly, for 10-hexyl-3-formyl phenothiazine, the singlet excited state peaks were located at 482 and 718 nm; the peak was over 100 nm red-shifted compared to pristine phenothiazine (610 nm) as a result of macrocycle functionalization at the C-3 position. Decay of singlet peaks at 718 nm resulted in a red-shifted peak in the 732 nm range (see spectrum recorded at 3 ns delay time) which could be ascribed to triplet state transition of the probe formed upon undergoing the intersystem crossing process.


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Figure 4. Femtosecond transient absorption spectra (excited at 400 nm of 100 fs laser pulses) at the indicated delay times of (a) phenothiazine and (b) 10-hexyl-3-formyl phenothiazine in Arsaturated benzonitrile. For the dyads, three solvents of varying polarity, viz., toluene, DCB and benzonitrile were employed. It may be mentioned here that at the excitation wavelength of 400 nm, about 80% of phenothiazine and 20% of fulleropyrrolidine had absorbance contributions (see Figure 2a), that is, at this wavelength, phenothiazine entity of the dyad was mainly excited. Figure 5 shows the femtosecond transient absorption spectra for the dyads in benzonitrile at different delay times. For dyads 1 and 2, immediately after excitation (within 5 ps delay time), peaks at 553, 742, 880, and 1025 nm were observed. No strong peak in the 800 nm corresponding to the singlet excited fulleropyrrolidine33 was observed indicating dominant excitation of phenothiazine entity. With time peaks corresponding to singlet excited states of the sensitizers diminished in intensity with a well-developed peak at 1020 nm (see spectrum at delay time of 10 ps) characteristic of C60•− indicating charge separation in these dyads. The 553 and 742 nm peaks had contributions of


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phenothiazine radical cation, PTZ•+, as validated, with small energy shifts, also by the spectroelectrochemical analysis in Figure 2b. The decay of the C60•− peak representing charge recombination was accompanied by a red-shifted peak with peak maxima at 590 nm. The decay of C60•− peak was completed by 250 ps, however, the peak at 590 nm persisted beyond 3 ns (monitoring time window of our instrument) suggesting that this could be due to populating triplet state of either phenothiazine or fullerene. Since 3C60* peaks appear at 700 and 825 nm,40 and based on the results obtained from the control experiments, this peak has been attributed to triplet state of phenothiazine, 3PTZ*. Interestingly, for dyad 3, the initial singlet excited state features of 1PTZ* were at 549, 589 and 750 nm within the first 5 ps. The PTZ•+-C60•− peaks were better defined and appeared at 595, 750 and 1020 nm. The spectral features survived beyond 3 ns, however, at this delay time, the PTZ•+ peak revealed a graduate red-shift and appeared at 610 nm, albeit populating the 3PTZ*. Similar spectral observations, that is, occurrence of charge separation upon photoexcitation, were also made in DCB (see Figure S2).


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Figure 5. Femtosecond transient absorption spectra (excited at 400 nm of 100 fs laser pulses) at the indicated delay times of dyads (a) 1, (b) 2 and (c) 3 in Ar-saturated benzonitrile. The right hand side panel shows time profile of the C60•− monitored at 1020 nm. As shown in Figure 6, charge separation in nonpolar toluene was also witnessed for all three dyads. That is, the initial instantaneously formed 1PTZ* revealed peaks at 528 and 745 nm. Decay of the singlet excited peaks resulted in new bands at 1020 nm corresponding to C60•− (see spectrum recorded at 20 ps delay time in Figures 6a and b) and in the visible region corresponding to that of PTZ•+. However, the time constant for the development of radical ionpair peaks were much longer than that observed in benzonitrile implying a relatively slower charge separation process. The decay of the radical ion-pair peaks resulting in populating 3PTZ* as evidenced by long-lived peaks in the 500-625 nm range.


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Figure 6. Femtosecond transient absorption spectra (excited at 400 nm of 100 fs laser pulses) at the indicated delay times of dyads (a) 1, (b) 2 and (c) 3 in Ar-saturated toluene. The right hand side panel shows time profile of the C60•− monitored at 1020 nm. The rate of charge separation, kCS and charge recombination, kCR were evaluated by analyzing the time profile of the C60•− peak at 1020 nm which was far from other singlet and triplet state peaks of the PTZ and C60 entities. Such time profiles are shown at the right hand side panel of each transient spectra in Figures 5 and 6. The kCS and kCR thus obtained are given in Table 1. The following trends in kCS and kCR were observed: (i) the magnitude of kCS was high in


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polar benzonitrile implying faster charge separation compared to that in less polar solvents; (ii) for a given dyad and solvent system, the kCR was an order of magnitude smaller compared to kCS; and (iii) the kCS and kCR values for dyad 3 were lower than that observed for dyads 1 and 2, due to increased distance between the donor and acceptor entities (phenyl ring spacer against direct linkage). The faster charge recombination in these dyads could be attributed to the intermediate population of 3PTZ* by the charge separated state prior returning to the ground state. Table 1. Charge-separation (kCS) and charge recombination (kCR) rate constants from the femtosecond transient spectral measurements for the dyads in solvents of varying polarity. Dyad

Solvent

Dielectric constant

kCS × 1010 s-1

kCR × 109 s-1

1

PhCN DCB Toluene PhCN DCB Toluene PhCN DCB Toluene

26.0 9.93 2.38 26.0 9.93 2.38 26.0 9.93 2.38

49.2 1.2 6.1 43.8 1.5 0.65 7.6 3.5 1.5

54 1.1 4.8 65 1.3 1.2 0.7 2.9 7.8

2

3

Having established efficient charge separation in these dyads, next their utility in building light energy harvesting devices was explored. 3.3. Studies on inverted BHJ cells. The energy level scheme of the devices, derived from the CV measurements is shown in Figure 7a. The HOMO–LUMO energies of the fullerene derivatives are presented in the earlier section. The differently substituted phenothiazine derivatives mainly affect the HOMO level leaving LUMO, associated with the fullerene core, nearly unchanged with respect to LUMO of PCBM (3.8 eV). Hence the Voc generated by an organic solar cell is based on HOMO (donor) – LUMO (acceptor) difference, and the device structure is otherwise the same, the voltage produced by the model OPVs is expected to be similar to the PCBM based standard device, about 0.5 – 0.6 V. The LUMO levels for all the fullerene derivatives are 0.1 eV higher than that of PCBM (Figure 7a). The phenothiazine entity on fulleropyrrolidine increases the absorption range to some extent (Figure 3a) compared to


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pristine PCBM. The changes in absorption occur mainly at wavelengths shorter than 400 nm thus leading similar absorption spectra for all the studied OPVs, as illustrated in Figure S3. Moreover, lower HOMO levels of dyads 1, 2 and 3, with respect to that of P3HT,41 support favorable charge transfer through the layered system. (a) LUMO

-3.2 eV -3.7 eV

energy level

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


-4.1 eV

dyad 1, 2, 3

-4.7 eV ITO

-5.1 eV

P3HT -5.0 eV AgO

-4.8 eV

ZnO

HOMO -7.5 eV

(b)

load

Ag Alq3

PCBM

P3HT: dyad 1, 2, 3, PCBM

Alq3

N

ZnO

O

ITO

N

O Al O

N

glass *

S

* n

hv

P3HT

C6 H13

Figure 7. (a) The energy level scheme of the materials illustrating the HOMO–LUMO levels of the dyads and work function of the electrodes used in the model OPVs. (b) Schematic presentation of the studied solar cell structure and chemical structures of the commercial substances used.

Film morphology and film thickness of model phenothiazine-C60 based BHJ solar cells were characterized first with FE-SEM and compared to a reference cell with PCBM as an acceptor. The films with PCBM and dyads 1 and 2 looked smooth and uniform when scanned with a 1 kV acceleration voltage to get information from the very surface (Figure S4). Large aggregates are


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visible on the film containing dyad 3 which is due to the lack of hexyl or glycol chain that enhances the solubility of dyads 1 and 2. Next, the acceleration voltage 5 kV was used to get information deeper than just from the surface (Figure S5). In the film made of PCBM, a pattern that resembles an interpenetrating network of two different materials was observed.42 A very similar pattern is noticed also for the film with dyad 2, having clear structural difference to the smooth film of dyad 1. Despite the large aggregates observed in a film containing dyad 3, a network of two different materials is visible. Cross section FE-SEM images revealed that all the films are approximately 145 nm thick as shown in Figure S6, in which a typical cross profile of the studied OPVs is presented. In order to gain more detailed information about the surface morphology, topographical AFM scans of the model devices were performed (Figure 8). The topography images indicated that the films containing dyad 2 or PCBM have hills with lateral diameter of roughly 200 nm. The film with dyad 1 seems more intermixed with the polymer resulting in network structure with more mingled acceptor and donor entities. The film containing dyad 3 has hills with higher lateral density, with more defined shape, and with a lateral diameter of approximately 100 nm. This indicates that the dyad 3 without hexyl or glycol chain results in different film morphology with more isolated donor and acceptor domains, as seen also in the FE-SEM micrographs. All the films are very flat and relatively smooth in 1 by 1 µm2 and 10 by 10 µm2 scans (Figure S7). The root-mean-squared roughness in 1 by 1 µm2 scan is 1.726, 0.828, 1.869, and 1.276 for film containing 1, 2, 3, and PCBM, respectively. The RMS roughness values for 10 by 10 µm2 scans are 2.622, 0.918, 2.615, and 2.208 for film containing 1, 2, 3, and PCBM, respectively. The films containing compound 1 or 3 have different morphology in microscale with respect to PCBM, which is best seen in the 1 by 1 µm2 scans. In terms of the FE-SEM and AFM micrographs, we conclude that glycol chain in dyad 2 allowed the formation of a BHJ with similar interpenetrating network structure with respect to PCBM, and hence showing the most promising morphology for charge transfer across the film. The hexyl chain in dyad 1 caused the compound to be blended too well with the polymer, and thus decreasing the formation of interpenetrating network structure between the dyad and the polymer. For dyad 3 the lack of glycol or hexyl chain produced aggregates in the BHJ film potentially diminishing the charge transfer efficiency of the system.


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Figure 8. AFM topography images for a) 1, b) 2, c) 3, and d) PCBM containing BHJ films.

Next, the phenothiazine-C60 dyads were tested in BHJ solar cells with a cell structure presented in Figure 7b. The IV-curves of the best reference and phenothiazine-C60 OPVs in dark and under 1 sun illumination are presented in Figure 9. The figures of merit of the prepared solar cells43 are presented in Table 2. The error limits of the photovoltaic parameters are presented in the Table as a standard deviation value (s) (see SI). From the three tested dyads those where the phenothiazine unit was bound directly to the Nmethyl-pyrrolidine (dyads 1 and 2) were more efficient than the one with the phenyl spacer (dyad 3). The best dyad was found to be compound 2, as it outperformed the reference device in both fill factor (FF) and open circuit voltage (Voc). However the power conversion efficiency (PCE) remained slightly below than that of the reference, 3.6 ± 0.7 %, due to lower photocurrent density. An optimization round was done for the most promising dyad (compound 2). The donoracceptor ratio in the active layer was set to 1:0.8 m% to increase the amount of P3HT in the active layer relative to fullerene in order to enhance charge separation and transport in the active layer. This resulted in an improvement in FF and Voc, producing PCE of 3.5 ± 0.7 % for the cell with dyad 2. It may be mentioned here that cells constructed using only phenothiazine-C60 dyads resulted in much lower efficiency due to poor absorbance of the dyad in the visible region, necessitating the importance of P3HT in the cell construction.


19


15 PCBM dyad 2

2

10

J, mA/cm

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

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5 dark

0 -5 -10

1 sun

-15 -0.2

0.0

0.2

0.4

0.6

U, V

Figure 9. IV characteristics of the best performing phenothiazine-C60 (red) and PCBM (blue) based OPVs. Table 2. Photovoltaic parameters with standard deviation values (average of five electrodes) with standard deviation values of the OPVs with phenothiazine-fullerene dyads as acceptor materials. Acceptor

PCE (%)

FF (%)

PCBM 3.6 ± 0.7 45.2 ± 3.3 dyad 1 2.1 ± 0.3 39.7 ± 2.5 dyad 2 3.3 ± 0.3 50.0 ± 3.2 dyad 3 0.6 ± 0.1 37.2 ± 1.8 dyad 2* 3.5 ± 0.1 57.2 ± 0.9 *optimization experiment with mass ratio 0.8:1

Voc (V) 0.57 ± 0.01 0.63 ± 0.01 0.63 ± 0.01 0.49 ± 0.02 0.64 ± 0.01

Isc (mA/cm2) 13.8 ± 1.5 8.3 ± 0.9 10.6 ± 0.6 3.1 ± 0.3 9.7 ± 0.2

4. SUMMARY In the present study, utilization of new-type of phenothiazine-C60 electron donor-acceptor dyads, with appreciable charge separated state lifetimes, as an active material in the construction of inverted BHJ solar cell is demonstrated. Charge separation and recombination rates for dyads 1 and 2 were found to be higher compared to that for dyad 3 due to close proximity between the phenothiazine donor and fullerene acceptor moieties, irrespective of the nature of the employed solvent (polar or nonpolar). The inverted organic BHJ solar cells constructed using the dyads and an additional electron donor material P3HT revealed photovoltaic performance close to that


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observed for PCBM, at least in the case of dyad 2 (PCE of 3.5 ± 0.1 %) having better miscibility. It is likely that the charge separated state generated in the dyad upon PTZ excitation (at lower wavelengths of < 450 nm) is helping in promoting electron migration.44 That is, by converting P3HT:PTZ•+-C60•− to P3HT•+:PTZ-C60•− within OPV assembly, the intermediate phenothiazine would act as a spacer blocking the charge recombination process. At higher wavelengths (wavelengths > 450 nm) where photoexcited P3HT would act as an electron donor, the fullerene entity of the dyad would act as electron acceptor with the possibility of appropriately positioned phenothiazine acting as a spacer blocking charge recombination process. In either case, intramolecular charge recombination is suggested to be diminished in such a BHJ structure due to the presence of phenothiazine. These findings show that donor-acceptor dyads are an attractive alternative to PCBM acceptor material for OPV applications, as evidenced by enhanced Voc and FF of the dyad based OPVs. Further studies to fully understand the mechanistic details of utilizing donor-acceptor dyads instead of PCBM in improving BHJ OPVs are in progress in our laboratories. ACKNOWLEDGMENTS This work is supported by National Science Foundation (Grant No. 1401188 to FD), and the Academy of Finland and National Doctoral Programme in Nanoscience (NGS-NANO). ASSOCIATED CONTENT Supporting Information Femtosecond transient spectra the dyads in DCB, FE-SEM images of the electrodes, 1H and 13

C NMR, FT-IR, MALDI-Mass spectra, and additional, and X-ray structural details. This

material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATIONPT Corresponding authors a*

E-mail: [email protected]; fax: +(940)565-4318; tel: (940)369-8832

b*

E-mail: [email protected]; fax: +358 3 3641 392; tel: +358 40 1981 125


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Notes The authors declare no competing financial interests. REFERENCES 1) Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Nguyen, T.-Q.; Dante, M.; Heeger, A. J. Efficient Tandem Polymer Solar Cells Fabricated by All-Solution Processing. Science 2007, 317, 222–225. 2) Liang, Y.; Feng, D.; Wu, Y.; Tsai, S.-T.; Li, G.; Ray, C.; Yu, L. Highly Efficient Solar Cell Polymers Developed via Fine-Tuning of Structural and Electronic Properties. J. Am. Chem. Soc. 2009, 131, 7792–7799. 3) Liang, Y.; Xu, Z.; Xia, J.; Tsai, S.-T.; Wu, Y.; Li, G.; Ray, C.; Yu, L. For the Bright Future—Bulk Heterojunction Polymer Solar Cells with Power Conversion Efficiency of 7.4%. Adv. Energy Mater. 2010, 22, E135–E138. 4) Hains, A. W.; Liang, Z.; Woodhouse, M. A.; Gregg, B. A. Molecular Semiconductors in Organic Photovoltaic Cells. Chem. Rev. 2010, 110, 6689–6735. 5) Po, R.; Maggini, M.; Camaioni, N. Polymer Solar Cells: Recent Approaches and Achievements. J. Phys. Chem. C 2010, 114, 695–706. 6) Günes, S.; Neugebauer, H.; Sariciftci, N. S. Conjugated Polymer-Based Organic Solar Cells. Chem. Rev. 2007, 107, 1324–1338. 7) Krebs, F. C. Fabrication and Processing of Polymer Solar Cells: A Review of Printing and Coating Techniques. Sol. Energy Mater. Sol. Cells 2009, 93, 394–412. 8) Krebs, F. C.; Tromholt, T.; Jorgensen, M. Upscaling of Polymer Solar Cell Fabrication using Full Roll-to-Roll Processing. Nanoscale 2010, 2, 873–886. 9) Umeyama, T.; Imahori, H. Design and Control of Organic Semiconductors and their Nanostructures for Polymer-Fullerene-Based Photovoltaic Devices, J. Mater. Chem. A, 2014, 2, 11545-11560. 10) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. Polymer Photovoltaic Cells: Enhanced Efficiencies via a Network of Internal Donor-Acceptor Heterojunctions. Science 1995, 270, 1789–1791. 11) Yu, G.; Heeger, A. J. Charge Separation and Photovoltaic Conversion in Polymer Composites with Internal Donor/Acceptor Heterojunctions. J. Appl. Phys. 1995, 78, 4510– 4515. 12) Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Efficient Photodiodes from Interpenetrating Polymer Networks. Nature 1995, 376, 498–500. 13) Hoppe, H.; Sariciftci, N. S. Organic Solar Cells: An Overview. J. Mater. Res. 2004, 19, 1924–1945.


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31) Kawauchi, H.; Suzuki, S.; Kozaki, M.; Okada, K.; Islam, D.-M. S.; Araki, Y.; Ito, O.; Yamanaka, K. Photoinduced Charge Separation and Charge Recombination Process of Fullerene Dyads Covalently Connected with Phenothiazine and its Trimer, J. Phys. Chem. A 2008, 112, 5878-5885. 32) Kremer, A.; Bietlot, E.; Zanelli, A.; Malicka, J. M.; Armaroli, N.; Bonifazi, D. Versatile Bisetheynylfulleropyrrolidine Scaffolds for Mimicking Artificial Light Harvesting Photoreaction Centers, Chem. Eur. J. 2014, 20, 1-11. 33) KC, C. B.; Lim, G. N.; Nesterov, V. N.; Karr, P. A.; D’Souza F. Phenothiazine-BODIPYFullerene Triads as Photosynthetic Reaction Center Models: Substitution and Solvent Polarity Effects on Photoinduced Charge Separation and Charge Recombination. Chem. Eur. J. 2014, 20, 17100-17112. 34) Mi, D.; Kim J.H.; Xu, F.; Lee, S.H.; Yoon, S. C.; Shin, W. S.; Moon, S. J.; Lee, C.; Hwang, D. H. Synthesis and Characterization of a Novel Fullerene Derivative for use in Organic Solar Cell, Sol. Energy Mater. Sol. Cells, 2011, 95, 1182-1187. 35) Mi, D.; Kim, H. U.; Kim, J. H.; Xu, F.; Jin, S. H.; Hwang, D. H. Synthesis of a Soluble Fulleropyrrolidine Derivative for use as an Electron Acceptor in Bulk Heterojunction Polymer Solar Cell, Synth. Met., 2012, 162, 483-489. 36) Kaunisto, K.; Subbaiyan, N. K.; KC, C. B.; Chukharev, V.; Hakola, H.; Vuorinen, T.; Manninen, V.; Tkachenko, N.; Lemmetyinen, H.; D’Souza F. The Effect of Thiophene Substituents of Fulleropyrrolidine Acceptors on the Performance of Inverted Organic Solar Cells. Synth. Met. 2014, 195, 193-200. 37) CCDC- 1060093 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via ww.ccdc.cam.ac.uk/data_-request/cif. 38) Rehm, D.; Weller, A. Kinetics of Fluorescence Quenching by Electron and Hydrogen-atom Transfer, Isr. J. Chem. 1970, 7, 259-271. 39) ES = (1240/λAbs + 1240/λFl)/2 ECS = e(EOx – ERed) where ES = singlet energy of phenothiazine (2.48 eV) and fulleropyrrolidine (1.72 eV); ECS = energy of charge separated state, EOx = first oxidation potential of phenothiazine and ERed = first reduction potential of fullerene. Solvent stabilization energies were neglected in these calculations. 40) D. M. Guldi and P. V. Kamat in Fullerenes Eds.: K. M. Kadish, R. S. Ruoff, Wiley, New York, 2000, Chapter 5, pp 225-281. 41) Hou, J.; Tan, Z.; Yan, Y.; He, Y.; Yang, C.; Li, Y. Synthesis and Photovoltaic Properties of Two-Dimensional Conjugated Polythiophenes with Bi(thienylenevinylene) Side Chains. J Am. Chem. Soc. 2006, 128, 4911–4916. 42) Yu, G.; Gao, J.; Hummelen, C.; Wudl, F.; Heeger, A. J. Polymer Photovoltaic Cells: Enhanced Efficiencies via a Network of Internal Donor-Acceptor Heterojunctions. Science, 1995, 270, 1789-1791.


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43) Advanced Characterization Techniques for Thin Film Solar Cells, Eds., Rau, U.; Abou-Ras, D.; Kirchartz, T. Wiley-VCH, 2011 44) Kaunisto, K. M.; Vivo, P.; Dubey, R. K.; Chukharev, V. I.; Efimov, A.; Tkachenko, N. V.; Lemmetyinen, H. J. Charge Transfer Dynamics in P3HT:Perylenediimide-C60 Blend Films Studied by Ultra-Fast Transient Absorption, J. Phys. Chem. C, 2014, 118, 10625-10630.


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