photovoltaic devices
2-1 Introduction
The advance of organic/inorganic hybrid materials based on π-conjugated polymers
and inorganic semiconductors has attracted great interests over the past decades due to its potential usage in optoelectronic applications such as light-emitting diodes, photodetectors, solar cells and others. In particular, significant consideration has been focused on the solution-processed hybrid solar cells consisting of electron-donating conducting polymers such as poly(3-hexylthiophene) (P3HT) and electron-accepting nanocrystals (e.g., CdS, CdSe, ZnO, and TiO2). While power conversion efficiencies (PCEs) for these hybrid bulk heterojunction (BHJ) solar cells are constantly being improved, they still suffer from structure control issues with the fine adjustment of electron donors/acceptors distribution. Efficient charge separation relies upon molecular interfaces to separate charge, and the domain size of the organic donor and inorganic acceptor materials should be comparable or smaller than the exciton diffusion length (~
5-20 nm) to increase the probability of exciton dissociation across the donor-acceptor interfaces.1 However, coagulation problems with inorganic nanocrystals often occur
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during the device fabrication process; that is, the blended organic/inorganic components may undergo macrophase separation that reduce the interfacial interactions between the polymer and inorganic materials, resulting in a decrease in the quantum efficiency, thereby degrading the device performance.2 Consequently, controllable nanomorphology, well-structured interfaces, and superior optoelectronic interactions are critical factors for developing highly efficient hybrid solar cells.
To this end, a number of processing methodologies have been developed to achieve a high miscibility of organic/inorganic components and prepare a favorable dispersion of nanocrystals in hybrid photovoltaic devices. In a typical way, solubility of the nanocrystals can be achieved by using capping ligands, e.g. oleylamine, trioctylphosphite, 2-mercaptoethanol, but the modifying surfactants could be regarded as an isolating layer that hinder charge separation and charge transport.3, 4 To overcome these drawbacks, it is necessary to remove excess capping ligands by additional procedures5 or replace them with conjugated ones.6, 7 Other routes also include the deposition of inorganic quantum dots on π-conjugated polymers via nanocrystal–polymer interactions.8 Prasad et al. designed a novel strategy by using thermally cleavable ligands to coat quantum dots, following a thermal annealing procedure on the as-fabricated photoactive layer to eliminate the ligands.9 However, these methodologies often require complicated treatments, which could be too
time-consuming to be applied to potential large-scale production.
An alternative strategy, which involves a π-conjugated polymer matrix such as
P3HT containing a precursor of an inorganic component required for the synthesis of organic/inorganic nanohybrids, has recently received a great deal of attention on the fabrication of organic photovoltaic devices.10 This is because the so-called “in situ”
formation routes possess important advantages in that the effects of the capping ligands on charge exchange are eliminated and do not require a separate nanoparticle synthesis step, achieving direct combination of nanocrystals and polymer materials.11 So far, a number of inorganic semiconductors (PbS, CdS, CdSe, ZnO and etc.) directly synthesized in conjugated polymers have been reported in several studies.11-16 The correlation between the morphology, charge transport, and photovoltaic properties of the resulting BHJ systems has been also reported.17-19 For the in situ synthetic process, developing a controllable morphology of nanohybrids and a well-defined interface between inorganic semiconductors and the conjugated polymers continues to be an important research topic. Our group previously reported a facile in situ method to fabricate highly elongated P3HT nanowires lined along their long fibril axis, with continuous ZnO nanocrystal pathways, where a “pre-crystallization” approach was first utilized to simultaneously organize organic P3HT molecules and inorganic zinc precursors into highly ordered nanowires with micrometer-scale lengths, followed by a
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thermal oxidation treatment to directly grow discrete ZnO nanocrystals on the existing nanofibrillar template at the ambient pressures.20
Despite continuous improvements in hybridization strategies, the in situ generation of metal oxide semiconductor inside the dry-state, π-conjugated polymer film remains
suboptimal. Conversion and high crystallinity of inorganic semiconductors are strongly limited by the low annealing temperature used for the sake of preventing degradation of polymer components. Thus, the quality of nanocrystals as prepared is relatively poor, less crystalline than those produced from calcination or colloidal synthesis.21, 22 Furthermore, only a few types of inorganic semiconductors with high crystallinity could be prepared successfully below the decomposition temperature of polymers via in situ synthetic process, for instance, a high-temperature heating treatment above 450 °C is required to transform titanium precursors, such as titanium tetraisopropoxide (TTIP) or titanium tetrachloride (TiCl4) into crystalline TiO2.23 However, TiO2 as an electron acceptor for the use in photovoltaics has the advantages of resistance to acid and base, good chemical and photochemical stability, non-toxicity, low cost, and better charge separation properties.24 These advantages have attracted intensive interest focusing on TiO2.24-29 Fortunately, hydrothermal synthetic technique provides a prospective route to prepare a well-crystalline and phase-pure oxide in one step in a tightly closed stainless steel autoclave under controlled temperatures (T < 200 °C) and/or pressures (p < 10
MPa).30, 31 O’Regan and Grätzel synthesized anatase TiO2 nanoparticles by hydothermal synthesis from titanium (IV) isopropoxide at 200 °C.32 Imai et al. developed a new method of preparation of anatase TiO2 films at low temperature by exposure to water vapor.33, 34 Langlet et al. also reported on the possibility to crystallize TiO2 thin films on polymer substrates and thermally sensitive substrates by using an autoclave under ethanol-water pressure.35, 36 Besides, a high-pressure crystallization process developed by Lu's group has been utilized to successfully crystallize TiO2 thin films at a temperature considerably lower than that required in a commonly adopted atmospheric pressure annealing process.37 Therefore, the hydrothermal technique of TiO2 preparation provides us with a practical approach to apply an in situ synthetic method for fabricating TiO2/conjugated polymer nanohybrids with a controllable nanostructure.
Herein, we report on a novel in situ growth strategy to fabricate P3HT/TiO2
nanohybrids, where a one-step approach was used to simultaneously organize organic P3HT chains and inorganic titanium precursors into highly-elongated nanofibrils, followed by the hydrothermal synthesis process at an elevated pressure in an autoclave to directly grow highly-crystallized TiO2 nanoparticles on the existing P3HT nanofibrils.
The TiO2 embossed nanofibrillar structure provides enhanced interfacial area for charge separation as well as efficient pathways for charge transport, thereby enhancing optoelectronic properties and device performance. We believe that the study offers a
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scalable and modular approach towards the co-assembly of disparate materials into close association in hybrid systems.
2-2 Material and equipments
In this study to in situ template synthesize a P3HT/TiO2 nanohybrid systems, the materials, the provider and the purity, which are used in solutions and thin films, and the all equipments for characteristic analyses are listed in Table 2-1 and in Table 2-2, respectively.
Table 2-1 Materials
No. Name Provider Purity
1 Chlorobenzene (CB) Acros 99.8%
2 Anisole (C6H5OCH3) Acros 99%
3 Methanol Acros 99.8%
4 Tetrahydrofuran (THF) Mallinckrodt ACS, stabilized 5 Titanium isopropoxide (TTIP) Sigma-Aldrich 97%
6 Bromine (Br2) Acros 99%
7 1-Bromohexane Acros 99%
8 3-Bromothiophene Acros 97%
9 N-bromosuccinimide (NBS) Acros 99%
10 Chloroform (CHCl3) Mallinckrodt ACS, stabilized 11 Dichloromethane (CH2Cl2) Mallinckrodt ACS, stabilized
12 Ether Mallinckrodt ACS, stabilized
13 n-Hexane Mallinckrodt ACS, stabilized
14
Isopropylmagnesium chloride (i-PrMgCl)
Acros 2.0M solution in THF
15 Ni(dppp)Cl2 Acros 99%
16 Iodine (I2) Mallinckrodt ACS, stabilized
17 Magnesium Acros 99.9%, turnings
18 Sodium metal Sigma-Aldrich 99 wt% dispersion in paraffin
19 Silica gel Merck 230-400 mesh, for
column chromatography
20 Sodium chloride (NaCl) SHOWA Reagent grade 21 Hydrochloric acid (HCl) Sigma-Aldrich 37%
22 Magnesium sulfate (MgSO4) SHOWA anhydrous
23 PEDOT:PSS Heraeus Clevios™ PH 500
24 Fluorine-doped glass (FTO) Pilkington 15 Ω/square
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Table 2-2 Equipments
No. Name Provider Type
1 Rotavapor EYELA N-1000V
2 Ultrasonic cleaner Elma T660H
3 NMR Bruker AVANCE 400
4 GPC Waters Breeze system
5 TEM JEOL JEM-1230EX
6 UV-VIS Agilent Technologies Cary 100
7 PL Perkin-Elmer LS55
8 WAXS NSRRC Beamline 13A1 and 17A1
9 SAXS NSRRC Beamline 23A1
10 Autoclave Atlas Machinery AT-23
11 Spin coater Laurell WS-400E-6NPP-LITE
12 Glove box MBraub UNILAB 2000
13 Solar cell electrometer Keithley 2400 14 TRPL laser source PicoQuant GmbH PDL 800-B
15 XPS Thermo Fisher Scientific Theta Probe
2-3 Synthesis of P3HT homopolymer via Grignard metathesis method
All synthetic reactions were carried out under purified nitrogen. High purity nitrogen was purchased (99.995% pure) and further purified by passing it through a column of molecular sieves and a BTS (Fluka) catalytic oxygen trap. THF was dried with sodium metal and benzophenol (Acros, 99%) was used as the indicator for dryness.[1,3-Bis(diphenylphosphino)propane]dichloronickel(II) (Ni(dppp)Cl2, Acros, 99%) and
isopropylmagnesium (i-PrMgCl, Acros, 2.0 M solution in THF) were used as received without purification.
2-3-1 Synthesis of 2,5-dibromo-3-hexylthiophene monomer
2,5-dibromo-3-hexylthiophene monomers were synthesized according to the reported procedures from the literature.38-40 The procedure for synthesizing 2,5-dibromo-3-hexylthiophene is shown in Figure 2-1. First, magnesium was added in a two-neck round bottom flask with dried ether. Then, 1-bromohexane was drop slowly into the flask and the solution was stirred under dry nitrogen at ambient temperature.
After 1 hr, the resulting solution was transferred into another flask containing 3-bromothiophene and Ni(dppp)Cl2 in dried ether and the mixture was reacted for 1 day.
The desired organic product was obtained from extraction with 1 M sodium chloride solution. The obtained organic layer was filtrated and concentrated after extraction. The
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crude product was further distilled under reduced pressure (65-70 oC (0.28 torr)).
Subsequently, N-bromosuccinimide (NBS) was added into a three-neck round bottom flask containing as synthesized 3-hexylthiophene in dried THF and the mixture was stirred under dry nitrogen for 2 hrs. The resulting mixture was then concentrated and filtrated by silica gel column chromatography with hexanes as eluent. The residual product was further distilled by reduced pressure distillation (145 oC (0.021 torr)).
2-3-2 Polymerization of P3HT homopolymer
The P3HT polymerization was prepared by following literature procedures.20, 41, 42 The synthetic route for the P3HT homopolymer is shown in Figure 2-2. All glass apparatuses were dried prior to use. The synthesized monomers were added into a round-bottomed flask equipped with a high vacuum Rotaflo stopcock, which was then evacuated under reduced pressure to remove water and oxygen. Freshly distilled THF were then added into the flask via a double-tipped stainless steel needle under the protection of nitrogen, and the solution was mixed with a magnetic stirrer under dry nitrogen. After a complete mixing, i-PrMgCl was added via syringe and the resulting mixture was refluxed at 70 °C for 2 hr. Then, the solution was cooled to room temperature, followed by adding a suspension of Ni(dppp)Cl2 in THF into the flask via a syringe. Upon stirring for polymerization for 2 hr, the reaction was quenched by
pouring of HCl (aq., 30 wt%) into the mixture. The polymer mixture was extracted with CHCl3 and dried over anhydrous MgSO4. The crude product was precipitated in methanol and washed by using Soxhlet apparatus with methanol, hexane and chloroform sequentially. The residual solvent was removed by evaporation to give a solid product and then stored under nitrogen atmosphere.
Br
Figure 2-1 The synthesis scheme of 2,5-dibromo-3-hexylthiophene.
S n
Figure 2-2 Schematic illustrations of polymerization of P3HT homopolymer.
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2-4 Characteristics of P3HT homopolymer
2-4-1 1H Nuclear magnetic resonance (1H NMR)
All compounds were checked by 1H NMR and the synthesized compounds were dissolved in CDCl3 and their 1H NMR spectra were recorded on a Bruker AC-400 NMR at room temperature. Tetramethylsilane (TMS) was used as the standard reference substance. Figure 2-3 represents the NMR spectrum of 3-hexylthiophene, and it shows six different peaks at 0.87 ppm (f); 1.30 ppm (e); 1.61 ppm (d); 2.61 ppm (c); 6.91 ppm (b); 7.21 ppm(a) which represent the H signals of 3-hexylthiophene monomer. The spectroscopic characterization was in accord with that found in the literature. The NMR spectrum confirmed that this compound was successfully synthesized. Furthermore, comparing Figure 2-4 to Figure 2-3, the peak at 7.21 ppm (a) appearing in Figure 2-3 vanished in Figure 2-4, which meant 2,5-dibromo-3-hexylthiophene monomer was successfully synthesized by the bromination of 3-hexylthiophene, and the product was in accord with that found in the theoretical information. Additionally, the obtained P3HT homopolymer was also characterized by 1H NMR spectroscopy, and the related NMR spectrum is shown in Figure 2-5. The 1H NMR spectrum reveals that the synthesized P3HT homopolymer has a head-to-tail regioregularity of ~94%.
S
a
a
b
b c b
c d
d e
e e f
f e
(a)
Figure 2-1 1H NMR spectra and the structural assignments of 3-hexylthiophene in CDCl3.
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S Br
a
b a
b c
c d
d d
d
e
e
(b)
Figure 2-4 1H NMR spectra and the structural assignments of 2,5-dibromo-3- hexylthiophene in CDCl3.
Br
a
b c
d e
S m Br
a b c d d
d e
Figure 2-5 1H NMR spectrum of P3HT homopolymer in CDCl3.
2-4-2 Gel permeation chromatography (GPC)
The molecular weight and its distribution of the P3HT sample were measured by using a GPC (Waters 2695) equipped with two Styragel columns (HR3 and HR4E), a refractive index detector (Waters 2414) and a photodiode array absorbance detector (Waters 2996). THF was used as the mobile phase at a flow rate of 1 mL/min. Figure 2-6 shows the GPC result of the synthesized P3HT sample. Based on the GPC data, it can be observed that the P3HT sample has an Mn of 56,200 g/mol and a polydispersity (PDI) of ~1.48. Therefore, the conjugated P3HT homopolymer used in this study was
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assigned as P3HT56.
abs. (a.u.)
Time (min)
0 2 4 6 8 10 12 14 16 18
Figure 2-6 GPC trace of the P3HT56 homopolymer synthesized by using the Grignard metathesis method.
2-5 In situ synthesis of P3HT/TiO
2nanohybrids with a nanowire structure
TiO2 nanocrystals were synthesized in situ on P3HT nanowires with the addition of TiO2 precursor titanium (IV) tetraisopropoxide (TTIP) in the P3HT solution. The detailed synthesis procedure is listed as follows. First, P3HT56 homopolymer was added to chlorobenzene (CB) solution to prepare a 1.0 wt% solution at 60 oC, followed by the addition of TTIP (97.0% pure, Aldrich) to the P3HT/CB solution in different molar ratios of 3-hexylthiophene monomer units to Ti (3HT/Ti) of 1:0.33, 1:0.66, 1:1 and 1:2. After stirring for 2 days, nanowire assembly of the polymeric complexes was carried out via the addition of anisole, a poor solvent to P3HT, into the mixture to obtain a 0.5 wt% solution. This was followed by stirring for another day. Subsequently, the P3HT/inorganic hybrid complexes were spin-coated or drop-casted onto variant substrates, e.g. quartz pieces, silicon wafers, or copper grids. Then, the in situ growth of TiO2 nanocrystals on P3HT nanowires were achieved by using an elevated pressure hydrothermal synthesis process on the coated film. In details, the coated polymer/complex thin film was first put into an enclosed stainless-steel autoclave with an inner chamber volume of 20 ml. A solution of ~3 ml of H2O/MeOH in a beaker (1:1 vol%) had also been placed in the autoclave, followed by placing the enclosed autoclave inside an oven for heating. A high pressure, steam saturated environment was created by
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thermally heating the enclosed stainless-steel autoclave filled with deionized water and methanol at 130 oC. For different treatments presented hereafter, stable temperature and pressure were reached within ca. 30 mins. Autoclaving treatments were performed in a range of 2 to 9 h. After the hydrothermal treatment, the autoclave was rapidly opened in order to discharge the vapor and to avoid its condensation on the film surface, followed by cooling down the samples to room temperature. During the hydrothermal process, the pressure inside the enclosed chamber was measured to be around 7.2 bars as indicated by a manometer pressure gauge. Finally, the resulting P3HT/TiO2 hybrids with D/A (donor/acceptor) network nanowire structures were obtained by performing the above-mentioned high-pressure hydrothermal treatment.
Figure 2-7 Procedure for preparation of P3HT/TiO2 hybrids with nanowire structures.
P3HT in CB stirred at 60 oC for 1 hr
Adding TTIP into the mixture
Stirred at 60 oC for 2 days
Adding anisole to induce nanofibril assembly
Stirred at 60 oC for another day
Coating onto variant substrates
The samples were put into an autoclave.
A solution of H2O/MeOH (1:1 vol%) had also been placed in the autoclave
Placing the enclosed autoclave inside an oven for heating to 130 oC for several hours
Obtaining P3HT/TiO2hybrid nanowires
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Table 2-3 Formula for each solution of P3HT/TiO2 precursor hybrid system.
Sample 3HT units
(molar ratio)
TTIP (molar ratio)
Solvent
Pristine P3HT56 1 0 CB/anisole
P3HT/TTIP_1:0.33 1 0.33 CB/anisole P3HT/TTIP_1:0.66 1 0.66 CB/anisole
P3HT/TTIP_1:1 1 1 CB/anisole
P3HT/TTIP_1:2 1 2 CB/anisole
2-6 Characterization methods of synthesized P3HT/TiO
2nanohybrids
2-6-1 X-ray photoelectron spectroscopy (XPS)
XPS studies were performed on a Theta Probe (Thermo Fisher Scientific, UK) equipped with a monochromatic Al K-alpha X-ray source (hν = 1,486.6 eV) operating at 15 kV, 100 W with a spot size of 400 μm in diameter and a quartz monochromator under UHV (2 x 10-9 torr) conditions. The C 1s peak located at 284.6 eV was assigned to aliphatic carbon atoms as the criterion to rectify binding energies of XPS spectra. A Shirley type nonlinear background was subtracted and the peak
deconvolution was performed using 90% Gaussian and 10% Lorentzian character. The final high-resolution S 2p spectrum was acquired by summing 60 spectra at a pass energy of 20 eV in order to obtain a sufficient signal-to-noise ratio. The photoelectrons were analyzed by using a concentric hemispherical analyzer.
2-6-2 Transmission electron microscopy (TEM)
To investigate the nanostructure and the embedded nanoparticle morphology of the P3HT/TiO2 hybrids in the thin film state prepared by using the current in situ hydrothermal process, transmission electron microscopy (TEM) was employed.
Samples for TEM measurements were prepared by depositing a droplet of the P3HT/inorganic hybrid complex solution onto a carbon-coated copper grid, followed by drying the sample in air for 6 h. Then, the P3HT/TiO2 hybrid sample underwent the hydrothermal crystallization process. Bright field images were recorded on a JEOL 1230EX operating at 100 kV with a Gaten Dual Vision CCD Camera.
2-6-3 Grazing incidence small-angle and wide-angle X-ray scattering (GISAXS and GIWAXS)
Samples for X-ray scattering measurements were prepared by following the same sample preparation procedure for TEM, except that the coating was performed on
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silicon wafers instead of on copper grids. GISAXS and GIWAXS experiments were performed separately at the 23A1 and 17A1 endstations at the National Synchrotron Radiation Research Center (NSRRC) in Taiwan. The wavelength of the incident beam was 1.1809 Å for the GISAXS and 1.3213 Å for the GIWAXS measurements. For the GISAXS measurements, an incident angle of 0.2° was used, and the scattering patterns were collected on a Mar-CCD with a diameter of 165 mm. The one-dimensional scattering profiles along the qxy axis (in-plane) were reported as plots of scattering intensity, I, versus the scattering vector, q. The scattering vector q was calibrated using a standard sample of Ag-behenate. For the GIWAXS measurements, the incident angle was set to 0.2° as well, and the scattering patterns were collected using a Mar3450 image plate detector. The two-dimensional XRD patterns observed for the measured samples were also circularly averaged to obtain one-dimensional diffraction profiles along the qxy andqz axis (out-of-plane) as plots of the scattering intensity versus the scattering vector, with the value of q calibrated using standard samples of Ag-behenate and Si wafer.
2-6-4 Photophysical property analysis
The UV-Vis absorption measurements of the hybrid samples were conducted using a Cary 100 UV-visible spectrophotometer at room temperature. The thin film samples
used in the analysis were prepared by coating hybrid samples on quartz pieces using the same sample preparation for the GIXS measurements. UV-Vis absorption was performed in a scanning range from 230 to 750 nm.
The photoluminescence (PL) properties were examined at the excitation wavelength of 550 nm on a Perkin Elmer LS55 luminescence spectrometer, for which the sample was prepared by spin-coating (a spin rate of 700 rpm) hybrid samples solution (0.5 wt% P3HT) onto a quartz plate. The band width was 2.5 nm for both excitation and emission measurement.
The photoluminescence lifetimes spectra (TRPL) in the solid state were obtained by exciting film samples (~80 nm in thickness) at 478 nm monochromatic light with an average power of 1 mW, operating at 25 MHz in a duration of 70 ps. The PL signal was collected using a Pico Quant avalanche photodiode with the detection wavelength of 650 nm.
2-7 Solar cell device fabrication and characterization
In our study, solar cells were fabricated with a so-called “inverse” device structure19, 43, 44 of FTO/c-TiO2/active layer/P3HT/PEDOT:PSS/Au. The photovoltaic device structure was illustrated in Figure 2-8.
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2-7-1 Preparation of FTO-coated glass substrates
Firstly, FTO-coated glass was cut into 2 cm x 2 cm pieces. Then, the FTO-coated glass substrates were cleaned by the neutral detergent in order to wipe off the oil. After that, the substrates were rinsed with deionized water, acetone and isopropyl alcohol by ultrasonication for 15 min in each step. Finally, the rinsed substrates were dried by blowing of nitrogen flow.
Firstly, FTO-coated glass was cut into 2 cm x 2 cm pieces. Then, the FTO-coated glass substrates were cleaned by the neutral detergent in order to wipe off the oil. After that, the substrates were rinsed with deionized water, acetone and isopropyl alcohol by ultrasonication for 15 min in each step. Finally, the rinsed substrates were dried by blowing of nitrogen flow.