J. Phys. Chem. C 2008, 112, 4761-4766
4761
Ultrafast Electron Transfer between Conjugated Polymer and Antimony-Doped Tin Oxide (ATO) Nanoparticles Jianchang Guo, Chunxing She, and Tianquan Lian* Department of Chemistry, Emory UniVersity, Atlanta, Georgia 30322 ReceiVed: September 26, 2007; In Final Form: January 15, 2008
Nanocomposites of conjugated polymers and wide band gap metal oxide semiconductors have received intense interests in recent years because of their potential application in solar cells. Photoinduced charge separation and recombination in conjugated poly[2-methoxy-5-(2-ethyl-hexyloxy)-(phenylene vinylene)] (MEH-PPV) sensitized Sb:SnO2 (ATO) nanocrystalline thin films were investigated by ultrafast transient IR spectroscopy. For ATO films with 0%, 1%, 3%, and 5% Sb dopants, the rates of electron injection from MEH-PPV to ATO were similar (ranging from 0.77 to 0.46 ps), while the rates of the charge recombination increased at higher doping levels. The effects of the doping level on the injection and the recombination rates were discussed.
1. Introduction Conjugated polymer-based nanocomposite materials have received intense interest in recent years because of their potential application in efficient and low-cost solar cells.1-24 Nanocomposites of conjugated polymers and wide band gap metal oxide semiconductors are particularly interesting because the latter can form interconnected three-dimensional nanostructures of high surface area and high charge mobility.16-24 In these composites, the absorption of solar photons by polymers generates excitons, which may dissociate at the polymer oxide interface, transferring an electron to the semiconductor nanoparticle and leaving a positive polaron on the polymer.16-18 A high-incident photon-to-current conversion efficiency requires efficient charge separation at the interface (fast charge separation and slow recombination rates) and charge transport through the composite materials. The charge separation and recombination processes in these composites have been investigated in recent years,16-24 although they are much less well understood than the corresponding processes in dye-sensitized nanocrystalline metal oxides.25,26 Doped wide band gap oxide materials such as Sb:SnO2 (ATO), F:SnO2 (FTO), and Sn:In2O3(ITO) form the conductive layers of transparent conducting glass electrodes. They have been used extensively in polymeric and molecular photovoltaic devices27,28 because of their high conductivity and transparency. In principle, nanocrystalline-doped metal oxides can also be used to fabricate photovoltaic nanocomposites with conjugated polymers. Despite these applications, charge separation dynamics at the polymer/conducting glass electrode interface and their dependence on doping level have not been reported. Recently, we showed that at a molecular adsorbate/nanocrystalline ATO interface the charge injection rate from the molecular excited state to ATO was not affected by the Sb doping level, but the charge recombination rate increased at higher doping.29 In this paper, we present a study of the electron injection and recombination dynamics in MEH-PPV sensitized ATO nanocrystalline thin films by ultrafast transient mid-IR spectroscopy. MEH-PPV was chosen because it is one of the most widely used materials in polymer-based photovoltaic devices. * Corresponding author. E-mail:
[email protected].
The electron density in the conduction band was controlled by the amount of Sb dopant in ATO. At the doping levels examined, the injection dynamics was similar to that of SnO2, but the charge recombination rate increased with the doping level. The effect of doping on the charge separation kinetics and possible charge recombination mechanisms are discussed. 2. Experimental Section Sample Preparations. Poly[2-methoxy-5-(2-ethyl-hexyloxy)(phenylene vinylene)] (MEH-PPV, Mn: 40 000-70 000) was purchased from Aldrich and used directly without further purification. Colloidal ATO was synthesized according to a published procedure.30 Briefly, 30 g (∼85 mmol) of SnCl4‚5H2O (98%, from Aldrich) was dissolved in 500 mL of H2O (Millipore, 18.3 MΩ/cm), to which a solution of SbCl3 (98%, from Aldrich) dissolved in 20 mL of HCl (37 wt %) was added dropwise in an ice bath under rapid stirring. The doping level was controlled by the amount of SbCl3 solution added, and three samples with Sb/Sn molar ratios of 0.01:1, 0.03:1, and 0.05:1 (referred to as 1%, 3%, and 5% ATO, respectively) were prepared. The resulting clear colorless solution was stirred for 30 min before aqueous ammonia (25%) was added to adjust the pH to 3.5-4.0 and was allowed to settle overnight in the dark. The precipitate was washed at least three times with water and resuspended in water. The suspension was adjusted to pH 9.5-10, stirred vigorously overnight, and dialyzed against 10 L of aqueous ammonia at pH 10 to produce clear ATO colloidal solution. The ATO colloidal solution was refluxed for 4 h. This colloid (120 mL) was poured into an autoclave and heated at 150 °C for 1 h and at 270 °C for 16 h. The colloid was then concentrated to 60 mL. Five milliliters of the solution and 2 drops of TritonX100 (from Aldrich) were mixed and stirred for 1 day. The resulting solution was cast onto sapphire windows, dried in air, and then baked at 400 °C for 1 h in an oven to produce nanoporous crystalline thin films. SnO2 nanocrystalline thin films were prepared by a method published previously.22 A detailed characterization of the ATO films prepared in our laboratory by X-ray diffraction and scanning electron microscopy were described in a previous publication.29 Finally, the films were submerged in a solution of MEH-PPV in chloroform
10.1021/jp077712x CCC: $40.75 © 2008 American Chemical Society Published on Web 03/04/2008
4762 J. Phys. Chem. C, Vol. 112, No. 12, 2008
Guo et al.
Figure 1. Absorption spectra of SnO2 (diamond, dash-dot line), 1% (hex, solid line), 2% (circle, dashed line), 3% (square, dotted line), and 5% ATO (triangle, dash-dot line) films from 0.3 to 6 µm. The UV/visible and FTIR spectra were collected in two different spectrometers, leaving a gap between 0.9 and 1.5 µm. The lines in the right side of the figure are simulated plasmon band absorption spectra of ATO (see main text). The inset shows the UV-visible spectra of 5% ATO films with and without adsorbed MEH-PPV.
(0.5 mg/mL) for several minutes, washed with chloroform, and dried in air prior to transient absorption spectrum measurement. Ultrafast Infrared Transient Absorption Measurements. Transient absorption measurements were carried out in a visible pump/mid-IR probe scheme. The tunable infrared spectrometer used was based on a Coherent Legend laser system (800 nm, 150 fs, 2.5 mJ/pulse) and two optical parametric amplifiers (OPAs). A Coherent IR-OPA (OPerA) was pumped at 1 mJ/ pulse to produce signal and idler outputs with 175 (1380 nm) and 90 (1903 nm) µJ of energy per pulse, respectively. The signal and idler were mixed in an AgGaS2 crystal to produce, by difference frequency generation, tunable mid-IR probe pulses with a full width at half-maximum (fwhm) of ∼120 cm-1 and pulse energy of 5 µJ at 5000 nm. The IR probe pulses were attenuated with ND filters and chirp-corrected with Ge windows before the sample. Tunable pump pulses ranging from 460 to 650 nm were generated by sum frequency mixing (in a BBO crystal) of the 800 nm pulse and the signal output (∼80 µJ/pulse) of a ClarkMXR IR-OPA (pumped with 1 mJ of 800 nm pulse). The typical pulse energy and beam diameter at the sample were ∼1 µJ and ∼250 µm for the pump pulses at 530 nm and ∼10 nJ and 180 µm for the mid-IR probe pulses. After the sample, the probe (centered at 2000 cm-1) was dispersed in a spectrometer and detected with a 32-element mercury cadmium telluride (MCT) array detector with a spectral resolution of 15 nm (5.4 cm-1 at 2000 cm-1). Every other pump pulse was blocked by a synchronized chopper (New Focus 3500) at 500 Hz, and the absorbance change was calculated from the pumped versus unpumped probe pulse intensities. Zero time delay and the instrument response function were determined with a thin silicon wafer, which gives an instantaneous mid-IR absorption response after excitation at 532 nm. Typically, the instrument response function after the sample was well fit by a Gaussian function with 160 fs full width at halfmaximum (fwhm). During the data collection, samples were constantly translated at a speed around 5 mm/min to avoid permanent photodamage. Although this speed was not sufficient to completely move the sample to a new spot before the next laser pulse (at 500 Hz) and the possibility of some photooxidation could not be excluded, under the experimental conditions, multiple measurements of each film yielded reproducible results.
3. Results and Discussion Electron Density and Energy Level of ATO Films. The FTIR absorption spectra of 1%, 2%, 3%, and 5% ATO films and undoped SnO2 film are shown in Figure 1. In addition to a strong valence-to-conduction band transition at
