Photoelectric Properties of Silicon Nanocrystals/P3HT Bulk-Heterojunction Ordered in Titanium Dioxide Nanotube Arrays
© to the authors 2009
Received: 24 June 2009
Accepted: 5 August 2009
Published: 18 August 2009
A silicon nanocrystals (Si-ncs) conjugated-polymer-based bulk-heterojunction represents a promising approach for low-cost hybrid solar cells. In this contribution, the bulk-heterojunction is based on Si-ncs prepared by electrochemical etching and poly(3-hexylthiophene) (P3HT) polymer. Photoelectric properties in parallel and vertical device-like configuration were investigated. Electronic interaction between the polymer and surfactant-free Si-ncs is achieved. Temperature-dependent photoluminescence and transport properties were studied and the ratio between the photo- and dark-conductivity of 1.7 was achieved at ambient conditions. Furthermore the porous titanium dioxide (TiO2) nanotubes’ template was used for vertical order of photosensitive Si-ncs/P3HT-based blend. The anodization of titanium foil in ethylene glycol-based electrolyte containing fluoride ions and subsequent thermal annealing were used to prepare anatase TiO2nanotube arrays. The arrays with nanotube inner diameter of 90 and 50 nm were used for vertical ordering of the Si-ncs/P3HT bulk-heterojunction.
Most of the solar cells installed today are made of silicon. The vision is to produce solar cells from cheap materials in low-cost processes, thus resulting in lower cost electricity production. It is believed that bulk-heterojunction solar cells made of from conjugated polymer blends offer such a possibility [1, 2]. Organic semiconductors propose simple device fabrication technologies (i.e., screen printing and spin casting) at low temperature and therefore are very attractive for photovoltaic industry . The bulk-heterojunction concept for solar cells has a potential to be considerably improved [2–4]. One possible route is the introduction of organic electron acceptors into a conjugated polymer matrix. Silicon in the form of tiny Si-ncs, which exhibit quantum confinement effects, offers unique properties: i.e., the Si-ncs can serve as electron acceptors [5, 6] and can produce multiple electrons per photon due to the carrier multiplication that can increase photocurrent generation . The bulk-heterojunction based on Si-ncs can present new possibilities to design new types of absorber materials for low-cost hybrid solar cells [5–7].
The photovoltaic performance of Si-ncs/conductive polymer bulk-heterojunction relies on mesoscopic arrangement of both nanocrystals and polymer conjugation chains. The morphology of the bulk-heterojunction can be significantly affected by various fabrication parameters during the device formation . Functional nanotubes fabrication and novel synthetic strategies for generating nanotubes from inorganic materials have been recently widely investigated and reported in literature . An incorporation of novel 1D nanostructures into nano-porous templates and an enhancement of the solar cells performance based on 1D configurations have been recently demonstrated [10, 11]. It is expected that a fiber- and/or vertical 1D-like order of photosensitive bulk-heterojunction gives considerable advantages over the thin film technology, because it provides larger interfacial area for efficient exciton dissociation and straight path for photogenerated carries. Furthermore, fiber arrays have much lower reflectance and enable fabrication of thicker devices with increased absorption compared with thin films. As a result, fibers help to avoid circuit shorts and interruption of percolation paths for carriers to their respective electrodes . Till date several nanotubular architectures have been investigated for potential enhancement of electron percolation pathways in bulk-heterojunction [8, 12]. One of such possibilities is vertically oriented titanium dioxide (TiO2) nanotube arrays. An efficient charge collection in photoelectrochemical devices after blend infiltration into transparent TiO2 nanotube films has been shown . In addition, such architecture allows an improvement in light harvesting as thicker devices can be produced in order to increase the optical density . A relatively easy fabrication technique of TiO2 nanotube arrays by anodization offers highly ordered nano-templates at low cost [15, 16].
In this study, the bulk-heterojunction based on Si-ncs prepared by electrochemical etching and poly(3-hexylthiophene) (P3HT) polymer is investigated. Firstly, the photo-transport properties of the bulk-heterojunction fabricated in parallel configuration on interdigitated electrodes are studied. Next, a vertical 1D-like configuration of the bulk-heterojunction through infiltration into a nanotubular template of anodic TiO2is performed. Anodization of titanium foil followed by annealing has been used to fabricate the crystalline anatase nanotubular templates. The optoelectrical properties of photoconductive Si-ncs/P3HT bulk-heterojunction aligned in TiO2nanotubes with inner diameter of 90 and 50 nm are compared and discussed in details.
Firstly, in order to infiltrate the bulk-heterojunction, the TiO2 nanotubes with two deferent inner diameters were prepared as follows. A 99.99% pure titanium foil was anodized in 0.3 wt% solution of NH4F in ethylene glycol in a two-electrode configuration under constant potentials of 40 and 20 V at room temperature, respectively . The titanium foil served as anode and platinum mesh as the counter electrode. The transformation of amorphous as-grown anodic TiO2 into crystalline anatase was performed by annealing in air at 450 °C .
Then, the dried Si-ncs were used for bulk-heterojunction fabrication. Commercially available (ALDRICH, Nakayama, Tsukuba-branch, Japan) P3HT polymers were dissolved in chlorobenzene (14 mg/mL). An aliquot of 400 mg of the polymer solution and 2 mg of Si-ncs were mixed. The photoelectric properties of the blend were investigated in two configurations. For parallel photo-conductivity measurements, the Si-ncs/P3HT blend was coated on interdigitated platinum contact evaporated on glass . For perpendicular measurements, the Si-ncs/P3HT blend was incorporated into the TiO2 nanotubes and covered on top by a transparent conductive oxide (TCO) electrode, which was spin-coated with poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), to form TCO/PEDOT:PSS/(Si-ncs/P3HT)/TiO2-NTs device. Another device with only pure P3HT polymer was prepared for comparison. All samples were dried at 140 °C for 30 min in vacuum.
The temperature-dependent PL measurements were run from 4 to 300 K by placing the samples into a cryostat. The He:Cd CW laser (325 nm or 3.82 eV) has been used as an excitation source. The conductivity and photo-conductivity measurements were performed in ambient conditions. A constant potential from a regulated dc power supply was applied and the resulting current was measured with an ampere meter (Sub-Femtoamp, Keithley 6430). For the photo-conductivity measurements, a white light of 1.5 AM was used to irradiate the samples.
Results and Discussions
Bulk-Heterojunction in Parallel Configuration
Figure 1b shows PL spectra measured for Si-ncs/P3HT blend film under excitation at 325 nm taken at 4 and 300 K. The introduction of the Si-ncs into the conjugated polymer results in change of the PL characteristics . Contrary to the pure polymer films, the red shift of the PL band as a function of temperature is recorded. A similar temperature dependence of the PL behavior was previously observed for Si-ncs with quantum confinement size effect, which was introduced into MEH-PPV polymer . After exciton transfer from the polymer to the Si-ncs, the state filling effect is responsible for the shift of the PL band . An increase in temperature decreases the emission intensity at higher emission energies and leads to a ~40-nm red-shift of PL spectrum maxima in temperature ranging from 4 to 300 K (Fig. 1b).
Figure 2b displays the current–voltage (I V) curves for Si-ncs/P3HT blend in dark (black line) and under AM1.5 illumination (red line). An introduction of the Si-ncs into P3HT polymer increases the overall dark-conductivity (more than two times) through the polymer. Furthermore, under AM1.5 illumination, the photocurrent generation is observed. The ratio of the photo-conductivity to the dark-conductivity is about 1.7. We attribute the photocurrent generation to the formation of the bulk-heterojunction in the Si-ncs/P3HT blend. To form the bulk-heterojunction, the photosensitive material requires proper energy band alignment. Fortunately, the energy levels of Si-ncs match the energy levels of the conjugated P3HT polymer. The highest occupied molecular orbital for P3HT is −5 eV below the vacuum, whereas the lowest unoccupied molecular orbital is −3 eV. The band gap of Si-ncs is about (~2 eV) , which allows a proper adjustment of the bands for the e h separation . The different electron affinities and ionization potentials provide a driving force for the e h dissociation when the excitons are generated under AM1.5 irradiation. We assume that a large fraction of the excitons dissociates at Si-ncs/polymer interface. Dissociated excitons leave the electrons in the nanocrystals and the holes in the polymer.
Alignment of Bulk-Heterojunction in TiO2Nanotubes
It is clear that the nanofiber morphology of the blend embedded into nanotubular templates can contribute to high open-circuit voltage as well. The bulk-heterojunction performance depends on mesoscopic arrangement of both nanocrystals and polymer chains. The fiber-like alignment provides larger interfacial area, which results in enhanced exciton dissociation. It has to be noted that compared to thin films, the fiber arrays configuration has lower reflectance. This is also most likely the reason in the case of TiO2nanotubes with larger diameter that allow infiltration of larger amount of bulk-heterojunction into nanotubes, and form thicker device. As a result, increased absorption and active surface area for exciton separation augment photovoltage generation. However, for a clear picture of the origin of an increase in the open-circuit voltage, further experiments have to be performed.
We have investigated photoelectric properties of the Si-ncs P3HT polymer-based bulk-heterojunction. The room temperature luminescent Si-ncs with large optical band gap (~2 eV) were prepared by electrochemical etching. The presence of the Si-ncs in the polymer increases the transport and assures an interaction between electronic states of the Si-ncs and P3HT polymer. As a result, the exciton transfer from the polymer and bulk-heterojunction formation were achieved. The exciton transfer and quantum confinement effect in Si-ncs lead to PL maxima shift around ~40 meV in the temperature region 4–300 K. The ratio between photo- and dark-conductivity around 1.7 in parallel configuration has been demonstrated. Furthermore, we showed that the titanium dioxide (TiO2) nanotube arrays with tunable nanotube diameter can be efficient for a vertical 1D-like order of Si-ncs/P3HT photosensitive blend. The arrangement of the Si-ncs/P3HT bulk-heterojunction within ordered TiO2nanotubes prepared perpendicular to the contact facilitates excitons separation and charge transfer along nanotubes. It is expected that the optimization of the morphology of the Si-ncs/P3HT blend infiltrated into TiO2nanotubes can lead to an improvement in the solar cell performance.
This study was supported by the New Energy Development Organization (NEDO) of Japan.
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