Template method for fabricating interdigitate p-n heterojunction for organic solar cell
© Hu et al.; licensee Springer. 2012
Received: 27 July 2012
Accepted: 9 August 2012
Published: 21 August 2012
Anodic aluminum oxide (AAO) templates are used to fabricate arrays of poly(3-hexylthiophene) (P3HT) pillars. This technique makes it possible to control the dimensions of the pillars, namely their diameters, intervals, and heights, on a tens-of-nanometer scale. These features are essential for enhancing carrier processes such as carrier generation, exciton diffusion, and carrier dissociation and transport. An interdigitated p-n junction between P3HT pillars and fullerene (C60) exhibits a photovoltaic effect. Although the device properties are still preliminary, the experimental results indicate that an AAO template is an effective tool with which to develop organic solar cells because highly regulated nanostructures can be produced on large areas exceeding 100 mm2.
KeywordsAnodic aluminum oxide P3TH pillars P-n junction Photovoltaic effect
Bulk heterojunction (BHJ) solar cells [1, 2] are superior to single-  and double-layer cells . The BHJ structure can be formed simply by mixing a donor and acceptor solution. This straightforward technique is advantageous in terms of increasing the donor/acceptor (D/A) interface, which provides the exciton dissociation sites. Meanwhile, a weak point as regards BHJs is that the pathways of the generated carriers are not ensured because of the random phase separation of the respective materials. To ensure exciton dissociation and carrier collection, continuous percolation pathways are required.
An ideal structure would be an interdigitated interface, where the donor and acceptor phases are separate. The diameter and interspatial distance of the pillars should preferably be comparable to the diffusion length of the excitons, which is of the order of 10 nm. Then, the excitons can diffuse to the D/A interface during their lifetime . Furthermore, the interdigitated structure must be aligned perpendicularly to connect with the electrodes so as to provide direct pathways for efficient charge transportation [6, 7]. Meanwhile, the film thicknesses should be around 100 to 200 nm to absorb the incident light and to confine the series resistance [8, 9]. For these reasons, the dimensions of the interdigitated structures should be carefully designed to enhance photovoltaic effects. Interdigitated structures have been obtained using different techniques including self-organization and nanoimprinting [10, 11]. However, there is still room for further optimization of the dimensions .
An AAO template was prepared by a conventional procedure using Al sheet (1 mm in thickness) with three main steps: first anodization, removal of the oxide layer, and second anodization . For the first anodization, a constant voltage of 40 V was applied for 12 h in 0.3 M oxalic acid solution at 0 °C. The alumina pores thus grown were etched away in a mixed solution of phosphoric acid (6% H3PO4) and chromic acid (1.8% CrO3) for 12 h at 60 °C. These procedures were needed to obtain a regular array of alumina dimples. The second anodization was initiated from these dimples and resulted in a highly ordered array of pores. The pore depth can be adjusted with the second anodization, which is performed under the same conditions as the first anodization. A subsequent widening process in 10% (v/v) phosphoric acid allowed fine-tuning of the pore diameters (Figure 1a).
To prepare the P3HT nanopillars, a P3HT solution (3 wt.% in chlorobenzene) was spin coated on the AAO template at a rotation speed of 3,500 rpm (Figure 1b). The P3HT solution penetrated into the pores through capillary force to form P3HT nanopillars (Figure 1c) . The alumina and Al substrate were removed by immersing the sample in 3 M NaOH solution for 45 min, and as a result, a self-standing P3HT film was obtained with a pillar structure on its surface (Figure 1d). Then, the P3HT film was soaked in a 3 M NaOH solution and rinsed in pure water to remove any remaining alumina particles and impurities.
To fabricate a photovoltaic device, the reverse side of the P3HT pillar film should be attached to an indium tin oxide (ITO) substrate. For this purpose, the surface tension of the ITO substrate was modified by soaking it in chloroform for 40 min . The chloroform treatment increased the affinity of the ITO surface for the P3HT films and made it easier to attach them to the ITO substrates (Figure 1e; see Additional file 1:Figure S1).
C60 molecules were deposited on the P3HT pillar surface to form a p-n junction in vacuum with a background pressure of 1 × 10−6 Pa (Figure 1f). The deposition rate was 10 to 20 nm/h, which was controlled by the temperature of a Knudsen cell. Then, a 120-nm-thick Al electrode was deposited on the C60 film (Figure 1g). The P3HT/C60 p-n junction thus prepared was annealed in a vacuum at 180 °C for 20 min. Scanning electron microscope (SEM) images were obtained with an SU8000 Hitachi scanning electron microscope (Minato-ku, Japan). The I-V curve was measured with a WXS-90S-L2 super solar simulator (WACOM, Fukaya-shi, Japan; Figure 1h). All measurements were performed under AM 1.5 irradiation (100 mW/cm2) with a 0.04 cm2 active surface area.
Results and discussion
Dimensional control of P3HT pillar by AAO template
Importantly, the P3HT pillar heights were very uniform at about 100 nm regardless of diameter. The height should be optimized to maintain mechanical stability and to enhance light absorption. If the pillars are too tall, aggregation and collapse occur (see Additional file 2: Figure S2). Meanwhile, the pillars should be tall enough to promote light absorption. The height of 100 nm was optimized to satisfy these requirements by adjusting the second anodization time to 70 s. Pillar height uniformity is another essential factor as regards device operation. Such highly regulated P3TH pillars were observed over the template area. Consequently, the AAO template was shown to be a powerful technique for controlling the nanoscale dimensions of the P3HT pillars, namely diameter, interval, and height, as well as their uniformity over a wide area of about 100 mm2.
Fabrication of P3HT/C60 interdigitated p-n heterojunction and its photovoltaic property
We described an AAO template technique for fabricating regular arrays of P3HT pillars and interdigitated p-n junctions of P3HT/C60. The feature of this technique is the high controllability of the nanoscale dimensions, such as the diameter, interval, height, and uniformity of the P3HT pillars. That is, these dimensions can be tailored to improve effective light absorption and carrier dissociation and transport. The device properties in this study were preliminary, and there is still room for further improvement. However, the technique we demonstrated here has great potential for use in developing a practical device because nanoscale structures can be fabricated in a large area exceeding 100 mm2.
anodic aluminum oxide
indium tin oxide
scanning electron microscope.
This work supported by the World Premier International Center for Materials Nanoarchitectonics (MANA) of the National Institute for Materials Science (NIMS), Tsukuba, Japan.
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