Quantum-dot-based hybrid materials for nanophotonic applications
Quantum dots (QDs) have attracted significant interest in nanoscience since the early 1990s. They have also been studied intensively in the first decade of the new century because of their potential for use in electronic applications such as field-effect transistors, photodetectors, displays, and photovoltaic cells. Because of their very small size, 3D-confined QDs have a large Bohr radius, which allows tuning of their optical properties as a function of size to target specific regions of the solar spectrum. They can also produce multiple excitons by absorbing a single photon, which could enable development of highly efficient photonic devices.1
We recently demonstrated near-IR photodetectors using various QD/polymer composites. One approach to enhance device performance is through conduction boosting by incorporation of pentacene into lead selenide (PbSe) QD/poly(vinyl carbazole) (also known as PVK) photodetector devices. Our results show that the device's external quantum efficiency can be improved by simply blending the pentacene.2 Similarly, we used polypyrrole and single-walled carbon nanotubes (SWCNTs) as conduction boosters instead of pentacene. We maximized the photodetection efficiency of devices containing SWCNTs by chemically attaching the PbSe QDs to SWCNTs.3,4
To further investigate the significance of QDs, we focused on their inorganic chemistry because the bulky ligand surrounding the QDs plays an important role in the charge transport in the structure.5,7 We developed photo-patternable QD arrays with enhanced conducting properties by incorporating stimuli-responsive ligands in the QD structure. We stabilized the QDs with ligands protected with tert-butoxycarbonyl (t-BOC), which is both thermal and photocleavable in the presence of an appropriate photo-acid generator (PAG). A schematic overview of this photocleaving is shown in Figure 1(a). We prepared cadmium telluride (CdTe), cadmium selenide (CdSe), and PbSe QDs functionalized with t-BOC-protected ligands. These nanocrystals, along with a very small amount of PAG, can be spin cast and photocured through a mask to create patterns with a resolution of a few micrometers. We next annealed and developed these patterns with suitable solvents to yield conducting micropatterns constituting semiconductor nanocrystals: see Figure 1(b–d). In Figure 1(e) we summarize the results for the photoconductivity of the unpatterned (initial) and photopatterned CdSe films, demonstrating that the photocurrent increases after the t-BOC group has been cleaved.
We subsequently extended the thermal deprotection technique of t-BOC-capped QDs to our hybrid photovoltaic device. The latter included poly(3-hexylthiophene) (P3HT) and t-BOC-protected CdSe nanorods of length 12–18nm, width ~6nm, and with aspect ratios less than three as active layer.6 We annealed the active layers to induce thermal cleavage of the t-BOC groups. Thermogravimetric analysis of the t-BOC-protected nanorods showed that t-BOC cleavage starts at approximately 150°C. The device structure we employed comprised indium tin oxide/poly(3,4-ethylene dioxythiophene) (PEDOT)/P3HT:CdSe-t-BOC/aluminum. The photocurrent density increased with higher annealing temperatures for a particular applied voltage. For a 10:90 P3HT:CdSe blend, the current density increased with the higher thermal annealing in dark and illuminated conditions: see Figure 1(f). A semilogarithmic plot of the same curve is shown in the inset of this figure. We annealed the active layers of these devices at 100 and 200°C. At the lower temperature, heating only results in removal of the solvent used for processing, while the higher annealing temperature triggers cleavage of the protecting group to improve charge separation at the nanoparticle-polymer interface. For the sample annealed at 200°C, the power-conversion efficiency increased by more than two orders of magnitude compared to that annealed at 100°C.
We studied the effect of varying ratios of P3HT and CdSe nanocrystals for devices annealed at 150°C. We found that for an increased concentration of nanocrystals, the device performance improved: see Figure 1(g). We studied the effect of increasing the annealing temperature by fabricating photovoltaic devices with 10:90 P3HT:CdSe films annealed at temperatures ranging from 150 to 300°C. Devices annealed at 240°C showed the maximum power-conversion efficiency of 0.44%, perhaps because this is close to the melting point of P3HT (which contributes to the formation of optimum bulk heterojunctions).
In summary, we have explored the scope and potential of employing QDs and carbon nanotubes in photodetection and photovoltaic devices. We found that by tuning their sizes and shapes, their optical properties can be tailored to suit the needs of various applications. The concept of patterned semiconductor nanocrystal structures is new and promises to yield many interesting applications in photonics. Although the efficiencies reported here for devices employing CdSe nanorods are lower than previous determinations in the literature, we note that the aspect ratios of the nanorods are much smaller than those used in earlier studies. Thus, there is much scope for improvement in these devices by increasing the nanorod aspect ratio. We can possibly also employ nanocrystals with multipod structures. We are continuing to work at achieving enhanced power-conversion efficiencies.
This work was supported in part by a grant from the Asian Office of Aerospace and Development, the Air Force Office of Scientific Research, and the National Research Foundation of Korea.
Kwang-Sup Lee is dean of the College of Life Sciences and Nanotechnology. He is also a research professor at the Institute for Lasers, Photonics, and Biophotonics at the State University of New York at Buffalo. He studies organic optoelectronic materials for nonlinear optics, solar cells, and organic semiconducting materials.
