Cover Image: Issue 7-8, Materials Today.Recent awareness of our limited natural resources has fueled the search for alternative forms of energy. A very promising area of research has focused on solar energy, our most abundant yet underutilized resource. Use of organic materials for solar cell fabrication is of particular interest due to their low-cost and solution-processability, large-scale synthesis, and molecular and electronic tunability. Solar cells have evolved through different architectures in efforts to optimize energy harvesting and charge transport. In principle, the most effective architecture would employ pillar-like vertical arrays of p- and n-type semiconductor materials. Because this particular architecture has been very difficult to achieve with organic semiconductors, subjacent solar cell architectures are currently employed, for the time being.
It is well known that charge transport in organic semiconductors is fundamentally governed by the degree of crystallinity. In devices such as transistors, crystallization along the plane of the substrate is preferred since charge delocalization is along the pi-stack direction. Major breakthroughs have already been accomplished with organic single-crystal nanowire transistors [1]. In devices such as solar cells, however, vertical crystallization is preferred since charges are collected at the top and bottom electrodes. This has already been demonstrated with inorganic semiconductors, such as ZnO nanowires [2]. There are several reports on polycrystalline nanostructured organic solar cells with the preferred architecture [3], but polycrystalline devices often suffer from defects, grain boundaries, and irreproducible device fabrication.
One approach to growing vertically oriented single-crystal nanowires is to employ organic materials that pack face-to-face, and thus have a preferential growth direction. There is of great interest in molecules that exhibit face-to-face stacking as it has been theorized that this packing results in high mobility along the nanowire growth direction. This phenomenon is the result of an increased overlap between the electronic wave functions of neighboring molecules in the stack. A large carrier-mobility is an important element to consider when employing organic semiconductors in solar cells. The feat of growing vertical single-crystal nanowires has been difficult to accomplish, mainly because the appropriate materials and/or growth techniques have not been utilized. The small molecule diketopyrrolopyrrole (DPP), is a prime example of a molecular building block that exhibits face-to-face pi-stacking and ideal electronic properties. Its electronic structure, tunability, and synthetic versatility mean that this material can be utilized in high-performance devices such as organic solar cells. DPP is particularly promising for large-scale manufacturing as it has demonstrated flexibility in device fabrication, through techniques such as doctor-blading and spin coating. Its electronic flexibility has brought about a large range of DPP-based devices with custom-designed properties. The Nguyen group at the University of California, Santa Barbara pioneered the use of DPP-thiophene analogues for applications in bulk heterojunction solar cells [4]; these devices have yielded power conversion efficiencies as high as 4.4 %.
Although there have been promising advancements in DPP bulk heterojunction solar cell devices, DPP single-crystal solar cells have seen little progress to date. We are currently actively pursuing the vertical crystallization of DPP and other organic semiconductor materials. The method of crystallization developed by us involves the growth of single crystal nanowires on a variety of substrates by physical vapor transport. This month's cover shows a scanning electron microscope (SEM) image of DPP crystals grown on a silicon dioxide substrate. Vertical DPP nanostructures were crystallized on a number of substrates prepared with several surface treatments. The resulting vertical crystals have a stunning resemblance to grass, thus, the moniker “crystal lawns.” Further studies on the electronic and molecular properties of the crystals will be carried out with particular focus on energy-level determination and morphology control.
From a fundamental point of view, DPP is important for understanding nanoscopic crystallization. Because crystallinity is an important factor that governs device performance, from a technical point of view, crystallization of DPP is imperative for producing high performance solar cells over large areas using a low cost fabrication method. At present the nucleation and growth kinetics of DPP small molecule crystallization are not well understood. This is important for elucidating and determining the intrinsic charge-transport properties of this class of materials. The “crystal lawn” demonstrates nanoscopic crystallization of vertically grown DPP nanowires, and marks the first step towards the production of single-crystal vertically-oriented nanowire solar cells.
Further Reading
[1] Briseño et al. Mater Today, 11 (4) (2008), p. 38
[2] Yang et al. Adv Func Mat, 12 (2002), p. 5
[3] Li et al. Appl Phys Lett, 95 (2009), p. 12
[4] Walker et al. Chem Mater, 23 (2011), p. 3