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Technology

Cryscade Solar Ltd. develops a concept of “n-peller” materials opening a way to high-performance organic solar cells (OSC) with 30% target efficiency based on molecular level charge separation and segregated transport of charges to their corresponding electrodes. OSC can be produced by low-cost printing technology.

Cryscade Solar Ltd. improves and optimizes Cascade Crystallization technology, which was developed by Optiva Inc. for manufacturing Thin Crystal Films (TCF™) polarizers. At present the patent portfolio of former Optiva Inc. on TCF™ polarizers is a property of Nitto Denko, a leading manufacturer of LCD polarizing films.

Cryscade Solar Ltd. modifies Cascade Crystallization procedure for producing ‘n-peller’ based active layers for OSC with a particular molecular ordering which makes possible extremely efficient light harvesting and charge transport.

Cryscade Solar Ltd. modifies and develops Cascade Crystallization process for manufacturing Cryscade™ photosensitive organic thin films [1,2] promising to increase the power conversion efficiency of organic photovoltaic devices.

The basic Cascade Crystallization technology is reviewed, analyzed and summarized by Dr. Pavel Lazarev, the founder of Optiva Inc., and the CEO and founder of Cryscade Solar Ltd., in the manuscript “Cascade Crystallization” (in preparation for publication). We present below in the section "Cascade Crystallization" the extracts from the manuscript devoted to the main principles, features and advantages of Cascade Crystallization technology.


Molecular level pn-junction materialsare initially developed by a group of researchers in Northwestern University (USA) lead by Prof. Michael Wasielewski [3].
What do we expect from a solar cell? A solar cell is a device that converts photons from the sun into electricity. Generally speaking, this device needs to fulfill two functions: photogeneration of charge carriers in a light-absorbing material as well as separation and delivering of the negative and positive charges to conductive contacts. It is desirable that, being separated, carriers might live without recombination during sufficiently long time period.

In accordance with the said requirements Cryscade Solar Ltd. is aiming for high-performance organic solar cells based on molecular level pn-junctions.

We take 'n-peller' molecule comprising electron donor and electron acceptor structural units — polycyclic organic compounds with conjugated pi-systems — linked via a bridge, designed to promote one-way electron transfer. Such molecules are proven to provide charge separation and demonstrate long lifetime of separated charges [4].


Then we gather these molecules into columnar stacks — supramolecules, and sew them together via pi-pi-interaction: donor to donor and acceptor to acceptor. So we obtain a ‘natural habitat’ for each type of carriers in the form of pipes, supporting segregated transport of electrons (pi-linked acceptors) and holes (pi-linked donors).

The main advantage with respect to ordinary organic solar cells here is that the photogenerated excitons have no need to run somewhere looking for the place to dissociate. The design of the molecule provides them with the opportunity to split off at the birthplace.

In order to achieve the most efficient light absorption in a handy and easy-assembly device supramolecules are to be placed vertically on a substrate. Modified Cascade Crystallization technique allows so called homeotropic alignment of molecular stacks.

After all, we close up molecular stacks with rectifying layers and electrodes to protect corresponding contacts from a penetration of alien carriers.

Using of organics makes possible to avoid expensive vacuum and high temperature processes. The active layer may be deposited from a solution by means of just a brush.

Cryscade’s solar cells combine efficient charge separation and lossless single-type carrier transport in an ordered mesoscopic supramolecular structure. They consolidate the charge transport properties of a crystalline structure with the processibility of polymeric materials. Low-temperature and therefore minimal energy-consuming processing requires less capital investment than conventional vacuum deposition fabrication techniques.

N-peller based organic solar cells will be no less effective than inorganic ones, but substantially less expensive.

Cascade Crystallization

Basic phenomena of Cascade Crystallization might be described as a crystalline film growth that does not depend upon substrate because crystalline order is introduced in liquid state and transferred without loss or with small losses of order onto a substrate. Substrate surface defects can not dictate the local order and so the global order introduced by deposition remains. This technology is limited to the class of conjugated aromatic organic molecules mainly with flat plate like molecular structure.

Until recently, there were not many crystal film growth techniques that would allow to control the direction of crystal growth and direction of crystallographic axes in order to produce anisotropic crystalline component of useful size. We know of just one, and it is epitaxial growth [5-6]. Technology presented here is the second.

Crystalline films have attractive anisotropic properties that might be used in optical and semiconductor industries.

Pre-ordering material in liquid phase - Lyotropic Liquid Crystals

In order to avoid effect of the substrate in crystal film growth we developed technique that pre-orders material in liquid state by self-assembly of molecules into supramolecules and forming Lyotropic Liquid Crystal with local crystalline order.

The tendency of conjugated aromatic molecules to aggregate into columns is present even in dilute solution (just as for amphiphilic systems, where micelle formation occurs before the mesophase is formed). However, although there may be a threshold concentration before aggregation begins to occur; there is no optimum column length and hence no critical concentration directly analogous to a Critical Micelle Concentration. A further distinction is the absence of a Krafft temperature. Since the process of mesophase formation does not depend on the presence of flexible alkyl chains, there is no threshold temperature below which mesophases cannot be produced because the vital flexibility of the molecules has been frozen out [7-10].

Basic principle of Cascade Crystallization

We developed the method for thin crystal film manufacturing which we refer to as Cascade Crystallization [11-18]. Cascade Crystallization process involves a chemical modification step and four steps of ordering during the crystal film formation. The chemical modification step introduces hydrophilic groups on the periphery of the molecule in order to impart amphiphilic properties to the molecule. Amphiphilic molecules stack together into supramolecules, which is the first step of ordering. By choosing specific concentration, supramolecules are converted into a liquid-crystalline state to form a lyotropic liquid crystal, which is the second step of ordering. The lyotropic liquid crystal is deposited under the action of a shear force (or meniscus force) onto a substrate, so that the shear force (or the meniscus) direction determines the crystal axis direction in the resulting solid crystal film. This shear-force- assisted directional deposition is the third step of ordering, representing the global ordering of the crystalline or polycrystalline structure on the substrate surface. The last fourth step of the Cascade Crystallization process is drying/crystallization, which converts the lyotropic liquid crystal into a solid crystal film. We will use the term Cascade Crystallization process to refer to the chemical modification and four ordering steps as a combined process demonstrated in Fig 1.

Fig. 1: Supramolecular formation and globalization of order

The film produced by the Cascade Crystallization process has a global order. The global order means that the direction of the crystallographic axis of the film over the entire substrate surface is controlled by the deposition process and, with a limited influence of the substrate surface. Molecules of the deposited material are packed into lateral supramolecules with a limited freedom of diffusion or motion. The lyotropic liquid crystal is characterized by an interplanar spacing of 3.4 ± 0.3 A in the direction of one of the optical axes.

Inventing of electric field of a special configuration on the stage when molecules undergo self-assembling in lyotropic liquid crystal allows a controlling of a molecular stacks orientation. Drying of a fresh-coated film under electric field may result in homeotropic orientation of supramolecules. This approach is currently in development.

New technique for growth of semiconductor organic crystalline films

Cascade Crystallization process is very attractive for manufacturing photosensitive and semiconducting thin crystalline films [19]. The advantage of Cascade Crystallization process is that this method allows to create devices with practically unrestricted area of surface.
Owing to anisotropic properties of the photoelectric layers employed in the organic photosensitive optoelectronic devices, these devices can be used as detectors of linearly polarized electromagnetic radiation. In other words, the response of photosensitive optoelectronic devices made by means of Cascade Crystallization process depends on mutual orientation of an optical transmission axis of organic photoelectric layer and polarization vector of linearly polarized electromagnetic radiation that is incident onto optoelectronic device.

Another advantage of thin crystalline films in photovoltaic devices is their lightfastness and environmental stability that was demonstrated in experiments with polarizing films [11, 16, 17].

Conclusion

Technological space of crystal growth technologies has received one more technique – thin crystalline film growth which employs visco-elastic properties of thixotropic supramolecular lyotropic liquid crystals in order to eliminate the influence of substrate surface defects on the orientation of seed crystals in the early stages of film crystallization.

This technique is limited to a specific class of compounds, which can form supramolecules, but are almost unaffected by surface properties of the substrate.

This technique is named Cascade Crystallization in order to reflect the presence of separated stages of order acquisition in subsequent steps of the process.

The advantage of Cascade Crystallization Process is that this method allows to create devices with practically unrestricted area of surface.

References

1. P. Lazarev, “Organic photovoltaic layer, organic photovoltaic device and method of manufacturing thereof”, Patent application WO 2007/012835.
2. P. Lazarev et al., “Organic compound, organic photovoltaic device, semiconductor crystal film and method of producing thereof”, Patent application WO 2007/020442.
3. M.R. Wasielewski, “Energy, Charge, and Spin Transport in Molecules and Self-Assembled Nanostructures Inspired by Photosynthesis”, J. Org. Chem., 71 (14), 5051-5066 (2006).
4. Patent pending.
5. J.H. van der Merwe, "Recent Developments in the Theory of Epitaxy", Chemistry and Physics of Solid Surfaces V, 365-401, Eds. R. Vanselow and R. Howe, Springer-Verlag, N.Y., (1984).
6. "Growth from the Vapor Phase", Modern Theory of Crystal Growth I, Ch. 8, Ed. A. A. Chernov, Springer-Verlag , N.Y. (1983).
7. A.S. Vasilevskaya, E.V. Generalova, A.S. Sonin. "Chromonic mesophases", Russian Chemical Reviews, 58, 904; translated from Usp. Khim., 57, 1575-1596 (1989).
8. J.-M. Lehn, "Supramolecular Chemistry, Concepts and Perspectives", VCH, Weinheim (1995).
9. J. Lydon, "Chromonics", Handbook of Liquid Crystals 2B, 981-1007 (1998).
10. J. Lydon, “Chromonic mesophases”, Curr. Opin. Colloid Interface Sci., 8 (6), 480-490 (2004).
11. K.I. Gvon et al., “Method and materials for Thermostable and lightfast dichroic light polarizers,” Patent US 5,739,296 (1998).
12. P. Lazarev, V. Nazarov, N. Ovchinnikova, “Method of Obtaining Anisotropic Crystalline Films and Devices for Implementation of the Method”, Patent US6913783.
13. P. Lazarev, K. Lokshin, V. Nazarov, “X-ray Diffraction by Large Area Organic Crystalline Nano-films”, Molecular Materials, 14 (4), 303-311(2001).
14. Dembo, A. Ionov, P. Lazarev, A. Manko, V. Nazarov, “Lyotropic Dye-water Mesophases Formed by Rod-like Supramolecules”, Molecular Materials, 14 (4), 275-290 (2001).
15. Y. Bobrov, "Spectral properties of Thin Crystal Film Polarizers", Molecular Materials, 14 (3), 191-203 (2001).
16. Y. Bobrov, L. Blinov, L. Ignatov, G. King, P. Lazarev, V. Nazarov, N. Ovchinnikova, S. Remizov, “Environmental and optical testing of Thin Crystal FilmTM polarizers” (2003), Journal of the SID, 11/1, 63-70.
17. P. Lazarev, V. Nazarov, N. Ovchinnikova, S. Remizov, “Printed Optical Components for Liquid Crystal Displays”, 12th International Symposium, Advanced Display Technologies: Basic Studies of Problems in Information Display (FLOWERS’2003), August 25-28, p.186-189, 2003.
18. P. Lazarev, V. Nazarov, “Anisotropic Film Manufacturing”, Patent application WO2004065524.
19. P. Lazarev, V. Nazarov, "Organic Photosensitive Optoelectronic Device", Patent application US2004067324.