Here below a short description of some of the most relevant results obtained on self-assembly and characterization at the nanoscale of organic semiconductors.
Solvent Vapour Annealing: controlling the self-assembly from nano- to macro-scale
Conjugated discotic molecules such as perylenes, coronenes, etc. are suitable systems for the development of functional materials exhibiting high charge mobilities for organic electronics. Typically these molecules are deposited from solution on surfaces using simple procedures like spin-coating or drop-casting. The obtained films are usually poorly uniform as they feature a low degree of order on the mesoscopic scale. The order can be increased exploiting methods such as physical/chemical vapor deposition, or zone-casting, which however can hardly be applied for large scale production.
We demonstrate that solvent vapor annealing (SVA) can be used to tailor millimeter long fibers or mesoscopic crystals starting from disordered perylenes or hexabenzocoronenes layers. The SVA process, whose mechanism is governed by nucleation, features both a long-range mass transport of the molecules over hundreds of microns distances, and a reversible character as obtained by changing the solvent, allowing thereby to switch the material morphology from ordered fibers to amorphous droplets. In view of its versatility proven by the use on different molecules, substrates and solvents, SVA can be regarded as a new, general method to self-assemble molecular-based architectures for application in material science.
1. E. Treossi, A. Liscio, X.L. Feng, V. Palermo, K. Muellen, and P. Samori, Temperature-Enhanced Solvent Vapor Annealing of a C-3 Symmetric Hexa-peri-Hexabenzocoronene: Controlling the Self-Assembly from Nano- to Macroscale. Small, (2009) 5, 112.
2. G. De Luca, A. Liscio, F. Nolde, L.M. Scolaro, V. Palermo, K. Mullen, and P. Samori, Self-assembly of discotic molecules into mesoscopic crystals by solvent-vapour annealing. Soft Matter, (2008) 4, 2064.
SOLAR CELLS AND THIN FILM TRANSISTORS BASED ON PERYLENE-FUNCTIONALIZED POLYMERS
Two main classes of semiconducting molecules are commonly used for opto-electronic applications: polymers, featuring an easy solution-processability in thin and uniform yet poorly ordered films, and small poly-aromatic molecules, forming highly defined (liquid)-crystalline architectures with excellent charge mobility. Herein, we combine the two material types by employing structurally well-defined polyisocyanopeptide polymers (PIC) as scaffolds to precisely arrange thousands of electron accepting molecules, i.e. perylene-bis(dicarboximides) (PDIs), in hundreds of nanometer long, well-defined chromophoric wires. The PIC polymer backbone enforces high control over the spatial location of PDI dyes favoring both enhanced exciton and charge transfer.
By adding to a monomeric PDI polycrystalline film just 17% of the PIC-PDI polymer, acting as percolation pathways for charge transfer between the different crystals, we obtained a two orders of magnitude increase in charge carrier mobility within the film, as measured in thin film transistor (TFT) devices .
Furthermore, the photovoltaic activity of this polymeric PIC-PDI shows a significant improvement, respect to monomeric PDI, when blended with regio-regular poly(3-hexylthiophene) (P3HT) in solar cells.For the first time we visualized by Kelvin Probe Force Microscopy (KPFM) the photovoltaic activity occurring in such a blend, thus allowing to gain quantitative insight into the correlation between architecture and function with a nanoscale resolution.
These multi-chromophoric wires represent a new class of versatile building blocks for nanoelectronics.
1. R. Dabirian, V. Palermo, A. Liscio, E. Schwartz, M.B.J. Otten, C.E. Finlayson, E. Treossi, R.H. Friend, G. Calestani, K. Müllen, R.J.M. Nolte, A.E. Rowan, and P. Samorì, The Relationship between Nanoscale Architecture and Charge Transport in Conjugated Nanocrystals Bridged by Multichromophoric Polymers. Journal of the American Chemical Society, (2009) 131, 7055.
2. C.E. Finlayson, R.H. Friend, M.B.J. Otten, E. Schwartz, J.J.L.M. Cornelissen, R.L.M. Nolte, A.E. Rowan, P. Samori, V. Palermo, A. Liscio, K. Peneva, K. Mullen, S. Trapani, and D. Beljonne, Electronic Transport Properties or Ensembles of Perylene-Substituted Poly-isocyanopeptide Arrays. Advanced Functional Materials, (2008) 18, 3947.
3. V. Palermo, M.B.J. Otten, A. Liscio, E. Schwartz, P.A.J. de Witte, M.A. Castriciano, M.M. Wienk, F. Nolde, G. De Luca, J.J.L.M. Cornelissen, R.A.J. Janssen, K. Mullen, A.E. Rowan, R.J.M. Nolte, and P. Samori, The Relationship between Nanoscale Architecture and Function in Photovoltaic Multichromophoric Arrays as Visualized by Kelvin Probe Force Microscopy. Journal of the American Chemical Society, (2008) 130, 14605.
4. E. Schwartz, V. Palermo, C.E. Finlayson, Y.-S. Huang, M.B.J. Otten, A. Liscio, S. Trapani, I. González-Valls, P. Brocorens, J.J.L.M. Cornelissen, K. Peneva, K. Müllen, F. Spano, A. Yartsev, S. Westenhoff, R.H. Friend, D. Beljonne, R.J.M. Nolte, P. Samorì, and A.E. Rowan, Helter-Skelter-Like Perylene Polyisocyanopeptides. Chemistry a European Journal, (2009) 15, 2536.
SPRAY DEPOSITION OF ORGANIC SEMICONDUCTORS
Micrometer thick uniform layers of a polymeric semiconductor (poly(3-hexylthiophene), P3HT) have been fabricated from solution by spray deposition making use of a commercial airbrush. Multiscale characterization by optical microscopy and atomic force microscopy revealed the formation of smooth layers featuring reproducible patterns of spatially correlated micron-sized holes. This morphology was found to be uniform over the whole sample surface, on the tens of millimeters scale. On this micro-patterned P3HT layer an orthogonal solvent (i.e. a solvent which does not dissolve the P3HT) has been employed to deposit either by spin-coating or by drop casting a second organic semiconductor. While spin-coated films exhibited nano-crystals of an alkylated perylene tetracarboxy diimide (PDI) preferentially grown into the microfabricated holes, drop-cast films displayed crystalline PDI fibers adsorbed on the patterned surface in random positions.
1. E. Treossi, A. Liscio, X.L. Feng, V. Palermo, K. Mullen, and P. Samori, Large-area bi-component processing of organic semiconductors by spray deposition and spin coating with orthogonal solvents. Applied Physics a-Materials Science & Processing, (2009) 95, 15.
Electronic Characterization of Organic Thin Films by Kelvin Probe Force Microscopy
We use Kelvin Probe Force Microscopy (KPFM) to simultaneously study structural and electronic properties of functional surfaces and interfaces. This is of paramount importance since it is well established that a solid surface possesses different properties than the bulk material. The versatility of the technique allows one to carry out investigations in a non-invasive way for different environmental conditions and sample types with resolutions of a few nanometers and some millivolts. KPFM can be used to acquire a wide knowledge of the overall electronic and electrical behavior of a sample surface. Moreover, by KPFM it is possible to study complex electronic phenomena in supramolecular engineered systems and devices. The combination of such a methodology with external stimuli, e.g., light irradiation, opens new doors to the exploration of processes occurring in nature or in artificial complex architectures. Therefore, KPFM is an extremely powerful technique that permits the unraveling of electronic (dynamic) properties of materials, enabling the optimization of the design and performance of new devices based on organic-semiconductor nanoarchitectures.
1. V. Palermo, M.B.J. Otten, A. Liscio, E. Schwartz, P.A.J. de Witte, M.A. Castriciano, M.M. Wienk, F. Nolde, G. De Luca, J.J.L.M. Cornelissen, R.A.J. Janssen, K. Mullen, A.E. Rowan, R.J.M. Nolte, and P. Samori, The Relationship between Nanoscale Architecture and Function in Photovoltaic Multichromophoric Arrays as Visualized by Kelvin Probe Force Microscopy. Journal of the American Chemical Society, (2008) 130, 14605.
2. A. Liscio, V. Palermo, and P. Samori, Probing local surface potential of quasi-one-dimensional systems: A KPFM study of P3HT nanofibers. Advanced Functional Materials, (2008) 18, 907.
3. A. Liscio, V. Palermo, K. Mullen, and P. Samori, Tip-Sample Interactions in Kelvin Probe Force Microscopy: Quantitative Measurement of the Local Surface Potential. Journal of Physical Chemistry C, (2008) 112, 17368.
4. A. Liscio, G. De Luca, F. Nolde, V. Palermo, K. Muellen, and P. Samori, Photovoltaic charge generation visualized at the nanoscale: A proof of principle. Journal of the American Chemical Society, (2008) 130, 780.
5. V. Palermo, G. Ridolfi, A.M. Talarico, L. Favaretto, G. Barbarella, N. Camaioni, and P. Samori, A Kelvin probe force microscopy study of the photogeneration of surface charges in all-thiophene photovoltaic blends. Advanced Functional Materials, (2007) 17, 472.
6. V. Palermo, M. Palma, and P. Samori, Electronic characterization of organic thin films by Kelvin probe force microscopy. Advanced Materials, (2006) 18, 145.