July 2, 2008
The Thin-Film Deposition of Conjugated Molecules for Organic Electronics
By Jin, Michael H-C
Device-quality conjugated organic thin films are now routinely prepared in many different ways to fabricate light-emitting diodes, thin-film transistors, and photovoltaic devices. Understanding how to design molecules through versatile synthetic chemistry and the mechanisms of phase transformation and chemical reaction that occur during the thin-film deposition process becomes especially vital for the performance of the applications. This article reviews the current understanding of various thin-film deposition technologies for the conjugated organic molecules primarily used in optoelectronics, particularly in photovoltaic applications. INTRODUCTION
The practical importance of the conducting organic molecules amplified by the discovery of highly conductive polymers and the remarkable advancement in current organic electronics are more than enough to discuss in a few pages of a review article. While many readers enjoy colorful pictures of flat panel displays fabricated with organic light-emitting diodes (OLED8) or generating electricity from flexible polymer solar cells to charge their batteries, the author is amazed by the endless effort made by scientists and engineers to make all these possible. Electrical conductivity in a given material can be understood by two physical attributes- delocalized energy states available to free carriers to transport through and the availability of the free carriers themselves. Therefore, high electrical conductivity requires both good charge carrier mobility and a good number of free carriers. This also explains that electrical conductivity as high as metal is made possible by externally creating free carriers in the material through a process called doping, although the charge carrier mobility of the organic material-as an aggregate of many numbers of organic molecules-is fundamentally limited by the inevitable hopping action between molecules.
The nature of the delocalized energy states becomes more important for OLEDs and organic solar cells because light emission and electricity generation respectively from those applications are the direct consequence of extra carriers that are injected (or generated) into the device by external electrical bias and photon absorption, respectively. For example, both electron and hole are injected separately through respective electrodes of an OLED and they need to travel through the device and combine together to emit light. Assuming hopping between molecules does not limit this process, delocalized energy states within individual molecules are prerequisite to this process. In the same manner, organic solar cells also rely on these delocalized energy states available for the transport of the free carriers generated by photon absorption for the ultimate electricity generation.
Basic physical chemistry predicts that only atoms (within a molecule) with pi lectrons participate in the formation of this delocalized molecular orbital-a benzene and polyacetylene molecule, for example. Conventionally, unsaturated bonds in the molecular structure indicate the existence of the orbital. These are referred to as conjugated molecules. The order and proximity among individual atoms and molecules determine the overlap between the orbitals and determine the entire conjugation of the material in a very complex manner. This eventually explains the importance of ordered structure in the material to achieve a good conjugation for charge transport. It should be noted that most optoelectronic phenomena from a given conjugated system are often represented and explained by the conjugation scheme and the electron distribution of one molecule because the interaction between molecules is often negligible, particularly for small organic molecules. In contrast, the electronic states of the polymer material are more sensitive to the order and the interaction among molecules-a different optical bandgap was measured from one polymer to the other depending on the level of interaction among molecules in the material.1 Figure 1 shows the schematic energy diagram for a conjugated molecule.2 The lowest energy electronic transition takes place between the highly occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). This pi-pi* transition energy is often referred to as a bandgap of the conjugated molecule.
For multilayer devices like OLEDs and organic solar cells, the energetics between neighboring layers is another critical element that governs the flow of the charge carriers and the recombination between electrons and holes. The careful design of the multilayer structure together with the fabrication of the individual layer that has good electronic properties maximizes the performance of the devices. The conjugated molecules are typically categorized into two kinds based on molecular weight: small molecules and polymers. This classification is based on the difference in their vapor pressure because it generally determines the method of thin-film deposition- small molecules allow their thermal evaporation, but the big polymer molecules cannot be easily evaporated and utilize a solution process instead to make thin films. Today, most commercial OLEDs and organic thin-film transistors (OTFTs) are manufactured by vacuum thermal evaporation.
There are numerous books and articles that cover a broad spectrum of organic electronics.^6 This review will primarily focus on the various conjugated polymers and their thin-film deposition methods studied in the field of OTFTs, OLEDs, and organic solar cells.
Various conjugated polymers are available and one unique aspect of pristine conjugated polymers-conjugated polymers without any functional group extrinsically added to the primary backbone of the molecule-is that they are generally insoluble and infusible due to their rigid backbone structure. They could be turned into soluble polymers by adding functional groups into their molecular structure, changing the electron distribution in the molecule. In addition, the overall change in the electronic states of the molecule could either increase or decrease the bandgap of the materials.1
The most common approach to making conjugated polymer thin film is to first synthesize the soluble version of the pristine conjugated polymers, followed by the dissolution of the polymer in a solvent for any solution thin-film deposition process such as spin- coating, direct casting, and various printing techniques. All these solution methods are very simple to perform and the films can be deposited on top of many different types of substrates, possibly with complicated shapes. Because the minimum thermal budget is only determined by the evaporation of the solvent to form the polymer thin film, the film is often deposited below 150[degrees]C allowing a variety of material selections for the substrate.
Interest in the deposition of pristine conjugated polymers is rising due to their superior thermal stability and potential tolerance to photo-degradation. Due to their insolubility, the typical solution process cannot be used. Instead, the thin polymer films were synthesized and also directly deposited on top of the substrate at the same lime through free radical polymerization. The monomer molecules used need to be excited to active radical intermediates. This can be achieved electrochemically, thermally, optically, or by using plasma. The authors will review the conjugated polymer thin films deposited by a thermal chemical vapor deposition polymerization method. Figure 2 displays examples of common conjugated polymers used in organic electronics.7
Solution Process of Soluble Conjugated Polymers
Poly(3-hexylthiophene) (P3HT) is one of the most common conjugated polymers typically deposited by a solution process for both OTFTs and organic solar cells. Although vacuumdeposited pentacene-based OTFTs exhibit a field effect mobility two orders higher than that of solution-processed P3HT-based OTFTs,8 the solution process always provides a more economical route to fabricate thin-film devices. P3HT shows the best mobility among other solution-deposited organic thin films and this can be explained by its unique microstructure and its influence on charge transport.7 Figure 3 shows the crystalline structure of P3HT and the configuration of the chain packing.7 While charge transport in the alkyl stacking direction is poor, the molecules are stacked cofacially causing a partial overlap between pi orbitals permitting good charge transport in the pi-pi stacking direction. This anisotropy predicted different carrier mobility of OTFT devices when there are different chain packing configurations.7 The packing at the interface between polymer thin film and the substrate often determines the overall packing in the thin film and it can be engineered by preparing a self-assembled mono-layer prior to the polymer deposition.9
P3HT is also often used in organic solar cells as a form of bulk heterojunction (Figure 4) by depositing a layer that has a random mixture between P3HT and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) molecules using a solution process.10 The absorption of a photon creates a Frenkel exciton and the interface between two molecules allows the exciton to dissociate into free carriers; the electron transfers to PCBM due to its lower LUMO level. The nano- scale random mixture of the molecules assures the exciton dissociation before lheir recombination. The dissociated charges also need to travel through the film to reach the respective charge- extracting electrodes without recombining with the opposite charge. In order to maximize the device performance, well-defined bicontinuous interpenetrating phase nanostructures in the film must be achieved. The electron microscopic study (Figure 5) showed post- production annealing at 150[degrees]C for more than 30 min. was necessary to develop the desirable nanomorphology." The annealing also increased the crystallinity of P3HT confirmed by x-ray diffraction. H. Hoppe and N.S. Sariciftci made very systematic reviews on the deposition of the bulk heterojunction layer elaboration on the effect of the solvent, blending ratio between polymer and PCBM, solution concentration, annealing, and chemical structure on the nanomorphology.12 Many different combinations of conjugated polymer and PCBM, two different polymers, or polymer and inorganic nanoparticles have been tried to fabricate the bulk heterojunction organic solar cells mostly using spin coating in the laboratory. However, none have surpassed the performance of the P3HT/ PCBM combination yet. A P3HT/PCBM bulk heteroj unction solar cell has achieved the power conversion efficiency of about 5% from a single junction cell,13 and more than 6% from a multijunction bulk heteroj unction structure.14 Another frequent attempt to improve the efficiency is to fabricate the ordered bulk heteroj unction solar cell in which the conjugated polymer is infiltrated into the ordered nanostructure of inorganic electron accepting materials.15 Even though the performance of the ordered bulk heteroj unction has not been great yet, the structure provides a good electron transport and better charge separation, and minimizes the electron-hole recombination. It should be noted that when the regioregular P3HT infiltrated into the straight, vertical pores of the alumina membrane, the hole mobility improved by a factor of 20. This was possibly due to the alignment of polymer chains along the wall of the inorganic material.16
Conjugated polymer is not only found in the hydrocarbon molecules, but also in biomolecules. In the past decade, there has been much effort to utilize biological molecules for photonic/ electronic applications. Deoxyribose nucleic acid (DNA) as a conducting wire, in particular?, has been one of the most interesting topics in the field.17 The conjugation along the z axis of the DNA molecule due to the overlap between electron orbital? of the base molecules inside DNA creates the delocalized orbital for electrical conduction although the exact electrical conduction mechanism is not clear yet. Among others, there have been a few studies that reported the use of DNA-based thin film as an integral component in OLEDs.18,19 In those studies, complex between DNA and hexadecyl trimethyl ammonium (CTMA) chloride was dissolved in an organic solvent and the solution was spin-coated on a substrate creating DNA-CTMA thin films. K. Hirata et al. reported that the DNA complex layer possesses both hole and electron transport abilities and preferentially transports holes due to a shallow LUMO level that prohibits an efficient electron injection from an adjacent carrier transport layer.18 More recently, J.A. Hagen et al. demonstrated that the presence of a DNA complex layer increased the brightness of OLEDs 30 times and the efficiency 10 times more than the OLEDs without the DNA complex layer (Figure 6).'9 The author has also demonstrated its potential usage as a hole transport layer and an electron blocking layer in P3HT/PCBM bulk heterojunction solar cells due to its high band gap, proven hole conduction, and shallow LUMO level.20
Thin-Film Deposition of Insoluble Conjugated Polymers
Although most polymer-based electronic devices are fabricated using soluble polymers, pristine poly(p-phenylene vinylene) (PPV) is one of the insoluble conjugated polymers often studied in the field for its high solvent resistance, excellent mechanical properties, and thermal stability. In particular, its deposition using thermal chemical vapor deposition polymerization (CVDP) has been demonstrated by a few studies21-23 including the recent report that showed the synthesis of the various shapes of PPV-made nano- features including tubes, rods, and fibers in addition to plain thin films without the use of solvents and catalysts often found as contaminants in the final polymer products.23 The synthetic chemistry of CVDP-deposited PPV was studied in detail previously.21,22 As shown in Figure 7, the thermal activation of the monomer, alpha,alpha'-dichloro-p-xylene creates quinohalodimethane. The polymerization reaction of those intermediates creates the precursor polymer thin film, and the subsequent halogen removal from the precursor thin film completes the formation of the pristine PPV film. Although earlier study demonstrated electroluminescence and photoluminescence from the PPV, the halogen removal process at elevated temperature (300[degrees]C) caused the damage to the underlying indium tin oxide layer and also defect formation in PPV films.21 In contrast, another recent study has confirmed the pristine PPV is stable over 400[degrees]C without reaching decomposition temperature.34 Further study is necessary to determine the true potential of the pristine PPV deposited by CVDP.
Another example of CVDP-deposited conjugated polymer was studied targeting photovoltaic applications by the Photovoltaic Materials Laboratory at the University of Texas at Arlington led by the author. Figures 8 and 9 show the schematic of CVDP used and synthetic chemistry involved to deposit insoluble poly(isothianaphthene-3,6-diyl) (PITN(3,6)) thin-film.23 The cyclic aromatization of the diethynyl thiophene monomer was expected from the principle of the well-known Bergman cyclization mechanism.26 The chemistry of Bergman cyclization is basically utilized in the gas phase reaction instead of the conventional wet chemistry. A lower bandgap than that of PPV (~2.1 eV) was expected from an idea that the quinoid state of the PITN(3,6) could be stabilized by the thiophene right fused into the phenyl ring where the backbone of the conjugation goes through.1 In fact, the bandgap of PITN(3.6) was about 1.78 eV being lower than that of the pristine PPV. The study also demonstrated that the conformal coating of any feature size is possible with CVDP. This would allow the deposition of the conjugated polymer into the pores of the ordered nanostructure of electron-accepting material in order to fabricate an ordered bulk heterojunction organic solar cell.15
Although it is not common, CVDP can be also used to synthesize a soluble conjugated polymer. A Massachusetts Institute of Technology group has recently demonstrated the deposition of doped poly(3,4- ethylenedioxythiophene) (PEDOT) thin films by introducing 3,4- ethylenedioxythiophene molecules as monomers, and Fe(III)Cl^sub 3^ as an oxidizing agent for doping.27 As the substrate temperature for the deposition increased, the doping level was increased from 17 at.% to 33 at.%. The work function was also controllable between 5.1 eV and 5.4 eV.
SMALL CONJUGATED MOLECULES
The conventional vacuum thermal evaporation is limited by its scalability, complicated control, and high cost. Organic vapor phase deposition (OPVD) introduced by Forrest separated the evaporation process of source material and its condensation on the surface of substrate in order to improve controllability.211 Figure 10 shows the example of OPVD, and it resembles conventional CVD except that the film formation results simply from the condensation of the source materials. Carrier gas delivers source materials to the condensation zone without a need of continuous pumping to get high vacuum.
The asymmetric tandem organic photovoltaic cells with hybrid planarmixed molecular heterojunctions were fabricated by OVPD and its power conversion efficiency was close to 6%.29 Figure 11 shows both the top and bottom cells have the copper phthalocyanine (CuPc)/ C^sub 60^ heterojunction layer sandwiched between the donor (CuPc) and acceptor layers (C^sub 60^). Thin layers of both 3,4,9,10-pery lenetetracarboxylic-bisbenzimidazole (PTCBI) and bathocuproine (BCP) are used as the exciton-blocking layer. Previously, Forrest also demonstrated 4% efficient organic solar cell without the donor- acceptor mixed layer.10
A family of organic molecules shows the conjugation among pi orbitals in their molecular structure forming delocalized energy states available for electrical conduction throughout the molecule and further in the material. Various soluble conjugated polymer thin films are routinely deposited to fabricate various optoelectronic applications including OLEDs, OTFTs, and solar cells. The chain packing, crystallinity, and nanomorphology of donor and acceptor mixture determine the electronic states of the molecules and the device performance heavily relies on them. Insoluble pristine conjugated polymer thin films can be deposited using chemical vapor deposition polymerization and they exhibit strong chemical resistance and thermal stability. Organic vapor phase deposition of small organic molecules is entering into the commercial sector for the manufacturing of OLED.
The author thanks Dr. Jung-II Jin of Korea University for discussions on conjugated polymers, James Grote of Air Force Research Laboratory for valuable comments on DNA studies, and Dr. Aloysius Hepp and Mr. Jeremiah McNatt of NASA Glenn Research Center for equipment support for the polymer thin-film deposition. The author also wishes to acknowledge the financial support from the University of Texas at Arlington for the CVDP study. How would you...
...describe the overall significance of this paper?
This paper provides a concise review on various thin-film deposition techniques to fabricate organic electronic devices. The performance of the device depends on the structural and electrical properties of the thin films. This paper explains which thin-film deposition method is preferred for a given type of device.
...describe this work to a materials science and engineering professional with no experience in your technical specialty?
This work provides an entry-level introduction of conjugated thin- film deposition and devices. It is interesting to learn how the conjugated molecules are deposited as thin films and what the technical challenges are.
...describe this work to a layperson?
This work introduces some of the popular conducting organic materials used in electronic devices and how those materials can be synthesized as thin films for the devices.
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Michael H.-C. Jin is an assistant professor in the Department of Materials Science and Engineering, the University of Texas at Arlington, Arlington, TX 76019, and can be reached at [email protected]
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