High-quality semiconductor heterostructures, which are junctions of two dissimilar materials in contact, are crucial to the proper operation of many organic semiconductor devices, including light-emitting diodes and photovoltaics. The materials in contact may differ in one or more of the following opto-electronic properties: the hole and electron transport levels, refractive indices, excited state energy levels.
By selecting the appropriate materials which are in contact, one can form e.g., a charge-confinement interface which transmits carriers of one sign but blocks carriers of the opposite sign. This may be achieved by adopting at least one of the following: the appropriate energy offsets in the hole and electron transport levels; a charge-injecting interface for efficient injection of charge of one sign by having a graded energy level; an exciton-confinement interface to prevent the exciton from wandering into the neighbouring layer by imposing a higher energy there; a photon-confinement interface that prevents photons from travelling away into the neighbouring layer by imposing the condition for total internal reflection at the interface; a charge-carrier-generation interface to create electrons and holes upon absorption of a photon by having the suitable offsets in both the electron and hole energy levels for exciton dissociation; a cascaded set of energy levels for these electrons and holes for even more efficient charge-carrier generation; or a charge-carrier-recombination interface to create excitons by the capture of electrons and holes electrically injected into the device by having the suitable offsets in both the electron and hole energy levels for exciton generation.
An organic light emitting diode (OLED) consists of a cathode, an emissive layer, and an anode in a sandwich structure. The anode usually consists of a transparent indium tin oxide (ITO) substrate coated with a layer of conducting polymer. The emissive layer consists of electron transporting, hole transporting and emissive materials. These materials may be molecules, oligomers or polymers or segments of polymers, or nanocrystals, or nanowires and nanosheets, with a π-π* gap of 1-4 electronvolts (eV). The cathode usually consists of a low work function material such as calcium, or a combination of an insulator such as lithium fluoride and a metal such as aluminium.
When a negative bias is applied to the cathode and is larger than the built-in potential, electrons are injected into the electron transporting material, while holes are injected into the hole transporting material. The electron transporting and hole transporting materials may be the same or different. The electrons are transported into the lowest unoccupied molecular orbital (LUMO) of the emissive material, and holes are transported into the highest occupied molecular orbital (HOMO) of the emissive material. The recombination of these electrons and holes gives a photon with energy corresponding to the LUMO-HOMO gap.
An organic photovoltaic (OPV) device consists of an electron collector (cathode), an absorption and charge-generating layer and a hole collector (anode). The electron collector usually consists of a low work function material such as calcium or aluminium. The absorption and charge-generation layer consists of absorbing materials that absorb the light to give an exciton state, charge dissociation materials that dissociate this exciton state to give the electron and hole, and charge transporting materials that transport the electrons and holes away from the dissociation sites. These materials may be the same or different. They may be molecules, oligomers or polymers or segments of polymers, or nanocrystals or nanowires or nanosheets, with a π-π* gap of 1-4 electronvolts (eV).
Upon absorption of a photon with energy similar to the absorption gap of the organic material, an exciton is formed. Excitons are coulombically-bound electron-hole pairs. Excitons need to be separated into electrons and holes, and collected at the cathode and anode respectively. The efficiency of separation of these photo-excited excitons into free electrons and holes is therefore critical to the efficiency of OPVs. This separation is achieved at the interface between two materials with appropriate energy offsets in both the LUMO and HOMO levels and is crucial for the efficient dissociation of the excitons to provide efficient OPVs. One of these materials is a hole transporting material, and the other is an electron transporting material. Holes and electrons are therefore respectively transported through these materials to the respective electrodes to be collected.
For a number of applications, particularly for charge-carrier-recombination in light-emitting devices, and for charge-carrier-generation in photovoltaics, it is desirable to have a distributed heterostructure, as opposed to a planar heterostructure. A planar heterostructure is flat. A distributed heterostructure creates a large interfacial area between the two materials in contact, by having “fingers” of one material in contact with the other material or one material embedded in the other, so that the two or more materials are in intimate contact. Having a large heterostructure interfacial area can improve e.g., charge-carrier generation efficiency in photovoltaic devices.
It is further desirable for the two charge-conducting materials that continuous paths exist between all locations in the hole transporting material to its proper contact (positively-biased contact for injection of holes in light-emitting diodes and negatively-biased contact for collection of holes in photovoltaics), and likewise for the electron transporting material. In addition, such paths should preferentially lie along the most direct route; otherwise the transport of these carriers would be obstructed, and the resistance of the device increases undesirably.
Therefore, for these applications, the heterostructure not only has to have a large surface area but its morphology should ideally be columnar (i.e., the shapes, such as either voids or columns, pass through the thickness of the film), which may be referred to herein as the “columnar distributed heterostructure”. Columnar distributed heterostructures are widely expected to be beneficial, but so far their fabrication has proved problematic. Controlling the morphology of such structures during formation is not straightforward.
In particular, for organic photovoltaic devices, the required lateral dimensions of the heterostructure in the directions along the plane of the film (i.e., the lateral length scale of the distributed heterostructure) is related to the exciton diffusion length scale. Absorption of light creates an exciton that diffuses about in the material. It is essential that this exciton can reach the charge-carrier-generating interface within its lifetime, in order to produce electrons and holes that can be separated. For typical exciton diffusion length scales of a few to tens of nanometers, it is useful to have the lateral length scale of similar dimensions, so that the majority of the excitons generated can reach the dissociation interface. With the charge-carriers so created, it is then essential for them to be transported to the respective collection electrodes. Up till now, it has proven difficult to fabricate such heterostructures of such fineness in a suitable morphology. It is one of the objectives of this invention to provide ways to achieve this without resorting to use of an electron beam or other lithographic methods.
It has been shown by Friend and co-workers (Nano. Lett. 2002, 2, 1353-1357), and others, that when two semiconducting polymers are co-dissolved in a common solvent, and the mixture deposited to give a film by spin-coating or ink-jet printing, the film formed will be naturally phase-separated. The character and length scale of the phase separation depends amongst other factors on the solvent and its evaporation rate (influenced e.g., by heating or the presence of a high vapor pressure of the solvent). These are useful parameters in providing a degree of control over the phase separation length scale and morphology, and hence of the distributed heterostructure, which has been shown to be useful for photovoltaic applications.
The typical phase separation length scale is of the order of a few micrometers to tens of micrometers. Although the solvent and its evaporation rate can influence this somewhat, it is ultimately strongly related to the character of the polymers. Very fine heterostructures below 1 micron in lateral length scale cannot readily be expected, particularly if the polymers are sufficiently incompatible that they phase separate out early in the solution drying process. In this manner, a hierarchy of phase separation occurs in both the lateral and vertical dimensions, possibly forming isolated phases within the polymer thin film, which often leads to a so-called “Russian doll” morphology in which material A for example is completely occluded in material B.
As a result, the connectivity is broken; holes, for example, cannot flow out of the occluded phases of the hole-conducting material. Furthermore, it is possible for certain polymers to develop “wetting” layers so that they may form a partial or complete overlayer or underlayer at the interfaces of the polymer thin film but not necessarily in the most desirable way. In the context of photovoltaic devices, this will mean that a significant fraction of the charges generated within these phases cannot successfully reach the electrodes, thereby limiting the efficiency of the photovoltaic devices.
It has also been shown by Steiner and co-workers (Nature, 1998, 391, 877-879) that the phase separation of polymer blends can be spatially influenced and directed by patterns pre-formed on the substrate. Such patterns include patterns in the surface energy, created e.g. by chemical reaction through a pattern-generating method such as contact printing, or by photolithography. It has further been shown that polymer blends have preferred phase-separation length scales of the order of a few to tens of microns, and application of the phase-separation pattern to the underlying chemical pattern is not possible if the length scales are very different. These methods require the formation of a chemical pre-pattern on the substrate, and then phase separation of the desired polymer combination over the pattern.
Bulk distributed heterostructures are known in the art and are used in polymer-based photovoltaic devices. Bulk distributed heterostructures can be made by the natural phase separation of the desired final polymers. In such heterostructures, the phases are not continuous from one electrode to the other. Phase occlusions and polymer wetting layers invariably occur during such phase separation and lead to trapping of the charge carriers, and hence less efficient light-emitting diodes and photovoltaic diodes. Furthermore, the control of length scales of the phase separation within a bulk distributed heterostructure is limited (sub-micrometer length scales are difficult to obtain) and is dependent on the phase separation of the polymers.
These two approaches to generate micron-scale heterostructures in polymer thin films do not provide enough control to form polymer heterostructures with the desired composition profile at will in both the lateral and vertical directions. These approaches tend to lead to the formation of occluded phases and wetting layers that are not always desirable.
A desirable morphology that cannot be readily formed, for example, is a columnar nanostructure in which the desired charge-dissociation materials, and electron transporting and hole transporting materials are arranged appropriately in close proximity laterally, which is immensely beneficial for OPVs. Similarly, a columnar nanostructure in which the desired emissive material, and electron transporting and hole transporting materials are arranged appropriately in close proximity laterally can be useful for OLEDs. Developing a method that can control the variation (or modulation) of the lateral composition profile of the materials is hence desirable.
Another desired morphology is the graded composition profile in the vertical direction. A graded composition profile is one in which the composition of the materials varies systematically in the film thickness direction, from being rich in material A at the bottom face, to being poor in material A at the top face. The top face could further be bound by a film of pure material B. In other words, the heterostructure interface becomes diffuse with respect to materials A and B. This morphology could be useful, for example, for OPVs in which the electron transport material is graded into the hole transport material. Such a vertical variation in composition may be considered as another form of modulated composition. Therefore, it is an objective of this invention to provide heterostructures with such modulated composition profiles, a method of making them, and devices incorporating them.
It is also an objective of this invention to create organic polymer semiconductor heterostructures on the sub-micron scale suitable for use in semiconductor devices, e.g. photovoltaic devices and light-emitting diodes, which are at least as, and preferably more efficient than, existing polymer heterostructures and which preferably exhibit improved performance.
Further objectives of this invention include: (1) the creation of organic solvent-processable heterostructures with one or more diffuse or abrupt interface; (2) the creation of abrupt nanostructures of controlled vertical and horizontal dimensions; (3) the creation of multi-layer polymer heterostructures with high aspect ratios; (4) the creation of tall polymer nanostructures that are self-organized with respect to the underlying nanotemplates; and (5) new methods for making the structures.