Optoelectronic devices interact with radiation and electric current. Solar cells are a particular example of a useful class of optoelectronic devices. Organic solar cell technology has been actively pursued in the research community due to its promise of lower cost, easier manufacturability, and other potential advantages such as flexible sheets of solar cells and various novel form factors.
Unlike Silicon solar cells, where photon absorption results in the formation of a free electron and hole, photoexcitation in organic semiconductors leads to the formation of a bound electron/hole pair (an “exciton”). In most semiconducting (conjugated) polymers or small molecules, excitons form constantly under sunlight but cannot serve as a source for external electricity since the excitons have a very short lifetime, with the electron spontaneously recombining within an exciton diffusion path length of typically 10 nm. However, this number can vary, depending on the specific organic compound, between 2 nm and several hundred nm.
To serve as a source for electrical energy, the electron and the hole comprising an exciton in an organic material must be separated so that the charges can be collected at different electrodes. To do so in an optimal way, a charge-splitting and transporting network must be structured where the interfaces between electron- and hole-accepting materials are spaced in, e.g., respective 10-nm arrays within the active area of the solar cell device. At such interfaces, the electrons transfer into and move through the electron-accepting material, while the holes travel through the hole-accepting material.
Until recently, there have been only a few attempts to create a more optimal charge-splitting and transporting network in an organic or plastic solar cell.
For example, Halls et al (Nature vol. 376, p 498, 1995) constructed an interpenetrating mixture of organic polymers to increase the surface area between the electron and hole accepting materials. In particular, they mixed a blend of the conjugated polymers (i) soluble MEH-PPV (as a hole-acceptor) and (ii) CN-PPV (as an electron acceptor) in a ˜1:1 ratio to create an active layer in an organic photovoltaic device that showed an external quantum efficiency (EQE) of 6%. This EQE was two orders of magnitude higher than the single layer structures of MEH-PPV (0.04%) and CN-PPV (0.001%). Higher efficiencies were not obtained since the phase separating network was essentially random with isolated “islands”, phases/features that were too large (10-100 nm) and poor connectivity with the respective electrodes.
More recently, Huynh et al. (Science, vol. 295, pp. 2425-2427, 2002) have reported the fabrication of hybrid nanorod-polymer solar cells. These cells have an EQE of 54%, a polychromatic efficiency of 1.7%, and are composed of a random assembly of CdSe nanorods in poly-3(hexylthiophene). The totally random and highly inefficient placement of the nanorods lowered the solar cell efficiency from what would be expected if the charge-separating network was ordered and interconnected on the desired 10-nm scale.
Granstrom et al. (Cavendish Laboratory) have shown that phase separation on a scale of about 50 nm can be obtained through lamination of two semiconductive polymers giving polychromatic efficiency of 1.9% (Nature, vol. 385, pp. 257-260). The interpenetrating network obtained this way is still not on the optimal size scale (about 10 nm) for these polymers. Conjugated polymers are known to be better hole conductors than electron conductors.
Similarly, in recent work at Cambridge University, Schmidt-Mende et al. (Science, vol. 293, pp. 1119-1122, 2002) made spatially mixed thin films of perylene dye with a liquid crystal polymer, and achieved an EQE of 34%, a 1.9% polychromatic cell efficiency; however the efficiency was low owing to the 100-200-nm scale of the interpenetrating dye/polymer mixture used as a crude charge separating network.
In the solar cell devices constructed by these and other groups, the device architectures are sub-optimal compared to that needed for higher-efficiency devices. These prior art devices are limited by the extent to which excitons can be harvested to perform useful work. This is due to two key factors:
First, in cells created with semiconducting nanorods, the nanorods within the solar cells were randomly arranged within a medium of conducting polymer. Since many nanorods were only partially aligned and large clusters of nanorods (interspersed with areas of few rods) were present in the devices, many excitons traveling through the active layers of these devices did not reach an electron affinity junction before spontaneously recombining. As the spacing of the nanorods was random, some areas of the device had many nanorods within 10 nm of one another, while many other areas of the device had no nanorods at all within 10 nm of one another (resulting in “dead” absorption space). This factor decreased the efficiency of both electron and hole transfer at differential electron affinity junctions between nanorods and conducting polymer.
Second, in cells composed of mixtures of perylene dye and liquid crystal polymers, the rough 100-200 nm scale of the interpenetrating dye/polymer interface resulted in low interfacial surface area, and thus the failure of many excitons traveling through such devices to reach an electron affinity junction before spontaneously recombining.
Furthermore, the movement of electrons through the material required regularly and continuously spaced nanorods, which could collect and transport free electrons to the outer boundary of the nanorod layer. This factor decreased the hole and electron collection efficiency. All of these factors combine to reduce the efficiency of the device, and therefore the potential electric power that can be produced by the solar cell.
An alternative approach to building an organic solar cell has been developed by Michael Graetzel and his colleagues, who have constructed dye-sensitized, nanocrystalline TiO2 based solar cells using a liquid electrolyte (Kohle et al., Advanced Materials, vol. 9, p. 904, 1997). In this device structure, referred to herein as the “Graetzel cell”, 20 nm diameter nanospheres of TiO2 are chemically bonded to Ruthenium pigment molecules. Upon absorbing light, the Ruthenium pigment molecules inject an electron into the titanium dioxide, which becomes positively charged as a result. Unfortunately, the Graetzel cell is relatively thick, e.g., several microns in thickness. The electric field in the Graetzel cell is directly proportional to the cell voltage and inversely proportional to the cell thickness. Since the voltage of the cell is essentially fixed and the cell is thick, the electric field of the Graetzel cell is not large enough to dominate charge migration. To overcome this, the TiO2 nanospheres are immersed in an electrolyte. By immersing such a TiO2 “paste” into a liquid redox electrolyte with I−/I2 species, the positive charge of the pigment molecules is neutralized, closing the circuit. The Graetzel cell is known to be able to generally reach 10% polychromatic efficiency. The shortcoming of the Graetzel cell is its lack of long-term stability, with no solution being known to effective seal the cell with the liquid I−/I2 electrolyte. Furthermore, the three-dimensional charge splitting network in a Graetzel cell is essentially random, presenting many curves for the liquid electrolyte to penetrate. Therefore, even if a Graetzel cell uses a solid electrolyte, the pore size distribution, pore spacing and pore filling are less than optimal.
Thus, there is a need in the art for optoelectronic devices, including solar cells that overcome the above disadvantages and a corresponding need for methods and apparatus for producing such devices.