SEMICONDUCTOR NON-TRADITIONAL ENERGY SOURCES

MESOSCOPIC DYE-SENCITIZED SOLAR CELLS

     Conventional solar cells require relatively pure absorbers to produce electrical current, whereas nanostructured absorbers can circumvent this limitation by enabling collection of carriers in a direction orthogonal to that of the incident light. Such systems have produced test devices having up to 10% efficiency, but typical devices yield 35% efficiencies over large areas and have longterm stability issues. New absorber combinations, control over the nanostructure of such systems, and a fundamental understanding of the operating principles of such devices are needed to enable a new generation of systems having two- to five-fold improvement in efficiency, low cost, and long-term stability.
     The absorber thickness is dictated by the absorption properties of the semiconductor being used; for example, 100 μm of Si or 13 μm of GaAs are required to absorb fully incident sunlight, so that incident photons are not wasted by virtue of being transmitted through the entire device assembly. In turn, the absorber must be sufficiently pure that the excited states produced by light absorption can survive for the required time and distance to be collected in an external circuit and do not instead recombine to produce heat. The required absorption length therefore dictates the minimum purity and cost needed to achieve the required carrier collection lengths. The use of nanostructured and possibly nanoporous systems, however, offers an opportunity to satisfy these two constraints, by collecting carriers in a direction that is orthogonal (nominally perpendicular) to the one in which light is absorbed, as illustrated in the right figure. In this way, such an approach offers the potential for obtaining high energy conversion efficiency from relatively impure, and therefore relatively inexpensive, photoconverters. One important example of such a structure is provided by mesoscopic dye-sensitized solar cells, which generally involve use of a highly porous film of randomly ordered nanoparticles of a transparent nanocrystalline oxide, such as TiO2, coated with an ultrathin layer of light absorber (e.g., dye molecules or semiconductor quantum dots). When photo-excited, the absorber injects electrons into the oxide nanoparticles and creates a positive charge in the absorber. After electron injection, the positive charge is neutralized by electron transfer to the oxidized dye from a liquid or solid medium that permeates the porous structure; this regenerates the absorber and completes the cycle. Another example of such nanostructured devices is the use of semiconductor nanowires or nanorods to absorb light and transfer the charge carriers over a very short distance to the collecting phase, which can be a conducting polymer, an electrolyte (a liquid charge conductor), or an inorganic conductor. Yet another example is an interpenetrating network of n-type and p-type organic semiconductors that form heterojunction-type solar cells.
     One important example of such a structure is provided by mesoscopic dye-sensitized solar cells, which generally involve use of a highly porous film of randomly ordered nanoparticles of a transparent nanocrystalline oxide, such as TiO2, coated with an ultrathin layer of light absorber (e.g., dye molecules or semiconductor quantum dots). When photo-excited, the absorber injects electrons into the oxide nanoparticles and creates a positive charge in the absorber. After electron injection, the positive charge is neutralized by electron transfer to the oxidized dye from a liquid or solid medium that permeates the porous structure; this regenerates the absorber and completes the cycle. Another example of such nanostructured devices is the use of semiconductor nanowires or nanorods to absorb light and transfer the charge carriers over a very short distance to the collecting phase, which can be a conducting polymer, an electrolyte (a liquid charge conductor), or an inorganic conductor. Yet another example is an interpenetrating network of n-type and p-type organic semiconductors that form heterojunction-type solar cells. An exciting aspect of this approach is that the generic concept of the nanostructured cell can be extended to a range of novel configurations involving different light absorbers and electron-hole conducting phases. A key property of the thin nanostructured film is that since charge carrier pairs are generated only near the interfaces and are separated rapidly into two different phases, bulk recombination and semiconductor instability are avoided. Junction recombination does, however, have to be minimized, and the surfaces of such systems need to be controlled to ensure that they have a relatively low level of defect-driven electrical recombination sites, to allow carriers to actually be collected from such devices. Fabrication of these types of cells can be remarkably simple, and efficiencies over 11% have already been reported for some dyesensitized nanostructured systems. There is considerable potential for increasing this performance to 20% by imaginative approaches that exploit the rapidly growing field of nanoscience. This efficiency objective provides a strong motivation for a program of basic research that aims to understand and control all the factors that determine cell performance in nanostructured systems. Building this knowledge base will provide the platform from which to launch an effort to achieve efficiencies beyond the Shockley-Queisser limit by incorporation of approaches such as multijunction cells and photon up-conversion.
Current mesoporous nanocrystalline films used in dye-sensitized solar cells consist of a random nanoparticle network and a disordered pore structure. Such films are characterized by slow electron transport. Moreover, because of the wide particle distribution and disordered nature of the pores, not all of the internal surface area of a film is accessible to the sensitizer. Also, it is difficult to fill the pores completely with viscous, quasi-solid, or solid ionically or electronic conductors, which serve to transfer photogenerated holes away from the sensitizers following charge separation. Development of ordered nanostructured, inorganic electrodes could lead to more effective incorporation of ionically or electronically conducting materials (ionic gels, polymers, etc.) within the pore structure and potentially to faster charge transport. Also, more uniformly sized particles coupled with periodic order could facilitate films favoring preferred crystal faces for optimizing charge separation. Developing new stable, near-infrared absorbing molecular and quantum confined sensitizers with increased red absorbance would allow for thinner TiO2 layers, which would result in lower charge recombination and higher overall efficiency. Confining photons to a high-refractive-index sensitized nanostructured oxide film is another approach to enhance the red response of the cells. For instance, a two-layer structure consisting of submicron spheres and a nanoparticulate TiO2 layer has been used to enhance light collection owing to multiple scattering. Incorporation of more advanced light management strategies, such as photonic band gaps, also offers promise for enhancing the red response of the cell. Also, relatively unexplored are self-assembling molecular, supermolecular, and inorganic interface layers having, for example, a broad spectral response and/or the electronic capability of directing the resulting energy vectorially as excitons or charges toward the nanostructure ( ltdt figure} One important example of such a structure is provided by mesoscopic dye-sensitized solar cells (Gratzel 2000; Gratzel 2001), which generally involve use of a highly porous film of randomly ordered nanoparticles of a transparent nanocrystalline oxide, such as TiO2, coated with an ultra-thin layer of light absorber (e.g., dye molecules or semiconductor quantum dots). interface for charge separation. For instance, a self-assembling, thin inorganic charge-mediating layer with appropriate electronic levels, covering the nanostructured surface, could allow for vectorial charge transfer from the sensitizer to the conduction band of the semiconducting oxide, while blocking the back electron transfer to the oxidized sensitizer or to the hole-carrying species in the pore structure. Development of ionic or electronic conductors with high charge mobility will be required to transmit the holes rapidly to the collecting electrode. Designing and developing novel materials and fabrication methodologies that are compatible for highthroughput, low-cost fabrication would also be useful.
         Figure on the right shows quadruple junction solar cell comprising sensitized mesoscopic oxides of different color or thin-film photovoltaic cells as light-absorbing layers. The theoretical conversion efficiency of such a device is close to 50%. Multijunction nanostructured injection solar cells can be designed by appropriate choice of the absorber to absorb and quantitatively convert incident photons to electric current in selective spectral regions of the solar emission, while maintaining high transparency in the remaining wavelength range (see right figure). Absorbers (sensitizers in the case of dye-sensitized solar cells; other inorganic or organic compounds in the cases of nanowires, nanorods, nanocylinders, or organic bulk heterojunctions) with appropriate excitation energies and charge injection properties will need to be developed and characterized. Also, there will be a need to determine the fundamental factors influencing the incident photon-to-current conversion efficiency of the sensitized layer. The conditions for forming appropriate multilayered structures by techniques such as screen-printing, to facilitate the fabrication and optimization of multijunction structures, will need to be developed and studied.

 

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