Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session L2: The Physics of Next Generation Photovoltaics |
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Sponsoring Units: DCMP DMP Chair: Arthur Nozik, National Renewable Energy Laboratory Room: Morial Convention Center LaLouisiane C |
Tuesday, March 11, 2008 2:30PM - 3:06PM |
L2.00001: Nano-Structured Silicon Thin Films for Photovoltaic Applications Invited Speaker: The current technology for thin-film silicon photovoltaic panels is based on hydrogenated amorphous silicon and related alloys, such as silicon-germanium and silicon-carbon. Currently there is great interest in using some form of thin-film silicon that includes nano-structured components. This interest is driven in part by the potential for decreased cost, increased efficiency, and increased stability. Also driving this interest is the abundance of silicon as an element and its lack of toxicity. I will review various structures that have been suggested, and discuss recent results on inhomogeneous films of hydrogenated amorphous silicon that contain nanocrystalline inclusions. In particular, I will describe the mechanisms for optical absorption, carrier transport and the role of defects. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:42PM |
L2.00002: Novel ultra high efficiency concepts in solar cells Invited Speaker: The limit efficiency of conventional solar cells is about 45{\%} as obtained in 1960 by Shokley and Queisser. Besides the multijuntion solar cells, other novel concepts have been proposed in the last years with efficiency limit in the range of 85{\%}. They will be reviewed now. The intermediate band (IB) solar cell attempts to increase the photocurrent of a solar cell without reducing the voltage. For it an electronic band is fabricated in the mid of the bandgap, the IB, so that electron hole pairs are created by the absorption of two sub-band-gap photons using the IB as a relay. Furthermore, the preservation of the voltage requires that a new quasi Fermi level appears in the IB, different to those in the valence band (VB) and in the conduction band (CB). So far IB materials have been produced, either using the confined states of quantum dots or by alloys. The basic principles, both of electron hole creation by double photon absorption and the appearance of a third quasi Fermi level have been experimentally proven. The principle of the multiple generation solar cells is based on the creation of several electron-hole pairs by photons whose energy is well above the bandgap. So far up to seven electro-hole pairs have been experimentally proven from a single photon in structures with PbSe quantum dots. Solar cells based on the two preceding concepts have been fabricated although with performances still low. In the hot carrier solar cells what is intended is to recover the energy of gas of electrons excited by the flux of photons before they thermalise with the lattice. Basic requirements of this device are being understood. They require a medium in which electrons and phonons are very decoupled and narrow contacts for extraction the hot electrons. The role of the quantum dots may be important. Experimental research in this field is about to start. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 4:18PM |
L2.00003: Using Multijunction Solar Cell Designs to Achieve High Efficiency Invited Speaker: Achieving high-efficiency requires minimizing the absorption and carrier-thermalization losses in solar cells. Multijunction solar cells do this by using multiple materials and matching their band gaps with the corresponding portions of the solar spectrum. The p-n junctions formed from each material must be near-perfect so as to avoid non-radiative recombination. The efficiency can be further increased by concentrating the incident light, which increases the generation rate of electron-hole pairs per semiconductor volume. Mirrors or lenses can concentrate the light onto a small area or light trapping can be used to concentrate the light into a thinner layer. The talk will describe the physics of how the different aspects of the design of multijunction cells contribute to achieving high efficiency. This abstract is subject to government rights. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:54PM |
L2.00004: Dye-Sensitized Approaches to Photovoltaics Invited Speaker: Sensitization of wide band-gap semiconductors to photons of energy less than the band-gap is a key step in two technically important processes - panchromatic photography and photoelectrochemical solar cells. In both cases the photosensitive species is not the semiconductor - silver halide or metal oxide - but rather an electrochemically active dye. The gap between the highest occupied molecular level (HOMO) and the lowest unoccupied molecular level (LUMO) is less than the band-gap of the semiconductor with which it is associated. It can therefore absorb light of a wavelength longer than that to which the semiconductor itself is sensitive. The electrochemical process is initiated when the dye molecule relaxes from its photoexcited level by electron injection into the semiconductor, which therefore acts as a photoanode. If the dye is in contact with a redox electrolyte, the negative charge represented by the lost electron can be recovered from the reduced state of the redox system, which in return is regenerated by charge transfer from a cathode. An external load completes the electrical circuit. The system therefore represents a conversion of the energy of absorbed photons into an electrical current by a regenerative device in every functional respect analogous to a solid-state photovoltaic cell. As in any engineering system, choice of materials, their optimization and their synergy are essential to efficient operation. While a semiconductor-electrolyte contact is analogous to a Schottky contact, in that a barrier is established between two materials of different conduction mechanism, with the possibility of optical absorption, charge carrier pair generation and separation, it should be remembered that the photogenerated valence band hole in the semiconductor represents a powerful oxidizing agent. Given that the band-gap is related to the strength and therefore the stability of chemical bonding within the semiconductor, for narrow-gap materials the most likely reaction of such a hole is the photocorrosion of the semiconductor itself. However, only relatively narrow band-gap materials have an effective optical absorption through the visible spectrum, towards and into the infra-red. Materials with an optimal band-gap match to the solar spectrum, of the order of 1.5eV, are therefore electrochemically unstable. A stable photoelectrochemical cell, without some process of optical sensitization, and necessarily using a wide-gap semiconductor is sensitive only to the ultra-violet limit of the visible spectrum. Over recent years a suitable combination of semiconductor and sensitizer has been identified and optimized, so that now a solar spectrum conversion efficiency of over 11{\%} has been verified in a sensitized photoelectrochemical device. One key to such an efficient system is the suppression of recombination losses. When the excited dye relaxes by electron loss, the separated charge carriers find themselves on opposite sides of a phase barrier -- the electron within the solid-state semiconductor, the positive charge externally, in association with the dye molecule. There is no valence---band involvement in the process, so the system represents a majority-carrier device, avoiding one of the major loss mechanisms in conventional photovoltaics. In consequence also a highly-disordered, even porous, semiconductor structure is acceptable, enabling surface adsorption of a sufficient concentration of the dye to permit total optical absorption of incident light of photon energy greater than the HOMO-LUMO gap of the dye molecule. The accepted wide-band semiconductor for photoelectrochemical applications is titanium dioxide in the anatase crystal structure. The size of the nanocrystals making up the semiconductor photoanode can be determined by hydrothermal processing of a precursor sol, and the film can be deposited on a transparent conducting oxide (TCO) substrate by any convenient thin-film process such as screen printing or tape casting. The preferred dye system is inspired by the natural processes involving chlorophyll, the coloring material in plants on which all earthly life depends. Chlorophyll is an organometallic dye, with a metal ion, Mg, within a porphyrin cage of nitrogen atoms. The synthetic chemist of course can select any convenient metal within the periodic table, and experience shows that ruthenium has the optimal properties expected. A ruthenium-pyridyl complex provides the chromophore of the dye, with the HOMO-LUMO gap, and thence the absorption spectrum bring modified by substitution with thiocyanide groups. Chemisorptive attachment of the dye to the metal oxide surface is obtained by carboxyl groups attached to the pyridyl components. The energetics of the dye is such that the LUMO level is just above the conduction band edge of the semiconductor, enabling relaxation by electron injection as required. A satisfactory electroactive dye structure, with good attachment properties and a wide optical absorption spectrum is therefore a sophisticated molecular engineering product. The electrolyte is also an optimized electrochemical system. The basic redox behavior is provided by the iodine/iodide system, with the advantage that the ions, both oxidized and reduced are relatively small, and therefore mobile in the supporting electrolyte. Energy losses due to slow diffusion are minimized. Early experiments used aqueous electrolytes, though with limited cell lifetime due to hydrolysis of the chemisorptive dye---semiconductor bond. A wide range of organic systems were therefore investigated, with the present favored formulation being based on imidazole salts. These have the additional advantage of low vapor pressure, very necessary as the photoactive sites under mid---day sun illumination may reach 80\r{ }C or higher. Low losses at the cathode counterelectrode are also a requirement for cell efficiency. The cathode is not necessarily transparent, and prototype cells on thin metal foils have been produced. However a TCO on glass or polymer counterelectrode is widely used. In either case suitable electrocatalytic behavior is required and frequently a nanodispersed Pt precipitated from haxachloride solution is employed. It is by now evident that the achievement of an industrially-competitive sensitized photoelectrochemical solar cell is the result of the optimization of several components, associated obviously with their effective synergy. Each change of a single component has repercussions on the choice and performance of others. However as already mentioned an efficiency of over 11{\%} has now been certified, and a stability of over 14,000 hours under accelerated testing with continuous simulated AM1.5 illumination was recently reported. In consequence there is increasing confidence on the part of industry. Several licensees of EPFL patents on dye---sensitized photovoltaic systems are now preparing for large-scale production. G24 Innovations PLC in Wales is commissioning a manufacturing plant, and Dyesol PLC in Australia is making available the required materials on an industrial scale. In conclusion, then, it can be stated that the DSC system is much more than a fascinating scientific artifact illustrating charge-transfer mechanisms at electrochemical interfaces; an efficiency and reliability with industrial credibility have been demonstrated and verified, and a significant role in competition with other photosystems can be foreseen. [Preview Abstract] |
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