We describe the most efficient semiconductor/liquid junction solar cell reported to date. Under W-halogen (ELH) illumination, the device is a 14% efficient two-electrode solar cell fabricated from ann-type silicon photoanode in contact with a nonaqueous electrolyte solution. The cell's central feature is an ultrathin electrolyte layer which simultaneously reduces losses which result from electrode polarization, electrolyte light absorption, and electrolyte resistance. The thin electrolyte layer also eliminates the need for forced convection of the redox couple and allows for precise control over the amount of water (and other electrolyte impurities) exposed to the semiconductor. After one month of continuous operation under ELH light at 100 mW /cm 2 , which corresponds to the passage of over 70 000 C/cm 2 , thin-layer cells retained over 90% of their efficiency. In addition, when made with Wacker Silso cast polycrystalline Si, cells yield an efficiency of9.8% under simulated AMI illumination. The thin-layer cells employ no external compensation yet surpass their corresponding experimental (three-electrode) predecessors in efficiency.Much attention has been focused on semiconductor/ liquid junction solar cells as an alternative to solid state devices. '2 Liquid junction cells offer potential cost advantages over their solid-state counterparts. For example, the processing required to form a diffused junction in a solid-state device is replaced by the simple immersion of the semiconductor in a liquid. In 1978 Heller et a!. reported the first highly efficient ( 12%) liquid junction solar cell. Another approach to limiting photocorrosion has been to employ nonaqueous solvents, J-s but until recently, efficiencies in these solvents were disappointingly low. Many of the early nonaqueous results were ascribed to the presence of electronic states at the semiconductor/liquid interface.6 However, from the current-voltage characteristics of experimental cells developed utilizing potentiostatic control, it was concluded that (I) in several systems, interface states were not limiting the cell efficiencies, and (2) that high efficiency junctions could be realized by drastically reducing the uncompensated cell resistance. Based on these ideas, studies of passivated systems have demonstrated that nonaqueous solvents can provide media where systematic design of highly efficient, nearly ideal semiconductor/liquid interfaces is possible. -11Although efficient aqueous two-electrode cells have been demonstrated by Heller and others, the only efficient nonaqueous systems to date have been three-electrode cells which utilize external potentiostatic control and do not necessarily represent practical prototypes. Nonaqueous twoelectrode cell design entails many difficulties including the following: limited stability ofboth forms of the redox couple, excessive absorption oflight by the solution, low solubility of the redox pair, insufficient conductivity of the electrolyte, and the presence of trace amounts of corrosive water. • 13The thin...

n-Type Si electrodes in MeOH solvent with 0.2 M (1-hydroxyethyl)ferrocene, 0.5 mM (1-hydroxyethyl)ferricenium, and 1.0 M LiClO4 exhibit air mass 2 conversion efficiencies of 10.1% for optical energy into electricity. We observe open-circuit voltages of 0.53 V and short-circuit quantum efficiencies for electron flow of nearly unity. The fill factor ofthe cell does not decline significantly with increases in light intensity, indicating substantial reduction in efficiency losses in MeOH solvent compared to previous nonaqueous n-Si systems. Matte etch texturing of the Si surface decreases surface reflectivity and increases photocurrent by 50% compared to shiny, polished Si samples. The high values of the open-circuit voltage observed are consistent with the presence of a thin oxide layer, as in a Schottky meta-insulator-semiconductor device, which yields decreased surface recombination and increased values of open-circuit voltage and short-circuit current. The n-Si system was shown to provide sustained photocurrent at air mass 2 levels (20 mA/cm2) for charge through the interface of >2,000 C/cm2. The n-Si/MeOH system represents a liquid junction cell that has exceeded the 10% barrier for conversion of optical energy into electricity. Nonaqueous solvent systems have been shown to be useful in suppressingelectrode corrosion or passivation processes at semiconductor photoanodes (1-4). However, the efficiencies of most systems under solar irradiation conditions generally have been modest (<6%). This has been ascribed to the presence of states at the liquid/solid interface that act as recombination centers and can seriously limit the efficiency of such systems (4-7 Fig. la. The presence of a depletion layer in the semiconductor results in the separation of photogenerated electron-hole pairs, with the electrons being driven into the bulk and the holes migrating to the solid/liquid interface. These holes are then consumed by a solution reductant, resulting in a flow of current. The voltage difference between the energy at the edge of the semiconductor conduction band and the redox potential of the solution determines the barrier height of the device. Fig. lb depicts a possible situation for the interface energetics when recombination sites are present at the surface (9). Here, the fill factor and short-circuit quantum yield may be lower than optimal due to the presence of these states at the interface. Thus, one method of improving the efficiency of a given semiconductor/liquid junction may be to treat chemically the surface in order to eliminate the deleterious recombination sites.Another commonly appreciated difficulty in surface barrier devices such as Schottky cells and solid/liquid junction cells is the large value of dark current arising from thermionic emission of carriers at the interface (10,11). This large dark current leads to values of the open-circuit voltage (VO) at typical solar intensities which differ substantially from the barrier height of these types of devices. For example, for an ideal Si/Au...

Semiconductor/liquid junctions derived from 0.5 ~m thick films of amorphous hydrogenated silicon, a-Si:H, have ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 165.123.34.86 Downloaded on 2015-06-30 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 165.123.34.86 Downloaded on 2015-06-30 to IP Vol. 131, No. 12 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 165.123.34.86 Downloaded on 2015-06-30 to IP

After etching, n-type cast polycrystalline silicon photoanodes immersed in a solution of methanol and a substituted ferrocene reagent exhibit photoelectrode efficiencies of 7.2%±0.7% under simulated AM2 illumination. Scanning laser spot data indicate that the grain boundaries are active; however, the semiconductor/liquid contact does not display the severe shunting effects which are observed at a polycrystalline Si/Pt Schottky barrier. Evidence for an interfacial oxide on the operating polycrystalline Si photoanode is presented. Some losses in short circuit current can be ascribed to bulk semiconductor properties; however, despite these losses, photoanodes fabricated from polycrystalline substrates exhibit efficiencies comparable to those of single crystal material. Two major conclusions of our studies are that improved photoelectrode behavior in the polycrystalline silicon/methanol system will primarily result from changes in bulk electrode properties and from grain boundary passivation, and that Fermi level pinning by surface states does not prevent the design of efficient silicon-based liquid junctions.

Es wird die Halbleiter/Flüssigkeits‐Grenzfläche in folgendem System untersucht: a‐Si:H/0.02 M FeCp2/0.5 mM FeCp2+/1.5 M LiClO4/MeOH (FeCp2: Ferrocen).