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Visible electroluminescence (EL) from cathodically biased n‐Si in an electrolyte solution containing has been studied. EL from the porous n‐Si samples fabricated by photoelectrochemical etching over a wide range of current densities shows both blueshift and peak broadening with increasing , which can be attributed to the progressive participation of more silicon hydride species, , with larger n in the emitting progress. Based on analysis of spectral changes and resolution of the EL peak into components, a revised mechanism of the EL, where silicon hydrides are considered to donate electrons to sulfate radical anions, is proposed. EL is produced by recombination of the resulting radical, , with an accumulated electron in the conduction band. The radicals would lead to competing reactions that strip off hydrogens from the surface silicon hydrides. with larger n is subject to the earlier EL. © 1999 The Electrochemical Society. All rights reserved.

Electrochromism of nanocrystalline TiO{sub 2} films under cathodic bias in propylene carbonate containing LiClO{sub 4} was studied by time-dependent absorption and fluorescence spectroscopies. The absorption spectra revealed two absorption bands near 700 and 420 nm. The absorption band near 700 nm showed a blue shift and an increase in absorbance with time. The blue shift seemed to be due to the progressive involvement of the trapped electrons in the lower energy levels within the shallow surface states. The newly observed band near 420 nm appeared to have originated from the deep surface states which, lying 1.3--1.8 eV below the conduction band, were gradually filled with electrons in the shallow surface states.

Spectral shifts of visible electroluminescence (EL) from cathodically biased n-Si in electrolyte solution containing persulfate ions are described. The electrochemically etched n-Si cathodes exhibit a blue shift of the EL with the increase of applied potential. It is explained that the blue shift is caused by the formation of an accumulation layer and the Fermi level pinning in the porous Si. A study of the EL as a function of time at a constant potential shows a red shift, which can be attributed to the bandgap lowering caused by the conversion of the predominant surface bonds Si-H to Si-Si. Electro-polymerization of aniline on the porous Si surface enhanced the stability of the EL possibly due to the strengthening of the surface bonds.

Current-voltage (J-V) curves of n-GaAs/KOH(aq)-Se{sub 2}{sup 2{minus}}/Se{sup 2{minus}} and n-Si/acetone-LiClO{sub 4}-Fc{sup +/0} (ferrocenium ion/ferrocene) solar cells exhibit sigmoidal shapes near the open-circuit potential. J-V and flatband potential measurements indicate that the deviation of the J-V curves from the ideal square-like shape to S shape is due to the light-induced displacement of the bandedges. Analytical expression describing the dependence of light-induced bandedge shifts on the anodic photocurrent, the density of surface states, and the interfacial charge-transfer velocity are derived and verified experimentally. Application of the expressions to J-V data yields quantitative information on the kinetics of interfacial charge transfer.

The open-circuit photovoltage (V[sub oc]) of semiconductor/liquid junction solar cells is a critical parameter in determining the energy conversion efficiency. A simple, quantitative relationship between the semiconductor surface recombination velocity (S[sub r]) and the open-circuit photovoltage (V[sub oc]) of photoelectrochemical systems was derived and verified experimentally. Experimental results obtained for the n-Si/acetone-FeCp[sub 2][sup +/0]-LiClO[sub 4] junction indicate that V[sub oc] is controlled by surface recombination. Quantitative analysis of the results using the derived expression yields values of the barrier height and S[sub r] that compare favorably with published data. Application of the equation to other semiconductor/liquid junction cells suggests that the expression may be important in evaluating and understanding the behavior of a wide range of photoelectrochemical systems.

Beneficial effects of in situ photopolymerized poly[di-(ethylene glycol)-2-ethyl hexyl ether acrylate] (PDEA) in the pores of dye-coated TiO 2 film on the photovoltaic characteristics of dye-sensitized solar cells (DSSCs)are reported. We have found remarkable enhancements of short-circuit photocurrent (J s c ) by 39% and of open-circuit voltage (V O C ) by 40 mV for the PDEA-modified DSSC compared with those of a conventional DSSC. This led to an enhancement in the overall solar energy conversion efficiency of 4.9% for the modified cell over 4.0% for a cell without PDEA at 100 mW/cm 2 . Moreover, the PDEA modification has led to improved long-term stability of the DSSC. Analyses of optical absorption and transient photocurrent of the cells suggest that the increased concentration of photo-injected electrons as a result of blocking of trap states in TiO 2 contributes to the enhancement in the J S C of PDEA-modified DSSCs. The V O C increase is attributed to an improved screening of the conduction band electrons from I 3 - ions in the presence of PDEA adhered to TiO 2 particles.

The open-circuit voltage increases substantially and stability of dye-sensitized TiO 2 solar cells is improved when polymer gel containing poly(vinylidenefluoride-co-hexafluoropropylene) (PVdF-HFP) in N-methyl-2-pyrrolidone replaces commonly used solvents such as acetonitrile (ACN) and 3-methoxypropionitrile (MPN). The energy conversion efficiency of the cell with 6 wt % PVdF-HFP is comparable to that obtainable in ACN and in MPN. The promising effect of PVdF-HFP as a gelator is attributed to its interaction with Li + ions.

The influence of a buffer layer of a TiO 2 -WO 3 composite material located between a TCO substrate and a TiO 2 (P25) layer on photocurrent-voltage characteristics of dye-sensitized solar cells was compared. The buffer layer effectively isolated dye molecules and electrolytes from directly contacting the conducting substrate. In the presence of the buffer layer having 15-75 mol % WO 3 , both open-circuit photovoltage (V oc ) and short-circuit photocurrent (J sc ) were enhanced. Analyses of scanning electron microscopy data and bandedge positions of the TiO 2 -WO 3 films indicate that the V oc and J sc increases are correlated with favorable energy levels for the electron injection from the P25 layer, and with improved interparticle electrical contacts at both the TCO/TiO 2 -WO 3 and the TiO 2 -WO 3 /P25 junctions.

The effect of electrodepositing polypyrrole (PPy) onto dye-coated TiO 2 surface on the photocurrent-voltage (J-V) characteristics of dye-sensitized solar cells was studied. It is observed that both open-circuit voltage (V oc ) and short-circuit photocurrent (J sc ) of PPy-anchored TiO 2 (PPy/TiO 2 ) solar cells increase, compared to those of a TiO 2 cell. Furthermore, the stability of the J-V characteristics is enhanced with the modification. The combined effects of reduced back-electron-transfer to I 3 ions and negative shift of flatband potential explain the V oc increase. Analyses of electrochromic measurements suggest that the J sc increase is related to the enrichment of conduction band electrons in the PPy/TiO 2 electrode.

Time-dependent changes in the mass of nanocrystalline TiO 2 thin film electrodes coupled with absorbance and current-voltage variations were monitored in propylene carbonate containing LiClO 4 . The onset potential for the absorbance at 800 nm lagged behind that for the mass increase by about 0.3 V, which is related to the energy required to desolvate Li + ions adsorbed on the TiO 2 surface prior to intercalation into the TiO 2 lattice. The intercalated Li + ions can accumulate in TiO 2 by repetitive scanning between 0.0 V and an intercalation-occurring potential. Absorption of UV light by TiO 2 affected both the Li + ion desorption from the electrode and possibly the chemical nature of the TiO 2 surface. The potential dependence on the irreversible mass decrease under UV illumination is linked to the flatband potential.

The spectral shifts of visible electroluminescence from chemically modified porous Si under cathodic bias in an electrolyte containing S 2 O 2- 8 have been studied by luminescence and Fourier transform infrared measurements. Modification of porous Si with soluble polyaniline and silane compounds results in blueshifts, whereas redshifts are observed with H + and O + ion implantations. Based on a recently proposed mechanism that silicon hydride acts as an electron injector to sulfate radical anion, the blueshift is linked to the lowering of the energy levels of silicon hydrides on the porous Si surface with the bandedges being invariant with the chemical modifications. The redshift is seen as a result of the conversion of silicon dihydride to silicon monohydride, with the energy levels of silicon monohydride lying higher than silicon dihydride.

The variation of photoelectrochemical behavior in the early stage of a Ru(II)‐dye‐sensitized solar cell with the addition of carboxylic acid in acetonitrile solution is described. In the presence of carboxylic acid, photocurrent‐voltage curves exhibit dramatic enhancement in stability, increase in photocurrent, and decrease in photovoltage compared with those in the absence of carboxylic acid. The changes were studied by measurements of flatband potential, UV‐visible absorption and reflectance spectra, and ac impedance spectra. The measurements revealed that adsorption of carboxylic acid on the surface plays a major role in variations of the photocurrent‐voltage characteristics. © 2000 The Electrochemical Society. All rights reserved.