The charge-recombination kinetics and band edge movement in dye-sensitized nanocrystalline TiO2 solar cells are investigated by intensity modulated photovoltage spectroscopy (IMVS). A theoretical model of IMVS for dye-sensitized nanocrystalline semiconductor electrodes is developed, and analytical expressions for the frequency dependence of the photovoltage response at open circuit are derived. The model considers charge trapping/detrapping and electron transfer from the conduction band and surface states of the semiconductor to redox species at the solid/solution interface. IMVS is shown to be valuable in elucidating the contributions of band edge shift and recombination kinetics to changes of the open-circuit photovoltage (V oc) resulting from surface modifications of the semiconductor. IMVS measurements indicate that surface treatment of [RuL2(NCS)2] (L = 2,2‘-bipyridyl-4,4‘-dicarboxylic acid)-sensitized TiO2 electrodes with 4-tert-butylpyridine or ammonia leads to a significant band edge shift concomitant with a more negative V oc. Surface-modified dye-covered TiO2 electrodes exhibit a much higher photovoltage, for a given concentration of accumulated photogenerated electrons, than the unmodified dye-covered electrode. The accumulated charge in the TiO2 electrode is not sufficient to induce a major potential drop across the Helmholtz layer and cannot thus explain the observed photovoltage. The surface charge density is also not sufficient to support an accumulation layer strong enough to have a major influence on the photovoltage. The movement of the Fermi level of the TiO2 electrode, arising from the accumulation of photogenerated electrons in the conduction band, accounts for the observed V oc. The second-order nature of the recombination reaction with respect to I3 - concentration is confirmed. Furthermore, the IMVS study indicates that recombination at the nanocrystallite/redox electrolyte interface occurs predominantly via trapped electrons in surface states.

Low bandgap lead-tin mixed perovskite solar cells have achieved high power conversion efficiency in excess of 17%. However, methylammonium (MA) cation was usually contained, and the thermal stability of MA is always a great concern. In this work, accoring to composition engineering, we tried to explore a nearly formamidinium (FA) based lowbandgap Pb-Sn mixed perovskite FAPb 0.75 Sn 0.25 I 3 as the absorber layer. Combined with interface enginering by replacing PEDOT:PSS layer with NiO x as hole transport layer, a power conversion efficiency of 17.25% has been obtained. This low-bandgap perovskite solar cell maintains about 91% of its original efficiency at 80 O C for 20 hours, and 92% of its initial performance after 46 days storage at the room temperature. The good thermal stability of nearly FA based low-bandgap perovskite could be good for delivering efficient and stable perovskite-perovskite tandem solar cells.

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The four complex, almost degenerate, second-order eigenmodes of a two-mode fiber having a circular core are reduced to two nondegenerate, linearly polarized second-order eigenmodes with stable-intensity lobe positions in a highly elliptical core fiber. Existing two-mode-fiber devices can be improved by this stabilization of the second-order modes. Practical sensors employing the two spatial modes as the two arms of an interferometer are described. The two arms of an interferometer of this type can have the same group delays, while the difference in phase delays is large.