Photosynthetic oxygen production by photosystem II (PSII) is responsible for the maintenance of aerobic life on earth. The production of oxygen occurs at the PSII oxygen-evolving complex (OEC), which contains a tetranuclear manganese (Mn) cluster. Photo-induced electron transfer events in the reaction center lead to the accumulation of oxidizing equivalents on the OEC. Four sequential photooxidation reactions are required for oxygen production. The oxidizing complex cycles among five oxidation states, called the S n states, where n refers to the number of oxidizing equivalents stored. Oxygen release occurs during the S 3-to-S0 transition from an unstable intermediate, known as the S4 state. In this report, we present data providing evidence for the production of an intermediate during each S state transition. These proteinderived intermediates are produced on the microsecond to millisecond time scale and are detected by time-resolved vibrational spectroscopy on the microsecond time scale. Our results suggest that a protein-derived conformational change or proton transfer reaction precedes Mn redox reactions during the S 2-to-S3 and S 3-to-S0 transitions.manganese cluster ͉ photosynthesis ͉ photosystem II ͉ time-resolved IR ͉ water oxidation T ime-resolved vibrational spectroscopy can detect chemical intermediates formed during enzymatic catalysis. Advantages include the technique's exquisite structural sensitivity and its high temporal resolution. Recent advances in methodology and interpretation have produced insights into the catalytic mechanism in several biological systems (for examples, see refs. 1-4).In this paper, we report the use of time-resolved IR spectroscopy to investigate the mechanism of photosynthetic water oxidation. Photosystem II (PSII) catalyzes the oxidation of water and the reduction of bound plastoquinone. Photoexcitation of PSII leads to the oxidation of the chlorophyll donor, P 680 , and the sequential reduction of a pheophytin (Fig. 1A, reaction 1) and a plastoquinone, Q A ( Fig. 1 A, reaction 2), in picoseconds. Q A reduces Q B to generate a semiquinone radical, Q B Ϫ , on the microsecond time scale (Fig. 1 A, reaction 3 ) (reviewed in ref. 5).A second photoexcitation leads to the reduction and protonation of Q B Ϫ to form the quinol Q B H 2 . The rate of reduction of Q B is faster than the rate of reduction of Q B Ϫ (see ref. 6 and references therein), which gives rise to a characteristic period-2 oscillation in kinetics originating on the PSII acceptor side (7).The primary chlorophyll donor, P 680 , oxidizes a tyrosine, Y Z (Y161 in the D1 subunit), on the nanosecond to microsecond time scale (Fig. 1 A, reaction 4). In turn, tyrosine Y Z ⅐ oxidizes the oxygen-evolving complex (OEC) on every flash (Fig. 1 A, reaction 5) (8). Four sequential photooxidation reactions are required for oxygen production, and the oxidizing complex cycles among five oxidation states, called the S n states, where n refers to the number of oxidizing equivalents stored (9). The rate of OEC oxidation slows as o...

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Photosystem II (PSII) catalyzes the oxidation of water to O2 at the manganese-containing, oxygen-evolving complex (OEC). Photoexcitation of PSII results in the oxidation of the OEC; four sequential oxidation reactions are required for the generation and release of molecular oxygen. Therefore, with flash illumination, the OEC cycles among five Sn states. Chloride depletion inhibits O2 evolution. However, the binding site of chloride in the OEC is not known, and the role of chloride in oxygen evolution has not as yet been elucidated. We have employed reaction-induced FT-IR spectroscopy and selective flash excitation, which cycles PSII samples through the S state transitions. On the time scale employed, these FT-IR difference spectra reflect long-lived structural changes in the OEC. Bromide substitution supports oxygen evolution and was used to identify vibrational bands arising from structural changes at the chloride-binding site. Contributions to the vibrational spectrum from bromide-sensitive bands were observed on each flash. Sulfate treatment led to an elimination of oxygen evolution activity and of the FT-IR spectra assigned to the S3 to S0 (third flash) and S0 to S1 transitions (fourth flash). However, sulfate treatment changed, but did not eliminate, the FT-IR spectra obtained with the first and second flashes. Solvent isotope exchange in chloride-exchanged samples suggests flash-dependent structural changes, which alter protein dynamics during the S state cycle.

Screen-printed thick-film Ag metallization has become highly successful in crystalline Si (c-Si) photovoltaics. However, a complete understanding of the mechanism resulting in low resistance contact is still lacking. In order to shed light on this mechanism for current-generation Ag paste, Si solar cells were fabricated using a range of emitter doping densities and contact firing conditions. Low resistance contact was found to vary as a function of emitter surface P concentration ([P su rface ]) and peak firing temperature. Scanning electron microscope (SEM) analysis revealed thin interfacial glass films (IGF) under the bulk Ag gridline. SEM analysis also showed increasing Ag crystallite density as both emitter [P su rface ] and peak firing temperature increased. Two mechanisms are proposed in forming low resistance contact to highly doped emitters: 1) formation of ultrathin IGF and/or nano-Ag colloids at low firing temperature, and 2) formation of Ag crystallites at high firing temperature. However, on lightly doped emitters, low resistance contact was achieved only at higher firing temperatures, concomitant with increasing Ag crystallite density, and suggests that thin IGF decorated with nano-Ag colloids may not be sufficient for low resistance contact to lightly doped emitters.

High throughput, low cost, and high efficiency are the keys to reducing the cost of photovoltaic electricity. To realize high efficiency, the quality of emitter is critical. The emitter can be formed either by batch deposition in a tube using liquid POCl 3 source or by inline spray deposition of phosphoric acid (P 2 O 5 ). The POCl 3 process has been optimized over the years to give reproducible performance, whereas the inline diffusion has not been systematically optimized. This paper reports on solar cell results with the inline emitters. Sheet resistance mapping tool has been used to quantify the emitter uniformity in conjunction with junction depth and phosphorus surface concentration to characterize the inline emitters. We have achieved average efficiencies of 17.1 and 17.4%, respectively, on 2 V-cm textured CZ with 45 and 60 V/sq emitters. Average fill factor of 0.772 and ideality factor of 1.06 support the front contact firing optimization for the inline emitters. Personal computer one dimensional (PC1D) modeling of the 60 V/sq emitter showed efficiency of >18% is achievable by reducing the front surface recombination velocity (FSRV) from 120 000 to 80 000 cm/s and increasing the fill factor to 79.5%.