Performance Of DSSC Based On Pigment-protein Complexes And Photopigments From Purple Bacteria

Dye-sensitized solar cells (DSSC), a new type of green solar cell, is considered ofone of the most promising solar cells in the future due to its low-price, simplefabrication process and stability of photoelectric properties. In DSSC, the sensitizersare very important in the process of photoelectric conversion. The ruthenium complexdyes, known as high conversion efficiency sensitizers of DSSC, has been widelyapplied in the DSSC. However, the high cost, degradation in presence of water andcomplicated preparation of these synthetic dyes can not be ignored, which hinders itsapplication and development of DSSC. Thus, it will become a promising direction ofthe current research that seeking and developing natural sensitizer with low-cost,broad spectrum responsive, efficient and eco-friendly. Anoxygenic phototrophicbacteria (APB) is a class of bacteria which conducts anoxygenic photosynthesis underanaerobic conditions. And the purple bacteria in APB are good model organisms forstuding photosynthesis mechanism. Compared with oxygenic photosyntheticorganisms, such as algaes and plants, APB has a rich variety of pigment-proteincomplexes (PPC) as well as different structural properties natural photosyntheticpigment. These photosynthetic elements have wide spectrum of response (250nm~1100nm), excellent photoelectric conversion performance and environmentalfriendliness, which has important application value in development of photoelectricelements and will be promising natural sensitizer candidates in fabricatingvisible-infrared responsive DSSC. However, until now there are few studies of DSSCbased on the natural pigments and PPC derived from purple bacteria. In this paper, wesystematically studied the photoelectric conversion performance ofbacteriochlorophyll a (BChl a) and its derivatives, three types of typical carotenoid(Car) and four types of typical PPC from APB in DSSC, screened the photosyntheticcomponents with excellent optoelectronic properties. After optimization of theadsorption condition, the thickness and surface morphology of photoanode, andco-sensitization of natural sensitizers, the performance of DSSC were improved. Themain results are as follows: Using the rapid and effective method for separation of pigment, thephotoelectrochemical properties of seven different natural photosynthetic pigments ofbacteriochlorophyll a (BChl a) and carotenoids (Car), and three modified BChl aderivatives from Rhodopseudomonas palustris CQV97, Rhodobacter azotoformansR7and Marichromatium sp.283-1were investigated in DSSC. The results concludedthat under simulated sunlight intensity of100mW/cm2and without the spacer, naturalBChl a had better photoelectric conversion efficiency of0.76%for its near-infraredabsorption. Purified rhodopin Car had higher conversion efficiency than that ofmixture-Car and other purfied Car. Comparing with BChl a in solution, theabsorption spectrum of TiO2electrode sensitized by BChl a red-shifted, and thefluorescence at800nm could be quenched, indicating that the electron ejected byBChl a entered into TiO2conduction band. In the DSSC based on BChl a, it wasdemonstrated that improving the surface morphology of TiO2films, and optimizingthe film thickness and BChl a concentration could increase the saturation adsorptionquantity of BChl a on the photoanodes, thereby the performance obtained was better.When the TiO2film of10μm thickness was treated by TiCl4and BChl aconcentration was0.48mg/ml, the maximum conversion efficiency was up to1.67%.The TiO2electrodes co-sensitized by BChl a and other natural pigments, which hascomplementary absorption spectrum, exhibited wide spectrum response from visiblelight to near-infrared region, the short circuit current Iscand the photoelectricconversion efficiency η of co-sensitizing solar cells were increased. When the finalconcentration ratio of BChl a and rhodopin was3:1, Iscand η were increased by12%and7.3%, respectively. Meanwhile, the η of BChl a and chlorophyll a co-sensitizedDSSC was2.2times of single BChl a sensitized DSSC.Using the light-harvesting complex2(LH2) from CQV97strains as a referencestandard, the adsorption equilibrium time was72h, the optimum concentration ofLH2was around46.8μg BChl/ml and the optimum film thickness was10μm forDSSC based on LH2. In this case, the maximum Iscreached1.46mA/cm2, and thephotocurrent response remained stable after alternating light and darkness by severaltimes. Better still, the η was0.49%, which approximate two folds higher than spinachLHC-based DSSC to date. For four kinds of typical PPCs, RC-sensitized solar cell reached the Iscof1.24mA/cm2and η of0.57%, which over three orders of magnitudehigher than any RC-based photovoltaic cells, up to now. The results illustrated thatsingle LHC from APB also could be used as photosensitive element in photovoltaicsolar cells, and DSSC was maybe a better systems for taking full advantage of thePPC’s photoelectric properties.Carboxylation multi-walled carbon nanotubes (WMNTs) had good biologicalcompatibility. Compared with pure TiO2electrode, PPC had a better stability onWMNTs/TiO2heterogeneity nanocomposite electrode. An appropriate concentrationof WMNTs in TiO2film electrodes, which could enhance the photoelectric propertiesof LH2sensitized DSSC, especially greatly increase of Isc, which the maximumgrowth rate was143%. At the same time, the adsorption capacity of PPC on electrodeand the electron transfer and capture efficiency in the semiconductor had beenimproved, the probability of electronic composite and the resistance of electrontransport in the semiconductor conduction band were decreased, in other words, theloss of electron was reduced in nanocomposite.