The Optimization Of Photoanode Of Dye-sensitized Solar Cells

Energy crisis is a worldwide problem of human beings facing. With the exhaustionof fossil fuels and environmental pollution caused by the large use of fossil fuels, it is aserious threat to survival and development of human beings. The large-scale solarenergy application is a key to solve the energy lack and environmental problems.Dye-sensitised soalr cells （DSSC） is thought to be a candidate to realize the large-scaleuse of solar energy for it has many advantages such as non-toxic, high efficiency ofconversion, low cost, etc. The photoanode is the most important part of DSSC. Thecomposition and structure of the photoanode of DSSC directly influence the conversionefficiency and long-term stability. It has attracted world wide attentions in recent yearsto improve the performance of DSSC based on varied nanometer materials andstructures.The goal of this paper is to find out optimizing material and nanostrcture for DSSCby testing and comparing the performance of DSSC derived from varied materials anddifferent nanostructures. The main contents are list as follows:①TiO₂nanoparticles （NPs）, ZnO NPs and SnO₂aggregates composed of particleswith crystal size of about20nm were choosed to fabricate the photoanode of DSSC. Itis found that the performance of TiO₂, ZnO, SnO₂cell is the best, second and thirdrespectively. As the conductive band （CB） potential of SnO₂is much lower than that ofTiO₂and ZnO, more free energy lose when electron jump from excited dye to the CB ofSnO₂which results in a low open circuit-voltage （VOC）. Thus, the pure SnO₂is not agood photoanode material for DSSC.②TiO₂nanotube arrays （NTs）, TiO₂nanowire arrays （NWs）, ZnO NWs and thehybrid of ZnO NWs and ZnO NPs were used as the photoanode of DSSC. It is foundthat the conversion efficiency of DSSC derived from TiO₂NTs synthesized byanodization is limited by the light absorption when photons pass through the light blackplatinized counter electrode and the brown electrolyte since a back-light-injection formmust be chosen to fabricate the DSSC due to the titanium substrate is opaque. As thediameters of the TiO₂nanotubes is limited in the range of50-300nm and the bottom ofthe arrays is compacted when the lenth is larger than about5m, so the surface isinsufficient to absorb enough dye molecules for light absorption and thus, a lowconversion efficiency is reached. The insufficient to absorb for ZnO NWs is similar to that of TiO₂NWs except that the lenth could be much larger than5m in which thesurface of ZnO NWs is not largely promoted. It is difficult to efficiently fillnanoparticles into the interstice of ZnO NWs to increase the working surface. So theperformance of DSSC based on the hybrid of ZnO NWs and ZnO BPs is not good.③According to the comparision of the I-V plot and darkcurrent-voltage plot ofZnO NW DSSC with those of DSSC based on TiO₂NPs, ZnO NPs and TiO₂-coatedZnO NW arrays, and the results of the VOC-time decay plot in the dark and the life timeof electrons in electrode, a conclusion can be draw that the low fill factor （FF） of ZnONW cell is not result from a rapid recombination of electrons with the redox species inthe electrolyte but is ascribed to the decrease of electron injection efficiency. With theincrease of injected electron density in electrode, the injection efficiency of lattercoming electrons is resisted by electrostatic exclusion. The light induced current dropsrapidly with the increase of applied bias. By coating ZnO NW with a thin layer of TiO₂,the FF of ZnO NWs cell can be greatly improved and the conversion efficiency can bepromoted largly. This is the first innovation point of the article.④Submicrometer ZnO porous aggregates and nanoparticles with size of20nmwere synthesized and used in DSSC. About20%higher conversion efficiency isachieved in ZnO aggregate cell compared with ZnO nanoparticle cell. The improvementof conversion efficiency is ascribed to the strong scattering function of aggregates as thecrystal sizes of both the samples are almost equal. This indicates that aggreagtes aresuitable to be used in photoanodes of DSSC than nanoparticles.⑤The long term stability of DSSC based on ZnO aggregates sensitized by dyeN719is excellent in a mild environment where the temperature is between-5-45oC.The cells can maintain about90%of their initial conversion efficiency after half a yearrunning. This indicates that DSSC based on ZnO aggregates has potential to be used inpractice. This is the second innovation point of the article.⑥By analyzing the FFs of different DSSCs based on ZnO crystals with varied size,we found that the DSSC with size of particles larger than50nm can not reach high FF.The band bendings on varied particle sizes are calculated according to the space chargelayer model. The results show that with the increase of applied bias, the band bendingon particles increases and the electron injection efficiency decrease accordingly andthen a low FF is obtained. Nanoparticles with size of12-30nm or nanowire arrays withdiameter less than30nm could effectively avoid the influence of the band bendingwhich should be chosen to fabricate the photoanode of DSSC to reach figh FF. This is the third innovation point of the article.⑦From our calculation, nanowire arrays with30nm in wire diameter,26m inlenth and5nm in interval, has surface area as large as that of nanoparticle film withthickness of10m and a particle size of15nm. Such nanowire arrays based DSSC maybreak through the conversion efficiency of the DSSC based on the submicrometer TiO₂porous aggregates which is highest efficiency achieved currently.