Page - 170 - in Photovoltaic Materials and Electronic Devices

Figure5ashowstheJ-Vcurvefor theDSSCscontainingtheZnONRsobtained from the 9-, 18-, and 27-h reactions, indicating that the short-circuit current density (Jsc) and cell performance significantly increase with the NR length. As revealed in the figures, the photovoltaic performances of our DSSCs employing ZnO NRs are comparable to published literature [22–24]. A higher amount of dye was adsorbed on longer NRs than on shorter NRs, indicating that longer NRs improve photon absorption and carrier generation. These results indicate that cell performance is strongly dependent on the electrode surface area. Increasing the NR length results inalargersurfacearea,whichleadstoahigheradsorptionofdyesaswellasahigher conversion efficiency. Furthermore, the Voc of the longer ZnO NRs was higher than that of the shorter ZnO NRs. This higher Voc is attributable to a reduction in recombinationlossesatZnO/dyeinterfaces. Regardingtheperformanceofthecells containingtheZnONRsgrownforvariousperiods, thecellcontainingthe27-hZnO NRs demonstrated optimal performance with a conversion efficiency (η) of 0.64%, Vocof0.62V,Jscof2.56mA/cm2, andfill factorof0.42. TheNRsalsoprovidedirect pathways from the point of photogeneration to the conducting substrate. These pathways ensure the rapid collection of carriers generated throughout the device. Figure 5b depicts the IPCE spectra of the DSSCs (D-719 dye) containing the 9-, 18-, and 27-h ZnO NRs, indicating a strong peak at 520 nm; this peak is attributable to the characteristic excitations of the D-719 dye. Our ZnO-based DSSCs show poor conversion efficiencies when compared to conventional TiO2-based DSSCs, as shownintheinsetofFigure5a. Themainreasonis thecorrosionofZnOonreacting withanacidandthe lowamountsofdyes thatareadsorbedduringtheproduction. Duringtheprocess,anamountofZn2+ ionsaredissolvedintothesolutionfromthe surfaceoftheZnOnanorods. Subsequently,aggregationofZn2+ ionswithsensitizer dyes occurs, and the phenomenon was reported for several organic sensitizer dyes as well as ruthenium complexes [25,26]. Once aggregation takes place in DSSCs, the power conversion efficiency will dramatically decrease [27]. Despite thelowerefficiencies inourZnO-basedDSSCs, theuseofZnOnanorodsstill shows high potential because of its better crystallinity and higher electron mobility. To overcomethechemical instabilityofZnO,theintroductionofnon-ruthenium-based sensitizers and the utilization of different nanotechnological architectures of ZnO mightbepracticalapproaches. 170