ZnO-based Solar Cells Research Articles

Effects of rare earth ion modifications on the photoelectrochemical properties of ZnO-based dye-sensitized solar cells

Dye-sensitized solar cells (DSSCs) were fabricated by using porous ZnO film electrodes derived from home-made ZnO nanoparticles. Electrochemical impedance spectroscopy and open-circuit voltage decay curve measurements were performed to investigate the photoelectrochemical characteristics of the ZnO films modified with different rare earth (La, Ce, Nd, Sm, and Gd) ions. The experimental results indicate that the rare earth oxides covered on the electrode surfaces can form energy barrier and maintain a lower charge recombination probability, and some rare earth ion modifications can passivate the surface states of ZnO electrode. Among these rare earth ions tested, the Nd-, Sm- and Gd-ion modifications can obviously enhance the open-circuit photovoltage and fill factor of the ZnO-based solar cell; whereas the La-, Ce-, Nd-, and Sm-ion modificaions lead to a decreased short-circuit photocurrent. The optimal conversion efficiency is obtained from the Gd-ion modified ZnO-based cells, with a 44.5% improvement in the efficiency as compared to the unmodified one, indicating this rare earth ion modification is promising in the ZnO-based solar cell.

Synthesis of highly‐ordered hierarchical ZnO nanostructures and their application in dye‐sensitized solar cells

AbstractIn order to improve the performance of ZnO‐based solar cells, highly‐ordered hierarchical ZnO nanostructures were design and fabricated. The hierarchical nanostructures were grown on FTO (fluorine doped tin oxide, SnO2:F) glass substrates via a facile, low‐temperature, and low‐cost chemical route. The morphology and structure of the obtained products has been confirmed by field‐emission scanning electron microscopy and X‐ray diffraction measurements. The performance investigation of the prepared dye‐sensitized solar cells (DSSCs) demonstrates that the hierarchical ZnO nanostructure‐based solar cell shows a higher short‐circuit current density compared with the ZnO nanowire counterpart. The enhanced current density may be due to the fact that the surface area of the hierarchical nanostructures is increased. These results indicate that hierarchical ZnO nanostructures are more suitable for the application as photoelectrode of DSSCs. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

TiO 2- and ZnO-based solar cells using a chlorophyll a derivative sensitizer for light-harvesting and energy conversion

TiO 2- and ZnO-based solar cells sensitized by a chlorophyll a derivative (methyl trans-3 2-carboxy-pyropheophorbide a) were fabricated and compared. The TiO 2-based solar cell produces higher values for the short-circuit photocurrent ( J sc), open-circuit photovoltage ( V oc), and energy-to-electricity conversion efficiency ( η) than the ZnO-based solar cell. The observed ATR-FTIR data on the dye-sensitized semiconductor electrodes and the spectra estimated from the density functional theory (DFT) suggest that the dye sensitizer is bound to TiO 2 with both the bidentate chelating and monodentate modes but is bound to ZnO with the monodentate mode exclusively. The frontier orbitals of the dye molecule bound to semiconductors suggest that the HOMO-2 and LUMO + 2 orbitals of the dye sensitizer do not participate in electron transfer processes for the dye–ZnO system, resulting in a lower J sc value and a relatively narrow response for the incident photon-to-current conversion efficiency in the solar cell. Transition component analysis based upon the time-dependent DFT results explains well the experimental UV–vis spectra and difference in the η values between the dye-sensitized solar cells based upon TiO 2 and ZnO nanocrystalline electrode materials.

Numerical Simulation of the Current−Voltage Curve in Dye-Sensitized Solar Cells

A theoretical model based on the numerical integration of the continuity equation for electrons with trap-limited density-dependent diffusion and recombination constants is implemented to describe the functioning of dye-sensitized solar cells (DSSC). The application of the model combines recent theory on charge transport in nanocrystalline materials with parameters extracted from five simple measurements: the UV/vis spectrum of the dye in solution, the steady-state current−voltage curve, the open circuit photovoltage versus light intensity curve, photocurrent transient upon switching on an illumination source, and open-circuit voltage decay upon switching off the illumination source. As a novel feature not previously included in this kind of calculations, the model includes an additional term that accounts for the charge transfer from the transparent conducting oxide (TCO) substrate to the electrolyte solution. The general applicability of the model is illustrated by applying it to two different types of cell: a TiO2-based solar cell with an organic solvent electrolyte and a ZnO-based solar cell with a solvent-free electrolyte. It is found that the numerical model is capable of adequately fitting all data for both systems, resulting in quantitative estimates for the main parameters controlling solar cell functioning and efficiency. The results show that it is possible to provide a global description of DSSCs based on fundamental theories for trap-limited transport and recombination using simple experimental techniques available to every solar cell laboratory. The present paper tries to help fill the gap between pure theoreticians and experimentalists working on this kind of system.

Comparison of Dye-Sensitized ZnO and TiO2 Solar Cells: Studies of Charge Transport and Carrier Lifetime

Nanocrystalline particles of ZnO and TiO2 of approximately equal size (∼15 nm) were used to prepare mesoporous electrodes for dye-sensitized solar cells. Electron transport in the solar cells was studied using intensity-modulated photocurrent spectroscopy and revealed very similar results for ZnO and TiO2. Apparent activation energies for electron transport in nanostructured ZnO of ≤0.1 eV were calculated from the temperature dependence of transport times under short-circuit conditions. The lifetime of electrons in the nanostructured semiconductors was evaluated from open-circuit voltage decay and intensity-modulated photovoltage spectroscopy. Significantly longer lifetimes were obtained with ZnO. Despite the reduced recombination, ZnO-based solar cells performed worse than TiO2 cells, which was attributed to a lower electron injection efficiency from excited dye molecules and/or a lower dye regeneration efficiency. The internal voltage in the nanostructured ZnO film under short-circuit conditions was abo...