Cation Exchange at Semiconducting Oxide Surfaces: Origin of Light-Induced Performance Increases in Porphyrin Dye-Sensitized Solar Cells (original) (raw)
Related papers
The Journal of Physical Chemistry C, 2010
Varying Li + concentration in the electrolyte of dye-sensitized solar cells equipped with compact TiO 2 blocking layers is found to alter the mean slopes of semilogarithmic open-circuit photovoltage-intensity and dark current-voltage plots. Almost identical values of ideality factor or transfer coefficient are required to fit data in the dark and under illumination for each Li + concentration. It is found that cell characteristics become progressively more "ideal" as Li + concentration is increased, with a transfer coefficient of ca.1 for 1 M Li + in the electrolyte. We find that trends in photovoltage-intensity data are well fitted using a model which assumes that electron transfer to acceptor species in the electrolyte occurs from both the conduction band of the TiO 2 and an exponential distribution of band gap surface states. Changes in the mean ideality factor and linearity of semilogarithmic photovoltage-intensity plots can be rationalized by considering the variation in overlap between occupied donor states (conduction band and surface states) with electron acceptor states in the electrolyte, as the conduction band edge is shifted positive by increasing Li + concentration. In accordance with previous studies, this positive shift in conduction band edge is also found to cause a dramatic increase in the photocurrent generation efficiency of the cells, especially in the long-wavelength region of the photocurrent action spectrum. It is argued that this improvement in photocurrent is predominantly due to an increase in wavelength-dependent electron injection efficiency, as opposed to an increase in electron collection efficiency.
Rsc Advances, 2014
The performance of dye-sensitized solar cells (DSSCs) with an electrolyte including mixed cations was evaluated, and the relevant carrier dynamics were investigated by the heterodyne transient grating method. The performance of the DSSCs showed maximum conversion efficiency for an Li + cation ratio of 75% with 25% other cations. † Electronic supplementary information (ESI) available: Transient absorption spectra measured for a DSSC, and the dependence of HD-TG responses on the mixed cation ratio for low concentrations of TBA + (0-25%). See
Three porphyrin dyes, P1, P2 and P3, bearing one, two and four pyridyl groups, respectively, in the meso positions, acting as electron acceptor anchoring groups, were synthesized, characterized and investigated as sensitizers for the fabrication of dye sensitized solar cells (DSSCs). The overall power conversion efficiencies (PCEs) of DSSCs based on these dyes lay in the range 2.46-3.9% using a 12 mm thick TiO 2 photoanode. Porphyrin P2 achieved the maximum performance, which can be rationalized by the high dye loading, efficient electron injection, dye regeneration process and longer electron lifetime, as demonstrated by the electrochemical impedance spectroscopy (EIS) measurements. The PCE of the DSSC based on the P2 sensitizer when the photoanode was treated with formic acid, showed an enhanced efficiency of 5.23%. This improvement, attributed to multifunctional properties such as higher dye uptake, reduced recombination process and enhanced charge collection efficiency. Deoxycholic acid (DCA) was also used as a coadsorbent in order to prevent dye aggregation and it was found that the PCE improved up to 6.12% for sensitizer P2 and the modified TiO 2 photoanode, which can be attributed to further improvement in the electron injection efficiency and charge collection efficiency.
ChemPlusChem, 2018
Dye design for panchromatic light absorption has attracted much attention in the field of dye-sensitized solar cells. An approach to enhance the panchromatic light absorption utilizes mixture of complementary light absorbing dyes besides dyes with specific anchoring groups that facilitate interfacial charge transfer with TiO 2. Dipole-dipole interaction between the dye molecules on surface leads to broaden the spectrum which leads to decreased DSSC device performance. However controlled aggregation of dyes facilitate both broadening the spectral profile along with enhanced photocurrent generation. To control the dye-dye interaction, dimeric dyes with different dipole lengths D 1-D sq , D sq-D sq were systematically designed and synthesized. Photo-physical, electrochemical properties were evaluated for determining the E HOMO and E LUMO levels that determines the electron injection from E LUMO of the dye to E CB of TiO 2 and regeneration of oxidized dye by the electrolyte, respectively. Absorption spectra of D sq-D sq , D 1-D sq were broadened in solution compared to model dye D sq indicates the dye-dye interaction is prominent in solution. In D 1-D sq excitation energy transfer between photoexcited D 1 and D sq was explained by using Förster resonance energy transfer (FRET). Homodimeric dye showed the device performace of 2.8 % (V oc 0.607, J sc 6.62 mA/cm 2 , ff 69.3%), whereas the heterodimeric dye D 1-D sq showed the device performance of 3.9% (V oc 0.652 V, J sc 8.89 mA/cm 2 , ff 68.8%). The increased photocurrent for D 1-D sq is due to the panchromatic IPCE response compared to D sq-D sq. The increased V oc is due to the effective passivation of TiO 2 surface by the spiro-linker, and the effective dipole moment that shift the conduction band on TiO 2. Hence, the open circuit potential, V oc , for the devices of D sq , D 1-D sq and D sq-D sq have been systematically modulated by controlling the intermolecular π−π and intramolecular dipole-dipole interactions of the dimeric dyes.
ChemistrySelect, 2018
We have designed and synthesized three new donor (D)-linker (p)-acceptor (A) type b-functionalized imidazole-fused porphyrins, 4 a-4 c by grafting the acceptor such as cyanoacrylic acid/ carboxylic acid at 2 nd position of imidazole fused to the b,b' edge of porphyrin donor via p-linker such as thiophene/ phenylene and examined them as sensitizers for dye-sensitized solar cells (DSSCs). The structure-activity relationships of the dyes are systematically investigated by using absorption spectroscopy, cyclic voltammetry, and density functional theory calculations. Dyes, 4 a and 4 b possessing cyanoacrylic acid acceptor exhibited broadened Soret bands compared with that of dye 4 c containing carboxylic acid acceptor, which led to good light-harvesting properties and thereby broader IPCE curves. The DSSCs of the dye, 4 a possessing thiophene linker and cyanoacrylic acid acceptor exhibit higher power conversion efficiency than the analogous dyes, 4 b and 4 c with phenylene p-linker and cyanoacrylic acid or carboxylic acid acceptor. The hike in the efficiency comes from the increased short-circuit current and open-circuit voltage due to reduced dye aggregation. Electrochemical impedance spectroscopy studies reveal that the lifetime of the injected electrons in the conduction band of TiO 2 and charge recombination rate at the TiO 2 /dye/ electrolyte interface mainly dependent on nature of p-linker and acceptor of the dyes.
The Journal of Physical Chemistry C, 2010
Two new sensitizers for dye sensitized solar cells have been designed consisting of thiophene units asymmetrically functionalized by N-aryl carbazole. The di(tert-butyl) carbazole moieties acts as an electron donor group, the thiophene chain as a bridge group, and the cyanoacrylic acid as an anchoring and electron acceptor group. An increase of the conjugation length produces two main effects: first, it leads to a red-shift of the optical absorption of the dyes, resulting in an improved overlap of the absorption with the solar spectrum. Second, the oxidation potential decreases. The photovoltaic performance of this set of dyes as sensitizers in mesoporous TiO 2 solar cells was investigated using electrolytes containing the iodide/triiodide redox couple. The dye with the best absorption characteristics showed the highest photocurrent but lower open circuit voltage due to more losses by recombination. A trend between structure (molecule dyes size) and recombination is demostrated using an analysis procedure based on -recombination model that combines impedance spectroscopy and density current-voltage data.
The influence of cations on charge accumulation in dye-sensitized solar cells
Journal of Electroanalytical Chemistry, 2007
The relation between open-circuit voltage, V OC , light intensity, /, and accumulated charge, Q, has been studied for dye-sensitized solar cells (DSCs) containing different counterions to the iodide/triiodide redox couple. At higher light intensities, V OC scaled in the order Cs + > K + > Na + > Li + , which was caused in part by shifts in the conduction band edge. The relation between V OC and Q was fitted to an exponential trap model. It was found that inclusion of a capacitive term improved the fit significantly. The determined values of C were found to be relatively large, up to 75 lF cm À2 , and dependent of cation. Physically, the largest fraction of C could be ascribed to the TiO 2 bulk or TiO 2 /dye/electrolyte interface. The interpretation of the trap distribution broadening parameter, b, was found to be dependent of fitting model. Using the model including the linear CV OC term, b was independent of cation and could be viewed as a TiO 2 material parameter, while in the model excluding CV OC , b was dependent of cation. Voltage decay experiments were performed to study the cationic influence on recombination. Electron lifetimes were calculated from the voltage decay curves and it was found that the DSC containing Li + yielded by far the shortest lifetime followed by the DSCs containing Na + , K + and Cs +. Voltage decay curves include the effect of TiO 2 conduction band shifts in the comparison of electron lifetimes with different cations. We therefore suggest that the electron lifetimes should be calculated from the corresponding charge decay curves. From such a comparison, it was found that the DSC containing Li + yielded the shortest lifetime whereas the DSCs containing Na + , K + or Cs + showed approximately identical lifetimes.
The Journal of Physical Chemistry C, 2012
Femtosecond time-gated fluorescence and nanosecond flash photolysis studies of seven complete, real titania nanoparticle solar cells sensitized with an efficient organic dye (TH305) were performed in order to investigate the role of the electrolyte composition on the charge transfer dynamics. The electron injection rate constants were found to range from 0.4 to 3.5 ps −1 in iodide-based electrolyte, and they well correlate with the shift of the conduction band edge potential of titania. The lithium cation additives resulted in 2 times faster electron injection rate constant (3.55 ps −1 ) with respect to that when larger sodium cations were used (1.86 ps −1 ). However, in the presence of a pyridine derivative component in the electrolyte solution, the electron injection rate constant decreased several times (0.38 ps −1 for Li + and 0.54 ps −1 for Na + ), while the electron injection efficiency was found to be still very high, 96−100%. The dye regeneration by the redox couple under relatively low fluence of excitation beam (0.4 mJ/cm 2 giving about 4 electrons per titania nanoparticle) proceeds with an average rate constant of about 40 × 10 3 s −1 and efficiency close to 100%, independent of the electron composition. However, for a larger fluence (2 mJ/cm 2 ) excitation, a titania−dye electron recombination process competes with the dye regeneration and lowers the solar cell efficiency. The effect of self-quenching, high vibrational levels of the dye excited state, and the neat solvent on the electron injection process are also discussed. This study clearly shows that for TH350-based DSSCs the best performance is obtained using Li + and TBP as additives to the iodide electrolyte, giving the highest open circuit voltage and almost 100% efficiency of electron injection and dye regeneration.