Effect of Posttreatment of Titania Mesoscopic Films by TiCl₄ in Solid-State Dye-Sensitized Solar Cells: A Time-Resolved Spectroscopy Study

Posttreatment of mesoporous titanium dioxide films by TiCl₄ solutions is commonly applied during the fabrication of solid-state dye-sensitized solar cells (ssDSCs), as this operation markedly improves the performance of the photovoltaic device. The effect of the posttreatment upon the charge carrier dynamics was scrutinized in an ssDSC aiming at unraveling its mechanism. Kinetic studies carried out using femtosecond and nanosecond transient absorption spectroscopies showed that a biphasic electron injection from the dye excited state is observed, for both treated and nontreated films, whose kinetics is not significantly affected by the surface modification step. However, hole injection in the hole transport material (HTM) spiro-OMeTAD and charge recombination were found to be markedly slower in TiCl₄-treated films. These findings are rationalized by a model describing the interaction at the interface between TiO₂, the dye-sensitizer, and spiro-OMeTAD. Rather than resulting from a modification of the energetics of the conduction band of the oxide, the effect of the TiCl₄ posttreatment appears to be associated with a subtle change of the film morphology. Results emphasize the importance of controlling the contact at the heterojunction between the HTM and the sensitized semiconductor oxide network

Extremely Slow Spontaneous Electron Trapping in Photodoped <i>n</i>‑Type CdSe Nanocrystals

The trapping dynamics of conduction-band electrons in colloidal degenerately doped <i>n</i>-CdSe nanocrystals prepared by photochemical reduction (photodoping) were measured by direct optical methods. The nanocrystals show spontaneous electron trapping with distributed kinetics that extend to remarkably long time scales. Shifts in nanocrystal band-edge potentials caused by quantum confinement and surface ion stoichiometry were also measured by spectroelectrochemical techniques, and their relationship to the slow electron trapping is discussed. The very long electron-trapping time scales observed in these measurements are more consistent with atomic rearrangement than with fundamental electron-transfer processes. Such slow and broadly distributed electron-trapping dynamics are reminiscent of the well-known distributed dynamics of nanocrystal photoluminescence blinking, and potential relationships between the two phenomena are discussed

Tunneling in the Delayed Luminescence of Colloidal CdSe, Cu⁺‑Doped CdSe, and CuInS₂ Semiconductor Nanocrystals and Relationship to Blinking

The photoluminescence decay dynamics of colloidal CdSe, Cu⁺:CdSe, and CuInS₂ nanocrystals have been examined as a function of temperature and magnetic field. All three materials show photoluminescence decay on time scales significantly longer than the intrinsic lifetimes of their luminescent excited states, i.e., delayed luminescence, involving formation of a metastable trapped excited state followed by detrapping to re-form the emissive excited state. Surprisingly, the delayed luminescence decay kinetics are nearly identical for these three very different materials, suggesting they reflect universal properties of the delayed luminescence phenomenon in semiconductor nanocrystals. By measuring luminescence decay over 8 decades in time and 6 decades in intensity, we observe for the first time a clear deviation from power-law dynamics in delayed luminescence. Furthermore, for all three materials, the delayed luminescence decay dynamics are observed to be nearly independent of temperature between 20 K and room temperature, reflecting tunneling as the dominant mechanism for detrapping from the metastable state. A kinetic model is introduced that invokes a log-normal distribution of tunneling rates and reproduces the full range of delayed luminescence decay dynamics well. These findings are discussed in relation to photoluminescence blinking, with which delayed luminescence appears closely associated

Single-Particle Photoluminescence Spectra, Blinking, and Delayed Luminescence of Colloidal CuInS₂ Nanocrystals

Single-nanocrystal and ensemble photoluminescence measurements on CuInS₂ semiconductor nanocrystals reveal luminescence bandshapes that are broad compared to those typical of individual II–VI or related semiconductor nanocrystals. This finding is consistent with the hypothesis of strong electron–phonon coupling in the emissive excited state of these CuInS₂ semiconductor nanocrystals. Blinking is observed that resembles that of other semiconductor nanocrystals. Ensemble luminescence measurements also reveal the existence of a remarkably long-lived excited state in these nanocrystals that continues to emit photons over several orders of magnitude in time following the excitation pulse. The delayed luminescence overlaps in time and shows similar distributed kinetics to the blinking “off” times of the same nanocrystal sample, supporting the proposal that these two phenomena arise from the same microscopic carrier-trapping and -detrapping processes. Excitation power dependence measurements illustrate that the delayed luminescence saturates at very low emission intensities under the excitation power densities used in the single-nanocrystal measurements, consistent with this metastable charge-trapped state being the “off” state of the luminescence blinking cycle

Kinetics of the Regeneration by Iodide of Dye Sensitizers Adsorbed on Mesoporous Titania

Regeneration of dye sensitizer molecules by reducing species contained in the electrolyte is a key mechanism in liquid dye-sensitized solar cells because it competes kinetically with a detrimental charge recombination process. Kinetics of the reduction by iodide ions of the oxidized states (S⁺) of two Ru^II complex dyes and four organic π-conjugated bridged donor–acceptor sensitizers were examined as a function of the electrolyte concentration. Results show that two different cases can be distinguished. A sublinear behavior of the regeneration rate and a plateau value reached at high bulk iodide concentrations were found for N820 ruthenium dye and interpreted as being due to an associative interaction involving the formation of (S⁺, I^–)···I^– surface complexes prior to the reaction. On the other hand, feeble reaction rates at low electrolyte concentrations and a superlinear behavior are observed predominantly for the organic dyes, pointing to a repulsive interaction between the dyed surface and iodide anions. At higher iodide bulk concentration, a linear behavior is reached, providing an estimate of a second-order rate constant. A correlation of these two opposite behaviors with the structure of the dye is observed, emphasizing the role of sulfur atoms in the association of I^– anions in the dye-sensitized layer. These findings allow for a better understanding of the dye–electrolyte interaction and of the effect of the iodide concentration on the photovoltaic performances of dye-sensitized solar cells

Strong Dependence of Quantum-Dot Delayed Luminescence on Excitation Pulse Width

Delayed luminescence involving charge-carrier trapping and detrapping has recently been identified as a widespread and possibly universal phenomenon in colloidal quantum dots. Its near-power-law decay suggests a relationship with blinking. Here, using colloidal CuInS₂ and CdSe quantum dots as model systems, we show that short (nanosecond) excitation pulses yield less delayed luminescence intensity and faster delayed luminescence decay than observed with long (millisecond) square-wave excitation pulses. Increasing the excitation power also affects the delayed luminescence intensity, but the delayed luminescence decay kinetics appear much less sensitive to excitation power than to excitation pulse width. An idealized four-state kinetic model reproduces the major experimental trends and highlights the very slow approach to steady state during photoexcitation, stemming from extremely slow detrapping of the metastable charge-separated state responsible for delayed luminescence. The impacts of these findings on proposed relationships between delayed luminescence and blinking are discussed

Strong Dependence of Quantum-Dot Delayed Luminescence on Excitation Pulse Width

Delayed luminescence involving charge-carrier trapping and detrapping has recently been identified as a widespread and possibly universal phenomenon in colloidal quantum dots. Its near-power-law decay suggests a relationship with blinking. Here, using colloidal CuInS₂ and CdSe quantum dots as model systems, we show that short (nanosecond) excitation pulses yield less delayed luminescence intensity and faster delayed luminescence decay than observed with long (millisecond) square-wave excitation pulses. Increasing the excitation power also affects the delayed luminescence intensity, but the delayed luminescence decay kinetics appear much less sensitive to excitation power than to excitation pulse width. An idealized four-state kinetic model reproduces the major experimental trends and highlights the very slow approach to steady state during photoexcitation, stemming from extremely slow detrapping of the metastable charge-separated state responsible for delayed luminescence. The impacts of these findings on proposed relationships between delayed luminescence and blinking are discussed