Ultrafast selective extraction of hot holes from cesium lead iodide perovskite films

Lead halide perovskites have some unique properties which are very promising for optoelectronic applications such as solar cells. LEDs and lasers. One important and expected application of perovskite halide semiconductors is solar cell operation including hot carriers. This advanced solar cell concept allows overcoming the Shockley-Queisser efficiency limit, thereby achieving energy conversion efficiency as high as 66% by extracting hot carriers. Understanding ultrafast photoexcited carrier dynamics and extraction in lead halide perovskites is crucial for these applications. Here, we clarify the hot carrier cooling and transfer dynamics in all-inorganic cesium lead iodide （CsPbI3） perovskite using transient absorption spectroscopy and Al2O3, poly（3-hexylthiophene-2,5-diyl） （P3HT） and TiO2 as selective contacts. We find that slow hot carrier cooling occurs on a timescale longer than 10 ps in the cases of CsPbI3/AI203 and CsPbI3/TiO2, which is attributed to hot phonon bottleneck for the high photoexcited carrier density. An efficient ultrafast hole transfer from CsPbI3 to the P3HT hole extracting layer is observed. These results suggest that hot holes can be extracted by appropriate selective contacts before energy dissipation into the halide perovskite lattice and that CsPbl3 has a potential for hot carrier solar cell applications.

Photovoltaic Properties of CdSe Quantum Dot Sensitized Inverse Opal TiO₂Solar Cells: The Effect of TiCl₄Post Treatment

Recently, semiconductor quantum dot (QD) sensitized solar cells (QDSSCs) are expected to achieve higher conversion efficiency because of the large light absorption coefficient and multiple exciton generation in QDs. The morphology of TiO2 electrode is one of the most important factors in QDSSCs. Inverse opal (IO) TiO2 electrode, which has periodic mesoporous structure, is useful for QDSSCs because of better penetration of electrolyte than conventional nanoparticulate TiO2 electrode. In addition, the ordered three dimensional structure of IO-TiO2 would be better for electron transport. We have found that open circuit voltage Voc of QDSSCs with IO-TiO2 electrodes was much higher (0.2 V) than that with nanoparticulate TiO2 electrodes. But short circuit current density Jsc was lower in the case of IO-TiO2 electrodes because of the smaller surface area of IO-TiO2. In this study, for increasing surface area of IO-TiO2, we applied TiCl4 post treatment on IO-TiO2 and investigated the effect of the post treatment on photovoltaic properties of CdSe QD sensitized IO-TiO2 solar cells. It was found that Jsc could be enhanced due to TiCl4 post treatment, but decreased again for more than one cycle treatment, which indicates excess post treatment may lead to worse penetration of electrolyte. Our results indicate that the appropriate post treatment can improve the energy conversion efficiency of the QDSSCs.

Characterization of hot carrier cooling and multiple exciton generation dynamics in PbS QDs using an improved transient grating technique

Multiple exciton generation （MEG） dynamics in colloidal PbS quantum dots （QDs） characterized with an im- proved transient grating （TG） technique will be reported. Only one peak soon after optical absorption and a fast decay within 1 ps can be observed in the TG kinetics when the photon energy of the pump light hv is smaller than 2.7Eg （Eg： band gap between LUMO and HOMO in the QDs）, which corresponds to hot carrier cooling. When hv is greater than 2.7Eg, however, after the initial peak, the TG signal decreases first and soon increases, and then a new peak appears at about 2 to 3 ps. The initial peak and the new peak correspond to hot carriers at the higher excited state and MEG at the lowest excited state, respectively. By proposing a theoretical model, we can calculate the hot carrier cooling time constant and MEG occurrence time constant quantitatively. When MEG does not happen for hv smaller than 2.7Eg, hot carrier cools with a time con- stant of 400 fs. When MEG occurs for hv larger than 2.7Eg, hot carrier cools with a time constant as small as 200 fs, while MEG occurs with a time constant of 600 fs. The detailed hot carrier cooling and MEG occurrence dynamics characterized in this work would shed light on the further understanding of MEG mechanism of various type of semiconductor QDs.