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PDの丁超さんが日中科学技術交流協会より、中国人留学生研究奨励賞を授与されました。
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昨年の張さんの受賞に続き、本研究室では2年連続の受賞となります。
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Fig. Correlation between QD center-to-center distance and charge transfer rate constants for MAA ligands treated films
Effect of the conduction band offset on interfacial recombination behavior of the planar perovskite solar cells

Understanding the behaviour of multiple exciton dissociation in quantum dot (QD) solid films is of fundamental interest and paramount importance for improving the performance of quantum dot solar cells (QDSCs). Unfortunately, the charge transfer behaviour of photogenerated multiple exciton in QD solid films is not clear to date. Herein, we systematically investigate the multiple exciton charge transfer behaviour in PbS QD solid films by using ultrafast transient absorption spectroscopy. We observe that the multiple exciton charge transfer rate within QD ensembles is exponentially enhanced as the interparticle distance between the QDs decreases. Biexciton and triexciton dissociation between adjacent QDs occurs via a charge transfer tunneling effect just like single exciton, and the charge tunneling constants of the single exciton (β1: 0.67 ± 0.02 nm−1), biexciton (β2: 0.68 ± 0.05 nm−1) and triexciton (β3: 0.71 ± 0.01 nm−1) are obtained. More importantly, for the first time, the interparticle distance limit (≤4.3 nm) for multiple exciton charge transfer between adjacent QDs is found for the extraction of multiple excitons rapidly before the occurrence of Auger recombination. This result points out a vital and necessary condition for the use of multiple excitons produced in PbS QD films, especially for their applications in QDSCs.
  

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Naoki Nakazawa, Yaohong Zhang, Feng Liu, Chao Ding, Kanae Hori, Taro Toyoda, Yingfang Yao, Yong Zhou, Shuzi Hayase, Ruixiang Wang, Zhigang Zoub and Qing Shen




Fig. Experimentally determined diagram of energy levels of Zn1-xMgxO films (x=0–0.20) and MAPbI3 film
Effect of the conduction band offset on interfacial recombination behavior of the planar perovskite solar cells

The effects of the conduction band offset (CBO) between the electron selective layer (ESL) and the perovskite layer in planar-heterojunction perovskite solar cells (PSCs) have been systematically investigated for the first time. To obtain different values of CBO, Magnesium doped zinc oxide (Zn1-xMgxO (ZMO)) thin films with a tunable conduction band energy level were employed as a model ESL in planar PSCs. We found that the charge recombination at the interface between the ESL and perovskite is strongly dependent on the CBO values: When the cliff structure is formed, i.e., when the conduction band minimum (CBM) of the ESL is lower than that of the perovskite, the interface recombination became dominant, and the open-circuit voltage (Voc) worsened. When the spike structure is formed, i.e., when the CBM of the ESL is higher than that of the perovskite, the interfacial recombination is largely suppressed, which leads to an increased Voc of the solar cells. Additionally, we found that an appropriate amount of Mg doping in ZnO to form ZMO reduced carrier concentration and improved carrier mobility, thereby enhancing the charge collection efficiency of the photoexcited electrons by the FTO electrode and, consequently, the short-circuit current density (Jsc). Using transient absorption (TA) measurements, we have revealed for the first time that the electron injection from photoexcited MAPbI3 to FTO through a ZMO compact layer occurs in the timescale of a few nanoseconds in planar PSCs. PSCs based on the optimized Zn0.9Mg0.1O-ESL exhibited a considerable increase (~35%) in power conversion efficiencies (PCE) compared with that of the control device.
  

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Chao Ding, Yaohong Zhang, Feng Liu, Yukiko Kitabatake, Shuzi Hayase, Taro Toyoda, Kenji Yoshino, Takashi Minemoto, Kenji Katayama and Qing Shen




Fig. SEM images of CdSe QD deposited IO-TiO2 with 5 cycles of ZnS passivation
Interface Passivation Effects on the Photovoltaic Performance of Quantum Dot Sensitized Inverse Opal TiO2 Solar Cells

Quantum dot (QD)-sensitized solar cells (QDSSCs) are expected to achieve higher energy conversion efficiency than traditional single-junction silicon solar cells due to the unique properties of QDs. An inverse opal (IO)-TiO2 (IO-TiO2) electrode is useful for QDSSCs because of its three-dimensional (3D) periodic nanostructures and better electrolyte penetration compared to the normal nanoparticles (NPs)-TiO2 (NPs-TiO2) electrode. We find that the open-circuit voltages Voc of the QDSSCs with IO-TiO2 electrodes are higher than those of QDSSCs with NPs-TiO2 electrodes. One important strategy for enhancing photovoltaic conversion efficiency of QDSSCs with IO-TiO2 electrodes is surface passivation of photoanodes using wide-bandgap semiconducting materials. In this study, we have proposed surface passivation on IO-TiO2 with ZnS coating before QD deposition. The efficiency of QDSSCs with IO-TiO2 electrodes is largely improved (from 0.74% to 1.33%) because of the enhancements of Voc (from 0.65 V to 0.74 V) and fill factor (FF) (from 0.37 to 0.63). This result indicates that ZnS passivation can reduce the interfacial recombination at the IO-TiO2/QDs and IO-TiO2/electrolyte interfaces, for which two possible explanations can be considered. One is the decrease of recombination at IO-TiO2/electrolyte interfaces, and the other one is the reduction of the back-electron injection from the TiO2 electrode to QDs. All of the above results are effective for improving the photovoltaic properties of QDSSCs.
  

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Kanae Hori, Yaohong Zhang, ID , Pimsiri Tusamalee, Naoki Nakazawa, Yasuha Yoshihara, Ruixiang Wang, Taro Toyoda, Shuzi Hayase and Qing Shen




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