Fig. Correlation between QD center-to-center distance and charge transfer rate constants for
MAA ligands treated films
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Effect of the conduction
band offset on interfacial recombination behavior of the planar perovskite solar
cells
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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.
[See more]
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
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Effect of the conduction
band offset on interfacial recombination behavior of the planar perovskite solar
cells
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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.
[See more]
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
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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.
[See more]
Kanae Hori, Yaohong Zhang, ID , Pimsiri Tusamalee, Naoki Nakazawa,
Yasuha Yoshihara, Ruixiang Wang, Taro Toyoda, Shuzi Hayase and Qing Shen
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