In spite of the impressive
progresses regarding perovskite-type
solar cells, a clear understanding about underlying mechanisms therein
is still sparse, especially because of the absence of spatially resolved
device characteristics which should be linked to exciton formation
efficiency, morphology, and crystallinity being estimated as functions
of positions within active layers. Here, the planar CH3NH3PbI3 (MAPbI3) perovskite solar
cells (PeSCs) with ZnO as the electron-transporting layer (ETL) were
fabricated. By varying the wide range of MAPbI3 active-layer
thickness, we estimate their device parameters and external quantum
efficiencies in addition to internal absorption spectra (Q) by means
of the transfer matrix method. Furthermore, the spectrally and spatially
resolved internal quantum efficiencies (IQEs) as a function of the
active-layer thickness within PeSCs were calculated, and the relationship
between IQE and device parameters extracted from the current–voltage
(J–V) behaviors was discussed.
It was found that the PeSC with MAPbI3 film thickness around
303 nm has a relatively high IQE and PCE, indicating that there is
more power loss of PeSCs when the thickness of the MAPbI3 layer is either less or more than about 300 nm. Furthermore, time-resolved
photoluminescence together with the thickness-dependent morphology
and crystallinity of the MAPbI3 film demonstrate that there
are two power loss processes in the fabricated PeSCs: one at the ZnO/MAPbI3 interface and the other in bulk. The present research is
beneficial for further understanding of the fundamental physics for
the PeSCs based on the ZnO ETL.
To achieve intrinsically light-weight flexible photovoltaic devices, a bulk-heterojunction-type active layer with a narrow-bandgap polymer is still considered as one of the most important candidates. Therefore, detailed information about the charge transfer efficiency from a photo-excited species on an electron-donating polymer to an electron acceptor is an important factor, given that it is among the most fundamental quantitative measures to understand the solar power conversion efficiency, in particular at the initial stage followed by primary exciton formation. To obtain accurate information in this regard, wide-range acceptor concentration-dependent transient absorption spectroscopy with femtosecond laser pulse excitation was performed using a representative narrow-bandgap polymer, commonly known as PTB7. The investigated acceptor concentration range covered was from 0.01 wt% up to 10 wt%, in addition to a 0 wt% pristine polymer sample and a sample with a conventional acceptor concentration of 60 wt%, which is important for high efficiency. From the kinetic data, an almost two orders of magnitude faster acceptor-induced charge transfer rate constant in addition to the native primary exciton lifetime of about 100 picoseconds could be extracted. These data were used to verify the suggested kinetic model and compare with device properties that show no meaningful loss during the extraction of photo-generated charge carriers.
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