ABSTRACT
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.
ABSTRACT
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.
