Abstract
The power conversion efficiency of perovskite solar cells (PSCs) has risen steadily in recent years; however, one important aspect of the puzzle remains to be solved—the long-term stability of the devices. We believe that understanding the underlying reasons for the observed instability and finding means to circumvent it is crucial for the future of this technology. Not only the perovskite itself but also other device components are susceptible to thermal degradation, including the materials comprising the hole-transporting layer. In particular, the performance-enhancing oxidized hole-transporting materials have attracted our attention as a potential weak component in the system. Therefore, we performed a series of experiments with oxidized spiro-OMeTAD to determine the stability of the material interfaced with five most popular perovskite compositions under thermal stress. It was found that oxidized spiro-OMeTAD is readily reduced to the neutral molecule upon interaction with all five perovskite compositions. Diffusion of iodide ions from the perovskite layer is the main cause for the reduction reaction which is greatly enhanced at elevated temperatures. The observed sensitivity of the oxidized spiro-OMeTAD to ion diffusion, especially at elevated temperatures, causes a decrease in the conductivity observed in the doped films of spiro-OMeTAD, and it also contributes significantly to a drop in the performance of PSCs operated under prolonged thermal stress.
Highlights
Perovskite solar cells (PSCs) have shown an impressive increase in device efficiency from 3.8 to 25.5% in recent years.[1]The observed improvement is due to a combination of advantageous properties, such as long diffusion lengths for electrons and holes, high absorption coefficient, low material cost,[2−4] and relative simplicity in the device fabrication.[5]
As mentioned in the Introduction, the interaction of oxidized spiro-OMeTAD with iodide ions migrating from MAPI at elevated temperatures leads to dedoping of the hole-transporting materials (HTMs), which is a major problem for the long-term stability of MAPI based PSCs
We decided to perform a series of experiments under different conditions to investigate the long-term stability of the oxidized HTM when interfaced with these new perovskites
Summary
Perovskite solar cells (PSCs) have shown an impressive increase in device efficiency from 3.8 to 25.5% in recent years.[1]. Diffusion of the mobile ions into the charge-transporting layers (CTLs) is another potential problem that is magnified at elevated temperatures.[30] It has been shown that the migration of these ions can negatively affect the long-term stability of the PSC devices.[31−35] It has been reported that halides such as iodine and bromine are among the main culprits in elemental diffusion The migration of these mobile ions beyond the perovskite/CTL interfaces can lead to an undesirable reaction between mobile ions and the CTL materials or metal electrode.[34,36,37]. Due to the complex nature of PSC devices and the multitude of possible processes occurring simultaneously, we have chosen to focus mostly on a specific problem of spiro-OMeTAD dedoping during the interaction between perovskite and oxidized spiro-OMeTAD in thin films at elevated temperatures, rather than attempting to evaluate the entire system at once
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