The stability of perovskite solar cells (PSCs) is currently a hot topic, but the investigation as well as the understanding of the degradation mechanisms remain incomplete. We present the intrinsic degradation mechanisms of a traditional monocation perovskite in photovoltaic devices performed with various electron transport layers (ETLs). The monocation perovskite material is a Cl-doped CH 3 NH 3 PbI 3 system, known to provide a favorable morphology leading to improved efficiency. With the long-term view of developing low-temperature processes for PSCs, two emerging ETLs compatible with TiO 2 substitution were chosen in order to study both the initial perovskite state and performance, along with their stability after aging. Aluminumdoped zinc oxide (AZO) and tin oxide (SnO 2 ) were thus selected and placed as the ETL in a planar NIP solar cell architecture, leading to different n-type substrates that can imply deviations in the formation and/or degradation of the perovskite layer. As a result, the overall performance and stability for the designed devices were strongly impacted using AZO as compared to SnO 2 . A detailed investigation using complementary characterization techniques helped in the understanding of the initial compositions and morphologies of the perovskite according to the underlying ETL layer used and their unalike evolution during mild aging conditions (inert atmosphere, dark, 35 °C). Infrared spectroscopy, X-ray diffraction, UV−visible absorption, photoluminescence, scanning electron microscopy, and current−voltage characteristics brought a new understanding of the local degradation mechanisms and their consequences on the macroscopic functional properties of PSC devices. Two different degradation mechanisms specific to the ETL have been distinguished. The ETL nature controls the perovskite microstructure and thereby the performance and stability of the complete device.
In the field of photovoltaics, perovskite solar cells have attracted great interest due to their high efficiency combined with a strong potential for low cost and good versatility. One of the main issues concerns the intrinsic stability of these cells. To develop mitigation strategies, there is a critical need for a better understanding of the most plausible degradation mechanisms. This work focuses on the impact of the hole transporting layer (HTL) on the stability of planar NIP perovskite solar cells based on MAPbI 3-x Cl x . From the comparison of two different HTLs (P3HT and PTAA), the crucial role of interfacial materials on the stability of a complete device is demonstrated. Even if PTAA-based devices presented better performances in the initial state, their degradation under mild aging conditions (35 °C, under dark and inert conditions) is more pronounced than that with the P3HT counterpart. Thanks to complementary characterization tools (infrared spectroscopy, X-ray diffraction, UV−visible absorption, and photoluminescence) applied to different stages of the stack assembly (with respectively three, four, or five layers), a degradation mechanism was identified at the perovskite−PTAA interface. These devices consist of several extremely thin layers; the interfaces play an important role on the performances and stability of the complete cells. It is a pioneering work in the community, which could be transposed to other devices and architectures.
This Article first addresses the need for intrinsic electroluminescence
and photoluminescence imaging characterizations. Both are always performed
in arbitrary units preventing system comparisons as a function of
chemistry, processing, aging time, or between different authors. We
propose a simple equation to extract the intrinsic contribution of
the sample in a broad range of measurement conditions. This work will
enable authors to vary the measurement features to optimize the outcomes
in resolution/saturation for photonic characterization. This method
will also help model the electroluminescence and photoluminescence
mechanisms with quantitative data. It should therefore be possible
to carry out a much broader and systematic study to help the future
development of solar cells. The developed quantification method was
applied to organic and perovskite solar cells. According to the selected
analysis, filters must be added to isolate the photovoltaic mechanisms
coming from the active layer itself from that of the active layer–electrode
interfaces. It was even possible to compare the organic and perovskite
technologies and, thereby, better appreciate their functioning principle
and its impact on the overall photovoltaic performances.
A significant current challenge for perovskite solar technology is succeeding in designing devices all by low temperature processes. This could help for both rigid devices industrialisation and flexible devices development. The depositions of nanoparticles from colloidal suspensions consequently emerge as attractive approaches, especially due to their potential for low temperature curing not only for the photoactive perovskite layer but also for charge transporting layers. Here, NIP solar cells based on aluminium doped zinc oxide (AZO) electron transport layer were fabricated using a low temperature compatible process for AZO deposition. For the extensively studied perovskites based on methylammonium lead halides (MAPbI3-xClx), the chloride/iodide equation is widely proposed to follow an optimal value corresponding to an introduced MAI:PbCl2 ratio of 3:1. However, the perovskite formulation should be considered as a key parameter for the optimization of power conversion efficiency when exploring new perovskite sub-layers. We here propose a systematic method for the structural determination of the optimal ratio. It may depend on the sublayer and results from structural changes around the optimal value. The functional properties gradually increase with the addition of chlorine as long as it remains intercalated in a single phase. Above the optimal ratio, the appearance of two phases degrades the system.
The stability of an uncharted FA0.95Cs0.05Pb(I0.83Br0.17)3 mixed perovskite
optimized for a low-temperature processable device architecture is
explored. The gold electrode was found to generate a safeguard mechanism.
Both electron- and hole-transporting layers, respectively, made of
SnO2 nanoparticles and a doped poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine]
polymer were selected for their compatibility with solution processes
and good final transparency. This allows a large range of possible
applications. Three aging conditions were chosen to decorrelate the
effects of temperature, oxygen, and humidity. A remarkable stability
of the system was evidenced with, for instance, a low power conversion
efficiency loss below 25% after 500 h at 85 °C in the dark. This
stability was however jeopardized with more severe aging conditions.
An in-depth study with microstructural, optical, optoelectronic characterizations,
combined with advanced imaging techniques, allowed to identify the
degradation mechanisms. We also showed that the perovskite experienced
a spatial distribution in the degradation intensity. This could be
attributed to an unanticipated protective effect of gold electrodes.
The latter could develop a complex with the corrosive tert-butylpyridine doping agent, preventing its diffusion and detrimental
consequences on the entire setup.
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