Lead halide perovskite solar cells have shown a tremendous rise in power conversion efficiency with reported record efficiencies of over 20% making this material very promising as a low cost alternative to conventional inorganic solar cells. However, due to a differently severe "hysteretic" behaviour during current density-voltage measurements, which strongly depends on scan rate, device and measurement history, preparation method, device architecture, etc., commonly used solar cell measurements do not give reliable or even reproducible results. For the aspect of commercialization and the possibility to compare results of different devices among different laboratories, it is necessary to establish a measurement protocol which gives reproducible results
Despite the high efficiency of over 21% reported for emerging thin film perovskite solar cells, one of the key issues prior to their commercial deployment is to attain their long term stability under ambient and outdoor conditions. The instability in perovskite is widely conceived to be humidity induced due to the water solubility of its initial precursors, which leads to decomposition of the perovskite crystal structure; however, we note that humidity alone is not the major degradation factor and it is rather the photon dose in combination with humidity exposure that triggers the instability. In our experiment, which is designed to decouple the effect of humidity and light on perovskite degradation, we investigate the shelf-lifetime of CH3NH3PbI3 films in the dark and under illumination under high humidity conditions (Rel. H. > 70%). We note minor degradation in perovskite films stored in a humid dark environment whereas upon exposure to light, the films undergo drastic degradation, primarily owing to the reactive TiO2/perovskite interface and also the surface defects of TiO2. To enhance its air-stability, we incorporate CH3NH3PbI3 perovskite in a polymer (poly-vinylpyrrolidone, PVP) matrix which retained its optical and structural characteristics in the dark for ∼2000 h and ∼800 h in room light soaking, significantly higher than a pristine perovskite film, which degraded completely in 600 h in the dark and in less than 100 h when exposed to light. We attribute the superior stability of PVP incorporated perovskite films to the improved structural stability of CH3NH3PbI3 and also to the improved TiO2/perovskite interface upon incorporating a polymer matrix. Charge injection from the polymer embedded perovskite films has also been confirmed by fabricating solar cells using them, thereby providing a promising future research pathway for stable and efficient perovskite solar cells.
engineering has shown to play an essential role to
optimize recombination losses in perovskite solar cells; however,
an in-depth understanding of the various loss mechanisms is still
underway. Herein, we study the charge transfer process and reveal
the primary recombination mechanism at inorganic electron-transporting
contacts such as TiO2 and its modified organic rivals.
The modifiers are chemically ([6,6]-phenyl C61 butyric
acid, PC60BA) or physically ([6,6]-phenyl C61 butyric acid methyl ester, PC60BM and C60)
attached fullerene to the TiO2 surface to passivate the
density of surface states. We do not observe any change in morphology,
crystallinity, and bulk defect density of halide perovskite (CH3NH3PbI3 in this case) upon interface
modification. However, we observe compelling results via photoluminescence
and electroluminescence studies that the recombination dynamics at
both time scales (slow and fast) are largely influenced by the choice
of the selective contact. We note a strong correlation between the
hysteresis and the so-called slow charge dynamics, both significantly
influenced by the characteristics of the selective contact, for example,
the presence of surface traps at the selective contact not only shows
a larger hysteresis but also leads to higher charge accumulation at
the interface and distinguishable slow dynamics (a slower stabilization
of recombination dynamics at a time scale of several minutes).
ZnO is a widely used metal-oxide semiconductor for photovoltaic application. In solar cell heterostructures they not only serve as a charge selective contact, but also act as electron acceptor. Although ZnO offers a suitable interface for exciton dissociation, charge separation efficiencies have stayed rather poor and conceptual differences to organic acceptors are rarely investigated. In this work, we employ Sn doping to ZnO nanowires in order to understand the role of defect and surface states in the charge separation process. Upon doping we are able to modify the metal-oxide work function and we show its direct correlation with the charge separation efficiency. For this purpose, we use the polymer poly(3-hexylthiophene) as donor and the squaraine dye SQ2 as interlayer. Interestingly, neither mobilities nor defects are prime performance limiting factor, but rather the density of available states around the conduction band is of crucial importance for hybrid interfaces. This work highlights crucial aspects to improve the charge generation process of metal-oxide based solar cells and reveals new strategies to improve the power conversion efficiency of hybrid solar cells.
Abstract:In this work, we describe the role of the different layers in perovskite solar cells to achieve reproducible,~16% efficient perovskite solar cells. We used a planar device architecture with PEDOT:PSS on the bottom, followed by the perovskite layer and an evaporated C 60 layer before deposition of the top electrode. No high temperature annealing step is needed, which also allows processing on flexible plastic substrates. Only the optimization of all of these layers leads to highly efficient and reproducible results. In this work, we describe the effects of different processing conditions, especially the influence of the C 60 top layer on the device performance.
the intense research effort on metal halide perovskites,
the fundamental correlation between the crystal structure and optoelectronic
properties remains unclear. As many intriguing phenomena are expected
to be based on the dipolar character of the rotating organic cations,
an improved understanding of the material’s polarizability
is of high relevance. Here, we study the orthorhombic–tetragonal
phase transition of methylammonium lead iodide to gain insight into
the polarization mechanism at low temperatures and the resulting effects
on solar cell performances. Using thermally stimulated current measurements,
we detect polarization currents when cooling or heating across the
phase transition. These (de)polarization currents are found to correlate
with a sudden change in rotational freedom of the organic cations,
with a temperature range of 20 K separating polarizing and depolarizing
processes. The nature of this cation polarization within the orthorhombic
phase is investigated with respect to its intrinsic and extrinsic
polarizability. Although the amount of unamplified polarization is
determined to be only in the order of a few nC/cm2, our
results show significant increases in solar cell performances upon
polarization, highlighting the intimate relationship between the alignment
of organic cations and optoelectronic properties in metal halide perovskites.
We report a procedure for large scale, reproducible and fast synthesis of polycrystalline, dense, vertically aligned α-MoO3 nanostructures on conducting (FTO) and non-conducting substrates (Si/SiO2) by using a simple, low-cost hydrothermal technique.
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