ABSTRACT
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ABSTRACT
We report significant improvements in the optoelectronic properties of lead halide perovskites with the addition of monovalent ions with ionic radii close to Pb2+. We investigate the chemical distribution and electronic structure of solution processed CH3NH3PbI3 perovskite structures containing Na+, Cu+, and Ag+, which are lower valence metal ions than Pb2+ but have similar ionic radii. Synchrotron X-ray diffraction reveals a pronounced shift in the main perovskite peaks for the monovalent cation-based films, suggesting incorporation of these cations into the perovskite lattice as well as a preferential crystal growth in Ag+ containing perovskite structures. Furthermore, the synchrotron X-ray photoelectron measurements show a significant change in the valence band position for Cu- and Ag-doped films, although the perovskite bandgap remains the same, indicating a shift in the Fermi level position toward the middle of the bandgap. Such a shift infers that incorporation of these monovalent cations dedope the n-type perovskite films when formed without added cations. This dedoping effect leads to cleaner bandgaps as reflected by the lower energetic disorder in the monovalent cation-doped perovskite thin films as compared to pristine films. We also find that in contrast to Ag+ and Cu+, Na+ locates mainly at the grain boundaries and surfaces. Our theoretical calculations confirm the observed shifts in X-ray diffraction peaks and Fermi level as well as absence of intrabandgap states upon energetically favorable doping of perovskite lattice by the monovalent cations. We also model a significant change in the local structure, chemical bonding of metal-halide, and the electronic structure in the doped perovskites. In summary, our work highlights the local chemistry and influence of monovalent cation dopants on crystallization and the electronic structure in the doped perovskite thin films.
ABSTRACT
We explore the degradation behaviour under continuous illumination and direct oxygen exposure of inverted unencapsulated formamidinium(FA)0.83Cs0.17Pb(I0.8Br0.2)3, CH3NH3PbI3, and CH3NH3PbI3-xClx perovskite solar cells. We continuously test the devices in-situ and in-operando with current-voltage sweeps, transient photocurrent, and transient photovoltage measurements, and find that degradation in the CH3NH3PbI3-xClx solar cells due to oxygen exposure occurs over shorter timescales than FA0.83Cs0.17Pb(I0.8Br0.2)3 mixed-cation devices. We attribute these oxygen-induced losses in the power conversion efficiencies to the formation of electron traps within the perovskite photoactive layer. Our results highlight that the formamidinium-caesium mixed-cation perovskites are much less sensitive to oxygen-induced degradation than the methylammonium-based perovskite cells, and that further improvements in perovskite solar cell stability should focus on the mitigation of trap generation during ageing.
ABSTRACT
Metal halide perovskites are of great interest for various high-performance optoelectronic applications. The ability to tune the perovskite bandgap continuously by modifying the chemical composition opens up applications for perovskites as coloured emitters, in building-integrated photovoltaics, and as components of tandem photovoltaics to increase the power conversion efficiency. Nevertheless, performance is limited by non-radiative losses, with luminescence yields in state-of-the-art perovskite solar cells still far from 100 per cent under standard solar illumination conditions. Furthermore, in mixed halide perovskite systems designed for continuous bandgap tunability (bandgaps of approximately 1.7 to 1.9 electronvolts), photoinduced ion segregation leads to bandgap instabilities. Here we demonstrate substantial mitigation of both non-radiative losses and photoinduced ion migration in perovskite films and interfaces by decorating the surfaces and grain boundaries with passivating potassium halide layers. We demonstrate external photoluminescence quantum yields of 66 per cent, which translate to internal yields that exceed 95 per cent. The high luminescence yields are achieved while maintaining high mobilities of more than 40 square centimetres per volt per second, providing the elusive combination of both high luminescence and excellent charge transport. When interfaced with electrodes in a solar cell device stack, the external luminescence yield-a quantity that must be maximized to obtain high efficiency-remains as high as 15 per cent, indicating very clean interfaces. We also demonstrate the inhibition of transient photoinduced ion-migration processes across a wide range of mixed halide perovskite bandgaps in materials that exhibit bandgap instabilities when unpassivated. We validate these results in fully operating solar cells. Our work represents an important advance in the construction of tunable metal halide perovskite films and interfaces that can approach the efficiency limits in tandem solar cells, coloured-light-emitting diodes and other optoelectronic applications.
ABSTRACT
We present a proof of concept demonstration of a novel optical element: a light-responsive aperture that can track a moving light beam. The element is created using a thermally-activated transparency-switching material composed of paraffin wax and polydimethylsiloxane (PDMS). Illumination of the material with a focused beam causes the formation of a localized transparency at the focal spot location, due to local heating caused by absorption of a portion of the incident light. An application is proposed in a new design for a self-tracking solar collector.
ABSTRACT
The incorporation of noble metal nanoparticles, displaying localized surface plasmon resonance, in the active area of donor-acceptor bulk-heterojunction organic photovoltaic devices is an industrially compatible light trapping strategy, able to guarantee better absorption of the incident photons and give an efficiency improvement between 12% and 38%. In the present work, we investigate the effect of Au and Ag nanoparticles blended with P3HT: PCBM on the P3HT crystallization dynamics by synchrotron grazing incidence X-ray diffraction. We conclude that the presence of (1) 80 nm Au, (2) mix of 5 nm, 50 nm, 80 nm Au, (3) 40 nm Ag, and (4) 10 nm, 40 nm, 60 nm Ag colloidal nanoparticles, at different concentrations below 0.3 wt% for Au and below 0.1% for Ag in P3HT: PCBM blends, does not affect the behaviour of the blends themselves.
ABSTRACT
The growth-kinetics of [6,6]-phenyl C61-butyric acid methyl ester (PCBM) crystals, on two different length-scales, is shown to be controlled by the thickness of the polymer layer within a PCBM-polymer bilayer. Using a model amorphous polymer we present evidence, from in situ optical microscopy and grazing-incidence X-ray diffraction (GIXD), that an increased growth-rate of nanoscale crystals impedes the growth of micron-sized, needle-like PCBM crystals. A combination of neutron reflectivity and GIXD measurements, also allows us to observe the establishment of a liquid-liquid equilibrium composition-profile between the PCBM layer and a polymer-rich layer, before crystallization occurs. While the interfacial composition-profile is independent of polymer-film-thickness, the growth-rate of nanoscale PCBM crystals is significantly larger for thinner polymer films. A similar thickness-dependent behavior is observed for different molecular weights of entangled polymer. We suggest that the behavior may be related to enhanced local-polymer-chain-mobility in nanocomposite thin-films.
ABSTRACT
Grazing incidence X-ray diffraction (GI-XRD) is used to characterize the crystallographic dynamics of low molecular weight (LMW) and high molecular weight (HMW) poly(3-hexylselenophene) (P3HS) films and blend films of P3HS with [6-6-]-phenyl-C(61) -butyric acid methyl ester (PCBM) as a function of 'step-by-step' thermal annealing, from room temperature to 250 °C. The temperature-dependent GIXRD data show how the melting point of P3HS crystallites is decreased by the presence of PCBM. P3HS crystallite domain sizes dramatically increase upon annealing to the P3HS melting temperature. The formation of well-oriented HMW P3HS crystallites with the (100) plane parallel to the substrate (edge-on orientation), when cooled from melt, are observed. We compare the behaviour of P3HS pure and blend films with that of poly(3-hexyl)thiophene (P3HT) pure and PCBM blended films and suggest that the similar temperature dependent behaviour we observe may be a common to polythiophene and related polymers and their blends.