Cesium Enhances Long-Term Stability of Lead Bromide Perovskite-Based Solar Cells

Direct comparison between perovskite-structured hybrid organic–inorganic methylammonium lead bromide (MAPbBr₃) and all-inorganic cesium lead bromide (CsPbBr₃), allows identifying possible fundamental differences in their structural, thermal and electronic characteristics. Both materials possess a similar direct optical band gap, but CsPbBr₃ demonstrates a higher thermal stability than MAPbBr₃. In order to compare device properties, we fabricated solar cells, with similarly synthesized MAPbBr₃ or CsPbBr₃, over mesoporous titania scaffolds. Both cell types demonstrated comparable photovoltaic performances under AM1.5 illumination, reaching power conversion efficiencies of ∼6% with a poly aryl amine-based derivative as hole transport material. Further analysis shows that Cs-based devices are as efficient as, and more stable than methylammonium-based ones, after aging (storing the cells for 2 weeks in a dry (relative humidity 15–20%) air atmosphere in the dark) for 2 weeks, under constant illumination (at maximum power), and under electron beam irradiation

Molecular Length, Monolayer Density, and Charge Transport: Lessons from Al–AlOx/Alkyl–Phosphonate/Hg Junctions

A combined electronic transport–structure characterization of self-assembled monolayers (MLs) of alkyl–phosphonate (AP) chains on Al–AlOx substrates indicates a strong molecular structural effect on charge transport. On the basis of X-ray reflectivity, XPS, and FTIR data, we conclude that “long” APs (C14 and C16) form much denser MLs than do “short” APs (C8, C10, C12). While current through all junctions showed a tunneling-like exponential length-attenuation, junctions with sparsely packed “short” AP MLs attenuate the current relatively more efficiently than those with densely packed, “long” ones. Furthermore, “long” AP ML junctions showed strong bias variation of the length decay coefficient, β, while for “short” AP ML junctions β is nearly independent of bias. Therefore, even for these simple molecular systems made up of what are considered to be inert molecules, the tunneling distance cannot be varied independently of other electrical properties, as is commonly assumed

Impedance Spectroscopic Indication for Solid State Electrochemical Reaction in (CH₃NH₃)PbI₃ Films

Halide perovskite-based solar cells still have limited reproducibility, stability, and incomplete understanding of how they work. We track electronic processes in [CH₃NH₃]­PbI₃(Cl) (“perovskite”) films <i>in vacuo</i>, and in N₂, air, and O₂, using impedance spectroscopy (IS), contact potential difference, and surface photovoltage measurements, providing direct evidence for perovskite sensitivity to the ambient environment. Two major characteristics of the perovskite IS response change with ambient environment, viz. -1- appearance of negative capacitance <i>in vacuo</i> or post<i>-vacuo</i> N₂ exposure, indicating for the first time an electrochemical process in the perovskite, and -2- orders of magnitude decrease in the film resistance upon transferring the film from O₂-rich ambient atmosphere to vacuum. The same change in ambient conditions also results in a 0.5 V decrease in the material work function. We suggest that facile adsorption of oxygen onto the film dedopes it from n-type toward intrinsic. These effects influence any material characterization, i.e., results may be ambient-dependent due to changes in the material’s electrical properties and electrochemical reactivity, which can also affect material stability

Mobility–Lifetime Products in MAPbI₃ Films

Photovoltaic solar cells operate under steady-state conditions that are established during the charge carrier excitation and recombination. However, to date no model of the steady-state recombination scenario in halide perovskites has been proposed. In this Letter we present such a model that is based on a single type of recombination center, which is deduced from our measurements of the illumination intensity dependence of the photoconductivity and the ambipolar diffusion length in those materials. The relation between the present results and those from time-resolved measurements, such as photoluminescence that are commonly reported in the literature, is discussed

What Is the Mechanism of MAPbI₃ p‑Doping by I₂? Insights from Optoelectronic Properties

Obtaining insight into, and ultimately control over, electronic doping of halide perovskites may improve tuning of their remarkable optoelectronic properties, reflected in what appear to be low defect densities and as expressed in various charge transport and optical parameters. Doping is important for charge transport because it determines the electrical field within the semiconducting photoabsorber, which strongly affects collection efficiency of photogenerated charges. Here we report on intrinsic doping of methylammonium lead tri-iodide, MAPbI₃, as thin films of the types used for solar cells and LEDs, by I₂ vapor at a level that does not affect the optical absorption and leads to a small (<20 meV, ∼9 nm) red shift in the photoluminescence peak. This I₂ vapor treatment makes the films 10× more electronically conductive in the dark. We show that this change is due to p-type doping because we find their work function to increase by 150 mV with respect to the ionization energy (valence band maximum), which does not change upon I₂ exposure. The majority carrier (hole) diffusion length increases upon doping, making the material less ambipolar. Our results are well-explained by I₂ exposure decreasing the density of donor defects, likely iodide vacancies (V_I) or defect complexes, containing V_I. Invoking iodide interstitials, which are acceptor defects, seems less likely based on calculations of the formation energies of such defects and is in agreement with a recent report on pressed pellets

Light-Induced Increase of Electron Diffusion Length in a p–n Junction Type CH₃NH₃PbBr₃ Perovskite Solar Cell

High band gap, high open-circuit voltage solar cells with methylammonium lead tribromide (MAPbBr₃) perovskite absorbers are of interest for spectral splitting and photoelectrochemical applications, because of their good performance and ease of processing. The physical origin of high performance in these and similar perovskite-based devices remains only partially understood. Using cross-sectional electron-beam-induced current (EBIC) measurements, we find an increase in carrier diffusion length in MAPbBr₃(Cl)-based solar cells upon low intensity (a few percent of 1 sun intensity) blue laser illumination. Comparing dark and illuminated conditions, the minority carrier (electron) diffusion length increases about 3.5 times from <i>L</i>_n = 100 ± 50 nm to 360 ± 22 nm. The EBIC cross section profile indicates a p–n structure between the n-FTO/TiO₂ and p-perovskite, rather than the p–i–n structure, reported for the iodide derivative. On the basis of the variation in space-charge region width with varying bias, measured by EBIC and capacitance–voltage measurements, we estimate the net-doping concentration in MAPbBr₃(Cl) to be 3–6 × 10¹⁷ cm^–3

High-Work-Function Molybdenum Oxide Hole Extraction Contacts in Hybrid Organic–Inorganic Perovskite Solar Cells

We investigate the effect of high work function contacts in halide perovskite absorber-based photovoltaic devices. Photoemission spectroscopy measurements reveal that band bending is induced in the absorber by the deposition of the high work function molybdenum trioxide (MoO₃). We find that direct contact between MoO₃ and the perovskite leads to a chemical reaction, which diminishes device functionality. Introducing an ultrathin spiro-MeOTAD buffer layer prevents the reaction, yet the altered evolution of the energy levels in the methylammonium lead iodide (MAPbI₃) layer at the interface still negatively impacts device performance

Interface-Dependent Ion Migration/Accumulation Controls Hysteresis in MAPbI₃ Solar Cells

Hysteresis in the current–voltage characteristics of hybrid organic–inorganic perovskite-based solar cells is one of the fundamental aspects of these cells that we do not understand well. One possible cause, suggested for the hysteresis, is polarization of the perovskite layer under applied voltage and illumination bias, due to ion migration <i>within the perovskite</i>. To study this problem systemically, current–voltage characteristics of both regular (light incident through the electron conducting contact) and so-called inverted (light incident through the hole conducting contact) perovskite cells were studied at different temperatures and scan rates. We explain our results by assuming that the effects of scan rate and temperature on hysteresis are strongly correlated to ion migration within the device, with the rate-determining step being ion migration at/across the interfaces of the perovskite layer with the contact materials. By correlating between the scan rate with the measurement temperature, we show that the inverted and regular cells operate in different hysteresis regimes, with <i>different</i> activation energies of 0.28 ± 0.04 eV and 0.59 ± 0.09 eV, respectively. We suggest that the differences observed between the two architectures are due to different rates of ion migration close to the interfaces, and conclude that the diffusion coefficient of migrating ions in the inverted cells is 3 orders of magnitude higher than in the regular cells, leading to different accumulation rates of ions near the interfaces. Analysis of <i>V</i>_OC as a function of temperature shows that the main recombination mechanism is trap-assisted (Shockley-Read Hall, SRH) in the space charge region, similar to what is the case for other thin film inorganic solar cells