Clara Aranda

Interfacial Degradation of Planar Lead Halide Perovskite Solar Cells

The stability of perovskite solar cells is one of the major challenges for this technology to reach commercialization, with water believed to be the major degradation source. In this work, a range of devices containing different cathode metal contacts in the configuration ITO/PEDOT:PSS/MAPbI3/PCBM/Metal are fully electrically characterized before and after degradation caused by steady illumination during 4 h that induces a dramatic reduction in power conversion efficiency from values of 12 to 1.8%. We show that a decrease in performance and generation of the S-shape is associated with chemical degradation of the metal contact. Alternatively, use of Cr2O3/Cr as the contact enhances the stability, but modification of the energetic profile during steady illumination takes place, significantly reducing the performance. Several techniques including capacitance-voltage, X-ray diffraction, and optical absorption results suggest that the properties of the bulk perovskite layer are little affected in the device degradation process. Capacitance-voltage and impedance spectroscopy results show that the electrical properties of the cathode contact are being modified by generation of a dipole at the cathode that causes a large shift of the flat-band potential that modifies the interfacial energy barrier and impedes efficient extraction of electrons. Ionic movement in the perovskite layer changes the energy profile close to the contacts, modifying the energy level stabilization at the cathode. These results provide insights into the degradation mechanisms of perovskite solar cells and highlight the importance to further study the use of protecting layers to avoid the chemical reactivity of the perovskite with the external contacts.

Surface Polarization Model for the Dynamic Hysteresis of Perovskite Solar Cells

The dynamic hysteresis of perovskite solar cells consists of the occurrence of significant deviations of the current density-voltage curve shapes depending on the specific conditions of measurement such as starting voltage, waiting time, scan rate, and other factors. Dynamic hysteresis is a serious impediment to stabilized and reliable measurement and operation of the perovskite solar cells. In this Letter, we formulate a model for the dynamic hysteresis based on the idea that the cell accumulates a huge quantity of surface electronic charge at forward bias that is released on voltage sweeping, causing extra current over the normal response. The charge shows a retarded dynamics due to the slow relaxation of the accompanying ionic charge, that produces variable shapes depending on scan rate or poling value and time. We show that the quantitative model provides a consistent description of experimental results and allows us to determine significant parameters of the perovskite solar cell for both the transient and steady-state performance.

On Mott-Schottky analysis interpretation of capacitance measurements in organometal perovskite solar cells

Capacitance response of perovskite-based solar cells (PSCs) can be exploited to infer underlying physical mechanisms, both in the materials bulk and at outer interfaces. Particularly interesting is applying the depletion layer capacitance theory to PSCs, following common procedures used with inorganic and organic photovoltaic devices. Voltage-modulation of the depletion layer width allows extracting relevant parameters as the absorber defect density and built-in potential by means of the Mott-Schottky (MS) analysis. However, the uncritical use of the MS technique may be misleading and yields incorrect outcomes as a consequence of masking effects that accumulation capacitances, commonly observed in PSCs, produce on the measured capacitance value. Rules are provided here to select the measuring frequency that allows extracting depletion layer capacitance, and the voltage range in which it dominates, avoiding accumulation capacitive parasitic contributions. It is noted that the distinction of the depletion c...

Coordination Chemistry Dictates the Structural Defects in Lead Halide Perovskites.

We show the influence of species present in precursor solution during formation of lead halide perovskite materials on the structural defects of the films. The coordination of lead by competing solvent molecules and iodide ions dictate the type of complexes present in the films. Depending on the processing conditions all PbIS5 (+) , PbI2 S4, PbI3 S3 (-) , PbI4 S2 (2-) , PbI5 S2 (3-) , PbI6 (4-) and 1D (Pb2 I4 )n chains are observed by absorption measurements. Different parameters are studied such as polarity of the solvent, concentration of iodide ions, concentration of solvent molecules and temperature. It is concluded that strongly coordinating solvents will preferentially form species with a low number of iodide ions and less coordinative solvents generate high concentration of PbI6 (-) . We furthermore propose that all these plumbate ions may act as structural defects determining electronic properties of the photovoltaic films.

Formation criteria of high efficiency perovskite solar cells under ambient conditions

The field of lead halide perovskites for solar cell applications has recently reported an impressive power conversion efficiency (PCE) above 21% using complex mixed cation formulations. Very importantly, the highest PCE has been obtained using totally dry environmental conditions thereby increasing the processing costs (i.e. use of a glovebox). In this work devices processed in air under different ambient conditions are prepared with a PCE approaching 19% for the simplest lead halide perovskite (MAPbI3, MA = methyl ammonium). It is shown that the PbI2 : MAI : additive complex needs to be generated in the correct stoichiometry where additives are any highly polar molecules that are able to stabilize the complex (i.e. H2O or dimethylsulphoxide (DMSO)). Under high humidity conditions H2O is incorporated into the complex and only small concentrations of further additives are needed. Precursor formulations not adequately balanced for the humidity conditions lead to films with poor morphology as evidenced by SEM. These films show negative multiiodide plumbate chemical defects as observed by absorbance measurements. These chemical defects act as recombination centers thereby reducing the photocurrent and fill factor in photovoltaic devices. In addition, the undesirable high conductivity of the perovskite hydrates (σ = 8 × 10−1 Scm−1), up to seven orders of magnitude higher than the pure MAPbI3, is shown, indicating that the presence of hydrates may act as shunting pathways that can significantly reduce the open circuit potential.

Effects of Frequency Dependence of the External Quantum Efficiency of Perovskite Solar Cells

Perovskite solar cells are known to show very long response time scales, on the order of milliseconds to seconds. This generates considerable doubt over the validity of the measured external quantum efficiency (EQE) and consequently the estimation of the short-circuit current density. We observe a variation as high as 10% in the values of the EQE of perovskite solar cells for different optical chopper frequencies between 10 and 500 Hz, indicating a need to establish well-defined protocols of EQE measurement. We also corroborate these values and obtain new insights regarding the working mechanisms of perovskite solar cells from intensity-modulated photocurrent spectroscopy measurements, identifying the evolution of the EQE over a range of frequencies, displaying a singular reduction at very low frequencies. This reduction in EQE is ascribed to additional resistive contributions hindering charge extraction in the perovskite solar cell at short-circuit conditions, which are delayed because of the concomitant large low-frequency capacitance.

Defect tolerant perovskite solar cells from blade coated non-toxic solvents

The process of crystallization of lead halide perovskites by industrially relevant techniques involving non-toxic solvents is not understood completely and needs improvement. To this date, devices with the highest efficiency are prepared by deposition of the perovskite layer using non-scalable techniques, toxic solvents and/or require additional processing steps. In this work, we show that perovskite solar cells can be obtained efficiently in one step by doctor blade. The perovskite film is formed under a supersaturation regime from non-toxic solvents following spherulitic growth. This method results in highly crystalline perovskite films with preferential crystal orientation. Co-local photoluminescence and light-beam induced current experiments show that the generated chemical defects are confined at the boundary of spherulites and do not have a negative effect on the extracted photocurrent. Strikingly, spherulite formation, rather than being detrimental, could lead to better photovoltaic performance in hybrid perovskite films. This is further confirmed in the photovoltaic devices, which were fabricated using non-toxic solvents by doctor blade process, with record efficiencies of 18.0% for MAPbI3 (MA = methyl ammonium). Moreover, devices with large area (1.53 cm2) fabricated using doctor blade show remarkable efficiencies (14.2%) reinforcing the viability of this solar technology towards industrialization.