Costantino Mauro

Correlation between Electronic Defect States Distribution and Device Performance of Perovskite Solar Cells

Abstract In the present study, random current fluctuations measured at different temperatures and for different illumination levels are used to understand the charge carrier kinetics in methylammonium lead iodide CH3NH3PbI3‐based perovskite solar cells. A model, combining trapping/detrapping, recombination mechanisms, and electron–phonon scattering, is formulated evidencing how the presence of shallow and deeper band tail states influences the solar cell recombination losses. At low temperatures, the observed cascade capture process indicates that the trapping of the charge carriers by shallow defects is phonon assisted directly followed by their recombination. By increasing the temperature, a phase modification of the CH3NH3PbI3 absorber layer occurs and for temperatures above the phase transition at about 160 K the capture of the charge carrier takes place in two steps. The electron is first captured by a shallow defect and then it can be either emitted or thermalize down to a deeper band tail state and recombines subsequently. This result reveals that in perovskite solar cells the recombination kinetics is strongly influenced by the electron–phonon interactions. A clear correlation between the morphological structure of the perovskite grains, the energy disorder of the defect states, and the device performance is demonstrated.

Unravelling the low-temperature metastable state in perovskite solar cells by noise spectroscopy

The hybrid perovskite methylammonium lead iodide CH3NH3PbI3 recently revealed its potential for the manufacturing of low-cost and efficient photovoltaic cells. However, many questions remain unanswered regarding the physics of the charge carrier conduction. In this respect, it is known that two structural phase transitions, occurring at temperatures near 160 and 310 K, could profoundly change the electronic properties of the photovoltaic material, but, up to now, a clear experimental evidence has not been reported. In order to shed light on this topic, the low-temperature phase transition of perovskite solar cells has been thoroughly investigated by using electric noise spectroscopy. Here it is shown that the dynamics of fluctuations detect the existence of a metastable state in a crossover region between the room-temperature tetragonal and the low-temperature orthorhombic phases of the perovskite compound. Besides the presence of a noise peak at this transition, a saturation of the fluctuation amplitudes is observed induced by the external DC current or, equivalently, by light exposure. This noise saturation effect is independent on temperature, and may represent an important aspect to consider for a detailed explanation of the mechanisms of operation in perovskite solar cells.

Universal crossover of the charge carrier fluctuation mechanism in different polymer/carbon nanotubes composites

Carbon nanotubes added to polymer and epoxy matrices are compounds of interest for applications in electronics and aerospace. The realization of high-performance devices based on these materials can profit from the investigation of their electric noise properties, as this gives a more detailed insight of the basic charge carriers transport mechanisms at work. The dc and electrical noise characteristics of different polymer/carbon nanotubes composites have been analyzed from 10 to 300 K. The results suggest that all these systems can be regarded as random resistive networks of tunnel junctions formed by adjacent carbon nanotubes. However, in the high-temperature regime, contributions deriving from other possible mechanisms cannot be separated using dc information alone. A transition from a fluctuation-induced tunneling process to a thermally activated regime is instead revealed by electric noise spectroscopy. In particular, a crossover is found from a two-level tunneling mechanism, operating at low temperatures, to resistance fluctuations of a percolative network, in the high-temperature region. The observed behavior of 1/f noise seems to be a general feature for highly conductive samples, independent on the type of polymer matrix and on the nanotube density.

A noise model for the evaluation of defect states in solar cells

A theoretical model, combining trapping/detrapping and recombination mechanisms, is formulated to explain the origin of random current fluctuations in silicon-based solar cells. In this framework, the comparison between dark and photo-induced noise allows the determination of important electronic parameters of the defect states. A detailed analysis of the electric noise, at different temperatures and for different illumination levels, is reported for crystalline silicon-based solar cells, in the pristine form and after artificial degradation with high energy protons. The evolution of the dominating defect properties is studied through noise spectroscopy.

Kondo-like transport and magnetic field effect of charge carrier fluctuations in granular aluminum oxide thin films

Granular aluminum oxide is an attractive material for superconducting quantum electronics. However, its low-temperature normal state transport properties are still not fully understood, while they could be related to the unconventional phenomenon of the superconductivity in this material. In order to obtain useful information on this aspect, a detailed study of charge carrier fluctuations has been performed in granular aluminum oxide films. The results of electric noise measurements indicate the presence of a Kondo-type spin-flip scattering mechanism for the conducting electrons in the normal state, at low temperatures. Moreover, the magnetic field dependence of the noise amplitude suggests that interface magnetic moments are the main source of fluctuations. The identification of the nature of fluctuation processes is a mandatory requirement for the improvement of quality and performance of quantum devices.

Author Correction: Kondo-like transport and magnetic field effect of charge carrier fluctuations in granular aluminum oxide thin films

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