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.

Defects in compound semiconductors caused by molecular nitrogen.

The interaction of nitrogen molecules (N2) with the host lattice of compound semiconductors is investigated using first-principles density-functional calculations. In ZnO it is found that N2 causes localized states in the band gap either by forming an N2O molecule or by breaking a Zn-O bond. This mechanism contributes to the observed low nitrogen doping efficiency in ZnO. The appearance of localized states caused by N2 was also found in other semiconductors such as MgO and NaCl.

Hydrogen local vibrational modes in zinc oxide.

Using Raman backscattering spectroscopy we have observed six local vibrational modes in as-grown state-of-the-art nominally undoped zinc oxide single crystals. The local vibrational modes are located at nu=2854, 2890, 2918, 2948, 2988, and 3096 cm(-1). Some specimens were annealed up to 950 degrees C to remove hydrogen. A subsequent Raman backscattering measurement revealed that the local vibrational modes disappeared. This establishes that the observed local vibrational modes are caused by the presence of hydrogen in the ZnO crystals.