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Photovoltaic interfases

Photovoltaic interfases

Hybrid organic-inorganic perovskites have changed drastically the photovoltaic field by 2010 due to their low cost and their high light absorption. This perovskites have a structure with MX62- inorganic octahedra, where M is a divalent metal (often Pb2+) and X is a halogen. In the interstices among octahedra organic molecules are hosted. Since these perovskites have been proposed for photovoltaic in 2009 with an efficiency of 3,8% at the time, their efficiency has been multiplied by more than a factor five in only some years ! This technological success can certainly be improved with a better comprehension of their fundamental properties, as the experimental determination of the electronic structure, which controls the optical absorption and the charge carrier transport, or the dynamical response to photoexcitation.

Band structure and tetragonal/cubic phase transition

One of the most promising perovskites for solar cells is CH3NH3PbI3, MAPbI3 or MAPI. Its experimental band structure was rather unknown until our measurements with wavevector resolution. Moreover, our results show that the photoemission spectral weight follows the periodicity of the high temperature cubic phase (above 50°C) even when the system is structurally in a tetragonal phase. This explains the insensitivity of solar cells towards the tetragonal/cubic transition, despite the transition temperature being in the operation temperature range.

MAPI band structure with wave vector resolution. The measurement was done at 170 K in the tetragonal phase. The spectral weight follows the cubic symmetry instead of the tetragonal cone. Figure from Min-I Lee et al. J. Phys. D : Appl. Phys. 50, 26LT02 (2017).

Sub-picosecond electronic relaxation dynamics

The electronic relaxation dynamics is particularly important in solar cells. If the charge carriers relax fast, they will not reach the external circuit, reducing therefore the photovoltaic efficiency. Two-photon photoemission allows to study the relaxation dynamics and to simulate the absorption of a solar photon. A first photon excites the systems and populates the conduction band. Afterwards, a second photon, with a temporal delay with respect to the first one, studies the relaxation dynamics. Our measurements indicate, among other results, that defects trap electrons within few picoseconds, in agreement with the efficiency reduction in aged hybrid perovskite solar cells.

Photoelectron intensity map as a function of kinetic energy and pump-probe delay for (a) an as-grown sample, and samples annealed at (b) 100°C and (c) 200°C. Figure from Z. Chen et al., Phys. Rev. Mat. 1, 045402 (2017). Copyright 2017 by American Physical Society.