Iodine-Doping Effects on MAPbI3 Charge Transport
Igal Levine a, Arava Zohar a, Satyajit Gupta, David Cahen
Proceedings of Perovskite Thin Film Photovoltaics (ABXPV17)
València, Spain, 2017 March 1st - 2nd
Organizers: Henk Bolink and David Cahen
Oral, Arava Zohar, presentation 049
Publication date: 18th December 2016

Halide perovskite (HaP)-based photovoltaic (PV) devices have reached >21% small area power conversion efficiency. However, HaP layers still suffer from limited basic knowledge of the effects of their defect chemistry and physics on their PV behavior and in particular the charge transport mechanism(s). Understanding the electronic, ionic and in particular the coupled electronic-ionic processes involved in the opto-electronic characteristics for the films, used for devices, can provide guidance for improvements in cell preparation and device performance.

Work function measurements on MAPbI3 layers show that changing the atmosphere in which the sample is stored / measured, affects its Fermi level position. The Fermi level position in the forbidden gap will influence charge recombination and Voc values of PV cells, made with those layers. Surface potentials of  MAPbI3 layers that were exposed to high vacuum (HV; ~10-6 mbar), O2, humid N2 , or to  I2 in dry inert carrier gas environment were characterized by Kelvin probe, measurements of the contact potential difference (CPD), UPS/ XPS and I-V measurements. During O2 exposure the WF increased by about 100 mV toward p-type. This effect is fully reversible. Humid N2 causes a dramatic increase of MAPbI3 film resistance up to a point that the film behaves as a capacitor. MAPbI3 layers which were exposed to I2 were doped permanently to 4.8eV, ~150 mV shift towards the VB. We postulate that the I2 decreases the density of I vacancies [VI-] in the lattice. The electrical characteristics of TiO2 / MAPbI3 / Spiro-OMeTAD/ Au PV cell, decrease after I2 doping which may hint at a positive role of ionic conduction. 

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