Formation, Distribution and Beneficial Role of PbI2 in Lead Halide Perovskite Solar Cells
Martyn McLachlan a c, Jiaqi Zhang a c, Tian Du a c, Claire Burgess a c, Jinhyun Kim b c, James Durrant b c
Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics
Proceedings of International Conference Asia-Pacific Hybrid and Organic Photovoltaics (AP-HOPV17)
Yokohama-shi, Japan, 2017 February 2nd - 4th
Organizers: Tsutomu Miyasaka and Iván Mora-Seró
Oral, Tian Du, presentation 069
Publication date: 7th November 2016

Herein we describe our work investigating the formation, distribution and beneficial role of lead iodide (PbI2) in methylammonium lead iodide (CH3NH3PbI3) photovoltaics. These devices have created major interest and excitement since they were first introduced a little more than 5-years ago. PbI2 secondary is often found in CH3NH3PbI3 primary phase in case of excessively thermally annealing the as-deposited films. Until now there has remained uncertainty over the role of PbI2 secondary phase, owing primarily to the uncertainty of the precise location of PbI2 in the lead halide perovskite active layers and the lack of understanding of how the PbI2 forms and in what quantity it is either beneficial or detrimental to device performance.

We use a simple thermal treatment to initiate PbI2 formation and combine X-ray diffraction, to monitor the formation of PbI2 as a function of temperature, and correlate the processing temperature with device performance measurements. We identify a temperature (130 °C) at which PbI2 is formed in the films and where excellent devices with low hysteresis are formed. Using high-resolution transmission electron microscopy (HRTEM) to probe the microstructure and composition of the films we show for the first time that PbI2 is formed in the grain boundary regions of the CH3NH3PbI3 films and that growth is occurring from the compact TiO2 electron transport layer through the film. Finally using transient absorption techniques we show, again for the first time, the presence of a photobleaching peak that we have unambiguously correlated with the thermally grown PbI2.

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