In situ
TEM
analysis of structural changes in metal‐halide perovskite solar cells under electrical bias
Authors: Jeangros Quentin , Duchamp Martial , Werner Jérémie , Dunin‐Borkowski Rafal , Niesen Björn , Ballif Christophe , Hessler‐Wyser Aïcha 
Published: December 2016

Organic‐inorganic metal‐halide perovskite solar cells are emerging as a promising photovoltaic technology to harvest solar energy, with latest efficiencies now surpassing 22%
1
‐ an impressive increase from the first reported value of 3% in 2009.
2
In addition to low manufacturing costs, the optical properties of such cells can be tailored to form efficient tandems when combined with high‐efficiency silicon solar cells.
3
A typical perovskite cell structure as investigated here is based on a methylammonium lead trihalide absorber (MAPbI
3
) that is placed between hole‐ (Spiro
‐
OMeTAD) and electron‐selective contacts (a fullerene‐based material).

While new record efficiencies are frequently reported, the commercial application of this solar cell technology remains hindered by issues related to thermal and operational stability. Different mechanisms that are still debated modify cell properties with time, temperature, illumination and general operating conditions.
4
In order to correlate applied voltage (
V
) and resulting current (
I
) to changes in active layer chemistry and structure on the nanometre scale, we performed both
ex situ
and
in situ
transmission electron microscopy (TEM) experiments, involving (scanning) TEM (STEM) imaging, selected‐area electron diffraction, energy‐dispersive X‐ray spectroscopy and electron energy‐loss spectroscopy. Samples were prepared by focused ion beam (FIB) milling, with exposure to air during transfer to the TEM minimised to <5 minutes to reduce any degradation of MAPbI
3
.

First, the effects of exposure to air and electron beam irradiation were assessed in relation to FIB final thinning parameters. Once adequate sample preparation and observation conditions were identified, changes in morphology during cell characterisation were assessed
ex situ
by comparing lamellae extracted from as‐manufactured and tested cells and then
in situ
by contacting FIB‐prepared samples to a microelectromechanical systems (MEMS) chip mounted in a TEM specimen holder
5
(Fig. 1a). Cell manufacturing parameters led to iodine diffusion into the hole collector, with the width of this diffused layer remaining constant during
I
‐
V
characterisation. Similarly to
ex situ
experiments, the MAPbI
3
/Spiro interface was observed to delaminate during
in situ
electrical measurements, resulting in the presence of a ~5 nm Pb‐rich layer on the hole‐transparent‐layer side (Figs. 1b‐c). In addition, PbI
2
nanoparticles were observed to nucleate within the MAPbI
3
layer at the hole‐collector interface and at the positions of structural defects (Figs. 1b‐d).

Overall, the active MAPbI
3
layer was observed to be sensitive to sample preparation, exposure to air, observation conditions and
I
‐
V
stimulus, resulting in the need for great care to deconvolute each effect. Different mechanisms that may all contribute to the decrease in efficiency of the cell were identified both
ex situ
and
in situ
, including ionic migration, PbI
2
formation and local delamination of interfaces.