Gold electrode mitigation impact: elucidation of both degradation and safeguard mechanisms in a mixed-ion perovskite solar device

Lara Perrin^a, Manon Spalla^a,b, Emilie Planès^a, Muriel Matheron^b, Solenn Berson^b, Lionel Flandin^a

^aUniv. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, Grenoble INP, LEPMI, 38000 Grenoble, France

^bUniv. Grenoble Alpes, CEA, LITEN, INES, 73375 Le Bourget du Lac, France

In the photovoltaic field, perovskite devices have already proved their potential to overcome the performance limits of current technologies and achieve low cost and high versatility. Several challenging developments are nowadays in progress on, for instance, low temperature processes for flexible devices compatibility, semi-transparent devices for tandem applications … Nevertheless, this technology is known to be sensitive to environmental factors as temperature, oxygen and humidity.

The degradation behaviour of a selected pertinent mixed-ion perovskite device has been here draft when submitted to incremental stress conditions (see adjacent figure). The presented device architecture “ITO/SnO₂/mixed-ion perovskite/PTAA/Au” is potentially compatible for both flexible and tandem applications. In order to find possible mitigation strategies, a careful study was conducted allowing to elucidate occurring degradation mechanisms thanks to an optimized characterization set composed of either imaging techniques (light beam induced current and photoluminescence), physico-chemical analyses (X-ray diffraction, UV-visible absorption and photoluminescence spectroscopy) and photovoltaic parameters measurements (power conversion and external quantum efficiencies).

As exposed on the presented figure (top view photographs of solar devices), it is possible to detect a strong difference in the perovskite layer already with an unaided eye, according to applied constraints (temperature only, additional air, followed by additional moisture). First of all, the talk will detail the degradation mechanisms highlighted using several complementary physico-chemistry characterizations. In particular, let us examine pictures presented on the right side (devices with gold electrode): the close yellow color resulting from conditions 1 and 3 were in fact proved to originate from a totally different downgrading product. Deeper imaging techniques were also employed and proved to be relevant to investigate the homogeneity of both the perovskite layer and its interfaces integrity.

Secondly, as evidenced on pictures on the left side (devices after top gold electrode delamination), it is obvious that gold plays a protective role against both air and moisture: degradation deceleration through a reduced permeation rate resulting in different perovskite layer chemical compositions and mechanical strengths. As regards condition 1 aging, at first sight it could seem obvious and rational that the electrode has no potential for protection against a sole temperature stress. However, advanced examinations brought to light a strong difference in the perovskite layer chemical evolution when scouting either under or apart the top electrode and here again the perovskite occurring degradation mechanism was found to be significantly lowered in the presence of gold electrode. This time gold has been proved to collaborate through an interaction with the hole transporting layer constituents. To give an idea of this significant contribution, devices were both aged with and without top electrode: completed devices loose 72% of photovoltaic efficiency after 500 h of aging at 85°C without gold, against 28% when aged with gold.