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Nature Communications (NPG), Vol. 7, 11574 (2016)

Accumulation of charges in photovoltaic cells perovskites : Solar cells that self-repair in the dark !

Light-activated photocurrent degradation and self-healing in perovskite solar cells

The "perovskite fever" that has taken hold of the photovoltaic community continues its momentum with record conversion rates exceeding 22% at the beginning of 2016. The problem of the stability of materials and performance over time is now essential to consider the transition from demonstration to industrial production. The two CNRS teams FOTON-OHM (J. Even) and ISCR-CTI (C. Katan), pioneers in the theoretical field on this subject, have recently joined several US laboratories to address these issues, notably with the Los Alamos National Laboratory (New Mexico) as part of the CINT program. The first joint work has just been accepted in the journal Nature Communication (W. Nie et al, 2016 DOI : 10.1038 / ncomms11574).

This work presents the first experimental demonstration and the detailed analysis of the self-healing of perovskite cells after a very slow degradation of photovoltaic yields under solar radiation. These cells are based on the three-dimensional perovskite lattice CH3NH3PbI3 material, at the origin of the decisive advances in the field since 2012. The various physicochemical processes that can be at the origin of the phenomena of slow degradation and self-healing are discussed.

The cells manufactured by the Los Alamos National Laboratory contain an exceptionally high quality CH3NH3PbI3 material ("wide grains", Science 2015). In the majority of cases, competing research teams observe degradations much faster and irreversible, the electrical characteristics being moreover tainted by a phenomenon of hysteresis. These irreversible degradations are linked to various processes associated with the presence of numerous grain boundaries, defects or even of poor quality interface. The excellent crystal quality of the CH3NH3PbI3 material obtained in this study, associated with the passivation of the interfaces thanks to the PCBM material, make it possible to delay the degradation of the photovoltaic efficiency over much longer time scales (of the order of the hour). This phenomenon of ultra-slow degradation, but reversible and in a(ultra-)fast way in the dark (<1 min), can be considered as an ultimate limitation related to the material itself. Moreover, this degradation depends strongly on the temperature because it is stopped at 0 °C. The polarization conditions of the cell (short circuit versus open circuit) also play a preponderant role. The authors of the article were thus led to consider an intrinsic physical process to explain all the phenomena.

In particular, Claudine KATAN (ISCR) and Jacky EVEN (FOTON) proposed an original interpretation based on the notion of a hybrid polaron in strong interaction with the inorganic and organic components of the CH3NH3PbI3 material. The vibrations of the inorganic crystal lattice and the disordered molecular rotations cooperate to slow down certain charge carriers produced by the photovoltaic effect. In a simplistic image of the phenomenon, the carriers of electrical charges are trapped by the deformations of the crystal lattice, a phenomenon accentuated by the cloud of molecules that surround it. The load is literally trapped in a spider’s web and moves with extreme slowness. The ultra-slow accumulation, over several tens of hours, of these charges would be at the origin of the degradations of performance, ending by disrupting the continuous flow of free charge carriers produced by photovoltaic effect. These free charges which constitute the electric current produced by the cell would be slowed down due to the presence of charged areas in the material. This theoretical hypothesis has the advantage of explaining the ultra-fast self-healing of the material and the solar cells, after a few minutes in the dark. The article presents experimental characterizations compatible with the existence of these « small » polarons, but further studies are still necessary.

The success of these studies is based on a multidisciplinary approach, combining physics, chemistry, materials science and technology. The first steps in putting these complementary skills available in the two ISCR and FOTON Research Units, notably through a transversal inter-UMR dynamic at the Rennes site, date back to 2010 and are now reflected in international recognition for its contribution to the understanding of the physics of « hybrid perovskites ».

Jacky EVEN, Fonctions Optiques pour les Technologies de l’informatiON (FOTON)
+33 2 23 23 82 95
Claudine KATAN, Institut des Sciences Chimiques de Rennes (ISCR)
+33 2 23 23 56 82

Light-activated photocurrent degradation and self-healing in perovskite solar cells, Wanyi Nie, Jean-Christophe Blancon, Amanda J. Neukirch, Kannatassen Appavoo, Hsinhan Tsai, Manish Chhowalla, Muhammad A. Alam, Matthew Y. Sfeir, Claudine Katan, Jacky Even, Sergei Tretiak, Jared J. Crochet, Gautam Gupta and Aditya D. Mohite. Nature communications, Vol. 7, 11574, may 2016, DOI : 10.1038/ncomms11574

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