Heat-induced metal migration damages perovskite solar cells
Although perovskite solar cells (PSCs) are less expensive, easier to manufacture, and more efficient than most modern alternatives, their propensity to lose efficiency makes commercialization something of a mystery. Now, a team from École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland has demonstrated how metal contacts in the cells are one culprit behind this degradation.
“Perovskites are a new technology, but they’ve achieved high power conversion efficiencies [22%] much faster than other materials. That’s why they are promising and exciting,” says Juan Pablo Correa Baena of EPFL, who supervised the project and worked alongside PhD candidate Konrad Domanski in Anders Hagfeldts’ and Michael Graetzel’s groups. “The question is whether the device can remain stable and withstand long-term use.”
PSCs comprise hybrid organic–inorganic materials. A lead- or tin-based material acts as the light harvesting active layer. On top of this, an organic “hole-transporting” layer interfaces with the electrode. The electrode, typically a metal contact, is made mainly of gold or silver. Until now, stability studies have suggested that the organic component or perovskite itself might be responsible for the cells’ rapid efficiency loss over time. Other studies observed different levels of degradation for different electrode materials, but did not explain why. Little attention has been given to the behavior of the electrode metal in contact with the organic hole transport layer.
The EPFL team wanted to explain the efficiency decrease that was observed when testing a gold contact device under full illumination and power transmission. To do so, the researchers controlled the local temperature of the PSC itself. When heated to 70oC, it rapidly dropped to 20% of its original power conversion efficiency. The experiment was repeated in light and dark environments, with the same results.
“That was surprising,” Correa Baena says. “People have been unknowingly heating their samples during testing. We’ve connected the dots by finding out what’s happening, but we don’t yet understand the mechanism of metal migration to the perovskite layer.”
By controlling PSC temperatures in situ, the team demonstrated that heating causes the degradation. Critically, this disentangled the effect of unintentionally heating a sample due to illumination. Scanning electron microscope (SEM) images of the top electrode showed the layer that separates the gold contact from the perovskite had become porous when heated.
Next, the researchers added “buffer” layers of alumina nanoparticles or chromium between the gold contact and the perovskite/hole transport layers. Doing so hindered the interaction between gold and perovskite. This led to the conclusion that the performance degradation is due to gold reacting with perovskite creating non-radiative recombination centers which, in turn, causes the loss of photogenerated electrons.
Michael McGehee, professor in Stanford University’s Materials Science & Engineering Department, who was not involved in the research, calls the study “very important...because perovskite solar cells can be [easily] processed and have extraordinary power conversion efficiency, but typically have poor stability. This manuscript provides valuable insight on why the stability in many of the cells is poor.” McGehee’s recent work has shown that replacing the metal contact in a PSC with indium tin oxide improves stability by several orders of magnitude, consistent with Correa Baena’s study.
Juan Bisquert, professor of applied physics at Universitat Jaume I de Castelló, Spain, who was also not involved in this work, agrees that work is significant. “It alerts researchers that perovskite solar cells may be plagued by inter-diffusion phenomena, since the structure is so open.” An important trend will be to develop specific barriers that secure each piece of material in its place, especially in the contacts regions.
The take-home message, however, is that researchers need to be careful about the temperatures at which they test PSCs for stability. The non-reactive metal Au is in fact not a stable electrode under operational conditions, and heat-induced metal ion migration causes degradation in the device. Further studies will explore the effects of different material compositions, and hopefully elucidate the mechanism.