Perovskite solar cells: What do you mean when you say “stable”

With record efficiencies above 26% for single junction and above 34% for silicon–perovskite tandem, perovskite solar cells (PSCs) have been demonstrated to be a superb player for the energy supply of our future. However, soon after the emergence of this technology back in 2009–2012, the research community recognized the need for the understanding and management of degradation mechanisms to boost cell device stability, module reliability, and, in general, device lifetime. The exponentially increased number of published works in the past 10 yr dedicated to this matter clearly demonstrates this effort [see Fig. 1(a)]. Although we are still in a learning process, the basic degradation mechanisms of this technology are being investigated and some of them have already been elucidated.

The PSC technology has benefited from the learning experience on stability assessment from other photovoltaic (PV) technologies such as organic photovoltaics (OPV), which in 2011 published the ISOS protocols for OPVs.¹ These ISOS protocols (different from the IEC stability standards required for certified and ready-to-commercialize modules²) were made as a guide for the academic community on how to carry out analysis and characterization of OPV stability. This facilitates the comparison and reproducibility of PV devices between different laboratories and locations around the globe. These have been also the basis for the publication, in 2020, of the corresponding ISOS protocols updated for PSCs where issues specifically observed for PSC (e.g., day/night cycling, bias voltage, etc.) have been considered.³ 

The ISOS protocols are the result of the gathering efforts of many international research scientists from the PV community who, back in 2008, came together to discuss reproducibility and stability issues of these PV technologies. That group of scientists established the International Summits on the Stability of Organic and Perovskite Photovoltaics (ISOS), originally dedicated to OPVs and upgraded to include PSCs in 2014.⁴ The ISOS summits are international conferences that have been organized at different locations, including three continents [see Fig. 1(b)]. This year it is the celebration of its 15th edition (ISIOS-15) to be held at the end of September 2024 at the Helmholtz-Zentrum Berlin (Germany).⁵ The effort this community gives to gender equality is interesting, demonstrating the participation of 50% female leaders in their activities or projects and the organization of a dedicated one-day female scientist day at the ISOS-14 edition that took place in Yokohama, Japan in 2023.⁶ 

Besides describing the different environment and conditions that solar cells should withstand to evaluate their stability, the ISOS protocols also identify what is a “stable” solar cell. Only if a PSC can survive these conditions can we call the PSC “stable.” When modifying any material, a thin film layer or an interface of our PSC via compositional, additive, solvent, or charge transport engineering, among many other methods, if the final efficiency of our device after stability assessment under continuous light irradiation is not within the 10% of the initial efficiency, then, we cannot call it “stable.” We can say that our PSC is more stable than the control device, but the final modified PSC device cannot be called a “stable” solar cell.

When “engineering” PSCs with the objective to enhance device performance, researchers usually focus on increasing efficiency, and then they proceed to carry out stability assessment. However, many published works demonstrate that this strategy does not always result in “stable” devices. We should not forget the relation between deep defect passivation and a device’s efficiency (non-radiative recombination and Voc) and shallow defect passivation and a PSC’s stability (ion suppression/immobilization).⁷ To enhance PSC stability, we should develop methods to passivate shallow defects which lead to immobilize or suppress ion migration. Many times, defect passivation results in devices with similar or slightly lower PCE if compared to the control device, especially when strong bonding between the additive and the halide perovskite (HP) takes place. Instead of leaving those solar cells behind, we should test them for stability. Shallow defect passivation does not affect the device’s Voc and its efficiency but can impact stability to a great extent. A holistic approach, where both deep and shallow defect passivation takes place, is thus the best strategy.

Published works of the PSC’s stability assessment under continuous light irradiation (ISOS-L) and outdoor conditions (ISOS-O) are shown in Figs. 2(a) and 2(b), respectively. We can observe stable PSCs lasting more than 4000 h for indoor tests (only works published in 2024 are shown) and more than 20 000 h for outdoor tests. What are the features that make these solar cells so stable? Most of the HPs employed in those tests are the classical triple and quadruple Pb-based HPs containing MA, FA, Cs, and/or Rb. In some cases, we observe the use of SnO₂ instead of TiO₂ or PCBM and C₆₀ working together with the electron transport layers (ETLs). There seems to be no difference between the PSC configuration: n-i-p, p-i-n, and carbon-based PSCs can be found in those results. Even the use of Spiro-OMeTAD, known for its unstable properties, is observed in some of these stable PSCs maintaining above 90% stability after 1000 h in both indoor and outdoor analysis. Interestingly, the most stable PSCs employ self-assembly monolayers (SAMs) at the interfaces. Some of these are phosphonic acid-based SAMs containing the phosphonate group in their structure [e.g., ((2-(9H-carbazol-9-yl)ethyl)phosphonic acid) or 2PACz, ((2-(3,6-Dimethoxy-9H-carbazol-9-yl)ethyl)phosphonic acid) or MeO-2PACz], which display strong bonding with the HP, but also with the semiconductor oxide used as a transport layer.

Whereas much more data on stability analysis are required to reach clear conclusions, which can also be employed by artificial intelligence tools, we are starting to observe a bright future for the long-term stability of PSCs and, thus, their commercial deployment. Just now, major PV companies are reporting the commercialization of their PSC modules with efficiencies above 16% (average efficiency offered by commercial silicon photovoltaics). Toshiba has claim 16.6% efficiency of their PSC module.²⁸ Oxford PV has just announced the commercialization of its tandem perovskite/Si modules with 24.5% efficiency, which can generate 20% more efficiency than silicon solar cells.²⁹ Utmo Light (China) said their panels have passed all IEC testing for solar modules and can withstand a 2300-h UV bath at 1000 W/m² and 60 °C, for 12 yr of operation without degradation.³⁰ GCL (China) reported that their perovskite and perovskite–silicon tandem solar modules with efficiencies above 19% and 26%, respectively.³¹ They announced that their perovskite solar panel had passed the IEC 61215 and IEC 61739² certification tests employed for silicon solar cells and guarantee that the 10-yr end power output will be at least 90% of the nominal output power, which decreases to 80% after 25 yr.³² These are impressive news reports for PSC technology, and we will be carefully looking at how these commercial devices withstand real long-term operational conditions. In the meantime, research articles on stability should carefully consider the publication of actual stable PSCs and seriously consider the term “stable” in the article title.