Perovskite Solar Cells: Causes of Degradation


Written by Mary O'Kane, a PhD student in Perovskite Solar Cells at the University of Sheffield, in collaboration with Ossila Ltd.

Perovskite solar cells show impressive efficiencies and offer “a different kind of solar cell” that could be cheap to manufacture and could be semi-transparent, lightweight, and flexible. For an overview of perovskite solar cells and why they are exciting, check out our guide Perovskites and Perovskite Solar Cells: An Introduction. Figure 1 provides a comparison between perovskite and silicon solar cells.


Figure 1. The three pillars of successful commercial solar cells, and how Si and Perovskites compare.

As shown, one of the major challenges in this field is the improvement of perovskite solar cell stability and durability. This article focuses primarily on the sources behind perovskite instabilities. A future article will discuss methods to increase the stability of perovskite solar cells.

Perovskite Solar Cell Durability

In order to enter the current market, the durability of perovskite solar cells must be improved. Some common standards for developing solar cells are as follows:

  • Initially, a rule for any “commercialisable” solar cell was to maintain a power conversion efficiency of 10% for 10 years.
  • Today, to realistically compete with crystalline-Si solar, a cell needs to last for 25 years in outdoor conditions to be marketable, and have similar efficiencies.1
  • According to the International Electrotechnical Commision's (IEC) standards 2, solar cells must perform well under non-laboratory conditions, such as in damp conditions i.e. 85% humidity at 85 degrees Celsius. They must withstand these for more than 1000 hours consistently.  

Currently, perovskite solar cells still do not reach these standards. A recent review presents a detailed summary of various studies that have been done on perovskite stability.1 This includes a triple cation perovskite withstanding 85% humidity for 250 hours3, and a methylammonium lead iodine (MAPbI3) perovskite solar cell withstanding 55% humidity for 480 hours.1


Factors Causing Degradation

The first step towards improving perovskite solar cell stability is to understand the exact causes of their instabilities. Many things can cause the perovskite crystal to undergo degradation, and these factors can broadly be split into two categories - extrinsic and intrinsic factors. Both are discussed below.

Extrinsic Factors

Moisture

Ambient humidity can cause rapid degradation of perovskite films, especially in MAPbI3. The rate of deterioration will increase when high humidity is combined with UV light, high temperatures, or the application of an electric field.4


MAPbI3 perovskite crystal structure interrupted by water molecules
Figure 2. Illustration of the basic crystal structure of MAPbI3. For more information about the perovskite crystal structure, check out the guide Perovskites and Perovskite Solar Cells: An Introduction.

 

 

The organic cations used in perovskite solar cells are very hygroscopic. It has been suggested that water molecules form weak hydrogen bonds with the cations1,5,6 and that this compromises the structural stability of the crystal. This can lead to the formation of a hydrated perovskite phase. This change is reversible.

 

However, it has been suggested that with enough moisture penetration, the perovskite crystal decomposes. Shown below are a chain of reactions that could be responsible for non-reversible degradation in MAPbI3 perovskites.1


MAPbI perovskite chain reactions, catalysed by water
Figure 3. Chain of reactions from MAPbI perovskites, catalysed by water. Adapted from Emami et al.'s 2018 paper.1

This illustrates the conversion of MAPbI3 to an aqueous form of methylammonium lead iodide (MAI) and lead iodide (PbI2). From this, hydroiodic acid (HI) and methylamine are produced. In the presence of oxygen, more water is then produced. This creates a cyclic process leading to further moisture degradation. The presence of excess PbI2 can also influence the formation of the perovskite active layer.

One way to reduce this effect is to increase the strength of the bonds between the organic component and the metal halides. It is extremely important that active layers are deposited in an environment where humidity is controlled. Hydrophobic interlayers can also be introduced to help protect the perovskite from ambient moisture.4


UV

The effect of UV light on perovskite solar cell stability is most significant when combined with other factors (e.g. moisture or oxygen exposure). However, it has been shown that MAPbI3 will degrade to PbI2 under UV light without moisture or oxygen present7. One of the main advantages of formamidinium lead iodide (FAPbI3) solar cells is that they have better photostability than MAPbI3 counterparts.8 Additionally, a common electron-transport layer, TiO2, is very susceptible to degradation under UV, reducing the durability of the perovskite solar cell.1


Temperature

Exposure to elevated temperatures can cause degradation. This is seen in both MAPbI3 and MAPbClxI(3-x).9 It is thought that PbI2 can form from MAPbI3 at elevated temperatures, via the reactions outlined in Figure 2. This can happen without oxygen or water present, but at a much slower rate (compared to when oxygen and water are present).9,10


Oxygen-rich/Ambient conditions

Any combination of UV light, high temperature, high humidity, and oxygen causes rapid degradation of MAPbI3 perovskite films. Coning et al’s 2015 study10 focuses on the films created in oxygen-rich and ambient conditions at elevated temperatures.

In oxygen-rich conditions, large PbI2 structures can then be seen in the perovskite film, whereas the same is not true for layers that have been prepared in a nitrogen-rich atmosphere.10 However, the layers prepared in ambient conditions are by far the most inhomogeneous. 


Perovskite solar cell device standard layered structure
Figure 4. Standard layered structure of a perovskite solar cell. More information about perovskite device structure available in this guide: Perovskites and Perovskite Solar Cells: An Introduction.

Sometimes, the layers surrounding the perovskite can contribute to their structural failure. For example, under exposure to UV light, trap states can be induced in the electron-transport layer (ETL) TiO2.11 This will drastically reduce performance of a solar cell. Using an Al-doped TiO2 or SnO2 ETL can negate this effect.6

Another layer that has a huge impact on a solar cell’s functionality is the hole-transport layer (HTL). Perovskite solar cells can have inorganic HTLs (e.g. P3HT) and organic HTLs (e.g.  Spiro-OMeTAD). However, there is a trade-off between the two. Spiro-OMeTAD is an effective HTL, but it needs two additives to give it suitable HTL properties. It has been suggested that this doped Spiro-OMeTAD layer does not form the necessary barrier to protect the perovskite from external factors (air, moisture etc.).6 An alternative is the organic P3HT, which creates a more stable solar cell. Unfortunately, these cells have lower efficiencies. The impact of different transport layers on device performance is discussed more thoroughly in Wang et al.’s 2016 study.6


Intrinsic Factors

During perovskite film formation, it is common that there are vacancies in the perovskite structure. These are defects and can encourage ion migration through the perovskite film (as shown in Figure 5).


Ion migration through 3D perovskite structure (due to iodide & metal vacancies)Holemigration through 3D perovskite structure (due to iodide & metal vacancies)
Figure 5. Illustrations showing ion and hole migration of due to iodide vacancies (left) and metal vacancies (right) through the 3D perovskite structure. Adapted from Aspiroz et al.'s 2015 paper.12

Ion migration within the perovskite layer can lead to the poor or fluctuating performance of a PV device. For example, a reaction between hole-transport layer Spiro-OMeTAD and migrating iodine ions can reduce the HTL’s conductivity. This quickly hampers the performance of the perovskite solar cell.4

It has been suggested that this ion migration can lead to the formation of a local electric field at the perovskite material interface. This can lead to deprotonation of the organic cations, and ultimate deterioration of the perovskite solar cell.4

Migration of perovskite material is not the only thing that causes problems. Ions from conductive contacts can migrate through the perovskite layer. This creates shunt pathways for electrons, short-circuiting the solar cell.


Summary

There are many factors that affect perovskite solar cell stability, but they fall into two broad categories:

Intrinsic factors (perovskite stoichiometry, ion migration, strength of bonds between cations and anions) and extrinsic factors (degradation due to air, moisture, temperature). This article has discussed how these factors cause instabilities and a following article “Methods to Increase Perovskite Solar Cell Stability“ will discuss approaches to increase perovskite solar cell stability and durability.


Mary’s notes: In researching this article, I found these reviews very helpful: Reference [1], [3], and [5]. I have tried to summarise key studies discussed in them, but for a more in-depth discussion of the degradation processes, these papers are a good place to start.


References

1. Emami, S., Andrade, L., & Mendes, A. Recent Progress in Long-term Stability of Perovskite Solar Cells. U.Porto Journal Of Engineering, 2018, 1(2), 52-62.

2. International Standard IEC61215. Terrestrial Photovoltaic (PV) Modules—Design Qualification and Type Approval—Part 1: Test Requirements and—Part 2: Test Procedures, 1.0 ed.; TC 82—Solar Photovoltaic Energy Systems; IEC: Geneva, Switzerland, 9 March 2016.

3. Saliba, M., Matsui, T., Seo, J., Domanski, K., Correa-Baena, J., & Nazeeruddin, M. et al. Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency. Energy & Environmental Science, 2016, 9(6), 1989-1997.

4. Rajagopal, A., Yao, K., & Jen, A. Toward Perovskite Solar Cell Commercialization: A Perspective and Research Roadmap Based on Interfacial Engineering. Advanced Materials, 2018, 30(32), 1800455.

5. Eperon, G., Habisreutinger, S., Leijtens, T., Bruijnaers, B., van Franeker, J., & deQuilettes, D. et al. . The Importance of Moisture in Hybrid Lead Halide Perovskite Thin Film Fabrication. ACS Nano, 2015, 9(9), 9380-9393.

6. Wang, D., Wright, M., Elumalai, N., & Uddin, A. Stability of perovskite solar cells. Solar Energy Materials And Solar Cells, 2016, 147, 255-275.

7. Lee, S., Kim, S., Bae, S., Cho, K., Chung, T., & Mundt, L. et al. UV Degradation and Recovery of Perovskite Solar Cells. Scientific Reports, 2016, 6(1).

8. Lee, J., Kim, D., Kim, H., Seo, S., Cho, S., & Park, N. Formamidinium and Cesium Hybridization for Photo- and Moisture-Stable Perovskite Solar Cell. Advanced Energy Materials, 2015, 5(20), 1501310.

9. Philippe, B., Park, B., Lindblad, R., Oscarsson, J., Ahmadi, S., Johansson, E., & Rensmo, H. Chemical and Electronic Structure Characterization of Lead Halide Perovskites and Stability Behavior under Different Exposures—A Photoelectron Spectroscopy Investigation. Chemistry Of Materials, 2015, 27(5), 1720-1731.

10. Conings, B., Drijkoningen, J., Gauquelin, N., Babayigit, A., D'Haen, J., & D'Olieslaeger, L. et al. . Intrinsic Thermal Instability of Methylammonium Lead Trihalide Perovskite. Advanced Energy Materials, 2015, 5(15), 1500477.

11. Pathak, S., Abate, A., Ruckdeschel, P., Roose, B., Gödel, K., & Vaynzof, Y. et al. Performance and Stability Enhancement of Dye-Sensitized and Perovskite Solar Cells by Al Doping of TiO2. Advanced Functional Materials, 2014, 24(38), 6046-6055.

12. Azpiroz, J., Mosconi, E., Bisquert, J., & De Angelis, F. Defect migration in methylammonium lead iodide and its role in perovskite solar cell operation. Energy & Environmental Science, 2015, 8(7), 2118-2127.