Perovskite Solar Cells: Passivation Techniques


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

What is Passivation?

Perovskite solar cells (PSCs) have demonstrated impressive device metrics, including open-circuit voltages (Voc) of up to 1.2V1. However, in order for PSCs to achieve their theoretical best efficiencies, all non-essential recombination pathways should be eliminated2. Considering that the defect density of perovskites is quite high3, the amount of non-radiative recombination is remarkably low.

Passivation refers to making a material “passive”. In physical chemistry, this generally refers to ensuring a material has less interaction with its environment. With PSCs, there are two main types of passivation layers (the first type will be the main focus of this article):

  1. Chemical - which passivate by filling trap states
  2. Physical - to isolate layers of the PSC from the external environment.10

As perovskites are usually solution-processed, there are many chances for defects – or trap states –  to form during their crystallisation4,5. Two types of traps are mentioned in this article: vacancies and interstitials.

Interstitials

Iodide interstitials appear to act as non-radiative recombination centres6 and can reduce the Voc of PSCs. This is an interstitial iodine atom that inserts itself between two of the iodine molecules that form the iodide octahedral (as shown in Figure 1b).

Vacancies

Vacancies occur when the perovskite crystal structure is missing an element (an iodide vacancy is shown in Figure 1c). These defects do not have a huge impact on device performance, especially when compared to other solar cells (such as Cadmium Telluride)3. It is suspected that the band energies of most defects are very close to (or within) the band edges of the perovskite, and so will not have a massive effect on optoelectronic properties.


Illustration of perovskite crystal structures (normal vs. iodide interstitial defect vs. iodide vacancy)
Figure 1. Illustrations showing crystal structures of the perovskite. A “normal” structure (a), a structure with an iodide interstitial defect shown in red (b), and an iodide vacancy shown as a white circle (c).

Migration Through the Perovskite Layer

Vacancies and interstitials can also migrate through the perovskite layer. This migration of defects is thought to be responsible for the hysteresis seen in J-V sweep. It can lead to charge build-up at the interfacial layers – affecting charge transport5. Research shows that the bulk of charge recombination centres (or at least the ones that are most detrimental to device performance) are localised to the surface of perovskite crystals5,7,8. These can act as traps for separated electrons or holes, reducing charge extraction (see Figure 2). By passivating these surface traps, charge-carrier lifetime can be increased and PSC performance can be improved.

Photo electron-hole pair, which are separated into separate transport layers. Adapted from Shao et al (2014)
Figure 2. A photon creating an electron-hole pair, which are separated to their respective transport layers. If there are trap states (shown circled in red), then effective charge transport is blocked and device performance suffers.

How Can You Measure Defect Density?

Photovoltaic Performance

Ultimately, trap states affect device performance. Therefore, device metrics and J-V curves are a good indication of whether passivation techniques are successfully reducing defect density. High Voc losses are seen when surface recombination is prevalent5. Defect passivation at layer interfaces should show an improvement in Voc 11.

A high defect density can also encourage vacancy-assisted ion migration within the perovskite crystal. This can lead to a build-up of ions at interfacial barriers of the device, which has complex effects on device performance5. One consequence thought to result from this is the hysteresis effect seen in J-V curves. It has been shown that hysteresis can be reduced by passivating vacancies in the perovskite.

Photoluminescence Measurement 

One thing to examine when exploring passivation techniques is steady-state or time-resolved photoluminescence (PL) data. From steady-state PL, quenching effects can be seen. PL-quenching refers to a decrease of photoluminescence (as shown in Figure 3). If charge-transport layers are in contact with the perovskite layer, PL-quenching will occur if there is good charge extraction between them12. The passivation of surface defects will lead to an improvement in charge extraction, which results in increased PL-quenching.   

Graph of photoluminescence intensity as a function of wavelength for steady-state measurement.
Figure 3. Graph showing PL intensity as a function of wavelength for a steady-state measurement. Quenching of the PL intensity will occur when the perovskite (blue line) is in contact with a charge-transport layer.

Average charge-carrier lifetime can be determined from time-resolved PL (TRPL). With TRPL, perovskite layers are exposed to light until they are fully excited. Then, the photoluminescence is measured over a very short period of time. From this exponential decay, carrier lifetimes can be calculated.

External Radiative Efficiency

A perfect solar cell should also be a good emitter at Voc, as this indicates that there is no occurrence of unwanted recombination. The external radiative efficiency (ERE) is defined as the fraction of dark recombination current that results in light emission2,13. This is essentially used as a measure of non-radiative recombination, with the best-performing cells being at around 1%.

Thermally-Stimulated Current

The above methods are all indirect ways of measuring trap-state density. Thermally-stimulated current has been shown to identify electronic trap states directly in perovskite materials.14 First, the device is cooled to very low temperatures, then illuminated to create charge carriers. While being held at these low temperatures, the carriers can relax inside the density of states and occupy trap states. The temperature is then slowly raised, and the carriers are released from their traps and produce current. Shallow traps are released at low temperatures. The deeper the trap, the higher the temperature before it is released.


Where Can Passivation Be Used?

It has been shown that there are noticeable non-recombination losses at interfaces between the charge transport and perovskite layers,15 and at grain boundaries within the perovskite.5 There are many passivation techniques that are being investigated at this time, but a few different approaches are outlined here.

Passivation Between the Electron-Transport Layer and Perovskite

TiO2 is a common electron-transport material (ETL) and has a high trap-state density. These surface traps can be partially passivated by introducing an interlayer (such as Phenyl-C61-butyric acid methyl ester (PCBM)), between the perovskite layer and the ETL.10 This has been shown to reduce defect density in the TiO2, reduce hysteresis effects and make the device more stable under UV radiation.

Another passivation technique on TiO2 is to use a self-assembled monolayer (SAM) of organic molecules. These monolayers usually contain nitrogen atoms, which are thought to form hydrogen-bonds with the methylamine groups of the perovskite,10,12 leading to increased film stability.

Grain-Boundary Passivation

Defects at grain boundaries within the perovskite film can encourage trap-assisted recombination. It has been shown that a passivation layer (such as PCBM) placed on top of a perovskite layer can infiltrate the perovskite layer. This can help passivate grain boundaries,9,10 as shown in Figure 4.


PCBM passivation layers seeping into grain boundaries of a perovskite layer
Figure 4. Image showing how passivation layers (in this case PCBM) can seep into the grain boundaries (GB) within the perovskite layer.

Grain boundaries and TiO2 traps10  can been passivated by introducing a layer of PbI2 around the perovskite. The exact process of how this works is still being debated, but it has been suggested that the band misalignment introduced by having a PbI2 passivation layer decreases the likelihood of change recombination.15 Figure 5 illustrates several theories on the mechanisms behind this process. It has been suggested that by introducing PbI2 interlayers between perovskite and TiO2 ETLs, recombination due to trap states is intercepted. It has also been suggested that band misalignment introduced by having a PbI2 passivation layer decreases the likelihood of change recombination between the perovskite and hole-transport layer (HTL).10,16


Illustration of the mechanisms behind how lead iodide reduces charge-carrier recombination
Figure 5. Proposed mechanisms of how PbI2 reduces charge carrier recombination. 

It has also been suggested that excess methylammonium iodide (MAI) could self-assemble into a passivating layer around the perovskite if PbI2 is deficient in the solutions.10 The use of excess MAI can produce perovskites with higher Voc and fill factor. Along with time-resolved photoluminescence measurements, Son et al concludes that MAI reduces trap states at the grain perovskite grain boundaries.17

Perovskite Film Surface Passivation

It has been suggested that the deposition of a fullerene layer (such as PCBM) on top of the perovskites can reduce trap density by up to two orders of magnitude.17 PL data shows that this layer increases carrier lifetime, and infers that surface recombination is reduced due to the passivation of PbI3 anti-site traps.

Introducing Lewis bases (e.g. iodopentafluorobenzene (IPFP)) on top of the perovskite has also been proven to passivate defects. Under-coordinated halide anions can act as electron traps. Lewis bases bind to these uncoordinated halogen ions – effectively screening the charge, thus reducing recombination.9,18


IPFB molecules binding to uncoordinated exposed halogen ions. Adapted from Abate et al (2014).
Figure 6. IPFB molecules bind to uncoordinated exposed halogen ions. 

Reports show that small molecule or polymer interlayers with electron-rich side groups can bond with uncoordinated Pb ions,5 thus reducing defect density. These polymer layers can also protect the perovskite from extrinsic factors (such as moisture or oxygen penetration).10

During Perovskite Deposition

Usually, anything added to the perovskite precursor is considered an additive rather than a passivation technique. However, F-PDI introduced in the solvent quenching phase of perovskite formation has been reported to chelate with Pb2+ at grain boundaries, effectively passivating the perovskite layer as it forms.19


Conclusion

PSCs already show fantastic device performance without requiring much effort to reduce defect density. However, if PSCs are to achieve their full potential, any factors that can lead to non-radiative recombination should be addressed. Defect passivation is therefore an important topic in perovskite research.


Mary’s Notes: Currently, there is plenty of literature surrounding different passivation techniques, and I have only summarised a few here.  Reference 10 is a fantastic place to start – but keep an eye out for further reviews about this topic. I suspect there will be some in the next year.


References

[1] Xiao Fu, Klaus J. Weber, and  Thomas P. White (2018),  Characterization of trap states in perovskite films by simultaneous fitting of steady-state and transient photoluminescence measurements, Journal of Applied Physics, 124, 073102; https://doi.org/10.1063/1.5029278

[2] H. Snaith (2018), Present status and future prospects of perovskite photovoltaics, Nature Materials, volume 17, pages 372–376.

[3] D. A. Egger, A. Bera, D. Cahen, G. Hodes, T. Kirchartz, L. Kronik, R. Lovrincic , A. M. Rappe, D. R. Reichman and O. Yaffe (2018), What Remains Unexplained about the Properties of Halide Perovskites? Advanced Materials Volume 30, Issue 20, https://doi.org/10.1002/adma.201800691

[4] J. Jean,  P. R. Brown,  R. L. Jaffe, T. Buonassisi and V. Bulović (2015), Pathways for solar photovoltaics, Energy Environ. Sci., 2015, 8, 1200-1219. https://doi.org/10.1039/C4EE04073B

[5] A. Rajagopal  K. Yao and A. K.‐Y. Jen (2018), Toward Perovskite Solar Cell Commercialization: A Perspective and Research Roadmap Based on Interfacial Engineering, Applied Materials, Vol 30, Issue 32. https://doi.org/10.1002/adma.201800455

[6] J. M. Azpiroz, E. Mosconi, J. Bisquertcd and F. De Angelis, Defect migration in methylammonium lead iodide and its role in perovskite solar cell operation, Energy Environ. Sci., 2015,8, 2118-2127.  https://doi.org/10.1039/C5EE01265A

[7] R.J. Stewart, C. Grieco, A. V. Larsen, J. J. Maier and J. B. Asbury, Approaching Bulk Carrier Dynamics in Organo-Halide Perovskite Nanocrystalline Films by Surface Passivation, J. Phys. Chem. Lett., 2016, 7 (7), pp 1148–1153.  https://doi.org/10.1021/acs.jpclett.6b00366

[8] W-J Yin, T. Shi, and Y. Yan, Unusual defect physics in CH3NH3PbI3 perovskite solar cell absorber, Appl. Phys. Lett. 104, 063903 (2014); https://doi.org/10.1063/1.4864778.

[9] Y. Shao, Z. Xiao, C. Bi, Y. Yuan & J. Huang, Origin and elimination of photocurrent hysteresis by fullerene passivation in CH3NH3PbI3 planar heterojunction solar cells, Nature Communications Volume 5, Article number: 5784 (2014).

[10] P. Zhao, B. Jo and K. SukJung, Passivation in perovskite solar cells: A review. Material Energy, Vol 7, March 2018, Pages 267-286, https://doi.org/10.1016/j.mtener.2018.01.004

[11] W. S. Yang, B-W Park, E H Jung, N J Jeon, Y C Kim, D U Lee, S S Shin, J Seo, E K Kim, Jun-H Noh, S. Seok. Iodide management in formamidinium-lead-halide–based perovskite layers for efficient solar cells, Science, 2017, Vol. 356, Issue 6345, pp. 1376-1379, DOI: 10.1126/science.aan2301

[12] L. Jiang, S. Cong,  Y-H Lou, Q-H Yi, J-T Zhu, H Mab and G-F Zou. Interface engineering toward enhanced efficiency of planar perovskite solar cells. J. Mater. Chem. A, 2016,4, 217-222. https://doi.org/10.1039/C5TA09231K

[13] Green, M. (2011). Radiative efficiency of state-of-the-art photovoltaic cells. Progress In Photovoltaics: Research And Applications, 20(4), 472-476. doi: 10.1002/pip.1147

[14] Baumann, A., Väth, S., Rieder, P., Heiber, M., Tvingstedt, K., & Dyakonov, V. (2015). Identification of Trap States in Perovskite Solar Cells. The Journal Of Physical Chemistry Letters, 6(12), 2350-2354. doi: 10.1021/acs.jpclett.5b00953

[15] Martin Stolterfoht, Christian M. Wolff, José A. Márquez, Shanshan Zhang, Charles J. Hages, Daniel Rothhardt, Steve Albrecht, Paul L. Burn, Paul Meredith, Thomas Unold & Dieter Neher, Visualization and suppression of interfacial recombination for high-efficiency large-area pin perovskite solar cells, Nature Energyvolume 3, pages847–854 (2018)

[16] Chen, Q., Zhou, H., Song, T., Luo, S., Hong, Z., & Duan, H. et al. (2014). Controllable Self-Induced Passivation of Hybrid Lead Iodide Perovskites toward High Performance Solar Cells. Nano Letters, 14(7), 4158-4163. doi: 10.1021/nl501838y

[17] Son, D., Lee, J., Choi, Y., Jang, I., Lee, S., & Yoo, P. et al. (2016). Self-formed grain boundary healing layer for highly efficient CH3NH3PbI3 perovskite solar cells. Nature Energy, 1(7). doi: 10.1038/nenergy.2016.81.

 [18] Abate, A., Saliba, M., Hollman, D., Stranks, S., Wojciechowski, K., & Avolio, R. et al. (2014). Supramolecular Halogen Bond Passivation of Organic–Inorganic Halide Perovskite Solar Cells. Nano Letters, 14(6), 3247-3254. doi: 10.1021/nl500627x

[19] Yang, J., Liu, C., Cai, C., Hu, X., Huang, Z., & Duan, X. et al. (2019). High‐Performance Perovskite Solar Cells with Excellent Humidity and Thermo‐Stability via Fluorinated Perylenediimide. Advanced Energy Materials, 1900198. doi: 10.1002/aenm.201900198