A PhD Student Condenses: Factors Influencing OPV Stability

Part of a series titled "A PhD Student Condenses...", where the latest academic articles in materials science will be condensed and reviewed by one of Ossila's academic collaborators - Emma Spooner, a first-year PhD student the University of Sheffield. If you have any feedback or want to submit a topic request for future blogposts in this series, you may do so via this online form.
Emma Spooner | Ossila - University of Sheffield PhD Student

My name is Emma Spooner and I am currently pursuing a PhD in Fullerene-Free Photovoltaic Devices at the University of Sheffield, in collaboration with Ossila Ltd. As part of this collaboration, I will review recent papers encountered in my research to provide a concise & helpful summary for Ossila’s customers.

For the fourth article in this series, I will be discussing the recent work by Rafique et al., entitled ‘Fundamentals of bulk heterojunction organic solar cells: An overview of stability/degradation issues and strategies for improvement’. A summary of the fundamental principles of solar cells can be found in this Ossila guide on solar cell theory & measurement, whilst this discussion will focus on factors influencing their stability and degradation.

 

Condensed Summary

Title:  Fundamentals of bulk heterojunction organic solar cells: An overview of stability/degradation issues and strategies for improvement.

Citation: S. Rafique et al., Renew. Sust. Energ. Rev. (84), 43-53 (2018).

DOI: 010.1016/j.rser.2017.12.008

Learning point: Bulk heterojunction organic solar cells are susceptible to a wide range of degradation mechanisms, including both obvious extrinsic factors, such as oxygen and humidity, and intrinsic factors. Encapsulation alone is not always sufficient to produce stable devices

 

The Stability of Organic Photovoltaics

Typically, organic photovoltaics (OPVs) are manufactured in the form of a bulk heterojunction (BHJ) cell, where the active layer consists of a blend of donor and acceptor materials with various interfacial layers and electrodes (seen in Figure 1). For OPVs to be commercially competitive with existing solar technologies, a ’10/10’ target must be reached, which refers to a 10% power conversion efficiency (PCE) and 10-year lifetime. Whilst the 10% PCE target has now been significantly exceeded by many groups,2 stability studies assessing the operational lifetime of OPVs still lag behind.

Whilst accelerated stability tests have extrapolated OPV lifetimes of up to 15 years,3 the conditions used are not fully representative of those experienced in outdoor installations. Even vigorous indoor testing (such as damp heat testing) does not incorporate the daily and seasonal fluctuations of temperature and light intensity that occur in an outdoor setting. Several outdoor stability studies that have attempted to replicate real-life conditions,4–7 but these have mostly been confined to the period of a year, and are therefore difficult to extrapolate accurately to longer periods of operation.

It is clear that further research needs to take place to extend the operational lifetime of OPVs. For this to be successful, there needs to be a deeper understanding of the mechanisms behind degradation. There have been several reviews published on this topic8,9 - the most recent being by Rafique and co-workers,1 which is discussed here.

Conventional stack geometry of a bulk heterojunction organic solar cell
Figure 1: An approximation of a bulk heterojunction organic solar cell, in a conventional stack geometry.

Lifetime Testing

Most OPVs will show a ‘burn-in’ upon initial testing. This refers to an exponential loss of efficiency (the length of which varies between systems, but is often within the first 100 hours) followed by a linear decay. After the burn-in period, PCE typically stabilises at an efficiency around 25-50% of the initial value.10 Generally, the ‘lifetime’ of an OPV refers to its T80, meaning the time taken for the PCE of a device to reach 80% of its post burn-in efficiency.11 The exact mechanism of efficiency loss during the burn-in period has not yet been completely established, but has been variously attributed to the generation of sub-band gap states,12 fullerene dimerisation,13 and other factors.

Emma’s comments: It is worth noting that recently a ‘burn-in free’ OPV was shown10 using a non-fullerene acceptor (NFA). Indeed, many of the degradation mechanisms of OPVs can be attributed to fullerene,14 and it seems likely that NFAs hold an advantage over their fullerene counterparts - both in efficiency and stability.

The linear decay after burn-in can be attributed to a number of different inter-related mechanisms that often happen at the same time, leading to a complex system that is difficult to optimise. To encourage consistency between different research groups, the International Summit on OPV Stability (ISOS) have published established lifetime testing standards, and most stability tests will attempt to replicate one of the three levels of testing protocol.8

Burn-in approximation for typical OPV device
Figure 2: Approximation of burn-in for a typical OPV device.

Extrinsic Degradation Factors

Rafique and co-workers divide the influences affecting degradation into both extrinsic factors (meaning environmental forces such as humidity and oxygen) and intrinsic factors (meaning the stability of the materials within the device itself).

Of the various extrinsic factors, oxygen and water are the most obvious. Oxygen causes photo-oxidation of layers within the device (especially the active layer), disrupts charge transport processes, and oxidises some cathodes, forming an insulating metal oxide barrier layer. Typically, it has been found that oxygen content and loss in device performance are linearly related.9

In contrast, device performance has been found to exponentially decrease with relative humidity.9 Water causes corrosion of metal electrodes, induces morphological changes, and swells water-soluble layers - often resulting in delamination of layers within the stack (and eventual failure).

Emma’s comments: Encapsulation of OPVs is very common in research, often done using epoxy and glass coverslips over the top of the cathode to reduce the diffusion of oxygen and water into the device. However, this cannot prevent ingress through the sides of the stack (which is often accelerated by water-soluble layers like PEDOT:PSS) and is therefore not completely effective. Glass encapsulation is also not compatible with flexible OPVs, meaning there are still significant areas for improvement.

Morphological changes can encourage formation of pinholes in one layer, which will in turn increase diffusion of oxygen or water into the next layer, furthering device deterioration. Whilst encapsulation can reduce the impact of extrinsic factors, current encapsulants will not protect devices permanently.

Other extrinsic factors discussed by Rafique et al. include mechanical stress (particularly relevant for flexible OPVs), processing conditions, light, and temperature. The latter two factors are closely related, as extended illumination will raise the temperature of the device. This has been shown to cause thermally-induced fullerene aggregation, and light itself will encourage photo-oxidation and photogeneration of tra p states. Little correlation has been found between light intensity and loss of device performance,9 but it is clear that even ambient lighting can introduce degradation mechanisms.14

Degradation factors affecting organic photovoltaic (OPV) stability
Figure 3: A summary of the most significant extrinsic and intrinsic degradation factors impacting the stability of an OPV.

Intrinsic Degradation

Low band-gap polymers (such as PTB7) have performed well in research, but have shown photochemical instability15 in comparison to other polymers such as P3HT. The materials used for interfacial layers (such as the hole-transport (HTL) and electron-transport layer (ETL)) will also be significant.

The most common HTL of all, PEDOT:PSS, shows intrinsic instabilities. It is acidic and hygroscopic, meaning it will absorb water, and encourage the diffusion of water into the device from the edges. As a result, it can corrode the commonly-used anode ITO, causing indium diffusion into the active layer,16 leading to accelerated charge recombination. This will occur even after encapsulation, and likely cannot be prevented with the use of PEDOT:PSS and ITO.

Other common materials (such as the ETL/cathode combination of LiF/Al) are very sensitive to oxidation, resulting in significantly unstable devices. Materials introduced into the stack (like solvent additives like diiodooctane (DIO)) can also cause degradation. DIO is light-sensitive and can be trapped by fullerene moieties or react with a conjugated polymer backbone, accelerating photo-oxidation of the active layer.17

Improving Stability

Intrinsic instability can be tackled in some ways via careful component choice. An example of this is tackling the indium diffusion at the ITO-PEDOT:PSS interface by either replacing the ITO,17 or replacing the PEDOT:PSS. Rafique and co-workers refer to alternative HTLs such as NiO, V2O5, WO3 and MoO3. Similarly, more stable ETL alternatives include CrOx, Cs2CO­3 and TiO2, and in literature there has been a move away from Al electrodes to more stable Ag.

Emma’s comments: Eventual large-scale manufacture should be kept in mind when replacing components in OPVs, as highly efficient materials may not have good stability. Solution processable interfacial layers are more favourable as these can be incorporated into a roll to roll manufacturing process.

Conclusions

It is worth noting that degradation factors that affect solution-processed BHJ cells will not affect other types of OPV in the same way (such as planar heterojunction cells5 and roll-to-roll processed cells.6) This is significant as future commercialisation of OPVs will likely require compatibility with roll-to-roll production, so stability in this context is particularly relevant.

It is likely that future work in improving OPV stability will focus on scalable manufacture, air-stable electrodes, suitable active-layer component choice, better encapsulation, and investigations that focus specifically on the mechanisms behind degradation (and how they can be prevented).


References

  1. Fundamentals of bulk heterojunction organic solar cells: An overview of stability/degradation issues and strategies for improvement, S. Rafique et al., Renew. Sustain. Energy Rev. (84), 43-53 (2018); DOI: 10.1016/j.rser.2017.12.008.
  2.   Molecular Optimization Enables over 13% Efficiency in Organic Solar Cells, W. Zhao et al., J. Am. Chem. Soc. (139), 7148-7151 (2017); DOI:10.1021/jacs.7b02677.
  3. Minimal long-term intrinsic degradation observed in a polymer solar cell illuminated in an oxygen-free environment, W. R. Mateker et al., Chem. Mater. (27), 404-407 (2015); DOI: 10.1021/cm504650a.
  4. PCDTBT based solar cells: One year of operation under real-world conditions, Y. Zhang et al.,  Sci. Rep. (6), 21632 (2016) , DOI:10.1038/srep21632.
  5. Outdoor Performance and Stability of Boron Subphthalocyanines Applied as Electron Acceptors in Fullerene-Free Organic Photovoltaics, D. S. Josey et al., ACS Energy Lett. (2), 726-732 (2017); DOI: 10.1021/acsenergylett.6b00716.
  6. Interlaboratory outdoor stability studies of flexible roll-to-roll coated organic photovoltaic modules: Stability over 10,000 h, S. A. Gevorgyan et al., Sol. Energy Mater. Sol. Cells (116), 187-196 (2013); DOI: 10.1016/j.solmat.2013.04.024.
  7. Flexible organic P3HT:PCBM bulk-heterojunction modules with more than 1 year outdoor lifetime, J. A. Hauch, Sol. Energy Mater. Sol. Cells (92), 727-731 (2008); DOI: 10.1016/j.solmat.2008.01.004.
  8. Stability of polymer solar cells,  M. Jørgensen et al., Adv. Mater. (24), 580-612 (2012); DOI: 10.1002/adma.201104187.
  9. Recent progress in degradation and stabilization of organic solar cells, H. Cao et al., J. Power Sources (264), 168-183 (2014); DOI: 10.1016/j.jpowsour.2014.04.080.
  10. An Efficient, ‘Burn in’ Free Organic Solar Cell Employing a Nonfullerene Electron Acceptor, H. Cha et al., Adv. Mater. (29), 1701156 (2017); DOI:10.1002/adma.201701156.
  11. High efficiency polymer solar cells with long operating lifetimes, C. H. Peters et al., Adv. Energy Mater. (1), 491-494 (2011); DOI: 10.1002/adma.201701156.
  12. The mechanism of burn-in loss in a high efficiency polymer solar cell, C. H. Peters et al., Adv. Mater. (24), 663-668 (2012); DOI: 10.1002/adma.201103010.
  13. Morphological and electrical control of fullerene dimerization determines organic photovoltaic stability, T. Heumueller et al., Energy Environ. Sci. (9), 247-256 (2016); DOI: 10.1039/C5EE02912K.
  14. The role of fullerenes in the environmental stability of polymer:fullerene solar cells, H. K. H. Lee et al., Energy Environ. Sci., (2018); DOI:10.1039/C7EE02983G.
  15. High-efficiency and air-stable P3HT-based polymer solar cells with a new non-fullerene acceptor, S. Holliday et al., Nat. Commun. (7), 1-11 (2016); DOI:10.1038/ncomms11585.
  16. Layer by layer characterisation of the degradation process in PCDTBT:PC71BM based normal architecture polymer solar cells, S. Rafique et al., Org. Electron. (40), 65-74 (2017); DOI: 10.1016/j.orgel.2016.10.029.
  17. Photoinduced degradation from trace 1,8-diiodooctane in organic photovoltaics, I. E. Jacobs, J. Mater. Chem. C (6), 219-225 (2018); DOI: 10.1039/c7tc04358a.
  18. Promising long-term stability of encapsulated ITO-free bulk-heterojunction organic solar cells under different aging conditions, S. B. Sapkota et al., Sol. Energy Mater. Sol. Cells (130), 144-150 (2014); DOI: 10.1016/j.solmat.2014.07.004.

 


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