A PhD Student Condenses: The Impact of OPV Processing Conditions

This is the first of a 6-part series titled "A PhD Student Condenses...",  where recent academic articles will be condensed & reviewed by Ossila's newest academic collaborator - Emma Spooner, a PhD student at the University of Sheffield.
Emma Spooner | Ossila - University of Sheffield PhD Student

My name is Emma Spooner and I am 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 be reviewing recent papers encountered in my research to provide a concise summary for Ossila’s customers.

The first paper I will review in this series is an article by Singh, Dey & Iyer featured in the December 2017 issue of Organic Electronics. It is based on optimising organic photovoltaic (OPV) processing conditions - a topic important in maximising power conversion efficiency (PCE) values, and therefore of particular interest to researchers in the field of OPVs.


 

Condensed Summary

Title: Influence of Molar Mass Ratio, Annealing Temperature and Cathode Buffer Layer on Power Conversion Efficiency of P3HT:PC71BM-based Organic Bulk Heterojunction Solar Cell

Citation: A. Singh et. al, Organ. Electron., 2017, 51, 428-434.

Learning point: The processing conditions used for one bulk-heterojunction OPV stack may not be the most optimal for a different stack.

 

The impact of OPV processing conditions

While a large part of research into the bulk heterojunction morphology of organic solar cells focuses on component choice,1 the morphology is also tuned by a host of processing conditions. Recently, Singh et al.2 published work into optimising three of these conditions; the molar mass ratio of the donor:acceptor blend, the annealing temperature of the active layer and the cathode buffer layer choice. The OPV cell used was a conventional stack of ITO-coated glass/PEDOT:PSS/active layer/cathode buffer layer/Al, where the active layer was a blend of regioregular P3HT and PC71BM.

Tuning the molar mass ratio and annealing temperature of the blend aims to create a favourable nanoscale morphology in the heterojunction. Additionally, adjusting the cathode buffer layer aims to block excess holes from reaching the cathode, hence reducing charge-carrier recombination at the interface.

 

OPV Stack (Singh et al, 2017)
Figure 1: Representation of the OPV stack used by Singh et al. (not to scale).

 

Results

1) Molar mass ratio of donor:acceptor

The authors found that as the amount of PC71BM increased, the UV-Vis absorption of the blend was quenched, seen in Table 1, which was attributed to the interaction between the donor and acceptor materials. A ratio of 1:0.8 P3HT:PC71BM was found to have the highest power conversion efficiency (PCE),

 

OPV performance table for varied donor:acceptor molar mass ratios
Table 1: Summary of the OPV performance where molar mass ratio of donor:acceptor was varied.

 

Emma's comments: The 1:0.8 P3HT:PC71BM ratio appears to be the best balance between high absorption and sufficient acceptor for efficient exciton dissociation.

 

2) Active layer annealing temperature

From 100 to 150°C, it was found that as annealing temperature was increased, UV-Vis absorption increased and was red-shifted. Above 150°C, absorption decreased and blue-shifted, seen in Table 2.

The initial increase in absorption was attributed to an increase in P3HT crystallinity, as a result of PC71BM diffusion caused by the higher temperature. This led to greater inter-chain interactions between P3HT chains, increasing the π-π* absorption and reducing the π-π* band gap causing the redshift.

The decrease in absorption after 175°C was attributed to phase separation between donor & acceptor, also reducing the inter-chain interactions and hence causing a blueshift.

 

OPV performance with varied annealing temperature of the active layer
Table 2: Summary of the OPV performance where annealing temperature of the active layer was varied.

 

Emma's comments: The optimum annealing temperature depends on the blend's composition; the glass transition temperature is typically greater for compositions with a higher PCBM content. Heating a film blend too far above the glass transition temperature can reduce its performance.

 

3) Cathode buffer layer

In combination with the Al cathode, the cathode buffer layer was varied between LiF, Ca, tris (8-hydoxyqinolinato) aluminium (Alq3), Bathophenanthroline (BPhen) and Bathocuproine (BCP). These have varying band-gaps, which dictate their effectiveness. BCP was found to be the most effective layer.

 

Band-gaps of cathode buffer layers vs band-gaps of active layer components
Figure 2: Representation of the band gaps of the tested cathode buffer layers, in comparison to that of the active layer components. The work function of the layer/Al is given for LiF and Ca.

 

OPV performance for varied cathode buffer layer
Table 3: Summary of the OPV performance where the cathode buffer layer was varied.

 

Emma's comments: BCP has a higher band-gap than the other materials. Therefore it is more effective at hole blocking and reducing recombination. This illustrates the importance of matching band gaps throughout devices.

 

Final Thoughts

The work by Singh et al. clearly demonstrates the significant impact of processing conditions on bulk heterojunction morphology and determines the optimum conditions for P3HT:PC71BM. These findings are important to keep in mind during OPV manufacture in research, as conditions used for one stack may not be the most optimal for another. Several reviews are available to gain a deeper understanding of the impact of the donor and acceptor choice, and processing conditions, on the morphology and performance of bulk heterojunctions.3, 4, 5

 

Materials Mentioned In This Paper

If you are interested in conducting a similar experiment, some of these materials are available from Ossila.

References

The paper discussed in this blogpost was published online in September 2017, and can be found here: http://www.sciencedirect.com/science/article/pii/S1566119917304743

  1. L. Lu, T. Zheng, Q. Wu, A. M. Schneider, D. Zhao and L. Yu, Chem. Rev., 2015, 115, 12666-12731.
  2. A. Singh, A. Dey and P. K. Iyer, Organ. Electron., 2017, 51, 428-434.
  3. C. J. Brabec, M. Heeney, I. McCulloch and J. Nelson, Chem. Soc. Rev., 2011, 40, 1185-1199.
  4. C. J. Brabec, S. Gowrisanker, J. J. Halls, D. Laird, S. Jia and S. P. Williams, Adv. Mater., 2010, 22, 3839-3856.
  5. H. Hoppe and N. S. Sariciftci, J. Mater. Chem., 2006, 16, 45-61.

 

 


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