A PhD Student Condenses: Introduction to Ternary Organic Solar Cells


Part of a series titled "A PhD Student Condenses...", where recent academic articles in materials science will be condensed and reviewed by one of Ossila's academic collaborators - Emma Spooner, a PhD student the University of Sheffield.
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 be running this monthly series where I review recent papers encountered in my research to provide a concise & helpful summary for Ossila’s customers.

For the fifth article in my series, I will be discussing the recent work by R. Yu et al., entitled ‘Recent Progress in Ternary Organic Solar Cells Based on Nonfullerene Acceptors’1. An overview of non-fullerene acceptors in organic photovoltaics can be found in previous articles by Ossila and as part of this series.

 

 

Condensed Summary

Title: Recent Progress in Ternary Organic Solar Cells Based on Nonfullerene Acceptors

Citation: R. Yu et al., Adv. Energ. Mater., 1702814 (2018); 

DOI: 10.1002/aenm.201702814.

Learning point: Ternary organic solar cells can show boosted efficiencies, morphologies, and stability in comparison to their binary counterparts. They are especially notable for improving the performance of lower-efficiency donors. 

 

Introduction to Ternary Organic Solar Cells

The majority of organic photovoltaics (OPVs) in research are based upon a binary active-layer mixture (of donor and acceptor materials) in the form of a bulk heterojunction (BHJ). Ternary organic solar cells extend this principle to three-component active layers, typically in the form of two donors and one acceptor, or one acceptor and two donors. The first ternary OPV was reported in 2009.2 For several years, the majority that followed this focused on a system of one fullerene-based acceptor with two donors.3

The most common motivation for introducing a third component into the active layer of an OPV is to extend solar absorption. Currently, the maximum efficiency of binary OPVs is limited by the narrow absorption bands of most organic semiconductors (OSCs), especially in fullerene-based systems where the donor is the main light absorber. Ternary cells can be designed with complementary absorption bands to maximise the range of the solar spectrum that the cell absorbs, and thus increase the short circuit current density, JSC, of the device.

Emma’s comments: A 2007 study4 found that for an ideal solar cell, theoretically expanding the absorption window from ~200nm to ~400nm, would result in a relative efficiency improvement of up to 35%. 

This approach is especially viable in systems that use non-fullerene acceptors (NFAs), as these are typically designed to absorb well in the visible region, and are easily tuneable - meaning a system of components with complementary band gaps can be generated. The review summarised here focuses specially on NFA-based ternary cells, the first of which was reported in 2016.5

Comparison of OPV stacks (Binary, tandem, ternary)
Figure 1: A binary OPV (a) compared to a tandem (b), and ternary (c) system.

Emma’s comments: Tandem solar cells are also designed to overcome narrow absorption of components (by stacking components with complementary absorption) but are often more complicated to manufacture than ternary cells. For those seeking further information,6,7  several comprehensive reviews can be found on high-efficiency tandem OPVs 

 

The Third Component

Yu and co-workers state that in addition to the principle motivation of extending light harvesting, introducing a third component into the active layer of an OPV can perform a range of other useful functions, such as improving charge transport and dissociation, and optimising the morphology of the active layer.

In terms of morphology, in many cases the third component of a ternary cell has been used to  control the crystallinity of the active layer, either by improving it8 or restricting it9, to give a more favourable BHJ for charge transport. It is worth noting that the morphology of the overall blend is dictated by the miscibility of the components with each other. In true ternary systems all three components are immiscible, whereas in pseudobinary systems two of the components can be miscible and form an ‘alloy’, i.e. a single phase of multiple components.10 The latter of these is more favourable, as it is more likely to form a suitable BHJ for charge transport, so often third components are designed to be miscible with the matching component in the active layer.

Emma’s comments: The morphology of binary active layer blends can often change unfavourably upon heating above the glass transition temperature of the blend. In some cases, introducing a third component has improved the thermal stability of the active layer by inhibiting phase separation and crystallization through formation of an alloy.10 Further discussion of OPV stability can be found in the previous article in this series.

Whilst a combination of a fullerene-based acceptor and two donors dominated ternary cells for many years, with the advent of NFAs has come other possible combinations, such as cells using two NFAs with a donor, or cells using a hybrid combination of one PCBM derivative, and one NFA. The latter of these can combine the advantages of each type of acceptor so can show significant improvements over a binary system,11 albeit without truly capitalising on the advantages of NFAs.

Using a ternary active layer can also allow performance parameters to be improved simultaneously in a way that is not possible with binary cells. This can allow the efficiencies of lower performing organic semiconductors (OSCs) to be boosted, a common example being that of P3HT, which has been used to great effect with a combination of two non-fullerene acceptors.9

Emma’s comments: As well as small molecules and conjugated polymers, there have also been some organic ternary systems proposed using dye sensitisers, nanoparticles, and nanostructures to improve light harvesting.

 

The Physics of Ternary Systems

Yu et al. break down the basic mechanisms of three-component systems into three  intertwined categories: charge transfer, energy transfer, and parallel-linkage transfer.

The first of these, charge transfer, refers to an ‘energy cascade’ of components, whereby the third material fits between or around the band gap offset of the binary system (pictured below). Here the ternary system both generates excitons and provides extra charge-transport pathways for dissociated free charges, and thus boosts overall performance.

NFA energy cascade system with 1 donor & 2 acceptors
Figure 2: A published example of an energy cascade system using one donor (PBTA-BO) and two acceptors (IFBR and PC61BM).12

 

In systems where energy transfer dominates over charge transfer, the ternary component of the active layer will not generate free charges, but instead generate excited states and transfer these to one or both other components of the system. This energy transfer will occur by long-distance förster resonance (FRET) or short-distance Dexter energy transfer. In order for this to occur, there must be overlap between the emission spectrum of the energy donor and the absorption spectrum of the energy acceptor (which should be distinguished from the donor and acceptor of the charge carriers). In ideal cases, energy transfer will boost charge generation (and photocurrent) and overall efficiency.

Energy transfer system with PCDTBT, PBDTTT-EF-T donor & N2200 acceptor
Figure 4: A published example of an energy transfer system, where the PCDTBT transfers photogenerated excitons to both the PBDTTT-EF-T donor and N2200 acceptor through FRET.13

 

In the final mechanism detailed by Yu et al., the third component of the active layer will form a charge transfer network, or electronic alloy, with the other component of the same form, e.g. the two donors. In this way, the ternary cell functions as a parallel-linked tandem cell, with the network optimising morphology and light absorption beyond that of a simple binary mixture.

Parallel-linked system with a PDBT-T1 donor & popular NFA ITIC-Th,
Figure 4: A published example of a parallel-linked system using a PDBT-T1 donor and popular NFA ITIC-Th, that forms a single acceptor phase with another NFA., SdiPBI-Se.14

 

Final Thoughts

An interesting extension of ternary solar cells is to add even more components. There has been at least one study published using a quaternary active layer. One example15 utilised common OSCs in a mix of PTB7:PCDTBT:PC61BM:PC71BM in an attempt to fully maximise solar spectrum capture, albeit at the cost of increased complexity.

Despite the many possible advantages of ternary cells, few systems16 have exceeded the highest-performing binary systems.17 It is possible that this is due to the added complexities that a three-component active layer brings, such as the difficulty in optimising miscibility and morphologies, and that optimisation has proved too difficult to truly capitalise on the potential of a ternary system. However, looking past raw efficiencies, ternary cells have significant potential for improving stability and active layer morphology of OPVs, suggesting that further improvements are within reach. It seems likely that ternary cells require informed design, and that an understanding of the impact of the third component on charge transport, exciton dissociation, morphology, and stability is important to achieve progression in performance.

 

References

  1. Recent Progress in Ternary Organic Solar Cells Based on Nonfullerene Acceptors, R. Yu et al., Adv. Energy Mater., 1702814 (2018); DOI: 0.1002/aenm.201702814.
  2. Near IR Sensitization of Organic Bulk Heterojunction Solar Cells: Towards Optimization of the Spectral Response of Organic Solar Cells, M. Koppe et al., Adv. Funct. Mater. (20), 338–346 (2010); DOI: 10.1002/adfm.200901473.
  3. Organic ternary solar cells: A review, T. Ameri et al., Adv. Mater. (25), 4245–4266 (2013); DOI: 10.1002/adma.201300623.
  4. Efficiency Potential of Organic Bulk Heterojunction Solar Cells, B. Minnaert et al., Prog. Photovoltaics Res. Appl. (15), 741–748 (2007); DOI: 10.1002/pip.797.
  5. Nonfullerene Polymer Solar Cells with 8.5% Efficiency Enabled by a New Highly Twisted Electron Acceptor Dimer, Y. J. Hwang et al., Adv. Mater. (28), 124–131 (2016); DOI: 10.1002/adma.201503801.
  6. Highly efficient organic tandem solar cells: a follow up review, T. Ameri et al., Energy Environ. Sci. (6), 2390-2413 (2013); DOI: 10.1039/c3ee40388b.
  7. Solution-processed organic tandem solar cells with power conversion efficiencies >12%, M. Li et al., Nat. Photonics (11), 85–90 (2017); DOI: 10.1038/nphoton.2016.240.
  8. High-Performance Ternary Organic Solar Cell Enabled by a Thick Active Layer Containing a Liquid Crystalline Small Molecule Donor, G. Zhang et al., J. Am. Chem. Soc. (139), 2387–2395 (2017); DOI: 10.1021/jacs.6b11991.
  9. Reducing the efficiency-stability-cost gap of organic photovoltaics with highly efficient and stable small molecule acceptor ternary solar cells, D. Baran et al., Nat. Mater. (16), 363-370( 2017); DOI:10.1038/nmat4797.
  10. Glass Forming Acceptor Alloys for Highly Efficient and Thermally Stable Ternary Organic Solar Cells, A. D. de Zerio et al., Adv. Energy Mater., 1702741 (2018); DOI: 10.1002/aenm.201702741.
  11. Ternary Polymer Solar Cells based on Two Acceptors and One Donor for Achieving 12.2% Efficiency, W. Zhao et al., Adv. Mater. (29), 1604059 (2017); DOI:10.1002/adma.201604059.
  12. Improved Performance of Ternary Polymer Solar Cells Based on A Nonfullerene Electron Cascade Acceptor, B. Fan et al., Adv. Energy Mater., 1602127 (2017); DOI:10.1002/aenm.201602127.
  13. High-performance ternary blend all-polymer solar cells with complementary absorption bands from visible to near-infrared wavelengths, H. Benten et al., Energy Environ. Sci. (9), 135–140 (2016); DOI: 10.1039/C5EE03460D.
  14. Ternary Organic Solar Cells Based on Two Compatible Nonfullerene Acceptors with Power Conversion Efficiency >10%, T. Liu et al., Adv. Mater. (28), 10008-10015 (2016); DOI:10.1002/adma.201602570.
  15. Long-term efficient organic photovoltaics based on quaternary bulk heterojunctions, M. Nam et al., Nat. Commun. (8), 1–10 (2017); DOI: 10.1038/ncomms14068.
  16. Ternary organic solar cells offer 14% power conversion efficiency, Z. Xiao et al., Sci. Bull. (62), 1562–1564 (2017); DOI: 10.1016/j.scib.2017.11.003.
  17. 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.