A PhD Student Condenses: Introduction to Ternary Organic Solar Cells

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Introduction to Ternary Organic Solar Cells

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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, J_SC, of the device.

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

The Third Component

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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.

Whilst a combination of a fullerene-based acceptor and two donors dominated ternary cells for many years, with the advent of NFAs, there 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

The Physics of Ternary Systems

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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.

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.

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.

Final Thoughts

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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.