This paper outlines experimental results on a model system of PCBM and PS for control parameter exploration of PCBM crystallisation. PS layer attributes, and annealing temperature amongst others are the variables explored.
Research Article
Nucleation Front Proliferation in Bi-modal PCBM Crystals, the spherulite – axial transition
https://doi.org/10.21203/rs.3.rs-2030998/v1
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This paper outlines experimental results on a model system of PCBM and PS for control parameter exploration of PCBM crystallisation. PS layer attributes, and annealing temperature amongst others are the variables explored.
PCBM can crystallise when heated above the glass transition (Y. Yang et al. 2014). Various crystal morphologies are observed depending on the substrate, solvent and the temperature (G. Li et al. 2008; Dang, Hirsch, and Wantz 2011; Volonakis, Tsetseris, and Logothetidis 2012). Two forms of PCBM crystal are reported in the literature: micron-sized crystals (needles or branched crystals) and nano-scale crystals (Yang et al. 2004; H. H. Lee et al. 2013; Môn et al. 2015). Micron-sized crystals can rupture the carefully developed BHJ morphology, drastically reduce the interfacial (heterojunction) area between the donor and acceptor, and decrease the PCE of a cell (Woo et al. 2008).
Mon et al.( Môn et al 2015) found evidence for both nano-scale and micron-sized PCBM crystals in annealed PCBM/PS bilayers (the PS layer on top), with the growth of both kinds of crystal strongly dependent on the thickness of the PS layer (for thicknesses below around 30 nm).
This shows grazing-incidence x-ray diffraction (GIXD) patterns from two bilayers (labelled h and i) and also an annealed PCBM single layer (labelled k).
This k shows the presence of crystallisation (characteristic Bragg peaks, due to PCBM crystals that have been seen by previous workers (Verploegen et al. 2010; Hopkinson et al. 2011)) that is not visible by optical microscopy, as the inset image is an optical micrograph showing no features on the sample surface while the GIXD pattern shows Bragg peaks indicative of extensive crystalline ordering.
This h and i show similar levels of coverage by micron-sized PCBM crystals to one another (observable via optical microscopy as in the inset image), but very different GIXD patterns. The Bragg peaks seen in
This h and i suggest the presence of significant nano-scale crystals (as opposed to the micron sized crystals visible by optical microscopy) for the thinner (8nm) PS layer sample, in comparison to the thicker (25nm) PS layer sample (which just shows a ring-like scattering pattern from amorphous PCBM). Mon et al. proposed a mechanism for the growth rate of both micron-sized needles and nanocrystals, involving the interaction between these processes, as a function of PS film thickness.
In the liquid-solid transition, the state of the order changes discontinuously, making it a first order phase transition. There is a change in entropy, related to the latent heat that is released, , and the melting temperature, :
If a liquid is held at exactly its melting temperature, however, it will not crystallise. This is because the formation of a crystal creates interfacial energy, , between the liquid and the solid. is defined as the temperature at which the free energies of the liquid and the solid are equal. Because of the energetic penalty from the interface it is necessary to undercool a liquid to achieve crystallisation. It is also necessary to give the system time to sample the microstates of the least energetic configuration, below this temperature, the resulting crystalline phase will be in a more highly ordered state than the liquid. Crystallisation usually occurs via a nucleation and growth mechanism, it might take some time for a sufficiently large initial crystalite to form that overcomes the penalty for the crystal/melt interface.
The increase in free energy on melting is greater further from ; which leads to an increase in nucleation as T is lowered. There is also an opposing factor that the mobility of the molecules is lowered as T is lowered and so nucleation and growth are slower. Thus there is a maximum region of temperature between the melting and glass transition temperatures where nucleation is at its greatest, which has been shown for PCBM to be at T = 150 ˚-170˚C (Lindqvist et al. 2013).
Crystal growth in PCBM can take many forms; the fullerene can pack in different ways each representing a local energy minimum and with each structure having different electronic properties (Volonakis, Tsetseris, and Logothetidis 2012). PCBM has been shown to form both nanometre-sized crystals(Môn et al. 2015; Hopkinson et al. 2011; Verploegen et al. 2010) and needle-like micron-sized crystals (Swinnen et al. 2006). It is evident in these images that there is some branching of the needles when cast from a 1:2 solution and annealed at 125˚C. This data is from a blend of P3HT:PCBM.
Such branching may be an early indication of the formation of a spherulite. A spherulite is a roughly spherical (or circular in a confined 2D geometry) formation of a crystal, usually produced by nucleation fronts growing in all directions from the nucleation site, but they can also be formed from needle-like crystals where there is a secondary growth front nucleated along the crystal length.
This occurs when the reorientation of the crystallising molecules is much slower than the interface propagation leading to difficulty aligning with the parent crystal (Granasy et al. 2005)
In this section we are looking at the formation of PCBM crystals in PCBM/PS bilayers. Whereas in the preceding chapters the annealing times were carefully controlled to minimise PCBM crystallisation (both nano-crystal and micron-sized), the purpose of this chapter is to explicitly examine the morphology of micron-sized crystals. This is achieved by using longer annealing times at higher temperatures. The results are found by optical microscopy and AFM.
This shows the growth of PCBM crystals in a bilayer following annealing. There were two types of crystal observed in this system which was seen to be dependent on the film thickness. This figure also highlights the effect of temperature on crystal growth rate as at higher temperatures we get much faster growth however with a lower nucleation density. The top morphology will be referred to as ‘needle’ and the bottom as ‘branched’ or ‘fan’. In this section results from a study on this behaviour are shown with an emphasis on the fans as there has been significant work done on needle crystals by others (Môn et al. 2015). The primary interest here is not the growth-rate behaviour of the crystals (investigated extensively as a function of film thickness by Môn et al. 2015), but the nature of the morphology. In particular whether the crystals grow as needles or fans, and the branching behaviour within the fans. The crystal structure itself is not investigated in this study, but the needle-like crystals found in PCBM/PS bilayers (Môn et al. 2015) and PCBM/polymer blends (G. Li et al. 2008) are shown to be single crystals (using selected-area electron diffraction measurements). Both needles and branched crystallites have been observed in pure PCBM films annealed at 220Cand 240C respectively by Zheng et. al (Zheng et al. 2011).
To probe the effect of film thickness on the formation of crystals of differing morphology, a sequence of samples was prepared with a PCBM nominal thickness of 15, 25, 30, 35 and 55nm and PS thicknesses of 20, 30, 40, 50 and 60nm. The thicknesses measured by AFM are shown in the appendix to this chapter. These samples used both 20k and 300k MW PS. Two temperatures were looked at: 170˚C and 180˚C.
The samples were annealed in-situ on the optical microscope for different times, chosen to allow the crystals to form to a visible degree without impinging on the other crystals. Annealing took place in the dark, with the microscope shutter opened periodically to take images. It was observed that annealing time does not cause a transition between needles and fans but rather the morphology is decided from the initial nucleation with, in some cases, very small (10–20µm) fans appearing with the distinctive branching of the crystal front. Typical optical micrographs of PCBM crystals for different layer thicknesses are shown, with corresponding morphological categorisations (‘phase diagrams’) given. Examination of these figures reveals that the PS thickness does not have a major effect on whether the crystal morphology is needle-like or fan-like, nor does a 10 oC change in the temperature or the PS MW. There are some differences between the morphological categorisations with temperature and PS layer thickness, but the clearest finding is the overall transition from needle-like crystals to fan-like crystals as the PCBM thickness is increased, with the appearance of fans for PCBM thicknesses of around 30nm or above. Other studies have found the occurrence of both needles and fans in PCBM/polymer blends but have reported the transition between these two morphologies to be controlled by annealing temperature (Wong et al. 2014) or the ratio of the blend (Swinnen et al. 2006). The ratio of the layer thickness in this study was not found to have a discernible effect but rather the only acting parameter was the PCBM thickness.
In this section the effect of temperature on the crystal morphology will be presented; in particular the morphology of the fan-like crystals. Figures 4.5–10 and 4.5–11 show a series of optical microscopy and AFM images from samples annealed at a range of different temperatures. The fans grow from an initial stem that resembles a needle in optical microscopy but when looked at in AFM it is clear there are ridges on the uppermost surface, as in, that then split into separate branches or fibrils. Temperature clearly has a strong effect on the size and growth rate of these crystals.
This shows OM images of fan crystals showing the increase in size and decrease in nucleation density of the crystals as the temperature is increased. The focus of this chapter is to qualitatively categorise crystal morphology as either a needle or a fan. Quantifying the characteristics of the crystals poses problems. Measurement of the width of fan crystals (trough to trough distance as shown in at the nucleation site (the ‘stem width’; reveals similar behaviour with temperature to that found by Mon et al for needles(Môn et al. 2015). The other parameter that could be measured is the width of the fibrils within the branching crystal microstructure, however the branching causes variations in the fibril width so this parameter is difficult to quantify.
Mostly the branching fibrils appear to fill the space between the outermost arms of the fan crystals. Fibril branching has been the subject of a number of theoretical studies, with proposed branching mechanisms including tip-splitting instabilities (due for example to the behaviour of temperature or impurity gradients that occurs when growing planar crystal front are subject to a pertubation) (Langer 1980; Crist and Schultz 2016) and non-crystallographic branching (due to nucleation of new crystallographic orientations at the growing crystal front) (Granasy et al. 2014). However, the fibril width is not easy to quantify experimentally. There are a range of fibril widths within a particular crystal (there is a range of at least a factor of two, either side of the point where a single fibril branches into two fibrils, and some crystals show a much broader range of feature sizes). The branching also seems to be halted within some samples, where non-branching fibrils overtake branching fibrils after some initial period of time. This is shown in Fig. 4.5–15; branching fibrils closer to the nucleation site are overtaken by fibrils that do not branch (even though those near to the growing tip do not appear to be impeded by neighbouring fibrils). Careful inspection of Fig. 4.5–15 also shows some kind of ‘frustrated bifurcation’ in the non-branching fibrils, where an indentation runs along the centre of the fibrils. Finally, this non-branching behaviour is also associated with the depressions either side of the fibrils (regions from which the bottom PCBM layer has been depleted to form the growing crystal (Môn et al. 2015)) going right down to a constant depth (indicative of full depletion of the bottom PCBM layer, leaving only the PS layer). This is shown in Fig. 4.5–19 where non-branching fibrils are all bounded by fully depleted depressions, in regions that are sufficiently far from the crystal nucleation site. We would expect from some theories (Gránásy et al. 2004) that the fans might be precursors to spherulites, however, we do not see much evidence of significant curvature in the crystal edges in any samples. Interestingly, the branching density is significantly lower in some of the higher temperature samples (eg; the 230˚C sample.. However, there is not sufficient data to state such temperature-dependent effects categorically.
Overall, varied crystal morphologies have been observed, with a systematic dependence of this on PCBM layer thicknesss. At present it is not clear exactly why this thickness dependence occurs, or what controls the value of the transition thickness between fans and needles (at around 30nm).
Crystal length for individual crystals is shown as a function of annealing time and temperature respectively. These results are in broad agreement with measurements made by Mon et. al, (who see an approximately constant growth rate on these timescales in films with PS thicknesses of around 40 nm or above). The annealing temperature is seen to have an effect on the rate of growth, with a higher growth-rate at higher temperature. For a more detailed analysis see Mon et al.
In this investigation, it appears that the presence of the layer of PS provides an energetic nucleus that continusously allows that needle-like crystal formation by enabling the crystal fron to proliferate with ease due to the contact with the PS layer owing to the crystal upper surface rupturing both layers (see fig X) in the bi-layers. Accordinginly, the smalller the initially nucleated stem, them less energetic barrier lowering occurs as there is less contact with the PS which acts as in impurity. Layer nucleation is known as in Graszy (reff).
Contributions;
The overall structure of the project was designed by EH and EH was responsible for the development of sample preparation methodologies and the design of experiments. EH performed the sample preparationand wrote the manuscript. The experiments were performed at Swansea University College of Engineering.
Funding and/or Conflicts of interests/Competing interests
Funding provided by Swansea University. No conflicts of interest.
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