The morphology of the active layer in OPV devices is widely recognized as being crucial for their photovoltaic performance [1-4]. The physics of the system dictates that excitons must dissociate efficiently at a donor-acceptor interface, and that sufficient pathways for charge transport to the electrodes are also required. Conjugated polymer crystals are considered to be the primary hole carrier and thus are essential for effective charge transport. With this in mind, the ideal morphology for an organic photovoltaic BHJ film was often considered until a few years ago to be a bicontinuous, interpenetrating network morphology composed of pure P3HT and pure PCBM phases, with both phases of order ∼20 nm in size [5, 6] and numerous cartoon depictions have helped to propagate this view, as the one shown in Figure 1.
In this idealized model, the two pure phases of donor and acceptor within the bulk heterojunction are interdigitated in percolated highways with an average length scale of around 10-20 nm, equal to or less than the exciton diffusion length, to ensure exciton dissociation and high mobility charge carrier transport with reduced recombination. Furthermore, a pure donor phase at the hole collecting electrode and a pure acceptor phase at the electron collecting electrodes should exist in order to minimize the losses by recombination of opposite charges or acting as diffusion barriers for the opposite sign charge carriers at the respective electrodes. The presence of mixed phases in these BHJ were considered to be counterproductive to device performance, since isolated molecules could act as traps for separated charges and centers for charge recombination within the percolation pathways.
Many efforts have used this ideal model to design studies that examine the effect of the chemical structure of conjugated polymer, composition, and processing methods on the ability to achieve this ideal interpenetrating two-phase system . Furthermore, when describing the device physics of such organic photovoltaic devices, theoretical models have been developed which mainly relied on the assumption that the components existed as two separated pure phases [7-10].
Experimentally, although several different techniques have been used to study the morphology of these systems, part of the difficulties in the past in determining the precise composition of phases, interfacial structure, and morphology of bulk heterojunctions has been the limitations of contrast between the phases. For instance in standard electron-based techniques, crystalline P3HT and PCBM offer sufficient contrast between those two phases; however their amorphous counterparts are almost indistinguishable. Consequently conventional x-ray diffraction methods are unable to probe the amorphous regions in these conjugated polymer-fullerene mixtures. On the other hand, AFM techniques only allow the study of the morphology of surfaces and this might be very different from the morphology of the underlying bulk of the film. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) have also been used but they can only provide indirect information about the morphology of these BHJ. For these reasons, more recently neutron and soft x-ray scattering techniques have started being used to provide heretofore unavailable information concerning the bulk morphology of these bulk heterojunctions. Additionally techniques utilizing specific atomic or group specific contrast such as secondary ion mass spectroscopy (SIMS) have added further information to the growing wealth of morphological information about BHJs. One clear advantage on the use of neutrons lies in the fact that for instance in the case of P3HT/PCBM systems, the scattering length density (SLD) difference between P3HT (0.83x10-6 Å-2) and PCBM (4.3x10-6 Å-2) is sufficiently high, and no additional deuteration of one component is necessary.
Over approximately the last five years a reinterpretation of the existing idealized model has lead to a new understanding on the thermodynamics and the morphology of these bulk heterojunction systems. This chapter reviews some of the relevant literature relating to this new emerging understanding with concentration on the system of P3HT and PCBM, which has been the most widely studied OPV system to date.
2. The Thermodynamics and Phase Behavior of Conjugated Polymer-Fullerene Systems
Until a few years ago, the conventional wisdom was that the most widely studied OPV system of P3HT:PCBM was a simple two phase bulk heterojunction with well-defined interfaces between regions of approximately pure P3HT and pure PCBM. Then, from 2008 onwards several works in the literature began to show that this model was deficient in a number of ways since it did not account for the phase behavior of the mixtures or explain what the interface was between the phases or the two electrode interfaces. However, the results reported are often contradictory or conflicting as explained below.
One of the early studies to refute the concept of simple pure two phase behavior in BHJ systems was made by Muller
Additional studies by Kim
A further measurement of the phase diagram of P3HT:PCBM blends was made by Zhao
The effects of polymer crystallinity on miscibility have also been studied for P3HT and MDMO-PPV blended with PCBM by Collins
Small-angle neutron scattering (SANS) studies on 1:1 P3HT:PCBM blend thin films by Kiel
where Tm is the melting point at a solvent (i.e. PCBM) volume fraction , is the melting point of pure polymer,
Small-angle x-ray scattering (SAXS) studies of the phase separation of P3HT-PCBM thick films showed that a bulk heterojunction system blend of 1:0.7 P3HT:PCBM is not just a simple two-phase system with well-defined interfaces . As other workers have shown the phase behavior was shown to be a much more complicated system incorporating regions of crystalline P3HT and PCBM, and a mixed phase of amorphous P3HT and PCBM. The phase separation length scale was found to be ∼25 nm both before and after thermal annealing. The thermal annealing was also shown to cause a reduction in the phase separation associated with diffusion of PCBM into the amorphous P3HT, as previously also reported by Kiel
The influence played by the electrode interfaces on BHJ phase behavior has been studied by Chen
A direct comparison between conventionally prepared P3HT/PCBM BHJs (C-BHJ) and an active layer created by solution processed, layer-by-layer, sequential deposition (layer evolved BHJ, LE-BHJ) has been studied by Moon
A systematic study to understand how the P3HT molecular weight influences the P3HT/PCBM phase behavior and the corresponding device performance has been undertaken by Nicolet
Molecular mixing in P3HT/PCBM thin films annealed at 140ºC was also the focus of a grazing incidence small-angle neutron scattering (GISANS) study by Ruderer
As discussed above, the thermodynamics and phase behavior of conjugated polymer/fullerene systems is very complex and except in specific examples (such as P3HT/PCBM) it is still poorly understood. Even in the highly studied example of P3HT/PCBM there are conflicting results about the phase behavior. For example while some consider that these systems have an eutectic type of phase behavior [11-13] (as is usually found in metal mixtures and salt mixtures), others  are more in favor of the classical Flory-Huggins phase behavior common in amorphous polymer systems . There is also disagreement in the P3HT/PCBM miscibility limits (i.e. in the amount of molecularly dispersed PCBM in the P3HT phase) reported in the literature. For instance, while Kozub
In most of the studies described above, the main focus has been the study of the thermodynamics of the bulk and therefore the experimental samples were thick enough to avoid any relevant effect from the substrate’s surface energy. It is however known that in very thin polymer films (~100 nm) as used in OPVs, the thin film morphology and phase behavior is also affected by the substrate’s surface energy. This phenomenon has also been the object of intense research and it is known that the substrate’s free energy can induce vertical phase segregation (normal to the substrate’s surface). However, just like reports of bulk BHJ behavior there are also contradictory reports in the literature concerning the effect of the substrate’s surface energy on the final film morphology.
The compositional depth profiles in thin films of APFO-3:PCBM blends spin-coated from chloroform solutions onto silicon has been studied by Björström
Vertical segregation behavior in P3HT:PCBM (1:1) blend films were also studied by Kim
The importance of film thickness has been investigated by Verploegen
Further NR studies of morphology of the P3HT:PCBM thin films either freshly cast, or after solvent and thermally annealing have been made by Parnell
The use of ToF-SIMS has been able to add to the morphological model derived by NR measurements by examining both the vertical as well as the lateral distribution of PCBM and P3HT in BHJ films . In 150 nm thick films spun coated from a 1:1 weight ratio of P3HT:PCBM chlorobenzene solutions, ToF-SIMS imaging showed that the lateral phase separation (within the limit of the micron resolution of SIMS imaging) was similar before and after an annealing treatment at 140ºC for 30 min. However, depth profiling clearly shows a vertical phase separation of P3HT:PCBM on the pristine blend (before annealing), with a higher concentration of PCBM close to the PEDOT:PSS interface. On the other hand, after annealing, the cross-sectional images of PCBM and P3HT are both uniform along the vertical axis showing that the annealing treatment suppresses the vertical phase segregation. Using low voltage, high resolution TEM, Beal
Theoretical descriptions indicate a segregation preference for a typical photovoltaic device is where the donor (P3HT) is concentrated close to the substrate, and the acceptor (PCBM) next to the top surface, onto which the cathode (for example Al) is deposited. This distribution of components is expected to enhance the selectivity of the contacts towards one type of charge carrier and so reduce charge leakage. As discussed above, there are clearly contradictory results in the experimental literature concerning the exact nature of this vertical phase segregation. However, there is agreement that annealing leads to an increase of the PCBM concentration closer to the cathode [30, 33-35], and this has been pointed out as being one of the reasons for the improvement of device efficiencies that is usually observed upon annealing treatments. The timing of the annealing process, i.e. before or after the deposition of a metal electrode, is also known to influence the results. Post-production thermal annealing can improve the evolution of well-ordered nanoscale morphology because of a limitation of PCBM overgrowth, that is, due to the confinement effect.
3. Kinetic Considerations
As shown in Figure 2, Muller
The effect of drying kinetics on the resultant P3HT:PCBM blend has a profound effect on the final device characteristics , where it was found that films dried slowly had better performance characteristics (higher external quantum efficiency, higher power conversion efficiency, higher fill factor, and lower series resistance) than the rapidly dried films. The charge carrier mobility of holes and electrons in P3HT:PCBM thick films was shown to have more balanced transport properties and non-dispersive dynamics for the slowly dried films, where as the rapidly dried film displayed dispersive dynamics and unbalanced transport. All these differences in performance were explained by the rate of solvent evaporation, as fast solvent loss quenches the phase separation process, and conversely the longer the blend is mobile and contains solvent the more the mixture will proceed to a more phase separated state. Campoy-Quiles
The time-dependent morphology evolution of blend films of P3HT and PCBM, was investigated by Jo
The reasons for the irreproducibility of the performance of P3HT:PCBM BHJ solar cells fabricated using nominally identical conditions has been investigated by de Villers
The rate of evaporation is clearly affected by the boiling point and vapor pressure of the solvents used. The influence solvent boiling point on the morphology and photovoltaic performance of P3HT:PCBMBHJ films produced via spin-coating, has been studied by Ruderer
The rate of cooling, after annealing, also has a dramatic influence on the final morphology of the film. A slower cooling rate leads to a greater extent of crystallization, when semi-crystalline polymers or crystalline nano-particles are involved. Despite this fact, only recently have some authors drawn attention to this important factor .
Recent detailed studies of the phase behavior of conjugated polymer:fullerene blends have begun to provide important information for understanding the property-processing relationships in organic solar cell blends. Until these studies conventional wisdom suggested that the P3HT:PCBM system was a simple two phase bulk heterojunction with well-defined interfaces between regions of pure P3HT and pure PCBM. The latest results clearly demonstrate that this assumption is not correct, but instead unequivocally show that BHJ materials are more complex systems incorporating regions of crystalline P3HT, PCBM, and intermixed regions of amorphous P3HT and PCBM. Upon annealing there is considerable interdiffusion of PCBM into the amorphous P3HT. It has also been shown that in the case of thin BHJ films (~100 nm) such as those used in OPVs, the substrate’s surface energy can have a crucial role on the final morphology.
Furthermore, the kinetics as manifested in the processing routes and conditions can also play a dominant role in the observed structure/morphology. For all these reasons it is very difficult to predict the structure of the BHJ since apparently the same process used by different groups can end to with different results.
Whilst significant insight has been derived in understanding the behavior of a limited number of systems, with P3HT:PCBM the most widely studied system by far, there is still no predictive understanding in even this system. If technologically meaningful device efficiencies are to be achieved significant additional work must be undertaken to derive predictive capability for these complex BHJ systems. This will ultimately require a global effort of complementary experimental measurements combined with theoretical and computational modeling.
AFM – Atomic Force Microscopy
BHJ – Bulk Hetero-Junction
DMTA – Dynamic Mechanical Thermal Analysis
GISANS - Grazing Incidence Small-Angle Neutron Scattering
GIXS – Grazing Incidence X-ray Scattering
GIWAXS/GIXD – Grazing Incidence Wide-Angle X-ray Scattering/x-ray diffraction
ITO – Indium Tin Oxide
MEH-PPV - poly [2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylene-vinylene]
MDMO-PPV – poly(2-methoxy-5-(3’,7’-dimethyloctyloxy)-1,4-phenylenevinylene)
NEXAFS – Near-Edge X-ray Absorption Fine Structure
NR – Neutron Reflectivity
OPV – Organic Photo-Voltaic
P3HT – poly(3-hexylthiophene)
PCBM – phenyl-C61-butyric acid methyl ester
PEDOT:PSS – Poly(3,4-EthyleneDiOxyThiophene):Poly(StyreneSulfonate)
SANS – Small Angle Neutron Scattering
SAXS – Small-Angle X-ray Scattering
SIMS – Secondary Ion Mass Spectrometry
TEM – Transmission Electron Microscopy
ToF-SIMS – Time-of-Flight Secondary Ion Mass Spectrometry
UCST – Upper Critical Solubility Temperature
UV-Vis – Ultra-Violet Visible
XRD – X-ray diffraction
WAXS – Wide-Angle X-ray Scattering
Gabriel Bernardo acknowledges financial support from the IPC's (Institute for Polymers and Composites) strategic project: "PEst-C/CTM/LA0025/2011" (Projecto Estratégico—LA 25—2011-2012—Strategic Project—LA 25—2011-2012).
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