**3. Kinetic Considerations**

The use of ToF-SIMS has been able to add to the morphological model derived by NR meas‐ urements by examining both the vertical as well as the lateral distribution of PCBM and P3HT in BHJ films [35]. 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 sep‐ aration (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 verti‐ cal axis showing that the annealing treatment suppresses the vertical phase segregation. Us‐ ing low voltage, high resolution TEM, Beal *et al.* [36] revealed some details of the mechanism of PCBM domain migration associated with the vertical segregation within P3HT:PCBM so‐

Xue *et al.* [37] have used the variation of the post-annealing cooling rate to create a series of "snapshots" of the reorganization processes that occur upon annealing. P3HT:PCBM blend devices exhibit a complex vertical stratification of both crystallinity and blend composition. Using a combination of UV-vis spectroscopy, XRD, NEXAFS, AFM, and contact angle meas‐ urements, they showed that annealing resulted in the formation of three distinct vertical lay‐ ers. Diffusion of PCBM from the interfaces into the bulk of the film results in the formation of (a) a P3HT rich substrate interfacial (wetting) layer, (b) a homogeneous 'bulk'central lay‐ er, and (c) a P3HT-rich air interfacial (capping) layer. The orientation of the P3HT molecules was shown to vary from *c*-axis P3HT alignment in the wetting layer at the substrate inter‐ face to an *a*-axis aligned in the capping layer at the air interface and also in the bulk layer. The data showed that by slowing the post-annealing cooling rate devices with significantly enhanced efficiencies can be prepared. This improvement in device performance was corre‐ lated with the observed increased crystallinity and hence polymer alignment, and also phase segregation both at the interfaces and in the bulk film. In particular, they found that slow cooling resulted in an aligned interfacial active layer/substrate structure that is benefi‐

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 distribu‐ tion 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 contradic‐ tory 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 anneal‐ ing 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

lar cells by giving direct confirmation of P3HT and PCBM crystallization.

cial for charge transport.

218 Optoelectronics - Advanced Materials and Devices

PCBM overgrowth, that is, due to the confinement effect.

As shown in Figure 2, Muller *et al.* [11] and Kim *et al.* [12] reported an eutectic composition (Ce) of Ce ≈ 35 – 40 wt% PCBM and Zhao *et al.* [13] a value of Ce≈ 60 wt% PCBM. However, whilst these authors were trying to extract the thermodynamic behavior of the blends, kinet‐ ic factors are clearly affecting the resulting observations. It is these types of discrepancies in observed phase behavior that highlights the difficulty in understanding the fundamental relationship between morphology and device performance. Although the morphology of the BHJ is ultimately determined by thermodynamic factors, the observed thin film morphology can be dominated by the kinetics of the film preparation methods and parameters used, including solvent evaporation/drying rates and by the kinetics of cooling after annealing. Herein lies some of the answer as to the reason for the contradictions in the observed phase behavior and morphology of these systems, since in putting together a device, the process‐ ing conditions between different groups often vary and also almost certainly do not allow the system to reach a global free energy minima and hence true a thermodynamic state. Changes to processing conditions are well known to make big variations in device performance, but there are also much subtler effects associated with the well-known irreproducibility in other‐ wise identical processing conditions, which are problematic for OPV development.

The effect of drying kinetics on the resultant P3HT:PCBM blend has a profound effect on the final device characteristics [38], where it was found that films dried slowly had better per‐ formance characteristics (higher external quantum efficiency, higher power conversion effi‐ ciency, 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 *et al.* [29] have studied the morphology changes induced by slow dry‐ ing and vapour annealing and showed that the PCBM concentration profile changes as the spin-coating speed (and hence the rate of drying) is reduced. Slow drying has a qualitatively similar effect to thermal annealing, whereby the composition gradient becomes more pro‐ nounced and the surface-segregated PCBM concentration increases.

The time-dependent morphology evolution of blend films of P3HT and PCBM, was investi‐ gated by Jo *et al.* [39], using two different annealing treatments with different morphology evolution time scales, i.e. a high-temperature thermal annealing (150 ºC), and a room-tem‐ perature solvent annealing. Comparing the morphological changes of the blend films after the two annealing treatments, solvent annealing resulted in a more favorable BHJ morpholo‐ gy than thermal annealing. The poor BHJ morphology after thermal annealing under these experimental conditions was attributed to the relatively fast diffusion and aggregation of the PCBM molecules during P3HT crystallization, which interfered with the growth of the elon‐ gated fibrillar P3HT crystals and subsequent evolution of the well-ordered BHJ morpholo‐ gy. These results are however, seemingly contradictory to those of Parnell *et al.* [34], suggesting that there are other factors that are contributing to the observed behaviors.

Figure 6, was compared with the structural changes during crystallization of the ternary P3HT:PCBM:solvent system observed in real time by grazing incidence x-ray diffraction (GIXD). It was shown that PCBM only crystallizes at the final stage of drying although its

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**Figure 6.** a) Schematic of the experimental setup for simultaneous real time GIXD and laser reflectometry of doctorbladed thin films in a controlled drying environment. (b) Schematic of P3HT unit cell. (c) Ternary phase diagram of P3HT-PCBM-DCB; the star symbols denote phase transitions in the binary cases. From reference [43]. "Reprinted with

Sobkowicz *et al.* [44] have measured the time- and temperature dependent aggregation of P3HT in o-dichlorobenzene solutions using rheometry and small-angle neutron scattering (SANS). They tried to set a starting point for understanding important aspects of morpholo‐ gy development in drying BHJ films. They found that the presence of PCBM dramatically slows the aggregation of P3HT in solution. Concentration and temperature dependencies were identified, with the latter being the more sensitive parameter. Analysis of the SANS data showed a strong affinity between P3HT and DCB with an interaction parameter that becomes more negative with increasing concentration, and elevated temperature dissolution suggesting UCST behavior in this polymer−solvent system. Modest cooling of P3HT solu‐ tions even in "good" solvents such as DCB resulted in rapid aggregation, although the ag‐ gregated solutions do not completely phase separate, but rather form physical gels consisting of anisotropic structures. An increase in solution modulus of several orders of

permission from (ACS Nano 5 (2011) 8579). Copyright (2012) American Chemical Society."

solubility limit is reached a very early stage of solvent evoporation.

The reasons for the irreproducibility of the performance of P3HT:PCBM BHJ solar cells fab‐ ricated using nominally identical conditions has been investigated by de Villers *et al.* [40]. They showed that this irreproducibility is the result of the occurrence of vertical phase seg‐ regation of P3HT to the top surface, which is controlled by subtle factors in the kinetics of solvent evaporation during spin-coating. When this type of vertical phase separation occurs, electron extraction is hindered by the poor contact between the PCBM component of the BHJ and the cathode.

Wang *et al.* [41] have used in-situ ellipsometry and grazing incidence x-ray scattering (GI-XS) to study molecular self-organization in P3HT and PCBM blend films in real time, during the drying process as they are cast from solution. They have identified three stages in film drying: (I) rapid solvent-evaporation, (II) moderate solvent-evaporation and rapid crystalli‐ zation, and (III) slow solvent-evaporation and slow crystallization. They showed that the on‐ set of fast crystallization commences when the volume fraction of P3HT:PCBM in a wet film reaches a critical volume fraction of 50%. The observed crystallization growth-mechanism is consistent with a heterogeneous nucleation process in which defects or impurities act as nu‐ cleation sites.

The rate of evaporation is clearly affected by the boiling point and vapor pressure of the sol‐ vents used. The influence solvent boiling point on the morphology and photovoltaic per‐ formance of P3HT:PCBMBHJ films produced via spin-coating, has been studied by Ruderer *et al*. [42]. The four solvents considered were chloroform (CF), toluene, chlorobenzene (CB) and xylene. Solar cells made using these solvents had different photovoltaic performances. Using a wide range of experimental techniques it was shown that solubility-driven cluster formation of PCBM occurred in these systems. In films made using solvents with poor solu‐ bility of PCBM many more clusters were formed. As-spun films showed no P3HT crystallin‐ ity, independent of the solvent used. After annealing, P3HT crystals formed with edge-on configuration relative to the substrate as the main orientation, with crystal lattice constants that were also independent of the solvent used. However the crystal sizes increased with in‐ creasing boiling point (i.e. decreasing evaporation rates) of the solvents used, which was at‐ tributed to the increased drying time during spin-coating and residual solvent in the BHJ films. For toluene-, CB-, and xylene-made films, lateral nanostructures were found. In the vertical direction P3HT enrichment layers were detected for toluene- and CB-made films and PCBM enrichment layers for the films made using toluene and xylene. Nevertheless films made using toluene, CB, and xylene showed similar photovoltaic performance. Con‐ versely, films made using chloroform presented a layered structure with a disadvantageous material distribution, with the P3HT hole conductor at the top electrode and the PCBM elec‐ tron conductor at the electron-blocking layer, consequently producing efficiencies that were significantly lower.

Schmidt-Hansberg *et al.* [43] have investigated the dynamics and thermodynamics of molec‐ ular ordering, in P3HT/PCBM mixtures, during film drying from 1,2-dichlorobenzene (DCB). The pathway through the phase diagrams of P3HT and PCBM solutions, as shown in Figure 6, was compared with the structural changes during crystallization of the ternary P3HT:PCBM:solvent system observed in real time by grazing incidence x-ray diffraction (GIXD). It was shown that PCBM only crystallizes at the final stage of drying although its solubility limit is reached a very early stage of solvent evoporation.

gy. These results are however, seemingly contradictory to those of Parnell *et al.* [34], suggesting that there are other factors that are contributing to the observed behaviors.

The reasons for the irreproducibility of the performance of P3HT:PCBM BHJ solar cells fab‐ ricated using nominally identical conditions has been investigated by de Villers *et al.* [40]. They showed that this irreproducibility is the result of the occurrence of vertical phase seg‐ regation of P3HT to the top surface, which is controlled by subtle factors in the kinetics of solvent evaporation during spin-coating. When this type of vertical phase separation occurs, electron extraction is hindered by the poor contact between the PCBM component of the

Wang *et al.* [41] have used in-situ ellipsometry and grazing incidence x-ray scattering (GI-XS) to study molecular self-organization in P3HT and PCBM blend films in real time, during the drying process as they are cast from solution. They have identified three stages in film drying: (I) rapid solvent-evaporation, (II) moderate solvent-evaporation and rapid crystalli‐ zation, and (III) slow solvent-evaporation and slow crystallization. They showed that the on‐ set of fast crystallization commences when the volume fraction of P3HT:PCBM in a wet film reaches a critical volume fraction of 50%. The observed crystallization growth-mechanism is consistent with a heterogeneous nucleation process in which defects or impurities act as nu‐

The rate of evaporation is clearly affected by the boiling point and vapor pressure of the sol‐ vents used. The influence solvent boiling point on the morphology and photovoltaic per‐ formance of P3HT:PCBMBHJ films produced via spin-coating, has been studied by Ruderer *et al*. [42]. The four solvents considered were chloroform (CF), toluene, chlorobenzene (CB) and xylene. Solar cells made using these solvents had different photovoltaic performances. Using a wide range of experimental techniques it was shown that solubility-driven cluster formation of PCBM occurred in these systems. In films made using solvents with poor solu‐ bility of PCBM many more clusters were formed. As-spun films showed no P3HT crystallin‐ ity, independent of the solvent used. After annealing, P3HT crystals formed with edge-on configuration relative to the substrate as the main orientation, with crystal lattice constants that were also independent of the solvent used. However the crystal sizes increased with in‐ creasing boiling point (i.e. decreasing evaporation rates) of the solvents used, which was at‐ tributed to the increased drying time during spin-coating and residual solvent in the BHJ films. For toluene-, CB-, and xylene-made films, lateral nanostructures were found. In the vertical direction P3HT enrichment layers were detected for toluene- and CB-made films and PCBM enrichment layers for the films made using toluene and xylene. Nevertheless films made using toluene, CB, and xylene showed similar photovoltaic performance. Con‐ versely, films made using chloroform presented a layered structure with a disadvantageous material distribution, with the P3HT hole conductor at the top electrode and the PCBM elec‐ tron conductor at the electron-blocking layer, consequently producing efficiencies that were

Schmidt-Hansberg *et al.* [43] have investigated the dynamics and thermodynamics of molec‐ ular ordering, in P3HT/PCBM mixtures, during film drying from 1,2-dichlorobenzene (DCB). The pathway through the phase diagrams of P3HT and PCBM solutions, as shown in

BHJ and the cathode.

220 Optoelectronics - Advanced Materials and Devices

cleation sites.

significantly lower.

**Figure 6.** a) Schematic of the experimental setup for simultaneous real time GIXD and laser reflectometry of doctorbladed thin films in a controlled drying environment. (b) Schematic of P3HT unit cell. (c) Ternary phase diagram of P3HT-PCBM-DCB; the star symbols denote phase transitions in the binary cases. From reference [43]. "Reprinted with permission from (ACS Nano 5 (2011) 8579). Copyright (2012) American Chemical Society."

Sobkowicz *et al.* [44] have measured the time- and temperature dependent aggregation of P3HT in o-dichlorobenzene solutions using rheometry and small-angle neutron scattering (SANS). They tried to set a starting point for understanding important aspects of morpholo‐ gy development in drying BHJ films. They found that the presence of PCBM dramatically slows the aggregation of P3HT in solution. Concentration and temperature dependencies were identified, with the latter being the more sensitive parameter. Analysis of the SANS data showed a strong affinity between P3HT and DCB with an interaction parameter that becomes more negative with increasing concentration, and elevated temperature dissolution suggesting UCST behavior in this polymer−solvent system. Modest cooling of P3HT solu‐ tions even in "good" solvents such as DCB resulted in rapid aggregation, although the ag‐ gregated solutions do not completely phase separate, but rather form physical gels consisting of anisotropic structures. An increase in solution modulus of several orders of magnitude accompanies the aggregation and the pure polymer solution modulus becomes frequency-independent. Although the percent crystallinity in the aggregated solutions can‐ not be calculated directly from these results, it appears that the crystalline fraction is lower in the solutions with PCBM present. These results also lend insight into the film drying process and will ultimately lead to improved processing procedures in pursuit of an opti‐ mized morphology for bulk heterojunction devices.

DMTA – Dynamic Mechanical Thermal Analysis

GIXS – Grazing Incidence X-ray Scattering

ITO – Indium Tin Oxide

NR – Neutron Reflectivity

OPV – Organic Photo-Voltaic

P3HT – poly(3-hexylthiophene)

PCBM – phenyl-C61-butyric acid methyl ester

SANS – Small Angle Neutron Scattering

SIMS – Secondary Ion Mass Spectrometry

TEM – Transmission Electron Microscopy

UCST – Upper Critical Solubility Temperature

2011-2012—Strategic Project—LA 25—2011-2012).

ToF-SIMS – Time-of-Flight Secondary Ion Mass Spectrometry

SAXS – Small-Angle X-ray Scattering

UV-Vis – Ultra-Violet Visible

WAXS – Wide-Angle X-ray Scattering

XRD – X-ray diffraction

**Acknowledgements**

GISANS - Grazing Incidence Small-Angle Neutron Scattering

NEXAFS – Near-Edge X-ray Absorption Fine Structure

GIWAXS/GIXD – Grazing Incidence Wide-Angle X-ray Scattering/x-ray diffraction

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MDMO-PPV – poly(2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene)

MEH-PPV - poly [2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylene-vinylene]

PEDOT:PSS – Poly(3,4-EthyleneDiOxyThiophene):Poly(StyreneSulfonate)

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—

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-crystal‐ line polymers or crystalline nano-particles are involved. Despite this fact, only recently have some authors drawn attention to this important factor [37].
