**1. Introduction**

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206 Optoelectronics - Advanced Materials and Devices

93, 1914-1917.

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 numer‐ ous 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 hetero‐ junction 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 collect‐ ing 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 counterpro‐ ductive 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 abil‐

© 2013 Bernardo and Bucknall; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Bernardo and Bucknall; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ity to achieve this ideal interpenetrating two-phase system [1]. Furthermore, when describ‐ ing 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 sepa‐ rated pure phases [7-10].

emerging understanding with concentration on the system of P3HT and PCBM, which has

Recent Progress in the Understanding and Manipulation of Morphology in Polymer: Fullerene Photovoltaic Cells

http://dx.doi.org/10.5772/51115

209

Until a few years ago, the conventional wisdom was that the most widely studied OPV sys‐ tem of P3HT:PCBM was a simple two phase bulk heterojunction with well-defined interfa‐ ces 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 re‐

One of the early studies to refute the concept of simple pure two phase behavior in BHJ sys‐ tems was made by Muller *et al.* [11], who studied the phase behavior of PCBM with P3BT, P3HT and P3DDT using a combination of DSC, optical microscopy and X-ray diffraction (XRD) and related this to the solar cell efficiency for a series of devices with different blend compositions. These binary systems were shown to feature simple eutectic phase behavior (Figure 2(a)), with a eutectic composition of 35 wt% PCBM in the PCBM:P3HT system, and that the optimum composition for device performance is slightly hypoeutectic when ex‐

Additional studies by Kim *et al.* [12], again using a combination of XRD and DSC, investigat‐ ed the phase behavior of PCBM with P3HT, MDMO-PPV and MEH-PPV. They observed both P3HT melting point depression and glass transition temperature elevation in the P3HT:PCBM blends as a function of increasing PCBM wt%. However, as shown in Figure 2, the phase diagram they obtained differs from that of Muller *et al.* [11]. The determined solu‐ bility limits of PCBM in P3HT (Figure 2(b)), MDMO-PPV and MEH-PPV were 30, 40 and 50 wt% respectively. By measuring field effect conduction in transistors, a strong correlation was found between the phase behavior and the charge transport in the studied conjugated polymer/fullerene blends with hole-only transport being observed below the solubility limit

A further measurement of the phase diagram of P3HT:PCBM blends was made by Zhao *et al.* [13] (Figure 2(c)) using conventional and modulated temperature DSC (DSC and MTDSC). Again the phase behavior differs from both Muller *et al* and Kim *et al*. Zhao and co-workers observed a single glass transition temperature (Tg) for all compositions, which increases with increasing concentration of PCBM from 12.1ºC for pure P3HT to 131.2ºC for pure PCBM. It was found that the film morphology of the blends results from a dual crystal‐ lization behavior in which the crystallization of each component is hindered by the other component. The phase diagram also showed that the morphology of the blends with 45-50 wt% PCBM, whilst giving the highest device performance, is intrinsically unstable at the de‐ sired maximum operating temperature of 80ºC, as the Tg is less than 40ºC. More recently, the

**2. The Thermodynamics and Phase Behavior of Conjugated Polymer-**

sults reported are often contradictory or conflicting as explained below.

been the most widely studied OPV system to date.

pressed in terms of the polymer component.

and ambipolar transport at higher PCBM fractions.

**Fullerene Systems**

**Figure 1.** The ideal structure of a bulk heterojunction solar cell as represented by Sariciftci *et al* [5]. "Reprinted with permission from (Chemical Reviews 107 (2007) 1324). Copyright (2012) American Chemical Society."

Experimentally, although several different techniques have been used to study the morphol‐ ogy of these systems, part of the difficulties in the past in determining the precise composi‐ tion of phases, interfacial structure, and morphology of bulk heterojunctions has been the limitations of contrast between the phases. For instance in standard electron-based techni‐ ques, crystalline P3HT and PCBM offer sufficient contrast between those two phases; how‐ ever 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 under‐ lying 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 scat‐ tering techniques have started being used to provide heretofore unavailable information concerning the bulk morphology of these bulk heterojunctions. Additionally techniques uti‐ lizing specific atomic or group specific contrast such as secondary ion mass spectroscopy (SIMS) have added further information to the growing wealth of morphological informa‐ tion about BHJs. One clear advantage on the use of neutrons lies in the fact that for in‐ stance 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 addition‐ al 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 het‐ erojunction 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.
