Nanostructure of self-assembled rod-coil block copolymer films for photovoltaic applications
Introduction
In organic photovoltaic devices, the nanoscale morphology of the light harvesting thin film is of fundamental importance. This is particularly true for bulk heterojunction solar cells, where the interface area between electron–donor and electron–acceptor interpenetrating domains plays a key role in the generation of free carriers. High photocurrents are reached only if the domain size is, in at least one dimension, comparable to the exciton diffusion length, and if the intradomain molecular arrangement induces high free carrier mobilities [1]. The bulk heterojunctions most investigated at present are based on blends of donor and acceptor molecules, such as poly(p-phenylenevinylene) (PPV) and fullerene derivatives. In these devices, the active layer is commonly in a non-equilibrium state and has a morphology that depends strongly on the deposition process. Moreover, during low temperature annealing, separation of donor and acceptor elements may occur, leading to larger domain sizes and correspondingly lower photovoltaic efficiencies [2], [3].
A better control over the morphology and a higher thermal stability can in principle be reached if diblock copolymers composed of two covalently connected polymers (or blocks) with appropriate electrochemical properties are used instead of blends [2], [3], [4]. If the two blocks possess sufficient repulsive energy, they will microphase-separate, producing microdomains rich in one block or the other. The resulting morphology is largely determined by the relative block lengths and may consist of spheres, cylinders or lamellae, with domain cross-sections of the order of the block length [5]. In the case of lamellae or cylinders with a dominant vertical orientation, the morphology is close to that of an ideal bulk heterojunction solar cell. In addition, since microphase separation is preserved under equilibrium conditions, the diblock copolymer films have an improved thermal stability in comparison to blends.
Self-assembled photovoltaic devices making use of rod–coil diblock copolymers consisting of a PPV based semiconducting polymer as the rigid block and a polystyrene chain functionalized with fullerene moieties as the flexible block have been previously studied [4], [6]. Strong luminescence quenching and higher photocurrents than in blends of both blocks were obtained and attributed to an enhanced donor–acceptor interface with efficient charge transfer statistics. Despite this encouraging data, the energy conversion efficiencies reported so far for block copolymer devices remain below the typical values obtained for blends [6], [7]. Absence of a bi-continuous donor–acceptor network and correspondingly low free carrier mobilities is one of the possible reasons that may cause limited performance. In previously reported devices [4], [6], the diblock copolymer layers were indeed obtained by spin-coating without further high temperature treatment, leading presumably to an inappropriate non-equilibrium disordered morphology.
To improve the photovoltaic conversion efficiency of diblock copolymer cells, the microphase separation kinetics during film deposition and subsequent heat treatment needs to be investigated. This is particularly true for rod–coil diblock copolymers, where the equilibrium phase diagram, which describes supramolecular ordering versus molecular structure, is still poorly understood [8], [9]. In addition, special emphasis needs to be placed on the relationship between charge transport and molecular ordering. In this article, we report atomic force microscopy and transmission electron microscopy investigations of the supramolecular structure of a series of rod–coil diblock copolymers in spin-coated films as a function of thermal annealing. Coil blocks with two different glass transition temperatures and lengths are used and allow us to investigate the influence of coil flexibility and block length ratio on the self-assembling process. Homopolymer thin films of the rigid conjugated block were also considered for comparison. Using this approach, we aim to obtain a better insight into the experimental conditions necessary to form a supramolecular structure that is appropriate for photovoltaic devices.
Section snippets
Experimental details
Molecular structure and thermal properties of the materials used for the present study are summarized in Table 1. For the conjugated rigid block we used a 2,5-di(2′-ethyl)hexyloxy poly(p-phenylenevinylene) (DEH-PPV) of low molecular weight (Mn = 4000 g/Mol). DEH-PPV is a hairy mesogenic PPV derivative with low polydispersity (< 1.2) that exhibits a liquid crystal behaviour between 65 and 200 °C [10]. It has a molecular geometry that should allow strong π stacking of the conjugated backbone in the
Results and discussion
The AFM phase images obtained on films after deposition are shown in Fig. 1. In the DEH-PPV homopolymer film (Fig. 1a), nanorod-like features are observed, which correspond likely to one-dimensional fibrillar structures or vertically oriented lamellae. The cross-section of these structures (approximately 10 nm) is close to the length of the DEH-PPV blocks (calculated value of 7.5 nm). Other, somewhat larger elongated structures are also present and could correspond to differently oriented
Conclusion
In conclusion, the thin film morphology of rod–coil block-copolymers, consisting of a DEH-PPV rod segment and either a styrene (ST) based or a butyl-acrylate (BA) based flexible coil block, has been shown to depend strongly on the molecular structure of the coil block. Only BA based polymers exhibit a significant molecular ordering already after deposition, and experience the formation of large nano-structured domains during annealing. We attribute this behaviour to the low glass transition
Acknowledgments
Financial support by the FAMOUS (Fullerene-Based Advanced Materials for Optoelectronic Utilizations) European network, by the Agence de l'Environnement et de la Maîtrise de l'Energie (ADEME) and by the University Louis Pasteur are gratefully acknowledged.
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