Elsevier

Thin Solid Films

Volumes 511–512, 26 July 2006, Pages 483-485
Thin Solid Films

Absorption and crystallinity of poly(3-hexylthiophene)/fullerene blends in dependence on annealing temperature

https://doi.org/10.1016/j.tsf.2005.12.064Get rights and content

Abstract

Composite films of P3HT/PCBM (poly[3-hexylthiophene-2,5-diyl]/[6,6]-phenyl C61 butyric acid methyl ester) are widely used as active layer in plastic solar cells. It was shown that the annealing of the active layer leads to increased optical absorption and therefore to improved performance of solar cells. However the origin of the improved optical absorption is not clear yet. In this work, structural and optical properties of such films were studied by X-ray diffraction and spectroscopic ellipsometry, respectively. The influence of the annealing temperature on film crystallinity and on optical absorption was studied. A direct correlation between optical absorption and film crystallinity was demonstrated, i.e. increased crystallinity leads to an increased optical absorption. The highest crystallinity and the highest optical absorption were observed for films annealed at 125 °C.

Introduction

Bulk heterojunction polymer solar cells based on P3HT/PCBM composites were intensively studied during last years because of their relatively high power conversion efficiencies [1], [2], [3]. The active layer of these devices consists of a combination of P3HT as donor and PCBM as acceptor. It was shown that the optical properties of P3HT/PCBM composites are strongly affected by the annealing step [4], [5], [6]. For example a significant increase in optical absorption of P3HT/PCBM thin films upon annealing at 130 °C up to a factor of 3 at λ = 500 nm was reported [5]. At the same time, the solar cell efficiency is improved from 0.4% to 2.5% upon annealing [1]. Since the increase in optical absorption was not sufficient to explain this unproportional increase of the solar cell efficiency, [4] it was explained by simultaneous increase of charge carrier mobilities as well. It was presumed that this is due to enhanced crystallisation of the P3HT during the annealing [1]. In fact, it is known that polythiophenes tend to form crystalline domains in pristine films [7], and Yang et al. showed by electron diffraction that P3HT is also capable of a crystalline organisation in blends with methanofullerenes, recently [8].

In this work, the structural changes of P3HT/PCBM blends upon annealing and their impact on the optical absorption of P3HT/PCBM thin films were investigated. Furthermore the influence of annealing temperature on the crystallinity of P3HT/PCBM blends was studied.

Section snippets

Experimental details

The samples studied in this work consist of thin P3HT/PCBM films on Si substrates. The film thickness was (65 ± 2) nm for all investigated films. The concentration of PCBM in blend was 67 wt.%. The films were prepared by spin coating from chlorobenzene solution. Some of the films were annealed at 100, 125 and 150 °C in air for 5 min.

The structural properties of the films were studied by X-ray diffraction in grazing incidence geometry as described in [9]. Their optical properties were studied by

Results and discussion

The diffractograms of the untreated and the annealed P3HT/PCBM films (at 100, 125 and 150 °C) are shown in Fig. 1a. The samples show a single diffraction peak at 2θ = 5.4°. The corresponding lattice constant d = (1.64 ± 0.03) nm can be calculated using Bragg's law. The detected peak originates from the P3HT crystallites with a-axis orientation (polymer backbone parallel and side chains perpendicular to the substrate [9], as shown in Fig. 2). The diffraction peaks corresponding to P3HT crystallites

Conclusions

The influence of the annealing on film crystallinity and on optical absorption of thin P3HT/PCBM films was studied. It was found that the film crystallinity is increased upon annealing. The increase of the film crystallinity results in an improvement of the optical absorption of the films in the low photon energy region and in a red shift of the absorption spectrum. A direct correlation between optical absorption and film crystallinity was demonstrated, i.e. increased crystallinity leads to an

Acknowledgement

Financial support by Thuringian Ministry of Culture (“NANORG1” — contact number 20101276) is gratefully acknowledged. We also thank P. Schilinsky, C. Waldauf and C. Brabec for helpful discussions.

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